Spheroidene and spheroidenone are two naturally-occurring carotenoids that function as light-harvesting molecules and triplet state quenchers in several strains of photosynthetic bacteria. Both carotenoids have ten π-electron conjugated carbon-carbon double bonds, but spheroidenone has a carbonyl group terminating one end of the conjugated chain. Transient grating (TG) spectroscopy of the carotenoids was carried out using 40 fs pulses to examine the role of the carbonyl in controlling the electronic and vibrational dynamics of deactivation of the photoexcited molecules. The time evolution of the TG signals of the two carotenoids differed primarily in two ways: (1) The rise time of the TG signal of spheroidene, which is associated with the lifetime of the S2 state, was approximately two times longer that of spheroidenone. This suggests that the presence of a carbonyl group in conjugation enhances the rate of depopulation of the S2 state; and (2) Coherent oscillations were observed in the kinetic profile of the TG signal from spheroidenone, but not from spheroidene. The frequency of these oscillations is ~145 cm-1, which corresponds to the difference in energy between ground state configurations of spheroidene. Therefore, one hypothesis being explored is that the presence of a carbonyl group in the conjugated π-electron system of a carotenoid facilitates conformational twisting of the polyene chain that gives rise to oscillations in the TG decay profile.

Transient grating spectroscopy of spheroidene and spheroidenone: The role of the carbonyl group in the excited state electronic and vibrational dynamics of carotenoids

253rd National Meeting (2017)

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