Side-chain engineering of highly efficient solid-state red fluorophore based on diaminoterephthalate with steric ortho-substituents

A series of 2,5-diaminoterephthalate-based fluorophores (Me, Et, iPr, and tBu) with steric ortho-substituents were designed and synthesized. There is an optical changing tendency of the four crystals with distinct red-shifted emissions. The crystal characteristics suggest that the enhanced anti-aromaticity of the central benzene rings of four compounds in the ground state might be an important reason for achieving long-wavelength fluorescence emission, which matches well with the changing tendency of C–N bond lengths. Moreover, all compounds exhibit highly efficient solid-state emission with fluorescence quantum efficiency reaching up to 69%.