Twin states and conical intersections in linear polyenes

We suggest for linear conjugated polyenes a twin state model which represents the ground state (S₀) and first excited state (S₁) as a superposition of mainly two mesomeric structures, the fully spin-paired one and a diradical. This model rationalizes why the bond-length alternation, which is pronounced in S₀, more or less disappears in S₁ and why the bond-alternation vibration (highest frequency C=C stretch) is raised in S₁ and depressed in S₀. The similarity to the Peierls effect and Kohn anomaly in one-dimensional metals is emphasized. Moreover, the conical intersection between S₂ and S₁ is qualitatively predicted, and invoking in addition, some spectroscopic and other observations and the phase-change rule, that between S₁ and S₀ can also be predicted. Compared with the consideration of densities of states and matrix elements, these intersections more satisfactorily explain the S₂/S₁ and S₁/S₀ internal conversions and their dependence on chain length, substituents, solvent and temperature and is furthermore consistent with photochemistry. This also includes an exponential gap rule for the internal-conversion rates, which is derived from a proposed dependence of the energy at the intersection on the S₁-S₀ energy gap. (C) 2000 Elsevier Science B.V.