Publications

Ultrafast Relaxations in Ruthenium Polypyridyl Chromophores Determined by Stochastic Kinetics Simulations

ABSTRACT

Maximizing the efficiency of solar energy conversion using dye assemblies rests on understanding where the energy goes following absorption. Transient spectroscopies in solution are useful for this purpose, and the time-resolved data are usually analyzed with a sum of exponentials. This treatment assumes that dynamic events are well separated in time, and that the resulting exponential prefactors and phenomenological lifetimes are related directly to primary physical values. Such assumptions break down for coincident absorption, emission, and excited state relaxation that occur in transient absorption and photoluminescence of tris(2,2'-bipyridine)ruthenium(2+) derivatives, confounding the physical meaning of the reported lifetimes. In this work, we use inductive modeling and stochastic chemical kinetics to develop a detailed description of the primary ultrafast photophysics in transient spectroscopies of a series of Ru dyes, as an alternative to sums of exponentials analysis. Commonly invoked 3-level schemes involving absorption, intersystem crossing, and slow non-radiative relaxation and incoherent emission to the ground state cannot re-produce the experimentally measured spectra. The kinetics simulations reveal that ultrafast decay from the singlet excited state manifold to the ground state competes with intersystem crossing to the triplet excited state, whose efficiency as determined to be less than unity. The populations predicted by the simulations are used to estimate the magnitudes of transition dipoles for excited state excitations, and evaluate the influence of specific ligands. The mechanistic framework and methodology presented here are entirely general, applicable to other dye classes, and can be extended to include charge injection by molecules bound to semiconductor surfaces.

A Kinetic Description of Ultrafast Excitation, Relaxation, and Charge Transfer in Ru Dye-Semiconductor Systems

ABSTRACT

We describe a predictive kinetic framework for ultrafast photophysics of ruthenium polypyridyl dyes in solution and on solid surfaces to probe how kinetic processes such as absorption, relaxation, and intersystem crossing affect excited state lifetimes. Employing a form of kinetic Monte Carlo that produces an absolute time base, we compute transient absorption (TA) signals and find excellent agreement with experimental spectroscopic data. We compare dye photophysics in solution to that of sensitized metal oxide films, where charge injection from the excited states may occur. Dye molecules have similar excitation and decay kinetics in solution and on ZrO₂ films where there is no charge transfer. In contrast, charge transfer to the semiconductor competes with intramolecular transitions for dyes bound to TiO₂. Comparison of simulated TA spectra to experiment allow rate coefficients for charge injection to be estimated. The kinetic framework is readily integrated into multiscale models for dye-sensitized light harvesting systems.

*Supported by the U S Department of Energy, Office of Basic Energy Sciences, Solar Photochemistry Program (No. DE-AC02-05CH11231) and the UNC EFRC Center for Solar Fuels, an Energy Frontier Research Center by (No. DE-SC0001011).

Simulating Radiative and Non-Radiative Decay Pathways of Photoexcited Ruthenium Polypyridyl Complexes

ABSTRACT

Bottlenecks in the generation of photoinduced currents in dye-sensitized solar cells are not fundamentally understood due to the lack of detailed information on initial ultrafast processes in photoexcited dyes that compete with charge injection. The ultrafast photophysics that drive solar energy conversion are typically reported in terms of phenomenological lifetimes, yet competing transitions occurring on comparable timescales obscures the relationship between such time constants and fundamental rate coefficients. Knowledge of primary kinetics will reveal dye design strategies that may improve sensitivity and efficiency. We employ stochastic simulations, which are a form of kinetic Monte Carlo that produce an absolute time base, to model explicit photoexcitation and the subsequent relaxation pathways of a series of ruthenium polypyridyl chromophores. The initial work focuses on isolated dyes in solution, producing a scheme that can be extended to dyes adsorbed on metal oxide surfaces. We predict transient absorption signals for comparison to spectroscopic data from multiple studies, and find that common assumptions about the sub-picosecond photophysics such as singlet-triplet energy transfer efficiency do not correctly reproduce experimental observations.

Susceptibility of two-dimensional resonance Raman spectroscopies to cascades involving solute and solvent molecules

ABSTRACT

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.

