PROTON-TRANSFER SPECTROSCOPY OF 2-(2'-HYDROXYPHENYL)IMIDAZOLE AND 2-(2'-HYDROXYPHENYL)BENZIMIDAZOLE DYES

The spectral parameters of the fluorescence emission, produced during the photoinduced intramolecular transfer of a proton (or hydrogen atom) in derivatives of 2-(2'-hydroxyphenyl)imidazole and 2-(2'-hydroxyphenyl)benzimidazole, were studied in several solvents at room temperature. In dry, non-polar solvents, only a single fluorescence band is observed, in the blue-green range, and is attributed to the tautomer produced by the shift of a hydrogen atom along the intramolecular hydrogen bond joining the phenolic oxygen and the imino N atom. In weak hydrogen-bonding solvents an additional UV (approximately 350 nm) band can be detected in both families of dyes. This is due to a small fraction of ground state open-enol form, which does not yield the excited state phototautomerization because the intramolecular hydrogen bond is missing. Several chromophores described here present good properties as ''proton transfer'' lasers, when pumped with pulses from an XeCl (308 nm) laser, e.g. the compound 4,5-dimethyl-2-(2'-hydroxyphenyl)imidazole, which shows an energy efficiency of 12%. The synthetic and analytical details of all the dyes studied are described.