Highly unusual effects of pi-conjugation extension on the molecular linear and quadratic nonlinear optical properties of ruthenium(II) ammine complexes

We have used several techniques, including hyper-Rayleigh scattering and Stark spectroscopy, to investigate the effects of polyene chain length on the optical properties of complexes containing ruthenium(II) electron donor groups and pyridinium electron acceptors. In marked contrast with all other known donor-acceptor polyenes, conjugation extension beyond a single double bond in the dipolar complexes studied leads to blue-shifting of the intramolecular charge-transfer absorptions. Furthermore, the static first hyperpolarizabilities beta0 become maximized with trans-1,3-butadienyl linkages and then decrease in complexes with three CH=CH bonds. Our results clearly demonstrate that the molecular engineering criteria for metal-containing nonlinear optical chromophores can differ dramatically from those for purely organic compounds.     

Syntheses and spectroscopic and quadratic nonlinear optical properties of extended dipolar complexes with ruthenium(II) ammine electron donor and N-methylpyridinium acceptor groups

In this paper, we describe the extremely unusual optical properties of Ru(II)-based electron donor-acceptor (D-A) polyene and some closely related chromophores. For three different polyene series, the intense, visible d-->pi* metal-to-ligand charge-transfer bands unexpectedly blue-shift as the number of E-ethylene units (n) increases from 1 to 3, and the static first hyperpolarizabilities beta(0) determined via hyper-Rayleigh scattering and Stark spectroscopy maximize at n = 2, in marked contrast to other known D-A polyenes in which beta(0) increases steadily with n. Time-dependent density-functional theory and finite field calculations verify these empirical trends, which arise from the orbital structures of the complexes. This study illustrates that transition metal-based nonlinear optical chromophores can show very different behavior when compared with their more thoroughly studied purely organic counterparts.     

Two-dimensional, pyrazine-based nonlinear optical chromophores with ruthenium(II) ammine electron donors

Six new nonlinear optical (NLO) chromophores with pyrazinyl-pyridinium electron acceptors have been synthesized by complexing a known pro-ligand with electron donating {Ru(II)(NH(3))(5)}(2+) or trans-{Ru(II)(NH(3))(4)(py)}(2+) (py = pyridine) centers. These cationic complexes have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible d → π* metal-to-ligand charge-transfer (MLCT) absorptions gain intensity on increasing the number of Ru(II) centers from one to two, but remain at constant energy. One or two Ru(III/II) redox processes are observed which are reversible, quasi-reversible, or irreversible, while all of the ligand-based reductions are irreversible. Molecular first hyperpolarizabilities β have been determined by using hyper-Rayleigh scattering (HRS) at 1064 nm, and depolarization studies show that the NLO responses of the symmetric species are strongly two-dimensional (2D) in character, with dominant "off-diagonal" β(zyy) components. Stark (electroabsorption) spectroscopic measurements on the MLCT bands also allow the indirect determination of estimated static first hyperpolarizabilities β(0). Both the HRS and the Stark-derived β(0) values increase on moving from mono- to bimetallic complexes, and substantial enhancements in NLO response are achieved when compared with one-dimensional (1D) and 2D monometallic Ru(II) ammine complexes reported previously.     

Contrasting linear and quadratic nonlinear optical behavior of dipolar pyridinium chromophores with 4-(dimethylamino)phenyl or ruthenium(II) ammine electron donor groups

In this article, we contrast the optical properties of dipolar chromophores having 4-(dimethylamino)phenyl electron donor (D) and pyridinium acceptor (A) groups with those of closely related cations having pyridyl-coordinated Ru(II) donors. A range of physical data, including that from Stark (electroabsorption) spectroscopy, permits unprecedented quantitative comparisons, most notably regarding the effects of extension of bridging polyene chains. The purely organic compounds display normal optical properties in that their intense, visible pi --> pi intramolecular charge-transfer (ICT) bands red-shift as the number of E-ethylene units (n) increases from 1 to 3 and the associated static first hyperpolarizabilities beta(0) increase steadily with n. The related Ru(II) complexes show intense, visible d --> pi metal-to-ligand charge-transfer (MLCT) bands, which are found to lower energy when compared with the ICT transitions of the corresponding organics. Abnormally, these MLCT bands blue-shift as n increases, and beta(0) maximizes at n = 2. Time-dependent density-functional theory and finite field calculations verify these empirical trends for both types of compound, which can be rationalized as arising from the differing orbital structures of the chromophores and the associated degrees of D-A electronic coupling.     

