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Inhibition of Light-Induced Tautomerization of 7-Azaindole by Phenol: Indications of Proton-Coupled Electron/Energy Transfer Quenching Moitrayee Mukherjee, Shreetama Karmakar, and Tapas Chakraborty* Department of Physical Chemistry and Raman Centre for Atomic Molecular and Optical Sciences, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India ABSTRACT: The photophysical behavior of a 1:1 complex between phenol and 7-azaindole (7AI) has been investigated in methylcyclohexane solutions at temperatures in the range of 27 to -50 °C. A linear Benesi-Hildebrand plot associated with changes in absorbance of the complex with phenol concentration in the solutions ensures 1:1 stoichiometry of the produced complex. Our estimate for the value of the association constant (Ka) of the complex is ∼120 M-1 at 27 °C, and it is nearly twice compared to that for 1:1 complex between 7AI and ethanol measured under the same condition. The complexation results in dramatic quenching of the normal fluorescence of 7AI and the process is accelerated upon lowering of temperature. The measured spectra show no indication that phenol promotes tautomerization of 7AI in the excited state. We have argued that the hydrogen bonding between pyridinic N and phenolic O-H (N 3 3 3 O-H) is a vital structural factor responsible for quenching of 7AI fluorescence, and this idea has been corroborated by showing that under same condition the fluorescence of 7AI is enhanced in the presence of anisole. As a plausible mechanism of quenching, we have invoked a proton-coupled electron transfer (PCET) process between phenol and excited 7AI, which outweighs the competing tautomerization process. An analysis in terms of RemmWeller model reveals that the PCET process involving phenol and excited 7AI could be energetically favorable (ΔG0ET< 0). An alternative mechanism, where quenching can occur via electronic energy transfer from the excited protonated 7AI to phenoxide ion, following a proton transfer along the N 3 3 3 O-H hydrogen bond, is also discussed.
1. INTRODUCTION Hydrogen bonding is a vital molecular interaction that governs the dynamics of energy dissipation of excited molecules in condensed phases as well as in molecular complexes isolated in the gas phase.1 In the past couple of decades, doubly hydrogenbonded complexes of 7-azaindole (7AI) have been subjected to extensive investigations from the viewpoints of photophysical interests, both in hydrocarbon solutions2-19 and in cold environment of supersonic jet expansion.20-32 On UV excitation to the lowest excited state, the monomer and doubly hydrogenbonded dimer of this molecule decay via two radically different photophysical pathways. While the monomer emits intense UV fluorescence, the dimer undergoes a nonradiative transition almost exclusively resulting in formation of a tautomeric configuration via exchange of two protons/H-atoms. According to the mechanism suggested for the process, both moieties of the dimer are tautomerized simultaneously and emit green fluorescence from the excited tautomeric configuration. An apparent reason of the overwhelming interests shown in studying the details of this photophysical process is due to the notion that similar tautomerization within the base pairs are responsible for mutagenesis in DNA replications.33,34 In a hydrocarbon solution at room temperature, the transition from the normal to tautomeric configuration of the homodimer (7AI2) is found to be almost exclusive.3 No distinct fluorescence r 2011 American Chemical Society
of the normal dimer can be recorded in ultraviolet by measuring the steady-state fluorescence spectrum and this is consistent with the finding of the time-resolved studies, where an ultrafast growth of the green tautomer fluorescence is observed.4,7 An extensive degree of experimental as well as theoretical efforts has been devoted to resolve the other pertinent issue, that is, whether the double proton/H atom transfer occur in concerted or stepwise manner.8 Temperature variation studies at low temperatures revealed an apparently very small barrier (∼1.5 kcal/ mol) to tautomerization.3 However, the origin of the barrier does not appear to be very obvious. Recently, by measuring fluorescence spectra in a series of aprotic liquids at different temperatures, Catalan suggested that some properties of the media, for example, solvent viscosity, volume cavity, and so forth, could be responsible for the observed barrier.35 Very efficient tautomerization of 7AI also occurs in 1:1 doubly hydrogen bonded complexes with carboxylic acids.17,18 In this case, carboxylic acids act as efficient catalysts. Because of higher natural acidity of the carboxylic acids, strongly bound doubly H-bonded cyclic complexes are produced, which are structurally similar to the homodimer. In fact, the value of the association constant (Ka) of 1:1 Received: September 4, 2010 Revised: December 29, 2010 Published: February 17, 2011 1830
