Proton transfer and internal conversion of o-hydroxybenzaldehyde: coherent versus statistical excited-state dynamics

Simulating ultrafast transient absorption spectra from first principles using a time-dependent configuration interaction probe

Transient absorption spectroscopy (TAS) is among the most common ultrafast photochemical experiments, but its interpretation remains challenging. In this work, we present an efficient and robust method for simulating TAS signals from first principles. Excited-state absorption and stimulated emission (SE) signals are computed using time-dependent complete active space configuration interaction (TD-CASCI) simulations, leveraging the robustness of time-domain simulation to minimize electronic structure failure. We demonstrate our approach by simulating the TAS signal of 1'-hydroxy-2'-acetonapthone (HAN) from ab initio multiple spawning nonadiabatic molecular dynamics simulations. Our results are compared to gas-phase TAS data recorded from both jet-cooled (T ∼ 40 K) and hot (∼403 K) molecules via cavity-enhanced TAS (CE-TAS). Decomposition of the computed spectrum allows us to assign a rise in the SE signal to excited-state proton transfer and the ultimate decay of the signal to relaxation through a twisted conical intersection. The total cost of computing the observable signal (∼1700 graphics processing unit hours for ∼4 ns of electron dynamics) was markedly less than that of performing the ab initio multiple spawning calculations used to compute the underlying nonadiabatic dynamics.

Proton coupled isomerization in double-hydrogen-bonding-center salicylaldehyde azine

The proton transfer dynamics of Schiff bases are of great importance due to the application potential. The excited state intramolecular proton transfer (ESIPT) of photochromic salicylaldehyde azine (SAA) in solutions was investigated by experimental and computational methods. Two distinguished spectral bands are observed in transient absorption spectra upon the photoexcitation with 400 nm. From the delayed stimulated emission signals, the ESIPT process is determined to be within <200 fs. Subsequently, the photoprotonated products undergo the vibrational relaxation with several picoseconds and the following isomerization with tens of picoseconds. These processes are significantly affected by the polarity of the solvents. The longest component with nanosecond scale can be explained to the relaxation to the enol structure. According to our observations, the SAA molecules with symmetric double-hydrogen-bonding-centers undergo single proton transfer rather than double proton transfer and subsequent intramolecular isomerization.

Excitation Energy-Dependent, Excited-State Intramolecular Proton Transfer-Based Dual Emission in Poor Hydrogen-Bonding Solvents

2-(2'-Hydroxyphenyl)benzoxazole (HBO) substituted at the 5'-position with bipyridylvinylene phenylenevinylene (compound 2) produces both normal and, via an excited-state intramolecular proton transfer (ESIPT) reaction, tautomer emissions in solvents that preserve intramolecular hydrogen bonds. The abundance of the tautomer emission from compound 2 in a poor hydrogen-bonding solvent increases in response to the application of a higher excitation energy. Based on quantum chemical calculations, the excitation-dependent dual emission is consistent with a model in which the ESIPT reaction is more favored in the S₂ than in the S₁ electronically excited state.

Nature of Fast Relaxation Processes and Spectroscopy of a Membrane-Active Peptide Modified with Fluorescent Amino Acid Exhibiting Excited State Intramolecular Proton Transfer and Efficient Stimulated Emission

A fluorescently labeled peptide that exhibited fast excited state intramolecular proton transfer (ESIPT) was synthesized, and the nature of its electronic properties was comprehensively investigated, including linear photophysical and photochemical characterization, specific relaxation processes in the excited state, and its stimulated emission ability. The steady-state absorption, fluorescence, and excitation anisotropy spectra, along with fluorescence lifetimes and emission quantum yields, were obtained in liquid media and analyzed based on density functional theory quantum-chemical calculations. The nature of ESIPT processes of the peptide's chromophore moiety was explored using a femtosecond transient absorption pump-probe technique, revealing relatively fast ESIPT velocity (∼10 ps) in protic MeOH at room temperature. Efficient superluminescence properties of the peptide were realized upon femtosecond excitation in the main long-wavelength absorption band with a corresponding threshold of the pump pulse energy of ∼1.5 μJ. Quantum-chemical analysis of the electronic structure of the peptide was performed using the density functional theory/time-dependent density functional theory level of theory, affording good agreement with experimental data.

