Bond length alternation in ground and HOMO→LUMO excited states in polyenes. Dynamic Stokes shift?

. Complete-active-space self-consistent-field calculation of the reorganisation energy, λ, corresponding to the strongly allowed HOMO→LUMO transition in planar polyenes in the trans form ( C 2 h symmetry), gives λ>0.5 eV. This large λ depends on the fact that the short and long bond lengths of the excited 1 B u (or 3 B u ) state compared to the 1 A g ground state are almost cancelled. The emission redshift (Stokes shift) in molecules with the same type of π system is quite small, however, which suggests that the Stokes shift may be dynamic, owing to the presence of another excited state at lower or about the same energy.