DOI: 10.1002/chem.201402027
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& Boron Compounds
Organic Fluorescent Thermometers Based on Borylated Arylisoquinoline Dyes Vnia F. Pais,[a] Jos M. Lassaletta,[b] Rosario Fernndez,[c] Hamdy S. El-Sheshtawy,[d] Abel Ros,*[b] and Uwe Pischel*[a] sensitivity even reaches values of up to 2.4 % K1. The thermometer function is interpreted as the interplay between excited ICT states of different geometry. In addition, the formation of an intramolecular Lewis pair can be followed by 11 B NMR spectroscopy. This provides a handle to monitor temperature-dependent ground-state geometry changes of the dyes. The role of steric hindrance is addressed by the inclusion of a derivative that lacks the Lewis pair formation.
Abstract: Borylated arylisoquinolines with redshifted internal charge-transfer (ICT) emission were prepared and characterized. Upon heating, significant fluorescence quenching was observed, which forms the basis for a molecular thermometer. In the investigated temperature range (283–323 K) an average sensitivity of 1.2 to 1.8 % K1 was found for the variations in fluorescence quantum yield and lifetime. In the physiological temperature window (298–318 K) the average
Introduction
Previous studies of organic compounds in which charge transfer was exploited for the photophysical design of molecular thermometers include rhodamine dyes (rhodamine B and rhodamine 3B),[3, 6] 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) derivatives,[7, 8] 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan),[8] N,N-dimethylaminobenzonitrile,[9] and a dipyren-1yl(2,4,6-triisopropylphenyl)borane.[10] However, on other occasions, the temperature-controlled formation of exciplexes,[11, 12] excimers in bis-pyrenes[13] or bis-napthalenes,[14] and the phenomenon of delayed fluorescence, observed for C70 fullerene[15, 16] or the acridine yellow dye,[17] was implemented with equal success for the purpose of molecular thermometry. Fluorophore-modified organic polymers in which temperature-dependent conformational changes and connected variations of the microenvironment of the fluorophore constitute the basic design principle were also used abundantly.[4, 18–24] Inorganic luminescent compounds and materials such as nanoparticles,[25–29] metal–organic frameworks,[30] metal clusters,[31] and organometallic complexes[32–38] have received increased attention as well. Recently, we reported on a new class of internal chargetransfer (ICT) fluorophores that are based on borylated arylisoquinoline (BAI) dyes.[39, 40] These compounds show redshifted fluorescence that can be fine-tuned by electron-donor substitution. The structural integration of a boronic ester moiety confers photophysical properties to these dyes that are not observed for the nonborylated analogues. The ICT character of the dyes was established by correlations of fluorescence with redox properties, solvatochromic effects, and density functional theory (DFT) calculations.[39] Akin to the observations made for the archetypal NBD fluorophore,[7] it was anticipated that the temperature-dependent interconversion between fluorescent ICT states and nonfluorescent “twisted” intramolecular charge-transfer (TICT) states
Temperature is a fundamental physical parameter that needs to be known and controlled in the most diverse applications, from engineering to life sciences. In recent years the exploitation of the temperature dependence of photophysical phenomena has given rise to the development of molecular thermometers that deliver an optical output response.[1, 2] The key principle for such devices lies in the temperature-controlled equilibration or competition between two states that have differentiated optical signatures; these are most often fluorescence or phosphorescence. Unlike the majority of conventional thermometers, chemical systems provide the appealing potential for small-scale integration that has found applications in, for example, microfluidic systems[3] or for temperature mapping in cells.[4, 5]
[a] Dr. V. F. Pais, Dr. U. Pischel CIQSO—Center for Research in Sustainable Chemistry and Department of Chemical Engineering, Physical Chemistry and Organic Chemistry, University of Huelva Campus El Carmen s/n, 21071 Huelva (Spain) E-mail: [email protected] [b] Prof. Dr. J. M. Lassaletta, Dr. A. Ros Institute for Chemical Research (CSIC-US) c/ Amrico Vespucio 49, 41092 Seville (Spain) E-mail: [email protected] [c] Prof. Dr. R. Fernndez Department of Organic Chemistry, Universidad of Seville C/Prof. Garca Gonzlez 1, 41012 Seville (Spain) [d] Dr. H. S. El-Sheshtawy Chemistry Department, Faculty of Science South Valley University, 83523 Qena (Egypt) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402027. Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper could be at the origin of a similar effect for the BAI dyes 1–3 investigated herein. Furthermore, we expected that the BN interaction (see below), which can be followed by the charac-
BN interactions for these dyes in solution. On the other hand, compound 3 contains a methyl group in the ortho-aryl position, and consequently the formation of an intramolecular Lewis pair is sterically hindered. In agreement with this connotation, the 11B NMR spectroscopic signal for dye 3 is downfieldshifted to d = 30.7 ppm (which corresponded to trigonal planar B), and the 1H NMR spectroscopic signal of the pinacolate methyl groups was found upfield-shifted at d = 0.79 ppm.
Optical properties The UV/Vis absorption and fluorescence spectra of the investigated dyes at room temperature are shown in Figure 1 (for numerical data see Table 1). Dyes 1 and 2 show significantly redshifted and broad absorption bands with maxima at 377 and 420 nm, respectively. In accordance with a recent report,[40] these are assigned to charge-transfer (CT) bands. However, the absorption maxima vary dramatically with the possibility to engage in BN interactions, and such a ground-state CT band was not seen for the sterically hindered 3 (see above). Hence, the UV/Vis absorption spectrum of 3 features a band at 283 nm that corresponds to localized p–p* transitions of the aryl residue and a sharp peak at 321 nm that is typical of isoquinolines. Furthermore, dyes 1–3 correspond well with the generally established excited-state ICT character of BAI dyes.[39] They show broad emissions in the turquoise to green-yellow
teristic 11B NMR spectroscopic signal,[41, 42] would provide a convenient handle to allow the direct monitoring of the temperature-dependent ground-state structural changes of the fluorophores.
Results and Discussion Synthesis and structural characterization of BAI dyes 1–3
Dyes 1–3 were prepared by borylation of their respective precursors A1–A3 by using a recently reported synthetic approach that is based on the IrIII-catalyzed nitrogen-directed ortho-CH borylation of arylisoquinolines.[43] The borylation reaction was carried out at 80 8C and afforded the dyes in excellent yields for products 1 and 2 (89 and 94 %) and in a moderate yield for 3 (43 %). Precursor A1 was prepared by means of a Pd0-catalyzed Suzuki coupling of 1-chloroisoquinoline with 4-methoxyphenylboronic acid.[40] For the preparation of A2, the methoxy group of A1 was treated with lithium 4-methylpiperazin-1-ide in a nucleophilic aromatic substitution.[40] Finally, A3 was obtained in a Suzuki coupling of 1chloroisoquinoline with 2methyl-4-methoxyphenylboronic acid. The described synthetic procedures are summarized in Scheme 1 (see the Experimental Section also). Dyes 1 and 2 show their 11 B NMR spectroscopic signals (in CD3CN at 293 K) at around d = 14–15 ppm, typical of tetracoordinated boron.[41, 42] Furthermore, in the 1H NMR spectra of these compounds, the pinacolate methyl groups were observed as Scheme 1. Reaction conditions: a) [Pd(PPh3)4] (3 mol %), Na2CO3 (2 equiv), THF/MeOH/H2O (see composition of sola single signal at d 1.30 ppm. vent mixture in the Experimental Section), 110 8C, overnight; b) 2-pyridinecarboxaldehyde N,N-dibenzyl hydrazine Both observations corroborate (1 mol %), [{Ir(m-OMe)cod}2] (cod = cyclooctadiene; 0.5 mol %), 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBPin; unambiguously the existence of 5 mol %), THF, 80 8C; and c) THF, argon, reflux, overnight. &
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Figure 1. Long-wavelength UV/Vis absorption bands (full lines) and corresponding fluorescence spectra (dashed lines) of the investigated compounds (1: blue; 2: red; 3: green) at 298 K. The spectra are normalized to 1 at their respective maxima.
spectral range that corresponds to fluorescence maxima between 470 and 560 nm (see Figure 1) and typically large Stokes shifts (91 nm for 1, 139 nm for 2, and 213 nm for 3). In comparison to the methoxy group in 1 and the methyl and methoxy groups in 3, the stronger electron-donating piperazinyl substituent in 2 lowers the energy of the emitting ICT state, thereby resulting in a significantly more redshifted emission. The involvement of CT in the lowest-energy transitions was corroborated by density functional theory (DFT) calculations. The results confirm that the HOMO is mainly located on the electron-donating aryl residue, whereas the LUMO has its major contribution from the isoquinolyl moiety that plays the role of the electron acceptor (see contour plots of frontier orbitals in Figure 2). The HOMO–LUMO energy gap DE qualitatively follows the trend of emission energies (see fluorescence maxima in Table 1) of the excited ICT states: DE(2) < DE(3) < DE(1). The fluorescence quantum yields of the dyes vary between 0.05 and 0.16 (see Table 1). Interestingly, dye 1 has an emission quantum yield that is three times higher than the sterically hindered dye 3; electronically they differ by only one methyl substituent. This observation proposes a relation between BN
Figure 2. Contour plots of HOMO and LUMO orbitals of the dyes 1–3 and their energies [eV].