Effects of π-expansion, an additional hydroxyl group, and substitution on the excited state single and double proton transfer of 2-hydroxybenzaldehyde and its relative compounds: TD-DFT static and dynamic study

The effects of π-expansion, an extra hydroxyl group, and substituents on the photophysical properties, the excited state single proton transfer and the double proton transfer of 2-hydroxybenzaldehyde and its relatives have been theoretically investigated using TD-DFT.

Ultrafast dynamics of the ESIPT photoswitch N-(3-pyridinyl)-2-pyridinecarboxamide

Molecular switches based on proton transfer that are photochromic and can be interconverted by light at different wavelengths back and forth between two thermodynamically stable tautomeric states in solution at room temperature are rare to date. We report on a study of the ultrafast conversion of the bistable proton transfer switch N-(3-pyridinyl)-2-pyridinecarboxamide (NPPCA) to a corresponding iminol after photoexcitation at λ_pump ≈ 265 nm by means of femtosecond time-resolved broad-band and single-colour transient electronic absorption spectroscopy (TEAS), transient fluorescence spectroscopy (TFLS), and transient vibrational absorption spectroscopy (TVAS) in acetonitrile solution. The interpretation of the data was accompanied by ab initio quantum chemical calculations of the excited electronic states and the vibrational frequencies of the reactant and product in their ground electronic state. The TEAS experiments provided four time constants, τ₁ = 0.09 ± 0.01 ps, τ₂ = 0.61 ± 0.01 ps, τ₃ = 5.10 ± 0.80 ps, and τ₄ = 20.0 ± 1.0 ps. The first two agree well with the measured TFLS lifetimes, τ_1,TFL < 0.18 ps and τ_2,TFL = 0.50 ± 0.01 ps. τ₁ is related to the relaxation of the initially excited Franck-Condon (FC) state of the pyridinecarboxamide, followed by the excited-state intramolecular proton transfer (ESIPT) step to the neighbouring pyridine. The subsequent return of the molecules to the electronic ground state takes place within τ₂, mediated by a conical intersection (CI) at a twisted configuration of the pyridinecarboxamide moiety. The main components in all TEAS time profiles feature a rise with τ₂ and a decay with τ₄ and describe subsequent molecular transformations in the electronic ground state. τ₃ is ascribed to vibrational cooling of the molecules. The final iminol exhibits a permanent UV absorption at λ = 247 nm, where its absorbance is stronger than that of the carboxamide reactant. The iminol structure is unambiguously identified by the TVA spectra, which show the build-up of corresponding vibrational bands with τ_4,TVA = 23 ± 2 ps after the initial bleach of the reactant vibrational bands, in excellent agreement with the TEAS data. Its lifetime is >10 ns.

Dynamics of excited state proton transfer in nitro substituted 10-hydroxybenzo[h]quinolines

The ground state tautomerism and excited state intramolecular proton transfer (ESIPT) of 10-hydroxybenzo[h]quinoline (HBQ) and its nitro derivatives, 7-nitrobenzo[h]quinolin-10-ol (2) and 7,9-dinitrobenzo[h]quinolin-10-ol (3), have been studied in acetonitrile using steady state as well as time dependent spectroscopy and quantum-chemical calculations. In addition to the enol form absorbance in the range 360-390 nm, the absorption spectra of 2 and 3 exhibit a red shifted band at ∼450 nm. Chemometric data processing, based on individual band decomposition, allowed us to estimate the position of the ground state enol-keto tautomeric equilibrium (ΔG values of 1.03 and 0.62 kcal mol^-1 respectively for 2 and 3). The fluorescence stems from the keto form even if the enol form is optically excited as proven by the shape of the excitation spectra indicating that ESIPT takes place. The Stokes shift of the substituted compounds is substantially lower compared to HBQ, which follows from the fact that the substitution occurs in the formal cyclohexa-2,4-dienone moiety and leads to a decrease of the HOMO level of the keto tautomer. The pump-probe experiments show that in the nitro substituted HBQs 2 and 3 ESIPT occurs with a time constant of 0.89 ps and 0.68 ps, respectively. In both cases a mixture of the enol and proton transfer forms is optically excited. The enol form exhibits then the ESIPT and subsequently both fractions take the same relaxation path. We propose that in 2 and 3 the ESIPT path exhibits a potential energy barrier resulting in an incoherent rate governed process while in HBQ the ESIPT proceeds as a ballistic wavepacket motion along a path without significant barriers. The theoretical calculations (M06-2X/TZVP) confirm the existence of a barrier in the ground and excited states as result of the substitution.

