Stepwise versus Concerted Mechanism of Photoinduced Proton

Nov 18, 2014 - *E-mail: [email protected]. This article is part of the Photoinduced Proton Transfer in Chemistry and Biology Symposium special iss...
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Stepwise versus Concerted Mechanism of Photoinduced Proton Transfer in sec-1,2-Dihydroquinolines: Effect of Excitation Wavelength and Solvent Composition Tatiana D. Nekipelova,*,†,§ Ivan S. Shelaev,‡ Fedor E. Gostev,‡ Victor A. Nadtochenko,‡,§ and Vladimir A. Kuzmin†,§ †

Emanuel Institute of Biochemical Physics, RAS, 4 Kosygin Street, Moscow 119334, Russia Semenov Institute of Chemical Physics, RAS, 4 Kosygin Street, Moscow 119991, Russia § Moscow State University, Faculty of Chemistry, 1 Leninskie Gory, Moscow 119992, Russia ‡

S Supporting Information *

ABSTRACT: Excited-state proton transfer in 2,2,4,6-tetramethyl-1,2-dihydroquinoline was studied in dependence on solvent composition in binary mixtures of MeOH with nonpolar pentane and polar acetonitrile by an ultrafast pump−probe technique with λpump = 350 nm and spectroscopic detection. The concerted mechanism was observed earlier at λpump = 308 nm, whereas the stepwise mechanism is clearly observed at λpump = 350 nm. The “fast” (200 fs) component is attributed to PT from MeOH molecule to the C(3) of the heterocycle to give the carbocationic structure in the highest vibrational ground state. This process is succeeded by the “slow” PT from the NH group to the MeO− anion in the cluster of MeOH molecules to give corresponding 2,3-dihydroquinoline. The former process is independent of the solvent composition, whereas the latter depends on the solvent composition in the MeOH−C5H12 mixtures with a characteristic time increasing from 15 ps in bulk MeOH to 150 ps in the mixture containing 2 vol % MeOH. This result is explained in terms of stabilization of the ionic species in the less polar microenvironment.

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containing an OH group, which exhibits the features of “super” photoacids in the excited state, is studied intensely because the investigation of excited-state “super” photoacids makes it possible to reveal important information on the mechanism of reversible proton transfer reactions in biochemical systems.12−14 In all the examples given above, the product of the proton transfer reaction is formed in the excited state, and reverse PT to give the reactant occurs in the ground state after radiative or nonradiative deactivation of the product in the excited state. Recently, we have studied a new photochemical reaction of alcohol and water addition to the double bond of 1,2dihydroquinolines (1,2-DHQs) to give 4-alkoxy- or 4hydroxy-1,2,3,4-tetrahydroquinolines, respectively (see the ref 15 review and references therein). The reaction is irreversible and quantitative. The detailed investigation of the reaction showed that the final product was formed in the interaction of an intermediate carbocation with a molecule of protic solvents, alcohol, or water.16 The carbocation formation mechanism depends on the structure of 1,2-DHQ (Scheme 1): in the case

INTRODUCTION Photoinduced proton transfer (PT) is one of the most important reactions in chemistry and biology. It is wellestablished that PT in hydrogen-bonded systems often occurs with high rate constants both in thermal and photoinduced reactions.1,2 The intramolecular PT takes place in bifunctional organic molecules that contain hydrogen-donating and hydrogen-accepting groups in close proximity, allowing the formation of an intramolecular hydrogen bond in the ground state. Such molecules can undergo excited state intramolecular PT (ESIPT).3−5 Keto−enol or keto−imine tautomerization is one of the fastest processes in nature and occurs usually adiabatically on the excited state PES.1 Dual fluorescence is typical of these processes.4−6 Methyl salicylate is a compound for which this phenomenon was observed for the first time almost 60 years ago,4 and PT in this compound has been investigated until now (see ref 5 and references therein). For compounds in which the groups involved in the PT reaction are structurally separated, proton transfer occurs in protic solvents (water or alcohols) via a bridge of hydrogen-bonded solvent molecules. This case has been more thoroughly studied for 7hydroxyquinoline (7HQ), for which this mechanism was suggested for the first time almost 30 years ago, and 7HQ continues to attract the attention of researchers up to the present.6−11 Such a bridge of solvent molecules interconnected by hydrogen bonding may be considered as a proton wire. Photoinduced proton transfer from the excited molecules © XXXX American Chemical Society

