Photochromic metal complexes: photoregulation of both the nonlinear optical and luminescent properties

A series of dithienylethene (DTE)-containing 2,2'-bipyridine ligands and their zinc(II) diacetate, zinc(II) dichloro, rhenium(I) tricarbonyl bromo, and ruthenium(II) bis(bipyridine) complexes have been designed and synthesized, and their photochromic, photophysical, and quadratic nonlinear optical properties have been studied. Upon UV irradiation at 350 nm, the ligands and complexes undergo ring closure of the DTE units, with a good to excellent photocyclization yield. In the case of the Re(I) and Ru(II) complexes, the photocyclization of the DTE units can also be triggered using visible light, upon excitation into the metal-to-ligand charge-transfer (MLCT) bands at 400 and 490 nm, respectively. Molecular quadratic nonlinear optical (NLO) responses of the complexes have been determined by using either the electrical field induced second harmonic generation (EFISH) or harmonic light scattering (HLS) technique at 1910 nm. These studies reveal a large increase of the second-order NLO activity after UV irradiation and subsequent formation of the ring-closed isomers. This efficient enhancement clearly reflects the delocalization of the π-electron system and the formation of strong push-pull chromophores in the closed forms. The combination of the photochromic DTE-based bipyridine ligand with luminescent Re(I) and Ru(II) fragments also allows the photoregulation of the emission, leading to an efficient quenching of the ligand-based 77 K luminescence and demonstrating that the photocontrol of two optical properties, linear and nonlinear, could be achieved by using the same photochromic ligand.     

A novel isopolytungstate functionalized by ruthenium: [HW(9)O(33)Ru(II)(2)(dmso)(6)](7-)

The ruthenium-supported isopolyanion [HW(9)O(33)Ru(II)(2)(dmso)(6)](7-) (1) is composed of a nonatungstate wheel stabilized by two Ru(dmso)(3) groups, representing the first structurally characterized Ru-coordinated polyoxotungstate and a novel class of isopolyanions supporting photochromic moieties.     

Efficient energy conversion in photochromic ruthenium DMSO complexes

The photochromic compounds trans- and cis-[Ru(tpy)(Mepic)(dmso)](OSO2CF3) (2 and 3, respectively; tpy is 2,2':6',2"-terpyridine; Mepic is 6-methyl-2-pyridinecarboxylate; dmso is dimethyl sulfoxide) and cis-[Ru(tpy)(Brpic)(dmso)](PF6) (4; Brpic is 6-bromo-2-pyridinecarboxylate) were prepared and characterized by single-crystal X-ray crystallography, electrochemistry, NMR, IR, and UV-vis spectroscopy. The geometry labels refer to the relationship between the carboxylate oxygen of the picolinate ligand and dmso. Electrochemical studies reveal that only the trans isomer shows S-to-O isomerization following oxidation of Ru(II) and O-to-S isomerization following reduction of Ru(III). The cis isomers of both complexes feature reversible one-electron Ru(III/II) couples. All complexes undergo phototriggered S-to-O isomerization following MLCT (metal-to-ligand charge transfer) excitation with quantum yields (Phi(S-->O)) of 0.79 (2), 0.011 (3), and 0.014 (4). The methyl group in 2 promotes isomerization by hindering rotation of the dmso ligand about the Ru-S bond. Computational results support this role for the methyl group. Relative energy calculations show that the barrier to rotation is approximately 8 kcal mol(-1). These results suggest that rotation is an important vibration for isomerization in photochromic ruthenium-dmso complexes.     

