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1. Torsionally broken symmetry assists infrared excitation of biomimetic charge-coupled nuclear motions in the electronic ground state

   https://doi.org/10.1039/D2SC02133A  

   Chatterjee, Gourab; Jha, Ajay; Blanco-Gonzalez, Alejandro; Tiwari, Vandana; Manathunga, Madushanka; Duan, Hong-Guang; Tellkamp, Friedjof; Prokhorenko, Valentyn I.; Ferré, Nicolas; Dasgupta, Jyotishman; et al (August 2022, Chemical Science)

   The concerted interplay between reactive nuclear and electronic motions in molecules actuates chemistry. Here, we demonstrate that out-of-plane torsional deformation and vibrational excitation of stretching motions in the electronic ground state modulate the charge-density distribution in a donor-bridge-acceptor molecule in solution. The vibrationally-induced change, visualised by transient absorption spectroscopy with a mid-infrared pump and a visible probe, is mechanistically resolved by ab initio molecular dynamics simulations. Mapping the potential energy landscape attributes the observed charge-coupled coherent nuclear motions to the population of the initial segment of a double-bond isomerization channel, also seen in biological molecules. Our results illustrate the pivotal role of pre-twisted molecular geometries in enhancing the transfer of vibrational energy to specific molecular modes, prior to thermal redistribution. This motivates the search for synthetic strategies towards achieving potentially new infrared-mediated chemistry. 

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2. Shedding light on ultrafast ring-twisting pathways of halogenated GFP chromophores from the excited to ground state

   https://doi.org/10.1039/D1CP02140K  

   Boulanger, Sean A.; Chen, Cheng; Tang, Longteng; Zhu, Liangdong; Baleeva, Nadezhda S.; Myasnyanko, Ivan N.; Baranov, Mikhail S.; Fang, Chong (July 2021, Physical Chemistry Chemical Physics)

   null (Ed.)

   Since green fluorescent protein (GFP) has revolutionized molecular and cellular biology for about three decades, there has been a keen interest in understanding, designing, and controlling the fluorescence properties of GFP chromophore ( i.e. , HBDI) derivatives from the protein matrix to solution. Amongst these cross-disciplinary efforts, the elucidation of excited-state dynamics of HBDI derivatives holds the key to correlating the light-induced processes and fluorescence quantum yield (FQY). Herein, we implement steady-state electronic spectroscopy, femtosecond transient absorption (fs-TA), femtosecond stimulated Raman spectroscopy (FSRS), and quantum calculations to study a series of mono- and dihalogenated HBDI derivatives (X = F, Cl, Br, 2F, 2Cl, and 2Br) in basic aqueous solution, gaining new insights into the photophysical reaction coordinates. In the excited state, the halogenated “floppy” chromophores exhibit an anti-heavy atom effect, reflected by strong correlations between FQY vs. Franck–Condon energy ( E FC ) or Stokes shift, and k nr vs. E FC , as well as a swift bifurcation into the I-ring (major) and P-ring (minor) twisting motions. In the ground state, both ring-twisting motions become more susceptible to sterics and exhibit spectral signatures from the halogen-dependent hot ground-state absorption band decay in TA data. We envision this type of systematic analysis of the halogenated HBDI derivatives to provide guiding principles for the site-specific modification of GFP chromophores, and expand design space for brighter and potentially photoswitchable organic chemical probes in aqueous solution with discernible spectral signatures throughout the photocycle. 

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3. From Coherence to Function: Exploring the Connection in Chemical Systems

   https://doi.org/10.1021/acs.accounts.4c00312  

   Rather, Shahnawaz R; Scholes, Gregory D; Chen, Lin X (September 2024, Accounts of Chemical Research)

