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

Abstract

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.