DOI: 10.1002/asia.201500328
Full Paper
Electrocyclic Reactions
Photon-Quantitative 6p-Electrocyclization of a Diarylbenzo[b]thiophene in Polar Medium Ruiji Li,[a] Takuya Nakashima,*[a] Olivier Galangau,[a, b] Shunsuke Iijima,[a] Rui Kanazawa,[a] and Tsuyoshi Kawai*[a, b] Abstract: The high reactivity of 6p-electrocyclization in polar solvents has remained one of the important challenges for diarylethenes because of the emergence of a twisted intramolecular charge transfer (TICT) state at the excited state in such polar media, which usually quenches the photocyclization reaction. Herein we report on the preparation and highly efficient photocyclization of 2,3-diarylbenzo[b]thiophenes with nonsymmetric side-aryl units in a polar solvent. While the dithiazolylbenzo[b]thiophene showed a suppressed
Introduction Photoresponsive molecular materials have attracted much interest because their physicochemical properties are modulated with precise spatiotemporal control in a remote and noninvasive manner.[1–7] In particular, photoswitching systems based on diarylethenes[8, 9] and their aromatic analogs[10] have been extensively studied because they undergo thermally irreversible photoisomerization based on 6p-electrocyclization even in the solid state. Recent studies on these classes of molecules have highlighted their high photosensitivity in the electrocyclization reaction.[11–16] These contributions made an extensive effort to control the molecular folding geometry suitable for the photocyclization reaction proceeding in a conrotatory manner.[8a] For example, we have recently reported a dithiazolylbenzo[b]thiophene derivative, which showed a photon-quantitative cyclization reaction in hexane.[11a] This extremely high photochromic reactivity of the molecule was explained in terms of noncovalent interactions such as S/N and CH/N interactions, which spe[a] R. Li, Dr. T. Nakashima, Dr. O. Galangau, S. Iijima, R. Kanazawa, Prof. T. Kawai Graduate School of Materials Science Nara Institute of Science and Technology, NAIST 8916-5 Takayama, Ikoma, Nara 630-0192 (Japan) E-mail: [email protected] [email protected] [b] Dr. O. Galangau, Prof. T. Kawai NAIST-CEMES International Collaborative Laboratory for Supraphotoactive Systems Centre d’Êlaboration de Mat¦riaux et d’Etudes Structurales, CEMES 29, rue Jeanne Marvig, BP 94347, Toulouse 31055 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500328. Chem. Asian J. 2015, 10, 1725 – 1730
quantum yield of 6p-electrocyclization of 54 % in methanol, the replacement of a thiazole unit with a thiophene ring led to a photon-quantitative 6p-cyclization reaction. The nonsymmetrical modification into the side-aryl units was considered to enhance the CH/p interactions between side-aryl units to support a photoreactive conformation in methanol. The stabilization of the photochromic reactive conformation is expected to suppress the formation of the TICT state at the excited state, leading to highly efficient photoreactivity.
cifically stabilize the photochromic reactive conformation in less polar solvents. A related strategy has also been employed by Nakatani, Tian, and Zhu.[16] These methodologies to regulate the rotational configuration with intra- and intermolecular interactions are in parallel with the design strategy for oligoarylene molecules forming specific foldamers.[17, 18] Therefore, the appropriate placement of interacting moieties including heteroatoms in a molecule is of major importance to control the conformational behavior. However, the high reactivity in polar solvents such as methanol has remained one of the important challenges for this class of molecules because of the lack of an effective design to control the molecular geometry of photochromes in polar solvents, wherein the hydrogen bonding interaction is less effective. Moreover, the introduction of polar units into diarylethene-based photochromes to enhance the solubility in polar solvents often results in the formation of a twisted intramolecular charge transfer (TICT) state in the excited state, which adversely affects the efficiency of the ringcyclization reaction.[19] Herein we report the design of photochromic molecules based on diarylbenzo[b]thiophenes (Figure 1) which exhibit a highly efficient 6p-electrocyclization reaction in methanol. Although the dithiazolylbenzothiophene 1 a[11a] with symmetrically modified side aryl units showed an ordinary photocyclization quantum yield of 54 % in methanol, the nonsymmetrical introduction of side-aryl units improves the reactivity up to unity at most. The effect of chemical modifications on the conformational preference is discussed in terms of the contribution of noncovalent interactions including CH/p interactions between side-aryl units to stabilize the photoreactive conformation in particular.
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Full Paper The spectroscopic properties of 1 a–3 a were evaluated in hexane and methanol (Table 1). Upon UV irradiation, both solutions of 2 a and 3 a in methanol underwent a spectral change with a decrease in absorbance at around 300 nm and an increase of the band at 610 nm, in a manner similar to that of the parent molecule 1 a[11a] (Figure 2). Each spectral change was accompanied by an isosbestic point at 315 and 320 nm for 2 and 3, respectively. These spectral shifts are a clear indication of the formation of closed-ring isomers 2 b and 3 b, which was confirmed by 1H NMR spectroscopy in CDCl3 (Figures S5 and S8 in the Supporting Information) after isolation by reverse phase HPLC using methanol as an eluent. Next, the optimized geometries and the frontier molecular orbitals (FMOs) for 1 a were compared with those of 2 a and 3 a by DFT calculations using the Gaussian 09 suite.[20] For the calculations, the wB97XD DFT functional[21] with the 6-31G(d)
Figure 1. Photochromic reaction and chemical structures of diarylbenzo[b]thiophenes.
Table 1. Absorption maxima and coefficients of the open- and closedring isomers of 1-3, together with the quantum yields in solutions.
Results and Discussion
Isomer
Fo¢c[d]
Fc¢o[e]
One of the side-aryl thiazole units of 1 a was replaced with a phenylthiophene unit in 2 a and 3 a (Figure 1) to investigate specific intramolecular interactions affecting the conformational behavior. 2,3-Diarylbenzo[b]thiophenes 2 a and 3 a were synthesized as shown in Scheme 1. Newly synthesized molecules
lmax [nm] (e [104 M¢1 cm¢1])
1 a[a] 1 b[a] 2a 2b 3a 3b
307 (2.9)[b] 597 (0.95)[b] 290 (3.4)[c] 612 (1.1)[c] 300 (3.2)[c] 615 (0.92)[c]
0.98,[b] 0.54[c] – 0.50,[b] 0.91[c] – 0.71,[b] 0.99[c] –
– 0.008[b] – 0.007[b] – 0.036[b]
[a] Ref. [11a]. [b] In hexane. [c] In methanol. [d] lirrad. = 313 nm.[e] lirrad. = 480 nm.
Scheme 1. Synthesis of 2,3-diarylbenzo[b]thiophenes: a) 2 m K3PO4 aq., [Pd(PPh3)4], 1,4-dioxane.
were characterized by 1H and 13C NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis (see the Supporting Information for details). The 2,3-diarylbenzo[b]thiophenes were synthesized via simple Suzuki–Miyaura cross-coupling reactions between the corresponding aryl units under identical conditions. The sequence of connection in the tri-arylene structures was controlled by exploiting the different reactivities of 2- and 3-positions in 2,3-dibromobenzo[b]thiophene. A thienyl pinacol borate (Ar1) was first reacted with dibromobenzothiophene followed by the coupling with a thiazolyl borate (Ar2) to form 2 a and vice versa for the synthesis of 3 a. The sequence of aryl units was confirmed by X-ray single-crystal analysis for all molecules. Chem. Asian J. 2015, 10, 1725 – 1730
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Figure 2. UV/Vis absorption spectral change of 2 and 3 in methanol (concentration: 2.0 Õ 10¢5 m) upon UV irradiation (l = 365 nm). Dotted line: open form, bold line: photostationary state.
