Investigation of the S-1/ICT -> S-0 internal conversion lifetime of 4 '-apo-beta-caroten-4 '-al and 8 '-apo-beta-caroten-8 '-al: Dependence on conjugation length and solvent polarity

The ultrafast internal conversion (IC) dynamics of aldehyde-substituted apocarotenoids (n'-apo-beta-caroten-n'-als with n = 4, 8 and 12) have been investigated in a systematic variation of conjugation length and solvent polarity using time-resolved femtosecond transient absorption spectroscopy. After excitation to the S-2 state with different excess energies, the subsequent intramolecular dynamics were investigated at several probe wavelengths covering the S-0 -> S-2 and S-1/ICT -> S-n absorption bands. Time constants tau(1) for the internal conversion process S-1/ICT -> S-0 of 4'-apo-beta-caroten-4'-al and 8'-apo-beta-caroten-8'-al have been newly measured. We compared these results with our earlier measurements for 12'-apo-beta-caroten-12'-al (D.A. Wild, K. Winkler, S. Stalke, K. Oum, T. Lenzer Phys. Chem. Chem. Phys. 2006, 8, 2499). In the case of the aldehyde with the longest conjugation (4'-apo-beta-caroten-4'-al), tau(1) is almost independent of solvent polarity (4-5 ps), whereas a significant reduction of tau(1) from 22.7 to 8.6 ps for the shorter 8'-apo-beta-caroten-8'-al and an even more pronounced reduction from 220 to 8.0 ps for 12'-apo-beta-caroten-12'-al were observed when the solvent medium was changed from n-hexane to methanol, respectively. In n-hexane, tau(1) of the apocarotenals is strongly dependent on the conjugation length and this can be well understood in terms of an energy gap law description where the S-1-S-0 energy differences were estimated from their steady-state fluorescence spectra. In highly polar solvents, the IC to S-0 is very fast, irrespective of the conjugation length. This is probably due to the stabilization of an intramolecular charge transfer (ICT) state in 12'-apo-beta-caroten-12'-al and 8'-apo-beta-caroten-8'-al. In the case of 4'-apo-beta-caroten-4'-al, such an influence of an ICT state is presumably less important than for the other two apocarotenals.