Two-dimensional coherent electronic spectrometer with switchable multi-color configurations

Broadband implementation of two-dimensional electronic spectroscopy (2DES) is a desirable goal for numerous research groups, yet achieving it presents considerable challenges. An effective strategy to mitigate these challenges is the utilization of two-color approaches, effectively broadening the spectral bandwidth accessible with 2DES. Here, we present a simple approach to include multi-color configurations based on adjustable mirror mounts. This enables seamless toggling between single-color, two-color, and transient 2DES within the same spectroscopic apparatus, which is benchmarked on two common laser dyes, Rhodamine 6G and Nile blue. Upon mixing the dyes, single-color 2DES shows overlapping signals, whereas a high selectivity toward Nile blue responses is maintained in two-color and transient 2DES, owing to the fully resonant excitation that is spectrally shifted relative to the detection window. This method is readily implemented in other setups with similar experimental layouts and can be used as a simple solution to overcome existing bandwidth limitations. With the inclusion of transient 2DES, additional insights into excited-state processes can be gained due to its increased sensitivity toward excited-state coherences.

Uncovering the Functional Role of Coherent Phonons during the Photoinduced Phase Transition in a Molecular Crystal

The atomic motions that make up phonons and molecular vibrations in molecular crystals influence their photophysical and electronic properties, including polaron formation, carrier mobility, and phase transitions. Discriminating between spectator and driving motions is a significant challenge hindering optimization. Unlocking this information and developing fine-tuned controls over actively participating phonon modes would not only lead to a stronger understanding of photochemistry but also provide a significant new tool in controlling solid state chemistry. We present a strategy using rationally designed double pulses to unveil the unique function of specific excited state phonon modes. Using ultrafast spectroscopy, we identified 50 and 90 cm^-1 phonons involved in modulating the photoinduced spin-Peierls melting of potassium tetracyanoquinodimethane crystals. We show that the 50 cm^-1 phonon specifically corresponds to the coherent nuclear wavepacket involved in the charge transfer component of the overall spin-Peierls phase melting process, while the 90 cm^-1 phonon facilitates the phase transition component.

Spectral Signatures of Ground- and Excited-State Wavepacket Interference after Impulsive Excitation

Impulsive transient absorption spectroscopy is used to track the formation and evolution of vibrational coherences in cresyl violet perchlorate under different excitation conditions. Resonant and off-resonant pump pulses result in the selective formation of excited (S₁)- and ground (S₀)-state wavepackets. Partially resonant and broadband excitation conditions lead to the simultaneous formation of wavepackets in the ground and excited states. The wavepackets are characterized by the phase-flips in the coherent signal associated with wavepacket motion across the absorption and emission maxima and by a red shift of 2-10 cm^-1 in the Raman features of the excited state compared to the ground-state wavepacket. We observe that, when wavepackets are simultaneously excited on the ground- and excited-state surfaces, interference on a picosecond timescale between coherent oscillations in the two wavepackets gives rise to features that cannot be attributed to the passage of a wavepacket through a conical intersection, such as shifting phase-flips and zero-amplitude nodes. Wavepacket filtering using windowed Fourier transforms highlights these interference effects and demonstrates that special care must be taken in order to properly interpret data that have been processed in this manner.

Excited-State Vibronic Dynamics of Bacteriorhodopsin from Two-Dimensional Electronic Photon Echo Spectroscopy and Multiconfigurational Quantum Chemistry

Owing to the ultrafast time scale of the photoinduced reaction and high degree of spectral overlap among the reactant, product, and excited electronic states in bacteriorhodopsin (bR), it has been a challenge for traditional spectroscopies to resolve the interplay between vibrational dynamics and electronic processes occurring in the retinal chromophore of bR. Here, we employ ultrafast two-dimensional electronic photon echo spectroscopy to follow the early excited-state dynamics of bR preceding the isomerization. We detect an early periodic photoinduced absorptive signal that, employing a hybrid multiconfigurational quantum/molecular mechanical model of bR, we attribute to periodic mixing of the first and second electronic excited states (S₁ and S₂, respectively). This recurrent interaction between S₁ and S₂, induced by a bond length alternation of the retinal chromohore, supports the hypothesis that the ultrafast photoisomerization in bR is initiated by a process involving coupled nuclear and electronic motion on three different electronic states.

