Optical properties of conjugated poly-phenylene-vinylene polymers

ABSTRACT Electron energy structure and optical response functions of semiconducting conjugated polymers, Poly-Phenylene-Vinylenes (PPV) are studied by first principle modeling ba sed on the density functional theory (DFT). Equilibrium atomic geometries are obtained throug h total energy minimization. Electron en ergy structure and optical properties are calculated using generalized gradient approximation method with ab initio pseudopotentials. Predicted data are compared with measured optical absorp tion spectra of PPV. Dominant contribution of the optical excitations related to the delocalized -electrons within the PPV chains is demonstrat ed by comparative analysis of experimental and theoretical data. Optical absorption spectra of acceptor type PPV chains show substantial blue shift with respect to the donor type of the PPV polymer. Comparison between calc ulated and measured optical spectra demonstrates that the shift could be attributed to the structural deformations of the benzene rings of the host PPV unit due to the interaction with the atoms of the derivative groups. Keywords: polymers, first principle modeling, geometry optimization, optical absorption INTRODUCTION The Kohn-Sham density functional theory (DFT) realistically pr edicts electronic structure of different systems, such as atoms, molecules, and solids [1 to 3]. Various generalized gradient approximation (GGA) methods describing exchange and correlation (XC) interaction have been shown to systematically improve the local density approximation (LDA) predictions of equilibrium atomic geometries [3]. In atomic systems (like large single molecules, inorganic solids etc.) the geometry optimization study primarily requires accurate prediction of the ground state which is well reproduced within a standard DFT approach. Electronic excitations are key points of most of the commonly measured optical spectra of organic materi als (see [4] and references therein). The first principle studies of excited states however requir e much larger effort than computations of the ground state. The DFT eigen energies underestimate gaps between bonding and anti-bonding states which requires quasi-particle (QP) correction [2,3]. It has been proved that in inorganic solids [5] and organic polymers [2] the QP correction results in substantial overestimates of the gaps. The situation is substantially improved by additional inclusion of excitonic (many-body) interaction into Hamiltonian of the system [2,3]. This st ate-of-the-art DFT+QP many-body theory predicts excitation energies in organic polymers with a g ood accuracy [2]. Such ap proach however requires large scale computations and it is very time consuming. On the other hand the corrections for QP and exciton interaction in electron eigen energies could be included through the scissors operator [3 ,6]. This substantially simplifies optical calculations still presenting the advantages of first principle theory for understanding at microscopic level the physics and chemistry of very complex systems. In this work a first principle method to predict optical functions of complex organic molecules and molecular complexes based on ab initio pseudopotential (PP) theory is described. Corrections for QP shift and excitonic effects are incorporated through the scissor operator. This method could be used to study electronic structure and optical functions of complex molecules and complexes which is demonstrated for organic polymer calculations. COMPUTATIONAL METHOD Optical absorption spectra of organic conj ugated PPV polymers w ith linear decacyl (R=C