Masashi Nitani

π-Extended thiadiazoles fused with thienopyrrole or indole moieties: synthesis, structures, and properties.

We report the syntheses, structures, photophysical properties, and redox characteristics of donor-acceptor-fused π-systems, namely π-extended thiadiazoles 1-5 fused with thienopyrrole or indole moieties. They were synthesized by the Stille coupling reactions followed by the PPh(3)-mediated reductive cyclizations as key steps. X-Ray crystallographic studies showed that isomeric 1b and 2b form significantly different packing from each other, and 1a and 4a afford supramolecular networks via multiple hydrogen bonding with water molecules. Thienopyrrole-fused compounds 1b and 2b displayed bathochromically shifted intramolecular charge-transfer (CT) bands and low oxidation potentials as compared to indole-fused analog 3b and showed moderate to good fluorescence quantum yields (Φ(f)) up to 0.73. In 3b-5b, the introduction of electron-donating substituents in the indole moieties substantially shifts the intramolecular CT absorption maxima bathochromically and leads to the elevation of the HOMO levels. The Φ(f) values of 3-5 (0.04-0.50) were found to be significantly dependent on the substituents in the indole moieties. The OFET properties with 1b and 2b as an active layer were also disclosed.

Short Synthesis of Alkyl-Substituted Acenes Using Carbonyl-Directed C–H and C–O Functionalization

A convenient method for the synthesis of tetraalkylanthracenes and -pentacenes by means of ruthenium-catalyzed regioselective C-H alkylation of the corresponding acenequinones was developed. Dialkyldiarylpentacene was also synthesized using chemoselective tandem C-H alkylation/C-O arylation of dimethoxypentacenequinone. It was suggested that a tetraalkylpentacene is stable under air in the dark and possesses an appropriate HOMO level as active material for p-type organic field-effect transistors (OFETs).

A Series of π‐Extended Thiadiazoles Fused with Electron‐Donating Heteroaromatic Moieties: Synthesis, Properties, and Polymorphic Crystals

π-Extended thiadiazoles 4-8 fused with various electron-donating heteroaromatic moieties have been designed and synthesized. Just like thiadiazoles 1-3 synthesized previously, 4-8 exhibit intramolecular charge-transfer (CT) interactions, moderate-to-good fluorescence quantum yields of up to 0.78, and electrochemical amphoterism. In comparison with 1-3, the benzannulation in thiadiazoles 4-7 moderately extends the π conjugation and significantly increases the stability of the cationic species formed upon electrochemical oxidation. The fluorescence quantum yields increase remarkably from 3 to 6 and 7 due to the efficient suppression of nonradiative intersystem crossing resulting from the benzannulation. The properties of 4-8 strongly reflect the different species annulated to the pyrrole rings, namely benzothiophene, naphthalene, and benzofuran. Eleven crystals, including poly- and pseudopolymorphic crystals of 1 (1-Crys.(Y) and 1-Crys.(G)), 2 (2-Crys.(O) and 2-Crys.(G)), 4 (4-Crys.(O) and 4-Crys.(G)), and 6 (6-Crys.(O) and 6-Crys.(G)), were obtained and characterized by X-ray crystallography. The fluorescence colors and efficiencies are distinct for each poly- and pseudopolymorph of 1, 2, 4, and 6. It has been suggested that both the extent of the electronic interactions in the π-stacked dimers and the presence of excitonic interactions originating in the 1D face-to-face slipped columns affect the fluorescence wavelengths of the poly- and pseudopolymorphs.

Arenedithiocarboxyimide-containing extended π-conjugated systems with high electron affinity

A series of new π-conjugated systems bearing arenedithiocarboxyimides (dithioimides) as electron-accepting terminal units were prepared utilizing thionation of the imide compounds in the final step of the synthesis. The thermal properties of the dithioimide compounds demonstrated that they had a weak crystallization nature, and their photophysical and electrochemical properties were significantly different from those of their imide analogs. As a result, the dithioimide compounds had narrower highest occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) energy gaps, and lower LUMO energy levels than those of the corresponding imide compounds. Organic field-effect transistors (OFETs) based on the dithioimide compounds showed good electron-transporting characteristics. Furthermore, the observed OFET performances were dramatically improved compared to those for the crystalline films of the corresponding imide derivatives, despite their tendency to form amorphous films. This unexpected phenomenon could be attributed to the presence of strong intermolecular electronic interactions for the dithioimide compounds, which induced the construction of a non-directional charge-transport pathway. Thus, the increase in electron mobilities for the dithioimide compounds was attributed to the combined effect of the low-lying LUMO energy level and the strong intermolecular electronic interactions in the solid state. Organic photovoltaics based on poly(3-hexylthiophene) as the hole-transporting material and the dithioimide compounds as the electron-transporting material exhibited poorer performances due to the high miscibility between the two compounds.

