Origin of electronic absorption spectra of MLCT-excited and one-electron reduced 2,2′-bipyridine and 1,10-phenanthroline complexes

Ab initio multireference calculation of electronic spectra of the osmium complexes, [Os(bpy) 3 ] 2 + and [Os(phen) 3 ] 2 +

The spin-orbit coupling corrected absorption spectra of osmium complexes, [Os(bpy)   3 ]   2 + and [Os(phen)   3 ]   2 + , were calculated by using ab initio multireference perturbation method (NEVPT2) with relativistic effects taken into account throughout ZORA approximation and corresponding all-electron basis sets. For the same purpose, the time-dependent DFT techniques were used. A very good agreement between NEVPT2 and experimental spectra should be highlighted, especially for the MLCT transitions that occur in visible and near-UV regions ( 16 , 000 - 33 , 000 cm   - 1 ). Moreover, the present study offers description of excited states of titled osmium complexes and their spectra interpretation using molecular orbitals.

Bifurcation of Excited-State Population Leads to Anti-Kasha Luminescence in a Disulfide-Decorated Organometallic Rhenium Photosensitizer

We report a rhenium diimine photosensitizer equipped with a peripheral disulfide unit on one of the bipyridine ligands, [Re(CO)3(bpy)(S-Sbpy4,4)]+ (1+, bpy = 2,2'-bipyridine, S-Sbpy4,4 = [1,2]dithiino[3,4-c:6,5-c']dipyridine), showing anti-Kasha luminescence. Steady-state and ultrafast time-resolved spectroscopies complemented by nonadiabatic dynamics simulations are used to disclose its excited-state dynamics. The calculations show that after intersystem crossing the complex evolves to two different triplet minima: a (S-Sbpy4,4)-ligand-centered excited state (3LC) lying at lower energy and a metal-to-(bpy)-ligand charge transfer (3MLCT) state at higher energy, with relative yields of 90% and 10%, respectively. The 3LC state involves local excitation of the disulfide group into the antibonding σ* orbital, leading to significant elongation of the S-S bond. Intriguingly, it is the higher-lying 3MLCT state, which is assigned to display luminescence with a lifetime of 270 ns: a signature of anti-Kasha behavior. This assignment is consistent with an energy barrier ≥ 0.6 eV or negligible electronic coupling, preventing reaction toward the 3LC state after the population is trapped in the 3MLCT state. This study represents a striking example on how elusive excited-state dynamics of transition-metal photosensitizers can be deciphered by synergistic experiments and state-of-the-art calculations. Disulfide functionalization lays the foundation of a new design strategy toward harnessing excess energy in a system for possible bimolecular electron or energy transfer reactivity.

Engineering of metal-organic frameworks (MOFs) for thermometry

Metal-organic frameworks (MOFs ) are excellent candidates for use in chemistry, material sciences and engineering thanks to their interesting qualitative features and potential applications. Quite interestingly, the luminescence of MOFs can be engineered by regulation of the ligand design, metal ion selection and encapsulation of guest molecules within the MOF cavity. Temperature is a very crucial physical parameter and the market share of temperature sensors is rapidly expanding with technology and medicinal advancement. Among the wide variety of available temperature sensors, recently MOFs have emerged as potential temperature sensors with the capacity to precisely measure the temperature. Lanthanide-based thermometry has advantages because of its ratiometric response ability, high quantum yield and photostability, and therefore lanthanide-based MOFs were initially focused on to construct MOF thermometers. As science and technology have gradually changed, it has been observed that with the inclusion of dye, quantum dots, etc. within the MOF cavity, it is possible to develop MOF-based thermometry. This review consolidates the recent advances of MOF-based ratiometric thermometers and their mechanism of energy transfer for determining the temperature (thermal sensitivity and temperature uncertainty). In addition, some fundamental points are also discussed, such as concepts for guiding the design of MOF ratiometric thermometers, thermometric performance and tuning the properties of MOF thermometers.

Photocatalytic Reduction of Nicotinamide Co-factor by Perylene Sensitized RhIII Complexes

The ambitious goal of artificial photosynthesis is to develop active systems that mimic nature and use light to split water into hydrogen and oxygen. Intramolecular design concepts are particularly promising. Herein, we firstly present an intramolecular photocatalyst integrating a perylene-based light-harvesting moiety and a catalytic rhodium center (Rh^III phenPer). The excited-state dynamics were investigated by means of steady-state and time-resolved absorption and emission spectroscopy. The studies reveal that photoexcitation of Rh^III phenPer yields the formation of a charge-separated intermediate, namely Rh^II phenPer⋅⁺ , that results in a catalytically active species in the presence of protons.

