Dynamics of rhenium photocatalysts revealed through ultrafast multidimensional spectroscopy

NOESY-Like 2D-IR Spectroscopy Reveals Non-Gaussian Dynamics

We have identified an unexpected signature of non-Gaussian dynamics in a conventional 2D IR measurement on a system with rapid intermolecular vibrational energy transfer. In a ternary mixture of the CO₂ reduction photocatalyst, ReCl(bpy)(CO)₃, NaSCN, and THF solvent, preferential association between the metal carbonyl catalyst and the NaSCN ion pairs facilitates intermolecular energy transfer on a few picoseconds time scale. Monitoring the cross peak between the highest frequency metal carbonyl band and the CN bands of NaSCN contact ion pairs, we find a striking time evolution of the cross-peak position on the detection axis. This frequency shift, which is due to spectral diffusion following intermolecular energy transfer, occurs with a time scale that is distinct from either the donor or acceptor spectral diffusion measured simultaneously. We argue that the energy transfer, a second-order Förster process, effectively increases the dimensionality of the 2D-IR spectroscopy and thus enables sensitivity to non-Gaussian dynamics.

Femtosecond studies: observing transition states and other ultra-short-lived species

To celebrate 2015 as the 'International Year of Light', this article offers a short survey of the progress made since the award of the Nobel Prize of 1999 to Professor Ahmed Zewail for his pioneering work on taking the timescale for observation of light-induced events down to the femtosecond level. Developments have included the extension of studies (i) to larger molecules, leading up to biological systems; (ii) the increased range of detection methods of transient species from the UV-Vis to the infrared region; (iii) the introduction of Raman spectroscopy to augment IR studies; (iv) examination of combination events to supplement dissociation events; (v) the interrogation of transient structures by X-ray absorption spectroscopy; (vi) the study of reactions taking place at solid surfaces.

Photocatalytic reduction of CO2 and H2O to CO and H2 with a cobalt chlorin complex adsorbed on multi-walled carbon nanotubes

The photocatalytic reduction of CO₂ and H₂O with triethylamine occurred using a cobalt(ii) chlorin complex adsorbed on multi-walled carbon nanotubes as a catalyst to yield CO and H₂ with a high turnover number of 710.

Short-Range Catalyst-Surface Interactions Revealed by Heterodyne Two-Dimensional Sum Frequency Generation Spectroscopy

Heterodyne 2D sum frequency generation spectroscopy is used to study a model CO2 reduction catalyst, Re(diCN-bpy) (CO)3Cl, as a monolayer on a gold surface. We show that short-range interactions with the surface can cause substantial line-shape differences between vibrational bands from the same molecules. We explain this interaction as the result of couplings between CO vibrational modes of the catalyst molecules and the image dipoles on gold surface, which are sensitive to the relative distance between the molecule and the surface. Thus, by analysis of HD 2D SFG line-shape differences and polarization dependences of IR spectra, we can unambiguously determine the ensemble-averaged orientation of the molecules on the surface. The high sensitivity of HD 2D SFG spectra to short-range interactions can be applied to many other adsorbate-substrate interactions and therefore can serve as a unique tool to determine adsorbate orientations on surfaces.

Real-Space Imaging of Carrier Dynamics of Materials Surfaces by Second-Generation Four-Dimensional Scanning Ultrafast Electron Microscopy

In the fields of photocatalysis and photovoltaics, ultrafast dynamical processes, including carrier trapping and recombination on material surfaces, are among the key factors that determine the overall energy conversion efficiency. A precise knowledge of these dynamical events on the nanometer (nm) and femtosecond (fs) scales was not accessible until recently. The only way to access such fundamental processes fully is to map the surface dynamics selectively in real space and time. In this study, we establish a second generation of four-dimensional scanning ultrafast electron microscopy (4D S-UEM) and demonstrate the ability to record time-resolved images (snapshots) of material surfaces with 650 fs and ∼5 nm temporal and spatial resolutions, respectively. In this method, the surface of a specimen is excited by a clocking optical pulse and imaged using a pulsed primary electron beam as a probe pulse, generating secondary electrons (SEs), which are emitted from the surface of the specimen in a manner that is sensitive to the local electron/hole density. This method provides direct and controllable information regarding surface dynamics. We clearly demonstrate how the surface morphology, grains, defects, and nanostructured features can significantly impact the overall dynamical processes on the surface of photoactive-materials. In addition, the ability to access two regimes of dynamical probing in a single experiment and the energy loss of SEs in semiconductor-nanoscale materials will also be discussed.

Ultrafast charge transfer dynamics in supramolecular Pt(ii) donor–bridge–acceptor assemblies: the effect of vibronic coupling

Thanks to major advances in laser technologies, recent investigations of the ultrafast coupling of nuclear and electronic degrees of freedom (vibronic coupling) have revealed that such coupling plays a crucial role in a wide range of photoinduced reactions in condensed phase supramolecular systems. This paper investigates several new donor–bridge–acceptor charge-transfer molecular assemblies built on a trans-Pt(ii) acetylide core. We also investigate how targeted vibrational excitation with low-energy IR light post electronic excitation can perturb vibronic coupling and affect the efficiency of electron transfer (ET) in solution phase. We compare and contrast properties of a range of donor–bridge–acceptor Pt(ii) trans-acetylide assemblies, where IR excitation of bridge vibrations during UV-initiated charge separation in some cases alters the yields of light-induced product states. We show that branching to multiple product states from a transition state with appropriate energetics is the most rigid condition for the type of vibronic control we demonstrate in our study.