Synthesis, structure, spectroscopy and redox chemistry of square-planar nickel(II) complexes with tetradentate o-phenylenedioxamidates and related ligands

A series of four-coordinate square-planar nickel(II) complexes of o-phenylenebis(N'-methyloxamidate)(L1) and related o-phenylene(N'-methyloxamidate)oxamate (L2) and o-phenylenebis(oxamate)(L3) tetradentate ligands have been synthesized and characterized structurally, spectroscopically and electrochemically. The parent nickel(II)-L1 complex presents an intense MLCT band in the UV region (lambda max = 357 nm) and a distinctive 1 s --> 4p CT satellite in the Ni K-edge XANES spectrum (E = 8339.2 eV). These features together with the short Ni-N(amidate) bond lengths (1.85-1.93 A) as revealed by the analysis of the Ni K-edge EXAFS spectrum and confirmed by single-crystal X-ray diffraction are typical of square-planar low spin (S = 0) Ni(II) ions. The dianionic nickel(II) complexes, [Ni(II)L(i)](2-)(i = -3), experience two redox processes in acetonitrile at 25 degrees C. The first redox process, at moderately low potentials (E1 = 0.12-0.52 V vs. SCE), is a reversible one-electron metal-centered oxidation to the corresponding monoanionic nickel(III) complexes, [Ni(III)L(i)]-. The second redox process, at relatively high potentials (E2 = 0.86-1.04 V vs. SCE), is a quasireversible to irreversible one-electron oxidation largely centered on the o-benzenediamidate fragment of the non-innocent ligand, yielding the corresponding neutral nickel(iii) complexes with a o-benzosemiquinonediimine pi-cation radical ligand, [Ni(III)(L(i))*+]. The singly and doubly oxidized species of the parent nickel(II)-L1 complex have been prepared by chemical oxidation and characterized spectroscopically in acetonitrile at -40 degrees C. The stable singly oxidized nickel(III)-L1 species presents an intense LMCT band in the NIR region (lambda max = 910 nm) and a rhombic X-band EPR spectrum (g1 = 2.193, g2 = 2.080 and g3 = 2.006) characteristic of square-planar low spin (S = 1/2) Ni(III) ions. The unstable double oxidized nickel(III)-L1 pi-cation radical species exhibits a rather intense visible band (lambda max = 645 nm) that is tentatively assigned as a MLCT transition from the Ni(III)-benzosemiquinone type ground state to the Ni(IV) excited state.     

Chemistry and electrochemistry of the heterodinuclear complex ClPd(dppm)2PtCl: a M-M' bond providing site selectivity

The heterodinuclear d(9)-d(9) title compound 1, whose crystal structure has been solved, reacts with dppm [bis(diphenylphosphino)methane] in the presence of NaBF4 to generate the salt [ClPd(mu-dppm)2Pt(eta(1)-dppm)][BF4] (2a), which contains a Pt-bound dangling dppm ligand. 2a has been characterized by 1H and 31P NMR, Fourier transform Raman [nu(Pd-Pt) = 138 cm(-1)], and UV-vis spectroscopy [lambda(max)(dsigma-dsigma*) = 366 nm]. In a similar manner, [ClPd(mu-dppm)2Pt(eta(1)-dppm=O)][BF4] (2b), ligated with a dangling phosphine oxide, has been prepared by the addition of dppm=O. The molecular structure of 2b has been established by an X-ray diffraction study. 2a reacts with 1 equiv of NaBH4 to form the platinum hydride complex [(eta(1)-dppm)Pd(mu-dppm)2Pt(H)][BF4] (3). Both 2a and 3 react with an excess of NaBH4 to provide the mixed-metal d(10)-d(10) compound [Pd(mu-dppm)3Pt] (4). The photophysical properties of 4 were studied by UV-vis spectroscopy [lambda(max)(dsigma-dsigma*) = 460 nm] and luminescence spectroscopy (lambda(emi) = 724 nm; tau(e) = 12 +/- 1 micros, 77 K). The protonation of 1 and 4 leads to [ClPd(mu-dppm)2(mu-H)PtCl]+ (5) and 3, respectively. Stoichiometric treatment of 1 with cyclohexyl or xylyl isocyanide yields [ClPd(mu-dppm)2Pt(CNC6H11)]Cl (6a) and [ClPd(mu-dppm)2Pt(CN-xylyl)]Cl (6b) ligated by terminal-bound CNR ligands. In contrast, treatment of 1 with the phosphonium salt [C[triple bond]NCH2PPh3]Cl affords the structurally characterized A-frame compound [ClPd(mu-dppm)2(mu-C=NCH2PPh3)PtCl]Cl (6c), spanned by a bridging isocyanide ligand. The electrochemical reduction of 2a at -1.2 V vs SCE, as well as the reduction of 5 in the presence of dppm, leads to a mixture of products 3 and 4. Further reduction of 3 at -1.7 V vs SCE generates 4 quantitatively. The reoxidation at 0 V of 4 in the presence of Cl- ions produces back complex 2a. The whole mechanism of the reduction of 1 has been established.     

