MLCT Band Research Articles

Chemical and photophysical properties of amine functionalized bis‐NHC‐pyridine‐RuII complexes

AbstractThe effects of backbone amine functionalization in three new homoleptic C^N^C type ruthenium(II) complexes bearing a tridentate bis‐imidazole‐2‐ylidene pyridine ligand framework are characterized and studied by single crystal diffraction, electrochemistry, optical spectroscopy and transient absorption spectroscopy in combination with ab initio DFT calculations. Functionalization by dimethylamine groups in 4‐position of the pyridine backbone significantly influences the properties of the complexes as revealed by comparison with the unfunctionalized references. As a result of the amine functionalization, a higher molar absorption coefficient of the MLCT bands, a decreased photoluminescence quantum yield at room temperature together with a shortened excited state lifetime but an improved photostability is observed. Introduction of electron donating and withdrawing groups at the NHC unit modifies the electronic and optical properties, such as the oxidation potential, absorption and emission properties, and the lifetimes of the excited states.

Kinetics and mechanism of the oxidation of hydrazine with [RuIII(pic)3] in aqueous solution

The kinetics and mechanism of the oxidation of hydrazine by [RuIII(pic)3] (pic- = picolinate anion) were studied spectrophotometrically as a function of hydrazine concentrations at a constant pH of 5.5. The rate of the reaction was followed by monitoring the appearance of the MLCT band of [RuII(pic)3]- at 466 nm. The reaction rate was found to be first-order for Ru(III)-complex concentration. The observed rate constant (k obs) values increased linearly with increase in the hydrazine concentration. The oxidation reaction was studied at varied temperature (20–40 °C). The ΔH≠ (73 ± 3 kJ mol−1 kJ·mol−1) and the negative ΔS≠ (34 ± 7 J·deg−1mol−1) values are consistent with the proposed outer-sphere electron transfer process operative in the oxidation of hydrazine with [RuIII(pic)3]. A working mechanism involving the formation of an active diazene (HN = NH) intermediate followed by its rapid two-electron oxidation by [RuIII(pic)3] in a kinetically inconsequential step producing N2 as sole product is proposed for an overall four-electron oxidation of hydrazine to dinitrogen.

Benzotriazole/indazole-appended phenanthroline-based NNN-tridentate ligand-supported Ru(II) complexes and their application in catalytic transfer hydrogenation

Two discrete Ru(II) complexes [Ru(L1)(bpy)Cl]Cl (Ru-1) and [Ru(L2)(bpy)Cl]Cl (Ru-2) bearing benzotriazole/indazole-appended phenanthroline-based NNN-tridentate ligands L1 and L2 respectively, with one bipyridine and one chloride as co-ligands, were synthesized and characterized fully via spectroscopic and crystallographic techniques. The crystal structures of the complexes revealed the discrete monomeric structures. Both the complexes exhibited MLCT bands in the range of 400–500 nm. The RuII/III redox potential of Ru-1 was found to be more positive (by 155 mV) than that of Ru-2, indicating the stronger π-acceptor property of the ligand L1 than L2. Both the complexes have been found to be efficient precatalysts in transfer hydrogenation of aldehydes and ketones.

Photophysical, electrochemical and electrochromic behavior of transition metal complexes with 1,10-phenanthroline ligands

