Jianwu Shi

Ambipolar organic field-effect transistors with air stability, high mobility, and balanced transport

Ambipolar organic field-effect transistors (OFETs) based on the organic heterojunction of copper-hexadecafluoro-phthalocyanine (F16CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T) were fabricated. The ambipolar OFETs eliminated the injection barrier for the electrons and holes though symmetrical Au source and drain electrodes were used, and exhibited air stability and balanced ambipolar transport behavior. High field-effect mobilities of 0.04cm2∕Vs for the holes and 0.036cm2∕Vs for the electrons were obtained. The capacitance-voltage characteristic of metal-oxide-semiconductor (MOS) diode confirmed that electrons and holes are transported at F16CuPc and BP2T layers, respectively. On this ground, complementary MOS-like inverters comprising two identical ambipolar OFETs were constructed.

Novel thiophene-aryl co-oligomers for organic thin film transistors

A series of thiophene-aryl co-oligomers with phenyl-based central units, i.e. phenyl, biphenyl, fluorene and phenanthrene, were synthesized, and their optical properties and charge carrier transporting properties were characterized. The co-oligomers with biphenyl and phenanthrene units exhibited significantly improved thermal stability. Absorption measurements and electrochemical characterization revealed that all co-oligomers have broader band gap and lower HOMO levels than the corresponding thiophene oligomer, α,ω-dihexylsexithiophene (DHα6T). Introduction of biphenyl and phenanthrene units afforded co-oligomers with the broadest band gap and lowest HOMO levels in comparison with the counterparts of phenyl and fluorene central units. Depending on the structure of the central units, the field-effect mobility (μTFT) of the co-oligomers from top-contact thin film transistors is in the range of (1–6.7) × 10−2 cm2 V−1 s−1. The rigid and planar central unit is favorable for good device performance. Among the co-oligomers, 2,7-bis(5′-hexyl-2,2′-bithien-5-yl)phenanthrene (DH-TTPhTT) exhibits the highest μTFT of 6.7 × 10−2 cm2 V−1 s−1, comparable to that of DHα6T in the same device configuration and fabrication conditions.

Geometries and electronic properties of AunPdm (n=1–4, m=−1, 0, 1) clusters

Abstract Possible conformers for Au n Pd m ( n =1–4, m =−1, 0, 1) clusters have been presented and studied by use of density functional theory. The results indicate that for n =2, linear conformer with C ∞v symmetry is the most stable for anion species, while for cation and neutral species, conformer with C 2v symmetry is the most stable. For n =3, 4, conformers with C 2v symmetry (kite-shape) are energetically favored. The calculated electron affinities (EAs) and vertical detachment energies (VDEs) are in good agreement with experiments for n =1–4. It is also interesting to note that for even n ( n =2, 4), the most stable conformers do not give the best agreement between calculated and experimental EA and VDE values, while for odd n ( n =3), the lowest energy conformer also gives the best agreement. The ionization potentials (IPs) of Au n Pd clusters are calculated as well.

Organic semiconductors based on annelated β-oligothiophenes and its application for organic field-effect transistors

Organic field-effect transistors based on two derivatives of annelated β-oligothiophenes are fabricated. High mobility of 2.2 cm2 V−1 s−1 is obtained for 2,5-distyryl-dithieno[2,3-b:3′,2′-d]thiophene (DEP-DTT) while 2,5-diphenyl-dithieno[2,3-b:3′,2′-d]thiophene (DP-DTT) presents no field-effect characteristics. The existence of S–π intermolecular interaction in DEP-DTT, which is introduced by CC double bonds, plays an important role in the molecular arrangement both in single crystal and thin film structures, and the charge transport of organic field-effect transistors.

Isomers of organic semiconductors based on dithienothiophenes: the effect of sulphur atoms positions on the intermolecular interactions and field-effect performances

2,5-Distyryl-dithieno[2,3-b:2′,3′-d]thiophene (DEP-bt-DTT), an isomer of 2,5-distyryl-dithieno [2,3-b:3′,2′-d]thiophene (DEP-bb-DTT) and 2,5-distyryl-dithieno[3,2-b:2′,3′-d]thiophene (DEP-tt-DTT), was synthesized. Organic field-effect transistors (OFETs) based on these three isomers were fabricated. The structure cell parameters and the formation of intermolecular interactions in their single crystals show regular change when the positions of sulphur atoms vary from top–bottom–top in DEP-tt-DTT to bottom–bottom–top in DEP-bt-DTT, then to bottom–bottom–bottom in DEP-bb-DTT. Combining the results of theoretical calculations and OFET performances, it reveals that: (1) the positions of sulphur atoms determine the contribution extent of sulphur atoms to the molecular conjugation and the formation of intermolecular interactions; (2) the existence of the intermolecular interactions, especially for S–π, benefits for the charge transport; (3) the field-effect mobility (μ) increases with increasing of the sulphur atom contribution to the molecular conjugation.