Synthesis, structure, and characterisation of a new phenolato-bridged manganese complex [Mn2(mL)2]2+: chemical and electrochemical access to a new mono-mu-oxo dimanganese core unit

The dinuclear phenolato-bridged complex [(mL)Mn(II)Mn(II)(mL)](ClO(4))(2) (1(ClO(4))(2)) has been obtained with the new [N(4)O] pentadentate ligand mL(-) (mLH=N,N'-bis-(2-pyridylmethyl)-N-(2-hydroxybenzyl)-N'-methyl-ethane-1,2-diamine) and has been characterised by X-ray crystallography. X- and Q-band EPR spectra were recorded and their variation with temperature was examined. All spectra exhibit features extending over 0-800 mT at the X band and over 100-1450 mT at the Q band, features that are usually observed for dinuclear Mn(II) complexes. Cyclic voltammetry of 1 exhibits two irreversible oxidation waves at E(1)(p)=0.89 V and E(2)(p)=1.02 V, accompanied on the reverse scan by an ill-defined cathodic wave at E(1')(p)=0.56 V (all measured versus the saturated calomel electrode (SCE)). Upon chemical oxidation with tBuOOH (10 equiv) at 20 degrees C, 1 is transformed into the mono-mu-oxo species [(mL)Mn(III)-(mu-O)-Mn(III)(mL)](2+) (2), which eventually partially evolves into the di-mu-oxo species [(mL)Mn(III)-(mu-O)(2)-Mn(IV)(mL)](n+) (3) in which one of the aromatic rings of the ligand is decoordinated. The UV/Vis spectrum of 2 displays a large absorption band at 507 nm, which is attributed to a phenolate-->Mn(III) charge-transfer transition. The cyclovoltammogram of 2 exhibits two reversible oxidation waves, at 0.65 and 1.16 V versus the SCE, corresponding to the Mn(III)Mn(III)/Mn(III)Mn(IV) and Mn(III)Mn(IV)/Mn(IV)Mn(IV) oxidation processes, respectively. The one-electron electrochemical oxidation of 2 leads to the mono-mu-oxo mixed-valent species [(mL)Mn(III)-(mu-O)-Mn(IV)(mL)](3+) (2 ox). The UV/Vis spectrum of 2 ox exhibits one large band at 643 nm, which is attributed to the phenolate-->Mn(IV) charge-transfer transition. 2 ox can also be obtained by the direct electrochemical oxidation of 1 in the presence of an external base. The 2 ox and 3 species exhibit a 16-line EPR signal with first peak to last trough widths of 125 and 111 mT, respectively. Both spectra have been simulated by using colinear rhombic Mn-hyperfine tensors. Mechanisms for the chemical formation of 2 and the electrochemical oxidation of 1 into 2 ox are proposed.     

Complexed nitrogen heterosuperbenzene: the coordinating properties of a remarkable ligand

