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.     

Spider-like oligothiophenes

Careful analysis and comparison of optical and electrochemical data available in recent literature for multi-thiophene molecular assemblies suggested a few basic rules for the design of structurally simple and easily accessible oligothiophenes endowed with properties not far from those exhibited by much more complex and synthetically demanding architectures. The synthesis and computational investigation of three examples of a class of oligothiophenes (spider-like) tailored according to these indications are reported together with their exhaustive optical and electrochemical characterization. The new compounds (T9 5, T14 6, T19 7) are characterized by a thiophene, a 2,2'-bithiophene and a 2,2',5',2'-terthiophene unit (the spider body) fully substituted with 5-(2,2'-bithiophen)yl pendants (the spider legs). Absorption and electrochemical data are in good agreement and point to a high pi-conjugation level, comparable to those displayed by much larger assemblies. Electrode potential cycling in proximity of the first oxidation peak affords fast and reproducible formation of conducting, highly stable [TXn]m films, mainly consisting of dimers (m=2). Electrooxidation kinetic experiments on deuterium-labelled T9 5, coupled to laser-desorption-ionization mass spectroscopy on the resulting dimer demonstrated that the coupling process is extremely regioselective in the alpha positions of the more conjugated pentathiophene chain. The optical and the electrochemical properties of the films are reported and discussed. A peculiar feature is their impressive charge-trapping ability. Spider-like oligothiophenes are promising materials for applications as active layers in multifunctional organic devices.     

Study of hopping transport in long oligothiophenes and oligoselenophenes: dependence of reorganization energy on chain length

Internal reorganization energies for self-exchange hole-transfer process were calculated at the B3LYP/6-31G(d) level of theory for a series of oligothiophenes and oligoselenophenes up to the 50-mers. This is the first study of reorganization energy in very long pi-conjugated systems. We observed a linear correlation between reorganization energy and the reciprocal chain length for these long pi-conjugated heterocyclic oligomers, which can be explained by the changes in bond length that occur between the neutral and cation radical species and by the charge distribution in the cation radicals. In contrast to the saturation behavior observed for the HOMO-LUMO gaps of long pi-conjugated heterocyclic oligomers, the reorganization energy does not show saturation behavior for any length of the oligomers in this study, even up to the 50-mers. Interestingly, the reorganization energy approaches zero for infinite numbers of oligomer units (at the B3LYP/6-31G(d) level of theory), that is, for polythiophene and polyselenophene. The absolute values of the reorganization energies of oligoselenophenes, and the changes that occur in those energies with chain length, are similar to those found for oligothiophenes.     

The variety of reactions of radical cations derived from 2-diphenylaminothiophene oligomers

ESR spectroelectrochemical measurements of 2-diphenylamino-substituted oligothiophenes 8_m proved the existence of radical cations upon oxidation. Their stability and dimerization depend significantly on the number m of thiophene units. The radical cations and are very reactive and dimerize spontaneously to yield either 2,5-bis(diphenylamino)-2,2′-bithiophene 10₂ or 2,5-bis(diphenylamino)-5,5′-bis(2-thienyl)-3,3′-bithiopene 11₂, respectively. In contrast, the radical cations are highly stable and do not dimerize at all.     

A–D–A‐Type Oligothiophenes for Small Molecule Organic Solar Cells: Extending the π‐System by Introduction of Ring‐Locked Double Bonds

A series of novel acceptor–donor–acceptor oligothiophenes terminally substituted with the 1‐(1,1‐dicyanomethylene)‐cyclohex‐2‐ene (DCC) acceptor has been synthesized. Structural, thermal, optoelectronic, and photovoltaic properties of the π‐extended DCCnTs (n = 1–4) are characterized and contrasted to the trends found for the series of parent dicyanovinyl (DCV)‐substituted oligothiophenes DCVnT. The optoelectronic properties reveal the influence of the additional exocyclic, sterically fixed double bonds in trans‐configuration in the novel DCCnT derivatives. A close correspondence for derivatives with equal number of double bonds, that is, DCCnTs and DCV(n + 1)Ts, is identified. Despite having the same energy gap, the energy levels of the frontier orbitals, HOMO and LUMO, for the DCC ‐ derivatives are raised and more destabilized due to the aromatization energy of a thiophene ring versus two exocyclic double bonds indicating improved donor and reduced acceptor strength. DCC‐terthiophenes give good photovoltaic performance as donor materials in vacuum‐processed solar cells (power conversion efficiencies ≤ 4.4%) clearly outperforming all comparable DCV4T derivatives.     

