Solvent - dependent ultrafast internal conversion dynamics of n 0 - apo - b - carotenoic - n 0 - acids ( n = 8 , 10 , 12 )

The ultrafast internal conversion dynamics of 120-apo-b-carotenoic-120-acid (120CA), 100-apo-bcarotenoic-100-acid (100CA) and 80-apo-b-carotenoic-80-acid (80CA) have been investigated by femtosecond pump–probe spectroscopy. The three apocarotenoic acids were excited to the S2 state with different excess energies. Time constants t1 for the IC process S1/ICT S0 were measured by probing the dynamics at 390 nm (S0 S2), 575 nm (S1/ICT Sn), 850, 860 and 890 nm (S2 Sn and S1/ICT S0). In nonpolar solvents, the observed reduction of the t1 values with increasing conjugation length of the acids is consistent with a reduction of the energy gap between the S1/ICT and S0 states. The values are in good agreement with those of the corresponding apocarotenals studied previously in our groups. In polar solvents, a pronounced reduction of t1 values was observed for 120CA, however the behavior was different from that observed for the respective aldehyde 120-apo-b-caroten-120-al studied previously: First, the degree of t1 reduction in methanol was milder for 120CA (218 55 ps) than for 120-apo-b-caroten-12 0-al (220 8 ps). Secondly, for 120CA the plateau of solvent independent t1 values extended further into the midpolar range (up to 0.5 on the Df scale) than previously observed for the 120-aldehyde. For 100CA the polarity effect on the t1 values was weaker (B71 ps in n-hexane and 34 ps in methanol) and for 80CA it disappeared completely (B24 ps averaged over all solvents). The polarity-induced reduction of t1 is likely due to the stabilization of an intramolecular charge transfer state in polar solvents. This S1/ICT state is also responsible for the stimulated emission in the near IR, which has been observed in this specific class of carotenoids with a terminal carboxyl group for the first time. The occurrence of stimulated emission in the near IR region is also consistent with the steady-state fluorescence spectra which are reported along with the absorption spectra of these species. Possible reasons for the different behavior of the apocarotenoic acids compared to the respective aldehydes are discussed.