Dense and Acidic Organelle-Targeted Visualization in Living Cells using a Viscosity-Responsive Fluorescent Probe Independent of the Twisted Intramolecular Charge Transfer Process

Abstract

Cell-imaging methods with functional fluorescent probes are an indispensable technique to evaluate physical parameters in cellular microenvironments. In particular, molecular rotors, which take advantage of the twisted intramolecular charge transfer (TICT) process, have helped evaluate microviscosity. However, the involvement of charge-separated species in the fluorescence process potentially limits the quantitative evaluation of viscosity. Herein we developed viscosity-responsive fluorescent probes for cell imaging that are not dependent on the TICT process. We synthesized AnP2-H and AnP2-OEG, both of which contain 9,10-di(piperazinyl)anthracene, based on 9,10-bis(N,N-dialkylamino)anthracene that is an aggregation-induced emission luminogen (AIEgen). AnP2-H and AnP2-OEG exhibited enhanced fluorescence as the viscosity increased, with sensitivities comparable to those of conventional molecular rotors. In living cell systems, AnP2-OEG showed low cytotoxicity and, reflecting its viscosity-responsive property, allowed specific visualization of dense and acidic organelles such as lysosomes, secretory granules and melanosomes under washout-free conditions. These results provide a new direction for developing functional fluorescent probes targeting dense organelles.

Fluorescent Flippers: Small‐Molecule Probes to Image Membrane Tension in Living Systems

“…[151] Applications to bioimaging are of interest because non-radiative decay occurs through conical intersections rather than the multiresponsive hypothetical TICT states, but are so far still in their infancies. [152] The porphyrin dimers 47 and 48 with a dialkyne twister is unique for several reasons (Figure 14). [122,153] Twisting occurs not because of steric or electrostatic repulsion, but because of the two alternatives available to conjugate with the two π bonds of the triple bonds.…”