ESIPT and Photodissociation of 3-Hydroxychromone in Solution

Oct 1, 2013 - The spectral properties of fluorescence sensors such as 3-hydroxychromone (3-HC) and its derivatives are sensitive to interaction with t...
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ESIPT and Photodissociation of 3‑Hydroxychromone in Solution: Photoinduced Processes Studied by Static and Time-Resolved UV/ Vis, Fluorescence, and IR Spectroscopy Katharina Chevalier,† Anneken Grün,‡ Anke Stamm,‡ Yvonne Schmitt,‡ Markus Gerhards,*,‡ and Rolf Diller*,† †

Department of Physics and ‡Department of Chemistry and Research Center OPTIMAS, University of Kaiserslautern, 67663 Kaiserslautern, Germany S Supporting Information *

ABSTRACT: The spectral properties of fluorescence sensors such as 3-hydroxychromone (3-HC) and its derivatives are sensitive to interaction with the surrounding medium as well as to substitution. 3-HC is a prototype system for other derivatives because it is the basic unit of all flavonoides undergoing ESIPT and is not perturbed by a substituent. In this study, the elementary processes and intermediate states in the photocycle of 3-HC as well as its anion were identified and characterized by the use of static and femtosecond time-resolved spectroscopy in different solvents (methylcyclohexane, acetonitrile, ethanol, and water at different pH). Electronic absorption and fluorescence spectra and lifetimes of the intermediate states were obtained for the normal, tautomer and anionic excited state, while mid-IR vibrational spectra yielded structural information on ground and excited states of 3-HC. A high sensitivity on hydrogen-bonding perturbations was observed, leading to photoinduced anion formation in water, while in organic solvents, different processes are suggested, including slow picosecond ESIPT and contribution of the trans-structure excited state or a different stable solvation state with different direction of OH. The formation of the latter could be favored by the lack of a substituent increasing contact points for specific solute−solvent interactions at the hydroxyl group compared to substituted derivatives. The effect of substituents has to be considered for the design of future fluorescence sensors based on 3-HC.

1. INTRODUCTION 3-Hydroxychromone (3-HC) and derivatives have attracted considerable interest in recent years due to their photochemistry and potential applications especially as molecular probes based on their spectrally separated dual fluorescence.1−9 The primary process leading to this dual fluorescence is an excited-state intramolecular proton transfer (ESIPT). ESIPT reactions have been observed in a variety of chemical systems, including compounds undergoing ESIPT to nitrogen, such as 2(2′-hydroxyphenyl)-benzothiazole (HBT)10−16 and 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole (Tinuvin-P),17 and compounds undergoing ESIPT to oxygen, such as 3-HC and derivatives, 4, 6, 1 8−28 3-HF, 10, 2 9−53 and 3-HF derivatives.9,30,33,36,44,52,54−56 For 3-HF and 3-HC, especially the attachment of electron-donating groups leads to a solventstabilized charge-transfer species prior to ESIPT, which has been reviewed recently.4 Among the 3-HC derivatives, 3-hydroxyflavone (3-HF) is the most frequently studied molecule. Like 3-HC,24 3-HF exhibits an ESIPT involving two forms in the excited state, a “normal” excited state (S1, hydroxyl attached to C1; for labeling, see Scheme 1) and a tautomer excited state (S1′, often referred to as “keto” form, hydroxyl attached to C5) with distinct transient absorption and emission properties.48,50 Due to the weak © 2013 American Chemical Society

Scheme 1. Possible Structures of 3-HF with R = Phenyl and 3-HC with R = H

charge-transfer character of both species, the emission wavelengths (at 400 nm from S1 and 525 nm from S1′) only slightly depend on the solvent, but ESIPT time constants differ significantly.4 Despite ultrafast ( 450 nm) in conjunction with a rise of absorption at low wavelength (negative amplitude A1 for λ < 400 nm). The combination of positive sign in the near-IR and negative sign in the near-UV of component A1 is consistent with formation of a relaxed excited state accompanied by a blue shift of excitedstate absorption due to IVR (note that, as mentioned above, the DAS does not take into account the continuous distribution of intermediate states.) This component is observed in all solvents with a similar time constant and relative amplitudes including water, with a corresponding time constant of about 1 ps. For 3HC/ACN-d3 and 3-HC/EtOH, oscillating features are