Ruthenium Assemblies for CO2 Reduction and H2 Generation: Time Resolved Infrared Spectroscopy, Spectroelectrochemistry and a Photocatalysis Study in Solution and on NiO

Two novel supramolecular complexes RuRe ([Ru(dceb)₂(bpt)Re(CO)₃Cl](PF₆)) and RuPt ([Ru(dceb)₂(bpt)PtI(H₂O)](PF₆)₂) [dceb = diethyl(2,2′-bipyridine)-4,4′-dicarboxylate, bpt = 3,5-di(pyridine-2-yl)-1,2,4-triazolate] were synthesized as new catalysts for photocatalytic CO₂ reduction and H₂ evolution, respectively. The influence of the catalytic metal for successful catalysis in solution and on a NiO semiconductor was examined. IR-active handles in the form of carbonyl groups on the peripheral ligand on the photosensitiser were used to study the excited states populated, as well as the one-electron reduced intermediate species using infrared and UV-Vis spectroelectrochemistry, and time resolved infrared spectroscopy. Inclusion of ethyl-ester moieties led to a reduction in the LUMO energies on the peripheral bipyridine ligand, resulting in localization of the ³MLCT excited state on these peripheral ligands following excitation. RuPt generated hydrogen in solution and when immobilized on NiO in a photoelectrochemical (PEC) cell. RuRe was inactive as a CO₂ reduction catalyst in solution, and produced only trace amounts of CO when the photocatalyst was immobilized on NiO in a PEC cell saturated with CO₂.

A fundamental role of solvent polarity and remote substitution of 2-(4-R-phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline framework in controlling of ground- and excited-state properties of Re(I) chromophores [ReCl(CO)3(R-C6H4-imphen)]

A series of Re(I) carbonyl chromophores with 1H-imidazo[4,5-f][1,10]phenanthroline (imphen) ligand functionalized with electron-donating amine groups attached to the imidazole ring via phenylene linkage was designed to investigate the impact of...

Computational Design of Rhenium(I) Carbonyl Complexes for Anticancer Photodynamic Therapy

New Re(I) carbonyl complexes are proposed as candidates for photodynamic therapy after investigating the effects of the pyridocarbazole-type ligand conjugation, addition of substituents to this ligand, and replacement of one CO by phosphines in [Re(pyridocarbazole)(CO)₃(pyridine)] complexes by means of the density functional theory (DFT) and time-dependent DFT. We have found, first, that increasing the conjugation in the bidentate ligand reduces the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap of the complex, so its absorption wavelength red-shifts. When the enlargement of this ligand is carried out by merging the electron-withdrawing 1H-pyrrole-2,5-dione heterocycle, it enhances even more the stabilization of the LUMO due to its electron-acceptor character. Second, the analysis of the shape and composition of the orbitals involved in the band of interest indicates which substituents of the bidentate ligand and which positions are optimal for reducing the HOMO–LUMO energy gap. The introduction of electron-withdrawing substituents into the pyridine ring of the pyridocarbazole ligand mainly stabilizes the LUMO, whereas the HOMO energy increases primarily when electron-donating substituents are introduced into its indole moiety. Each type of substituents results in a bathochromic shift of the lowest-lying absorption band, which is even larger if they are combined in the same complex. Finally, the removal of the π-backbonding interaction between Re and the CO trans to the monodentate pyridine when it is replaced by phosphines PMe₃, 1,4-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA), and 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP) causes another extra bathochromic shift due to the destabilization of the HOMO, which is low with DAPTA, moderate with PMe₃, but especially large with CAP. Through the combination of the PMe₃ or CAP ligands with adequate electron-withdrawing and/or electron-donating substituents at the pyridocarbazole ligand, we have found several complexes with significant absorption at the therapeutic window. In addition, according to our results on the singlet–triplet energy gap, all of them should be able to produce cytotoxic singlet oxygen.

Computations are crucial for proposing new Re(I) carbonyl complexes with very interesting features that make them promising compounds for photodynamic therapy.

Two-dimensional ultrafast transient absorption spectrograph covering deep-ultraviolet to visible spectral region optimized for biomolecules

We report on the implementation of a multi-kHz single-shot referenced non-coherent two-dimensional UV spectrograph based on conventional pump-probe geometry. It has the capability to cover a broad spectral region in excitation from 270-to-380 nm and in the detection from 270-to-390 nm and 320-to-720 nm. Other setups features are: an unprecedented time resolution of 33 fs (standard deviation); signals are photometrically corrected; a single-shot noise of <1 mOD. It has the capability to operate with sample volumes as small as few μl which is an accomplishment in studying biological or biomimetic systems. To show its performances and potentials, we report two preliminary studies on the photophysics of phenanthrenes hosted in a multichromophoric antenna system and of aromatic amino acids in a blue-copper azurin.

