Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 4174−4181
pubs.acs.org/JPCL
Ultrafast Ground-State Intramolecular Proton Transfer in Diethylaminohydroxyflavone Resolved with Pump−Dump−Probe Spectroscopy Zhuoran Kuang,†,‡,∥ Qianjin Guo,†,∥ Xian Wang,†,‡ Hongwei Song,†,‡ Mark Maroncelli,*,§ and Andong Xia*,†,‡
Downloaded via UNIV OF SOUTH DAKOTA on July 13, 2018 at 03:03:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: 4′-N,N-Diethylamino-3-hydroxyflavone (DEAHF), due to excited-state intramolecular proton transfer (ESIPT) reaction, exhibits two solvent-dependent emission bands. Because of the slow formation and fast decay of the ground-state tautomer, its population does not accumulate enough for its detection during the normal photocycle. As a result, the details of the ground-state intramolecular proton-transfer (GSIPT) reaction have remained unknown. The present work uses femtosecond pump−dump−probe spectroscopy to prepare the short-lived ground-state tautomer and track this GSIPT process in solution. By simultaneously measuring femtosecond pump−probe and pump−dump−probe spectra, ultrafast kinetics of the ESIPT and GSIPT reactions are obtained. The GSIPT reaction is shown to be a solvent-dependent irreversible two-state process in two solvents, with estimated time constants of 1.7 ps in toluene and 10 ps in the more polar tetrahydrofuran. These results are of great value in both fully describing the photocycle of this four-level proton transfer molecule and for providing a deeper understanding of dynamical solvent effects on tautomerization.
E
indicators of solvent polarity, can be used to characterize different physical properties of the microenvironment.18 A few studies on hydroxyflavone derivatives have proposed that the lifetime of the ground-state tautomer (T) form might extend from picoseconds to long-lived, depending on molecular structure and surrounding environment.20−25 Because of the slow formation and fast decay of the tautomer, the concentration of T is extremely low because it does not accumulate during the intrinsic photocycle. Given such a low concentration, the GSIPT reaction of DEAHF is very difficult to detect using conventional spectroscopic methods. The unknown GSIPT reaction, which is the final step of the normal photocycle, may play an important role in determining the chemical reactivity of DEAHF.26 Hence a comprehensive study of the GSIPT is required to fully elucidate the proton-transfer mechanisms in the photocycle of DEAHF. Our primary concern in this work is to track the ultrafast GSIPT reaction in DEAHF and construct a rational kinetic scheme for the complete photocycle. To render the dynamics of the ground-state tautomer (T) detectable, we employ pump− dump−probe (PDP) transient absorption spectroscopy. This technique, built upon conventional pump−probe (PP) spectroscopy,
xcited-state intramolecular proton-transfer (ESIPT) is considered to be a fundamental photoreaction of importance in chemical and living systems. As such, ESIPT has received considerable attention theoretically and experimentally.1−6 4′-N,N-Diethylamino-3-hydroxyflavone (DEAHF; see Scheme 1) is a prototype molecule that undergoes ESIPT following optical excitation.7,8 In solution, the equilibrated ground state of DEAHF only exists as the “normal” (N) form, which absorbs in the near-UV region. Excitation of the ground-state (N) molecule immediately produces the excited “normal” (N*) form, which exhibits significant intramolecular charger-transfer (ICT) character. The N* form may undergo ESIPT generating the “tautomer” (T*) form in S1. Dual emission from N* and T* has been observed in most solvents. Once formed via relaxation of S1 to S0, the ground-state “tautomer” (T) reforms N through the ground-state intramolecular proton transfer (GSIPT) reaction. The solvent effect on the ESIPT reaction of DEAHF is predicted theoretically and observed experimentally via steadystate fluorescence, time-resolved fluorescence, and transient absorption spectroscopies.9−18 As a consequence of the large difference in dipole moments between the N* and T* states, their relative energetics depends on solvent polarization,14,19 leading to a solvent-induced energy barrier to this protoncoupled charge-transfer reaction. Spectroscopic parameters, such as the positions of N* and T* emission maxima and the intensity ratio of the two emission bands, which are sensitive © XXXX American Chemical Society
Received: June 12, 2018 Accepted: July 10, 2018 Published: July 10, 2018 4174
DOI: 10.1021/acs.jpclett.8b01826 J. Phys. Chem. Lett. 2018, 9, 4174−4181
Letter
The Journal of Physical Chemistry Letters
GSIPT (∼1.7 ps in toluene and ∼10 ps in THF) being both faster and more solvent-dependent than ESIPT. The present work constitutes an important first step toward fully understanding the photocycle of this prototypical four-level ESIPT molecule. Steady-State Spectra. Steady-state absorption and emission spectra of DEAHF in two selected solvents with different polarities, toluene (dielectric constant ε = 2.38) and THF (ε = 7.58),38 are shown in Figure 1. Both toluene and THF are regarded as
Scheme 1. Molecular Structure of 4′-N,N-Diethylamino3-hydroxyflavone (DEAHF) and Its Photocyclea
“N” and “T” stand for the “normal” and “tautomer” forms, respectively. The asterisks (*) denote S1 states.
