RESONANCE RAMAN SPECTRUM OF THE EXCITED 21A_g STATE OF ß‐CAROTENE

Abstract

Abstract— Resonance Raman (RR) bands assignable to the 21Ag excited state of ß‐carotene are recorded using picosecond time‐resolved resonance Raman (PTR3) spectroscopy. The RR spectrum contains bands in both the C‐C (1204 cm−1, 1243 cm−1, and 1282 cm−1) and C=C (1777 cm−1) stretching regions. The time‐dependent intensities of these RR features, measured with ˜ 30 ps. resolution, are found (i) to closely correlate with picosecond transient absorption (PTA) data recorded at 575 nm on the same sample and (ii) inversely correlate with the time‐dependent intensities of RR bands assigned to the 11Ag ground state. Both of these observations support the assignment of these four RR features to the 21Ag excited state. These results remove uncertainties associated with earlier experiments in which excited‐state RR scattering from (3‐carotene was not observed in spite of predicted trends emanating from studies of shorter polyene compounds. The observed C=C band position also agrees with a recent report of this feature.

Photophysical Characterization of Natural cis-Carotenoids¶

“…In a fluorescence study of diphenylpolyenes a similar observation was made for diphenylhexatriene with a fluorescent S 1 state lying about 1600 cm Ϫ1 below the allowed S 2 state in toluene (39). The emission rate was increased by a factor of two when the solvent was exchanged from n-hexane to dioxane (n ϭ 1.4165 20 ). Therefore we suggest that the forbidden state is located very close to the allowed one, but at lower energy, as for all linear polyenes and carotenoids investigated so far with N Ն 3 (3-6,11).…”

“…This excited singlet state is mainly deactivated by fast internal conversion to a lower-lying singlet excited state (S 1 ) labeled 2 1 A g . The 2 1 A g → 1 1 A g transition is forbidden by symmetry, whereupon the excited molecules undergo an efficient internal conversion back to ground state associated with a large change in the CϭC stretching frequency (3,20,21). From this latter observation, together with the fact that the S 1 lifetime is only weakly affected on going from room-temperature solvents to low-temperature highly viscous environments (22,23), it is generally believed that the CϭC stretching vibrations act as promoting and accepting modes between S 1 and S 0 , and that the involvement of low-frequency, large-amplitude vibrations, such as out-of-plane skeletal twisting, bending and torsional modes is negligible.…”

Singlet Excited States and the Light‐Harvesting Function of Carotenoids in Bacterial Photosynthesis

“…The abnormally high frequency of the S, Raman line (1777 cm-') was ascribed to the vibronic coupling between the S, (2A,-) and So (A,-) states through the particular a,-type C=C stretching vibration in relation to the case of linear polyenes (32,33). More convincing evidence was obtained by the observation that the time dependence in the intensities of the S, Raman lines closely correlates with the time dependence of the S, t S, transient absorption and that they inversely correlate with the time dependence of the intensities of the So Raman lines (34). The S, Raman spectrum of all-trans-p-carotene in the entire fingerprint region was subsequently reported (34,35).…”

Light‐harvesting function of carotenoids in photo‐synthesis: The roles of the newly found 11B state

“…In the picosecond region, a simplified spectral pattern appears again with a key Raman line above 1740 cm −1 . This extremely high frequency was ascribed to the vibronic coupling between the 2 1 A and the 1 1 A (ground) states when it was first detected 22, 23. Obviously, the spectral changes originate from the Raman spectrum of the 2 1 A state 24.…”