Metal-to-ligand Charge Transfer Band Research Articles

Mer-M(CO)3(PNP)0/+ pincer complexes (M = W(0) or Re(I); PNP = 4,5-bis(diphenylphosphino)acridine): synthesis, spectroscopy and anti-Kasha emission.

Two isoelectronic and isostructural W(0) and Re(I) complexes mer-W(CO)3(PNP) (1) and [mer-Re(CO)3(PNP)]Cl (2) (PNP = 4,5-bis(diphenylphosphino)acridine) were synthesized and characterized by X-ray diffraction, infrared, electronic absorption and emission spectroscopy, and cyclic voltammetry. Structures of these complexes show a metal center bonded to the pincer ligand and two axial CO and one equatorial CO ligands. DFT calculations showed that the LUMOs of both complexes are the lowest energy π* orbitals localized in the acridine part of the ligand. The HOMO of 1 is dominated by the dπ orbital of W(0) while the HOMO of 2 has a substantial contribution from the highest energy π orbital of the acridine ring. TD-DFT calculations were performed to assist assignment of the UV-vis absorption spectra. The UV-vis absorption spectrum of 1 shows a very low energy W → π* (acridine) metal-to-ligand-charge-transfer (MLCT) absorption band that ranges from visible (500 nm) to near-infrared (>900 nm) regions and an intense acridine π → π* absorption band at 410 nm. There is a blue-green window in the ∼450-500 nm range between the π → π* and W → π*(acridine) MLCT absorptions. The absorption spectrum of 2, dominated by intense π → π* absorptions, shows no distinct low energy MLCT band. Complex 1 is luminescent, displaying acridine-based ππ* fluorescence at 501 nm which is anti-Kasha as it is higher in energy than the lowest energy excited state.

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 the Ultrafast Interligands Electron Transfers in Dye-Sensitized Solar Cells.

Charge-transfer dynamics and interligand electron transfer (ILET) phenomena play a pivotal role in dye-sensitizers, mostly represented by the Ru-based polypyridyl complexes, for TiO2 and ZnO-based solar cells. Starting from metal-to-ligand charge-transfer (MLCT) excited states, charge dynamics and ILET can influence the overall device efficiency. In this letter, we focus on N34- dye ( [Ru(dcbpy)2(NCS)2]4-, dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) to provide a first direct observation with high time resolution (<20 fs) of the ultrafast electron exchange between bpy-like ligands. ILET is observed in water solution after photoexcitation in the ∼400 nm MLCT band, and assessment of its ultrafast time-scale is here given through a real-time electronic dynamics simulation on the basis of state-of-the-art electronic structure methods. Indirect effects of water at finite temperature are also disentangled by investigating the system in a symmetric gas-phase structure. As main result, remarkably, the ILET mechanism appears to be based upon a purely electronic evolution among the dense, experimentally accessible, MLCT excited states manifold at ∼400 nm, which rules out nuclear-electronic couplings and proves further the importance of the dense electronic manifold in improving the efficiency of dye sensitizers in solar cell devices.

Novel Metallo-Supramolecular Polymers with 1-Thioxophosphole Main-Chain Units and Remarkable Photoinduced Changes in Their Resonance Raman Spectra.

New low-bandgap unimers, with the central thiophene-(1-thioxophosphole)-thiophene (TPT) ring sequence and 2,2':6',2″-terpyridin-4'-yl (tpy) end groups connected to the central unit via conjugated linkers of different size, are prepared and assembled with Zn(II) and Fe(II) ions to metallo-supramolecular polymers (MSPs) that are studied regarding their properties. The most interesting feature of Zn-MSPs is the luminescence extended deep into the NIR region. Fe-MSPs not only show the metal-to-ligand charge transfer (MLCT) manifested by the MLCT band (an expected feature) but also an as-yet-undescribed remarkable phenomenon: specific damping of the bands of the TPT sequence in the resonance Raman spectra taken from solid Fe-MSPs using the excitation to the MLCT band (532 nm). The damping is highly reversible at the low laser power of 0.1 mW but gradually becomes irreversible as the power reaches ca. 5 mW. The revealed phenomenon is not shown by the same Fe-MSPs in solutions, nor by Fe-MSPs containing no phosphole units. A hypothesis is proposed that explains this phenomenon and its dependence on the irradiation intensity as a result of the interplay of three factors: (i) enhancement of the MLCT process by excitation radiation, (ii) the electron-acceptor character of the 1-thioxophosphole ring, and (iii) morphological changes of the lattice and their dependence on the population of new structures in the lattice.