Photochromic dithienylethene derivatives containing Ru(II) or Os(II) metal units. Sensitized photocyclization from a triplet state

Efficient photocyclization from a low-lying triplet state is reported for a photochromic dithienylperfluorocyclopentene with Ru(bpy)3 units attached via a phenylene linker to the thiophene rings. The ring-closure reaction in the nanosecond domain is sensitized by the metal complexes. Upon photoexcitation into the lowest Ru-to-bpy 1MLCT state followed by intersystem crossing to emitting 3MLCT states, photoreactive 3IL states are populated by an efficient energy-transfer process. The involvement of these 3IL states explains the quantum yield of the photocyclization, which is independent of the excitation wavelength but decreases strongly in the presence of dioxygen. This behavior differs substantially from the photocyclization of the nonemissive dithienylperfluorocyclopentene free ligand, which occurs from the lowest 1IL state on a picosecond time scale and is insensitive to oxygen quenching. Cyclic voltammetric studies have also been performed to gain further insight into the energetics of the system. The very high photocyclization quantum yields, far above 0.5 in both cases, are ascribed to the strong steric repulsion between the bulky substituents on the dithienylperfluorocyclopentene bridge bearing the chelating bipyridine sites or the Ru(bpy)3 moieties, forcing the system to adopt nearly exclusively the reactive antiparallel conformation. In contrast, replacement of both Ru(II) centers by Os(II) completely prevents the photocyclization reaction upon light excitation into the low-lying Os-to-bpy 1MLCT state. The photoreaction can only be triggered by optical population of the higher lying 1IL excited state of the central photochromic unit, but its yield is low due to efficient energy transfer to the luminescent lowest 3MLCT state.     

Orientation tuning of a polypyridyl Ru(II) complex immobilized on a clay surface toward chiral discrimination

The present work reports an attempt to elucidate a stereoselective energy-transfer system by immobilizing a chiral metal complex on a clay surface. The metal complex used was [Ru(bpy)2L(i)]2+ with L1 = bpy (2,2'-bipyridine), L2 = 4,4'-diundecyl-2,2'-bipyridine, and L3 = 5,5'-diundecyl-2,2'-bipyridine. The adsorption structure of [Ru(bpy)2L(i)]2+ was studied by means of electric dichroism measurements on an aqueous dispersion of a colloidal clay. It was found that the molecular orientation of the adsorbed Ru(II) complex was affected remarkably by the positions of the alkyl chains on the bpy ligand; that is, the angle of the 3-fold or pseudo-3-fold symmetry axis of the Ru(II) complex with respect to the surface normal was obtained to be 24 degrees, 30 degrees, and 52 degrees for i = 1, 2, and 3, respectively. The efficiency of the energy-transfer was determined by photoluminescence quenching measurements between the adsorbed Ru(II) complex and [Ru(acac)3] (acac = acetylacetonate) in solution. As a result, stereoselectivity appeared most for the case of [Ru(bpy)2L3]2+ in which its two helically twisted bpy ligands were projected in an outward direction.     

Chemical modulation of the luminescence of a DNA-bound diruthenium(II) complex by copper(II) ion and EDTA

A method for the chemical modulation of the photoluminescence of a DNA-bound diruthenium(II) complex, [(bipy)₂Ru(bpib)Ru(bipy)₂]⁴⁺ (bipy = 2,2′-bipyridine, bpib = 1,4-bis([1, 10]phenanthroline [5,6-d]imidazol-2-yl) benzene) by the introduction of Cu²⁺ ion and EDTA has been developed. The diruthenium(II) complex showed strong photoluminescence both in buffer solutions and on an indium-tin oxide (ITO) surface, which was not modulated by Cu²⁺ or EDTA. The DNA-bound [(bipy)₂Ru(bpib)Ru(bipy)₂]⁴⁺ with a binding constant of 3.8 × 10⁴ M⁻¹ showed an enhancement in the luminescence based on the electrostatic interaction between the complex and DNA. The presence of Cu²⁺ was found to quench the luminescence of DNA-bound [(bipy)₂Ru(bpib)Ru(bipy)₂]⁴⁺, but the quenched luminescence was recovered by addition of an equimolar concentration of EDTA. Hence, the photoluminescence of DNA-bound [(bipy)₂Ru(bpib)Ru(bipy)₂]⁴⁺ depends strongly on the introduction of Cu²⁺ and EDTA.     

