Photochemical and Thermal Isomerization of a Ruthenium(II)−Alloxazine Complex Involving an Unusual Coordination Mode

Binding of Stimuli-Responsive Ruthenium Aqua Complexes with 9-Ethylguanine

Stimuli-responsive ruthenium complexes proximal- and distal-[Ru(C10tpy)(C10pyqu) OH2]2+ (proximal-1 and distal-1; C10tpy = 4'-decyloxy-2,2':6',2″-terpyridine and C10pyqu = 2-[2'-(6'-decyloxy)-pyridyl]quinoline) were experimentally studied for adduct formation with a model DNA base. At 303 K, proximal-1 exhibited 1:1 adduct formation with 9-ethylguanine (9-EtG) to yield proximal-[Ru(C10tpy)(C10pyqu)(9-EtG)]2+ (proximal-RuEtG). Rotation of the guanine ligand on the ruthenium center was sterically hindered by the presence of an adjacent quinoline moiety at 303 K. Results from 1H NMR measurements indicated that photoirradiation of a proximal-RuEtG solution caused photoisomerization to distal-RuEtG, whereas heating of proximal-RuEtG caused ligand substitution to proximal-1. The distal isomer of the aqua complex, distal-1, was observed to slowly revert to proximal-1 at 303 K. In the presence of 9-EtG, distal-1 underwent thermal back-isomerization to proximal-1 and adduct formation to distal-RuEtG. Kinetic analysis of 1H NMR measurements showed that adduct formation between proximal-1 and 9-EtG was 8-fold faster than that between distal-1 and 9-EtG. This difference may be attributed to intramolecular hydrogen bonding and steric repulsion between the aqua ligand and the pendant moiety of the bidentate ligand..

Tetramethylalloxazines as efficient singlet oxygen photosensitizers and potential redox-sensitive agents

Tetramethylalloxazines (TMeAll) have been found to have a high quantum yield of singlet oxygen generation when used as photosensitizers. Their electronic structure and transition energies (S0 → Si, S0 → Ti, T1 → Ti) were calculated using DFT and TD-DFT methods and compared to experimental absorption spectra. Generally, TMeAll display an energy diagram similar to other derivatives belonging to the alloxazine class of compounds, namely π,π* transitions are accompanied by closely located n,π* transitions. Photophysical data such as quantum yields of fluorescence, fluorescence lifetimes, and nonradiative rate constants were also studied in methanol (MeOH), acetonitrile (ACN), and 1,2-dichloroethane (DCE). The transient absorption spectra were also analyzed. To assess cytotoxicity of new compounds, a hemolytic assay was performed using human red blood cells (RBC) in vitro. Subsequently, fluorescence lifetime imaging experiments (FLIM) were performed on RBC under physiological and oxidative stress conditions alone or in the presence of TMeAll allowing for pinpointing changes caused by those compounds on the intracellular environment of these cells.

Formation of the silver-flavin coordination polymers and their morphological studies

This Communication describes the formation of 1D-coordination polymeric motifs involving modified flavin analog connected together through intervening silver ions. Rare bidentate coordination mode for model flavin was achieved with silver...

A Visible-Light and Temperature Responsive Host-Guest System: Photoisomerization of a Ruthenium Complex and Inclusion Complex Formation with Cyclodextrins

In the present study, we investigated the visible-light- and thermal-stimuli-responsive properties of a host–guest system based on proximal- and distal-[Ru(C10tpy)(C10pyqu)OH2]2+ (proximal and distal-1, C10tpy = 4’-decyloxy-2,2’;6’,2”-terpyridine, C10pyqu = 2-[2’-(6’-decyloxy)-pyridyl]quinoline). The...

Photoisomerization and thermal isomerization of ruthenium aqua complexes with chloro-substituted asymmetric bidentate ligands

Introduction of a chloro substituent to the bidentate ligand of ruthenium aqua complexes enhanced photoisomerization and thermal back-isomerization.

Synthesis and photophysical properties of ruthenium(ii) polyimine complexes decorated with flavin

Phosphorescent emission from a flavin localized triplet excited state (³IL) is observed for the first time in a flavin decorated tris(dipyridine) Ru(ii) complex with strong visible light absorption.

Photoisomerization of ruthenium(ii) aquo complexes: mechanistic insights and application development

The perspective article highlights a new strategic synthesis of dinuclear ruthenium(ii) complexes acting as active water oxidation catalysts and also reports the development of unique visible-light-responsive giant vesicles, both of which are achieved based on photoisomerization.