A panchromatic graphite-derived-carbon-dot TiO2 photocatalyst: Broad visible light absorption via ligand-to-metal charge transfer

In recent decades, solar-to-chemical conversion catalysts for energy and environmental applications have drawn significant attention. Catalysts composed of nano-size TiO2 and various dye-based photosensitizers can absorb solar light efficiently and produce promising photocatalysts, but their photoactive region is limited. In this study, we demonstrate that nanocomposites composed of graphite-derived carbon dots (GCDs) and TiO2 are capable of ligand-to-metal charge transfer (LMCT), which was active across the entire visible-light spectrum. The effect of GCDs on the photocatalytic activity of TiO2 was determined through the production of hydrogen peroxide and the degradation of pollutants. On the basis of chemical and optical investigation, we demonstrate the formation of an LMCT band in the TiO2/GCD nanocomposites, which are responsible for photoactivation below the bandgap of each component. Our research provides conclusive evidence for the presence of the LMCT band in TiO2/GCD and reveals a novel method for expanding the light-absorption spectrum of the photocatalyst.

Luminescence color tuning of the four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes with imidazolylidene ligand functionalized by thiazole/benzoxazole moiety

Here, we report a series of the four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes with imidazolylidene and POP ligands, [Cu(BenzThiazolyl-Im)(POP)]PF6 (P1), [Cu(BenzThiazolyl-BenIm)(POP)]PF6 (P2), [Cu(BenzBenzoxazolyl-Im)(POP)]PF6 (P3), and [Cu(BenzBenzoxazolyl-BenIm)(POP)]PF6 (P4), (BenzThiazolyl-Im=3-benzyl-1-(thiazol-2-yl)-1H-imidazolylidene, BenzThiazolyl-BenIm=3-benzyl-1-(thiazol-2-yl)-1H-Benzoimidazolylidene BenzBenzoxazolyl-Im=3-benzyl-1-(benzoxazol-2-yl)-1H-imidazolylidene, BenzBenzoxazolyl-BenIm=3-benzyl-1-(benzocazol-2-yl)-1H-imidazolylidene, POP=bis[2-diphenylphosphino]-phenyl)ether). Thiazole/benzoxazole moiety were functionalized onto imidazolylidene ligand to tune the photophysical properties of the corresponding complexes. P1 and P2 exhibit the metal-to-ligand charge transfer (MLCT) absorption band with the onset at 406–412 nm, while P3 and P4 have similar MLCT band with the onset at 380–389 nm. The emission wavelengths of all NHCCu(I) complexes can be fine-tuned to cover the spectral range of 517–551 nm with the phosphorescent origin in PMMA films. P2 show the highest photoluminescence efficiency of 40.9% with the red-shifted emission wavelength at 551 nm, whereas P4 has the lowest photoluminescence efficiency of 1.80% with the blue-shifted emission wavelength at 517 nm. Density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations were employed to rationalize the differences for photophysical properties of the NHCCu(I) complexes.

Charge-Transfer Spectroscopy of Bisaxially Coordinated Iron(II) Phthalocyanines through the Prism of the Lever's EL Parameters Scale, MCD Spectroscopy, and TDDFT Calculations.

