A concise modular synthesis of 2,5-diethynyl-3,4-dibutyl-thiophene-bridged back-to-back terpyridine ligands

Alcohol- and water-soluble bis(tpy)quaterthiophenes with phosphonium side groups: new conjugated units for metallo-supramolecular polymers

Bis(tpy)quaterthiophenes with symmetrically distributed two and four 6-bromohexyl side groups were prepared and modified by the reaction with triethylphosphine to give the corresponding ionic species. Both ionic and non-ionic bis(tpy)quaterthiophenes (unimers) were assembled with Zn(2+) and Fe(2+) ions to conjugated metallo-supramolecular polymers (MSPs), of which the ionic ones are soluble in alcohols and those derived from tetrasubstituted unimers are soluble even in water. The differences in assembly are specified between systems with (i) ionic and non-ionic unimers, (ii) Zn(2+) and Fe(2+) ion couplers, and (iii) methanol and water solvents. A substantial decrease in the stability of Fe-MSPs and a surprisingly high red shift of the luminescence band of Zn-MSPs were observed on going from methanol to aqueous solutions.

Synthesis, photophysical and electrochemical characterization of terpyridine-functionalized dendritic oligothiophenes and their Ru(II) complexes

Pd-catalyzed Sonogashira cross-coupling reactions were used to synthesize novel π-conjugated oligothienylene-ethynylene dendrons and their corresponding terpyridine-based ligands. Their complexation with Ru(II) led to interesting novel metallodendrimers with rich spectroscopic properties. All new compounds were fully characterized by ¹H and ¹³C NMR, as well as MALDI–TOF mass spectra. Density functional theory (DFT) calculations performed on these complexes gave more insight into the molecular orbital distributions. Photophysical and electrochemical studies were carried out in order to elucidate structure–property relationships and the effect of the dendritic structure on the metal complexes. Photophysical studies of the complexes revealed broad absorption spectra covering from 250 to 600 nm and high molar extinction coefficients. The MLCT emission of these complexes were significantly red-shifted (up to 115 nm) compared to the parent [Ru(tpy)₂]²⁺ complex.

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.

Giant Macrocycles Composed of Thiophene, Acetylene, and Ethylene Building Blocks

Fully conjugated giant macrocyclic oligothiophenes with 60pi, 90pi,120pi, 150pi, and 180pi frames (1, 2, 3, 4 and 5) have been designed, and their butyl-substituted derivatives (1a, 2a, 3a, 4a, and 5a) have been synthesized using modified Sonogashira and McMurry coupling reactions as key steps. The 60-180pi systems 1-5 are circular with 1.8-6 nm inner cavities and 3.3-7.5 nm outside molecular diameters. Compound 1a containing ten 3,4-dibutyl-2,5-thienylene, eight ethynylene, and two vinylene units has been converted into macrocyclic oligo(3,4-dibutyl-2,5-thienylene-ethynylene) 6a using bromination/dehydrobromination procedure. Giant macrocycles 1a-6a exhibit a red shift of their absorption spectra and a fairly strong fluorescence with a large Stokes shift as compared to a linear conjugated counterpart having five thiophene rings. Compounds 1a-6a exhibit multistep reversible redox behaviors with fairly low first oxidation potentials, reflecting their cyclic conjugation. Furthermore, chemical oxidation of 1a-6a with FeCl3 shows drastic changes of spectroscopic properties due to intramolecular and intermolecular pi-pi interactions. Doping of 1a-3a with iodine forms semiconductor due to its pi-donor properties and pi-pi stacking ability. X-ray analysis of 1a confirmed a round, planar structure with nanoscale inner cavity, and revealed host ability for alkanes and unique packing structure. Interestingly, 2a and 3a self-aggregate in the solid state to form "molecular wires," which are about 200 nm thick and more than 1 mm long. The internal structures of fibrous aggregates have been investigated by optical microscope, scanning electron microscopy, atomic force microscopy, and X-ray diffraction analyses.

