Dynamics of Intramolecular Excited State Proton Transfer in Emission Tunable, Highly Luminescent Imidazole Derivatives

The enol-keto excited state dynamics of a series of emission tunable imidazole derivatives undergoing excited state intramolecular proton transfer (ESIPT) were determined by means of steady state and time-resolved spectroscopic techniques in different solvents at room temperature and at 77 K. Examination of the corresponding non-ESIPT compounds, with the proton transfer function deliberately blocked, was carried out for comparison. At room temperature, the ESIPT process in the examined samples, determined by picosecond streak camera experiments, had lifetimes ranging from less than 10 ps to ca. 100 ps, and the resulting keto forms deactivated with lifetimes less than 100 ps up to a few nanoseconds. Delayed luminescence detection at 77 K in solid glasses allowed the identification of the phosphorescence of the enolic form and, in a few cases, P-type delayed fluorescence was also seen. The phosphorescence lifetimes were in the range of seconds at 77 K. The enolic triplet excited state absorption at RT, determined by nanosecond laser flash-photolysis, displayed 460-500 nm and lifetimes on the order of tens of microseconds. In a few cases, a broad band with a maximum around 420 nm was detected and tentatively ascribed to the triplet excited state of the keto form. Reaction rates with oxygen on the order of (2-4) x 10(9) M-1 s(-1) were measured.