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

Multireference Moller-Plesset perturbation theory with complete active space configuration interaction (CASCI-MRMP) is applied to the study of the valence pi --> pi* 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 greater than or equal to 7 and that the 3(1)A(g)(-) state also becomes lower than the 1(1)B(u)(+) state at n greater than or equal to (C) 2004 Elsevier B.V. All rights reserved.