Excited states of s-indacene

The electronic properties of 1,3,5,7-tetra-tert-butyl-s-indacene (TTBI); the stable derivative of antiaromatic s-indacene, have been studied via absorption and resonance Raman spectroscopy at low temperature. The two strongest absorption regions, at approximate to 550 and 320 nm, show Franck-Condon structures suggesting change of excited-state geometry with respect to S-o Large enhancement factors have been measured for the most active Raman modes when the excitation frequency goes through the S-2 vibronic structure. By means of semiempirical QCFF-PI calculations ground- and excited-state optimized geometries and vibrational frequencies have been obtained for a quantitative estimate of vibronic Franck-Condon activities in the S-2 and S-6 states. The effect of normal-mode rotation in the excited state (Duschinsky effect) and alkyl substitution on the electronic and resonance Raman spectra of TTBI is discussed.