Photoinduced Electron Transfer Reactions of Ruthenium(II)-Complexes Containing Amino Acid with Quinones

With the aim of mimicking, at basic level the photoinduced electron transfer process in the reaction center of photosystem II, ruthenium(II)-polypyridyl complexes, carrying amino acids were synthesized and studied their photoinduced electron transfer reactions with quinones by steady state and time resolved measurements. The reaction of quinones with excited state of ruthenium(II)-complexes, I–V in acetonitrile has been studied by luminescence quenching technique and the rate constant, k_q, values are close to the diffusion controlled rate. The detection of the semiquinone anion radical in this system using time-resolved transient absorption spectroscopy confirms the electron transfer nature of the reaction. The semiclassical theory of electron transfer has been successfully applied to the photoluminescence quenching of Ru(II)-complexes with quinones.

Binding and Fluorescence Resonance Energy Transfer (FRET) of Ruthenium(II)-Bipyridine-Calixarene System with Proteins—Experimental and Docking Studies

The investigation of the interaction of ruthenium(II)-bipyridine-tert-butylcalix[4]arene complexes (Rubc2 and Rubc3) with proteins (BSA and ovalbumin) using absorption, emission, excited state lifetime and circular dichroism techniques and by docking studies show that luminophore-receptor system bind strongly with proteins. An enhancement of absorption as well as emission intensity of Ru(II)-calixarene complexes in the presence of proteins, but the quenching of the emission intensity of proteins in the presence of Ru(II)-calixarene complexes are the interesting observations. The enhancement of emission intensity of Ru(II)-calixarene complex, in the presence of proteins, is due to the fluorescence resonance energy transfer (FRET) from protein to Ru(II)-calixarene complex. Among the two Ru(II)-calixarene complexes synthesized Rubc3 has more efficient binding and energy transfer than Rubc2 and BSA, with a large cavity size, has the advantage for binding over ovalbumin. Docking studies reveal that the presence of tert-butylcalix[4]arene moiety in Ru(II)-calixarene complexes facilitates binding with proteins. After the binding of Rubc2 and Rubc3 with proteins, the nearby fluorophores present in proteins are in optimal distance from the ruthenium centre for efficient FRET process to occur.     

EPR investigations of synthetic manganese complexes as bio-mimics of the water oxidation complex in photosystem II

Research in the Swedish Consortium for Artificial Photosynthesis aims to construct a supramolecular system containing synthetically connected D (electron donor), S (photosensitizer), and A (electron acceptor) compartments. These are intended to carry out catalytic water oxidation on the donor side and catalytic hydrogen formation on the acceptor side, driven by light energy absorbed by the photosensitizer. In this minireview, we focus our attention on our spectroscopic and electrochemical studies of a series of manganese complexes partially mimicking the water-oxidizing manganese complex in the natural photosystem II (PSII), using ruthenium(II) tris(bypyridine) as the photosensitizer. The manganese complexes we discuss fall in three categories: monomeric manganese systems covalently linked to the ruthenium(II) tris(bypyridine) center, dimeric manganese complexes that are not covalently connected to ruthenium(II) tris(bypyridine) and dimeric manganese complexes covalently bound to a ruthenium(II) tris(bypyridine) center via an amide bound. The review focuses on the use of electron paramagnetic resonance spectroscopy in the studies of our manganese compounds.     

Mixed-ligand complexes of Pt(II) and Pd(II) with 2-phenylbenzothiazole

The mixed-ligand cyclometalated [M(Bt)(μ-Cl)]₂ and [(M(N∧N))(Bt)]⁺ complexes (M = Pd(II), Pt(II); Bt^? is the deprotonated form of 2-phenylbenzothiazole; and ( N∧N) is ethylenediamine (En) and orthophenanthroline (Phen)) are studied and described by ¹H NMR spectroscopy, electronic absorption and emission spectroscopy, and voltammetry. The one-electron reduction of complexes is attributed to the electron transfer to the π * orbitals of both diimine and cyclometalated ligands. The long-wavelength absorption bands and vibrationally structured luminescence bands are assigned to optical transitions that are localized mainly on the M(Bt) metal-complex fragment.     