A series of non-noble transition metal complexes were synthesized by coordinating Fe2+, Co2+, and Ni2+ with ligands having a parent structure of 4,7-diphenyl-1,10-phenanthroline (L1 and L2) and dipyrido[3,2-a:2′,3′-c]phenazine (L3–L5), and their photophysical, electrochemical, and electrochromic properties were studied. Both transition metal ions and ligands produce more or less influence on these properties. Electrochromic properties such as color, optical contrast, response time, coloration efficiency and switching stabilities can be tuned by selecting proper metal ions, parent structure of ligands, and substituent groups. Theory calculation results indicate that electrons flow from the metallic center to the ligands. The UV–vis spectra of all five Fe(II) complexes showed a broad and strong MLCT band at ca. 530 nm. The UV–vis spectra of Fe(II), Co(II) and Ni(II) complexes with L2 ligand showed a strong LC (π–π*) band at ca. 325 nm. Electrochromic process caused a change in the intensity of LC and MLCT absorption. The ligands produced little effect on the bleached and colored states of Fe(II) complexes, all of them showed a bleached state of red and a colored state of purple-red probably due to the reversible Fe(II)/Fe(III) transition. Both Co(II) and Ni(II) complexes showed a bleached state of pale yellow, however, the ligands produced significant effect on their colored states probably due to the partially electron density displacement between the metal ions and the ligands. When the parent structure of ligand is 4,7-diphenyl-1,10-phenanthroline, electron-donating substituent such as –OCH3 is beneficial to electrochromic properties, and Fe(II) complexes have higher optical contrast and coloration efficiency compared to Co(II) and Ni(II) complexes. When the parent structure of ligand is dipyrido[3,2-a:2′,3′-c]phenazine, –CN substituent markedly enhances the optical contrast and coloration efficiency of Fe(II), Co(II) and Ni(II) complexes.

Iron(III)-Complexes with N-Phenylpyrazole-Based Ligands

The use of iron as a replacement for noble metals in photochemical and photophysical applications is challenging due to the typically fast deactivation of short-lived catalytically active states. Recent success of a cyclometalated iron(III) complex utilizing a bis-tridentate ligand motif inspired the use of phenyl-1H-pyrazole as a bidentate ligand. Five complexes using the tris(1-phenylpyrazolato-N,C2)iron(III) complex scaffold are presented. In addition to the parent complex, four derivatives with functionalization in the meta-position of the phenyl ring are thoroughly investigated by single crystal diffractometry, UV-Vis-spectroscopy, and cyclic voltammetry. Advanced X-ray spectroscopy in the form of X-ray absorption and emission spectroscopy allows unique insights into the electronic structure as well as DFT calculations. The ligand design leads to overlapping MLCT and LMCT absorption bands, and emissive behavior is suppressed by low-lying MC states.

Rational Design of Iron Spin-Crossover Complexes Using Heteroscorpionate Chelates.

The optical, structural, and magnetic properties of iron(II,III) sandwich complexes, Fe(Tp')2n+ (Tp' = bis(3,5-dimethylpyrazolyl)benzotriazolylborate), are described. The intensely colored FeII(Tp')2 (orange) and FeIII(Tp')2+ (purple) show strong MLCT bands. Geometric isomerism for M(Tp')2 is established crystallographically in the racemate of chiral cis-Fe(Tp')2. For the first time, paramagnetic 11B NMR describes solution-phase low-spin (LS, S = 0) to high-spin (HS, S = 2) crossover behavior in Fe(Tp')2. Thermochemical parameters for solution-phase SCO of Fe(Tp')2 demonstrate the endothermic LS to HS conversion and entropic preference of the HS state. Entropy changes for both Fe(Tp')2 isomers are significantly larger than for the majority of iron scorpionate SCO systems. Solid-state magnetic and thermochemical measurements show cis-Fe(Tp')2 to be thermally stable up to 520 K, allowing experimental investigation of a solid-state SCO magnetic hysteresis of over 45 K. A large solution vs solid-state SCO difference was observed: cis-Fe(Tp')2 shows Tc ≈ 270 K (solution) and Tc ≈ 385 K (solid), with the remarkably wide ΔTc ≈ 115 K; trans-Fe(Tp')2 shows Tc ≈ 278 K (solution) and Tc ≈ 372 K (solid). Solid-state Tc values are among the highest seen for iron(II) molecular systems. The large solution/solid ΔTc difference is explained by "anchoring" intermolecular interactions in the solid state that prevent thermal expansion of the LS iron(II) coordination sphere in its transition to the HS state. DFT calculations, validated against LS cis-Fe(Tp')2 crystallography and LS to HS SCO thermochemical parameters, demonstrate the role the benzotriazole rings play in its structural and optical properties. The Lewis basicity of M(Tp')2 is shown with the structural characterization of the air-stable tin(II) adduct [cis-Fe(Tp')2-SnCl2]; tin(II) coordination does not alter the iron(II) spin state. The Tp' chelate adds functionality (asymmetry, chirality, chemical reactivity) to the array of iron SCO materials for potential incorporation into nanoscale magnetic switches and spintronic devices.

