The Synthesis, Properties And Applications Of Photoswitchable Fluorescent Diarylethenes

As the performance of organic photochromic compounds is based on the unit of single molecule, high-density optical information storage and high-precision information grab are thus able to be achieved. In a variety of existing molecular switch systems, diarylethene has unique thermal stability and excellent fatigue resistance, which makes it become one of the most promising photochromic compounds for applications. Accompanying with the "on" and "off" actions of diarylethene molecules, not only the primary color is changed, but also other physical properties such as the refractive index, conductivity, chiral, fluorescence and so on are reversibly changed. Among them, the fluorescent signal is widely studied and used, due to its super-high sensitivity and low detection limit. The coupling of diarylethene with fluorophore generates photoswitchable fluorescent molecule in which the fluorescence of fluorophore moiety is photo-regulated by the photochromic behavior of diarylethene moiety. Recently, the applications of photoswitchable fluorescent diarylethenes show a strong development momentum in optical storage, optical switches, all-optical transistor, chemical sensing, optoelectronic devices, bio-imaging and other fields. However, the performance of current photoswitchable fluorescent diarylethenes is far from satisfactory for practical applications because of the constraints of the current molecular structure and fluorescence properties. The basic research for their optical properties are also incomplete, and some of the potential applications such as super-resolution imaging are not available to be involved. Therefore, the design and synthesis of novel high-performance photoswitchable fluorescent diarylethenes from the source are crucial. Based on this idea, we designed and synthesized a series of novel photoswitchable fluorescent diarylethenes in the dissertation, and their photochromism and photoswitching fluorescence were also studied systematically. In addition, we explored their applications in erasable optical storage, all-optical transistor and super-resolution imaging. The specific research contents are as follows:(1) Typical bisthienyl-perfluorocyclopentene is chosen as the framework of diarylethene derivatives in the dissertation, and substituted with some high-performance fluorophores to form photoswitchable fluorescent diarylethenes. To establish a general synthetic pathway to them, we have designed and synthesized the halogenated diarylethene intermediates with Suzuki coupling reactivity. Thus, the derivatization of diarylethene is convenient to be accomplished via the coupling reaction of diarylethene intermediates with the boronic acid or ester substituents.(2) Tetraphenylethene (TPE) is one kind of aggregation-induced emission (AIE) active materials. In contrary to the conventional fluorescent dyes which show week fluorescence on the solid state because of the aggregation caused quenching (ACQ) phenomenon, TPE exhibits the special ability of fluorescence enhancement at solid state. In the dissertation, we designed and synthesized the TPE-diarylethene conjugate molecule, and its optical properties in different media are also investigated. The research results show that the open form isomer exhibit AIE properties, i.e., it has very weak fluorescence in the solution whereas the fluorescence intensity is enhanced>10times at solid state (including solid, nanoparticles, polymer media). This TPE-diarylethene conjugate compound shows reversible photochromic and fluorescent switching upon the alternate ultraviolet and visible light irradiation, with a maximum fluorescence quenching ratio of50as well as a good fatigue resistance. We constructed a rewritable optical storage model based on the use of its excellent fluorescent photoswitching ability at solid state. In addition, the super-resolution imaging in its doped polymer film was studied, and the images with a high-resolution below100nm was obtained.