The life and times of excited states of organometallic and coordination compounds

Photophysics and photochemistry of (n-Bu4N)2[Pt(NO3)6] in acetonitrile: ultrafast pump-probe spectroscopy and quantum chemical insight

The ultrafast processes caused by photoexcitation of (n-Bu4N)2[Pt(NO3)6] complex in acetonitrile were studied by means of transient absorption (TA) pump-probe spectroscopy and verified by quantum chemical calculations. The primary photochemical process was found to be an inner-sphere electron transfer followed by an escape of an •NO3 radical to the bulk solution. The reaction occurs via the dissociative triplet excited LMCT state of the initial complex. Based on the experimental data and quantum chemical calculations, the mechanism of ultrafast photophysical and photochemical processes is proposed.

Structure and spin of the low- and high-spin states of Fe2+(phen)3 studied by x-ray scattering and emission spectroscopy

The structure and spin of photoexcited Fe2+(phen)3 in water are examined by x-ray scattering and x-ray emission spectroscopy with 100 ps time resolution. Excitation of the low-spin (LS) ground state (GS) to the charge transfer state 1MLCT* leads to the formation of a high-spin (HS) state that returns to the GS in 725 ps. Density functional theory (DFT) predicts a Fe-N bond elongation in HS by 0.19 Å in agreement with the scattering data. The angle between the ligands increases by 5.4° in HS, which allows the solvent to get 0.33 Å closer to Fe in spite of the expansion of the molecule. The rise in solvent temperature from the return of photoproducts to the GS is dominated by the formation dynamics of HS, 1MLCT* → HS, which is followed by a smaller rise from the HS → GS transition. The latter agrees with the 0.61 eV energy gap E(HS)-E(LS) calculated by DFT. However, the temperature rise from the 1MLCT → HS transition is greater than expected, by a factor of 2.1, which is explained by the re-excitation of nascent HS* by the 1.2 ps pump pulse. This hypothesis is supported by optical spectroscopy measurements showing that the 1.2 ps long pump pulse activates the HS* → 5MLCT* channel, which is followed by the ultrafast return to HS* via intersystem crossing. Finally, the spins of the photoproducts are monitored by the Kβ emission and the spectra confirm that the spins of LS and HS states are 0 and 2, respectively.

Primary photophysical and photochemical processes for cerium ammonium nitrate (CAN) in acetonitrile

Cerium ammonium nitrate (CAN) is an important photolytic source of NO3• radicals in aqueous nitric acid solutions and in acetonitrile. In this work we performed the study of primary photochemical processes for CAN in acetonitrile by means of ultrafast TA spectroscopy and quantum chemical calculations. Photoexcitation of CAN is followed by ultrafast (< 100 fs) intersystem crossing; the vibrationally cooled triplet state decays to pentacoordinated Ce(III) intermediate and NO3• radical with the characteristic time of ca. 40 ps. Quantum chemical (QM) calculations satisfactorily describe the UV-vis spectrum of the triplet state. An important feature of CAN photochemistry in CH3CN is the partial stabilization of the radical complex (RC) [(NH4)2CeIII(NO3)5…NO3•], which lifetime is ca. 2 μs. The possibility of the RC stabilization is supported by the QM calculations.

Primary processes in photophysics and photochemistry of a potential light-activated anti-cancer dirhodium complex

Photophysics and photochemistry of a potential light-activated cytotoxic dirhodium complex [Rh2(µ-O2CCH3)2(bpy)(dppz)](O2CCH3)2, where bpy = 2,2'-bipyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine (Complex 1 or Rh2) in aqueous solutions was studied by means of stationary photolysis and time-resolved methods in time range from hundreds of femtoseconds to microseconds. According to the literature, Complex 1 demonstrates both oxygen-dependent (due to singlet oxygen formation) and oxygen-independent cytotoxicity. Photoexchange of an acetate ligand to a water molecule was the only observed photochemical reaction, which rate was increased by oxygen removal from solutions. Photoexcitation of Complex 1 results in the formation of the lowest triplet electronic excited state, which lifetime is less than 10 ns. This time is too short for diffusion-controlled quenching of the triplet state by dissolved oxygen resulting in 1O2 formation. We proposed that singlet oxygen is produced by photoexcitation of weakly bound van der Waals complexes [Rh2…O2], which are formed in solutions. If this is true, no oxygen-independent light-induced cytotoxicity of Complex 1 exists. Residual cytotoxicity deaerated solutions are caused by the remaining [Rh2…O2] complexes.