interaction and consequently the planarization of the biaryl system on the one hand and the fluorescence quantum yield on the other hand. However, although dye 2 clearly features a BN interaction, its relatively low-lying emissive charge-transfer state (most redshifted emission maximum) preferably deactivates through nonradiative pathways. Thereby the emission quantum yield is diminished. In accordance with the discussion of related BAI dyes, this finds its explanation in the energy-gap law.[39] These few notions demonstrate that structure–property relationships are certainly more complex than it appears at first sight. Temperature-dependent geometrical changes
To obtain insight into the ground-state structural changes of the compounds upon variation of the temperature, the 11 B NMR spectroscopic signal was followed. In Figure 3 the example of dye 2 is shown. Upon changing gradually from 283 to 323 K, an accentuated shift change of the signal toward Table 1. Photophysical properties of dyes 1–3 in acetonitrile. lower field (Dd = + 2.8 ppm) was observed in CD3CN. Similar SA [% K1][i] labs [nm][a,b] lf [nm][a,c] Ff[a,d] tf [ns] ([%])[a,e] ln(A [s1])[f] Ea [kJ mol1][g] Q [%][h] changes were monitored for dye Ff tf 1 (see the Supporting Informa1 377 468 0.16 0.97 (37) 29.5 23.8 73 1.8 1.5 tion). However, the sterically hin(9650) 2.56 (63) 2 420 559 0.07 0.59 (88) 26.7 13.6 52 1.2 1.2 dered compound 3 showed no (12 150) 5.39 (12) temperature effect on the 3 283 496 0.05 1.65 (92) 28.5 20.7 72 1.8 1.6 11 B NMR spectroscopic signal (6500) 4.60 (8) (see the Supporting Informa[a] Measured at 298 K. [b] UV/Vis absorption maximum; the corresponding molar absorption coefficient tion). This does not mean that e [m1 cm1] is given in parentheses. [c] Fluorescence maximum of corrected spectra. [d] Fluorescence quantum the torsional angle between the yield measured against quinine sulfate as standard. [e] Fluorescence lifetimes measured by single-photon counting. [f] Preexponential factor according to the Arrhenius equation [Eq. (1)]. [g] Activation energy for nonaryl planes in 3 is not altered radiative decay; r2 0.996. [h] Fluorescence quenching upon heating from 283 to 323 K. [i] Average sensitivity upon heating, but this process is (in the temperature range of 283–323 K) based on the temperature-dependent quantum yield or lifetime data. not seen in the 11B NMR spectra
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Figure 4. Temperature dependence of the UV/Vis absorption (left) and fluorescence spectra (right) of dye 2 in acetonitrile upon heating from 283 to 323 K.
Figure 3. Temperature dependence of the 11B NMR spectra of dye 2 in [D3]acetonitrile. The dashed red line is meant to guide the eye (see also Figure 4). The spectra were recorded between 283 and 323 K in steps of 5 K.
owing to the absence of BN interactions. As can be deduced from the fact that even at higher temperature (343 K) the 11 B NMR spectroscopic resonance signals of dyes 1 and 2 remain at lower positions (d < 20 ppm) than that of compound 3 (d 31 ppm), the temperature increase leads to an equilibrium between the more planar low-temperature form and the twisted high-temperature form (see Scheme 2 for the exam-
Scheme 2. Schematic representation of the temperature-dependent groundstate equilibrium between the planar (left) and twisted forms (right) of borylated arylisoquinolines (R = methoxy (1) or N-methylpiperazinyl (2)).
Figure 5. Temperature dependence of a) the fluorescence lifetime, b) emission quantum yield, c) and 11B NMR spectroscopic signal of dye 2 in acetonitrile. The lines are meant to guide the eye.
ples of dyes 1 and 2). The partial thermally induced conversion between the planar form (BN) interaction and a nonplanar form (with no BN interaction) is also expressed by a reduced intensity of the CT bands (see above) of dyes 1 and 2 in the UV/Vis absorption spectrum (see Figure 4 for dye 2, and the Supporting Information).
temperature effect is caused by the change in solvent properties that would ultimately lead to variations in the polarity function and solvatochromic effects.[39] The fluorescence modulation is reversible and upon cooling back to 283 K the initial emission signal level was restored in all cases. The cycling can be repeated at least ten times without significant differences being observed (see Figure 6 for the example of dye 2). Akin to the interpretations of related probes that are based on charge-transfer phenomena, we propose the conversion of a fluorescent ICT state into a nonfluorescent (or less fluorescent) TICT state in the rationalization of our observations.[7] The temperature-dependent changes of the ground-state geometry (see above) give a hint of the conformational flexibility of the dyes and increased torsional angles upon heating. Similar effects are expected for the excited state in which the population of a less fluorescent TICT state is promoted with increasing temperature. Experimentally this assumption is indirectly evi-
Molecular fluorescent thermometer In a further step we were interested in harnessing the changes of the fluorescence of the dyes as a convenient and readily detectable output signal. Upon gradual heating from 283 to 323 K, a pronounced quenching of the ICT emission intensity that varied between 52 and 73 % was observed (see Figures 4 and 5 for the example of BAI dye 2 and the Supporting Information for the other dyes). The fluorescence maxima are not shifted (dyes 1 and 3) or suffer only a minor blueshift by approximately 8 nm for dye 2. This observation excludes that the &
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SA ¼
denced by the aforementioned fluorescence quenching for dyes with larger torsional angles (compare dyes 1 and 3; Table 1). Thus, the TICT state provides an efficient channel of nonradiative excited-state deactivation and thereby leads to the observed reduction of the fluorescence quantum yield. In the same way as observed for the quantum yield (normally monitored through fluorescence intensity), the concentration-independent excited-state lifetime tf can also be used for temperature monitoring. This bears advantages such as the independency of the signal reading on the fluorophore distribution in heterogeneous media, which for fluorescence intensity measurements can be only overcome with ratiometrically responding systems. Akin to the observations made for the quantum yield, the fluorescence lifetime is reduced with increasing temperature (Figure 5 for dye 2). With the quantum yield and lifetime data at hand, the corresponding kinetic constants for the radiative (kf) and nonradiative (knr) excited-state deactivation were calculated for each temperature [see Eq. (1)]. The data for dye 2 are plotted in Figure 7 (for the other dyes, see the Supporting Information) [Eqs. (2) and (3)]: 1 kf þ knr
ð3Þ
On the one hand, the plot of ln(knr) versus 1/T according to the Arrhenius equation [Eq. (2) with the rate constant k(T) = knr(T) at temperature T, A as pre-exponential factor, R as gas constant, and Ea as activation energy] yields Arrhenius-type behavior for all investigated dyes (Figure 7, filled circles) and the resulting activation energies (Ea) vary between 13.6 (dye 2) and 23.8 kJ mol1 (dye 1) (see Table 1). Hence, the notion of the involvement of a thermally induced nonradiative decay pathway (such as the invoked TICT) is supported by the analysis of the temperature-dependent excited-state kinetic data. The order of magnitude of the activation energies is comparable with those for related ICT chromophores.[7] However, as often observed for organic fluorophores, the radiative rate constant kf is practically independent of the temperature [see Figure 7, empty circles; with k(T) = kf(T) in Eq. (2)]. The applicability of molecular thermometers is commonly rated by the average sensitivity SA of the fluorescence quantum yield (intensity) or lifetime toward the temperature change [Eq. (3) in which X: Ff or tf].[36] The calculated values for this parameter are listed in Table 1 and are in the order of 1.2 to 1.8 % K1 for the full investigated temperature range of 283–323 K. In the physiological temperature window (298– 318 K) even higher values apply: up to 2.4 % K1 (dye 1) for the fluorescence intensity/quantum yield measurements and up to 2.0 % K1 (dye 3) for the lifetime-based thermometer. These data compare nicely with some of the best-performing molecular optical thermometers based on lanthanide complexes or those based on quantum dot luminescence.[2] On the basis of a 1 % precision of the measurement of the fluorescence intensity, a temperature resolution of approximately 0.4–0.6 8C can be estimated for the most sensitive dyes, 1 and 3.
Figure 6. Cycling of dye 2 for ten consecutive cycles of heating (to 323 K) and cooling (to 283 K).
Ff ¼ kf tf with tf ¼
1 DX X ref DT
ð2Þ
ð1Þ
Conclusion In conclusion, borylated arylisoquinolines show a pronounced dependence of their ICT fluorescence on the temperature. The sensitive (SA between 1.2 to 1.8 % K1) molecular thermometers were tested in the range of 283–323 K, including the physiologically relevant temperature window. Some of the thermometers can be operated with visible-light excitation and provide spectrally well-separated emission outputs. The interplay between planar and twisted ground-state geometries was studied by taking advantage of the 11B NMR spectroscopic signal as an indicator for the formation of an intramolecular Lewis pair. The temperature-dependent excited-state behavior, as monitored by fluorescence, is rationalized by the interplay between ICT and TICT states, which has precedence in the literature.[7] The analysis of the temperature-dependent kinetic constants provides evidence for the thermal activation of a nonradiative excited-state decay channel. The implication of
Figure 7. Arrhenius plot for the radiative (kf, empty circles) and nonradiative rate constants (knr, filled circles) of excited-state deactivation of dye 2 in acetonitrile. The major lifetime component was analyzed. Chem. Eur. J. 2014, 20, 1 – 9
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planar versus nonplanar geometries is further corroborated by observations of a reduced fluorescence quantum yield of a sterically hindered derivative. Potentially, the boronic ester opens possibilities to functionalize biologically relevant carbohydrates with the thermometer function. By building on the photophysical foundation that was laid in this work, such studies are currently underway in our laboratories.