Special Issue: Photoinduced Proton Transfer in Chemistry and Biology Symposium Received: August 6, 2014 Revised: November 5, 2014

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and H2O molecules.16,25 If there is no substituent at C(4), the PT reaction takes place also with other normal alcohols and alcohols with branching at the third C atom (EtOH, PrOH, TFE, and BuiOH) but not with PriOH and ButOH.25,26 This implies that structural factors, rather than acidity or polarity of a protic solvent, determine the formation of the complex with hydrogen bonds between the 1,2-DHQ and alcohol molecules, which is active in the PT reaction. Interestingly that in the case of 2,2,4,6-tetramethyl-1,2-dihydroquinoline, the absorption spectrum in EtOH is red-shifted by 1−2 nm, whereas the spectrum in MeOH is blue-shifted by 5−7 nm relative to the spectrum in alkanes 16 (Figure S1 of the Supporting Information), indicating hydrogen bonding of different types for these alcohols. The H-bond for MeOH is formed between the H atom of the MeOH hydroxyl group and the π-system of the heterocycle, and the H-bond in the case of EtOH is formed between the amino group of 1,2-DHQ and the hydroxyl oxygen of EtOH (N−H---O). In this contribution, we report experimental results on the dynamics of photoinduced PT in sec-1,2-DHQ, 2,2,4,6tetramethyl-1,2-hydroquinoline, using another excitation wavelength, λpump = 350 nm, in bulk MeOH and in binary mixtures of MeOH with nonpolar C5H12 and polar MeCN, solvents that are inert in the PT reaction. Both solvents form homogeneous solutions with MeOH throughout the entire range of concentrations at room temperature. Unlike the case of excitation with λpump = 308 nm, stepwise dynamics were observed upon excitation at λpump = 350 nm. The effect of the solvent composition and the cosolvent nature was also revealed.

Scheme 1. Photolysis of 2,2,4,6-Tetramethyl- and 1,2,2,4,6Pentamethy-1,2-dihydroquinolines in Protic Solvents

of secondary 1,2-DHQ with the NH function, the primary intermediate product observed by conventional flash photolysis is 2,3-dihydroquinoline (2,3-DHQ), the tautomer of 1,2-DHQ. Formally, this product is formed as a result of PT from the NH group to the C(3) atom of the heterocycle (ene−imine tautomerization). However, this tautomerization is irreversible, occurs only under irradiation in protic solvents and does not take place in the ground state even at elevated temperatures in protic solvents. 2,3-DHQ interacts with a protic solvent to give the carbocation as a result of PT from the solvent. In the case of tertiary N-methyl-substituted 1,2-DHQ, the carbocation is the primary intermediate species, which is formed as a result of PT from the solvent to the C(3) atom of the excited 1,2-DHQ molecule. The lifetimes of 2,3-DHQ and the carbocation are on the millisecond timescale, and these intermediates were examined by microsecond conventional flash photolysis.15,16 The study of the PT reaction upon femtosecond excitation with λpump = 308 nm showed the formation of the primary transient species for both sec-1,2-DHQ and tert-1,2-DHQs on the femtosecond timescale (200−500 fs) from the nonrelaxed excited state in the nonadiabatic process with the formation of 2,3-DHQ or the carbocation, respectively, in the ground state.17,18 The features of the excited state proton transfer in 1,2-DHQ, which distinguishs this reaction from photoinduced proton transfer in the aforementioned cases, 7-hydroxyquinoline among them, are as follows. First, the proton transfer in 1,2DHQ is an example of the PT reaction, in which the product is formed in the ground rather than the excited state as a result of proton transfer with the participation of an excited molecule. No dual fluorescence was observed for 1,2-DHQ. The photoinduced PT in 1,2-DHQ differs from the nonadiabatic PT considered in the literature. The nonadiabatic PT was observed experimentally for the contact radical ion pairs of substituted benzophenones/N,N-diethylaniline19 and for the PT reaction of diphenylmethyl carbanion with a variety of acids20 and was considered theoretically21,22 as proton tunneling through a barrier with the involvement of a solvent reorganization coordinate. The formation of the products in the excited state was accepted for these systems. Second, 1,2-DHQs become C-base in the first excited state, and a proton is transferred from a protic solvent to the C(3) atom of the heterocycle (i.e., the basic site is the C-atom, rather than a heteroatom as in the case of 7HQ). The formation of C-bases in the excited state was discussed in a review23 and observed for biphenyls.24 Third, the reaction under study is extremely sensitive to a solvent. For 4-methyl-substituted 1,2-DHQs, photoinduced PT occurs only with the participation of MeOH

