DOI: 10.1002/chem.201402448
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& Molecular Switches
A Multifunctional Photoswitch: 6p Electrocyclization versus ESIPT and Metalation Juliette Gurin,[a] Anne Laustic,[a] Stphanie Delbaere,[b] Jrme Berthet,[b] Rgis Guillot,[a] Cyril Ruckebusch,[c] Rmi Mtivier,[d] Keitaro Nakatani,[d] Maylis Orio,*[c] Michel Sliwa,*[c] and Pei Yu*[a]
Abstract: A terthiazole-based molecular switch associating 6p electrocyclization, excited state intramolecular proton transfer (ESIPT), and strong metal binding capability was prepared. The photochemical and photophysical properties of this molecule and of the corresponding nickel and copper complexes were thoroughly investigated by steady-state and ultrafast absorption spectroscopy and rationalized by
Introduction Molecules capable of switching between two molecular states under an external stimulus have been drawing widespread interest as they can act as switching elements to regulate various properties and functions.[1] Of the different kinds of molecular switches investigated so far, photochromic diarylethenes are of particular interest because not only they can undergo fast, photo-resistant 6p electrocyclization and cycloreversion between (at least) two thermally stable isomeric states upon light excitation of appropriate wavelengths,[2] but such a photochromic reaction is also tolerant toward various structural patterns.[3] Various multifunctional molecular switches have thus been designed either by using appropriately functionalized
DFT/TDDFT calculations. The switch behaves as a biphotochrome with time-dependent photochemical outcome and displays efficient ESIPT-based fluorescence photoswitching. Both photochemical reactions are suppressed by nickel or copper metalation, and the main factors contributing to the quenching of the electrocyclization are discussed.
ethenic linkers in place of classical ones (perfluorocyclopentene, cyclopentene, maleic anhydride, or maleimide)[4–9] or by associating organic diarylethenes with metal ions.[10–17] The latter approach is very attractive as it provides ready access to new properties and functions with variously altered photochemical reactivities. We report herein on a novel multifunctional switch (1 a, Scheme 1), derived from photochromic terarylenes developed in recent years because of their good photochromic features.[18] The appeal of such a molecular switch is twofold. First, owing to the structural similarity of 4,4’-bis(2-o-hydroxyphenylthiazole) moiety of 1 a to salen (N,N’-bis(salicylidene)ethylenediamine) or salophen (N,N’-bis(salicylidene)-1,2-phenylenedi-
[a] Dr. J. Gurin, Dr. A. Laustic, Dr. R. Guillot, Dr. P. Yu Institut de Chimie Molculaire et des Matriaux d’Orsay UMR 8182, Universit Paris-Sud, Bt. 420 91405 Orsay Cedex (France) Fax: (+ 33) 169-15-4754 E-mail: [email protected] [b] Prof. S. Delbaere, Dr. J. Berthet Universit Lille Nord de France, UDSL, CNRS UMR 8516 3, rue du Professeur Laguesse, 59006 Lille cedex (France) [c] Dr. C. Ruckebusch, Dr. M. Orio, Dr. M. Sliwa Laboratoire de Spectrochimie Infrarouge et Raman (LASIR) CNRS UMR 8516/Universit Lille Nord de France Universit Lille1 - Sciences et Technologies/Chemistry Department bt C5/59655 Villeneuve d’Ascq Cedex (France) Fax: (+ 33) 320-43-6755 E-mail: [email protected] [email protected] [d] Dr. R. Mtivier, Prof. K. Nakatani PPSM, ENS Cachan, UMR 8531, 61 av. Prsident Wilson 94235 Cachan cedex (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402448. Chem. Eur. J. 2014, 20, 12279 – 12288
Scheme 1. A novel multifunctional switch combining 6p electrocyclization, excited state intramolecular proton transfer (ESIPT), and a metal coordination site (M = Ni2 + or Cu2 + ).
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Full Paper amine), ligands widely used in coordination chemistry, 1 a is likely to be an excellent ligand for a wide range of metal ions, thereby permitting investigations of potential synergistic effects between the photochromism of the ligand and metal-centered properties. Secondly, 1 a on its own is interesting as its 4,4’-bis(2-hydroxyphenylthiazole) moiety is known to display excitedstate intramolecular proton transfer (ESIPT) and act as a ratiometric fluorescent probe for metal ions.[19] ESIPT is an important and ultrafast photochemical process in solution[20] and solid state,[21] and has been extensively investigated for its potential applications in various fields.[22–27] Scarcely investigated, association of the two photochemical processes, ESIPT and 6p electrocyclization of diarylethenes, within the same moleScheme 2. Synthetic route to 1 a and 2 a. NBS = N-bromosuccinimide. cule has yet led to appealing features such as chemically gated photochromism in few dithienylmaleimide derivatives[28] The key intermediate 3 was easily obtained by reacting 2and also the design of a dithienylmaleimide-based probe for methoxy-thiobenzamide and 1,4-dibromopentane-2,3-dione. a naked-eye detection of volatile neurotoxic organophosThe latter was generated by slowly adding two equivalents of phates.[29] A better understanding of the interplay between the bromine into pure 2,3-pentanedione and was used without further purification. After bromination and Suzuki cross-coutwo important photochemical processes is of great interest pling with 2-phenyl-4-B(pin)-5-Me-thiazole,[18b] terthiazole 2 a not only from an academic viewpoint, but also for designing novel photoswitchable systems. 1 a is an excellent candidate was obtained in fairly good yield. Deprotection of methoxy for such studies because two ESIPT processes are possible, begroups of 2 a provided 1 a. All of the new organic compounds tween the hydrogen atom of each phenolic hydroxy group were characterized by 1H NMR and 13C NMR spectroscopy and and the nitrogen atom of the neighboring thiazolyl group, and HRMS. Note that the assignment of the two different OH the two potential ESIPT sites are closely conjugated with the groups in 1 a is not trivial, but can be carried out by using 6p electrocyclization backbone, which would favor strong inHMBC (heteronuclear multi-bond correlation) spectroscopy teractions between the two photochemical processes. Herein (see the Supporting Information). we report the synthesis, photochemical, and photophysical 1 a, 1 b, and 2 a were also characterized by single-crystal properties of 1 a as well as those of two of its metal comstructure analyses (see the Supporting Information). The moplexes. The dramatic impact of ESIPT process and metalation lecular structures of 1 a and 2 a are shown in Figure 1. on the photochemical reactivity of the 6p electrocyclization of None of the compounds was found to display 6p electrocycthe backbone is observed and rationalized by DFT calculations, lization-based photochromism in the crystalline state because, ultrafast spectroscopy, and comparison with 2 a, a close derivafor 1 a the two side thiazole groups are arranged in a photoitive of 1 a in which any ESIPT process is absent. nactive parallel conformation with respect to the central thiazole ring, while for 2 a, despite the potential photoactive antiparallel conformation, the CC distance between the two reactive carbon atoms (5.32 ) is much larger than the known upper limit (4.2 ) to allow such a cyclization.[30] Note also that Results and Discussion 1 a adopts the expected cis-enol geometry, with a hydrogen Syntheses and characterizations of the multifunctional bond between each hydroxy group and the nitrogen atom of switch the neighboring thiazole ring. Selected X-ray data together with those derived from DFTStarting from commercially available 2-methoxybenzonitrile, optimized structures are listed in Table 1 (see the Supporting 1 a was prepared according to Scheme 2. Chem. Eur. J. 2014, 20, 12279 – 12288
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Figure 1. ORTEP plots of molecular structures of 1 a and 2 a (ellipsoids set at 50 % probability; H atoms shown as small spheres of arbitrary radii).
Table 1. Selected experimental and calculated structural data of 1 and 2 and the calculated energy differences between their different forms and conformations.
1 a exptl 1 b exptl 2 a exptl 1 a (p) calcd 1 a (ap) calcd 1 b calcd 2 a (p) calcd 2 a (ap) calcd 2 b calcd
z1 [8]
z2 [8]
d(CC) []
z(NNOO) [8]
Erel [kcal mol1]
49 7 95 -49 47 7 -52 48 7
141 7 153 134 50 7 133 54 7
4.84 1.52 5.32 4.44 3.70 1.54 4.41 3.71 1.54
25.8 3.8 9.7 27.3 27.5 0.4 4.4 21.3 30.9
– – – 0.06 0 18.32 0.13 0 20.47
Figure 2. Spectral changes of a) 1 a and b) 2 a in MeCN at room temperature: open form a (c) and closed form b (a). Computed spectra of c) 1 and d) 2: parallel (p) and antiparallel (ap) open conformers and closed form (b).
