Geometry and steric effects on the electronic states of aryl-o-carboranes

Abstract

o-Carborane was used to alter the electronic states of pi-conjugated organic aryls, and demonstrated as an effective electron control unit, to tune HOMO and LUMO energies for the electron transfer. Two different types of bis-aryl compounds were selected: one with di-phenyls and its fused fluorenyl, and another with two isomers of naphthyls, alpha-and beta-forms. Among the series of aryl compounds, the second type of naphthyls were more susceptible to electronic alteration and alpha-isomer of naphthyl showed the most perturbed electronic state. The ground state structures confirmed by crystallographic measurements provide a close correlation of structure and electronic property between photo-responsive aryl groups and o-carborane. In addition to the distance effect, within similar proximity of the aryl functional groups, namely naphthyl case, the steric factor controlled the altered electronic state. HOMO and LUMO energies were estimated and confirmed by cyclic voltammograms and DFT calculations, respectively, and established the authenticity of the electronic alteration. Excited states were calculated by the TD-DFT correlations with the corresponding absorption spectra to illustrate the electronic perturbation in a systematic fashion suggesting that excited states were lowered in regard to the electronic perturbation. Therefore, within bis-aryls-o-carborane series, the following decreasing energy order is observed; alpha-Np, beta-Np, Flu, diPh, and Ph. The final verification of the lowered energy was made according to the electron transfer efficiency between the well-studied iridium based photosensitizer, Ir(ppy)(3) and two typical bis-aryls-o-carboranes, alpha-Np and Ph. The electron transfer efficiency of alpha-Np is 10 times greater than that of Ph (1.231 x 10(7) M(-1)s(-1) for alpha-Np and 9.275 x 10(5) M(-1)s(-1) for Ph). (c) 2018 Elsevier B.V. All rights reserved.