Probing Structure and Reaction Dynamics of Proteins Using Time-Resolved Resonance Raman Spectroscopy

The mechanistic understanding of protein functions requires insight into the structural and reaction dynamics. To elucidate these processes, a variety of experimental approaches are employed. Among them, time-resolved (TR) resonance Raman (RR) is a particularly versatile tool to probe processes of proteins harboring cofactors with electronic transitions in the visible range, such as retinal or heme proteins. TR RR spectroscopy offers the advantage of simultaneously providing molecular structure and kinetic information. The various TR RR spectroscopic methods can cover a wide dynamic range down to the femtosecond time regime and have been employed in monitoring photoinduced reaction cascades, ligand binding and dissociation, electron transfer, enzymatic reactions, and protein un- and refolding. In this account, we review the achievements of TR RR spectroscopy of nearly 50 years of research in this field, which also illustrates how the role of TR RR spectroscopy in molecular life science has changed from the beginning until now. We outline the various methodological approaches and developments and point out current limitations and potential perspectives.

Effect of point mutations on the ultrafast photo-isomerization of Anabaena sensory rhodopsin

Anabaena sensory rhodopsin (ASR) is a particular microbial retinal protein for which light-adaptation leads to the ability to bind both the all-trans, 15-anti (AT) and the 13-cis, 15-syn (13C) isomers of the protonated Schiff base of retinal (PSBR). In the context of obtaining insight into the mechanisms by which retinal proteins catalyse the PSBR photo-isomerization reaction, ASR is a model system allowing to study, within the same protein, the protein-PSBR interactions for two different PSBR conformers at the same time. A detailed analysis of the vibrational spectra of AT and 13C, and their photo-products in wild-type ASR obtained through femtosecond (pump-) four-wave-mixing is reported for the first time, and compared to bacterio- and channelrhodopsin. As part of an extensive study of ASR mutants with blue-shifted absorption spectra, we present here a detailed computational analysis of the origin of the mutation-induced blue-shift of the absorption spectra, and identify electrostatic interactions as dominating steric effects that would entail a red-shift. The excited state lifetimes and isomerization reaction times (IRT) for the three mutants V112N, W76F, and L83Q are studied experimentally by femtosecond broadband transient absorption spectroscopy. Interestingly, in all three mutants, isomerization is accelerated for AT with respect to wild-type ASR, and this the more, the shorter the wavelength of maximum absorption. On the contrary, the 13C photo-reaction is slightly slowed down, leading to an inversion of the ESLs of AT and 13C, with respect to wt-ASR, in the blue-most absorbing mutant L83Q. Possible mechanisms for these mutation effects, and their steric and electrostatic origins are discussed.

Point Mutation of Anabaena Sensory Rhodopsin Enhances Ground-State Hydrogen Out-of-Plane Wag Raman Activity

The interaction between the retinal protonated Schiff base (RPSB) and surrounding protein residues inside the retinal pocket is believed to play a major role in the ultrafast isomerization of the former. Coherent time-resolved vibrational spectroscopic techniques are applied to reveal the effect of changes in the protein architecture by point mutations (V112N and L83Q) close to the RPSB in Anabaena sensory rhodopsin (ASR). Our study reveals that such point mutations have a minor effect on the low-frequency (<400 cm^-1) torsional modes but dramatically influence the ground-state vibrational Raman activity of the C₁₄-H out-of-plane (HOOP) wag mode (800-820 cm^-1). In mutated ASR, the increase of HOOP Raman activity in the ground state is experimentally observed for the all- trans RPSB, which has shorter excited-state lifetime than in wild-type ASR. This indicates that predistortion of the RPSB inside the mutated retinal pocket is a major factor in the acceleration of the isomerization rate.