Tetra-peri-(tert-butyl-benzo)-di-peri-(pyrimidino)-coronene 1, the parent compound of the nitrogen heterosuperbenzene family N-HSB, is employed as a novel monotopic ligand in the formation of [Pd(eta3-C3H5)(1)]PF6 2 and [Ru(bpy)2(1)](PF6)2 (where bpy = 2,2'-bipyridine 3a and d8-2,2'-bipyridine 3b). These N-coordinated complexes are fully characterized by 1H NMR and IR spectroscopy and ESI-MS. Metal coordination has a profound effect on both the absorption and the emission properties of 1. Pd(II) coordination causes a red-shift in the low-energy absorptions, a decrease in the intensity of the n-pi absorptions, and a quenching of the emission. Ru(II) coordination causes absorption throughout the visible region and creates two new complexes that join an elite group of compounds known as "black" absorbers. 3a and 3b possess two discernible 1MLCT bands. The one of exceptionally low energy (lambda(max) = 615 nm) has an associated (3)MLCT emission (lambda(max) = 880 nm) due to the unprecedented electron delocalization and acceptor properties of the rigid aromatic N-HSB 1. Both Ru(II) complexes are near-IR emitters with unusually protracted emission lifetimes of 320 ns at 77 K. They are photochemically inert, and their electrochemical properties are consistent with the presence of a low-lying pi orbital on 1. The first two reversible reductions (E(1/2) (CH3CN), -0.54 V, -1.01 V vs SCE) are due to the stepwise reduction of 1 and are anodically shifted as compared to [Ru(bpy)3]2+. Temperature- and concentration-dependent NMR studies on 2 and 3a suggest extensive aggregation is occurring in solution.     

Tris-cyclometalated iridium(III) complexes of carbazole(fluorenyl)pyridine ligands: synthesis, redox and photophysical properties, and electrophosphorescent light-emitting diodes

Using ligands synthesized by Suzuki cross-coupling methodology, new phosphorescent homoleptic tris-cyclometalated complexes have been obtained, namely fac-[Ir(Cz-2-Fl(n)Py)(3)] (1 d-f) and fac-[Ir(Cz-3-Fl(n)Py)(3)] (2 d-f), which are solution-processible triplet emitters (Cz denotes N-hexylcarbazole, n is the number of 9,9'-dihexylfluorene (Fl) units (n=0,1,2) and Py is pyridine). In all cases, Py and Fl are substituted at the 2- and 2,7-positions, respectively, and Cz moieties are substituted by either Py or Fl at the 2- or 3-positions, in series 1 and 2, respectively. The oxidation potential of 1 d studied by cyclic voltammetry ({E{{{\rm ox}\hfill \atop 1/2\hfill}}}=0.14 V, versus Ag/AgNO(3), CH(2)Cl(2)) is less positive (i.e. raised HOMO level) compared to that of the isomer 2 d ({E{{{\rm ox}\hfill \atop 1/2\hfill}}}=0.30 V), where the Cz-nitrogen is meta to the Ir center. Ligand-centered oxidations occur at more positive potentials, leading to 7+ oxidation states with good chemical reversibility and electrochemical quasi-reversibility, for example, for 2 f {E{{{\rm ox}\hfill \atop {\rm pa}\hfill}}} =0.45 (1e), 0.95 (3e), 1.24 V (3e). Striking differences are seen in the solution-state photophysical data between complexes [Ir(Cz-2-Py)(3)] (1 d) and [Ir(Cz-3-Py)(3)] (2 d), in which the Cz moiety is bonded directly to the metal center: for the latter there is an 85 nm blue-shift in emission, a decrease in the luminescence lifetime and an increase in the PLQY value. Organic light emitting devices were made by spin-coating using polyspirobifluorene:bis(triphenyl)diamine (PSBF:TAD) copolymer as host and the complexes 1 d or 2 d as dopants. Turn-on voltages are low (3-4 V). With 1 d orange light is emitted at lambda(max)=590 nm with an EQE of 1.3 % (at 7.5 mA cm(-2)) and an emission intensity (luminance) of 4354 cd m(-2) (at 267 mA m(-2)). The green emission from 2 d devices (lambda(max)=500 nm) is due to the reduced electron-donating ability of the carbazole unit in 2 d. Recording the EL spectra of the 1 d device at 6 V (current density, 100 mA cm(-2)) established that the time to half brightness was about 9 h under continuous operation with no change in the spectral profile, confirming the high chemical stability of the complex.     

A novel phosphate chemosensor utilizing anion-induced fluorescence change

[structure: see text] The neutral receptor N,N'-bis[3,5-di[(1-pyrenylmethyl)carbamoyl]benzyl] pyridine-2,6-dicarbamide (2) provides a pseudo-tetrahedron cleft and multiple hydrogen bondings to form a 1:1 complex with phosphate ion in a highly selective manner, by comparison with other anions (F(-), Cl(-), Br(-), SCN(-), AcO(-), NO(3)(-), ClO(4)(-), and HSO(4)(-)). The binding strength can be inferred from the emission intensity ratio of the pyrene monomer (lambda(max) 377 nm) to the excimer (lambda(max) 477 nm). Fluorescence titration, X-ray analysis, and NMR studies support a proposed complexation model.     

