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

Computational study on zinc porphyrin analogs for use in dye-sensitized solar cells

The density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches have been applied to obtain the optimized geometries, electronic structures, molecular orbitals and absorption spectra of a series of meso-substituted zinc porphyrin analogs with phenyl and thiophene groups as the π bridging unit and cyanoacrylic acid as the acceptor unit. The results showed that the introduction of thiophene group increases the orbital splitting and changes the absorption spectra properties significantly. It is indicated that when there is only one thiophene group included in the π bridge, the oscillator strength of B absorption band is much stronger. The increasing length of thiophene chain just changes the scope of specific absorption enhancement. The effect of attaching an additional electron-donating group diphenylamine instead of phenyl to the porphyrin core also has been shown. It is found that the diphenylamine group reduces the band gap, and leads to facile intramolecular charge transfer from diphenylamine and porphyrin ring unit to acceptor unit. These kinds of zinc porphyrin analogs have the LUMO energy close to the conduction band of TiO 2 and more red-shifted absorption spectrum compared with phenyl substituted analogs.

Ethynyl thiophene-appended unsymmetrical zinc porphyrin sensitizers for dye-sensitized solar cells

Zn porphyrin sensitizers with ethynyl thiophene linker, pyrene/fluorene donors and cyanoacrylic or malonic acid anchoring group were synthesized for DSSC applications. Conversion efficiency up to 3.14% was exhibited.

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.

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.

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.

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.

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.