The position of the experimentally observed (in the UV-vis and magnetic circular dichroism (MCD) spectra) low-energy metal-to-ligand charge-transfer (MLCT) band in low-spin iron(II) phthalocyanine complexes of general formula PcFeL2, PcFeL'L″, and [PcFeX2]2- (L, L', or L″ are neutral and X- is an anionic axial ligand) was correlated with the Lever's electrochemical EL scale values for the axial ligands. The time-dependent density functional theory (TDDFT)-predicted UV-vis spectra are in very good agreement with the experimental data for all complexes. In the majority of compounds, TDDFT predicts that the first degenerate MLCT band that correlates with the MCD A-term observed between 360 and 480 nm is dominated by an eg (Fe, dπ) → b1u (Pc, π*) single-electron excitation (in traditional D4h point group notation) and agrees well with the previous assignment discussed by Stillman and co-workers[ Inorg. Chem. 1994, 33, 573-583]. The TDDFT calculations also suggest a small energy gap for b1u/b2u (Pc, π*) orbital splitting and closeness of the MLCT1 eg (Fe, dπ) → b1u (Pc, π*) and MLCT2 eg (Fe, dπ) → b2u (Pc, π*) transitions. In the case of the PcFeL2 complexes with phosphines as the axial ligands, additional degenerate charge-transfer transitions were observed between 450 and 500 nm. These transitions are dominated by a2u (Pc + L, π) → eg (Pc, π*) single-electron excitations and are unique for the PcFe(PR3)2 complexes. The energy of the phthalocyanine-based a2u orbital has large axial ligand dependency and is the reason for a large energy deviation for B1 a2u (Pc + L, π) → eg (Pc, π*) transition. The energies of the axial ligand-to-iron, axial ligand-to-phthalocyanine, iron-to-axial ligand, and phthalocyanine-to-axial ligand charge-transfer transitions were discussed on the basis of TDDFT calculations.

Luminescent copper(I) complexes bearing benzothiazole-imidazolylidene ligand with various substituents: Synthesis, photophysical properties and computational studies

The synthesis and photophysical investigation of a series of copper(I) complexes bearing benzothiazole-imidazolylidene-type N-heterocyclic carbene (NHC) ligands are reported. Their photophysical properties are tuned by adjusting the different substituens at 6-position of the benzothiazole ring. C1 and C2 exhibit the lowest-lying metal-to-ligand charge transfer (MLCT) absorption bands at 325–400 nm, while the MLCT band of C3 is almost unresolved owing to the blue shift of this band. Upon excitation with UV light all the complexes emit bright yellow-green light (λem = 543–551 nm) with higher emission quantum yields of 24.9–44.1% and excited state lifetimes on the microsecond scale (τ = 13.7–32.7 μs) in PMMA films. Additionally, the electronic properties, absorption and emission profiles of these complexes were further explored using DFT/TDDFT methods.

Enhanced luminescence of tris(carboxylato)uranyl(VI) complexes and energy transfer to Eu(III): a combined spectroscopic and theoretical investigation.

Complex formation between uranyl and carboxylate ligands (benzoate, nicotinate and isonicotinate) has been studied extensively by absorption and luminescence spectroscopy in acetonitrile medium. Experimental data had indicated the existence of stable and enhanced luminescent tris(carboxylato) uranyl(VI) complexes i.e. [UO2(L)3]- with D3h symmetry. The high luminescence of these complexes was due to the sensitization of the Oyl → U ligand to metal charge transfer (LMCT) emission by extremely intense equatorial (carboxylate ligands) LMCT bands. The variation in the experimentally observed parameters such as intensity of equatorial LMCT bands, luminescence lifetimes, quantum yields and structural parameters among tris(carboxylato) uranyl(VI) complexes are affirmed by quantum chemical calculations using density functional theory and the computational results are found to be in good agreement with experimental findings. Interestingly, in a very dilute mixture of [UO2(L)3]- and Eu(III), energy transfer from uranyl to Eu(III) is observed and it leads to the detection of europium at trace levels. This is an intriguing observation as none of the previous studies have reported such a low level of detection limit of Eu(III) by means of energy transfer from any metal.

