Transient Vibrational Spectroscopy Research Articles

Vibrational Dynamics at Aqueous–Mineral Interfaces

The dynamics of water at mineral surfaces has attracted much attention due to the marked differences, compared to the bulk, in the ability of interfacial water populations to redistribute vibrational energy, largely due to perturbations in the local hydrogen-bonding environments at interfaces. However, many unanswered questions persist regarding these geochemically and technologically relevant systems. The evolution of our understanding and current state-of-the-art interpretation are reviewed for three important mineral/aqueous interfaces (Al2O3, SiO2, and CaF2). While we focus on time-resolved vibrational Sum Frequency Generation (vSFG), as it is inherently surface specific, we include complementary time-resolved techniques such as IR and THz spectroscopies, which combined can provide a broader picture of interfacial dynamics at mineral surfaces. We show that vibrational dynamics are uniquely positioned to inform on structure at interfaces, which could be missed using conventional static vibrational spectra. Insights presented here shine light on previous successes and suggest future avenues for transient vibrational spectroscopy at mineral/aqueous interfaces.

Excited-state symmetry breaking in 9,10-dicyanoanthracene-based quadrupolar molecules: the effect of donor-acceptor branch length.

Excited-state symmetry breaking is investigated in a series of symmetric 9,10-dicyanoanthracenes linked to electron-donating groups on the 2 and 6 positions via different spacers, allowing for a tuning of the length of the donor–acceptor branches. The excited-state properties of these compounds are compared with their dipolar single-branch analogues. The changes in electronic structure upon their optical excitation are monitored by transient electronic spectroscopy in the visible and near-infrared regions as well as by transient vibrational spectroscopy in the mid-infrared. Our results reveal that, with the shortest branches, electronic excitation remains distributed almost symmetrically over the molecule even in polar environments. Upon increasing the donor–acceptor distance, excitation becomes unevenly distributed and, with the longest one, it fully localises on one branch in polar solvents. The influence of the branch length on the propensity of quadrupolar dyes to undergo excited-state symmetry breaking is rationalised in terms of the balance between interbranch coupling and solvation energy.

Transient FTIR Measurements at Nanoseconds Resolution: Correlating between Faceting and Photocatalytic Activity in BiOCl

Transient IR measurements of photo-excited bismuth oxychloride (BiOCl) were performed by a step-scan technique at a resolution of 2.5 nanoseconds. Six types of faceted BiOCl particles, which differed also in their average size, were used. Following excitation, an increase in the intensity of the Bi-O signal, which lasted for 70–130 nanoseconds was observed. A negative correlation between the characteristics of the transient signal (duration of signal and intensity) versus the photocatalytic activity toward the reduction of Cr(VI) was found. In parallel, a positive correlation was found between the duration of the transient signal and the specific surface area of the various types of particles. The increased transient intensity is explained in terms of formation of deep traps, most likely at the surface of the particles. The results demonstrate that transient vibrational spectroscopy may serve as an important tool for studying photocatalytic materials.

Charge-transfer and impulsive electronic-to-vibrational energy conversion in ferricyanide: ultrafast photoelectron and transient infrared studies.

The photophysics of ferricyanide in H2O, D2O and ethylene glycol was studied upon excitation of ligand-to-metal charge transfer (LMCT) transitions by combining ultrafast photoelectron spectroscopy (PES) of liquids and transient vibrational spectroscopy. Upon 400 nm excitation in water, the PES results show a prompt reduction of the Fe3+ to Fe2+ and a back electron transfer in ∼0.5 ps concomitant with the appearance and decay of a strongly broadened infrared absorption at ∼2065 cm-1. In ethylene glycol, the same IR absorption band decays in ∼1 ps, implying a strong dependence of the back electron transfer on the solvent. Thereafter, the ground state ferric species is left vibrationally hot with significant excitation of up to two quanta of the CN-stretch modes, which completely decay on a 10 ps time scale. Under 265 nm excitation even higher CN-stretch levels are populated. Finally, from a tiny residual transient IR signal, we deduce that less than 2% of the excited species undergo photoaquation, in line with early flash photolysis experiments. The latter is more significant at 265 nm compared to 400 nm excitation, which suggests photodissociation in this system is an unlikely statistical process related to the large excess of vibrational energy.

