The origin of the dark S1 state in carotenoids: a comprehensive model

In carotenoids, by analogy to polyenes, the symmetry of the p-electron system is often invoked to explain their peculiar electronic features, in particular the inactivity of the S-0 -> S-1 transition in one-photon excitation. In this review, we verify whether the molecular symmetry of carotenoids and symmetry of their pi-electron system are supported in experimental and computational studies. We focus on spectroscopic techniques which are sensitive to the electron density distribution, including the X-ray crystallography, electronic absorption, two-photon techniques, circular dichroism, nuclear magnetic resonance, Stark and vibrational spectroscopies, and on this basis we seek for the origin of inactivity of the S-1 state. We come across no experimental and computational evidence for the symmetry effects and the existence of symmetry restrictions on the electronic states of carotenoids. They do not possess an inversion centre and the C-2h symmetry approximation of carotenoid structure is by no means justified. In effect, the application of symmetry rules (and notification) to the electronic states of carotenoids in this symmetry group may lead to a wrong interpretation of experimental data. This conclusion together with the results summarized in the review allows us to advance a consistent model that explains the inactivity of the S-0 -> S-1 transition. Within this model, S-1 is never accessible from S-0 due to the negative synergy of (i) the contributions of double excitations of very low probability, which elevate S-1 energy, and (ii) a non-verticality of the S-0 -> S-1 transition, due to the breaking of Born-Oppenheimer approximation. Certainly, our simple model requires a further experimental and theoretical verification.