a
is referred to as stimulated emission pumping (SEP). It has been proven to be a powerful approach for studying complex photoinduced dynamics involving branched relaxation pathways with spectral overlap.24,25,27−36 In the PDP experiment applied to detecting the GSIPT of DEAHF (see Scheme 1), an ultrafast pump pulse first brings the chromophore to the N* state. After allowing time for the N* → T* reaction, a dump pulse, resonant with the emission of T*, serves as an effective means of preparing a significant population of the ground-state tautomer. The evolution of the T population and thus the kinetics of the GSIPT reaction can then be detected by a broadband probe pulse. Combining the PP and PDP techniques provides full spectroscopic access into both the groundand excited-state dynamics of DEAHF. Despite many previous studies of the mechanism of excitedstate proton transfer of this prototypical reaction, almost nothing is known about ground-state reaction of DEAHF. Questions to be addressed are the following: (1) Is the GSIPT reaction better described as a barrier-less relaxation, a simple two-state kinetic process, or does it involve intermediates, such as solute−solvent complexes or torsional conformers? (2) What is the time scale of the GSIPT reaction, and what does this time scale indicate about the reversibility of the reaction? (3) Is the GSIPT reaction sensitive to the solvent polarity in a manner similar to that of the ESIPT reaction? In this Letter, we address these questions from a comprehensive study of DEAHF as a function of solvent polarity by employing the PDP technique combined with quantum-chemical calculations. Toluene and tetrahydrofuran (THF), which both have a rapid solvation response of a few picoseconds but different solvation strengths, are selected to provide weakly polar and strongly polar solvent environments, respectively.37 Density functional theory (DFT) calculations of the energies of the ground-state species predict that the GSIPT reaction (T → N) is irreversible in these solvents, which is consistent with analysis of the experimental spectra. Calculations of reaction profiles for both ESIPT and GSIPT in continuum solvent suggest that GSIPT should be less sensitive to solvent than ESIPT; however, this prediction is not upheld by experiment. Global analysis of the femtosecond PP and PDP spectra enables us to unravel the kinetics of the entire photocycle of DEAHF in toluene and THF. We find significant solvent dependence on both reactions, with
Figure 1. Normalized absorption and emission spectra for DEAHF in THF and toluene and the spectra of the pump (dark shaded) and the dump (light shaded) laser pulses.
aprotic solvents when considering the solute−solvent interaction for DEAHF in solution.17 As shown in Figure 1, the absorption lineshapes are nearly identical in these two solvents. The absorption band around 400 nm is attributed to the N → N* ICT transition. (Calculated transition properties are shown in Table S1.) No evidence of ground-state tautomer (T) is detected in steady-state spectra. Dual emission is clearly seen in the fluorescence of DEAHF. The emission bands around 480 and 580 nm arise from N* and T* emission, respectively.18,39 Both emission bands shift bathochromically in the higher polarity solvent, but the shift is much larger for the N* emission. These results can be explained by the much larger dipole moment of the N* state compared with the remaining states, which leads to a stronger solvatochromic response of the N ↔ N* transition. The N*/T* intensity ratio is also much larger in the more polar solvent THF compared with toluene, reflecting the shift of the ESIPT equilibrium toward the N* state with increasing solvent polarity.14,16,19 Theoretical Approaches. Geometry-optimized structures and dipole moments of the N, N*, T, and T* states of DEAHF in toluene, calculated using DFT and TD-DFT, are shown in Figure 2a. Calculated electric dipole moments in THF, frontier molecular orbitals, and electron−hole distributions for S1 states are depicted in Figures S2−S4 of the Supporting Information (SI). There are significant differences in the dipole moment vectors calculated for the N (7.3 D, 39°) and N* states (22.9 D, 8°). Dipole moments of T*, T, and N differ slightly in magnitude but vary significantly in orientation. The remarkable ICT character of the N* state leads to a solvent-dependent N* → N emission. The relaxed T* geometry is an ESIPT tautomer with a small dipole moment (10.6 D) in comparison with N* (22.9 D). Therefore, N* and T* are subjected to different solvent polarization configurations, which results in a solvation-induced barrier to the ESIPT reaction, N* → T*.1,12,14 The T* fluorescence 4175
DOI: 10.1021/acs.jpclett.8b01826 J. Phys. Chem. Lett. 2018, 9, 4174−4181
Letter
The Journal of Physical Chemistry Letters
Figure 2. (a) Optimized structures of DEAHF in toluene, selected critical bond lengths, and calculated dipole moment vectors of N, N*, T, and T*. (Arrows show components along the x−y plane; the z axis is perpendicular to this sheet; the angle specified is clockwise to the x inertial axis.) (b) Potential energy curves of the S0 and S1 states of DEAHF along with the O−H distance in toluene and THF. The inset shows the stepwise scanned O−H distance.
