An intramolecular charge transfer state of carbonyl carotenoids: implications for excited state dynamics of apo-carotenals and retinal

Abstract

Excited state dynamics of two apo-carotenals, retinal and 12'-apo-β-carotenal, were studied by femtosecond transient absorption spectroscopy. We make use of previous knowledge gathered from studies of various carbonyl carotenoids and suggest that to consistently explain the excited-state dynamics of retinal in polar solvents, it is necessary to include an intermolecular charge transfer (ICT) state in the excited state manifold. Coupling of the ICT state to the A(g)(-) state, which occurs in polar solvents, shortens lifetime of the lowest excited state of 12'-apo-β-carotenal from 180 ps in n-hexane to 7.1 ps in methanol. Comparison with a reference molecule lacking the conjugated carbonyl group, 12'-apo-β-carotene, demonstrates the importance of the carbonyl group; no polarity-induced lifetime change is observed and 12'-apo-β-carotene decays to the ground state in 220 ps regardless of solvent polarity. For retinal, we have confirmed the well-known three-state relaxation scheme in n-hexane. Population of the B(u)(+) state decays in <100 fs to the A(g)(-) state, which is quenched in 440 fs by a low-lying nπ* state that decays with a 33 ps time constant to form the retinal triplet state. In methanol, however, the A(g)(-) state is coupled to the ICT state. This coupling prevents population of the nπ* state, which explains the absence of retinal triplet formation in polar solvents. Instead, the coupled A(g)(-)/ICT state decays in 1.6 ps to the ground state. The A(g)(-)/ICT coupling is also evidenced by stimulated emission, which is a characteristic marker of the ICT state in carbonyl carotenoids.

Collisional relaxation of apocarotenals: identifying the S* state with vibrationally excited molecules in the ground electronic state S0*

“…For higher initial excitation energies, zeaxanthin will enter S 0 with much higher excess energy which gives rise to more pronounced S* absorption features. 41,45 Because formation of the S 1 /ICT state leads to a substantial change in dipole moment, solvation dynamics will induce a transient Stokes shift of the S 1 /ICT stimulated emission and possibly also excited state absorption features, as already demonstrated by us previously.…”

“…We therefore believe that extensions of our kinetic model will be only required in special cases: for instance, systems such as b-apo-12 0 -carotenal are known to exhibit significant charge transfer character in polar solvents, including the appearance of substantial stimulated emission and a drastic reduction of the S 1 lifetime: 28,34,35,40,41 the first electronically excited state is then better termed ''S 1 /ICT'', [42][43][44] but the basic model of three electronic states survives. This can be explained in the same way as above.…”

A comprehensive picture of the ultrafast excited-state dynamics of retinal

“…In n-butanol, stimulated emission (SE) was observed above 620 nm, in agreement with SE kinetics previously detected in other polar solvents. 14,15 Later on, Polívka et al 7 ascribed this red emission to an ICT state, based on an analogous assignment of SE features observed for b-apo-12 0 -carotenal, b-apo-8 0 -carotenal [5][6][7] and b-apo-12 0 -carotenoic acid. 16 Although these experiments have improved the understanding of retinal photochemistry, important pieces of information are still missing, such as (a) the complete spectral characterisation Universität Siegen, Physikalische Chemie, Adolf-Reichwein-Str.…”

“…For instance, systems such as b-apo-12 0 -carotenal, show a strong decrease of the excited-state lifetime with increasing solvent polarity, in contrast to nonpolar carotenoids. [4][5][6][7] This is ascribed to the formation of considerable intramolecular charge transfer (ICT) character in the S 1 state. [8][9][10] Earlier studies suggest that the photochemistry of retinal is more complex.…”

Excited‐State Evolution of Keto‐Carotenoids after Excess Energy Excitation in the UV Region

“…The S 1 ‐S 0 relaxation in keto‐carotenoids with N <10 is governed by coupling of the S 1 state to an intramolecular charge transfer (ICT) state [15,25,26] . This coupling increases with solvent polarity and in most polar solvents such as methanol or acetonitrile the lifetime of the coupled S 1 /ICT can be order of magnitude faster than in a non‐polar solvent [27,28] …”