A potential novel and general route for bromide replacement in diimine ReI tricarbonyl complexes leading to carboxylates: The effect in luminescence

The reaction of the 3,5-dimethylpyrazolyl-pyridazine-6-carboxylic acid (Me2LCOOH) or pyrazolyl-pyridazine-6-carboxylic acid (LCOOH) with [(THF)(CO)3Re(µ-Br)2Re(CO)3(THF)] in DMF followed by the addition of traces of water leads to a series of dehalided rhenium(I) tricarbonyl carboxylates: [(Me2LCOO)(H2O)Re(CO)3] and [(LCOO)(H2O)Re(CO)3] instead of the normal rhenium(I) tricarbonyl bromide obtained by reaction in chloroform, [(Me2LCOOH)Re(CO)3Br]. The reaction is believed to occur through adventitious amine in the reaction media which leads to the carboxylic acid deprotonation and reaction. To test this idea the diimine-rhenium(I) tricarbonyl bromide [(phen)Re(CO)3Br] and [(bppdz)Re(CO)3Br] (phen: 1,10-phenanthroline; bppdz: 3,6-bis(1H-pyrazol-1-yl)pyridazine) were reacted with glacial acetic acid and triethylamine in dry DMF at room temperature. These reaction lead to the corresponding rhenium(I) acetates [(phen)Re(CO)3(OOCCH3)] and [(bppdz)Re(CO)3(OOCCH3)] in good to high yields using soft reaction conditions and without the use of a silver salt. Thiethylammonium bromide is also obtained from the reaction. Moreover, the fact that this unexpected rhenium(I) tricarbonyl carboxylates were obtained in as N,N and N,O coordination isomers for [(Me2LCOO)(H2O)Re(CO)3] and the comparison of the newly synthetized [(phen)Re(CO)3(OOCCH3)] and [(bppdz)Re(CO)3(OOCCH3)] compounds with their corresponding bromides allow to stablish that the replacement of the halide by a carboxylate hypsochromically shift the energy of the MLCT band while the emission experiences the opposite, it is bathochromically shifted. The emission quantum yields are slightly diminished. Interestingly the [(phen)Re(CO)3(OOCCH3)] and [(bppdz)Re(CO)3(OOCCH3)] acetates are dual emitters.

Probing the strong electron-acceptor property of pyridine-appended 1,2,3-benzotriazole ligand in tris-homoleptic Ru(II) and Fe(II) complexes

In order to probe the σ-donor/π-acceptor properties of the fused triazolo-based pyridine-appended bidentate ligand, 1-(pyridin-2-yl)-1H-benzo[d][1–3]triazole (BTZ-2P), two discrete octahedral tris-homoleptic complexes, [Ru(BTZ-2P)3](PF6)2 (1) and [Fe(BTZ-2P)3](ClO4)2 (2) were synthesized and well characterized for the first time. Crystal structures of the complexes revealed the key geometrical parameters which highlighted the metal-ligand bonding characteristics in the complexes, especially the strong π-acceptor property of the ligand. The bonding with the BTZ-2P ligand resulted in simultaneous stabilization of the HOMO and destabilization of the LUMO of both the metal complexes, leading to anodic shift of the MII/III redox potential and blue shift of the MLCT band in absorption spectroscopy in comparison to their analogous Ru(II) and Fe(II) tris-bipyridine complexes. Experimental results and theoretical calculations confirmed that in addition to being a good σ-donor, the BTZ-2P ligand exhibits strong π-acceptor property as well compared to the classic 2,2′-bipyridine ligand. This disclosure can have future implications in stabilizing low-valent transition metal complexes with the BTZ-2P ligand.