band is characterized as locally excited emission because of the relatively small difference in dipole moments between T* and T, in line with the much smaller frequency shift of the T* emission with solvent polarity. These dipole moments agree well with TDDFT-RISM calculations performed by Sato and coworkers but are a little larger in magnitude than the previous CIS calculations by Chou and coworkers.10,12 Nemkovich et al. also characterized the significant changes in dipole moment upon optical excitation by electro-optical absorption measurements.40 Finally, we note that both absorption of N and emission of T* result in a shortened and presumably strengthened intramolecular hydrogen bond. For the GSIPT reaction, the dipole moment vectors of T*, T, and N are all relevant. In the PDP process, the ground-state reactant (T) is populated from the deactivated T* state by the radiative transition. The difference between T* and T dipole moments means that the solvent polarization immediately after the T* → T transition will not be in equilibrium, suggesting that the GSIPT reaction is coupled to solvent reorientation in much the same way as is the ESIPT reaction when initiated by N → N* excitation. Calculations predict a much more modest difference in the T and N dipole moments, but this difference could still have significant effects on the kinetics of the GSIPT reaction by virtue of small changes to the reaction barrier. To investigate this possibility and make comparisons to the ESIPT reaction, the potential curves of DEAHF in the S0 and S1 states were scanned based on constrained optimizations at different O−H distances. As shown in Figure 2b, the energy of the S0 state increases as the O−H distance lengthens, and a potential barrier of ca. 12 kcal/mol to the N → T isomerization is obtained. In contrast, the barrier in the T → N direction is only ca. 2 kcal/mol, even smaller than the calculated barrier of the ESIPT (N* → T*) reaction (∼4 kcal/mol). This suggests that GSIPT may be faster than ESIPT, whose rate constant has been estimated to be between 3 × 1010 and 3 × 1011 s−1 in many solvents.15,16 Thus it is expected that the ground-state tautomer (T) of DEAHF in toluene and THF is short-lived. In addition, the great disparity between forward and backward GSIPT barriers indicates that the GSIPT reaction (T → N) is irreversible. Conventional Pump−Probe Spectroscopy. Broadband PP transient absorption spectra of DEAHF in toluene and THF are shown in Figure 3. Excited-state absorption (ESA), groundstate bleaching (GSB), and stimulated emission (SE) all contribute
Figure 3. Femtosecond time-resolved pump−probe spectra of DEAHF excited at 400 nm in (a) toluene and (b) THF at different probe delays. The blue and red shaded areas represent the corresponding steady-state absorption and emission spectra, respectively.
these spectra and must be accounted for to properly resolve contributions from multiple transient species. Consider first the spectra in THF (Figure 3b), which shows a more distinct dual-emission feature. At a delay time of 0.15 ps, the transient spectrum exhibits broad ESA across most of the spectral range. The negative signal below 430 nm is ascribed to the GSB, in accordance with the steady-state absorption spectrum. The dip around 470 nm is assigned to the stimulated N* emission, which shifts to 490 nm within 3 ps. A new SE band concurrently rises around 580 nm, in line with the steady-state T* emission. In addition, there is a positive peak around 660 nm, which is attributed to the ESA of the T* state. At progressively 4176
DOI: 10.1021/acs.jpclett.8b01826 J. Phys. Chem. Lett. 2018, 9, 4174−4181
Letter
The Journal of Physical Chemistry Letters
consistent with the contribution of some T → T* absorption. At subsequent probe delays, the outline of the PDP spectrum gradually recovers to that of PP spectrum, with the only difference being overall magnitude. These observations suggest that the ground-state tautomer produced has a significantly shorter lifetime than N* and T* states. Similar PDP spectra are observed for DEAHF in THF (Figure S5 in the SI), where the dump pulse induces a loss of SE (580 nm) and ESA (660 nm) and generates a short-lived ground-state intermediate, which we attribute to T, as in toluene. To better illustrate the temporal effects of dumping on the transient absorption spectra, kinetic traces at selected wavelengths are shown in Figure 5. Up to the dump at 36 ps, the
increasing time delays, the intensity ratio of the observed SE bands at 580 and 480 nm significantly increases until ca. 200 ps when the spectral line shape becomes stable and thereafter decays uniformly on a much longer time scale. The general behavior of the PP spectra in toluene (Figure 3a) is very similar to that in THF. The first SE band appears at 460 nm at a delay time of 0.5 ps. At longer delay times, the line shape of the SE band appears to undergo a slight bathochromic shift to 470 nm and rapidly decrease in magnitude. Subsequently, the second SE band around 580 nm grows up, accompanied by the decay of the first SE band around 470 nm. The main difference between the two solvents lies in the different evolution rates of the SE bands of the N* and T* states. Whereas the uniform decrease in the intensity of both bands occurs in ca. 40 ps in toluene, this uniform decay does not occur in THF until ca. 200 ps. There is also a broad ESA band across the spectra overlapping with the two SE bands in both solvents. The PP data indicate the formation of T* on the scale of tens of picoseconds in toluene and THF. We therefore set the dump delay in PDP measurements to 36 ps, a time when the T* state has a considerable population, to observe the kinetics of the GSIPT reaction. Pump−Dump−Probe Spectroscopy. Ultrafast population transfer from an emissive excited state to the ground state can be realized by an intense “dump” pulse resonant with an emission band.28,29 In the case of DEAHF, we apply a 600 nm dump pulse to effect the T* → T transfer. Time-resolved spectra of DEAHF in toluene at selected time delays are shown in Figure 4 in the presence and absence of a dump pulse at
Figure 5. Selected PP and PDP traces of DEAHF in (a) toluene and (b) THF at 560 and 660 nm. Points show the experimental data and lines are the fits from global analysis. For more traces at different wavelengths, see Figure S6 in the SI. Figure 4. Femtosecond time-resolved absorbance difference spectra of DEAHF in toluene at the indicated delays in the presence (black line, PDP) and absence (gray line, PP) of a dump pulse arriving at 36 ps after the initial 400 nm excitation. The dotted line denotes a double-difference spectrum, in which ΔΔOD = PDP − 0.63 × PP.
