Yulia A. Getmanenko

Unprecedented Negishi Coupling at C−Br in the Presence of a Stannyl Group as a Convenient Approach to Pyridinylstannanes and Their Application in Liquid Crystal Synthesis

The 2-bromo-5(or 6)-tri-n-butylstannylpyridines, prepared from dibromopyridines and i-PrMgCl at room temperature, undergo Negishi coupling with either alkyl or arylzinc chlorides. The new alkyl- and aryl-substituted pyridylstannanes produced are shown to be suitable for further functionalization by Stille coupling. A group of new liquid crystalline materials with aromatic cores comprised of pyridine and thiophene rings were prepared utilizing these new pyridinylstannanes as key intermediates.

Polymethine materials with solid-state third-order optical susceptibilities suitable for all-optical signal-processing applications

Judicious substitution of chalcogenopyrylium-terminated polymethine dyes with sterically demanding groups ameliorates the deleterious effects of aggregation on the optical properties of these materials in the solid state, facilitating high-number-density films that exhibit an unprecedented combination of nonlinear optical properties with low linear and nonlinear losses.

Pyrrole[3,2-d:4,5-d′]bisthiazole-bridged bis(naphthalene diimide)s as electron-transport materials

A series of materials with 2,6-disubstituted-N-alkyl-pyrrole[3,2-d:4,5-d′]bisthiazole (PBTz) with triisopropylsilyl- (TIPS), bromo- and naphthalene diimide (NDI) groups were synthesized. The electronic properties of 2,6-bis-TIPS- and 2,6-dibromo-N-hexyl-PBTz were studied by cyclic voltammetry and by density functional theory (DFT) calculations, and their solid-state packing was examined by the single crystal X-ray structural analysis. DFT calculations and the electrochemical data revealed that this core is both a weak donor and a weak acceptor. Small molecules with bis(NDI)-substituted N-alkyl-PBTz architecture were studied by differential pulse voltammetry, UV-vis absorption spectroscopy, and differential scanning calorimetry, and their electrical properties were examined in n-channel organic field-effect transistors using solution-processed films. The electron mobility value μe as high as 0.13 cm2 V−1 s−1 with a Ion/Ioff ratio of 5 × 105 and threshold voltage Vth = 4.9 V was observed for PBTz-bridged bis(naphthalene diimide) with hexyl chains on pyrrole and NDI nitrogen atoms, while the material with longer dodecyl groups showed μe up to 0.19 cm2 V−1 s−1 with a Ion/Ioff ratio of 7 × 104 and Vth = 7.9 V in a 1 : 1 polystyrene matrix. Finally, compounds with electron-withdrawing acetyl groups at position 6 of the NDI units were examined by electrochemistry and in OFET configurations.

2,5-Dibromopyridine as a key building block in the synthesis of 2,5-disubstituted pyridine-based liquid crystals

Fifteen 2,5‐disubstituted pyridine based liquid crystals were synthesized exploiting the different reactivities of the bromine atoms in 2,5‐dibromopyridine under Negishi coupling conditions. Convenient approaches to both 2‐iodo‐5‐alkyl‐pyridines and 2‐alkyl‐5‐bromopyridines were also developed. The liquid crystalline behaviour of the synthesized materials was investigated using DSC and polarizing microscopy. The charge mobility of 2‐(4‐heptyloxyphenyl)‐5‐heptylpyridine was measured using the time of flight technique.

Benzo[1,2-b:6,5-b′]dithiophene(dithiazole)-4,5-dione derivatives: synthesis, electronic properties, crystal packing and charge transport