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EXPERIMENTAL SECTION 2,2,4,6-Tetramethyl-1,2-dihydroquinoline (1,2-DHQ, Reakhim, Russia) was sublimed under reduced pressure before use. Methanol (Merck, for spectroscopy, 99.9%), acetonitrile (Reakhim, Russia, analytical grade, 99.9%), and n-pentane (Acros Organics, pure, 99.0%) were purified and dried according to the standard procedures.27 Transient absorption spectra were measured by the femtosecond pump to supercontinuum probe setup. The output of a Ti:sapphire oscillator (800 nm, 80 MHz, 80 fs, Tsunami, Spectra-Physics) was amplified by a regenerative amplifier system (Spitfire, Spectra-Physics) at a repetition rate of 1 kHz as described earlier.28 The amplified pulses were split into two beams. One of the beams was directed into a noncollinearly phase-matched optical parametric amplifier. Its output centered at 700 nm was compressed by a pair of quartz prisms. The gauss pulse of 25 fs at 350 nm with the bandwidth of ∼32 nm (full width at half-maximum) was used as a pump. The second beam was focused onto a 500 μm quartz cell with H2O to generate supercontinuum probe pulses. The pump and probe pulses were time-delayed with respect to each other using a computer-controlled delay stage. They were then attenuated, recombined, and focused onto the sample cell. The pump and probe light spots had the diameters of 300 and 120 μm, respectively. The pump pulse energy was attenuated at 240 nJ to get optimal excitation on a linear part of the light curve. Experiments were carried out at 298 K in a flow optical cell with a path length of 0.5 mm. Frequency control of laser pulses was produced by regular device synchronization and a control amplifier SDG II Spitfire 9132 manufactured by SpectraPhysics. Measurements were done at a frequency of 50 Hz. Analysis of the kinetic curves was instrument-limited at a zero time delay and was carried out starting from 100 fs, because B

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there is a nonlinear coherent peak with a characteristic time of 100 fs masking the true kinetics.

Table 1. Time Constants Determined from Global Analysis of Kinetic Data of the Laser Photolysis of 1,2-DHQ in Dependence on the Composition of MeOH−C5H12 and MeOH−MeCN Binary Mixtures (λpump = 350 nm)

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RESULTS AND DISCISSION Spectral Dynamics in Bulk MeOH. The stepwise spectral dynamics is observed in MeOH after a femtoseconds pulse with λpump = 350 nm (Figures 1 and 2). The initial spectrum is

solvent

MeOH vol %

τ1 (fsa)

τ2 (psb)

τ3(fluo) (nsc)

MeOH−C5H12

100 10 2 0 50 20 0

200 ± 50 200 ± 50 200 ± 50 − 200 ± 50 200 ± 50 −

15 ± 2 71 ± 8 150 ± 20 − 16 ± 2 18 ± 2 −

1.0 (1.1) 2.2 (2.2) 3.0 (2.9) 4.8 (4.7) 4.8 (4.9), 0.7 7.0 (6.7), 0.7 7.5 (7.5)

MeOH−MeCN

a

Rise at 400−450 nm. bRise at 400−450 nm and decay at 470−600 nm. cIn brackets, the fluorescence lifetimes for the given mixtures measured in ref 29 are given.

