Carotenoids forbidden transition

The analysis of carotenoid identity, conformation, and binding in vivo should allow further progress to be made in understanding of the functions of these pigments in the photosynthetic machinery. One of the obvious steps toward improvement could be the use of continuously tuneable laser systems in order to obtain more detailed resonance Raman excitation profiles (Sashima et al 2000). This technique will be suitable for the investigation of in vivo systems with more complex carotenoid composition. In addition, this method may be applied for the determination of the energy of forbidden Sj or 2 Ag transition. This is an important parameter, since it allows an assessment of the energy transfer relationship between the carotenoids and chlorophylls within the antenna complex. 

In relative isolation the BChl-a molecules absorb at 772 nm (the Qy band), 575 nm (the band), and 360 and 390 nm (the B bands). The carotenoids have a strong So S2 absorption in the region 450-550 nm, while their So Si transition is dipole-forbidden and it is not found in the one-photon absorption spectrum. 

Hsu et al. [1] investigated the origin of the substantial Coulombic coupling between the carotenoid Si state and Bchls in LH2. They found that a significant contribution could be attributed to mixing of the 2Ag and the B carotenoid states, induced by distortion of the carotenoid structure. However, even for a completely planar carotenoid molecule, with a forbidden Sq Si transition, the... 

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]. 

Fig. 2. The ordering of low-energy singlet states in polyenes and carotenoids. Labels refer to molecules belonging to the Cji, point group. Arrows with x s refer to symmetry-forbidden electronic transitions.

In summary, theory gives at least a semi-quantitative explanation for why Fig. 2 applies to a wide range ofpolyene systems and provides a useful guide for understanding relative energies, the symmetry-forbidden nature of the So<-4S, transitions, and the re-arrangement of r-bond orders in carotenoid... 

The transition from S to the lowest excited singlet state of carotenoids, S, is electric dipole forbidden (Kohler, 1991), and is not observed in the usual single-photon absorption experiment. S, is readily populated by relaxation from Sj. The lifetime of the S state, as measured by transient absorption techniques, is on the order of 10-40 ps for most carotenoids (Wasielewski and Kispert, 1986 Trautman et al 1990). The state decays almost totally via in ternal conversion. Fluorescence from S, is virtually undetectable (quantum yield <10" ), and intersystem crossing to the triplet state is not observed. Because the forbidden nature of the S, S, transition... 

Excited carotenoid molecules can transfer energy rapidly to nearby chlorophyll molecules in photosynthetic antenna complexes [57]. It has been suggested that this process involves exchange coupling, because the radiative transitions from the lowest excited singlet states of the carotenoids to the ground states are forbidden by molecular symmetry (Box 4.12). However, the rate could possibly be explained by considering the full expression for the direct interaction (Eq. 7.14) instead of just dipole-dipole interactions [58]. 

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