Chemiluminescence reactions with cationic, neutral, and anionic ruthenium(II) complexes containing 2,2′-bipyridine and bathophenanthroline disulfonate ligands

Ruthenium complexes containing 4,7-diphenyl-1,10-phenanthroline disulfonate (bathophenanthroline disulfonate; BPS) ligands, Ru(BPS)₃⁴⁻, Ru(BPS)₂(bipy)²⁻ and Ru(BPS)(bipy)₂, were compared to tris(2,2′-bipyridine)ruthenium(II) (Ru(bipy)₃²⁺), including examination of the wavelengths of maximum absorption and corrected emission intensity, photoluminescence quantum yield, stability of their oxidised ruthenium(III) form, and relative chemiluminescence intensities and signal-to-blank ratios with cerium(IV) sulfate and six analytes (codeine, morphine cocaine, potassium oxalate, furosemide and hydrochlorothiazide) in acidic aqueous solution. The presence of BPS ligands in the complex increased the photoluminescence quantum yield, but decreased the stability of the oxidised form of the reagent. In contrast to previous evidence showing much greater electrochemiluminescence intensities using Ru(BPS)₂(bipy)²⁻ and Ru(BPS)(bipy)₂, these complexes did not provide superior chemiluminescence signals than their homoleptic analogues.     

Preparation and Properties of Electrospun Sheath-core Modified-PMMA Nanofibers with Photoluminescence and Photochromic Functions

Multi-functional nanofibers are playing an important role in the optical field, and are widely used in fluorescence indication, product anti-counterfeit identification and smart clothing. Nanofibers with photoluminescence and photochromic functions are already attracting more interest from researchers. In this work, based on electrospun technology, the modified-PMMA nanofibers[PMMA=poly(methyl methacrylate)] with photoluminescence and photochromic functions were prepared through the design of the sheath-core structure(SCNFs 1-4). Compared with other samples, SCNF-4 shows outstanding photoluminescence and photochromic functions. SCNF-4 can produce green light and its fluorescence intensity and fluorescence lifetime can reach 7144 a.u. and 1031.32 μs, respectively. In photochromic functions, SCNF-4 can show purple in 1 min under the 365 nm ultraviolet light, and the color can be preserved for more than 4 h under the sunlight. When SCNF-4 is irradiated by far infrared light, the color of the samples can fade quickly in 40 s. Under the irradiation of ultraviolet light of different wavelengths, SCNF-4 can display multi-color fluorescence and achieve a reversible transition between white and purple. The design of the sheath-core structure realizes the complementarity of photoluminescence and photochromic functions of the electrospun modified-PMMA nanofibers, which is important to promote the wide application of multi-functional nanofibers in the optical field.     

Synthesis and characterization of a Ru(II) complex with functionalized phenanthroline ligands having single-double linked anthracenyl and 1-methoxy-1-buten-3-yne moieties

Two series of bidentate polypyridine ligands, made of phenanthroline chelating subunits having substituted mono-and di-anthracenyl groups, and 1-methoxy-1-buten-3-yne at the 4 and 7-positions with the corresponding heteroleptic Ru(II) complex have been synthesized and characterized. The complex is formulated as [(Ru(L(1))(L(2))(NCS)(2))], (where L(1 )= 4-(9-dianthracenyl-10-(2,3-dimethylacrylic acid)-7-(9-anthracenyl-10-(2,3-dimethylacrylic acid)-1,10-phenanthroline and L(2) = 4,7-bis(1-methoxy-1-buten-3-yne)-1,10-phenanthroline). The Ru(II) complex shows characteristic broad and intense metal-to-ligand charge transfer (MLCT) bands absorption and appreciable photoluminescence spanning the visible region. The ligands and complex were characterized by FT-IR, 1H, 13C NMR spectroscopy, UV-Vis, photoluminescence and elemental analysis (see in supplementary materials). The anchoring groups in both ligands have allowed an extended delocalization of acceptor orbital of the metal-to-ligand charge-transfer (MLCT) excited state.     

Photoluminescence and electrochemiluminescence of a Ru(II)(bpy)3-quencher dual-labeled oligonucleotide probe

A molecular beacon oligonucleotide probe covalently labeled with Ru(II)(bpy)3 and Black Hole Quencher-2 is synthesized, and hybridization assays are performed using photoluminescence and electrochemiluminescence methods of excitation.     