reaction was completed to the time delay of 200−500 fs.17 It should be emphasized that, whatever the excitation wavelength is, the final product 1,2,3,4-tetrahydroquinoline is formed quantitatively, indicating that the nature of final transient species generated as a result of the PT reaction does not depend on the excitation wavelength. The results obtained set two main problems which should be considered: (i) the cause of the difference in PT dynamics at different excitation wavelengths and (ii) interpretation of the stepwise dynamics for λpump = 350 nm. It should be pointed out that the concerted mechanism is observed up to λpump = 320 nm. The ultrafast excited state proton transfer usually occurs in the hydrogen-bonded complex formed in the ground state. We have shown that a complex of the 1,2-DHQ molecule with at least six MeOH molecules is required for the reaction under study.29,30 MeOH possesses both acid and base properties and thus can form complexes with sec-1,2-DHQ as the proton donor and acceptor. The analysis of the sec-1,2-DHQ spectra in binary mixtures of MeOH with alkanes shows that methanol molecules at a volume fraction of MeOH ≥ 2% act as proton donors forming the H-bond with an electronic π-system of the heterocycle, which manifests itself in a blue shift of the longwavelength absorption band (Figure S1 of the Supporting Information).29 Quantum-chemical calculations of the 1,2DHQ molecule in excited states in the gas phase by the CNDO/S method showed that the S1 state is a CT state, with the change of the Mulliken charges in comparison with the S0 state being ΔδN = +0.230 at the nitrogen atom and ΔδC(3) = −0.105 at the C(3) atom31 (i.e., the charge redistribution in the excited singlet state results in the partial localization of the negative charge at the C(3) atom promoting the PT reaction from protic solvents to this position). A schematic diagram of energy levels for the S0 and S1 states of 1,2-DHQ and 2,3-DHQ is depicted in Figure S2 of the Supporting Information with the use of spectral data and the data on the energy of the S1 state for 1,2-DHQ31 (Table S1 of the Supporting Information). The energy of the S0 state of 2,3DHQ is higher by 15−20 kcal/mol than that of 1,2-DHQ. Therefore, taking into account that for 2,3-DHQ λmax = 420 nm, the energy level of the S1 state for 2,3-DHQ relative to the zero energy level is close or higher than that for 1,2-DHQ (i.e., the conical intersection between the S1 state of 1,2-DHQ and the S0 state of 2,3-DHQ is possible upon excitation with λpump = 308 nm not upon excitation with λpump = 350 nm ) (Figure S2 of the Supporting Information). Upon excitation by λpump = 350 nm, there is no intersection with the PES for the complex of

Figure 1. Transient absorbance at different time delays in the femtosecond laser photolysis of 1,2-DHQ in MeOH, λpump 350 nm. Inset: kinetics of transient absorbance at different λprobe.

Figure 2. Kinetics of transient absorbance at different λprobe in different time windows, λpump = 350 nm.

similar to those observed earlier in hexane upon excitation with λpump = 308 nm with a maxima at 450 and 700 nm.17 The transient absorbance strongly increases in the range 390−410 nm and slightly increases at 410−440 nm for 200 ps (Figure 1). Then the absorbance at 500−600 nm decays with a characteristic time of 15 ps giving rise to the absorption at 400−440 nm (Figure 2A, Table 1). After completion of this process, the transient absorbance of the relaxed excited singlet state of 1,2-DHQ decays with a time constant of ∼1 ns equal to the fluorescence lifetime of 1,2DHQ in MeOH (Figure 2B).29 This stepwise dynamics of the transient absorbance after the femtosecond pulse with λpump = 350 nm crucially differs from the dynamics observed for λpump = 308 nm, when the PT C

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Scheme 2. Stepwise PT in 2,2,4,6-Tetramethyl-1,2-dyhydroquinoline in MeOH at λpump 350 nma

τ1 characterizes the first step of PT and vibrational relaxation (VR) of the nonrelaxed S1v, τ2 characterizes PT from the NH group to the negatively charged MeOH cluster, τ3 characterizes the internal conversion (IC) and fluorescence of the relaxed S1 state a