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The Journal of Physical Chemistry A complex with acetic acid in hydrocarbon solution at room temperature has been found to be much larger (Ka = 1.8 104 M-1) compared to the homodimer (7AI2) formation (Ka = 2.2 103 M-1).18 Furthermore, no UV fluorescence from the normal form of the 7AI-acetic acid complex has been detected, and the tautomeric form of the complex is the only emitting species, as in the case of homodimer. On the other hand the protic solvents display a large variation in tautomerization efficiency, which depends also on the nature of the reaction medium. No green fluorescence has been detected from pure aqueous solutions of 7AI.9,12 However, such fluorescence does appear when water is finely dispersed in diethyl ether or tetrahydrofuran,16 and under such conditions water has been proposed to form 1:1 complexes. However, in the bulk water, owing to presence of extensive H-bonded networks, probability for identification of the 1:1 complex turns out to be small. On the other hand, a highly red-shifted normal fluorescence (λmax ∼ 385 nm) is observed from the fully solvated species in the excited state. To account for the red-shifted emission, the possibility for exciplex formation has also been suggested.9 On the other hand, under a supersonic jet expansion condition, no green fluorescence from the 1:1 water complex is observed, but occurrence of such emission from a complex containing two water molecules (1:2 type) has been reported very recently.36 In contrast to the behavior of the water complex, green fluorescence has been observed from a neat alcohol solution of 7AI9 and also when the alcohols are diluted in hydrocarbon solvents at room temperature.10,14 It has been suggested that a significant extent of 1:1 complex is also present in neat alcohol solutions because of their lower tendency toward self-aggregations. In dilute hydrocarbon solution, 1:1 nature of the complex between methanol and 7AI was ascertained from BenesiHildebrand plot.14 However, unlike the homodimer and/or 1:1 complexes with carboxylic acids, alcohol-assisted tautomerization is much slower, and the complexes display distinct normal emission in the ultraviolet with λmax at ∼350 nm.14 This spectral behavior is consistent with the experimental estimate of a small value of association constant (Ka = 50 M-1) and also from the theoretical prediction for a geometry of the 1:1 complex in which the H-bonds are much weaker compared to those in the homodimer and 1:1 complex with acetic acid. Analyzing the rate of tautomerization assisted by a series of alcohols having different acid-base characters, Kwon et al.14 have shown recently that the double proton/H-atom transfer rates are increased profoundly with the acidic character of the alcohols, and this led the authors to propose that 7AI tautomerization is triggered by the transfer of a proton from alcohol to the pyridinic nitrogen atom along the N 3 3 3 H-O hydrogen bond. Second, the tautomerization is significantly slowed down on isotopic (O-H/O-D) substitution of methanol and that indicates that the proton transfer is the rate limiting step in the process. In the present paper, we report the photophysical behavior of a 1:1 complex of 7AI with phenol. The motivations of the study are the following. First, although catalytic influence of various aliphatic alcohols has been explored extensively, no study, to our knowledge, has been reported until the date on the influence of aromatic alcohols. The acid dissociation constant of phenol (pKa ∼ 10) being several orders of magnitude larger compared to, for example, ethanol (pKa ∼ 16), the catalytic effect of the former is expected to be much larger. Second, the S1rS0 electronic excitation energy of isolated phenol is about 1700
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cm-1 larger compared to that of 7AI.22,37 Therefore, in an isolated complex the electronic interaction between the two molecular moieties could be small if 7AI is excited to lowest available electronic energy level. However, in a hydrocarbon solution at room temperature, because of spectral broadening and structural flexibility in a supporting liquid medium, new photophysical pathways could be evolved, which can compete with the tautomerization dynamics in the excited state. The results presented below show that such a competing deactivation channel dominates over the tautomerization channel.
