Excited-State Dynamics of Monomeric and Aggregated Carotenoid 8′-Apo-β-carotenal

Abstract

Excited-state properties of monomeric and aggregated carbonyl carotenoid 8′-apo-β-carotenal were studied by means of femtosecond transient absorption spectroscopy. For monomers, the polarity-dependent behavior characteristic of carotenoids with conjugated carbonyl group was observed. In <i>n</i>-hexane the S₁ lifetime is 25 ps, but it is shortened to 8 ps in methanol. This shortening is accompanied by the appearance of new spectral bands in transient absorption spectrum. On the basis of analysis of the transient absorption spectra of monomeric 8′-apo-β-carotenal in <i>n</i>-hexane and methanol, we propose that the polarity-induced spectral bands are due to the S₁(A_g^–)–S₃(A_g⁺) transition, which is enhanced upon breaking the symmetry of the molecule. This symmetry breaking is caused by the conjugated carbonyl group; it is much stronger in polar solvents where the S₁ state gains significant charge-transfer character. Upon addition of water to methanol solution of 8′-apo-β-carotenal we observed formation of aggregates characterized by either blue-shifted (H-aggregate) or red-shifted (J-aggregate) absorption spectrum. Both aggregate types exhibit excited-state dynamics significantly different from those of monomeric 8′-apo-β-carotenal. The lifetime of the relaxed S₁ state is 20 and 40 ps for the H- and J-aggregates, respectively. In contrast to monomers, aggregation promotes formation of triplet state, most likely by homofission occurring between tightly packed molecules within the aggregate