Excited-state properties of the central-cis isomer of the carotenoid peridinin

The central-cis isomer of the carotenoid peridinin, presumably 13-cis, was separated and studied with spectroscopic methods including static absorption, fluorescence and femtosecond time-resolved absorption. The investigations exposed differences in the photophysical properties of this isomer in respect to all-trans peridinin. Steady-state absorption spectroscopy revealed the presence of an additional weak absorption band at the long wavelength tail of the main S-0 -> S-2 transition. Modelling of the hypothetical vibronic progression of the S-0 -> S-1 electronic transition demonstrated that this weak band can be associated with a higher (0-2) vibronic band of the transition and that lower vibronic bands have negligible intensities due to a large displacement between the S-0 and S-1 states energy curves as also suggested by the spectral shape of steady-state fluorescence emission. Transient absorption studies demonstrated that the lifetime of the S-1 state of the central-cis isomer is shorter compared to the all-trans counterpart by 6-16%, depending on the polarity of the solvent. On the other hand, molecular isomerization negligibly affects the lifetime of intramolecular charge transfer (ICT), which for both isomers is similar to 10 ps in the polar solvent methanol.