Temperature-dependent study of the ground-state reverse proton transfer of 3-hydroxyflavone

Ultrafast Ground-State Intramolecular Proton Transfer in Diethylaminohydroxyflavone Resolved with Pump-Dump-Probe Spectroscopy

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.

Empirical valence bond models for reactive potential energy surfaces: a parallel multilevel genetic program approach

We describe a new method for constructing empirical valence bond potential energy surfaces using a parallel multilevel genetic program (PMLGP). Genetic programs can be used to perform an efficient search through function space and parameter space to find the best functions and sets of parameters that fit energies obtained by ab initio electronic structure calculations. Building on the traditional genetic program approach, the PMLGP utilizes a hierarchy of genetic programming on two different levels. The lower level genetic programs are used to optimize coevolving populations in parallel while the higher level genetic program (HLGP) is used to optimize the genetic operator probabilities of the lower level genetic programs. The HLGP allows the algorithm to dynamically learn the mutation or combination of mutations that most effectively increase the fitness of the populations, causing a significant increase in the algorithm's accuracy and efficiency. The algorithm's accuracy and efficiency is tested against a standard parallel genetic program with a variety of one-dimensional test cases. Subsequently, the PMLGP is utilized to obtain an accurate empirical valence bond model for proton transfer in 3-hydroxy-gamma-pyrone in gas phase and protic solvent.

Energy transfer, charge transfer, and proton transfer in molecular composite systems

Models for the fundamental mechanisms of excitation energy transfer, including cases involving singlet oxygen states, twisting-intramolecular-charge-transfer (TICT) states, and intramolecular proton transfer, are described in terms of elementary concepts and energy diagrams. Three limiting cases of energy transfer are distinguished, Davydov free excitons (Simpson and Peterson strong coupling) and localized excitons (weak coupling), and the Förster mechanism of vibrational-relaxation energy transfer. The prominent rôle of the singlet molecular oxygen states is described, together with the rôle of simultaneous transitions for molecular oxygen pairs. The origin of sudden polarization via the TICT-state potential is discussed and the generality of this phenomenon emphasized. The intramolecular proton-transfer phenomenon is outlined, and its role in molecular excitation transient phenomena is described. The complex interaction of all of these excitation mechanisms in determining photochemical and radiation chemical pathways is suggested.