Visible light-induced palladium-carbon bond weakening in catalytically relevant T-shaped complexes

Triggering one-electron redox processes during palladium catalysis holds the potential to unlock new reaction mechanisms and synthetic methods not previously accessible in the typical two-electron reaction manifolds that dominate palladium catalysis. We report that T-shaped organopalladium(ii) complexes coordinated by a bulky monophosphine, a class of organometallic intermediate featured in a range of contemporary catalytic reactions, undergo blue light-promoted bond weakening leading to mild and efficient homolytic cleavage of strong Pd(ii)-C(sp3) bonds under ambient conditions. The origin of light-triggered radical formation in these systems, which lack an obvious ligand-based chromophore (i.e., π-systems), was investigated using a combination of DFT calculations, photoactinometry, and transient absorption spectroscopy. The available data suggest T-shaped organopalladium(ii) complexes manifest unusual blue light-accessible Pd-to-C(sp3) transition. The quantum efficiency and excited state lifetime of this process were unexpectedly superior compared to a prototypical (α-diimine)Pd(ii) complex featuring a low-lying, ligand-centered LUMO (π*). These results suggest coordinatively-unsaturated organopalladium(ii) compounds, catalysts in myriad catalytic processes, have untapped potential for one-electron reactivity under visible light excitation.

Primary and Secondary Processes in the Ultraviolet Photodissociation of CpCo(CO)2 (Cyclopentadienylcobalt Dicarbonyl)

We investigated the photodissociation dynamics of CpCo(CO)2 (cyclopentadienylcobalt dicarbonyl) in metal-to-ligand charge transfer (MLCT) bands. By employing DFT calculations, the absorption band (210-240 nm) was characterized as a charge transfer from the Co center to the Cp (cyclopentadienyl, C5H5) ligand. Ion imaging was utilized to analyze the CO fragments and coordinatively unsaturated complexes (CpCoCO, CpCo, and CoC3H3) across the entire MLCT band. Measuring the production yields of individual unsaturated complexes as a function of photolysis wavelength by considering wavelength dependence indicated the involvement of several photochemical pathways: the first photodissociation and sequential dissociation of CpCo(CO)2, and the second photodissociation of unsaturated intermediates within the pulse duration of the photolysis laser. The recoil velocity shifts of CpCo and CoC3H3 were attributed to the onset of the sequential dissociation of CpCoCO. Evidence for the second photodissociation of CpCoCO was obtained through the matching of linear momenta between the CO(v = 0, 1) and CpCo fragments. The DFT calculations performed to determine the electronic structures and potential energy curves for photoinduced CO loss in CpCo(CO)2 and CpCoCO supported our interpretation of the experimental results. This study presents a practical approach to selectively detecting specific processes among the mixture of products and intermediates when photolyzing transition-metal carbonyls, as their concurrent generation is unavoidable in laser-based experiments.

Nature of Linear Spectral Properties and Fast Relaxations in the Excited States and Two-Photon Absorption Efficiency of 3-Thiazolyl and 3-Phenyltiazolyl Coumarin Derivatives

Coumarin-based fluorescent agents play an important role in the manifold fundamental scientific and technological areas and need to be carefully studied. In this research, linear photophysics, photochemistry, fast vibronic relaxations, and two-photon absorption (2PA) of the coumarin derivatives, methyl 4-[2-(7-methoxy-2-oxo-chromen-3-yl)thiazol-4-yl]butanoate (1) and methyl 4-[4-[2-(7-methoxy-2-oxo-chromen-3-yl)thiazol-4-yl]phenoxy]butanoate (2), were comprehensively analyzed using stationary and time-resolved spectroscopic techniques, along with quantum-chemical calculations. The steady-state one-photon absorption, fluorescence emission, and excitation anisotropy spectra, as well as 3D fluorescence maps of 3-hetarylcoumarins 1 and 2 were obtained at room temperature in solvents of different polarities. The nature of relatively large Stokes shifts (∼4000-6000 cm^-1), specific solvatochromic behavior, weak electronic π → π* transitions, and adherence to Kasha's rule were revealed. The photochemical stability of 1 and 2 was explored quantitatively, and values of photodecomposition quantum yields, on the order of ∼10^-4, were determined. A femtosecond transient absorption pump-probe technique was used for the investigation of fast vibronic relaxation and excited-state absorption processes in 1 and 2, while the possibility of efficient optical gain was shown for 1 in acetonitrile. The degenerate 2PA spectra of 1 and 2 were measured by an open aperture z-scan method, and the maximum 2PA cross-sections of ∼300 GM were obtained. The electronic nature of the hetaryl coumarins was analyzed by quantum-chemical calculations using DFT/TD-DFT level of theory and was found to be in good agreement with experimental data.

