Noise suppression in femtosecond mid-infrared light sources

Determination of the rotational isomerization rate along carbon-carbon single bonds in solution

The temperature- and viscosity-dependent rotational isomerization time constant (τrot) along the C-C˙ bond of CF2BrCF2 radical in solution was measured using femtosecond infrared spectroscopy after photodissociating the I atom from CF2BrCF2I. Three density functional theory (DFT) functionals, ωB97XD, APFD, and B3LYP were used with the aug-cc-pVTZ basis set to calculate the required parameters in calculating τrot using Kramers' theory of reaction rates. The measured τrot was consistent with the value calculated using the vibrational frequencies and rotational barriers of the related compounds calculated by DFT method with ωB97XD/aug-cc-pVTZ. Kramers' theory calculation of τrot was further verified by an experimental measurement for CF3CF2CF2˙ in CCl4 at 293 K. The τrot along the C-C(˙) bond of ethyl radical and ethane derivatives in solution can be reliably estimated by Kramers' theory combined with DFT calculations using the ωB97XD functional and aug-cc-pVTZ basis set.

FullThrOTTLE-TrIR: Time-Resolved IR Spectroscopy of Electrochemically Generated Species Using a Full Throughput Optically Transparent Thin-Layer Electrochemical Cell

An optically transparent thin-layer electrochemical cell with stopped-flow sample transport has been developed for optical-pump infrared-probe transient absorption spectroscopy of prereduced or preoxidized molecules. Time-resolved IR-spectra of Re(bpy)(CO)3X (X = Cl, Br) complexes in different oxidation states are presented as a proof-of-principle application for this combined electrochemical and spectroscopic tool. The excited-state lifetimes and IR-spectroscopic signatures of various oxidation states of the molecule, including follow-up reaction intermediates, are disentangled by kinetic sorting, using lifetime density analysis. The method can be applied to assign and differentiate molecular intermediates in photo- and electrochemical reactions, adding new analytic coordinates to classical FTIR- and UV-vis-spectroelectrochemistry.

Unexpected Redox Chemistry of P∩N- and As∩N-Rhenium(I) Tricarbonyl Complexes in the Presence of CO2 Acting as an Acid

This study reports on Re tricarbonyl complexes bearing 8-(diphenylphosphanyl)quinoline, P∩N, and 8-(diphenylarsanyl)quinoline, As∩N, as bidendate ligands. We studied the reactivity of these complexes in comparison with fac-Re(N∩N)(CO)3Cl (with N∩N = 2,2'-bipyridine or 4,4'-dimethyl-2,2'-bipyridine). We used a combination of electrochemical and spectroelectrochemical methods with time-resolved spectroscopy over 10 orders of magnitude (100 ps-1 s) to investigate the peculiar reactivity of one-electron-reduced Re(CO)3(P∩N)Cl and Re(CO)3(As∩N)Cl complexes also in the presence of protons.

Adding a second AgGaS2 stage to Ti:sapphire/BBO/AgGaS2 setups increases mid-infrared power twofold

We present a method for increasing the power of mid-infrared laser pulses generated by a conventional beta-barium borate (BBO) optical parametric amplifier (OPA) and AgGaS2 difference frequency generation (DFG) pumped by a Ti:sapphire amplifier. The method involves an additional stage of parametric amplification with a second AgGaS2 crystal pumped by selected outputs of the conventional DFG stage. This method does not require additional pump power from the Ti:sapphire laser source and improves the overall photon conversion efficiency for generating mid-infrared light. It merely requires an additional AgGaS2 crystal and dichroic mirrors. Following difference frequency generation, the method reuses near-infrared light (∼1.9 µm), typically discarded, to pump the additional AgGaS2 stage and amplifies the mid-infrared light twofold. We demonstrate and characterize the power, spectrum, duration, and noise of the mid-IR pulses before and after the second AgGaS2 stage. We observe small changes in center frequencies, bandwidth, and pulse duration for ∼150-fs pulses between 4 and 5 µm.

Versatile Femtosecond Laser Synchronization for Multiple-Timescale Transient Infrared Spectroscopy

Several ways to electronically synchronize different types of amplified femtosecond laser systems are presented based on a single freely programmable electronics hardware: arbitrary-detuning asynchronous optical sampling (ADASOPS), as well as actively locking two femtosecond laser oscillators, albeit not necessarily to the same round-trip frequency. They allow us to rapidly probe a very wide range of timescales, from picoseconds to potentially seconds, in a single transient absorption experiment without the need to move any delay stage. Experiments become possible that address a largely unexplored aspect of many photochemical reactions, in particular in the context of photo-catalysis as well as photoactive proteins, where an initial femtosecond trigger very often initiates a long-lasting cascade of follow-up processes. The approach is very versatile and allows us to synchronize very different lasers, such as a Ti:Sa amplifier and a 100 kHz Yb-laser system. The jitter of the synchronization, and therewith the time-resolution in the transient experiment, lies in the range from 1 to 3 ps, depending on the method. For illustration, transient IR measurements of the excited state solvation and decay of a metal carbonyl complex as well as the full reaction cycle of bacteriorhodopsin are shown. The pros and cons of the various methods are discussed, with regard to the scientific question one might want to address, and also with regard to the laser systems that might be already existent in a laser lab.