PDP trace overlaps with the unmodified PP trace. At 36 ps, the instrument-limited decrease in SE at 560 nm reflects the formation of T. The 560 nm PDP signal initially decays in a few picoseconds in toluene and with a longer time constant in THF. These fastest components are associated with the groundstate T → N reaction. They are absent in the 660 nm traces, where, in both solvents, the initial rise after the dump occurs in the tens of picoseconds range. These slower times in the PDP signals at 660 nm are comparable to the times associated with the N* → T* transition found in the PP traces. We attribute these rises to equilibration of the N* and T* populations after perturbation by the dumping pulse.16 The PDP spectra of DEAHF thus appear to involve a superposition of the kinetics of the excited state species N* and T* together with that of the newly formed ground-state T. To analyze the PDP data and disentangle the underlying kinetic components, we turn to a
36 ps. Also shown is the double-difference spectrum, ΔΔOD (dotted line) obtained by subtracting the PP spectrum, scaled by a factor of 0.63 from the corresponding PDP spectrum.29 This scaling reflects an estimated fraction of 0.37, whereas the T* population is dumped to the ground state. (See Section S2 in the SI for details.) It is clearly seen from the difference spectrum taken at a delay of 37 ps (i.e., 1 ps after the dump pulse) that the SE (540 to 630 nm) and ESA (near 660 nm) bands of the T* state are depleted, indicating a population loss by the T* state. The ΔΔOD spectrum at this time indicates a slight shift in the position of the SE band to shorter wavelengths, 4177
DOI: 10.1021/acs.jpclett.8b01826 J. Phys. Chem. Lett. 2018, 9, 4174−4181
Letter
The Journal of Physical Chemistry Letters
solvation configuration between N*relaxed and T* results in an appreciable ESIPT barrier. Therefore, the kinetics of the ESIPT reaction of DEAHF exhibits a marked solvent polarity dependence. To apply global analysis to the PDP and PP spectra, the continuous processes in Scheme 2a are simplified to the stateto-state kinetic model shown in Scheme 2b.30,41 This model describes the following dynamical processes: (1) solvation and vibrational relaxation from N*FC to N*relaxed (denoted as k1), (2) reversible ESIPT reaction (denoted as k2, k3, and k4) and spontaneous decays of N*relaxed (k5) and T* (k6), and (3) dump-induced ground-state dynamics corresponding to the GSIPT reaction from T to N (k7). The model presented in Scheme 2b contains the minimum possible number of components required for fitting the PP and PDP data to within expected uncertainties. Attempts to model these data using fewer components failed to provide satisfactory fits. Figure 5 and Figure S6 show the quality of the simultaneous fits to the PP and PDP kinetic traces at different wavelengths. Excited-State Dynamics. Figure 6 shows species-associated difference spectra (SADSs) and corresponding temporal concentrations obtained from global analysis of Scheme 2b. On the basis of the state-to-state evolution approximation, each SADS represents a transient species whose concentration varies with time according to coupled differential rate equations.30 The SADSs are similar in the two solvents, and the main features of these spectra are reasonably consistent with what might be expected from the steady-state spectra. For example, all SADSs end in a negative feature at short wavelengths due to the fact that the population of N*, T*, and T entails the loss of N population and thus a GSB. The spectra of N*relaxed in both solvents exhibit a negative-going signal at the
global analysis of the spectra based on a phenomenological kinetic model. Phenomenological Model. The excited-state dynamics of DEAHF in aprotic solvents are typical of chromophores that undergo ESIPT upon actinic excitation. Numerous studies support a description of such ESIPT reactions using the relaxation pathways shown in Scheme 2a.12−16,19 In brief, optical excitation Scheme 2. Phenomenological Model of DEAHFa
a
(a) Proposed excited-state relaxation pathways of DEAHF. (b) Target evolution model used in global analysis. FC denotes the Franck−Condon state.
