Transient Infrared Absorption Spectroscopy Research Articles

Accessing the Activation Mechanisms of Ethylene Photo-Polymerization under Pressure by Transient Infrared Absorption Spectroscopy.

The ambient temperature photoinduced polymerization of compressed (P < 1 GPa) fluid ethylene was characterized by transient infrared absorption spectroscopy with a resolution of few nanoseconds, 3 orders of magnitude higher than previously reported. The reaction has been studied under both one- and two-photon excitation evidencing in the latter case its occurrence only in the presence of different transition metal oxides. Their photocatalytic activity is ascribed to the stabilization of the excited biradicals through electron density exchange between the d orbitals of the metal and the π antibonding orbitals of ethylene which lengthens the lifetime of the biradicals. In both one- and two-photon activation cases the polymerization is characterized by an initial step distinguished by a molecularity of 0.15 ± 0.02 identified as the activation step of the reaction lasting, in the one-photon excitation case, a few hundreds of nanoseconds. Using pulsed excitation the reaction evolves toward a free radical polymerization only under one-photon excitation whereas the critical concentration of radicals required to propagate the reaction is never achieved in the two-photon excitation case. Comparison with continuous wave excitation unambiguously identifies in the average power released to the sample the key factor to drive quantitatively and qualitatively the polymerization.

Solvated iron metallogrid complex exhibits reversible photocycle

Measurement with transient optical infrared absorption and X-ray emission spectroscopy reveal independent intra-site transitions, pointing toward future use in advanced light-driven reactions.

Direct Observation of Ylide and Enol Intermediates Formed in Competition with Wolff Rearrangement of Photoexcited Ethyl Diazoacetoacetate.

The photoexcitation of α-diazocarbonyl compounds produces ketenes by both concerted and stepwise Wolff rearrangements. The stepwise mechanism proceeds through singlet carbene intermediates which can also participate in bimolecular reactions such as ylide formation with nucleophiles. Here, ultrafast transient infrared absorption spectroscopy is used to show competitive production of singlet carbene and ketene intermediates from the photoexcitation of ethyl diazoacetoacetate. We provide direct spectroscopic evidence for ylide formation by singlet α-carbonyl carbene capture in aprotic nucleophilic solvents (with ylide bands at 1625 cm-1 in acetonitrile and 1586 and 1635 cm-1 in tetrahydrofuran) and report an enol-mediated pathway for singlet α-carbonyl carbene reaction with alcohols (ethanol or tert-butanol) identified by an absorption band at 1694 cm-1; however, we find no evidence for a previously proposed ylide pathway. The α-carbonyl carbene is monitored by using a band with solvent-dependent wavenumber in the range 1627-1645 cm-1. A computed two-dimensional cut of the potential energy surface for the reaction of the singlet α-carbonyl carbene with methanol shows that the enol forms without a barrier and that this reaction is promoted by an intermolecular hydrogen bond from methanol to the carbonyl oxygen atom. The corresponding ylide structure lies higher in energy, with a barrierless downhill path to isomerization to the enol.

Radical Formation in the Photoactivated Adenylate Cyclase OaPAC Revealed by Ultrafast Spectroscopy

