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We report the design and synthesis of a tetraphenylethene substituted with naphthalimide at the 4, 6 positions, named NI-2TPE. NI-2TPE exhibits strong solvent-dependent emission properties with combined ICT and AIE characteristics in THF–H2O systems. This probe was used directly on test papers to distinguish normal organic solvents using their emission colours under UV light based on its AIE and ICT nature. Thanks to the vinyl group in NI-2TPE, we synthesized a copolymer of NIPAM and NI-2TPE, termed P(NIPAM-co-NI-2TPE). The resulting polymer is highly soluble and fluorescent in water (ΦF = 15.4%). Due to the well-known thermo-responsive character of NIPAM, P(NIPAM-co-NI-2TPE) exhibits an interesting fluorescence change in response to various temperatures. Due to the thermo-induced shrinking of the PNIPAM chain, the fluorescence intensity gradually increased from 20 to 34 °C. As the temperature further increased from 34 to 90 °C, the fluorescence intensity decreased sharply, which was caused by the well-known thermal effects. Furthermore, we synthesized a P(HEA-co-NI-2TPE–TPP acrylate) copolymer, in which HEA is a hydrophilic unit, TPP is a mitochondria label and NI-2TPE a fluorescent probe. The corresponding polymer probe is highly soluble in water with FLQY = 7% and we have further applied this probe as a mitochondria targeted imaging tracker in HeLa cells successfully.

In this article, we report the design and synthesis of a fluorescent molecular switch based on perylenemonoimide–hexaarylbiimidazole dyad (PMI–N–HABI), which exhibits not only photoswitchable near-infrared (NIR) fluorescence but also photoswitchable electron paramagnetic resonance (EPR) resulting from photo-induced twin radicals. The emission of PMI–N–HABI varies with different polarities of solvents due to the D–π–A molecular architecture, displaying strong NIR emission (715 nm) in methylene chloride with fluorescence quantum yield as high as 0.29. The photo-induced cleavage of the C–N bond between the two imidazole groups produces twin radicals efficiently upon UV irradiation and the colored species recovered to the initial state spontaneously by the collision of the two intramolecular TPIRs in a few seconds, with decay (or fluorescence recovery) half-life time (τ1/2) of 1.3 s in toluene at 297 K. The radical nature of the colored species is verified by EPR measurements. The emission of the perylenemonoimide could be reversibly quenched by the photo-induced radicals through the Forster resonance energy transfer (FRET) mechanism. The fluorescence-switching dye is applied to the optical nanoimaging of polymer nanowires prepared using the typical electrospinning method. The average diameter of about 160 nm is in accordance with the scan electron microscopic (SEM) images. Compared with the traditional SEM technique, we provide a convenient alternative to SEM with practical imaging resolution for polymer nanostructures.