Langmuir-Blodgett films of a clay mineral and ruthenium(II) complexes with a noncentrosymmetric structure

Mono- and multilayers of amphiphilic [Ru(phen)(2)(dcC12bpy)](2+) (phen = 1,10-phenanthroline, dcC12bpy = 4,4'-caboxyl-2,2'-bipyridyl didodecyl ester) hybridized with a clay mineral have been prepared by a modified Langmuir-Blodgett method, and their structures and properties have been investigated. Formation of a hybrid monolayer of the Ru(II) complex cations and the clay platelets at an air-clay suspension interface was confirmed by surface pressure-molecular area (pi-A) isotherm measurement and atomic force microscopic (AFM) observation. Multilayers were fabricated by depositing the hybrid monolayers onto glass substrates. The absorbance at 492 nm due to the Ru(II) complex cation in the multilayer increased linearly with the increase in the layer number, indicating layer-by-layer deposition of the hybrid monolayers. Because no increase in the second-harmonic generation (SHG) signal from the multilayers against the layer number was observed, the orientation of the Ru(II) complex cations in the layer would be disturbed. The hydrophilic surface of the transferred hybrid monolayer can be converted to a hydrophobic surface by dipping it in an aqueous solution of octadecylammonium chloride (ODAH(+)Cl(-)). The multilayers modified with ODAH(+) showed a quadratic relation between the SHG intensity and the layer number. This means that the Ru(II) complex cations in the multilayer are successfully oriented in a noncentrosymmetric way by the conversion of the surface property. Both a racemic mixture and an enantiomer of the Ru(II) complex cations were employed to examine the chiral effect on the film properties. The chiral contribution to the SHG signal was enhanced in the multilayer modified with ODAH(+).     

Syntheses and quadratic optical nonlinearities of ruthenium(II) complexes with ethynyl-connected N-methylpyridinium electron acceptors

We have prepared a number of new dipolar complexes containing ethynyl or buta-1,3-diynyl units linking electron-rich {Ru(II)(NH3)5}2+, trans-{Ru(II)(NH3)4L}+ (L = pyridine or N-methylimidazole), or trans-{Ru(II)Cl(pdma)2}+ [pdma = 1,2-phenylenebis(dimethylarsine)] centers to pyridinium electron acceptors. In acetonitrile solutions at 295 K, the new complexes display unusual blue-shifting of their metal-to-ligand charge-transfer (MLCT) bands as the conjugation is extended, in a fashion similar to that of the corresponding ethenyl systems. Hyper-Rayleigh scattering (HRS) and Stark spectroscopic measurements provide direct and indirect estimates of static first hyperpolarizabilities beta0, and both the linear and nonlinear optical (NLO) properties are temperature- and medium-dependent. Thus, at 77 K in butyronitrile glasses, the MLCT bands display more normal red shifts upon conjugation extension. While the Stark-derived beta0 values generally increase as n (the number of ethynyl units) increases from 0 to 2, the HRS data show maximization at n = 1 for two of the ammine series but an increase upon moving from n = 1 to 2 for the pdma complexes. Comparisons with the analogous ethenyl chromophores show that the latter generally display larger beta0 values, whether determined via HRS or Stark data, and the inferiority of the ethynyl systems in terms of NLO response is more pronounced when n = 2. This differing behavior is attributable primarily to larger increases in the transition dipole moment mu12 (and, hence, donor-acceptor pi-electronic coupling) on elongation in the ethenyl chromophores.     

Surface-based molecular self-assembly: Langmuir-Blodgett films of amphiphilic Ln(III) complexes

The unique photophysical properties of the Ln(III) series has led to significant research efforts being directed towards their application in sensors. However, for “real-life” applications, these sensors should ideally be immobilised onto surfaces without loss of function. The Langmuir-Blodgett (LB) technique offers a promising method in which to achieve such immobilisation. This mini-review focuses on synthetic strategies for film formation, the effect that film formation has on the physical properties of the Ln(III) amphiphile, and concludes with examples of Ln(III) LB films being used as sensors.     