Intensity noise in difference frequency generation-based tunable femtosecond MIR sources

We characterize the intensity noise of two mid-infrared (MIR) ultrafast tunable (3.5-11 μm) sources based on difference frequency generation (DFG). While both sources are pumped by a high repetition rate Yb-doped amplifier delivering 200 μJ 300 fs at a central wavelength of 1030 nm, the first is based on intrapulse DFG (intraDFG), and the second on DFG at the output of an optical parametric amplifier (OPA). The noise properties are assessed through measurement of the relative intensity noise (RIN) power spectral density and pulse-to-pulse stability. The noise transfer mechanisms from the pump to the MIR beam is empirically demonstrated. As an example, improving the pump laser noise performance allows reduction of the integrated RIN (IRIN) of one of the MIR source from 2.7% RMS down to 0.4% RMS. The intensity noise is also measured at various stages and in several wavelength ranges in both laser system architectures, allowing us to identify the physical origin of their variation. This study presents numerical values for the pulse to pulse stability, and analyze the frequency content of the RINs of particular importance for the design of low-noise high repetition rate tunable MIR sources and future high performance time-resolved molecular spectroscopy experiments.

MCL-1 promiscuity and the structural resilience of its binding partners

The allosteric protein MCL-1 and its natural inhibitors, the BH3-only proteins PUMA, BIM, and NOXA regulate apoptosis by interacting promiscuously within an entangled binding network. Little is known about the transient processes and dynamic conformational fluctuations that are the basis for the formation and stability of the MCL-1/BH3-only complex. In this study, we designed photoswitchable versions of MCL-1/PUMA and MCL-1/NOXA, and investigated the protein response after an ultrafast photo-perturbation with transient infrared spectroscopy. We observed partial α-helical unfolding in all cases, albeit on strongly varying timescales (1.6 ns for PUMA, 9.7 ns for the previously studied BIM, and 85 ns for NOXA). These differences are interpreted as a BH3-only-specific "structural resilience" to defy the perturbation while remaining in MCL-1's binding pocket. Thus, the presented insights could help to better understand the differences between PUMA, BIM, and NOXA, the promiscuity of MCL-1, in general, and the role of the proteins in the apoptotic network.

The molecular mechanism of light-induced bond formation and breakage in the cyanobacteriochrome TePixJ

Cyanobacteriochromes (CBCRs) are small and versatile photoreceptor proteins with high potential for biotechnological applications. Among them, the so-called DXCF-CBCRs exhibit an intricate secondary photochemistry: miliseconds after activation with light, a covalent linkage between a conserved cysteine residue and the light-absorbing tetrapyrrole chromophore is reversibly formed or broken. We employed time-resolved IR spectroscopy over ten orders of magnitude in time in conjunction with 2D-IR spectroscopy to investigate the molecular mechanism of this intriguing reaction in the DXCF-CBCR model system TePixJ from T. elongatus. The crosspeak pattern in the 2D-IR spectrum facilitated the assignment of the dominant signals to vibrational modes of the chromophore, which in turn enabled us to construct a mechanistic model for the photocycle reactions from the time-resolved IR spectra. Here, we assigned the time-resolved signals to several proton transfer steps and distinct geometric changes of the chromophore. We propose a model that describes how these events lead to the rearrangement of charges in the chromophore binding pocket, which serves as the trigger for the light-induced bond formation and breakage with the nearby cysteine.

Vibrationally resolved two-photon electronic spectra including vibrational pre-excitation: Theory and application to VIPER spectroscopy with two-photon excitation

Following up on our previous work on vibrationally resolved electronic absorption spectra including the effect of vibrational pre-excitation [von Cosel et al., J. Chem. Phys. 147, 164116 (2017)], we present a combined theoretical and experimental study of two-photon-induced vibronic transitions in polyatomic molecules that are probed in the VIbrationally Promoted Electronic Resonance experiment using two-photon excitation (2P-VIPER). In order to compute vibronic spectra, we employ time-independent and time-dependent methods based on the evaluation of Franck-Condon overlap integrals and Fourier transformations of time-domain correlation functions, respectively. The time-independent approach uses a generalized version of the FCclasses method, while the time-dependent approach relies on the analytical evaluation of Gaussian moments within the harmonic approximation, including Duschinsky rotation effects. For the Coumarin 6 dye, two-dimensional 2P-VIPER experiments involving excitation to the lowest-lying singlet excited state (S₁) are presented and compared with corresponding one-photon VIPER spectra. In both cases, coumarin ring modes and a CO stretch mode show VIPER activity, albeit with different relative intensities. Selective pre-excitation of these modes leads to a pronounced redshift of the low-frequency edge of the electronic absorption spectrum, which is a prerequisite for the VIPER experiment. Theoretical analysis underscores the role of interference between Franck-Condon and Herzberg-Teller effects in the two-photon experiment, which is at the root of the observed intensity distribution.

Cu(Proline)2 Complex: A Model of Bio-Copper Structural Ambivalence

Complexes of Cu²⁺(d⁹) with proline may be considered a simple model to address the structural flexibility and electronic properties of copper metalloproteins. To discuss optical electronic spectra and infrared spectral responses, we use quantum chemistry applied to model systems prepared under different geometries and degree of hydration. A comparison of experimental data with calculations indicates that first explicit neighbor water clustering next to the Cu²⁺(d⁹) complex is critical for a correct description of the electronic properties of this system. We deduce that the moderately hydrated trans conformer is the main structural form of the complex in water. Further, we suggest that the antisymmetric stretching mode of the carbonyl moieties of the conformer is dominant in the spectrally broadened infrared resonance at 1605 cm⁻¹, where inhomogeneity of the transition at the blue side can be ascribed to a continuum of less optimal interactions with the solvent. Extracted structural properties and hydration features provide information on the structural flexibility/plasticity specific to Cu²⁺(d⁹) systems in correlation with the electronic behavior upon photoexcitation. We discuss the role and the nature of the axial ligand in bio-copper structural ambivalence and reactivity.

