Excited states of s-Indacene

The electronic properties of 1,3,5,7-tetra-tert-butyl-s-indacen(eT TBI), 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 e550 and 320 nm, show Franck-Condon structures suggesting change of excited-state geometry with respect to SO. Large enhancement factors have been measured for the most active Raman modes when the excitation frequency goes through the S2 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 S2 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.