Absorption and fluorescence spectroscopy of 3-hydroxy-3-phenyl-1-o-carboxyphenyltriazene and its copper (II), nickel (II) and zinc (II) complexes: a novel fluorescence sensor

Absorption and fluorescence spectroscopic properties of 3-hydroxy-3-phenyl-1-o-carboxyphenyltriazene (HT) are studied. The mechanism of photo-induced electron transfer (PET) followed by energy transfer process of the ligand and the Cu (II), Ni (II) and Zn (II) metal complexes have been investigated. The excited state photo induced intramolecular hydrogen transfer from N-OH to triazene 1-nitrogen atom is explained. The effect of pH, solvent and concentration on the absorption and fluorescence of the ligand is studied and it has been found that the absorption and fluorescence of HT is highly pH, solvent and concentration dependent. Participation of the N-OH proton of HT in the solvent assisted O to N-proton transfer has also been proposed. The fluorescence band shift and changes in intensity is modulated by protonation and complexation with metal ions. This fluorophore can thus be used as a pH dependent and M⁽ⁿ⁺¹⁾⁺/Mⁿ⁺ redox on/off switchable molecular sensor.     

DFT study of electronic structure and optical properties of some Ru- and Rh-based complexes for dye-sensitized solar cells

The paper reports Time Dependent Density Functional Theory (TD DFT) calculations providing the structure, electronic properties and spectra of [Ru(II)(bpy)_{3? n} (dcbpy) _n ]²⁺ and [Rh(III)(bpy)_{3? n} (dcbpy) _n ]³⁺ complexes, where bpy?=?2,2′-bipyridyl, dcbpy?=?4,4′-dicarboxy-2,2′-bipyridyl, and n?=?0,?1,?2,?3, studied as possible pigments for dye-sensitized solar cells. The role of the metallic ion and of the COOH groups on the optical properties of these complexes are compared and contrasted and their relevance as dyes for hybrid organic–inorganic photovoltaic cells is discussed. It was found that the optical spectra are strongly influenced by the metallic ion, with visible absorption bands for the Ru(II) complexes and only ultraviolet bands for the Rh(III) complexes. Upon excitation, the extra positive charge of the Rh³⁺ centre tends to draw electrons towards the metal ion, facilitating some charge transfer from the ligand to the metal, whereas in the case of the Ru²⁺ ion the electron transfer is clearly from the metal to the ligand. The carboxyl groups play an important role in strengthening the absorption bands in solution in the visible region. Of the complexes studied, the most suited as pigments for dye-sensitized solar cells are the [Ru(II)(bpy)_{3? n} (dcbpy) _n ]²⁺ complexes with n?=?1 and 2. This is based on the following arguments: (i) their intense absorption band in the visible region, (ii) the presence of the anchoring groups allowing the bonding to the TiO₂ substrate and the charge transfer, and (iii) the good energy level alignment with the conduction band edge of the semiconducting substrate and the redox level of the electrolyte.     

DFT study of the effect of different metals on structures and electronic spectra of some organic-metal compounds as sensitizing dyes

Ruthenium polypyridined-derivative complexes are used in dye-sensitized solar cell [DSSC] as a light to current conversion sensitizer. In order to lower the cost of the DSSC the normal transition metals were used to replace the noble metal ruthenium, and some compounds [ML₂L′] (M = Pt, Fe, Ni, Zn; L = isonicotinic acid, L′ = maleonitriledithiolate, I = PtL₂L′, II = FeL₂L′, III = NiL₂L′, IV = ZnL₂L′) were selected as the replacement. The geometries, electronic structures and optical absorption spectra of these compounds have been studied by using density functional theory (DFT) calculation at the B3LYP/LANL2DZ, B3P86/LANL2DZ, B3LYP/GEN level of theory. All the geometric parameters are close to the experimental values. The HOMOs are mainly on the maleonitriledithiolate groups mixed with fewer characters of the metal atom, the LUMOs are mainly on the two pyridine ligands. This means that the electron transition is attributed to the LLCT. The maximum absorptions of complexes are found to be at 351 nm, 806 nm for compound I, and 542 nm for compound II. The maximum absorptions of complexes are found to be at 884 nm for compound III, and 560 nm for compound IV. This means that those compounds may be as a suitable sensitizer for solar energy conversion applications.     