Rotationally-Resolved Two-Dimensional Infrared Spectroscopy of CO2(g): Rotational Wavepackets and Angular Momentum Transfer

Angular momentum transfer and wavepacket dynamics of CO₂(g) were measured on the picosecond time scale using polarization-resolved two-dimensional infrared (2D-IR) spectroscopy. The dynamics of rotational levels up to J_max ≈ 50 are observed simultaneously at room temperature. Rotational wavepackets launched by the pump pulses cause oscillations in the intensity of individual peaks and beating patterns in the 2D-IR spectra. The structure of the rotationally resolved 2D-IR spectrum is explained using nonlinear response function theory. Spectral diffusion of the rotationally resolved 2D-IR peaks reveals information about angular momentum transfer. We demonstrate the ability to directly measure inelastic angular momentum dynamics simultaneously across the ∼50 thermally excited rotational levels over several hundred picoseconds.

Structure, Organization, and Heterogeneity of Water-Containing Deep Eutectic Solvents

The spectroscopy and structural dynamics of a deep eutectic mixture (KSCN/acetamide) with varying water content is investigated from 2D IR (with the C-N stretch vibration of the SCN^- anions as the reporter) and THz spectroscopy. Molecular dynamics simulations correctly describe the nontrivial dependence of both spectroscopic signatures depending on water content. For the 2D IR spectra, the MD simulations relate the steep increase in the cross-relaxation rate at high water content to the parallel alignment of packed SCN^- anions. Conversely, the nonlinear increase of the THz absorption with increasing water content is mainly attributed to the formation of larger water clusters. The results demonstrate that a combination of structure-sensitive spectroscopies and molecular dynamics simulations provides molecular-level insights into the emergence of heterogeneity of such mixtures by modulating their composition.

Photoacid-induced aqueous acid-base reactions probed by femtosecond infrared spectroscopy

Acid-base reactions involving an excited photoacid have typically been investigated at high base concentrations, but the mechanisms at low base concentrations require clarification. Herein, the dynamics of acid-base reactions induced by an excited photoacid, pyranine (DA), were investigated in the presence of azide ion (N₃^-) in D₂O solution using femtosecond infrared spectroscopy. Specifically, the spectral characteristics of four species (DA, electronically excited DA (DA*), the conjugate base of DA* (A*^-), and the conjugate base of DA (A^-)) were probed in the spectral region of 1400-1670 cm^-1 in the time range of 1 ps-1 μs. This broad timescale encompassed all the acid-base reactions initiated by photoexcitation at 400 nm; thus, reactions related to both DA* and A^- could be probed. Furthermore, changes in the populations of N₃^- and DN₃ were monitored using the absorption bands at 2042 and 2133 cm^-1, respectively. Following excitation, approximately half of DA* relaxed to DA with a time constant of 0.44 ± 0.04 ns. The remainder underwent an acid-base reaction to produce A*^-, which relaxed to A^- with a time constant of 3.9 ± 0.3 ns. The acid-base reaction proceeded via two paths, namely, proton exchange with the added base or simple deuteron release to D₂O (protolysis). Notably, all the acid-base reactions were well described by the rate constant at the steady-state limit. Thus, although the acid-base reactions at low base concentrations (< 0.1 M) were diffusion controlled, they could be described using a simple rate equation.

Long seed, short pump: converting Yb-doped laser radiation to multi-µJ few-cycle pulses tunable through 2.5-15 µm

We present a setup for generating broadband (up to 1050 cm^-1) and broadly tunable (2.5-15 µm) mid-infrared pulses using an Yb-doped femtosecond laser as the pump source. Our scheme, comprising two parametric amplifiers and a mixing stage, exploits favorable group velocity matching conditions in GaSe pumped at 2 µm to directly produce sub-70 fs pulses throughout the tuning range without any additional dispersion compensation, while 30-50 fs pulse durations are achieved with simple dispersion compensation by propagation through thin bulk media. The generated pulses have sub-1% short- and long-term energy noise, as well as stable spectral parameters, while delivering 0.5-2 W average mid-IR power. We expect the source to be useful for various spectroscopic applications in the mid-IR.

Photogeneration of quinone methide from adamantylphenol in an ultrafast non-adiabatic dehydration reaction

The ultrafast photochemical reaction of quinone methide (QM) formation from adamantylphenol was monitored in real time using femtosecond transient absorption spectroscopy and fluorescence upconversion in solution at room temperature. Experiments were complemented by theoretical studies simulating the reaction pathway and elucidating its mechanism. Excitation with sub-20 fs UV pulses and broadband probing revealed ultrafast formation of the long-lived QM intermediate directly in the ground state, occurring with a time constant of around 100 fs. UV-vis transient absorption data covering temporal dynamics from femtoseconds to hundreds of milliseconds revealed persistence of the absorption band assigned to QM and partially overlapped with other contributions tentatively assigned to triplet excited states of the adamantyl derivative and the phenoxyl radical that are clearly distinguished by their evolution on different time scales. Our data, together with the computations, provide evidence of a non-adiabatic photodehydration reaction, which leads to the formation of QM in the ground state via a conical intersection, circumventing the generation of a transient QM excited state.

Spatio-spectral couplings in saturated collinear OPCPA

Ultrafast laser pulses featuring both high spatio-temporal beam quality and excellent energy stability are crucial for many applications. Here, we present a seed laser with high beam quality and energy stability, based on a collinear optical parametric chirped pulse amplification (OPCPA) stage, delivering 46 µJ pulses with a 25 fs Fourier limit at 1 kHz repetition rate. While saturation of the OPCPA stage is necessary for achieving the highest possible energy stability, it also leads to a degradation of the beam quality. Using simulations, we show that spectrally dependent, rotationally symmetric aberrations dominate the collinear OPCPA in saturation. We experimentally characterize these aberrations and then remove distinct spatial frequencies to greatly improve the spectral homogeneity of the beam quality, while keeping an excellent energy stability of 0.2 % rms measured over 70 hours.

