Photochromic diarylethenes (DTE) are well-known as its thermally irreversible photoisomerization and excellent fatigue resistance,whichhave aroused extensive interest in many applications, such as optical memory and fluorescence imaging.One fundamental limitation to develop photoswitchable DTE is the necessity of UV light to trigger photocyclization. Compared with visible lights, the UV light owns short wavelength, high energy and is more broadly absorbable by the molecules in samples so that it is often invasive to the dye molecules and samples especially the biological ones.Also, UV light shows relatively poorer penetration than that of visible lights. Visible-light driven DTE probes are more practical in optical applications especially the fluorescence imaging. It is of great significance to develop visible light driven photoswitchable fluorescent DTEinstead of ultraviolet light.
Although visible light activated DTE photochromic system have attracting much interest recently, only few examples about visible light controlled fluorescence photoswitching of DTE have been reported. The situation for photoswitchable fluorescent DTE molecular system is different from the only photochromic one, as a fluorophore is usually introduced to the parent DTE structure or the photoswitchable molecule is designed fluorescent itself. Photoswitchable fluorescent DTE systems are more complicated than photochromic one. The possible pathways of energy or electron transfer between the fluorophore and DTE parent structure will interfere the photoisomerization process. The concentration-caused fluorescence quenching, or aggregation caused quenching (ACQ), as a classic problem in organic solid state luminescence, is also faced byphotoswitchablefluorescentmolecules, which would inevitably limit their application in high-density optical systems. Aggregation induced emission (AIE), as theoppositefluorescencephenomenon to ACQ, was reported by Tang and coworkers in 2001 which provide important ideas for solving the ACQ problem of fluorescent molecular switches. Great progress has been made about AIE materialsinrecenttwentyyears. Currently, DTE has made certain progress in both the visible light-driven photochromic reaction and AIE. However, how to achieve both functions and obtain a fluorescent molecular switch with high fluorescent switch contrast is still a challenge.
Figure 1. Chemical structures and photochromic reactions of TPE-2DTE and OTPE-2DTE.
Herein, Li and Zhu’s group reported two novelhigh-performance diarylethenes with aggregated-induced emission (AIE)by conjugating two diarylethene groups on one AIE-gen,i.e., TPE-2DTE(blue-greenfluorescent)and OTPE-2DTE(orange fluorescence)(Figure 1).Their open forms show enhanced fluorescence in aggregated state and solid state,which is a typical AIE character. In aggregated state or solid state, OTPE-2DTE with oxacyclo-tetraphenylethene (OTPE) fluorophore initsstructurehasafluorescence emission peakat560 nm, whilethatofTPE-2DTE containing only tetraphenylethylene(TPE)islocatedat500 nm.They exhibits the larges stokes shifts of > 200 nm, which are favorable for fluorescence detection in practical applications.Meanwhile, tetraphenylethylene group is a kind of electron-giving conjugated group, which effectively reduces the energy gap and promotes the absorption and responseoftheDTE structure to visible light.Remarkably, 405 nm purple irradiation gives the fluorescence ON/OFF ratios of 1196: 1 and 1983: 1 (Figure 2a and 2b) for TPE-2DTE and OTPE-2DTE, respectively. The reverse process can be accomplished after another longer wavelength irradiation such as 621 nm andthetwocompoundsshow considerable fatigue resistance(Figure 2c and 2d).
Figure 2.Fluorescence photoswitching properties upon 405 nm visible light irradiation. The fluorescence spectra of (a) TPE-2DTE and (b) OTPE-2DTE in nanoparticle state.The first 10 photoswitching cycles of the fluorescence intensity of (c) TPE-2DTE at 490 nm and (d) OTPE-2DTE at 550 nm upon alternating405nm and 621 nm light irradiations in the PMMA films.The photographs of (e) TPE-2DTE and (f) OTPE-2DTE ink (CHCl3solution containing PMMA) written on the glass slides (words ‘TPE’ and‘OTPE’, respectively) with a Chinese brush under alternating 405 nm and 621 nmirradiations.
The excellent visible light-driven fluorescent switch properties of TPE-2DTE and OTPE-2DTEwill be vitally important for optical applications with visible light as the working wavelength. For example, TPE-2DTE or OTPE-2DTE can be applied as a kind of invisible inkwhichcan achieve reversible cloaking, photochromism, and fluorescence switches at 405 and 621 nm lights irradiations (Figure 2e and 2f).Based on the high-performance photoswitching at solid state under visible light irradiations, TPE-2DTE and OTPE-2DTE were used as super-resolution imaging probes to stain and then image the cylindrical micelles of PST-b-PEO block copolymer. A high imaging resolution of about 50 nm (Figure 3) was achieved when 405 nm is deactivation wavelength while the 561 nm is the activation wavelength. The conventional fluorescence imaging can only show the rough morphology of the cylindrical micelles without clear edges, and the full width at half maximum (FWHM) is calculated to be about 400 nm. By analyzing the profile of the micelles in the super-resolution images, the FWHM is calculated to be as low as 37 nm, which is very closed to the reported values of about 40 nm observed by SEM. The resolution is nearly improved by 10 times of that of the conventional imaging. As a novel kind of fluorescent-switchable materials, TPE-2DTE and OTPE-2DTE conjugates have great potential in localization-based super-resolution imaging utilizing visible lights regulation.
Figure 3. Super-resolution imaging of cylindrical PSt-b-PEO micelles using TPE-2DTE probes. (a)Conventional fluorescence imaging(upper part) and Super-resolution imaging(lower part). (b) Cross-sectional profiles diameter of micelles along the dashed lines at the same place in (a). (c) The overall resolution of (c) determined by Fourier ring correlation (FRC).
In summary, TPE-2DTE and OTPE-2DTE are a kind of high-performance visible-light-driven diarylethenes with aggregated-induced emission.They have important practical valueasthey can avoid the adverse effects of ultraviolet light irradiation and ACQ phenomenonandthusare more suitable for optical applications such as super-resolution imaging. This work offers a guidance to design bright-emission and high-performance visible light controlled diarylethene photoswitches for practical applications.
This work was recently published onACS Applied Materials & Interfaces by Chong Li (co-first author and co-corresponding author), Kai Xiong, Ying Chen, Cheng Fan, Ya-Long Wang, Ming-Qiang Zhu (co-corresponding author) from Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology. This work was supported by the National Basic Research Program (973) of China, the National Science Foundation of China, the Fundamental Research Funds for Central Universities, and the Natural Science Foundation of Hubei Province.
Fulltext link:https://pubs.acs.org/doi/abs/10.1021/acsami.0c03122
Article title: Visible-Light-Driven Photoswitching of AIE-Active Diarylethenes for Super Resolution Imaging
