Temperature dependence of the lowest excited singlet-state lifetime of all-trans-β-carotene and fully deuterated all-trans-β-carotene

A 4 ps, 450 nm laser pulse was used to electronically excite all-trans-β-carotene and all-trans-β-carotene-d₅₆ in 3-methylpentane. The transient absorption spectra of these molecules were measured as a function of temperature down to 20 K. In all cases the 400-500 nm electronic absorption band of each carotene bleaches and a new absorption band near 560 nm appears immediately upon excitation. These bands recover with single exponential kinetics: τ = 8.1 ± 0.5 ps for all-trans-β- carotene, and τ = 10.5 ± 0.6 ps for all-trans-β-carotene-d ₅₆ at 294 K. These recovery times increase by about a factor of 2 in glassy 3-methylpentane, and are nearly independent of temperature from 100 to 20 K. The weak dependencies of the lowest excited single-state lifetime of all-trans-β-carotene on deuteration and temperature are discussed in terms of nonradiative decay mechanisms within carotenoids.