Information for details). A pretty good agreement is found between the experimental data of 1 a, 1 b, and the computed data (1 a (p), 1 b), while the structure of 2 a, which is probably due to solid-state interactions, is rather close to that of computed 2 a (ap*), the second most stable antiparallel conformation (Supporting Information, Table S3). Such a general agreement validates the present theoretical calculations. It is also of interest to point out that the computed energy difference between the photoinactive parallel (p) and photoactive antiparallel (ap) conformations is very small for both molecules, suggesting the co-existence of photoreactive conformations in solution for both compounds. Photochromic and fluorescence properties of 1 a and 2 a Photochromic behaviors of 1 a and 2 a were investigated in two solvents with different polarities (MeCN and toluene) as the photodynamics of ESIPT process is solvent-sensitive.[31–33] The spectral changes of 1 a and 2 a upon alternate UV and visible-light irradiation in MeCN are shown in Figure 2 a, b and compared with TDDFT calculated spectra (Figure 2 c, d). The solution of 1 a (2 a) is colorless and is characterized by an intense absorption band at 332 nm (324 nm). Upon UV irradiation at 335 nm, the colorless solution of 1 a (2 a) turns blue, indicating the formation of the closed form 1 b (2 b) characterized by a broad absorption band around 600 nm (624 nm). The Chem. Eur. J. 2014, 20, 12279 – 12288
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blue colored closed form 1 b (2 b) is stable in the dark at room temperature in a deoxygenated atmosphere, but can be brought back to the open form 1 a (2 a) by visible light irradiation (575 nm), with the restoration of the initial spectrum of 1 a (2 a). Actually, studies of the thermal bleaching rate of 1 b and 2 b at various temperatures (Supporting Information, Figure S4) led to a half-life of about 5 years and an associated activation energy (Ea) of 136 kJ mol1 for 2 b, and a few hundred years of half-life and an activation energy of 153 kJ mol1 for 1 b, respectively. These photochromic features are qualitatively quite similar to those reported for the terphenylthiazole (112 kJ mol1 without methoxy or hydroxy substituent).[18b] The main photochromic data of 1 and 2 are gathered in Table 2 together with the relevant data obtained from TDDFT calculations. Satisfactory agreement is found between the experimental and computed data in terms of band energy, shape, and relative intensity (Figure 2; see the Supporting Information for details). Focusing on the antiparallel forms (the most stable forms) of both 1 a and 2 a, TDDFT calculations predict main electronic absorptions at 354 and 342 nm, which match well with the experimental values of 332 and 324 nm, respectively. These two electronic transitions are of the same nature (charge transfer, CT) and same origin (HOMO!LUMO + 1). However, the electronic density distribution of the orbitals involved (Figure 3) is quite different due to the relative orientation of the OH/OMe groups in their respective open forms (Figure 1). For 1 a (ap), the donor orbital is well delocalized while the acceptor orbital is mainly localized on the lateral hydroxyphenylthiazolyl group. Concerning 2 a (ap), the donor orbital is mainly centered on the hexatriene moiety while the acceptor orbital is delocalized onto the lateral phenylthiazolyl- and the central methoxyphenyl-substituted thiazolyl groups. Finally, in line with their ob-
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Full Paper to the conjunction of this decreased cyclization efficiency and lmax [nm] lmax calcd [nm] F aa!b t trans-keto [ms] a more important 1 b to 1 a back-reaction, favored by a com1a 332 354 (ap) 0.04[b](Tol.) 0.76(Tol.) paratively much higher absorp356 (p) 0.01[b](MeCN) 0.41(MeCN) 1a 450 453 0.02[d](Tol.) 408, 86(Tol.) tion coefficient of 1 b than that trans-keto 0.06[d](MeCN) 270, 69(MeCN) of 2 b at the irradiation wave[c] 1b 585 576 0.01 (Tol.) length (335 nm; Figure 2). The 626 0.01[c](MeCN) ratio of photoactive conformer 2a 324 342 (ap) 0.15[b](Tol.) 0.91(Tol.) 346 (p) 0.11[b](MeCN) 0.94(MeCN) (ap) to photoinactive one (p) in 2b 624 606 0.01[c](Tol.) solution is known to greatly [c] 0.01 (MeCN) impact the cyclization quantum [a] aa!b : 6p electrocyclization conversion ratio; ttransketo : lifetime of ESIPT photoproduct. [b] Cyclization quanyield.[2] In the present case, the tum yield. [c] Ring opening quantum yield. [d] 1 a trans-keto formation quantum yield. 1 solution H NMR spectrum shows only one set of methyl signals at room temperature for both 1 a and 2 a, indicating either the presence of only one conformation (ap) or fast interconversion between the two conformations at room temperature. Given the negligible computed energy difference between the two conformers (Table 1), the latter, with ap and p conformations equally populated, is more likely. As shown below, the presence of competing ESIPT process in 1 a is the main reason for its much decreased cyclization quantum yield compared to 2 a. Nanosecond pump–probe absorption measurements were also carried out for 1 a as short-lived photochromic species are usually generated in ESIPT chromophores.[34–36] Indeed, it is well-established that ESIPT occurs within a few tens of femtoseconds from the Frank–Condon excited enol state (enol*) to produce the cis-keto excited state (cis-keto*). The latter may decay to the corresponding cis-keto ground state by fluorescence (see below) followed by an ultrafast reverse proton transfer to the initial enol ground state (enol). Besides the radiative decay, another deactivation channel for the cis-keto* is the isomerization to the trans-keto form, which relaxes back to enol in a few hundreds of microseconds owing to the existence of an energy barrier. After nanosecond excitation at 330 nm, the transient absorption spectra of 1 a recorded at different time delays and time decays are shown in Figure 4 a, b, respectively. Results obtained for similar experiments performed for 2 a are provided in Figure 4 c, d. Figure 3. Orbitals involved in the lowest-energy transition calculated by For 1 a, roughly three different kinds of signals can be identiTDDFT for antiparallel 1 a (a) and antiparallel 2 a (b). fied: one negative band at 340 nm and two positive bands centered around 450 and 610 nm, respectively. The negative absorption band is due to the ground state bleaching of 1 a in served solution photochromic reactivity, the acceptor and its enol form, and the broad positive absorption band, which donor orbitals of both molecules (Figure 3) feature contribudoes not vary with time delays, corresponds to the formation tions from the two reactive carbon atoms and they also presof the closed form 1 b. Based on literature data[34–36] and ent the adapted symmetry for the conrotatory cyclization to form the single CC bond. TDDFT calculations (Table 2; see the Supporting Information As it can be seen from Table 2, the most striking difference for details), the transient positive absorption band peaking at between the two molecules concerns their respective electro450 nm is assigned to the trans-keto form resulting from the cyclization efficiency. Indeed, the cyclization quantum yield of ultrafast cis–trans isomerization of cis-keto* of one of the two 1 a is found to be, regardless of the solvent, considerably ESIPT sites. The decay rate of trans-keto is identical to the reposmaller than that of 2 a, while the cycloreversion quantum pulation of the ground enol state (Figure 4 b). However, two yield remains in the same range for the two compounds exponential time constants (Table 2) in acetonitrile (toluene), (Table 2). The lower 1 a to 1 b conversion ratio (a1a!1b) is due 69 ms (86 ms) and 270 ms (408 ms) with similar contributions, Table 2. Main photochromic data of 1 and 2 at room temperature.[a]
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Figure 4. a) Absorption spectra of 1 a and 1 b, and nanosecond transient absorption spectra at different time delays for 1 a in acetonitrile after 330 nm excitation. b) Different decays for 1 a at different wavelengths. c) Nanosecond transient absorption spectra at different time delays for 2 a in acetonitrile after 355 nm excitation. d) Normalized decays at 460 nm for 2 a under N2 (dashed line) or O2 saturated atmosphere.
are needed to fit the experimental data, and these two time constants could be assigned to different trans-keto forms, coming from antiparallel and parallel open forms or a twistedketo form.[34] The re-population of enol state is not complete because of the existence of the stable closed form 1 b. It is worth noting that the amount of the cyclized 1 b and that of the short-lived trans-keto form are directly proportional to the corresponding absorbance changes at 340 nm. Using the yield formation of 1 b it is possible to determine the formation yield of trans-keto (Table 2). Of the two ESIPT sites present in 1 a, it is difficult to determine which one contributes the most to the observed transient absorption band. Nevertheless, the ESIPT process involving the central phenol group may be favored over the terminal one because of its stronger acidity, revealed by a more downfield-shifted 1H NMR signal (see the Supporting Information). This can be qualitatively explained by the electro-attracting effect of the phenylthiazole group as compared with the methyl group. This stronger acidity is expected to be preserved in the excited state, therefore making the proton transfer easier than that of the terminal site.
Concerning 2 a, as expected, no trans-keto absorption band is observed. Instead, a triplet-state contribution is clearly seen (Figure 4 c), with a lifetime of about 268 ns, which is shortened in dioxygen-saturated solution (Figure 4 d). This is in agreement with the literature data, that is, the photoinactive parallel conformer is deactivated by fluorescence and intersystem crossing.[37–39] The triplet state is not observed for 1 a because ESIPT is the main deactivation process of the enol* state for both its parallel and antiparallel forms. Another major difference between 1 a and 2 a is their fluorescence properties in solution, which are summarized in Table 3. Compound 2 a exhibits only weak fluorescence (Ff = 0.01) in the solvents considered, with an emission maximum around 415 nm. Emission decays were measured in acetonitrile and toluene with the laser excitation set at 330 nm. The decays are well-fitted using a double-exponential model with a main contribution with a characteristic time of about 90 ps, irrespective of solvent polarity. In agreement with literature data,[37–39] such a weak fluorescence is ascribed to the photoinactive parallel conformer of 2 a. The minor component is found to strongly depend on solvent polarity. This could tentatively be assigned to a charge-transfer excited state of a non-reactive conformer (parallel open form). However, such assumption should be considered cautiously and will not be discussed further. In contrast, a large Stokes-shifted fluorescence signal is observed for 1 a. This is the signature of an ESIPT process which is here characterized by a quantum yield (Ff) varying from 0.07 in toluene (slightly polar solvent) to less than 0.01 in acetonitrile (polar solvent). The strong solvent dependence of the fluorescence of 1 a is also consistent with emission signals observed for ESIPT: in apolar solvents the emitting cis-keto* prevails, whereas in polar solvents cis–trans isomerization to trans-keto, as well as the existence of some geometrical isomers, such as twisted enol*, are favored. The latter form is characterized by a residual fluorescence around 420 nm in MeCN.[33] Decay traces obtained for 1 a were fitted with a double-exponential function. The major time decay is solvent-dependent and can be clearly ascribed to the cis-keto* emission. On the contrary, the minor contribution is only slightly affected by solvent polarity and could thus be tentatively attributed to the emission of a relaxed enol* form or a twisted cis-keto* form with a lower radiative rate constant.[40] Finally, the 6p electrocyclization of 1 a to 1 b in toluene leads to an important photo-modulation of the fluorescence (ca. 80 %), as shown in Figure 5. This decrease in the fluores-
Table 3. Fluorescence properties of 1 a and 2 a excited at 335 nm.[a]
lem (Dn/cm1)
1a Ff
toluene (er = 2.38)
524 nm (10410)
0.07
MeCN (er = 37.5)
523 nm (11000)
0.01
t [ps] (a)
lem (Dn/cm1)
2a Ff
885 (0.87) 214 (0.13) 76 (0.94) 262 (0.06)
415 nm (6710)
0.01
415 (6710)
0.01
t [ps] (a) 94 (0.98) 900 (0.02) 89 (0.79) 191 (0.21)
[a] t is the emission decay time of the excited species and a represents the pre-exponential factor.
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Figure 5. Fluorescence spectra of 1 a in toluene at room temperature and its evolution upon UV irradiation (lex = 350 nm).
cence is in good agreement with the 0.76 conversion ratio observed in the steady state absorption experiment considering the same solvent (Table 2).