Excited State Vibrational Spectra of All- trans Retinal Derivatives in Solution Revealed By Pump-DFWM Experiments

The ultrafast structural changes during the photoinduced isomerization of the retinal-protonated Schiff base (RPSB) is still a poorly understood aspect in the retinal's photochemistry. In this work, we apply pump-degenerate four-wave mixing (pump-DFWM) to all- trans retinal (ATR) and retinal Schiff bases (RSB) to resolve coherent high- and low-frequency vibrational signatures from excited electronic states. We show that the vibrational spectra of excited singlet states in these samples exhibit pronounced differences compared to the relaxed ground state. Pump-DFWM results indicate three major features for ATR and RSB. (i) Excited state vibrational spectra of ATR and RSB consist predominately of low-frequency modes in the energetic range 100-500 cm^-1. (ii) Excited state vibrational spectra show distinct differences for excitation in specific regions of electronic transitions of excited state absorption and emission. (iii) Low-frequency modes in ATR and RSB are inducible during the entire lifetime of the excited electronic states. This latter effect points to a transient molecular structure that, following initial relaxation between different excited electronic states, does not change anymore over the lifetime of the finally populated excited electronic state.

Mapping the ultrafast vibrational dynamics of all-trans and 13-cis retinal isomerization in Anabaena Sensory Rhodopsin

Discrepancies in the isomerization dynamics and quantum yields of the trans and cis retinal protonated Schiff base is a well-known issue in the context of retinal photochemistry. Anabaena Sensory Rhodopsin (ASR) is a microbial retinal protein that comprises a retinal chromophore in two ground state (GS) conformations: all-trans, 15-anti (AT) and 13-cis, 15-syn (13C). In this study, we applied impulsive vibrational spectroscopic techniques (DFWM, pump-DFWM and pump-IVS) to ASR to shed more light on how the structural changes take place in the excited state within the same protein environment. Our findings point to distinct features in the ground state structural conformations as well as to drastically different evolutions in the excited state manifold. The ground state vibrational spectra show stronger Raman activity of the C14-H out-of-plane wag (at about 805 cm-1) for the 13C isomer than that for the AT isomer, which hints at a pre-distortion of 13C in the ground state. Evolution of the Raman frequency after interaction with the actinic pulse shows a blue-shift for the C[double bond, length as m-dash]C stretching and CH3 rocking mode for both isomers. For AT, however, the blue-shift is not instantaneous as observed for the 13C isomer, rather it takes more than 200 fs to reach the maximum frequency shift. This frequency blue-shift is rationalized by a decrease in the effective conjugation length during the isomerization reaction, which further confirms a slower formation of the twisted state for the AT isomer and corroborates the presence of a barrier in the excited state trajectory previously predicted by quantum chemical calculations.

Coherent phenomena in photosynthetic light harvesting: part two-observations in biological systems

Considerable debate surrounds the question of whether or not quantum mechanics plays a significant, non-trivial role in photosynthetic light harvesting. Many have proposed that quantum superpositions and/or quantum transport phenomena may be responsible for the efficiency and robustness of energy transport present in biological systems. The critical experimental observations comprise the observation of coherent oscillations or "quantum beats" via femtosecond laser spectroscopy, which have been observed in many different light harvesting systems. Part Two of this review aims to provide an overview of experimental observations of energy transfer in the most studied light harvesting systems. Length scales, derived from crystallographic studies, are combined with energy and time scales of the beats observed via spectroscopy. A consensus is emerging that most long-lived (hundreds of femtoseconds) coherent phenomena are of vibrational or vibronic origin, where the latter may result in coherent excitation transport within a protein complex. In contrast, energy transport between proteins is likely to be incoherent in nature. The question of whether evolution has selected for these non-trivial quantum phenomena may be an unanswerable question, as dense packings of chromophores will lead to strong coupling and hence non-trivial quantum phenomena. As such, one cannot discern whether evolution has optimised light harvesting systems for high chromophore density or for the ensuing quantum effects as these are inextricably linked and cannot be switched off.