Quadratic nonlinear optical properties of ruthenium (II)-bipyridine complexes in crystalline powders

Quadratic nonlinear optical properties for the crystalline powders of two types of ruthenium-bipyridine [Ru(bipy)₃] complexes were investigated. The nonlinear optical processes markedly depended on the molecular structures of the ruthenium complexes. Second harmonic generation (SHG) and very weak two-photon emission were observed for the alkylated ruthenium-bipyridine compexes with two long alkyl chains attached via amide bonds (RuC_nB), whereas only two-photon emission was observed for Ru(bipy)₃. The existence of two amide bonds in one bipyridine ligand for RuC_nB complexes most probably enhanced the molecular hyperpolarizability as compared with Ru(bipy)₃. The SHG intensity from RuC_nB complexes increased in the order RuC₁₈B < RuC₁₂B < RuC₁₆B. The order of SHG intensity from RuC_nB was ascribed to the difference in size of each crystalline powder estimated by X-ray diffraction methods.     

Syntheses, spectroscopic and molecular quadratic nonlinear optical properties of dipolar ruthenium(II) complexes of the ligand 1,2-phenylenebis(dimethylarsine)

Six new complex salts trans-[Ru(II)Cl(pdma)2L][PF6]n [pdma = 1,2-phenylenebis(dimethylarsine); L = (E,E,E)-1,6-bis(4-pyridyl)hexa-1,3,5-triene (bph), n= 1, 5; L =N-methyl-4-[(E)-2-(4-pyridyl)ethenyl]pyridinium (Mebpe+), n= 2, 7; L =N-methyl-4-[(E,E)-4-(4-pyridyl)buta-1,3-dienyl]pyridinium (Mebpb+), n= 2, 8; L =N-methyl-4-[(E,E,E)-6-(4-pyridyl)hexa-1,3,5-trienyl]pyridinium (Mebph+), n= 2, 9; L = bis(4-pyridyl)acetylene (bpa), n= 1, 10; L =N-methyl-4-[2-(4-pyridyl)ethynyl]pyridinium (Mebpa+), n= 2, 11] have been prepared. The electronic absorption spectra of 5 and 7-11 display intense, visible metal-to-ligand charge-transfer (MLCT) bands, with lambdamax values in the range 434-492 nm in acetonitrile. Cyclic voltammetric studies reveal reversible Ru(III/II) waves with E(1/2) values in the range 1.06-1.15 V vs. Ag-AgCl, together with irreversible L-based reduction processes. Along with a number of previously reported related compounds (B. J. Coe et al., J. Chem. Soc., Dalton Trans., 1996, 3917; 1997, 591; 2000, 797), salts 5 and 7-11 have been investigated by using Stark (electroabsorption) spectroscopy in butyronitrile glasses at 77 K. These studies have afforded dipole moment changes Deltamu12 for the MLCT transitions which have been used to calculate molecular static first hyperpolarizabilities beta0 according to the two-state equation beta0= 3Deltamu12(mu12)2/(Emax)2 (mu12 = transition dipole moment, Emax = MLCT energy). MLCT absorption and electrochemical data show that a trans-[Ru(II)Cl(pdma)2]+ centre is considerably less electron-rich than a [Ru(II)(NH3)5]2+ unit. Although the beta0 responses of the pdma complexes are only a little smaller than those of their [Ru(II)(NH3)5]2+ analogues, this result is partly attributable to unexpected changes in the relative mu12 values on freezing. Thus, substantial increases in mu12 for the arsine compounds act to partially offset the beta0-decreasing influence of their higher Emax values when compared with the analogous pentaammine species. Single crystal X-ray structures have been obtained for the salts 1(.)2.5MeCN, 4(.)2MeCN, 7 and 11, but only 1(.)2.5MeCN adopts a non-centrosymmetric space group (Fdd2) such as may show bulk NLO effects.     

