Unveiling ultrafast dynamics in bridged bimetallic complexes using optical and X-ray transient absorption spectroscopies

Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes

Utilization of renewable energies for catalytically generating value-added chemicals is highly desirable in this era of rising energy demands and climate change impacts. Artificial photosynthetic systems or photocatalysts utilize light to convert abundant CO2, H2O, and O2 to fuels, such as carbohydrates and hydrogen, thus converting light energy to storable chemical resources. The emergence of intense X-ray pulses from synchrotrons, ultrafast X-ray pulses from X-ray free electron lasers, and table-top laser-driven sources over the past decades opens new frontiers in deciphering photoinduced catalytic reaction mechanisms on the multiple temporal and spatial scales. Operando X-ray spectroscopic methods offer a new set of electronic transitions in probing the oxidation states, coordinating geometry, and spin states of the metal catalytic center and photosensitizers with unprecedented energy and time resolution. Operando X-ray scattering methods enable previously elusive reaction steps to be characterized on different length scales and time scales. The methodological progress and their application examples collected in this review will offer a glimpse into the accomplishments and current state in deciphering reaction mechanisms for both natural and synthetic systems. Looking forward, there are still many challenges and opportunities at the frontier of catalytic research that will require further advancement of the characterization techniques.

Stimulated X-ray emission spectroscopy

We describe an emerging hard X-ray spectroscopy technique, stimulated X-ray emission spectroscopy (S-XES). S-XES has the potential to characterize the electronic structure of 3d transition metal complexes with spectral information currently not reachable and might lead to the development of new ultrafast X-ray sources with properties beyond the state of the art. S-XES has become possible with the emergence of X-ray free-electron lasers (XFELs) that provide intense femtosecond X-ray pulses that can be employed to generate a population inversion of core-hole excited states resulting in stimulated X-ray emission. We describe the instrumentation, the various types of S-XES, the potential applications, the experimental challenges, and the feasibility of applying S-XES to characterize dilute systems, including the Mn4Ca cluster in the oxygen evolving complex of photosystem II.

Photodriven electron-transfer dynamics in a series of heteroleptic Cu(I)-anthraquinone dyads

Solar fuels catalysis is a promising route to efficiently harvesting, storing, and utilizing abundant solar energy. To achieve this promise, however, molecular systems must be designed with sustainable components that can balance numerous photophysical and chemical processes. To that end, we report on the structural and photophysical characterization of a series of Cu(I)-anthraquinone-based electron donor-acceptor dyads. The dyads utilized a heteroleptic Cu(I) bis-diimine architecture with a copper(I) bis-phenanthroline chromophore donor and anthraquinone electron acceptor. We characterized the structures of the complexes using x-ray crystallography and density functional theory calculations and the photophysical properties via resonance Raman and optical transient absorption spectroscopy. The calculations and resonance Raman spectroscopy revealed that excitation of the Cu(I) metal-to-ligand charge-transfer (MLCT) transition transfers the electron to a delocalized ligand orbital. The optical transient absorption spectroscopy demonstrated that each dyad formed the oxidized copper-reduced anthraquinone charge-separated state. Unlike most Cu(I) bis-phenanthroline complexes where increasingly bulky substituents on the phenanthroline ligands lead to longer MLCT excited-state lifetimes, here, we observe a decrease in the long-lived charge-separated state lifetime with increasing steric bulk. The charge-separated state lifetimes were best explained in the context of electron-transfer theory rather than with the energy gap law, which is typical for MLCT excited states, despite the complete conjugation between the phenanthroline and anthraquinone moieties.

Site-specific electronic structure of covalently linked bimetallic dyads from nitrogen K-edge x-ray absorption spectroscopy

A nitrogen K-edge x-ray absorption near-edge structure (XANES) survey is presented for tetrapyrido[3,2-a:2',3'-c:3″,2″-h:2‴,3‴-j]phenazine (tpphz)-bridged bimetallic assemblies that couple chromophore and catalyst transition metal complexes for light driven catalysis, as well as their individual molecular constituents. We demonstrate the high N site sensitivity of the N pre-edge XANES features, which are energetically well-separated for the phenazine bridge N atoms and for the individual metal-bound N atoms of the inner coordination sphere ligands. By comparison with the time-dependent density functional theory calculated spectra, we determine the origins of these distinguishable spectral features. We find that metal coordination generates large shifts toward higher energy for the metal-bound N atoms, with increasing shift for 3d < 4d < 5d metal bonding. This is attributed to increasing ligand-to-metal σ donation that increases the effective charge of the bound N atoms and stabilizes the N 1s core electrons. In contrast, the phenazine bridge N pre-edge peak is found at a lower energy due to stabilization of the low energy electron accepting orbital localized on the phenazine motif. While no sensitivity to ground state electronic coupling between the individual molecular subunits was observed, the spectra are sensitive to structural distortions of the tpphz bridge. These results demonstrate N K-edge XANES as a local probe of electronic structure in large bridging ligand motifs, able to distinctly investigate the ligand-centered orbitals involved in metal-to-ligand and ligand-to-ligand electron transfer following light absorption.