Adenylate cyclases (AC) form a class of enzymes which are involved in the conversion of ATP to cAMP, playing an important role in many regulatory processes as the cAMP produced is involved in signalling pathways controlled by cAMP binding proteins. There is a very special subgroup of AC enzymes called photoactivated adenylate cyclases (PAC) where the ATP to cAMP conversion is controlled by light. Interestingly the photoregulation is controlled by BLUF (Blue light sensing using FAD) domains which are responsible for light regulation in many processes like phototoaxis, photophobia etc. Light regulation of BLUF domains is governed by the reorganization of the hydrogen bond network around the flavin and it is reflected in a ∼10 nm red shift of the flavin absorption peak. The first discovered photo-activated adenylate cyclase was found in a unicellular flagellate Euglenagracilis where the flagellar movement is regulated via a photoactivated AC. Another photoactivated adenylate cyclase called bPAC was discovered in the sulfide-oxidizing bacterium Beggiatoa sp. where the light regulation is solved by the BLUF domain located at the N-terminal part of the protein, the cAMP conversion is accomplished by a class IIIb type adenylyl cyclase at the C-terminal. Another promising PAC was discovered recently in the photosynthetic cyanobacterium Oscillatoriaacuminata which shows substantial activation (20-fold increase in the cAMP level in light compared to the dark) upon light illumination. OaPAC is a 366-aa residue long homodimer having a class III adenylate cyclase domain at the C-terminal and a BLUF domain at the N-terminal. In this work we characterized the primary photochemistry of OaPAC by the means of ultrafast (femtosecond to nanosecond) spectroscopy. Ultrafast transient visible and transient infrared absorption spectroscopy reveals that neutral flavin radical is formed after the blue light absorption.

Transient IR spectroscopy identifies key interactions and unravels new intermediates in the photocycle of a bacterial phytochrome.

Phytochromes are photosensory proteins in plants, fungi, and bacteria, which detect red- and far-red light. They undergo a transition between the resting (Pr) and photoactivated (Pfr) states. In bacterial phytochromes, the Pr-to-Pfr transition is facilitated by two intermediate states, called Lumi-R and Meta-R. The molecular structures of the protein in these states are not known and the molecular mechanism of photoconversion is not understood. Here, we apply transient infrared absorption spectroscopy to study the photocycle of the wild-type and Y263F mutant of the phytochrome from Deinococcus radiodurans (DrBphP) from nano- to milliseconds. We identify two sequentially forming Lumi-R states which differ in the local structure surrounding the carbonyl group of the biliverdin D-ring. We also find that the tyrosine at position 263 alters local structure and dynamics around the D-ring and causes an increased rate of Pfr formation. The results shed new light on the mechanism of light-signalling in phytochrome proteins.

Superheating and Homogeneous Melting Dynamics of Bulk Ice.

Homogeneous melting of crystals is a complex multistep process involving the formation of transient states at temperatures considerably higher than the melting point. The nature and persistence of these metastable structures are intimately connected to the melting process, and a precise definition of the temporal boundaries of these phenomena is not yet available. We set up a specifically designed experiment to probe by transient infrared absorption spectroscopy the entire dynamics, ranging from tens of picoseconds to microseconds, of superheating and melting of an ice crystal. In spite of a large excess of energy provided, only about 30% of the micrometric crystal liquefies in the first 20-25 ns because of the long persistence of the superheated metastable phase that extends for more than 100 ns. This behavior is ascribed to the population of low-energy states that trap a large amount of energy, favoring the formation of a metastable, likely plastic, ice phase.

Femtosecond infrared spectroscopy of chlorophyll f-containing photosystem I.

The recent discovery of extremely red-shifted chlorophyll f pigments in both photosystem I (PSI) and photosystem II has led to the conclusion that chlorophyll f plays a role not only in the energy transfer, but also in the charge separation processes [Nürnberg et al., Science, 2018, 360, 1210-1213]. We have employed ultrafast transient infrared absorption spectroscopy to study the contribution of far-red light absorbing chlorophyll f to energy transfer and charge separation processes in far-red light-grown PSI (FRL-PSI) from the cyanobacterium Chroococcidiopsis thermalis PCC 7203. We compare the kinetics and spectra of FRL-grown PSI excited at 670 nm and 740 nm wavelengths to those of white light-grown PSI (WL-PSI) obtained at 675 nm excitation. We report a fast decay of excited state features of chlorophyll a and complete energy transfer from chlorophyll a to chlorophyll f in FRL-PSI upon 670 nm excitation, as indicated by a frequency shift in a carbonyl absorption band occurring within a 1 ps timescale. While the WL-PSI measurements support the assignment of initial charge separation to A-1+˙A0-˙ [Di Donato et al., Biochemistry, 2011, 50, 480-490] from the kinetics of a distinct cation feature at 1710 cm-1, in the case of FRL-PSI, small features at 1715 cm-1 from the chlorophyll cation are present from sub-ps delays instead, supporting the replacement of the A-1 pigment with chlorophyll f. Comparisons of nanosecond spectra show that charge separation proceeds with 740 nm excitation, which selectively excites chlorophyll f, and modifications in specific carbonyl absorption bands assigned to P700+˙ minus P700 and A1-˙ minus A1 indicate dielectric differences of FRL-PSI compared to WL-PSI in one or both of the two electron transfer branches of FRL-PSI.