   Conspectus: The role of quantum mechanical coherences or coherent superposition states in excited state processes has received considerable attention in the last two decades largely due to advancements in ultrafast laser spectroscopy. These coherence effects hold promise for enhancing the efficiency and robustness of functionally relevant processes, even when confronted with strong energy disorder and environmental fluctuations. Understanding coherence deeply drives us to unravel mechanisms and dynamics controlled by order and synchronization at a quantum mechanical level, envisioning optical control of coherence to enhance functions or create new ones in molecular and material systems. In this frontier, the interplay between electronic and vibrational dynamics, specifically the influence of vibrations in directing electronic dynamics, has emerged as the leading principle. Here, two energetically disparate quantum degrees of freedom work in-sync to dictate the trajectory of an excited state reaction. Moreover, with the vibrational degree being directly related to the structural composition of molecular or material systems, new molecular designs could be inspired by tailoring certain structural elements. In the realm of chemical kinetics, our understanding of the dynamics of chemical transformations is underpinned by fundamental theories such as transition state theory, activated rate theory, and Marcus theory. These theories elucidate reaction rates by considering the energy barriers that must be overcome for reactants to transform into products. Those barriers are surmounted by the stochastic nature of energy gap fluctuations within reacting systems, emphasizing that the reaction coordinate—the pathway from reactants to products—is not rigidly defined by a specific vibrational motion but encompasses a diverse array of molecular motions. While less is known about the involvement of specific intramolecular vibrational modes, their significance in certain cases cannot be overlooked. In this Account, we summarize key experimental findings that offer deeper insights into the complex electronic-vibrational trajectories encompassing excited states afforded from state-of-the-art ultrafast laser spectroscopy in three exemplary processes: photo-induced electron transfer, singlet-triplet intersystem crossing, and intramolecular vibrational energy flow in molecular systems. We delve into rapid decoherence—loss of phase and amplitude correlations—of vibrational coherences along promoter vibrations during a sub-picosecond intersystem crossing dynamics in a series of binuclear platinum complexes. This rapid decoherence illustrates the vibration-driven reactive pathways from Franck-Condon state to the curve crossing region. We also explore the generation of new vibrational coherences induced by impulsive reaction dynamics—rather than by the laser pulse—in these systems, which sheds light on specific energy dissipation pathways and thereby on the progression of the reaction trajectory in the vicinity of the curve crossing on the product side. Another property of vibrational coherences, amplitude, reveals how energy can flow from one vibration to another in the electronic excited state of a terpyridine-molybdenum complex hosting a nonreactive dinitrogen substrate. A slight change in vibrational energy triggers a quasi-resonant interaction, leading to constructive wavepacket interference and ultimately intramolecular vibrational redistribution from a Franck-Condon active terpyridine vibration to dinitrogen stretching vibration, energizing the dinitrogen bond. 

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4. Energy- and conformer-dependent excited-state relaxation of an E / Z photoswitchable thienyl-ethene

   https://doi.org/10.1039/c9cp01226e  

   Young, Jamie D.; Honick, Chana R.; Zhou, Jiawang; Pitts, Cody R.; Ghorbani, Fereshte; Peters, Garvin M.; Lectka, Thomas; Tovar, John D.; Bragg, Arthur E. (July 2019, Physical Chemistry Chemical Physics)

   Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E / Z photoisomerization following absorption of visible light. cis -to- trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565–420 nm) transient absorption (TA) studies to probe the photophysics of cis -to- trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (∼4 ps) and slow (∼40 ps) components of spectral dynamics observed at low excitation energies (>470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (P A and P B ) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the P B conformer is found to contribute to triplet population and the switching of cis -4TCE: high-energy excitation (<470 nm) of P B involves direct excitation to S 2 , relaxation from which prepares an ISC-active S 1 geometry (ISC QY 0.4–0.67, k ISC ∼ 1.6–2.6 × 10 −9 s −1 ) that is the gateway to triplet population and isomerization. We ascribe low cis -to- trans isomerization yield to excitation of the nonreactive P A conformer (75–85% loss) as well as loses along the P B S 2 → S 1 → T 1 cascade (10–20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive “antiparallel” ring conformation. 

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5. Simultaneous single-shot rotation–vibration non-equilibrium thermometry using pure rotational fs/ps CARS coherence beating

   https://doi.org/10.1364/OL.453272  

   Chen, Timothy_Y; Liu, Ning; Kliewer, Christopher_J; Dogariu, Arthur; Kolemen, Egemen; Ju, Yiguang (March 2022, Optics Letters)

   We report the development of a simple and sensitive two-beam hybrid femtosecond/picosecond pure rotational coherent anti-Stokes Raman scattering (fs/ps CARS) method to simultaneously measure the rotational and vibrational temperatures of diatomic molecules. Rotation–vibration non-equilibrium plays a key role in the chemistry and thermalization in low-temperature plasmas as well as thermal loading of hypersonic vehicles. This approach uses time-domain interferences between ground state and vibrationally excited N₂molecules to intentionally induce coherence beating that leads to apparent non-Boltzmann distributions in the pure rotational spectra. These distortions enable simultaneous inference of both the rotational and vibrational temperatures. Coherence beating effects were observed in single-shot fs/ps CARS measurements of a 75 Torr N₂DC glow discharge and were successfully modeled for rotational and vibrational temperature extraction. We show that this method can be more sensitive than a pure rotational fs/ps CARS approach using a spectrally narrow probe pulse. Lastly, we experimentally measured the beat frequencies via Fourier transform of the time-domain response and obtained excellent agreement with the model. 

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