Figure 4. Photoinduced IR difference spectra of 3-HC dissolved in (a) MCH-d14 and (b) ACN-d3 for selected delay times and (c) 3-HF dissolved in MCH-d14 and ACN-d338 at 100 ps pump−probe delay. The dotted curves in (a) and (b) display the inverted steady-state IR spectra of 3-HC dissolved in MCH-d14 and ACN-d3 (solvent subtracted), scaled for comparability.

the structural differences between 3-HC and 3-HF resulting in a lower number of vibrational bands for 3-HC, the difference spectra of 3-HC at 100 ps delay time resemble those of 3-HF, which consist of ground-state bleach signals and positive IR absorption bands assigned to S1′.38 Apart from similarities in ground-state absorption, all difference spectra display a characteristic doublet at 100 ps with absorption bands at around 1440 and 1460 cm−1 and three positive bands between 1600 and 1500 cm−1 as well as between 1420 and 1300 cm−1. This analogy between 3-HC and 3-HF supports the assignment of the state that is formed within 100 ps after photoexcitation of 11239

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3-HC to S1′. In particular, the IR bands assigned to S1 of 3-HF/ ACN-d3 (shown in ref 38) systematically do not coincide with the bands observed for 3-HC at 100 ps pump−probe delay. However, especially the characteristic features in the difference spectra of both solutes in MCH-d14 at 100 ps delay time are in good agreement, for example, the doublet and triplet of positive bands above 1420 cm−1 (see above). This supports the assignment of the positive bands of 3-HC at the 100 ps delay time to S1′. By comparison with (TD-)DFT calculations on isolated 3-HC molecules as well as 3-HC/ACN aggregates (B3LYP functional, TZVP basis set; for further details as well as the mode analysis, see the Supporting Information), strong ground-state vibrational bands with frequencies above 1600 cm−1 can be assigned to CO/CC stretching, and those with frequencies below 1500 cm−1 can be assigned to O−H/ C−H bending modes. For S1′, the strongest bands can be attributed to have a contribution of O−H bending. Both S0 and S1′ IR absorption bands of 3-HC are narrower in MCH-d14 than those in ACN-d3. In ACN-d3, bands are broader, slightly shifted, and have partially different relative intensities compared to MCH-d14 (especially at around 1480 cm−1), which can be attributed to interaction with solvent molecules. The changes in relative intensities are more pronounced for 3-HC than those for 3-HF, which could be due to the lack of the bulky phenyl group, which for 3-HF, partially restrains interaction of solvent molecules with the γ-pyrone ring. For 3-HF, comparison of IR results on S1 with quantum chemical calculations indicated that the slow ESIPT in ACN (τ ≈ 3 ps) is mediated by solute− solvent interaction via a H-bond with the hydroxyl group of 3HF.38 The difference spectra of 3-HC in MCH-d14 further indicate that ultrafast ESIPT occurs in MCH followed by relaxation processes because absorption bands of S1′ slightly shift and change in intensity between 0.7 and 100 ps. On a nanosecond time scale, all difference signals decay, consistent with the fluorescence lifetime of 600 ps, implying a decay of S1′ with subsequent ultrafast ground-state back proton transfer as predicted by quantum chemical calculations by the lack of a barrier for this process.19 Striking is the fact that ground-state bleach signals (especially at around 1650 cm−1) are also reduced on a picosecond time scale. Information about picosecond processes was deduced from a global analysis of the IR data up to 200 ps pump−probe delay. It yielded the time constants τ1 = 5.3 and τ2 = 21 ps for 3-HC/ MCH-d14 and τ1 = 5.2 and τ2 = 38 ps for 3-HC/ACN-d3 as well as the corresponding DAS depicted in Figure 5. The time constants observed in IR experiments coincide well with those of UV/Vis experiments. The ultrafast subpicosecond time constant observed in the UV/Vis experiments was not found to be due to the lower time resolution of the IR experiment. Static spectra, transients, and observed relative amplitudes of the DAS did not change when a sample with lower concentration was probed (see the Supporting Information); thus, formation of dimers can be excluded. Furthermore, upon deuteration of the aprotic (nonexchanging) solvents ACN and MCH, no direct kinetic isotope effect for the proton transfer of 3-HC is expected. Processes involved in the photocycle of 3-HC can be identified on the basis of the DAS. Component A2 in MCH-d14 can be attributed to vibrational cooling because it displays qualitatively the blue shift of bands assigned to S1′ by a pairing of a high-energy negative lobe and a low-energy positive lobe at positions with S1′ absorption. Although a global fit gives solely

