Laser spectroscopic studies of compounds undergoing an intramolecular excited state proton transfer reaction
A. Vdovin,   E. Luzina
The project main topic is the investigation of phenyl-oxazole family of compounds, possessing the proton donor and acceptor centers within one molecule. Optical excitation in these systems results in the redistribution of the electronic charges and fast reorganization of molecular structure leading to a tautomeric form (excited state intramolecular proton transfer reaction, ESIPT).

The goal of the project is to study the vibronic spectra of the series of phenyloxazole derivatives and their van der Waals complexes with solvent molecules in supersonic molecular beams. The aim is to test the influence of substituency and complexation on the spectroscopic and dynamical aspects of proton transfer process.
Among the methods applied are laser induced fluorescence (LIF) excitation and emission spectroscopy, two-color fluorescence depletion spectroscopy, and resonance enhanced two-color photoionization (R2PI) spectroscopy with time-of-flight mass-selective ion detection. The molecules are investigated under conditions of supersonic jet cooling.
The DBP molecule, the close derivative of BBHQ, exhibits the single largely Stokes-shifted fluorescence; excitation spectrum is characterized by a broad fluorescence background underlying some weak, sharp features [ 1 ].  A spectacular deuterium substitution effect has been observed: after deuteration of phenolic group a new, well-structured spectrum with reduced background appears. From the analysis of the width of the vibronic bands of DBP-d1 the upper limit of the excited state deuteron transfer rate constant have been estimated as kDT=1.7x1012 s-1.
Vibronic spectra of BBHQ molecule, its deuterated forms and van der Waals complexes with argon atoms have been studied using R2PI techniques with mass-selective detection in time-of-flight mass spectrometer [to be publ.]. Ionization thresholds of deuterated forms and molecular complexes have been obtained for the first time. Combined application of double resonance vibronic hole-burning technique and R2PI spectroscopy allowed to perform the complete analysis of vibrational spectra of mono- and doubly- deuterated BBHQ molecule. Mass-selective spectra of van der Waals complexes of BBHQ with argon atoms allow to analyze the homogeneous broadening of spectral lines of individual complexes and estimate the rate of proton transfer reaction.
These studies are of importance in understanding the dynamics of the excited state proton transfer reaction in molecular clusters. The obtained results will be also a challenge for the forthcoming studies on proton transfer dynamics of BBHQ by femtosecond pump-probe two photon ionization experiment. Laser spectroscopy studies of other dibenzoxazolylphenol derivatives and their molecular complexes are now in progress.

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