Fluorinated diphenylpolyenes: crystal structures and emission properties

(E,E,E)-1,6-Diaryl(Ar)-1,3,5-hexatrienes (2, Ar = 4-fluorophenyl; 3, Ar = 2,4-difluorophenyl; 4, Ar = 2,4,6-trifluorophenyl; 5, Ar = perfluorophenyl) and (E,E,E)-1-perfluorophenyl-6-phenyl-1,3,5-hexatriene (6) were prepared. The absorption and fluorescence spectra in methylcyclohexane solution showed only a small dependence on the fluorine ring substituent, and were similar to those of the unsubstituted parent compound (1, Ar = phenyl). The solid-state absorption and fluorescence spectra shifted to red relative to those in solution and strongly depended on the substituent. The emission from crystals 1-5 originated mainly from monomeric species with the maximum wavelength (lambda f(max)) of 440-465 nm, which overlapped the emission from molecular aggregates (1-4) or excimeric species (5) in the red region. Crystal 6 exhibited red-shifted (lambda f(max) = 530 nm) and structureless emission due to excimers. The cocrystal of 1 and 5 (1/5) showed red-shifted (lambda f(max) = 558 nm) and distinctly structured emission, not from exciplexes but from the excited states of molecular aggregates in which molecules 1 and 5 strongly interact already in the ground state. These assignments were confirmed by the results of fluorescence lifetime and quantum yield measurements in the solid state. Single-crystal X-ray structure analyses showed that the molecules were basically planar in each crystal, whereas the crystal packing was strongly substituent-dependent. Weak pi-pi interactions in the herringbone (1 and 2) and in the pi-stacked but largely offset structures (3 and 4) account for their predominantly monomeric origin of emission. The observation of excimer fluorescence from 5 was rather unexpected, since the molecules in this crystal were arranged in an offset stacking fashion due to perfluorophenyl-perfluorophenyl (C6F5...C6F5) interaction. The structures of 6 and 1/5 considerably resembled each other, in which molecules were pi-stacked with more face-to-face geometries than those in 5, as a result of strongly attractive perfluorophenyl-phenyl (C6F5...C6H5) interaction. Nevertheless, the fluorescence origin was clearly different for 6 and 1/5. This can be ascribed to the difference in the strength of orbital-orbital interaction between molecular pi-planes in the ground and excited states in crystals.     

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.     

Synthesis of highly soluble and oxidatively stable tetraceno[2,3-b]thiophenes and pentacenes

A comparative study of suitably functionalized, highly soluble tetraceno[2,3-b]thiophenes (1-3) and pentacenes (4-6) that show higher photoxidative stability than that of unfunctionalized corresponding acenes is reported. The absorption and emission of 1-3 (Amax = 624-656 nm, lambda max = 634-672 nm, PhiF approximately 10%) and 4-6 (Amax = 672-704 nm, lambda max = 682-718 nm, PhiF approximately 10%) were found to be systematically red-shifted by the substitution in the order of the tert-butylethynyl < triisopropylsilylethynyl < phenylethynyl groups. The oxidation potentials of these compounds were similar (E1/2 approximately 0.70 V), except for 4, which showed lower oxidation potential (E1/2 approximately 0.63 V).     

Synthesis and photophysical characteristics of 2,7-fluorenevinylene-based trimers and their electroluminescence

Three new 2,7-fluorenevinylene-based trimers were synthesized and characterized. The synthesis was carried out by the Heck coupling reaction of 9,9-dihexyl-2,7-divinylfluorene with 2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole, N,N-diphenyl-4-bromoaniline, or 3-bromopyrene to afford the trimers OXD, TPA, and PYR, respectively. All the trimers were readily soluble in common organic solvents such as tetrahydrofuran, dichloromethane, chloroform, and toluene. Their glass transition temperatures ranged from 33 to 60 degrees C. The UV-vis spectra showed an absorption maximum at lambda(a,max) = 379-417 nm with optical band gap of Eg = 2.47-2.66 eV. In solution, they emitted strong blue-green photoluminescence (PL) with PL maximum at lambda(f,max) = 455-565 nm and fluorescence quantum yield of Phi(f) = 0.65-0.74. On the other hand, in their spin-coated films, the PL efficiencies significantly decreased due to the presence of concentration quenching. All samples showed nanosecond transient lifetime containing two components, suggesting excimer formation. The organic light-emitting diodes (OLEDs) with OXD and TPA showed green emission with electroluminescence (EL) quantum efficiencies of eta(EL) approximately 10(-2)%, while very weak EL efficiency of eta(EL) approximately 10(-5)% was observed with PYR. The highest occupied molecular orbital (HOMO) levels of the films were found to be 5.05-5.75 eV.     

