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Two-dimensional resonance Raman (2DRR) spectroscopies have been used to investigate the structural heterogeneity of ensembles and chemical reaction mechanisms in recent years. Our previous work suggests that the intensities of artifacts may be comparable to the desired 2DRR response for some chemical systems and experimental approaches. In a type of artifact known as a "cascade," the four-wave mixing signal field radiated by one molecule induces a four-wave mixing process in a second molecule. We consider the susceptibility of 2DRR spectroscopy to various types of signal cascades in the present work. Calculations are conducted using empirical parameters obtained for a molecule with an intramolecular charge-transfer transition in acetonitrile. For a fully impulsive pulse sequence, it is shown that "parallel" cascades involving two solute molecules are generally more intense than that of the desired 2DRR response when the solute's mode displacements are 1.0 or less. In addition, we find that the magnitudes of parallel cascades involving both solute and solvent molecules (i.e., a solute-solvent cascade) may exceed that of the 2DRR response when the solute possesses small mode displacements. It is tempting to assume that solute-solvent cascades possess negligible intensities because the off-resonant Raman cross sections of solvents are usually 4–6 orders of magnitude smaller than that of the electronically resonant solute; however, the present calculations show that the difference in solute and solvent concentrations can fully compensate for the difference in Raman cross sections under common experimental conditions. Implications for control experiments and alternate approaches for 2DRR spectroscopy are discussed. [ABSTRACT FROM AUTHOR]

The experimental design and theoretical description of a novel five-pulse laser spectroscopy is presented with an application to a pyridinium charge transfer complex in acetonitrile and methanol. In field-resolved polarizability response spectroscopy (PORS), an electronically resonant laser pulse first excites a solvated chromophore (reactant) and off-resonant Raman spectra of the resulting nuclear motions are measured as a function of the reaction time. The present apparatus differs from our earlier design by performing the Raman probe measurement (with fixed pulse delays) in the frequency domain. In addition, the full electric fields of the signals are measured by spectral interferometry to separate nonresonant and Raman responses. Our theoretical model shows how the PORS signal arises from nuclear motions that are displaced/driven by the photoinduced reaction. The field-resolved off-resonant (of the solute’s electronic transitions) probing favors detection of solvent (as opposed to solute) dynamics coupled to the reaction. The sign of the signal represents the relative strengths of polarization responses associated with the ground and photoexcited solutions. Signatures of nonresonant and PORS signal contributions to the experimental results are analyzed with numerical calculations based on a theoretical model we have developed for reaction-induced PORS. Our model identifies two mechanisms of PORS signal generation: (i) structural relaxation induced resonance; (ii) dephasing induced resonance. In the charge transfer reaction investigated, the solvent-dependent and time-evolving (solvent) polarizability spectral density (PSD) is readily obtained. The general trend of an initial broadband inertial nuclear response followed by a decrease in the linewidth of the PSD establishes that the measured PSD is inconsistent with the approximation of a linear response. Furthermore, the explicit time evolution of the PSD is important for properly describing solvent control of reactions that do not satisfy the time-scale separability inherent to nonadiabatic kinetic models. [ABSTRACT FROM AUTHOR]

Electric field-resolved transient grating measurements are used to distinguish the four-wave mixing signal emission from a resonant solute and a non-resonant solvent. The two components of the solution (i.e., solute and solvent) emit signal fields at different times with respect to the arrival of the probe pulse to the sample. This gives rise to a recurrence in the temporal profile of the total signal field. We show that the origin of this interference is the difference in relaxation time scales of the holographic gratings associated with the solute and solvent. The grating of the resonant solute relaxes on the time scale of a few picoseconds due to depopulation of its excited electronic state, whereas the electronic polarizability response of the solvent relaxes on the femtosecond time scale. This separability of responses is a general phenomenon that is particularly useful for studying weakly absorbing solute dynamics in polarizable solvents. [ABSTRACT FROM AUTHOR]

Optoelectronic properties of a polyphenylenevinylene-based oligomer and its paracylophane-linked dimer are studied using a variety of experimental and theoretical techniques. Despite the symmetrical structure and redshifted absorption of the dimer versus the monomer, an exciton picture is not the most appropriate. Electronic structure calculations establish changes in charge density upon optical excitation and show localized excitations that cannot be accounted for by a simple Frenkel exciton model. Visible frequency pump-probe anisotropy measurements suggest that the dimer should be considered as a three-level system with a fast, ∼130 fs, internal conversion from the higher to lower energy excited electronic state. Signatures of nuclear relaxation processes are compared for electric field-resolved transient grating and two-dimensional photon echo spectra. These measurements reveal that nuclear relaxation occurs on similar time scales for the monomer and dimer. The connection between the spectral phase of four-wave mixing signals and the time dependent width of a nuclear wave packet is discussed. Semiempirical electronic structure and metropolis Monte Carlo calculations show that the dominant line broadening mechanisms for the monomer and dimer are associated with inter-ring torsional coordinates. Together, the theoretical calculations and electric field-resolved four-wave mixing experiments suggest that while the structure of dimer is more rigid than that of monomer, the difference in their rigidities is not sufficient to slow down excited state relaxation of dimer with respect to the monomer. [ABSTRACT FROM AUTHOR]

Distinct hydrogen bonding patterns are predicted in the amide I and amide A vibrational bands of four dominant conformations of antamanide using anharmonic vibrational Hamiltonians constructed at the DFT and AM1 levels. We show how these conformations may be distinguished using coherent three pulse infrared measurements with several pulse polarization configurations in the amide I region. The amide A hydrogen bonded N-H stretching modes are highly localized and have conformation-dependent frequencies, but their anharmonicities are insensitive to local structure at the hydrogen bond distances in antamanide. [ABSTRACT FROM AUTHOR]

