Combined Raman and Computational Study of a Novel Series of Macrocyclic π-Conjugated Diacetylene-Bridged α-Linked Oligothiophenes

Small Structural Changes in the Alkyl Substituents of Macrocyclic π-Extended Thiophene Oligomers Causes a Key Effect on Their Stacking and Functional Properties

Three new macrocyclic π-extended thiophene hexamers composed of four thienylene-ethynylene and two thienylene-vinylene units with or without four alkyl substituents have been synthesized. Despite similar shape-persistent structures in solution, the alkyl substituents control the solid-state structures and morphologies. The unsubstituted hexamer exhibited a planar conformation with a theoretically predicted structure in the solid state; however, the planar hexamer with four ethyl substituents formed a closely stacked columnar crystal structure to exhibit π-π interactions. Interestingly, the hexamer with four butyl substituents adopted both planar and twisted conformations in the solid state, exhibiting polymorphism based on induced-fit stacking of molecules. Thus, the butyl-substituted hexamer produces a mixture of yellow, orange, and red single crystals from toluene/acetone, and X-ray analysis revealed six different conformations. Consequently, the small structural difference in the macrocycles causes a key effect on their functional properties in the solid state, and their morphology governs electrical conductivity and organic field-effect-transistor activity. The polymorphism of the hexamers was applied to the switching of film morphology.

A synergic approach of X-ray powder diffraction and Raman spectroscopy for crystal structure determination of 2,3-thienoimide capped oligothiophenes

This work presents a Raman based approach for the rapid identification of the molecular conformation in a series of new 2,3-thienoimide capped quaterthiophenes.

Triindole-bridge-triindole dimers as models for two dimensional microporous polymers

New dimers with two triindole subunits bound together through different linkers (p-phenylene or diacetylene groups) have been synthesized and studied as model systems to determine the differences in the electron transfer ability of the two bridging units. Our results show that whereas a p-phenylene bridge nearly isolates the two subunits of the dimers a diacetylene bridge allows a high level of electronic connection between them.

Designing new symmetrical facial oligothiophene amphiphiles

In this study we designed a new class of symmetrical facial oligothiophene amphiphiles, which could be obtained in fewer steps than for previously reported analogues, but still possess the specific substituent sequence to control their backbone curvature. This novel design allows the late-stage introduction of hydrophilic groups, aiding both purification and ease of structure variation. Following the new synthetic scheme, symmetrical ter- and sexi-thiophenes were synthesized, analysed and their properties were compared to their non-symmetrical analogues. Surprisingly, the self-assembly behaviour in water, aggregate morphologies and photo-physical properties turned out to be significantly different despite the same ratio of hydrophilic and hydrophobic substituents. The new substitution pattern resulted in a drastic decrease of the critical aggregation concentration and an increase of the aggregate size. The symmetrical positioning of the substituents also heavily influenced the photo-physical properties. The changes were observed as large blue shifts in the absorption and emission spectra in water when compared to similar regio-regular oligothiophene amphiphiles.

Thiophene- and Selenophene-Based Heteroacenes: Combined Quantum Chemical DFT and Spectroscopic Raman and UV−Vis−NIR Study

In this article, we report the characterization of a series of thiophene- and selenophene-based heteroacenes, materials with potential applications in organic electronics. In contrast to the usual alpha-oligothiophenes, these annelated oligomers have a larger band gap than most semiconductors currently used in the fabrication of organic field-effect transistors (OFETs) and therefore they are expected to be more stable in air. The synthesis of these fused-ring molecular materials was motivated by the notion that a more rigid and planar structure should reduce defects (such as torsion about single bonds between alpha-linked units or S-syn defects) and thus improve pi-conjugation for better charge-carrier mobility. The conjugational properties of these heteroacenes have been investigated by means of FT-Raman spectroscopy, revealing that pi-conjugation increases with the increasing number of annelated rings. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G** level, to assess information regarding the minimum-energy molecular structure, topologies, and absolute energies of the frontier molecular orbitals around the gap, vibrational normal modes related to the main Raman features, and vertical one-electron excitations giving rise to the main optical absorptions.

Fourier Transform Raman and DFT Study of Three Annulated Oligothiophenes with Different Molecular Shapes

Herein, we study the conjugation properties of three different thienoacenes, each of which has three or four fused thiophene rings, by means of Fourier transform Raman spectroscopy. The B3LYP/6-31G** vibrational analysis of all of the collected spectroscopic data evidences that the selective enhancement of a limited number of Raman scatterings is related to the occurrence in the three thienoacenes of a vibronic coupling between the lowest unoccupied frontier molecular orbital (LUMO) and some Raman-active skeletal nu(C==C) stretching modes of 1600-1300 cm(-1).

Modulation of electronic properties in neutral and oxidized oligothiophenes substituted with conjugated polyaromatic hydrocarbons

The structural and electronic properties of neutral and oxidized terthiophenes substituted with polyaromatic systems have been investigated using a combination of both Raman and electronic absorption spectroscopy in conjunction with density functional theory calculations. Naphthylethenyl terthiophene exhibits structural and electronic properties, in both the neutral and oxidized species, that are dominated by the terthiophene backbone, in a manner similar to that previously reported for phenylethenylterthiophene. Anthracenylethenyl terthiophene, on the other hand, displays properties that are dominated by the anthracene group. Unlike both phenylethenyl and naphthylethenyl terthiophene, which have electronic absorption spectra dominated by transitions between molecular orbitals that are delocalized throughout the molecules, the absorption spectrum of anthracenylethenyl terthiophene consists of a simple addition of the absorption bands of the separate terthiophene and anthracenylethene chromophores. This is the result of a spatial partitioning of its molecular orbitals that effectively electronically decouples the anthracene and terthiophene moieties. Upon oxidation, the naphthylethenylterthiophene sigma-dimerizes to form sexithiophene charged species and spectral signatures of the sexithiophene backbone are evident in both the electronic absorption and resonance Raman spectra. In contrast, these signatures are absent in the corresponding spectra of the oxidized anthracenylethenylterthiophene, suggesting that the anthracene group is the primary site of the structural changes induced by oxidation.