A Molecular Photosensor Based on Photoswitching of Charge Carrier Mobility

Polymer photoelectronic device based on interaction between π-conjugated polymer matrices and photochromic molecules was fabricated. The theoretical and experimental studies proved that the photochromic reaction in studied devices should eventuate in changes of optical and electrical properties of polymers such as luminescence and conductivity. The quantum chemical calculations showed that the presence of dipolar species in the vicinity of a polymer chain modifies the on-chain site energies and consequently increases the width of the distribution of hopping transport states. Optical switching was studied using standard absorption and photoluminescence spectroscopy. A strong photoluminescence quenching after the photochromic conversion caused by radiative energy transfer was observed. The influence of photoswitchable charge carrier traps on charge transport were evaluated by Space Charge Limited Current (SCLC) method. It was shown that deep traps may significantly affect the energy of the transport level, and thus modulate the transport of charge carriers.     

Synthesis and characterization of tris(heteroleptic) Ru(II) complexes bearing styryl subunits

We have developed and optimized a well-controlled and refined methodology for the synthesis of substituted π-conjugated 4,4'-styryl-2,2'-bipyridine ligands and also adapted the tris(heteroleptic) synthetic approach developed by Mann and co-workers to produce two new representative Ru(II)-based complexes bearing the metal oxide surface-anchoring precursor 4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine. The two targeted Ru(II) complexes, (4,4'-dimethyl-2,2'-bipyridine)(4,4'-di-tert-butyl-2,2'-bipyridine)(4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine) ruthenium(II) hexafluorophosphate, [Ru(dmbpy)(dtbbpy)(p-COOMe-styryl-bpy)](PF(6))(2) (1) and (4,4'-dimethyl-2,2'-bipyridine)(4,4'-dinonyl-2,2'-bipyridine)(4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine) ruthenium(II) hexafluorophosphate, [Ru(dmbpy)(dnbpy)(p-COOMe-styryl-bpy)](PF(6))(2) (2) were obtained as analytically pure compounds in high overall yields (>50% after 5 steps) and were isolated without significant purification effort. In these tris(heteroleptic) molecules, NMR-based structural characterization became nontrivial as the coordinated ligand sets each sense profoundly distinct magnetic environments greatly complicating traditional 1D spectra. However, rational two-dimensional approaches based on both homo- and heteronuclear couplings were readily applied to these structures producing quite definitive analytical characterization and the associated methodology is described in detail. Preliminary photoluminescence and photochemical characterization of 1 and 2 strongly suggests that both molecules are energetically and kinetically suitable to serve as sensitizers in energy-relevant applications.     

Steric influence on the excited-state lifetimes of ruthenium complexes with bipyridyl-alkanylene-pyridyl ligands

The structural effect on the metal-to-ligand charge transfer (MLCT) excited-state lifetime has been investigated in bis-tridentate Ru(II)-polypyridyl complexes based on the terpyridine-like ligands [6-(2,2'-bipyridyl)](2-pyridyl)methane ( 1) and 2-[6-(2,2'-bipyridyl)]-2-(2-pyridyl)propane ( 2). A homoleptic ([Ru( 2) 2] (2+)) and a heteroleptic complex ([Ru(ttpy)( 2)] (2+)) based on the new ligand 2 have been prepared and their photophysical and structural properties studied experimentally and theoretically and compared to the results for the previously reported [Ru( 1) 2] (2+). The excited-state lifetime of the homoleptic Ru (II) complex with the isopropylene-bridged ligand 2 was found to be 50 times shorter than that of the corresponding homoleptic Ru (II) complex of ligand 1, containing a methylene bridge. A comparison of the ground-state geometries of the two homoleptic complexes shows that steric interactions involving the isopropylene bridges make the coordination to the central Ru (II) ion less octahedral in [Ru( 2) 2] (2+) than in [Ru( 1) 2] (2+). Calculations indicate that the structural differences in these complexes influence their ligand field splittings as well as the relative stabilities of the triplet metal-to-ligand charge transfer ( (3)MLCT) and metal-centered ( (3)MC) excited states. The large difference in measured excited-state lifetimes for the two homoleptic Ru (II) complexes is attributed to a strong influence of steric interactions on the ligand field strength, which in turn affects the activation barriers for thermal conversion from (3)MLCT states to short-lived (3)MC states.     