Vibrational exciton Hamiltonians for the amide I and amide A modes of both the α- and 3[sub 10]-helical conformations of a fifteen unit polyalanine oligomer CH[sub 3]-CO(Ala)[sub 15]-NHCH[sub 3] are constructed using density-functional calculations for smaller model peptides. Energy levels as well as the transition dipoles of all singly and doubly excited-state manifolds are calculated. A variety of C[sub 13]-substituted isotopic derivatives are examined with respect to their ability to reveal differences in local secondary structures in two-dimensional infrared spectra in the amide I region. Amide mode anharmonicities are predicted to be valid indicators of secondary helical structures. [ABSTRACT FROM AUTHOR]

The complete anharmonic cubic and quartic force field of the two carbonyl stretching vibrations of a rhodium di-carbonyl complex is calculated at the density functional level and used to simulate the third-order vibrational response function. The infrared photon echo spectrum calculated using the diagonalized resulting exciton Hamiltonian is in qualitative agreement with measured values. Quartic terms in the potential are critical for reproducing the experimental transition energies and transition dipoles. [ABSTRACT FROM AUTHOR]

The linear absorption spectra and absolute resonance Raman excitation profiles of the “push-pull” chromophore julolidinemalononitrile have been measured in cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol solution at excitation wavelengths spanning the strong visible charge-transfer absorption band. Numerical simulation of the spectra using time-dependent wave-packet propagation methods yields the excited-state geometry changes along the ∼15 strongly Raman-active vibrations as well as the solvent reorganization energies. The distribution of the total vibrational reorganization energy among the various normal modes is solvent dependent, indicating solvent polarity effects on the electronic structure. These results are compared with those previously obtained for two other push-pull chromophores, p-nitroaniline and julolidinyl-n-N,N[sup ′]-diethylthiobarbituric acid. The frequency dispersion of the molecular first hyperpolarizability, β, is also calculated in each solvent using a time-domain form of the standard Oudar–Chemla two-state model modified to incorporate solvent reorganization, inhomogeneous broadening, and the vibronic structure of the charge-transfer state. We show that accurate extrapolation of β measured at frequencies in the near-infrared to zero frequency requires a realistic description of the excited state as the measuring wavelength approaches a two-photon resonance. This is particularly relevant to the high chromophore concentrations needed for device applications, where intermolecular interactions can strongly perturb the electronic transitions. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Low-dimensional organic/inorganic hybrid perovskites (OIHPs) are a promising class of materials with a wide range of potential applications in optoelectronics and other fields since these materials can synergistically combine individual features of organic molecules and inorganics into unique properties. Non-covalent interactions are commonly observed in OIHPs, in particular, π-effect interactions between the organic cations. Such non-covalent interactions can significantly influence important properties of the low-dimensional OIHPs, including dielectric confinement, bandgap, photoluminescence, quantum efficiency, charge mobility, trap density, stability, and chirality. This perspective reviews recent studies of non-covalent interactions involving the π systems of organic cations in low-dimensional OIHPs. The analysis of crystal structures of low-dimensional OIHPs offers significant insight into understanding such non-covalent interactions and their impacts on specific properties of these OIHPs. The developed structure–property relationships can be used to engineer non-covalent interactions in low-dimensional OIHPs for applications. [ABSTRACT FROM AUTHOR]

Conventional time-of-flight methods can be used to determine carrier mobilities for photovoltaic cells in which the transit time between electrodes is greater than the RC time constant of the device. To measure carrier drift on sub-ns timescales, we have recently developed a two-pulse time-of-flight technique capable of detecting drift velocities with 100-ps time resolution in perovskite materials. In this method, the rates of carrier transit across the active layer of a device are determined by varying the delay time between laser pulses and measuring the magnitude of the recombination-induced nonlinearity in the photocurrent. Here, we present a related experimental approach in which diffractive optic-based transient grating spectroscopy is combined with our two-pulse time-of-flight technique to simultaneously probe drift and diffusion in orthogonal directions within the active layer of a photovoltaic cell. Carrier density gratings are generated using two time-coincident pulse-pairs with passively stabilized phases. Relaxation of the grating amplitude associated with the first pulse-pair is detected by varying the delay and phase of the density grating corresponding to the second pulse-pair. The ability of the technique to reveal carrier diffusion is demonstrated with model calculations and experiments conducted using MAPbI3 photovoltaic cells. [ABSTRACT FROM AUTHOR]

We resolve information about the dynamics of simple liquids that has been obscured in prior frequency and time-domain measurements by way of full field-resolved polarization emission time (FR-PET) measurements of carbon disulfide. The amplitude and phase of the field-resolved transient-grating signal is used to calculate a spectrogram of the signal field at each delay between the transient-grating (TG) pump and probe pulses. The temporal maximum of the spectrogram, defined to be the signal emission time, varies with pump−probe delay; it follows the convolution of the TG pulses while the pulses overlap and exhibits recurrences at times when the nuclear dynamics are the main component of the liquid material response. Since this is a third-order nonlinear spectroscopic method, the isotropic and anisotropic signals are constructed from the polarization tensor components. The frequency-integrated anisotropic component of the signal is equivalent to the signal measured in optical Kerr Effect (OKE) experiments. The FR-PET determination of the signal emission times is a direct measurement of the third-order nonlinear (polarizability) polarization and, hence provides new strong constraints on appropriate models of the liquid dynamics. Models for the material response function are used to calculate the signal emission times. In particular, we show that the proper treatment of the time-correlation function for orientational motion gives the best fit to the FR-PET data for rotational diffusional motion. We also establish that librational motion is not a short-time (coherent) motion that leads to rotational diffusion. Finally, we find that the Bucaro−Litovitz form for interaction-induced dynamics is not entirely correct for the CS2 liquid we study. We suggest that the failing may result from the implicit assumption of two-body interactions, which is only appropriate for gases. [ABSTRACT FROM AUTHOR]