and H2O.16,25,26 Therefore, isopropanol is the inert solvent for the reaction. The solvent compositions for the femtosecond experiments have been chosen with allowance for the experimental results on the relative quantum yields in binary mixtures,30 which show that the PT reaction occurs at a MeOH volume content exceeding 1% (0.25 M) in the MeOH−C5H12 mixtures and 10% (2.5 M) in the MeOH−MeCN mixtures. The reasons for this dependence on the binary mixtures composition were discussed in detail elsewhere.30 This finding substantially differs from the results for the photoinduced PT in 7-hydroxyquinoline, for which it has been established that PT is solvent-mediated and occurs via a proton wire of two water or alcohol molecules6−11 or three ammonia molecules32 interconnecting the acidic and basic sites of the 7HQ molecule. Therefore, the investigation of PT in 7HQ was carried out at the concentration of alcohols in alkanes (30 mM),8,9 an order of magnitude below the lower limit for 1,2DHQ in pentane. The different behavior of the transient absorbance of 1,2DHQ after excitation with a femtosecond pulse at λpump = 350 nm is observed in pure inert solvents: (i) in pentane (Figure S4A of the Supporting Information), the absorbance decays with a characteristic time equal to the lifetime of fluorescence in C5H12 (4.8 ns29) without observable changes in the shape of the spectrum, which corresponds to the spectrum of the S1 state with maxima at 450 and 700 nm similarl to the excitation with λpump = 308 nm17 and (ii) in acetonitrile (Figure S4B of the Supporting Information) the spectrum observed at a time delay of 0.1 ps has a maximum at 700 nm and a shoulder at 450 nm at the shorter wavelengths then the absorbance decays in the range 440−400 nm, with the absorbance at 400 nm becoming zero at a time delay of 500 ps. As a result, the maximum at 450 nm appears. This process occurs with two characteristic times of 8 and 350 ps. The whole spectrum decays with a characteristic time of 7.5 ns corresponding to the fluorescence lifetime in MeCN (Table 1).29 Similar to the 1,2-DHQ solution in bulk MeOH, the stepwise dynamics of transient absorbance is observed in the mixtures (see Figures 3 and 4, where the decay kinetics of the absorbance at λprobe = 420 and 450 nm are compared for the MeOH−C5H12 and MeOH−MeCN binary mixtures on the timescale up to 500 ps). The dynamics in the time-window of 0.1−2 ps for the mixtures with C5H12 and MeCN is almost the same as for pure MeOH (Figures 1 and 2 and Table 1) and differs significantly from those in pure inert solvents (Figure S5 of the Supporting Information). In the pure inert solvents, the

2,3-DHQ, but this may be with the highest vibrational levels of the carbocation ground state, and the species with the carbocationic structure is formed (i.e. first, the proton of the MeOH molecule is transferred to the C(3) atom of the heterocycle). This is indicated by the appearance of the absorbance at 550 nm (the vibrationally relaxed carbocation absorbs at 480 nm). Simultaneously with the formation of the carbocationic species, the MeO− anion is generated. The vibrationally excited carbocation in the negatively charged cluster undergoes PT from the N−H bond to the methoxy anion to afford 2,3-DHQ either in the direct reaction or via PT with participation of the hydrogen-bonded MeOH molecules (Scheme 2). We do not consider the reversed order of the reactions when the first step is PT from the NH group to MeOH and then is completed by PT from MeOH2+ to C(3) atom via the bridge of the MeOH molecules. The formation of the carbocation as the only transient species in the case of tert-DHQ with the N−CH3 group15−18 supports the suggested mechanism. Moreover, the absence of the PT reaction in 4-methyl-substituted sec-1,2DHQ in the solutions of higher alcohols (EtOH and higher), for which the formation of the hydrogen bond of the N−H-O−C type was shown,16 is another argument in favor of this mechanism. Effect of Solvent Composition in Binary Mixtures of MeOH with C5H12 and MeCN. The photolysis of 1,2-DHQ in MeOH is a remarkable reaction (Scheme 1). Despite its multistep character, the yield of each transient species and the final product tetrahydroquinoline is equal to the yield of the initial PT reaction because of the irreversibility of every step and absence of byproducts. This allowed us to measure the relative quantum yields of photoinduced PT in 1,2-DHQ in the binary mixtures of MeOH with inert solvents by measuring the relative yields of 2,3-DHQ by conventional flash photolysis (Figure S3 of the Supporting Information).30 Nonpolar pentane and polar acetonitrile was chosen as inert solvents in ref 30 and in this study. Albeit experimentally, it is difficult to employ pentane as a solvent because of its high volatility at ambient temperature (bp = 36.1 °C), the miscibility of C5H12 with MeOH over the entire range of solvent compositions is an advantage of this solvent over hexane and higher alkanes. Figure S3 of the Supporting Information also gives the relative quantum yields for the C6H14−MeOH and PriOH−MeOH mixtures. As was mentioned above, the reaction under study is very sensitive to a solvent, and photoinduced PT for 4substituted 1,2-DHQ occurs only in the presence of MeOH D