Synthetic procedures Compounds A1, A2, and 2 were prepared following described methodologies.[40]
Experimental Section
1-(4-Methoxy-2-methylphenyl)isoquinoline (A3): A Schlenk tube was charged with a solution of the corresponding 1-chloroisoquinoline (197 mg, 1.2 mmol), 2-methyl-4-methoxyphenylboronic acid (239 mg, 1.44 mmol), Na2CO3 (254 mg, 2.4 mmol), and [Pd(PPh3)4] (41 mg, 3 mol %). After three cycles of vacuum–argon, toluene (1 mL), MeOH (0.2 mL), and H2O (0.25 mL) were added sequentially. The reaction mixture was stirred at 110 8C overnight, cooled to room temperature, quenched with H2O (10 mL), and extracted with CH2Cl2 (3 10 mL). The organic layer was dried over MgSO4, filtered, concentrated, and the residue was purified by flash chromatography on silica gel (EtOAc/n-hexane 1:3!1:1) to give A3 (220 mg, 74 %) as a light yellow viscous oil. 1H NMR (400 MHz, CDCl3): d = 8.61 (d, J = 6.0 Hz, 1 H; ArH), 7.88 (d, J = 8.4 Hz, 1 H; ArH), 7.71– 7.65 (m, 3 H; 3 ArH), 7.49 (t, J = 7.2 Hz, 1 H; ArH), 7.27 (d, J = 3.2 Hz, 1 H; ArH), 6.90–6.86 (m; 2 ArH), 3.89 (s, 3 H; OCH3), 2.08 ppm (s, 3 H; CH3); 13C NMR (100 MHz, CDCl3): d = 161.3 (C), 159.6 (C), 142.2 (CH), 138.1 (C), 136.4 (C), 131.7 (C), 130.8 (CH), 130.0 (CH), 127.8 (C), 127.6 (CH), 127.1 (C), 126.8 (CH), 119.7 (CH), 115.7 (CH), 110.9 (CH), 55.0 (OCH3), 20.1 ppm (CH3); HRMS (EI): m/z calcd for C17H14NO: 248.1075 [M + 1]; found: 248.1069.
Materials and methods 1
H and 13C NMR spectra were recorded at 400 and 100 MHz, respectively, using the solvent peak for CDCl3 as the internal reference (d = 7.26 and 77.0 ppm for 1H and 13C, respectively). 11B NMR spectra were recorded with complete proton decoupling at 160 MHz by using BF3·Et2O (d = 0.00 ppm) as internal standard. Electrospray ionization (EI) mass spectra and high-resolution mass spectra were recorded using a QTRAP mass spectrometer (hybrid triple quadrupole/linear ion trap mass spectrometer). Flash column chromatography was carried out on silica gel (35–70 or 70– 200 mm).
All reactions were carried out in oven-dried Schlenk tubes under an argon atmosphere. Anhydrous 1,4-dioxane was obtained by distillation from sodium using benzophenone as indicator. Anhydrous tetrahydrofuran (THF) was obtained by using Grubbs-type solvent drying columns. Ethyl acetate (EtOAc), methanol (MeOH), n-hexane, dichloromethane (CH2Cl2), and toluene were purchased from commercial suppliers and used without further purification. Reagents, metallic precursors, and ligands for the syntheses were purchased from commercial suppliers and used as received.
General procedure for the Ir-catalyzed borylation: Following a recently described method,[43] a dried Schlenk tube was charged with the substrate and B2Pin2 (1 equiv). After three cycles of vacuum– argon flushing, catalyst stock solution (1 mL)[47] per 0.5 mmol substrate and HBPin (5 % mol) were added. The reaction mixture was stirred at 80 8C until quantitative consumption of the starting material was achieved. The mixture was cooled to room temperature, concentrated to dryness, and purified by column chromatography or by precipitation.
The photophysical measurements were performed on air-equilibrated acetonitrile solutions, typically adjusting an optical density of approximately 0.1 at the excitation wavelength and using quartz cuvettes with 1 cm optical path length. Room-temperature (298 K) UV/Vis absorption spectra were recorded using a UV-1603 spectrophotometer from Shimadzu, and the fluorescence spectra were measured using a Cary Eclipse fluorimeter from Varian. Steady-state temperature-dependent photophysical measurements were carried out using a Perkin–Elmer Lambda 750 UV/Vis spectrophotometer or a fluorimeter from Edinburgh instruments with a Peltier system for temperature control (10–50 8C). For the temperature-dependent fluorescence measurements the samples were excited at wavelengths of minimal absorbance variation throughout the experiment. The lifetime measurements were accomplished using time-correlated single-photon-counting (Edinburgh instruments FLS 920). As excitation source a picosecond-pulsed UV-LED (EPLED 280, l = 283.2 nm, pulse width fwhm 706.5 ps) or picosecond-pulsed diode lasers (EPLED 330, l = 334.6 nm, pulse width fwhm 758.7 ps; EPLED 360, l = 367.4 nm, pulse width fwhm 745.5 ps) were used. Deconvolution analysis of the decay kinetics yielded the fluorescence lifetimes. The instrument response function was obtained with a light-scattering Ludox solution. The fluorescence quantum yields (error 10 %) were determined with quinine sulfate (Ff = 0.55 in 0.05 m sulfuric acid)[44, 45] as reference and corrected for refractive index differences between water and acetonitrile. The calculations were performed with the Gaussian 03 program.[46] The ground-state geometries were calculated by applying the Kohn–Sham density-functional theory (DFT) with the Becke3-Lee– Yang–Parr hybrid functional (B3LYP) method using the 6-31+G(d,p) basis set for the full structural optimization. Analytical frequency
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1-[4-Methoxy-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]isoquinoline (1): Following a general procedure for Ir-catalyzed borylation, starting from A1 (59 mg, 0.25 mmol) and carrying out the reaction at 80 8C, precipitation from the reaction crude with pentane, trituration, and filtration afforded 1 (85 mg, 94 %) as a yellow-green solid. 1H NMR (400 MHz, CDCl3): d = 8.83 (d, J = 8.0 Hz, 1 H; ArH), 8.48 (d, J = 6.4 Hz, 1 H; ArH), 8.18 (d, J = 8.4 Hz, 1 H; ArH), 7.89 (d, J = 8.0 Hz, 1 H; ArH), 7.81 (dd, J = 8.0, 7.2 Hz, 1 H; ArH), 7.71 (dd, J = 8.0, 7.2 Hz, 1 H; ArH), 7.60 (d, J = 6.4 Hz, 1 H; ArH), 7.34 (d, J = 2.4 Hz, 1 H; ArH), 6.87 (dd, J = 8.4, 2.4 Hz, 1 H; ArH), 3.92 (s, 3 H; OCH3), 1.41 ppm (s, 12 H; 4 CH3); 13C NMR (100 MHz, CDCl3): d = 162.2 (C), 157.4 (C), 139.1 (C), 134.5 (CH), 132.4 (CH), 131.9 (C), 128.6 (CH), 127.7 (2 CH), 127.3 (CH), 125.1 (C), 120.0 (CH), 116.6 (CH), 113.5 (CH), 80.3 (2 C), 55.3 (OCH3), 27.2 ppm (4 CH3) (CB not observed); 11B NMR (160 MHz, CDCl3): d = 13.1 ppm (br s); HRMS (EI): m/z calcd for C22H24BNO3 : 360.1768 [M + 1]; found: 360.1771. 1-[4-Methoxy-2-methyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]isoquinoline (3): Following a general procedure for Ir-catalyzed borylation, starting from A3 (62 mg, 0.25 mmol) and carrying out the reaction at 80 8C, flash chromatography on
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Full Paper silica gel (EtOAc/n-hexane 1:2) gave 3 (40 mg, 43 %) as a yellow viscous oil. 1H NMR (400 MHz, CDCl3): d = 8.55 (d, J = 5.6 Hz, 1 H; Ar H), 7.83 (d, J = 5.6 Hz, 1 H; ArH), 7.63–7.59 (m, 2 H; 2 ArH), 7.51 (d, J = 8.4 Hz, 1 H; ArH), 7.39 (t, J = 8.0 Hz, 1 H; ArH), 7.25 (br s, 1 H; ArH), 6.96 (br s, 1 H; ArH), 3.89 (s, 3 H; OCH3), 2.01 (s, 3 H; CH3), 0.78 ppm (s, 12 H; 4 CH3); 13C NMR (100 MHz, CDCl3): d = 162.5 (C), 158.7 (C), 141.7 (CH), 137.7 (C), 137.2 (C), 136.0 (C), 129.5 (CH), 129.1 (C), 127.4 (CH), 126.5 (CH), 126.4 (CH), 119.0 (CH), 118.6 (CH), 116.5 (CH), 83 (2 C), 55.3 (OCH3), 24.1 (4 CH3), 20.0 ppm (CH3) (CB not observed); 11B NMR (160 MHz, CDCl3): d = 30.7 ppm (br s); HRMS (EI): m/z calcd for C23H25BNO3 : 374.1927 [M + 1]; found: 374.1926.
