Elucidating transient molecular structural transformation steps from the reactant to product state during chemical reactions has been a grand challenge for physical chemists, biophysicists, and materials scientists. The main hurdle lies in the spatial and/or time resolution to spectroscopically characterize reaction pathways because the intrinsic atomic motions occur on ultrafast (i.e., femtosecond to picosecond) timescales. To guide the rational design of functional materials and biomolecules, we have developed a structural dynamics tool called femtosecond stimulated Raman spectroscopy (FSRS)^1-4 with wavelength-tunable Raman pump and probe pulses to capture structural snapshots of functional molecules in condensed phase, from aqueous solution, organic solvent to protein environment. Using low-frequency FSRS with broadband up-converted multicolor array (BUMA) technology,^4,5 we track the aqueous aluminum speciation as a function of pH during electrolytic synthesis of flat Al₁₃ nanoclusters that are versatile solution precursors to make environment-friendly Al₂O₃ thin films.⁶ Upon incorporation of an actinic 400 nm pump pulse, we monitor the initial structural evolution of photoexcited chromophore inside the protein pocket of a series of non-invasive, genetically encodable fluorescent protein biosensors for Ca²⁺ imaging.^7,8 The fluorescence modulation mechanism has been revealed to strongly correlate with ultrafast conformational dynamics of the embedded three-residue chromophore that exhibits different excited-state proton transfer capabilities in various microenvironments. The versatility and unique resolving power of broadband tunable FSRS, aided by femtosecond transient absorption, our newly developed time-resolved third-harmonic generation spectroscopy, quantum calculations and molecular dynamics simulations, provide a new powerful toolset to elucidate the multidimensional reaction coordinate in the electronic ground and excited state.

References

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     (5)   Liu, W.; Zhu, L.; Wang, L.; Fang, C. Opt. Lett. 2013, 38, 1772-1774.

     (6)   Wang, W.; Liu, W.; Chang, I.-Y.; Wills, L. A.; Zakharov, L. N.; Boettcher, S. W.; Cheong, P. H.-Y.; Fang, C.; Keszler, D. A. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 18397-18401.

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     (8)   Tang, L.; Liu, W.; Wang, Y.; Zhao, Y.; Oscar, B. G.; Campbell, R. E.; Fang, C. Chem. Eur. J. 2015, 21, 6481-6490.