Imidazo[1,2- a ]pyridine and Imidazo[1,5- a ]pyridine: 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, imidazo[1,2- a ]pyridine and imidazo[1,5- a ]pyridine 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. Imidazo[1,2- a ]pyridine derivatives Ia and IIa were purely dipolar, while imidazo[1,5- a ]pyridine 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 imidazo[1,5- a ]pyridine compared to that of imidazo[1,2- a ]pyridine. Furthermore, crystalline powders of imidazo[1,2- a ]pyridine 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.