Synthesis of a Ru(II) Complex with a Naphthoquinone-Annelated Imidazole Ligand Exhibiting Proton-Responsive Redox and Luminescent Behavior

A mononuclear ruthenium complex, [RuII(L)(bpy)2](PF6), with a naphthoquinone-annelated imidazole ligand HL (2-(pyridin-2-yl)-1H-naphtho[2,3-d]imidazole-4,9-dione) was synthesized and structurally characterized. Electrochemical study reveals that the Ru complex shows four reversible redox waves at +0.98 V, −1.13 V, −1.53 V, and −1.71 V versus SCE in acetonitrile, which are assigned to Ru(II)/Ru(III), L−/L•2−, and two bpy/bpy•− redox couples, respectively. The redox potential of Ru(II)/Ru(III) was positively shifted upon the addition of trifluoromethanesulfonic acid due to protonation of the L− moiety, leading to stabilization of the Ru 4d orbital. In UV-vis absorption measurements for the Ru complex in acetonitrile, a metal-to-ligand charge transfer (MLCT) band was observed at 476 nm, which was shifted to 450 nm by protonation, which might be due to a decrease in the electron delocalization and stabilization of the π orbitals in L−. The blue shift of the MLCT band by protonation was associated with a shift of an emission band from 774 nm to 620 nm, which could be caused by the decreased electronic delocalization in the MLCT excited state. These electrochemical and spectroscopic changes were reversible for the protonation/deprotonation stimuli.

Nanostructured polymer assemblies stabilize photoactivatable anticancer ruthenium complexes under physiological conditions

Photoactivatable ruthenium (Ru) complexes are promising compounds for anticancer phototherapy. They must be stable under physiological conditions before they are transported to cancer cells. In this paper, we systematically studied the stabilities of two Ru-containing block copolymers (Ru complexes as side group or in main chain, respectively) and their corresponding Ru complexes in different media, including saline, bovine serum albumin (BSA) solution, Dulbecco's Modified Eagle's Medium (DMEM, pH 6.5) and DMEM (pH 5.5) with 1.0 mM glutathione (GSH). Their stabilities were studied by monitoring the metal-to-ligand charge transfer (MLCT) bands of the Ru moieties via UV–Vis absorption spectroscopy. The MLCT bands of Ru complexes changed to varying degrees within a day in the above-mentioned media, indicating that they were instable. In contrast, the MLCT bands of Ru-containing polymer assemblies did not change significantly under the same conditions. These results showed that the self-assembled nanostructures protected the Ru moieties and improved their stability. In addition, these nanostructured polymer assemblies could be activated by red light. Therefore, the studied Ru-containing polymer assemblies are more suitable for in vivo applications than their low-molecular weight analogues.

Selective C-C Bond Scission of Ketones via Visible-Light-Mediated Cerium Catalysis

Summary Here, we report a general catalytic manifold for the selective C–C bond scission of ketones via the exploitation of the ligand-to-metal charge transfer (LMCT) excitation mode. Through a cooperative utilization of Lewis acid catalysis and LMCT catalysis, the C–C bond of ketones could be selectively and effectively cleaved, enabling the installation of different functionalities at each carbon of the cleaved C–C bond through a sequential and orthogonal manner. This reaction manifold serves as a photocatalytic alternative to the Norrish type I reaction with the combination of visible light and inexpensive cerium salts. Under operationally simple conditions, a wide range of acyclic and cyclic ketones, from simple strained cyclobutanones to complex androsterone with less strained cyclopentanone moiety, could be successfully transformed into versatile chemical building blocks.

Photoselective MLCT to d-d pathways for light-induced excited spin state trapping.

We use femtosecond optical pump-probe spectroscopy to study the Light Induced Excited Spin State Trapping (LIESST) dynamics in an FeII spin-crossover material. In these systems, LIESST derives from fast molecular switching induced by light from low (LS, S = 0) to high spin (HS, S = 2) states, as reported for molecules in solution as well as in the solid state. Since the direct LS-to-HS conversion is forbidden by selection rules, the switching dynamics involves intermediate electronic states such as metal-to-ligand charge transfer (MLCT) or ligand-field excited states of singlet or triplet nature. In addition, the HS state is structurally trapped by the elongation of the metal-ligand bond, which is accompanied by the coherent activation and damping of the molecular breathing mode. The ultrafast LIESST dynamics was mainly investigated in FeN6 ligand field systems with almost octahedral symmetry, under MLCT excitation. Our recent study on the FeII(pap-5NO2)2 spin-crossover material, with a FeIIN4O2 ligand field of C2 symmetry, has shown that in addition to MLCT bands, optical excitation, through quite intense and low-energy shifted d-d bands, can also drive LIESST. Compared to MLCT, d-d excitation involves shorter-lived intermediates, drives faster LS-to-HS switching, and enhances the coherent structural dynamics. In this paper, we present an ultrafast study of the pump wavelength dependence of LIESST and we evidence a photoselective crossover from the MLCT to the d-d pathways.

