Interfacial charge-transfer switch: ruthenium-dppz compounds anchored to nanocrystalline TiO2

The compound Ru(bpy)2(dppz-R)(PF6)2, where bpy is 2,2'-bipyridine and dppz-R is 11-(diethoxyphosphorylmethyl)dipyrido[3,2-a:2',3'-c]phenazine, was prepared and anchored to mesoporous nanocrystalline (anatase) TiO2 thin films as a probe of the effects of interfacial water on excited-state charge transfer processes at semiconductor interfaces. In nitrogen-saturated fluid acetonitrile, the Ru(bpy)2(dppz-R)(PF6)2 compound was found to be highly photoluminescent. Water was found to quench the excited state by a mechanism adequately described by the Perrin model, from which the radius of quenching was abstracted, 75 +/- 2 A. The Ru(bpy)2(dppz-R)(PF6)2 compounds were found to bind to the TiO2 thin films in high surface coverages, 5 x 10(-8) mol cm(-2). When these films were immersed in acetonitrile, long-lived excited states (tau = 825 ns) that were quenched by the addition of water were observed. About 30% of the excited states could not be quenched by water. Efficient electron injection, phi(inj) = 0.8, was observed after light excitation of Ru(bpy)2(dppz-R)/TiO2 in a 0.1 M LiClO4/acetonitrile solution. The addition of large concentrations of water, >0.5 M, was found to decrease the injection yield to phi(inj) = 0.3.     

Static and dynamic quenching of Ru(II) polypyridyl excited states by iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(bpy)(2)(deeb)(PF(6))(2), where bpy is 2,2-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, in dichloromethane were found to be efficiently quenched by iodide at room temperature. The ionic strength dependence of the UV-visible absorption spectra gave evidence for ion pairing. Iodide was found to quench the excited states by static and dynamic mechanisms. Stern-Volmer and Benesi-Hildebrand analysis of the spectral data provided a self-consistent estimate of the iodide-Ru(bpy)(2)(deeb)(2+) adduct in dichloromethane, K = 59 700 M(-1). Transient absorption studies clearly demonstrated an electron-transfer quenching mechanism with transient formation of I(2)(*)(-) in high yield, phi = 0.25 for 355 or 532 nm excitation. For Ru(bpy)(2)(deeb)(PF(6))(2) in acetonitrile, similar behavior could be observed at higher iodide concentrations than that required in dichloromethane. The parent Ru(bpy)(3)(2+) compound also ion pairs with iodide in CH(2)Cl(2), and light excitation gave a higher I(2)(*)(-) yield, phi = 0.50. X-ray crystallographic, IR, and Raman data gave evidence for interactions between iodide and the coordinated deeb ligand in the solid state.     

New dicarboxylic acid bipyridine ligand for ruthenium polypyridyl sensitization of TiO2

An ambidentate dicarboxylic acid bipyridine ligand, (4,5-diazafluoren-9-ylidene) malonic acid (dfm), was synthesized for coordination to Ru(II) and mesoporous nanocrystalline (anatase) TiO(2) thin films. The dfm ligand provides a conjugated pathway from the pyridyl rings to the carbonyl carbons of the carboxylic acid groups. X-ray crystal structures of [Ru(bpy)(2)(dfm)]Cl(2) and the corresponding diethyl ester compound, [Ru(bpy)(2)(defm)](PF(6))(2), were obtained. The compounds displayed intense metal-to-ligand charge transfer (MLCT) absorption bands in the visible region (ε > 11,000 M(-1) cm(-1) for [Ru(bpy)(2)(dfm)](PF(6))(2) in acetonitrile). Significant room temperature photoluminescence, PL, was absent in CH(3)CN but was observed at 77 K in a 4:1 EtOH:MeOH (v:v) glass. Cyclic voltammetry measurements revealed quasi-reversible Ru(III/II) electrochemistry. Ligand reductions were quasi-reversible for the diethyl ester compound [Ru(bpy)(2)(defm)](2+), but were irreversible for [Ru(bpy)(2)(dfm)](2+). Both compounds were anchored to TiO(2) thin films by overnight reactions in CH(3)CN to yield saturation surface coverages of 3 × 10(-8) mol/cm(2). Attenuated total reflection infrared measurements revealed that the [Ru(bpy)(2)(dfm)](2+) compound was present in the deprotonated carboxylate form when anchored to the TiO(2) surface. The MLCT excited states of both compounds injected electrons into TiO(2) with quantum yields of 0.70 in 0.1 M LiClO(4) CH(3)CN. Micro- to milli-second charge recombination yielded ground state products. In regenerative solar cells with 0.5 M LiI/0.05 M I(2) in CH(3)CN, the Ru(bpy)(2)(dfm)/TiO(2) displayed incident photon-to-current efficiencies of 0.7 at the absorption maximum. Under the same conditions, the diethylester compound was found to rapidly desorb from the TiO(2) surface.     