2. EXPERIMENT 7-Azaindole and phenol were purchased from Sigma-Aldrich and purified further by vacuum sublimation before use. Anisole was procured from Spectrochem Pvt. Ltd. and purified by vacuum distillation. UV grade methylcyclohexane and ethanol were also obtained from Spectrochem and used as supplied after testing their purity by measuring fluorescence spectra. The spectrum of pure methylcyclohexane recorded by exciting at 300 nm is displayed in Figure 4, trace 5. Except for the two Raman bands, there is no other emission. The concentration of the stock solution of phenol was 2 M, and it was added gradually to the said 7AI solution to get its effective concentration in the range of 3.33 10-3 to 3.33 10-2 M. To prepare the 7AIethanol and 7AI-anisole complexes, pure ethanol and anisole were added to the said 7AI solution. The electronic absorption spectra of all the solutions were recorded using a Shimadzu UV2410 spectrometer and the fluorescence spectra of the same sets of solutions were recorded using a Jobin Yvon FluoroMax-3 spectrofluorimeter after correcting with respect to instrument response parameters. For measuring the spectra at low temperatures (absorption as well as fluorescence), a home-built doublewalled quartz dewar was used. The space in between the two walls of the dewar was evacuated continuously by a vacuum pump. The sample was taken in a round quartz cell and inserted into the dewar and cooled by regulated flow of liquid nitrogen vapor. The temperature inside the cell was monitored and controlled using a home-built temperature controller with an accuracy of (1 °C. 3. RESULTS AND DISCUSSION 3a. Structure of the Complex. The fully optimized structure corresponding to the most favored conformation (doubly hydrogen-bonded cyclic geometry) of the 7AI-phenol complex (1:1) in the ground state is displayed in Figure 1A. The basis set superposition error (BSSE)-corrected binding energy is 9.9 kcal/ mol, and it is smaller compared to the 1:1 complex with acetic acid (Figure 1B), which is known to be a very effective catalyst for tautomerization of 7AI in the excited state. The optimized geometric parameters of the two complexes indicates that the hydrogen bond lengths in the latter complex are much shorter compared to those in the former, and this happens because the O-H group of one phenol molecule cannot be packed properly between the widely separated pyridinic N and pyrrolic N-H groups of 7AI to form two strong hydrogen bonds. 3b. Electronic Absorption Spectra of 7AI-Phenol Complex. The complex is easily produced upon mixing phenol and 7AI in a hydrocarbon solution. In Figure 2, we have shown the changes of the longest wavelength segment of the S1rS0 absorption system of 7AI in a dilute methylcyclohexane owing to a successive increase of phenol concentration in the solution. 1831
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Figure 1. Optimized structures of (A) 7AI-phenol and (B) 7AI-acetic acid complexes predicted by calculation at the DFT/B3LYP/6-311þ G** level. The hydrogen bond lengths (Å) in complex A is predicted to be longer compared to those in complex B.
Figure 2. Changes in the longest wavelength segment of S1rS0 absorption spectrum of 7AI upon complexation with phenol (PhOH) in methylcyclohexane at room temperature. The traces 1-4 denote the spectra recorded for 7AI solution (10-5 M) with different concentration of phenol as mentioned in the figure. The longest wavelength segments of the spectra of pure phenol solutions for the lowest and highest concentrations used are shown using dashed lines, traces 5 and 6, respectively. The depicted absorption spectra of the mixed solutions (traces 2-4) are obtained after subtracting the absorbance of pure phenol solution from the recorded absorbance of the mixed solutions over the wavelength range shown.