Figure 5. Decay-associated IR spectra of 3-HC dissolved in (a) MCHd14 and (b) ACN-d3 from a global fit of the transient IR data up to 200 ps with two time constants.

a monoexponential approximation of the nonexponential vibrational cooling process, global fitting has already been successfully used to describe vibrational cooling/relaxation processes in the IR spectral region.74−78 A similar pattern was observed in the transient IR data of 3-HF dissolved in ACN-d3 and MCH-d14 with time constants of 12−17 and 13 ps (from a global fit of 3-HF/MCH-d14 data; not published), respectively.38 The time constant found for 3-HC is thus longer than that of 3-HF, which might be due to the smaller extent of lowenergy vibrational modes like the phenyl torsion of 3-HF that couple with solvent modes. Furthermore, the pattern of this component implies that no significant IR absorption of S1′ is superimposed by ground-state bleach signals above 1600 cm−1. IR component A1 in MCH-d14 has a more complex structure. On the one hand, it indicates like the corresponding vis-DAS a rise of S1′ population because it displays negative bands with similar amplitude at positions with hot/unrelaxed S1′ vibrational bands, that is, positive lobes of component A2 (cf. Figure 5 at 1575, 1430, 1405, and 1275 cm−1). Additionally, a negative band resembling the strongest bleach signals above 1600 cm−1 contributes to component A1 in MCH-d14. The latter suggests a partial repopulation of the ground state on a picosecond time scale (rotational diffusion can be excluded because all data were collected in magic angle configuration). Part of the positive peaks (1471, 1437, 1413, 1366, and 1285 cm−1) of component A1 might be caused by the time constants τ1 and τ2 differing by a factor of only four. If we assume a sequential two-step reaction scheme (A → B → C, with τ1 = 5.3 and τ2 = 21 ps), the faster component A1 contains, apart from the corresponding spectral information about species A and B, positive bands at positions with absorption of C. These are mirror images of those bands that appear with negative sign and amplitudes τ2/τ1 ≈ 4 times stronger in component A2. In such a reaction model, solely the positive peaks at 1504 and 1324 cm−1 of component A1 and a fraction of the amplitudes of the peaks at 1471 and 1285 cm−1 would characterize species A. To sum up, the UV/ Vis and IR characteristics of the spectra associated with a time constant of 5 ps in MCH-d14 imply the formation of additional S1′ absorption and thus a picosecond ESIPT as well as partial ground-state repopulation from a state characterized by IR bands at 1504 and 1324 and possibly 1471 and 1285 cm−1. The origin of this slow ESIPT is discussed below. The time constants found by a global analysis of the IR data on 3-HC/ACN-d3 are better separated (τ2/τ1 ≈ 7). In analogy 11240

