Molecular engineering enabling reversible transformation between helical and planar conformations by cyclization of alkynes

Described herein is a molecular engineering enabling reversible transformation between helical and planar conformations. Starting from easily available 2-(pyridin-2-yl)anilines and alkynes, a one-pot strategy is set up for the synthesis of aza[4]helicenes via two successive rhodium-catalyzed C−H activation/cyclizations. Helical pyrrolophenanthridiziniums can be transformed to planar conformations through the cleavage of acidic pyrrole N–H, leading to turn-off fluorescence. NMR spectra, single crystal X-ray diffraction and DFT calculations demonstrate that the formation of intramolecular C‒H∙∙∙N hydrogen bond is beneficial to stabilize the pyrrole nitrogen anion of planar molecule and endow an increased planarity. The reversible conformation transformations can be finely adjusted by the electron-donating and -withdrawing groups on the π+-fused pyrrole skeleton in the physiological pH range, thus affording an opportunity for pH-controlled intracellular selective fluorescence imaging. Pyrrolophenanthridiziniums show turn-on fluorescence in lysosomes owing to the acidic environment of lysosomes and turn-off fluorescence out of lysosomes, indicating the occurrence of deprotonation reaction outside lysosomes and the corresponding transformation from helical to planar conformations.