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Full Paper basis set was employed. The solvent polarity was taken into account by using the polarizable continuum model (PCM).[22] All molecules gave similar optimized structures with the photoreactive conformation as the most stable geometries (Figure S12 in the Supporting Information). A negligible difference between 1 a, 2 a, and 3 a was also found in the shape of FMOs (HOMO and LUMO) (Figures S14–S16 in the Supporting Information). The HOMOs were delocalized in the entire molecule, whereas the LUMOs were rather localized in a thiazolyl sidearyl group for all molecules. The energy differences between the open-ring isomers a-form and the closed-ring isomers bform were also similar for all molecules (Figure S12). These three molecules successfully gave single crystals[23–25] from methanol solution and exhibited photoreactive conformations with distances between the photoreactive carbon atoms below 3.7 æ[26] (Figure 3, see also Figure S21, Tables S1
Figure 3. Crystal structures of (a) 1 a, (b) 2 a and (c) 3 a with the indication of close contacts of noncovalent interactions (dotted lines). The dots indicate the centroids of five-membered aryl rings.
and S2 in the Supporting Information). Since the packing modes of molecules in the crystals were different from each other with different space groups and intermolecular packing manners (Table S1 in the Supporting Information), the atomic distances of intramolecular interactions could not be directly compared. Compound 1 a gave photoreactive conformers very similar to that found in the single crystal prepared from hexane solution.[11a] The CH/S and CH/N interactions between those atoms on the central benzothiophene and the side-aryl units were suggested from the close atomic contacts (H5-S1: 3.058; H4-N1: 2.665 æ in Figure 3 b) for 2 a that appear to stabilize the specific conformation with the aid of CH/p interactions between methyl groups and aryl rings with the CH–aryl disChem. Asian J. 2015, 10, 1725 – 1730
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tance in the range of 2.8–3.1 æ.[27] On the other hand, significantly tight contacts between the sulfur atom in the central benzothiophene and the thiazolyl nitrogen atom in a side-aryl unit (S1–N1, S1’–N1’ < 3.0 æ in Figure 3 c) was observed for 3 a, suggesting that the S/N interaction harnesses the rotation about the benzothiophene-thiazole bond. Although no specific interaction was suggested between the central and the Ar2 units for 3 a, the CH/p interactions with the CH–aryl distance in the range of 2.7–3.2 æ[27] between the side-aryl units seem to support the photoreactive conformations. Overall, ordinary DFT calculations and X-ray crystallography data failed to highlight the effect of the substitution of a phenylthiazole with a phenylthiophene on the conformational behavior of 2,3-diarylbenzo[b]thiophenes. However, the ring-cyclization quantum yield (FO¢C) in hexane dropped from 0.98 for the parent compound to 0.50 and 0.71 for 2 a and 3 a, respectively. Although CH/S and CH/N interactions were expected between the central and side-aryls in 2 a as suggested in the Xray crystallographic data, the contribution of an unreactive conformation with a flipped Ar1-unit stabilized by the Me/S interaction may decrease the FO¢C in hexane.[10a,c] The absence of CH/N interaction in 3 a also resulted in a decreased FO¢C in hexane. Interestingly, the FO¢C jumped in methanol from 0.54 of 1 a to 0.91 and > 0.99 for 2 a and 3 a, respectively. The reason for the low reactivity of 1 a in methanol was attributed, at least partly, to a weakening of the H-bond-like CH/N interaction between the 4-proton on the central benzothiophene and the nitrogen in the Ar2-thiazolyl ring (Figure 1) in a protic solvent,[11a] leading to a larger conformational fluctuation regardless of the photoreactive conformation in the single crystal. The NMR chemical shift of the 4-proton (H4) (7.97 ppm) was almost identical for both 1 a and 2 a in CD3OD (Figure S18 in the Supporting Information). Therefore, the CH/N interaction was expected to have a small effect on the conformational preference of both molecules in methanol. To gain further insight into the reason for the higher photocyclization efficiency of 2 a and 3 a in comparison to 1 a, the conformational preferences for these diarylbenzothiophenes were studied by 1H NMR measurements in CD3OD (Figure S18 in the Supporting Information). Here we focus on the values of chemical shift of two methyl groups and the splitting between them (Dd), which efficiently probe the substantial ring-current effect operating on these methyl groups (Table 2) and relative stability of the highly symmetric C2-conformation around the photoreactive 6p system.[11a] Each methyl peak was assigned to either Ar1- or Ar2-methyl groups based on differential nuclear Overhauser effect (1D NOE) measurements (Figure S20 in the Supporting Information). For 1H NMR spectral simulations, the GIAO model at the PBEPBE/6-311 + + G(2d,p) level was used.[28] Compound 2 a gave a set of peaks at 2.06 (Ar2-thiazole) and 2.28 ppm (Ar1-thiophene) with Dd of 0.22 ppm. The former peak appeared more upfield than that of 1 a (2.08 ppm). On the other hand, 3 a, which exhibited a photon-quantitative reactivity in methanol, provided the peaks at 2.11 (Ar1-thiazole) and 2.10 ppm (Ar2-thiophene) with a surprisingly small peak splitting of only Dd = 0.01 ppm. The thiazolyl-methyl peak at 2.11 ppm was a more upfield signal compared to the Ar1-thia-
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Full Paper Table 2. Summary of the chemical shifts of methyl groups of 1 a–3 a in CD3OD together with simulated values[a,b] (in parentheses) based on the GIAO model at the PBEPBE/6-311 + + G(2d,p) level.
Compd.
Ar1-Me [ppm]
Ar2-Me [ppM]
Dd [ppm]
1a
2.17 (2.183)[c] , (2.166)[d] 2.28 (2.112)[c] , (3.002)[d] 2.11 (2.144)[c] , (2.172)[d]
2.08 (2.243)[c] , (2.976)[d] 2.06 (2.236)[c] , (2.051)[d] 2.10 (2.166)[c] , (2.885)[d]
0.09 (0.06)[c] , (0.81)[d] 0.22 (0.124)[c] , (0.951)[d] 0.01 (0.022)[c] , (0.713)[d]
2a 3a
Figure 4. Modes of CH/p interactions working in diarylbenzo[b]thiophenes (parts of structures were given for clarity) with symmetrically and nonsymmetrically modified side-aryl units.
[a] Solvent polarity was taken into account by using the polarizable continuum model. [b] Average values of three protons to take the rotation of methyl groups into consideration. [c] Based on the most stable photoreactive conformation (see Figure S12 in the Supporting Information). [d] Based on the non-photoreactive conformation with a flipped aryl unit (see Figure S19 in the Supporting Information).