Enhancement of metal-metal coupling at a considerable distance by using 4-pyridinealdazine as a bridging ligand in polynuclear complexes of rhenium and ruthenium

Novel polynuclear complexes of rhenium and ruthenium containing PCA (PCA = 4-pyridinecarboxaldehyde azine or 4-pyridinealdazine or 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) as a bridging ligand have been synthesized as PF(6-) salts and characterized by spectroscopic, electrochemical, and photophysical techniques. The precursor mononuclear complex, of formula [Re(Me(2)bpy)(CO)(3)(PCA)](+) (Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine), does not emit at room temperature in CH(3)CN, and the transient spectrum found by flash photolysis at lambda(exc) = 355 nm can be assigned to a MLCT (metal-to-ligand charge transfer) excited state [(Me(2)bpy)(CO)(3)Re(II)(PCA(-))](+), with lambda(max) = 460 nm and tau < 10 ns. The spectral properties of the related complexes [[Re(Me(2)bpy)(CO)(3)}(2)(PCA)](2+), [Re(CO)(3)(PCA)(2)Cl], and [Re(CO)(3)Cl](3)(PCA)(4) confirm the existence of this low-energy MLCT state. The dinuclear complex, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(II)(NH(3))(5)](3+), presents an intense absorption in the visible spectrum that can be assigned to a MLCT d(pi)(Ru) --> pi(PCA); in CH(3)CN, the value of lambda (max) = 560 nm is intermediate between those determined for [Ru(NH(3))(5)(PCA)](2+) (lambda(max) = 536 nm) and [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](4+) (lambda(max) = 574 nm), indicating a significant decrease in the energy of the pi-orbital of PCA. The mixed-valent species, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(III)(NH(3))(5)](4+), was obtained in CH(3)CN solution, by bromine oxidation or by controlled-potential electrolysis at 0.8 V in a OTTLE cell of the [Re(I),Ru(II)] precursor; the band at lambda(max) = 560 nm disappears completely, and a new band appears at lambda(max) = 483 nm, assignable to a MMCT band (metal-to-metal charge transfer) Re(I) --> Ru(III). By using the Marcus-Hush formalism, both the electronic coupling (H(AB)) and the reorganization energy (lambda) for the metal-to-metal intramolecular electron transfer have been calculated. Despite the considerable distance between both metal centers (approximately 15.0 Angstroms), there is a moderate coupling that, together with the comproportionation constant of the mixed-valent species [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](5+) (K(c) approximately 10(2), in CH(3)CN), puts into evidence an unusual enhancement of the metal-metal coupling in the bridged PCA complexes. This effect can be accounted for by the large extent of "metal-ligand interface", as shown by DFT calculations on free PCA. Moreover, lambda is lower than the driving force -DeltaG degrees for the recombination charge reaction [Re(II),Ru(II)] --> [Re(I),Ru(III)] that follows light excitation of the mixed-valent species. It is then predicted that this reverse reaction falls in the Marcus inverted region, making the heterodinuclear [Re(I),Ru(III)] complex a promising model for controlling the efficiency of charge-separation processes.     

A nuclear isotope effect for interfacial electron transfer: excited-state electron injection from Ru ammine compounds to nanocrystalline TiO2

The coordination compounds Ru(deeb)(NH3)4(PF6)2 and Ru(deeb)(NH2(CH2)2NH2)4(PF6)2, where deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine, were synthesized and attached to optically transparent nanocrystalline (anatase) TiO2 films. The compounds were found to be nonemissive in fluid acetonitrile and when attached to TiO2 with excited-state lifetimes <10 ns. Infrared measurements showed the expected isotopic substitution of the deuterated compounds on TiO2 thin films. A small 10-15 mV shift in the RuIII/II reduction potentials was measured upon deuteration. Metal-to-ligand charge-transfer (MLCT) excitation resulted in interfacial electron transfer into the TiO2 semiconductor with quantum yields that were dependent on the excitation wavelength and deuteration of the ammine ligands. The quantum yields were optimized with blue light excitation (417 nm) and deuterium substitution. In contrast, the kinetic rate constants for charge recombination were insensitive to deuteration and the excitation wavelength. Control experiments with Ru(deeb)(bpy)2(PF6)2 indicated that deuteration of the TiO2 surface alone does not affect the injection or recombination processes. A model is proposed wherein electron injection occurs in competition with vibrational relaxation and/or intersystem crossing of the excited states. Exchange of hydrogen by deuterium slows vibrational relaxation and/or intersystem crossing, resulting in higher injection yields.     