Spiers Memorial Lecture. Introductory lecture: the impact of structure on photoinduced processes in nucleic acids and proteins

Light is an important environmental variable and most organisms have evolved means to sense, exploit or avoid it and to repair detrimental effects on their genome. In general, light absorption is the task of specific chromophores, however other biomolecules such as oligonucleotides also do so which can result in undesired outcomes such as mutations and cancer. Given the biological importance of light-induced processes and applications for imaging, optogenetics, photodynamic therapy or photovoltaics, there is a great interest in understanding the detailed molecular mechanisms of photoinduced processes in proteins and nucleic acids. The processes are typically characterized by time-resolved spectroscopic approaches or computation, inferring structural information on transient species from stable ground state structures. Recently, however, structure determination of excited states or other short-lived species has become possible with the advent of X-ray free-electron lasers. This review gives an overview of the impact of structure on the understanding of photoinduced processes in macromolecules, focusing on systems presented at this Faraday Discussion meeting.

Ultrafast structural molecular dynamics investigated with 2D infrared spectroscopy methods

Ultrafast, multi-dimensional infrared (IR) spectroscopy has been advanced in recent years to a versatile analytical tool with a broad range of applications to elucidate molecular structure on ultrafast timescales, and it can be used for samples in a many different environments. Following a short and general introduction on the benefits of 2D IR spectroscopy, the first part of this chapter contains a brief discussion on basic descriptions and conceptual considerations of 2D IR spectroscopy. Outstanding classical applications of 2D IR are used afterwards to highlight the strengths and basic applicability of the method. This includes the identification of vibrational coupling in molecules, characterization of spectral diffusion dynamics, chemical exchange of chemical bond formation and breaking, as well as dynamics of intra- and intermolecular energy transfer for molecules in bulk solution and thin films. In the second part, several important, recently developed variants and new applications of 2D IR spectroscopy are introduced. These methods focus on (i) applications to molecules under two- and three-dimensional confinement, (ii) the combination of 2D IR with electrochemistry, (iii) ultrafast 2D IR in conjunction with diffraction-limited microscopy, (iv) several variants of non-equilibrium 2D IR spectroscopy such as transient 2D IR and 3D IR, and (v) extensions of the pump and probe spectral regions for multi-dimensional vibrational spectroscopy towards mixed vibrational-electronic spectroscopies. In light of these examples, the important open scientific and conceptual questions with regard to intra- and intermolecular dynamics are highlighted. Such questions can be tackled with the existing arsenal of experimental variants of 2D IR spectroscopy to promote the understanding of fundamentally new aspects in chemistry, biology and materials science. The final part of the chapter introduces several concepts of currently performed technical developments, which aim at exploiting 2D IR spectroscopy as an analytical tool. Such developments embrace the combination of 2D IR spectroscopy and plasmonic spectroscopy for ultrasensitive analytics, merging 2D IR spectroscopy with ultra-high-resolution microscopy (nanoscopy), future variants of transient 2D IR methods, or 2D IR in conjunction with microfluidics. It is expected that these techniques will allow for groundbreaking research in many new areas of natural sciences.

Femtosecond spectroscopic study of photochromic reactions of bacteriorhodopsin and visual rhodopsin

Photochromic ultrafast reactions of bacteriorhodopsin (H. salinarum) and bovine rhodopsin were conducted with a femtosecond two-pump probe pulse setup with the time resolution of 20-25fs. The dynamics of the forward and reverse photochemical reactions for both retinal-containing proteins was compared. It is demonstrated that when retinal-containing proteins are excited by femtosecond pulses, dynamics pattern of the vibrational coherent wave packets in the course of the reaction is different for bacteriorhodopsin and visual rhodopsin. As shown in these studies, the low-frequencies that form a wave packets experimentally observed in the dynamics of primary products formation as a result of retinal photoisomerization have different intensities and are clearer for bovine rhodopsin. Photo-reversible reactions for both retinal proteins were performed from the stage of the relatively stable photointermediates that appear within 3-5ps after the light pulse impact. It is demonstrated that the efficiency of the reverse phototransition K-form→bacteriorhodopsin is almost five-fold higher than that of the Batho-intermediate→visual rhodopsin phototransition. The results obtained indicate that in the course of evolution the intramolecular mechanism of the chromophore-protein interaction in visual rhodopsin becomes more perfect and specific. The decrease in the probability of the reverse chromophore photoisomerization (all-trans→11-cis retinal) in primary photo-induced rhodopsin products causes an increase in the efficiency of the photoreception process.