To avoid formation of 7AI homodimer and maximizing the concentration of the mixed dimer (1:1 complex), the concentration of 7AI was kept low (10-5 M) and the added phenol concentration in the final solution was varied in the range of 3.33 10-3 to 3.33 10-2 M. The longest wavelength absorption profiles of only PhOH solution corresponding to the lowest (3.33 10-3 M) and highest (3.33 10-2 M) concentrations used in the mixed solution are also displayed (traces 5 and 6, respectively). The 1:1 nature of the complex is ascertained by observing a linear Benesi-Hildebrand plot associated with changes in the absorption of the complex at 304 nm with increasing concentration of phenol (Figure 3A). A direct nonlinear plot, depicting the increase in absorbance of the complex (Acomplex) with phenol concentration for probing at 304 nm is presented in part B of the Figure 3. The association constant (Ka) of the complex estimated by analyzing the nonlinear plot following the eq 3 in ref 18 is 120 ( 15 M-1. Under the same
Figure 3. Benesi-Hildebrand plot (part A) showing changes in absorbance at 304 nm of the 7AI-phenol complex with an increase in concentration of the phenol in the methylcyclohexane solution of 7AI (10-5 M). The nonlinear plot showing the increase in absorbance of the complex with phenol concentration is presented in part B. A more accurate value of association constant (Ka) is estimated from the nonlinear plot.
condition, the value of Ka estimated for 7AI-ethanol complex is 62 ( 5 M-1, and it is somewhat similar to what has been estimated for the 7AI-methanol complex (50 M-1) in nheptane by Kwon et al.14 Evidently, the higher value of Ka for the former case is because of higher acidity of phenol compared to methanol or ethanol. Nevertheless, Ka of 7AI-phenol complex is much smaller compared to Ka found for 1:1 complex with acetic acid (∼2 104 M-1),14 and the difference is consistent with the prediction of the electronic structure calculation that the binding energy of 7AI-acetic acid complex (15.3 kcal/mol) is larger compared to that of 7AI-phenol complex, and accordingly, the hydrogen bond lengths in the former case are predicted to be much shorter compared to those in the latter. Thus, with respect to both the estimated thermodynamic parameter (Ka) and theoretical predictions, the present system under study is a loosely bound hydrogen-bonded complex. 3c. Complexation Effects on Fluorescence Spectra. To investigate the catalytic effect of phenol on the excited-state tautomerization of 7AI in 1:1 doubly hydrogen-bonded complex configuration, we have measured a set of fluorescence spectra with mixed solutions containing 7AI (10-5 M) and phenol of different concentrations in the range of 3.33 10-3 to 3.33 10-2 M. Three such spectra are presented in Figure 4 (traces 2-4) along with that of the pure solution of 7AI (trace 1). All the spectra are measured at room temperature by exciting at 300 nm. 1832
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The Journal of Physical Chemistry A It is worth mentioning that the sharp bands displayed on each of the spectra at 312 and 324 nm are the Raman bands of the solvent, because they show up in the same way in the emission spectrum of pure methylcyclohexane (trace 5). The wavelength positions of these bands undergo shifting with change in excitation wavelength, which confirms the assignments, and the same assignments were suggested also in earlier studies.17
Figure 4. Fluorescence spectra of 7AI in methylcyclohexane (10-5 M) at room temperature in the presence of three different concentrations of phenol (traces 1-4). The spectrum recorded in presence of ethanol of concentration 2.85 10-2 M in the same 7AI solution is shown (trace 6) for a comparison. Excitation wavelength, λexc = 300 nm was kept fixed to record all the emission spectra. The lowermost trace denotes the emission spectrum of pure solvent. The sharp band at 324 nm is a Raman band of the solvent. The fluorescence excitation spectra for the same set of mixed and pure 7AI solutions are presented in the inset (traces 1-4). The excitation spectra are recorded by probing the emission at 380 nm.