Nanoparticles Functionalised with Re(I) Tricarbonyl Complexes for Cancer Theranostics

Globally, cancer is the second (to cardiovascular diseases) leading cause of death. Regardless of various efforts (i.e., finance, research, and workforce) to advance novel cancer theranostics (diagnosis and therapy), there have been few successful attempts towards ongoing clinical treatment options as a result of the complications posed by cancerous tumors. In recent years, the application of magnetic nanomedicine as theranostic devices has garnered enormous attention in cancer treatment research. Magnetic nanoparticles (MNPs) are capable of tuning the magnetic field in their environment, which positively impacts theranostic applications in nanomedicine significantly. MNPs are utilized as contrasting agents for cancer diagnosis, molecular imaging, hyperfusion region visualization, and T cell-based radiotherapy because of their interesting features of small size, high reactive surface area, target ability to cells, and functionalization capability. Radiolabelling of NPs is a powerful diagnostic approach in nuclear medicine imaging and therapy. The use of luminescent radioactive rhenium(I), 188/186Re, tricarbonyl complexes functionalised with magnetite Fe3O4 NPs in nanomedicine has improved the diagnosis and therapy of cancer tumors. This is because the combination of Re(I) with MNPs can improve low distribution and cell penetration into deeper tissues.

Carbazole effect on ground- and excited-state properties of rhenium(i) carbonyl complexes with extended terpy-like ligands

The ground- and excited-state properties of three novel complexes [ReCl(CO)₃(Lⁿ-κ²N)] bearing 2,2':6',2''-terpyridine, 2,6-di(thiazol-2-yl)pyridine and 2,6-di(pyrazin-2-yl)pyridine functionalized with 9-carbazole attached to the central pyridine ring of the triimine core via phenylene linkage were investigated by spectroscopic and electrochemical methods and were simulated using density functional theory (DFT) and time-dependent DFT. To get a deeper and broader understanding of structure-property relationships, the designed Re(i) carbonyl complexes were compared with previously reported analogous systems - without any groups attached to the phenyl ring and bearing pyrrolidine instead of 9-carbazole. The results indicated that attachment of the N-carbazolyl substituent to the triimine core has less influence on the nature of the triplet excited state of [ReCl(CO)₃(Lⁿ-κ²N)] than the pyrrolidine group. Additionally, the impact of the ligand structural modifications on the light emission of the Re(i) complexes under external voltage was preliminarily examined with electroluminescence spectra of diodes containing the synthesized new molecules in an active layer.

Optical and Infrared Spectroelectrochemical Studies of CN-Substituted Bipyridyl Complexes of Ruthenium(II)

Ruthenium(II) polypyridyl complexes [Ru(CN-Me-bpy)_x(bpy)_3-x]²⁺ (CN-Me-bpy = 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine, bpy = 2,2'-bipyridine, and x = 1-3, abbreviated as 1²⁺, 2²⁺, and 3²⁺) undergo four (1²⁺) or five (2²⁺ and 3²⁺) successive one-electron reduction steps between -1.3 and -2.75 V versus ferrocenium/ferrocene (Fc⁺/Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 2²⁺ and 3²⁺ being separated by 150-210 mV; reduction of the unsubstituted bpy ligand in 1²⁺ and 2²⁺ occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Me-bpy ligand shifts the ν(C≡N) IR band by ca. -45 cm^-1, enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2⁺, 3⁺, and 3⁰ show distinct ν(C≡N) features due to the presence of both CN-Me-bpy and CN-Me-bpy^•-, confirming that each reduction is localized on a single ligand. NIR spectra of 1⁰, 1^-, and 2^- exhibit a prominent band attributable to the CN-Me-bpy^•- moiety between 6000 and 7500 cm^-1, whereas bpy^•--based absorption occurs between 4500 and 6000 cm^-1; complexes 2⁺, 3⁺, and 3⁰ also exhibit a band at ca. 3300 cm^-1 due to a CN-Me-bpy^•- → CN-Me-bpy interligand charge-transfer transition. In the UV-vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 1²⁺ is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(C≡N) IR band is shifted by -38 cm^-1 and 5-7-fold-enhanced relative to the ground state.

Strong Impact of Intramolecular Hydrogen Bonding on the Cathodic Path of [Re(3,3'-dihydroxy-2,2'-bipyridine)(CO)3Cl] and Catalytic Reduction of Carbon Dioxide

