Synthesis and properties of functional dyes with squaraine–naphthalene diimide hybrid structure

Naphthalene diimides (NDIs) are promising candidate for electron acceptors due to their low-lying HOMOs and LUMOs. The functinalization of soluble NDIs at the 2,6-position affects the absorption and electrochemical properties. In this study, NDI-based hybrid dyes NDI-SQ-A, B fused with squaraine chromophore were designed and synthesized in order to elucidate the effects of the substitution on their optical and electrochemical properties. These dyes were successfully synthesized by Stille coupling reactions using 3-stannylcyclobutenediones and brominated NDI derivative, followed by a condensation reaction. DFT calculation predicts that the present dyes adopt distorted structures coming from a steric hindrance between semisquaraine and NDI moieties. The hybrid dyes show low-lying LUMOs due to the introduction of electron-deficient NDI moiety and broad absorption spectra in the far-red region. The absorption spectra of their thin films were bathochromically shifted relative to those in solution, indicating that hybrid dyes formed J aggregates.