Using azobenzene photocontrol to set proteins in motion

Controlling the activity of proteins with azobenzene photoswitches is a potent tool for manipulating their biological function. With the help of light, it is possible to change binding affinities, control allostery or manipulate complex biological processes, for example. Additionally, owing to their intrinsically fast photoisomerization, azobenzene photoswitches can serve as triggers that initiate out-of-equilibrium processes. Such switching of the activity initiates a cascade of conformational events that can be accessed with time-resolved methods. In this Review, we show how the potency of azobenzene photoswitching can be combined with transient spectroscopic techniques to disclose the order of events and experimentally observe biomolecular interactions in real time. This strategy will further our understanding of how a protein can accommodate, adapt and readjust its structure to answer an incoming signal, revealing more of the dynamical character of proteins.

Folding and Unfolding of the Tryptophan Zipper in the Presence of Two Thioamide Substitutions

We studied the stability and folding and unfolding kinetics of the tryptophan zipper, containing different double thioamide subsitutions. Conformation change was triggered by photoisomerization of an integrated AMPP photoswitch in the turn region of the hairpin, and transient spectra were recorded in the deep UV and the mid-IR, covering the time window of the (un)folding transition from picoseconds to tens of microseconds. Thio-substitution of inward-pointing backbone carbonyls was found to strongly destabilize the β-hairpin structures, whereas molecules with two outward pointing thio-carbonyls showed similar or enhanced stability with respect to the unsubstituted sequence, which we attribute to stronger interstrand hydrogen bonding. Thiolation of the two Trp residues closest to the turn can even prevent the opening of the hairpin after cis-trans isomerization of the switch. The circular dichroism due to the two thioamide ππ* transitions is spectrally well-separated from the aromatic tryptophan signal. It changes upon photoswitching, reflecting a local change in coupling and geometry.

CH Mode Mixing Determines the Band Shape of the Carboxylate Symmetric Stretch in Apo-EDTA, Ca2+-EDTA, and Mg2+-EDTA

The infrared spectra of EDTA complexed with Ca²⁺ and Mg²⁺ contain, to date, unidentified vibrational bands. This study assigns the peaks in the linear and two-dimensional infrared spectra of EDTA, with and without either Ca²⁺ or Mg²⁺ ions. Two-dimensional infrared spectroscopy and DFT calculations reveal that, in both the presence and absence of ions, the carboxylate symmetric stretch and the terminal CH bending vibrations mix. We introduce a method to calculate participation coefficients that quantify the contribution of the carboxylate symmetric stretch, CH wag, CH twist, and CH scissor in the 1400-1550 cm^-1 region. With the help of participation coefficients, we assign the 1400-1430 cm^-1 region to the carboxylate symmetric stretch, which can mix with CH modes. We assign the 1000-1380 cm^-1 region to CH twist modes, the 1380-1430 cm^-1 region to wag modes, and the 1420-1650 cm^-1 region to scissor modes. The difference in binding geometry between the carboxylate-Ca²⁺ and carboxylate-Mg²⁺ complex manifests as new diagonal and cross-peaks between the mixed modes in the two complexes. The small Mg²⁺ ion binds EDTA tighter than the Ca²⁺ ion, which causes a redshift of the COO symmetric stretches of the sagittal carboxylates. Energy decomposition analysis further characterizes the importance of electrostatics and deformation energy in the bound complexes.

Sequence of Events during Peptide Unbinding from RNase S: A Complete Experimental Description

The phototriggered unbinding of the intrinsically disordered S-peptide from the RNase S complex is studied with the help of transient IR spectroscopy, covering a wide range of time scales from 100 ps to 10 ms. To that end, an azobenzene moiety has been linked to the S-peptide in a way that its helicity is disrupted by light, thereby initiating its complete unbinding. The full sequence of events is observed, starting from unfolding of the helical structure of the S-peptide on a 20 ns time scale while still being in the binding pocket of the S-protein, S-peptide unbinding after 300 μs, and the structural response of the S-protein after 3 ms. With regard to the S-peptide dynamics, the binding mechanism can be classified as an induced fit, while the structural response of the S-protein is better described as conformational selection.

Simple carrier-envelope phase control and stabilization scheme for difference frequency generation-based systems

We report about a setup for carrier-envelope phase (CEP) control and stabilization in passive systems based on difference frequency generation (DFG). The principle of this approach relies on the amplitude to phase modulation transfer in the white-light generation process. A small modulation of the pump laser intensity is used to obtain a DFG output modulated in CEP. This technique is demonstrated in a CEP-stable system pumped by an Yb-doped fiber amplifier. It is first characterized by measuring CEP modulations produced by applying arbitrary waveforms. The CEP actuator is then used for slow drifts correction in a feedback loop. The results show the capability of this simple approach for OPA/OPCPA CEP-stabilized setups.

The Speed of Allosteric Signaling Within a Single-Domain Protein

While much is known about different allosteric regulation mechanisms, the nature of the allosteric signal and the time scale on which it propagates remains elusive. The PDZ3 domain from postsynaptic density-95 protein is a small protein domain with a terminal third α-helix, i.e., the α3-helix, which is known to be allosterically active. By cross-linking the allosteric helix with an azobenzene moiety, we obtained a photocontrollable PDZ3 variant. Photoswitching triggers its allosteric transition, resulting in a change in binding affinity of a peptide to the remote binding pocket. Using time-resolved infrared and UV/vis spectroscopy, we follow the allosteric signal transduction and reconstruct the timeline in which the allosteric signal propagates through the protein within 200 ns.

