UC Berkeley

I have used femtosecond time resolved resonance Raman spectroscopy to probe vibrational dynamics during ultrafast photochemical reactions in synthetic and biological materials, revealing nuclear motions that play key roles in the photoactivity of these systems. These results expose mechanistically significant structural details important for advancing our fundamental understanding of photochemical processes and our ability to design improved photoactive materials. In performing these studies I have improved the capabilities of tunable femtosecond stimulated Raman spectroscopy (FSRS) by developing high-throughput detection techniques and by redesigning the optical layout of the instrument for improved performance, tune-ability, and time-resolution.

My initial studies were performed on a synthetic photoactive system relevant for electron transfer. The iron(II) complex, [Fe(tren(py)₃)]²⁺, is a spin-crossover compound that undergoes an ultrafast ∆S = 2 transition upon excitation of its metal-to-ligand charge transfer band at ~ 560 nm. Using time resolved FSRS I was able to record the vibrational dynamics of this intersystem crossing during the 5 ps following actinic initiation of the photochemistry. Analysis of the time resolved vibrational spectra show that the spin-crossover process takes place in < 200 fs, and is intimately associated with the expansion of iron-ligand bonds, providing important temporal and structural characterization of the photoreactivity of this compound.

I have also completed a study of the structural dynamics of the biological photoreceptor photoactive yellow protein (PYP). Upon absorption of light, the PYP chromophore, para-hydroxy-cinnamic acid, undergoes trans-cis isomerization in < 3 ps defining the primary photochemistry of the PYP photocycle. The constraints on the chromophore imposed by the protein binding pocket suggest an isomerization mechanism that involves the out-of-plane rotation of the chromophore's C₉=O carbonyl. Using FSRS I was able to acquire vibrational spectra of the PYP chromophore from 0 fs to 300 ps following photoexcitation, recording the dynamics of the C₉=O out-of-plane vibration during the initiation of the PYP photocycle for the first time. Following excitation, these data show structural evidence for a ~ 150 fs charge shift in the chromophore excited state preceding isomerization. The frequency of the C₉=O out of plane vibration downshifts in ~ 800 fs as the excited chromophore decays to the early cissoid photocycle intermediate, I₀, confirming the key role of carbonyl motion for entering the active photocycle. Following formation of I₀, the structure of the chromophore is highly distorted. The relaxation of this distortion is likely a key driving force for the continuation of the PYP photocycle.

The results presented in this work provide important benchmarks for the two systems studied, as well as laying the groundwork for future mechanistic studies on a variety of photochemical systems. The technique and methodology presented may be applied to the development of synthetic photoactive materials in a straightforward way, providing chemists with a direct means of identifying structural elements that promote the desired photoactivity. These experiments demonstrate the value of time resolved structural characterization in developing a clear understanding of ultrafast chemical processes, in addition to demonstrating FSRS' potential as a valuable tool for revealing the relationship between structure and function in photochemical systems.