Metal Complexes of Singly, Doubly and Triply Linked Porphyrins and Corroles: An Insight into the Physicochemical Properties

Metal complexes of multi‐porphyrins and multi‐corroles are unique systems that display a host of extremely interesting properties. Availability of free meso and β positions allow formation of different types of directly linked bis‐porphyrins giving rise to intriguing optical and electronic properties. While the fields of metalloporphyrin and corroles monomer have seen exponential growth in the last decades, the chemistry of metal complexes of bis‐porphyrins and bis‐corroles remain rather underexplored. Therefore, the impact of covalent linkages on the optical, electronic, (spectro)electrochemical, magnetic and electrocatalytic activities of metal complexes of bis‐porphyrins and ‐corroles has been summarized in this review article. This article shows that despite the (still) somewhat difficult synthetic access to these molecules, their extremely exciting properties do make a strong case for pursuing research on these classes of compounds.

Highly Planar and Completely Insulated Oligothiophenes: Effects of π-Conjugation on Hopping Charge Transport

Elucidating the nature of long-range intramolecular charge transport in π-conjugated molecules is of great importance for the development of organic electronic materials. However, the effects of the degree of π-conjugation on the hopping charge transport have not been experimentally explored so far owing to the lack of π-conjugated backbones with different conjugation degrees and several-nanometer lengths. Here we develop highly planar and completely insulated oligothiophenes between 0.85 and 9.64 nm in length. As compared to distorted oligothiophenes, single-molecule conductance measurements of the planar molecules show (i) a smaller activation energy and larger electrical conductance in the hopping transport regime and (ii) a shift in crossover between tunneling and hopping conduction toward a short molecular length. Theoretical calculations indicate that small reorganization energies and narrow energy gaps derived from the planar backbones result in these superior characteristics. This study reveals that the planarity of π-conjugation has significant advantages for hopping charge transport.

Facile synthesis of oligo(3-hexylthiophene)s conductive wires with charge-transfer functions

A series of fully conjugated oligo(3-hexylthiophene)s bearing different starting- and end-groups have been synthesized by means of externally initiated Kumada catalyst-transfer polymerization (KCTP) and Grignard Metathesis Polymerization (GRIM).

Electrostatically promoted dynamic hybridization of glucans with cationic polythiophene

A hetero-triplex composed of a glucan and a cationic polythiophene was dynamic and disassembled into a hetero-duplex in basic solutions.

Thieno[3,4-b]pyrazine-based oligothiophenes: simple models of donor–acceptor polymeric materials

Thieno[3,4-b]pyrazine-based oligomers have been thoroughly investigated in order to correlate the effects of conjugation length and ‘donor–acceptor’ properties on the resulting optical and electronic properties.

Molecular tuning in highly fluorescent dithieno[3,2-b:2',3'-d]pyrrole-based oligomers: effects of N-functionalization and terminal aryl unit

A series of eight conjugated oligomers consisting of central dithieno[3,2-b:2',3'-d]pyrroles (DTPs) end-capped with either thienyl or phenyl groups have been prepared from N-alkyl-, N-aryl-, and N-acyl-dithieno[3,2-b:2',3'-d]pyrroles via Stille and Suzuki cross-coupling. The DTP-based quaterthiophene, N-phenyl-2,6-bis(2-thienyl)dithieno-[3,2-b:2',3'-d]pyrrole was characterized via X-ray crystallography and was found to crystallize in the orthorhombic space group Pna2(1) with a = 10.8666(3) Å, b = 22.8858(6) Å, c = 7.4246(2) Å, and Z = 4. The full oligomeric series was thoroughly investigated via photophysical, electrochemical, and DFT calculations in order to correlate the cumulative effects of both aryl end-groups and N-functionalization on the resulting optical and electronic properties. Through such molecular tuning, it was found to be possible to modulate the HOMO energy by as much as 0.32 V and to generate highly fluorescent oligomers with solution fluorescence efficiencies as high as 92%.

Functional oligothiophenes toward molecular wires in single-molecular electronics

The synthesis of 3-hexylthiophene-based oligothiophenes with a length of approximately 10 nm bearing anchor units at both terminal positions has been accomplished. In addition, we have designed and synthesized completely encapsulated oligothiophenes to investigate single-molecule conductance. Their properties are evaluated by UV–vis absorption spectra and cyclic voltammetry (CV) measurements. The conductance of a single-molecule junction for thiol-introduced oligothiophenes was measured by 10-nm-scale nanogap gold electrodes or modified scanning tunneling microscope (STM) techniques.