[18] S. Uchiyama, N. Kawai, A. P. de Silva, K. Iwai, J. Am. Chem. Soc. 2004, 126, 3032 – 3033. [19] S. Uchiyama, Y. Matsumura, A. P. de Silva, K. Iwai, Anal. Chem. 2004, 76, 1793 – 1798. [20] K. Iwai, Y. Matsumura, S. Uchiyama, A. P. de Silva, J. Mater. Chem. 2005, 15, 2796 – 2800. [21] Y. Shiraishi, R. Miyamoto, X. Zhang, T. Hirai, Org. Lett. 2007, 9, 3921 – 3924. [22] C. Gota, S. Uchiyama, T. Yoshihara, S. Tobita, T. Ohwada, J. Phys. Chem. B 2008, 112, 2829 – 2836. [23] Y. Shiraishi, R. Miyamoto, T. Hirai, Langmuir 2008, 24, 4273 – 4279. [24] Z. Q. Guo, W. H. Zhu, Y. Y. Xiong, H. Tian, Macromolecules 2009, 42, 1448 – 1453. [25] C. D. S. Brites, P. P. Lima, N. J. O. Silva, A. Milln, V. S. Amaral, F. Palacio, L. D. Carlos, Adv. Mater. 2010, 22, 4499 – 4504. [26] H. S. Peng, M. I. J. Stich, J. B. Yu, L. N. Sun, L. H. Fischer, O. S. Wolfbeis, Adv. Mater. 2010, 22, 716 – 719. [27] E. J. McLaurin, V. A. Vlaskin, D. R. Gamelin, J. Am. Chem. Soc. 2011, 133, 14978 – 14980. [28] C. D. S. Brites, P. P. Lima, N. J. O. Silva, A. Milln, V. S. Amaral, F. Palacio, L. D. Carlos, Nanoscale 2012, 4, 4799 – 4829. [29] J. Feng, L. Xiong, S. Q. Wang, S. Y. Li, Y. Li, G. Q. Yang, Adv. Funct. Mater. 2013, 23, 340 – 345. [30] Y. J. Cui, H. Xu, Y. F. Yue, Z. Y. Guo, J. C. Yu, Z. X. Chen, J. K. Gao, Y. Yang, G. D. Qian, B. L. Chen, J. Am. Chem. Soc. 2012, 134, 3979 – 3982. [31] D. Cauzzi, R. Pattacini, M. Delferro, F. Dini, C. Di Natale, R. Paolesse, S. Bonacchi, M. Montalti, N. Zaccheroni, M. Calvaresi, F. Zerbetto, L. Prodi, Angew. Chem. 2012, 124, 9800 – 9803; Angew. Chem. Int. Ed. 2012, 51, 9662 – 9665. [32] M. Engeser, L. Fabbrizzi, M. Licchelli, D. Sacchi, Chem. Commun. 1999, 1191 – 1192. [33] J. Stehr, J. M. Lupton, M. Reufer, G. Raschke, T. A. Klar, J. Feldmann, Adv. Mater. 2004, 16, 2170 – 2174. [34] S. M. Borisov, A. S. Vasylevska, C. Krause, O. S. Wolfbeis, Adv. Funct. Mater. 2006, 16, 1536 – 1542. [35] L. H. Fischer, M. I. J. Stich, O. S. Wolfbeis, N. Tian, E. Holder, M. Schferling, Chem. Eur. J. 2009, 15, 10857 – 10863. [36] L. N. Sun, J. B. Yu, H. S. Peng, J. Z. Zhang, L. Y. Shi, O. S. Wolfbeis, J. Phys. Chem. C 2010, 114, 12642 – 12648. [37] J. B. Yu, L. N. Sun, H. S. Peng, M. I. J. Stich, J. Mater. Chem. 2010, 20, 6975 – 6981. [38] K. Miyata, Y. Konno, T. Nakanishi, A. Kobayashi, M. Kato, K. Fushimi, Y. Hasegawa, Angew. Chem. 2013, 125, 6541 – 6544; Angew. Chem. Int. Ed. 2013, 52, 6413 – 6416. [39] V. F. Pais, H. S. El-Sheshtawy, R. Fernndez, J. M. Lassaletta, A. Ros, U. Pischel, Chem. Eur. J. 2013, 19, 6650 – 6661. [40] V. F. Pais, M. Lineros, R. L pez-Rodrguez, H. S. El-Sheshtawy, R. Fernndez, J. M. Lassaletta, A. Ros, U. Pischel, J. Org. Chem. 2013, 78, 7949 – 7961. [41] L. Zhu, S. H. Shabbir, M. Gray, V. M. Lynch, S. Sorey, E. V. Anslyn, J. Am. Chem. Soc. 2006, 128, 1222 – 1232. [42] B. E. Collins, S. Sorey, A. E. Hargrove, S. H. Shabbir, V. M. Lynch, E. V. Anslyn, J. Org. Chem. 2009, 74, 4055 – 4060. [43] A. Ros, B. Estepa, R. L pez-Rodrguez, E. lvarez, R. Fernndez, J. M. Lassaletta, Angew. Chem. 2011, 123, 11928 – 11932; Angew. Chem. Int. Ed. 2011, 50, 11724 – 11728. [44] W. H. Melhuish, J. Phys. Chem. 1960, 64, 762 – 764. [45] W. H. Melhuish, J. Phys. Chem. 1961, 65, 229 – 235. [46] Gaussian 03 (Revision E.01), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, 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, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
Acknowledgements Financial support from the Spanish MINECO (CTQ2011-28390 for U.P., CTQ2010-15297 and CTQ2010-14974 for A.R.), the European Union (FEDER), and the Junta de Andaluca (2008/FQM3685 for U.P., 2008/FQM-3833 and 2009/FQM-4537 for A.R., PhD fellowship for V.F.P.) is gratefully acknowledged. A.R. was supported by a Marie Curie Reintegration Grant (FP7-PEOPLE2009-RG-256461) and by the Consejo Superior de Investigaciones Cientficas with a JAE-Doc Fellowship. H.S.E.-S. thanks the Computational Laboratory for Analysis, Modeling, and Visualization (Jacobs University Bremen, Germany) for access to computation resources. Keywords: boron · charge transfer · fluorescence · molecular devices · Lewis pairs [1] S. Uchiyama, A. P. de Silva, K. Iwai, J. Chem. Educ. 2006, 83, 720 – 727. [2] X. D. Wang, O. S. Wolfbeis, R. J. Meier, Chem. Soc. Rev. 2013, 42, 7834 – 7869. [3] D. Ross, M. Gaitan, L. E. Locascio, Anal. Chem. 2001, 73, 4117 – 4123. [4] C. Gota, K. Okabe, T. Funatsu, Y. Harada, S. Uchiyama, J. Am. Chem. Soc. 2009, 131, 2766 – 2767. [5] K. Okabe, N. Inada, C. Gota, Y. Harada, T. Funatsu, S. Uchiyama, Nat. Commun. 2012, 3, 705. [6] J. A. B. Ferreira, S. M. B. Costa, L. F. V. Ferreira, J. Phys. Chem. A 2000, 104, 11909 – 11917. [7] S. Fery-Forgues, J.-P. Fayet, A. Lopez, J. Photochem. Photobiol. A 1993, 70, 229 – 243. [8] C. F. Chapman, Y. Liu, G. J. Sonek, B. J. Tromberg, Photochem. Photobiol. 1995, 62, 416 – 425. [9] I. D. Figueroa, A. El Baraka, E. QuiÇones, O. Rosario, M. Deumi, Anal. Chem. 1998, 70, 3974 – 3977. [10] J. Feng, K. J. Tian, D. H. Hu, S. Q. Wang, S. Y. Li, Y. Zheng, Y. Li, G. Q. Yang, Angew. Chem. 2011, 123, 8222 – 8226; Angew. Chem. Int. Ed. 2011, 50, 8072 – 8076. [11] F. Pragst, H.-J. Hamann, K. Teuchner, M. Naether, W. Becker, S. Daehne, Chem. Phys. Lett. 1977, 48, 36 – 39. [12] N. Chandrasekharan, L. A. Kelly, J. Am. Chem. Soc. 2001, 123, 9898 – 9899. [13] G. A. Baker, S. N. Baker, T. M. McCleskey, Chem. Commun. 2003, 2932 – 2933. [14] M. T. Albelda, E. Garca-EspaÇa, L. Gil, J. C. Lima, C. Lodeiro, J. Seixas de Melo, M. J. Melo, A. J. Parola, F. Pina, C. Soriano, J. Phys. Chem. B 2003, 107, 6573 – 6578. [15] C. Baleiz¼o, S. Nagl, S. M. Borisov, M. Schferling, O. S. Wolfbeis, M. N. Berberan-Santos, Chem. Eur. J. 2007, 13, 3643 – 3651. [16] V. Augusto, C. Baleiz¼o, M. N. Berberan-Santos, J. P. S. Farinha, J. Mater. Chem. 2010, 20, 1192 – 1197. [17] J. C. Fister III, D. Rank, J. M. Harris, Anal. Chem. 1995, 67, 4269 – 4275. Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wallingford, CT, 2004. [47] The catalyst stock solution was prepared by dissolving 2-pyridinecarboxaldehyde N,N-dibenzyl hydrazone (37.6 mg, 0.125 mmol) and [{Ir(mOMe)(cod)}2] (41 mg, 0.063 mmol) in dry tetrahydrofuran to give a solution with a total volume of 25 mL (sonication for one hour was em-
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ployed to facilitate dissolution). The resulting red-brown solution was kept under argon.
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FULL PAPER & Boron Compounds
Getting warmer: A series of molecular fluorescent thermometers with good to very good sensitivity is reported. Borylated arylisoquinolines show pronounced fluorescence quenching upon heating (see figure), which can be reverted by cooling. The analysis of kinetic parameters revealed the involvement of a thermally activated nonradiative excited-state decay channel.
