Isoindigo dyes functionalized with terminal electron-withdrawing groups: Computational, optical and electrical characterization

Abstract

Eight novel isoindigo (iI) based small molecules have been successfully synthesized. Their molecular structure consists of an electron acceptor iI core symmetrically linked to two furan (F-series) or thiophene (T-series) rings and end-functionalized with four auxiliary electron withdrawing groups (EWGs) of different strength. The optical properties of the dyes in chloroform solution are uniformly modulated by the terminal EWGs so that absorption maxima wavelengths move to higher values as the EWG's strength increases. A computational (DFT level) analysis provides useful information on the electronic structure of the dyes: upon photoexcitation, the electron density moves away from iI core or towards it according to the different EWG considered. Optical analysis is performed on dyes' thin films as well and a general broadening and red shift of the absorption is observed as compared to the behaviour in solution; all the dye's thin films are characterized by narrow bandgaps (<1.60 eV) and diffused absorption of most of the visible light. From XRD diffraction analysis performed on drop casted films of the dyes, it is possible to observe a lamellar organization in the solid phase with lamellae width clearly linked to the nature of the terminal EWG. HOMO and LUMO energies of the dyes, determined by cyclic voltammetry analysis performed on dyes' thin films, show very stable LUMO and HOMO energy levels, suggesting, respectively, a tendency to act as n-type semiconductors and a very good thermo-oxidative stability. The dyes are finally employed as active layers in organic field-effect transistors to study their charge transport properties: all of them display unipolar n-type charge transport with the presence of the electron accumulation phenomenon under the application of positive gate voltages. For one of the dye, mobility (μ) up to 10−2 cm2/V∙s was measured, whereas values around 10−3 cm2/V∙s were found for the others.