Solution‐Processable n‐Type Organic Field‐Effect Transistor (OFET) Materials Based on Carbonyl‐Bridged Bithiazole and Dioxocyclopentene‐Annelated Thiophenes

A series of electronegative π-conjugated compounds composed of carbonyl-bridged bithiazole and alkyl-substituted dioxocyclopenta[b]thiophene were synthesized as a candidate for solution-processable n-type organic semiconductor materials and characterized on the basis of photophysical and electrochemical properties. Cyclic voltammetry measurements showed that the first half-wave reduction potentials of these compounds are between -0.97 and -1.14 V versus ferrocene/ferrocenium, which corresponds to lowest unoccupied molecular orbital energy levels between -3.83 and -3.66 eV. Thanks to hexyl or dodecyl groups in the molecules, the compounds are sufficiently soluble to realize the fabrication of their thin films through a spin-coating method. As a result, the prepared organic field-effect transistors based on these newly developed compounds exhibited n-channel characteristics not only under vacuum but also in air, and the best field-effect electron mobility observed under vacuum was 0.011 cm(2) V(-1) s(-1) with an on/off ratio of 10(8) and a threshold voltage of 16 V.

Tetraalkoxyphenanthrene-Fused Thiadiazoloquinoxalines: Synthesis, Electronic, Optical, and Electrochemical Properties, and Self-Assembly

π-Extended thiadiazoloquinoxaline (TQ) derivatives 1a,b-3a,b, in which a tetraalkoxyphenanthrene moiety is annulated with the TQ core and benzene rings are incorporated via the ethynylene spacer, were synthesized. They display absorption bands reaching into 750 nm and possess the electron-affinity comparable to [60]fullerene. The CF3- and OMe-substituents on the benzene rings have moderate effects on modulation of the HOMO and LUMO levels. Tetraalkoxyphenanthrene-fused TQs 1a,b-3a,b aggregate in the solid state and assemble in solution through π-π stacking interactions. The self-assembly of 1a,b-3a,b into 1D superstructures was confirmed, and the difference in the alkoxy groups and the solvents for self-assembly proved to change their morphology. Comparison of the properties of 1a and those of reference compounds 4 and 5 clarified the effects of both the fusion of the phenanthrene moiety and the introduction of ethynylene spacers on the properties.

Electron-Accepting π-Conjugated Molecules with Fluorine-Containing Dicyanovinylidene as Terminal Groups: Synthesis, Properties, and Semiconducting Characteristics

A series of electron-accepting π-conjugated molecules having fluorine-containing dicyanovinylidene as terminal groups has been synthesized for the application to electron-transporting semiconductors. This terminal group can be easily incorporated into π-conjugated frameworks. Electrochemical measurements indicated that these compounds showed low-lying lowest unoccupied molecular orbital energy levels, which could be fine-tuned by the combination of central unit. The thin films fabricated by solution process showed typical electron-transporting characteristics in field-effect transistors.

Tetraalkoxyphenanthrene-Fused Hexadecadehydro[20]- and Tetracosadehydro[30]annulenes: Syntheses, Aromaticity/Antiaromaticity, Electronic Properties, and Self-Assembly

Tetraalkoxyphenanthrene-fused hexadecadehydro[20]- and tetracosadehydro[30]annulenes possessing octatetrayne linkages were synthesized and their properties together with those of phenanthrene-fused octadehydro[12]- and dodecadehydro[18]annulenes have been investigated. Various spectroscopic and electrochemical measurements as well as quantum chemical calculations support that planar [20]- and [30]annulenes are weakly antiaromatic and nonaromatic, respectively. The detailed concentration- and temperature-dependent 1H NMR and UV-vis data of present dehydroannulenes provided evidence for the enhancement of π-π stacking interactions by extension of the acetylenic linkages. Dehydroannulenes formed self-assembled clusters and their morphology and crystallinity proved to depend on the length of acetylenic linkages, the shape of dehydroannulene core, and the bulkiness of alkoxy groups appended to the phenanthrene moieties.