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absorbance slowly decays with a lifetime of fluorescence. The addition of MeOH results in a fast increase in the absorbance within the first 200 fs at 390−430 nm independent of the inert solvent, but the following “slow” increase in the absorbance at this spectral range depends on the solvent composition and the cosolvent (Figures 3 and 4). As the concentration of MeOH in the binary mixtures with C5H12 and MeCN increases, the absorbance at 420 nm becomes higher than that at 450 nm. This characterizes not only the changes in transient species spectra with variation of their microenvironment but also the formation of 2,3-DHQ (λmax 420 nm), the product of the PT reaction (Figures 3 and 4). Interestingly, the difference in the absorbance at 420 and 450 nm in the MeOH−C5H12 mixtures, containing 10 and 100 vol % MeOH is almost the same at a time delay of 500 ps, whereas this difference is much lower for the MeOH−MeCN mixtures even for 50 vol % MeOH. These data are consistent with the values of the relative quantum yield of 2,3-DHQ (see Figure S3 of the Supporting Information) obtained by the conventional flash photolysis in these binary mixtures.30 In the MeOH−C5H12 mixtures, the stepwise dynamics depends on the MeOH concentration: the increase in the MeOH concentration almost does not affect the dynamics of the first, “fast” step of the process, whereas the characteristic time of the “slow” growth decreases significantly from 150 to 15 ps with an increase in the volume fraction of MeOH from 2 to 50 vol % MeOH, respectively (Figure 3 and Table 1). At MeOH higher concentrations, the time constant of the “slow” growth almost does not depend on the solvent composition. Then, the absorbance decays with a characteristic time equal to the fluorescence lifetime for the given solvent composition.29 In the MeOH−MeCN binary mixtures, the dynamics of both “fast” and “slow” steps at 420 nm are almost independent of the MeOH concentration in the concentration range in which the PT reaction takes place ([MeOH] ≥ 20 vol %, compare Figures 3D and 4D and Table 1). The results obtained in the binary mixtures can be interpreted in terms of the peculiarities of MeOH aggregation in these solvent mixtures and the distribution of the 1,2-DHQ molecules between the MeOH clusters and the inert solvent. As mentioned above, the photoinduced PT reaction in 1,2-DHQ occurs at the methanol concentrations providing the formation of a complex with a number of alcohol molecules higher than six.29 This result agrees with the known experimental data and the quantum-chemical calculations, which show that PT is facilitated by cooperative effects in the clusters of alcohol, water, and ammonia molecules containing more than five solvent molecules.33−35 The consideration of the data on the relative quantum yields of proton transfer in the binary mixtures from this standpoint (Figure S3 of the Supporting Information) indicates that the complex with six or more MeOH molecules in the configuration corresponding to the photoinduced proton transfer is formed only at [MeOH] ≥ 0.5 M (2 vol %) and 2.5 M (10 vol %) in the MeOH mixtures with C5H12 and MeCN, respectively. The increase in the MeOH concentration necessary for the proton transfer reaction in the mixtures with MeCN by an order of magnitude in comparison with the MeOH−C5H12 mixtures is the result of several factors. First, the formation of the MeOH aggregates in nonpolar alkanes is observed at lower methanol concentrations than in acetonitrile, and these aggregates are tighter and involve only alcohol molecules.36,37 In MeCN, the MeOH aggregates are looser, contain a lower number of alcohol molecules and may

Figure 3. Kinetics of transient absorbance at λprobe = 420 (red) and 450 (black) nm in the femtoseconds laser photolysis (λpump = 350 nm) of 1,2-DHQ in the binary mixtures MeOH−C5H12 of different composition.

Figure 4. Kinetics of transient absorbance at λprobe = 420 (red) and 450 (black) nm in the fs laser photolysis (λpump = 350 nm) of 1,2DHQ in the binary mixtures MeOH−MeCN of different composition.

relaxation of the primarily formed absorbance occurs during the first 100 fs in pentane and 200 fs in acetonitrile, and then the E

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Figure 5. Absorption spectra of excited-state 1,2-DHQ in (A) pure MeCN and MeOH and in (B and C) the MeOH−MeCN binary mixtures of different composition.