Ruthenium(II)-polyazine light absorbers bridged to reactive cis-dichlororhodium(III) centers in a bimetallic molecular architecture

Bimetallic complexes of the form [(bpy)(2)Ru(BL)RhCl(2)(phen)](PF(6))(3), where bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, and BL = 2,3-bis(2-pyridyl)pyrazine (dpp) or 2,2'-bipyrimidine (bpm), were synthesized, characterized, and compared to the [{(bpy)(2)Ru(BL)}(2)RhCl(2)](PF(6))(5) trimetallic analogues. The new complexes were synthesized via the building block method, exploiting the known coordination chemistry of Rh(III) polyazine complexes. In contrast to [{(bpy)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5) and [{(bpy)(2)Ru(bpm)}(2)RhCl(2)](PF(6))(5), [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) and [(bpy)(2)Ru(bpm)RhCl(2)(phen)](PF(6))(3) have a single visible light absorber subunit coupled to the cis-Rh(III)Cl(2) moiety, an unexplored molecular architecture. The electrochemistry of [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) showed a reversible oxidation at 1.61 V (vs Ag/AgCl) (Ru(III/II)), quasi-reversible reductions at -0.39 V, -0.74, and -0.98 V. The first two reductive couples corresponded to two electrons, consistent with Rh reduction. The electrochemistry of [(bpy)(2)Ru(bpm)RhCl(2)(phen)](PF(6))(3) exhibited a reversible oxidation at 1.76 V (Ru(III/II)). A reversible reduction at -0.14 V (bpm(0/-)), and quasi-reversible reductions at -0.77 and -0.91 V each corresponded to a one electron process, bpm(0/-), Rh(III/II), and Rh(II/I). The dpp bridged bimetallic and trimetallic display Ru(dpi)-->dpp(pi*) metal-to-ligand charge transfer (MLCT) transitions at 509 nm (14,700 M(-1) cm(-1)) and 518 nm (26,100 M(-1) cm(-1)), respectively. The bpm bridged bimetallic and trimetallic display Ru(dpi)-->bpm(pi*) charge transfer (CT) transitions at 581 nm (4,000 M(-1) cm(-1)) and 594 nm (9,900 M(-1) cm(-1)), respectively. The heteronuclear complexes [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) and [{(bpy)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5) had (3)MLCT emissions that are Ru(dpi)-->dpp(pi*) CT in nature but were red-shifted and lower intensity than [(bpy)(2)Ru(dpp)Ru(bpy)(2)](PF(6))(4). The lifetimes of the (3)MLCT state of [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) at room temperature (30 ns) was shorter than [(bpy)(2)Ru(dpp)Ru(bpy)(2)](PF(6))(4), consistent with favorable electron transfer to Rh(III) to generate a metal-to-metal charge-transfer ((3)MMCT) state. The reported synthetic methods provide means to a new molecular architecture coupling a single Ru light absorber to the Rh(III) center while retaining the interesting cis-Rh(III)Cl(2) moiety.     

Large Molecular Quadratic Hyperpolarizabilities in Donor/Acceptor-Substituted trans-Tetraammineruthenium(II) Complexes

