Dithienopyrrolobenzothiadiazole and Naphthalimide-Based Donor–Acceptor Triads for Charge Transport and Photocatalytic Synthesis of Benzimidazoles

Herein, acceptor–donor–acceptor π-conjugated small molecules, namely, triad 1 and triad 2, consisting of dithienopyrrolobenzothiadiazole and 1,8-naphthalimide, both with and without a thiophene spacer, were synthesized. Their absorption characteristics, redox properties, charge carrier mobilities, and their potential for metal-free photocatalytic transformations were investigated. Triad 1 and triad 2 showed wide absorption spectra in the range of 300–600 nm in solutions and 350–700 nm in thin films, along with narrow electrochemical bandgaps of 2.05 and 1.83 eV, respectively. Futhermore, the charge carrier mobilities of both triads were assessed through space-charge limited current measurements. The calculated hole mobilities for triad 1 and triad 2 were 1.26 × 10–3 and 8.38 × 10–4 cm2/V s, respectively, while the electron mobilities for triad 1 and triad 2 were 6.73 × 10–4 and 4.18 × 10–4 cm2/V s, respectively. Moreover, triad 1 and triad 2 exhibited efficient photocatalytic behavior toward selective metal-free condensation cyclization reactions of o-phenylenediamine/1,2-diamino-4,5-difluorobenzene and aromatic aldehydes with wide substrate scope under blue LED light to give a variety of pharmaceutically relevant benzimidazole derivatives.