Binuclear wirelike dimers based on ruthenium(II)-bipyridine units linked by ethynylene-oligothiophene-ethynylene bridges

The syntheses, structural characteristics, electrochemical behavior, ground-state spectra, photophysical properties, and transient absorption (TA) spectra in CH(3)CN solvent are reported for binuclear [(bpy)(2)Ru(bpy-E(T)(n)()E-bpy)Ru(bpy)(2)](4+) complexes, Ru(bpyT(n)bpy)Ru, where the Ru-based units are connected by alternating 3,4-dibutylthiophene (DBT')/thiophene (T') fragments linked via ethynyl groups (E) to bpy ligands at the 5-position (bpy is 2,2'-bipyridine). The ligand bpyT(3)bpy represents a module containing DBT'/T'/DBT' subunits, and bpyT(5)bpy accounts for a DBT'/T'/DBT'/T'/DBT' pattern. The syntheses and electrochemical and spectroscopic (emission and TA) properties in CH(2)Cl(2) solvent of the bpyT(n)()bpy ligands are likewise reported. The behavior of the Ru(bpyT(n)bpy)Ru dimers has been compared to that of the bpyT(n)bpy ligands and to that of a related mononuclear complex, [(bpy)(2)Ru(bpy-E-DBT')](2+), Ru(bpyDBT'). For the dimeric complexes, the electrochemical results show that the first reduction step takes place at the bpy ligand(s) bearing an ethynylene group, the first oxidation step is thiophene-centered, and further oxidation involves the metal centers, which are only weakly interacting. The photophysical and TA results for the Ru(bpyT(n)bpy)Ru dimers account for the presence of low-lying oligothiophene-centered (3)pi,pi levels, while higher-lying metal-ligand charge transfer ((3)MLCT) levels are thermally accessible only for the case of Ru(bpyT(3)bpy)Ru; the possible role of charge separation (CS) levels (from oxidation at the thiophene bridge and reduction at one of the coordinated bpy's) is also discussed.

Nanosized Gradient π-Conjugated Thienylethynylene Dendrimers for Light Harvesting: Synthesis and Properties

[reaction: see text] A family of pi-conjugated dendrimers based on truxene and thienylethynylene units are synthesized via a mixed divergent/convergent growth approach. These dendrimers possess an intrinsic energy gradient from the periphery to the core through branches and thus show a broad absorption in the UV-vis range and an efficient energy transfer to the lower-energy center. The molecules hence have the potential to be used as light harvesting materials.

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

The preparation, structural features, electrochemical behavior, and optical properties (at room temperature and at 77 K) are reported for a series of thiophene-containing hybrids based on the bent conjugated backbone of a rigid ditopic ligand, the dimeric moiety 3,4-dibutyl-2,5-bis{5'-[(3,4-dibutylthien-2-ylethynyl)-2,2'-bipyridin-5-yl]ethynyl}thiophene (TBTBT). Within the dimer, the diethynyl-2,2'-bipyridine units (bpy, the coordination sites) alternate with three 3,4-dibuthylthiophene units and coordination of the [Re(CO)3Cl], [Ru(bpy)2]2+, and [Os(bpy)2]2+ centers results in the mononuclear species RuTBTBT and OsTBTBT and the binuclear species RuTBTBTRu, OsTBTBTOs, RuTBTBTOs, and ReTBTBTOs. At room temperature, the emitting states obtained by photoexcitation are of 3MLCT nature, and vibronic analysis of the emission spectra indicates that they are largely delocalized over the TBTBT ligand. In the binuclear species, the intermetal separation is ca. 17 A, and for RuTBTBTOs, an efficient Ru --> Os excitation transfer takes place, resulting solely in an Os-based emission. The process is ascribed to double-electron transfer (Dexter), as mediated by the TBTBT ligand; a similar conclusion holds for the case of ReTBTBTOs. For RuTBTBTOs, the process is discussed in some detail also with regard to the possibility of disentangling the constituent hole and electron-transfer events.