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to the UV/Vis results, the IR DAS A1, A2, and A0 display mirror-image features. Bands with a positive sign in the IR component A0 (A2) appear with a negative sign and comparable amplitude in component A2 (A1). This corroborates a sequential reaction scheme (A → B → C, with τ1 = 5.2 and τ2 = 38 ps), where component A2 displays the formation of S1′ and hence a slow ESIPT. It possibly comprises vibrational cooling features occurring on a similar time scale but cannot be solely explained by this process due to the lack of low-energy positive lobes paired with negative peaks in A2. Initial and intermediate states in such a sequential reaction are discussed below. (d). Intermediate States in the Photocycle of 3-HC. In the preceding section, diverse processes and kinetic components were described for 3-HC in different solvents. The question of corresponding intermediate states is addressed now. As intermediate states, the following could play a role. Apart from the normal cis-structure in the ground (S0) and excited state (S1) as well as the tautomeric cis-structure in the ground (S0′) and excited state (S1′), the normal trans-species in its ground (S0T) and excited state (S1T) could be relevant. On the basis of simple energy considerations, the ground-state species S0T and S0′ can directly be excluded as intermediate states within the photocycle of 3-HC prior to formation of S1′, the species prevailing at 100 ps. Apart from the singlet states, triplet states were proposed to intervene in the dynamics of ESIPT in 3-HC,67 but the time constants observed in our study are rather short. Due to the typically small spin−orbit couplings of organic molecules, intersystem crossing usually takes place on the order of nanoseconds to microseconds.79 Thus, triplet states are disregarded for the discussion. Furthermore, a contribution of dimers was excluded by repeating the UV/Vis and IR experiments at different 3-HC concentrations. The contribution of dimers should be higher at higher concentrations, but because all experiments yielded the same result (cf. Supporting Information), dimer formation can be neglected. By comparison to the experiments in water at different pHs, anion formation can also be excluded in the organic solvents due to the missing features of the anionic excited-state species in the UV/Vis results (cf. Figures 2 and 3). In the next step, solvent−solute interaction and the resulting aggregated states are considered. In nonpolar aprotic solvents like MCH, a specific interaction of the carbonyl or hydroxyl group of 3-HC with solvent molecules is negligible; no solvent−solute aggregates with typically slowed down proton transfer rates are formed in MCH. However, the process with time constant 5.3 ps in MCH-d14 partially populates S1′, indicating a slow picosecond ESIPT. Solely S1 and S1T are left as candidates for the initial state of this slow ESIPT process in MCH. In contrast, specific interactions of solute and solvent were proposed to be responsible for the picosecond ESIPT of 3-HF in polar protic and polar aprotic solvents like EtOH and ACN.30,37,38,49 Thus, the corresponding states with specific solvent interaction, that is, solvent−solute aggregates in S1, S1T, and S1′, have to be included in the discussion of intermediate states in ACN. Due to the lower complexity in MCH, possible intermediate states in MCH are discussed first. As already mentioned, a slow ESIPT, as observed with 5.3 ps in the IR results on 3-HC/MCH-d14, cannot be caused by solvent−solute interaction. However, a slow ESIPT has also been observed and predicted by quantum chemically extended molecular dynamics simulations for 3-HF in MCH.34 The