Abstract: A formalism is presented for a novel fifth-order spectroscopy designed to measure the spectrum of nuclear modes coupled to photoinduced processes. In this description of polarizability response spectroscopy, a two-level system first interacts (twice) with an electronically resonant laser pulse to create ground and excited state wavepackets that evolve until the polarizability spectrum is probed by three off-resonant pulses and a fourth local oscillator probe field. In the experiment and in the present formalism, heterodyne detection is accomplished by tuning the relative phase of a local oscillator field with respect to the signal field. Full field-resolved signal detection by spectral interferometry is also described with the formalism of this paper. Terms in the response function involving electronic ground and excited state populations are shown to be 180 degrees out-of-phase. We present a model for which signal generation in the presence of these interfering terms results from two mechanisms: structural relaxation induced resonance and dephasing-induced resonance. Our model shows that interference between ground and excited state terms in the PORS response function may be interpreted as arising from the overlap of hole and particle nuclear wavepackets in coordinate space. These results are discussed in the context of experimental measurements for Coumarin 153-Solvent systems. Comparison of theoretical and experimental signals suggests that structural relaxation-induced resonance of the solvent is the primary origin of the measured signals in both solvation and charge transfer processes. [Copyright &y& Elsevier]

Electron transfer rate were measured for methyl viologen-hexacyanoferrate and methyl viologen-hydroquinone complexes in solution and in trehalose–water glass matrices. Rates measured in the glass were a factor of 2.5–3 smaller than those observed in aqueous solution, whereas linear absorption spectra were shifted by less than 300 cm-1 with only minor differences in line shape. Our data in the trehalose–water glass are consistent with a predominantly aqueous local environment. Electron transfer rate data are modeled by allowing 10% of the solvent reorganization in the glass to proceed diffusively. [Copyright &y& Elsevier]

Julolidinemalononitrile, p-nitroaniline, and julolidinyl-n-N,N′-diethylthiobarbituric acid are studied with ground and excited state molecular dynamics simulations in conjunction with the collective electronic oscillator formalism and Onsager’s cavity model. Ground and excited state geometries are calculated in the gas phase and four solvents. The results are interpreted in the context of a two-state valence bond model for charge-transfer transitions of conjugated organic molecules, and are compared to recent resonant Raman experimental results. The calculated geometries are qualitatively consistent with both the two-state model and experiment. In addition, calculated transition density matrices are presented to visualize the changes in charge distribution accompanying photoexcitation. [Copyright &y& Elsevier]

The linear absorption spectra and absolute resonance Raman excitation profiles of [(2E,4E,6E,8E)-9-[4-(dibutylamino)phenyl]-2,4,6,8-nonatetraenylidene]propanedinitrile, an electron donor–acceptor push–pull chromophore with a polyenic inker group, have been measured in cyclohexane and acetonitrile solution. Numerical simulation of the spectra yields solvent reorganization energies and the excited-state geometry changes along the strongly Raman-active vibrations. The ground-state vibrational frequencies and excited-state reorganization energies are considered within the context of two-state valence bond models. Comparison of the results for this chromophore with p-nitroaniline, which has a purely aromatic linker group, suggests that the two-state model is more applicable to molecules with polyenic rather than aromatic linkers. [Copyright &y& Elsevier]

A kinetic framework for the ultrafast photophysics of tris(2,2-bipyridine)ruthenium(II) phosphonated and methyl-phosphonated derivatives is used as a basis for modeling charge injection by ruthenium dyes into a semiconductor substrate. By including the effects of light scattering, dye diffusion, and adsorption kinetics during sample preparation and the optical response of oxidized dyes, quantitative agreement with multiple transient absorption datasets is achieved on timescales spanning femtoseconds to nanoseconds. In particular, quantitative agreement with important spectroscopic handles—the decay of an excited state absorption signal component associated with charge injection in the UV region of the spectrum and the dynamical redshift of a ∼500 nm isosbestic point—validates our kinetic model. Pseudo-first-order rate coefficients for charge injection are estimated in this work, with an order of magnitude ranging from 1011 to 1012 s−1. The model makes the minimalist assumption that all excited states of a particular dye have the same charge injection coefficient, an assumption that would benefit from additional theoretical and experimental exploration. We have adapted this kinetic model to predict charge injection under continuous solar irradiation and find that as many as 68 electron transfer events per dye per second take place, significantly more than prior estimates in the literature. [ABSTRACT FROM AUTHOR]

Conventional time-of-flight (TOF) measurements yield charge carrier mobilities in photovoltaic cells with time resolution limited by the RC time constant of the device, which is on the order of 0.1–1 µs for the systems targeted in the present work. We have recently developed an alternate TOF method, termed nonlinear photocurrent spectroscopy (NLPC), in which carrier drift velocities are determined with picosecond time resolution by applying a pair of laser pulses to a device with an experimentally controlled delay time. In this technique, carriers photoexcited by the first laser pulse are "probed" by way of recombination processes involving carriers associated with the second laser pulse. Here, we report NLPC measurements conducted with a simplified experimental apparatus in which synchronized 40 ps diode lasers enable delay times up to 100 µs at 5 kHz repetition rates. Carrier mobilities of ∼0.025 cm2/V/s are determined for MAPbI3 photovoltaic cells with active layer thicknesses of 240 and 460 nm using this instrument. Our experiments and model calculations suggest that the nonlinear response of the photocurrent weakens as the carrier densities photoexcited by the first laser pulse trap and broaden while traversing the active layer of a device. Based on this aspect of the signal generation mechanism, experiments conducted with co-propagating and counter-propagating laser beam geometries are leveraged to determine a 60 nm length scale of drift velocity dispersion in MAPbI3 films. Contributions from localized states induced by thermal fluctuations are consistent with drift velocity dispersion on this length scale. [ABSTRACT FROM AUTHOR]