Water-Assisted Hole Trapping at the Highly Curved Surface of Nano-TiO2 Photocatalyst.

Heterogeneous photocatalysis is vital in solving energy and environmental issues that this society is confronted with. Although photocatalysts are often operated in the presence of water, it has not been yet clarified how the interaction with water itself affects charge dynamics in photocatalysts. Using water-coverage-controlled steady and transient infrared absorption spectroscopy and large-model (∼800 atoms) ab initio calculations, we clarify that water enhances hole trapping at the surface of TiO2 nanospheres but not of well-faceted nanoparticles. This water-assisted effect unique to the nanospheres originates from water adsorption as a ligand at a low-coordinated Ti–OH site or through robust hydrogen bonding directly to the terminal OH at the highly curved nanosphere surface. Thus, the interaction with water at the surface of nanospheres can promote photocatalytic reactions of both oxidation and reduction by elongating photogenerated carrier lifetimes. This morphology-dependent water-assisted effect provides a novel and rational basis for designing and engineering nanophotocatalyst morphology to improve photocatalytic performances.

Fusion of Ultraviolet-Visible and Infrared Transient Absorption Spectroscopy Data to Model Ultrafast Photoisomerization.

Ultrafast photoisomerization reactions generally start at a higher excited state with excess of internal vibrational energy and occur via conical intersections. This leads to ultrafast dynamics which are difficult to investigate with a single transient absorption spectroscopy technique, be it in the ultraviolet-visible (UV-vis) or infrared (IR) domain. On one hand, the information available in the UV-vis domain is limited as only slight spectral changes are observed for different isomers. On the other hand, the interpretation of vibrational spectra is strongly hindered by intramolecular relaxation and vibrational cooling. These limitations can be circumvented by fusing UV-vis and IR transient absorption spectroscopy data in a multiset multivariate curve resolution analysis. We apply this approach to describe the spectrodynamics of the ultrafast cis-trans photoisomerization around the C-N double bond observed for aromatic Schiff bases. Twisted intermediate states could be elucidated, and isomerization was shown to occur through a continuous complete rotation. More broadly, data fusion can be used to rationalize a vast range of ultrafast photoisomerization processes of interest in photochemistry.

Direct Visualization of Excited-State Symmetry Breaking Using Ultrafast Time-Resolved Infrared Spectroscopy.

Most symmetric quadrupolar molecules designed for two-photon absorption behave as dipolar molecules in the S1 electronic excited state. This is usually explained by a breakup of the symmetry in the excited state. However, the origin of this process and its dynamics are still not fully understood. Here, excited-state symmetry breaking in a quadrupolar molecule with a D-π-A-π-D motif, where D and A are electron donating and accepting units, is observed in real time using ultrafast transient infrared absorption spectroscopy. The nature of the relaxed S1 state was found to strongly depend on the solvent polarity: (1) in nonpolar solvents, it is symmetric and quadrupolar; (2) in weakly polar media, the quadrupolar state observed directly after excitation transforms to a symmetry broken S1 state with one arm bearing more excitation than the other; and (3) in highly polar solvents, the excited state evolves further to a purely dipolar S1 state with the excitation localized entirely on one arm. The time scales associated with the transitions between these states coincide with those of solvation dynamics, indicating that symmetry breaking is governed by solvent fluctuations.