Herein, we present the cathodic paths of the Group-7 metal complex [Re(3,3'-DHBPY)(CO)₃Cl] (3,3'-DHBPY = 3,3'-dihydroxy-2,2'-bipyridine) producing a moderately active catalyst of electrochemical reduction of CO₂ to CO. The combined techniques of cyclic voltammetry and IR/UV-vis spectroelectrochemistry have revealed significant differences in the chemistry of the electrochemically reduced parent complex compared to the previously published Re/4,4'-DHBPY congener. The initial irreversible cathodic step in weakly coordinating THF is shifted toward much less negative electrode potentials, reflecting facile reductive deprotonation of one hydroxyl group and strong intramolecular hydrogen bonding, O-H···O^-. The latter process occurs spontaneously in basic dimethylformamide where Re/4,4'-DHBPY remains stable. The subsequent reduction of singly deprotonated [Re(3,3'-DHBPY-H⁺)(CO)₃Cl]^- under ambient conditions occurs at a cathodic potential close to that of the Re/4,4'-DHBPY-H⁺ derivative. However, for the stabilized 3,3'-DHBPY-H⁺ ligand, the latter process at the second cathodic wave is more complex and involves an overall transfer of three electrons. Rapid potential step electrolysis induces 1e^--reductive cleavage of the second O-H bond, triggering dissociation of the Cl^- ligand from [Re(3,3'-DHBPY-2H⁺)(CO)₃Cl]^2-. The ultimate product of the second cathodic step in THF was identified as 5-coordinate [Re(3,3'-DHBPY-2H⁺)(CO)₃]^3-, the equivalent of classical 2e^--reduced [Re(BPY)(CO)₃]^-. Each reductive deprotonation of the DHBPY ligand results in a redshift of the IR ν(CO) absorption of the tricarbonyl complexes by ca. 10 cm^-1, facilitating the product assignment based on comparison with the literature data for corresponding Re/BPY complexes. The Cl^- dissociation from [Re(3,3'-DHBPY-2H⁺)(CO)₃Cl]^2- was proven in strongly coordinating butyronitrile. The latter dianion is stable at 223 K, converting at 258 K to 6-coordinate [Re(3,3'-DHBPY-2H⁺)(CO)₃(PrCN)]^3-. Useful reference data were obtained with substituted parent [Re(3,3'-DHBPY)(CO)₃(PrCN)]⁺ that also smoothly deprotonates by the initial reduction to [Re(3,3'-DHBPY-H⁺)(CO)₃(PrCN)]. The latter complex ultimately converts at the second cathodic wave to [Re(3,3'-DHBPY-2H⁺)(CO)₃(PrCN)]^3- via a counterintuitive ETC step generating the 1e^- radical of the parent complex, viz., [Re(3,3'-DHBPY)(CO)₃(PrCN)]. The same alternative reduction path is also followed by [Re(3,3'-DHBPY-H⁺)(CO)₃Cl]^- at the onset of the second cathodic wave, where the ETC step results in the intermediate [Re(3,3'-DHBPY)(CO)₃Cl]^•- further reducible to [Re(3,3'-DHBPY-2H⁺)(CO)₃]^3- as the CO₂ catalyst.

Time-Resolved Femtosecond Stimulated Raman Spectra and DFT Anharmonic Vibrational Analysis of an Electronically Excited Rhenium Photosensitizer

Time-resolved femtosecond stimulated Raman spectra (FSRS) of a prototypical organometallic photosensitizer/photocatalyst ReCl(CO)₃(2,2'-bipyridine) were measured in a broad spectral range ∼40-2000 (4000) cm^-1 at time delays from 40 fs to 4 ns after 400 nm excitation of the lowest allowed electronic transition. Theoretical ground- and excited-state Raman spectra were obtained by anharmonic vibrational analysis using second-order vibrational perturbation theory on vibrations calculated by harmonic approximation at density functional theory-optimized structures. A good match with anharmonically calculated vibrational frequencies allowed for assigning experimental Raman features to particular vibrations. Observed frequency shifts upon excitation (ν(ReCl) and ν(CC inter-ring) vibrations upward; ν(CC, CN) and ν(Re-C) downward) are consistent with the bonding/antibonding characters of the highest occupied molecular orbital and the lowest unoccupied molecular orbital involved in excitation and support the delocalized formulation of the lowest triplet state as ReCl(CO)₃ → bpy charge transfer. FSRS spectra show a mode-specific temporal evolution, providing insights into the intersystem crossing (ISC) mechanism and subsequent relaxation. Most of the Raman features are present at ∼40 fs and exhibit small shifts and intensity changes with time. The 1450-1600 cm^-1 group of bands due to CC, CN, and CC(inter-ring) stretching vibrations undergoes extensive restructuring between 40 and ∼150 fs, followed by frequency upshifts and a biexponential (0.38, 21 ps) area growth, indicating progressing charge separation in the course of the formation and relaxation of the lowest triplet state. Early (40-150 fs) restructuring was also observed in the low-frequency range for ν(Re-Cl) and δ(Re-C-O) vibrations that are presumably activated by ISC. FSRS experimental innovations employed to measure low- and high-energy Raman features simultaneously are described and discussed in detail.

Unconventional two-step spin relaxation dynamics of [Re(CO)3(im)(phen)]+ in aqueous solution

Changes of molecular spin are ubiquitous in chemistry and biology. Among spin flip processes, one of the fastest is intersystem crossing (ISC) in transition metal complexes. Here, we investigate the spin relaxation dynamics and emission spectrum of [Re(CO)3(im)(phen)]+ (im = imidazole, phen = phenanthroline) using extensive full-dimensional excited-state dynamics simulations in explicit aqueous solution. Contrary to what has been observed in other transition metal complexes, the transition from the singlet to triplet states occurs via a two-step process, with clearly separable electronic and nuclear-driven components with two different time scales. The initially excited electronic wave function is a "molecular spin-orbit wave packet" that evolves almost instantaneously, with an 8 fs time constant, into an approximate 25 : 75 singlet-to-triplet equilibrium. Surprisingly, this ISC process is an order of magnitude faster than it was previously documented for this and other rhenium(i) carbonyl diimine complexes from emission spectra. Simulations including explicit laser field interactions evidence that few-cycle UV laser pulses are required to follow the creation and evolution of such molecular spin-orbit wave packets. The analysis of the dynamics also reveals a retarded ISC component, with a time constant of 420 fs, which can be explained invoking intramolecular vibrational energy redistribution. The emission spectrum is shown to be characterized by ISC convoluted with internal conversion and vibrational relaxation. These results provide fundamental understanding of ultrafast intersystem crossing in transition metal complexes.