Photophysics of the adsorbed bipyridyl complexes of ruthenium(II) with phosphines

Luminescence of the ruthenium(II) complexes cis-Ru(bpy)₂(CN)₂ (I), cis-[Ru(bpy)₂(PPh₃)CN](BF₄) (II), and cis-Ru(bpy)(dppe)(CN)₂ (III)[bpy=2.2′-bipyridyl, PPh₃=triphenylphosphine, dppe=1,2-bis(diphenylphosphino)ethane], adsorbed on silicon oxide (Aerosil) were studied at a temperature of 77 K. The luminescence spectra, decay times, and quantum yields were measured, and the intermolecular rate constants of radiative transitions and nonradiative decay of the excited electronic state with the metal-to-ligand charge transfer (MLCT) were determined. It is found that the adsorption of the complex is accompanied by a decrease in the energy of the radiative MLCT state and by a considerable acceleration of its nonradiative decay. It is concluded that the interaction of the complexes with the surface adsorption centers occurs via formation of a strong hydrogen bond with a hydroxyl-hydrate cover, the interaction of complexes in the ³MLCT state being stronger than in the ground state. The additive (in the number of phosphorus atoms coordinated to the central ruthenium ion), a shift of the absorption and luminescence bands to shorter wavelengths in the sequence of complexes I–III, is retained when the complexes transform from solutions to the absorbed state.     

Spectroscopy and quantum-chemical calculations of nitro-bis-bipyridyl complexes of ruthenium(II) with 4-substituted pyridine ligands

The luminescence, absorption, and luminescence excitation spectra of complexes cis-[Ru(bpy)₂(L)(NO₂)]⁺ [bpy = 2,2′-bipyridyl, L = pyridine, 4-aminopyridine, 4-dimethylaminopyridine, 4-picoline, isonicotinamide, or 4,4′-bipyridyl] in alcoholic (4 : 1 EtOH–MeOH) solutions are studied at 77 K. A linear correlation is established between the energy of the lowest electronically excited metal-toligand charge transfer state d_π(Ru) → π*(bpy) of the complexes and the pK_a parameter of the free 4-substituted pyridines used as ligands L. The B3LYP/[6-31G(d)+LanL2DZ(Ru)] hybrid density functional method is used to optimize the geometry of complexes and calculate their electronic structure and the charge distribution on the atoms of the nearest environment of ruthenium(II) ions. It is shown that there exists a mutually unambiguous correspondence between the charge on the nitrogen atom of ligands L coordinated in the complex and the pK_a parameter of ligands. The calculated energies of the electronically excited metal-to-ligand charge transfer states of complexes linearly (correlation coefficient 0.99) depend on the charge on the nitrogen atom of ligands L, which completely agrees with the experimental data.     

Spectroscopy and photophysics of chloro-bis-bipyridyl complexes of ruthenium(II) with pyridine ligands

The absorption, luminescence, and luminescence excitation spectra of ruthenium(II) complexes cis-[Ru(bpy)₂(L)Cl]⁺[bpy=2,2′-bipyridyl; L=NH₃, pyrazine, pyridine, 4-aminopyridine, 4-picoline, isonicotinamide, 4-cyanopyridine, 4,4′-bipyridyl, or trans-1,2-bis(4-pyridyl)ethylene] in alcoholic (4: 1 EtOH-MeOH) solutions are studied. At 77 K, the quantum yields and decay times of the luminescence of the complexes are measured and the deactivation rate constants of the lowest electronically excited metal-to-ligand charge transfer state (³MLCT) are determined. The linear correlation between the energy of the lowest state ³MLCT d _π(Ru)>π*(bpy) of the cis-[Ru(bpy)₂(L)Cl]⁺ complexes and the parameter pK_a of the free 4-substituted pyridines and pyrazine used as ligands is established.     

Optical Recognition of Anions by Ruthenium(II)-Bipyridine-Calix[4]Arene System

The two t-butylcalix[4]arene attached ruthenium(II)-bipyridine complexes (Rubc2 and Rubc3) has been synthesized and the anion recognition studies have been carried out using emission techniques. The binding of anions, which are sensed by the complexes, are studied by UV-visible and emission techniques. The complex Rubc2 recognizes the Cl^?, H₂PO₄ ^? and AcO^? anions. The complex Rubc3 recognizes the Br^? and AcO^? anions. The AcO^? quenches the emission intensity of both two complexes but the other anion increases the emission intensity of the complexes. The excited state lifetime and transient absorption studies were carried out the AcO^? facilitates non radiative pathway. The other anions stabilize the excited state and facilitate the radiative pathway.     