An On-the-Fly Surface-Hopping Program JADE for Nonadiabatic Molecular Dynamics of Polyatomic Systems: Implementation and Applications

Nonadiabatic dynamics simulations have rapidly become an indispensable tool for understanding ultrafast photochemical processes in complex systems. Here, we present our recently developed on-the-fly nonadiabatic dynamics package, JADE, which allows researchers to perform nonadiabatic excited-state dynamics simulations of polyatomic systems at an all-atomic level. The nonadiabatic dynamics is based on Tully's surface-hopping approach. Currently, several electronic structure methods (CIS, TDHF, TDDFT(RPA/TDA), and ADC(2)) are supported, especially TDDFT, aiming at performing nonadiabatic dynamics on medium- to large-sized molecules. The JADE package has been interfaced with several quantum chemistry codes, including Turbomole, Gaussian, and Gamess (US). To consider environmental effects, the Langevin dynamics was introduced as an easy-to-use scheme into the standard surface-hopping dynamics. The JADE package is mainly written in Fortran for greater numerical performance and Python for flexible interface construction, with the intent of providing open-source, easy-to-use, well-modularized, and intuitive software in the field of simulations of photochemical and photophysical processes. To illustrate the possible applications of the JADE package, we present a few applications of excited-state dynamics for various polyatomic systems, such as the methaniminium cation, fullerene (C20), p-dimethylaminobenzonitrile (DMABN) and its primary amino derivative aminobenzonitrile (ABN), and 10-hydroxybenzo[h]quinoline (10-HBQ).

Photophysics of Derivatives of 3-Hydroxybenzo[c]coumarin

The photophysical studies of two phenols, derivatives of 3-hydroxybenzo[c]coumarin, were performed in n-nonane matrix at 5 K. Unstructured fluorescence spectrum of the derivative bearing a salicylaldehyde moiety, whose onset is shifted by ca. 3000 cm(-1) to lower energy in respect to that of absorption, and short decay time of this emission (0.75 ns) suggested the occurrence of excited-state intramolecular proton transfer (ESIPT). The experimental results were interpreted with the aid of quantum chemistry calculations performed with the DFT and TDDFT/B3LYP/6-31++G(d,p) methods.

Time-dependent density functional theory study on the excited-state intramolecular proton transfer in salicylaldehyde

Time-dependent density functional theory method was performed to investigate the excited state intramolecular hydrogen bond dynamics of salicylaldehyde (SA). The geometric structures and IR spectra in the ground state S0 state and the excited state S1 state of SA are calculated using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods, respectively. In addition, the absorption and fluorescence peaks are also calculated using TDDFT methods. It is noted that the calculated large Stokes shift is in good agreement with the experimental results. Furthermore, our results have demonstrated that the excited state intramolecular proton transfer (ESIPT) process happens upon photoexcitation, which are distinct monitored by the formation and disappearance of the characteristic peaks of IR spectra involved in the formation of hydrogen bonds in different states and in the potential energy curves. We find that the hydrogen bonded quasi-aromatic chelating ring in the excited state becomes smaller which can facilitate the ESIPT process. The results presented here suggest that the ESIPT process of the SA molecule in the excited state can be attributed to the electronegativity change of O1 induced by excitation.

Excited-state free energy surfaces in solution: time-dependent density functional theory∕reference interaction site model self-consistent field method

Constructing free energy surfaces for electronically excited states is a first step toward the understanding of photochemical processes in solution. For that purpose, the analytic free energy gradient is derived and implemented for the linear-response time-dependent density functional theory combined with the reference interaction site model self-consistent field method. The proposed method is applied to study (1) the fluorescence spectra of aqueous acetone and (2) the excited-state intramolecular proton transfer reaction of ortho-hydroxybenzaldehyde in an acetonitrile solution.

Excited-state diproton transfer in [2,2'-bipyridyl]-3,3'-diol: the mechanism is sequential, not concerted

