Tuning the “critical polarity” of TICT dyes: Construction of polarity-sensitive platform to distinguish duple organelles

Abstract

Polarity-sensitive dyes have demonstrated potential applications in bioimaging experiments. However, the twisted intramolecular charge transfer (TICT) mechanism to explain the polarity-responsive behavior still requires in-depth exploration. Herein, we proposed a new concept, “critical polarity”, for the in-depth explanation of TICT mechanism. The polarity below “critical polarity” was defined as “low polarity”, in which the fluorophore would present in local excited (LE) states. The polarity above “critical polarity” was defined as “high polarity”, in which the fluorophore would present in TICT states. Both proper “critical polarity” and large emission difference between LE and TICT states are required to construct desirable dual-emissive fluorophores highly sensitive to polarity. Three fluorescent dyes have been fabricated by modifying different electron donors on naphthalene imide, and S-NAT with modest electron donor demonstrated the highest sensitivity to polarity. Theoretical analysis together with the experimental results proved the validation of “critical polarity” mechanism. Finally, two fluorescent probes have been constructed based on the polarity-sensitive fluorophore. Amongst, the S-NAT could distinguish endoplasmic reticulum (ER) and lipid droplets (LDs) in dual emission colors, while S-NAT-M enabled discrimination of lysosomes and LDs in different emission colors. The two fluorescent probes with high quantum yield and low cytotoxicity could be employed to study the interaction between duple organelles, and the “critical polarity” mechanism can serve as a validate strategy to direct the construction of ratiometric dyes for polarity.