(3) To further extend the application of the photoswitchable fluorescent diarylethenes in the field of living cell super-resolution imaging, we designed and synthesized a fluorescent probe DTE-NI monomer with the ability to highlight lysosomes in living cells. DTE-NI monomer has a diarylethene moiety containing a polymerizable vinyl group, which can be copolymerized with the water-soluble N-isopropylacrylamide to generate a copolymer PNIPAM-DTE enabling photoswitchable fluorescent diarylethenes to be water-soluble and biocompatible. The another component of DTE-NI is a naphthalimide moiety substituted by a methyl-piperazinyl group at its peri-position. The piperazinyl group can be protonated in the acidic micro-environment of lysosome and inhibit the photoinduced charge transfer (PET) process, which makes the increase of the fluorescence quantum yield of DTE-NI from3.3%(before protonation) to14.5%. This is the reason why DTE-NI can be able to " highlight" the positions of lysosomes in cells. DTE-NI and PMIPAM-DTE exhibit reversible photochromic and fluorescent switching upon the alternate ultraviolet and visible light irradiation, with a maximum fluorescence quenching ratio of110as well as a excellent fatigue resistance. The experiment of living cell imaging shows that water-soluble PNIPAM-DTE is not only selectively located in lysosomes, but also enables super-resolution imaging to be achieved.with a high spatial resolution of40nm. This is the first example that diarylethene derivative is successfully applied in the super-resolution imaging of living cells.(4) In light of the fact that diarylethenes with bright fluorescence are scarce, I constructed a series of perylene monoimide (PMI)-diarylethene (DTE) conjugate molecules with bright fluorescence by coupling diarylethene moiety with the PMI which is famous for the excellent photostability and high fluorescence quantum yield. The another terminal of diarylethene is linked with different groups with various electronic effects to regulate the photoswitching performance of PMI-DTE conjugates. The result demonstrates that electron-donating group can improve the performance of photochromsm and fluorescence photoswitching, whereas the electron-withdrawing group will reduce that. The polymer medium can diminish the speed of photochromism, but rise the fluorescence quenching ratio up to107. Tetraphenylethene group here in this molecule system loses its AIE activity, but is found exhibiting a moderate electron-donating ability which is comparable to octyloxyphenyl group. The open form of TPE-DTE-PMI compound displays solvatochromic fluoresence caused by intramolecular charge transfer (ICT), while its closed form shows fluorescence quenching caused by intramolecular energy transfer (ET). Thus, DTE in TPE-DTE-PMI is declared to act as a reversible adjuster for intramolecular charge transfer in ring-opening isomer and photochromic energy transfer in ring-closing isomer upon alternating UV/visible light irradiation, respectively. That affords a new approach to switch two kinds of physical processes in one molecule.(5) The performance of current diarylethene molecules is far from satisfactory because of the scarcity of high-speed switching capability and large fluorescence on-off ratio in situ. With this in mind, integrating multiple diarylethene moieties on a single fluorephore is the strategy that we put forward to enhance the modulation of fluorescence. Specially, a series of novel photoswitchable fluorescent molecules PMI-nDTE (n-1,2,3) were synthesized by coupling one to three diarylethene moieties with PMI at bay-or perz-positons through oxygen bridge atoms. The effects of molecule structure, solvent polarity and solid media on photochromism and fluorescence photoswitching properties were also investigated. Moreover, via a proof-of-principle study, the quantitative functions between the fluorescence quenching efficiencies, fluorescence on/off switching ratio, cyclizaiton yields, and UV irradiation time in dilute solution are deduced theoretically and verified experimentally for the first time. As expected, PMI-3DTE has certainly the best performance and become a "super" fluorescence photoswithing molecule. Its fluorescence is quenched nearly100%in seconds of UV irradiation and the fluorescence on/off switching ratio is up to3024in dilute toluene. Although the photochromic process in the solid media is relatively slower, the fluorescence photoswitching speed is not influenced and the fluorescence on/off switching ratio goes precipitously up to10000, and also a good fatigue resistance is exhibited. In addition, due to its such high fluorescence on/off switching ratio, a "dark state" is able to be obtained for PMI-3DTE. That extremely benefits the high noise-signal ratio and low threshold readout. The "super" molecule PMI-3DTE performs very well in the application of erasable optical storage and all-optical transistors model experiments. Especially, block copolymer vesicles stained with PMI-3DTE are observed via optical nanoimaging with a sub-100nm resolution, portending a bright future of dithienylethenes applying in super-resolution imaging.