Resonance Raman intensities of p-nitroaniline, a prototypical “push–pull” chromophore with a large first hyperpolarizability (β), have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong near-ultraviolet charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the five or six principal Raman-active vibrations as well as estimates of the solvent reorganization energies. The total vibrational reorganization energy decreases and the solvent reorganization energy increases with increasing solvent polarity in all solvents except methanol, where specific hydrogen-bonding interactions may be important. The dimensionless normal coordinate geometry changes obtained from the resonance Raman analysis are converted to actual bond length and bond angle changes with the aid of normal mode coefficients from a ground-state density functional theory calculation. The geometry changes upon electronic excitation involve predominantly the C[sub phenyl]–N[sub nitro], N–O, and phenyl C[sub 2]–C[sub 3] bond lengths, with little involvement of the amino group. Nonresonant Raman spectra in 1,4-dioxane, dichloromethane, ethyl acetate, acetone, acetonitrile, and methanol show only a very small solvent dependence of the vibrational frequencies. This suggests that changing the solvent affects the excited state more than the ground state, calling into question two-state models that treat the ground and charge-transfer excited states as linear combinations of neutral and zwitterionic basis states with solvent dependent coefficients. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mixtures of layered perovskite quantum wells with different sizes form prototypical light-harvesting antenna structures in solution-processed films. Gradients in the bandgaps and energy levels are established by concentrating the smallest and largest quantum wells near opposing electrodes in photovoltaic devices. Whereas short-range energy and charge carrier funneling behaviors have been observed in layered perovskites, our recent work suggests that such light-harvesting processes do not assist long-range charge transport due to carrier trapping at interfaces between quantum wells and interstitial organic spacer molecules. Here, we apply a two-pulse time-of-flight technique to a family of layered perovskite systems to explore the effects that interstitial organic molecules have on charge carrier dynamics. In these experiments, the first laser pulse initiates carrier drift within the active layer of a photovoltaic device, whereas the second pulse probes the transient concentrations of photoexcited carriers as they approach the electrodes. The instantaneous drift velocities determined with this method suggest that the rates of trap-induced carrier deceleration increase with the concentrations of organic spacer cations. Overall, our experimental results and model calculations suggest that the layered perovskite device efficiencies primarily reflect the dynamics of carrier trapping at interfaces between quantum wells and interstitial organic phases. [ABSTRACT FROM AUTHOR]

Femtosecond laser spectroscopies are used to examine the electronic structures of two proteins found in the phycobilisome antenna of cyanobacteria, allophycocyanin (APC) and C-phycocyanin (CPC). The wave function composition involving the pairs of phycocyanobilin pigments (i.e., dimers) found in both proteins is the primary focus of this investigation. Despite their similar geometries, earlier experimental studies conducted in our laboratory and elsewhere observe clear signatures of exciton electronic structure in APC but not CPC. This issue is further investigated here using new experiments. Transient grating (TG) experiments employing broadband quasicontinuum probe pulses find a redshift in the signal spectrum of APC, which is almost twice that of CPC. Dynamics in the TG signal spectra suggest that the sub-100 fs dynamics in APC and CPC are respectively dominated by internal conversion and nuclear relaxation. A specialized technique, intraband electronic coherence spectroscopy (IECS), photoexcites electronic and nuclear coherences with nearly full suppression of signals corresponding to electronic populations. The main conclusion drawn by IECS is that dephasing of intraband electronic coherences in APC occurs in less than 25 fs. This result rules out correlated pigment fluctuations as the mechanism enabling exciton formation in APC and leads us to propose that the large Franck–Condon factors of APC promote wave function delocalization in the vibronic basis. For illustration, we compute the Hamiltonian matrix elements involving the electronic origin of the α84 pigment and the first excited vibronic level of the β84 pigment associated with a hydrogen out-of-plane wagging mode at 800 cm-1. For this pair of vibronic states, the -51 cm-1 coupling is larger than the 40 cm-1 energy gap, thereby making wave function delocalization a feasible prospect. By contrast, CPC possesses no pair of vibronic levels for which the intermolecular coupling is larger than the energy gap between vibronic states. This study of APC and CPC may be important for understanding the photophysics of other phycobiliproteins, which generally possess large vibronic couplings. [ABSTRACT FROM AUTHOR]

We measure and simulate electric field distortions resulting from propagation of mid-infrared pulses that are resonant with the OH stretch vibration through optically dense HDO:D2O. These distortions are characterized experimentally by full-field-resolved time- and frequency-domain measurements, specifically cross-correlation frequency-resolved optical gating and spectral interferometry, establishing amplitude and phase of the signal fields. Correlation-function finite-difference time-domain (CF-FDTD) simulations using response functions for the OH-stretching vibration, obtained from nonlinear spectroscopic studies reported by others, show that details of the line shape functions are manifested in the measured (linear-response) spectrograms. The degree of homogeneous or inhomogeneous broadening present in the various model correlation functions is readily apparent in the measured and simulated signals. Surprisingly, the published correlation functions are shown to range from modest inhomogeneous to homogeneous line broadening. The present experimental and simulation approach is very useful for establishing the correct form of energy gap correlation functions and dephasing dynamics of IR and optical transitions. In the case of HDO:D2O, correlation functions with modest inhomogeneous broadening better reflect our measured responses. [ABSTRACT FROM AUTHOR]

Experimental methods based on a wide range of physical principles are used to determine carrier mobilities for light-harvesting materials in photovoltaic cells. For example, in a time-of-flight experiment, a single laser pulse photoexcites the active layer of a device, and the transit time is determined by the arrival of carriers at an acceptor electrode. With inspiration from this conventional approach, we present a multidimensional time-of-flight technique in which carrier transport is tracked with a second intervening laser pulse. Transient populations of separate material components of an active layer may then be established by tuning the wavelengths of the laser pulses into their respective electronic resonances. This experimental technique is demonstrated using photovoltaic cells based on mixtures of organohalide perovskite quantum wells. In these "layered perovskite" systems, charge carriers are funneled between quantum wells with different thicknesses because of staggered band alignments. Multidimensional time-of-flight measurements show that these funneling processes do not support long-range transport because of carrier trapping. Rather, our data suggest that the photocurrent is dominated by processes in which the phases of the thickest quantum wells absorb light and transport carriers without transitions into domains occupied by quantum wells with smaller sizes. These same conclusions cannot be drawn using conventional one-dimensional techniques for measuring carrier mobilities. Advantages and disadvantages of multidimensional time-of-flight experiments are discussed in the context of a model for the signal generation mechanism. [ABSTRACT FROM AUTHOR]