“Charge transfer Complexes of Indolyldiene Aniline derivatives with p-benzoquinone derivatives”

Abstract

Charge transfer (CT) complexes of p-benzoquinone derivatives with Indolyldiene aniline derivatives have been prepared and investigated by Elemental analysis, IR, ¹H-NMR and electronic absorption spectroscopy. The spectral changes revealed that acidic acceptors form complexes with π - π? electronic interaction and proton transfer while non-acidic acceptors yield complexes having π - π transition only. The formation of 1:2 (D:A) complexes is also ascertained. The ionization potential and electron affinity are determined from the electronic absorption spectra for both the donors and acceptors respectivily.     

Investigations on Photoinduced Interaction of 9-Aminoacridine with Certain Catechols and Rutin

The fluorescence quenching of 9-aminoacridine by certain biologically important catechols and rutin was investigated using absorption, steady state and time resolved fluorescence measurements. The in vitro-antioxidant activities of the above compounds were studied using deoxyribose degradation assay and nitric oxide scavenging assay. The experimental results showed that the fluorescence of 9-aminoacridine was quenched by quencher molecules via forming ground state complex. The bimolecular quenching rate constant k(q), binding constant (K) and number of binding sites (n) were calculated at different temperatures from relevant fluorescence data. Static quenching mechanism was supported by lifetime measurement. The free energy change (ΔG(et)) for electron transfer process was calculated by Rehm-Weller equation. The binding distance of 4-nitrocatechol with 9-aminoacridine was obtained according to Forster's non-radiative energy transfer theory. Nature of binding forces and their interactions was probed based on thermodynamic parameters.     

Spectroscopic properties of 2-phenylbenzothiazole and 1-phenylpyrazole luminophores metalated with platinum metals

The effect of metalation of 2-phenylbenzothiazole and 1-phenylpyrazole with Pd(II), Pt(II), and Rh(III) on the structure and optical properties of luminophore complexes is studied by ¹H NMR spectroscopy, IR spectroscopy, and electronic absorption and emission spectroscopy. It is shown that metalation of luminophores leads to the formation of five-membered {M(C??N)} fragments in the composition of square planar and octahedral complexes, which exhibit a long-wavelength charge-transfer band. The luminescence properties of the complexes are characterized by quenching of the fluorescence and by enhancement of the phosphorescence from the mainly intraligand excited state. Fluorescence quenching of complexes is attributed to the thermally activated energy transfer to metal-centered states and their efficient nonradiative decay.     

Optical and electrochemical characteristics of ethylenediamine complexes of Pt(II) and Ir(III) with metalated 2-phenyl- and 2-naphthylbenzothiazole

The cyclometalated complexes [Pt(С^N)En]PF₆ and [Ir(C^N)₂En]PF₆ ((C^N)^– are deprotonated forms of 2-phenylbenzothiazole or 2-naphthylbenzothiazole and En is ethylenediamine) are studied by ¹Н NMR, IR, electronic absorption, and emission spectroscopy, as well as by voltammetry. Metalation of heterocyclic ligands leads to the formation of five-membered {M(C^N)} cycles in the composition of squareplanar Pt(II) complexes and octahedral Ir(III) complexes of the cis-С,С structure. A bathochromic shift of the metal-to-cyclometalated ligand charge transfer bands and a decrease in the potential difference between the single-electron waves of metal-centered oxidation and ligand-centered reduction of complexes upon substitution of 2-phenylbenzothiazole by 2-naphthylbenzothiazole and of Pt(II) by Ir(II) are shown. The phosphorescence of complexes in the visible region is assigned to the radiative transition from the metal-modified intraligand electronic excited state.     