Fabrication of Self‐Assembled Tris(1,10‐phenanthroline) ruthenium/12‐Molybdophosphoric Acid Films with Bifunctional Electrocatalytic and Photoluminescent Properties

The thin films containing transition metal complex tris(1,10‐phenanthroline) ruthenium(II) Ru(phen)₃Cl₂ (abbr Ru(phen)₃, phen=1,10‐phenanthroline), and 12‐molybdophosphoric acid [PMo₁₂O₄₀]_3? (abbr PMo₁₂) were fabricated on quartz, silicon and ITO substrates by layer‐by‐layer (LBL) method. The LBL films were characterized by the UV‐vis spectroscopy, X‐ray photoelectron spectroscopy, atomic force microscopy and cyclic voltammetry. The films can catalyze both the reduction of ClO , BrO , IO , and the oxidation of C₂O due to the presence of bifunctional composite, and the redox potentials depend on pH as a result of protonation. The photoluminescence of films were also investigated. The films exhibited photoluminescence arising from π*–t_2g ligand‐to‐metal transition of Ru(phen)₃.     

Catalytic Activation of H(2) and C-H Bonds by Electron-Deficient Ruthenium(II) Porphyrins

The Ru(2) and RuNi derivatives of 1,8-bis(10,15,20-trimesityl-5-porphyrinato)anthracene-a recently reported cofacial diporphyrin ligand comprising two hindered porphyrins spanned by an anthracene bridge-have been synthesized. Both Ru(2)(DPAHM) and RuNi(DPAHM) are extremely reactive species that apparently contain 14-electron Ru(II) centers and, as is the case for their monoporphyrin analog, (5,10,15,20-tetramesitylporphyrinato)ruthenium [Ru(TMP)], must be rigorously protected from oxygen, nitrogen, and other ligating agents. In addition, these electron-deficient Ru(II) porphyrins all appear to bind aromatic solvents such as benzene and toluene, the weakest ligating solvents in which these Ru(II) porphyrins have been found soluble. Ru(TMP) and its metallodiporphyrin analogs, Ru(2)(DPAHM) and RuNi(DPAHM), catalyze H(2)/D(2) exchange in benzene solution and as solids. When adsorbed on a particularly nonpolar carbon support, these Ru(II) porphyrins all manifest significant activity with respect to catalytic H(2)/D(2) exchange [approximately 40 turnovers s(-)(1), when normalized for Ru(II) content]. In addition, these molecules slowly catalyze the exchange of H(2) into deuterated aromatic hydrocarbons and, in the absence of solvent, the exchange of D(2) into CH(4). Kinetic studies of H(2)/D(2) exchange catalyzed by these Ru(II) porphyrins on carbon supports indicate that exchange is likely to be effected by one face of a single Ru(TMP) moiety. The activity of each supported catalyst was suppressed by the presence of ligands, either exogenous (CO irreversibly and N(2) reversibly) or from polar functionalities on the surface of the supporting matrix.     