Heteroleptic Copper(I) complexes of bipyridine glycoluril and phosphine ligands: Photophysical and computational studies

A series of heteroleptic copper(I) complexes of the type [Cu(bpg)(PP)]PF6 (1–4) have been synthesized where bpg = [4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo[4,5-f] [1,10]-phenanthroline-6,13-dione] and PP are phosphine derived ancillary ligands (triphenyl phosphine (PPh3), 1,2 bis(diphenylphosphino)ethane (dppe), bis[(2-diphenylphosphino)phenyl]ether (POP) and 1,3 bis(diphenylphosphino)propane (dppp)). All copper(I)-phosphine complexes were thoroughly characterized by elemental analyses, IR, 1H NMR, 13C NMR, 31P NMR and ESI-MS, UV–visible, emission spectroscopy and DFT calculations. The crystal structure of 1 contains two Cu(I) molecules in asymmetric unit which localizes in distorted tetrahedral geometry with N2P2 coordination core. The UV–visible spectra indicated that ancillary phosphine ligands significantly affect the MLCT band of the copper(I) complexes. The bands in the UV–visible spectra fit well with the TD-DFT calculations and are assigned to the contribution of HOMO-2 → LUMO, HOMO-1 → LUMO and HOMO → LUMO orbitals. These Cu(I) complexes exhibit green emission in CH2Cl2 at room temperature with emission wavelengths at 545–572 nm. Cyclic voltammetric data revealed that steric hindrance of diphosphine ligands affected the oxidation potential of Cu(I) complexes.

Spectral and Electrochemical Sensing Studies of Novel Tetradentate Schiff bases, having Enhanced Antifungal activity, containing Ferrocene and Aromatic moieties attached Imines

Multi metal ion sensing unsymmetrical Schiff bases containing ferrocene group attached imine at one side and aromatic moiety connected azomethine on the other side have been synthesized. Titration studies coupled with UV-Visible spectrophotometer exposes the binding aptitude of new receptors. Development of MLCT band near 457 nm for the coordination of Cu2+ ions with new ligands is also noticed. Electrochemical studies of receptor solutions with added metal ions expose quasi reversible process by giving superfluous ΔEP values (146-161 mV, than the expected 59 mV). Concentration of metal ions required for effective sensing is calculated from the percentage amount of ΔIpa extracted from the Ipa data. Results obtained in computational molecular docking studies and in- vitro analysis invite more focused research by pharmacist to develop new formulations for antifungal medicines.

Effect of Alkyl Chain Length on the Photophysical, Photochemical, and Photobiological Properties of Ruthenium(II) Polypyridyl Complexes for Their Application as DNA-Targeting, Cellular-Imaging, and Light-Activated Therapeutic Agents.