Ultrafast photodynamics for 1 a and 1 b To gain more insight into the dynamics of the two photochemical processes involved, femtosecond transient absorption spectra were recorded for 1 a and 2 a in acetonitrile after 325 nm excitation. Figure 6 shows the transient absorption
Figure 6. Transient absorption spectra of 1 a (right) and 2 a (left) in acetonitrile obtained for different time delays after 325 nm excitation. Inset: normalized spectra obtained by nanosecond transient absorption (1 ms after excitation) and femtosecond transient absorption (800 ps after excitation). Chem. Eur. J. 2014, 20, 12279 – 12288
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spectra for 1 a and 2 a for time delays in the range of 0 to 400 ps. For 2 a (Figure 6 left), two main bands grow with the excitation pulse (instrumental response function about 200 fs): a broad absorption band covering the 340–700 nm domain and a negative band in the 320–340 domain. The negative band is assigned undoubtedly to the ground state depopulation. The broad absorption band has four maxima: 375, 455, 535, and 620 nm. At 400 ps, only 375 and 620 nm absorption bands remain and the spectrum is similar to the stationary one of the closed form 2 b. Therefore these two bands can be assigned to the absorption of closed form 2 b. This is typical of diarylethenes, with a ring-closing reaction through a conical intersection within a few hundreds of femtoseconds.[38] After 0.6 ps, the 455 nm band decays in 1.5 ps (characteristic time constant of 1.36 0.1 ps) while 375 and 620 nm bands increase concomitantly with the appearance of two isosbestic points at 400 and 560 nm. During this process the ground-state bleach does not change. This process is thus assigned to the formation of closed form 2 b, occurring through an intermediate absorbing at 455 nm. Finally the 535 nm band disappears in 100 ps (characteristic time constant of 111 5 ps), whereas the ground state depopulation decays simultaneously to a stationary value. This time constant is similar to the emission time decay measured by the single photon counting method. The 535 nm band is thus assigned to the S1 state of the parallel conformer of 2 a, which, being not active in ring-closing reaction, relaxes partly by fluorescence and intersystem crossing in about a hundred picoseconds. Contrary to classical report on normal diarylethenes, a second pathway is found for the formation of the closed form through an intermediate which has a lifetime of 1.5 ps. However, it was already mentioned for different diarylethenes that excitation of a higher excited state can lead to the closed form not only via the classical mechanism, that is, ultrafast internal conversion to S1 state and then to the closed form through a conical intersection in a few hundreds of femtoseconds, but also via the formation of a different S1 state, which has a geometry similar to the closed form and relaxes in a few picoseconds.[41] For 1 a, two main bands also grow within the excitation pulse, the depopulation band and a broad positive band with different maxima in comparison to 2 a: 395 nm, 455 nm (shoulder), 610 nm (broad tail). After 0.6 ps, 395 and 455 nm bands are found to grow (time constant of 1.8 0.1 ps) together with the appearance of a negative broad band above 500 nm and no change of depopulation band. The final spectrum at 10 ps shows the ground state depopulation band, one absorption band with two maxima (395 and 455 nm) with a higher contribution for the shoulder at 455 nm and a negative band from 500 nm to 700 nm. This spectrum is characteristic of cis-keto* fluorescent state and this step can be assigned to a relaxation from hot cis-keto* state formed just after excitation within 200 fs.[35–36, 42] Finally the entire spectra decay in 84 5 ps and the transient spectrum remains then stable and similar to the one obtained by nanosecond transient absorption experiments, with trans-keto and 1 b as major and minor
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Full Paper photoproducts, respectively. This time constant is equal to the fluorescence lifetime measured by the single photon counting method (Table 3). The relaxation of cis-keto* is mainly a non-radiative process, as only a proportion of about 30 % goes to the trans-keto form (evaluated by the recovery of the depopulation band). As the ground-state depopulation and experimental conditions are the same for 1 a and 2 a, the absorption band at 610 nm indicates that the formation of 1 b is about ten times less efficient than that of 2 b, and this is in good agreement with their respective formation quantum yields (Table 2). Therefore, despite the fact that 1 a shares the same hexatriene backbone with 2 a, the ultrafast ESIPT process, which occurs within 50 fs with almost no geometrical changes, prevails on the comparatively slow backbone 6p electrocyclization (a few hundred fs). The overall photodynamic schemes that can be drawn for 1 a and 2 a are given in Scheme 3.
Figure 8. Experimental (c) and computed (a) absorption spectra of a) Ni-(1a-2 H), b) Cu-(1a-2 H), c) Ni-(1b-2 H), and d) Cu-(1b-2 H) in CHCl3 at room temperature.
Scheme 3. Overall main photoinduced processes for 1 a and 2 a.
Metal complexes of 1 a and metal-gated photochromism First trials of metalation have been carried out with first-row transition-metal ions, such as nickel and copper. Metalation of 1 a with one equivalent of Ni(OAc)2·4H2O or Cu(OAc)2·2H2O in refluxing MeOH gave Ni-(1 a-2 H) and Cu-(1 a-2 H), respectively. The crystal structures of the two metal complexes have been determined, and their molecular structures are shown in Figure 7. The two metal complexes are isostructural and characterized both by a square-planar coordination sphere with NiN and NiO distances of about 1.90 and CuN and CuO of about 1.84 . Note that, contrary to the always-paramagnetic Cu2 + complex, the Ni2 + complex is diamagnetic in such a squareplanar environment.
Figure 7. Molecular structure of Ni-(1a-2 H) (left) and Cu-(1a-2 H) (right) (ellipsoids set at 50 % probability; H atoms shown as small spheres of arbitrary radii). Chem. Eur. J. 2014, 20, 12279 – 12288
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The solution absorption spectra of the two metal complexes are also qualitatively quite similar, as shown in Figure 8. In comparison to 1 a, the absorption spectra of the two complexes are both characterized by: 1) a blue-shifted and intense ligand centered band around 300 nm; and 2) a rather intense charge-transfer band, peaking at 416 nm for Ni-(1 a-2 H) and 382 nm for Cu-(1 a-2 H). The latter also shows a very weak absorption band at about 600 nm, which is likely attributed to the copper d–d transition. When submitted to steady-state UV irradiation at either 405 nm or 313 nm, no significant spectral changes can be detected for both metal complexes. Despite the loss of photochromism, relatively high thermal stability of 1 b allowed to prepare Ni-(1 b-2 H) and Cu-(1 b-2 H). Monitoring of the UV/Vis spectral evolution upon addition of nickel (copper) acetate into a UV (365 nm) irradiated 1 a solution (Supporting Information, Figure S5) in deoxygenated environment and at room temperature showed that a new species was formed (Figure 8), with a new band at 450 nm and a slight bathochromic shift of its large band in the visible. These spectra were corroborated by TDDFT calculations and assigned to the metalated closed form (see also the Supporting Information, Figure S15 and Table S8). It is also of interest to note that 1 b interacts much more quickly with copper acetate than with nickel acetate as the formation of the band at 450 nm was over after a few minutes in the case of copper while that of nickel took more than 14 h. Similarly to the metalated open form, photochromic activity is lost as no detectable ring-opening reaction under visible irradiation (600 nm) can be observed. On the other hand, its demetalation, easily achieved by shaking a solution of M-(1 a-2 H) or M-(1 b-2 H) in dichloromethane with an aqueous HCl solution, allows its initial photochemical and photophysical properties to be recovered. Thus,
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Full Paper a metal-binding based gated photochromism is realized in such a system. To gain more insight into the inhibition of photochromic reactivity observed for metalated complexes, DFT calculations and ultrafast spectroscopy have been performed. In Table 4, some selected experimental data (structure, absorption) of the two metal complexes are reported together with those derived from DFT calculations (see the Supporting Information for details). A general good agreement is found
Table 4. Selected experimental and calculated data of the two metal complexes. z1 [8] z2 [8] d(CC) [] z(NNOO) [8] Erel [kcal mol1] Ni-(1 a-2 H) exptl Ni-(1 a-2 H) calcd Ni-(1 b-2 H) calcd Cu-(1 a-2 H) exptl Cu-(1 a-2 H) calcd Cu-(1 b-2 H) calcd
69 59 13.5 91 57 11.9
4 11 10.3 6 17 8.3
3.83 3.81 1.57 4.17 3.74 1.57
1.3 0.5 1.0 1.3 2.7 8.1
– 0 28.13 – 0 25.04
between the experimental and calculated data. From a structural viewpoint, it is reasonable to think that the conformation of both metal complexes in solution would be close to that of their respective solid structure. For Ni-(1 a-2 H) and Cu-(1 a2 H), this conformation appears to be neither antiparallel nor parallel because the two thiazole groups are tightly locked within the same plane by metal coordination. Furthermore, even though the distance between the two potentially photoreactive carbon atoms is short enough for the 6p electrocyclization (< 4.2 , Table 4), metalation would greatly hinder the rotation of one of the two terminal thiazole groups around the thiazole–thiazole single CC bond, which is necessary for the conrotatory ring-closing reaction. Finally, from the electronic considerations, TDDFT calculations predict that the low-energy band of both metal complexes is of the same origin (HOMO! LUMO + 1) and similar metal-to-ligand charge transfer (MLCT) nature (Table 5, Figure 9, and the Supporting Information for details). Except few cases where the MLCT band proved to be able to enhance the electrocyclization quantum yields through efficient excited-state intramolecular energy transfer,[43–46] the presence of such a band is generally not favorable to the 6p electrocyclization process as the metal ion is often responsible for additional deactivation channels of the excited state. It is particularly true in the present cases where the metal coordination site and the photoactive site are very close and strongly
Table 5. Selected experimental and calculated transitions observed in the metal complexes.
Ni-(1 a-2 H) Cu-(1 a-2 H) Ni-(1 b-2 H) Cu-(1 b-2 H)
Transition
lcalc. [nm] fosc
lexp. [nm] Assignment
HOMO!LUMO + 1 HOMO!LUMO + 1 HOMO!LUMO + 1 HOMO!LUMO + 1
416 396 452 443
416 382 460 456
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0.118 0.087 0.224 0.360
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MLCT MLCT MLCT MLCT
Figure 9. Molecular orbitals involved in the first electronic transition obtained by TDDFT calculations for a) Ni-(1a-2 H) and b) Cu-(1a-2 H).
coupled electronically. Experimentally, this is also supported by femtosecond transient absorption, recorded in dichloromethane for Ni-(1 a-2 H) (Cu-(1 a-2 H) is not sufficiently soluble). Indeed, after 400 nm (325 nm) femtosecond excitation, along with the ground-state depopulation band, only a weak and very broad band between 300 to 700 nm appears within 100 fs and then decays biexponentially in 2.5 and 34 ps (0.3 and 4.5 ps) respectively (see the Supporting Information). This transient band and biexponential decay, tentatively attributed to the non-radiative relaxation of a MLCT state combined with some cooling processes, deactivates the excited state which can lead to photochromic photoproduct and prevents the formation of any cyclized metal complex. Finally, it appears that the visible band for the metalated closed form has a strong MLCT contribution and will also inhibit in the same way the back photochromic reaction (Supporting Information, Figure S15).
Conclusion We have designed a terthiazole-based molecular switch that not only combines two important photochemical reactions, that is, 6p electrocyclization and ESIPT, but also offers the possibility of binding a wide range of metal ions. The photochemical and photophysical properties of this switch have been investigated in detail by steady-state and ultrafast spectroscopy, and rationalized by DFT calculations. It is found that the molecular switch behaves as a biphotochrome with a time-dependent photochemical outcome. The 6p electrocyclization-based
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Full Paper photochromic reaction of the switch is impeded to a large extent by the ultrafast ESIPT process. Conversely, the ESIPTbased fluorescence can be efficiently photo-switched by the 6p electrocyclization. By combining DFT calculations and ultrafast spectroscopy, we have also shown that the quenching of the 6p electrocyclization observed for the nickel and copper complexes can probably be accounted for by the conjunction of two factors: the lack of structural flexibility and MLCT-related ultrafast desactivation of the excited state. These findings will help to rationally design novel metal complexes with desired photoresponsive behaviors.