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Trace 1 indicates that at such low concentration of 7AI (10-5 M), the fluorescence of the pure solution appears almost exclusively in the ultraviolet region, and it is emitted from the locally excited state (S1) of the monomer. The concentration of the dimer being low, the visible tautomer fluorescence, which is emitted exclusively by this species, is also very small. Here the key observation is that there is no indication for enhancement of tautomer fluorescence with increase in phenol concentration in the solutions; rather, the intensity of the UV fluorescence of the monomer is sharply decreased. The monomer fluorescence almost disappears when the phenol concentration is 3.33 10-2 M. In contrast, the trace 6 indicates that in the presence of nearly the same concentration of ethanol, there is a significant enhancement of tautomer fluorescence, and a distinct red-shifted UV fluorescence with the maximum at ∼350 nm also appear. The latter is the normal fluorescence of the 7AI-ethanol complex, emitted from the locally excited state (S1).14 Thus, the results presented here imply that although phenol is a stronger proton donor compared to ethanol, it does not assist in tautomerization of 7AI; rather it induces efficient deactivation of 7AI exited state via some other nonradiative channels. The fluorescence excitation spectra corresponding to the solutions 1-4, recorded by monitoring the fluorescence at 380 nm, are presented in the inset (Figure 4). The basic features of all the spectra look similar except of diminishing intensity with increase of phenol concentration, and no distinct band shows up at wavelengths longer than 300 nm that can correspond to the 7AI-phenol complex. Thus, in longer wavelength region (λ > 300 nm) the features of the excitation and absorption spectra (Figure 2) are different, and this indicates again that the complex is nonfluorescent. To study the thermal effects on the excited state behavior of the complex, we have measured the emission spectra of a mixed solution by lowering its temperature up to -50 °C, and the results are presented in Figure 5 (part A). The concentrations of 7AI and phenol in the mixed solution were 10-5 and 1.66 10-3 M, respectively, and the excitation wavelength was 300 nm. It is
Figure 5. Fluorescence spectra recorded at six different temperatures (27 to -50 °C) of (A) mixed solution containing phenol (1.66 10-3 M) in the 10-5 M 7AI solution and (B) pure 7AI solution of same concentration. Excitation wavelength was 300 nm in recording all the spectra. The corresponding excitation spectra recorded by probing tautomeric emission at 500 nm are shown in inset I and inset II respectively. 1833
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Figure 6. Absorption spectra of a mixed solution of 7AI (10-5 M) containing phenol of concentration 1.66 10-3 M in methylcyclohexane recorded at four different temperatures in the range of 27 to -20 °C. The inset shows the set of spectra recorded with the pure 7AI solution (10-5 M).
seen that upon temperature lowering, the UV fluorescence of the monomer is sharply depleted, and there is a noticeable development of the tautomer fluorescence in the visible region. At lower temperatures, because of higher values of the association/dimerization constants, the concentrations of both 7AI2 and 7AIPhOH complex would be relatively larger compared to when the solution is at room temperature. Since the light of 300 nm is absorbed by both (7AI)2 and 7AI-PhOH complex, the observed enhanced tautomer fluorescence at lower temperatures can be contributed either solely by the homodimer or partly also by the complex. To distinguish between these two possibilities, the fluorescence spectra of pure 7AI solution (10-5 M) are also measured by varying temperatures identically, and the results are shown in part B in the same intensity scale. The fluorescence excitation spectra recorded by probing the visible tautomer fluorescence of the two solutions are shown in the respective insets. The identical features of the two sets of FE spectra indicate that the emitting species at 500 nm is the same under the two conditions, and it must be the tautomeric form of the homodimer. The enhancement of the tautomer fluorescence of pure 7AI solution is much larger compared to that of the mixed solution, and this happens with much less depletion of monomer fluorescence in the former case. At -50 °C, complete quenching of the monomer fluorescence of the mixed solution occurs because of formation of 7AI-PhOH complex at a large concentration, and the homodimer population under such condition being comparatively low, the intensity of the tautomer fluorescence must be smaller. This observation supports further that the 7AI-PhOH complex is a nonemitting species. To observe the distinct longer wavelength features of the nonfluorescent 7AI-PhOH complex, we have measured the absorption spectra of the same mixed solution at several low temperature and the results are shown in Figure 6. The inset shows the changes in absorption spectra of the pure solution of 7AI for the same effect. Thus, although the mixed solution spectrum (trace 1) is barely different from that of pure 7AI solution (dotted trace) at room temperature, cooling has a very pronounced effect on concentration of the complex. The trace 4 shows that the maximum of the first band of the monomer at
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Figure 7. The changes of the fluorescence spectra of 7AI (10-5 M) in presence of anisole at several concentrations (mentioned on the figure). The corresponding changes of the absorption spectra are shown in the inset.
