Theoretical and jet spectroscopic investigations of energetics and structures for the low-lying singlet states of fluorene and 9,9′-spirobifluorene

Ab initio, semiempirical and spectroscopic studies of fluorene (FR) and 9,9′-spirobifluorene (SBF) were performed to elucidate π-orbital interaction between two fluorenyl rings of SBF and to predict the energies of the low-lying singlet electronic states of the molecules. Energies and symmetries of π-orbitals of FR and SBF molecules were determined by 3-21 G and 6-31 G*calculations on the optimized structures. The INDO/S-CIS semiempirical method was applied to predict the excited state energies, the transition dipole moments and the oscillator strengths for the optical transitions. Laser-induced fluorescence (LIF) excitation spectra were measured for FR and SBF cooled in pulsed supersonic expansions of He in the ranges 283.7-296.7 nm and 289.1-305.6 nm, respectively. In the LIF excitation spectra of FR and SBF, highly resolved vibronic bands were observed having the band origins of 33791 and 33047 cm^-1, respectively. The spectral shift of the 0-0 band of SBF to red by 744 cm^-1 may be attributed to the spiroconjugation arising from the interaction of four p_π orbitals in the different planes.