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Title: Why is the Energy of the Singly Occupied Orbital in Some Radicals below the Highest Occupied Orbital Energy?
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Author(s) / Creator(s):
; ; ;
Date Published:
Journal Name:
Chemistry of Materials
Page Range / eLocation ID:
3678 to 3691
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    Although cyclic voltammetry (CV) measurements in solution have been widely used to determine the highest occupied molecular orbital energy (EHOMO) 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 onEHOMOby combining density functional theory and molecular dynamics calculations for four donor molecules with a common backbone moiety. We compare the experimentalEHOMOvalues to the calculated values obtained from either implicit or first solvation shell theories. We find that the first solvation shell method can capture theEHOMOvariation 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 theEHOMOobtained using an implicit method is insensitive to solvent choice. The first solvation shell, however, producesEHOMOvalues 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 thatEHOMOobtained by CV measurements should account for the influence of solvent when the results are reported, interpreted, or compared to other molecules.

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