In this article, we present quantum chemical calculations, based on density functional theory (DFT), performed to investigate the geometries and the opto-electronic properties of a new synthesized graft copolymer based on poly(N-vinylcarbazole) (PVK) and poly(3-methylthiophene) (PMeT) named PVK-3MeT. First, we have theoretically computed and compared the structural, optical, and vibrational parameters of both neutral and doped states. In addition, the excited state was theoretically obtained by the ab initio RCIS/STO-3G method. To assign the absorption and emission peaks observed experimentally, we computed the energies of the lowest singlet excited state with the time-dependent density functional theory (TD-DFT) method. Electronic parameters such as the HOMO-LUMO band gap, the ionization potential (IP), and electron affinity (EA) are extracted. Calculations show that the PVK-3MeT copolymer is nonplanar in its ground neutral state. Meanwhile, upon doping or photoexcitation, an enhancement of the planarity is observed, resulting on a decrease of the inter-ring torsion angle between 3-methylthiophene units. Such modifications in the geometric parameters induce a dramatic change on the HOMO and LUMO orbitals in the doped or excited states. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.