The excited-state mono- and diproton transfer has been investigated in the S(1) state of [2,2'-bipyridyl]-3,3'-diol using the quantum mechanical resolution-of-identity second-order approximate coupled-cluster (RI-CC2) and time-dependent density functional theory (TDDFT) methods. Static investigation of stationary points and scans of the pi pi* and n pi* energy surfaces have been performed. These calculations show that the concerted diproton transfer in S(1) proceeds along a ridge thus making this process highly unlikely since it will stabilize toward the unsymmetrical monoproton transfer. A small energy barrier of about 0.11 eV (RI-CC2 result) between the mono- and diketo structures is obtained allowing rapid continuation of the proton transfer to the diketo form. On-the-fly dynamics simulations performed at the RI-CC2 level confirm this picture. The first proton transfer step is so fast (7 fs) that it probably cannot be resolved by experimental techniques. Important participation of the n pi* state is predicted. The present results shed a completely new light on the interpretation of the experimental results. The simulations clearly show that what has been experimentally determined as concerted transfer is in fact a combination of two sequential proton transfers separated by a small delay below the present experimental resolution. Concerning the second step of the sequential proton transfer the dynamics calculations indicate the existence of a highly dynamic system. Both the forward and reverse reactions of a monoketo/diketo equilibrium were found within the 300 fs period of the simulation. Environmental effects will certainly lead to a substantial cooling of the initially hot molecule and a concomitant decrease in the monoketo/diketo conversion rates, which will result in the experimentally observed overall time scale of 10 ps for the second proton transfer step.

Ultrafast internal conversion pathway and mechanism in 2-(2'-hydroxyphenyl)benzothiazole: a case study for excited-state intramolecular proton transfer systems

We study the ultrafast electronic relaxation of the proton transfer compound 2-(2'-hydroxyphenyl)benzothiazole (HBT) in a joint approach of femtosecond pump-probe experiments and dynamics simulations. The measurements show a lifetime of 2.6 ps for the isolated molecule in the gas phase in contrast to approximately 100 ps for cyclohexane solution. This unexpected decrease by a factor of 40 for the gas phase is explained by ultrafast internal conversion to the ground state promoted by an inter-ring torsional mode. The quantum chemical calculations based on multireference configuration interaction clearly demonstrate that a S(0)/S(1) conical intersection at a 90 degrees twisted structure exists and is responsible for the ultrafast decay. The reaction path leading from the keto form of HBT to this intersection is practically barrierless on the S(1) surface. The on-the-fly dynamics simulations using time-dependent density functional theory show that after electronic excitation to the S(1) state and after fast excited-state proton transfer (30-50 fs), HBT reaches the region of the S(1)/S(0) crossing within about 500 fs, which will lead to the observed 2.6 ps deactivation to the ground state. After the internal conversion, HBT branches in two populations, one that rapidly closes the proton transfer cycle and another (trans-keto) that takes approximately 100 ps for that step.

Theoretical Studies of Conjugation Effects on Excited State Intramolecular Hydrogen-Atom Transfer Reactions in Model Systems

Intramolecular hydrogen-atom transfer dependence on electronic conjugation of curcumin and related molecular models in the ground state and 1pipi* excited state are computationally studied at the first-principles electronic structure level. The larger, more conjugated, systems exhibit a lower reaction barrier in the ground state but a higher barrier in the excited state. This is associated with a smaller increase in the conjugation upon excitation in the larger systems. Our studies provide a detailed description and analysis of these energy trends as well as an insight into the physical nature of the intramolecular hydrogen-atom transfer reactions.

Electronic and quantum dynamical insight into the ultrafast proton transfer of 1-hydroxy-2-acetonaphthone

The ultrafast proton-transfer dynamics of 1-hydroxy-2-acetonaphthone has been theoretically analyzed in the ground and first singlet excited electronic states by density functional theory calculations and quantum dynamics. The potential energies obtained in the ground electronic state reveal that the proton-transfer process does not lead to a stable keto tautomer unless the transfer of the hydrogen from the enol form is accompanied by an internal rotation of the newly formed O-H bond. Calculations in the first singlet excited electronic state point to a very low barrier for the formation of the keto tautomer. The analysis of the calculated frequencies of the two tautomers in the excited state unveils a coupling of the skeletal motions (low frequency modes) with the proton-transfer process, as it has been stated from time-resolved experiments. The electronic energies obtained by the time-dependent density functional theory formalism have been fitted to a monodimensional potential energy surface in order to perform an exact quantum dynamics study of the process. Our results show that the proton-transfer process is completed within 25.5 fs, in remarkable good agreement with experiments.