Controlled Synthesis of Multinuclear Metal Complex Arrays by Cross Coupling of Coordinated Ligands

Conditions for efficient cross linking of chelating organic ligands within different metal-containing building blocks have been developed, allowing the preparation of heteronuclear Ru/Os complexes in a controlled manner. Critical to the success of this protocol is the introduction of an Os center early in the synthesis and the use of CuI under polar conditions. Noteworthy features of this method are the tolerance of the metal centers in both reaction partners and the absence of any metal scrambling during the C-C formation or ultimate complexation procedures.

Oligomeric ligands incorporating multiple 5,5'-diethynyl-2,2'-bipyridine moieties bridged and end-capped by 3,4-dibutylthiophene units

In this work, we detail the synthesis and photophysical properties of a series of soluble polybipyridine ligands comprising one to five bipyridine units sandwiched between rigid carbon-carbon triple bonds substituted by 3,4-dibutylthiophene repeating units. The dual Sonogashira coupling reaction of 5,5'-dibromo-2,2'-bipyridine with TMS- and (CH3)2C(OH)-protected acetylene allows the synthesis of a dissymmetrically functionalized building block which was selectively deprotected at either the TMS or 2-hydroxyprop-2-yl site. Various combinations allow the interconnection of the terminal alkyne to 3,4-dibutyl-2,5-diiodothiophene or 3,4-dibutyl-2-iodothiophene leading to bipyridine frameworks bearing two acetylene-protected groups or one acetylenethiophene/one acetylene-protected function. It is possible therefore to construct dimeric to pentameric bipyridine ligands where the chelating subunit is bridged by a 3,4-dibutyl-2,5-diethynylthiophene spacer and end-capped by a 3,4-dibutyl-2-ethynylthiophene stopper. All cross-coupling reactions are promoted with palladium(0) tetrakistriphenylphosphine under mild conditions. Spectroscopic data for the new oligomers are discussed in terms of the extent of pi-electron conjugation. Upon increasing the number of pi-electrons from 24 to 104, there is a progressive lowering in the energy of absorption and fluorescence transitions, while the emission quantum yields remain essentially constant. The LUMO levels of these large molecules, estimated by cyclic voltammetry, lie in the range -3.06 to -3.18 eV.

Temperature-Induced Switching of the Mechanism for Intramolecular Energy Transfer in a 2,2‘:6‘,2‘ ‘-Terpyridine-Based Ru(II)−Os(II) Trinuclear Array

The synthesis and photophysical properties of a linear 2,2':6',2' '-terpyridine-based trinuclear Ru(II)-Os(II) nanometer-sized array are described. This array comprises two bis(2,2':6',2' '-terpyridine) ruthenium(II) terminals connected via alkoxy-strapped 4,4'-diethynylated biphenylene units to a central bis(2,2':6',2' '-terpyridine) osmium(II) core. The mixed-metal linear array was prepared using the "synthesis at metal" approach, and the Ru(II)-Ru(II) separation is ca. 50 A. Energy transfer occurs with high efficiency from the Ru(II) units to the Os(II) center at all temperatures. Forster-type energy transfer prevails in a glassy matrix at very low temperature, but this is augmented by Dexter-type electron exchange at higher temperatures. This latter process, which is weakly activated, involves long-range superexchange interactions between the metal centers. In fluid solution, a strongly activated process provides for fast energy transfer. Here, a charge-transfer (CT) state localized on the bridge is populated as an intermediate species. The CT triplet does not undergo direct charge recombination to form the ground state but transfers energy, possibly via a second CT state, to the Os(II)-based acceptor. The short tethering strap constrains the geometry of the linker, especially in a glassy matrix, such that low-temperature electron exchange occurs across a particular torsion angle of 37 degrees . The probability of triplet energy transfer depends on temperature but always exceeds 75%.

Exciton-like energy collection in an oligothiophene wire end-capped by Ru- and Os-polypyridine chromophores

Ru(II)- and Os(II)-polypyridine termini are linked by a quinquethiophene bridge (the inter-metal separation is ca. 1.9 nm) wherein excitation energy flows into the luminescent Os-based unit by way of a conductive level.

