Bhupinder Sandhu

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.

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.

Absolute Configuration and Polymorphism of 2-Phenylbutyramide and α-Methyl-α-phenylsuccinimide

Crystal structures of racemic and homochiral forms of 2-phenylbutyramide (1) and 3-methyl-3-phenylpyrrolidine-2,5-dione (2) were investigated in detail by a single crystal X-ray diffraction study. Absolute configurations of the homochiral forms of 1 and 2, obtained by chromatographic separation of racemates, were determined. It was revealed that racemate and homochiral forms of 1 are very similar in terms of supramolecular organization (H-bonded ribbons) in crystal, infrared (IR) spectral characteristics, and melting points. The presence of two different molecular conformations in homochiral forms of 1 allowed mimicking of crystal packing of the H-bonded ribbons in racemate 1. Two polymorph modifications (monoclinic and orthorhombic) comprising very similar H-bonded zigzag-like chains were found for the homochiral forms of compound 2 that were significantly different in terms of crystal structure, IR spectra, and melting points from the racemic form of 2. Unlike compound 1, homochiral forms of compound 2 have a higher density than the corresponding racemate which contradicts the Wallach rule and indicates that, in this case, homochiral forms are more stable than racemate forms.

Pyrimidine-2,4-diamine acetone monosolvate.

In the title compound, C4H6N4·C3H6O, the pyrimidine-2,4-diamine molecule is nearly planar (r.m.s. deviation = 0.005 Å), with the endocyclic angles covering the range 114.36 (10)–126.31 (10)°. In the crystal, N—H⋯N and N—H⋯O hydrogen bonds link the molecules into ribbons along [101], and weak C—H⋯π interactions consolidate further the crystal packing.

6-Methyl­pyridin-2-amine

In the title molecule, C6H8N2, the endocyclic angles are in the range 118.43 (9)–122.65 (10)°. The molecular skeleton is planar (r.m.s. deviation = 0.007 Å). One of the two amino H atoms is involved in an N—H⋯N hydrogen bond, forming an inversion dimer, while the other amino H atom participates in N—H⋯π interactions between the dimers, forming layers parallel to (100).

Pyridine-4-carbaldehyde-fumaric acid (2/1).

In the title co-crystal, 2C6H5NO·C4H4O4, two crystallographically different hydrogen-bonded trimers are formed, one in which the components occupy general positions, and one generated by an inversion centre. This results in the uncommon situation of Z = 3 for a triclinic crystal. In the formula units, molecules are linked by O—H⋯N hydrogen bonds.

Supramolecular synthesis based on piperidone derivatives and pharmaceutically acceptable co-formers.

The target complexes, bis{(E,E)-3,5-bis[4-(diethylamino)benzylidene]-4-oxopiperidinium} butanedioate, 2C27H36N3O(+)·C4H4O4(2-), (II), and bis{(E,E)-3,5-bis[4-(diethylamino)benzylidene]-4-oxopiperidinium} decanedioate, 2C27H36N3O(+)·C10H16O4(2-), (III), were obtained by solvent-mediated crystallization of the active pharmaceutical ingredient (API) (E,E)-3,5-bis[4-(diethylamino)benzylidene]-4-piperidone and pharmaceutically acceptable dicarboxylic (succinic and sebacic) acids from ethanol solution. They have been characterized by melting point, IR spectroscopy and single-crystal X-ray diffraction. For the sake of comparison, the structure of the starting API, (E,E)-3,5-bis[4-(diethylamino)benzylidene]-4-piperidone methanol monosolvate, C27H35N3O·CH4O, (I), has also been studied. Compounds (II) and (III) represent salts containing H-shaped centrosymmetric hydrogen-bonded synthons, which are built from two parallel piperidinium cations and a bridging dicarboxylate dianion. In both (II) and (III), the dicarboxylate dianion resides on an inversion centre. The two cations and dianion within the H-shaped synthon are linked by two strong intermolecular N(+)-H···(-)OOC hydrogen bonds. The crystal structure of (II) includes two crystallographically independent formula units, A and B. The cation geometries of units A and B are different. The main N-C6H4-C=C-C(=O)-C=C-C6H4-N backbone of cation A has a C-shaped conformation, while that of cation B adopts an S-shaped conformation. The same main backbone of the cation in (III) is practically planar. In the crystal structures of both (II) and (III), intermolecular N(+)-H···O=C hydrogen bonds between different H-shaped synthons further consolidate the crystal packing, forming columns in the [100] and [101] directions, respectively. Salts (II) and (III) possess increased aqueous solubility compared with the original API and thus enhance the bioavailability of the API.

3,4-Di-amino-pyridinium hydrogen malonate.

In the title salt, C5H8N3 +·C3H3O4 −, the 3,4-diaminopyridinium cation is almost planar, with an r.m.s. deviation of 0.02 Å. The conformation of the hydrogen malonate anion is stabilized by an intramolecular O—H⋯O hydrogen bond, which generates an S(6) ring. In the crystal, N—H⋯O hydrogen bonds link cations and anions into layers parallel to the ab plane.