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V. F. Pais, J. M. Lassaletta, R. Fernndez, H. S. El-Sheshtawy, A. Ros,* U. Pischel* && – && Organic Fluorescent Thermometers Based on Borylated Arylisoquinoline Dyes
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Organic Fluorescent Thermometers Based on Borylated Arylisoquinoline Dyes Vnia F. Pais,[a] Jos M. Lassaletta,[b] Rosario Fernndez,[c] Hamdy S. El-Sheshtawy,[d] Abel Ros,*[b] and Uwe Pischel*[a] sensitivity even reaches values of up to 2.4 % K1. The thermometer function is interpreted as the interplay between excited ICT states of different geometry. In addition, the formation of an intramolecular Lewis pair can be followed by 11 B NMR spectroscopy. This provides a handle to monitor temperature-dependent ground-state geometry changes of the dyes. The role of steric hindrance is addressed by the inclusion of a derivative that lacks the Lewis pair formation.
Abstract: Borylated arylisoquinolines with redshifted internal charge-transfer (ICT) emission were prepared and characterized. Upon heating, significant fluorescence quenching was observed, which forms the basis for a molecular thermometer. In the investigated temperature range (283–323 K) an average sensitivity of 1.2 to 1.8 % K1 was found for the variations in fluorescence quantum yield and lifetime. In the physiological temperature window (298–318 K) the average
Introduction
Previous studies of organic compounds in which charge transfer was exploited for the photophysical design of molecular thermometers include rhodamine dyes (rhodamine B and rhodamine 3B),[3, 6] 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) derivatives,[7, 8] 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan),[8] N,N-dimethylaminobenzonitrile,[9] and a dipyren-1yl(2,4,6-triisopropylphenyl)borane.[10] However, on other occasions, the temperature-controlled formation of exciplexes,[11, 12] excimers in bis-pyrenes[13] or bis-napthalenes,[14] and the phenomenon of delayed fluorescence, observed for C70 fullerene[15, 16] or the acridine yellow dye,[17] was implemented with equal success for the purpose of molecular thermometry. Fluorophore-modified organic polymers in which temperature-dependent conformational changes and connected variations of the microenvironment of the fluorophore constitute the basic design principle were also used abundantly.[4, 18–24] Inorganic luminescent compounds and materials such as nanoparticles,[25–29] metal–organic frameworks,[30] metal clusters,[31] and organometallic complexes[32–38] have received increased attention as well. Recently, we reported on a new class of internal chargetransfer (ICT) fluorophores that are based on borylated arylisoquinoline (BAI) dyes.[39, 40] These compounds show redshifted fluorescence that can be fine-tuned by electron-donor substitution. The structural integration of a boronic ester moiety confers photophysical properties to these dyes that are not observed for the nonborylated analogues. The ICT character of the dyes was established by correlations of fluorescence with redox properties, solvatochromic effects, and density functional theory (DFT) calculations.[39] Akin to the observations made for the archetypal NBD fluorophore,[7] it was anticipated that the temperature-dependent interconversion between fluorescent ICT states and nonfluorescent “twisted” intramolecular charge-transfer (TICT) states
Temperature is a fundamental physical parameter that needs to be known and controlled in the most diverse applications, from engineering to life sciences. In recent years the exploitation of the temperature dependence of photophysical phenomena has given rise to the development of molecular thermometers that deliver an optical output response.[1, 2] The key principle for such devices lies in the temperature-controlled equilibration or competition between two states that have differentiated optical signatures; these are most often fluorescence or phosphorescence. Unlike the majority of conventional thermometers, chemical systems provide the appealing potential for small-scale integration that has found applications in, for example, microfluidic systems[3] or for temperature mapping in cells.[4, 5]
[a] Dr. V. F. Pais, Dr. U. Pischel CIQSO—Center for Research in Sustainable Chemistry and Department of Chemical Engineering, Physical Chemistry and Organic Chemistry, University of Huelva Campus El Carmen s/n, 21071 Huelva (Spain) E-mail: [email protected] [b] Prof. Dr. J. M. Lassaletta, Dr. A. Ros Institute for Chemical Research (CSIC-US) c/ Amrico Vespucio 49, 41092 Seville (Spain) E-mail: [email protected] [c] Prof. Dr. R. Fernndez Department of Organic Chemistry, Universidad of Seville C/Prof. Garca Gonzlez 1, 41012 Seville (Spain) [d] Dr. H. S. El-Sheshtawy Chemistry Department, Faculty of Science South Valley University, 83523 Qena (Egypt) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402027. Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper could be at the origin of a similar effect for the BAI dyes 1–3 investigated herein. Furthermore, we expected that the BN interaction (see below), which can be followed by the charac-
BN interactions for these dyes in solution. On the other hand, compound 3 contains a methyl group in the ortho-aryl position, and consequently the formation of an intramolecular Lewis pair is sterically hindered. In agreement with this connotation, the 11B NMR spectroscopic signal for dye 3 is downfieldshifted to d = 30.7 ppm (which corresponded to trigonal planar B), and the 1H NMR spectroscopic signal of the pinacolate methyl groups was found upfield-shifted at d = 0.79 ppm.
Optical properties The UV/Vis absorption and fluorescence spectra of the investigated dyes at room temperature are shown in Figure 1 (for numerical data see Table 1). Dyes 1 and 2 show significantly redshifted and broad absorption bands with maxima at 377 and 420 nm, respectively. In accordance with a recent report,[40] these are assigned to charge-transfer (CT) bands. However, the absorption maxima vary dramatically with the possibility to engage in BN interactions, and such a ground-state CT band was not seen for the sterically hindered 3 (see above). Hence, the UV/Vis absorption spectrum of 3 features a band at 283 nm that corresponds to localized p–p* transitions of the aryl residue and a sharp peak at 321 nm that is typical of isoquinolines. Furthermore, dyes 1–3 correspond well with the generally established excited-state ICT character of BAI dyes.[39] They show broad emissions in the turquoise to green-yellow
teristic 11B NMR spectroscopic signal,[41, 42] would provide a convenient handle to allow the direct monitoring of the temperature-dependent ground-state structural changes of the fluorophores.
Results and Discussion Synthesis and structural characterization of BAI dyes 1–3
Dyes 1–3 were prepared by borylation of their respective precursors A1–A3 by using a recently reported synthetic approach that is based on the IrIII-catalyzed nitrogen-directed ortho-CH borylation of arylisoquinolines.[43] The borylation reaction was carried out at 80 8C and afforded the dyes in excellent yields for products 1 and 2 (89 and 94 %) and in a moderate yield for 3 (43 %). Precursor A1 was prepared by means of a Pd0-catalyzed Suzuki coupling of 1-chloroisoquinoline with 4-methoxyphenylboronic acid.[40] For the preparation of A2, the methoxy group of A1 was treated with lithium 4-methylpiperazin-1-ide in a nucleophilic aromatic substitution.[40] Finally, A3 was obtained in a Suzuki coupling of 1chloroisoquinoline with 2methyl-4-methoxyphenylboronic acid. The described synthetic procedures are summarized in Scheme 1 (see the Experimental Section also). Dyes 1 and 2 show their 11 B NMR spectroscopic signals (in CD3CN at 293 K) at around d = 14–15 ppm, typical of tetracoordinated boron.[41, 42] Furthermore, in the 1H NMR spectra of these compounds, the pinacolate methyl groups were observed as Scheme 1. Reaction conditions: a) [Pd(PPh3)4] (3 mol %), Na2CO3 (2 equiv), THF/MeOH/H2O (see composition of sola single signal at d 1.30 ppm. vent mixture in the Experimental Section), 110 8C, overnight; b) 2-pyridinecarboxaldehyde N,N-dibenzyl hydrazine Both observations corroborate (1 mol %), [{Ir(m-OMe)cod}2] (cod = cyclooctadiene; 0.5 mol %), 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBPin; unambiguously the existence of 5 mol %), THF, 80 8C; and c) THF, argon, reflux, overnight. &
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Figure 1. Long-wavelength UV/Vis absorption bands (full lines) and corresponding fluorescence spectra (dashed lines) of the investigated compounds (1: blue; 2: red; 3: green) at 298 K. The spectra are normalized to 1 at their respective maxima.
spectral range that corresponds to fluorescence maxima between 470 and 560 nm (see Figure 1) and typically large Stokes shifts (91 nm for 1, 139 nm for 2, and 213 nm for 3). In comparison to the methoxy group in 1 and the methyl and methoxy groups in 3, the stronger electron-donating piperazinyl substituent in 2 lowers the energy of the emitting ICT state, thereby resulting in a significantly more redshifted emission. The involvement of CT in the lowest-energy transitions was corroborated by density functional theory (DFT) calculations. The results confirm that the HOMO is mainly located on the electron-donating aryl residue, whereas the LUMO has its major contribution from the isoquinolyl moiety that plays the role of the electron acceptor (see contour plots of frontier orbitals in Figure 2). The HOMO–LUMO energy gap DE qualitatively follows the trend of emission energies (see fluorescence maxima in Table 1) of the excited ICT states: DE(2) < DE(3) < DE(1). The fluorescence quantum yields of the dyes vary between 0.05 and 0.16 (see Table 1). Interestingly, dye 1 has an emission quantum yield that is three times higher than the sterically hindered dye 3; electronically they differ by only one methyl substituent. This observation proposes a relation between BN
Figure 2. Contour plots of HOMO and LUMO orbitals of the dyes 1–3 and their energies [eV].
