Imidazo1,2-apyridine and Imidazo1,5-apyridine: Electron Donor Groups in the Design of D–π–A Dyes

This work investigates the electron-donating capabilities of two 10-π electron nitrogen bridgehead bicyclic 5,6-fused ring systems, imidazo1,2-apyridine and imidazo1,5-apyridine rings. Eight compounds with varying positions of electron-withdrawing moieties (TCF or DCI) coupled to the imidazopyridine ring were synthesized and studied. DCI-containing compounds (Ib–IVb) exhibited a purely dipolar nature with broad absorption bands, weak fluorescence, large Stokes shifts, and strong solvatochromism. In contrast, TCF-containing compounds (Ia–IVa) demonstrated diverse properties. Imidazo1,2-apyridine derivatives Ia and IIa were purely dipolar, while imidazo1,5-apyridine derivatives IIIa and IVa displayed a cyanine-like character with intense absorption and higher quantum yields of emission. The observed gradual red shift in optical properties with changing electron-donor groups (IIb < Ib < IIIb < IVb) and (IIa < Ia < IIIa < IVa) underscores the stronger electron-donor character of imidazo1,5-apyridine compared to that of imidazo1,2-apyridine. Furthermore, crystalline powders of imidazo1,2-apyridine derivatives exhibited fluorescence despite minimal emission in solution. Two compounds (Ib and IVa) were successfully formulated into nanoparticles for potential in vivo imaging applications in zebrafish embryos.