Heteronuclear d-d and d-f Ru(ii)/M complexes [M = Gd(iii), Yb(iii), Nd(iii), Zn(ii) or Mn(ii)] of ligands combining phenanthroline and aminocarboxylate binding sites: combined relaxivity, cell imaging and photophysical studies

A ligand skeleton combining a 1,10-phenanthroline (phen) binding site and one or two heptadentate N3O4 aminocarboxylate binding sites, connected via alkyne spacers to the phen C3 or C3/C8 positions, has been used to prepare a range of heteronuclear Ru·M and Ru·M2 complexes which have been evaluated for their cell imaging, relaxivity, and photophysical properties. In all cases the phen unit is bound to a {Ru(bipy)2}2+ unit to give a phosphorescent {Ru(bipy)2(phen)}2+ luminophore, and the pendant aminocarboxylate sites are occupied by a secondary metal ion M which is either a lanthanide [Gd(iii), Nd(iii), Yb(iii)] or another d-block ion [Zn(ii), Mn(ii)]. When M = Gd(iii) or Mn(ii) these ions provide the complexes with a high relaxivity for water; in the case of Ru·Gd and Ru·Gd2 the combination of high water relaxivity and 3MLCT phosphorescence from the Ru(ii) unit provides the possibility of two different types of imaging modality in a single molecular probe. In the case of Ru·Mn and Ru·Mn2 the Ru(ii)-based phosphorescence is substantially reduced compared to the control complexes Ru·Zn and Ru·Zn2 due to the quenching effect of the Mn(ii) centres. Ultrafast transient absorption spectroscopy studies on Ru·Mn (and Ru·Zn as a non-quenched control) reveal the occurrence of fast (<1 ns) PET in Ru·Mn, from the Mn(ii) ion to the Ru(ii)-based 3MLCT state, i.e. MnII-(phen˙-)-RuIII → MnIII-(phen˙-)-RuII; the resulting MnIII-(phen˙-) state decays with τ ≈ 5 ns and is non-luminescent. This occurs in conformers when an ET pathway is facilitated by a planar, conjugated bridging ligand conformation connecting the two units across the alkyne bridge but does not occur in conformers where the two units are electronically decoupled by a twisted conformation of the bridging ligand. Computational studies (DFT) on Ru·Mn confirmed both the occurrence of the PET quenching pathway and its dependence on molecular conformation. In the complexes Ru·Ln and Ru·Ln2 (Ln = Nd, Yb), sensitised near-infrared luminescence from Nd(iii) or Yb(iii) is observed following photoinduced energy-transfer from the Ru(ii) core, with Ru → Nd energy-transfer being faster than Ru → Yb energy-transfer due to the higher density of energy-accepting states on Nd(iii).

Charge-Transfer versus Charge-Separated Triplet Excited States of [ReI (dmp)(CO)3 (His124)(Trp122)]+ in Water and in Modified Pseudomonas aeruginosa Azurin Protein

A computational investigation of the triplet excited states of a rhenium complex electronically coupled with a tryptophan side chain and bound to an azurin protein is presented. In particular, by using high-level molecular modeling, evidence is provided for how the electronic properties of the excited-state manifolds strongly depend on coupling with the environment. Indeed, only upon explicitly taking into account the protein environment can two stable triplet states of metal-to-ligand charge transfer or charge-separated nature be recovered. In addition, it is also demonstrated how the rhenium complex plus tryptophan system in an aqueous environment experiences too much flexibility, which prevents the two chromophores from being electronically coupled. This occurrence disables the formation of a charge-separated state. The successful strategy requires a multiscale approach of combining molecular dynamics and quantum chemistry. In this context, the strategy used to parameterize the force fields for the electronic triplet states of the metal complex is also presented.

Metal-mediated diradical tuning for DNA replication arrest via template strand scission