New Ru(II) chromophores with extended excited-state lifetimes

We describe the synthesis, electrochemical, and photophysical properties of two new luminescent Ru(II) diimine complexes covalently attached to one and three 4-piperidinyl-1,8-naphthalimide (PNI) chromophores, [Ru(bpy)(2)(PNI-phen)](PF(6))(2) and [Ru(PNI-phen)(3)](PF(6))(2), respectively. These compounds represent a new class of visible light-harvesting Ru(II) chromophores that exhibit greatly enhanced room-temperature metal-to-ligand charge transfer (MLCT) emission lifetimes as a result of intervening intraligand triplet states ((3)IL) present on the pendant naphthalimide chromophore(s). In both Ru(II) complexes, the intense singlet fluorescence of the pendant PNI chromophore(s) is nearly quantitatively quenched and was found to sensitize the MLCT-based photoluminescence. Excitation into either the (1)IL or (1)MLCT absorption bands results in the formation of both (3)MLCT and (3)IL excited states, conveniently monitored by transient absorption and fluorescence spectroscopy. The relative energy ordering of these triplet states was determined using time-resolved emission spectra at 77 K in an EtOH/MeOH glass where dual emission from both Ru(II) complexes was observed. Here, the shorter-lived higher energy emission has a spectral profile consistent with that typically observed from (3)MLCT excited states, whereas the millisecond lifetime lower energy band was attributed to (3)IL phosphorescence of the PNI chromophore. At room temperature the data are consistent with an excited-state equilibrium between the higher energy (3)MLCT states and the lower energy (3)PNI states. Both complexes display MLCT-based emission with room-temperature lifetimes that range from 16 to 115 micros depending upon solvent and the number of PNI chromophores present. At 77 K it is apparent that the two triplet states are no longer in thermal equilibrium and independently decay to the ground state.     

Comparative study of ruthenium(II) tris(bipyridine) derivatives for electrochemiluminescence application

Several [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine and its derivatives) complexes were synthesized and compared electrochemically and spectroscopically in the search for better luminophores for electrochemiluminescence (ECL)-based analytical applications. ECL measurement in [Ru(bpy)3]2+/tripropylamine (TPA) aqueous buffer solutions has led to a conclusion that due to the complexity of the ECL generation process, the photoluminescence efficiency cannot be used to predict ECL intensity and there is no obvious relationship between the photoluminescence quantum yield and the ECL intensity. Under the present experimental condition, when compared with the pristine [Ru(bpy)3]2+, the ethoxycarbonyl-substituted derivative, [Ru(bpy-COOEt)3]2+, one of the most efficient luminophores under photoexcitation, did not generate reasonably intense ECL, whereas luminophores with lower photoluminescence quantum yields demonstrated higher ECL. These findings are useful for further efforts in the search for more efficient ECL luminophores.     

Syntheses, characterization, and photochromic studies of spirooxazine-containing 2,2'-bipyridine ligands and their zinc(II) thiolate complexes

A series of photochromic spirooxazine-containing zinc(II) diimine bis-thiolate complexes were successfully synthesized, and their photophysical and photochromic properties were studied. The X-ray crystal structure of complex 1a has also been determined. Upon excitation by UV light at 330 nm, all the ligands and complexes exhibit photochromic behavior. The thermal bleaching kinetics of the ligands and the complexes were studied in dimethylformamide at various temperatures. The photochemical quantum yields for the photochromic reactions of the ligands and complexes were also determined.     

Development of a triple channel detection probe for hydrogen peroxide

The rapid and reliable measurement of hydrogen peroxide (H₂O₂) is imperative for many areas of technology, including pharmaceutical, clinical, food industry and environmental applications. In this work, a novel multifunctional complex, [Ru(bpy)₂(luminol-bpy)](PF₆)₂ (bpy: 2,20'-bipyridine), was designed and synthesized by incorporating a Ru(II) complex with a luminal group. In the presence of horseradish peroxidase (HRP), reaction of [Ru(bpy)₂(luminol-bpy)]²⁺ with H₂O₂ can be monitored by three sensing channels including photoluminescence (PL), chemiluminiscence (CL) and eletrochemiluminiscence (ECL). The quantitative assays for H₂O₂ in aqueous solutions using [Ru(bpy)₂(Luminalbpy)]( PF₆)₂ as a probe were established with PL, ECL and CL signal output modes, respectively.     

instant "base-promoted" generation of roper's-type Ru(0) complexes Ru(CO)2(PR3)3 from a simple carbonylchlororuthenium(II) precursor

An uncommon synergism in the concerted action of OH- and PR3 toward the simple Ru(II) complex Ru(CO)3Cl2(thf) allows a highly efficient reduction of the metal in ethanol or acetonitrile solution at 0 degrees C, with selective production of the corresponding Roper's-type Ru(0) complexes Ru(CO)2(PR3)3 in high yields within 10 min.