RADIATIONLESS PROCESSES IN ANTIAROMATIC MOLECULES - THE PHOTOPHYSICS OF S-INDACENE STUDIED BY ULTRASHORT LIGHT-PULSES

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 S0→S1 absorption is observed in the wavelength range from 600 to 1100 nm. Intense S0→S2 and S0→Sn bands occur around 500 nm and in the ultraviolet. The S2 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 S1 and S2 states resulting in short lifetimes of 18 and 2.5 ps, respectively. The radiationless S2→S0 decay channel is more effective than S2→S1 internal conversion as supported by CNDO/SDCI calculations of the molecular geometry in the S0, S1 and S2 states. © 1990.