A series of M(PyED)·X (X = 2Cl-, SO42-) pyridine-metalloenediyne complexes [M = Cu(II), Fe(II), or Zn(II)] and their independently synthesized, cyclized analogs have been prepared to investigate their potential as radical-generating DNA-damaging agents. All complexes possess a 1:1 metal-to-ligand stoichiometry as determined by electronic absorption spectroscopy and X-ray diffraction. Solution structural analysis reveals a pπ Cl [Formula: see text] Cu(II) LMCT (22,026 cm-1) for Cu(PyED)·2Cl, indicating three nitrogens and a chloride in the psuedo-equatorial plane with the remaining pyridine nitrogen and solvent in axial positions. EPR spectra of the Cu(II) complexes exhibit an axially elongated octahedron. This spectroscopic evidence, together with density functional theory computed geometries, suggest six-coordinate structures for Cu(II) and Fe(II) complexes and a five-coordinate environment for Zn(II) analogs. Bergman cyclization via thermal activation of these constructs yields benzannulated product indicative of diradical generation in all complexes within 3 h at 37 °C. A significant metal dependence on the rate of the reaction is observed [Cu(II) > Fe(II) > Zn(II)], which is mirrored in in vitro DNA-damaging outcomes. Whereas in situ chelation of PyED leads to considerable degradation in the presence of all metals within 1 h under hyperthermia conditions, Cu(II) activation produces >50% compromised DNA within 5 min. Additionally, Cu(II) chelated PyED outcompetes DNA polymerase I to successfully inhibit template strand extension. Exposure of HeLa cells to Cu(PyBD)·SO4 (IC50 = 10 μM) results in a G2/M arrest compared with untreated samples, indicating significant DNA damage. These results demonstrate metal-controlled radical generation for degradation of biopolymers under physiologically relevant temperatures on short timescales.

Raman Spectroscopy as a Method to Investigate Catalytic Intermediates: CO2 Reducing [Re(Cl)(bpy-R)(CO)3] Catalyst

Complexes of the type [Re(Cl)(bpy-R)(CO)3] (1, bpy = bipyridine, R = (t)Bu, H, CF3) show high catalytic activity for electrochemical CO2 reduction. Application of Raman spectroscopy to these complexes as well as to the doubly reduced species [Re(bpy-R)(CO)3](-) (3), which are the postulated active species, and the monoreduced complex [Re(Cl)(bpy-CF3)(CO)3](-) (2) and comparison with state-of-the-art quantum chemical calculations allows accurate investigation of electronic structures as well as geometries. For doubly reduced complexes, calculations point out a formal closed-shell singlet state only compatible with a formal {Re(I)(bpy-R)(2-)} moiety. In contrast, based on molecular orbital analysis and the change of the actual charge distribution during the overall two-electron reduction, the system is better described as {Re(0)(bpy-R(•))(-)}. Interestingly, the Raman spectra of the monoreduced and doubly reduced complexes with the CF3-substituted bpy ligand are virtually identical, which points to the same overall electronic structure of the bpy species in both complexes. Additional Raman experiments and calculations of [Re(COOH)(bpy)(CO)3] (4) and [Re(bpy)(CO)4]OTf (5), which are proposed to be intermediates of the catalytic cycle for CO2 reduction, confirm the presence of neutral bpy showing that the reducing equivalent stored at the bidentate ligand is involved in the activation of CO2. As such, Raman spectroscopy combined with quantum chemical calculations is an ideal tool to investigate catalysts with redox active ligands, since the spectra give local information about the electronic and geometric structure of the molecule.

Ultrafast excited state dynamics of [Cr(CO)4(bpy)]: revealing the relaxation between triplet charge-transfer states

Ultrafast excited state dynamics of [Cr(CO)₄(bpy)] upon metal-to-ligand charge-transfer (¹MLCT) transition have been studied by pump-probe absorption spectroscopy and DFT calculation.

Synthesis and Photophysical Study of a [NiFe] Hydrogenase Biomimetic Compound Covalently Linked to a Re-diimine Photosensitizer

The synthesis, photophysics, and photochemistry of a linked dyad ([Re]-[NiFe₂]) containing an analogue ([NiFe₂]) of the active site of [NiFe] hydrogenase, covalently bound to a Re-diimine photosensitizer ([Re]), are described. Following excitation, the mechanisms of electron transfer involving the [Re] and [NiFe₂] centers and the resulting decomposition were investigated. Excitation of the [Re] center results in the population of a diimine-based metal-to-ligand charge transfer excited state. Reductive quenching by NEt₃ produces the radically reduced form of [Re], [Re]⁻ (k_q = 1.4 ± 0.1 × 10⁷ M^–1 s^–1). Once formed, [Re]⁻ reduces the [NiFe₂] center to [NiFe₂]⁻, and this reduction was followed using time-resolved infrared spectroscopy. The concentration dependence of the electron transfer rate constants suggests that both inter- and intramolecular electron transfer pathways are involved, and the rate constants for these processes have been estimated (k_inter = 5.9 ± 0.7 × 10⁸ M^–1 s^–1, k_intra = 1.5 ± 0.1 × 10⁵ s^–1). For the analogous bimolecular system, only intermolecular electron transfer could be observed (k_inter = 3.8 ± 0.5 × 10⁹ M^–1 s^–1). Fourier transform infrared spectroscopic studies confirms that decomposition of the dyad occurs upon prolonged photolysis, and this appears to be a major factor for the low activity of the system toward H₂ production in acidic conditions.