Pentameric Thiophene‐Based Ligands that Spectrally Discriminate Amyloid‐β and Tau Aggregates Display Distinct Solvatochromism and Viscosity‐Induced Spectral Shifts

A wide range of neurodegenerative diseases are characterized by the deposition of multiple protein aggregates. Ligands for molecular characterization and discrimination of these pathological hallmarks are thus important for understanding their potential role in pathogenesis as well as for clinical diagnosis of the disease. In this regard, luminescent conjugated oligothiophenes (LCOs) have proven useful for spectral discrimination of amyloid‐beta (Aβ) and tau neurofibrillary tangles (NFTs), two of the pathological hallmarks associated with Alzheimer’s disease. Herein, the solvatochromism of a library of anionic pentameric thiophene‐based ligands, as well as their ability to spectrally discriminate Aβ and tau aggregates, were investigated. Overall, the results from this study identified distinct solvatochromic and viscosity‐dependent behavior of thiophene‐based ligands that can be applied as indices to direct the chemical design of improved LCOs for spectral separation of Aβ and tau aggregates in brain tissue sections. The results also suggest that the observed spectral transitions of the ligands are due to their ability to conform by induced fit to specific microenvironments within the binding interface of each particular protein aggregate. We foresee that these findings might aid in the chemical design of thiophene‐based ligands that are increasingly selective for distinct disease‐associated protein aggregates.     

α‐Oligofurans: A Computational Study

Recently, α‐oligofurans have emerged as interesting and promising organic electronic materials that have certain advantages over α‐oligothiophenes. In this work, α‐oligofurans were studied computationally, and their properties were compared systematically with those of the corresponding oligothiophenes. Although the two materials share similar electronic structures, overall, this study revealed important differences between α‐oligofurans and α‐oligothiophenes. Twisting studies on oligofurans revealed them to be significantly more rigid than oligothiophenes in the ground state and first excited state. Neutral α‐oligofurans have more quinoid character, higher frontier orbital energies, and higher HOMO–LUMO gaps than their α‐oligothiophene counterparts. The theoretical results suggest that oligofurans (and subsequently polyfuran) have lower ionization potentials than the corresponding oligothiophenes (and polythiophene), which in turn predicts that oligofurans can be lightly doped more easily than oligothiophenes. Oligofuran dications (8 F²⁺–14 F²⁺) of medium‐sized and longer chain lengths show a polaron‐pair character, and the polycations of α‐oligofurans cannot accommodate high positive charges as easily as their thiophene analogues.     

Synthesis and properties of monodisperse multi‐triarylamine‐substituted oligothiophenes and 4,7‐bis(2′‐oligothienyl)‐2,1,3‐benzothiadiazoles for organic solar cell applications

Two novel series of monodisperse multi‐triarylamine‐substituted oligothiophenes, G ₂ ‐ OT ( n )‐ G ₂ with thiophene unit (n) varying from 6 to 8, and 4,7‐bis(2′‐oligothienyl)‐2,1,3‐benzothiadiazoles G ₂ ‐ OT ( n ) BTD ‐ G ₂ (n = 2, 4, 6) have been synthesized by the Suzuki coupling reactions. With an elongation of alkyl‐substituted oligothiophene core or an incorporation of benzothiadiazole into the central core, the absorption and emission spectra of G ₂ ‐ OT ( n )‐ G ₂ and G ₂ ‐ OT ( n ) BTD ‐ G ₂ series red‐shift substantially with the optical gap reducing to 1.95 eV for G ₂ ‐ OT ( 6 ) BTD ‐ G ₂ . Alkyl‐substitution onto oligothiophene backbone not only improves the solubility of the highly extended dendrimers but also renders coplanarity of the dendritic oligothiophene backbone at the excited state, which results in the enhancement of fluorescence quantum efficiency. The bulk heterojunction solar cells using these newly synthesized dendritic oligothiophenes as a donor material and [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) as an acceptor material were fabricated and investigated which showed an increase in device performance as compared with those of the lower homologues. On increasing the loading of PCBM from 1.5 to 3 times in the active layer, there was also an enhancement in device performance with power conversion efficiencies of as‐fabricated solar cells increasing from 0.18% to 0.32%. In addition, proper annealing procedure could significantly improve the device performance of the dendrimer‐based photovoltaic cell. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 137–148, 2009