Effects of single atom doping on the ultrafast electron dynamics of M1Au24(SR)18 (M = Pd, Pt) nanoclusters.

Atomically precise, doped metal clusters are receiving wide research interest due to their synergistic properties dependent on the metal composition. To understand the electronic properties of doped clusters, it is highly desirable to probe the excited state behavior. Here, we report the ultrafast relaxation dynamics of doped M1@Au24(SR)18 (M = Pd, Pt; R = CH2CH2Ph) clusters using femtosecond visible and near infrared transient absorption spectroscopy. Three relaxation components are identified for both mono-doped clusters: (1) sub-picosecond relaxation within the M1Au12 core states; (2) core to shell relaxation in a few picoseconds; and (3) relaxation back to the ground state in more than one nanosecond. Despite similar relaxation pathways for the two doped nanoclusters, the coupling between the metal core and surface ligands is accelerated by over 30% in the case of the Pt dopant compared with the Pd dopant. Compared to Pd doping, the case of Pt doping leads to much more drastic changes in the steady state and transient absorption of the clusters, which indicates that the 5d orbitals of the Pt atom are more strongly mixed with Au 5d and 6s orbitals than the 4d orbitals of the Pd dopant. These results demonstrate that a single foreign atom can lead to entirely different excited state spectral features of the whole cluster compared to the parent Au25(SR)18 cluster. The detailed excited state dynamics of atomically precise Pd/Pt doped gold clusters help further understand their properties and benefit the development of energy-related applications.

The influence of surface passivation on electronic energy relaxation dynamics of CdSe and CdSe/CdS nanocrystals studied using visible and near infrared transient absorption spectroscopy.

Charge carrier relaxation dynamics of electronically excited CdSe and CdSe/CdS core/shell nanocrystals (NCs) were studied using femtosecond time-resolved transient absorption spectroscopy, employing both visible and near-infrared (NIR) probe laser pulses. Following 400 nm excitation, the combination of visible and NIR laser probe pulses were used to determine the influence of surface passivation on electronic relaxation dynamics for nanocrystals overcoated with either organic ligands or inorganic semiconductors. In particular, low-energy NIR photons were used to isolate transient absorption signals due to either electron and hole intraband transitions. Four relaxation components were detected for CdSe NCs passivated by organic molecules: (1) picosecond hole relaxation; (2) electron deep trapping; (3) electron surface trapping; and (4) exciton radiative recombination. Based on TA data collected over a broad energy range, electron deep trapping at Se(2-) sites was suppressed for CdSe NCs passivated by inorganic (CdS) semiconducting materials. By comparing the time-dependent transient absorption data of a series of CdSe/CdS NCs with different shell thicknesses, evidence for the transition from Type-I to quasi Type-II NCs was obtained. These data illustrate the sensitivity of femtosecond time-resolved transient absorption measurements carried out over visible and near infrared probe energies for determining the influence of nanocrystal structure on electronic relaxation dynamics.

Extremely rapid self-reaction of the simplest Criegee intermediate CH2OO and its implications in atmospheric chemistry.

Criegee intermediates, which are carbonyl oxides produced when ozone reacts with unsaturated hydrocarbons, play an important role in the formation of OH and organic acids in the atmosphere, but they have eluded direct detection until recently. Reactions that involve Criegee intermediates are not understood fully because data based on their direct observation are limited. We used transient infrared absorption spectroscopy to probe directly the decay kinetics of formaldehyde oxide (CH2OO) and found that it reacts with itself extremely rapidly. This fast self-reaction is a result of its zwitterionic character. According to our quantum-chemical calculations, a cyclic dimeric intermediate that has the terminal O atom of one CH2OO bonded to the C atom of the other CH2OO is formed with large exothermicity before further decomposition to 2H2CO + O2((1)Δg). We suggest that the inclusion of this previously overlooked rapid reaction in models may affect the interpretation of previous laboratory experiments that involve Criegee intermediates.