A family of six Ru(II) polypyridyl complexes (1-6) which contain phenanthroline-based ligands functionalized with alkyl chains of different lengths (one methyl group, 10 and 21 carbon alkyl chains) and either 1,10-phenanthroline (phen) or 1,4,5,8-tetraazaphenanthrene (TAP) as ancillary ligands have been synthesized and characterized. The influence of the alkyl chain length on their photophysical and photochemical properties as well as in their photobiological applications has been elucidated by monitoring the changes in their MLCT-centered absorption and emission bands. The presence of one methyl group or 10 carbon alkyl chains does not seem to significantly affect the photophysical and photochemical properties of the resulting Ru(II) complexes when compared to the well-known [Ru(phen)3]2+ and [Ru(TAP)2phen]2+. However, an effect on their emission properties and in their ability to photosensitize singlet oxygen is observed for the Ru(II) complexes containing 21 carbon alkyl chains. The binding of these complexes to salmon testes DNA (stDNA) was investigated by observing the changes in the photophysical properties. Complexes 1, 2, 4, and 5 all showed changes in their MLCT bands that could be analyzed using conventional fitting methods, such as the Bard equation. In contrast, complexes 3 and 6, possessing long aliphatic chains, gave rise to nonclassic behavior. In addition to these analyses, both thermal denaturation and circular dichroism studies of 1-6 were carried out in the presence of stDNA which confirmed that these complexes bind to DNA. Confocal microscopy and viability studies in HeLa cervical cancer cells reveal an alkyl chain-length dependence on the cellular uptake and cytotoxicity of the resulting Ru(II) complexes due to an enhancement of their lipophilicity with increasing alkyl chain length. Thus, complexes containing 10 and 21 carbon alkyl chains are rapidly taken up into HeLa cells and, in particular, those with 21 carbon alkyl chains show a significant phototoxicity against the same cell line. Therefore, this study provides further insight into the possible modulation of the photophysical, photochemical, and photobiological properties of Ru(II) polypyridyl complexes by varying the length of the alkyl chains attached to the polypyridyl ligands coordinated to the Ru(II) center and the nature of the auxiliary groups, which we show has a significant effect on photophysical and biological properties.

The case of [(bpm)Re(CO)3Br] and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] (bpm: 2,2′-bipyrimidine) towards multi-metallic ReI species and their properties

The reaction of [Re(CO)3(THF)Br]2 and 2,2′-bipyrimidine (bpm) in dichloromethane at room temperature employing a stoichiometric 2:1 ratio leads to a mixture of [(bpm)Re(CO)3Br] (21%) and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] (79%). When a 1:1 ratio is used, only anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] is obtained in a 89% yield. The reaction of [Re(CO)5Br] and (bpm) leads to [(bpm)Re(CO)3Br] as a single product. [(bpm)Re(CO)3Br] and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] display one and two rhenium(I) centers of distorted octahedral coordination geometry respectively, each with a facial ligand correlation. The intramolecular metal to metal distance is 5.8508(16) Å in anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. The HOMO-LUMO gap for [(bpm)Re(CO)3Br] results 0.74 eV larger (3.03 vs 2.29 eV) than the computed for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. In the same sense, the anti-isomer is computed to be 0.14 eV (13.4 kJ mol−1) more stable compared to the hypothetical syn-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. Cyclic voltammetry shows that the difference between the ReI/Re0 reduction and its ReI/ReII oxidation in terms of potential is about 3.46 V for [(bpm)Re(CO)3Br], while this value diminishes to 2.62 V for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. Following this tendency, the MLCT band for [(bpm)Re(CO)3Br] is centered at 385 nm (DMF), while it is red-shifted almost 100 nm for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. While [(bpm)Re(CO)3Br] emits at 680 nm (excited at 380 nm), anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] remains non-emissive, which is attributed to a favored vibrational deactivation path.

6,6'-Ditriphenylamine-2,2'-bipyridine: Coordination Chemistry and Electrochemical and Photophysical Properties.

