The remarkable ability of B3LYP/3-21G(*) calculations to describe geometry, spectral and electrochemical properties of molecular and supramolecular porphyrin–fullerene conjugates

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Abstract
In recent years, application of calculated density functional theory Kohn-Sham (DFT-KS) orbitals and eigenvalues have gained increased popularity due to their ability to display and predict physico-chemical properties of large systems. Our interest in this area has been to employ the exchange-correlation hybrid functional B3LYP with a small 3-21 G(*) basis to determine the geometry of large donor-acceptor assemblies followed by comparison of the computed KS orbital energies with measured electrochemical and spectral properties. The surprisingly localized orbitals allow prediction of the site of electron transfer during electrochemical oxidation and reduction of covalently bonded or self assembled porphyrin-fullerene donor-acceptor systems. The highest occupied orbitals (HOMOs) track oxidation potentials while the lowest unoccupied orbitals (LUMOs) track the reduction potentials of these compounds. Such studies are important to determine the position of energy levels of these donor-acceptor systems for understanding the pathways of photo initiated electron or energy transfer processes. The geometry and association energy of supramolecular assemblies (including loosely bound complexes self-assembled by H-bonding, electrostatics, or by pi-pi polphyrin-fullerene interactions) are also well represented, but the association energy must be scaled to agree with experimental values. The unusual success of the B3LYP/3-21G(*) method may be attributable, in part, to fortuitous shadowing of the interaction energy by significant basis set superposition effects. Results of several molecular and supramolecular systems at this computational level, comprised of porphyrin-fullerene donor-acceptor entities, developed in our laboratory, are reviewed.