Transient CO desorption from thin Pt films induced by mid-IR pumping

Resonant and off-resonant mid-infrared pump-probe spectroscopy is used to measure the vibrational dynamics of CO adsorbed to thin (0.2 nm, 2 nm, and 10 nm) heterogeneous Pt layers in an aqueous solution. The transient signals observed with resonant pumping are dominated by vibrational relaxation of the CO internal stretch vibration with a lifetime of T₁ ∼ 3 ps in all cases. Off-resonant pumping suppresses that contribution to the signal and singles out a signal, which is attributed to heating of the metal layer as well as transient desorption of the CO molecules. Due to the small photon energy (0.2 eV) used as pump pulses, the mechanism of desorption must be thermal, in which case the desorption yield depends exclusively on the fluence of absorbed light and not its wavelength. The thin Pt layers facilitate CO desorption, despite a relatively low pump pulse fluence, as they concentrate the absorbed energy in a small volume.

Intramolecular Vibrational Energy Relaxation of CO2 in Cross-Linked Poly(ethylene glycol) Diacrylate-Based Ion Gels

Ultrafast two-dimensional infrared spectroscopy (2D-IR) and Fourier transform infrared spectroscopy (FTIR) were used to measure carbon dioxide (CO₂) in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf₂N]), cross-linked low-molecular-weight poly(ethylene glycol) diacrylate (PEGDA), and an ion gel composed of a 50 vol % blend of the two. The center frequency of the antisymmetric stretch, ν₃, of CO₂ shifts monotonically to lower wavenumbers with increasing polymer content, with the largest line width in the ion gel (6 cm^-1). Increasing polymer content slows both spectral diffusion and vibrational energy relaxation (VER) rates. An unexpected excited-state absorbance peak appears in the 2D-IR of cross-linked PEGDA due to VER from the antisymmetric stretch into the bending mode, ν₂. Thirty-two response functions are necessary to describe the observed features in the 2D-IR spectra. Nonlinear least-squares fitting extracts both spectral diffusion and VER rates. In the ion gel, CO₂ exhibits spectral diffusion dynamics that lie between that of the pure compounds. The kinetics of VER reflect both fast excitation and de-excitation of the bending mode, similar to the ionic liquid (IL), and slow overall vibrational population relaxation, similar to the cross-linked polymer. The IL-like and polymer-like dynamics suggest that the CO₂ resides at the interface of the two components in the ion gel.

Coordination Environment Prevents Access to Intraligand Charge-Transfer States through Remote Substitution in Rhenium(I) Terpyridinedicarbonyl Complexes

Six rhenium(I) κ³N-dicarbonyl complexes with 4'-(4-substituted phenyl)terpyridine ligands were evaluated in their ground and excited states. These complexes, bearing substituents of different electron-donating strengths-from CN to NMe₂-were studied by a combination of transient IR (TRIR), electrochemistry, and IR spectroelectrochemistry, as well as time-dependent density functional theory (TD-DFT). They exhibit panchromatic absorption and can act as stronger photoreductants than their tricarbonyl counterparts. The ground- and excited-state potentials, absorption maxima, and lifetimes (250-750 ps) of these complexes correlate well with the Hammett σ_p substituent constants, showing the systematic effect of remote substitution in the ligand framework. TRIR spectroscopy allowed us to assign the lowest singlet and triplet excited states to a metal-to-ligand charge-transfer (MLCT) character. This result contrasts our previous report on analogous κ²N-tricarbonyl complexes, where remote substitution switched the character from MLCT to intraligand charge transfer. With the help of TD-DFT calculations, we dissect the geometric and electronic effects of coordination of the third pyridine, local symmetries, and increasing conjugation length. These results give valuable insights for the design of complexes with long-lived triplet excited states and enhanced absorption throughout the visible spectrum, while showcasing the boundaries of the excited-state switching strategy via remote substitution.

Flexible to rigid: IR spectroscopic investigation of a rhenium-tricarbonyl-complex at a buried interface

This work explores the solid-liquid interface of a rhenium-tricarbonyl complex embedded in a layer of zirconium oxide deposited by atomic layer deposition (ALD). Time-resolved and steady state infrared spectroscopy were applied to reveal the correlations between the thickness of the ALD layer and the spectroscopic response of the system. We observed a transition of the molecular environment from flexible to rigid, as well as limitations to ligand exchange and excited state quenching on the embedded complexes, when the ALD layer is roughly of the same height as the molecules.

A closer look into the distance dependence of vibrational energy transfer on surfaces using 2D IR spectroscopy

Vibrational energy transfer (VET) between two isotopologues of [Re(dcb)(CO)₃Br] immobilized on a TiO₂ surface is studied with the help of 2D IR spectroscopy in dependence of surface coverage. To dilute the molecules on the surface, and thereby control the intermolecular distances, two different diluents have been used: a third isotopologue of the same molecule and 4-cyanobenzoic acid. As expected, the VET rate decreases with dilution. For a quantitative investigation of the distance dependence of the VET rate, we analyze the data based on an excitonic model. This model reveals the typical 1/r⁶-distance dependence for a dimer of a donor and acceptor, similar to the nuclear Overhauser effect in NMR spectroscopy or Förster resonant energy transfer in electronic spectroscopy. However, VET becomes a collective phenomenon on the surface, with the existence of a network of coupled molecules and its disappearance below a percolation threshold, dominating the concentration dependence of the VET rate.

