Palladium-Catalyzed [3+2] Annulation of Naphthalimide Acceptors and Thiophene Donors

Donor-acceptor (D-A) dyes constitute one of the fundamental structural motifs of functional organic materials. In most cases, the donor and acceptor moieties are connected by a single bond, which could potentially be replaced by a fused aromatic ring to enhance the rigidity and conjugation of the dye moieties. However, there is still a lack of synthetic methodologies for such fused D-A systems. Here we report the synthesis of D-A and A-D-A dyes that possess fully annulated donor and acceptor moieties based on palladium (Pd)-catalyzed [3+2] annulation reaction between bromo-chloro-naphthalene dicarboximide and thiophene- and indole-based boronic esters. Thus, a series of fused D-A and A-D-A conjugated dyes were synthesized in good to high yields by a cascade of Pd-catalyzed Suzuki-Miyaura cross-coupling and direct arylation reactions. The newly synthesized fused D-A and A-D-A dyes with one or two naphthalimide units fused to five-membered electron-rich heterocyles were systematically investigated by ultraviolet-visible spectroscopy, cyclic and square wave voltammetry, and density functional theory calculations. These dyes possess desirable optical and electrochemical properties for application as organic electronic materials as they show absorption up to the near-infrared region, undergo up to 4-fold reduction processes, and have low-lying LUMO energy levels down to -3.62 eV.