involve 1−2 MeCN molecules due to the H-bonding between MeOH and MeCN molecules.38 Second, 1,2-DHQ is weakly soluble in nonpolar solvents; therefore, the solute molecules are associated preferentially with MeOH even at low concentrations of the latter in C5H12, a fact that was shown by the analysis of the fluorescence spectra in the mixtures (Figure S1 of the Supporting Information).29 As has also been found, the 1,2-DHQ molecules that are not associated with MeOH in the MeOH−MeCN mixtures are observed even at a methanol concentration up to 95 vol %, owing to the good solubility of 1,2-DHQ in MeCN, the possibility of hydrogen bonding between 1,2-DHQ and MeCN molecules, and the existence of free MeCN molecules at these MeOH concentrations. Results of this study support this observation. The addition of MeOH to MeCN results in the appearance of two components in the transient absorbance corresponding to the decay of two excited states with different times. The spectra of these components (Figure 5) calculated from the global analysis are similar to those observed in pure MeCN and MeOH after vibrational relaxation of the excitation (compare Figure 5A with Figure 1 and Figure S4B of the Supporting Information). The proportion of the component corresponding to the complex with MeOH increases with an increase in the MeOH concentration, as well as its lifetime from ∼700 ps at 10−50 vol % MeOH to 1 ns in bulk MeOH, whereas the lifetime of the component assigned to the 1,2-DHQ molecules in the MeCN environment decreases from 7.5 to 4.9 ns in pure and 50 vol % MeCN, respectively (Figure 5, panels B and C), coinciding with the data on the fluorescence lifetimes (Table 1).29 This indicates the existence of at least two forms of 1,2-DHQ molecules in the MeOH−MeCN mixtures: in the MeCN and MeOH microenvironment, respectively. The former is inert in the PT reaction, and the latter undergoes photoinduced PT in the clusters of MeOH molecules. Thus, the first step of the stepwise PT in 1,2-DHQ is independent of the inert solvent and is determined by the formation of the complex of appropriate composition and configuration with the MeOH cluster in the ground state. But there is a question left. Why does the second step of the reaction depend so dramatically on the nature of the inert solvent and the solvent composition? It is well-known that ionic species are stabilized in nonpolar solvents.39 From this standpoint the carbocation and the MeO− anion, generated as a result of the primary PT reaction in the MeOH−C5H12

mixtures, are stabilized, and this may be the reason for the deceleration of the second step of proton transfer in the less polar microenvironment as the fraction of C5H12 increases. On the other hand, the polarity of MeOH and MeCN are close to each other, and this factor should not strongly affect the kinetics of the proton transfer in the mixture.

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CONCLUSIONS In summary, photoinduced PT in methyl-substituted sec-1,2DHQ exhibits the wavelength dependence of the spectral dynamics, which changes from concerted to stepwise on going from excitation with λpump = 308 to 350 nm. We suppose that upon excitation of the complex 1,2-DHQ−nMeOH by λpump = 308 nm to the higher vibrational levels of the S1 state, there is a conical intersection with the higher vibrational levels of the 2,3DHQ−nMeOH complex in the ground state. This results in the concerted mechanism for the formation of the tautomer. Upon excitation by λpump = 350 nm, there is no intersection with the PES for the complex of 2,3-DHQ, and first, the proton of the MeOH molecule is transferred to the C(3) atom to afford the carbocationic species in the higher vibrational levels of the ground state in the negatively charged (MeOH)n−1MeO− cluster. The second step is PT from the NH group of the carbocationic species to the cluster. If the first “fast” step is independent of the solvent composition, the following “slow” PT crucially depends on the solvent composition in the MeOH−C5H12 binary mixture. We attributed this result to the stabilization of the ionic species in the less polar microenvironment. This phenomenon may be of importance for the PT reactions of small organic molecules in the complex with biomacromolecules. For these systems, the micropolarity of the environment can vary significantly depending on the binding site.

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ASSOCIATED CONTENT

* Supporting Information S

The following materials are provided: absorption and fluorescence spectra of 1,2-DHQ in MeOH, C5H12, MeCN and their binary mixtures; schematic diagram of electronic levels of 1,2-DHQ and 2,3-DHQ; dependence of the quantum yield of the formation of 2,3-DHQ in binary solvents on the solvent composition; spectral evolution for pure inert solvents C5H12 and MeCN after fs laser pulse; and kinetics of transient absorbance at λprobe = 420 nm in the femtosecond laser photolysis (λpump = 350 nm) of 1,2-DHQ in inert solvents and F

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their mixtures with MeOH on the timescale 0−2 ps. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program “Theoretical and Experimental Study of the Nature of Chemical Bond and Mechanisms of the Important Chemical Reactions and Processes” of the Division of Chemistry and Material Sciences RAS. In the part of study related to the femtosecond laser spectroscopy setup, the work was supported by grants of the Russian Foundation for Basic Research Nos. 13-02-12433, 1203-91056 and 14-03-00546.

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