Light‐harvesting function of carotenoids in photo‐synthesis: The roles of the newly found 1¹B state

Abstract

This minireview article highlights the energetics and the dynamics of the 1(1)B(u)(-) and 3(1)A(g)(-) states of carotenoids discovered very recently. Those "hidden" covalent states have been revealed by measurements of resonance-Raman excitation profiles of crystalline carotenoids. The dependence of the energies of the low-lying singlet states, including the 1(1)B(u)(+), 3(1)A(g)(-), 1(1)B(u)(-), and 2(1)A(g)(-) states, on the number of conjugated double bonds (n) is in agreement with the extrapolation of those state energies calculated by Tavan and Schulten for shorter polyenes (P. Tavan and K. Schulten, Journal of Chemical Physics, 1986, vol. 85, pp. 6602-6609). It has also been shown that the internal-conversion processes among those singlet states take place in accord with the state ordering, i.e., 1(1)B(u)(+) --> 1(1)B(u)(-) --> 2(1)A(g)(-) --> 1(1)A(g)(-) (the ground state) for carotenoids having n = 9 and 10, whereas 1(1)B(u)(+) --> 3(1)A(g)(-) --> 1(1)B(u) (-) --> 2(1)A(g)(-) --> 1(1)A(g)(-) for carotenoids having n = 11-13. Radiative transitions of 1(1)B(u)(+) --> 2(1)A(g)(-) and 1(1)B(u)(-) --> 2(1)A(g)(-) as well as a branching into the triplet manifold of 1(1)B(u)(-) --> 1(3)A(g) --> 1(3)B(u) have also been found. Those low-lying singlet states of all-trans carotenoids can facilitate multiple channels of singlet-energy transfer to bacteriochlorophyll in the LH2 antenna complexes of purple photosynthetic bacteria. Thus, the newly found 1(1)B(u)(-) and 3(1)A(g)(-) states of carotenoids need to be incorporated into the picture of carotenoid-to-bacteriochlorophyll singlet-energy transfer.

Ultra-broadband 2D electronic spectroscopy of carotenoid-bacteriochlorophyll interactions in the LH1 complex of a purple bacterium

“…On the contrary, we observe the presence of an intense negative peak around 940 nm, which is a well-known feature of the Car and is associated with PA from S 2 to higher lying states. 18 This signal extends to longer excitation wavelengths where there is a weak tail of the ground state absorption spectrum of the Car. At t = 20 fs, very short time after the end of the pump pulse, we observe a strong increase of the PB(Q x ) features, while PA(S 2 ) does not increase significantly, an indication that loss towards S x partially occurs already during the excitation pulse which continuously creates it.…”

“…An intermediate state with 1B u − symmetry (named S x ) mediating the S 2 → S 1 IC process has been first postulated theoretically and then experimentally demonstrated. [11][12][13][14][15][16][17][18][19][20] Recently, it has been proposed that when Cars are embedded in the LH2 complexes, S x might play an active role in the EET towards the higher excited state Q x of BChls. 21 Moreover, experimental evidence demonstrated the existence of another intermediate state, named S * , involved in the IC process of the Cars, whose origin and nature are still debated.…”

Electronic excitations in long polyenes revisited

“…21,23,35 It may be, however, that the presence of the 1B − u singlet state below the 1B + u state has been actually observed in pure polyenes much earlier. In 1985 Snyder et al 36 interestingly observed a drastic qualitative change of the polyene fluorescence spectrum upon increasing the size of an all-transpolyene from N = 12 π -electrons to N = 14.…”

Quantum Chemical Description of Absorption Properties and Excited‐State Processes in Photosynthetic Systems

“…Reviews on (proposed) excited-state dynamics and optically forbidden states can be found in refs. [157,171,183,189,190,206].…”