Effect of graphene substrate on the spectroscopic properties of   photovoltaic molecules: role of the in-plane and out-of-plane pi-bonds
  The electronic structure of pentacene decorated with dipole groups (d-pentacene) and adsorbed onto a graphene substrate has been studied within the density functional theory. Three reference configurations have been considered, namely the ideal molecule without distortions, the actual molecule including intramolecular distortions and the molecule adsorbed onto graphene. Calculations show a noticeable charge redistribution within the d-pentacene + graphene system due to molecular distortion, as well as the formation of weak $\pi$-bonds between the molecule and the substrate. Additionally, the effect of the chemical modification of the terminal saturation with --H by --OH and =O is checked to explore the possibility of "levels engineering". The imaginary part of the dielectric function of d-pentacene in the ideal and distorted conformations and the molecule adsorbed at graphene were calculated within the random phase approximation. Results show that, even though molecular distortions change apreciably the absorption spectrum of isolated d-pentacene, the adsorbed molecule exhibits an optical spectrum which mimics quite much that of single graphene. 
M. Wawrzyniak-Adamczewska, M. Wierzbowska and J. J. Melendez
cond-mat.mtrl-sci