of the chromophore results in an essentially instantaneous population of the Franck−Condon (FC) state, N*FC. Subsequently, rapid solvent relaxation occurs, leading to an energetically more favorable solvated configuration, denoted N*relaxed. In the case of DEAHF, the fact that solvent stabilization of N*relaxed is much greater than that of N*FC and T* leads to a competition between the rate of ESIPT and solvent relaxation. After reaching the N*relaxed state, the significantly different
Figure 6. Global analysis results derived from simultaneous analysis of PP and PDP data: species-associated difference spectra (SADSs) of DEAHF in (a) toluene and (b) THF and temporal concentrations of corresponding species as a function of delay time in (c) toluene and (d) THF. The blue and red shaded areas in panels a and b represent the corresponding steady-state absorption and emission spectra, respectively. Dotted lines in panels c and d correspond to concentrations after the dump. The dumping efficiencies are estimated as ca. 0.37 and 0.30 for DEAHF in toluene and THF, respectively. 4178
DOI: 10.1021/acs.jpclett.8b01826 J. Phys. Chem. Lett. 2018, 9, 4174−4181
Letter
The Journal of Physical Chemistry Letters
the GSIPT reaction is essentially irreversible. At least in these two solvents, the T → N rate constant is much more sensitive to solvent polarity than the N* → T* reaction, with the rate ratios in toluene/THF being ∼5 for k7 compared with ∼2 for k3. Given the much larger difference in N versus T dipole moments calculated in S1 compared with S0, this greater solvent sensitivity is unexpected. The cyclic proton shuttling process (N → N* → T* → T → N) favors the formation of a real four-level scheme for realizing population inversion and negligible self-absorption, making ESIPT emitters attractive as laser dyes.43 The presence of a fast, radiationless decay from T to N, that is, the GSIPT reaction, leads to a negligible population in the T state. Because an appreciable population accumulates in the T* state, the T* acting as the upper laser level will form a population inversion with respect to the lower laser level, T. The ultrafast GSIPT reaction (ca. 2−10 ps), which is faster than the ESIPT reaction (ca. 16−30 ps), should enable optically pumped lasing with DEAHF.44 In this sense, DEAHF can be regarded as an attractive model system to understand the basic ESIPT and GSIPT reactions of a real four-level molecule. In conclusion, we have investigated the excited- and groundstate dynamics of a solvatochromic intramolecular protontransfer prototype, DEAHF, using femtosecond PP and PDP spectroscopy combined with theoretical calculations. Unlike the ESIPT, the large energy difference (ca. 10 kcal/mol) between T and N in the S0 states indicates that the GSIPT reaction is intrinsically irreversible. Through the use of stimulated emission pumping, we have observed the ground-state reaction for the first time. Combining PP and PDP spectroscopy with global analysis methods, the reaction rate constants in the complete DEAHF photocycle are estimated in toluene and THF. The GSIPT reaction is shown to be a solvent-dependent, irreversible, two-state process. The time constant of the GSIPT reaction is estimated to be ∼1.7 ps in toluene, whereas it is retarded to ∼10 ps in THF. This large difference in rates is surprising given the nearly identical dipole moments and solvation energies calculated for the N and T states, and it deserves further study. The GSIPT reaction is also significantly faster than the ESIPT reaction, which suggests DEAHF as an interesting candidate for a four-level laser dye. Finally, this work demonstrates that broadband PDP spectroscopy, which can actively control the reaction path, is a powerful method in tracking ultrafast groundstate dynamics, in particular, for molecules involving photoisomerization.
location of the N* steady-state emission (460 and 484 nm). In THF this feature dominates the SADS of N*relaxed, whereas it is less prominent in toluene. The main feature in the T* SADS also occurs approximately in the location of the steadystate emission (∼575 nm), accompanied by a positive feature near 650 nm assigned to ESA. The SADS of the ground-state species T shows strong positive signals peaked ∼650 cm−1 to the blue of the steady state T* emission band. Such a shift is regarded as the Stokes shift of the T form of DEAHF. Finally, the features of the SADS of N*FC are not well-defined, and their interpretation is far from obvious, which is unsurprising considering that this short-lived state is used to approximate the distributed dynamics of solvation and vibrational relaxation. Table 1 summarizes the reciprocal rate constants estimated from the global analysis. Consider first the excited-state reaction. Table 1. Time Constants Estimated from Global Analysis solvent process excited-state evolution
