The Forbidden 1 1 B u - Excited Singlet State in Peridinin and Peridinin Analogues.

Theoretical studies have predicted the presence of a forbidden 11 Bu - state in proximity to the strongly allowed 11 Bu + excited state in polyenes and carotenoids. The 11 Bu - state is invariably predicted to have a very low oscillator strength, which precludes direct optical spectroscopic assignment. We report here a direct UV-vis optical spectroscopic feature assigned to the 11 Bu - state of S-2-peridinin, a synthetic analogue of the naturally occurring carotenoid, peridinin. The shift of the ground state dipole of S-2-peridinin compared to natural peridinin enhances the oscillator strength of absorption from the ground state to the 11 Bu - state by 2 orders of magnitude relative to peridinin. It is postulated that this is due to a quadrupolar electrostatic field generated from the more central location of the lactone ring along the polyene chain in S-2-peridinin. MNDO-PSDCI and EOM-CCSD calculations provide a theoretical basis for this assignment and explain the unique properties of the 11 Bu - state and why the transition from the ground state to this state has such a low oscillator strength in most other polyenes and carotenoids.