Authors: Adina I. Ciuciu, Kamil Skonieczny, Dominik Koszelewski, Daniel T. Gryko, Lucia Flamigni
The enol-keto excited state dynamics of a series of emission tunable imidazole derivatives undergoing excited state intramolecular proton transfer (ESIPT) has been 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, has lifetimes ranging from less than 10 ps to ca.100 ps and the resulting keto forms deactivate with lifetimes lower than 100 ps up to few nanoseconds. Delayed luminescence detection at 77 K in solid glasses allows to identify the phosphorescence of the enolic form and, in a few cases, a P-type delayed fluorescence is also evidenced. The phosphorescence lifetimes are in the range of seconds at 77 K. The enolic triplet excited state absorption at RT, determined by nanosecond laser flash-photolysis, displays a maximum around 460-500 nm and lifetimes of the order of tens of microseconds. In a few cases a broad band with maximum around 420 nm is detected and tentatively ascribed to the triplet excited state of the keto form. Reaction rates with oxygen of the order of 2 - 4 × 109 M-1 s-1 are measured.