Shayne Sorenson

Solvation Reaction Field at the Interface Measured by Vibrational Sum Frequency Generation Spectroscopy

Interfacial electric fields are important in several areas of chemistry, materials sciences, and device physics. However, they are poorly understood, partly because they are difficult to measure directly and model accurately. We present both a spectroscopic experimental investigation and a theoretical model for the interfacial field at the junction of a conductor and a dielectric. First, we present vibrational sum frequency generation (VSFG) results of the nitrile (CN) stretch of 4-mercaptobenzonitrile (4-MBN) covalently attached to a gold surface and in contact with a variety of liquid dielectrics. It is found that the CN stretch frequency red-shifts with increasing dielectric constant. Second, we build a model in direct analogy to the well-known Onsager reaction field theory, which has been successful in predicting vibrational frequency shifts in bulk dielectric media. Clearly, due to the asymmetric environment, with metal on one side and a dielectric on the other, the bulk Onsager model is not applicable at the interface. To address this, we apply the Onsager model to the interface accounting for the asymmetry. The model successfully explains the red-shift of the CN stretch as a function of the dielectric constant and is used to estimate the reaction field near the interface. We show the similarities and differences between the conventional bulk Onsager model and the interfacial reaction field model. In particular, the model emphasizes the importance of the metal as part of the solvation environment of the tethered molecules. We anticipate that our work will be of fundamental value to understand the crucial and often elusive electric fields at interfaces.

Ultrafast Intramolecular Electron and Proton Transfer in Bis(imino)isoindole Derivatives.

Concerted motion of electrons and protons in the excited state is pertinent to a wide range of chemical phenomena, including those relevant for solar-to-fuel light harvesting. The excited state dynamics of small proton-bearing molecules are expected to serve as models for better understanding such phenomena. In particular, for designing the next generation of multielectron and multiproton redox catalysts, understanding the dynamics of more than one proton in the excited state is important. Toward this goal, we have measured the ultrafast dynamics of intramolecular excited state proton transfer in a recently synthesized dye with two equivalent transferable protons. We have used a visible ultrafast pump to initiate the proton transfer in the excited state, and have probed the transient absorption of the molecule over a wide bandwidth in the visible range. The measurement shows that the signal which is characteristic of proton transfer emerges within ∼710 fs. To identify whether both protons were transferred in the excited state, we have measured the ultrafast dynamics of a related derivative, where only a single proton was available for transfer. The measured proton transfer time in that molecule was ∼427 fs. The observed dynamics in both cases were reasonably fit with single exponentials. Supported by the ultrafast observations, steady-state fluorescence, and preliminary computations of the relaxed excited states, we argue that the doubly protonated derivative most likely transfers only one of its two protons in the excited state. We have performed calculations of the frontier molecular orbitals in the Franck-Condon region. The calculations show that in both derivatives, the excitation is primarily from the HOMO to LUMO causing a large rearrangement of the electronic charge density immediately after photoexcitation. In particular, charge density is shifted away from the phenolic protons and toward the proton acceptor nitrogens. The proton transfer is hypothesized to occur both due to enhanced acidity of the phenolic proton and enhanced basicity of the nitrogen in the excited state. We hope this study can provide insight for better understanding of the general class of excited state concerted electron-proton dynamics.

Electronic State-Resolved Electron–Phonon Coupling in an Organic Charge Transfer Material from Broadband Quantum Beat Spectroscopy

The coupling of electron and lattice phonon motion plays a fundamental role in the properties of functional organic charge-transfer materials. In this Letter we extend the use of ultrafast vibrational quantum beat spectroscopy to directly elucidate electron-phonon coupling in an organic charge-transfer material. As a case study, we compare the oscillatory components of the transient reflection (TR) of a broadband probe pulse from single crystals of quinhydrone, a 1:1 cocrystal of hydroquinone and p-benzoquinone, after exciting nonresonant impulsive stimulated Raman scattering and resonant electronic transitions using ultrafast pulses. Spontaneous resonance Raman spectra confirm the assignment of these oscillations as coherent lattice phonon excitations. Fourier transforms of the vibrational quantum beats in our broadband TR measurements allow construction of spectra that we show report the ability of these phonons to directly modulate the electronic structure of quinhydrone. These results demonstrate how coherent ultrafast processes can characterize the complex interplay of charge transfer and lattice motion in materials of fundamental relevance to chemistry, materials sciences, and condensed matter physics.

Intermolecular electron transfer from intramolecular excitation and coherent acoustic phonon generation in a hydrogen-bonded charge-transfer solid

Organic materials that produce coherent lattice phonon excitations in response to external stimuli may provide next generation solutions in a wide range of applications. However, for these materials to lead to functional devices in technology, a full understanding of the possible driving forces of coherent lattice phonon generation must be attained. To facilitate the achievement of this goal, we have undertaken an optical spectroscopic study of an organic charge-transfer material formed from the ubiquitous reduction-oxidation pair hydroquinone and p-benzoquinone. Upon pumping this material, known as quinhydrone, on its intermolecular charge transfer resonance as well as an intramolecular resonance of p-benzoquinone, we find sub-cm(-1) oscillations whose dispersion with probe energy resembles that of a coherent acoustic phonon that we argue is coherently excited following changes in the electron density of quinhydrone. Using the dynamical information from these ultrafast pump-probe measurements, we find that the fastest process we can resolve does not change whether we pump quinhydrone at either energy. Electron-phonon coupling from both ultrafast coherent vibrational and steady-state resonance Raman spectroscopies allows us to determine that intramolecular electronic excitation of p-benzoquinone also drives the electron transfer process in quinhydrone. These results demonstrate the wide range of electronic excitations of the parent of molecules found in many functional organic materials that can drive coherent lattice phonon excitations useful for applications in electronics, photonics, and information technology.

Evidence of Ultrafast Charge Transfer Driven by Coherent Lattice Vibrations

We report evidence that intermolecular vibrations coherently drive charge transfer between the sites of a material on ultrafast time scales. Following a nonresonant stimulated Raman pump pulse that excites the organic material quinhydrone, we observe the initial appearance of oscillations due to intermolecular lattice vibrations and then the delayed appearance of a higher-frequency oscillation that we assign to a totally symmetric intramolecular vibration. We use the coherent dynamics of the transient reflectivity signal to propose that coherence transfer drives excitation of this intramolecular vibration. Furthermore, we conclude that the dynamical frequency shift of the intramolecular vibration reports the formation of a quasi-stable charge-separated state on ultrafast time scales. We calculate model dynamics using the extended Hubbard Hamiltonian to explain coherence transfer due to vibrationally driven charge transfer. These results demonstrate that the coherent excitation of low-frequency vibrations can drive charge transfer in the solid state and control material properties.

Coherent multimode electron-phonon coupling dynamics

Upon non-resonant ultrafast pumping of a charge-transfer molecular material, at least two phonons dynamically couple to electron transfer transitions. This coupling mechanism provides insight into multi-mode electron-phonon coupling in both organic and inorganic solids.