Spectroscopic and dynamic properties of the peridinin lowest singlet excited states

Spectroscopic properties as well as excited state dynamics of the carotenoi d peridinin in several solvents of different polarities were investigated b y time-resolved fluorescence and transient absorption techniques. A strong dependence of the peridinin lowest excited states dynamics on solvent polar ity was observed after excitation into the strongly allowed S-2 state. Peri dinin relaxes to the ground state within 10 ps in the strongly polar solven t methanol, while in the nonpolar solvent n-hexane a 160 ps lifetime was ob served, thus confirming the previous observations revealed by transient abs orption spectroscopy in the visible region (Bautista, J. A.; et al. J. Phys . Chem. B 1999, 103, 8751). In addition, the solvent dependence in the near -IR region is demonstrated by a strong negative feature in the transient ab sorption spectrum of peridinin in methanol, which is not present in n-hexan e. This band, characterized by a 1 ps rise time, is ascribed to stimulated emission from an intramolecular charge-transfer (ICT) state. Time-resolved fluorescence data support assignment of this band to the emissive singlet s tate, whose dynamic characteristics depend strongly on the dielectric stren gth of the medium. On the basis of all our time-resolved measurements combi ned with simulations of the observed kinetics, we propose the following mod el: the initially populated S-2 state decays to the S-1 state within less t han 100 fs for both solvents. Then, the population is transferred from the S-1 state to the S-0 and ICT states. The S1 --> ICT transfer is controlled by a solvent polarity dependent barrier. In n-hexane the barrier is high en ough to prevent the S-1 --> ICT transfer and only S-1 --> S-0 relaxation ch aracterized by a time constant of 160 ps is observed. An increase of solven t polarity leads to a significant decrease of the barrier, enabling a direc t quenching of the S-1 state by means of the S-1 --> ICT transfer, which is characterized by a time constant of 148 ps for tetrahydrofuran, 81 ps for 2-propanol, and 11 ps for the most polar solvent methanol. The ICT state is then rapidly depopulated to the ground state. This relaxation also exhibit s solvent dependence, having a time constant of 1 ps in methanol, 2.5 ps in 2-propanol, and 3.5 ps in tetrahydrofuran.