Dipolar ruthenium(II) ammine complexes as electron transfer mediators of amperometric glucose sensors

The mediation of dipolar ruthenium(II) ammine complexes containing pyridinium ions [Ru(NH(3))(5)(L(+))](3+)(L(+): pyridinium ions) in glucose oxidation has been investigated by a voltammetric method. These ruthenium(II) complexes had appropriate redox potentials of 0.10-0.18 V vs. Ag/AgCl and high k(s) values of 5.7-17 x 10(6) M(-1) s(-1) which are the second-order rate constants for electron transfer from glucose oxidase in reduced form to [Ru(NH(3))(5)(L(+))](4+). In particular, the k(s) values for [Ru(NH(3))(5)(L(+))](3+) were greater than those of osmium(II)-polypyridine complexes possessing similar redox potentials which are most commonly used. All the dipolar ruthenium(II) complexes used in this study are therefore concluded to be useful for the electron transfer mediators of amperometric glucose sensors.     

Monomolecular Langmuir-Blodgett layers and films based on an amphiphilic nickel(II) macrocyclic complex

A study has been made on the behavior of a complex formed by nickel(II) with the amphiphilic macrocyclic ligand 6-n-octadecyl-1,4,6,8,11-pentaazacyclotetradecane on the surface of water, and the Langmuir-Blodgett technology has been used to deposit films of that compound on glass substrates with and without electrically conducting coatings (SnO₂). Some optical characteristics of these sandwich structures have been examined.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 30, No. 1, pp. 45–49, January–February, 1994.     

Langmuir-Blodgett films of amphiphilic bis(tetrathiafulvalene) macrocycles with four alkyl chains

Amphiphilic bis(tetrathiafulvalene) [bis(TTF)] macrocycles with four alkyl chains were fabricated as novel electrically active Langmuir-Blodgett (LB) films. Two TTF units were linked via [24]crown-8, [21]crown-7, and [18]crown-6 macrocycles, forming charge-transfer (CT) salts with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane (F4-TCNQ) at the air-water interface and on solid substrates. The CT salt of the amphiphilic bis(TTF)-macrocycle having a [24]crown-8 ring system formed a uniform surface morphology on mica. Using single-crystal X-ray structural analysis, the layer structure between the hydrophobic chains and the one-dimensional pi-pi stack of the CT salt was confirmed. Our results show that the bis(TTF)-macrocycle was folded at the flexible [24]crown-8 moiety, forming intramolecular pi-pi dimer structures and one-dimensional intermolecular pi-pi stacks with F4-TCNQ dimers. The open-shell electronic structure of the LB films was determined by electronic spectra, electrical conductivity, and electron spin resonance analyses. Asymmetry was introduced into the bis(TTF)-macrocycle by changing the ring size from [24]crown-8 to [21]crown-7. The surface morphology of the CT salts with F4-TCNQ was established as two-dimensional round-shape domains on mica. Further reduction of the macrocyclic ring from [21]crown-7 to [18]crown-6 resulted in a CT salt of the bis(TTF)-macrocycle with F4-TCNQ with a leaf-shape domain morphology and a typical dimension of approximately 1 microm2 on mica. In general, decreasing the macrocyclic ring size from [24]crown-8 to [21]crown-7 or [18]crown-6 affected the inter- and intramolecular interactions and the surface morphologies of LB films.     

Oxygen quenching of tris(2,2′-bipyridine) ruthenium(II) complexes in thin organic films

A study is presented of the quenching, by oxygen, of the luminescence of tris(2,2′-bipyridine) ruthenium(II) complexes immobilized in thin, transparent, polymer-based films. The film media consist of a water-insoluble linear polymer plasticized with a trialkylphosphate ester, in which the complex ruthenium cations are solubilized by ion pairing with organophilic anions such as tetraphenylborate.

Luminescence lifetimes were studied in relation oxygen concentration in a gas stream contiguous with the film medium, film thickness and concentration of the metal complex within the film medium. It is shown that the microheterogeneous environment of the luminescent complex, which has recently been implicated in the non-linear quenching responses of polymer-immobilized, transition metal complex oxygen sensors, may arise simply as a consequence of the limited solubility of the complex in the film medium. When solubility is limited, the partial precipitation of the complex results in a colloidal of luminescent particles which exhibit non- uniform susceptibilities to quenching by oxygen. Good solubility, and therefore linear quenching characteristics, are promoted by methyl substitution of the bipyridyl ligand and by use of a plasticizer (tributylphosphate) with marked cation solvating powers.     