Ligand effects on luminescence of new type blue light-emitting mono(2-phenylpyridinato)iridium(III) complexes

Newly prepared hydrido iridium(III) complexes [Ir(ppy)(PPh3)2(H)L](0,+) (ppy = bidentate 2-phenylpyridinato anionic ligand; L = MeCN (1b), CO (1c), CN(-) (1d); H being trans to the nitrogen of ppy ligand) emit blue light at the emission lambda(max) (452-457, 483-487 nm) significantly shorter than those (468, 495 nm) of the chloro complex Ir(ppy)(PPh3)2(H)(Cl) (1a). Replacing ppy of 1a-d with F2ppy (2,4-difluoro-2-phenylpyridinato anion) and F2Meppy (2,4-difluoro-2-phenyl-m-methylpyridinato anion) brings further blue-shifts down to the emission lambda(max) at 439-441 and 465-467 nm with CIE color coordinates being x = 0.16 and y = 0.18-0.20 to display a deep-blue photoemission. No significant blue shift is observed by replacing PPh3 of 1a with PPh2Me to produce Ir(ppy)(PPh2Me)2(H)(Cl) (1aPPh 2Me), which displays emission lambda max at 467 and 494 nm. The chloro complexes, [Ir(ppy)(PPh3)2(Cl)(L)](0,+) (L = MeCN (2b), CO (2c), CN(-) (2d)) having a chlorine ligand trans to the nitrogen of ppy also emit deep-blue light at emission lambda(max) 452-457 and 482-487 nm.     

Free radical chemistry of flavan-3-ols: determination of thermodynamic parameters and of kinetic reactivity from short (ns) to long (ms) time scale

The physical chemistry and the free radical chemistry of the most abundant polyphenolic flavan-3-ols in food, catechin, its methylated metabolites, and several methylated analogues, have been investigated by laser flash photolysis and cyclic voltammetry studies. Two independent phenoxyl radicals formed upon oxidation of flavan-3-ols have been characterized and identified unambiguously: a short-lived resorcinol-like radical characterized by an absorption band at lambda = 495 nm and a long-lived catechol-like transient absorbing at lambda = 380 nm. The determination of all the thermodynamic constants of each phenolic function of flavan-3-ols, namely, redox potential (E degrees (3') = 0.13(5) V/SCE, E degrees (4') = 0.11(0) V/SCE, E degrees (5) = 0.28(5) V/SCE) and microscopic dissociation constants (pK(a3') = 9.02, pK(a4') = 9.12, pK(a5) = 9.43, pK(a7) = 9.58) were performed. These values are discussed and compared to the prediction of the density functional theory calculations made on the different species catechin, catechin phenoxyl, and catechin phenolate for each phenolic site.     

Triplet excimers of fluoroquinolones in aqueous media

Generation of triplet eximers of 6-fluoro-7-piperazinyl-quinolone-3-carboxylic acids (FQs) have been detected in aqueous media using laser flash photolysis (LFP). These transient species (SS) are generated by self-quenching reactions of FQ triplet excited states such as pefloxacin (PFX), norfloxacin (NFX), the N-acetylated form of NFX (ANFX), and its methyl ester (EANFX) with their ground states. In this context, self-quenching rate constants in the range of (1-7) × 10(8) M(-1) s(-1) were determined. The triplet excimers show transient absorption spectra with λ(max) ca. 710 nm for SS(NFX), 740 nm for SS(PFX), and 620 nm for SS(ANFX) and E(ANFX), which are red-shifted with respect to their predecessors triplet excited states. These excimers can be also observed in the presence of phosphate buffer (PB). Experiments performed with NFX and ANFX at different PB concentrations showed that deprotonation processes are not involved in the generation of SS. The triplet multiplicity of the FQ excimers was confirmed by energy transfer reactions with naproxen. The correlation between fluorescence, intersystem crossing, excimer and photodegradation quantum yields of (A)NFX indicated that FQ self-quenching reactions are mainly a deactivation pathway. On the other hand, generation of FQ radical anions absorbing at λ(max) ca. 620 nm has been observed by an efficient electron transfer reaction from Trp to NFX, PFX, and ANFX (rate constants ca. 1 × 10(9) M(-1) s(-1)).     