Experimental Section General 1 H NMR and 13C NMR spectra were obtained on either a Bruker AM 360 (360 MHz), DRX 300 (300 MHz), or DPX 250 (250 MHz) spectrometer and calibrated to the residual solvent peaks (CDCl3, 7.26 ppm and 77 ppm). The data are reported as chemical shift (d, ppm). UV/Vis steady-state absorption spectra were recorded on a Varian Cary 5000 spectrometer equipped with a temperature control unit. An Oriel Hg (Xe) 200 W lamp equipped with appropriate interference filters was used for sample irradiation. Solvents and reagents are used as received unless otherwise stated. Elemental analyses were performed by Service de Microanalyse, ICSN, 91198, Gif sur Yvette cedex, France.
(300 MHz, CDCl3): d = 8.52 (d, J = 7.9 Hz, 2 H), 8.04 (s, 1 H), 7.39 (m, 2 H), 7.09 (m, 4 H), 4.07 (s, 3 H), 4.04 (s, 3 H), 3.00 ppm (s, 3 H); elemental analysis calcd (%) for C21H18N2O2S2 : C 63.93, H 4.60, N 7.10; found: C 63.92, H 4.72, N 7.13. 1,4-Dibromopentane-2,3-dione: Under stirring, Br2 (5.1 mL, 100 mmol) was slowly added to 2,3-pentanedione (5.2 mL, 50 mmol) during 0.5 h. After 1 h of stirring, Ar was flushed into the mixture to remove HBr. The resulting brown oil was a mixture of several isomers but was used without further purification. 4-(5-Methyl-2-o-methoxyphenylthiazole-4-yl)-5-bromo-2-o-methoxyphenylthiazole (4): After complete dissolution of 3 (0.5 g, 1.3 mmol) in DMF (40 mL) at 80 8C, NBS (0.25 g, 1.4 mmol) was added and the solution was stirred at 80 8C overnight. The reaction was quenched with water and the solid was filtered and washed with water and dried under vacuum to afford 4 (0.575 g, 96 %) as an off-white solid. 1H NMR (250 MHz, CDCl3): d = 8.52 (d, J = 7.5 Hz, 1 H), 8.44 (d, J = 8.0 Hz, 1 H), 7.38 (m, 2 H), 7.06 (m, 4 H), 4.06 (s, 3 H), 4.04 (s, 3 H), 2.73 ppm (s, 3 H); elemental analysis calcd (%) for C21H18N2O2S2·0.2 H2O: C 52.88, H 3.68, N 5.87; found: C 52.91, H 3.70, N 5.95.
Synthesis and characterization
4-(5-Methyl-2-o-methoxyphenylthiazole-4-yl)-5-(5-methyl-2-phenylthiazole-4-yl)-2-o-methoxyphenylthiazole (2 a): 4 (0.45 g, 1 mmol), 2-pheny-4-Bpin-5-Me-thiazole[18b] (0.36 g, 1.2 mmol), CsF (0.38 g, 2.5 mmol), and [Pd(PPh3)4] (0.05 g, 0.04 mmol) were dissolved in dioxane (40 mL) under Ar, and the reaction mixture was heated to reflux for 6 h. After the addition of water (20 mL), the mixture was extracted with CHCl3 (3 20 mL) and the organic layers were washed with brine, dried over Na2SO4, and filtered. After evaporation of the solvent, the residue was purified by column chromatography (CH2Cl2). The resulting solid was dissolved in a minimum of CH2Cl2 and MeOH. Slow evaporation of CH2Cl2 afforded 2 a (0.423 g, 78 %) as an off-white solid. 1H NMR (300 MHz, CDCl3): d = 8.57 (d, J = 8.0 Hz, 1 H), 8.12 (d, J = 7.6 Hz, 1 H), 7.97 (m, 2 H), 7.42 (m, 5 H), 7.00 (m, 4 H), 4.07 (s, 3 H), 4.00 (s, 3 H), 2.58 (s, 3 H), 2.06 ppm (s, 3 H); 13C NMR (63 MHz, CDCl3): d = 164.0, 161.3, 157.9, 156.8, 156.3, 146.9, 144.8, 134.0, 133.2, 132.2, 130.8, 130.2, 129.8, 129.0, 128.7, 128.6, 126.6, 122.8, 121.2, 121.1, 111.6, 111.4, 55.9, 55.7, 12.6, 12.5 ppm; elemental analysis calcd (%) for C31H25N3O2S3·0.3 CH3OH: C 65.11, H 4.57, N 7.28; found: C 64.94, H 4.50, N 7.40.
2-Methoxythiobenzamide: A solution of P4S10 (8.5 g, 20 mmol) in EtOH (30 mL) was stirred for 1 h at 0 8C. 2-methoxybenzonitrile (2.4 mL, 20 mmol) was added and the resulting solution was heated to reflux overnight. The resulting mixture was stirred at room temperature with aqueous NaOH solution (1 m, 50 mL) and EtOAc (50 mL) for about 3 h. The aqueous layer was extracted with EtOAc (3 20 mL) and the combined organic layers were washed with brine (3 20 mL), dried over Na2SO4 and filtered. After evaporation of the solvent, the residue was purified by column chromatography (CH3Cl) to afford 2-methoxythiobenzamide (2.3 g, 70 %) as a pale yellow solid. 1H NMR (300 MHz, CDCl3): d = 9.06 (s, 1 H), 8.64 (d, J = 8.0 Hz, 1 H), 8.10 (s, 1 H), 7.47 (m, 1 H), 7.06 (m, 1 H), 6.96 (d, J = 8.3 Hz, 1 H), 3.97 ppm (s, 3 H); 13C NMR (63 MHz, CDCl3): d = 199.4, 156.3, 136.4, 133.7, 125.1, 121.3, 111.6, 56.2 ppm; elemental analysis calcd (%) for C8H9NOS: C 57.46, H 5.42, N 8.38; found: C 56.95, H 5.43, N 8.16. 4-(5-Methyl-2-o-methoxyphenylthiazole-4-yl)-2-o-methoxyphenylthiazole (3): 2-methoxythiobenzamide (0.85 g, 5 mmol) and 1,4dibromopentane-2,3-dione (0.65 g, 2.5 mmol) were heated to reflux in MeOH (30 mL) for 5 h. After cooling, the solid was filtered and washed with MeOH and dried under vacuum. The compound was obtained as an off-white solid (0.627 g, 63 %). 1H NMR
4-(5-Methyl-2-o-hydroxyphenylthiazole-4-yl)-5-(5-methyl-2-phenylthiazole-4-yl)-2-o-hydroxyphenylthiazole (1 a): 2 a (0.20 g, 0.4 mmol) was dissolved in dry CH2Cl2 (20 mL) under Ar, and then cooled at 78 8C. After slow addition of BBr3 (1 m, 3.5 mL, 3.5 mmol), the reaction mixture was allowed to slowly warm up to room temperature overnight under Ar. After addition of water (20 mL), the aqueous phase was neutralized with NaHCO3 and the mixture was stirred for 1 h. The aqueous layer was extracted with CH2Cl2 (3 20 mL) and the combined organic phase was washed with brine (3 20 mL), dried over Na2SO4, and filtered. After evaporation, the resulting solid was dissolved in a minimum of CH2Cl2 and MeOH. Slow evaporation of CH2Cl2 gave 1 a (0.073 g, 52 %) as an off-white solid. m.p. 198–200 8C; 1H NMR (360 MHz, CDCl3): d = 12.15 (s, 1 H), 11.30 (s, 1 H), 7.90 (m, 2 H), 7.68 (d, J = 7.9 Hz, 1 H), 7.51 (d, J = 8.4 Hz, 1 H), 7.38 (m, 4 H), 7.20 (m, 1 H), 7.09 (d, J = 8.3 Hz, 1 H), 6.92 (m, 3 H), 2.60 (s, 3 H), 2.15 ppm (s, 3 H); 13C NMR (63 MHz, CDCl3): d = 168.5, 165.7, 165.6, 157.4, 157.2, 144.2, 143.3, 142.4, 133.5, 133.0, 132.3, 131.7, 131.2, 130.2, 129.0, 128.1, 127.6, 127.2, 126.7, 126.6, 119.8, 119.2, 118.0, 116.8, 116.7, 12.8, 12.4 ppm; HRMS (ESI): m/z: 540.0846 [M + H] + , 562.0665 [M + Na] + ; elemental analysis calcd (%) for C29H21N3O2S3·0.4 CH3OH: C 63.91, H 4.12, N 7.61; found: C 63.90, H 4.19, N 7.72.
Quantum yield determination: The photochromic reaction was induced by a continuous wavelength irradiation Hg/Xe lamp (Hamamatsu, 200 W) equipped with narrow-band interference filters of appropriate wavelengths. The irradiation power was measured by a photodiode from Ophir (PD300-UV). The photochromic quantum yields were determined by probing the sample with a xenon lamp during the photochromic reaction. Absorption changes were monitored by a CCD camera mounted with a spectrometer (Princeton instruments). Kinetic profiles were analyzed by an Igor implemented home-made software.
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Full Paper Cu-(1 a-2 H): 1 a (30 mg, 0.06 mmol) and copper(II) acetate monohydrate (11 mg, 0.06 mmol) were stirred in refluxed MeOH (10 mL) under Ar for 6 h. After filtration, the resulting solid was dissolved in a minimum of CH2Cl2 and MeOH. Slow evaporation of CH2Cl2 gave Cu-(1 a-2 H) (23 mg, 64 %) as a crystalline green solid. MS (ESI): m/z: 623.07 [M + Na] + ; elemental analysis calcd (%) for C29H19CuN3O2S3·1 H2O: C 56.25, H 3.42, N 6.79; found: C 56.36, H 3.41, N 6.75. Ni-(1 a-2 H): This compound was prepared by the same procedure by using nickel(II) acetate tetrahydrate. Ni-(1 a-2 H) (20 mg, 60 %) was obtained as an crystalline orange solid. 1H NMR (360 MHz, CDCl3): d = 7.92 (m, 2 H), 7.47 (m, 3 H), 7.32 (dd, J = 1.72 Hz, J = 7.94 Hz, 1 H), 7.22–7.13 (m, 4 H), 6.60–6.54 (m, 2 H), 2.74 (s, 2 H), 1.88 ppm (s, 2 H); HRMS (ESI): m/z: 596.0058 [M + H] + , 617.9879 [M + Na] + ; elemental analysis calcd (%) for C29H19N3NiO2S3 : C 58.41, H 3.21, N 7.05; found: C 57.37, H 3.28, N 6.84. Single crystals suitable for structure determination of 1 a, 1 b, 2 a, Ni-(1 a-2 H), and Cu-(1 a-2 H) were obtained by slow evaporation of their respective CH2Cl2–MeOH solution.