simulations supported the idea that ultrafast IVR depopulates proton-transfer-promoting modes, for example, the O−O inplane and C−H out-of-plane bending modes are crucial in all investigated solvents (MCH, ACN, and methanol), and thus causes a picosecond ESIPT in MCH. The observation of a slow ESIPT of 3-HC in MCH-d14 could analogously be caused by depopulation of ESIPT-promoting modes through an IVR process. The formation of such a relaxed state could also favor a partial ground-state repopulation by a higher population of vibrational modes in the excited state overlapping with groundstate vibrational modes. In this case, the rate of slow ESIPT (kESIPT) and ground-state repopulation (kGS) occurring from one and the same initial state are merged to one time constant τ = 1/(kESIPT + kGS) in a global fit. A different explanation is based on the fact that 3-HC is missing the bulky phenyl group compared to 3-HF. This together with excess energy after photoexcitation could facilitate the formation of a normal-trans species in the excited state (S1T). The initially populated Franck−Condon state could relax into two species, the normal-cis state (S1) undergoing an ultrafast ESIPT in MCH (which is supported by the transient data; see above) and the normal-trans species (S1T), where the hydroxyl group has first to rotate around the C1−O2 bond to enable the ESIPT process. TD-DFT calculations (B3LYP functional, TZVP basis set; for further details, see the Supporting Information) predict that in contrast to 3-HF (isolated molecule, S1: 3173 cm−1; S1T: 5719 cm−1; relative energies refer to the most stable structure in the excited state: S1′: 0 cm−1), for 3-HC, the normal-cis (S1: 4050 cm−1) and normal-trans species (S1T: 4339 cm−1) are both stable (local minima within the S1 potential surface) and have almost the same energy, whereas in the ground state of 3-HC, the transstate is not favored (S0T: 2915 cm−1).18 For the cis- and transstructures of 3-HC aggregated with one ACN molecule, we also found structures with very similar energies (see Supporting Information Figure S9, traces b and f). Thus, S1 and S1T could both be formed simultaneously from a Franck−Condon state depending on the character of vibrational modes populated by photoexcitation. While the molecules in the S1 state exhibit an ultrafast ESIPT toward S1′, the molecules in S1T could deactivate via two pathways. They could either relax toward S1′ by rotation of the hydroxyl group followed by ultrafast ESIPT, or they could decay first into S0T followed by a back rotation of the hydroxyl group forming S0. The back rotation of the hydroxyl group in the ground state might be barrierless and ultrafast because the energy of S0 is far below that of S0T. This could explain why the process occurring with 5.3 ps in MCHd14 leads to a higher S1′ as well as ground-state population simultaneously (S1T → S0 and S1′). For 3-HC dissolved in ACN-d3 and EtOH, the data strongly suggest a sequential reaction (no branching) with two time constants and two intermediate states. Because the sequential reaction leads to the tautomer excited state (S1′), the candidates for the intermediate states are the normal-cis (S1) and normal-trans excited (S1T) states as well as aggregates with the solvent. A dissociative process of solvent−solute aggregates is not expected to lead to large shifts throughout the entire fingerprint region, in contrast to the features observed in ACNd3 (cf. Figure 5). Therefore, dissociation of solvent−solute aggregates in the S1′ leading to unperturbed, excited-state tautomeric 3-HC molecules can be excluded as the origin for one of the time constants under discussion. It is rather plausible that the normal-cis or normal-trans states play a role in the 11241

