Spectroscopic properties of phenolic and quinoid carotenoids: a combined theoretical and experimental study

Abstract

For the natural carotenoid 3,3′-dihydroxyisorenieratene (DHIR) and two synthetic derivatives, 3,3′-dihydroxy-16,17,18,16′,17′,18′-hexanor-Φ,Φ-carotene (DHHC) and Φ,Φ-carotene-3,3′-dione (DHIRQ, isorenieratene-3,3′-dione), steady state absorption experiments and combined density functional and multi-reference configuration interaction calculations were carried out. In addition, femtosecond transient absorption spectra were recorded for DHIR. Due to their marked out-of-plane distortion in DHIR, the phenolic end groups participate only partially in the conjugation system. In the low-energy regime its absorption spectrum with the maximum at 21 700 cm⁻¹ in acetone solution therefore closely resembles that of β-carotene, the same as for the T₁ energy. Further similarities are also found for the decay kinetics of the optically bright 1¹B_u⁺ state of these compounds. After femtosecond excitation, the 1¹B_u⁺ population of DHIR decays with a lifetime of 110 fs to the vibrationally hot 2¹A_g⁻,v state which in turn relaxes to the 2¹A_g⁻,0 state within 500 fs. Decay of the 2¹A_g⁻,0 state to the S₀ state occurs at a time scale of 12 ps. Demethylation of the phenolic end groups alleviates the steric repulsion by the polyene chain and causes a small red shift (1000 cm⁻¹) comparing the absorption spectra of DHHC and DHIR. Oxidation of DHIR leads to drastic changes of the electronic and geometric properties. The quinoid end groups of DHIRQ are fully integrated into the conjugation system, shifting the absorption maximum to 17 800 cm⁻¹ in acetone solution which thus takes a blue color. The results of the quantum chemical calculations indicate that, in addition to the 2¹A_g⁻ (S₁) state, two dark internal charge-transfer singlet states and the 1¹B_u⁻ state might be located energetically below the optically bright 1¹B_u⁺ (S₅) state of DHIRQ.