Real-time observation of ligand-induced allosteric transitions in a PDZ domain

While allostery is of paramount importance for protein regulation, the underlying dynamical process of ligand (un)binding at one site, resulting time evolution of the protein structure, and change of the binding affinity at a remote site are not well understood. Here the ligand-induced conformational transition in a widely studied model system of allostery, the PDZ2 domain, is investigated by transient infrared spectroscopy accompanied by molecular dynamics simulations. To this end, an azobenzene-derived photoswitch is linked to a peptide ligand in a way that its binding affinity to the PDZ2 domain changes upon switching, thus initiating an allosteric transition in the PDZ2 domain protein. The subsequent response of the protein, covering four decades of time, ranging from ∼1 ns to ∼μs, can be rationalized by a remodeling of its rugged free-energy landscape, with very subtle shifts in the populations of a small number of structurally well-defined states. It is proposed that structurally and dynamically driven allostery, often discussed as limiting scenarios of allosteric communication, actually go hand-in-hand, allowing the protein to adapt its free-energy landscape to incoming signals.

Living Long and Prosperous: Productive Intraligand Charge-Transfer States from a Rhenium(I) Terpyridine Photosensitizer with Enhanced Light Absorption

The ground- and excited-state properties of six rhenium(I) κ²N-tricarbonyl complexes with 4'-(4-substituted-phenyl)terpyridine ligands bearing substituents of different electron-donating abilities were evaluated. Significant modulation of the electrochemical potentials and a nearly 4-fold variation of the triplet metal-to-ligand charge-transfer (³MLCT) lifetimes were observed upon going from CN to OMe. With the more electron-donating NMe₂ group, we observed in the κ²N complex the appearance of a very strong absorption band, red-shifted by ca. 100 nm with respect to the other complexes. This was accompanied by a dramatic enhancement of the excited-state lifetime (380 vs 1.5 ns), and a character change from ³MLCT to intraligand charge transfer (³ILCT), despite the remote location of the substituent. The dynamics and character of the excited states of all complexes were assigned by combining transient IR spectroscopy, IR spectroelectrochemistry, and (time-dependent) density functional theory calculations. Selected complexes were evaluated as photosensitizers for hydrogen production, with the κ²N-NMe₂ complex resulting in a stable and efficient photocatalytic system reaching TON_Re values of over 2100, representing the first application of the ³ILCT state of a rhenium(I) carbonyl complex in a stable photocatalytic system.

A Correction Scheme for Fano Line Shapes in Two-Dimensional Infrared Spectroscopy

The asymmetry of Fano line shapes observed for metal-adsorbate systems is reflected in two-dimensional infrared (2D IR) spectroscopy as a distorted spectrum. A phenomenological correction scheme is proposed that transforms distorted 2D IR spectra into conventional spectra. To that end, a phase correction factor is first derived from the IR absorption spectrum of the sample by symmetrizing the asymmetric line shape and subsequently applied to the distorted 2D IR spectra. The concept is illustrated for a model system consisting of an organic molecule (p-mercaptobenzonitrile) adsorbed on a sputter-coated metal layer (Au). The correction scheme reveals conventional, easily interpretable 2D IR spectra.

Edge-pixel referencing suppresses correlated baseline noise in heterodyned spectroscopies

Referencing schemes are commonly used in heterodyned spectroscopies to mitigate correlated baseline noise arising from shot-to-shot fluctuations of the local oscillator. Although successful, these methods rely on careful pixel-to-pixel matching between the two spectrographs. A recent scheme introduced by Feng et al. [Opt. Express 27(15), 20323-20346 (2019)] employed a correlation matrix to allow free mapping between dissimilar spectrographs, leading to the first demonstration of floor noise limited detection on a multichannel array used in heterodyned spectroscopy. In addition to their primary results using a second reference spectrometer, Feng et al. briefly demonstrated the flexibility of their method by referencing to same-array pixels at the two spectral edges (i.e., edge-pixel referencing). We present a comprehensive study of this approach, which we term edge-pixel referencing, including optimization of the approach, assessment of the performance, and determination of the effects of background responses. We show that, within some limitations, the distortions due to background signals will not affect the 2D IR line shape or amplitude and can be mitigated by band narrowing of the pump beams. We also show that the performance of edge-pixel referencing is comparable to that of referencing to a second spectrometer in terms of noise suppression and that the line shapes and amplitudes of the spectral features are, within the measurement error, identical. Altogether, these results demonstrate that edge-pixel referencing is a powerful approach for noise suppression in heterodyned spectroscopies, which requires no new hardware and, so, can be implemented as a software solution for anyone performing heterodyned spectroscopy with multichannel array detectors already.

Probing the primary processes of a triazido-cobalt(III) complex with femtosecond vibrational and electronic spectroscopies. Photochemical selectivity and multi-state reactivity

The elementary dynamics following 355 nm-excitation of the complex, mer-[Co(dien)(N3)3], were studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond-ultraviolet-pump/mid-infrared-to-near-ultraviolet probe spectroscopy in conjunction with electronic structure calculations based on density functional theory. Following the initial N3--to-Co charge transfer excitation, the parent complex undergoes an ultrafast metal-to-ligand back electron transfer (BET) within 2 ps thereby populating a metal-centered singlet excited state, 1MC, which can either repopulate the electronic ground state or cleave an azido ligand from the ligand sphere surrounding the metal center. From the asymptotic ground-state bleaching signal after 1 ns, a primary quantum yield for ligand loss of ca. 13% is estimated. The IR-spectrum of the product demonstrates that the photodissociation occurs selectively from the equatorial binding site thereby leading exclusively to the solvolysis product, mer-trans-[Co(dien)(N3)2(DMSO)]+, which features the solvent ligand in the equatorial coordination plane and the azides in the two axial positions. The remarkable photochemical selectivity is traced back to the initial BET and the nature of the intermediate state, 1MC, whose electronic structure entails occupancy of the σ-antibonding d(x2-y2)-orbital. A stereochemical scrambling at the stage of the primary penta-coordinated diazido product is kinetically inhibited on the singlet surface by an energy barrier of roughly 27 kJ mol-1. Primary penta-coordinated products that may be born on the triplet surface are funneled to their singlet ground-state preferentially from geometries with trans-oriented azido ligands thereby also preventing a stereochemical isomerization that could possibly arise from an intersystem crossing.

