The π → π* excited states of long linear polyenes studied by the CASCI-MRMP method

Multireference Møller–Plesset perturbation theory with complete active space configuration interaction (CASCI-MRMP) is applied to the study of the valence π → π* excited states of all-trans linear polyenes C2nH2n+2 (n = 3–14). Our focus was to determine the nature of the four lowest-lying singlet excited states, 2 1 A g - , 1 1 B u + , 1 1 B u - and 3 1 A g - and their ordering. The ionic 1 1 B u + state is the lowest optically allowed excited state, while the covalent 2 1 A g - , 1 1 B u - and 3 1 A g - states are the optically forbidden states. Theory predicts that the 1 1 B u - state becomes lower than the 1 1 B u + state at n ⩾ 7 and that the 3 1 A g - state also becomes lower than the 1 1 B u + state at n ⩾ 11.