Radiationless processes in antiaromatic molecules: The photophysics of s-indacene studied by ultrashort light pulses

Abstract

1,3,5,7-tetra-tert-butyl-indacene (TTBI), a non-alternant antiaromatic hydrocarbon, is investigated in solution by steady-state and time-resolved spectroscopy. A weak symmetry-forbidden S 0→S 1 absorption is observed in the wavelength range from 600 to 1100 nm. Intense S 0→S 2 and S 0→S n bands occur around 500 nm and in the ultraviolet. The S 2 level shows fluorescence with a quantum yield of 9×10 −5. Internal conversion directly to the ground state represents the main radiationless deactivation process of the S 1 and S 2 states resulting in short lifetimes of 18 and 2.5 ps, respectively. The radiationless S 2→S 0 decay channel is more effective than S 2→S 1 internal conversion as supported by CNDO/SDCI calculations of the molecular geometry in the S 0, S 1 and S 2 states.