Interest in layered organohalide perovskites is motivated by their potential for use in optoelectronic devices. In these systems, the smallest and largest quantum wells are primarily concentrated near the glass and air interfaces of a film, thereby establishing a gradient in the average values of the bandgaps. It has been suggested that this layered architecture promotes the funneling of electronic excitations through space in a manner similar to light-harvesting processes in photosynthetic antennae. Whereas energy and charge transfer are difficult to distinguish by conventional transient absorption techniques, it has recently been shown that these competing relaxation mechanisms may be separately targeted with nonlinear fluorescence (NLFL) and photocurrent "action spectroscopies." Here, we present perturbative rate functions to describe NLFL experiments conducted on layered perovskite systems. The formulas reproduce the patterns of resonances observed in experimental measurements and show how signatures of energy transfer manifest in two-dimensional spectra. Overall, this work suggests that NLFL spectroscopy may be used to fully reveal the trajectories of electronic excitations by correlating ultrafast energy transfer pathways to fluorescence emission from the thickest quantum wells. [ABSTRACT FROM AUTHOR]

Femtosecond laser spectroscopies are used to examinea thyminefamily of systems chosen to expose the interplay between excited statedeactivation and two distinct vibrational energy transfer (VET) pathways:(i) VET from the base to the deoxyribose ring; (ii) VET between neighboringunits in a dinucleotide. We find that relaxation in the ground electronicstate accelerates markedly as the molecular sizes increase from thenucleobase to the dinucleotide. This behavior directly reflects growthin the density of vibrational quantum states on the substituent ofthe base. Excited state lifetimes are studied at temperatures rangingfrom 100 to 300 K to characterize the thermal fluctuations that connectthe Franck–Condon geometries and the conical intersectionsleading back to the ground state. An Arrhenius analysis yields anapproximate excited state energy barrier of 13 meV in the thyminedinucleotide. In addition, we find that the transfer of vibrationalenergy from the base to the substituent suppresses thermal fluctuationsacross this energy barrier. The possibility that the solvent viscosityimposes friction on the reaction coordinate is examined by comparingthymine and adenine systems. Experiments suggest that the solventviscosity has little effect on barrier crossing dynamics in thyminebecause the conical intersection is accessed through relatively smallout-of-plane atomic displacements. Overall, we conclude that the transferof vibrational quanta from thymine to the deoxyribose ring couplessignificantly to the internal conversion rate, whereas the neighboringunit in the dinucleotide serves as a secondary heat bath. In naturalDNA, it follows that (local) thermal fluctuations in the geometriesof subunits involving the base and deoxyribose ring are most importantto this subpicosecond relaxation process. [ABSTRACT FROM AUTHOR]

Two-dimensional resonance Raman (2DRR) spectroscopy has been developed for studies of photochemical reaction mechanisms and structural heterogeneity in complex systems. The 2DRR method can leverage electronic resonance enhancement to selectively probe chromophores embedded in complex environments (e.g., a cofactor in a protein). In addition, correlations between the two dimensions of the 2DRR spectrum reveal information that is not available in traditional Raman techniques. For example, distributions of reactant and product geometries can be correlated in systems that undergo chemical reactions on the femtosecond time scale. Structural heterogeneity in an ensemble may also be reflected in the 2D spectroscopic line shapes of both reactive and non-reactive systems. In this perspective article, these capabilities of 2DRR spectroscopy are discussed in the context of recent applications to the photodissociation reactions of triiodide and myoglobin. We also address key differences between the signal generation mechanisms for 2DRR and off-resonant 2D Raman spectroscopies. Most notably, it has been shown that these two techniques are subject to a tradeoff between sensitivity to anharmonicity and susceptibility to artifacts. Overall, recent experimental developments and applications of the 2DRR method suggest great potential for the future of the technique. [ABSTRACT FROM AUTHOR]

Analogues of 2D photon echo methods in which two population times are sampled have recently been used to expose heterogeneity in chemical kinetics. In this work, the two population times sampled for a transition metal complex are transformed into a 2D rate spectrum using the maximum entropy method. The 2D rate spectrum suggests heterogeneity in the vibrational cooling (VC) rate within the ensemble. In addition, a cross peak associated with VC and back electron transfer (BET) dynamics reveals correlation between the two processes. We hypothesize that an increase in the strength of solute-solvent interactions, which accelerates VC, drives the system toward the activationless regime of BET. [ABSTRACT FROM AUTHOR]

Two-dimensional resonance Raman (2DRR) spectroscopy has recently been developed as a tool for studies of structural heterogeneity and photochemical dynamics in condensed phases. In this paper, 2DRR spectroscopy is used to investigate line broadening mechanisms of both oxygen- and water-ligated myoglobins. General signatures of anharmonicity and inhomogeneous line broadening are first established with model calculations to facilitate signal interpretation. It is shown that the present quasi-degenerate version of 2DRR spectroscopy is insensitive to anharmonicity, because signal generation is allowed for harmonic modes. Rather, the key information to be gained from 2DRR spectroscopy pertains to the line broadening mechanisms, which are fairly obvious by inspection of the data. 2DRR signals acquired for both heme protein systems reveal significant heterogeneity in the vibrational modes local to the heme's propionic acid side chains. These side chains are known to interact with solvent, because they protrude from the hydrophobic pocket that encloses the heme. Molecular dynamics simulations suggest that the heterogeneity detected in our 2DRR experiments reflects fluctuations in the geometries of the side chains. Knowledge of such thermal motions will be useful for understanding protein function (e.g., ligand binding) because the side chains are an effective "gateway" for the exchange of thermal energy between the heme and solvent. [ABSTRACT FROM AUTHOR]

