Importance of a low-lying nπ*-state in the photo-isomerisation reaction of Z-methylfurylfulgide

Knowledge about the character of electronic states involved in ultrafast photoreactions is essential for understanding the dynamics leading to static and transient experimental spectra. Here we present a detailed study of the three lowest-energy electronic states of the photochromic switch methylfurylfulgide in its Z-form and of their changes as they approach the S1S0-conical intersection that facilitates the formation of E-methylfurylfulgide. We studied the molecular energy landscape by identifying important points along the reaction coordinate (Franck–Condon point, S1-energy minimum and S1S0-conical intersection) by state-averaged CASSCF(10,9)/6-31g(d,p) calculations, assigned the character of the three lowest electronic states at these points and verified our classifications by CASPT2(18,13) calculations in a basis of polarised triple-zeta quality for the atoms relevant to the reaction and polarised double-zeta for the others. In contrast to the E→Z interconversion, it turns out that a proper active space for the Z→E interconversion should consist not only of π orbitals but should contain also orbitals of n character. We expect this to experimentally manifest itself in visible differences between Z→E and E→Z behaviour.