A 2,2'-bipyridine with bulky triphenylamine substituents in the 6 and 6' positions of the ligand (6,6'-ditriphenylamine-2,2'-bipyridine, 6,6'-diTPAbpy) was generated. Despite the steric bulk, the ligand readily formed bis(homoleptic) complexes with copper(I) and silver(I) ions. Unfortunately, efforts to use the 6,6'-diTPAbpy system to generate heteroleptic [Cu(6,6'-diTPAbpy)(bpy)]+ complexes were unsuccessful with only the [Cu(6,6'-diTPAbpy)2](PF6) complex observed. The 6,6'-diTPAbpy ligand could also be reacted with 6-coordinate metal ions that featured small ancillary ligands, namely, the [Re(CO)3Cl] and [Ru(CO)2Cl2] fragments. While the complexes could be formed in good yields, the steric bulk of the TPA units does alter the coordination geometry. This is most readily seen in the [(6,6'-diTPAbpy)Re(CO)3Cl] complex where the Re(I) ion is forced to sit 23° out of the plane formed by the bpy unit. The electrochemical and photophysical properties of the family of compounds were also examined. 6,6'-diTPAbpy exhibits a strong ILCT absorption band (356 nm, 50 mM-1 cm-1) which displays a small increase in intensity for the homoleptic complexes ([Cu(6,6'-diTPAbpy)2]+; 353 nm, 72 mM-1 cm-1, [Ag(6,6'-diTPAbpy)2]+; 353 nm, 75 mM-1 cm-1), despite containing 2 equiv of the ligand, attributed to an increased dihedral angle between the TPA and bpy moieties. For the 6-coordinate complexes the ILCT band is further decreased in intensity and overlaps with MLCT bands, consistent with a further increased TPA-bpy dihedral angle. Emission from the 1ILCT state is observed at 436 nm (τ = 4.4 ns) for 6,6'-diTPAbpy and does not shift for the Cu, Ag, and Re complexes, although an additional 3MLCT emission is observed for [Re(6,6'-diTPAbpy)(CO)3Cl] (640 nm, τ = 13.8 ns). No emission was observed for [Ru(6,6'-diTPAbpy)(CO)2Cl2]. Transient absorption measurements revealed the population of a 3ILCT state for the Cu and Ag complexes (τ = 80 ns). All assignments were supported by TD-DFT calculations and resonance Raman spectroscopic measurements.

Probing the Mechanism for 2,4'-Dihydroxyacetophenone Dioxygenase Using Biomimetic Iron Complexes.

In this study, we report the synthesis and characterization of [Fe(T1Et4iPrIP)(2-OH-AP)(OTf)](OTf) (2), [Fe(T1Et4iPrIP)(2-O-AP)](OTf) (3), and [Fe(T1Et4iPrIP)(DMF)3](OTf)3 (4) (T1Et4iPrIP = tris(1-ethyl-4-isopropyl-imidazolyl)phosphine; 2-OH-AP = 2-hydroxyacetophenone, and 2-O-AP- = monodeprotonated 2-hydroxyacetophenone). Both 2 and 3 serve as model complexes for the enzyme-substrate adduct for the nonheme enzyme 2,4'-dihydroacetophenone (DHAP) dioxygenase or DAD, while 4 serves as a model for the ferric form of DAD. Complexes 2-4 have been characterized by X-ray crystallography which reveals T1Et4iPrIP to bind iron in a tridentate fashion. Complex 2 additionally contains a bidentate 2-OH-AP ligand and a monodentate triflate ligand yielding distorted octahedral geometry, while 3 possesses a bidentate 2-O-AP- ligand and exhibits distorted trigonal bipyramidal geometry (τ = 0.56). Complex 4 displays distorted octahedral geometry with 3 DMF ligands completing the ligand set. The UV-vis spectrum of 2 matches more closely to the DAD-substrate spectrum than 3, and therefore, it is believed that the substrate for DAD is bound in the protonated form. TD-DFT studies indicate that visible absorption bands for 2 and 3 are due to MLCT bands. Complexes 2 and 3 are capable of oxidizing the coordinated substrate mimics in a stoichiometric and catalytic fashion in the presence of O2. Complex 4 does not convert 2-OH-AP to products under the same catalytic conditions; however, it becomes anaerobically reduced in the presence of 2 equiv 2-OH-AP to 2.

Delocalization tunable by ligand substitution in [L2Al] n- complexes highlights a mechanism for strong electronic coupling.

Ligand-based mixed valent (MV) complexes of Al(iii) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I2P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I2P−)(I2P2−)Al and [(I2P2−)(I2P3−)Al]2− charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I2P−)(I2P2−)Al and [(I2P2−)(I2P3−)Al]2− show intervalence charge transfer (IVCT) transitions in the regions 6850–7740 and 7410–9780 cm−1, respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I2P2−)(I2P3−)Al]2− complexes containing –PhNMe2 or –PhF5 substituents. Minor localization of charge in [(I2P2−)(I2P3−)Al]2− was observed when –PhOMe substituents are included.