Excited-State Proton Transfer Controls Irreversibility of Photoisomerization in Mononuclear Ruthenium(II) Monoaquo Complexes: A DFT Study

The detailed DFT investigation clears the working mechanism of the irreversible photoisomerization of trans-[Ru(tpy)(pynp)(OH2)](2+) (TA) and cis-[Ru(tpy)(pynp)(OH2)](2+) (CA) complexes. Both TA and CA complexes present two types of low lying triplet states, one resulting from a triplet metal-ligand charge-transfer (TMLCT) and the other from a triplet metal-centered d-d transition (TMC). The vertical excitation of the singlet ground state of the complexes leads to a singlet excited state, which undergoes ultrafast decay to the corresponding TMLCT. For TA, this TMLCT transforms with a low barrier to a TMC state. The dissociative nature of the TMC state leads to easy water removal to produce a five-coordinate intermediate that can isomerize via rotation of a pynp ligand and proceed towards the CA product. For CA, however, during this excitation and intersystem crossing process, an excited-state proton transfer (ESPT) occurs and the resultant TMLCT is very much stabilized with a very strong Ru(II)-OH bond; the high barrier from this TMLCT blocks conversion to a TMC state and thus prevents isomerization from the cis to the trans isomer. This high barrier also prevents the possibility of the isomerization process from TA to CA solely on the adiabatic triplet pathway. Instead, crossing points (XMC-CB, XMC-CA) near the minimum of the triplet metal-centered state of the cis isomer provide nonadiabatic decay channels to the ground-state S0--CA, which completes the photoisomerization pathway from TA to CA.

New Series of Dinuclear Ruthenium(II) Complexes Synthesized Using Photoisomerization for Efficient Water Oxidation Catalysis

A new series of proximal,proximal-[Ru2(tpy)2(L)XY](n+) (p,p-Ru2XY, tpy = 2,2':6',2″-terpyridine, L = 5-phenyl-2,8-di(2-pyridyl)-1,9,10-anthyridine, X and Y = other coordination sites) were synthesized using photoisomerization of a mononuclear complex. The p,p-Ru2XY complexes undergo unusual reversible bridge-exchange reactions to generate p,p-Ru2(μ-Cl), p,p-Ru2(μ-OH), and p,p-Ru2(OH)(OH2) with μ-Cl, μ-OH, as well as hydroxo and aquo ligands at X and Y sites of p,p-Ru2XY, respectively. The geometric and electronic structures of these complexes were characterized based on UV-vis and (1)H NMR spectra, X-ray crystallography, and density functional theory (DFT) calculations. (1)H NMR data showed C2 symmetry of p,p-Ru2(OH)(OH2) with the distorted L chelate and nonequivalence of two tpy ligands, in contrast to the C2v symmetry of p,p-Ru2(μ-Cl) and p,p-Ru2(μ-OH). However, irrespective of the lower symmetry, p,p-Ru2(OH)(OH2) is predominantly formed in neutral and weakly basic conditions due to the specially stabilized core structure by multiple hydrogen-bond interactions among aquo, hydroxo, and backbone L ligands. The electrochemical data suggested that p,p-Ru2(OH)(OH2) (Ru(II)-OH:Ru(II)-OH2) is oxidized to the Ru(III)-OH:Ru(III)-OH state at 0.64 V vs saturated calomel electrode (SCE) and further to Ru(IV)═O:Ru(IV)-OH at 0.79 V by successive 1-proton-coupled 2-electron processes at pH 7.0. The cyclic voltammogram data exhibited that the p,p-Ru2(OH)(OH2) complex works more efficiently for electrocatalytic water oxidation, compared with a similar mononuclear complex distal-[Ru(tpy)(L)OH2](2+) (d-RuOH2) and p,p-Ru2(μ-Cl) and p,p-Ru2(μ-OH), showing that the p,p-Ru2 core structure with aquo and hydroxo ligands is important for efficient electrocatalytic water oxidation. Bulk electrolysis of the p,p-Ru2(OH)(OH2) solution corroborated the electrocatalytic cycle involving the Ru(III)-OH:Ru(III)-OH state species as a resting state. The mechanistic insight into O-O bond formation for O2 production was provided by the isotope effect on electrocatalytic water oxidation by p,p-Ru2(OH)(OH2) and d-RuOH2 in H2O and D2O media.