Efficient greenish blue electrochemiluminescence from fluorene and spirobifluorene derivatives

The spectroscopic and electrochemical behavior as well as electrogenerated chemiluminescence (ECL) of a series of donor-π-donor derivatives bearing triphenylamine groups as donor connected to a fluorene, 2,7-bis-(4-(N,N-diphenylamino)phen-1-yl)-9,9'-dimethylfluorene (1), or spirobifluorene core, 2,7-bis-(4-(N,N-diphenylamino)phen-1-yl)-9,9'-spirobifluorene (2) and 2,2',7,7'-tetrakis(4-(N,N-diphenylamino)phen-1-yl)-9,9'-spirobifluorene (3), were investigated. Besides a high photoluminescence (PL) quantum yield in solution (between 81 and 87%), an efficient radical ions annihilation process induces intense greenish blue ECL emission that could be seen with the naked eye. Only the tetrasubstituted spirobifluorene derivative (compound 3) shows weak ECL obtained by a direct annihilation mechanism. Because the energy of the annihilation reaction is higher than the energy required to form the singlet excited state, the S-route could be considered the pathway followed by the ECL process in these molecules. The ECL emissions recorded by direct ion-ion annihilation show two bands compared to the single structureless PL band. The ECL spectra obtained by a coreactant approach using benzoylperoxide as a coreagent show no differences relative to that produced by annihilation, except for an increasing of ECL intensity for all compounds.     

Synthesis of layered (2-D) V-based bimetallic oxalates from non-aqueous media that cannot be synthesized from aqueous media

The reaction of (NBu4)3[V(III)(ox)3] (1, ox = oxalate) and M(II) (M = Fe, Co, Ni, Cu) ions in MeCN, leads to the isolation of V-based coordination polymers of [N(n-Bu)4][Fe(II)V(II)I(ox)3].0.30[[N(n-Bu)4](BF4)] (2), [N(n-Bu)4][Co(II)V(III)(ox)3].0.75[[N(n-Bu)4](BF4)] (3), [N(n-Bu)4][Ni(II)V(III)(ox)3].0.20[[N(n-Bu)4](BF4)].0.20MeCN (4), and [N(n-Bu)4][Cu(II)V(III)(ox)2](BF4)2 (5) composition. Due to the lability of [V(III)(ox)3]3- to dissociate ox2-, these compounds cannot be prepared from aqueous media. 5 is best described as [N(n-Bu)4][V(III)Cu(II)(ox)2](BF4)2, and 2, 3, 4, and 5 are proposed to have a layered (2-D) motif for the MM(ox)x (x = 2, 3) extended framework. The [V(III)Cu(II)(ox)2] composition of 5 is reported for the first time for a bimetallic oxalate. 2 shows a weak antiferromagnetic interaction between Fe(II), S = 2 and V(III), S = 1 ions (theta = -9.4 K) within the 2-D layers. 3 and 5 do not magnetically order above 2 K. 4 magnetically order as ferromagnets below 2.55 K [taken as the onset of magnetization in chi'(T)], and has a glass transition temperature (chi'(max) at 1000 Hz) at 2.26 K.     

Electrogenerated chemiluminescence of platinum(II) alkynyl terpyridine complex with peroxydisulfate as coreactant

A Pt(II) alkynyl terpyridine complex containing a carbazole moiety, [Pt((t)Bu(3)tpy)(C≡C-C(6)H(4)-4-carbazole-9)](+) ((t)Bu(3)tpy = 4,4',4'-tri-tert-butyl-2,2':6',2'-terpyridine) 1, has been synthesized and characterized. The photophysical behavior has been studied, and the molecular structure has been determined by X-ray crystallography. The complex was found to exhibit intense electrogenerated chemiluminescence (ECL) using peroxydisulfate (S(2)O(8)(2-)) as coreactant in acetonitrile/water (1-25%, v/v) mixture at both glassy carbon and gold electrodes, representing the first ECL example of the Pt(II) alkynyl family. The ECL was produced at potential corresponding to the first reduction wave (-0.90 V vs SCE), significantly shifted by ～0.65 V toward more positive potential compared with that of [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine). The ECL spectrum was found to be identical to the photoluminescence spectrum recorded in the same medium, indicating the formation of the same excited state of dπ(Pt) → π*((t)Bu(3)tpy) (3)MLCT mixed with π(C≡CR) → π*((t)Bu(3)tpy) (3)LLCT in both cases. The ECL mechanism was proposed involving the formation of the strongly oxidizing intermediate, SO(4)(?-), mainly generated during the catalytic reduction of S(2)O(8)(2-) by the electrogenerated 1(-). Chemiluminescence of 1/S(2)O(8)(2-) based on reduction with Al metal is also described.     