A series of new Ru(II) complex salts trans-[Ru(NH(3))(4)(L(1))(L(2))](PF(6))(n) [n = 2, L(1) = 4-acetylpyridine (4-acpy) and L(2) = 4-(dimethylamino)pyridine (dmap) (1), 4-(dimethylamino)benzonitrile (dmabn) (2), 4-picoline (4-pic) (3), or 1-methylimidazole (1-MeIm) (4); n = 3, L(1) = N-methyl-4,4'-bipyridinium (MeQ(+)) and L(2) = dmap (6), dmabn (7), 1-MeIm (8), 4-acpy (9), or phenothiazine (PTZ) (10); n = 2, L(1) = dmap and L(2) = 4-pyridinecarboxaldehyde (pyca) (12) or ethyl isonicotinate (isne) (13)] have been synthesized and fully characterized. These complexes display intense, visible metal-to-ligand charge-transfer (MLCT) absorptions which are highly solvatochromic. An X-ray crystal structure determination has been carried out for trans-[Ru(NH(3))(4)(MeQ(+))(PTZ)](PF(6))(3).Me(2)CO (10.Me(2)CO). This salt, empirical formula C(26)H(38)F(18)N(7)OP(3)RuS, crystallizes in the hexagonal system, space group P6(3), with a = b = 17.853(4) ?, c = 21.514(6) ?, and Z = 6. The MeQ(+) ligand adopts an almost planar conformation, with a torsion angle of 9.6 degrees between the two pyridyl rings. The dipolar cations exhibit a strong projected component along the z axis, but crystal twinning precludes second-harmonic generation. Measurements of the first hyperpolarizability beta by using the hyper-Rayleigh scattering technique at 1064 nm yield very large values in the range (232-621) x 10(-30) esu, the largest being for trans-[Ru(NH(3))(4)(MeQ(+))(dmabn)](PF(6))(3) (7). These beta values are resonance enhanced via the MLCT excitations. A correlation between beta and the MLCT absorption energy confirms that this excitation is the primary contributor to beta. The two-level model yields static hyperpolarizabilities beta(0) in the range (10-130) x 10(-30) esu, with trans-[Ru(NH(3))(4)(MeQ(+))(dmap)](PF(6))(3) (6) having the largest. The beta(0) values of the complexes of the bipyridyl ligand MeQ(+) are larger than those of their analogues containing monopyridyl ligands because of extended conjugation. beta(0) correlates with the MLCT energy only when the MLCT absorption is sufficiently far from the second harmonic at 532 nm.     

Reactivity of radicals generated on irradiation of trans-[Ru(NH3)4(NO2)P(OEt)3](PF6)

The electronic absorption spectrum of trans-[Ru(NH(3))(4)(NO(2))(P(OEt)(3)](+) in aqueous solution is characterized by a strong absorption band at 334 nm (lambda(max) = 1800 mol(-1) L cm(-1)). On the basis of quantum mechanics calculations, this band has been assigned to a MLCT transition from the metal to the nitro ligand. Molecular orbital calculations also predict an LF transition at 406 nm, which is obscured by the intense MLCT transition. When trans-[Ru(NH(3))(4)(NO(2))(P(OEt)(3)](+) in acetonitrile is irradiated with a 355 nm pulsed laser light, the absorption features are gradually shifted to represent those of the solventocomplex trans-[Ru(NH(3))(4)(solv)(P(OEt)(3)](2+) (lambda(max) = 316 nm, epsilon = 650 mol(-1) L cm(-1)), which was also detected by (31)P NMR spectroscopy. The net photoreaction under these conditions is a photoaquation of trans-[Ru(NH(3))(4)(NO(2))(P(OEt)(3)](+), although, after photolysis, the presence of the nitric oxide was detected by differential pulse polarography. In phosphate buffer pH 9.0, after 15 min of photolysis, a thermal reaction resulted in the formation of a hydroxyl radical and a small amount of a paramagnetic species as detected by EPR spectroscopy. In the presence of trans-[Ru(NH(3))(4)(solv)P(OEt)(3)](2+), the hydroxyl radical initiated a chain reaction. On the basis of spectroscopic and electrochemical data, the role of the radicals produced is analyzed and a reaction sequence consistent with the experimental results is proposed. The 355 nm laser photolysis of trans-[Ru(NH(3))(4)(NO(2))(P(OEt)(3)](+) in phosphate buffer pH 7.4 also gives nitric oxide, which is readily trapped by ferrihemeproteins (myoglobin, hemoglobin, and cytochrome C), giving rise to the formation of their nitrosylhemeproteins(II), (NO)Fe(II)hem.     

Evidence for static quenching of MLCT excited states by iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(deeb)(bpy)(2)(PF(6))(2) [where bpy is 2,2-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine] in acetonitrile or dichloromethane were found to be quenched by iodide at room temperature. The ionic strength dependence of the optical spectra gave evidence for ion pairing. Iodide is found to quench the photoluminescence (PL) intensity and influence the spectral distribution of the emitted light. A static component to the time-resolved PL quenching provided further evidence for ground-state adduct. Stern-Volmer analysis of the static component provided an estimate of the iodide-Ru(deeb)(bpy)(2)(2+) adduct equilibrium constant in dichloromethane, K(sv) = 40,000 M(-)(1). Transient absorption studies clearly demonstrate that an electron-transfer quenching mechanism is operative and that I(2)(-)(*) can be photoproduced in high yield, phi = 0.25. For Ru(bpy)(3)(PF(6))(2) in acetonitrile, similar behavior could be observed at iodide concentrations >100 times that required for dichloromethane.     

