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

Abstract

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 3–21G and 6–31G∗ 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.