Synthesis and electronic properties of series of oligothiophene-[1,10]phenanthrolines

Organic semiconductors containing metal binding sites within their molecular backbones are of a general interest in organic materials chemistry. In this paper, we describe a straightforward synthetic procedure, which gives access to a series of 2-(oligothienyl)-[1,10]phenanthrolines (nT-phen), 2,9-bis(oligothienyl)-[1,10]phenanthrolines (nT-phen-nT) and 2,2'-(oligothienyl)bis-[1,10]phenanthrolines (phen-nT-phen). By a Negishi-type cross coupling of 2-iodo-[1,10]phenanthroline or 2,9-diiodo-[1,10]phenanthroline with in situ generated alpha-zinc derivatives of different mono-, ter-, and quinquethiophenes we were able to synthesize the corresponding oligothienyl-phenanthrolines in medium to excellent yields. Furthermore, characterization of the optical properties of the new materials indicated that the two subunits, oligothiophene and phenanthroline, are in pi-conjugation. Characterization of the redox properties revealed additional evidence for the role of [1,10]phenanthroline as a pi-bridging unit in the nT-phen-nT series.

Mononuclear and Binuclear Wirelike Ruthenium(II) Complexes with Oligo-diethynyl-thiophene Bridged Back-to-Back Terpyridine Ligands: Synthesis and Electrochemical and Photophysical Properties

The syntheses, structural features, electrochemical behavior, absorption spectra, and photophysical properties of five mononuclear complexes [(terpy)Ru(terpy-DEDBT(n)-terpy)](2+), RuT(n), and five binuclear complexes [(terpy)Ru(terpy-DEDBT(n)-terpy)Ru(terpy)](4+), RuT(n)Ru, are reported, where n varies from 1 to 5 so that the metal-metal distance is estimated to be 42 A for the largest binuclear complex, RuT(5)Ru (terpy is 2,2':6',2"-terpyridine and DEDBT is 2,5-diethynyl-3,4-dibutylthiophene). The metal-centered oxidation potentials for the mononuclear and binuclear species are slightly more positive than for the reference [Ru(terpy)(2)](2+) complex, owing to the withdrawing nature of the back-to-back terpyridine ligands incorporating the repeat diethynyl-thiophene units. Comparison of the reduction potentials for the mononuclear and binuclear complexes reveals that the reduction steps are localized either at the terpy fragments of the T(n) ligands or at the terpy peripheral ligands. The spectroscopic results (absorption spectra at room temperature, luminescence spectra and lifetimes at room temperature and at 77 K) in acetonitrile solvent are consistent with the establishment of electronic delocalization within the oligomeric diethynyl-thiophene fragments (DEDBT(n)) of the T(n) ligands; however, the results also indicate that the terpy units of these ligands and the DEDBT(n)fragments are not strongly coupled. Both at room temperature and at 77 K, the (3)metal-to-ligand charge-transfer luminescence of RuT(n) and RuT(n)Ru complexes is strongly depressed in the larger species with respect to what happens for n < or = 2 (where the luminescence quantum yield is phi approximately 10(-4)); this is discussed in terms of the possible intervention of triplet levels localized at the oligothiophene DEDBT(n)(fragments.

A Convenient Protocol for the Synthesis of Ligands from a 4-Methyl-3,5-diacylaminophenyl Platform

The synthesis of stable and highly organized phenanthroline, terpyridine, and pyridino-oxazoline ligands bearing one or two 4-methyl-3,5-diacylaminophenyl modules equipped with two lateral dialkoxyphenyl groups has been performed using EDC.HCl and DMAP reagents in the final coupling reaction. Evidently, in the final ligands and in the solid state intermolecular hydrogen bonding maintains the coherence of the tridimensional structure as clearly evidenced by FT-IR and X-ray diffraction spectroscopy in the cases of the methoxy ligands. The supramolecular packing is also maintained by additional pi-pi stacking interactions.