Toward ultrafast soft x-ray spectroscopy of organic photovoltaic devices

Novel ultrafast x-ray sources based on high harmonic generation and at x-ray free electron lasers are opening up new opportunities to resolve complex ultrafast processes in condensed phase systems with exceptional temporal resolution and atomic site specificity. In this perspective, we present techniques for resolving charge localization, transfer, and separation processes in organic semiconductors and organic photovoltaic devices with time-resolved soft x-ray spectroscopy. We review recent results in ultrafast soft x-ray spectroscopy of these systems and discuss routes to overcome the technical challenges in performing time-resolved x-ray experiments on photosensitive materials with poor thermal conductivity and low pump intensity thresholds for nonlinear effects.

The impact of aggregation of AIE and ACQ moiety-integrating material on the excited state dynamics

The investigation of the properties of aggregate materials is highly interesting because the process of aggregation can result in the disappearance of original properties and the emergence of new ones. Here, a novel fluorescent material (TPEIP), which synergistically combines aggregation-induced emission (AIE) and aggregation caused quenching (ACQ) moieties, was first synthesized by the cyclization reaction of 2,3-diamino-phenazine with 4-tetraphenylenthenealdehyde. We controlled the degree of aggregation of TPEIP to shed light on the impact of the aggregation on the excited state dynamics. TPEIP aggregation realized control over the Intersystem Crossing (ISC) rates and, in turn, the suppression of triplet excited states in MeOH, EtOH or via the simple addition of water to TPEIP solutions in DMSO. From global target analysis, the time scale was 966.2 ps for ISC for TPEIP in DMSO, but it was 860 ps in the case of TPEIP solutions featuring 5% water. The dynamics of TPEIP excited states undergo significant changes as the degree of aggregation increases. Notably, the lifetime of singlet excited states decreases, and there was a gradual diminishment in triplet states.

Synergistic performance of a new bimetallic complex supported on magnetic nanoparticles for Sonogashira and C-N coupling reactions

This paper describes the synthesis of a novel Cu-Ni bimetallic system comprising of magnetic nanoparticles, as the core, and 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole (4-ABPT), as a conjugated bridge, between nickel and copper species. With low Cu and Ni loading (0.06 mol% Ni, 0.08 mol% Cu), the resulting Fe3O4@SiO2@4-ABPT/Cu-Ni showed to be a highly efficient catalyst for the Sonogashira and C-N cross-coupling reactions. The developed catalyst was well characterized by FT-IR, XRD, EDX-mapping, FE-SEM, TEM, ICP, VSM, TGA/DTG/DTA, LSV, and XPS techniques. Fe3O4@SiO2@4-ABPT/Cu-Ni nanocatalyst was compatible with a wide range of amines and aryl halides in the Sonogashira and C-N cross-coupling reactions and offered desired coupling products in high to excellent yields under palladium- and solvent-free conditions. Based on the XPS results, the 4-ABPT ligand can adjust electron transfer between Ni and Cu in Fe3O4@SiO2@4-ABPT/Cu-Ni, promoting the formation and stabilization of Cu+ and Ni3+ species. Electronic interactions and the synergistic effect between these metals increased the selectivity and activity of Fe3O4@SiO2@4-ABPT/Cu-Ni catalyst in the Sonogashira and C-N cross-coupling reactions compared with its monometallic counterparts. Additionally, the magnetic properties of Fe3O4@SiO2@4-ABPT/Cu-Ni facilitated its separation from the reaction mixture, promoting its reuse for several times with no significant loss in its catalytic activity or performance.

Optical detection of alcohols with a Cu(I)HETPHEN complex by reversible aldehyde to hemiacetal conversion

A heteroleptic copper(I) bis(phenanthroline) complex with aldehyde groups at the 4,7 positions of the phenanthroline ligand was synthesized. The complex is responsive to alcohol, resulting in a distinct colour change caused by the facile reaction of the aldehyde group with alcohol, forming a hemiacetal product. The aldehyde species can be regenerated after heating the intermediate at 80 °C for 10 minutes, demonstrating the reusability of the complex for alcohol detection. This work presents a new strategy for applying transition metal complexes in small molecule sensing by installing functional groups in the secondary coordination sphere which reversibly react with analytes.