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4. CONCLUSIONS In summary, for the first time, static and femtosecond timeresolved spectroscopy were used to identify and characterize elementary processes such as ESIPT as well as intermediate states in the photocycle of 3-HC in protic, polar, and nonpolar solvents including MCH, ACN, EtOH, and neat water at different pHs. While static and transient UV/Vis and fluorescence spectroscopy provided electronic spectra of the ground and intermediate states as well as lifetimes of excited states, transient IR data yielded information on the related molecular structures and structural dynamics. In analogy to 3-HF, for 3-HC, three different observed fluorescence bands with maxima at 390, 493, and 467 nm could be assigned to the normal, tautomers and anionic excited states, respectively, with an excited-state lifetime of 600 ps for the tautomeric form (in MCH). Anionic fluorescence mostly dominates in water at pH 13. Transient UV/Vis data support the assignment of a shortlived (36 ps) anionic excited state upon direct excitation of the anionic ground state (e.g., in water at pH 13), exhibiting solvation on a subpicosecond time scale (τ1= 120 fs and τ2= 880 fs). Excitation of neutral 3-HC molecules in neat water (at pH 7) leads primarily to the formation of the short-lived anionic excited state within 3 ps and partially to the formation of the longer-living tautomer excited state S1′. These two processes seem to be competing deactivation channels, with the branching ratio in favor of dissociation, in contrast to 3-HF, where the opposite behavior was found. However, in organic solvents, the UV/Vis and IR results show that anion formation is negligible; instead, transient data and especially vibrational frequencies support the formation of the tautomer excited state (S1′) after photoexcitation. Additionally, observed ultrafast UV/Vis components with time constants of 0.2−0.4 ps in the organic solvents support the assignment to IVR. Two mechanisms are discussed for the picosecond ESIPT component (∼5.5 ps), with partial ground-state repopulation observed besides ultrafast ESIPT in MCH; on the one hand is IVR from the initially excited Franck−Condon state leading to a depopulation of proton-transfer-promoting modes, and on the other hand is partial and transient population of the transstructure excited state (S1T) with an energy barrier to S1 and S1′. Both processes result in a slow ESIPT. Additionally, vibrational cooling in S1′ (21 ps) was observed in MCH. For 3-HC in the polar solvents ACN and EtOH, transient spectra indicate the formation of S1′ in a sequential reaction on a several tens of picoseconds time scale with two preceding intermediate states (S1 → S1T → S1′). On the basis of a thorough discussion on intermediate states by means of quantum chemical calculations including solvent−solute aggregates, a contribution of a stable solvation state with an orientation of the OH group different from that of the S1 state as the intermediate state prior to ESIPT was proposed. The formation of S1T is favored, especially by the lack of the phenyl group compared to 3-HF. The bulky phenyl group is considered to reduce the extent of specific solvent−solute interaction on the hydroxyl group of 3-HF compared to 3-HC. Therefore, ESIPT is not controlled by phenyl torsion like earlier suggested36 but is facilitated by the presence of the phenyl group in a polar solvent. Thus, 3-HC takes an exceptional position in the group of 3HC derivatives. Most derivatives that were previously

excited-state dynamics because they are both equally stable (for the isolated molecule as well as solvent−solute aggregates with ACN), and the rotation of the hydroxyl group is not hindered by any bulky group like for 3-HF and other derivatives. Because photoexcitation promotes 3-HC molecules into the S1 state, a reaction S1 → S1T → S1′ is proposed as a reaction model for 3HC dissolved in ACN. The ESIPT could be slowed down by strong interaction of the hydroxyl group with polar solvent molecules, in contrast to 3-HF, where the phenyl group can reduce contact points with the solvent. This interaction inducing a barrier for ESIPT could lead to a favorable formation of the trans-state prior to ESIPT (e.g., by depopulation of the proton-transfer-promoting mode via IVR). The interaction with the solvent has to be interrupted for the back rotation of the hydroxyl group (trans → cis) followed by ESIPT to occur. Several stable solvation states with different directions of OH could occur in polar solvents, whereas the rather narrow IR absorption bands in the DAS in addition to the observation of one corresponding time constant indicate one rather stabilized state. This additional process and the higher number of solvent contact points would also explain the slower time constants observed for 3-HC than for 3-HF in polar solvents (slow ESIPT of 3-HF/ACN mediated by solvent proceeds with τ ≈ 3 ps).30,38 Note that in ref 24, a faster ESIPT rate is reported for 3-HC compared to 3-HF in the apolar solvent 3-methylpentane at low temperature (∼180 K) upon nanosecond excitation. Apart from that, the complexes of 3-HC with one or two waters or methanols were found to exhibit a slowed down ESIPT rate in argon matrix experiments at low temperature.35 The evidence for a sequential reaction scheme in ACN-d3 and EtOH strongly suggests the presence of an additional stable solvation state (which could be the S1T state) to the photocycle of 3-HC. This is due to the DAS of each of the two observed processes in ACN-d3, which show quite distinct respective positive and negative bands indicating generally differing respective initial and final states (cf. Figure 5 b). Additionally to the slow ESIPT with 36−38 ps, an unresolved fast ESIPT of unperturbed 3-HC molecules in the ACN-d3 analogue to 3-HF/ACN30,38 could contribute to the similarities in difference spectra at early delay times (cf. Figures 3 and 4). An intermediate charge-transfer state (CT*) stabilized by polar solvents has been observed in the excited state prior to proton transfer for several 3-HF derivatives with an electrondonating group instead of the phenyl group. However, for 3HC, the excited-state charge separation is small due to the lack of electron-donating substituents, and therefore, an intermediate charge-transfer state is not expected, in analogy to 3-HF.4 To sum up the discussion regarding intermediate states, the contribution of S1T to the photocycle of 3-HC in MCH and ACN is strongly suggested by the data presented here. A reaction scheme S1 → S1T → S1′ is proposed for 3-HC dissolved in ACN with solvent−solute interaction. The intermediate state is a stable solvation state with a different direction of the OH group than that in the S1 state. According to our considerations, the S1T state is favored as an intermediate state. In MCH, alternatively, IVR from the initially excited FC state leading to depopulation of proton-transfer-promoting modes in S1 or a partial population of S1T by relaxation from the FC state could explain the picosecond ESIPT and partial ground-state recovery with a time constant of 5.3 ps in the IR experiment. 11242