interaction and consequently the planarization of the biaryl system on the one hand and the fluorescence quantum yield on the other hand. However, although dye 2 clearly features a BN interaction, its relatively low-lying emissive charge-transfer state (most redshifted emission maximum) preferably deactivates through nonradiative pathways. Thereby the emission quantum yield is diminished. In accordance with the discussion of related BAI dyes, this finds its explanation in the energy-gap law.[39] These few notions demonstrate that structure–property relationships are certainly more complex than it appears at first sight. Temperature-dependent geometrical changes
To obtain insight into the ground-state structural changes of the compounds upon variation of the temperature, the 11 B NMR spectroscopic signal was followed. In Figure 3 the example of dye 2 is shown. Upon changing gradually from 283 to 323 K, an accentuated shift change of the signal toward Table 1. Photophysical properties of dyes 1–3 in acetonitrile. lower field (Dd = + 2.8 ppm) was observed in CD3CN. Similar SA [% K1][i] labs [nm][a,b] lf [nm][a,c] Ff[a,d] tf [ns] ([%])[a,e] ln(A [s1])[f] Ea [kJ mol1][g] Q [%][h] changes were monitored for dye Ff tf 1 (see the Supporting Informa1 377 468 0.16 0.97 (37) 29.5 23.8 73 1.8 1.5 tion). However, the sterically hin(9650) 2.56 (63) 2 420 559 0.07 0.59 (88) 26.7 13.6 52 1.2 1.2 dered compound 3 showed no (12 150) 5.39 (12) temperature effect on the 3 283 496 0.05 1.65 (92) 28.5 20.7 72 1.8 1.6 11 B NMR spectroscopic signal (6500) 4.60 (8) (see the Supporting Informa[a] Measured at 298 K. [b] UV/Vis absorption maximum; the corresponding molar absorption coefficient tion). This does not mean that e [m1 cm1] is given in parentheses. [c] Fluorescence maximum of corrected spectra. [d] Fluorescence quantum the torsional angle between the yield measured against quinine sulfate as standard. [e] Fluorescence lifetimes measured by single-photon counting. [f] Preexponential factor according to the Arrhenius equation [Eq. (1)]. [g] Activation energy for nonaryl planes in 3 is not altered radiative decay; r2 0.996. [h] Fluorescence quenching upon heating from 283 to 323 K. [i] Average sensitivity upon heating, but this process is (in the temperature range of 283–323 K) based on the temperature-dependent quantum yield or lifetime data. not seen in the 11B NMR spectra
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Figure 4. Temperature dependence of the UV/Vis absorption (left) and fluorescence spectra (right) of dye 2 in acetonitrile upon heating from 283 to 323 K.
Figure 3. Temperature dependence of the 11B NMR spectra of dye 2 in [D3]acetonitrile. The dashed red line is meant to guide the eye (see also Figure 4). The spectra were recorded between 283 and 323 K in steps of 5 K.
owing to the absence of BN interactions. As can be deduced from the fact that even at higher temperature (343 K) the 11 B NMR spectroscopic resonance signals of dyes 1 and 2 remain at lower positions (d < 20 ppm) than that of compound 3 (d 31 ppm), the temperature increase leads to an equilibrium between the more planar low-temperature form and the twisted high-temperature form (see Scheme 2 for the exam-
Scheme 2. Schematic representation of the temperature-dependent groundstate equilibrium between the planar (left) and twisted forms (right) of borylated arylisoquinolines (R = methoxy (1) or N-methylpiperazinyl (2)).
Figure 5. Temperature dependence of a) the fluorescence lifetime, b) emission quantum yield, c) and 11B NMR spectroscopic signal of dye 2 in acetonitrile. The lines are meant to guide the eye.
ples of dyes 1 and 2). The partial thermally induced conversion between the planar form (BN) interaction and a nonplanar form (with no BN interaction) is also expressed by a reduced intensity of the CT bands (see above) of dyes 1 and 2 in the UV/Vis absorption spectrum (see Figure 4 for dye 2, and the Supporting Information).
temperature effect is caused by the change in solvent properties that would ultimately lead to variations in the polarity function and solvatochromic effects.[39] The fluorescence modulation is reversible and upon cooling back to 283 K the initial emission signal level was restored in all cases. The cycling can be repeated at least ten times without significant differences being observed (see Figure 6 for the example of dye 2). Akin to the interpretations of related probes that are based on charge-transfer phenomena, we propose the conversion of a fluorescent ICT state into a nonfluorescent (or less fluorescent) TICT state in the rationalization of our observations.[7] The temperature-dependent changes of the ground-state geometry (see above) give a hint of the conformational flexibility of the dyes and increased torsional angles upon heating. Similar effects are expected for the excited state in which the population of a less fluorescent TICT state is promoted with increasing temperature. Experimentally this assumption is indirectly evi-
Molecular fluorescent thermometer In a further step we were interested in harnessing the changes of the fluorescence of the dyes as a convenient and readily detectable output signal. Upon gradual heating from 283 to 323 K, a pronounced quenching of the ICT emission intensity that varied between 52 and 73 % was observed (see Figures 4 and 5 for the example of BAI dye 2 and the Supporting Information for the other dyes). The fluorescence maxima are not shifted (dyes 1 and 3) or suffer only a minor blueshift by approximately 8 nm for dye 2. This observation excludes that the &
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SA ¼
denced by the aforementioned fluorescence quenching for dyes with larger torsional angles (compare dyes 1 and 3; Table 1). Thus, the TICT state provides an efficient channel of nonradiative excited-state deactivation and thereby leads to the observed reduction of the fluorescence quantum yield. In the same way as observed for the quantum yield (normally monitored through fluorescence intensity), the concentration-independent excited-state lifetime tf can also be used for temperature monitoring. This bears advantages such as the independency of the signal reading on the fluorophore distribution in heterogeneous media, which for fluorescence intensity measurements can be only overcome with ratiometrically responding systems. Akin to the observations made for the quantum yield, the fluorescence lifetime is reduced with increasing temperature (Figure 5 for dye 2). With the quantum yield and lifetime data at hand, the corresponding kinetic constants for the radiative (kf) and nonradiative (knr) excited-state deactivation were calculated for each temperature [see Eq. (1)]. The data for dye 2 are plotted in Figure 7 (for the other dyes, see the Supporting Information) [Eqs. (2) and (3)]: 1 kf þ knr
ð3Þ
On the one hand, the plot of ln(knr) versus 1/T according to the Arrhenius equation [Eq. (2) with the rate constant k(T) = knr(T) at temperature T, A as pre-exponential factor, R as gas constant, and Ea as activation energy] yields Arrhenius-type behavior for all investigated dyes (Figure 7, filled circles) and the resulting activation energies (Ea) vary between 13.6 (dye 2) and 23.8 kJ mol1 (dye 1) (see Table 1). Hence, the notion of the involvement of a thermally induced nonradiative decay pathway (such as the invoked TICT) is supported by the analysis of the temperature-dependent excited-state kinetic data. The order of magnitude of the activation energies is comparable with those for related ICT chromophores.[7] However, as often observed for organic fluorophores, the radiative rate constant kf is practically independent of the temperature [see Figure 7, empty circles; with k(T) = kf(T) in Eq. (2)]. The applicability of molecular thermometers is commonly rated by the average sensitivity SA of the fluorescence quantum yield (intensity) or lifetime toward the temperature change [Eq. (3) in which X: Ff or tf].[36] The calculated values for this parameter are listed in Table 1 and are in the order of 1.2 to 1.8 % K1 for the full investigated temperature range of 283–323 K. In the physiological temperature window (298– 318 K) even higher values apply: up to 2.4 % K1 (dye 1) for the fluorescence intensity/quantum yield measurements and up to 2.0 % K1 (dye 3) for the lifetime-based thermometer. These data compare nicely with some of the best-performing molecular optical thermometers based on lanthanide complexes or those based on quantum dot luminescence.[2] On the basis of a 1 % precision of the measurement of the fluorescence intensity, a temperature resolution of approximately 0.4–0.6 8C can be estimated for the most sensitive dyes, 1 and 3.
Figure 6. Cycling of dye 2 for ten consecutive cycles of heating (to 323 K) and cooling (to 283 K).
Ff ¼ kf tf with tf ¼
1 DX X ref DT
ð2Þ
ð1Þ
Conclusion In conclusion, borylated arylisoquinolines show a pronounced dependence of their ICT fluorescence on the temperature. The sensitive (SA between 1.2 to 1.8 % K1) molecular thermometers were tested in the range of 283–323 K, including the physiologically relevant temperature window. Some of the thermometers can be operated with visible-light excitation and provide spectrally well-separated emission outputs. The interplay between planar and twisted ground-state geometries was studied by taking advantage of the 11B NMR spectroscopic signal as an indicator for the formation of an intramolecular Lewis pair. The temperature-dependent excited-state behavior, as monitored by fluorescence, is rationalized by the interplay between ICT and TICT states, which has precedence in the literature.[7] The analysis of the temperature-dependent kinetic constants provides evidence for the thermal activation of a nonradiative excited-state decay channel. The implication of
Figure 7. Arrhenius plot for the radiative (kf, empty circles) and nonradiative rate constants (knr, filled circles) of excited-state deactivation of dye 2 in acetonitrile. The major lifetime component was analyzed. Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper calculations were performed to check for the stationary points on the potential-energy surface. The frontier orbital energies were calculated by the time-dependent density-functional theory (TD-DFT) at the B3LYP/6-31+G(d,p) level of theory.
planar versus nonplanar geometries is further corroborated by observations of a reduced fluorescence quantum yield of a sterically hindered derivative. Potentially, the boronic ester opens possibilities to functionalize biologically relevant carbohydrates with the thermometer function. By building on the photophysical foundation that was laid in this work, such studies are currently underway in our laboratories.
