Synthesis of novel π-extended 2,5-disubstituted indolizines and their absorption and fluorescence properties

We report herein the synthesis of novel π-extended fluorescent 2-aryl-indolizines bearing substituents such as chalcone, styrene and imine groups at the C-5 position of the indolizine. Absorption and fluorescence properties of these 2,5-substituted indolizines were significantly affected by the substitution pattern at the C-5 position, as evidenced by absorption spectra, Stokes shifts and fluorescence measurements. Among the indolizine derivatives, those containing chalcone exhibited a highly notable red shift in their maximum wavelength of fluorescence (λ em = 650 nm) compared to those containing imine (λ em = 487 nm) or olefin (λ em = 543 nm), accompanied by relatively large Stokes shifts (71-171 nm / 2955–6761 cm −1 ) and fluorescence quantum-yield values from 0.002 to 0.296. All synthesized indolizines demonstrated colorful in solution, while the imine- and olefin-substituted indolizines exhibited fluorescence both in solution and the solid state. The chalcone derivatives showcased the capability to regulate its fluorescence modulation. Although the presence of the carbonyl group extensively suppressed this effect, it was significantly restored upon reduction to the hydroxyl group, demonstrating a promisor “ off-on ” fluorescent sensors for monitoring oxidative processes within biological systems.