Ultrafast light-induced processes in molecular systems rely on the efficient conversion of photon energy into atomic motions and are tightly controlled by vibrational couplings between different normal modes. Upon light absorption the optical energy converted into vibrational motions is initially stored in an excited Franck-Condon manifold and is then funneled to different vibrational degrees of freedom. Vibrational Mode Coupling (VMC) is the bio-physical mechanism determining efficient and selective energy relaxation processes, characterizing the molecular multidimensional energy surface that describes how the potential energy surface (PES) of the system changes with modification of the geometrical configuration. Critically, establishing a spectroscopic protocol to access the VMCs that rule the ultrafast evolution of photoexcited molecules is typically hampered by the need of multidimensional spectroscopic probes detecting different energy scales with high temporal and frequency resolution and remains an open challenge. This project aims to precisely tackle this issue introducing a 2-Dimensional Femtosecond Coherent Raman Spectroscopy (2D-FCRS) scheme: the basic idea is to exploit a 8 fs pump pulse to selectively excite vibrational wave-packets in a desired electronic excited state, whose evolution is tracked by the joint action of a femtosecond probe pulse and a narrowband Raman pump. Since the evolution of these wave-packets is determined by the mode couplings in the vibrationally structured PES of the system, the proposed scheme will be the key to map the multidimensional energy surfaces involved in the process. 2D-FCRS will be applied to study the paradigmatic case of the Green Fluorescent Protein ultrafast dynamics, addressing the highly debated case of the excited state proton transfer in this essential bioimaging dye. We anticipate that the introduced approach will bring about impactful insights into the reaction dynamics of photo-active molecular compounds
The realization of this project will provide a novel spectroscopic tool for measuring vibrational mode couplings pertaining to Raman excitations in a desired electronic state. During the last decade, tremendous efforts have been devoted to the understanding of the basic mechanisms ruling ultrafast energy-transfer processes, fuelling much inter-disciplinary works between Physics and Biochemistry and inspiring novel artificial biochemical tool for visualizing gene expression and protein function in living cells [14]. In this respect, establishing a protocol for selectively accessing VMCs between different active sites in molecules has become a demanding task, in particular for the study of photo-active polyatomic molecules. In fact, during ultrafast chemical rearrangement low and high frequency modes can act as energy acceptors converting light into vibrational energy, which is then funneled to different vibrational degrees of freedom. For example, in Hemeproteins, the vibrational energy is initially stored in the highly excited Franck-Condon manifold and is then transferred on sub-picosecond time regimes into lower frequency modes, prior to a few picoseconds dissipation through the protein [4,5]. On a more general perspective, different low frequency modes, such as collective carbon-bond vibrational modes or protein phonon modes, have been shown to drive chemical reactions pointing to the crucial role of the mode coupling between the high and the low frequency manifolds [15-19]. VMCs are the bio-physical mechanisms ruling efficient and selective energy relaxation/redistribution processes and, in this respect, the Green Fluorescent Protein photo-physics has been object of fruitful scientific debates. GFP luminescence is known to arise from excited-state proton transfer (ESPT), evolving to a long-lived and highly fluorescent state; anyway, the microscopic details of the ESPT process are still not fully understood and recently, the role of the low frequency phenoxy-ring motions, which may optimize the geometry of the chromophore for ESPT, has been object of an open debate [3, 13].
If accepted, this proposal will be the first experimental realization of an 2D-FCRS setup for studying the role of vibrational couplings in photo-excited molecular complexes and we plan to apply the developed scheme to unravel the low frequency modes that drive the excited-state proton transfer in GFP, accessing their couplings with the high frequency vibrational manifold. Despite providing the chance to uncover the mechanism ruling efficient ESPT in GFP, this proposal promises to boost the ultrafast coherent Raman spectroscopic paradigm, developing a novel 2-dimensional powerful spectroscopic protocol.
In view of the wide interest within the scientific community of the molecular properties accessed by the present project and used for tackling debated topics [3, 4, 13], we believe that the whole scientific community will benefit from the outcomes of this proposal.
References:
[14] Salaris F., Brain Res., 1723, 146393 (2019).
[15] Liu D. et al., Phys. Rev. Lett. 101, 135501 (2008).
[16] Turton D. A., Nat. Commun. 5, 3999 (2014).
[17] Schnedermann C. et al., J. Am. Chem. Soc. 137, 2886 (2015).
[18] Schnedermann C. et al., J. Phys. Chem. A, 119, 36, 9506 (2015).
[19] Kuramochi H. et al, Nat. Chem. 9, 660 (2017).