3,5-Diarylimidazo[1,2-a]pyridines as Color-Tunable Fluorophores

A new protocol for the synthesis of color-tunable fluorescent 3,5-diarylimidazo[1,2-a]pyridines has been achieved via palladium-catalyzed C-H amination of pyridinium zwitterions. Based on experimental results and computational analysis, we extracted a high correlation of photophysical properties with the theoretical concept and predicted emission wavelengths of 3,5-diarylimidazo[1,2-a]pyridines. The emission wavelengths of imidazo[1,2-a]pyridines increase as a function of the electron-withdrawing nature of the substituent on the C5-aryl group of imidazo[1,2-a]pyridine as a result of inductive effects on the LUMO levels. Varying the substituent on the C3-aryl group imidazo[1,2-a]pyridine changes the HOMO levels. Combining these two sites, the HOMO and LUMO levels can be tuned fairly decoupled from each other. This conceptual trend is demonstrated across a series where the C3 and C5 positions were functionalized independently and then utilizes a combination strategy where both sites are used to prepare fluorophores with a large window of emission wavelengths. In view of the biological properties of imidazo[1,2-a]pyridines, the developed method provides an efficient approach for understanding and preparing strongly fluorescent bioprobes.