293.5 nm is shifted to 296 nm upon complex formation, and the longer wavelength segment of the absorption curve of the complex is extended up to 320 nm. In contrast, the temperature lowering to the same extent has visibly smaller effect on homodimer concentration in the pure solution (inset), and it is manifested only by increased intensity of the longer wavelength segment of the curves beyond 300 nm. Such comparison also indicates that at -20 °C, the homodimer population in the mixed solution is much smaller compared to that of the complex. To invoke a suitable model that explains quenching in the preformed complex, we attempt first to understand whether the hydrogen bonds of the complex have any key role to play. In the optimized structure of the complex shown in Figure 1, the phenolic O-H group is simultaneously a hydrogen bond donor to the pyridinic N and acceptor for the pyrrolic N-H group. According to the suggested mechanisms of the alcohol or acid catalyzed complexes, the first step is likely to be a proton transfer along N 3 3 3 H-O hydrogen bond from phenol to the excited 7AI moiety. For a naive verification of whether such a proton transfer is playing the role in the quenching process, we made a comparative study of the photophysical behavior of the 1:1 complex of 7AI with anisole (methoxy benzene). In this case, anisole is only the acceptor for the N-H 3 3 3 O hydrogen bond between pyrrolic N-H of 7AI and oxygen atom of the methoxy group. In Figure 7 we have presented the changes in fluorescence spectral feature of 7AI in presence of anisole of several concentrations in the solutions, and the corresponding absorption spectra are shown in the inset. It is seen that the overall fluorescence intensity from the locally excited state of 7AI (λex = 300 nm) is enhanced in presence of anisole. The absorption spectra (inset) show that in the presence of anisole, the tail of the lowest energy transition of 7AI is shifted further in the longest wavelengths (traces 1-6). However, unlike PhOH complex no distinct structure develops, although the concentrations of anisole used are much larger compared to phenol. This indicates that the hydrogen bond in 7AI-anisole complex is weaker compared to the phenol complex, and this happens because anisole acts only as an acceptor of a hydrogen bond. The traces 7 and 8 (dotted lines) denote that the longer wavelength tails of the absorption spectrum of pure anisole in methylcyclohexane for 1834
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The Journal of Physical Chemistry A the highest and lowest concentrations used, respectively, in recording the above-mentioned spectra of the complexes. It is important to mention that with increase in anisole concentration, the background absorption of the solution at 300 nm (the wavelength selected to record the fluorescence spectra) is also increased, and this partly contributes to the increased intensities of the absorption traces 1-6 of the mixed solutions. However, these increased background absorptions due to anisole hardly contribute to the enhanced fluorescence intensity of 7AI (traces 1-6), and in support of this we have presented the emission spectrum of pure anisole solution of the highest concentration used (1.38 10-1 M), trace 7. It shows clearly that for excitation at 300 nm, very little fluorescence is generated, and this certainly cannot be responsible for the enhanced intensities of traces 1-6. Thus, we infer that although phenol and anisole have the same aromatic chromophore, the ability of the former for a proton/Hatom transfer along the N 3 3 3 H-O hydrogen bond makes it an efficient quencher of the excited electronic state of 7AI. We suggest here two probable mechanisms for the quenching. First, a proton-coupled electron transfer (PCET) pathway. Following a proton transfer along N 3 3 3 H-O hydrogen bond from phenol to excited 7AI, the produced electron-rich phenoxide ion should be an efficient electron donor. The occurrence of excited state quenching of an aromatic chromophore by phenol in 1:1 complexes in liquids have been documented in a number of cases in the past.38-41 In a recent study, Prashanthi and Bangal have shown that phenol can quench the fluorescence of a pyridine-linked porphyrin derivative in dichloromethane solution at room temperature,41 and the authors interpreted the quenching in terms of a PCET process in the preformed 1:1 adduct of the two molecules, where the phenolic O-H binds with the pyridinic N through a hydrogen bond. In comparison, in the present system, the acceptor 7AI is directly excited. The electronic excitation energy of the molecule being much higher compared to porphyrin, the complex is in a more favorable situation for undergoing facile PCET. According to RehmWeller model,42 for facile occurrence of an electron transfer from phenol to excited 7AI, the thermodynamic driving force, ΔG0ET, must be negative. The lowest electronic excitation energy of 7AI chromophore is 4.27 eV and oxidation potential of phenol is 1.47 V. Neglecting the Coulombic term (as there is no net change in the charges in a PCET process), one estimates that for occurrence of the process the reduction potential of 7AI must be less than 2.8 V. To our knowledge, this parameter for 7AI has not been measured, although the values for a large number of other azaaromatic compounds have been reported.43 For example, the reduction potential of pyridine is 2.62 V, and for other substituted pyridines and most of azaaromatics the values are much less. On the basis of such correlation, one can guess that the reduction potential of 7AI should be smaller than that of pyridine, and this would result in a significantly negative value of ΔG0ET. Therefore, from a thermodynamic viewpoint, PCET in the present system is likely to be an effective mechanism for the occurrence of fluorescence quenching. A proton transfer followed by an electron transfer from PhOH to 7AI in ππ* state is effectively an H-atom transfer process. Such a configuration has been predicted to be stable also in the homodimer of 7AI in a recent theoretical study by Gelabert et al.44 Using a hybrid CIS/TDDFT theoretical approach proposed initially by Dreuw and Head-Gordon, it has been shown that charge-transfer ππ* state of 7AI homodimer has a deep potential energy profile with respect to a proton transfer coordinate.