Excitation of the [Re] center in the linked-dyad complex ([Re]-[NiFe₂]) populates the ³MLCT excited state, and reductive quenching by NEt₃ produces [Re]⁻. [Re]⁻ reduces the [NiFe₂] center to [NiFe₂]⁻ via inter- and intramolecular electron transfer pathways (k_inter = 5.9 ± 0.7 × 10⁸ M⁻¹ s⁻¹, k_intra = 1.5 ± 0.1 × 10⁵ s⁻¹). For the analogous bimolecular system, where only intermolecular electron transfer could be observed, k_inter = 3.8 ± 0.5 × 10⁹ M⁻¹ s⁻¹.

Cationic cluster formation versus disproportionation of low-valent indium and gallium complexes of 2,2'-bipyridine

Group 13 M^I compounds often disproportionate into M⁰ and M^III. Here, however, we show that the reaction of the M^I salt of the weakly coordinating alkoxyaluminate [Ga^I(C₆H₅F)₂]⁺[Al(OR^F)₄]⁻ (R^F=C(CF₃)₃) with 2,2'-bipyridine (bipy) yields the paramagnetic and distorted octahedral [Ga(bipy)₃]^2+•{[Al(OR^F)₄]⁻}₂ complex salt. While the latter appears to be a Ga^II compound, both, EPR and DFT investigations assign a ligand-centred [Ga^III{(bipy)₃}^•]²⁺ radical dication. Surprisingly, the application of the heavier homologue [^In^I(C₆H₅F)₂]⁺[Al(OR^F)₄]⁻ leads to aggregation and formation of the homonuclear cationic triangular and rhombic [In₃(bipy)₆]³⁺, [In₃(bipy)₅]³⁺ and [In₄(bipy)₆]⁴⁺ metal atom clusters. Typically, such clusters are formed under strongly reductive conditions. Analysing the unexpected redox-neutral cationic cluster formation, DFT studies suggest a stepwise formation of the clusters, possibly via their triplet state and further investigations attribute the overall driving force of the reactions to the strong In−In bonds and the high lattice enthalpies of the resultant ligand stabilized [M₃]³⁺{[Al(OR^F)₄]⁻}₃ and [M₄]⁴⁺{[Al(OR^F)₄]⁻}₄ salts.

Group 13 metals such as gallium and indium with oxidation states of +I tend to disproportionate into more stable 0 and +III states. Here, the authors report that with a 2,2'-bipyridine ligand Ga(+I) forms a paramagnetic mononuclear Ga(+III) complex, whereas the indium analogue aggregates forming cationic clusters retaining the +I state.

Induction of E/Z isomerization in a pendant metal-bound azobenzene: a synthetic, spectroscopic and theoretical study

Re(CO)3 conjugates 1 and 2 that incorporate azobenzenes can be readily generated via one-pot reactions using Schiff base reaction forming conditions. Excitation of the MLCT bands in 1 and 2 results in isomerization of the azobenzene moiety, and this process has been investigated via time-resolved photophysics and TDDFT calculations.

Ultrafast electronic and vibrational relaxations in mixed-ligand dithione-dithiolato Ni, Pd, and Pt complexes

Ultrafast excited-state dynamics of planar Pt, Pd, and Ni dithione-dithiolato complexes were investigated by transient absorption spectroscopy on the femtosecond-picosecond timescale. All studied complexes show a common photobehaviour, although individual kinetics parameters and quantum yields vary with the metal, the dithione ligand and, namely the solvent (DMF, MeCN). Laser pulse irradiation at 800 nm populates the lowest singlet excited state of a dithiolato → dithione charge transfer character, (1)LL'CT. The optically excited state undergoes a solvation-driven sub-picosecond electronic relaxation that enhances the dithione/dithiolato charge separation. The (1)LL'CT state decays with a 1.9-4.5 ps lifetime by two simultaneous pathways: intersystem crossing (ISC) to the lowest triplet state (3)LL'CT and non-radiative decay to the ground state. ISC occurs on a ∼6 ps timescale, virtually independent of the metal, whereas the rate of the non-radiative decay to the ground state decreases on going from Ni (2 ps) to Pd (3 ps) and Pt (∼10 ps). (3)LL'CT is initially formed as a vibrationally excited state. Its equilibration (cooling) takes place on a picosecond timescale and is accompanied by a competitive decay to the ground state. Equilibrated (3)LL'CT is populated with a quantum yield of less than 50%, depending on the metal: Pt > Pd > Ni. (3)LL'CT is long-lived for Pt and Pd (≫500 ps) and short-lived for Ni (∼15 ps). Some of the investigated complexes also exhibit spectral changes due to vibrational cooling of the singlet (2-3 ps, depending on the solvent). Rotational diffusion occurs with lifetimes in the 120-200 ps range. Changing the dithione (Bz2pipdt/(i)Pr2pipdt) as well as dithiolate/diselenolate (dmit/dsit) ligands has only small effects on the photobehavior. It is proposed that the investigated dithione-dithiolato complexes could act as photooxidants (*E(o) ≈ +1.2 V vs. NHE) utilizing their lowest excited singlet ((1)LL'CT), provided that the excited-state electron transfer is ultrafast, competitive with the picosecond decay. On the other hand, the efficiency of any triplet-based processes would be severely limited by the low quantum yield of the triplet population.