Coherent nuclear wavepacket motions in ultrafast excited-state intramolecular proton transfer: sub-30-fs resolved pump-probe absorption spectroscopy of 10-hydroxybenzo[h]quinoline in solution

The dynamics of the excited-state intramolecular proton transfer of 10-hydroxybenzo[h]quinoline (10-HBQ) and the associated coherent nuclear motion were investigated in solution by femtosecond absorption spectroscopy. Sub-picosecond transient absorption measurements revealed spectral features of the stimulated emission and absorption of the keto excited state (the product of the reaction). The stimulated emission band appeared in the 600-800-nm region, corresponding to the wavelength region of the steady-state keto fluorescence. It showed successive temporal changes with time constants of 350 fs and 8.3 ps and then disappeared with the lifetime of the keto excited state (260 ps). The spectral feature of the stimulated emission changed in the 350-fs dynamics, which was likely assignable to the intramolecular vibrational energy redistribution in the keto excited state. The 8.3-ps change caused a spectral blue shift and was attributed to the vibrational cooling process. The excited-state absorption was observed in the 400-600-nm region, and it also showed temporal changes characterized by the 350-fs and 8.3-ps components. To examine the coherent nuclear dynamics (nuclear wavepacket motion) in excited-state 10-HBQ, we carried out pump-probe measurements of the stimulated emission and absorption signals with time resolution as good as 27 fs. The obtained data showed substantially modulated signals due to the excited-state vibrational coherence up to a delay time of several picoseconds after photoexcitation. This means that the vibrational coherence created by photoexcitation in the enol excited state is transferred to the product. Fourier transform analysis indicated that four frequency components in the 200-700-cm(-1) region contribute to the oscillatory signal, corresponding to the coherent nuclear motions in excited-state 10-HBQ. Especially, the lowest-frequency mode at 242 cm(-1) is dephased significantly faster than the other three modes. This observation was regarded as a manifestation that the nuclear motion of the 242-cm(-1) mode is correlated with the structural change of the molecule associated with the reaction (the reaction coordinate). The 242-cm(-1) mode observed in excited-state 10-HBQ was assigned to a vibration corresponding to the ground-state vibration at 243 cm(-1) by referring to the results of resonance Raman measurements and density functional calculations. It was found that the nuclear motion of this lowest-frequency mode involves a large displacement of the OH group toward the nitrogen site as well as in-plane skeletal deformation that assists the oxygen and nitrogen atoms to come closer to each other. We discuss the importance of the nuclear wavepacket motion on a multidimensional potential-energy surface including the vibrational coordinate of the low-frequency modes.

Excited-state intramolecular proton transfer: a survey of TDDFT and RI-CC2 excited-state potential energy surfaces

TDDFT and RI-CC2 calculations have been performed on the excited-state intramolecular proton transfer in malonaldehyde, o-hydroxybenzaldehyde, salicylic acid, 7-hydroxy-1-indanone, and 2-(2'-hydroxyphenyl)benzothiazole. Vertical and adiabatic excitation energies have been computed for the npi and pipi states. Overall, we have found that both RI-CC2 and TDDFT methods are good candidates for the description of ESIPT potential energy surfaces. Proton transfer (PT) curves have been computed for both excited states. An essentially barrierless and very shallow energy profile has been found for the pi pi* state. For the n pi* state the keto minimum is more pronounced than for the pi pi* state and, depending on the case, energy barriers ranging from values <0.1 eV up to 0.5 eV were found. From the computed PT curves we conclude that extended crossing regions between the two excited states will occur.

Effect of alkyl substituents on excited state intramolecular proton transfer dynamics of jet-cooled bis(benzoxazolyl)phenoles

Structural factors affecting the dynamics of the excited state intramolecular proton transfer (ESIPT) are studied for alkyl derivatives of 2,5-bis(2-benzoxazolyl)phenol. Two fluorescence bands with equal decay times are observed in solution, while only one--emitted by the phototautomer--in supersonic jet. All evidence indicates the existence of a potential barrier in the S(1) state. Upon deuteration of the OH group the laser induced fluorescence (LIF) excitation spectra become much sharper as a result of slowing down the proton transfer reaction. Two conformers (rotamers) of each compound in the ground state were detected using hole burning technique. With a help of theoretical calculations three vibrations were identified as the most active ones in reducing the distance between two heavy atoms, N and O, involved in H-bond formation. The widths of (0,0) transitions in LIF excitation spectra decrease with increasing size or number of alkyl substituents at terminal aromatic rings. The corresponding calculated rate constants of ESIPT reaction ( approximately 10(12) s(-1)) decrease approximately three times upon the substituent effect. In contrast, model compound 2,5-bis(2-benzoxazolyl)-4-methoxyphenol (BBMP) with OCH(3) parasubstituent in central ring slows down the ESIPT reaction to such an extent that double, primary and phototautomeric, fluorescences coexist.