Traditional second-order kinetic theories fail to describe sub-picosecond photochemical reactions when solvation and vibrational dephasing undermine the assumption of equilibrium initial conditions. Four-wave mixing spectroscopies may reveal insights into such non-equilibrium processes but are limited by the single "population time" available in these types of experiments. Here, we use two-dimensional resonance Raman (2DRR) spectroscopy to expose correlations between coherent nuclear motions of the reactant and product in the photodissociation reaction of triiodide. It is shown that the transition of a nuclear wavepacket from the reactant (triiodide) to product (diiodide) states gives rise to a unique pattern of 2DRR resonances. Peaks associated with this coherent reaction mechanism are readily assigned, because they are isolated in particular quadrants of the 2DRR spectrum. A theoretical model in which the chemical reaction is treated as a vibronic coherence transfer transition from triiodide to diiodide reproduces the patterns of 2DRR resonances detected in experiments. These signal components reveal correlation between the nonequilibrium geometry of triiodide and the vibrational coherence frequency of diiodide. The 2DRR signatures of coherent reaction mechanisms established in this work may generalize to studies of ultrafast energy and charge transfer processes. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) resonance Raman spectroscopies hold great potential for uncovering photoinduced relaxation processes in molecules but are not yet widely applied because of technical challenges. Here, we describe a newly developed 2D resonance Raman experiment operational at the third-harmonic of a Titanium-Sapphire laser. High-sensitivity and rapid data acquisition are achieved by combining spectral interferometry with a background-free (six-pulse) laser beam geometry. The third-harmonic laser pulses are generated in a filament produced by the fundamental and second-harmonic pulses in neon gas at pressures up to 35 atm. The capabilities of the setup are demonstrated by probing ground-state wavepacket motions in triiodide. The information provided by the experiment is explored with two different representations of the signal. In one representation, Fourier transforms are carried out with respect to the two experimentally controlled delay times to obtain a 2D Raman spectrum. Further insights are derived in a second representation by dispersing the signal pulse in a spectrometer. It is shown that, as in traditional pump-probe experiments, the six-wave mixing signal spectrum encodes the wavepacket's position by way of the (time-evolving) emission frequency. Anharmonicity additionally induces dynamics in the vibrational resonance frequency. In all cases, the experimental signals are compared to model calculations based on a cumulant expansion approach. This study suggests that multi-dimensional resonance Raman spectroscopies conducted on systems with Franck-Condon active modes are fairly immune to many of the technical issues that challenge off-resonant 2D Raman spectroscopies (e.g., third-order cascades) and photon-echo experiments in the deep UV (e.g., coherence spikes). The development of higher-order nonlinear spectroscopies operational in the deep UV is motivated by studies of biological systems and elementary organic photochemistries. [ABSTRACT FROM AUTHOR]

Kinetic models based on Fermi's Golden Rule are commonly employed to understand photoinduced electron transfer dynamics at molecule-semiconductor interfaces. Implicit in such second-order perturbative descriptions is the assumption that nuclear relaxation of the photoexcited electron donor is fast compared to electron injection into the semiconductor. This approximation breaks down in systems where electron transfer transitions occur on 100-fs time scale. Here, we present a fourth-order perturbative model that captures the interplay between time-coincident electron transfer and nuclear relaxation processes initiated by light absorption. The model consists of a fairly small number of parameters, which can be derived from standard spectroscopic measurements (e.g., linear absorbance, fluorescence) and/or first-principles electronic structure calculations. Insights provided by the model are illustrated for a two-level donor molecule coupled to both (i) a single acceptor level and (ii) a density of states (DOS) calculated for TiO2 using a first-principles electronic structure theory. These numerical calculations show that second-order kinetic theories fail to capture basic physical effects when the DOS exhibits narrow maxima near the energy of the molecular excited state. Overall, we conclude that the present fourth-order rate formula constitutes a rigorous and intuitive framework for understanding photoinduced electron transfer dynamics that occur on the 100-fs time scale. [ABSTRACT FROM AUTHOR]

At the instant following the non-radiative deactivation of its ππ* electronic state, the vibrational modes of thymine possess a highly non-equilibrium distribution of excitation quanta (i.e., >4 eV in excess energy). Equilibrium is re-established through rapid (5 ps) vibrational energy transfer to the surrounding solvent. The mechanisms behind such vibrational cooling (VC) processes are examined here using femtosecond transient grating and two-dimensional photon echo spectroscopies conducted at 100 K and 300 K in a mixture of methanol and water. Remarkably, we find that this variation in temperature has essentially no impact on the VC kinetics. Together the experiments and a theoretical model suggest three possible mechanisms consistent with this behavior: (i) vibrational energy transfer from the solute to solvent initiates (directly) in intramolecular modes of the solute with frequencies >300 cm-1; (ii) the relaxation induced increase in the temperature of the environment reduces the sensitivity of VC to the temperature of the equilibrium system; (iii) the time scale of solvent motion approaches 0.1 ps even at 100 K. Mechanism (i) deserves strong consideration because it is consistent with the conclusions drawn in earlier studies of isotope effects on VC in hydrogen bonding solvents. Our model calculations suggest that mechanism (ii) also plays a significant role under the present experimental conditions. Mechanism (iii) is ruled out on the basis of long-lived correlations evident in the photon echo line shapes at 100 K. These insights into photoinduced relaxation processes in thymine are made possible by our recent extension of interferometric transient grating and photon echo spectroscopies to the mid UV spectral region. [ABSTRACT FROM AUTHOR]