excited-state decay ground-state evolution
reciprocal rate constant k1−1 k2−1 k3−1 k4−1 k5−1 k6−1 k7−1
toluene (ps) 3.0 3.3 16 450 1.8 3.0 1.7
± ± ± ± ± ± ±
0.3 0.8 2 50 0.2 0.5 0.2
THF (ps) 2.5 5.0 30 146 350 550 10.0
± ± ± ± ± ± ±
0.3 0.8 3 20 80 80 0.3
Rate constants k1 and k2 are both affected by the distributed kinetics of solvation and are therefore not well-defined quantities. In contrast, assuming that solvation is complete in a few picoseconds, k3 and k4 should provide good measures for the kinetics of ESIPT between the equilibrium solvation states of N* and T*. Equilibration between these two states is complete within ∼30 ps, after which time the entire S1 population decays uniformly to S0 at the average of rate constants k5 and k6.11 The ESIPT equilibrium constant K, estimated from K = k3/k4,16 significantly decreases between the weakly polar toluene (ca. 28) and the more polar THF (ca. 4.9). The ratio of K in these two solvents is in good agreement with the ratio of steady-state T*/N* emission intensities in toluene and THF reported by Klymchenko et al.17 These results indicate a shift of the ESIPT equilibrium in favor of the more polar N* state with increasing solvent polarity, as expected from the slight difference in N*−T* solvation energies in the DFT calculations as well as from prior work.16,18 The forward rate constant k3 decreases with increasing solvent polarity, as also predicted by the DFT estimates of solvent-dependent N* → T* activation energies in Figure 2b. Solvent-Dependent GSIPT Reaction. The evolution on the ground-state potential surface is the counter process of the excited-state evolution. According to the Franck−Condon principle, the configuration of T at the initial dumping time is almost the same as that of T*. The T that is generated by the deactivation of T* rearranges its nuclei from the T* configuration back to the equilibrated ground-state configuration.12,27,42 The T population created by the dump pulse is rapidly depleted in 1.7 ps in toluene and 10 ps in THF (Figure 6). As anticipated by the estimated ∼10 kcal/mol energy difference in DFT energies (Figure 2b), no back reaction N → T needs to be included in Scheme 2b to fit the PP and PDP spectra; that is,
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01826.
■
Materials and methods, target analysis procedures, additional TD-DFT results, PP and PDP data (PDF)
AUTHOR INFORMATION
Corresponding Authors
*A.X.: E-mail:
[email protected]. *M.M.: E-mail:
[email protected]. ORCID
Xian Wang: 0000-0003-3520-7397 Mark Maroncelli: 0000-0003-1633-1472 Andong Xia: 0000-0002-2325-3110 4179
DOI: 10.1021/acs.jpclett.8b01826 J. Phys. Chem. Lett. 2018, 9, 4174−4181
Letter
The Journal of Physical Chemistry Letters Author Contributions
(16) Shynkar, V. V.; Mély, Y.; Duportail, G.; Piémont, E.; Klymchenko, A. S.; Demchenko, A. P. Picosecond Time-Resolved Fluorescence Studies Are Consistent with Reversible Excited-State Intramolecular Proton Transfer in 4′-(Dialkylamino)-3-hydroxyflavones. J. Phys. Chem. A 2003, 107, 9522−9529. (17) Klymchenko, A. S.; Pivovarenko, V. G.; Demchenko, A. P. Elimination of the Hydrogen Bonding Effect on the Solvatochromism of 3-Hydroxyflavones. J. Phys. Chem. A 2003, 107, 4211−4216. (18) Klymchenko, A. S.; Demchenko, A. P. Multiparametric probing of intermolecular interactions with fluorescent dye exhibiting excited state intramolecular proton transfer. Phys. Chem. Chem. Phys. 2003, 5, 461−468. (19) Cheng, Y. M.; Pu, S. C.; Yu, Y. C.; Chou, P. T.; Huang, C. H.; Chen, C. T.; Li, T. H.; Hu, W. P. Spectroscopy and femtosecond dynamics of 7-N,N-diethylamino-3-hydroxyflavone. The correlation of dipole moments among various states to rationalize the excitedstate proton transfer reaction. J. Phys. Chem. A 2005, 109, 11696− 11706. (20) Brewer, W. E.; Studer, S. L.; Chou, P.