Elena Galán

Aromatic/Proaromatic Donors in 2‐Dicyanomethylenethiazole Merocyanines: From Neutral to Strongly Zwitterionic Nonlinear Optical Chromophores

Push-pull compounds, in which a proaromatic electron donor is conjugated to a 2-dicyanomethylenethiazole acceptor, have been prepared, and their properties compared to those of model compounds featuring an aromatic donor. A combined experimental (X-ray diffraction, (1) H NMR, IR, Raman, UV/Vis, nonlinear optical (NLO) measurements) and theoretical study reveals that structural and solvent effects determine the ground-state polarisation of these merocyanines: whereas 4H-pyran-4-ylidene- and 4-pyridylidene-containing compounds are zwitterionic and 1,3-dithiol-2-ylidene derivatives are close to the cyanine limit, anilino-derived merocyanines are essentially neutral. This very large range of intramolecular charge transfer (ICT) gives rise to efficient second-order NLO chromophores with μβ values ranging from strongly negative to strongly positive. In particular, pyranylidene derivatives are unusual in that they show an increase in the degree of ICT on lengthening the π-spacer, a feature that lies behind the very large negative μβ values they display. The linking of the formally quinoidal 2-dicyanomethylenethiazole moiety to proaromatic donors seems a promising approach towards the optimisation of zwitterionic NLO chromophores.

Linear and V-Shaped Nonlinear Optical Chromophores with Multiple 4H-Pyran-4-ylidene Moieties

A simple synthesis of dipolar one- and two-dimensional chromophores bearing two or three 4H-pyran-4-ylidene moieties is reported. Whereas the pyranylidene fragments acting as donors are proaromatic, the spacer one is not. In the linear derivatives, chain elongation gives rise to a sharp increase in the second order nonlinear optical responses, but some V-shaped derivatives display first hyperpolarizabilities (beta) lower than those of their linear analogues. This uncommon feature lends experimental support to previous theoretical studies on the relative contribution and sign of the beta-tensor components.

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.