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Received: March 3, 2014 Revised: April 9, 2014 Published online on August 5, 2014
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& Molecular Switches
A Multifunctional Photoswitch: 6p Electrocyclization versus ESIPT and Metalation Juliette Gurin,[a] Anne Laustic,[a] Stphanie Delbaere,[b] Jrme Berthet,[b] Rgis Guillot,[a] Cyril Ruckebusch,[c] Rmi Mtivier,[d] Keitaro Nakatani,[d] Maylis Orio,*[c] Michel Sliwa,*[c] and Pei Yu*[a]
Abstract: A terthiazole-based molecular switch associating 6p electrocyclization, excited state intramolecular proton transfer (ESIPT), and strong metal binding capability was prepared. The photochemical and photophysical properties of this molecule and of the corresponding nickel and copper complexes were thoroughly investigated by steady-state and ultrafast absorption spectroscopy and rationalized by
Introduction Molecules capable of switching between two molecular states under an external stimulus have been drawing widespread interest as they can act as switching elements to regulate various properties and functions.[1] Of the different kinds of molecular switches investigated so far, photochromic diarylethenes are of particular interest because not only they can undergo fast, photo-resistant 6p electrocyclization and cycloreversion between (at least) two thermally stable isomeric states upon light excitation of appropriate wavelengths,[2] but such a photochromic reaction is also tolerant toward various structural patterns.[3] Various multifunctional molecular switches have thus been designed either by using appropriately functionalized
DFT/TDDFT calculations. The switch behaves as a biphotochrome with time-dependent photochemical outcome and displays efficient ESIPT-based fluorescence photoswitching. Both photochemical reactions are suppressed by nickel or copper metalation, and the main factors contributing to the quenching of the electrocyclization are discussed.
ethenic linkers in place of classical ones (perfluorocyclopentene, cyclopentene, maleic anhydride, or maleimide)[4–9] or by associating organic diarylethenes with metal ions.[10–17] The latter approach is very attractive as it provides ready access to new properties and functions with variously altered photochemical reactivities. We report herein on a novel multifunctional switch (1 a, Scheme 1), derived from photochromic terarylenes developed in recent years because of their good photochromic features.[18] The appeal of such a molecular switch is twofold. First, owing to the structural similarity of 4,4’-bis(2-o-hydroxyphenylthiazole) moiety of 1 a to salen (N,N’-bis(salicylidene)ethylenediamine) or salophen (N,N’-bis(salicylidene)-1,2-phenylenedi-
[a] Dr. J. Gurin, Dr. A. Laustic, Dr. R. Guillot, Dr. P. Yu Institut de Chimie Molculaire et des Matriaux d’Orsay UMR 8182, Universit Paris-Sud, Bt. 420 91405 Orsay Cedex (France) Fax: (+ 33) 169-15-4754 E-mail: [email protected] [b] Prof. S. Delbaere, Dr. J. Berthet Universit Lille Nord de France, UDSL, CNRS UMR 8516 3, rue du Professeur Laguesse, 59006 Lille cedex (France) [c] Dr. C. Ruckebusch, Dr. M. Orio, Dr. M. Sliwa Laboratoire de Spectrochimie Infrarouge et Raman (LASIR) CNRS UMR 8516/Universit Lille Nord de France Universit Lille1 - Sciences et Technologies/Chemistry Department bt C5/59655 Villeneuve d’Ascq Cedex (France) Fax: (+ 33) 320-43-6755 E-mail: [email protected] [email protected] [d] Dr. R. Mtivier, Prof. K. Nakatani PPSM, ENS Cachan, UMR 8531, 61 av. Prsident Wilson 94235 Cachan cedex (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402448. Chem. Eur. J. 2014, 20, 12279 – 12288
Scheme 1. A novel multifunctional switch combining 6p electrocyclization, excited state intramolecular proton transfer (ESIPT), and a metal coordination site (M = Ni2 + or Cu2 + ).
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Full Paper amine), ligands widely used in coordination chemistry, 1 a is likely to be an excellent ligand for a wide range of metal ions, thereby permitting investigations of potential synergistic effects between the photochromism of the ligand and metal-centered properties. Secondly, 1 a on its own is interesting as its 4,4’-bis(2-hydroxyphenylthiazole) moiety is known to display excitedstate intramolecular proton transfer (ESIPT) and act as a ratiometric fluorescent probe for metal ions.[19] ESIPT is an important and ultrafast photochemical process in solution[20] and solid state,[21] and has been extensively investigated for its potential applications in various fields.[22–27] Scarcely investigated, association of the two photochemical processes, ESIPT and 6p electrocyclization of diarylethenes, within the same moleScheme 2. Synthetic route to 1 a and 2 a. NBS = N-bromosuccinimide. cule has yet led to appealing features such as chemically gated photochromism in few dithienylmaleimide derivatives[28] The key intermediate 3 was easily obtained by reacting 2and also the design of a dithienylmaleimide-based probe for methoxy-thiobenzamide and 1,4-dibromopentane-2,3-dione. a naked-eye detection of volatile neurotoxic organophosThe latter was generated by slowly adding two equivalents of phates.[29] A better understanding of the interplay between the bromine into pure 2,3-pentanedione and was used without further purification. After bromination and Suzuki cross-coutwo important photochemical processes is of great interest pling with 2-phenyl-4-B(pin)-5-Me-thiazole,[18b] terthiazole 2 a not only from an academic viewpoint, but also for designing novel photoswitchable systems. 1 a is an excellent candidate was obtained in fairly good yield. Deprotection of methoxy for such studies because two ESIPT processes are possible, begroups of 2 a provided 1 a. All of the new organic compounds tween the hydrogen atom of each phenolic hydroxy group were characterized by 1H NMR and 13C NMR spectroscopy and and the nitrogen atom of the neighboring thiazolyl group, and HRMS. Note that the assignment of the two different OH the two potential ESIPT sites are closely conjugated with the groups in 1 a is not trivial, but can be carried out by using 6p electrocyclization backbone, which would favor strong inHMBC (heteronuclear multi-bond correlation) spectroscopy teractions between the two photochemical processes. Herein (see the Supporting Information). we report the synthesis, photochemical, and photophysical 1 a, 1 b, and 2 a were also characterized by single-crystal properties of 1 a as well as those of two of its metal comstructure analyses (see the Supporting Information). The moplexes. The dramatic impact of ESIPT process and metalation lecular structures of 1 a and 2 a are shown in Figure 1. on the photochemical reactivity of the 6p electrocyclization of None of the compounds was found to display 6p electrocycthe backbone is observed and rationalized by DFT calculations, lization-based photochromism in the crystalline state because, ultrafast spectroscopy, and comparison with 2 a, a close derivafor 1 a the two side thiazole groups are arranged in a photoitive of 1 a in which any ESIPT process is absent. nactive parallel conformation with respect to the central thiazole ring, while for 2 a, despite the potential photoactive antiparallel conformation, the CC distance between the two reactive carbon atoms (5.32 ) is much larger than the known upper limit (4.2 ) to allow such a cyclization.[30] Note also that Results and Discussion 1 a adopts the expected cis-enol geometry, with a hydrogen Syntheses and characterizations of the multifunctional bond between each hydroxy group and the nitrogen atom of switch the neighboring thiazole ring. Selected X-ray data together with those derived from DFTStarting from commercially available 2-methoxybenzonitrile, optimized structures are listed in Table 1 (see the Supporting 1 a was prepared according to Scheme 2. Chem. Eur. J. 2014, 20, 12279 – 12288
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Figure 1. ORTEP plots of molecular structures of 1 a and 2 a (ellipsoids set at 50 % probability; H atoms shown as small spheres of arbitrary radii).
Table 1. Selected experimental and calculated structural data of 1 and 2 and the calculated energy differences between their different forms and conformations.
1 a exptl 1 b exptl 2 a exptl 1 a (p) calcd 1 a (ap) calcd 1 b calcd 2 a (p) calcd 2 a (ap) calcd 2 b calcd
z1 [8]
z2 [8]
d(CC) []
z(NNOO) [8]
Erel [kcal mol1]
49 7 95 -49 47 7 -52 48 7
141 7 153 134 50 7 133 54 7
4.84 1.52 5.32 4.44 3.70 1.54 4.41 3.71 1.54
25.8 3.8 9.7 27.3 27.5 0.4 4.4 21.3 30.9
– – – 0.06 0 18.32 0.13 0 20.47
Figure 2. Spectral changes of a) 1 a and b) 2 a in MeCN at room temperature: open form a (c) and closed form b (a). Computed spectra of c) 1 and d) 2: parallel (p) and antiparallel (ap) open conformers and closed form (b).
