Optical absorption and photoluminescence in pristine and photopolymerizedC60solid films

The optical absorption (OA) and photoluminescence (PL) spectra of pristine, oxygen-free ${\mathrm{C}}_{60}$ films in the vicinity of the absorption edge across the highest-occupied-molecular-orbital to lowest-unoccupied-molecular-orbital (HOMO-LUMO) gap are studied to elucidate the nature of these electronic states. Structures observed in both the OA and PL spectra are identified with Herzberg-Teller vibronic coupling of the respective singlet electronic states to intramolecular vibrational modes. Similarities and differences among the PL and OA spectra of ${\mathrm{C}}_{60}$ in solution, solid films, and single crystals are discussed. The larger linewidth of the ${\mathrm{C}}_{60}$ film spectra is attributed to the presence of defects, presumably associated with numerous grain boundaries. Finally, the OA and PL spectra of photopolymerized ${\mathrm{C}}_{60}$ solid films are presented. The features in both the OA and PL spectra are observed to broaden considerably. In addition, two of the OA bands blueshift slightly (400–800 ${\mathrm{cm}}^{\mathrm{-}1}$) and the PL features downshift by ∼330 ${\mathrm{cm}}^{\mathrm{-}1}$ upon polymerization. These results are consistent with the intermolecular oligomerization bond disorder and reduced symmetry of the ${\mathrm{C}}_{60}$ shell imposed by the covalent bonds between ${\mathrm{C}}_{60}$ molecules in the polymeric phase.