Lattice vibrations of γ - and β -coronene from Raman microscopy and theory

We combine polarization-resolved low-frequency Raman microscopy and dispersion-corrected density-functional calculations (DFT-D3) to study polymorph-dependent lattice vibrations in coronene, a model molecular system for nanographenes and disklike organic semiconductors that exhibits two crystalline structures with distinct electronic and optical properties. Changes in low-energy Raman-active lattice phonons are followed across the γ- to β-phase transition at 150 K. Raman frequencies calculated using DFT-D3 agree to within 4 cm-1, and on the basis of polarization dependence of peak positions and intensities we achieve a clear mode assignment. Studies of the associated atomic motions show how the pure librational and rotational modes of γ-coronene change into mixed roto-librations in the β-phase, explaining the remarkable differences in Raman spectra of the two phases.