Information for details). A pretty good agreement is found between the experimental data of 1 a, 1 b, and the computed data (1 a (p), 1 b), while the structure of 2 a, which is probably due to solid-state interactions, is rather close to that of computed 2 a (ap*), the second most stable antiparallel conformation (Supporting Information, Table S3). Such a general agreement validates the present theoretical calculations. It is also of interest to point out that the computed energy difference between the photoinactive parallel (p) and photoactive antiparallel (ap) conformations is very small for both molecules, suggesting the co-existence of photoreactive conformations in solution for both compounds. Photochromic and fluorescence properties of 1 a and 2 a Photochromic behaviors of 1 a and 2 a were investigated in two solvents with different polarities (MeCN and toluene) as the photodynamics of ESIPT process is solvent-sensitive.[31–33] The spectral changes of 1 a and 2 a upon alternate UV and visible-light irradiation in MeCN are shown in Figure 2 a, b and compared with TDDFT calculated spectra (Figure 2 c, d). The solution of 1 a (2 a) is colorless and is characterized by an intense absorption band at 332 nm (324 nm). Upon UV irradiation at 335 nm, the colorless solution of 1 a (2 a) turns blue, indicating the formation of the closed form 1 b (2 b) characterized by a broad absorption band around 600 nm (624 nm). The Chem. Eur. J. 2014, 20, 12279 – 12288
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blue colored closed form 1 b (2 b) is stable in the dark at room temperature in a deoxygenated atmosphere, but can be brought back to the open form 1 a (2 a) by visible light irradiation (575 nm), with the restoration of the initial spectrum of 1 a (2 a). Actually, studies of the thermal bleaching rate of 1 b and 2 b at various temperatures (Supporting Information, Figure S4) led to a half-life of about 5 years and an associated activation energy (Ea) of 136 kJ mol1 for 2 b, and a few hundred years of half-life and an activation energy of 153 kJ mol1 for 1 b, respectively. These photochromic features are qualitatively quite similar to those reported for the terphenylthiazole (112 kJ mol1 without methoxy or hydroxy substituent).[18b] The main photochromic data of 1 and 2 are gathered in Table 2 together with the relevant data obtained from TDDFT calculations. Satisfactory agreement is found between the experimental and computed data in terms of band energy, shape, and relative intensity (Figure 2; see the Supporting Information for details). Focusing on the antiparallel forms (the most stable forms) of both 1 a and 2 a, TDDFT calculations predict main electronic absorptions at 354 and 342 nm, which match well with the experimental values of 332 and 324 nm, respectively. These two electronic transitions are of the same nature (charge transfer, CT) and same origin (HOMO!LUMO + 1). However, the electronic density distribution of the orbitals involved (Figure 3) is quite different due to the relative orientation of the OH/OMe groups in their respective open forms (Figure 1). For 1 a (ap), the donor orbital is well delocalized while the acceptor orbital is mainly localized on the lateral hydroxyphenylthiazolyl group. Concerning 2 a (ap), the donor orbital is mainly centered on the hexatriene moiety while the acceptor orbital is delocalized onto the lateral phenylthiazolyl- and the central methoxyphenyl-substituted thiazolyl groups. Finally, in line with their ob-
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Full Paper to the conjunction of this decreased cyclization efficiency and lmax [nm] lmax calcd [nm] F aa!b t trans-keto [ms] a more important 1 b to 1 a back-reaction, favored by a com1a 332 354 (ap) 0.04[b](Tol.) 0.76(Tol.) paratively much higher absorp356 (p) 0.01[b](MeCN) 0.41(MeCN) 1a 450 453 0.02[d](Tol.) 408, 86(Tol.) tion coefficient of 1 b than that trans-keto 0.06[d](MeCN) 270, 69(MeCN) of 2 b at the irradiation wave[c] 1b 585 576 0.01 (Tol.) length (335 nm; Figure 2). The 626 0.01[c](MeCN) ratio of photoactive conformer 2a 324 342 (ap) 0.15[b](Tol.) 0.91(Tol.) 346 (p) 0.11[b](MeCN) 0.94(MeCN) (ap) to photoinactive one (p) in 2b 624 606 0.01[c](Tol.) solution is known to greatly [c] 0.01 (MeCN) impact the cyclization quantum [a] aa!b : 6p electrocyclization conversion ratio; ttransketo : lifetime of ESIPT photoproduct. [b] Cyclization quanyield.[2] In the present case, the tum yield. [c] Ring opening quantum yield. [d] 1 a trans-keto formation quantum yield. 1 solution H NMR spectrum shows only one set of methyl signals at room temperature for both 1 a and 2 a, indicating either the presence of only one conformation (ap) or fast interconversion between the two conformations at room temperature. Given the negligible computed energy difference between the two conformers (Table 1), the latter, with ap and p conformations equally populated, is more likely. As shown below, the presence of competing ESIPT process in 1 a is the main reason for its much decreased cyclization quantum yield compared to 2 a. Nanosecond pump–probe absorption measurements were also carried out for 1 a as short-lived photochromic species are usually generated in ESIPT chromophores.[34–36] Indeed, it is well-established that ESIPT occurs within a few tens of femtoseconds from the Frank–Condon excited enol state (enol*) to produce the cis-keto excited state (cis-keto*). The latter may decay to the corresponding cis-keto ground state by fluorescence (see below) followed by an ultrafast reverse proton transfer to the initial enol ground state (enol). Besides the radiative decay, another deactivation channel for the cis-keto* is the isomerization to the trans-keto form, which relaxes back to enol in a few hundreds of microseconds owing to the existence of an energy barrier. After nanosecond excitation at 330 nm, the transient absorption spectra of 1 a recorded at different time delays and time decays are shown in Figure 4 a, b, respectively. Results obtained for similar experiments performed for 2 a are provided in Figure 4 c, d. Figure 3. Orbitals involved in the lowest-energy transition calculated by For 1 a, roughly three different kinds of signals can be identiTDDFT for antiparallel 1 a (a) and antiparallel 2 a (b). fied: one negative band at 340 nm and two positive bands centered around 450 and 610 nm, respectively. The negative absorption band is due to the ground state bleaching of 1 a in served solution photochromic reactivity, the acceptor and its enol form, and the broad positive absorption band, which donor orbitals of both molecules (Figure 3) feature contribudoes not vary with time delays, corresponds to the formation tions from the two reactive carbon atoms and they also presof the closed form 1 b. Based on literature data[34–36] and ent the adapted symmetry for the conrotatory cyclization to form the single CC bond. TDDFT calculations (Table 2; see the Supporting Information As it can be seen from Table 2, the most striking difference for details), the transient positive absorption band peaking at between the two molecules concerns their respective electro450 nm is assigned to the trans-keto form resulting from the cyclization efficiency. Indeed, the cyclization quantum yield of ultrafast cis–trans isomerization of cis-keto* of one of the two 1 a is found to be, regardless of the solvent, considerably ESIPT sites. The decay rate of trans-keto is identical to the reposmaller than that of 2 a, while the cycloreversion quantum pulation of the ground enol state (Figure 4 b). However, two yield remains in the same range for the two compounds exponential time constants (Table 2) in acetonitrile (toluene), (Table 2). The lower 1 a to 1 b conversion ratio (a1a!1b) is due 69 ms (86 ms) and 270 ms (408 ms) with similar contributions, Table 2. Main photochromic data of 1 and 2 at room temperature.[a]
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Figure 4. a) Absorption spectra of 1 a and 1 b, and nanosecond transient absorption spectra at different time delays for 1 a in acetonitrile after 330 nm excitation. b) Different decays for 1 a at different wavelengths. c) Nanosecond transient absorption spectra at different time delays for 2 a in acetonitrile after 355 nm excitation. d) Normalized decays at 460 nm for 2 a under N2 (dashed line) or O2 saturated atmosphere.
are needed to fit the experimental data, and these two time constants could be assigned to different trans-keto forms, coming from antiparallel and parallel open forms or a twistedketo form.[34] The re-population of enol state is not complete because of the existence of the stable closed form 1 b. It is worth noting that the amount of the cyclized 1 b and that of the short-lived trans-keto form are directly proportional to the corresponding absorbance changes at 340 nm. Using the yield formation of 1 b it is possible to determine the formation yield of trans-keto (Table 2). Of the two ESIPT sites present in 1 a, it is difficult to determine which one contributes the most to the observed transient absorption band. Nevertheless, the ESIPT process involving the central phenol group may be favored over the terminal one because of its stronger acidity, revealed by a more downfield-shifted 1H NMR signal (see the Supporting Information). This can be qualitatively explained by the electro-attracting effect of the phenylthiazole group as compared with the methyl group. This stronger acidity is expected to be preserved in the excited state, therefore making the proton transfer easier than that of the terminal site.
Concerning 2 a, as expected, no trans-keto absorption band is observed. Instead, a triplet-state contribution is clearly seen (Figure 4 c), with a lifetime of about 268 ns, which is shortened in dioxygen-saturated solution (Figure 4 d). This is in agreement with the literature data, that is, the photoinactive parallel conformer is deactivated by fluorescence and intersystem crossing.[37–39] The triplet state is not observed for 1 a because ESIPT is the main deactivation process of the enol* state for both its parallel and antiparallel forms. Another major difference between 1 a and 2 a is their fluorescence properties in solution, which are summarized in Table 3. Compound 2 a exhibits only weak fluorescence (Ff = 0.01) in the solvents considered, with an emission maximum around 415 nm. Emission decays were measured in acetonitrile and toluene with the laser excitation set at 330 nm. The decays are well-fitted using a double-exponential model with a main contribution with a characteristic time of about 90 ps, irrespective of solvent polarity. In agreement with literature data,[37–39] such a weak fluorescence is ascribed to the photoinactive parallel conformer of 2 a. The minor component is found to strongly depend on solvent polarity. This could tentatively be assigned to a charge-transfer excited state of a non-reactive conformer (parallel open form). However, such assumption should be considered cautiously and will not be discussed further. In contrast, a large Stokes-shifted fluorescence signal is observed for 1 a. This is the signature of an ESIPT process which is here characterized by a quantum yield (Ff) varying from 0.07 in toluene (slightly polar solvent) to less than 0.01 in acetonitrile (polar solvent). The strong solvent dependence of the fluorescence of 1 a is also consistent with emission signals observed for ESIPT: in apolar solvents the emitting cis-keto* prevails, whereas in polar solvents cis–trans isomerization to trans-keto, as well as the existence of some geometrical isomers, such as twisted enol*, are favored. The latter form is characterized by a residual fluorescence around 420 nm in MeCN.[33] Decay traces obtained for 1 a were fitted with a double-exponential function. The major time decay is solvent-dependent and can be clearly ascribed to the cis-keto* emission. On the contrary, the minor contribution is only slightly affected by solvent polarity and could thus be tentatively attributed to the emission of a relaxed enol* form or a twisted cis-keto* form with a lower radiative rate constant.[40] Finally, the 6p electrocyclization of 1 a to 1 b in toluene leads to an important photo-modulation of the fluorescence (ca. 80 %), as shown in Figure 5. This decrease in the fluores-
Table 3. Fluorescence properties of 1 a and 2 a excited at 335 nm.[a]
lem (Dn/cm1)
1a Ff
toluene (er = 2.38)
524 nm (10410)
0.07
MeCN (er = 37.5)
523 nm (11000)
0.01
t [ps] (a)
lem (Dn/cm1)
2a Ff
885 (0.87) 214 (0.13) 76 (0.94) 262 (0.06)
415 nm (6710)
0.01
415 (6710)
0.01
t [ps] (a) 94 (0.98) 900 (0.02) 89 (0.79) 191 (0.21)
[a] t is the emission decay time of the excited species and a represents the pre-exponential factor.
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Figure 5. Fluorescence spectra of 1 a in toluene at room temperature and its evolution upon UV irradiation (lex = 350 nm).
cence is in good agreement with the 0.76 conversion ratio observed in the steady state absorption experiment considering the same solvent (Table 2).