zolyl-methyl one of 1 a (2.17 ppm). These upfield shifts of thiazolyl-methyl in comparison with 1 a could be attributed to the larger ring current effect of the oppositely facing thienyl ring on the thiazolyl-methyl groups for 2 a and 3 a, which also indicated a stronger CH/p interaction between the electron-deficient methyl and the electron-rich thiophene. The thienylmethyl peaks of 2.28 for 2 a and 2.11 ppm for 3 a were also found upfield in comparison with the synthetic intermediates of phenylthienyl parts (2.4–2.7 ppm), also supporting the operation of CH/p interaction. The DFT simulation of the 1H NMR spectrum of 3 a based on an optimized photoreactive conformation well reproduced the experimental result including a very small peak splitting Dd (Table 1). Thus, the conformation of 3 a is expected to be well controlled into the photoreactive one to achieve the photon-quantitative reactivity in methanol. It is suggested that the attractive interactions to harness the rotations around the single bond between the central and side-aryl units are not always necessary at both sides for controlling the molecular geometry. The results of 2 a and 3 a with enhanced photosensitivity in methanol let us to hypothesize that the nonsymmetrical modification into side-aryl units in terms of electron-donating or deficient properties may direct the side-aryl pair to reinforce the CH/p interactions. The importance of CH/p interaction on stabilizing the photoreactive conformation was partly verified by a decrease in FO¢C of a derivative of 1 a in which methyl groups on the reactive carbon atoms were substituted by a proton and a methoxy group (Figure S22 in the Supporting Information). The CH/p interaction is supposed to be composed of electrostatic and dispersion interactions as attractive forces.[29] While the contribution of the electrostatic force to this interaction has been thought to be relatively small, the enhanced electron-donating property in an aryl ring owing to the substitution of thiazole with thiophene should practically increase the strength of the CH/p interaction.[26] The nature of the CH/p interaction characterized by the dispersion force is most likely to work more efficiently in polar solvents than in less polar ones with the aid of the solvophobic effect.[30] The idea is that the opposed CH/p interactions between Ar1 and Ar2 equally operate for a symmetrically modified molecule like Chem. Asian J. 2015, 10, 1725 – 1730
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1 a, while the stronger CH/p interaction operating between the electron-deficient thiazolyl-methyl group and the electronrich thiophene ring in the nonsymmetrically modified one positions these moieties close together (Figure 4). As a result, another methyl-aryl pair is expected to come close to each other. As a related matter, we have recently demonstrated that a significant mono-directional CH/p interaction was still effective to fix a photoreactive conformation in a diarylethene and a terarylene derivative of nonsymmetrical structures.[31] To further support the above hypothesis, we additionally synthesized 2,3-diarylbenzo[b]thiophenes 4 a with an electrondonating methoxy (OMe) group to make one of the side-aryl units electron-richer relative to the other one (Figure 5). This compound showed a reversible photochromic reaction in methanol in a similar manner to the parent molecule 1 a (Figure S23 in the Supporting Information). Meanwhile, a photoreactive conformer was given in a single crystal of 4 a prepared from methanol solution (Figure 6, also see Table S1 in the Supporting Information). The noncovalent interactions including CH/N and S/N interactions with the atomic distances equiva-
Figure 5. Introduction of an electron-donating (OMe) group into 1 a to form 4 a.
Figure 6. Crystal structures of 4 a with the indication of close contacts of noncovalent interactions (dotted lines). The dots indicate the centroids of five-membered aryl rings.
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Full Paper lent to or shorter than the sum of van der Waals radii of each atom (H: 1.2, N: 1.55, S: 1.8 æ) are expected to support the photoreactive conformations in a manner similar to that of the parent molecule 1 a (Figure 3). It should be noted that the distances between the methyl groups and the centroids of opposing thiazolyl rings were shorter than 3.0 æ, corresponding to the effective distance of CH/p interaction.[27] The 1H NMR chemical shifts of two methyl groups on sidearyl units appeared at 2.04 and 2.15 ppm for 4 a in CD3OD (Figures S18 and S20 in the Supporting Information), showing upfield shifts in comparison with those of 1 a (2.06 and 2.17 ppm, respectively). These slight upfield shifts from 1 a to 4 a suggested that the stronger ring current effect operates on these methyl protons through the reinforced CH/p interactions in 4 a owing to the asymmetrization of aide aryl units in terms of the electron-rich or -deficient property. As a consequence, the ring-cyclization quantum yield in methanol substantially increased from 0.54 for 1 a to 0.70 for 4 a in methanol. This increase in FO¢C further supports the reinforcement of CH/p interactions between the pair of electron-deficient and -richer side aryl units (Figure 4), stabilizing the photoreactive conformation in methanol.
Conclusions We report highly sensitive photochromic molecules with nonsymmetrical modification on side-aryl units based on 2,3-dithiazolylbenzo[b]thiophene in methanol. The modifications on the diarylbenzothiophenes effectively increased the efficiency of the photocyclization reaction to unity at most in methanol. The combination of electron-deficient and electron-rich aromatics as side aryl units was considered to reinforce a CH/p interaction between the electron-rich aryl ring and the methyl group attached to the electron-deficient aryl unit, which is highlighted in methanol, stabilizing the photoreactive conformation. The present results would give a useful insight and demonstrate a wide flexibility in the molecular design of photon-quantitative photochromes based on a 6p-system.
Experimental Section 2,3-Diarylbenzo[b]thiophenes were synthesized through the sequential step-by-step Pd-catalyzed cross-coupling reactions between 2,3-dibromobenzo[b]thiophene and the corresponding arylpinacol borates (see the Supporting Information for details). 1 H NMR spectra were recorded on a JEOL AL-300 spectrometer (300 MHz). Temperature-dependent 1H NMR spectra were measured using a JEOL ECP 400 spectrometer (400 MHz). Separative HPLC was performed on a JASCO LC-2000 Plus Series. Mass spectra were measured with a JEOL JMS-700 mass spectrometer. Absorption spectra in solution were studied with a JASCO V-670 spectrophotometer. UV irradiation was carried out using a Panasonic Aicure UV curing system (LED, l = 365 nm) as the exciting light source. X-ray crystallographic analyses were carried out with a Rigaku R-AXIS RAPID/s Imaging Plate diffractometer with MoKa radiation. The structures were solved by direct method (SHELXL-97) and refined by the full-matrix least-squares on F2. All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were Chem. Asian J. 2015, 10, 1725 – 1730