The direction of electron flow in molecular optoelectronic devices is dictated by charge transfer between a molecular excited state and an underlying conductor or semiconductor. For those devices, controlling the direction and reversibility of electron flow is a major challenge.We describe here a single-molecule photodiode. It is based on an internally conjugated, bichromophoric dyad with chemically linked (porphyrinato)zinc(II) and bis(terpyridyl)ruthenium(II) groups. On nanocrystalline, degenerately doped indium tin oxide electrodes, the dyad exhibits distinct frequency-dependent, charge-transfer characters. Variations in the light source between red-light (-1.9 eV) and blue-light (-2.7 eV) excitation for the integrated photodiode result in switching of photocurrents between cathodic and anodic. The origin of the excitation frequency-dependent photocurrents lies in the electronic structure of the chromophore excited states, as shown by the results of theoretical calculations, laser flash photolysis, and steady-state spectrophotometric measurements. [ABSTRACT FROM AUTHOR]

Femtosecond transient grating experiments are used to investigate electronic structures and transport mechanisms in dye-sensitized nanocrystalline TiO2 films. This study examines two molecular sensitizers spanning the weak (a phosphonated Ruthenium complex) and strong (catechol) molecule-TiO2 coupling regimes. It is shown that strong molecule-TiO2 interactions give rise to photoinduced vibrational coherences at the interface between species. We suggest that the amplitudes of these coherences reflect the molecule-TiO2 coupling strength and signify the delocalization of excited state wavefunctions. [ABSTRACT FROM AUTHOR]

The prevalence of ultrafast electron-transfer processes in light-harvesting materials has motivated a deeper understanding of coherent reaction mechanisms. Kinetic models based on the traditional (equilibrium) form of Fermi's Golden Rule are commonly employed to understand photoinduced electron-transfer dynamics. These models fail in two ways when the electron-transfer process is fast compared to solvation dynamics and vibrational dephasing. First, electron-transfer dynamics may be accelerated if the photoexcited wavepacket traverses the point of degeneracy between donor and acceptor states in the solvent coordinate. Second, traditional kinetic models fail to describe electron-transfer transitions that yield products which undergo coherent nuclear motions. We address the second point in this work. Transient absorption spectroscopy and a numerical model are used to investigate coherent back-electron-transfer mechanisms in a transition metal complex composed of titanium and catechol, [Ti(cat)3]2-. The transient absorption experiments reveal coherent wavepacket motions initiated by the back-electron-transfer process. Model calculations suggest that the vibrationally coherent product states may originate in either vibrational populations or coherences of the reactant. That is, vibrational coherence may be produced even if the reactant does not undergo coherent nuclear motions. The analysis raises a question of broader significance: can a vibrational population-to-coherence transition (i.e., a nonsecular transition) accelerate electron-transfer reactions even when the rate is slower than vibrational dephasing? [ABSTRACT FROM AUTHOR]

The electronic structure of 2D semiconductors depends on their thickness, providing new opportunities to engineer semiconductors for energy conversion, electronics, and catalysis. Here we show how a 3D semiconductor, black phosphorus, becomes active for solar-to-chemical energy conversion when it is thinned to a 2D material. The increase in its band gap, from 0.3 eV (3D) to 2.1 eV (2D monolayer), is accompanied by a 40-fold enhancement in the formation of chemical products. Despite this enhancement, smaller flakes also have shorter excited state lifetimes. We deduce a mechanism in which recombination occurs at flake edges, while the "van der Waals" surface of black phosphorus bonds to chemical intermediates and facilitates electron transfer. The unique properties of black phosphorus highlight its potential as a customizable material for solar energy conversion and catalysis, while also allowing us to identify design rules for 2D photocatalysts that will enable further improvements in these materials. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) perovskite quantum wells are generating broad scientific interest because of their potential for use in optoelectronic devices. Recently, it has been shown that layers of 2D perovskites can be grown in which the average thicknesses of the quantum wells increase from the back to the front of the film. This geometry carries implications for light harvesting applications because the bandgap of a quantum well decreases as its thickness increases. The general structural formula for the 2D perovskite systems under investigation in this work is (PEA)2(MA)n−1[PbnI3n+1] (PEA = phenethyl ammonium, MA = methyl ammonium). Here, we examine two layered 2D perovskites with different distributions of quantum well thicknesses. Spectroscopic measurements and model calculations suggest that both systems funnel electronic excitations from the back to the front of the film through energy transfer mechanisms on the time scales of 100’s of ps (i.e., energy transfer from thinner to thicker quantum wells). In addition, the model calculations demonstrate that the transient absorption spectra are composed of a progression of single exciton and biexciton resonances associated with the individual quantum wells. We find that exciton dissociation and/or charge transport dynamics make only minor contributions to the transient absorption spectra within the first 1 ns after photo-excitation. An analysis of the energy transfer kinetics indicates that the transitions occur primarily between quantum wells with values of n that differ by 1 because of the spectral overlap factor that governs the energy transfer rate. Two-dimensional transient absorption spectra reveal a pattern of resonances consistent with the dominance of sequential energy transfer dynamics. [ABSTRACT FROM AUTHOR]

Medical hydrogel applications have expanded rapidly over the past decade. Implantation in patients by noninvasive injection is preferred, but this requires hydrogel solidification from a low viscosity solution to occur in vivo via an applied stimuli. Transdermal photo-cross-linking of acrylated biopolymers with photoinitiators and lights offers a mild, spatiotemporally controlled solidification trigger. However, the current short wavelength initiators limit curing depth and efficacy because they do not absorb within the optical window of tissue (600-900 nm). As a solution to the current wavelength limitations, we report the development of a red light responsive initiator capable of polymerizing a range of acrylated monomers. Photoactivation occurs within a range of skin type models containing high biochromophore concentrations. [ABSTRACT FROM AUTHOR]

Highlights: [•] We discuss the outlook for multidimensional spectroscopies in the deep UV. [•] Photophysics are examined in small DNA components at cryogenic temperatures. [•] Wavepacket motions are detected in ring-opening systems with 2DUV spectroscopy. [•] Measurements of electronic wavepacket motions in molecules are proposed. [ABSTRACT FROM AUTHOR]