Complete photodissociation dynamics of CF2I2in solution

Photoexcited CF₂I₂in c-C₆H₁₂undergoes various secondary reactions including complex and isomer formation, after ultrafast two- or three-body dissociations.

Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: III. Dynamics and Spectroscopy

In recent years, interest in carbon capture and sequestration has led to numerous investigations of the ability of ionic liquids to act as recyclable CO₂-sorbent materials. Herein, we investigate the structure and dynamics of a model physisorbing ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄C₁Im][PF₆]), from the perspective of CO₂ using two-dimensional (2D) IR spectroscopy and molecular dynamics simulations. A direct comparison of experimentally measured and calculated 2D IR line shapes confirms the validity of the simulations and spectroscopic calculations. Taken together, the simulations and experiments reveal new insights into the interactions of a CO₂ solute with the surrounding ionic liquid and how these interactions manifest in the 2D IR spectra. In particular, higher CO₂ asymmetric stretch vibrational frequencies are associated with softer, less populated solvent cages and lower frequencies are associated with tighter, more highly populated solvent cages. The CO₂ interacts most strongly with the anions, and these interactions persist for more than 1 ns. The second strongest interactions are with the imidazolium cation ring that last 100 ps, and the weakest interactions are with the cation butyl tail that persist for 10 ps. The principal contributors to spectral diffusion of the CO₂ asymmetric stretch vibrational frequency due to the dynamical evolution of the solvent are through Lennard-Jones interactions at short times and electrostatics at long times.

Photometrics of ultrafast and fast broadband electronic transient absorption spectroscopy: State of the art

The physical limits of the photometric resolution in broadband electronic transient absorption spectroscopy are discussed together with solutions for how to reach these limits in practice. In the first part, quantitative expressions for the noise contributions to the transient absorption signal are derived and experimentally tested. Experimental approaches described in the literature are discussed and compared on this basis. Guide-lines for designing a setup are established. In the second part, a method for obtaining nearly shot-noise limited kinetics with photometric resolution of the order of 100 μOD in overall measurement times of a few minutes from femtosecond to microsecond time scale is presented. The results are discussed in view of other experiments of step-scan type which are subject to a background or to correlated noise. Finally, detailed information is provided on how to obtain transient absorption spectra where counting statistics are the sole source of noise. A method for how to suppress outliers without introducing bias is discussed. An application example is given to demonstrate the achievable signal-to-noise level and the fast acquisition time.

Aqueous solvation from the water perspective

The response of water re-solvating a charge-transfer dye (deprotonated Coumarin 343) after photoexcitation has been measured by means of transient THz spectroscopy. Two steps of increasing THz absorption are observed, a first ∼10 ps step on the time scale of Debye relaxation of bulk water and a much slower step on a 3.9 ns time scale, the latter of which reflecting heating of the bulk solution upon electronic relaxation of the dye molecules from the S₁ back into the S₀ state. As an additional reference experiment, the hydroxyl vibration of water has been excited directly by a short IR pulse, establishing that the THz signal measures an elevated temperature within ∼1 ps. This result shows that the first step upon dye excitation (10 ps) is not limited by the response time of the THz signal; it rather reflects the reorientation of water molecules in the solvation layer. The apparent discrepancy between the relatively slow reorientation time and the general notion that water is among the fastest solvents with a solvation time in the sub-picosecond regime is discussed. Furthermore, non-equilibrium molecular dynamics simulations have been performed, revealing a close-to-quantitative agreement with experiment, which allows one to disentangle the contribution of heating to the overall THz response from that of water orientation.

Disentangling the photochemistry of benzocyclobutenedione

The ultrafast photophysics and photochemistry of benzocyclobutenedione (BCBD) dissolved in dichloromethane is investigated by transient absorption spectroscopy in both the IR and the UV/Vis regime. The molecule is excited at 300 nm to the S3 (ππ*) state and a time scale from roughly 100 fs to several nanoseconds is covered. The initially excited S3 deactivates quickly to the lower-lying S1 (nπ*) state. Three parallel photochemical reaction pathways starting in the S1 state that compete with deactivation to S0 are identified in the transient IR spectra, two of them consisting of a sequence of steps. DFT/TDDFT calculations of the normal modes of the reactant and various photoproducts support the analysis of the transient spectra. The rapid internal conversion (IC) to the S1 state of BCBD is followed by a sub-picosecond vibrational relaxation (VR) to S1 (ν = 0). In parallel BCBD loses one carbonyl group and forms benzocyclopropenone, which subsequently rearranges to cyclopentadienylidene ketene. Ring opening in the S1 (ν = 0) state produces vibrationally hot bisketene, which cools within 22 ps. This reaction competes with the intramolecular rearrangement to singlet oxacarbene, which subsequently converts into the triplet carbene via intersystem crossing (ISC). The late-time product identified in the transient UV/Vis spectra is probably due to dimerization of the carbene. Molecular dynamics (MD) simulations of the early-time photochemistry of BCBD successfully reproduce the formation of the three main photoproducts.