Ultrafast spectroscopy of RuII polypyridine complexes in the gas phase and the liquid phase: [Ru(2,2'-bipyridine)2(nicotinamide)2]2

[Ru(bipyridine)₂(nicotinamide)₂]²⁺ (1) and its monoaqua-complex [Ru(bipyridine)₂(nicotinamide)(H₂O)]²⁺ (2) were spectroscopically studied for the first time in the gas phase by static and time resolved UV photodissociation spectroscopy, observing nicotinamide and H₂O ligand dissociation for 1 and 2, respectively. Both processes and their ultrafast dynamics were investigated in parallel by transient absorption spectroscopy in aqueous solution. The latter data were newly acquired for the long-wavelength MLCT band excitation of 1 and provide novel ultrafast ligand dissociation results for 2, confirming the gas phase results, i.e., exclusive H₂O cleavage over nicotinamide loss. Similar apparent time constants in the sub-ps and few ps ranges were obtained for 1 in both phases, whereas a larger time constant of ca. two hundreds of ps for the ground state recovery was observed exclusively in the solution phase. Our reaction scheme accounts for faster dissociation dynamics in the gas phase by energetical lowering of the ³MC vs. the ³MLCT states by lack of solvent stabilization of the latter. Based on the apparent time constants, we favour, for the solution dynamics, a fast bimodal vibrational deactivation in the ³MLCT/³MC manifolds and a slow dissociation obfuscated by the ground state recovery. This is substantiated by a similar reaction scheme proposed for the ultrafast dynamics of 2, resulting in a new assignment for transient absorption features with λ > 550 nm to the ³MC manifold, and a common kinetic description for 1 and 2. Computations at the TD-DFT/cc-PVTZ/MDF28 level support our spectroscopic findings and the suggested deactivation pathways.

Molecular Wires for Efficient Long-Distance Triplet Energy Transfer

We propose design rules for building organic molecular bridges that enable coherent long-distance triplet-exciton transfer. Using these rules, we describe example polychromophoric structures with low inner-sphere exciton reorganization energies, low static and dynamic disorder, and enhanced π-stacking interactions between nearest-neighbor chromophores. These features lead to triplet-exciton eigenstates that are delocalized over several units at room temperature. The use of such bridges in donor-bridge-acceptor assemblies enables fast triplet-exciton transport over very long distances that is rate-limited by the donor-bridge injection and bridge-acceptor trapping rates.

Electrocatalytic water oxidation performance in an extended porous organic framework with a covalent alliance of distinct Ru sites

The rational synthesis of durable, earth-abundant efficient electrocatalysts for the oxygen evolution reaction (OER) from water is one of the most important routes for storing renewable energy and minimizing fossil fuel combustion. The prime hurdles for effectively utilizing commercial RuO₂ as (OER) electrocatalysts are its very low stability, catalyst deactivation, and high cost. In this work, we explored a Ru-integrated porous organic polymer (Ru@Bpy-POP) by a facile one-pot Friedel-Crafts alkylation strategy between redox-active (Ru(demob)3Cl2) and a carbazole unit, which is composed of unique features including an extended framework unit, isolated active sites, and tunable electrode kinetics. Ru@Bpy-POP can serve as a bridge between a Metal-Organic Framework (MOF) and POP-based catalytic systems with a balanced combination of covalent bonds (structural stability) and open metal sites (single site catalysis). Ru@Bpy-POP, deposited on a three-dimensional nickel foam electrode support, exhibits a promising electrocatalytic OER activity with an ultra-low ruthenium loading compared to a benchmark RuO₂ catalyst, providing an overpotential of about 270 mV to reach 10 mA cm^-2 in an alkaline medium. Moreover, a high current density of 248 mA cm^-2 was achieved for the Ru@Bpy-POP catalyst at only 1.6 V (vs. RHE), which is much higher than 91 mA cm^-2 for commercial RuO₂. The robust, albeit highly conjugated, POP framework not only triggered facile electro-kinetics but also suppressed aggregation and metallic corrosion during electrolysis. In particular, the benefits of covalent integration of distinct Ru sites into the framework can modulate intermediate adsorption and charge density, which contributes to its exceptional OER activity. All of the critical steps involved in OER are complemented by Density Functional Theory (DFT) calculations, which suggest that electrocatalytic water oxidation proceeds from a closed-shell configuration to open-shell electronic configurations with high-spin states. These open-shell configurations are more stable than their closed-shell counterparts by 1 eV, improving the overall catalytic activity.

