Polyene, spectroscopic transitions

RetinalS. The structure and photophysics of rhodopsins are intimately related to the spectroscopic properties of their retiny1-polyene chromophore in its protein-free forms, such as the aldehyde (retinal), the alcohol (retinol or vitamin A), and the corresponding Schiff bases. Since most of the available spectroscopic information refers to retinal isomers (48-55), we shall first center the discussion on the aldehyde derivatives. Three bands, a main one (I) around 380 nm and two weaker transitions at 280 nm and 250 nm (II and III), dominate the spectrum of retinals in the visible and near ultraviolet (Fig. 2). Assignments of these transitions are commonly made in terms of the lowest tt, tt excited states of linear polyenes, the spectroscopic theories of which have been extensively discussed in the past decade (56-60). In terms of the idealized C2h point group of, for example, all-trans butadiene, transitions are expected from the Ta ground state to B , A, and A" excited states... 

In the singlet manifold, carotenoids have, like all polyenes, an unusual electronic structure The hrst excited state (Si) has the same symmetry, A, as the ground state, and thus one-photon transitions from So to Si are forbidden. In other words, the Si state does not appear in the absorption (or emission) spectrum of carotenoids (with more than 9 double bonds), which is dominated by the very strong So S2 (B ) transition. Carotenoids also possess a state of symmetry, which may lie near S2, though evidence for the spectroscopic observation of this state remains controversial [132-135]. Finally, some unusual carotenoids with polar substituents, such as peridinin, may also have low-lying charge transfer states [42, 136, 137]. 

A detailed description of the chemical relaxation processes of electronically excited molecules depends directly on the knowledge and understanding of their spectroscopic properties, and of the excited states Involved. The last two decades have seen a large number of experimental and theoretical studies on the electronic transitions of conjugated polyenes. A thorough review of the state of affairs up to 1974 has been given by Hudson and Kohler (21). Of particular Interest to the study of... 

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