Donor-Acceptor Based 1,8-Naphthalimide Substituted Phenothiazines: Tuning of HOMO-LUMO Gap

Abstract

A set of donor-acceptor (D-A) substituted phenothiazine based chromophores (D-π-D-π-A, D′-π-D-π-A, D-A′-D-π-A and D′-A′-D-π-A) were designed and synthesized. The phenothiazine was substituted with the 4-ethynyl-1,8-naphthalimide (NPI) on one side and a variety of donors (phenothiazine, carbazole, ferrocene and triphenylamine) were introduced on the other side of phenothiazine via Pd-catalyzed Sonogashira cross-coupling reaction resulting PTZ8–11. In order to tune the optoelectronic properties of the phenothiazine chromophores, cyano-based acceptor 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) was incorporated by [2+2] cycloaddition-electrocyclic ring-opening reaction of phenothiazine derivatives PTZs 8–11 and tetracyanoethylene (TCNE). The photophysical and electrochemical studies showed that the incorporation of TCBD acceptor in the phenothiazine chromophores PTZs 12–15 facilitated the donor-acceptor strength to greater extent. The electronic absorption spectra exhibited red shifted absorption bands for the TCBD substituted phenothiazines as compared to the alkynylated phenothiazines which led to much lower optical band gaps in the formers. The experimental values were also supported by the DFT and TD-DFT calculations. © 2021 Wiley-VCH GmbH