Ultrafast dynamics of gas-phase excited-state intramolecular proton transfer in 1-hydroxy-2-acetonaphthone

Insights into the Excited-State Processes in 1-Hydroxy-2-acetonaphthone at ADC(2) and CASSCF Levels

1-Hydroxy-2-acetonaphthone (HAN) has been extensively studied both experimentally and computationally to ascertain the existence of the excited-state proton transfer process. However, the process of full photocycle including the nonradiative relaxation pathways is yet to be proposed. Therefore, in the present study, we aim at providing a comprehensive picture of the excited-state processes in HAN including the proton transfer and relaxation processes through electronic structure calculations at second-order algebraic diagrammatic construction (ADC(2)) and complete active space second-order perturbation theory (CASPT2)//complete active space self-consistent field (CASSCF) and dynamics simulations at ADC(2) levels. Our studies show that the proton transfer process in the S₁ state is barrierless and produces a stable keto form, which is in accordance with previous experimental and computational studies. Adiabatic dynamics simulations at the ADC(2) level confirmed the ultrafast process with an average proton transfer time of 43 fs. The resultant keto conformer then undergoes torsional rotation, leading to a conical intersection that mediates the internal conversion process to the ground state. Our dynamics simulation predicted that this deactivation process occurs at a time scale beyond 600 fs of simulation time. We also explored nonradiative relaxation from the enol Franck-Condon region, and this process was found to be improbable from the static point of view at both the ADC(2) and CASPT2 levels of theory due to a high energy barrier along the torsional coordinate.

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

Silica-based materials (SBMs) are widely used in catalysis, photonics, and drug delivery. Their pores and cavities act as hosts of diverse guests ranging from classical dyes to drugs and quantum dots, allowing changes in the photochemical behavior of the confined guests. The heterogeneity of the guest populations as well as the confinement provided by these hosts affect the behavior of the formed hybrid materials. As a consequence, the observed reaction dynamics becomes significantly different and complex. Studying their photobehavior requires advanced laser-based spectroscopy and microscopy techniques as well as computational methods. Thanks to the development of ultrafast (spectroscopy and imaging) tools, we are witnessing an increasing interest of the scientific community to explore the intimate photobehavior of these composites. Here, we review the recent theoretical and ultrafast experimental studies of their photodynamics and discuss the results in comparison to those in homogeneous media. The discussion of the confined dynamics includes solvation and intra- and intermolecular proton-, electron-, and energy transfer events of the guest within the SBMs. Several examples of applications in photocatalysis, (photo)sensors, photonics, photovoltaics, and drug delivery demonstrate the vast potential of the SBMs in modern science and technology.

Excited-state intramolecular proton transfer of 2-acetylindan-1,3-dione studied by ultrafast absorption and fluorescence spectroscopy

We employ transient absorption from the deep-UV to the visible region and fluorescence upconversion to investigate the photoinduced excited-state intramolecular proton-transfer dynamics in a biologically relevant drug molecule, 2-acetylindan-1,3-dione. The molecule is a ß-diketone which in the electronic ground state exists as exocyclic enol with an intramolecular H-bond. Upon electronic excitation at 300 nm, the first excited state of the exocyclic enol is initially populated, followed by ultrafast proton transfer (≈160 fs) to form the vibrationally hot endocyclic enol. Subsequently, solvent-induced vibrational relaxation takes place (≈10 ps) followed by decay (≈390 ps) to the corresponding ground state.

An abnormally slow proton transfer reaction in a simple HBO derivative due to ultrafast intramolecular-charge transfer events

We report on the steady-state, picosecond and femtosecond time-resolved studies of a charge and proton transfer dye 6-amino-2-(2'-hydroxyphenyl)benzoxazole (6A-HBO) and its methylated derivative 6-amino-2-(2'-methoxyphenyl)benzoxazole (6A-MBO), in different solvents. With femtosecond resolution and comparison with the photobehaviour of 6A-MBO, we demonstrate for 6A-HBO in solution, the photoproduction of an intramolecular charge-transfer (ICT) process at S1 taking place in ∼140 fs or shorter, followed by solvent relaxation in the charge transferred species. The generated structure (syn-enol charge transfer conformer) experiences an excited-state intramolecular proton-transfer (ESIPT) reaction to produce a keto-type tautomer. This subsequent proton motion occurs in 1.2 ps (n-heptane), 14 ps (DCM) and 35 ps (MeOH). In MeOH, it is assisted by the solvent molecules and occurs through tunneling for which we got a large kinetic isotope effect (KIE) of about 13. For the 6A-DBO (deuterated sample in CD3OD) the global proton-transfer reaction takes place in 200 ps, showing a remarkable slow KIE regime. The slow ESIPT reaction in DCM (14 ps), not through tunnelling as it is not sensitive to OH/OD exchange, has however to overcome an energy barrier using intramolecular as well as solvent coordinates. The rich ESIPT dynamics of 6A-HBO in the used solutions is governed by an ICT reaction, triggered by the amino group, and it is solvent dependent. Thus, the charge injection to a 6A-HBO molecular frame makes the ICT species more stable, and the phenol group less acidic, slowing down the subsequent ESIPT reaction. Our findings bring new insights into the coupling between ICT and ESIPT reactions on the potential-energy surfaces of several barriers.

Slow excited state phototautomerization in 3-hydroxyisoquinoline

In the present work we report the spectral and photophysical properties of 3-hydroxyisoquinoline in various protic/aprotic solvents. Our steady state and time resolved fluorescence data indicates that in the monomer form of 3HIQ phototautomerization can take place in the excited state through excited state intramolecular proton, while as per earlier suggestions phototautomerization in 3HIQ occurs in dimer or complex (in the presence of acetic acid) form. Moreover, we find rather slow tautomerization (occurring on the nanosecond scale). It is found that proton transfer occurs both in the ground as well as excited states and is controlled by the polarity of the solvent.