Tracking a Paternò–Büchi Reaction in Real Time Using Transient Electronic and Vibrational Spectroscopies

A detailed mechanistic investigation of the early stages of the Paternò-Büchi reaction following 267 nm excitation of benzaldehyde in cyclohexene has been completed using ultrafast, broadband transient UV-visible and IR absorption spectroscopies. Absorption due to electronically excited triplet state benzaldehyde decays on a 80 ps time scale via reaction with cyclohexene. The growth and subsequent decay of the biradical intermediate produced following C-O bond formation is followed by transient vibrational spectroscopy. The biradical decays by ring closure to an oxetane or by dissociating, reforming the ground state reactants. Detailed kinetic analysis allowed derivation of quantum yields and rate constants for these competing biradical decay processes, ϕ(oxetane) = 0.53, ϕ(diss) = 0.47, koxetane = 0.27 ± 0.09 ns(-1) and k(diss) = 0.24 ± 0.09 ns(-1). This study provides a striking illustration of the ways in which contemporary ultrafast transient absorption spectroscopy methods can be used to dissect the mechanism and kinetics of a classic photoreaction.

Diverse photoinduced dynamics in an organic charge-transfer complex having strong electron-phonon interactions.

CONSPECTUS: Phenomena that occur in nonequilibrium states created by photoexcitation differ qualitatively from those that occur at thermal equilibrium, and various physical theories developed for thermal equilibrium states can hardly be applied to such phenomena. Recently it has been realized that understanding phenomena in nonequilibrium states in solids is important for photoenergy usage and ultrafast computing. Consequently, much effort has been devoted to revealing such phenomena by developing various ultrafast observation techniques and theories applicable to nonequilibrium states. This Account describes our recent studies of diverse photoinduced dynamics in a strongly correlated organic solid using various ultrafast techniques. Solids in which the electronic behavior is affected by Coulomb interactions between electrons are designated as strongly correlated materials and are known to exhibit unique physical properties even at thermal equilibrium. Among them, many organic charge-transfer (CT) complexes have low dimensionality and flexibility in addition to strong correlations; thus, their physical properties change sensitively in response to changes in pressure or electric field. Photoexcitation is also expected to drastically change their physical properties and would be useful for ultrafast photoswitching devices. However, in nonequilibrium states, the complicated dynamics due to these characteristics prevents us from understanding and using these materials for photonic devices. The CT complex (EDO-TTF)2PF6 (EDO-TTF = 4,5-ethylenedioxytetrathiafulvalene) exhibits unique photoinduced dynamics due to strong electron-electron and electron-phonon interactions. We have performed detailed studies of the dynamics of this complex using transient electronic spectroscopy at the 10 and 100 fs time scales. These studies include transient vibrational spectroscopy, which is sensitive to the charges and structures of constituent molecules, and transient electron diffraction, which provides direct information on the crystal structure. Photoexcitation of the charge-ordered low-temperature phase of (EDO-TTF)2PF6 creates a new photoinduced phase over 40 fs via the Franck-Condon state, in which electrons and vibrations are coherently and strongly coupled. This new photoinduced phase is assigned to an insulator-like state in which the charge order differs from that of the initial state. In the photoinduced phase, translations of component molecules proceed before the rearrangements of intramolecular conformations. Subsequently, the charge order and structure gradually approach those of the high-temperature phase over 100 ps. This unusual two-step photoinduced phase transition presumably originates from steric effects due to the bent EDO-TTF as well as strong electron-lattice interactions.

Proteins in action: femtosecond to millisecond structural dynamics of a photoactive flavoprotein.