Synthetic procedures Compounds A1, A2, and 2 were prepared following described methodologies.[40]
Experimental Section
1-(4-Methoxy-2-methylphenyl)isoquinoline (A3): A Schlenk tube was charged with a solution of the corresponding 1-chloroisoquinoline (197 mg, 1.2 mmol), 2-methyl-4-methoxyphenylboronic acid (239 mg, 1.44 mmol), Na2CO3 (254 mg, 2.4 mmol), and [Pd(PPh3)4] (41 mg, 3 mol %). After three cycles of vacuum–argon, toluene (1 mL), MeOH (0.2 mL), and H2O (0.25 mL) were added sequentially. The reaction mixture was stirred at 110 8C overnight, cooled to room temperature, quenched with H2O (10 mL), and extracted with CH2Cl2 (3 10 mL). The organic layer was dried over MgSO4, filtered, concentrated, and the residue was purified by flash chromatography on silica gel (EtOAc/n-hexane 1:3!1:1) to give A3 (220 mg, 74 %) as a light yellow viscous oil. 1H NMR (400 MHz, CDCl3): d = 8.61 (d, J = 6.0 Hz, 1 H; ArH), 7.88 (d, J = 8.4 Hz, 1 H; ArH), 7.71– 7.65 (m, 3 H; 3 ArH), 7.49 (t, J = 7.2 Hz, 1 H; ArH), 7.27 (d, J = 3.2 Hz, 1 H; ArH), 6.90–6.86 (m; 2 ArH), 3.89 (s, 3 H; OCH3), 2.08 ppm (s, 3 H; CH3); 13C NMR (100 MHz, CDCl3): d = 161.3 (C), 159.6 (C), 142.2 (CH), 138.1 (C), 136.4 (C), 131.7 (C), 130.8 (CH), 130.0 (CH), 127.8 (C), 127.6 (CH), 127.1 (C), 126.8 (CH), 119.7 (CH), 115.7 (CH), 110.9 (CH), 55.0 (OCH3), 20.1 ppm (CH3); HRMS (EI): m/z calcd for C17H14NO: 248.1075 [M + 1]; found: 248.1069.
Materials and methods 1
H and 13C NMR spectra were recorded at 400 and 100 MHz, respectively, using the solvent peak for CDCl3 as the internal reference (d = 7.26 and 77.0 ppm for 1H and 13C, respectively). 11B NMR spectra were recorded with complete proton decoupling at 160 MHz by using BF3·Et2O (d = 0.00 ppm) as internal standard. Electrospray ionization (EI) mass spectra and high-resolution mass spectra were recorded using a QTRAP mass spectrometer (hybrid triple quadrupole/linear ion trap mass spectrometer). Flash column chromatography was carried out on silica gel (35–70 or 70– 200 mm).
All reactions were carried out in oven-dried Schlenk tubes under an argon atmosphere. Anhydrous 1,4-dioxane was obtained by distillation from sodium using benzophenone as indicator. Anhydrous tetrahydrofuran (THF) was obtained by using Grubbs-type solvent drying columns. Ethyl acetate (EtOAc), methanol (MeOH), n-hexane, dichloromethane (CH2Cl2), and toluene were purchased from commercial suppliers and used without further purification. Reagents, metallic precursors, and ligands for the syntheses were purchased from commercial suppliers and used as received.
General procedure for the Ir-catalyzed borylation: Following a recently described method,[43] a dried Schlenk tube was charged with the substrate and B2Pin2 (1 equiv). After three cycles of vacuum– argon flushing, catalyst stock solution (1 mL)[47] per 0.5 mmol substrate and HBPin (5 % mol) were added. The reaction mixture was stirred at 80 8C until quantitative consumption of the starting material was achieved. The mixture was cooled to room temperature, concentrated to dryness, and purified by column chromatography or by precipitation.
The photophysical measurements were performed on air-equilibrated acetonitrile solutions, typically adjusting an optical density of approximately 0.1 at the excitation wavelength and using quartz cuvettes with 1 cm optical path length. Room-temperature (298 K) UV/Vis absorption spectra were recorded using a UV-1603 spectrophotometer from Shimadzu, and the fluorescence spectra were measured using a Cary Eclipse fluorimeter from Varian. Steady-state temperature-dependent photophysical measurements were carried out using a Perkin–Elmer Lambda 750 UV/Vis spectrophotometer or a fluorimeter from Edinburgh instruments with a Peltier system for temperature control (10–50 8C). For the temperature-dependent fluorescence measurements the samples were excited at wavelengths of minimal absorbance variation throughout the experiment. The lifetime measurements were accomplished using time-correlated single-photon-counting (Edinburgh instruments FLS 920). As excitation source a picosecond-pulsed UV-LED (EPLED 280, l = 283.2 nm, pulse width fwhm 706.5 ps) or picosecond-pulsed diode lasers (EPLED 330, l = 334.6 nm, pulse width fwhm 758.7 ps; EPLED 360, l = 367.4 nm, pulse width fwhm 745.5 ps) were used. Deconvolution analysis of the decay kinetics yielded the fluorescence lifetimes. The instrument response function was obtained with a light-scattering Ludox solution. The fluorescence quantum yields (error 10 %) were determined with quinine sulfate (Ff = 0.55 in 0.05 m sulfuric acid)[44, 45] as reference and corrected for refractive index differences between water and acetonitrile. The calculations were performed with the Gaussian 03 program.[46] The ground-state geometries were calculated by applying the Kohn–Sham density-functional theory (DFT) with the Becke3-Lee– Yang–Parr hybrid functional (B3LYP) method using the 6-31+G(d,p) basis set for the full structural optimization. Analytical frequency
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1-[4-Methoxy-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]isoquinoline (1): Following a general procedure for Ir-catalyzed borylation, starting from A1 (59 mg, 0.25 mmol) and carrying out the reaction at 80 8C, precipitation from the reaction crude with pentane, trituration, and filtration afforded 1 (85 mg, 94 %) as a yellow-green solid. 1H NMR (400 MHz, CDCl3): d = 8.83 (d, J = 8.0 Hz, 1 H; ArH), 8.48 (d, J = 6.4 Hz, 1 H; ArH), 8.18 (d, J = 8.4 Hz, 1 H; ArH), 7.89 (d, J = 8.0 Hz, 1 H; ArH), 7.81 (dd, J = 8.0, 7.2 Hz, 1 H; ArH), 7.71 (dd, J = 8.0, 7.2 Hz, 1 H; ArH), 7.60 (d, J = 6.4 Hz, 1 H; ArH), 7.34 (d, J = 2.4 Hz, 1 H; ArH), 6.87 (dd, J = 8.4, 2.4 Hz, 1 H; ArH), 3.92 (s, 3 H; OCH3), 1.41 ppm (s, 12 H; 4 CH3); 13C NMR (100 MHz, CDCl3): d = 162.2 (C), 157.4 (C), 139.1 (C), 134.5 (CH), 132.4 (CH), 131.9 (C), 128.6 (CH), 127.7 (2 CH), 127.3 (CH), 125.1 (C), 120.0 (CH), 116.6 (CH), 113.5 (CH), 80.3 (2 C), 55.3 (OCH3), 27.2 ppm (4 CH3) (CB not observed); 11B NMR (160 MHz, CDCl3): d = 13.1 ppm (br s); HRMS (EI): m/z calcd for C22H24BNO3 : 360.1768 [M + 1]; found: 360.1771. 1-[4-Methoxy-2-methyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]isoquinoline (3): Following a general procedure for Ir-catalyzed borylation, starting from A3 (62 mg, 0.25 mmol) and carrying out the reaction at 80 8C, flash chromatography on
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Full Paper silica gel (EtOAc/n-hexane 1:2) gave 3 (40 mg, 43 %) as a yellow viscous oil. 1H NMR (400 MHz, CDCl3): d = 8.55 (d, J = 5.6 Hz, 1 H; Ar H), 7.83 (d, J = 5.6 Hz, 1 H; ArH), 7.63–7.59 (m, 2 H; 2 ArH), 7.51 (d, J = 8.4 Hz, 1 H; ArH), 7.39 (t, J = 8.0 Hz, 1 H; ArH), 7.25 (br s, 1 H; ArH), 6.96 (br s, 1 H; ArH), 3.89 (s, 3 H; OCH3), 2.01 (s, 3 H; CH3), 0.78 ppm (s, 12 H; 4 CH3); 13C NMR (100 MHz, CDCl3): d = 162.5 (C), 158.7 (C), 141.7 (CH), 137.7 (C), 137.2 (C), 136.0 (C), 129.5 (CH), 129.1 (C), 127.4 (CH), 126.5 (CH), 126.4 (CH), 119.0 (CH), 118.6 (CH), 116.5 (CH), 83 (2 C), 55.3 (OCH3), 24.1 (4 CH3), 20.0 ppm (CH3) (CB not observed); 11B NMR (160 MHz, CDCl3): d = 30.7 ppm (br s); HRMS (EI): m/z calcd for C23H25BNO3 : 374.1927 [M + 1]; found: 374.1926.