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placed using AFIX instructions. DFT calculations were performed with Gaussian 09 at the wB97XD/6–31G(d) level. Photoirradiation with UV and visible light under microscope observation was conducted using a mercury lamp for epi-illumination through BP330– 385 and BP520–550 filters, respectively. Absolute ring photoreaction yield (FO¢C and FC¢O) values were obtained on a Shimadzu QYM-01 setup.[33]
Acknowledgements The authors thank Mr. F. Asanoma, Mr. S. Katao and Ms. Y. Nishikawa at NAIST for their assistance with VT-NMR, X-ray crystallography and mass spectra, respectively. This work was partly supported by Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan under Green Photonics Project at NAIST and the Grand-in-Aid for on Innovative Areas of “PhotoSynergetics”, NAIST Green-Photonics Project and the Mazda Foundation. Keywords: crystals · noncovalent photochromism · quantum yield
interactions
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[1] H. Dìrr and H. Bouas-Laurent, Photochromism: Molecules and Systems; Elsevier, Amsterdam, 2003. [2] B. Feringa, Molecular Switches, Wiley-VCH, Weinheim, 2001. [3] I. Yildiz, E. Deniz, F. M. Raymo, Chem. Soc. Rev. 2009, 38, 1859 – 1867. [4] R. Gçstl, A. Senf, S. Hecht, Chem. Soc. Rev. 2014, 43, 1982 – 1996. [5] K. Mutoh, H. Arai, Y. Kobayashi, J. Abe, Pure Appl. Chem. 2015, 87, 511 – 523. [6] J. Gu¦rin, A. L¦austic, S. Delbaere, J. Berthet, R. Guillot, C. Ruckebusch, R. M¦tivier, K. Nakatani, M. Orio, M. Sliwa, P. Yu, Chem. Eur. J. 2014, 20, 12279 – 12288. [7] A. M. Asadirad, S. Boutault, Z. Erno, N. R. Branda, J. Am. Chem. Soc. 2014, 136, 3024 – 3027. [8] a) M. Irie, Chem. Rev. 2000, 100, 1685 – 1716; b) M. Irie, T. Fukaminato, K. Matsuda, S. Kobatake, Chem. Rev. 2014, 114, 12174 – 12277. [9] a) H. Tian, S. Yang, Chem. Soc. Rev. 2004, 33, 85 – 97; b) H. Tian, S. Wang, Chem. Commun. 2007, 781 – 792. [10] a) T. Nakashima, K. Atsumi, S. Kawai, T. Nakagawa, Y. Hasegawa, T. Kawai, Eur. J. Org. Chem. 2007, 3212 – 3218; b) S. Kawai, T. Nakashima, K. Atsumi, T. Sakai, M. Harigai, Y. Imamoto, H. Kamikubo, M. Kataoka, T. Kawai, Chem. Mater. 2007, 19, 3479 – 3483; c) S. Kawai, T. Nakashima, Y. Kutsunugi, H. Nakagawa, H. Nakano, T. Kawai, J. Mater. Chem. 2009, 19, 3606 – 3611. [11] a) S. Fukumoto, T. Nakashima, T. Kawai, Angew. Chem. Int. Ed. 2011, 50, 1565 – 1568; Angew. Chem. 2011, 123, 1603 – 1606; b) S. Fukumoto, T. Nakashima, T. Kawai, Eur. J. Org. Chem. 2011, 5047 – 5053; c) T. Nakashima, R. Fujii, T. Kawai, Chem. Eur. J. 2011, 17, 10951 – 10957. [12] S. Aloı¨se, M. Sliwa, Z. Pawlowska, J. Rehault, J. Dubois, O. Poizat, G. Buntinx, A. Perrier, F. Maurel, S. Yamaguchi, M. Takeshita, J. Am. Chem. Soc. 2010, 132, 7379 – 7390. [13] a) K. Morinaka, T. Ubukata, Y. Yokoyama, Org. Lett. 2009, 11, 3890 – 3893; b) H. Ogawa, K. Takagi, T. Ubukata, A. Okamoto, N. Yonezawa, S. Delbaere, Y. Yokoyama, Chem. Commun. 2012, 48, 11838 – 11840. [14] R. Gçstl, B. Kobin, L. Grubert, M. Patzel, S. Hecht, Chem. Eur. J. 2012, 18, 14282 – 14285. [15] S. Pu, C. Zheng, Q. Sun, G. Liu, C. Fan, Chem. Commun. 2013, 49, 8036 – 8038. [16] a) W. Li, C. Jiao, X. Li, Y. Xie, K. Nakatani, H. Tian, W. Zhu, Angew. Chem. Int. Ed. 2014, 53, 4603 – 4607; b) W. Li, X. Li, Y. Xie, Y. Wu, M. Li, X. Y. Wu, W. H. Zhu, H. Tian, Sci. Rep. 2015, 5, 9186. [17] G. Guichard, I. Huc, Chem. Commun. 2011, 47, 5933 – 5941. [18] a) T. Nakashima, K. Yamamoto, Y. Kimura, T. Kawai, Chem. Eur. J. 2013, 19, 16972 – 16980; b) T. Nakashima, K. Imamura, K. Yamamoto, Y. Kimura, S. Katao, Y. Hashimoto, T. Kawai, Chem. Eur. J. 2014, 20, 13722 – 13729.
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Full Paper [19] a) M. Irie, K. Sayo, J. Phys. Chem. 1992, 96, 7671 – 7674; b) M. Ohsumi, M. Hazama, T. Fukaminato, M. Irie, Chem. Commun. 2008, 3281 – 3283. [20] Gaussian 09, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. [21] J. D. Chai, M. Head-Gordon, Phys. Chem. Chem. Phys. 2008, 10, 6615 – 6620. [22] J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 2005, 105, 2999 – 3093. [23] Crystallographic data for 1 a: C28H20N2S3, a = 11.8375(6), b = 12.8279(7), c = 15.7808(8) æ, a = 95.9310(13), b = 101.9037(14), g = 92.0425(13)8, triclinic, space group P¢1(#2), Z = 4, V = 2328.2(2) æ3, 1calcd = 1.371 gcm¢3, 39714 reflections measured, 10666 unique reflections (Rint = 0.0738); structure solved by direct methods and refined with a full matrix against all F2 data; hydrogen atoms were calculated in riding positions; wR = 0.1729, R = 0.0625. CCDC-1051196 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. [24] Crystallographic data for 2 a: C29H21NS3, a = 23.2685(5), b = 10.1171(2), c = 10.1904(2) æ, a = b = g = 908, orthorhomibic, space group Pna21(#33), Z = 4, V = 2398.90(10) æ3, 1calcd = 1.328 gcm¢3 ; 22949 reflections measured, 5495 unique reflections (Rint = 0.0339); structure solved by direct methods and refined with a full matrix against all F2 data; hydrogen atoms were calculated in riding positions; wR = 0.0892, R = 0.0372. CCDC-1051198 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.
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[25] Crystallographic data for 3 a: C29H21NS3, a = 12.2985(3), b = 12.8668(3), c = 15.5198(3) æ, a = 98.5856(7), b = 103.0839(7), g = 93.3441(7)8, triclinic, space group P¢1(#2), Z = 1, V = 2354.49(9) æ3, 1calcd = 1.373 gcm¢3 ; 33647 reflections measured, 8601 unique reflections (Rint = 0.0367); structure solved by direct methods and refined with a full matrix against all F2 data; hydrogen atoms were calculated in riding positions; wR = 0.0977, R = 0.0414. CCDC-1051199 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. [26] S. Kobatake, K. Uchida, E. Tsuchida, M. Irie, Chem. Commun. 2002, 2804 – 2805. [27] H. Suezawa, T. Yoshida, Y. Umezawa, S. Tsuboyama, M. Nishio, Eur. J. Inorg. Chem. 2002, 3148 – 3155. [28] S. Aloı¨se, M. Sliwa, G. Buntinx, S. Delbaere, A. Perrier, F. Maurel, D. Jacquemin, M. Takeshita, Phys. Chem. Chem. Phys. 2013, 15, 6226 – 6234. [29] a) M. Nishio, M. Hirota, Y. Umezawa, The CH/p interaction, Wiley-VCH, New York, 1998; b) S. Tsuzuki, K. Honda, T. Uchimaru, M. Mikami, K. Tanabe, J. Am. Chem. Soc. 2000, 122, 3746 – 3753. [30] A. J. Goodman, E. C. Breinlinger, C. M. Mclntosh, L. N. Grimaldi, V. M. Rottelo, Org. Lett. 2001, 3, 1531 – 1534. [31] a) O. Galangau, Y. Kimura, R. Kanazawa, T. Nakashima, T. Kawai, Eur. J. Org. Chem. 2014, 7165 – 7173; b) O. Galangau, T. Nakashima, F. Maurel, T. Kawai, Chem. Eur. J. 2015, 21, 8471 – 8482. [32] Crystallographic data for 4 a: C29H22N2OS3, a = 7.4606(2), b = 28.2772(8), c = 11.7003(3) æ, a = 90, b = 96.0039(8), g = 908, monoclinic, space group P121/n1(#14), Z = 4, V = 2454.81(11) æ3, 1calcd = 1.382 gcm¢3 ; 41939 reflections measured, 5631 unique reflections (Rint = 0.0501); structure solved by direct methods and refined with a full matrix against all F2 data; hydrogen atoms were calculated in riding positions; wR = 0.09927, R = 0.0403. CCDC-1051197 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. [33] T. Sumi, Y. Takagi, A. Yagi, M. Morimoto, M. Irie, Chem. Commun. 2014, 50, 3928 – 3930. Manuscript received: April 1, 2015 Final article published: June 12, 2015
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Full Paper
Electrocyclic Reactions
