Abstract
All-trans carotenoids have low-lying singlet states with different symmetries. The energy transfer from carotenoid (Car) to bacteriochlorophyll (BChl) is governed by internal conversion and vibrational relaxation. In the present investigation, we focus our attention on the vibrational relaxation. The 1Bu+ state plays the most important role in Car-to-BChl singlet-energy transfer, in both shorter-chain and longer-chain Cars. We measured the time-resolved absorption spectrum of spirilloxanthin (the number of conjugated double bonds, n = 13), having the shortest lifetime in the 1Bu+ state, by the use of approximately 40 fs pulses. In the initial time evolution, two peaks appeared that is to be assigned to the 1Bu+(1) and 1Bu+(0) states; the former decayed much faster than the latter. The lifetime of vibrational relaxation is substantially longer than that of internal conversion, so that the Car-to-BChl singlet-energy transfer and the internal conversion can take place competitively from plural 1Bu+ vibronic levels.
