Peripheral Substituent Effects on Two-Photon Absorption in 3-Sulfonylated Imidazo[1,2- a ]pyridine Derivatives

Bioprobes, organic light-emitting diodes, optoelectronics, and molecular logic gates are among the main applications of imidazopyridine derivatives. This direct interface between molecular synthesis and applications drives a continuous need for studies focused on the optical characterization of these emerging compounds. In this context, we investigated how different peripheral groups affect the linear and nonlinear optical responses of derivatives of 2-phenyl-3-(phenylsulfonyl)imidazo[1,2-a]pyridine. To this end, we conducted experimental characterizations of linear absorption, fluorescence, transient absorption, and two-photon absorption (2PA), along with density functional theory (DFT) calculations. Despite a low fluorescence quantum yield, attributed to the high orientational mobility of the chemical groups attached to the C2 and C3 positions of the imidazo[1,2-a]pyridine core, the experimental results revealed a trend toward the formation of push–pull regions between the peripheral group and the sulfone moiety, thereby increasing the linear and nonlinear absorptive responses. In quantitative terms, we observed 2PA cross sections (σ2PA) of approximately 50 GM in the lowest-energy band, demonstrating the influence of the SO2 functional group on the increase in nonlinear absorptive response. We also observed excited-state absorption in samples with emissive properties, which can increase the effective 2PA cross section of the compounds. These results provide guidelines for molecular engineering strategies aimed at introducing structural modifications that enhance fluorescence emission while preserving the favorable nonlinear optical response.