S1 and T1 species of .beta.-carotene generated by direct photoexcitation from the all-trans, 9-cis, 13-cis, and 15-cis isomers as revealed by picosecond transient absorption and transient Raman spectroscopies

• DOI: 10.1021/j100161a022
• Publication Date: 2005-03-15T00:56:55Z
• Authors: Hideki Hashimoto; Yasushi Koyama; Yoshinori Hirata; Noboru Mataga
• Keywords: 01 natural sciences; 0104 chemical sciences

Time-resolved absorption spectra, upon excitation of the all-trans, 9-cis, 13-cis, and 15-cis isomers of {beta}-carotene, were recorded by using 20-25-ps, 355-nm pump and white-continuum interrogating pulses. The isomers showed the S{sub n} {l arrow} S{sub 1} absorption at 556 (all-trans), 565 (9-cis), 560 (13-cis), and 562 (15-cis) nm; no time-dependent changes of the absorptions were detected. (The S{sub 1} lifetimes for the all-trans and 15-cis isomers were determined precisely, by using 0.5-ps pump and interrogating pulses, to be 12.4 {plus minus} 0.5 and 14.0 {plus minus} 0.5 ps, respectively.) After the decay of the S{sub n} {l arrow} S{sub 1} absorption, a weak absorption ascribable to the T{sub n} {l arrow} T{sub 1} transition remained in the 510-520-nm region, from which the quantum yield of intersystem crossing was estimated to be on the order of 10{sup {minus}3}. Transient Raman spectra were recorded by using 355-nm pump and 532-nm probe mode-locked (76 MHz), Q-switched (800 Hz) pulse trains (pulse duration {approximately} 100 ps). An S{sub 1} Raman spectrum was extracted by subtraction between a pair of pump-and-probe spectra with different delay times, i.e. 0 and 500 ps. A T{sub 1} Raman spectrum was extracted by subtracting a probe-only spectrum from amore » pump-and-probe spectrum with the delay time 500 ps. Each isomer showed a unique S{sub 1} Raman spectrum, which was interpreted by assuming the configuration of the particular isomer. On the other hand, the 13-cis and 15-cis isomers showed T{sub 1} Raman spectra that are essentially the same as that shown by the all-trans isomer, the 9-cis isomer showed a unique T{sub 1} Raman spectrum. The above results indicate (1) that no isomerization takes place in the S{sub 1} state and (2) that the T{sub 1} state is generated through intersystem crossing.« less