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(7) Klymchenko, A. S.; Demchenko, A. P. 3-Hydroxychromone Dyes Exhibiting Excited-State Intramolecular Proton Transfer in Water with Efficient Two-Band Fluorescence. New J. Chem. 2004, 28, 687−692. (8) Klymchenko, A. S.; Duportail, G.; Ozturk, T.; Pivovarenko, V. G.; Mely, Y.; Demchenko, A. P. Novel Two-Band Ratiometric Fluorescence Probes with Different Location and Orientation in Phospholipid Membranes. Chem. Biol. (Oxford, U.K.) 2002, 9, 1199− 1208. (9) Klymchenko, A. S.; Mely, Y.; Demchenko, A. P.; Duportail, G. Simultaneous Probing of Hydration and Polarity of Lipid Bilayers with 3-Hydroxyflavone Fluorescent Dyes. Biochim. Biophys. Acta 2004, 1665, 6−19. (10) Barbara, P. F.; Walsh, P. K.; Brus, L. E. Picosecond Kinetic and Vibrationally Resolved Spectroscopic Studies of Intramolecular Excited-State Hydrogen-Atom Transfer. J. Phys. Chem. 1989, 93, 29−34. (11) Barbatti, M.; Aquino, A. J.; Lischka, H.; Schriever, C.; Lochbrunner, S.; Riedle, E. Ultrafast Internal Conversion Pathway and Mechanism in 2-(2′-Hydroxyphenyl)Benzothiazole: A Case Study for Excited-State Intramolecular Proton Transfer Systems. Phys. Chem. Chem. Phys. 2009, 11, 1406−1415. (12) Elsaesser, T.; Kaiser, W. Visible and Infrared Spectroscopy of Intramolecular Proton Transfer Using Picosecond Laser Pulses. Chem. Phys. Lett. 1986, 128, 231−237. (13) Mohammed, O. F.; Luber, S.; Batista, V. S.; Nibbering, E. T. Ultrafast Branching of Reaction Pathways in 2-(2′-Hydroxyphenyl)Benzothiazole in Polar Acetonitrile Solution. J. Phys. Chem. A 2011, 115, 7550−7558. (14) Rini, M.; Dreyer, J.; Nibbering, E. T. J.; Elsaesser, T. Ultrafast Vibrational Relaxation Processes Induced by Intramolecular Excited State Hydrogen Transfer. Chem. Phys. Lett. 2003, 374, 13−19. (15) Rini, M.; Kummrow, A.; Dreyer, J.; Nibbering, E. T. J.; Elsaesser, T. Femtosecond Mid-Infrared Spectroscopy of Condensed Phase Hydrogen-Bonded Systems as a Probe of Structural Dynamics. Faraday Discuss. 2003, 122, 27−40. (16) Schriever, C.; Lochbrunner, S.; Ofial, A. R.; Riedle, E. The Origin of Ultrafast Proton Transfer: Multidimensional Wave Packet Motion vs. Tunneling. Chem. Phys. Lett. 2011, 503, 61−65. (17) Chudoba, C.; Riedle, E.; Pfeiffer, M.; Elsaesser, T. Vibrational Coherence in Ultrafast Excited State Proton Transfer. Chem. Phys. Lett. 1996, 263, 622−628. (18) Stamm, A.; Weiler, M.; Brächer, A.; Schwing, K.; Pfister, K.; Gerhards, M. Combined IR/IR and IR/UV Spectroscopy on the Proton Transfer Coordinate of Isolated 3-Hydroxychromone in the Electronic Ground and Excited State, 2013, to be published. (19) Ash, S.; De, S. P.; Beg, H.; Misra, A. Excited State Intramolecular Proton Transfer in 3-Hydroxychromone: A DFTBased Computational Study. Mol. Simul. 2011, 37, 914−922. (20) Bouman, T. D.; Knobeloch, M. A.; Bohan, S. An Ab Initio Random Phase Approximation Study of the Excited-State Intramolecular Proton Transfer in 3-Hydroxychromone. J. Phys. Chem. 1985, 89, 4460−4464. (21) Brucker, G. A.; Kelley, D. F. Spectroscopy and Proton Transfer of Matrix-Isolated Hydrogen-Bonding 3-Hydroxychromone Complexes. J. Phys. Chem. 1987, 91, 2862−2866. (22) Itoh, M.; Fujiwara, Y. Two-Step Laser Excitation Fluorescence Study of the Ground- and Excited-State Proton Transfer in 3Hydroxyflavone and 3-Hydroxychromone. J. Phys. Chem. 1983, 87, 4558−4560. (23) Itoh, M.; Tanimoto, Y.; Tokumura, K. Transient Absorption Study of the Intramolecular Excited-State and Ground-State Proton Transfer in 3-Hydroxyflavone and 3-Hydroxychromone. J. Am. Chem. Soc. 1983, 105, 3339−3340. (24) Itoh, M.; Tokumura, K.; Tanimoto, Y.; Okada, Y.; Takeuchi, H.; Obi, K.; Tanaka, I. Time-Resolved and Steady-State Fluorescence Studies of the Excited-State Proton Transfer in 3-Hydroxyflavone and 3-Hydroxychromone. J. Am. Chem. Soc. 1982, 104, 4146−4150.