Photophysics of intramolecularly hydrogen-bonded aromatic systems: ab initio exploration of the excited-state deactivation mechanisms of salicylic acid

Excited state reaction paths and the corresponding energy profiles of salicylic acid have been determined with the CC2 method, which is a simplified version of singles-and-doubles coupled cluster theory. At crucial points of the potential energy hypersurfaces, single-point energy calculations have been performed with the CASPT2 method (second-order perturbation theory based on the complete active space self-consistent field reference). Hydrogen transfer along the intramolecular hydrogen bond as well as torsion and pyramidization of the carboxy group have been identified as the most relevant photochemical reaction coordinates. The keto-type planar S(1) state reached by barrierless intramolecular hydrogen transfer represents a local minimum of the S(1) energy surface, which is separated by a very low barrier from a reaction path leading to a low-lying S(1)-S(0) conical intersection via torsion and pyramidization of the carboxy group. The S(1)-S(0) conical intersection, which occurs for perpendicular geometry of the carboxy group, is a pure biradical. From the conical intersection, a barrierless reaction path steers the system back to the two known minima of the S(0) potential energy surface (rotamer I, rotamer II). A novel structure, 7-oxa-bicyclo[4.2.0]octa-1(6),2,4-triene-8,8-diol, has been identified as a possible transient intermediate in the photophysics of salicylic acid.

Photophysics of Organic Photostabilizers. Ab Initio Study of the Excited-State Deactivation Mechanisms of 2-(2‘-Hydroxyphenyl)benzotriazole

Excited-state reaction paths and the corresponding energy profiles of 2-(2'-hydroxyphenyl)benzotriazole (TIN-H) have been determined with the CC2 (simplified singles-and-doubles coupled-cluster) ab initio method. Hydrogen transfer along the intramolecular hydrogen bond, torsion of the aromatic rings and pyramidization of the central nitrogen atom are identified as the most relevant photochemical reaction coordinates. The keto-type planar S(1) state reached by barrierless intramolecular hydrogen transfer is found to be unstable with respect to torsion. The latter mode, together with a moderate pyramidization of the central nitrogen atom, provides barrierless access to a S(1)-S(0) conical intersection. Only the pi-type orbitals of the aromatic rings are involved in the open-shell structures. The S(1)-S(0) conical intersection, which occurs for perpendicular geometry of the aromatic rings, is a pure biradical. From the conical intersection, a barrierless reaction path steers the system back to the enol-type minimum of the S(0) potential-energy surface, thus closing the photocycle. This photophysical pathway accounts for the remarkable photostability of the molecule.

Ab Initio Molecular Dynamics of Excited-State Intramolecular Proton Transfer around a Three-State Conical Intersection in Malonaldehyde

Excited-state potential energy surface (PES) characterization is carried out at the CASSCF and MRSDCI levels, followed by ab initio dynamics simulation of excited-state intramolecular proton transfer (ESIPT) on the S2(pipi*) state in malonaldehyde. The proton-transfer transition state lies close to an S2/S1 conical intersection, leading to substantial coupling of proton transfer with electronic relaxation. Proton exchange proceeds freely on S2, but its duration is limited by competition with twisting out of the molecular plane. This rotamerization pathway leads to an intersection of the three lowest singlet states, providing the first detailed report of ab initio dynamics around a three-state intersection (3SI). There is a significant energy barrier to ESIPT on S1, and further pyramidalization of the twisted structure leads to the minimal energy S1/S0 intersection and energetic terminal point of excited-state dynamics. Kinetics and additional mechanistic details of these pathways are discussed. Significant depletion of the spectroscopic state and recovery of the ground state is seen within the first 250 fs after photoexcitation.

Ultrafast proton transfer of 1-hydroxy-2-acetonaphthone: Reaction path from resonance Raman and transient absorption studies

The intramolecular degrees of freedom contributing to the ultrafast excited-state intramolecular proton transfer of 1-hydroxy-2-acetonaphthone are determined. Thereto, resonance Raman studies are combined with transient absorption measurements with 30-fs time resolution. Enhanced Raman intensity is found in coordinates that are dominantly associated with deformations and bond length changes in the naphthalene chromophore. This indicates that the primary changes after the optical excitation are a geometric relaxation of the chromophore. A ringing of the molecule after the ultrafast proton transfer is observed by the transient absorption measurements. It reveals the nuclear coordinates contributing to the reaction path beyond the Franck-Condon region. There, planar H-chelate ring deformations changing the donor-acceptor distance are found to dominate. The difference in the observed vibrational signatures indicates a significant turn in the reaction path.