A series of dihalo- and bis-aroyl-substituted benzo[1,2-b:6,5-b′]dithiophene-4,5-diones were synthesized, and their electronic, electrochemical, and electrical properties investigated. Synthetic strategies to increase (i) the conjugation length of the base molecular structure – through introduction of thiophene units bearing electronically neutral substituents (hydrogen or alkyl groups) or strong electron-withdrawing pentafluorobenzoyl group(s) – and (ii) the electron affinity – by moving to a benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione structure – were developed. Molecular packing in the single crystal was studied by single-crystal X-ray structural analysis, and this information was subsequently used in the determination of the electronic band structures, densities of states (DOS), effective transfer integrals, and effective charge-carrier masses via density functional theory (DFT) methods. The charge-carrier transport properties of the benzo[1,2-b:6,5-b′]dithiophene-4,5-dione and benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione derivatives were investigated through the fabrication and characterization of organic field-effect transistors (OFETs) via both solution-processed and vacuum-deposited films. 2,7-Bis-pentafluorobenzoyl-benzo[1,2-b:6,5-b′]dithiophene-4,5-dione (10a) exhibited field-effect behavior with an average electron mobility μe = 4.4 (±1.7) × 10−4 cm2 V−1 s−1 when the active layer was vacuum-deposited, and a larger μe= 6.9 × 10−3 cm2 V−1 s−1 when the active layer was solution-processed. These results are in stark contrast with the DFT-determined electronic band structure and effective mass, which indicate that the material possesses good intrinsic charge-carrier transport characteristics. The combined results reveal the importance of thin-film processing and that further processing refinements could lead to improved device performance. Only one material with benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione core, 2,7-bis-(4-n-hexyl-thiophene-2-yl)-benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione (19d), showed average μe = 8.2 × 10−5 cm2 V−1 s−1 in OFET with solution-processed active layer. Unexpectedly, measurable hole transport was observed for 2,7-bis-(5-n-nonyl-thiophen-2-yl)-benzo[1,2-b:6,5-b′]dithiophene-4,5-dione (19b) (μh = 8.5 × 10−5 cm2 V−1 s−1) and 2,6-bis-(thiophen-2-yl)-3,5-di-n-hexyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophen-4-one (30a) (μh = 3.7 × 10−4 cm2 V−1 s−1).

Bis(naphthalene diimide) derivatives with mono- and dicarbonyl-fused tricyclic heterocyclic bridges as electron-transport materials

A series of triads in which two naphthalene diimide (NDI) units are bridged through their 2-positions by cyclopenta[2,1-b; 3,4-b′]dithiophene-4-one, cyclopenta[2,1-b;3,4-b′]dithiazole-4-one, benzo[1,2-b:6,5-b′]dithiophene-4,5-dione, and benzo[1, 2-b:6,5-b′]dithiazole-4,5-dione was synthesised, characterised by cyclic voltammetry and UV–vis absorption spectroscopy, modelled using density functional theory (DFT) calculations, and examined as solution-processed films in n-channel OFETs. The influence of the electron affinity and ionisation potential of the bridging fused heterocycle and of the length and position of alkyl chains in the triads on the electronic properties of the materials in solution and electrical properties in the thin film was explored. The nature of the bridge has a strong effect on the optical and electrochemical properties of materials. Use of increasingly electron-poor bridging groups led to hypsochromic shifts of the low-energy visible absorption (assigned to a transition from a bridge-based HOMO to a LUMO predominantly located on the NDI moieties) and an anodic shift of the first reduction potential from ca.−1.1 V to−0.7 V vs. Cp2Fe+/0. The average electron mobility, μe, varied from 0.17 cm2 V−1 s−1 to 2.3×10−4 cm2 V−1 s−1 with the largest mobility observed for the material with cyclopenta[2,1-b;3,4-b′]dithiophene-4-one bridge. An extension of the N-alkyl chains on the NDI units from n-hexyl to n-dodecyl led to a two-fold decrease in μe for materials with cyclopenta[2,1-b;3,4-b′]dithiophene-4-one bridge, while introduction of alkyl chains to the 3- and 5-positions of the bridge was even more detrimental for the electron transport. Despite the expected increased planarity (based on DFT calculations) and more facile first reduction potential of compounds based on the cyclopenta[2,1-b;3,4-b′]dithiazole-4-one bridge, a three-fold decrease in μe was observed when this bridge was used in place of its thiophene analogue, although the threshold voltage decreased from 11.5 to 7.3 V. Materials containing benzo[1,2-b:6,5-b′]dithiophene(or dithiazole)-4,5-dione bridges, which are easier to reduce than an isolated NDI, exhibited further decrease in μe to 7×10−3 cm2 V−1 s−1 and 2.3×10−4 cm2 V−1 s−1, respectively.

