Evidence for an intramolecular charge transfer state in 12 '-apo-beta-caroten-12 '-al and 8 '-apo-beta-caroten-8 '-al: Influence of solvent polarity and temperature

Abstract

The ultrafast excited-state dynamics of two carbonyl-containing carotenoids, 12'-apo-beta-caroten-12'-al and 8'-apo-beta-caroten-8'-al, have been investigated by transient absorption spectroscopy in a systematic variation of solvent polarity and temperature. In most of the experiments, 12'-apo-beta-caroten-12'-al was excited at 430 nm and 8'-apo-beta-caroten-8'-al at 445 or 450 nm via the S-0 -> S-2 (1(1)A(g)(-) -> 1(1)B(u)(+)) transition. The excited-state dynamics were then probed at 860 nm for 12'-apo-beta-caroten-12'-al and at 890 or 900 nm for 8'-apo-beta-caroten-8'-al. The temporal evolution of all transient signals measured in this work can be characterized by an ultrafast decay of the S-2 -> S-N absorption at early times followed by the formation of a stimulated emission (SE) signal, which subsequently decays on a much slower time scale. We assign the SE signal to a low-lying electronic state of the apocarotenals with intramolecular charge-transfer character (ICT -> S-0). This is the first time that the involvement of an ICT state has been detected in the excited-state dynamics of a carbonyl carotenoid in nonpolar solvents such as n-hexane or i-octane. The amplitude ratio of ICT-stimulated emission to S-2 absorption was weaker in nonpolar solvents than in polar solvents. We interpret the results in terms of a kinetic model, where the S-1 and ICT states are populated from S-2 through an ultrafast excited-state branching reaction (tau(2) < 120 fs). Delayed formation of a part of the stimulated emission is due to the transition S-1 -> ICT (tau(3) = 0.5-4.1 ps, depending on the solvent), which possibly involves a slower backward reaction ICT -> S-1. Determinations of tau(1) were carried out for a large set of solvents. Especially in 12'-apo-beta-caroten-12'-al, the final SE decay, assigned to the nonradiative relaxation ICT -> S-0, was strongly dependent on solvent polarity, varying from tau(1) = 200 ps in n-hexane to 6.6 ps in methanol. In the case of 8'-apo-beta-caroten-8'-al, corresponding values were 24.8 and 7.6 ps, respectively. This indicates an increasing stabilization of the ICT state with increasing solvent polarity, resulting in a decreasing ICT-S-0 energy gap. Tuning the pump wavelength from the blue wing to the maximum of the S-0 -> S-2 absorption band resulted in no change of tau(1) in acetone and methanol. Additional measurements in methanol after excitation in the red edge of the S-0 -> S-2 band (480-525 nm) also show an almost constant tau(1) with only a 10% reduction at the largest probe wavelengths. The temperature dependence of the tau(1) value of 12'-apo-beta-caroten-12'-al was well described by Arrhenius-type behavior. The extracted apparent activation energies for the ICT -> S-0 transitions were in general small (on the order of a few times RT), which is in the range expected for a radiationless process.