Excited-State Dynamics of 12′-Apo-β-caroten-12′-al and 8′-Apo-β-caroten-8′-al in Supercritical CO2, N2O, and CF3H

The ultrafast excited-state dynamics of the two carbonyl carotenoids 12'-apo-beta-caroten-12'-al (12'C) and 8'-apo-beta-caroten-8'-al (8'C) have been investigated in supercritical (sc) fluids by femtosecond transient absorption spectroscopy. CO2, N2O, and CF3H were employed as solvent media over the pressure range 85-300 bar and at the temperatures 308 and 323 K. The carotenoids were excited to the S-2 state at 390 nm, and the subsequent dynamics were probed at different wavelengths in the UV-vis (390, 545, 580, 600, and 650 nm) and near IR (780 nm) regions. Stimulated emission in the near IR signaled the presence of a state with intramolecular charge transfer character (S-1/ICT). For 12'C in scCO(2) and scN(2)O, the internal conversion (IC) time constant iota(1) for the S-1/ICT -> S-0 transition showed no systematic pressure dependence and yielded an average value of 190 ps. This is slightly smaller than the values in nonpolar organic solvents (ca. 220 ps) found in our previous studies and probably due to the substantial quadrupole moment of the nondipolar CO2 and the small dipole moment of N2O, which might slightly stabilize the S-1/ICT state relative to So. This results in an acceleration of the nonradiative rate in the simple framework of an energy gap law approach. In polar CF3H, a pronounced acceleration of the internal conversion rate was observed with increasing pressure, which can be explained by the polarity increase, as characterized by the parameter Delta f = (epsilon -1)/(epsilon + 2) - (n(2) - 1)/(n(2) + 2). We find scCF(3)H to be the first solvent where the S-1/ICT state of 12'C does not decay in a monoexponential fashion. This is most likely attributed to time-dependent solvation of the S-1/ICT state, vibrational cooling, or conformational relaxation processes in 12'C. In addition, we studied the dynamics of the longer conjugated species 8'C, where the decays of all transients in scCO(2) and scCF(3)H could be described well by monoexponential fits, in good agreement with previous results in organic solvents. Anisotropy decays from polarization spectroscopy of the 12'C species provided orientational relaxation time constants which were increasing with viscosity. The values in scCO(2) were extrapolated to a free rotor time of 4.6 ps, which is in good agreement with a value of 5.2 ps calculated on the basis of the rotational constants. We also report on pressure- and temperature-dependent steady-state absorption spectra of the two apocarotenals in scCO(2), scN(2)O, and scCF(3)H. The band position of the So - S2 transition correlates well with solvent polarizability, but-in contrast to our previous study Of C-40 carotenoids-a substantial influence of polarity was also observed. Specifically, we found indications for solvent clustering, resulting in a saturation of the solvent shift at lower densities.