Solvation-driven charge transfer and localization in metal complexes

In any physicochemical process in liquids, the dynamical response of the solvent to the solutes out of equilibrium plays a crucial role in the rates and products: the solvent molecules react to the changes in volume and electron density of the solutes to minimize the free energy of the solution, thus modulating the activation barriers and stabilizing (or destabilizing) intermediate states. In charge transfer (CT) processes in polar solvents, the response of the solvent always assists the formation of charge separation states by stabilizing the energy of the localized charges. A deep understanding of the solvation mechanisms and time scales is therefore essential for a correct description of any photochemical process in dense phase and for designing molecular devices based on photosensitizers with CT excited states.

In the last two decades, with the advent of ultrafast time-resolved spectroscopies, microscopic models describing the relevant case of polar solvation (where both the solvent and the solute molecules have a permanent electric dipole and the mutual interaction is mainly dipole–dipole) have dramatically progressed. Regardless of the details of each model, they all assume that the effect of the electrostatic fields of the solvent molecules on the internal electronic dynamics of the solute are perturbative and that the solvent–solute coupling is mainly an electrostatic interaction between the constant permanent dipoles of the solute and the solvent molecules. This well-established picture has proven to quantitatively rationalize spectroscopic effects of environmental and electric dynamics (time-resolved Stokes shifts, inhomogeneous broadening, etc.). However, recent computational and experimental studies, including ours, have shown that further improvement is required.

Indeed, in the last years we investigated several molecular complexes exhibiting photoexcited CT states, and we found that the current description of the formation and stabilization of CT states in an important group of molecules such as transition metal complexes is inaccurate. In particular, we proved that the solvent molecules are not just spectators of intramolecular electron density redistribution but significantly modulate it.

Our results solicit further development of quantum mechanics computational methods to treat the solute and (at least) the closest solvent molecules including the nonperturbative treatment of the effects of local electrostatics and direct solvent–solute interactions to describe the dynamical changes of the solute excited states during the solvent response.

Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses

Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor–acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.

Photoinduced electron transfer in solvated molecular assemblies occurs on the ultrafast timescale before full electronic and geometric relaxation take place. Here Canton et al. monitor this out-of-equilibrium process in a donor–acceptor bimetallic assembly using an X-ray free-electron laser.

Spin density distribution after electron transfer from triethylamine to an [Ir(ppy)2(bpy)]+ photosensitizer during photocatalytic water reduction

The photoreduction of the bis(2-phenylpyridinato-)(2,2'-bipyridine)iridium(III) ion ([Ir(ppy)2(bpy)](+)), used as a photosensitizer in photocatalytic water splitting, by triethylamine was studied by means of UV/VIS, XANES, and EPR spectroscopies, supported by theoretical calculations at density functional theory (DFT) and complete active space self-consistent field (CASSCF/CASPT2) levels. The combination of these methods suggests a predominant bpy localization of the spin-density of the unpaired electron with notable delocalization to the Ir center. This is particularly evident from EPR and theoretical results and leads to broad EPR lines and a large anisotropy of the g-factor.

Tryptophan-accelerated electron flow across a protein-protein interface

We report a new metallolabeled blue copper protein, Re126W122Cu(I) Pseudomonas aeruginosa azurin, which has three redox sites at well-defined distances in the protein fold: Re(I)(CO)3(4,7-dimethyl-1,10-phenanthroline) covalently bound at H126, a Cu center, and an indole side chain W122 situated between the Re and Cu sites (Re-W122(indole) = 13.1 Å, dmp-W122(indole) = 10.0 Å, Re-Cu = 25.6 Å). Near-UV excitation of the Re chromophore leads to prompt Cu(I) oxidation (<50 ns), followed by slow back ET to regenerate Cu(I) and ground-state Re(I) with biexponential kinetics, 220 ns and 6 μs. From spectroscopic measurements of kinetics and relative ET yields at different concentrations, it is likely that the photoinduced ET reactions occur in protein dimers, (Re126W122Cu(I))2 and that the forward ET is accelerated by intermolecular electron hopping through the interfacial tryptophan: *Re//←W122←Cu(I), where // denotes a protein-protein interface. Solution mass spectrometry confirms a broad oligomer distribution with prevalent monomers and dimers, and the crystal structure of the Cu(II) form shows two Re126W122Cu(II) molecules oriented such that redox cofactors Re(dmp) and W122-indole on different protein molecules are located at the interface at much shorter intermolecular distances (Re-W122(indole) = 6.9 Å, dmp-W122(indole) = 3.5 Å, and Re-Cu = 14.0 Å) than within single protein folds. Whereas forward ET is accelerated by hopping through W122, BET is retarded by a space jump at the interface that lacks specific interactions or water molecules. These findings on interfacial electron hopping in (Re126W122Cu(I))2 shed new light on optimal redox-unit placements required for functional long-range charge separation in protein complexes.