Twofold photoswitching of NIR fluorescence and EPR based on the PMI–N–HABI for optical nanoimaging of electrospun polymer nanowires

Abstract

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 (τ_1/2) 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.