Model ab initio study of excess charge carrier solvation on conjugated carbon chains.

摘要

When an excess charge carrier (electron or hole) is added to a semiconducting polymer chain, it is well known that the carrier may self-trap into a polaronic state accompanied by a bond length adjustment pattern. A different mechanism of self-localization is the solvation of charge carriers expected to take place when the polymer chain is immersed in polar media such as common solvents. We use density-functional computations in conjunction with the Polarizable Continuum Model to unequivocally demonstrate solvation-induced self-consistent charge localization into large-radius one-dimensional (1D) polarons on long CNH2 carbon chains with the polyynic structure. Within the computational framework used, the solvation results in a much more pronounced charge localization. When the lattice is allowed to relax, even larger degrees of charge localization are expected to occur. Our systematic study at various levels of ab initio theory investigates this conjecture. In all cases, solvation indeed increases charge localization however, within the framework of Hartree-Fock method we observe charge localization in the vacuum environment in the absence of lattice relaxation. In some of the cases, counterintuitively, lattice relaxation leads to slightly smaller degrees of localization in both HF and BHandHLYP methods. The ground state of odd-N polyynic oligomers C NH2 features kink-solitons in carbon-carbon bond-length alternation (BLA) patterns. We perform a systematic first-principles computational study of neutral and singly-charged kinks in long oligomers addressing relationships between BLA patterns, electron energy gaps, and accompanying distributions of spin and charge densities, both in vacuum and in the screening solvent environment. A quantitative comparison is made of the results derived with four different ab initio methods: from pure DFT to pure Hartree-Fock (HF) and including two popular hybrid density functionals, B3LYP and BHandHLYP. A clear correlation is demonstrated between the derived spatial extent of kinks and the amount of HF exchange used in the functionals. For charged kinks, we find a substantial difference in the behavior of charge and spin densities. We believe this work to be of relevance for various 1D semiconductors in polar environments.