One‐ and Two‐Photon‐Induced Photochemistry of Iron Pentacarbonyl [Fe(CO)5]: Insights from Coupled Cluster Response Theory

AbstractWe present herein the first comprehensive study of the one‐ and two‐photon absorption of Fe(CO)5 utilising a hierarchy of linear‐ and quadratic‐response coupled cluster (LR‐ and QR‐CC) methodologies to provide an in‐depth characterisation, as well as potential energy curves for axial and equatorial bond dissociations, highlighting the state crossings leading from the bright 1A2′′ state through to the dissociative 1E′ state. We have characterised a range of metal‐to‐ligand charge transfer (MLCT) and ligand field (LF) states that are in agreement with both previous studies and experiment, including the identification of a series of E′ states that present Rydberg character in the 5.9–7.2 eV region. Due to the rapid excited state dissociation of Fe(CO)5 through the low lying 1E′ and 2E′′ ligand‐field states, we have also included an LR‐CCSD analysis of the major dissociative product, Fe(CO)4. Analysis of the C2v geometry of Fe(CO)4 reveals four accessible ligand field states at 1.085, 1.684, 1.958, and 2.504 eV respectively, reinforcing the highly unstable nature of Fe(CO)4 along with a strong MLCT band between 4.300 and 5.573 eV. This band overlaps with one in the spectra of Fe(CO)5 suggesting that full fragmentation could proceed by two paths: two‐photon excitation leading to dissociation, or through sequential one‐photon absorption events, the first causing dissociation to and the second initiating further fragmentation of the complex.

Rational use of ligand to shift the UV–vis spectrum of Ru-complex sensitiser dyes for DSSC applications

Radiation is energy travelling through space. Sunlight radiation such as ultraviolet (UV) radiation are higher-energy radiation which are used in medicine and energy industry such as dye sensitized solar cells (DSSCs). We recently designed new Ru-complexes through novel ligands based on a recently synthesised high performance dye, i.e. [(η6-p-cymene)Ru(4,4′,5,5′-H4tcbpy)Cl]Cl [1] (A as L1-Ru-L2) for applications in DSSCs. Two electron rich top-ligands, η6-N, N, N′, N′-tetramethyl-p-phenylenediamine (new dye D) and η6- 1,4 di[(dimethylamino)ethyl] benzene (new dye E) outperform other ligands for the new high performance Ru-complexes. Our time dependent density functional theory (TD-DFT) using the CAM-B3LYP/6-311G+(d)/LANL2DZ functional and basis-set model reveals the apparent enhancement of the major UV–vis bands of metal to ligand charge transfer (MLCT) in the new dyes. The major UV–vis bands of the new dyes D and E are red shifted 35 nm and 78 nm, respectively, from the MLCT band of the reference dye A centred at 433 nm. We further discovered that the electron rich top-ligands (L1) are responsible to lift a cluster of occupied outer valence orbitals such as HOMO, HOMO-1 and HOMO-2 in the new D and E complexes, while the virtual orbitals which concentrate on the lower ligand L2 (4,4′,5,5′ H4 tcbpy) remain almost unchanged. The top-ligand L1 enhances the MLCT transition of the new dyes D and E through the dominant 7dz2 electrons of the transition metal Ru in their HOMOs. The present study provides the rational of ligand control for metal complexes in broader applications such as electron transfer and electronics.

Emission Tuning of Heteroleptic Arylborane-Ruthenium(II) Complexes by Ancillary Ligands: Observation of Strickler-Berg-Type Relation.

