Journal of Physical Chemistry A, Vol.108, No.40, 8279-8286, 2004
Excited states of bridged [14]annulenes
Fluorescence and Raman spectra of two representative bridged [14]annulenes, i.e., 1,6:8,13-ethanediylidene[14]annulene and 1,6:8,13-propanediylidene[14]annulene (1 and 2 in Figure 1, respectively) have been measured. The fluorescence spectra show a resolved Franck-Condon structure assigned to low-frequency totally symmetric modes and their overtones/combinations. The Raman spectra have been measured using several excitation wavelengths in pre- and/or near-resonance conditions with respect to the La and B-a states. A(1) and a(2) vibrational modes are resonantly enhanced in both cases. Excited-state equilibrium geometries and energies of 1 and 2 have been calculated by means of the MCSCF/CAS A initio procedure. Franck-Condon factors have been determined coupling structural data with normal mode displacements. On this basis fluorescence and Raman excitation profiles are reasonably well fitted to the experiment. It is concluded that the L-b, L-a, and B,, states of 1 and 2 have equilibrium structures accurately described by the present calculations.