[18] S. Uchiyama, N. Kawai, A. P. de Silva, K. Iwai, J. Am. Chem. Soc. 2004, 126, 3032 – 3033. [19] S. Uchiyama, Y. Matsumura, A. P. de Silva, K. Iwai, Anal. Chem. 2004, 76, 1793 – 1798. [20] K. Iwai, Y. Matsumura, S. Uchiyama, A. P. de Silva, J. Mater. Chem. 2005, 15, 2796 – 2800. [21] Y. Shiraishi, R. Miyamoto, X. Zhang, T. Hirai, Org. Lett. 2007, 9, 3921 – 3924. [22] C. Gota, S. Uchiyama, T. Yoshihara, S. Tobita, T. Ohwada, J. Phys. Chem. B 2008, 112, 2829 – 2836. [23] Y. Shiraishi, R. Miyamoto, T. Hirai, Langmuir 2008, 24, 4273 – 4279. [24] Z. Q. Guo, W. H. Zhu, Y. Y. Xiong, H. Tian, Macromolecules 2009, 42, 1448 – 1453. [25] C. D. S. Brites, P. P. Lima, N. J. O. Silva, A. Milln, V. S. Amaral, F. Palacio, L. D. Carlos, Adv. Mater. 2010, 22, 4499 – 4504. [26] H. S. Peng, M. I. J. Stich, J. B. Yu, L. N. Sun, L. H. Fischer, O. S. Wolfbeis, Adv. Mater. 2010, 22, 716 – 719. [27] E. J. McLaurin, V. A. Vlaskin, D. R. Gamelin, J. Am. Chem. Soc. 2011, 133, 14978 – 14980. [28] C. D. S. Brites, P. P. Lima, N. J. O. Silva, A. Milln, V. S. Amaral, F. Palacio, L. D. Carlos, Nanoscale 2012, 4, 4799 – 4829. [29] J. Feng, L. Xiong, S. Q. Wang, S. Y. Li, Y. Li, G. Q. Yang, Adv. Funct. Mater. 2013, 23, 340 – 345. [30] Y. J. Cui, H. Xu, Y. F. Yue, Z. Y. Guo, J. C. Yu, Z. X. Chen, J. K. Gao, Y. Yang, G. D. Qian, B. L. Chen, J. Am. Chem. Soc. 2012, 134, 3979 – 3982. [31] D. Cauzzi, R. Pattacini, M. Delferro, F. Dini, C. Di Natale, R. Paolesse, S. Bonacchi, M. Montalti, N. Zaccheroni, M. Calvaresi, F. Zerbetto, L. Prodi, Angew. Chem. 2012, 124, 9800 – 9803; Angew. Chem. Int. Ed. 2012, 51, 9662 – 9665. [32] M. Engeser, L. Fabbrizzi, M. Licchelli, D. Sacchi, Chem. Commun. 1999, 1191 – 1192. [33] J. Stehr, J. M. Lupton, M. Reufer, G. Raschke, T. A. Klar, J. Feldmann, Adv. Mater. 2004, 16, 2170 – 2174. [34] S. M. Borisov, A. S. Vasylevska, C. Krause, O. S. Wolfbeis, Adv. Funct. Mater. 2006, 16, 1536 – 1542. [35] L. H. Fischer, M. I. J. Stich, O. S. Wolfbeis, N. Tian, E. Holder, M. Schferling, Chem. Eur. J. 2009, 15, 10857 – 10863. [36] L. N. Sun, J. B. Yu, H. S. Peng, J. Z. Zhang, L. Y. Shi, O. S. Wolfbeis, J. Phys. Chem. C 2010, 114, 12642 – 12648. [37] J. B. Yu, L. N. Sun, H. S. Peng, M. I. J. Stich, J. Mater. Chem. 2010, 20, 6975 – 6981. [38] K. Miyata, Y. Konno, T. Nakanishi, A. Kobayashi, M. Kato, K. Fushimi, Y. Hasegawa, Angew. Chem. 2013, 125, 6541 – 6544; Angew. Chem. Int. Ed. 2013, 52, 6413 – 6416. [39] V. F. Pais, H. S. El-Sheshtawy, R. Fernndez, J. M. Lassaletta, A. Ros, U. Pischel, Chem. Eur. J. 2013, 19, 6650 – 6661. [40] V. F. Pais, M. Lineros, R. L pez-Rodrguez, H. S. El-Sheshtawy, R. Fernndez, J. M. Lassaletta, A. Ros, U. Pischel, J. Org. Chem. 2013, 78, 7949 – 7961. [41] L. Zhu, S. H. Shabbir, M. Gray, V. M. Lynch, S. Sorey, E. V. Anslyn, J. Am. Chem. Soc. 2006, 128, 1222 – 1232. [42] B. E. Collins, S. Sorey, A. E. Hargrove, S. H. Shabbir, V. M. Lynch, E. V. Anslyn, J. Org. Chem. 2009, 74, 4055 – 4060. [43] A. Ros, B. Estepa, R. L pez-Rodrguez, E. lvarez, R. Fernndez, J. M. Lassaletta, Angew. Chem. 2011, 123, 11928 – 11932; Angew. Chem. Int. Ed. 2011, 50, 11724 – 11728. [44] W. H. Melhuish, J. Phys. Chem. 1960, 64, 762 – 764. [45] W. H. Melhuish, J. Phys. Chem. 1961, 65, 229 – 235. [46] Gaussian 03 (Revision E.01), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, 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, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
Acknowledgements Financial support from the Spanish MINECO (CTQ2011-28390 for U.P., CTQ2010-15297 and CTQ2010-14974 for A.R.), the European Union (FEDER), and the Junta de Andaluca (2008/FQM3685 for U.P., 2008/FQM-3833 and 2009/FQM-4537 for A.R., PhD fellowship for V.F.P.) is gratefully acknowledged. A.R. was supported by a Marie Curie Reintegration Grant (FP7-PEOPLE2009-RG-256461) and by the Consejo Superior de Investigaciones Cientficas with a JAE-Doc Fellowship. H.S.E.-S. thanks the Computational Laboratory for Analysis, Modeling, and Visualization (Jacobs University Bremen, Germany) for access to computation resources. Keywords: boron · charge transfer · fluorescence · molecular devices · Lewis pairs [1] S. Uchiyama, A. P. de Silva, K. Iwai, J. Chem. Educ. 2006, 83, 720 – 727. [2] X. D. Wang, O. S. Wolfbeis, R. J. Meier, Chem. Soc. Rev. 2013, 42, 7834 – 7869. [3] D. Ross, M. Gaitan, L. E. Locascio, Anal. Chem. 2001, 73, 4117 – 4123. [4] C. Gota, K. Okabe, T. Funatsu, Y. Harada, S. Uchiyama, J. Am. Chem. Soc. 2009, 131, 2766 – 2767. [5] K. Okabe, N. Inada, C. Gota, Y. Harada, T. Funatsu, S. Uchiyama, Nat. Commun. 2012, 3, 705. [6] J. A. B. Ferreira, S. M. B. Costa, L. F. V. Ferreira, J. Phys. Chem. A 2000, 104, 11909 – 11917. [7] S. Fery-Forgues, J.-P. Fayet, A. Lopez, J. Photochem. Photobiol. A 1993, 70, 229 – 243. [8] C. F. Chapman, Y. Liu, G. J. Sonek, B. J. Tromberg, Photochem. Photobiol. 1995, 62, 416 – 425. [9] I. D. Figueroa, A. El Baraka, E. QuiÇones, O. Rosario, M. Deumi, Anal. Chem. 1998, 70, 3974 – 3977. [10] J. Feng, K. J. Tian, D. H. Hu, S. Q. Wang, S. Y. Li, Y. Zheng, Y. Li, G. Q. Yang, Angew. Chem. 2011, 123, 8222 – 8226; Angew. Chem. Int. Ed. 2011, 50, 8072 – 8076. [11] F. Pragst, H.-J. Hamann, K. Teuchner, M. Naether, W. Becker, S. Daehne, Chem. Phys. Lett. 1977, 48, 36 – 39. [12] N. Chandrasekharan, L. A. Kelly, J. Am. Chem. Soc. 2001, 123, 9898 – 9899. [13] G. A. Baker, S. N. Baker, T. M. McCleskey, Chem. Commun. 2003, 2932 – 2933. [14] M. T. Albelda, E. Garca-EspaÇa, L. Gil, J. C. Lima, C. Lodeiro, J. Seixas de Melo, M. J. Melo, A. J. Parola, F. Pina, C. Soriano, J. Phys. Chem. B 2003, 107, 6573 – 6578. [15] C. Baleiz¼o, S. Nagl, S. M. Borisov, M. Schferling, O. S. Wolfbeis, M. N. Berberan-Santos, Chem. Eur. J. 2007, 13, 3643 – 3651. [16] V. Augusto, C. Baleiz¼o, M. N. Berberan-Santos, J. P. S. Farinha, J. Mater. Chem. 2010, 20, 1192 – 1197. [17] J. C. Fister III, D. Rank, J. M. Harris, Anal. Chem. 1995, 67, 4269 – 4275. Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wallingford, CT, 2004. [47] The catalyst stock solution was prepared by dissolving 2-pyridinecarboxaldehyde N,N-dibenzyl hydrazone (37.6 mg, 0.125 mmol) and [{Ir(mOMe)(cod)}2] (41 mg, 0.063 mmol) in dry tetrahydrofuran to give a solution with a total volume of 25 mL (sonication for one hour was em-
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ployed to facilitate dissolution). The resulting red-brown solution was kept under argon.
Received: February 4, 2014 Published online on && &&, 0000
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Getting warmer: A series of molecular fluorescent thermometers with good to very good sensitivity is reported. Borylated arylisoquinolines show pronounced fluorescence quenching upon heating (see figure), which can be reverted by cooling. The analysis of kinetic parameters revealed the involvement of a thermally activated nonradiative excited-state decay channel.
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V. F. Pais, J. M. Lassaletta, R. Fernndez, H. S. El-Sheshtawy, A. Ros,* U. Pischel* && – && Organic Fluorescent Thermometers Based on Borylated Arylisoquinoline Dyes
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