Metal-ligand charge-transfer-promoted photoelectronic Bergman cyclization of copper metalloenediynes: photochemical DNA cleavage via C-4' H-atom abstraction

Metal-to-ligand charge-transfer (MLCT) photolyses (lambda > or = 395 nm) of copper complexes of cis-1,8-bis(pyridin-3-oxy)oct-4-ene-2,6-diyne (bpod, 1), [Cu(bpod)(2)]PF(6) (2), and [Cu(bpod)(2)](NO(3))(2) (3) yield Bergman cyclization of the bound ligands. In contrast, the uncomplexed ligand 1 and Zn(bpod)(2)(CH(3)COO)(2) compound (4) are photochemically inert under the same conditions. In the case of 4, sensitized photochemical generation of the lowest energy (3)pi-pi state, which is localized on the enediyne unit, leads to production of the trans-bpod ligand bound to the Zn(II) cation by photoisomerization. Electrochemical studies show that 1, both the uncomplexed and complexed, exhibits two irreversible waves between E(p) values of -1.75 and -1.93 V (vs SCE), corresponding to reductions of the alkyne units. Irreversible, ligand-based one-electron oxidation waves are also observed at +1.94 and +2.15 V (vs SCE) for 1 and 3. Copper-centered oxidation of 2 and reduction of 3 occur at E(1/2) = +0.15 and +0.38 V, respectively. Combined with the observed Cu(I)-to-pyridine(pi) MLCT and pyridine(pi)-to-Cu(II) ligand-to-metal charge transfer (LMCT) absorption centered near approximately 315 nm, the results suggest a mechanism for photo-Bergman cyclization that is derived from energy transfer to the enediyne unit upon charge-transfer excitation. The intermediates produced upon photolysis degrade both pUC19 bacterial plasmid DNA, as well as a 25-base-pair, double-stranded oligonucleotide. Detailed analyses of the cleavage reactions reveal 5'-phosphate and 3'-phosphoglycolate termini that are derived from H-atom abstraction from the 4'-position of the deoxyribose ring rather than redox-induced base oxidation.     

Blue and near-UV phosphorescence from iridium complexes with cyclometalated pyrazolyl or N-heterocyclic carbene ligands

Two approaches are reported to achieve efficient blue to near-UV emission from triscyclometalated iridium(III) materials related to the previously reported complex, fac-Ir(ppz)(3) (ppz = 1-phenylpyrazolyl-N,C(2)'). The first involves replacement of the phenyl group of the ppz ligand with a 9,9-dimethyl-2-fluorenyl group, i.e., fac-tris(1-[(9,9-dimethyl-2-fluorenyl)]pyrazolyl-N,C(2)')iridium(III), abbreviated as fac-Ir(flz)(3). Crystallographic analysis reveals that both fac-Ir(flz)(3) and fac-Ir(ppz)(3) have a similar coordination environment around the Ir center. The absorption and emission spectra of fac-Ir(flz)(3) are red shifted from those of fac-Ir(ppz)(3). The fac-Ir(flz)(3) complex gives blue photoluminescence (PL) with a high efficiency (lambda(max) = 480 nm, phi(PL) = 0.38) at room temperature. The lifetime and quantum efficiency were used to determine the radiative and nonradiative rates (1.0 x 10(4) and 2.0 x 10(4) s(-1), respectively). The second approach utilizes N-heterocyclic carbene (NHC) ligands to form triscyclometalated Ir complexes. Complexes with two different NHC ligands, i.e., iridium tris(1-phenyl-3-methylimidazolin-2-ylidene-C,C(2)'), abbreviated as Ir(pmi)(3), and iridium tris(1-phenyl-3-methylbenzimidazolin-2-ylidene-C,C(2)'), abbreviated as Ir(pmb)(3), were both isolated as facial and meridianal isomers. Comparison of the crystallographic structures of the fac- and mer-isomers of Ir(pmb)(3) with the corresponding Ir(ppz)(3) isomers indicates that the imidazolyl-carbene ligand has a stronger trans influence than pyrazolyl and, thus, imparts a greater ligand field strength. Both fac-Ir(pmi)(3) and fac-Ir(pmb)(3) complexes display strong metal-to-ligand-charge-transfer absorption transitions in the UV (lambda = 270-350 nm) and phosphoresce in the near-UV region (E(0)(-)(0) = 380 nm) at room temperature with phi(PL) values of 0.02 and 0.04, respectively. The radiative decay rates for fac-Ir(pmi)(3) and fac-Ir(pmb)(3) (5 x 10(4) s(-1) and 18 x 10(4) s(-1), respectively) are somewhat higher than that of fac-Ir(flz)(3), but the nonradiative rates are two orders of magnitude faster (i.e., (2-4) x 10(6) s(-1)).     

