Exploration of ground and excited electronic states of aromatic and quinoid S,S-dioxide terthiophenes. Complementary systems for enhanced electronic organic materials

Synthesis of p-Azaquinodimethane-based Quinoidal Fluorophores

Quinoidal π-conjugated systems are sought-after materials for semiconducting applications because of their rich optical and electronic characteristics. However, the analogous fluorescent compounds are extremely rare, with just two reports in the literature. Here, we present the design and development of a third series of quinoidal fluorophores [(2,5-diarylidene)-3,6-bis(hexyloxy)-2,5-dihydropyrazine (Q1-Q5)] that incorporates p-azaquinodimethane. The fluorophores are synthesized in a two-step synthetic approach employing Knoevenagel condensation of N,N-diacetyl-piperazine-2,5-dione with different aromatic aldehydes followed by O-alkylation in high yields. Q1-Q5 are strongly emissive, and by altering the aryl-substituents, the emission colors can be modulated from blue to orange. The compounds possess emission maxima (λem) at 475-555 nm in the solution state and 510-610 nm in the solid state, with fluorescence quantum yields of up to 60%. To the best of our knowledge, the reported systems are the first quinoidal dual-state emissive (solution- and solid-state) compounds. In trifluoroacetic acid, Q5 exhibits halochromic behavior, with a dramatic color change from yellow to blue. Furthermore, the preliminary fluorescent sensing studies demonstrated that Q5 could act as a selective turn-off fluorescence probe for electron-deficient picric acid (PA), with an emission quenching of >90% in the solution state. The thin-layer chromatography (TLC) strip sensor of Q5 was also designed to detect PA in water.

Double [4]Helicene-like Naphthobisbenzothiophene Diimides and Their Thienyl-S,S-dioxidized Derivatives with Attractive Solid-State Fluorescence and High Electron Affinity

A series of double [4]helicene-like naphthobisbenzothiophene diimides and their thienyl-S,S-dioxidized derivatives are synthesized via MoCl₅-catalyzed cyclization and m-CPBA-mediated oxidation reactions. The functional five-membered ring diimides show a helicene-like geometry, strong solid-state fluorescence, and deep LUMO of -4.37 eV.

π-Conjugated stannole copolymers synthesised by a tin-selective Stille cross-coupling reaction

The synthesis of four well-defined conjugated polymers TStTT1-4 containing unusual heterocycle units in the main chain, namely stannole units as building blocks, is reported. The stannole-thiophenyl copolymers were generated by tin-selective Stille coupling reactions in nearly quantitative yields of 94% to 98%. NMR data show that the tin atoms in the rings remain unaffected. Weight-average molecular weights (M w) were high (4900-10 900 Da and 9600-21 900 Da); and molecular weight distributions (M w/M n) were between 1.9 and 2.3. The new materials are strongly absorbing and appear blue-black to purple-black. All iodothiophenyl-stannole monomers St1-4 and the resulting bisthiophenyl-stannole copolymers TStTT1-4 were investigated with respect to their optoelectronic properties. The absorption maxima of the polymers are strongly bathochromically shifted compared to their monomers by about 76 nm to 126 nm in chloroform. Density functional theory calculations support our experimental results of the single stannoles St1-4 showing small HOMO-LUMO energy gaps of 3.17-3.24 eV. The optical band gaps of the polymers are much more decreased and were determined to be only 1.61-1.79 eV. Furthermore, both the molecular structures of stannoles St2 and St3 from single crystal X-ray analyses and the results of the geometry optimisation by DFT confirm the high planarity of the molecules backbone leading to efficient conjugation within the molecule.

Thieno[3,4-b]thiophene-Based Novel Small-Molecule Optoelectronic Materials

Because of the tailorable photoelectric properties derived from judicious molecular design and large-area and low-temperature processability especially on flexible substrates, design and synthesis of new organic π-functional materials is always a central topic in the field of organic optoelectronics, which siginificantly contributed to the development of high-performance optoelectronic devices such as organic photovoltaics (OPVs), organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs). Compared with polymers, small molecules with well-defined molecular structures benefit the establishment of structure-property relationships, which may provide valuable guidelines for the design of new optoelectronic materials to further promote the device performance. New building blocks are essential for the construction of optoelectronic materials. As is well recognized, thiophene-based functional materials have played an indispensable role in the development of organic optoelectronics. Compared with six-membered benzene, five-membered thiophene shows weaker aromaticity and lower steric hindrance and may provide extra sulfur-sulfur interactions in solid state. Among various thiophene building blocks, thieno[3,4-b]thiophene (TbT) is an asymmetric fused bithiophene containing four functionalization positions, in which the proaromatic thiophene can effectively stabilize the quinoidal resonance of the aromatic thiophene. Thus, TbT exhibits a unique characteristic of quinoid-resonance effect that is powerful to modulate electronic structures. Although the application of TbT in polymer donor materials represented by PTB-7 has achieved a great success, its application in small-molecule optoelectronic materials is almost an untouched field. In this Account, we summerize the rational design of a series of TbT-based small-molecule optoelectronic materials designed and optimized by quinoid-resonance effect, regiochemistry, and side-chain engineering and demonstrate the crucial effect of TbT building blocks on the electronic structures, photophysical and charge transport properties, and photovoltaic performance. With well-defined regioregular oligothieno[3,4-b]thiopenes, we revealed the quinoid-resonance effect of the TbT moiety and its geometric origin. TbT-based small molecules exhibit full-color tunable emissions in the visible to near-infrared regions and excellent performance in OFETs and OPVs. For instance, TbT-based quinoidal molecules with near-infrared fluorescence quantum yields up to 53.1% and TbT-based aromatic molecules with full-color-tunable emissions and high fluorescence quantum yields approaching 100% in polar solvent were designed and synthesized. Solution-processable ambient-stable n-channel organic thin-film transistors based on two-dimensional π-expanded quinoidal terthiophenes with distal or proximal sulfur orientations (2DQTTs) realized a record electron mobility of 5.2 cm² V^-1 s^-1. Furthermore, TbT-based electron donor and electron acceptor materials were successfully designed for OPV applications delivering high power conversion efficiencies up to 9.26% and 10.07%, respectively. We believe that new TbT-based small-molecule materials designed by a synergy of molecular engineering strategy may not only further promote OFET and OPV performance but also realize more unique applications.