Novel heteroleptic arylborane-ruthenium(II) complexes having a series of ancillary ligands L' ([Ru(B2bpy)L'2]2+) in CH3CN showed low-energy/intense metal-to-ligand charge transfer (MLCT)-type absorption and intense/long-lived emission compared to the reference complexes. The spectroscopic and photophysical properties of [Ru(B2bpy)L'2]2+ were shown to be manipulated synthetically by the electron-donating ability of the ancillary ligand(s). The intense and long-lived emission observed for [Ru(B2bpy)L'2]2+ in CH3CN at 298 K is responsible for the accelerated radiative and decelerated nonradiative decay processes, which are controllable through the electronic structures of the ancillary ligand(s). On the basis of the present systematic study, furthermore, we succeeded in demonstrating the Strickler-Berg-type relation between the molar absorption coefficients of the MLCT bands and the radiative rate constants of the complexes.

Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex.

Among the many metal-dinitrogen complexes synthesized, the end-on bridging (μ2, η1, η1-N2) coordination mode is notoriously unreactive for nitrogen fixation. This is principally due to the large activation energy for ground-state nitrogen-element bond formation and motivates exploration of the photoexcited reactivity of this coordination mode. To provide the foundation for this concept, the photophysics of a dinitrogen-bridged molybdenum complex was explored by ultrafast electronic spectroscopies. The complex absorbs light from the UV to near-IR, and the transitions are predominantly of metal-to-ligand charge transfer (MLCT) character. Five excitation wavelengths (440, 520, 610, 730, and 1150 nm) were employed to access MLCT bands, and the dynamics were probed between 430 and 1600 nm. Despite the large energy space occupied by electronic states (ca. 1.2 eV), the dynamics were independent of the excitation wavelength. In the proposed kinetic model, photoexcitation from a Mo-N═N-Mo centered ground state populates the π*-state delocalized over two terpyridine ligands. Due to a large terpyridine-terpyridine spatial separation, electronic localization occurs within 100 fs, augmented by symmetry breaking. The subsequent interplay of internal conversion and intersystem crossing (ISC) populates the lowest 3MLCT state in 2-3 ps. Decay to the ground state occurs either directly or via a thermally activated metal-centered (3MC) trap state having two time constants (10-15 ps, 23-26 ps [298 K]; 103 ps, 612 ps [77 K]). ISC between 1MLCT and 3MLCT involves migration of energized electron density from the terpyridine π* orbitals to the Mo-N═N-Mo core. Implication of the observed dynamics for the potential N-H bond forming reactivity are discussed.

Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts

Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO2) irreversibly activate hydrogen peroxide (H2O2) to form pools of metal-hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) species for alkene epoxidation. The product distributions from reactions with Z-stilbene, in combination with time-resolved UV–vis spectra of the reaction between H2O2-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η2-O2) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C8H8) epoxidation reveal that these materials first adsorb and then irreversibly activate H2O2 to form pools of interconverting M-OOH and M-(η2-O2) intermediates, which then react with styrene or H2O2 to form either styrene oxide or H2O2 decomposition products, respectively. Activation enthalpies (ΔH⧧) for C8H8 epoxidation and H2O2 decomposition decrease linearly with increasing heats of adsorption for pyridine or deuterated acetonitrile coordinated to Lewis acid sites, which suggests that materials with greater electron affinities (i.e., stronger Lewis acids) are more active for C8H8 epoxidation. Values of ΔH⧧ for C8H8 epoxidation and H2O2 decomposition also decrease linearly with the ligand-to-metal charge-transfer (LMCT) band energies for the reactive intermediates, which is a more relevant measure of the requirements for the active sites in these catalytic cycles. Epoxidation rates depend more strongly on the LMCT band energy than H2O2 decomposition rates, which shows that more electrophilic M-OOH and M-(η2-O2) species (i.e., those formed at stronger Lewis acid sites) give both greater rates and greater selectivities for epoxidations. Thermochemical analysis of ΔH⧧ for C8H8 epoxidation and adsorption enthalpies for C8H8 within the pores of *BEA and SiO2 reveal that the 0.7 nm pores within M-BEA preferentially stabilize transition states for C8H8 epoxidation with respect to the 5.4 nm pores of M-SiO2, while H2O2 decomposition is unaffected by the differences between these pore diameters due to the small Stokes diameter of H2O2. Thus, the differences in reactivity and selectivity between M-BEA and M-SiO2 materials is solely attributed to confinement of the transition state and not differences in the identity of the reactive intermediates, mechanism for alkene epoxidation, or intrinsic activation barriers. Consequently, the rates and selectivities for alkene epoxidation reflect at least two orthogonal catalyst design criteria—the electronegativities of the transition metal atoms that determine the electronic structure of the active complex and the mean diameters of the surrounding pores that can selectively stabilize transition states for specific reaction pathways.