Experimental and Theoretical Study of the Product Channels of the C2H + NO Reaction

The product channels of the C2H + NO reaction were studied by transient infrared absorption spectroscopy. HCN, CO, CN, and HNO (produced from HCO secondary chemistry), were successfully detected. Analysis of the obtained product yields provides an estimation of the branching fractions to be φ(1a)(HCN+CO) = 0.77 ± 0.04 and φ(1b)(HCO+CN) = 0.23 ± 0.04 at 298 K. The potential energy surface (PES) of the reaction was investigated by ab initio methods at CCSD(T)/6-311++G(d,p)//DFT-B3LYP/6-311++G(d,p) levels as well as higher levels of theory for selected critical points. The singlet potential energy surface shows a low-energy pathway only to the HCN + CO product channel, but a plausible route to HCO + CN products exists on the triplet surface. The computational results are in qualitative agreement with the experimental observations.

Probing the Photoisomerization of CHBr3 and CHI3 in Solution with Transient Vibrational and Electronic Spectroscopy

Transient infrared absorption spectroscopy monitors condensed-phase photodissociation dynamics of 30 mM CHBr3 and 50 mM CHI3 in liquid CCl4. The experiments have picosecond time resolution and monitor the C-H stretch region of both the parent polyhalomethanes and their photolytically generated isomers. The C-H stretching transitions of these isomers, in which the emergent halogen atom returns to form a C-X-X bonding motif, appear about 9 ps after photolysis for iso-CHBr2-Br and in about 46 ps for iso-CHI2-I. These time scales are consistent with, but differ from, the time evolution of the transient electronic absorption spectra of the same samples, highlighting the subtle differences between monitoring the vibrational and electronic chromophores. The specificity of using vibrational transitions to track condensed-phase reaction dynamics permits reassessment of the transient electronic spectrum of photolysis in neat CHBr3, which has an additional prompt feature near 400 nm. Calculations show that this feature, which arises from a precursor to the isomer, is a charge-transfer transition of a contact pair between the nascent Br fragment and a nearby CHBr3 molecule. Dilution and solvent studies show that transition is independent of the solvent. The iso-CHBr2-Br transition wavelength, however, shifts over the range of 400 to 510 nm depending on the solvent. Time-dependent density functional calculations faithfully reproduce these trends.

Product Channels of the CN + HCNO Reaction

The kinetics of the reaction of CN radical with fulminic acid (HCNO) was studied by transient infrared absorption spectroscopy with the primary goal of resolving whether the dominant product channel is NO + HCCN (1a) or HCN + NCO (1b). HCN, HCCN, and NO reaction products were directly detected. In some experiments, (15)N(18)O reagent was included in the reaction mixtures in order to suppress possible secondary chemistry due to NCO radicals. Several other possible secondary reactions were also investigated and found to be very slow. The resulting product branching fractions of φ(1a) = 0.98 ± 0.07 for NO + HCCN and φ(1b) ≤ 0.07 for HCN + NCO, respectively, were obtained at 298 K. The potential energy surface (PES) of the reaction was calculated by ab initio methods at several levels of coupled-cluster theory. The calculations show pathways to channels (1a) and (1b) with nearly identical energetics and a substantial dependence on the level of theory used, suggesting that multireference calculations are needed to accurately predict the experimental results.

A study of the pH dependence of electronically excited guanosine compounds by picosecond time-resolved infrared spectroscopy

The photophysical properties of 5'-guanosine monophosphate (5'-GMP) and polyguanylic acid {poly(G)} in D(2)O solutions of varying pH have been studied using picosecond transient infrared absorption spectroscopy. Whereas in neutral or weakly alkaline solution only the vibrationally excited electronic ground state of 5'-GMP is observed, in acidic solution the relatively long-lived (229 +/- 20 ps) electronic excited state of protonated 5'-GMP, which possesses strong absorptions at 1517 and 1634 cm(-1), could be detected. The picosecond transient behaviour of polyguanylic acid in acidic solution is also very different from that of the polynucleotide in neutral solution due not only to the protonation of guanine moieties yielding the protonated excited state but because of the disruption of the guanine stacks which are present in the species in neutral solution.

