IR spectra of photopolymerized C₆₀ films. Experimental and density functional theory study

IR spectra of photopolymerized fullerene films obtained by simultaneous deposition and UV irradiation were measured in the range of 1500-450 cm ^-1. The degree of the polymerization of the G₆₀ films was estimated to be about 95%. To assist the assignment of the experimental IR spectra of the films, quantum chemical calculations of the equilibrium structures of the C₆₀ dimers and trimers were performed at the DFT(B3LYP)/ 3-21G level of theory. Next, IR frequencies and intensities for those structures were calculated. For the five-trimer structures found in the calculations, the relative stabilities were determined at the B3LYP/4-31G * and B3LYP/6-31G* levels and used to select the lowest-energy trimers, which are Trimer A (angle between monomer centers is 90°) and Trimer B (angle between monomer centers is 120°). Next, the IR spectra of the polymerized fullerene films were compared with the calculated frequencies of the lowest-energy dimer and the two lowest-energy trimers. On the basis of this analysis and on the comparison of the film spectra with the IR spectra of the C₆₀ dimer and trimer spectra obtained by other methods, it was shown that the main components of the films are C₆₀ dimers and the orthorhombic (O) polymer phase. The tetragonal (T) and rhombohedral (R) polymers, as well as small amounts of monomers, were also found. Although vibrational frequencies of different C₆₀ phases are similar in most cases, we found several unique spectral features of the C₆₀ dimer and other polymers that may be used to determine the composition of the polymerized C₆₀ film.