Photon-Quantitative 6p-Electrocyclization of a Diarylbenzo[b]thiophene in Polar Medium Ruiji Li,[a] Takuya Nakashima,*[a] Olivier Galangau,[a, b] Shunsuke Iijima,[a] Rui Kanazawa,[a] and Tsuyoshi Kawai*[a, b] Abstract: The high reactivity of 6p-electrocyclization in polar solvents has remained one of the important challenges for diarylethenes because of the emergence of a twisted intramolecular charge transfer (TICT) state at the excited state in such polar media, which usually quenches the photocyclization reaction. Herein we report on the preparation and highly efficient photocyclization of 2,3-diarylbenzo[b]thiophenes with nonsymmetric side-aryl units in a polar solvent. While the dithiazolylbenzo[b]thiophene showed a suppressed
Introduction Photoresponsive molecular materials have attracted much interest because their physicochemical properties are modulated with precise spatiotemporal control in a remote and noninvasive manner.[1–7] In particular, photoswitching systems based on diarylethenes[8, 9] and their aromatic analogs[10] have been extensively studied because they undergo thermally irreversible photoisomerization based on 6p-electrocyclization even in the solid state. Recent studies on these classes of molecules have highlighted their high photosensitivity in the electrocyclization reaction.[11–16] These contributions made an extensive effort to control the molecular folding geometry suitable for the photocyclization reaction proceeding in a conrotatory manner.[8a] For example, we have recently reported a dithiazolylbenzo[b]thiophene derivative, which showed a photon-quantitative cyclization reaction in hexane.[11a] This extremely high photochromic reactivity of the molecule was explained in terms of noncovalent interactions such as S/N and CH/N interactions, which spe[a] R. Li, Dr. T. Nakashima, Dr. O. Galangau, S. Iijima, R. Kanazawa, Prof. T. Kawai Graduate School of Materials Science Nara Institute of Science and Technology, NAIST 8916-5 Takayama, Ikoma, Nara 630-0192 (Japan) E-mail: [email protected] [email protected] [b] Dr. O. Galangau, Prof. T. Kawai NAIST-CEMES International Collaborative Laboratory for Supraphotoactive Systems Centre d’Êlaboration de Mat¦riaux et d’Etudes Structurales, CEMES 29, rue Jeanne Marvig, BP 94347, Toulouse 31055 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500328. Chem. Asian J. 2015, 10, 1725 – 1730
quantum yield of 6p-electrocyclization of 54 % in methanol, the replacement of a thiazole unit with a thiophene ring led to a photon-quantitative 6p-cyclization reaction. The nonsymmetrical modification into the side-aryl units was considered to enhance the CH/p interactions between side-aryl units to support a photoreactive conformation in methanol. The stabilization of the photochromic reactive conformation is expected to suppress the formation of the TICT state at the excited state, leading to highly efficient photoreactivity.
cifically stabilize the photochromic reactive conformation in less polar solvents. A related strategy has also been employed by Nakatani, Tian, and Zhu.[16] These methodologies to regulate the rotational configuration with intra- and intermolecular interactions are in parallel with the design strategy for oligoarylene molecules forming specific foldamers.[17, 18] Therefore, the appropriate placement of interacting moieties including heteroatoms in a molecule is of major importance to control the conformational behavior. However, the high reactivity in polar solvents such as methanol has remained one of the important challenges for this class of molecules because of the lack of an effective design to control the molecular geometry of photochromes in polar solvents, wherein the hydrogen bonding interaction is less effective. Moreover, the introduction of polar units into diarylethene-based photochromes to enhance the solubility in polar solvents often results in the formation of a twisted intramolecular charge transfer (TICT) state in the excited state, which adversely affects the efficiency of the ringcyclization reaction.[19] Herein we report the design of photochromic molecules based on diarylbenzo[b]thiophenes (Figure 1) which exhibit a highly efficient 6p-electrocyclization reaction in methanol. Although the dithiazolylbenzothiophene 1 a[11a] with symmetrically modified side aryl units showed an ordinary photocyclization quantum yield of 54 % in methanol, the nonsymmetrical introduction of side-aryl units improves the reactivity up to unity at most. The effect of chemical modifications on the conformational preference is discussed in terms of the contribution of noncovalent interactions including CH/p interactions between side-aryl units to stabilize the photoreactive conformation in particular.
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Full Paper The spectroscopic properties of 1 a–3 a were evaluated in hexane and methanol (Table 1). Upon UV irradiation, both solutions of 2 a and 3 a in methanol underwent a spectral change with a decrease in absorbance at around 300 nm and an increase of the band at 610 nm, in a manner similar to that of the parent molecule 1 a[11a] (Figure 2). Each spectral change was accompanied by an isosbestic point at 315 and 320 nm for 2 and 3, respectively. These spectral shifts are a clear indication of the formation of closed-ring isomers 2 b and 3 b, which was confirmed by 1H NMR spectroscopy in CDCl3 (Figures S5 and S8 in the Supporting Information) after isolation by reverse phase HPLC using methanol as an eluent. Next, the optimized geometries and the frontier molecular orbitals (FMOs) for 1 a were compared with those of 2 a and 3 a by DFT calculations using the Gaussian 09 suite.[20] For the calculations, the wB97XD DFT functional[21] with the 6-31G(d)
Figure 1. Photochromic reaction and chemical structures of diarylbenzo[b]thiophenes.
Table 1. Absorption maxima and coefficients of the open- and closedring isomers of 1-3, together with the quantum yields in solutions.
Results and Discussion
Isomer
Fo¢c[d]
Fc¢o[e]
One of the side-aryl thiazole units of 1 a was replaced with a phenylthiophene unit in 2 a and 3 a (Figure 1) to investigate specific intramolecular interactions affecting the conformational behavior. 2,3-Diarylbenzo[b]thiophenes 2 a and 3 a were synthesized as shown in Scheme 1. Newly synthesized molecules
lmax [nm] (e [104 M¢1 cm¢1])
1 a[a] 1 b[a] 2a 2b 3a 3b
307 (2.9)[b] 597 (0.95)[b] 290 (3.4)[c] 612 (1.1)[c] 300 (3.2)[c] 615 (0.92)[c]
0.98,[b] 0.54[c] – 0.50,[b] 0.91[c] – 0.71,[b] 0.99[c] –
– 0.008[b] – 0.007[b] – 0.036[b]
[a] Ref. [11a]. [b] In hexane. [c] In methanol. [d] lirrad. = 313 nm.[e] lirrad. = 480 nm.
Scheme 1. Synthesis of 2,3-diarylbenzo[b]thiophenes: a) 2 m K3PO4 aq., [Pd(PPh3)4], 1,4-dioxane.
were characterized by 1H and 13C NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis (see the Supporting Information for details). The 2,3-diarylbenzo[b]thiophenes were synthesized via simple Suzuki–Miyaura cross-coupling reactions between the corresponding aryl units under identical conditions. The sequence of connection in the tri-arylene structures was controlled by exploiting the different reactivities of 2- and 3-positions in 2,3-dibromobenzo[b]thiophene. A thienyl pinacol borate (Ar1) was first reacted with dibromobenzothiophene followed by the coupling with a thiazolyl borate (Ar2) to form 2 a and vice versa for the synthesis of 3 a. The sequence of aryl units was confirmed by X-ray single-crystal analysis for all molecules. Chem. Asian J. 2015, 10, 1725 – 1730
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Figure 2. UV/Vis absorption spectral change of 2 and 3 in methanol (concentration: 2.0 Õ 10¢5 m) upon UV irradiation (l = 365 nm). Dotted line: open form, bold line: photostationary state.