Characterization of the Platinum-Hydrogen Bond by Surface-Sensitive Time-Resolved Infrared Spectroscopy

The vibrational dynamics of Pt-H on a nanostructured platinum surface has been examined by ultrafast infrared spectroscopy. Three bands are observed at 1800, 2000, and 2090 cm^-1, which are assigned to Pt-CO in a bridged and linear configuration and Pt-H, respectively. Lifetime analysis revealed a time constant of (0.8 ± 0.1) ps for the Pt-H mode, considerably shorter than that of Pt-CO because of its stronger coupling to the metal substrate. Two-dimensional attenuated total reflection infrared spectroscopy provided additional evidence for the assignment based on the anharmonic shift, which is large in the case of Pt-H (90 cm^-1), in agreement with the density functional theory calculations. The absorption cross section of Pt-H is smaller than that of the very strong Pt-CO vibration by only a modest factor of ∼1.5-3. Because Pt-H is transiently involved in catalytic water splitting on Pt, the present spectroscopic characterization paves the way for in-operando kinetic studies of such reactions.

Ultrafast dynamics of the ESIPT photoswitch N-(3-pyridinyl)-2-pyridinecarboxamide

Molecular switches based on proton transfer that are photochromic and can be interconverted by light at different wavelengths back and forth between two thermodynamically stable tautomeric states in solution at room temperature are rare to date. We report on a study of the ultrafast conversion of the bistable proton transfer switch N-(3-pyridinyl)-2-pyridinecarboxamide (NPPCA) to a corresponding iminol after photoexcitation at λ_pump ≈ 265 nm by means of femtosecond time-resolved broad-band and single-colour transient electronic absorption spectroscopy (TEAS), transient fluorescence spectroscopy (TFLS), and transient vibrational absorption spectroscopy (TVAS) in acetonitrile solution. The interpretation of the data was accompanied by ab initio quantum chemical calculations of the excited electronic states and the vibrational frequencies of the reactant and product in their ground electronic state. The TEAS experiments provided four time constants, τ₁ = 0.09 ± 0.01 ps, τ₂ = 0.61 ± 0.01 ps, τ₃ = 5.10 ± 0.80 ps, and τ₄ = 20.0 ± 1.0 ps. The first two agree well with the measured TFLS lifetimes, τ_1,TFL < 0.18 ps and τ_2,TFL = 0.50 ± 0.01 ps. τ₁ is related to the relaxation of the initially excited Franck-Condon (FC) state of the pyridinecarboxamide, followed by the excited-state intramolecular proton transfer (ESIPT) step to the neighbouring pyridine. The subsequent return of the molecules to the electronic ground state takes place within τ₂, mediated by a conical intersection (CI) at a twisted configuration of the pyridinecarboxamide moiety. The main components in all TEAS time profiles feature a rise with τ₂ and a decay with τ₄ and describe subsequent molecular transformations in the electronic ground state. τ₃ is ascribed to vibrational cooling of the molecules. The final iminol exhibits a permanent UV absorption at λ = 247 nm, where its absorbance is stronger than that of the carboxamide reactant. The iminol structure is unambiguously identified by the TVA spectra, which show the build-up of corresponding vibrational bands with τ_4,TVA = 23 ± 2 ps after the initial bleach of the reactant vibrational bands, in excellent agreement with the TEAS data. Its lifetime is >10 ns.

Probing the band gap of liquid ammonia with femtosecond multiphoton ionization spectroscopy

The solvated electron primary yield is used in a multiphoton-ionization action-spectroscopic experiment to explore the band gap of liquid ammonia.

Recent advances in multidimensional ultrafast spectroscopy

Multidimensional ultrafast spectroscopies are one of the premier tools to investigate condensed phase dynamics of biological, chemical and functional nanomaterial systems. As they reach maturity, the variety of frequency domains that can be explored has vastly increased, with experimental techniques capable of correlating excitation and emission frequencies from the terahertz through to the ultraviolet. Some of the most recent innovations also include extreme cross-peak spectroscopies that directly correlate the dynamics of electronic and vibrational states. This review article summarizes the key technological advances that have permitted these recent advances, and the insights gained from new multidimensional spectroscopic probes.

Ultrafast dynamics of ionic liquids in colloidal dispersion

Ionic liquid (IL)-surfactant complexes have significance both in applications and fundamental research, but their underlying dynamics are not well understood. We apply polarization-controlled two-dimensional infrared spectroscopy (2D-IR) to study the dynamics of [BMIM][SCN]/surfactant/solvent model systems. We examine the effect of the choice of surfactants and solvent, and the IL-to-surfactant ratio (W-value), with a detailed analysis of the orientation and structural dynamics of each system. Different surfactants create very different environments for the entrapped ILs, ranging from a semi-static micro-environment to a fluxional environment that evolves even faster than the bulk IL. The oil-phase also clearly affects the microscopic dynamics. The anisotropy decay for entrapped ILs completes within 10 ps, which is similar to free thiocyanate ion in water, while a significant reorientation-induced spectral diffusion (RISD) effect is observed. The entrapped ionic liquid are highly dynamic for all W-values, and no core-shell structure is observed. We hypothesize that, instead of an ionic liquid-reverse micelle (IL-RM), the microscopic structure of this system is small colloidal dispersions or pairs of IL and surfactants. A detailed analysis of the polarization-controlled 2D-IR spectra of AOT system reveals a potential ion-exchange mechanism.

General noise suppression scheme with reference detection in heterodyne nonlinear spectroscopy

We devised a novel two-step reference scheme that can greatly suppress the additive and convolutional noises in heterodyne nonlinear spectroscopy. To optimally remove additive noise, we fully utilized the spectral correlation in multi-channel reference data through a linear combination and regression algorithm. Using our pump-probe 2D IR spectrometer, we demonstrated that our scheme can improve the signal-to-noise ratio by 10-30 times and reach the noise floor of the signal detector. The new algorithm is guaranteed to reduce noise, enables the use of unmatched reference detectors, and does not introduce baseline shift or signal distortion. This scheme is applicable to many heterodyne spectroscopic techniques.