Sensitization and stabilization of TiO(2) photoanodes with electropolymerized overlayer films of ruthenium and zinc polypyridyl complexes: a stable aqueous photoelectrochemical cell

Overlayer thin films of vinylbipyridine (vbpy)-containing Ru and Zn complexes have been formed on top of ruthenium dye complexes adsorbed to TiO(2) by reductive electropolymerization. The goal was to create an efficient, water-stable photoelectrode or electrodes. An adsorbed-[Ru(vbpy)(2)(dcb)](PF(6))(2)/poly-[Ru(vbpy)(3)](PF(6))(2) surface composite displays excellent stability toward dissolution in water, but the added overlayer film greatly decreases incident photon-to-current conversion efficiencies (IPCE) in propylene carbonate with I(3)(-)/I(-) as the carrier couple. An ads-[Ru(vbpy)(2)(dcb)](PF(6))(2)/poly-[Zn(vbpy)(3)](PF(6))(2) composite displays no loss in IPCE compared to ads-[Ru(vbpy)(2)(dcb)](PF(6))(2) but is susceptible to film breakdown in the presence of water by solvolysis and loss of the cross-linking Zn(2+) ions. Success was attained with an ads-[Ru(vbpy)(2)(dcb)](PF(6))(2)/poly-[Ru(vbpy)(2)(dppe)](PF(6))(2) composite. In this case the electropolymerized layer is transparent in the visible. The composite electrode is stable in water, the IPCE in propylene carbonate with I(3)(-)/I(-) is comparable to the adsorbed complex, and a significant IPCE is observed in water with the quinone/hydroquinone carrier couple. The assembly [(bpy)(2)(CN)Ru(CN)Ru(vbpy)(2)(NC)Ru(CN)(bpy)(2)](PF(6))(2) ([Ru(CN)Ru(NC)Ru](PF(6))(2)) adsorbs spontaneously on TiO(2), and electropolymerization of thin layers of the assembly to give ads-[Ru(CN)Ru(NC)Ru](PF(6))(2)/poly-[Ru(CN)Ru(NC)Ru](PF(6))(2) enhances IPCE and has no deleterious effect on the IPCE/Ru.     

Nitric oxide production by visible light irradiation of aqueous solution of nitrosyl ruthenium complexes

[Ru(II)L(NH(3))(4)(pz)Ru(II)(bpy)(2)(NO)](PF(6))(5) (L is NH(3), py, or 4-acpy) was prepared with good yields in a straightforward way by mixing an equimolar ratio of cis-[Ru(NO(2))(bpy)(2)(NO)](PF(6))(2), sodium azide (NaN(3)), and trans-[RuL(NH(3))(4)(pz)] (PF(6))(2) in acetone. These binuclear compounds display nu(NO) at ca. 1945 cm(-)(1), indicating that the nitrosyl group exhibits a sufficiently high degree of nitrosonium ion (NO(+)). The electronic spectrum of the [Ru(II)L(NH(3))(4)(pz)Ru(II)(bpy)(2)(NO)](5+) complex in aqueous solution displays the bands in the ultraviolet and visible regions typical of intraligand and metal-to-ligand charge transfers, respectively. Cyclic voltammograms of the binuclear complexes in acetonitrile give evidence of three one-electron redox processes consisting of one oxidation due to the Ru(2+/3+) redox couple and two reductions concerning the nitrosyl ligand. Flash photolysis of the [Ru(II)L(NH(3))(4)(pz)Ru(II)(bpy)(2)(NO)](5+) complex is capable of releasing nitric oxide (NO) upon irradiation at 355 and 532 nm. NO production was detected and quantified by an amperometric technique with a selective electrode (NOmeter). The irradiation at 532 nm leads to NO release as a consequence of a photoinduced electron transfer. All species exhibit similar photochemical behavior, a feature that makes their study extremely important for their future application in the upgrade of photodynamic therapy in living organisms.     

Toward exceeding the Shockley-Queisser limit: photoinduced interfacial charge transfer processes that store energy in excess of the equilibrated excited state

