Tullio Scopigno

Perylene bisimides (PBIs) are prime building blocks for functional supramolecular architectures, yet mapping their salt-induced self-assembly kinetics under physiological conditions remains challenging, due to rapid aggregation and overwhelming fluorescence. Here, we combine fluorescence stopped-flow kinetics and stimulated Raman scattering (SRS) spectroscopy to decipher the electrolyte-controlled polymerization of a spermine-functionalized PBI. We found that self-assembly obeys an anticooperative K2–K pathway, where nucleation yields rapid and salt-independent dimers. In contrast, polymer growth is strongly salt-dependent, with polymer dissociation rates decreasing as NaCl concentration increases. Crucially, chloride ions selectively stabilize the growing assemblies by drastically suppressing the monomer release, establishing a Michaelis–Menten-type saturation profile. To link these kinetic trajectories with molecular-scale structural rearrangements, SRS is used to effectively suppress the fluorescence background, revealing that the relative intensity of low-frequency Raman modes, associated with high-mass displacement and structurally delocalized in nature, are drastically reduced upon aggregation. This vibrational suppression underscores the role of packing constraints and excitonic pressure within the cofacial H-aggregates. By directly bridging macroscale kinetic pathways with microscopic vibrational dynamics, this work offers a generalizable framework to rationally design and tune water-soluble supramolecular materials for biomedical and optoelectronic applications.

Accessing excited-state vibrations is crucial to elucidate physical and chemical properties of photoreactive molecules and solids. Time-resolved resonance Raman spectroscopy provides unique vibrational fingerprints of different electronic levels, but its spontaneous version often suffers from strong fluorescence backgrounds and has its spectral/temporal resolutions fundamentally limited by the Fourier principle, hindering spectral assignments and the investigation of ultrafast dynamics. Here, we discuss how these issues can be overcome using Impulsive Stimulated Raman spectroscopy, a pump-probe technique that tracks coherent vibrational motion on different electronic states in real time. Using the highly fluorescent cyanine dye IR-775 as an experimental benchmark, we show that broadband probe detection isolates Raman spectra across different resonances, enabling unambiguous excited-state assignment. The time-domain scheme effectively measures fluorescence-free spectra, revealing a low-frequency Raman mode at 130 cm−1 in the electronic ground and first excited states, which plays a critical role in vibronic coupling with higher-lying electronic levels. Finally, we highlight the technique's versatility in light of recent experiments and related perspectives. These include two-color schemes with pulses spanning from X-rays to the infrared, the use of pulse chirp as a powerful control knob to unveil specific spectroscopic signatures, and strategies to amplify the scattering cross section.

Water’s polarity and hydrogen-bond network give rise to its unique chemical and biochemical behaviour. Its vibrational motions, occurring on a few-femtosecond timescale, govern ultrafast energy transfer within the hydrogen-bond network. However, direct real-time observation of these motions has remained elusive due to the extreme temporal resolution required. Here, we investigate the ground-state vibrational dynamics of liquid water initiated by a sub-5 fs near-infrared (NIR) pump pulse via Impulsive Stimulated Raman Scattering (ISRS). Using few-fs ultraviolet (UV) probe pulses transmitted through a 5 µm-thick liquid jet, we monitor the coherent vibrational wave packet dominated by the OH stretch mode, exhibiting a 10 fs oscillation period and a 25 fs damping time. These results reveal the rapid dephasing of the OH stretch mode preceding its relaxation through coupling to the bending vibrations, highlighting the importance of intermolecular couplings of liquid water in the high frequency vibrational dynamics.

Water plays a critical role in chemistry and biochemistry due to its unique properties that arise from its polar nature and the hydrogen-bonding network it forms. Its vibrational motions occur on the few-femtosecond timescale and are pivotal for energy transfer within the hydrogen-bond network. Despite the latter, the real-time observation of these motions has not been achieved so far due to the high time resolution necessary to capture them. Here, we investigate the ground state vibrational dynamics of liquid water initiated by a sub-5 fs near-infrared (NIR) pump pulse via Impulsive Stimulated Raman Scattering (ISRS). By monitoring the transient transmission of few-fs ultraviolet (UV) probe pulses through a 5µm-thick liquid jet, we track in real-time the motion of the coherent vibrational wave packet, which is dominated by the high-frequency OH stretch mode, with a 10-fs oscillation period and a damping time of 25 fs. This reveals the rapid dephasing of the OH stretch mode that precedes its relaxation via coupling to the bend modes, highlighting the importance of intermolecular couplings of liquid water in the high frequency vibrational dynamics.