Organometallic complexes for nonlinear optics. 30.1 electrochromic linear and nonlinear optical properties of alkynylbis(diphosphine)ruthenium complexes

A combination of cyclic voltammetry, UV-vis-NIR spectroelectrochemistry, time-dependent density functional theory (TD-DFT), and Z-scan measurements employing a modified optically transparent thin-layer electrochemical (OTTLE) cell has been used to identify and assign intense transitions of metal alkynyl complexes at technologically important wavelengths in the oxidized state and to utilize these transitions to demonstrate a facile electrochromic switching of optical nonlinearity. Cyclic voltammetric data for the ruthenium(II) complexes trans-[RuXY(dppe)(2)] [dppe = 1,2-bis(diphenylphosphino)ethane, X = Cl, Y = Cl (1), Ctbd1;CPh (2), 4-Ctbd1;CC(6)H(4)Ctbd1;CPh (3); X = Ctbd1;CPh, Y = Ctbd1;CPh (4), 4-Ctbd1;CC(6)H(4)Ctbd1;CPh (5)] show a quasi-reversible oxidation at 0.50-0.60 V (with respect to ferrocene/ferrocenium 0.56 V), which is assigned to the Ru(II/III) couple. The ruthenium(III) complex cations trans-[RuXY(dppe)(2)](+) were obtained by the in situ oxidation of complexes 1-5 using an OTTLE cell. The UV-vis-NIR optical spectra of 1(+)-5(+) contain a low-energy band in the near-IR region ( approximately 8000-16 000 cm(-)(1)), in contrast to 1-5, which are optically transparent at wavelengths < 22 000 cm(-)(1). TD-DFT calculations have been applied to model systems trans-[RuXY(PH(3))(4)] [X = Cl, Y = Cl, Ctbd1;CPh, or 4-Ctbd1;CC(6)H(4)Ctbd1;CPh; X = Ctbd1;CPh, Y = Ctbd1;CPh or 4-Ctbd1;CC(6)H(4)Ctbd1;CPh] to rationalize the optical spectra of 1-5 and 1(+)-5(+). The important low-energy bands in the electronic spectra of 1(+)-5(+) are assigned to the promotion of an electron from either a chloride p orbital or an ethynyl p orbital to the partially occupied HOMO. These absorption bands have been utilized to demonstrate a facile switching of cubic nonlinear optical (NLO) properties at 12 500 cm(-)(1) (corresponding to the wavelength of maximum transmission in biological materials such as tissue) using the OTTLE cell, the first electrochromic switching of molecular nonlinear refraction and absorption, and the first switching of optical nonlinearity using an electrochemical cell.     

Langmuir-Blodgett films based on inorganic molecular complexes with magnetic or optical properties

Langmuir and Langmuir-Blodgett (LB) films of a great variety of molecular metal complexes with interesting magnetic or optical properties have been prepared in the last few years. Some of the results obtained by our group and others are summarized in this article.     

Electroluminescence in ruthenium(II) complexes

We have investigated the electrochemical, spectroscopic, and electroluminescent properties of a family of diimine complexes of Ru featuring various aliphatic side chains as well as a more extended pi-conjugated system. The performance of solid-state electroluminescent devices fabricated from these complexes using indium tin oxide (ITO) and gold contacts appears to be dominated by ionic space charge effects. Their electroluminescence efficiency was limited by the photoluminescence efficiency of the Ru films and not by charge injection from the contacts. The incorporation of di-tert-butyl side chains on the dipyridyl ligand was found to be the most beneficial substitution in terms of reducing self-quenching of luminescence.     

Study of nonlinear optical properties of multilayer Langmuir-Blodgett films containing bacteriorhodopsin

Multilayer oriented Langmuir-Blodgett films of bacteriorhodopsin were prepared and their nonlinear optical properties, including second harmonic generation and photoresponse at a two phase-modulated beams mixing, were investigated. The nonlinear component of refractive index of the films was measured.     