A Raman-pumpfrequency modulation scheme and an automated signal-processingalgorithm are developed for improved collection of time-resolved femtosecondstimulated Raman spectra. Together, these two advancements removethe broad background signals endemic to FSRS measurements and retrievesignals with high sensitivity. We apply this frequency-modulated femtosecondstimulated Raman spectroscopy (FM-FSRS) to the characterization ofultrafast energy transport in a copolymer comprised of polystyrenelinked oligo(phenylene–ethynylene) donor and thiophene–benzothiadiazoleacceptor chromophores. After photoexcitation of the donor, ultrafastenergy transfer is monitored by the decay of donor vibrational modesand simultaneous growth of acceptor modes. The FM-FSRS method showsclear advantages in signal-to-noise levels, mitigation of artifactfeatures, and ease of data processing over the conventional FSRS technique. [ABSTRACT FROM AUTHOR]

Photoinduced electrocyclic ring opening reactions in conjugated cylcoalkenes are among themost elementary processes in organic chemistry. One prototypical ring opening reaction transforms cyclohexadiene into hexatriene. It is known that a sequence of sub-100 fs internal conversion transitions precedes bond breaking in cyclohexadiene and some of its derivatives. However, these excited state dynamics have never been directly monitored in solution because of insufficient time resolution. Here we aim to uncover the extraordinary photophysics behind related ultrafast internal conversion processes in a derivative of cyclohexadiene, α-terpinene (α-TP), solvated in cyclohexane. Transient absorption anisotropy experiments conducted with 20 fs laser pulses at 267 nm expose non-exponential depopulation kinetics for the ππ* electronic state of α-TP. Our data show that population transfer rapidly accelerates within the first 100 fs after photoexcitation. In addition, recurrences in two-dimensional photon echo (2DPE) line shapes reveal strong vibronic coupling in a normal mode near 523 cm-1, which involves torsions of the C=C bonds and hydrogen out-of-plane (HOOP) wagging on a vinyl group. With the support of several experiments, we hypothesize that the excited state wavepacket in α-TP undergoes several recurrences in the C=C stretching coordinate before displacement along the C=C torsion/vinyl HOOP coordinate finally sets it free from the Franck-Condon region of the potential energy surface. The unconfined wavepacket departs the ππ* electronic state by way of a conical intersection with a lower energy excited state. The present observations are made possible by recent improvements to both the time resolution and detection sensitivity of our experimental setup. This work demonstrates that it is now possible to acquire 2DPE signals in the deep ultraviolet, which are comparable with high-quality measurements in the visible spectral region. These technical developments open the door to studies of many beautiful models for elementary chemical dynamics. [ABSTRACT FROM AUTHOR]

Femtosecond laser spectroscopies are used to investigate the influence of intramolecular nuclear modes on electronic relaxation in the cyanobacterial light harvesting protein, C-phycocyanin (CPC). Of particular interest are sub-ps dynamics localized on pairs of closely spaced phycocyanobilin pigments (i.e. dimers). Experiments conducted under different polarization conditions are used to distinguish isotropic and anisotropic vibrational modes within the dimers. Two isotropic nuclear modes are detected near 185 and 260 cm-1 using two-dimensional photon echo spectroscopy. In addition, a transient absorption anisotropy measurement reveals vibrational resonances associated with (out-of-plane) anisotropic nuclear modes near 640 and 815 cm-1. We investigate two possible origins for the recurrences in the anisotropy. A mechanism involving ground state nuclear coherences in the Condon approximation is ruled out by comparing the potential energy surfaces of the excitons to the direction of wavepacket motion. Electronic structure calculations suggest that non-Condon effects are the most likely explanation for the beats observed in the anisotropy. Such non-Condon effects also hold interesting implications for the vibronic exciton electronic structure of CPC. We calculate non-Condon intermolecular couplings in the dimer as large as 10 cm-1, which suggests that these effects are not negligible and deserve further consideration. Our findings provide additional insights into the sub-100 fs vibronic relaxation channel found in the closely related protein, allophycocyanin, whose pigment dimers possess nearly the same geometry and intermolecular Coulombic interactions as CPC. This study underscores the complex interplay of intramolecular vibronic coupling and site energy tuning in photosynthetic light harvesting. [ABSTRACT FROM AUTHOR]

The simultaneous, concerted transfer of electrons and protons—electron-proton transfer (EPT)—is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4'-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H+ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck-Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated +H-B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state. [ABSTRACT FROM AUTHOR]

Exciton sizes influence transport processes and spectroscopic phenomena in molecular aggregates and crystals. Thermally driven nuclear motion generally localizes electronic states in equilibrium systems. Exciton sizes also undergo dynamic changes caused by nonequilibrium relaxation in the lattice structure local to the photoexcitations (i.e., self-trapping). The α-phase of crystalline perylene is particularly well-suited for fundamental studies of exciton self-trapping mechanisms. It is generally agreed that a subpicosecond self-trapping process in α-perylene localizes photoexcited excitons onto pairs of closely spaced molecules (i.e., dimers), which then relax through excimer emission. Here, electronic relaxation dynamics in α-perylene single crystals are investigated using a variety of nonlinear optical spectroscopies in conjunction with a Frenkel exciton model. Linear absorption and photon echo spectroscopies suggest that excitons are delocalized over less than four unit cells (16 molecules) at 78 K prior to self-trapping. Stimulated Raman spectroscopies conducted on and off electronic resonance reveal significant vibronic coupling in a mode at 104 cm−1, which corresponds to the displacement between perylene molecules comprising a dimer. Strong vibronic coupling in this mode suggests that motion along the interdimer axis is instrumental in driving the self-trapping process. The results are discussed in the context of our recent study of tetracene and rubrene single crystals in which similar experiments and models were employed. [ABSTRACT FROM AUTHOR]