Electron-phonon coupling in negatively charged acene- and phenanthrene-edge-type hydrocarbon crystals

Vibronic interaction and its role in the occurrence of possible superconductivity in the monoanions of phenanthrene-edge-type aromatic hydrocarbons are studied. The vibrational frequencies and the vibronic coupling constants are computed and analyzed and the electron-phonon coupling constants are estimated. The results for phenanthrene-edge-type hydrocarbons are compared with those for acene-edge-type hydrocarbons. The lowest frequency mode and the C-C stretching modes of 1400-1600 cm(-1) afford large electron-phonon coupling constants in the monoanions of acene- and phenanthrene-edge-type hydrocarbons. The total electron-phonon coupling constants decrease with an increase in the number of carbon atoms in both acene- and phenanthrene-edge-type hydrocarbons, but those for the monoanions of phenanthrene-edge-type hydrocarbons are larger than those for the monoanions of acene-edge-type hydrocarbons. Possible superconducting transition temperatures T(c)s for the monoanions are estimated. The monoanions of phenanthrene-edge-type hydrocarbons would have higher T(c)s than the monoanions of acene-edge- type hydrocarbons if phenanthrene-edge-type hydrocarbons exhibit superconductivity. These results suggest that molecular edge structures as well as molecular sizes have relevance to the strength of electron-phonon coupling and T(c)s. The fragment molecular-orbital method (FMO) method successfully characterizes the distinct electronic structures of the two small polynuclear aromatic hydrocarbons (PAHs) with different type of edges such as anthracene and phenanthrene. (C) 2002 American Institute of Physics.