-T.; Orton, E. Temperature-dependent study of the ground-state reverse proton transfer of 3-hydroxyflavone. Chem. Phys. Lett. 1989, 158, 345−350. (21) Suda, K.; Terazima, M.; Kimura, Y. Anomalous ground-state proton transfer of 4′-N,N-diethylamino-3-hydroxyflavone in ionic liquids of imidazolium-based cations with tetrafluoroborate. Chem. Commun. 2013, 49, 3976−3978. (22) Bader, A. N.; Pivovarenko, V. G.; Demchenko, A. P.; Ariese, F.; Gooijer, C. Excited State and Ground State Proton Transfer Rates of 3-Hydroxyflavone and Its Derivatives Studied by Shpol’skii Spectroscopy: The Influence of Redistribution of Electron Density. J. Phys. Chem. B 2004, 108, 10589−10595. (23) Itoh, M.; Tanimoto, Y.; Tokumura, K. Transient Absorption Study of the Intramolecular Excited-State and Ground-State ProtonTransfer in 3-Hydroxyflavone and 3-Hydroxychromone. J. Am. Chem. Soc. 1983, 105, 3339−3340. (24) Dzugan, T. P.; Schmidt, J.; Aartsma, T. J. On the ground-state tautomerization of 3-hydroxyflavone. Chem. Phys. Lett. 1986, 127, 336−342. (25) Itoh, M.; Fujiwara, Y. The ground state tautomer in the excited state relaxation process of 3-hydroxyflavone at room temperature. Chem. Phys. Lett. 1986, 130, 365−367. (26) Brewer, W. E.; Studer, S. L.; Standiford, M.; Chou, P. T. Dynamics of the triplet state and the reverse proton transfer of 3hydroxyflavone. J. Phys. Chem. 1989, 93, 6088−6094. (27) Changenet-Barret, P.; Choma, C. T.; Gooding, E. F.; DeGrado, W. F.; Hochstrasser, R. M. Ultrafast Dielectric Response of Proteins from Dynamics Stokes Shifting of Coumarin in Calmodulin. J. Phys. Chem. B 2000, 104, 9322−9329. (28) Gai, F.; McDonald, J. C.; Anfinrud, P. A. Pump−Dump−Probe Spectroscopy of Bacteriorhodosin: Evidence for a Near-IR Excited State Absorbance. J. Am. Chem. Soc. 1997, 119, 6201−6202. (29) Kennis, J. T.; Larsen, D. S.; van Stokkum, I. H.; Vengris, M.; van Thor, J. J.; van Grondelle, R. Uncovering the hidden ground state of green fluorescent protein. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 17988−17993. (30) Larsen, D. S.; Vengris, M.; van Stokkum, I. H.; van der Horst, M. A.; de Weerd, F. L.; Hellingwerf, K. J.; van Grondelle, R. Photoisomerization and photoionization of the photoactive yellow protein chromophore in solution. Biophys. J. 2004, 86, 2538−2550. (31) Redeckas, K.; Voiciuk, V.; Zigmantas, D.; Hiller, R. G.; Vengris, M. Unveiling the excited state energy transfer pathways in peridininchlorophyll a-protein by ultrafast multi-pulse transient absorption spectroscopy. Biochim. Biophys. Acta, Bioenerg. 2017, 1858, 297−307. (32) Busby, E.; Carroll, E. C.; Chinn, E. M.; Chang, L.; Moulé, A. J.; Larsen, D. S. Excited-State Self-Trapping and Ground-State Relaxation Dynamics in Poly(3-hexylthiophene) Resolved with Broadband Pump−Dump−Probe Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 2764−2769. (33) Fitzpatrick, A. E.; Lincoln, C. N.; van Wilderen, L. J.; van Thor, J. J. Pump-dump-probe and pump-repump-probe ultrafast spectros-
∥
Z.K. and Q.G. contributed equally.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by NSFCs (nos. 21673252, 21333012, and 21773252) and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB12020200).
■
REFERENCES
(1) Demchenko, A. P.; Tang, K. C.; Chou, P. T. Excited-state proton coupled charge transfer modulated by molecular structure and media polarization. Chem. Soc. Rev. 2013, 42, 1379−1408. (2) Hsieh, C. C.; Jiang, C. M.; Chou, P. T. Recent experimental advances on excited-state intramolecular proton coupled electron transfer reaction. Acc. Chem. Res. 2010, 43, 1364−1374. (3) Hammes-Schiffer, S. Theory of Proton-Coupled Electron Transfer in Energy Conversion Processes. Acc. Chem. Res. 2009, 42, 1881−1889. (4) Hammes-Schiffer, S.; Stuchebrukhov, A. A. Theory of Coupled Electron and Proton Transfer Reactions. Chem. Rev. 2010, 110, 6939−6960. (5) Zhao, G. J.; Han, K. L. Hydrogen bonding in the electronic excited state. Acc. Chem. Res. 2012, 45, 404−413. (6) Hsieh, C. C.; Chou, P. T.; Shih, C. W.; Chuang, W. T.; Chung, M. W.; Lee, J.; Joo, T. Comprehensive studies on an overall proton transfer cycle of the ortho-green fluorescent protein chromophore. J. Am. Chem. Soc. 2011, 133, 2932−2943. (7) Chou, P. T.; Martinez, M. L.; Clements, J. H. Reversal of Excitation Behavior of Proton-Transfer Vs Charge-Transfer by Dielectric Perturbation of Electronic Manifolds. J. Phys. Chem. 1993, 97, 2618−2622. (8) Swinney, T. C.; Kelley, D. F. Proton transfer dynamics in substituted 