Higher-Order Effects in Condensed Phase Spectroscopy and Dynamics

ABSTRACT

Higher-Order Effects in Condensed Phase Spectroscopy and Dynamics (Under the direction of Andrew M. Moran) Researchers in the 1970s wondered whether traditional Raman experiments could distinguish homogeneous and inhomogeneous line broadening mechanisms. Since then, a feedback between experiment and theory has spawned and matured the field of multidimensional Raman spectroscopy and laid the groundwork for modeling nonlinear photoinduced reaction pathways. Here two-dimensional resonance Raman (2DRR) spectroscopy is developed to investigate photochemical reaction mechanisms and structural heterogeneity in condensed phase systems. Models are developed to understand 2DRR spectra and extended to incorporate non-radiative transitions. The photodissociation reaction of triiodide serves to uncover the capabilities of 2DRR. A unique pattern of 2DRR resonances is associated with the transition of a nuclear wavepacket from reactant to product. The pattern of resonances is reproduced by modeling the photodissociation as a vibronic coherence transfer. Transient absorption experiments performed on a transition metal complex composed of titanium and catechol, [Ti(cat)3]2-, exhibit signatures of coherent wavepacket motion initiated by back-electron transfer. The model used for triiodide photodissociation applies to this system, and calculations predict that vibrational coherences in the product are independent of whether the reactant undergoes coherent nuclear motion. Vibrational population-to-coherence transitions could accelerate the electron transfer (ET) process, regardless if vibrational dephasing is faster than the reaction rate--a prediction not captured by traditional ET models. 2DRR spectroscopy is further used to investigate oxygen- and water-ligated myoglobin line broadening mechanisms. Vibrational modes proximal to propionic acid side chains of the heme exhibit significant heterogeneity in the 2DRR spectra. A hydrophobic pocket encompasses the heme, but the side chains are exposed to solvent. Molecular dynamics (MD) simulations suggest that fluctuations in the side chain geometries are correlated with the heterogeneity. 2DRR spectra and MD simulations reveal that the side chains function as effective pathways for thermal relaxation. Despite progress, a major challenge still plagues multidimensional Raman spectroscopy. Cascading signals radiated in the same direction as the desired signal can render a signal impossible to analyze. Simulations of 2DRR, femtosecond stimulated Raman spectroscopy, and the accompanying artifacts suggest solute-solute and solute-solvent interactions can significantly affect measured signals if experimental parameters are not carefully selected.

Two-Dimensional Resonance Raman Signatures of Vibronic Coherence Transfer in Chemical Reactions

ABSTRACT

Two-dimensional resonance Raman (2DRR) spectroscopy has been developed for studies of photochemical reaction mechanisms and structural heterogeneity in condensed phase systems. 2DRR spectroscopy is motivated by knowledge of non-equilibrium effects that cannot be detected with traditional resonance Raman spectroscopy. For example, 2DRR spectra may reveal correlated distributions of reactant and product geometries 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 chapter, these capabilities of 2DRR spectroscopy are discussed in the context of recent applications to the photodissociation reactions of triiodide. We show that signatures of “vibronic coherence transfer” in the photodissociation process can be targeted with particular 2DRR pulse sequences. Key differences between the signal generation mechanisms for 2DRR and off-resonant 2D Raman spectroscopy techniques are also addressed. Overall, recent experimental developments and applications of the 2DRR method suggest that it will be a valuable tool for elucidating ultrafast chemical reaction mechanisms.

Perspective: Two-dimensional resonance Raman spectroscopy

ABSTRACT

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.

Communication: Uncovering correlated vibrational cooling and electron transfer dynamics with multidimensional spectroscopy

ABSTRACT

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.

Two-dimensional resonance Raman spectroscopy of oxygen-and water-ligated myoglobins

ABSTRACT

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.

Ultrafast Spectroscopic Signatures of Coherent Electron-Transfer Mechanisms in a Transition Metal Complex

ABSTRACT

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?

Elucidation of reactive wavepackets by two-dimensional resonance Raman spectroscopy

ABSTRACT

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.