Determination of ruthenium and osmium in each other's presence in chloride solutions by direct and third-order derivative spectrophotometry

Ruthenium and osmium (up to 20 mug Ru(Os) ml(-1)) can be determined in chloride solutions directly after absorption of RuO(4) and OsO(4) in hydrochloric acid. In 9 M HCl, RuO(4) and OsO(4) are quantitatively converted into RuCl(6)(2-) (lambda(max) = 480.0 nm, epsilon = 4.8 x 10(3) l mol(-1) cm(-1)) and OsCl(6)(2-) (lambda(max) = 334.8 nm, epsilon = 8.4 x 10(3) l mol(-1) cm(-1)) respectively. Osmium does not interfere with the determination of ruthenium in the form of the RuCl(6)(2-) complex by direct spectrophotometry. The absorbance of the obtained solution at lambda(max) = 480.0 nm corresponds only to the concentration of ruthenium. A derivative spectrophotometric method using numerical calculation of absorption spectra of the RuCl(6)(2-) and OsCl(6)(2-) complexes has been developed for the determination of osmium in a mixture with ruthenium. The interfering effect of ruthenium on the determination of osmium can be eliminated by measuring the value of a third-order derivative spectrum of the OsCl(6)(2-) complex at 350.0 nm ("zero-crossing point" of ruthenium). Simple and rapid determination of ruthenium and osmium in a calibration standard solution of the noble metals (Ru, Rh, Pd, Os, Ir, Pt and Au) for plasma spectroscopy using the proposed methods has been achieved.     

Carbomethoxychlorocarbene: spectroscopy, theory, chemistry and kinetics.

Photolysis (254 nm) of methyl 8-chloro-3a,7a-methanoindan-8-carboxylate (5) in argon at 14 K produces carbomethoxychlorocarbene (6) as a persistent species. The IR and UV-vis spectra of the carbene were recorded and analyzed with the aid of density functional calculations (B3-LYP/6-31G). The IR spectrum of 6 is consistent with the carbene having a nonplanar singlet ground state, in agreement with the G3(MP2)//B3-LYP calculations of Scott and Radom (accompanying paper). Irradiation (300 nm) of 5 in solution produces indane in 97% yield. In cyclohexane, carbene 6 is trapped by insertion into a CH bond, whereas in 2,3-dimethylbutene it adds to the double bond to form a cyclopropane. Laser flash photolysis of 5 (308 nm, 17 ns, XeCl excimer) produces carbene 6 which reacts with pyridine to form an ylide (lambda(max) = 440 nm). It was possible to resolve the growth of the ylide in Freon-113 (CF(2)ClCFCl(2)) to measure the lifetime (tau = 114 ns, ambient temperature) of the carbene and the absolute rate constant of its reaction with pyridine (k(pyr) = 2 x 10(9) M(-)(1) s(-)(1)). A plot of log(1/tau) versus 1/T in CF(2)ClCFCl(2) is linear with Arrhenius parameters E(a) = 10.9 +/- 0.8 kJ/mol and A = 10(9.1)(+/-)(0.2) s(-)(1). In perfluorohexane, a less reactive solvent than Freon-113, E(a) = 23.4 +/- 1.7 kJ/mol, A = 10(10.6)(+/-)(0.) s(-)(1), and tau = 354 ns at 293 K. It is argued that the activation barrier to carbene disappearance in perfluorohexane represents the lower limit to the barrier to Wolff rearrangement of the carbene.