100 fs photo-isomerization with vibrational coherences but low quantum yield in Anabaena Sensory Rhodopsin

Counter-intuitive photochemistry: in Anabaena Sensory Rhodopsin, the retinal 13-cis isomer isomerizes much faster than all-trans ASR, but with a 3-times lower quantum yield.

The photocycle and ultrafast vibrational dynamics of bacteriorhodopsin in lipid nanodiscs

The photocycle and vibrational dynamics of bacteriorhodopsin in a lipid nanodisc microenvironment have been studied by steady-state and time-resolved spectroscopies. Linear absorption and circular dichroism indicate that the nanodiscs do not perturb the structure of the retinal binding pocket, while transient absorption and flash photolysis measurements show that the photocycle which underlies proton pumping is unchanged from that in the native purple membranes. Vibrational dynamics during the initial photointermediate formation are subsequently studied by ultrafast broadband transient absorption spectroscopy, where the low scattering afforded by the lipid nanodisc microenvironment allows for unambiguous assignment of ground and excited state nuclear dynamics through Fourier filtering of frequency regions of interest and subsequent time domain analysis of the retrieved vibrational dynamics. Canonical ground state oscillations corresponding to high frequency ethylenic and C-C stretches, methyl rocks, and hydrogen out-of-plane wags are retrieved, while large amplitude, short dephasing time vibrations are recovered predominantly in the frequency region associated with out-of-plane dynamics and low frequency torsional modes implicated in isomerization.

Vibrationally coherent crossing and coupling of electronic states during internal conversion in β-carotene

Coupling of nuclear and electronic degrees of freedom mediates energy flow in molecules after optical excitation. The associated coherent dynamics in polyatomic systems, however, remain experimentally unexplored. Here, we combined transient absorption spectroscopy with electronic population control to reveal nuclear wave packet dynamics during the S2 → S1 internal conversion in β-carotene. We show that passage through a conical intersection is vibrationally coherent and thereby provides direct feedback on the role of different vibrational coordinates in the breakdown of the Born-Oppenheimer approximation.

Broad-Band Impulsive Vibrational Spectroscopy of Excited Electronic States in the Time Domain

We demonstrate that transient absorption spectroscopy performed with an ultrashort pump pulse and a chirped, broad-band probe pulse is capable of recording full vibrational spectra of excited electronic states in the time domain. The resulting spectra do not suffer from the nontrivial baselines and line shapes often encountered in frequency domain techniques and enable optimal and automated subtraction of background signatures. Probing the molecular dynamics continuously over a broad energy bandwidth makes it possible to confidently assign the vibrational coherences to specific electronic states and suggests the existence of mode-specific absorption spectra reminiscent of resonance Raman intensity analysis. The first observation of the nominally forbidden one-photon ground to first excited electronic state transition in β-carotene demonstrates the high sensitivity of our approach. Our results provide a first glimpse of the immense potential of broad-band impulsive vibrational spectroscopy (BB-IVS) to study ultrafast chemical reaction dynamics.

Coherent High-Frequency Vibrational Dynamics in the Excited Electronic State of All-Trans Retinal Derivatives

Coherent vibrational dynamics of retinal in excited electronic states are of primary importance in the understanding of photobiology. Using pump-DFWM, we demonstrate for the first time the existence of coherent double-bond high-frequency modulations (>1300 cm(-1)) in the excited electronic state of different retinal derivatives. All-trans retinal as well as retinal Schiff bases exhibit a partial frequency downshift of the C═C double-bond mode from ∼1580 cm(-1) in the ground state to 1510 cm(-1) in the excited state. In addition, a new vibrational band at ∼1700 cm(-1) assigned to the C═N stretching mode in retinal Schiff bases in the excited state is detected. The newly reported bands are observed only in specific spectral regions of excited-state absorption. Implications regarding the observation of vibrational coherences in naturally occurring retinal protonated Schiff bases in rhodopsins are discussed.