Synergic effect of unsaturated inner bridges and polymorphism for tuning the optoelectronic properties of 2,3-thieno(bis)imide based materials

Abstract

2,3-Thieno(bis)imide (N) ended oligomers are emerging as valuable molecular materials for applications in organic electronics. Here, we report the synthesis and characterization of three new 2,3-thieno(bis)imide ended oligothiophenes (T) bearing unsaturated ethylene (E), azomethine (I) and ethinyl (A) inner bridges (NTE, NTI and NTA, respectively). The effect of the unsaturated bridge on the π-conjugation extent, molecular conformation and overall aromaticity is related to the functional optoelectronic and morphological properties and compared to the properties of the linear analogue (NTT) with a bithiophene inner moiety. Optical spectroscopy and cyclovoltammetry analysis show a strong red shift of the absorption and an increased energy band gap on going from NTI and NTE to NTA. The HOMO level decreases in the order NTE > NTI > NTA. Moreover, while the LUMO of NTE and NTA have almost the same energy, NTI has a LUMO energy about 0.1 eV lower, likely due to the electron withdrawing effect of the azomethine moiety. Morphological investigation of solution cast thin deposits shows that the unsaturated bridges promote the formation of concomitant polymorphs with the simultaneous presence of microcrystals with different morphology and fluorescence properties. Moreover, irreversible conversion of one polymorph to the other was achieved by thermal treatments for NTA and NTE and by exploiting this feature, we realized a time temperature integrator (TTI) device based on NTE material. This device allowed to monitor temperature evolutions in the range between RT and 200 °C by means of a red to yellow fluorescence switch that was detectable by optical microscopy.