PNNL

Abstract

Electron affinities (EAs) and electronic structures of benzoquinone molecules (BQs) play a vital role in a wide range of applications involving these molecules from biological photosynthesis to energy conversion processes. In this article, we report a systematic spectroscopic probe on the electronic structures and EAs of all three isomers (o-, m-, and p- BQs) employing photodetachment photoelectron spectroscopy (PES) and ab initio electronic structure calculations. PES spectra of three radical anions (BQ?-) were taken with different photon energies and at low temperature in order to accurately determine the EAs and probe ground as well as the excited states of the corresponding neutral molecules. Similar spectral pattern was observed in the o- and p- BQ?- spectra, each revealing a broad ground state feature and a large band gap, followed by well-resolved excited states peaks. The EAs of o- and p- BQ are determined to be 1.90, 1.85 eV, and singlet-triplet band gaps to be 1.68, and 2.32 eV, respectively. In contrast, the spectrum of m- BQ?- is distinctly different from its two congeners, with no clear band gap and a much higher EA (2.89 eV). Accompanying theoretical calculations confirm experimental EAs and band gaps, and further unravel a triplet ground state for m-BQ in contrast to singlet ground states for o- and p- isomers. The diradical nature of m-BQ, consistent with its non-Kekulé structure, is primarily responsible for the observed high EA and also explains its nonexistence in bulk materials.