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Full Paper basis set was employed. The solvent polarity was taken into account by using the polarizable continuum model (PCM).[22] All molecules gave similar optimized structures with the photoreactive conformation as the most stable geometries (Figure S12 in the Supporting Information). A negligible difference between 1 a, 2 a, and 3 a was also found in the shape of FMOs (HOMO and LUMO) (Figures S14–S16 in the Supporting Information). The HOMOs were delocalized in the entire molecule, whereas the LUMOs were rather localized in a thiazolyl sidearyl group for all molecules. The energy differences between the open-ring isomers a-form and the closed-ring isomers bform were also similar for all molecules (Figure S12). These three molecules successfully gave single crystals[23–25] from methanol solution and exhibited photoreactive conformations with distances between the photoreactive carbon atoms below 3.7 æ[26] (Figure 3, see also Figure S21, Tables S1
Figure 3. Crystal structures of (a) 1 a, (b) 2 a and (c) 3 a with the indication of close contacts of noncovalent interactions (dotted lines). The dots indicate the centroids of five-membered aryl rings.
and S2 in the Supporting Information). Since the packing modes of molecules in the crystals were different from each other with different space groups and intermolecular packing manners (Table S1 in the Supporting Information), the atomic distances of intramolecular interactions could not be directly compared. Compound 1 a gave photoreactive conformers very similar to that found in the single crystal prepared from hexane solution.[11a] The CH/S and CH/N interactions between those atoms on the central benzothiophene and the side-aryl units were suggested from the close atomic contacts (H5-S1: 3.058; H4-N1: 2.665 æ in Figure 3 b) for 2 a that appear to stabilize the specific conformation with the aid of CH/p interactions between methyl groups and aryl rings with the CH–aryl disChem. Asian J. 2015, 10, 1725 – 1730
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tance in the range of 2.8–3.1 æ.[27] On the other hand, significantly tight contacts between the sulfur atom in the central benzothiophene and the thiazolyl nitrogen atom in a side-aryl unit (S1–N1, S1’–N1’ < 3.0 æ in Figure 3 c) was observed for 3 a, suggesting that the S/N interaction harnesses the rotation about the benzothiophene-thiazole bond. Although no specific interaction was suggested between the central and the Ar2 units for 3 a, the CH/p interactions with the CH–aryl distance in the range of 2.7–3.2 æ[27] between the side-aryl units seem to support the photoreactive conformations. Overall, ordinary DFT calculations and X-ray crystallography data failed to highlight the effect of the substitution of a phenylthiazole with a phenylthiophene on the conformational behavior of 2,3-diarylbenzo[b]thiophenes. However, the ring-cyclization quantum yield (FO¢C) in hexane dropped from 0.98 for the parent compound to 0.50 and 0.71 for 2 a and 3 a, respectively. Although CH/S and CH/N interactions were expected between the central and side-aryls in 2 a as suggested in the Xray crystallographic data, the contribution of an unreactive conformation with a flipped Ar1-unit stabilized by the Me/S interaction may decrease the FO¢C in hexane.[10a,c] The absence of CH/N interaction in 3 a also resulted in a decreased FO¢C in hexane. Interestingly, the FO¢C jumped in methanol from 0.54 of 1 a to 0.91 and > 0.99 for 2 a and 3 a, respectively. The reason for the low reactivity of 1 a in methanol was attributed, at least partly, to a weakening of the H-bond-like CH/N interaction between the 4-proton on the central benzothiophene and the nitrogen in the Ar2-thiazolyl ring (Figure 1) in a protic solvent,[11a] leading to a larger conformational fluctuation regardless of the photoreactive conformation in the single crystal. The NMR chemical shift of the 4-proton (H4) (7.97 ppm) was almost identical for both 1 a and 2 a in CD3OD (Figure S18 in the Supporting Information). Therefore, the CH/N interaction was expected to have a small effect on the conformational preference of both molecules in methanol. To gain further insight into the reason for the higher photocyclization efficiency of 2 a and 3 a in comparison to 1 a, the conformational preferences for these diarylbenzothiophenes were studied by 1H NMR measurements in CD3OD (Figure S18 in the Supporting Information). Here we focus on the values of chemical shift of two methyl groups and the splitting between them (Dd), which efficiently probe the substantial ring-current effect operating on these methyl groups (Table 2) and relative stability of the highly symmetric C2-conformation around the photoreactive 6p system.[11a] Each methyl peak was assigned to either Ar1- or Ar2-methyl groups based on differential nuclear Overhauser effect (1D NOE) measurements (Figure S20 in the Supporting Information). For 1H NMR spectral simulations, the GIAO model at the PBEPBE/6-311 + + G(2d,p) level was used.[28] Compound 2 a gave a set of peaks at 2.06 (Ar2-thiazole) and 2.28 ppm (Ar1-thiophene) with Dd of 0.22 ppm. The former peak appeared more upfield than that of 1 a (2.08 ppm). On the other hand, 3 a, which exhibited a photon-quantitative reactivity in methanol, provided the peaks at 2.11 (Ar1-thiazole) and 2.10 ppm (Ar2-thiophene) with a surprisingly small peak splitting of only Dd = 0.01 ppm. The thiazolyl-methyl peak at 2.11 ppm was a more upfield signal compared to the Ar1-thia-
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Full Paper Table 2. Summary of the chemical shifts of methyl groups of 1 a–3 a in CD3OD together with simulated values[a,b] (in parentheses) based on the GIAO model at the PBEPBE/6-311 + + G(2d,p) level.
Compd.
Ar1-Me [ppm]
Ar2-Me [ppM]
Dd [ppm]
1a
2.17 (2.183)[c] , (2.166)[d] 2.28 (2.112)[c] , (3.002)[d] 2.11 (2.144)[c] , (2.172)[d]
2.08 (2.243)[c] , (2.976)[d] 2.06 (2.236)[c] , (2.051)[d] 2.10 (2.166)[c] , (2.885)[d]
0.09 (0.06)[c] , (0.81)[d] 0.22 (0.124)[c] , (0.951)[d] 0.01 (0.022)[c] , (0.713)[d]
2a 3a
Figure 4. Modes of CH/p interactions working in diarylbenzo[b]thiophenes (parts of structures were given for clarity) with symmetrically and nonsymmetrically modified side-aryl units.
[a] Solvent polarity was taken into account by using the polarizable continuum model. [b] Average values of three protons to take the rotation of methyl groups into consideration. [c] Based on the most stable photoreactive conformation (see Figure S12 in the Supporting Information). [d] Based on the non-photoreactive conformation with a flipped aryl unit (see Figure S19 in the Supporting Information).