Living systems are fundamentally dependent on the ability of proteins to respond to external stimuli. The mechanism, the underlying structural dynamics, and the time scales for regulation of this response are central questions in biochemistry. Here we probe the structural dynamics of the BLUF domain found in several photoactive flavoproteins, which is responsible for light activated functions as diverse as phototaxis and gene regulation. Measurements have been made over 10 decades of time (from 100 fs to 1 ms) using transient vibrational spectroscopy. Chromophore (flavin ring) localized dynamics occur on the pico- to nanosecond time scale, while subsequent protein structural reorganization is observed over microseconds. Multiple time scales are observed for the dynamics associated with different vibrations of the protein, suggesting an underlying hierarchical relaxation pathway. Structural evolution in residues directly H-bonded to the chromophore takes place more slowly than changes in more remote residues. However, a point mutation which suppresses biological function is shown to ‘short circuit’ this structural relaxation pathway, suppressing the changes which occur further away from the chromophore while accelerating dynamics close to it.

Time-Resolved Studies of CN Radical Reactions and the Role of Complexes in Solution

Time-resolved studies using 100 fs laser pulses generate CN radicals photolytically in solution and probe their subsequent reaction with solvent molecules by monitoring both radical loss and product formation. The experiments follow the CN reactants by transient electronic spectroscopy at 400 nm and monitor the HCN products by transient vibrational spectroscopy near 3.07 microm. The observation that CN disappears more slowly than HCN appears shows that the two processes are decoupled kinetically and suggests that the CN radicals rapidly form two different types of complexes that have different reactivities. Electronic structure calculations find two bound complexes between CN and a typical solvent molecule (CH(2)Cl(2)) that are consistent with this picture. The more weakly bound complex is linear with CN bound to an H atom through the N atom, and the more strongly bound complex has a structure in which the CN bridges Cl and H atoms of the solvent. Fitting the transient absorption data with a kinetic model containing two uncoupled complexes reproduces the data for seven different chlorinated alkane solvents and yields rate constants for the reaction of each type of complex. Depending on the solvent, the linear complex reacts between 2.5 and 12 times faster than the bridging complex and is the primary source of the HCN reaction product. Increasing the Cl atom content of the solvents decreases the reaction rate for both complexes.

Time-Resolved Spectroscopic Study of the Reaction Cl + n-C5H12 → HCl + C5H11 in Solution

We study the hydrogen abstraction reaction from pentane by chlorine radicals using four different experimental approaches. We use two different solvents (CH2Cl2 and CCl4) and two different chlorine atom sources (photodissociation of dissolved Cl2 and two-photon photolysis of the solvent) to investigate their effects on the recombination and reactivity of the chlorine radical. All four experimental schemes involve direct probing of the transient chlorine population via a charge transfer transition with a solvent molecule. In one of the four approaches, photolysis of Cl2 in dichloromethane, we also monitor the nascent reaction products (HCl) by transient vibrational spectroscopy. Probing both the reactants and the products provides a comprehensive view of this bimolecular reaction in solution. Between one-third and two-thirds of the chlorine radicals that initially escape the solvent cage undergo diffusive geminate recombination with their partner radical (either another chlorine atom or the solvent radical). The rest react with pentane with the bimolecular rate constants k(bi) = (9.5 +/- 0.7) x 10(9) M(-1) s(-1) in CH2Cl2 and k(bi) = (7.4 +/- 2) x 10(9) M(-1) s(-1) in CCl4. The recombination yield phi(rec) depends on both the chlorine atom precursor and the solvent and is larger in the more viscous carbon tetrachloride solutions. The bimolecular reaction rate k(bi) depends only on the solvent and is consistent with a nearly diffusion-limited reaction.

Excited state structure of 4-(dimethylamino)benzonitrile studied by femtosecond mid-infrared spectroscopy and ab initio calculations

Abstract Combining femtosecond transient vibrational spectroscopy and high-level calculations is a powerful tool in the determination of excited-state structures. Striking differences in the experimental vibrational pattern of the locally excited states of 4-(dimethylamino)benzonitrile (DMABN) and 4-aminobenzonitrile (ABN) are explained on the basis of molecular structures obtained from ab initio complete-active-space self-consistent-field (CASSCF) calculations, giving evidence for a strong sensitivity of the molecular structure on modest changes in the substituents. The 4.0 ps charge-transfer time for DMABN in acetonitrile is resolved for the first time by tracking the downshifted CN stretching mode.