Nanocrystalline (anatase), mesoporous TiO2 thin films were functionalized with [Ru(bpy)2(deebq)](PF6)2, [Ru(bq)2(deeb)](PF6)2, [Ru(deebq)2(bpy)](PF6)2, [Ru(bpy)(deebq)(NCS)2], or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine, bq is 2,2'-biquinoline, and deeb and deebq are 4,4'-diethylester derivatives. These compounds bind to the nanocrystalline TiO2 films in their carboxylate forms with limiting surface coverages of 8 (+/- 2) x 10(-8) mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.70 V vs SCE) occurs prior to TiO2 reduction. Steady state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced sensitizer. The thermally equilibrated metal-to-ligand charge-transfer excited state and the reduced form of these compounds do not inject electrons into TiO2. Nanosecond transient absorption measurements demonstrate the formation of an extremely long-lived charge separated state based on equal concentrations of the reduced and oxidized compounds. The results are consistent with a mechanism of ultrafast excited-state injection into TiO2 followed by interfacial electron transfer to a ground-state compound. The quantum yield for this process was found to increase with excitation energy, a behavior attributed to stronger overlap between the excited sensitizer and the semiconductor acceptor states. For example, the quantum yields for [Os(bpy)2(dcbq)]/TiO2 were phi(417 nm) = 0.18 +/- 0.02, phi(532.5 nm) = 0.08 +/- 0.02, and phi(683 nm) = 0.05 +/- 0.01. Electron transfer to yield ground-state products occurs by lateral intermolecular charge transfer. The driving force for charge recombination was in excess of that stored in the photoluminescent excited state. Chronoabsorption measurements indicate that ligand-based intermolecular electron transfer was an order of magnitude faster than metal-centered intermolecular hole transfer. Charge recombination was quantified with the Kohlrausch-Williams-Watts model.     

Excited-state acid-base chemistry: evidence for a dissociative excited state

Excitation (410 nm) of the bimetallic [(bpy)(2)Ru(CN)(mu-CN)Rh(NH(3))(4)Br](2+) produces the MLCT state localized on the (bpy)(2)Ru(CN)(2) ligand. Photoinduced cleavage of the bimetallic occurs in the presence of [H(+)], and the dependence yields a K(a) equivalent to that for ground-state cis-(bpy)(2)Ru(CN)(2) implying separation of the bimetallic prior to relaxation. The pH dependence and the emissivity of a bimetallic composed of components that individually quench at a diffusion controlled rate suggest that rupture of the RuCN-Rh bond is due to the reduction in electron density at the cyano ligand that occurs on population of the MLCT state. Unlike known photoinduced metal ligand dissociations, where the excitation energy is consumed in the dissociation, the dissociated "(bpy)(2)Ru(CN)(2) ligand" remains excited.     

Incorporation of amphiphilic ruthenium(II) ammine complexes into Langmuir-Blodgett thin films with switchable quadratic nonlinear optical behavior

Nine nonlinear optical (NLO) chromophores with pyridinium electron acceptors have been synthesized by complexing new proligands with {Ru(II)(NH(3))(5)}(2+) electron-donor centers. The presence of long alkyl/fluoroalkyl chain substituents imparts amphiphilic properties, and these cationic complexes have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Each complex shows three reversible/quasireversible redox processes; a Ru(III/II) oxidation and two ligand-based reductions. The energies of the intense visible d → π* metal-to-ligand charge-transfer (MLCT) absorptions correlate to some extent with the ligand reduction potentials. (1)H NMR spectroscopy also provides insights into the relative electron-withdrawing strengths of the new ligands. Single crystal X-ray structures have been determined for two of the proligand salts and one complex salt, [Ru(II)(NH(3))(5)(4-C(16)H(33)PhQ(+))]Cl(3)·3.25H(2)O (PhQ(+) = N-phenyl-4,4'-bipyridinium), showing centrosymmetric packing structures in each case. The PF(6)(-) analogue of the latter complex has been used to deposit reproducibly high-quality, multilayered Langmuir-Blodgett (LB) thin films. These films show a strong second harmonic generation (SHG) response from a 1064 nm laser; their MLCT absorbance increases linearly with the number of layers (N) and I(2ω)/I(ω)(2) (I(2ω) = intensity at 532 nm; I(ω) = intensity at 1064 nm) scales quadratically with N, consistent with homogeneous deposition. LB films on indium tin oxide (ITO)-coated glass show electrochemically induced switching of the SHG response, with a decrease in activity of about 50% on Ru(II) → Ru(III) oxidation. This effect is reversible, but reproducible over only a few cycles before the signal from the Ru(II) species diminishes. This work extrapolates our original solution studies (Coe, B. J. et al. Angew. Chem., Int. Ed.1999, 38, 366) to the first demonstration of redox-switching of NLO activity in a molecular material.     

