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Empirical Relationship between Chemical Structure and Redox Properties: Mathematical Expressions Connecting Structural Features to Energies of Frontier Orbitals and Redox Potentials for Organic Molecules
A set of mathematical expressions
to predict redox potentials and
frontier orbital energy levels for organic molecules as a function
of structural features is proposed. This is achieved by using the
principal component regression method on reduction potential (<i>E</i><sub>red</sub>), oxidation potential (<i>E</i><sub>ox</sub>), highest occupied molecular orbital (HOMO), and lowest
unoccupied molecular orbital (LUMO) values calculated using density
functional theory (DFT) on a training set consisting of 77 547
molecules from PubChem database. The first set of expressions allows
prediction of <i>E</i><sub>red</sub>, <i>E</i><sub>ox</sub>, HOMO, and LUMO values using molecular fingerprints
alone with <i>R</i><sup>2</sup> of ca. 0.74, 0.82, 0.92,
and 0.85, respectively, which can be used for preliminary screening
of molecules before performing DFT calculations. In the second set
of expressions, when we include DFT-calculated HOMO and LUMO values
as additional descriptors, the <i>R</i><sup>2</sup> values
of <i>E</i><sub>ox</sub> and <i>E</i><sub>red</sub> predictions increase to 0.91 and 0.90, respectively. This more accurate
approach for redox potential predictions is still significantly more
computationally efficient compared to DFT calculations of redox potentials.
The potential of these approaches is demonstrated by using the examples
of polyacenes and quinoxaline family of molecules. These empirical
relations are ideally suited for high-throughput screening for a variety
of optoelectronic applications. The resultant tool, QSROAR, is made
available at https://github.com/piyushtagade/qsroar_version2