Investigation of Differential Photochemical and Electrochemical Isomerizations of Quinazolinespirohexadienone Photochromes

It has been observed that quinazolinespirohexadienone (QSHD) photochromes form two distinct long-wavelength (LW) products upon photochemical- and electrochemical-induced isomerization from the common short-wavelength (SW) form. It has been proposed that the electrochemical path (forming “eLW”) proceeds through the one-electron-reduced doublet intermediate state (D0) of SW while the photochemical path (forming “pLW”) proceeds through an excited intermediate state (S1 or T0) of SW. Using computational methods, geometry optimization, molecular orbital, bond order, and bond length calculations were performed on the proposed intermediate states of these two pathways to explain the experimental observation. Additionally, while S1 is likely to be the relevant photochemical excited state, we use T0 as the model. This is justified because ignoring spin-orbit coupling is assumed to introduce far less error than would comparison of data obtained from ground-state vs. excited-state computational methods. Computational results show that smaller bond orders and longer bond lengths have been calculated for the bonds that must be broken in order to yield the observed eLW and pLW products from their respective intermediates