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

The ultrafast excited-state dynamics of the two carbonyl carotenoids 12′-apo-caroten-12′-al (12′C) and 8′-apo-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 S2 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 (S1/ICT). For 12′C in scCO2 and scN2O, the internal conversion (IC) time constant τ1 for the S1/ICT f S0 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 S1/ICT state relative to S0. 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 ∆f ) (ε 1)/(ε + 2) (n2 1)/(n2 + 2). We find scCF3H to be the first solvent where the S1/ICT state of 12′C does not decay in a monoexponential fashion. This is most likely attributed to time-dependent solvation of the S1/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 scCO2 and scCF3H 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 scCO2 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 pressureand temperature-dependent steady-state absorption spectra of the two apocarotenals in scCO2, scN2O, and scCF3H. The band position of the S0 f S2 transition correlates well with solvent polarizability, butsin contrast to our previous study of C40 carotenoidssa 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.