Photocatalytic Water-Soluble Cationic Platinum(II) Complexes Bearing Quinolinate and Phosphine Ligands.

Cationic Pt(II) complexes ([Pt(QO/S)(P∧P)]X), having 8-oxy or 8-thioquinolinate (QO/S) and seven different mono- or bidentate phosphines as ligands, have been synthesized and characterized. The photophysical, stability, and photocatalytic properties of those complexes were studied and compared to that of the parent [Pt(QO/S)(dmso)(Cl)]. The coordination of phosphines induced a red-shift in the absorption energy of the MLCT band, whereas the emission wavelength of the complexes only depended on the nature of the quinolinate ligand. Moreover, the photocatalytic activity of the Pt(II) complexes was evaluated in the oxidation of sulfides using atmospheric oxygen as an oxidant. All the complexes were active photocatalysts for that transformation, with [Pt(QO)(BINAP)]Cl and [Pt(QO)(SEGPHOS)]Cl (BINAP: 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl, SEGPHOS: (4,4'-bibenzodioxole)-5,5'-diyldiphosphine) exhibiting high catalytic performance and stability. In addition, the enhanced water solubility of the complexes allowed performance of the photooxidation reaction under environmentally friendly conditions. In particular, the catalyst [Pt(QS)(dppe)]Cl, bearing 8-thioquinolinate and diphenylphosphinoethate (dppe) as ligands, successfully catalyzed the oxidation of a variety of sulfides using water as a solvent.

1,3,4-Oxadiazole based ruthenium amphiphile for Langmuir-Blodgett films and photo-responsive logic gate construction

Amphiphilic ruthenium bipyridine complexes based on 1,3,4-oxadiazole-pyridine ligands (L1, L2) were synthesized and characterized by 1H NMR, UV–Vis spectroscopy, electrochemistry and UV–Vis spectroelectrochemistry. The free ligands and the respective ruthenium(II) complexes exhibit an intraligand π−π∗ absorption band in the UV region that overlaps with the MLCT bands. The [Ru(bpy)2(L2)](PF6)2 complex with a bidentated L2 ligand did not form a stable monolayer in water-air interface demonstrating the relevance of coordination geometry on the molecular packing properties. In contrast, stable Langmuir-Blodgett films of the [Ru(bpy)2(L1)Cl]PF6 complex (RuL1) exhibiting reversible and fast spectroelectrochemical properties associated with the Ru(III/II) redox couple were obtained, where redox site inactivation become relevant above 20 monolayers where the current leveled up. Thin LB films were deposited on poly(bithiophene) to enhance its photoelectrochemical properties and realize photoresponsive AND and OR logic gate devices.

Spectroscopic characterization of valence tautomeric behavior in a cobalt-dioxolene complex using an ancillary ligand containing quinoline groups

Co-dioxolene complex [Co(DBCat)(pbqa)](PF6) (DBCat = 3,5-di-tert-butylcatechol, pbqa = (2-pyridylmethyl)bis(2-quinolylmethyl)amine), was prepared from methanol solution. Single crystal X-ray structural analysis revealed that the valence of the cobalt ion to be +III, as DBCat is formally a catecholate dianion, whereas the acetonitrile solution exhibits an MLCT absorption near 575 nm, suggesting it is present as hs-CoII(SQ). Of note, this MLCT band disappears with decreasing temperature. This behavior suggests that [Co(DBCat)(pbqa)](PF6) undergoes a temperature-dependent change from hs-CoII(SQ) to ls-CoIII(Cat). Magnetic susceptibility vs. temperature data for the compound displayed a gradual variation over 250 K, due to intramolecular electron transfer in the solid-state as well. [Co(DBCat)(pbqa)](PF6) demonstrates valence tautomerism due to a strong π-π interaction between the quinoline rings of the ancillary ligand. Comparison of this pbqa compound to similar Co compounds provides further evidence for these conclusions.