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Figure 8. TDDFT calculated electronic transition energies to S1 of 7AI, protonated 7AI (7AIHþ), phenol, and phenoxide ion (PhO-).
A photoactive ππ* excited state for the dimer has been defined to be a configuration where a proton has been transferred from the ππ* excited moiety to the other in the ground state. This configuration is preferentially stabilized with respect to intermoiety separation on simultaneous transfer of charge from the former to the latter moiety. The second probable mechanism that we propose here is an energy transfer quenching. Following light absorption by the complex, an initial proton transfer along the N 3 3 3 H-O hydrogen bond leaves the system in an ion-pair state, where the protonated 7AI is in the ππ* excited state and the anion (phenoxide ion) in the ground state. As stated before, although the energy transfer from excited 7AI to phenol is not energetically feasible in the neutral form, it could be facile in the said ionpair, and the reasons are the following. First, the excited state of the phenoxide ion is energetically lower compared to that of the protonated 7AI, and second, its electronic transition moment is much higher compared to its neutral precursor.45 The energies of the lowest excited states of 7AI, PhOH, 7AIHþ and PhO-, predicted by electronic structure calculation at TDDFT (6311þG**) level is shown schematically in Figure 8. A comparison of these values shows clearly that although the S1 of protonated 7AI is below to that of 7AI, the lowest excited state of phenoxide ion is much lower. Here, the proton transfer from PhOH to excited 7AI and energy transfer in the other way could also occur concertedly.
4. SUMMARY In this paper we have shown that 7AI forms a 1:1 hydrogenbonded complex with phenol in a hydrocarbon solution at room temperature. The 1:1 nature of the complex has been ascertained by observing a linear Benesi-Hildebrand plot, and the estimated value of the association constant is ∼120 M-1 at 25 °C. Electronic structure calculation predicts that the complex favors a doubly hydrogen-bonded cyclic structure. However, the hydrogen bonds are much weaker compared to those present in the cyclic 1:1 complex of 7AI with acetic acid. Upon complexation, phenol completely quenches the fluorescence of 7AI and shows no indication of 7AI tautomerization in the excited state. On the other hand, 7AI fluorescence in the hydrocarbon solution is found to be enhanced upon addition of anisole, which forms a singly hydrogen bonded (N-H 3 3 3 O) complex involving pyrrolic N-H group. The contrasts in photophysical behavior of the two complexing molecules indicate that the formation of N 3 3 3 O-H type hydrogen bond between phenol and pyidinic N atom is the key factor for quenching of the 7AI fluorescence. As 1835
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The Journal of Physical Chemistry A a plausible mechanism, we have suggested that a PCET process between phenol and excited 7AI is responsible for the observed quenching. In the framework of Rehm-Weller model, we have argued that the electron transfer from phenol to excited 7AI is an energetically favorable process (ΔG0ET < 0). We also have discussed the possibility for electronic energy transfer quenching from the excited state of protonated 7AI to the phenoxide ion, which is a nonfluorescent species.
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