Efficient Exciton Diffusion and Resonance-Energy Transfer in Multilayered Organic Epitaxial Nanofibers

Multilayered epitaxial nanofibers are exemplary model systems for the study of exciton dynamics and lasing in organic materials because of their well-defined morphology, high luminescence efficiencies, and color tunability. We use temperature-dependent continuous wave and picosecond photoluminescence (PL) spectroscopy to quantify exciton diffusion and resonance-energy transfer (RET) processes in multilayered nanofibers consisting of alternating layers of para-hexaphenyl (p6P) and α-sexithiophene (6T) serving as exciton donor and acceptor material, respectively. The high probability for RET processes is confirmed by quantum chemical calculations. The activation energy for exciton diffusion in p6P is determined to be as low as 19 meV, proving p6P epitaxial layers also as a very suitable donor material system. The small activation energy for exciton diffusion of the p6P donor material, the inferred high p6P-to-6T resonance-energy-transfer efficiency, and the observed weak PL temperature dependence of the 6T acceptor material together result in an exceptionally high optical emission performance of this all-organic material system, thus making it well suited, for example, for organic light-emitting devices.

The impact of thienothiophene isomeric structures on the optoelectronic properties and photovoltaic performance in quinoxaline based donor–acceptor copolymers

The importance of the monomer's isomeric structure in the design of new D–A copolymers for various applications.

Synthesis, spectroscopy, and computational analysis of photoluminescent bis(aminophenyl)-substituted thiophene derivatives

Substituted oligothiophenes have a long history in the field of organic electronics, as they often combine outstanding electro-optical properties with the ease of synthesis. To assist the rational selection of the most promising structures to be synthesized, there is the demand for tools that allow prediction of the properties of the materials. In this study, we present strategies for synthesis and computational characterization, with respect to the fluorescence behavior of oligothiophene-based materials for organoelectronic applications. In a combined approach, sophisticated computational methodologies are directly compared to experimental results. The M06-2X functional in combination with the polarizable continuum model in a state-specific formulation for excited-state solvation proved to be particularly reliable. In addition, a semiclassical approach for describing the vibrational broadening of the spectra is employed. As a result, a robust procedure for the prediction of the fluorescence spectra of oligothiophene derivatives is presented.

Supramolecular architecture in Langmuir and Langmuir-Blodgett films incorporating a chiral azobenzene

This article describes the synthesis and fabrication of Langmuir and Langmuir-Blodgett (LB) films incorporating a chiral azobenzene derivative, namely, ( S)-4- sec-butyloxy-4'-[5''-(methyloxycarbonyl)pentyl-1''-oxy]azobenzene, abbreviated as AZO-C4(S). Appropriate conditions for the fabrication of monolayers of AZO-C4(S) at the air-water interface have been established, and the resulting Langmuir films have been characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and UV-vis reflection spectroscopy. The results indicate the formation of an ordered trilayer at the air-water interface with UV-vis reflection spectroscopy showing a new supramolecular architecture for multilayered films as well as the formation of J aggregates. Films were transferred onto solid substrates, with AFM revealing well-ordered multilayered films without 3D defects. Infrared and UV-vis absorption spectroscopy indicate that the supramolecular architecture may be favored by the formation of H bonds between acid groups in neighboring layers and pi-pi intermolecular interactions. Circular dichroism spectra reveal chiro-optical activity in multilayered LB films.

Reverse Selectivity in m-CPBA Oxidation of Oligothiophenes to Sulfones

Oligothiophene sulfones of up to six rings can be conveniently prepared by the direct oxidation of butyl-substituted thiophene oligomers with m-CPBA in dichloromethane. Reverse selectivity of oxidized rings is observed relative to previously reported systems without beta-substitution. The selectivity in the trimer and hexamer is confirmed with single-crystal X-ray structure data. The sulfones possess red-shifted absorptions and increased electron affinities relative to the parent oligomers.

Torsional effects on excitation energies of thiophene derivatives induced by β-substituents: Comparison between time-dependent density functional theory and approximated coupled cluster approaches

The influence of methyl or phenyl substitution in beta-position of dioxygenated terthiophene and diphenylthiophene on the optical properties is investigated by first-principles calculations. We compare the approximated singles and doubles coupled cluster (CC2) approach with time-dependent density functional theory methods. CC2 reproduces experimental excitation energies with an accuracy of 0.1 eV. We find that the different substituents modify the inter-ring torsional angle which in turn strongly influences the excitation energies. The steric contribution to the excitation energies have been separated from the total substituent effects.