zolyl-methyl one of 1 a (2.17 ppm). These upfield shifts of thiazolyl-methyl in comparison with 1 a could be attributed to the larger ring current effect of the oppositely facing thienyl ring on the thiazolyl-methyl groups for 2 a and 3 a, which also indicated a stronger CH/p interaction between the electron-deficient methyl and the electron-rich thiophene. The thienylmethyl peaks of 2.28 for 2 a and 2.11 ppm for 3 a were also found upfield in comparison with the synthetic intermediates of phenylthienyl parts (2.4–2.7 ppm), also supporting the operation of CH/p interaction. The DFT simulation of the 1H NMR spectrum of 3 a based on an optimized photoreactive conformation well reproduced the experimental result including a very small peak splitting Dd (Table 1). Thus, the conformation of 3 a is expected to be well controlled into the photoreactive one to achieve the photon-quantitative reactivity in methanol. It is suggested that the attractive interactions to harness the rotations around the single bond between the central and side-aryl units are not always necessary at both sides for controlling the molecular geometry. The results of 2 a and 3 a with enhanced photosensitivity in methanol let us to hypothesize that the nonsymmetrical modification into side-aryl units in terms of electron-donating or deficient properties may direct the side-aryl pair to reinforce the CH/p interactions. The importance of CH/p interaction on stabilizing the photoreactive conformation was partly verified by a decrease in FO¢C of a derivative of 1 a in which methyl groups on the reactive carbon atoms were substituted by a proton and a methoxy group (Figure S22 in the Supporting Information). The CH/p interaction is supposed to be composed of electrostatic and dispersion interactions as attractive forces.[29] While the contribution of the electrostatic force to this interaction has been thought to be relatively small, the enhanced electron-donating property in an aryl ring owing to the substitution of thiazole with thiophene should practically increase the strength of the CH/p interaction.[26] The nature of the CH/p interaction characterized by the dispersion force is most likely to work more efficiently in polar solvents than in less polar ones with the aid of the solvophobic effect.[30] The idea is that the opposed CH/p interactions between Ar1 and Ar2 equally operate for a symmetrically modified molecule like Chem. Asian J. 2015, 10, 1725 – 1730
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1 a, while the stronger CH/p interaction operating between the electron-deficient thiazolyl-methyl group and the electronrich thiophene ring in the nonsymmetrically modified one positions these moieties close together (Figure 4). As a result, another methyl-aryl pair is expected to come close to each other. As a related matter, we have recently demonstrated that a significant mono-directional CH/p interaction was still effective to fix a photoreactive conformation in a diarylethene and a terarylene derivative of nonsymmetrical structures.[31] To further support the above hypothesis, we additionally synthesized 2,3-diarylbenzo[b]thiophenes 4 a with an electrondonating methoxy (OMe) group to make one of the side-aryl units electron-richer relative to the other one (Figure 5). This compound showed a reversible photochromic reaction in methanol in a similar manner to the parent molecule 1 a (Figure S23 in the Supporting Information). Meanwhile, a photoreactive conformer was given in a single crystal of 4 a prepared from methanol solution (Figure 6, also see Table S1 in the Supporting Information). The noncovalent interactions including CH/N and S/N interactions with the atomic distances equiva-
Figure 5. Introduction of an electron-donating (OMe) group into 1 a to form 4 a.
Figure 6. Crystal structures of 4 a with the indication of close contacts of noncovalent interactions (dotted lines). The dots indicate the centroids of five-membered aryl rings.
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Full Paper lent to or shorter than the sum of van der Waals radii of each atom (H: 1.2, N: 1.55, S: 1.8 æ) are expected to support the photoreactive conformations in a manner similar to that of the parent molecule 1 a (Figure 3). It should be noted that the distances between the methyl groups and the centroids of opposing thiazolyl rings were shorter than 3.0 æ, corresponding to the effective distance of CH/p interaction.[27] The 1H NMR chemical shifts of two methyl groups on sidearyl units appeared at 2.04 and 2.15 ppm for 4 a in CD3OD (Figures S18 and S20 in the Supporting Information), showing upfield shifts in comparison with those of 1 a (2.06 and 2.17 ppm, respectively). These slight upfield shifts from 1 a to 4 a suggested that the stronger ring current effect operates on these methyl protons through the reinforced CH/p interactions in 4 a owing to the asymmetrization of aide aryl units in terms of the electron-rich or -deficient property. As a consequence, the ring-cyclization quantum yield in methanol substantially increased from 0.54 for 1 a to 0.70 for 4 a in methanol. This increase in FO¢C further supports the reinforcement of CH/p interactions between the pair of electron-deficient and -richer side aryl units (Figure 4), stabilizing the photoreactive conformation in methanol.
Conclusions We report highly sensitive photochromic molecules with nonsymmetrical modification on side-aryl units based on 2,3-dithiazolylbenzo[b]thiophene in methanol. The modifications on the diarylbenzothiophenes effectively increased the efficiency of the photocyclization reaction to unity at most in methanol. The combination of electron-deficient and electron-rich aromatics as side aryl units was considered to reinforce a CH/p interaction between the electron-rich aryl ring and the methyl group attached to the electron-deficient aryl unit, which is highlighted in methanol, stabilizing the photoreactive conformation. The present results would give a useful insight and demonstrate a wide flexibility in the molecular design of photon-quantitative photochromes based on a 6p-system.
Experimental Section 2,3-Diarylbenzo[b]thiophenes were synthesized through the sequential step-by-step Pd-catalyzed cross-coupling reactions between 2,3-dibromobenzo[b]thiophene and the corresponding arylpinacol borates (see the Supporting Information for details). 1 H NMR spectra were recorded on a JEOL AL-300 spectrometer (300 MHz). Temperature-dependent 1H NMR spectra were measured using a JEOL ECP 400 spectrometer (400 MHz). Separative HPLC was performed on a JASCO LC-2000 Plus Series. Mass spectra were measured with a JEOL JMS-700 mass spectrometer. Absorption spectra in solution were studied with a JASCO V-670 spectrophotometer. UV irradiation was carried out using a Panasonic Aicure UV curing system (LED, l = 365 nm) as the exciting light source. X-ray crystallographic analyses were carried out with a Rigaku R-AXIS RAPID/s Imaging Plate diffractometer with MoKa radiation. The structures were solved by direct method (SHELXL-97) and refined by the full-matrix least-squares on F2. All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were Chem. Asian J. 2015, 10, 1725 – 1730