NMR spectroscopy on flavins and flavoproteins

(1)H-, (11)B-, (13)C-, (15)N-, (17)O-, (19)F-, and (31)P-NMR chemical shifts of flavocoenzymes and derivatives of it, as well as of alloxazines and isoalloxazinium salts, from NMR experiments performed under various experimental conditions (e.g., dependence of the chemical shifts on temperature, concentration, solvent polarity, and pH) are reported. Also solid-state (13)C- and (15)N-NMR experiments are described revealing the anisotropic values of corresponding chemical shifts. These data, in combination with a number of coupling constants, led to a detailed description of the electronic structure of oxidized and reduced flavins. The data also demonstrate that the structure of oxidized flavin can assume a configuration deviating from coplanarity, depending on substitutions in the isoalloxazine ring, while that of reduced flavin exhibits several configurations, from almost planar to quite bended. The complexes formed between oxidized flavin and metal ions or organic molecules revealed three coordination sites with metal ions (depending on the chemical nature of the ion), and specific interactions between the pyrimidine moiety of flavin and organic molecules, mimicking specific interactions between apoflavoproteins and their coenzymes. Most NMR studies on flavoproteins were performed using (13)C- and (15)N-substituted coenzymes, either specifically enriched in the pterin moiety of flavin or uniformly labeled flavins. The chemical shifts of free flavins are used as a guide in the interpretation of the chemical shifts observed in flavoproteins. Although the hydrogen-bonding pattern in oxidized and reduced flavoproteins varies considerably, no correlation is obvious between these patterns and the corresponding redox potentials. In all reduced flavoproteins the N(1)H group of the flavocoenzyme is deprotonated, an exception is thioredoxin reductase. Three-dimensional structures of only a few flavoproteins, mostly belonging to the family of flavodoxins, have been solved. Also the kinetics of unfolding and refolding of flavodoxins has been investigated by NMR techniques. In addition, (31)P-NMR data of all so far studied flavoproteins and some (19)F-NMR spectra are discussed.

Complete photochromic structural changes in ruthenium(II)-diimine complexes, based on control of the excited states by metalation

The thermal and photochemical reactions of a newly synthesized complex, [Ru(II)(TPA)(tpphz)](2+) (1; TPA=tris(2-pyridylmethyl)amine, tpphz=tetrapyrido[3,2-a:2',3'-c:3'',2''-h: 2''',3'''-j]phenazine), and its derivatives have been investigated. Heating a solution of complex 1 (closed form) and its derivatives in MeCN caused the partial dissociation of one pyridylmethyl moiety of the TPA ligand and the resulting vacant site on the Ru(II) center was occupied by a molecule of MeCN from the solvent to give a dissociated complex, [Ru(II)(η(3)-TPA)(tpphz)(MeCN)](2+) (1', open form), and its derivatives, respectively, in quantitative yields. The thermal dissociation reactions were investigated on the basis of kinetics analysis, which indicated that the reactions proceeded through a seven-coordinate transition state. Although the backwards reaction was induced by photoirradiation of the MLCT absorption bands, the photoreaction of complex 1' reached a photostationary state between complexes 1 and 1' and, hence, the recovery of complex 1 from complex 1' was 67%. Upon protonation of complex 1 at the vacant site of the tpphz ligand, the efficiency of the photoinduced recovery of complex 1+H(+) from complex 1'+H(+) improved to 83%. In contrast, dinuclear μ-tpphz complexes 2 and 3, which contained the Ru(II)(TPA)(tpphz) unit and either a Ru(II)(bpy)2 or Pd(II)Cl2 moiety on the other coordination edge of the tpphz ligand, exhibited 100% photoconversion from their open forms into their closed forms (2'→2 and 3'→3). These results are the first examples of the complete photochromic structural change of a transition-metal complex, as represented by complete interconversion between its open and closed forms. Scrutinization by performing optical and electrochemical measurements allowed us to propose a rationale for how metal coordination at the vacant site of the tpphz ligand improves the efficiency of photoconversion from the open form into the closed form. It is essential to lower the energy level of the triplet metal-to-ligand charge-transfer excited state ((3)MLCT*) of the closed form relative to that of the triplet metal-centered excited state ((3)MC*) by metal coordination. This energy-level manipulation hinders the transition from the (3)MLCT* state into the (3)MC* state in the closed form to block the partial photodissociation of the TPA ligand.

Synthesis, structure and photophysical properties of a flavin-based platinum(II) complex

We synthesized a thiosemicarbazone-functionalized flavin (Fl-(H)TSC: 2-[2-(3,4-dihydro-7,8-dimethyl-2,4-dioxobenzo[g]pteridin-10(2H)-yl)ethylidene]-hydrazinecarbothioamide) and its Pt(II) complex [Pt(Fl-TSC)(2)], and characterized it using X-ray diffraction, UV-visible absorption and luminescence spectroscopy. X-ray structural analysis for [Pt(Fl-TSC)(2)] revealed that the structure of the isoalloxazine part was almost the same as that in lumiflavin (7,8,10-trimethylisoalloxazine), and the thiosemicarbazone moiety acted as a bidentate ligand to form a PtS(2)N(2) planar conformation. UV-visible absorption and luminescence spectra of these compounds were very similar to those of riboflavin, but the emission intensity and the lifetime decreased considerably. Theoretical calculations suggested that the charge-separated state (Fl˙(-)-TSC˙(+)) contributed to the faster quenching from the (1)π-π* emission state.