Changing Directions: Influence of Ligand Electronics on the Directionality and Kinetics of Photoinduced Charge Transfer in Cu(I)Diimine Complexes

A key challenge to the effective utilization of solar energy is to promote efficient photoinduced charge transfer, specifically avoiding unproductive, circuitous electron-transfer pathways and optimizing the kinetics of charge separation and recombination. We hypothesize that one way to address this challenge is to develop a fundamental understanding of how to initiate and control directional photoinduced charge transfer, particularly for earth-abundant first-row transition-metal coordination complexes, which typically suffer from relatively short excited-state lifetimes. Here, we report a series of functionalized heteroleptic copper(I)bis(phenanthroline) complexes, which have allowed us to investigate the directionality of intramolecular photoinduced metal-to-ligand charge transfer (MLCT) as a function of the substituent Hammett parameter. Ultrafast transient absorption suggests a complicated interplay of MLCT localization and solvent interaction with the Cu(II) center of the MLCT state. This work provides a set of design principles for directional charge transfer in earth-abundant complexes and can be used to efficiently design pathways for connecting the molecular modules to catalysts or electrodes and integration into systems for light-driven catalysis.

Watching Excited State Dynamics with Optical and X-ray Probes: The Excited State Dynamics of Aquocobalamin and Hydroxocobalamin

Femtosecond time-resolved X-ray absorption (XANES) at the Co K-edge, X-ray emission (XES) in the Co Kβ and valence-to-core regions, and broadband UV-vis transient absorption are combined to probe the femtosecond to picosecond sequential atomic and electronic dynamics following photoexcitation of two vitamin B₁₂ compounds, hydroxocobalamin and aquocobalamin. Polarized XANES difference spectra allow identification of sequential structural evolution involving first the equatorial and then the axial ligands, with the latter showing rapid coherent bond elongation to the outer turning point of the excited state potential followed by recoil to a relaxed excited state structure. Time-resolved XES, especially in the valence-to-core region, along with polarized optical transient absorption suggests that the recoil results in the formation of a metal-centered excited state with a lifetime of 2-5 ps. This combination of methods provides a uniquely powerful tool to probe the electronic and structural dynamics of photoactive transition-metal complexes and will be applicable to a wide variety of systems.

Signatures of diradicals in x-ray absorption spectroscopy

Theoretical simulations are critical to analyze and interpret the x-ray absorption spectrum of transient open-shell species. In this work, we propose a model of the many-body core-excited states of symmetric diradicals. We apply this model to analyze the carbon K-edge transitions of o-, m-, and p-benzyne, three organic diradicals with diverse and unusual electronic structures. The predictions of our model are compared with high-level multireference computations of the K-edge spectrum of the benzynes obtained with the driven similarity renormalization group truncated to third order. Our model shows the importance of a many-body treatment of the core-excited states of the benzynes and provides a theoretical framework to understand which properties of the ground state of these diradicals can be extracted from their x-ray spectrum.

Bridge-Mediated Metal-to-Metal Electron and Hole Transfer in a Supermolecular Dinuclear Complex: A Computational Study Using Quantum Electron-Nuclear Dynamics

Bimetallic electron donor-acceptor complexes can facilitate electron and energy transfer with excellent structural control through synthetic design. In this work, we investigate the photochemical dynamics in a Ru-Cu bimetallic complex after photoexcitation of the Ru-centered charge transfer state. The physical underpinnings of the metal-to-metal directional charge transfer process are unraveled via analyses of the quantum electronic dynamics and electron-nuclear trajectories. The effects of molecular vibrations in the photoexcited state on the charge transfer processes are also analyzed.

Effect of electronic structure of energy transfer in bimetallic Ru(II)/Os(II) complexes

Novel monometallic (μ-LL')Ru, Ru(μ-LL'), homobimetallic Ru(μ-LL')Ru, and heterodimetallic Ru(μ-LL')Os and Os(μ-LL')Ru complexes based on two asymmetrical ligands LL' (where LL' = L¹L^1', L²L^2') have been synthesized and characterized. Spectroscopic analysis indicates that all complexes exhibit intense spin-allowed ligand-centered (LC) transitions at 288 nm and Ru-based moderate spin-allowed MLCT absorption between 440-450 nm. The Ru(μ-LL')Os and Os(μ-LL')Ru dinuclear complexes show Os-based unit absorption in the range of 565-583 nm. The Ru-based units of the complexes present different emission intensities due to differing steric hindrance at the coordination sites of the two bridging ligands. The Os(μ-LL')Ru dinuclear complexes present weaker emission intensity than their parent monometallic complexes (μ-LL')Ru. These results indicate that the emission of Os(μ-LL')Ru dinuclear complexes is quenched by the Os(II)-based units.