Photophysics and spectroscopy of tropolone and its van der Waals complexes

Abstract

Tropolone (TRN), a pseudo-aromatic molecule which undergoes intramolecular proton transfer in both its ground and excited states, has been chosen as a model for the study of excited state proton transfer processes in polyatomic molecules. TRN is an excellent model for studying the mechanism and dynamics of intramolecular excited state proton transfer because of its simple chemical structure, photochemical stability, and readily observable tunneling doublet splittings whose magnitudes are dependent on isotopic substitution, vibrational excitation, and solvation. The spectroscopy and photophysics of TRN in the vapour phase, in a variety of solvents, and in microscopic van der Waals solvent clusters have been investigated. Energy and time resolved spectroscopic techniques have been used to study the first excited singlet state of TRN in a supersonic jet expansion. The lifetime of TRN excited to the origin of the S1 1([pi],[pi]*) state is 1.17 ns and decreases rapidly with excess vibrational energy in the excited state. The data are interpreted in terms of an increased non-radiative decay rate which is a result of perturbations in the S1 potential energy surface resulting from enhanced vibronic coupling to a nearby 1(n,[pi]*) state. The absorption, emission, and excitation spectra of TRN have been recorded in a variety of polar, non-polar, and hydrogen bonding solvents and the fluorescence quantum yields have been determined. In perfluoro-n-hexane and in aqueous solution the lowest energy singlet state is of ([pi],[pi]*) character. In n-hexane, carbon tetrachloride, acetonitrile and methanol solutions it is postulated that an inversion of the two lowest excited singlet states of TRN occurs and that the lowest energy singlet state is of (n,[pi]*) character. The structures and excited state proton transfer properties of the vdW complexes of TRN with CO, n-alkanes, perfluoro-n-alkanes, CFH3, CF2H2, CF3H, and CO2 have been investigated using LIFE spectroscopy and empirical Lennard-Jones and ab initio theoretical methods. The solvent molecules are found to bind to TRN either by primarily dispersive intermolecular forces above the plane of the seven-membered ring, or by hydrogen bonding to the hydroxyl and keto moieties of the chromophore.