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placed using AFIX instructions. DFT calculations were performed with Gaussian 09 at the wB97XD/6–31G(d) level. Photoirradiation with UV and visible light under microscope observation was conducted using a mercury lamp for epi-illumination through BP330– 385 and BP520–550 filters, respectively. Absolute ring photoreaction yield (FO¢C and FC¢O) values were obtained on a Shimadzu QYM-01 setup.[33]
Acknowledgements The authors thank Mr. F. Asanoma, Mr. S. Katao and Ms. Y. Nishikawa at NAIST for their assistance with VT-NMR, X-ray crystallography and mass spectra, respectively. This work was partly supported by Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan under Green Photonics Project at NAIST and the Grand-in-Aid for on Innovative Areas of “PhotoSynergetics”, NAIST Green-Photonics Project and the Mazda Foundation. Keywords: crystals · noncovalent photochromism · quantum yield
interactions
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[1] H. Dìrr and H. Bouas-Laurent, Photochromism: Molecules and Systems; Elsevier, Amsterdam, 2003. [2] B. Feringa, Molecular Switches, Wiley-VCH, Weinheim, 2001. [3] I. Yildiz, E. Deniz, F. M. Raymo, Chem. Soc. Rev. 2009, 38, 1859 – 1867. [4] R. Gçstl, A. Senf, S. Hecht, Chem. Soc. Rev. 2014, 43, 1982 – 1996. [5] K. Mutoh, H. Arai, Y. Kobayashi, J. Abe, Pure Appl. Chem. 2015, 87, 511 – 523. [6] J. Gu¦rin, A. L¦austic, S. Delbaere, J. Berthet, R. Guillot, C. Ruckebusch, R. M¦tivier, K. Nakatani, M. Orio, M. Sliwa, P. Yu, Chem. Eur. J. 2014, 20, 12279 – 12288. [7] A. M. Asadirad, S. Boutault, Z. Erno, N. R. Branda, J. Am. Chem. Soc. 2014, 136, 3024 – 3027. [8] a) M. Irie, Chem. Rev. 2000, 100, 1685 – 1716; b) M. Irie, T. Fukaminato, K. Matsuda, S. Kobatake, Chem. Rev. 2014, 114, 12174 – 12277. [9] a) H. Tian, S. Yang, Chem. Soc. Rev. 2004, 33, 85 – 97; b) H. Tian, S. Wang, Chem. Commun. 2007, 781 – 792. [10] a) T. Nakashima, K. Atsumi, S. Kawai, T. Nakagawa, Y. Hasegawa, T. Kawai, Eur. J. Org. Chem. 2007, 3212 – 3218; b) S. Kawai, T. Nakashima, K. Atsumi, T. Sakai, M. Harigai, Y. Imamoto, H. Kamikubo, M. Kataoka, T. Kawai, Chem. Mater. 2007, 19, 3479 – 3483; c) S. Kawai, T. Nakashima, Y. Kutsunugi, H. Nakagawa, H. Nakano, T. Kawai, J. Mater. Chem. 2009, 19, 3606 – 3611. [11] a) S. Fukumoto, T. Nakashima, T. Kawai, Angew. Chem. Int. Ed. 2011, 50, 1565 – 1568; Angew. Chem. 2011, 123, 1603 – 1606; b) S. Fukumoto, T. Nakashima, T. Kawai, Eur. J. Org. Chem. 2011, 5047 – 5053; c) T. Nakashima, R. Fujii, T. Kawai, Chem. Eur. J. 2011, 17, 10951 – 10957. [12] S. Aloı¨se, M. Sliwa, Z. Pawlowska, J. Rehault, J. Dubois, O. Poizat, G. Buntinx, A. Perrier, F. Maurel, S. Yamaguchi, M. Takeshita, J. Am. Chem. Soc. 2010, 132, 7379 – 7390. [13] a) K. Morinaka, T. Ubukata, Y. Yokoyama, Org. Lett. 2009, 11, 3890 – 3893; b) H. Ogawa, K. Takagi, T. Ubukata, A. Okamoto, N. Yonezawa, S. Delbaere, Y. Yokoyama, Chem. Commun. 2012, 48, 11838 – 11840. [14] R. Gçstl, B. Kobin, L. Grubert, M. Patzel, S. Hecht, Chem. Eur. J. 2012, 18, 14282 – 14285. [15] S. Pu, C. Zheng, Q. Sun, G. Liu, C. Fan, Chem. Commun. 2013, 49, 8036 – 8038. [16] a) W. Li, C. Jiao, X. Li, Y. Xie, K. Nakatani, H. Tian, W. Zhu, Angew. Chem. Int. Ed. 2014, 53, 4603 – 4607; b) W. Li, X. Li, Y. Xie, Y. Wu, M. Li, X. Y. Wu, W. H. Zhu, H. Tian, Sci. Rep. 2015, 5, 9186. [17] G. Guichard, I. Huc, Chem. Commun. 2011, 47, 5933 – 5941. [18] a) T. Nakashima, K. Yamamoto, Y. Kimura, T. Kawai, Chem. Eur. J. 2013, 19, 16972 – 16980; b) T. Nakashima, K. Imamura, K. Yamamoto, Y. Kimura, S. Katao, Y. Hashimoto, T. Kawai, Chem. Eur. J. 2014, 20, 13722 – 13729.
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Full Paper [19] a) M. Irie, K. Sayo, J. Phys. Chem. 1992, 96, 7671 – 7674; b) M. Ohsumi, M. Hazama, T. Fukaminato, M. Irie, Chem. Commun. 2008, 3281 – 3283. [20] Gaussian 09, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. [21] J. D. Chai, M. Head-Gordon, Phys. Chem. Chem. Phys. 2008, 10, 6615 – 6620. [22] J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 2005, 105, 2999 – 3093. [23] Crystallographic data for 1 a: C28H20N2S3, a = 11.8375(6), b = 12.8279(7), c = 15.7808(8) æ, a = 95.9310(13), b = 101.9037(14), g = 92.0425(13)8, triclinic, space group P¢1(#2), Z = 4, V = 2328.2(2) æ3, 1calcd = 1.371 gcm¢3, 39714 reflections measured, 10666 unique reflections (Rint = 0.0738); structure solved by direct methods and refined with a full matrix against all F2 data; hydrogen atoms were calculated in riding positions; wR = 0.1729, R = 0.0625. CCDC-1051196 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. [24] Crystallographic data for 2 a: C29H21NS3, a = 23.2685(5), b = 10.1171(2), c = 10.1904(2) æ, a = b = g = 908, orthorhomibic, space group Pna21(#33), Z = 4, V = 2398.90(10) æ3, 1calcd = 1.328 gcm¢3 ; 22949 reflections measured, 5495 unique reflections (Rint = 0.0339); structure solved by direct methods and refined with a full matrix against all F2 data; hydrogen atoms were calculated in riding positions; wR = 0.0892, R = 0.0372. CCDC-1051198 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.
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[25] Crystallographic data for 3 a: C29H21NS3, a = 12.2985(3), b = 12.8668(3), c = 15.5198(3) æ, a = 98.5856(7), b = 103.0839(7), g = 93.3441(7)8, triclinic, space group P¢1(#2), Z = 1, V = 2354.49(9) æ3, 1calcd = 1.373 gcm¢3 ; 33647 reflections measured, 8601 unique reflections (Rint = 0.0367); structure solved by direct methods and refined with a full matrix against all F2 data; hydrogen atoms were calculated in riding positions; wR = 0.0977, R = 0.0414. CCDC-1051199 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. [26] S. Kobatake, K. Uchida, E. Tsuchida, M. Irie, Chem. Commun. 2002, 2804 – 2805. [27] H. Suezawa, T. Yoshida, Y. Umezawa, S. Tsuboyama, M. Nishio, Eur. J. Inorg. Chem. 2002, 3148 – 3155. [28] S. Aloı¨se, M. Sliwa, G. Buntinx, S. Delbaere, A. Perrier, F. Maurel, D. Jacquemin, M. Takeshita, Phys. Chem. Chem. Phys. 2013, 15, 6226 – 6234. [29] a) M. Nishio, M. Hirota, Y. Umezawa, The CH/p interaction, Wiley-VCH, New York, 1998; b) S. Tsuzuki, K. Honda, T. Uchimaru, M. Mikami, K. Tanabe, J. Am. Chem. Soc. 2000, 122, 3746 – 3753. [30] A. J. Goodman, E. C. Breinlinger, C. M. Mclntosh, L. N. Grimaldi, V. M. Rottelo, Org. Lett. 2001, 3, 1531 – 1534. [31] a) O. Galangau, Y. Kimura, R. Kanazawa, T. Nakashima, T. Kawai, Eur. J. Org. Chem. 2014, 7165 – 7173; b) O. Galangau, T. Nakashima, F. Maurel, T. Kawai, Chem. Eur. J. 2015, 21, 8471 – 8482. [32] Crystallographic data for 4 a: C29H22N2OS3, a = 7.4606(2), b = 28.2772(8), c = 11.7003(3) æ, a = 90, b = 96.0039(8), g = 908, monoclinic, space group P121/n1(#14), Z = 4, V = 2454.81(11) æ3, 1calcd = 1.382 gcm¢3 ; 41939 reflections measured, 5631 unique reflections (Rint = 0.0501); structure solved by direct methods and refined with a full matrix against all F2 data; hydrogen atoms were calculated in riding positions; wR = 0.09927, R = 0.0403. CCDC-1051197 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. [33] T. Sumi, Y. Takagi, A. Yagi, M. Morimoto, M. Irie, Chem. Commun. 2014, 50, 3928 – 3930. Manuscript received: April 1, 2015 Final article published: June 12, 2015
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