Vibrational, electronic, and vibronic excitations of polar C60F18 molecules: Experimental and theoretical study

Polarized optical conductivity spectra in the IR and visible range and variable-temperature photoluminescence spectra of the C60F18 single crystals in the 4-300 K temperature range are reported. Density-functional theory (DFT) calculations of vibrational spectra and time-dependent DFT calculations of the excitation energies of the C60F18 molecule are performed to interpret experimentally observed phenomena. Orientation of the C-3v-symmetric C60F18 molecules in the single crystal is revealed by comparison of the experimental polarized IR spectra and vibrational DFT calculations. A(2) symmetry is assigned to the lowest energy singlet excited state of C60F18, and hence pure electronic S-0 -> S-1 excitation is found to be dipole forbidden. Fine vibronic structure found at low temperature is interpreted in terms of Herzberg-Teller and Franck-Condon mechanisms. Considerable similarity with vibronic spectra of C-60 is found and explained by presumable localization of the lowest energy electronic excitation of C60F18 on the fullerenelike part of the molecule.