3-hydroxyflavones: Solvent polarization effects. J. Chem. Phys. 1993, 99, 211−221. (9) Rumble, C. A.; Breffke, J.; Maroncelli, M. Solvation Dynamics and Proton Transfer in Diethylaminohydroxyflavone. J. Phys. Chem. B 2017, 121, 630−637. (10) Hayaki, S.; Kimura, Y.; Sato, H. Ab initio study on an excitedstate intramolecular proton-transfer reaction in ionic liquid. J. Phys. Chem. B 2013, 117, 6759−6767. (11) Kimura, Y.; Fukuda, M.; Suda, K.; Terazima, M. Excited state intramolecular proton transfer reaction of 4′-N,N-diethylamino-3hydroxyflavone and solvation dynamics in room temperature ionic liquids studied by optical Kerr gate fluorescence measurement. J. Phys. Chem. B 2010, 114, 11847−11858. (12) Chou, P. T.; Pu, S. C.; Cheng, Y. M.; Yu, W. S.; Yu, Y. C.; Hung, F. T.; Hu, W. P. Femtosecond dynamics on excited-state proton/charge-transfer reaction in 4′-N,N-diethylamino-3-hydroxyflavone. The role of dipolar vectors in constructing a rational mechanism. J. Phys. Chem. A 2005, 109, 3777−3787. (13) Douhal, A.; Sanz, M.; Carranza, M. A.; Organero, J. A.; Santos, L. Femtosecond observation of intramolecular charge- and protontransfer reactions in a hydroxyflavone derivative. Chem. Phys. Lett. 2004, 394, 54−60. (14) Chou, P.-T.; Huang; Pu, S.-C.; Cheng, Y.-M.; Liu, Y.-H.; Wang, Y.; Chen, C.-T. Tuning Excited-State Charge/Proton Transfer Coupled Reaction via the Dipolar Functionality. J. Phys. Chem. A 2004, 108, 6452−6454. (15) Ameer-Beg, S.; Ormson, S. M.; Poteau, X.; Brown, R. G.; Foggi, P.; Bussotti, L.; Neuwahl, F. V. R. Ultrafast Measurements of Charge and Excited-State Intramolecular Proton Transfer in Solutions of 4′(N,N-Dimethylamino) Derivatives of 3-Hydroxyflavone. J. Phys. Chem. A 2004, 108, 6938−6943. 4180
DOI: 10.1021/acs.jpclett.8b01826 J. Phys. Chem. Lett. 2018, 9, 4174−4181
Letter
The Journal of Physical Chemistry Letters copy resolves cross section of an early ground state intermediate and stimulated emission in the photoreactions of the Pr ground state of the cyanobacterial phytochrome Cph1. J. Phys. Chem. B 2012, 116, 1077−1088. (34) Guo, X.; Wang, S.; Xia, A.; Su, H. Determination of the Formation of Dark State via Depleted Spontaneous Emission in a Complex Solvated Molecule. J. Phys. Chem. A 2007, 111, 5800−5805. (35) Brewer, W. E.; Martinez, M. L.; Chou, P. T. Mechanism of the ground-state reverse proton transfer of 2-(2-hydroxyphenyl)benzothiazole. J. Phys. Chem. 1990, 94, 1915−1918. (36) Chou, P. T.; Yu, W. S.; Chen, Y. C.; Wei, C. Y.; Martinez, S. S. Ground-state reverse double proton transfer of 7-azaindole. J. Am. Chem. Soc. 1998, 120, 12927−12934. (37) Reynolds, L.; Gardecki, J. A.; Frankland, S. J. V.; Horng, M. L.; Maroncelli, M. Dipole Solvation in Nondipolar Solvents: Experimental Studies of Reorganization Energies and Solvation Dynamics. J. Phys. Chem. 1996, 100, 10337−10354. (38) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. Subpicosecond measurements of polar solvation dynamics: Coumarin 153 revisited. J. Phys. Chem. 1995, 99, 17311−17337. (39) Ormson, S. M.; Brown, R. G.; Vollmer, F.; Rettig, W. Switching between charge- and proton-transfer emission in the excited state of a substituted 3-hydroxyflavone. J. Photochem. Photobiol., A 1994, 81, 65−72. (40) Nemkovich, N. A.; Pivovarenko, V. G.; Baumann, W.; Rubinov, A. N.; Sobchuk, A. N. Dipole Moments of 4′-Aminoflavonol Fluorescent Probes in Different Solvents. J. Fluoresc. 2005, 15, 29−36. (41) Ghosh, D.; Batuta, S.; Das, S.; Begum, N. A.; Mandal, D. Proton Transfer Dynamics of 4′-N,N-Dimethylamino-3-hydroxyflavone Observed in Hydrogen-Bonding Solvents and Aqueous Micelles. J. Phys. Chem. B 2015, 119, 5650−5661. (42) Vengris, M.; van Stokkum, I. H. M.; He, X.; Bell, A. F.; Tonge, P. J.; van Grondelle, R.; Larsen, D. S. Ultrafast Excited and GroundState Dynamics of the Green Fluorescent Protein Chromophore in Solution. J. Phys. Chem. A 2004, 108, 4587−4598. (43) Zhang, W.; Yan, Y.; Gu, J.; Yao, J.; Zhao, Y. S. Low-threshold wavelength-switchable organic nanowire lasers based on excited-state intramolecular proton transfer. Angew. Chem., Int. Ed. 2015, 54, 7125−7129. (44) Chou, P.-T.; Martinez, M. L.; Clements, J. H. The observation of solvent-dependent proton-transfer/charge-transfer lasers from 4′ -diethylamino-3-hydroxyflavone. Chem. Phys. Lett. 1993, 204, 395− 399.
4181
DOI: 10.1021/acs.jpclett.8b01826 J. Phys. Chem. Lett. 2018, 9, 4174−4181