Demystifying the Origin of Vibrational Coherence Transfer Between the S1 and T1 States of the Pt-pop Complex

We demonstrate that spin-vibronic coupling is the most significant mechanism in vibrational coherence transfer (VCT) from the singlet (S₁) to the triplet (T₁) state of the [Pt₂(P₂O₅H₂)₄]^4- complex. Our time-dependent correlation function-based study shows that the rate of intersystem crossing (k_ISC) through direct spin-orbit coupling is negligibly small, making VCT vanishingly small due to the ultrashort decoherence time (2.5 ps). However, the inclusion of the spin-vibronic contribution to the net k_ISC in selective normal modes along the Pt-Pt axis increases the k_ISC to such an extent that VCT becomes feasible. Our results suggest that k_ISC for the S₁ →T₂ (τ_ISC = 1.084 ps) is much faster than the S₁ → T₁ (τ_ISC = 763.4 ps) and S₁ → T₃ (τ_ISC = 13.38 ps) in CH₃CN solvent, indicating that VCT is possible from the low-lying excited singlet (S₁) to the triplet (T₁) state through the intermediate T₂ state. This is the first example where VCT occurs solely due to spin-vibronic interactions. This finding can pave the way for new types of photocatalysis.

Cyanate Formation via Photolytic Splitting of Dinitrogen

Light-driven N2 cleavage into molecular nitrides is an attractive strategy for synthetic nitrogen fixation. However, suitable platforms are rare. Furthermore, the development of catalytic protocols via this elementary step suffers from poor understanding of N-N photosplitting within dinitrogen complexes, as well as of the thermochemical and kinetic framework for coupled follow-up chemistry. We here present a tungsten pincer platform, which undergoes fully reversible, thermal N2 splitting and reverse nitride coupling, allowing for experimental derivation of thermodynamic and kinetic parameters of the N-N cleavage step. Selective N-N splitting was also obtained photolytically. DFT computations allocate the productive excitations within the {WNNW} core. Transient absorption spectroscopy shows ultrafast repopulation of the electronic ground state. Comparison with ground-state kinetics and resonance Raman data support a pathway for N-N photosplitting via a nonstatistically vibrationally excited ground state that benefits from vibronically coupled structural distortion of the core. Nitride carbonylation and release are demonstrated within a full synthetic cycle for trimethylsilylcyanate formation directly from N2 and CO.

1,7,9,10-Tetrasubstituted PMIs Accessible through Decarboxylative Bromination: Synthesis, Characterization, Photophysical Studies, and Hydrogen Evolution Catalysis

In this work, we present a new synthetic strategy for fourfold-substituted perylene monoimides via tetrabrominated perylene monoanhydrides. X-ray diffraction analysis unveiled the intramolecular stacking orientation between the substituents and semicircular packing behavior. We observed the remarkable influence of the substituent on the longevity and nature of the excited state upon visible light excitation. In the presence of poly(dehydroalanine)-graft-poly(ethylene glycol) graft copolymers as solubilizing template, the chromophores are capable of sensitizing [Mo3 S13 ]2- clusters in aqueous solution for stable visible light driven hydrogen evolution over three days.

