Adam D. Dunkelberger

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.

The influence of vibrational excitation on the photoisomerization of trans-stilbene in solution.

Preparing electronically excited trans-stilbene molecules in deuterated chloroform using both one-photon excitation and excitation through an intermediate vibrational state explores the influence of vibrational energy on excited-state isomerization in solution. After infrared excitation of either two quanta of C-H stretch vibration |2ν(CH)> at 5990 cm(-1) or the C-H stretch-bend combination |ν(CH) + ν(bend)> at 4650 cm(-1) in the ground electronic state, an ultraviolet photon intercepts the vibrationally excited molecules during the course of vibrational energy flow and promotes them to the electronically excited state. The energy of the infrared and ultraviolet photons together is the same as that added in the one-photon excitation. Transient broadband-continuum absorption monitors the lifetime of electronically excited molecules. The lifetime of excited-state trans-stilbene after one-photon electronic excitation with 33,300 cm(-1) of energy is (51 +/- 6) ps. The excited-state lifetimes of (55 +/- 9) ps and (56 +/- 7) ps for the cases of excitation through |2ν(CH)> and |ν(CH) + ν(bend)>, respectively, are indistinguishable from that for the one-photon excitation. Vibrational relaxation in the electronically excited state prepared by the two-photon excitation scheme is most likely faster than the barrier crossing, making the isomerization insensitive to the method of initial state preparation.

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 avobenzones 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.

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.