Blue-green emitting sulphonamido-imidazole derivatives: ESIPT based excited state dynamics

Abstract

Six imidazole derivatives characterized by the presence in their molecules of a sulphonamido group and able to display excited state intramolecular proton transfer (ESIPT) have been synthesized in a straightforward manner and the dynamics of their excited states investigated in detail in solvents of different polarity and proticity: toluene (TOL), dichloromethane (DCM) and methanol (MeOH). With the exception of one compound, these ESIPT-capable molecules are highly luminescent. The major emitting species at room temperature is the ketimine (K) tautomer but the weak emission from the enamide (E) form was detected in several cases. In general, the luminescence quantum yields (ϕ_fl) of the K form range between 0.4 and 0.6 with lifetimes of several nanoseconds, with radiative rate constants k_r of the order of 10⁸ s⁻¹. The lifetime of the E form, and hence of the ESIPT process, range between τ <10 ps up to 190 ps. At 77 K, in addition to hypsochromically shifted fluorescence bands from the E and/or K tautomers, E phosphorescence emissions with lifetimes in the range of seconds (0.4–2.5 s) are also detected. The triplet excited state absorbance of these molecules was probed in DCM and MeOH and both E (between 450–520 nm) and K (at about 420 nm) forms were identified. The triplet lifetimes at room temperature in air-free solutions are in the microsecond range, whereas the reaction rates with oxygen are of the order of 10⁹ M⁻¹ s⁻¹.