Ab initio electronic structure theory as an aid to understanding excited state hydrogen transfer in moderate to large systems

Hypocrellin and hypericin are naturally occurring polycyclic perylene quinones, and they have both been found to exhibit photoactivated antiviral and anticancer activity. One mode of action proposed involves excited-state hydrogen transfer. Consequently these compounds have been widely studied using spectroscopic methods, and are found to both absorb and emit in the visible region. Recently an analog dihydroxy perylene quinone was synthesized in order to examine its antiviral activity in relation to the naturally occurring compounds. Its UV-visible absorption and emission spectra are quite different to those of hypocrellin and hypericin, with very weak absorption and no visibility of emission at all. The ab inito excited-state methods, configuration interaction singles (CIS), state-averaged complete active space self-consistent field (SA–CASSCF), and SA-multireference perturbation theory (SA–MRMP2) are used to examine the origin of this different absorption and emission behavior. Owing to the size of these systems (between 24 and 40 heavy atoms) extensive use of parallel processor algorithms was made, especially a parallel atomic orbital-based CIS energy and gradient code developed at the ABCC. The performance of these methods, and possible ,as well as future directions and prospects are discussed.