Quantum-chemical study of effect of conjugation on structure and spectral properties of C105 sensitizing dye

The structure and spectral properties of two organic ruthenium complexes used as sensitizing dyes for solar batteries (well-known N3 dye and its selenophene-conjugated analogue C105 ([Ru(bpy)(bpysef)(COOH)₂(NCS)₂] (bpy = 2,2′-bipyridine, bpysef = 4,4′-bis(5-hexylselenophene-2-yl)2,2′-bipyridine)) are comparatively studied within the density functional method. It is shown that the conjugation of the bipyridine ligand with selenophene affects the electronic structure of the C105 dye. A multilevel model for interpreting the electronic spectra of dyes is proposed based on the analysis of the shapes of molecular orbitals. The nature of the absorption bands of these ruthenium complexes in the region of 300–800 nm is explained. It is found that, in the polar acetonitrile solvent, these dyes are negatively solvatochromic, which agrees with the current classical views on the effect of the solvent on the shape of electronic absorption spectra of related compounds.     

槲皮素-Sn(II)配合物荧光、紫外及红外光谱的测量与分析

以NH4Cl-NH3.H2O为缓冲液,十六烷基三甲基溴化铵(CTAB)为荧光增强剂,用荧光分光光度计分别采集槲皮素、槲皮素-Sn(II)配合物溶液、槲皮素-Sn(II)-NH4Cl-NH3.H2O、槲皮素-Sn(II)-NH4Cl-NH3.H2O-CTAB溶液以及将其溶液分别静置7h和21h后的荧光光谱,并对光谱进行分析。在不加CTAB的条件下,用紫外分光光度计分别测量加入缓冲液前后的紫外光谱。用漫反射方法测定配合物的红外光谱,并对其结构进行初步分析。在缓冲液的作用下,槲皮素-Sn(II)配合物的结构发生了变化;通过分析,发现与缓冲液发生反应的主要基团为酚羟基,红外光谱中酚羟基的消失和NH4+基团的出现,说明了配合物中的酚羟基与NH4+发生了取代反应。     

Electron dynamics in charged wet TiO2 anatase (001) surface functionalised by ruthenium ions

The surface of charged wet TiO₂ anatase (001) functionalised by ruthenium ion at ambient temperatures is studied by computational modelling. Response of this model to photoexcitations at ambient temperatures is explored with the Redfield density matrix equation of motion on the basis of Kohn–Sham orbitals. The parameters of the Redfield equation are on-the-fly non-adiabatic couplings for electronic degrees of freedom obtained along the ab initio molecular dynamics nuclear trajectories. The main results in this study are the following: (1) optical properties of the doped models such as light absorption intensity and transition energies can be tuned by modifying total charge; (2) electron and hole relaxation rates depend on the initial excitation; and (3) in the doped model, excitations of lower energy provide quicker relaxation. Results of computational modelling would benefit understanding of the mechanism of electron transfer processes on the surface of ruthenium-doped TiO₂.     

Magnetic Circular Dichroism Studies of the Low-Energy Absorption Bands of Mn(III) Porphyrin Complexes

Porphyrin manganese complexes are of interest, both from their possible involvement in oxygen formation in photosynthesis and as analogues to the biologically important iron porphyrin complexes. Recent studies by Bocher(^1–3) have indicated that porphyrin manganese III complexes exhibit a unique absorption spectrum indicative of an unusual electronic structure. Their spectra are considerably more complex than other metalloporphyrins, e.g., heme proteins, which typically exhibit visible absorption spectra containing two prominent bands (α,β) in the 16 to 20-kK region and a third, more intense band, the Soret band, at 23 to 25 kK.     

DFT/TDDFT investigation on the photophysical properties of a series of phosphorescent cyclometalated complexes based on the benchmark complex FIrpic

The photophysical properties of four Ir(III) complexes have been investigated by means of the density functional theory/time-dependent density functional theory (DFT/TDDFT). The effect of the electron-withdrawing and electron-donating substituents on charge injection, transport, absorption and phosphorescent properties has been studied. The theoretical calculation shows that the lowest-lying singlet absorptions for complexes 1–4 are located at 387, 385, 418 and 386 nm, respectively. For 1–4, the phosphorescence at 465, 485, 494 and 478 nm is mainly attributed to the LUMO → HOMO and LUMO → HOMO-1 transition configurations characteristics. In addition, ionisation potential (IP), electron affinities (EAs) and reorganisation energy have been investigated to evaluate the charge transfer and balance properties between hole and electron. The balance of the reorganisation energies for complex 3 is better than others. The difference between hole transport and electron transport for complex 3 is the smallest among these complexes, which is beneficial to achieve the hole and electron transfer balance in emitting layer.