The effect of solvent on determining highest occupied molecular orbital energies of semiconducting organic molecules:...

Although cyclic voltammetry (CV) measurements in solution have been widely used to determine the highest occupied molecular orbital energy (E_HOMO ) of semiconducting organic molecules, an understanding of the experimentally observed discrepancies due to the solvent used is lacking. To explain these differences, we investigate the solvent effects on E_HOMO by combining density functional theory and molecular dynamics calculations for four donor molecules with a common backbone moiety. We compare the experimental E_HOMO values to the calculated values obtained from either implicit or first solvation shell theories. We find that the first solvation shell method can capture the E_HOMO variation arising from the functional groups in solution, unlike the implicit method. We further applied the first solvation shell method to other semiconducting organic molecules measured in solutions for different solvents. We find that the E_HOMO obtained using an implicit method is insensitive to solvent choice. The first solvation shell, however, produces E_HOMO values that are sensitive to solvent choices and agrees with published experimental results. The solvent sensitivity arises from a hierarchy of three effects: (1) the solute electronic state within a surrounding dielectric continuum, (2) ambient temperature or solvent atoms changing the solute geometry, and (3) electronic interactions between the solute and solvents. The implicit method, on the other hand, only captures the effect of a dielectric environment. Our findings suggest that E_HOMO obtained by CV measurements should account for the influence of solvent when the results are reported, interpreted, or compared to other molecules.