Ultrafast processes: coordination chemistry and quantum theory

Coordination compounds, characterized by fascinating and tunable electronic properties, are capable of binding easily to proteins, polymers, wires and DNA. Upon irradiation, these molecular systems develop functions finding applications in solar cells, photocatalysis, luminescent and conformational probes, electron transfer triggers and diagnostic or therapeutic tools. The control of these functions is activated by the light wavelength, the metal/ligand cooperation and the environment within the first picoseconds (ps). After a brief summary of the theoretical background, this perspective reviews case studies, from 1st row to 3rd row transition metal complexes, that illustrate how spin-orbit, vibronic coupling and quantum effects drive the photophysics of this class of molecules at the early stage of the photoinduced elementary processes within the fs-ps time scale range.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Mechanistic study of the trans,cis,cis-[RuCl2(DMSO)2(H2O)2] complex photochemistry in aqueous solutions

It is known that trans,cis,cis-[RuCl2(DMSO)2(H2O)2] (1a) complexes, which are formed upon dissolution of trans-[RuCl2(DMSO)4] in water, demonstrate light-induced cytotoxicity. The mechanistic study of 1a photochemistry has been performed using ultrafast pump-probe spectroscopy, laser flash photolysis and stationary photolysis. The first stage of 1a photochemistry is the photoexchange of a DMSO ligand to a water molecule; its quantum yield is wavelength-dependent (estimating by values 0.3 and 0.04 upon irradiation at 308 and 430 nm, respectively). The mechanism of photoexchange is complicated involving at least four Ru(ii) intermediates. Two tentative mechanisms of the process are proposed.

Ultrafast photoinduced energy and charge transfer

After presenting the basic theoretical models of excitation energy transfer and charge transfer, I describe some of the novel experimental methods used to probe them. Finally, I discuss recent results concerning ultrafast energy and charge transfer in biological systems, in chemical systems and in photovoltaics based on sensitized transition metal oxides.

The MMCT excited state of a localized mixed valence cyanido-bridged RuII-Ru-RuII complex

To investigate MMCT excited states of MV complexes, two symmetrical tetranuclear cyanido-bridged localized MV complexes RuIICNRuIII,III2NCRuII have been designed and synthesized. The ultrafast time-resolved transient absorption (TA) spectroscopy experiment reveals that the MMCT rate of 1 and 2 is 0.18 × 1014 s-1 (τ = 5.7 × 10-14 s) and 0.29 × 1013 s-1 (τ = 3.46 × 10-13 s), respectively, which suggests that the MMCT rate or the lifetime of the MMCT excited state may be controlled by a slight change of the substituent group on the metal center.

A Fast Transient Absorption Study of Co(AcAc)3

The study of transition metal coordination complexes has played a key role in establishing quantum chemistry concepts such as that of ligand field theory. Furthermore, the study of the dynamics of their excited states is of primary importance in determining the de-excitation path of electrons to tailor the electronic properties required for important technological applications. This work focuses on femtosecond transient absorption spectroscopy of Cobalt tris(acetylacetonate) (Co(AcAc)₃) in solution. The fast transient absorption spectroscopy has been employed to study the excited state dynamics after optical excitation. Density functional theory coupled with the polarizable continuum model has been used to characterize the geometries and the electronic states of the solvated ion. The excited states have been calculated using the time dependent density functional theory formalism. The time resolved dynamics of the ligand to metal charge transfer excitation revealed a biphasic behavior with an ultrafast rise time of 0.07 ± 0.04 ps and a decay time of 1.5 ± 0.3 ps, while the ligand field excitations dynamics is characterized by a rise time of 0.07 ± 0.04 ps and a decay time of 1.8 ± 0.3 ps. Time dependent density functional theory calculations of the spin-orbit coupling suggest that the ultrafast rise time can be related to the intersystem crossing from the originally photoexcited state. The picosecond decay is faster than that of similar cobalt coordination complexes and is mainly assigned to internal conversion within the triplet state manifold. The lack of detectable long living states (>5 ps) suggests that non-radiative decay plays an important role in the dynamics of these molecules.

Quantum chemistry of OsCl62− photoaquation products and the reaction scheme

The mechanism of formation of the hydroxocomplex Os^IVCl₅(OH)²⁻ after photoexcitation of Os^IVCl⁶²⁻.