investigated are substituted at the 2-position directly adjacent to the hydroxyl group, which reduces contact points for specific solute−solvent interaction at the hydroxyl group and could thus inhibit the formation of the normal-trans species. 3-HC, instead of being a simpler system than 3-HF and other derivatives, shows a more complex and flexible behavior upon photoexcitation.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental results, including decay-associated spectra of 3HC dissolved in MCH-d14 and EtOH, IR transients of 3-HC in MCH-d14 at different concentrations, static UV/Vis spectra of 3-HC dissolved in ACN at different concentrations, and fluorescence excitation spectra of 3-HC in water at pH 7 and 13 at selected emission wavelengths along with quantum chemical calculations, including geometric parameters obtained for the ground and excited state of 3-HC−ACN aggregates with 3-HC in normal cis-, normal trans-, and tautomer cis-structure, comparison of the static FTIR ground-state IR spectrum to results from calculations, comparison of the IR spectrum of the long-living transient species of 3-HC (>100 ps) to results from calculations, characterization of vibrational modes of 3-HC− ACN aggregates for the ground state in a normal cis-structure, as well as the excited state in the tautomer cis-structure. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.D.). *E-mail: [email protected] (M.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support by the Deutsche Forschungsgemeinschaft (DFG) Grant GE 961/8-1 (M.G.) as well as support by the Research Initiative Membrane Biology “RIMB” (R.D.).

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REFERENCES

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