Tuning range-separated DFT functionals for modeling the peak absorption of MEH-PPV polymer in various solvents

Abstract

Density Functional Theory (DFT) and time-dependent Density Functional Theory (TD-DFT) are powerful tools for modeling orbital energy in conjugated molecules. Range Separated (RS) functionals vary the percentage of Hartree-Fock and DFT exchange for long-range and short-range interactions and are used in cases involving charge transfer excitation. The accuracy of the computed results using RS functionals depends on the range-separation parameter (ω) whose optimal values are not always known. Conjugated polymers add additional difficulty because the length of the polymer can vary. In this study, accurate values of ω for three lengths of MEH-PPV polymer (trimer, tetramer, and pentamer) in five different solvents (chloroform, chlorobenzene, xylene, Tetrahydrofuran, and dichloromethane) are reported using the RS functionals wB97XD and CAM-B3LYP. Range separation parameters are predicted and used for longer polymer chains. The differences in the ω for different solvents is statistically significant and gives insight into the polymer/solvent interaction.