Synthesis of dendrimers based on tetrakis(thiophene-2-yl)ethene as new dendron.

Two novel dendrimers, 16T and 20T, based on 1,1,2,2-tetra(thiophen-2-yl)ethene (4T) as a new dendron, were efficiently synthesized via carbonylation, Suzuki, and McMurry reactions. All intermediates and title compounds were fully characterized by (1)H NMR, (13)C NMR, and HRMS. 4T and 16T were confirmed by X-ray single crystal analyses. In addition, the absorption behaviors of two titled dendrimers are also described.

Thiophene-Based Double Helices: Syntheses, X-ray Structures, and Chiroptical Properties

We demonstrate facile and efficient construction of conjugated double helical ladder oligomers from the saddle-shaped cyclooctatetrathiophene (COTh) building blocks. The key step involves deprotonation of tetra[3,4]thienylene (β,β-COTh) with n-BuLi which displays remarkably high ipsilateral selectivity. Three racemic double helical ladder oligomers, rac-DH-1, rac-DH-2, and rac-DH-3, containing two, three, and five COTh annelated moieties are efficiently synthesized by diastereoselective coupling of the racemic precursors. The X-ray crystallographic studies of rac-DH-1, rac-DH-2 and rac-DH-3 unambiguously revealed that each double helical scaffold has two single helices intertwined with each other via the C-C single bonds. Following removal of TMS groups, double helical ladder oligomer rac-DH-1-D had sufficient solubility to be resolved via chiral HPLC, thus enabling determination of its chirooptical properties such as CD spectra and optical rotation. (+)-DH-1-D has a large barrier for racemization, with lower limit of ΔG(‡) > 48 kcal mol(-1), which may be compared to DFT-computed barrier of 51 kcal mol(-1). The enantiomers of DH-1-D show 1 order of magnitude stronger chirooptical properties than the carbon-sulfur [7]helicene, as determined by the anisotropy factor g = Δε/ε = -0.039, based on Δεmax = -11 and ε = 2.8 × 10(2) L mol(-1) cm(-1) in cyclohexane at 327 nm.

Synthesis and spectroscopic properties of propeller type 2,4,6-tri(anthracen-9-yl)-1,3,5-triazine

The novel propeller type compound, 2,4,6-tri(anthracen-9-yl)-1,3,5-triazine (TAT) with C3 symmetry was synthesized by using 9-anthryl lithium with 1,3,5-trichlorotriazine or 2-chloro-4,6-dimethoxy-1,3,5-triazine in the presence of palladium catalyst in 22–48% yield. Another two propeller-like compounds, 2-(anthracen-9-yl)-4,6-dimethoxy-1,3,5-triazine (3) and 2,4-di(anthracen-9-yl)-6-methoxy-1,3,5-triazine (4) were obtained at the same time. The propeller type structures of TAT, 3 and 4 were confirmed by crystal analysis and theoretical calculations. The intramolecular C–C bond rotations between the anthryl groups and the triazine units in molecules TAT, 3 and 4 were regarded as the key factor to understand their spectroscopic behaviors in fluid medium and rigid solution.

The efficient synthesis of dibenzo[d,d′]benzo[1,2-b:4,3-b′]dithiophene and cyclopenta[1,2-b:4,3-b′]bis(benzo[d]thiophen)-6-one

Summary With 3,3′-bi[benzo[b]thiophenyl] as starting material, dibenzo[d,d′]benzo[1,2-b:4,3-b′]dithiophene, a [5]heterohelicene, was synthesized efficiently in 60% yield via formylation and McMurry reaction. Cyclopenta[1,2-b:4,3-b′]bis(benzo[d]thiophen)-6-one, another interesting helical ketone, was also prepared in 79% yield via deprotonation and ketonization of 3,3′-bi[benzo[b]thiophenyl]. In addition, the single-crystal structure of dibenzo[d,d′]benzo[1,2-b:4,3-b′]dithiophene and UV–vis spectra of both title compounds are described.

From Saddle-Shaped to Planar Cyclic Oligothienoacenes: Stepped-Cyclization and Their Applications in OFETs

A series of cyclic oligothienoacenes (1, 3, and 4) have been successfully synthesized through a stepped-cyclization synthetic strategy from tetra[3,4-thienylene] (2). Single-crystal X-ray diffraction analysis combined with theoretical calculations elucidated that the molecular configuration transforms from the saddle shape of 2 to the quasi-saddle shape (3b, 3c), then to the quasi-helical shape (4b), and at last to the quasi-plane shape (1, 3a, 4a). Organic field-effect transistors (OFETs) based on 1, 3a, and 4a were fabricated. The field-effect mobilities of 7.5 × 10-4, 2.6 × 10-3, and 6.8 × 10-4 cm2 V-1 s-1 were obtained for 1, 3a, and 4a, respectively. Compounds 3a and 4a can act as air-stable p-type organic semiconductors, while 1 is sensitive to the air.