The wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS) has been known for its high selectivity and versatility to study steady-state and dynamical processes of photoactive chemical and biological systems. Using resonance Raman enhancement, one could selectively enhance the signal from transient species during an ultrafast chemical reaction. Recently, we have implemented tunable FSRS and femtosecond transient absorption spectroscopy to investigate two series of photoacids derived from the green fluorescent protein (GFP) core chromophore 4’-hydroxybenzylidene-1,2-dimethyl-imidazolinone (p-HBDI). By conformational locking and site-specific fluorination, the chromophore has been engineered to be highly fluorescent and of superphotoacidity in solution, enabling excited-state proton transfer (ESPT) to inert proton acceptors such as alcohols. The ESPT dynamics of superphotoacids were shown to be solvation-limited with fast proton transfer (~300 fs) and slower solvent reorientation (e.g., 5-8 ps for methanol). This study provides the mechanistic understanding and design strategy to achieve stronger photoacids and redder, brighter fluorophores with bioimaging applications. In particular, based on excited-state intramolecular charge transfer (ICT), one could separately tune the ground- and excited-state properties, e.g., energy and pKa, to synergistically achieve redder emission and superphotoacidity. Aided by systematic organic synthesis of locked and unlocked chromophore derivatives, we demonstrated that the electronic state properties as well as emission wavelengths can be altered by modifying the donor and acceptor moieties in an additive manner. Such powerful structural dynamics characterization can rationally guide the molecular engineering efforts.

Recent advances of tunable femtosecond stimulated Raman spectroscopy to design and develop superphotoacids and redder fluorophores

2019 Fall National Meeting

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