Bis(5-alkylthiophen-2-yl)arene liquid crystals as molecular semiconductors

Mesogens with a general structure bis(alkylthiophene)arene, in which the central arene moiety is 1,4-benzene, 2,5-pyridine, 3,6-pyrimidine, 2,6-naphthalene and 2,6-dithieno[3,2-b;2′,3′-d]thiophene, were synthesized, and the effect of the central unit and the length of the alkyl chains on the liquid crystalline behavior was studied by differential scanning calorimetry and polarized optical microscopy, and in the case of 1,4-bis(5-tridecylthiophen-2-yl)benzene (2a) and 2,6-bis(5-tridecylthiophen-2-yl)DTT by synchrotron X-ray analysis. Several 2,5-bis(5-alkylthiophen-2-yl)pyridines and 1,4-bis(5-tridecylthiophen-2-yl)benzene 2a exhibit a rare transition – formation of the SmF phase directly from the isotropic liquid; in the case of 2a, formation of higher ordered smectic phases was observed on further cooling. Hole-transporting properties of 2a in different mesophases were studied by the time-of-flight technique. The hole mobility, μh, in SmF reached 6 × 10−3 cm2 V−1 s−1 and increased with an increase of the order of the mesophase up to 0.02 cm2 V−1 s−1 in the S1 phase (tentatively assigned as the crystal G phase), ∼0.09 cm2 V−1 s−1 in the S2 phase (tentatively assigned as the crystal H phase), and 0.11 cm2 V−1 s−1 in the crystalline phase at 70 °C.

Radiation-induced trapping and charge transport in a smectic liquid crystal

We have studied changes in the hole transport properties of a smectic liquid crystalline semiconductor resulting from high-energy electron irradiation. The “radiation doping” results in sharply increased shallow trap densities and the onset of nearly trap-limited hopping conduction. A simple semiquantitative model provides estimates of the trap lifetime and the irradiation energy required to create a single trap. Experimental techniques to overcome parasitic effects due to increased molecular ion densities in irradiated samples are also discussed.

Characterisation of a dipolar chromophore with third-harmonic generation applications in the near-IR

E-2-Tricyanovinyl-3-n-hexyl-5-[4-{bis(4-n-butylphenyl)amino}-2-methoxystyryl]-thiophene, 1, has previously been used to demonstrate applications relying on frequency tripling of 1.55 μm light. Here we report the synthesis and chemical characterisation of 1, along with quantum-chemical calculations and additional experimental investigations of its third-order nonlinear properties that give more insight into its frequency tripling properties. Although 1 can be processed into amorphous films, crystals can also be grown by slow evaporation of solutions; the crystal structure determined by X-ray diffraction shows evidence of significant contributions from zwitterionic resonance forms to the ground-state structure, and reveals centrosymmetric packing exhibiting π–π and C–H⋯NC interactions. Both solutions and films of 1 exhibit near-infrared two-photon absorption into the low-lying one-photon-allowed state with a peak two-photon cross-section of ca. 290 GM (measured using the white-light continuum method with a pump wavelength of 1800 nm) at a transition energy equivalent to degenerate two-photon absorption at ca. 1360 nm; two related chromophores are also found to show comparable near-IR two-photon cross-sections. Closed-aperture Z-scan measurements and quantum-chemical calculations indicate that the nonlinear refractive index and third-harmonic generation properties of 1 are strongly dependent on frequency in the telecommunications range, due the aforementioned two-photon resonance.

5,5′-Bis-(alkylpyridinyl)-2,2′-bithiophenes: synthesis, liquid crystalline behaviour and charge transport

Liquid crystalline materials with 5,5′-bis-(alkylpyridinyl)-2,2′-bithiophene structures with different positions of the nitrogen atoms in the aromatic core were synthesized, characterized by differential scanning calorimetry, polarized optical microscopy and, in some cases, by synchrotron X-ray diffraction analysis and single crystal X-ray analysis. The molecular planarity and liquid crystalline (LC) behaviour of these materials are strongly influenced by the location of the nitrogens in the aromatic core, and LC behaviour is more complex than the mesomorphic behaviour of the known structurally related 5,5′-bis-(alkylphenyl)-2,2′-bithiophene LCs. Single crystal X-ray structural analysis of 5,5′-bis-(5-alkylpyridin-2-yl)-2,2′-bithiophenes 2 reveals that the aromatic core is nearly planar with s-cis conformation of the thiophene and pyridine rings and s-trans conformation of the thiophene rings of the central 2,2′-bithiophene unit. The length of the alkyl chains has a pronounced effect on the molecular planarity and the packing motifs. A preliminary study of the charge-transporting properties of 5,5′-bis-(5-alkylpyridin-2-yl)-2,2′-bithiophenes 2d and 2e with n-nonyl and n-decyl alkyl chains, respectively, by the time-of-flight technique shows that the hole mobility is temperature independent, electric field dependent and with a magnitude of ∼1.5 × 10−4 cm2 V−1 s−1 in the unidentified S2/Cr2 phase.