Interplay of light antenna and excitation "energy reservoir" effects in a bichromophoric system based on ruthenium-polypyridine and pyrene units linked by a long and flexible poly(ethylene glycol) chain

Steady-state and time-resolved spectroscopic properties of bichromophoric species containing [Ru(bpy)(3)](2+) and pyrene (pyr) units linked together by flexible poly(ethylene glycol) chains of variable length, [Ru(bpy)(2)(bpy-pyr)](PF(6))(2) (1) and [Ru(bpy)(2)(bpy-O6-pyr)](PF(6))(2) (2), have been investigated in acetonitrile solvent. The complexes were designed with the aim of examining the intercomponent energy-transfer processes taking place after light absorption at the two chromophores and the influence of the distance separation between them; in the case of complex 2, the linking chain in the extended conformation is as long as 21 A. Direct excitation of the pyrene unit (lambda(exc) = 410 nm) results in singlet-to-singlet energy transfer (an antenna effect) to the Ru-based component, (1)pyr --> (1)MLCT, which we analyze in terms of the F?rster mechanism taking place with unit efficiency. Analysis of the time-resolved pyrene fluorescence reveals that the actual center-to-center distance separation (d(cc)) between the photoactive centers changes according to a Gaussian distribution, with an average d(cc) = 13.6 A (distribution width, a = 2.8 A) and 12 A (a = 10.2 A), for 1 and 2, respectively; this is ascribed to folding of the poly(ethylene glycol) linking chain. In O(2)-free solvent at room temperature, after population of the (1)MLCT level (which takes place either because of direct excitation by using lambda(exc) > 355 nm or via the "antenna" effect) and subsequent intersystem crossing localized at the Ru center, (1)MLCT --> (3)MLCT, a triplet-triplet thermal equilibration is established which involves the physically separated centers, (3)MLCT <--> (3)pyr, with K(eq) = 11 (the energy gap between the two levels is 480 cm(-1), as determined from luminescence data obtained at 77 K). As a consequence of this equilibrium, the (3)MLCT luminescence lifetime becomes tau(Ru) approximately 9 micros both in 1 and 2, i.e., 1 order of magnitude longer than for the unsubstituted [Ru(bpy)(3)](2+) luminophore. In air-equilibrated solvent, diffusional quenching by O(2) effectively depletes the (3)pyr level and only the forward (3)MLCT --> (3)pyr energy transfer step is observed with k(en) = 4 x 10(8) and 2 x 10(8) s(-1) for 1 and 2, respectively. As briefly discussed, reasons for the high rate constants observed for the various triplet-triplet steps may be traced back to the folding properties of the linking chains.     

Reactions of acetonitrile coordinated to a nitrosylruthenium complex with H2O or CH(3)OH under mild conditions: structural characterization of imido-type complexes

The reaction of cis-[Ru(NO)(CH(3)CN)(bpy)(2)](3+) (bpy = 2,2'-bipyridine) in H(2)O at room temperature proceeded to afford two new nitrosylruthenium complexes. These complexes have been identified as nitrosylruthenium complexes containing the N-bound methylcarboxyimidato ligand, cis-[Ru(NO)(NH=C(O)CH(3))(bpy)(2)](2+), and methylcarboxyimido acid ligand, cis-[Ru(NO)(NH=C(OH)CH(3))(bpy)(2)](3+), formed by an electrophilic reaction at the nitrile carbon of the acetonitrile coordinated to the ruthenium ion. The X-ray structure analysis on a single crystal obtained from CH(3)CN-H(2)O solution of cis-[Ru(NO)(NH=C(O)CH(3))(bpy)(2)](PF(6))(3) has been performed: C(22)H(20.5)N(6)O(2)P(2.5)F(15)Ru, orthorhombic, Pccn, a = 15.966(1) A, b = 31.839(1) A, c = 11.707(1) A, V = 5950.8(4) A(3), and Z = 8. The structural results revealed that the single crystal consisted of 1:1 mixture of cis-[Ru(NO)(NH=C(O)CH(3))(bpy)(2)](2+) and cis-[Ru(NO)(NH=C(OH)CH(3))(bpy)(2)](3+) and the structural formula of this single crystal was thus [Ru(NO)(NH=C(OH(0.5))CH(3))(bpy)(2)](PF(6))(2.5). The reaction of cis-[Ru(NO)(CH(3)CN)(bpy)(2)](3+) in dry CH(3)OH-CH(3)CN at room temperature afforded a nitrosylruthenium complex containing the methyl methylcarboxyimidate ligand, cis-[Ru(NO)(NH=C(OCH(3))CH(3))(bpy)(2)](3+). The structure has been determined by X-ray structure analysis: C(25)H(29)N(8)O(18)Cl(3)Ru, monoclinic, P2(1)/c, a = 13.129(1) A, b = 17.053(1) A, c = 15.711(1) A, beta = 90.876(5) degrees, V = 3517.3(4) A(3), and Z = 4.     