Application of transient vibrational spectroscopies to the excited states of metal polypyridyl complexes

Application of transient vibrational spectroscopies to the excited states of metal polypyridyl complexes

Ultrafast investigation of condensed phase chemical reaction dynamics using transient vibrational spectroscopy: Geminate recombination, vibrational energy relaxation, and electronic decay of the iodine A′ excited state

The reaction dynamics of iodine geminate recombination on the excited A′ 3Π2u state are investigated using picosecond transient Raman spectroscopy. Vibrational energy relaxation and the dynamics of geminate recombination are directly observed in the transient Raman spectrum. Geminate recombination rates are significantly slower for the excited electronic state compared to the ground electronic state. This observation implies that a fundamentally different geminate recombination mechanism is responsible for the formation of the A′ 3Π2u state. Evidence is reported which suggests the possible role of iodine atom–solvent complexes in the reaction step leading from photodissociated atoms to formation of the excited state. The solvent dependence of vibrational energy relaxation suggests that the rate is dominated by V–T (and possibly V–R) energy relaxation which is also found to be significantly slower in the excited electronic state compared to the ground state. In n-hexane the vibrational relaxation rate appears to be enhanced by V–V energy transfer. Electronic relaxation is investigated and is found to be largely the result of the formation and decay of I2⋅solvent exciplexes.

ChemInform Abstract: Transient Vibrational Spectroscopy by Flash Photolysis Stimulated Emission Pumping: 3ν2 of Singlet Methylene.

ChemInformVolume 20, Issue 19 Physical Organic Chemistry ChemInform Abstract: Transient Vibrational Spectroscopy by Flash Photolysis Stimulated Emission Pumping: 3ν2 of Singlet Methylene. W. XIE, W. XIE Dep. Chem., Univ. Pa., Philadelphia, PA 19104, USASearch for more papers by this authorA. RITTER, A. RITTER Dep. Chem., Univ. Pa., Philadelphia, PA 19104, USASearch for more papers by this authorC. HARKIN, C. HARKIN Dep. Chem., Univ. Pa., Philadelphia, PA 19104, USASearch for more papers by this authorK. KASTURI, K. KASTURI Dep. Chem., Univ. Pa., Philadelphia, PA 19104, USASearch for more papers by this authorH.-L. DAI, H.-L. DAI Dep. Chem., Univ. Pa., Philadelphia, PA 19104, USASearch for more papers by this author W. XIE, W. XIE Dep. Chem., Univ. Pa., Philadelphia, PA 19104, USASearch for more papers by this authorA. RITTER, A. RITTER Dep. Chem., Univ. Pa., Philadelphia, PA 19104, USASearch for more papers by this authorC. HARKIN, C. HARKIN Dep. Chem., Univ. Pa., Philadelphia, PA 19104, USASearch for more papers by this authorK. KASTURI, K. KASTURI Dep. Chem., Univ. Pa., Philadelphia, PA 19104, USASearch for more papers by this authorH.-L. DAI, H.-L. DAI Dep. Chem., Univ. Pa., Philadelphia, PA 19104, USASearch for more papers by this author First published: May 9, 1989 https://doi.org/10.1002/chin.198919041AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume20, Issue19May 9, 1989 RelatedInformation

Transient vibrational spectroscopy by flash photolysis stimulated emission pumping: 3ν2 of singlet methylene

A vibration–rotation spectroscopic method based on stimulated emission pumping has been developed for transient molecules generated by flash photolysis. This method may be generally applicable to radicals with electronic excited states accessible by visible or UV laser pulses. This first demonstration on the bending v2 =3 level of ã 1A1 CH2 generated a vibrational term value of 3950.45(7) cm−1 and the rotational constants A=25.665(40), B=11.470(7), and C=6.7140(61) cm−1. These constants indicate that the 3ν2 level is well below the bending-potential barrier. Perturbations of several rotational levels by the triplet state are identified. In addition, and most interestingly, abnormally intense ΔKa =3 transitions were observed in the ã←b̃ 2163 band.