Vibrations play a crucial role in many photophysical and photochemical processes in which excitations reside on electronically excited states. Specific coupling between electronic and vibrational degrees of freedom rules structural rearrangement events and efficient energy transfer phenomena. However, despite huge efforts from different research fields, assigning vibrational signals from spectroscopic measurements with congested spectra uniquely to a specific electronic state, ground or otherwise, remains a demanding task. In order to tackle this issue, the present research proposes the development of a chirp modulator for Impulsive Vibrational Scattering (IVS), a powerful technique able to coherently stimulate and probe Raman-active modes using femtosecond broadband pulses. Recently, we have theoretically demonstrated that in IVS the signal originates from different physical processes interfering with each other in a mode-specific way. Building on our previous results and on an existing pump-probe setup, here we propose the realization of a femtosecond pulse shaper able to control the chirp of the ultrashort probe pulse, used for real-time monitoring the molecular vibrations in IVS. The proposed experimental scheme will be used for experimentally demonstrating how a fine tuning of the probe chirp can be used as a novel control knob, allowing us to selectively enhance desired vibrational features and to distinguish spectral components arising from different excited states. Harnessing the possibility to assign the pertaining electronic state to a given molecular vibration, we will be able to track the ultrafast dynamics of photosynthetic complexes, in order to address the highly debated case of vibrational relaxation and energy transfer processes in these essential reaction centres.
The realization of this project will provide a novel spectroscopic tool for assigning vibrational oscillations to the pertaining electronic state. During the last decade, establishing a protocol for discerning between ground and excited state coherences has become a demanding task, in particular for the study of photosynthetic complexes. In recent years, there have been tremendous efforts to understand the basic mechanisms ruling photosynthetic energy-transfer processes, inspiring much inter-disciplinary works, in view of technological application for alternative energy sources development [1]. Sophisticated nonlinear optical techniques, such as two-dimensional spectroscopy, have revealed vibrational signals in transient absorption spectra, whose origins are still topics of lively debates [2]. In fact, despite numerous efforts, overcoming the lack of experimental methods for assigning these vibrational signals to the corresponding electronic pathways remains a challenging task. This is a natural hindrance to the development of bio-inspired artificial systems, that remain still less efficient and less stable than their natural counterparts. Notably, for efficient photosynthesis, energy migrates through large chromophore assemblies to the active site of charge generation. Although this transfer is generally downhill in energy, every energy transfer step must obey the law of energy conservation. This means that vibrational or environmental modes take up the excess energy of each transfer step. For this reason, identifying and assigning excited state vibrations where energy is conveyed would enable to unveil the mechanisms ruling energy transfer processes.
One of the prototypical light-harvesting systems is represented by the Fenna-Matthews-Olson (FMO) complex, which is the mediation of the energy transfer from the chlorosome antenna to the reaction centre of green sulfur bacteria [3]. The FMO complex has a trimeric structure where each subunit contains eight bacteriochlorophyll pigments, strongly interacting with each other due to their arrangement within the protein scaffold. By exploiting two-dimensional electronic spectroscopy (2DES) long-lived vibrational coherences were observed [2]. Whether these signals originate in the ground or excited electronic states reflects different physical interpretations of the role of coherence in energy transfer. For instance, vibrations that couple strongly to electronic transitions give rise to long-lived oscillatory signals according to several theoretical calculations [2]. Unfortunately, underdamped vibrations in the ground electronic state also yield nearly identical signals, while these nuclear motions do not have influence on energy transfer that resides in the excited-state manifold. Moreover, the electronic spectra of these systems present a complex and congested structure, with overlapping features in the absorption profiles, which cannot be uniquely ascribed to ground or excited state. Critically, despite a decade of extensive studies, the pertaining electronic states of the vibrations detected in FMO complexes have not been assigned and how these coherences mediate energy transfer remains an open question.
If accepted, this proposal will be the first experimental realization of an IVS setup with tunable chirped probe pulses for studying the role of vibrations in photosynthetic complexes. As already emphasized, in order to realize efficient artificial photosystems, it is crucial to obtain a detailed picture of the electronic-vibrational couplings, relevant for energy transfer in natural light harvesters. To this aims, we will focus on the energy transfer between chlorosome antenna to the reaction centre of green sulfur bacteria mediated by FMO.
Despite providing the chance to uncover the mechanism ruling efficient energy transfer in this photosynthetic complex, this proposal promises to boost the IVS spectroscopic paradigm, developing a novel experimental protocol based on wisely chirping of the PP. In view of the wide use within the scientific community of IVS, which has been exploited for tackling highly debated topics, as testified by recent seminal works [4-7], we believe that the whole scientific community will benefit from the outcomes of this proposal.
References:
1-Huelga S.et al., Contemp. Phys. 54, 181 (2013)
2-Engel G. S. et al., Nature, 446, 782¿786, (2007)
3-Dostál J. et al., Nat. Chem. 8, 705 (2016)
4-Schnedermann C. et al., J. Am. Chem. Soc. 137, 2886 (2015)
5-Kuramochi H. et al, Nature Chemistry 2017
6- Fujisawa T. et al., J. Am. Chem. Soc. 138, 3942 (2016)
7- Monacelli L. et al, J. Phys. Chem. Lett., 8, 966 (2017)