Ultrafast photodynamics for 1 a and 1 b To gain more insight into the dynamics of the two photochemical processes involved, femtosecond transient absorption spectra were recorded for 1 a and 2 a in acetonitrile after 325 nm excitation. Figure 6 shows the transient absorption
Figure 6. Transient absorption spectra of 1 a (right) and 2 a (left) in acetonitrile obtained for different time delays after 325 nm excitation. Inset: normalized spectra obtained by nanosecond transient absorption (1 ms after excitation) and femtosecond transient absorption (800 ps after excitation). Chem. Eur. J. 2014, 20, 12279 – 12288
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spectra for 1 a and 2 a for time delays in the range of 0 to 400 ps. For 2 a (Figure 6 left), two main bands grow with the excitation pulse (instrumental response function about 200 fs): a broad absorption band covering the 340–700 nm domain and a negative band in the 320–340 domain. The negative band is assigned undoubtedly to the ground state depopulation. The broad absorption band has four maxima: 375, 455, 535, and 620 nm. At 400 ps, only 375 and 620 nm absorption bands remain and the spectrum is similar to the stationary one of the closed form 2 b. Therefore these two bands can be assigned to the absorption of closed form 2 b. This is typical of diarylethenes, with a ring-closing reaction through a conical intersection within a few hundreds of femtoseconds.[38] After 0.6 ps, the 455 nm band decays in 1.5 ps (characteristic time constant of 1.36 0.1 ps) while 375 and 620 nm bands increase concomitantly with the appearance of two isosbestic points at 400 and 560 nm. During this process the ground-state bleach does not change. This process is thus assigned to the formation of closed form 2 b, occurring through an intermediate absorbing at 455 nm. Finally the 535 nm band disappears in 100 ps (characteristic time constant of 111 5 ps), whereas the ground state depopulation decays simultaneously to a stationary value. This time constant is similar to the emission time decay measured by the single photon counting method. The 535 nm band is thus assigned to the S1 state of the parallel conformer of 2 a, which, being not active in ring-closing reaction, relaxes partly by fluorescence and intersystem crossing in about a hundred picoseconds. Contrary to classical report on normal diarylethenes, a second pathway is found for the formation of the closed form through an intermediate which has a lifetime of 1.5 ps. However, it was already mentioned for different diarylethenes that excitation of a higher excited state can lead to the closed form not only via the classical mechanism, that is, ultrafast internal conversion to S1 state and then to the closed form through a conical intersection in a few hundreds of femtoseconds, but also via the formation of a different S1 state, which has a geometry similar to the closed form and relaxes in a few picoseconds.[41] For 1 a, two main bands also grow within the excitation pulse, the depopulation band and a broad positive band with different maxima in comparison to 2 a: 395 nm, 455 nm (shoulder), 610 nm (broad tail). After 0.6 ps, 395 and 455 nm bands are found to grow (time constant of 1.8 0.1 ps) together with the appearance of a negative broad band above 500 nm and no change of depopulation band. The final spectrum at 10 ps shows the ground state depopulation band, one absorption band with two maxima (395 and 455 nm) with a higher contribution for the shoulder at 455 nm and a negative band from 500 nm to 700 nm. This spectrum is characteristic of cis-keto* fluorescent state and this step can be assigned to a relaxation from hot cis-keto* state formed just after excitation within 200 fs.[35–36, 42] Finally the entire spectra decay in 84 5 ps and the transient spectrum remains then stable and similar to the one obtained by nanosecond transient absorption experiments, with trans-keto and 1 b as major and minor
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Full Paper photoproducts, respectively. This time constant is equal to the fluorescence lifetime measured by the single photon counting method (Table 3). The relaxation of cis-keto* is mainly a non-radiative process, as only a proportion of about 30 % goes to the trans-keto form (evaluated by the recovery of the depopulation band). As the ground-state depopulation and experimental conditions are the same for 1 a and 2 a, the absorption band at 610 nm indicates that the formation of 1 b is about ten times less efficient than that of 2 b, and this is in good agreement with their respective formation quantum yields (Table 2). Therefore, despite the fact that 1 a shares the same hexatriene backbone with 2 a, the ultrafast ESIPT process, which occurs within 50 fs with almost no geometrical changes, prevails on the comparatively slow backbone 6p electrocyclization (a few hundred fs). The overall photodynamic schemes that can be drawn for 1 a and 2 a are given in Scheme 3.
Figure 8. Experimental (c) and computed (a) absorption spectra of a) Ni-(1a-2 H), b) Cu-(1a-2 H), c) Ni-(1b-2 H), and d) Cu-(1b-2 H) in CHCl3 at room temperature.
Scheme 3. Overall main photoinduced processes for 1 a and 2 a.
Metal complexes of 1 a and metal-gated photochromism First trials of metalation have been carried out with first-row transition-metal ions, such as nickel and copper. Metalation of 1 a with one equivalent of Ni(OAc)2·4H2O or Cu(OAc)2·2H2O in refluxing MeOH gave Ni-(1 a-2 H) and Cu-(1 a-2 H), respectively. The crystal structures of the two metal complexes have been determined, and their molecular structures are shown in Figure 7. The two metal complexes are isostructural and characterized both by a square-planar coordination sphere with NiN and NiO distances of about 1.90 and CuN and CuO of about 1.84 . Note that, contrary to the always-paramagnetic Cu2 + complex, the Ni2 + complex is diamagnetic in such a squareplanar environment.
Figure 7. Molecular structure of Ni-(1a-2 H) (left) and Cu-(1a-2 H) (right) (ellipsoids set at 50 % probability; H atoms shown as small spheres of arbitrary radii). Chem. Eur. J. 2014, 20, 12279 – 12288
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The solution absorption spectra of the two metal complexes are also qualitatively quite similar, as shown in Figure 8. In comparison to 1 a, the absorption spectra of the two complexes are both characterized by: 1) a blue-shifted and intense ligand centered band around 300 nm; and 2) a rather intense charge-transfer band, peaking at 416 nm for Ni-(1 a-2 H) and 382 nm for Cu-(1 a-2 H). The latter also shows a very weak absorption band at about 600 nm, which is likely attributed to the copper d–d transition. When submitted to steady-state UV irradiation at either 405 nm or 313 nm, no significant spectral changes can be detected for both metal complexes. Despite the loss of photochromism, relatively high thermal stability of 1 b allowed to prepare Ni-(1 b-2 H) and Cu-(1 b-2 H). Monitoring of the UV/Vis spectral evolution upon addition of nickel (copper) acetate into a UV (365 nm) irradiated 1 a solution (Supporting Information, Figure S5) in deoxygenated environment and at room temperature showed that a new species was formed (Figure 8), with a new band at 450 nm and a slight bathochromic shift of its large band in the visible. These spectra were corroborated by TDDFT calculations and assigned to the metalated closed form (see also the Supporting Information, Figure S15 and Table S8). It is also of interest to note that 1 b interacts much more quickly with copper acetate than with nickel acetate as the formation of the band at 450 nm was over after a few minutes in the case of copper while that of nickel took more than 14 h. Similarly to the metalated open form, photochromic activity is lost as no detectable ring-opening reaction under visible irradiation (600 nm) can be observed. On the other hand, its demetalation, easily achieved by shaking a solution of M-(1 a-2 H) or M-(1 b-2 H) in dichloromethane with an aqueous HCl solution, allows its initial photochemical and photophysical properties to be recovered. Thus,
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Full Paper a metal-binding based gated photochromism is realized in such a system. To gain more insight into the inhibition of photochromic reactivity observed for metalated complexes, DFT calculations and ultrafast spectroscopy have been performed. In Table 4, some selected experimental data (structure, absorption) of the two metal complexes are reported together with those derived from DFT calculations (see the Supporting Information for details). A general good agreement is found
Table 4. Selected experimental and calculated data of the two metal complexes. z1 [8] z2 [8] d(CC) [] z(NNOO) [8] Erel [kcal mol1] Ni-(1 a-2 H) exptl Ni-(1 a-2 H) calcd Ni-(1 b-2 H) calcd Cu-(1 a-2 H) exptl Cu-(1 a-2 H) calcd Cu-(1 b-2 H) calcd
69 59 13.5 91 57 11.9
4 11 10.3 6 17 8.3
3.83 3.81 1.57 4.17 3.74 1.57
1.3 0.5 1.0 1.3 2.7 8.1
– 0 28.13 – 0 25.04
between the experimental and calculated data. From a structural viewpoint, it is reasonable to think that the conformation of both metal complexes in solution would be close to that of their respective solid structure. For Ni-(1 a-2 H) and Cu-(1 a2 H), this conformation appears to be neither antiparallel nor parallel because the two thiazole groups are tightly locked within the same plane by metal coordination. Furthermore, even though the distance between the two potentially photoreactive carbon atoms is short enough for the 6p electrocyclization (< 4.2 , Table 4), metalation would greatly hinder the rotation of one of the two terminal thiazole groups around the thiazole–thiazole single CC bond, which is necessary for the conrotatory ring-closing reaction. Finally, from the electronic considerations, TDDFT calculations predict that the low-energy band of both metal complexes is of the same origin (HOMO! LUMO + 1) and similar metal-to-ligand charge transfer (MLCT) nature (Table 5, Figure 9, and the Supporting Information for details). Except few cases where the MLCT band proved to be able to enhance the electrocyclization quantum yields through efficient excited-state intramolecular energy transfer,[43–46] the presence of such a band is generally not favorable to the 6p electrocyclization process as the metal ion is often responsible for additional deactivation channels of the excited state. It is particularly true in the present cases where the metal coordination site and the photoactive site are very close and strongly
Table 5. Selected experimental and calculated transitions observed in the metal complexes.
Ni-(1 a-2 H) Cu-(1 a-2 H) Ni-(1 b-2 H) Cu-(1 b-2 H)
Transition
lcalc. [nm] fosc
lexp. [nm] Assignment
HOMO!LUMO + 1 HOMO!LUMO + 1 HOMO!LUMO + 1 HOMO!LUMO + 1
416 396 452 443
416 382 460 456
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0.118 0.087 0.224 0.360
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MLCT MLCT MLCT MLCT
Figure 9. Molecular orbitals involved in the first electronic transition obtained by TDDFT calculations for a) Ni-(1a-2 H) and b) Cu-(1a-2 H).
coupled electronically. Experimentally, this is also supported by femtosecond transient absorption, recorded in dichloromethane for Ni-(1 a-2 H) (Cu-(1 a-2 H) is not sufficiently soluble). Indeed, after 400 nm (325 nm) femtosecond excitation, along with the ground-state depopulation band, only a weak and very broad band between 300 to 700 nm appears within 100 fs and then decays biexponentially in 2.5 and 34 ps (0.3 and 4.5 ps) respectively (see the Supporting Information). This transient band and biexponential decay, tentatively attributed to the non-radiative relaxation of a MLCT state combined with some cooling processes, deactivates the excited state which can lead to photochromic photoproduct and prevents the formation of any cyclized metal complex. Finally, it appears that the visible band for the metalated closed form has a strong MLCT contribution and will also inhibit in the same way the back photochromic reaction (Supporting Information, Figure S15).
Conclusion We have designed a terthiazole-based molecular switch that not only combines two important photochemical reactions, that is, 6p electrocyclization and ESIPT, but also offers the possibility of binding a wide range of metal ions. The photochemical and photophysical properties of this switch have been investigated in detail by steady-state and ultrafast spectroscopy, and rationalized by DFT calculations. It is found that the molecular switch behaves as a biphotochrome with a time-dependent photochemical outcome. The 6p electrocyclization-based
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Full Paper photochromic reaction of the switch is impeded to a large extent by the ultrafast ESIPT process. Conversely, the ESIPTbased fluorescence can be efficiently photo-switched by the 6p electrocyclization. By combining DFT calculations and ultrafast spectroscopy, we have also shown that the quenching of the 6p electrocyclization observed for the nickel and copper complexes can probably be accounted for by the conjunction of two factors: the lack of structural flexibility and MLCT-related ultrafast desactivation of the excited state. These findings will help to rationally design novel metal complexes with desired photoresponsive behaviors.