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.

Combined Quantum Chemical Density Functional Theory and Spectroscopic Raman and UV−vis−NIR Study of Oligothienoacenes with Five and Seven Rings

In this article, we report the characterization of novel oligothienoacenes with five and seven fused thiophene rings, materials with potential applications in organic electronics. In contrast to usual alpha-linked oligothiophenes, these fused oligothiophenes have a larger band gap than most semiconductors currently used in the fabrication of organic field-effect transistors (OFETs) and therefore they are expected to be more stable in air. The synthesis of these fused-ring oligomers was motivated by the notion that a more rigid and planar structure should reduce defects (such as torsion about single bonds between alpha-linked units or S-syn defects) and thus improve conjugation for better charge-carrier mobility. The conjugational properties of these two molecular materials have been investigated by means of FT-Raman spectroscopy, revealing that conjugation still increases in passing from the five-ring oligomer to that with seven-rings. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G level, to assess information regarding the minimum-energy molecular structure, topologies, and absolute energies of the frontier molecular orbitals (MOs.) around the gap, vibrational normal modes related to the main Raman features, and vertical one-electron excitations giving rise to the main optical absorptions.

Synthesis and Characterization of Three Novel Perfluoro-oligothiophenes Ranging in Length from the Trimer to the Pentamer

In this Article, we report on the synthesis and full characterization of three perfluorinated oligothiophenes, ranging in length from the trimer to the pentamer (PF-nT, with n = 3, 4, or 5). The differential pulse voltammetry (DPV) analysis of the compounds showed that they can be both oxidized and reduced (i.e., they display a dual or amphoteric electrochemical behavior), with the reduction peaks positively shifted relative to those of the corresponding unsubstituted oligothiophenes. The electrochemically determined energy gaps are in agreement with those measured from the UV-vis-NIR absorption spectra in solution. The conjugational properties have been investigated by means of FT-Raman spectroscopy, both as pure solids and as dilute solutes in CH(2)Cl(2), revealing that: (i) pi-conjugation does not still reach saturation with chain length for the longest oligomer, and (ii) conformational distortions from a nearly coplanar arrangement of the successive thiophene units upon solution are not too large. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G level, to assess information about the optimized molecular structure, equilibrium atomic charges distribution, energies and topologies of the frontier molecular orbitals (MO) around the gap, vibrational normal modes associated with the most outstanding Raman scatterings, and vertical one-electron excitations that give rise to the main optical absorptions.

Quater-, Quinque-, and Sexithiophene Organogelators: Unique Thermochromism and Heating-Free Sol-Gel Phase Transition

A series of quater-, quinque-, and sexithiophene derivatives bearing two cholesteryl groups at the alpha-position, which are abbreviated as 4 T-(chol)(2), 5 T-(chol)(2), and 6 T-(chol)(2), respectively, have been synthesized. It has been found that these oligothiophene derivatives act as excellent organogelators for various organic fluids and show the unique thermochromic behaviors through the sol-gel phase transition. It was shown on the basis of extensive investigations, performed with UV-visible spectroscopy, circular dichroism (CD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM), that these gelators self-assemble into the one-dimensional structures in the organogels, in which the pi-block moieties of the oligothiophenes are stacked in an H-aggregation mode. Surprisingly, an AFM image shows that 4 T-(chol)(2) forms unimolecular fibers in a left-handed helical sense, whereby one pitch of the helical fiber is constructed by 400-540 4 T-(chol)(2) molecules. Very interestingly, the conformational change in the oligothiophene moieties can be visually detected: for example, 6 T-(chol)(2) shows a specific absorption maximum in the gel (lambda(max) = 389 nm) and in the solution (lambda(max) = 439 nm). In addition, a sol-gel phase transition of the 6 T-(chol)(2) gel was implemented by addition of oxidizing and reducing reagents such as FeCl(3) and ascorbic acid, respectively. The stimuli-responsive functionality of the oligothiophene-based organogels makes them promising candidates for switchable opto- and electronic soft materials.

π-Stacking in Thiophene Oligomers as the Driving Force for Electroactive Materials and Devices

The pi-stacking between aromatic oligomers has been extensively studied for many years, although the notion of exploiting this phenomenon as the driving force for molecular actuation has only recently emerged. In this work we examine with MP2 and Car-Parrinello ab initio calculations the actuation properties of a novel class of thiophene-based materials introduced by Swager et al. (Adv. Mater. 2002, 14, 368; J. Am. Chem. Soc. 2003, 125, 1142). The chemical ingredients of the assembly, calix[4]arenes and oligothiophenes, are screened separately to characterize the actuation mechanisms and design optimal architectures. In particular, ab initio methods are used to study pi-stacking in mixed-valence oligothiophene dimers, revealing strong interactions that can be turned on and off as a function of the electrochemical potential. We show how these interactions could be harnessed to achieve molecular actuation and investigate the response of an active unit in real time with first-principles molecular dynamics simulations.