Ligand-localized electron trapping at sensitized semiconductor interfaces

Nanocrystalline (anatase), mesoporous TiO2 thin films were derivatized with [Ru(bpy)2(deebq)](PF6)2 or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine and deebq is 4,4'-diethylester-2,2'-biquinoline. Both compounds bind to the nanocrystalline TiO2 films with typical limiting surface coverages of 7 (+/-2) x 10-8 mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.60 V vs SCE) occurs prior to TiO2 reduction. Steady-state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced compound, MII(deebq-)(bpy)2+/TiO2. Neither the photoluminescent excited states or the reduced forms of these compounds inject electrons efficiently into TiO2. Transient absorption measurements after a approximately 10-ns laser pulse, reveal greater than 80% MLCT excited states and a smaller fraction of extremely long-lived charge-separated state intermediates assigned to equal concentrations of MII(deebq-)(bpy)2+/TiO2 and MIII(deebq)(bpy)23+/TiO2. The results are consistent with a mechanism of ultrafast electron injection followed by ligand-localized trapping on a second compound. The quantum yield for formation of the charge-separated states (phiCSS) is excitation wavelength dependent. With 417 nm excitation, phiCSS(417) = 0.14 +/- 0.03, and this decreases with 532.5 nm excitation, phiCSS(532.5) = 0.08 +/- 0.03, and 683 nm excitation for M = Os, phiCSS(683) = 0.05 +/- 0.01. Electron transfer to yield ground-state products, MII(deebq-)(bpy)2+/TiO2 + MIII(deebq)(bpy)23+/TiO2 --> 2 MII(deebq)(bpy)22+/TiO2, occurs with a driving force of 2.05 eV for Ru/TiO2 and 1.64 eV for Os/TiO2. The dynamics of this process were quantified on a millisecond time scale and were found to follow second-order kinetics. The intermediates are sufficiently long-lived that continued pulsed excitation at 10 Hz leads to high concentrations and the formation of transient images on the semiconductor surface that are easily observed by the naked eye.     

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.     

Visible light induced photocleavage of DNA by a mixed-metal supramolecular complex: [[(bpy)(2)Ru(dpp)](2)RhCl2]5+

The mixed-metal supramolecular complex, [[(bpy)(2)Ru(dpp)](2)RhCl(2)](PF(6))(5) (bpy = 2,2'-bipyridine and dpp = 2,3-bis(2-pyridyl)pyrazine) coupling two ruthenium light absorbers (LAs) to a central rhodium, has been shown to photocleave DNA. This system possesses a lowest lying metal to metal charge transfer (MMCT) excited state in contrast to the metal to ligand charge transfer states (MLCT) of the bpm and Ir analogues. The systems with an MLCT excited state do not photocleavage DNA. [[(bpy)(2)Ru(dpp)](2)RhCl(2)](PF(6))(5) is the first supramolecular system shown to cleave DNA. It functions through an excited state previously unexplored for this reactivity, a Ru --> Rh MMCT excited state. This system functions when irradiated with low energy visible light with or without molecular oxygen.     

High-extinction ruthenium compounds for sunlight harvesting and hole transport

The compounds Ru(bpy) 2(BTL)(PF 6) 2 and Ru(deeb) 2(BTL)(PF 6) 2, where bpy is 2,2'-bipyridine, deeb is 4,4'-(C 2H 5CO 2) 2-bpy, and BTL is 9'-[4,5-bis(cyanoethylthio)]-1,3-dithiol-2-ylidene]-4',5'-diazafluorene, were found to have very high extinction coefficients in the visible region. In an acetonitrile solution, the extinction of Ru(deeb) 2(BTL)(PF 6) 2 was = 44 000 +/- 1000 M (-1) cm (-1) at lambda = 470 nm. Two quasi-reversible oxidation waves, E 1/2 = +0.88 and +1.16 V, and an irreversible reduction, E pr = -1.6 V, were observed versus ferrocene (Fc (+/0)). At -40 degrees C, a state was observed with spectroscopic properties characteristic of a metal-to-ligand charge-transfer excited state, tau = 25 ns. This same compound was found to photoinject electrons into TiO 2 with a quantum yield Phi = 0.3 +/- 0.2 for 532.5 or 417 nm light excitation in a 0.1 M LiClO 4/acetonitrile electrolyte. In regenerative solar cells, a sustained photocurrent was observed with a maximum incident photon-to-current efficiency of 0.4. The photocurrent action and absorptance spectra were in good agreement, consistent with injection from a single excited state.