Fluorescence excitation and excited state intramolecular proton transfer of jet-cooled naphthol derivatives: part 2. 2-Hydroxy-1-naphthaldehyde

The S(1)← S(0) fluorescence excitation spectrum of jet-cooled 2-hydroxy-1-naphthaldehyde (2H1N) with origin at 26,668 cm(-1) has been measured. Nine totally symmetric modes and three non-totally symmetric modes have been assigned in the excitation spectrum. Ab initio calculations indicate that 2H1N undergoes a planarity change upon excitation, which may account for the unusual intensity of non totally symmetric vibrational modes in the excitation spectrum. A number of low intensity features were observed on the low energy side of the origin which have been assigned to the 2H1N dimer rather than different ground state confomers of 2H1N. The origin of the S(1)← S(0) electronic transition of the dimer lies at ~26,401 cm(-1); combinations of two low frequency intermolecular modes of the dimer (59 cm(-1) and 17 cm(-1)) were also observed. The occurrence of excited state intramolecular proton transfer (ESIPT) in 2H1N cannot be proven on the basis of this work. A comparison of the (photo)physical properties of 2H1N with 1-hydroxy-2-naphthaldehyde (1H2N) [A. McCarthy and A.A. Ruth, PCCP, 2011, 13, 7485-7499 (Part 1)], however, indicate the plausibility of an ESIPT process in 2H1N. The strength of the intramolecular hydrogen bond (IMHB) in 2H1N was computed as ~10.6 kcal/mol, a value comparable to the IMHB strength of 1H2N. The establishment of a lower limit on the state lifetimes of 2H1N, of ~1.8 ps, indicates that any proposed ESIPT reaction in 2H1N may not proceed barrierlessly. Above an excess energy of ~1000 cm(-1), the intensity of the fluorescence excitation spectrum reduces significantly, indicating the onset of a non-radiative decay mechanism.

DFT based computational study on the excited state intramolecular proton transfer processes in o-hydroxybenzaldehyde

Potential energy (PE) curves for the intramolecular proton transfer in the ground (GSIPT) and excited (ESIPT) states of o-hydroxybenzaldehyde (OHBA) were studied using DFT-B3LYP/6-31G(d) and TD-DFT-B3LYP/6-31G(d) level of theory, respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer in this compound. Excited states PE calculations support the ESIPT process in OHBA. The contour PE diagram and the variation of oscillator strength along the proton transfer co-ordinate support the dual emission in OHBA. Our calculations also support the experimental observations of Nagaoka et al. [S. Nagaoka, U. Nagashima, N. Ohta, M. Fujita, T. Takemura, J. Phys. Chem. 92 (1988) 166], i.e. normal emission of the title compound comes from S(2) state and the red-shifted proton transfer band appears from the S(1) state. ESIPT process has also been explained in terms of HOMO and LUMO electron density of the enol and keto tautomer of OHBA and from the potential energy surfaces.

Chemical and biological caging effects on the relaxation of a proton-transfer dye

We report studies of the interaction between a proton-transfer dye (1'-hydroxy,2'-acetonaphthone, HAN), with the human serum albumin (HSA) protein and a beta-cyclodextrin derivative (DM-beta-CD) in neutral water solutions. We used steady-state and picosecond time-resolved emission spectroscopy to follow the structural changes of HAN due to the hydrophobicity and confinement effect of these nanocavities. Upon encapsulation, the fluorescence intensity of the 1:1 inclusion complex in both cavities increases, and the emission lifetimes become longer. For the DM-beta-CD complexes, we obtained 430 and 920 ps, whereas for the HSA complexes we obtained 630 ps and 2 ns. Picosecond anisotropy measurements show strong confinement due to protein docking. The rotational time for the CD complex is 660 ps, whereas for the protein complex we find 6 ns. The process of energy transfer from the excited triptophan 214 (Trp214) of HSA to the trapped HAN occurs with high efficiency (71%), and the calculated distance between both chromophores is 17 A. We believe that the results are important for a better understanding of the processes occurring in inclusion complexes such as those in nanopharmacodynamics.

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.

Ultrafast excited-state dynamics in photochromic N-salicylideneaniline studied by femtosecond time-resolved REMPI spectroscopy

Ultrafast processes in photoexcited N-salicylideneaniline have been investigated with femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The ion signals via the S(1)(n,pi( *)) state of the enol form as well as the proton-transferred cis-keto form emerge within a few hundred femtoseconds after photoexcitation to the first S(1)(pi,pi( *)) state of the enol form. This reveals that two ultrafast processes, excited-state intramolecular proton transfer (ESIPT) reaction and an internal conversion (IC) to the S(1)(n,pi( *)) state, occur on a time scale less than a few hundred femtoseconds from the S(1)(pi,pi( *)) state of the enol form. The rise time of the transient corresponding to the production of the proton-transferred cis-keto form is within 750 fs when near the red edge of the absorption is excited, indicating that the ESIPT reaction occurs within 750 fs. The decay time of the S(1)(pi,pi( *)) state of the cis-keto form is 8.9 ps by exciting the enol form at 370 nm, but it dramatically decreases to be 1.5-1.6 ps for the excitation at 365-320 nm. The decrease in the decay time has been attributed to the opening of an efficient nonradiative channel; an IC from S(1)(pi,pi( *)) to S(1)(n,pi( *)) of the cis-keto form promotes the production of the trans-keto form as the final photochromic products. The two IC processes may provide opposite effect on the quantum yield of photochromic products: IC in the enol form may substantially reduce the quantum yield, but IC in the cis-keto form increase it.