Imidazo[1,2-<i>a</i>]pyridine and Imidazo[1,5-<i>a</i>]pyridine: Electron Donor Groups in the Design of D–π–A Dyes
Léa Valla, Delphine Pitrat, Jean‐Christophe Mulatier, Tangui Le Bahers, Erwann Jeanneau, Lamiaa M. A. Ali, Christophe Nguyen, Magali Gary‐Bobo, Chantal Andraud, Yann Bretonnière
Abstract
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.