A non-innocent salen naphthalene ligand and its Co 2+ , Ni 2+ and Cu 2+ metal complexes: Structural, electrochemical, and spectroscopic characterization and computational studies

Abstract Three mononuclear metal complexes of Co2+, Ni2+ and Cu2+ with the Salen type ligand ((2,2′-[(1-methyl-1,2-ethanediyl) bis(nitriloethylidyne)] bis(1-naphthalenol)), H2Naphpren or L) were synthesized and structurally characterized. All metal complexes have square planar geometries and crystallize in the P21/c space group. Structural characterization of the ligand shows contribution of the bis(ketone) tautomer in the crystalline phase, in agreement with density functional theory (DFT) computations. However, according to the UV-vis spectrum, there is a stronger influence of the bis(phenol) tautomer in solution. Cyclic voltammetry experiments showed that the ligand undergoes a single irreversible oxidation process at 0.434 V (vs Fc/Fc+) while the complexes with Co2+, Ni2+ and Cu2+ suffer electroxidative process on the surface of the working electrode in addition to solution-assigned redox processes. The CoL oxidation is localized on the metal center, however, the CuL and NiL oxidations are considered as metal-ligand based processes. DFT calculations confirmed our experimental findings. The UV-vis spectra of the complexes illustrate ligand characteristic bands: in the case of the Cu2+ complex, the metal-to-ligand charge transfer (MLCT) band is present at 385 nm, while in the case of Co2+ complex this same type of MLCT band appears at 377 nm.

New Ru(ii) photocages operative with near-IR light: new platform for drug delivery in the PDT window.

A series of Ru(ii) complexes bearing the tridentate 2,6-di(quinolin-2-yl)pyridine (dqpy) ligand were designed to undergo photoinduced ligand dissociation with red/near-IR light. The complexes [Ru(dqpy)(L)(CH3CN)]2+, where L = 2,2'-bipyridine (bpy, 1), 4,4'dimethyl-2,2'-bipyridine (Me2bpy, 2), and 1,10-phenanthroline (phen, 3). Complexes 1-3 exhibit red-shifted lowest energy metal-to-ligand charge transfer (MLCT) absorption maxima at ∼600 nm, as compared to the corresponding tpy (2,2';6',2''-terpyridine) complexes with MLCT bands at ∼565 nm which appear as shoulders to the MLCT bands at ∼455 nm. This shift is attributed to the lower energy LUMO afforded by the dqpy ligand when compared to tpy, as evidenced by the shift of the first reduction wave to ∼0.3 V more positive potentials in the former. In addition, the lowest MLCT maximum of [Ru(dqpy)(acac)(CH3CN)]+ (4; acac- = acetylacetonate) is observed at 770 nm, attributed to the additional increase in energy of the HOMO afforded by the presence of the π-donating acac- ligand and supported by calculations. Complexes 1-3 undergo ligand substitution upon irradiation with red light, λ irr ≥ 610 nm, and the ligand substitution photochemistry of 4 is accessible with near-IR light, λ irr ≥ 715 nm and λ irr = 735 nm. Complexes 1-4 exhibit similar quantum yields of ligand exchange, Φ L, with 450 and 600 nm irradiation, however, that of 4 is 2-3 times greater than those measured for 1-3. This enhancement is explained by the difference in ligand contributions to the HOMO. Density functional theory calculations predict partial dqpy ππ* character in the MLCT states of 1-3 and a mixed Ru/acac- → dqpy metal/ligand-to-ligand charge transfer (ML-LCT) state in 4. The photoreactivity of 1-4 with tissue-penetrating red and near-IR light, together with their exceptional dark stability (>48 h), makes the new Ru(ii)-dqpy platform ideal for the development of new complexes for photoinduced drug release and for other applications that require broad absorption from the ultraviolet and visible ranges into the near-IR, such as solar energy conversion.