Spectroscopy and photophysics of tropolone in condensed media

Abstract

A systematic study of the spectroscopy and photophysics of tropolone in condensed media has been undertaken by measuring its UV–visible absorption, emission and emission–excitation spectra in a number of solvents of varying structure, at temperatures between 77 K and 295 K, and by measuring its quantum yields of emission, ϕem, and time-resolved emission decays as a function of temperature, T, and excitation wavelength, λex, in each solvent. In weakly interacting solvents, such as 3-methylpentane and perfluoro-2-n-butyltetrahydrofuran, the only species yielding significant fluorescence is the S1, 1(π,π*) state of the intramolecularly hydrogen-bonded neutral molecule. In acidic aqueous solution at pH <3, the emitting state is also 1(π,π*) S1, but the intramolecular hydrogen bond has been disrupted by the solvent. In basic aqueous solution at pH >8 the emitting species is the corresponding excited anion. In some solvents, emission from photochemical products prevents reliable measurements from being made at room temperature. Contrary to previous reports, no emission from solvent-stabilised 1(n,π*) excited states is observed. In glass-forming media the fluorescence quantum yields and the lifetimes of the emitting species increase in a sigmoidal fashion as the temperaure decreases, and are a factor of more than 100 greater at 77 K than at room temperature. The fluorescence quantum yields also decrease with increasing excitation energy in the UV, an effect which is most pronounced in more weakly interacting solvents at temperatures near room temperature. A mechanism is proposed. Both the spectra and the excited state temporal decays indicate that emission from short-lived, vibrationally unrelaxed S1 constitutes the majority of the fluorescence at room temperature, a small fraction of the total emission at 77 K, and an increasing fraction of the total emission with decreasing excitation wavelength. The variations of ϕem and lifetime with temperature are attributed to the effects of temperature-dependent solvent relaxation dynamics on the non-radiative decay, based on qualitative correlations between the observed parameters describing the effects in different media and known characteristics of the media, including their glass transition temperatures and polarity.