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'-hydroxyphe nyl)benzimidazole, were studied in several solvents at room temperatur e. In dry, non-polar solvents, only a single fluorescence band is obse rved, in the blue-green range, and is attributed to the tautomer produ ced by the shift of a hydrogen atom along the intramolecular hydrogen bond joining the phenolic oxygen and the imino N atom. In weak hydroge n-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 p hototautomerization because the intramolecular hydrogen bond is missin g. Several chromophores described here present good properties as ''pr oton transfer'' lasers, when pumped with pulses from an XeCl (308 nm) laser, e.g. the compound 4,5-dimethyl-2-(2'-hydroxyphenyl)imidazole, w hich shows an energy efficiency of 12%. The synthetic and analytical d etails of all the dyes studied are described.