Proximity effects in the excited state ordering and photophysics of thienyl-pyridyl ketones

Abstract

The triplet states of the six thienyl-pyridyl ketone isomers were investigated by stationary and time-resolved absorption and emission techniques. Phosphorescence was observed in both fluid solutions and frozen glasses. The intersystem crossing yields are unitary. In a rigid matrix (EPA) at low temperature (77 K), two types of phosphorescence emission were observed, depending on the position of the thienyl group. A short-lived one ( τ≈5 ms) was detected for the 3,n′-TPKs and assigned to n,π* transition; a longer-lived one ( τ≈100 ms) was observed for the 2,n′-TPKs and assigned to π,π* transition. In an EPI matrix at 77 K, the 2,n′-TPKs followed biexponential decay kinetics with a short-lived component ( τ≈5 ms) and a longer-lived component ( τ≈15-40 ms). From the emission excitation spectra, the spin-forbidden S 0→T 1 and S 0→T 2 transitions were detected. Spectra and decay kinetics of the transients produced by nanosecond laser flash photolysis and their reactivity in self-quenching processes indicate that the behavior of the 2,n′-TPKs is substantially different from that of the 3,n′-TPKs. The triplet state properties of these molecules are determined by the balance of the opposite effects of the thienyl (electron-donating) and pyridyl (electron-withdrawing) rings which cause the energies of the lowest n,π* and π,π* states to be very close. Consequently, for the 2,n′-TPKs, an excited state switching occurs going from the singlet (n,π*) to the triplet (π,π*) manifold. Due to the proximity of the lowest triplet states of different configuration, some unusual features were observed, such as room-temperature phosphorescence from molecules having the lowest π,π* triplet states and absorption spectra from T 1 and T 2 simultaneously populated under laser excitation.