Energy Transfer in Hybrids Based on a Thiophene-Substituted Ethynylbipyridine Dimer Decorated with Re(I), Ru(II), and Os(II) Units

2,3-Di(2-pyridyl)-5-phenylpyrazine: a NN-CNN-type bridging ligand for dinuclear transition-metal complexes

A new bridging ligand, 2,3-di(2-pyridyl)-5-phenylpyrazine (dpppzH), has been synthesized. This ligand was designed so that it could bind two metals through a NN-CNN-type coordination mode. The reaction of dpppzH with cis-[(bpy)2RuCl2] (bpy = 2,2'-bipyridine) affords monoruthenium complex [(bpy)2Ru(dpppzH)](2+) (1(2+)) in 64 % yield, in which dpppzH behaves as a NN bidentate ligand. The asymmetric biruthenium complex [(bpy)2Ru(dpppz)Ru(Mebip)](3+) (2(3+)) was prepared from complex 1(2+) and [(Mebip)RuCl3] (Mebip = bis(N-methylbenzimidazolyl)pyridine), in which one hydrogen atom on the phenyl ring of dpppzH is lost and the bridging ligand binds to the second ruthenium atom in a CNN tridentate fashion. In addition, the RuPt heterobimetallic complex [(bpy)2Ru(dpppz)Pt(C≡CPh)](2+) (4(2+)) has been prepared from complex 1(2+), in which the bridging ligand binds to the platinum atom through a CNN binding mode. The electronic properties of these complexes have been probed by using electrochemical and spectroscopic techniques and studied by theoretical calculations. Complex 1(2+) is emissive at room temperature, with an emission λmax = 695 nm. No emission was detected for complex 2(3+) at room temperature in MeCN, whereas complex 4(2+) displayed an emission at about 750 nm. The emission properties of these complexes are compared to those of previously reported Ru and RuPt bimetallic complexes with a related ligand, 2,3-di(2-pyridyl)-5,6-diphenylpyrazine.

A rational protocol for the synthesis of arylated bipyridine ligands via a cycloaddition pathway

A generic design principle for the preparation of a variety of substituted phenyl-polypyridine ligands is described. These ligands are readily prepared by a regioselective [4+2] cycloaddition between electron-deficient dienes, such as 2,6-disubstituted-1,3,4-triazines, and ethynyl-arenes or ethynyl-alkanes. Exceptional reactivity is found with electron-rich dienophiles bearing ethynylgallate or ethynylphenyldibutylamino groups. Two regioisomers are formed, the meta being preferred due to favorable pi-pi interactions in the transition state, while the para isomers are formed in low yields in most cases. The use of tert-butylacetylene or N,N-dimethylamino-2-propyne, however, drives the reaction exclusively to the para isomer. Di-N,N-dibutylaminophenyl or isoquinoline ligands can also be produced in a single step by reverse Diels-Alder reactions. Cross-coupling reactions of iodo-substituted ligands or their platinum(II) complexes under Pd(0) catalysis gives branched ligands and complexes bearing paraffin chains, electron-donor or electron-acceptor groups. The use of a chloro-Pt(II) complex of an iodo-functionalized ligand allows both halogens to be replaced by ethynyl groups by using different catalysts. This methodology readily accommodates various functional groups and has been successfully extended to systems containing a variety of donor/acceptor frameworks. All ligands strongly absorb in the near-UV and luminesce in solution at rt with quantum yields ranging from 0 to 66%. Excited state lifetimes are in the nanosecond range and the solvent effects are in keeping with singlet excited states mixed with charge-transfer character. As deduced from spectroscopic and electrochemical studies, the di-n-butylamino derivatives are strong reductants in the excited state.

Synthesis and Optical Properties of Bifunctional Thiophene Molecules Coordinated to Ruthenium

A series of unsymmetrical bi- and tetrathiophenes have been synthesized with bipyridine and phosphonic acid functional groups. To do this, phosphonic esters were bonded to thiophenes and the thiophenes coupled to bipyridine. After synthesis of the thienylbipyridines, bis(bipyridine) ruthenium was coordinated to them through the bipyridines. The thienylbipyridines absorb visible light and fluoresce; however, on attachment to ruthenium, both their fluorescence and that of ruthenium are quenched. An additional effect of coordinating ruthenium to the thiophenes is a new absorption band around 470 nm. Variation in oligothiophene length and bipyridine substitution position allowed comparison of the effect of these variables on electronic properties. The longer oligothiophenes display lower-energy absorptions and emissions than that of the shorter thiophenes. In contrast, the position of the bipyridine attachment does not have a large effect on the absorbance or emission wavelength, or on the fluorescence quantum yield.