Low-Temperature Spectra and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with Cyclometalated Ancillary Ligands: The Excited State Spin-Orbit Coupling Origin of Variations in Emission Efficiencies.

The 77 K emission spectra of cyclometalated ruthenium(II)-2,2'-bipyridine (CM-Ru-bpy) chromophores are very similar to those of related Ru-bpy complexes with am(m)ine or diimmine ancillary ligands, and density functional theory (DFT) modeling confirms that the lowest energy triplet metal to ligand charge transfer (3MLCT) excited states of CM-Ru-bpy and related Ru-bpy complexes have very similar electronic configurations. However, the phosphorescence decay efficiencies of CM-Ru-bpy excited states are about twice those of the conventional Ru-bpy analogues. In contrast to the similar 3MLCT excited state electronic configurations of the two classes of complexes, the CM-Ru-bpy chromophores have much broader visible region MLCT absorptions resulting from several overlapping transitions, even at 87 K. The emitting excited-state emission efficiencies depend on spin-orbit coupling (SOC) mediated intensity stealing from singlet excited states, and this work explores the relationship between the phosphorescence efficiency and visible region absorption spectra of Ru-bpy 3MLCT excited states in the weak SOC limit. The intrinsic 3MLCT emission efficiency, ιem, depends on mixing with singlet excited states whose RuIII-dπ-orbital angular momenta differ from that of the emitting state. DFT modeling of the 1MLCT excited-state electronic configurations that contribute significantly to the lowest energy absorption bands have RuIII-dπ orbitals that differ from those of their emitting 3MLCT excited states. This leads to a very close relationship between ιem and the lowest energy MLCT band absorptivities in Ru-bpy chromophores. Thus, the larger number of 1MLCT transitions that contribute to the lowest energy absorption bands accounts for the enhanced phosphorescence efficiency of Ru-bpy complexes with cyclometalated ancillary ligands.

Syntheses, structure and properties of dinuclear Co complexes with bis(catecholate) ligands – Effect of a quinoline ring in the terminal group

Two types of biscatechol, namely H4L1 (5,5′-(buta-1,3-diyne-1,4-diyl)bis(3-t-butylcatechol)) and H4L2 (5,5′-(ethyne-1,2-diyl)bis(3-t-butylcatechol)) were synthesized. In these ligands, two redox active catechol moieties are connected by one or two triple bonds. Also, tpa (tris(2-pyridylmethyl) amine), bpqa (bis(2-pyridylmethyl)(2-quinolylmethyl)amine) and pbqa ((2-pyridylmethyl)bis(2-quinolylmethyl)amine) were synthesized as terminal ligands of the tetracoordinated tripod type. In total, six dinuclear Co complexes were synthesized from these biscatechol and terminal ligands as follows: [Co2(L1)(tpa)2](BF4)2 (1), [Co2(L1)(bpqa)2](PF6)2 (2), [Co2(L1)(pbqa)2](PF6)2 (3), [Co2(L2)(tpa)2](BF4)2 (4), [Co2(L2)(bpqa)2](PF6)2 (5), [Co2(L2)(pbqa)2](PF6)2 (6). Of the six dinuclear Co complexes, complex 6, which was isolated as a mixed valent state CoII(HS)-[SQ-Cat]-CoIII(LS) compound, showed an absorption intensity at around 703 nm (MLCT bands) that increased with increasing temperature in acetonitrile solution. In addition, an investigation of the magnetic properties of the complex 6 with SQUID showed that the χMT value gradually increased as the temperature increased from 150 to 380 K. This suggests that a transition from CoIII(LS) (S = 0) to CoII(HS) (S = 3/2) accompanies the temperature rise. This means the steric hindrance and electronic effect of the quinolyl groups around the Co ion produce a coordination atmosphere weaker than that of pyridyl groups, with the result that the CoIII ions easily convert to CoII ions.