Design and synthesis of 1,8-naphthalimide functionalized benzothiadiazoles

Abstract

A series of multi acceptor based push–pull derivatives BTD2–BTD5 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling reaction followed by [2+2] cycloaddition–electrocyclic ring-opening reaction in which the benzothiadiazole (BTD, A1), naphthalimide (NPI, A2), tetracyanobutadiene (TCBD, A3) or cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (expanded TCBD, A4) moiety serves as the electron-withdrawing unit and triphenylamine (TPA) and ferrocene (Fc) are used as electron donating units. The incorporation of cyano based strong acceptors TCBD and expanded TCBD in the naphthalimide functionalized benzothiadiazole derivatives BTD1 and BTD4 results in improved light harvesting properties. The photophysical properties of the benzothiadiazoles BTD1–BTD5 revealed that the chromophore BTD3 exhibits broad electronic absorption spectra in the near IR regions compared to BTD1, BTD2, BTD4 and BTD5 due to the presence of the expanded TCBD unit. The electrochemical properties revealed that the benzothiadiazole derivatives BTD1–BTD5 exhibit multiple number of redox waves in cyclic voltammetry due to the presence of several acceptor units (BTD (A1), NPI (A2), TCBD (A3), expanded TCBD (A4)) and donor units (TPA (D1) and Fc (D2) units). The theoretical calculation demonstrates that the electron density of the HOMO energy level is mainly localized on the TPA and Fc units whereas LUMO is delocalized over the acceptor (BTD-NPI, TCBD-BTD and expanded TCBD-BTD) segments.