A cis,fac-[RuCl2(DMSO)3(H2O)] complex exhibits ultrafast photochemical aquation/rearrangement

It is known that both cis,fac-[RuCl2(DMSO)3(H2O)] (1a) and trans,cis,cis-[RuCl2(DMSO)2(H2O)2] (2a) complexes, which are formed on the dissolution of trans and cis-isomers of [RuCl2(DMSO)4] in water, demonstrate light-induced anticancer activity. The first stage of 1a photochemistry is its transformation to 2a occurring with a rather high quantum yield, 0.64 ± 0.17. The mechanism of the 1a → 2a phototransformation was studied by means of nanosecond laser flash photolysis and ultrafast pump-probe spectroscopy. The reaction occurs in the picosecond time range via the formation and decay of two successive intermediates interpreted as Ru(ii) complexes with different sets of ligands. A tentative mechanism of phototransformation is proposed.

Structural dynamics upon photoexcitation-induced charge transfer in a dicopper(i)–disulfide complex

A study of structural evolution upon photoinduced charge transfer in a dicopper complex with biologically relevant sulfur coordination.

Short-lived intermediates in photochemistry of an OsCl62− complex in aqueous solutions

Photoaquation of an Os^IVCl₆²⁻ complex occurs via a pentacoordinated Os^IVCl₅⁻ intermediate.

Directly Coupled Versus Spectator Linkers on Diimine PtII Acetylides-Change the Structure, Keep the Function?

Modification of light-harvesting units with anchoring groups for surface attachment often compromises light-harnessing properties. Herein, a series of [donor-acceptor-anchor] platinum(II) diimine (bis-)acetylides was developed in order to systematically compare the effect of conjugated versus electronically decoupled modes of attachment of protected anchoring groups on the photophysical properties of light-harvesting units. The first examples of "decoupled" phosphonate diimine Pt^II complexes are reported, and their properties are compared and contrasted to those of carboxylate analogues studied by a diversity of methods. Ultrafast time-resolved IR and transient absorption spectroscopy revealed that all complexes have a charge-transfer (CT) lowest excited state with lifetimes between 2 and 14 ns. Vibrational signatures and dynamics of CT states were identified; the assignment of electronic states and their vibrational origin was aided by TDDFT calculations. Ultrafast energy redistribution accompanied by structural changes was directly captured in the CT states. A significant difference between the structures of the electronic ground and CT excited states, as well as differences in the structural reorganisation in the complexes bearing directly attached or electronically decoupled anchoring groups, was discovered. This work demonstrates that decoupling of the anchoring group from the light-harvesting core by a saturated spacer is an easy approach to combine surface attachment with high reduction potential and ten times longer lifetime of the CT excited state of the light-absorbing unit, and retain electron-transfer photoreactivity essential for light-harvesting applications.

Primary photophysical and photochemical processes for hexachloroosmate(iv) in aqueous solution

The photoaquation of the Os^IVCl₆^2- complex was studied by means of stationary photolysis, nanosecond laser flash photolysis and ultrafast kinetic spectroscopy. The Os^IVCl₅(OH)^2- complex was found to be the only reaction product. The quantum yield of photoaquation is rather low and wavelength-dependent. No impact of redox processes on photoaquation was revealed. The total characteristic lifetime of the process is about 80 ps. Three intermediates were recorded in the femto- and picosecond time domains and assigned to different Os(iv) species. The nature of intermediates and possible mechanisms of photoaquation are discussed.

Synthesis and crystal structure of a homoleptic diruthenium complex containing tetra-2-pyridyl-1,4-pyrazine (tppz)

Treatment of hydrated ruthenium(III) chloride with tetra-2-pyridyl-1,4-pyrazine (tppz) in refluxing ethoxyethanol afforded the homoleptic dinuclear complex [(tppz)Ru(μ-tppz)Ru(tppz)]4+ (1) besides small amounts of the species [Ru(tppz)2]2+. The title complex 1 was obtained as purple crystals and characterized as its hexafluoridophosphate salt by NMR spectroscopy, mass spectrometry and microanalyses. The molecular structure of 1(PF6)4 has been established by X-ray crystallography.

