Red to Near-Infrared Fluorescence in the Solid-State of Alkoxy-Substituted Bisazomethine Dyes Possessing a Dibutylamino Terminal Group

Eleven novel bisazomethine dyes possessing a dibutylamino terminal group with diverse alkoxy substituents on the phenyl ring were prepared by a reaction of diaminomaleonitrile and aminobenzaldehydes. We observed that ten derivatives exhibited red-shifted fluorescence in a crystalline state with an improved quantum efficiency from that in a solution. They showed red to near-infrared fluorescence with a quantum efficiency in the range of 12 to 28%. X-ray crystal structure analysis was carried out for interpreting optical properties in a crystalline state. The solid-state fluorescence property of the derivatives with a long alkoxy side chain could be characterized by their structural feature, whereas no close correlation was found between the solid-state fluorescence property and structure feature in crystal structure in the derivatives with a short to middle alkoxy side chain.