Electropolymerization of vinylbipyridine complexes of ruthenium(II) and osmium(II) in SiO2 sol-gel films

PF(6)(-) salts of the complexes [Ru(vbpy)(3)](2+) and [Os(vbpy)(3)](2+) (vbpy = 4-methyl-4'-vinyl-2,2'-bipyridine) have been electropolymerized into the pores of SiO(2) sol-gel films deposited on conductive Tin(IV)-doped indium oxide-coated glass slides (ITO, In(2)O(3):Sn). The resulting transparent composites represent a new class of materials of general formulas ITO/SG-poly-[Ru(vbpy)(3)](PF(6))(2) and ITO/SG-poly-[Os(vbpy)(3)](PF(6))(2). The composites are stable with respect to loss of complexes to the external solution and demonstrate several interesting phenomena: (1) Sol-gel pores, serving as diffusion channels for the vbpy complexes and counterions, play a key role in the formation of the polymer and dictate the electrochemical properties of the resulting composite. (2) Dynamic polymer growth occurs within individual diffusion channels creating parallel structures of filled and unfilled channels. (3) Unidirectional charge transfer and a "bilayer" effect have been shown to operate in ITO/SG-poly-[Ru(vbpy)(3)](PF(6))(2) films exposed to [Os(vbpy)(3)](PF(6))(2) in the external solution. (4) Photophysical properties of the metal-to-ligand charge transfer (MLCT) excited states in ITO/SG-poly-[Ru(vbpy)(3)](PF(6))(2) composites are significantly modified compared to electropolymerized films on ITO or model monomeric complexes in solution.     

Dithiocarboxylate complexes of ruthenium(II) and osmium(II)

The ruthenium(II) complexes [Ru(R)(κ(2)-S(2)C·IPr)(CO)(PPh(3))(2)](+) (R = CH=CHBu(t), CH=CHC(6)H(4)Me-4, C(C≡CPh)=CHPh) are formed on reaction of IPr·CS(2) with [Ru(R)Cl(CO)(BTD)(PPh(3))(2)] (BTD = 2,1,3-benzothiadiazole) or [Ru(C(C≡CPh)=CHPh)Cl(CO)(PPh(3))(2)] in the presence of ammonium hexafluorophosphate. Similarly, the complexes [Ru(CH=CHC(6)H(4)Me-4)(κ(2)-S(2)C·ICy)(CO)(PPh(3))(2)](+) and [Ru(C(C≡CPh)=CHPh)(κ(2)-S(2)C·ICy)(CO)(PPh(3))(2)](+) are formed in the same manner when ICy·CS(2) is employed. The ligand IMes·CS(2) reacts with [Ru(R)Cl(CO)(BTD)(PPh(3))(2)] to form the compounds [Ru(R)(κ(2)-S(2)C·IMes)(CO)(PPh(3))(2)](+) (R = CH=CHBu(t), CH=CHC(6)H(4)Me-4, C(C≡CPh)=CHPh). Two osmium analogues, [Os(CH=CHC(6)H(4)Me-4)(κ(2)-S(2)C·IMes)(CO)(PPh(3))(2)](+) and [Os(C(C≡CPh)=CHPh)(κ(2)-S(2)C·IMes)(CO)(PPh(3))(2)](+) were also prepared. When the more bulky diisopropylphenyl derivative IDip·CS(2) is used, an unusual product, [Ru(κ(2)-SC(H)S(CH=CHC(6)H(4)Me-4)·IDip)Cl(CO)(PPh(3))(2)](+), with a migrated vinyl group, is obtained. Over extended reaction times, [Ru(CH=CHC(6)H(4)Me-4)Cl(BTD)(CO)(PPh(3))(2)] also reacts with IMes·CS(2) and NH(4)PF(6) to yield the analogous product [Ru{κ(2)-SC(H)S(CH=CHC(6)H(4)Me-4)·IMes}Cl(CO)(PPh(3))(2)](+)via the intermediate [Ru(CH=CHC(6)H(4)Me-4)(κ(2)-S(2)C·IMes)(CO)(PPh(3))(2)](+). Structural studies are reported for [Ru(CH=CHC(6)H(4)Me-4)(κ(2)-S(2)C·IPr)(CO)(PPh(3))(2)]PF(6) and [Ru(C(C≡CPh)=CHPh)(κ(2)-S(2)C·ICy)(CO)(PPh(3))(2)]PF(6).