Solvent Reorientation Process in the “Twisted” Intramolecular Charge-Transfer Process of Cyanophenyldisilane−(H2O)2 Cluster Investigated by Transient Infrared Spectroscopy

The solvent reorientation process of the intramolecular charge-transfer (ICT) process of the (p-cyanophenyl)pentamethyldisilane-(H2O)2 (CPDS-(H2O)2) cluster in the excited-state was investigated by transient infrared (IR) absorption spectroscopy. It was found that there are at least two isomers in the charge-transfer (CT) state: one of the isomers exhibits a band of a pi-hydrogen-bonded OH stretch of the water moiety. Analyses of the IR spectra in the dominant isomers revealed that water molecules are hydrogen-bonded with each other in the CT state. This indicates that the reorientation process of the water molecules takes place to form such a dimer structure during the ICT process.

Photooxidation of Guanine by a Ruthenium Dipyridophenazine Complex Intercalated in a Double‐Stranded Polynucleotide Monitored Directly by Picosecond Visible and Infrared Transient Absorption Spectroscopy

Transient species formed by photoexcitation (400 nm) of [Ru(dppz)(tap)2]2+ (1) (dppz = dipyrido[3,2-a:2',3'-c]phenazine; tap=1,4,5,8-tetraazaphenanthrene) in aqueous solution and when intercalated into a double-stranded synthetic polynucleotide, [poly(dG-dC)]2, have been observed on a picosecond timescale by both visible transient absorption (allowing monitoring of the metal complex intermediates) and transient infrared (IR) absorption spectroscopy (allowing direct study of the DNA nucleobases). By contrast with its behavior when free in aqueous solution, excitation of 1 when bound to [poly(dG-dC)]2 causes a strong increase in absorbance at 515 nm due to formation of the reduced complex [Ru(dppz)(tap)2]+ (rate constant=(2.0+/-0.2) x 10(9) s(-1)). The subsequent reformation of 1 proceeds with a rate constant of (1.1+/-0.2) x 10(8) s(-1). When the process is carried out in D2O, the rates of formation and removal of [Ru(dppz)(tap)2]+ are reduced (rate constants (1.5+/-0.3) x 10(9) and (0.7+/-0.2) x 10(8) s(-1) respectively) consistent with proton-coupled electron transfer processes. Picosecond transient IR measurements in the 1540-1720 cm(-1) region in D2O solution confirm that the reduction of 1 intercalated into [poly(dG-dC)]2 is accompanied by bleaching of IR ground-state bands of guanine (1690 cm(-1)) and cytosine (1656 cm(-1)), each with similar rate constants.

Direct observation of the solvent reorientation dynamics in the “twisted” intramolecular charge-transfer process of cyanophenyldisilane–water cluster by transient infrared spectroscopy

The solvent reorientation dynamics of the intramolecular charge-transfer (ICT) process of the (p-cyanophenyl)pentamethyldisilane-H(2)O (CPDS-H(2)O) cluster was investigated by transient infrared (IR) absorption spectroscopy. Transient IR bands of two distinct charge-transfer (CT) states appeared in both the OH and the CN-stretching vibration regions. Analyses of the IR spectra and the time profiles of the transient bands revealed that the ICT process of the CPDS-H(2)O cluster proceeds in two steps. The first step is a transition from a photo-prepared locally excited (LE) state to the CT state, which is accompanied by a minor reorientation of the H(2)O moiety. In contrast, the second step is an extensive reorientation process of the H(2)O molecule in the CT state. These two reorientation processes exhibit very distinct pico- and nano-second time scales. In the latter case, a relatively slow time constant of 2 ns was related to a large geometric change in the orientation.