Solvent Dependent Dynamics of Salicylidene Aniline in Binary Mixtures of Supercritical CO2 with 1-Propanol or Cyclohexane.

The role of different solvent environments in determining the behavior of molecules in solution is a fundamental aspect of chemical reactivity. We present an approach for exploring the influence of solvent properties on condensed-phase dynamics using ultrafast transient absorption spectroscopy in supercritical CO2. Using supercritical CO2 permits adjustment of the density, by varying the temperature and pressure, whereas varying the concentration or identity of a second solvent, the cosolvent, in a binary mixture allows for adjustments of the degree of interaction between the solute and the solvent. Salicylidene aniline, a prototypical excited-state intramolecular proton-transfer system, is the subject of this study. In this system, the decay rate of the transient absorption signal decreases as the fraction of the cosolvent (for both 1-propanol and cyclohexane) increases. The decay rate also decreases with an increase in the viscosity of the mixture, but the effect is much larger for the 1-propanol cosolvent than for cyclohexane. These observations illustrate that the decay rate of the photoexcited salicylidene aniline depends on more than just the solvent viscosity, suggesting that properties such as polarity also play a role in the dynamics.

Picosecond Dynamics of Avobenzone in Solution.

Avobenzone, a dibenzoylmethane compound commonly found in sunscreens, can photoisomerize after exposure to near-ultraviolet light. At equilibrium, avobenzone exists as a chelated enol characterized by a strong intramolecular hydrogen bond. Many nanosecond- to microsecond-scale experiments have shown that the photoisomerization involves several nonchelated enol (NCE) isomers and reaction paths, including some that reduce avobenzone's efficacy as a sunscreen. Because some of the NCE isomers are unstable, these experiments do not directly measure their spectroscopic signatures. Here, we report the dynamics of avobenzone on the picosecond time scale. We excite avobenzone at 350 nm and observe the formation and relaxation of new isomers and vibrationally excited species with broadband visible probe pulses and 266 nm probe pulses. Our results show the first direct evidence of two unstable NCE isomers and establish the lifetimes of and the branching ratio between these isomers.

Photoisomerization and relaxation dynamics of a structurally modified biomimetic photoswitch.

Recent experimental and theoretical studies on N-alkylated indanylidene pyrroline Schiff bases (NAIP) show that these compounds exhibit biomimetic photoisomerization analogous to that in the chromophore of rhodopsin. The NAIP compounds studied previously isomerize rapidly and often evolve coherently on the ground-electronic surface after reaction. We present the results of transient electronic absorption spectroscopy on dMe-OMe-NAIP, a newly synthesized NAIP analogue that differs from other NAIP compounds in the substituents on its pyrrolinium ring. Following excitation with 400 nm light, dMe-OMe-NAIP relaxes from the electronic-excited state in less than 500 fs, which is slower than in other analogues, and does not show the prominent oscillations observed in other NAIP compounds. A reduction in the amount of twisting between the rings caused by removal of the methyl group is likely responsible for the slower isomerization. Measurements in solvents of varying viscosity and structure suggest that intramolecular processes dominate the relaxation of nascent photoproducts.

UV photodissociation of η5-C5H5NiNO: an excited-state Jahn-Teller distortion produces a cartwheeling NO.

The 225 nm photodissociation of cyclopentadienylnickel nitrosyl was studied using velocity-mapped ion imaging with 1 + 1' REMPI detection of the NO (X (2)Π(1/2,3/2), v'' = 0) photofragment. The product recoil energy and angular distributions were measured for selected rotational states of NO. The NO product displays two speeds, a slow product peaked at the center of the ion image and a fast anisotropic product that has an inverted rotational population. In rotational states above J'' = 40.5, an even faster anisotropic NO photofragment appears, most likely because the metal-containing dissociation partner emerges in a lower electronic state, increasing the available energy. The µ-v-j vector correlations were measured and are consistent with the orientation µâˆ¥v⊥ j. The observed vector correlations arise from an excited-state Jahn-Teller distortion of the parent, a distortion that bends the Ni-NO coordinate prior to dissociation.