Probing structural evolution along multidimensional reaction coordinates with femtosecond stimulated Raman spectroscopy

Mapping out multidimensional potential energy surfaces has been a goal of physical chemistry for decades in the quest to both predict and control chemical reactivity. Recently a new spectroscopic approach called Femtosecond Stimulated Raman Spectroscopy or FSRS was introduced that can structurally interrogate multiple dimensions of a reactive potential energy surface. FSRS is an ultrafast laser technique which provides complete time-resolved, background-free Raman spectra in a few laser shots. The FSRS technique provides simultaneous ultrafast time (~50 fs) and spectral (~8 cm⁻¹) resolution, thus enabling one to follow reactive structural evolutions as they occur. In this perspective we summarize how FSRS has been used to follow structural dynamics and provide mechanistic detail on three classical chemical reactions: a structural isomerization, an electron transfer reaction, and a proton transfer reaction.This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the Royal Society of Chemistry and can be found at: http://pubs.rsc.org/en/journals/journalissues/cp

Quantifying the Ultrafast and Steady-State Molecular Reduction Potential of a Plasmonic Photocatalyst

Plasmonic materials are promising photocatalysts as they are well-suited to convert light into hot carriers and heat. Hot electron transfer is suggested as the driving force in many plasmon-driven reactions. However, to date there are no direct molecular measures of the rate and yield of plasmon-to-molecule electron transfer, or energy of these electrons on the timescale of plasmon decay. Here, we use ultrafast and spectroelectrochemical surface-enhanced Raman spectroscopy to quantify electron transfer from a plasmonic substrate to adsorbed methyl viologen molecules. We observe a reduction yield of 2.4 - 3.5 % on the picosecond timescale, with plasmon-induced potentials ranging from -3.1 to -4.5 mV. Excitingly, some of these reduced species are stabilized and persist for tens of minutes. This work provides concrete metrics toward optimizing material-molecule interactions for efficient plasmon-driven photocatalysis

Coherent Phonon Catalysts: Lattice Vibrations Drive a Photoinduced Phase Transition in a Molecular Crystal

The atomic motions that make up phonons and molecular vibrations in molecular crystals influence their photophysical and electronic properties including polaron formation, carrier mobility, and phase transitions. Discriminating between spectator and driving motions is a significant challenge hindering optimization. Unlocking this information and developing fine-tuned controls over actively participating phonon modes would not only lead to a stronger understanding of photochemistry but also provide a significant new tool in controlling solid-state chemistry. We present a strategy using rationally-designed double pulses to enhance the yield of a photoinduced phase transition in a molecular crystal through coherent control of individual phonons. Using ultrafast spectroscopy, we identified 50 cm-1 and 90 cm-1 phonons responsible for the photoinduced spin-Peierls melting of potassium tetracyanoquinodimethane crystals. We show that the 90 cm-1 phonon can be used to catalyze the phase transition process while the 50 cm-1 phonon enhances the yield of the initial charge transfer reaction.<br /

Continuous Wave Photon Upconversion from a Copper Selenide Nanocrystal Film

Photon upconversion is of great interest for improving the efficiency of silicon photovoltaic cells, for biological imaging, and for thermal management strategies. Currently, the vast majority of materials being developed for solar upconversion are composed of rare and expensive elemental compounds. Moving forward, the development of earth abundant, non-toxic materials that efficiently convert near infrared light into visible light would be ideal. Copper selenide-based materials meet these criteria, and are of great interest due to their unique thermoelectric and plasmonic properties. In particular, doped copper selenides (Cu2−xSe) have tunable near infrared localized surface plasmon resonances, large Seebeck coefficients, and low thermal conductivity, with a range of chemical and thermoelectric applications. Here, we observe another interesting application of this material in the upconversion of near infrared light from a silica xerogel film containing degenerately doped Cu2−xSe nanocrystals, with an onset flux of ∼ 1.96 ± 0.29 kW/cm^2 and at least 1% quantum yield. Our investigations suggest a plasmon-driven thermal mechanism likely plays a role in this upconversion process

Stimulated Raman Scattering: From Bulk to Nano

Stimulated Raman scattering (SRS) describes a family of techniques first discovered and developed in the 1960s. Whereas the nascent history of the technique is parallel to that of laser light sources, recent advances have spurred a resurgence in its use and development that has spanned across scientific fields and spatial scales. SRS is a nonlinear technique that probes the same vibrational modes of molecules that are seen in spontaneous Raman scattering. While spontaneous Raman scattering is an incoherent technique, SRS is a coherent process, and this fact provides several advantages over conventional Raman techniques, among which are much stronger signals and the ability to time-resolve the vibrational motions. Technological improvements in pulse generation and detection strategies have allowed SRS to probe increasingly smaller volumes and shorter time scales. This has enabled SRS research to move from its original domain, of probing bulk media, to imaging biological tissues and single cells at the micro scale, and, ultimately, to characterizing samples with subdiffraction resolution at the nanoscale. In this Review, we give an overview of the history of the technique, outline its basic properties, and present historical and current uses at multiple length scales to underline the utility of SRS to the molecular sciences

Competition between Reaction and Degradation Pathways in Plasmon-Driven Photochemistry

Plasmonic materials are exciting candidates for driving photochemical reactions, as they couple strongly with light across a wide range of the electromagnetic spectrum and can dramatically impact the photophysical properties of proximal molecular species. Plasmons have been shown to drive a number of photochemical reactions, but a detailed understanding of the mechanism is lacking in many cases. Here we investigate the effects of plasmonic field enhancement of the plasmon-driven conversion of 4-nitrobenzenethiol to 4,4′-dimercaptoazobenzene. By tuning the ensemble-averaged field enhancement of a plasmonic substrate, we quantify how the reaction yield and rate depend on the magnitude of the electric field. Surprisingly, we find no correlation of increased reaction rate or yield with greater field enhancement. Kinetic analysis of the reaction rate constants reveals a wide range of values, indicating that plasmonic excitation is not the rate-limiting step in this system. Additionally, we identify a competing degradation pathway that significantly contributes to the loss of reactant. This work identifies several factors that are critical in determining the overall efficiency of a plasmon-driven process and should help to lead to optimally designed plasmonic photocatalytic systems