Excited state properties of aryl carotenoids

Abstract

Excited-state properties of aryl carotenoids, important components of light harvesting antennae of green sulfur bacteria, have been studied by femtosecond transient absorption spectroscopy. To explore effects of the conjugated aryl group, we have studied a series of aryl carotenoids with conjugated ϕ-ring, chlorobactene, β-isorenieratene and isorenieratene, and compared them with their non-aryl counterparts γ-carotene and β-carotene, which contain β-ring. Changing β-ring to ϕ-ring did not reveal any changes in absorption spectra, indicating negligible effect of the ϕ-ring on the effective conjugation length. This observation is further supported by the carotenoid S₁ lifetimes. In n-hexane, the S₁ lifetime of chlorobactene having one ϕ-ring is 6.7 ps, while the S₁ lifetime of the β-ring analog, γ-carotene is 5.4 ps. The same effect is observed for the series β-carotene (two β-rings), β-isorenieratene (one β- and one ϕ-ring) and isorenieratene (two ϕ-rings) whose S₁ lifetimes in n-hexane are 8.2, 10.3 and 12.7 ps, respectively. The systematically longer lifetimes of aryl carotenoids show that the additional conjugated CC bonds at the ϕ-ring do not contribute to the conjugation length. The S₁ lifetimes of aryl carotenoids were slightly shortened in benzene, indicating π–π stacking interaction between the ϕ-ring and benzene.