Excited state relaxation processes of H2-evolving Ru-Pd supramolecular photocatalysts containing a linear or non-linear bridge: a DFT and TDDFT study

In this study, the early-time excited state relaxation processes of bimetallic Ru-Pd supramolecular photocatalysts containing a linear 2,2':5',2''-terpyridine or a nonlinear 2,2':6',2''-terpyridine bridging ligand (BL) were investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) approaches. The bridge based metal-to-ligand charge transfer triplet (³MLCT) state of the metal complex containing a linear bridging ligand was calculated to be the lowest energy triplet (T₁) state which is closely related to the photocatalytic H₂ production, while for that having a nonlinear bridging ligand, the T₁ state is a Ru metal-centered (MC) triplet (³MC_Ru) state that is short-lived and rapidly decays to the ground electronic state (S₀). Our simulation provides an alternative explanation for the smaller interligand electron transfer (ILET) rate in the Ru-Pd complex containing a linear bridge compared to the corresponding monometal Ru complex. Based on the calculation, we also suggest that the successive ³MLCT → ³MC_Ru → S₀ conversion is responsible for the inefficiency of the Ru-Pd complex containing nonlinear bridge as a photocatalyst for H₂ production. This study provides theoretical insights into the key steps of the photoinduced processes of the bimetallic H₂-evolving supramolecular photocatalyst.

Photoinduced structural distortions and singlet–triplet intersystem crossing in Cu(i) MLCT excited states monitored by optically gated fluorescence spectroscopy

Comprehensive analysis of the photo-induced structural distortions and singlet–triplet intersystem crossing dynamics of a series of Cu(i) phenanthroline chromophores.

Primary photochemical processes for Pt(iv) diazido complexes prospective in photodynamic therapy of tumors

A case study of chain photoaquation of mixed-ligand Pt(iv) diazido complexes tested in PDT of tumors is performed.

Tunable Self-Assembly and Morphology-Dependent Photoconductivity of a Donor-Acceptor-Structured Diruthenium Complex

A donor-acceptor-structured diruthenium complex, 1(PF₆)₄, that contains an electron-deficient bridging ligand and electron-rich distal diarylamines modified with long aliphatic chains has been synthesized. By varying the solvent environments and assembly conditions, we obtained three different self-assembled nanostructures of 1(PF₆)₄, including zero-dimensional nanospheres, one-dimensional nanofibers, and thin films with interconnected nanowire networks. These structures were investigated by scanning electron microscopy, transmission electron microscopy, dynamic light scattering, X-ray diffraction, and atomic force microscopy (AFM) analysis. Conductive AFM analysis shows that the nanowire networks exhibit a high conductivity of 0.023 S/cm and an enhanced photoconductivity of 0.59 S/cm under visible light irradiation.

Computational characterization of competing energy and electron transfer states in bimetallic donor-acceptor systems for photocatalytic conversion

The rapidly growing interest in photocatalytic systems for direct solar fuel production such as hydrogen generation from water splitting is grounded in the unique opportunity to achieve charge separation in molecular systems provided by electron transfer processes. In general, both photoinduced and catalytic processes involve complicated dynamics that depend on both structural and electronic effects. Here the excited state landscape of metal centered light harvester-catalyst pairs is explored using density functional theory calculations. In weakly bound systems, the interplay between structural and electronic factors involved can be constructed from the various mononuclear relaxed excited states. For this study, supramolecular states of electron transfer and excitation energy transfer character have been constructed from constituent full optimizations of multiple charge/spin states for a set of three Ru-based light harvesters and nine transition metal catalysts (based on Ru, Rh, Re, Pd, and Co) in terms of energy, structure, and electronic properties. The complete set of combined charge-spin states for each donor-acceptor system provides information about the competition of excited state energy transfer states with the catalytically active electron transfer states, enabling the identification of the most promising candidates for photocatalytic applications from this perspective.

Aromatic C-H Activation in the Triplet Excited State of Cyclometalated Platinum(II) Complexes Using Visible Light

The visible-light driven cyclometalation of arene substrates containing an N-donor heteroaromatic moiety as directing group by monocyclometalated Pt(II) complexes is reported. Precursors of the type [PtMe(C^N)(N^CH)], where N^CH is 2-phenylpyridine (ppyH) or related compunds with diverse electronic properties and C^N is the corresponding cyclometalated ligand, afford homoleptic cis-[Pt(C^N)2] complexes upon irradiation with blue LEDs at room temperature with evolution of methane. Heteroleptic derivatives cis-[Pt(ppy)(C'^N')] are obtained analogously from [PtMe(ppy)(N'^C'H)], where N'^C'H represents an extended set of heteroaromatic compounds. Experimental and computational studies demonstrate an unprecedented C-H oxidative addition, which is initiated by a triplet excited state of metal-to-ligand charge-transfer (MLCT) character and leads to a detectable Pt(IV) methyl hydride intermediate.