Because of their sub picosecond temporal resolution, coherent Raman spectroscopies have been proposed as a viable extension of spontaneous Raman thermometry, to determine dynamics of mode specific vibrational energy content during out of equilibrium molecular processes. Here we show that the presence of multiple laser fields stimulating the vibrational coherences introduces additional quantum pathways, resulting in destructive interference. This ultimately reduces the thermal sensitivity of single spectral lines, nullifying it for harmonic vibrations and temperature independent polarizability. We demonstrate how harnessing anharmonic signatures such as vibrational hot bands enables coherent Raman thermometry.

Pump-probe spectroscopy is a powerful tool to investigate light-induced dynamical processes in molecules and solids. Targeting vibrational excitations occurring on the time scales of nuclear motions is challenging, as pulse durations shorter than a vibrational period are needed to initiate the dynamics, and complex experimental schemes are required to isolate weak signatures arising from wavepacket motion in different electronic states. Here, we demonstrate how introducing a temporal delay between the spectral components of femtosecond beams, namely a chirp resulting in the increase of their duration, can counterintuitively boost the desired signals by 2 orders of magnitude. Measuring the time-domain vibrational response of permanganate ions embedded in a KClO4 matrix, we identify an intricate dependence of the vibrational response on pulse chirps and probed wavelength that can be exploited to unveil weak signatures of the doping ions-otherwise dominated by the nonresonant matrix-or to obtain vibrational excitations pertaining only to the excited state, suppressing ground-state contributions.

Noise manifests ubiquitously in nonlinear spectroscopy, where multiple sources contribute to experimental signals generating interrelated unwanted components, from random point-wise fluctuations to structured baseline signals. Mitigating strategies are usually heuristic, depending on subjective biases such as the setting of parameters in data analysis algorithms and the removal order of the unwanted components. We propose a data-driven frequency-domain denoiser based on a convolutional neural network to extract authentic vibrational features from a nonlinear background in noisy spectroscopic raw data. The different spectral scales in the problem are treated in parallel by means of filters with multiple kernel sizes, which allow the receptive field of the network to adapt to the informative features in the spectra. We test our approach by retrieving asymmetric peaks in stimulated Raman spectroscopy, an ideal test-bed due to its intrinsic complex spectral features combined with a strong background signal. By using a theoretical perturbative toolbox, we efficiently train the network with simulated datasets resembling the statistical properties and lineshapes of the experimental spectra. The developed algorithm is successfully applied to experimental data to obtain noise- and background-free stimulated Raman spectra of organic molecules and prototypical heme proteins.

Due to their sub picosecond temporal resolution, coherent Raman spectroscopies have been proposed as a viable extension of Spontaneous Raman (SR) thermometry, to determine dynamics of mode specific vibrational energy content during out of equilibrium molecular processes. Here we show that the presence of multiple laser fields stimulating the vibrational coherences introduces additional quantum pathways, resulting in destructive interference. This ultimately reduces the thermal sensitivity of single spectral lines, nullifying it for harmonic vibrations and temperature independent polarizability. We demonstrate how harnessing anharmonic signatures such as vibrational hot bands enables coherent Raman thermometry.

Ultrafast reactions activated by light absorption are governed by multidimensional excited-state (ES) potential energy surfaces (PESs), which describe how the molecular potential varies with the nuclear coordinates. ES PESs ad-hoc displaced with respect to the ground state can drive subtle structural rearrangements, accompanying molecular biological activity and regulating physical/chemical properties. Such displacements are encoded in the Franck-Condon overlap integrals, which in turn determine the resonant Raman response. Conventional spectroscopic approaches only access their absolute value, and hence cannot determine the sense of ES displacements. Here, we introduce a two-color broadband impulsive Raman experimental scheme, to directly measure complex Raman excitation profiles along desired normal modes. The key to achieve this task is in the signal linear dependence on the Frank-Condon overlaps, brought about by non-degenerate resonant probe and off-resonant pump pulses, which ultimately enables time-domain sensitivity to the phase of the stimulated vibrational coherences. Our results provide the tool to determine the magnitude and the sensed direction of ES displacements, unambiguously relating them to the ground state eigenvectors reference frame.