Synthesis, optical properties, and two-photon bioimaging evaluation of novel fluorescent cationic molecules with symmetrical long conjugated all- structures.

Five novel fluorescent molecules (PPy, BOPPy, CNPPy, BPPy, and BPIm), which possess symmetrical long conjugated all- structures and are capped with hydroxyethyl-bonded pyridinium or benzimidazolium cations, were designed, synthesized, and characterized by H NMR, C NMR, and HRMS. The systematic investigations of their linear and nonlinear optical properties in different solvents indicate that all the target compounds exhibit large Stokes shifts (71-152 nm) and four of them (PPy, CNPPy, BPPy, and BPIm) have satisfactory two-photon action cross-sections (45.2-112.4 GM in DMSO). The fluorescence stability experiments reveal that their fluorescence emission is insensitive within the biologically relevant pH range of 4.0-8.0, which may enable applications to be possible. Cytotoxicity assessments, together with one- and two-photon excited fluorescence imaging studies in live cells were performed to evaluate their application values in bioimaging. It is found that PPy is not only endowed with low cytotoxicity and good cell membrane permeability, but also shows bright intracellular fluorescence signals. The high comprehensive performance enables PPy to have a promising application prospect in living cell imaging.