TRANSIENT ABSORPTION OF STRONGLY FLUORESCENT PYRYLIUM-SALTS

The pyrylium salts are strongly fluorescent compounds which show a relatively weak intersystem crossing to the triplet state. The later has been characterized for compounds 1-6 by time-resolved laser spectroscopy of their triplet-triplet absorption; it appears that the absorption corresponding to the transition of lowest energy T1-T2 occurs at wavelengths higher than 700 nm. Several pyrylium salts show a concentration-dependent, but not an excitation intensity-dependent one, variation of their triplet lifetime, a process typical of a self-quenching reaction and not of a triplet-triplet annihilation one. The rate constant of this reaction varies from 2.5 10(7) M-1 s-1 for 3 to 4.2 10(8) M-1 s-1 for 6 and, depending on the compound, the lifetime of the triplet species at infinite dilution varies between 13 and > 150 mu-s. A self-quenching process of the singlet excited state also occurs for compound 6, but not for the others; this has been attributed to the presence of the electron-donating OCH3 group which interacts with the pyrylium ring of an other molecule.