Photophysical properties of novel [1,2,3]triazolo[4,5-d] pyridazine derivatives

Photostability, fluorescence properties and triplet yield of [1,2,3]triazolo[4,5-d]pyridazine derivatives have been studied in various solvents. The energy of the lowest singlet excited state significantly decreases in the series of 1, 2, and 3, and the latter compound emits negligible fluorescence. All substances are photochemically very stable. No photodecomposition occurs from singlet excited state and the triplet molecules do not react with alcohol showing that the lowest triplet state has ππ∗ character. Internal conversion is found to be the dominant singlet excited state depopulating process. The short fluorescence lifetime of 2 is attributed to the particularly rapid non-radiative transition to the ground state. Both 1 and 2 readily form hydrogen-bonded complex with hexafluoroisopropanol. Light absorption does not enhance the hydrogen bonding power, therefore; no fluorescence quenching but fluorescence yield enhancement is observed.