Experimental Section General 1 H NMR and 13C NMR spectra were obtained on either a Bruker AM 360 (360 MHz), DRX 300 (300 MHz), or DPX 250 (250 MHz) spectrometer and calibrated to the residual solvent peaks (CDCl3, 7.26 ppm and 77 ppm). The data are reported as chemical shift (d, ppm). UV/Vis steady-state absorption spectra were recorded on a Varian Cary 5000 spectrometer equipped with a temperature control unit. An Oriel Hg (Xe) 200 W lamp equipped with appropriate interference filters was used for sample irradiation. Solvents and reagents are used as received unless otherwise stated. Elemental analyses were performed by Service de Microanalyse, ICSN, 91198, Gif sur Yvette cedex, France.
(300 MHz, CDCl3): d = 8.52 (d, J = 7.9 Hz, 2 H), 8.04 (s, 1 H), 7.39 (m, 2 H), 7.09 (m, 4 H), 4.07 (s, 3 H), 4.04 (s, 3 H), 3.00 ppm (s, 3 H); elemental analysis calcd (%) for C21H18N2O2S2 : C 63.93, H 4.60, N 7.10; found: C 63.92, H 4.72, N 7.13. 1,4-Dibromopentane-2,3-dione: Under stirring, Br2 (5.1 mL, 100 mmol) was slowly added to 2,3-pentanedione (5.2 mL, 50 mmol) during 0.5 h. After 1 h of stirring, Ar was flushed into the mixture to remove HBr. The resulting brown oil was a mixture of several isomers but was used without further purification. 4-(5-Methyl-2-o-methoxyphenylthiazole-4-yl)-5-bromo-2-o-methoxyphenylthiazole (4): After complete dissolution of 3 (0.5 g, 1.3 mmol) in DMF (40 mL) at 80 8C, NBS (0.25 g, 1.4 mmol) was added and the solution was stirred at 80 8C overnight. The reaction was quenched with water and the solid was filtered and washed with water and dried under vacuum to afford 4 (0.575 g, 96 %) as an off-white solid. 1H NMR (250 MHz, CDCl3): d = 8.52 (d, J = 7.5 Hz, 1 H), 8.44 (d, J = 8.0 Hz, 1 H), 7.38 (m, 2 H), 7.06 (m, 4 H), 4.06 (s, 3 H), 4.04 (s, 3 H), 2.73 ppm (s, 3 H); elemental analysis calcd (%) for C21H18N2O2S2·0.2 H2O: C 52.88, H 3.68, N 5.87; found: C 52.91, H 3.70, N 5.95.
Synthesis and characterization
4-(5-Methyl-2-o-methoxyphenylthiazole-4-yl)-5-(5-methyl-2-phenylthiazole-4-yl)-2-o-methoxyphenylthiazole (2 a): 4 (0.45 g, 1 mmol), 2-pheny-4-Bpin-5-Me-thiazole[18b] (0.36 g, 1.2 mmol), CsF (0.38 g, 2.5 mmol), and [Pd(PPh3)4] (0.05 g, 0.04 mmol) were dissolved in dioxane (40 mL) under Ar, and the reaction mixture was heated to reflux for 6 h. After the addition of water (20 mL), the mixture was extracted with CHCl3 (3 20 mL) and the organic layers were washed with brine, dried over Na2SO4, and filtered. After evaporation of the solvent, the residue was purified by column chromatography (CH2Cl2). The resulting solid was dissolved in a minimum of CH2Cl2 and MeOH. Slow evaporation of CH2Cl2 afforded 2 a (0.423 g, 78 %) as an off-white solid. 1H NMR (300 MHz, CDCl3): d = 8.57 (d, J = 8.0 Hz, 1 H), 8.12 (d, J = 7.6 Hz, 1 H), 7.97 (m, 2 H), 7.42 (m, 5 H), 7.00 (m, 4 H), 4.07 (s, 3 H), 4.00 (s, 3 H), 2.58 (s, 3 H), 2.06 ppm (s, 3 H); 13C NMR (63 MHz, CDCl3): d = 164.0, 161.3, 157.9, 156.8, 156.3, 146.9, 144.8, 134.0, 133.2, 132.2, 130.8, 130.2, 129.8, 129.0, 128.7, 128.6, 126.6, 122.8, 121.2, 121.1, 111.6, 111.4, 55.9, 55.7, 12.6, 12.5 ppm; elemental analysis calcd (%) for C31H25N3O2S3·0.3 CH3OH: C 65.11, H 4.57, N 7.28; found: C 64.94, H 4.50, N 7.40.
2-Methoxythiobenzamide: A solution of P4S10 (8.5 g, 20 mmol) in EtOH (30 mL) was stirred for 1 h at 0 8C. 2-methoxybenzonitrile (2.4 mL, 20 mmol) was added and the resulting solution was heated to reflux overnight. The resulting mixture was stirred at room temperature with aqueous NaOH solution (1 m, 50 mL) and EtOAc (50 mL) for about 3 h. The aqueous layer was extracted with EtOAc (3 20 mL) and the combined organic layers were washed with brine (3 20 mL), dried over Na2SO4 and filtered. After evaporation of the solvent, the residue was purified by column chromatography (CH3Cl) to afford 2-methoxythiobenzamide (2.3 g, 70 %) as a pale yellow solid. 1H NMR (300 MHz, CDCl3): d = 9.06 (s, 1 H), 8.64 (d, J = 8.0 Hz, 1 H), 8.10 (s, 1 H), 7.47 (m, 1 H), 7.06 (m, 1 H), 6.96 (d, J = 8.3 Hz, 1 H), 3.97 ppm (s, 3 H); 13C NMR (63 MHz, CDCl3): d = 199.4, 156.3, 136.4, 133.7, 125.1, 121.3, 111.6, 56.2 ppm; elemental analysis calcd (%) for C8H9NOS: C 57.46, H 5.42, N 8.38; found: C 56.95, H 5.43, N 8.16. 4-(5-Methyl-2-o-methoxyphenylthiazole-4-yl)-2-o-methoxyphenylthiazole (3): 2-methoxythiobenzamide (0.85 g, 5 mmol) and 1,4dibromopentane-2,3-dione (0.65 g, 2.5 mmol) were heated to reflux in MeOH (30 mL) for 5 h. After cooling, the solid was filtered and washed with MeOH and dried under vacuum. The compound was obtained as an off-white solid (0.627 g, 63 %). 1H NMR
4-(5-Methyl-2-o-hydroxyphenylthiazole-4-yl)-5-(5-methyl-2-phenylthiazole-4-yl)-2-o-hydroxyphenylthiazole (1 a): 2 a (0.20 g, 0.4 mmol) was dissolved in dry CH2Cl2 (20 mL) under Ar, and then cooled at 78 8C. After slow addition of BBr3 (1 m, 3.5 mL, 3.5 mmol), the reaction mixture was allowed to slowly warm up to room temperature overnight under Ar. After addition of water (20 mL), the aqueous phase was neutralized with NaHCO3 and the mixture was stirred for 1 h. The aqueous layer was extracted with CH2Cl2 (3 20 mL) and the combined organic phase was washed with brine (3 20 mL), dried over Na2SO4, and filtered. After evaporation, the resulting solid was dissolved in a minimum of CH2Cl2 and MeOH. Slow evaporation of CH2Cl2 gave 1 a (0.073 g, 52 %) as an off-white solid. m.p. 198–200 8C; 1H NMR (360 MHz, CDCl3): d = 12.15 (s, 1 H), 11.30 (s, 1 H), 7.90 (m, 2 H), 7.68 (d, J = 7.9 Hz, 1 H), 7.51 (d, J = 8.4 Hz, 1 H), 7.38 (m, 4 H), 7.20 (m, 1 H), 7.09 (d, J = 8.3 Hz, 1 H), 6.92 (m, 3 H), 2.60 (s, 3 H), 2.15 ppm (s, 3 H); 13C NMR (63 MHz, CDCl3): d = 168.5, 165.7, 165.6, 157.4, 157.2, 144.2, 143.3, 142.4, 133.5, 133.0, 132.3, 131.7, 131.2, 130.2, 129.0, 128.1, 127.6, 127.2, 126.7, 126.6, 119.8, 119.2, 118.0, 116.8, 116.7, 12.8, 12.4 ppm; HRMS (ESI): m/z: 540.0846 [M + H] + , 562.0665 [M + Na] + ; elemental analysis calcd (%) for C29H21N3O2S3·0.4 CH3OH: C 63.91, H 4.12, N 7.61; found: C 63.90, H 4.19, N 7.72.
Quantum yield determination: The photochromic reaction was induced by a continuous wavelength irradiation Hg/Xe lamp (Hamamatsu, 200 W) equipped with narrow-band interference filters of appropriate wavelengths. The irradiation power was measured by a photodiode from Ophir (PD300-UV). The photochromic quantum yields were determined by probing the sample with a xenon lamp during the photochromic reaction. Absorption changes were monitored by a CCD camera mounted with a spectrometer (Princeton instruments). Kinetic profiles were analyzed by an Igor implemented home-made software.
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Full Paper Cu-(1 a-2 H): 1 a (30 mg, 0.06 mmol) and copper(II) acetate monohydrate (11 mg, 0.06 mmol) were stirred in refluxed MeOH (10 mL) under Ar for 6 h. After filtration, the resulting solid was dissolved in a minimum of CH2Cl2 and MeOH. Slow evaporation of CH2Cl2 gave Cu-(1 a-2 H) (23 mg, 64 %) as a crystalline green solid. MS (ESI): m/z: 623.07 [M + Na] + ; elemental analysis calcd (%) for C29H19CuN3O2S3·1 H2O: C 56.25, H 3.42, N 6.79; found: C 56.36, H 3.41, N 6.75. Ni-(1 a-2 H): This compound was prepared by the same procedure by using nickel(II) acetate tetrahydrate. Ni-(1 a-2 H) (20 mg, 60 %) was obtained as an crystalline orange solid. 1H NMR (360 MHz, CDCl3): d = 7.92 (m, 2 H), 7.47 (m, 3 H), 7.32 (dd, J = 1.72 Hz, J = 7.94 Hz, 1 H), 7.22–7.13 (m, 4 H), 6.60–6.54 (m, 2 H), 2.74 (s, 2 H), 1.88 ppm (s, 2 H); HRMS (ESI): m/z: 596.0058 [M + H] + , 617.9879 [M + Na] + ; elemental analysis calcd (%) for C29H19N3NiO2S3 : C 58.41, H 3.21, N 7.05; found: C 57.37, H 3.28, N 6.84. Single crystals suitable for structure determination of 1 a, 1 b, 2 a, Ni-(1 a-2 H), and Cu-(1 a-2 H) were obtained by slow evaporation of their respective CH2Cl2–MeOH solution.
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Received: March 3, 2014 Revised: April 9, 2014 Published online on August 5, 2014
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