Twisting of Conjugated Oligomers and Polymers: Case Study of Oligo- and Polythiophene

Interring twisting (change in the dihedral angle between conjugated rings) of polythiophene was studied theoretically using periodic boundary conditions (PBC) at the B3LYP/6-31G(d) level. We find that the band gap of polymers is strongly dependent on the interring twist angle; yet twisting requires very little energy. A twist of 30 degrees increases the band gap by 0.75 eV in polythiophene, while requiring only 0.41 kcal mol(-1) per monomer unit. Such a small energetic value is of the order of crystal packing or van der Waals forces. These results are compared with calculations performed on model oligomers. Sexithiophene, its radical cations, and its dication are optimized at 0-180 degrees end-to-end twist angles (which correspond to 0-36 degrees interring dihedral angles) using the B3LYP/6-31G(d) method. The theoretical results suggest that the HOMO-LUMO gap, ionization potential, and charge distribution of oligomers are strongly dependent on twisting, whereas, similar to the case of polythiophene, twisting of neutral oligothiophenes costs very little energy. In the case of the radical cation, the lowest energy transition is shifted to a longer wavelength region on twisting, while the second-lowest energy transition is shifted to a shorter wavelength region. This implies that twisted, doped conducting polymers (modeled here by an oligomer radical cation), in contrast to planar, doped polymers, should be transparent within a certain optical window (in the far-visible region, at approximately 1.5 eV). This observation is explained on the basis of changes in the shape and overlap of the frontier molecular orbitals.

Single molecule spectroscopy of poly 3-octyl-thiophene (P3OT)

We report on the spectroscopy of isolated chains of P3OT, in a highly dilute solution in the inert polymer host poly(methyl-methacrylate) (PMMA). This environment permits a detailed analysis of emission transitions in the 1.9-2.2 eV range by suppressing the formation of the lowest emitting-energy aggregated form of P3OT. Herein it is observed that the 1.9-2.2 eV band is in fact split into low (red) and high (blue) energy forms in a highly analogous situation to that found for the conjugated polymer MEH-PPV. Another focus of this work is an investigation of the interaction of singlet and triplet excitons in P3OT. The results indicate that, like in MEH-PPV, triplet excitons are highly efficient fluorescence quenchers for P3OT, strongly quenching the fluorescence of P3OT under even relatively low excitation intensities.

Solvent Effects on Electronic Structures and Chain Conformations of α-Oligothiophenes in Polar and Apolar Solutions

Solvent effects on electronic structures and chain conformations of alpha-oligothiophenes nTs (n = 1 to 10) are investigated in solvents of n-hexane, 1,4-dioxane, carbon tetrachloride, chloroform, and water by using density functional theory (DFT) and molecular dynamics (MD) simulations. Both implicit and explicit solvent models are employed. The polarized continuum model (PCM) calculations and MD simulations demonstrate the weak solvent effects on the electronic structures of alpha-oligothiophenes. The lowest dipole-allowed vertical excitation energies of nTs, obtained from time-dependent DFT/PCM calculations at the B3LYP/6-31G(d) level, exhibit a red shift as the solvent polarity increases, in agreement with experiments. The studied solvents have little impact on the state order of the low-lying excited states provided that the nTs are kept in C2h or C2v symmetry. The MD simulations demonstrate that the chain conformations are distorted to some extent in polar and nonpolar solvents. A qualitative picture of the distribution of solvent molecules around the solvated nTs is drawn by means of radial and spatial distribution functions. The S...H-O and pi...H-O solute-solvent interactions are insignificant in aqueous solution.

Synthesis of 10-nm Scale Oligothiophene Molecular Wires Bearing Anchor Units at Both Terminal Positions

Oligothiophenes with the length of ca.10 nm bearing anchor units (a protected thiol group or trimethylsilylethynyl) at both terminal positions in the conjugated backbone have been synthesized by the block-coupling synthetic strategy. Their electronic properties were clarified by spectroscopic and electrochemical measurements.

Synthesis of Dendritic Oligothiophenes and Their Self-Association Properties by Intermolecular π−π Interactions

[structure: see text] The addition of oligothiophene into a dendritic structure causes a self-association behavior by intermolecular pi-pi interactions in a solution and in a solid state. Increasing the generation of the dendritic structure gives not only a high association constant but also sufficient field-effect hole mobility, which indicates that the charge-transporting passes are formed by the strong pi-pi interactions.