Evaluation of acceptor strength in thiophene coupled donor-acceptor chromophores for optimal design of organic photovoltaic materials.

A series of thiophene coupled acceptors were systematically investigated at the density functional theory level to reveal structure-property relationships for building blocks of materials used in organic photovoltaic applications. All of the acceptor groups studied in this work retain their aromaticity when coupled to thiophene groups as estimated from their aromatic stabilization energies. However, pure chains of acceptors may adopt quinoidal geometry along the conjugated backbone depending on the structure of interest. Spearman rank order correlation has been used to assess the relationships between the computed variables such as highest occupied molecular orbital, lowest unoccupied molecular orbital, E(g), oscillator strength, exciton binding energy, aromatic stabilization energy, etc. The relative acceptor strengths were plotted and electrostatic potential maps were generated to examine the charge distribution over the chromophores. It has been found that there is no correlation between acceptor strength and electron withdrawing ability of the acceptor. Electron rich and highly electronegative atoms within acceptor groups mainly affect the charge distribution over the acceptor geometry. Exciton binding energy increases with the increasing aromatic character of the acceptor group. The acceptor strength is inversely correlated with the oscillator strength for the lowest excited state transition.