Bimetallic Iridium(III) Complexes Consisting of Ir(ppy)2 Units (ppy = 2-Phenylpyridine) and Two Laterally Connected N∧N Chelates as Bridge: Synthesis, Separation, and Photophysical Properties

Reaction of the precursor complex Ir2(ppy)4Cl2 (ppy = 2-phenylpyridine) with the bridging ligand 3,8-dipyridyl-4,7-phenanthroline (L) affords, in 94% yield, the cyclometalated iridium dinuclear complex [(ppy)2Ir(mu-L)Ir(ppy)2]2+ (12+) as a mixture of three stereoisomers. This mixture consists of a meso form Delta,Lambda and a racemic form (enantiomeric pair Delta,Delta and Lambda,Lambda) in the ratio 1:1.5. Single-crystal X-ray characterization of the perchlorate salt of the meso form reveals (i) the distortion of the bridging ligand from the planarity and (ii) the location of the two iridium subunits above and below the medium plane of the bridging ligand. Ion-pair chromatography with Delta-TRISPHAT anion (TRISPHAT = tris(tetrachlorobenzenediolato)phosphate(V)) as resolving anion permits the separation of the three stereoisomers. The 1H NMR spectroscopic analysis of each fraction indicates high diastereomeric purity. Electronic circular dichroism properties and comparison with literature data establish their absolute configuration. The absorption and emission properties of the three stereoisomers show only very small variations. The anisotropic properties can be interpreted as distinct interactions of the isomers with the chiral resolving Delta-TRISPHAT anion.

Spectroscopic and Redox Properties of Novel d6-Complexes Engineered from All Z-Ethenylthiophene-bipyridine Ligands

A series of quasilinear dinuclear complexes incorporating ruthenium(II)- and osmium(II)-tris(2,2'-bipyridine) units has been prepared in which the individual metal-containing moieties are separated by 3,4-dibutyl-2,5-diethenylthiophene spacers and end-capped by 3,4-dibutyl-2-ethenylthiophene subunits; related ruthenium(II) and osmium(II) mononuclear complexes have also been prepared where one bpy unit is likewise end-capped by 3,4-dibutyl-2-ethenylthiophene subunits [bpy = 2,2'-bipyridine]. Overall, mononuclear species, labeled here Ru and Os, and dinuclear species, RuRu, OsOs, and RuOs, have been prepared and investigated. Their electrochemical behavior has been studied in CH3CN solvent and reveals ethenylthiophene-centered oxidations (irreversible steps at > +1.37 V vs SCE), metal-centered oxidations (reversible steps at +1.30 V vs SCE for Ru(II/III) and +0.82 V vs SCE for Os(II/III)), and successive reduction steps localized at the substituted bpy subunits. The spectroscopic studies performed for the complexes in CH3CN solvent provided optical absorption spectra associated with transitions of ligand-centered nature (LC, from the bpy and ethenylthiophene subunits) and metal-to-ligand charge-transfer nature (MLCT), with the former dominating in the visible region (400-600 nm). While the constituent ethenylthiophene-bpy ligands are strong fluorophores (fluorescence efficiency in CH2Cl2 solvent, phi em = 0.49 and 0.39, for the monomer and the dimer, respectively), only weak luminescence is observed for each complex in acetonitrile at room temperature. In particular, (i) the complexes Ru and RuRu do not emit appreciably, and (ii) the complexes Os, OsOs, and RuOs exhibit triplet emission of 3Os --> L CT character, with phi em in the range from 10-3 to 10-4. These features are rationalized on the basis of the role of nonemissive triplet energy levels, 3Th, centered on the ethenylthiophene spacer. These levels appear to lie lower in energy than the 3Ru --> L CT triplet levels, and in turn higher in energy than the 3Os --> L CT triplet levels, along the sequence 3Ru --> L CT > 3Th > 3Os --> L CT.

Photophysical Properties of Ruthenium(II) Tris(2,2‘-bipyridine) Complexes Bearing Conjugated Thiophene Appendages

A small series of ruthenium(II) tris(2,2'-bipyridine) complexes has been synthesized in which ethynylated thiophene residues are attached to one of the 2,2'-bipyridine ligands. The photophysical properties depend on the conjugation length of the thiophene-based ligand, and in each case, dual emission is observed. The two emitting states reside in thermal equilibrium at ambient temperature and can be resolved by emission spectral curve-fitting routines. This allows the properties of the two states to be evaluated in both fluid butyronitrile solution and a transparent KBr disk. It is concluded that both emitting states are of metal-to-ligand charge-transfer (MLCT) character, and despite the presence of conjugated thiophene residues, there is no indication for a low-lying pi,pi*-triplet state that promotes nonradiative decay of the excited-state manifold. A key feature of these systems is that the conjugation length imposed by the thiophene-based ligand helps to control the rate constants for both radiative and nonradiative decay from the two MLCT triplet states.