Novel Azo-photoswitches:Design,Synthesis,Isomerization Properties And Potential Applications

Azobenzenes are a typical class of photoswitches,which can undergo reversible trans(?)cis photoisomerizations with large differences of the isomers in molecular configuration and property,and they have received widespread interest in developing photo-functional molecular materials.However,traditional azobenzenes generally show incomplete photoconversions and low thermal stability of cis isomers(readily relax back to trans isomers),which limit their photoswitch performance.Moreover,although azo-switches have been used in a wide range of fields,performance of the currently developed photoactive materials or devices are still far from good enough for pratical applications.To deal with these problems,we have designed and synthesized novel azo molecules with improved photoswitch properties and developed high-performance azo-materials for potential applications.The main studies work in this dissertation are summarized as follows:1.Synthesis,Derivatizations,Isomerization Properties of Pyrazolylazophenyl Ethers and Their Versatile Applications in Light-Responsive SystemsAlthough isomerization properties of azobenzene have been known for more than eighty years and a huge number of azobenzene derivatives have been prepared,photoisomerization yields above 90%in both directions have been seldom reported and thermal half-lives of cis isomers are generally not more than several days at room temperature.The integration of high bi-directional photoconversions and long thermal half-lives is a very challenging task in this field.In this work,we develop pyrazolylazophenyl ethers(pzAzo ethers)as a new kind of azo-switches that provide quantitative(>98%)trans?cis photoisomerization at 365 nm light,near-quantitative(95-96%)reverse isomerization at 532 nm light,and a long cis-isomer half-life of three months at 25?.They can be easily synthesized by routine routes in high yields and readily functionalized at one or both sides with a broad scope of substituent groups,allowing such superior azo-switches to be conveniently introduced into various chemical systems of interest.A photo-reversible host-guest system is developed using a water-soluble pzAzo ether derivative and cyclodextrin,and photoresponsive polymer film is fabricated by incorporating pzAzo ethers units into poly acrylate side chains,and a thiol-containing pzAzo ether is used as a photoresponsive ligand on gold surface for a photorswitchable solid-state nanogold film.Compared with traditional azobenzenes,pzAzo ethers can endow photoresponsive systems with high responsiveness,long-lived metastable states and robust reversibility.Such superior yet pragmatic azo switches will attract broad interests for many valuable applications,where traditional azo-switches can be replaced and upgraded photo-regulation of molecular systems will be realized.2.Room-Temperature Photochemical Crystal(?)Liquid Transitions of Small-Molecule Azoswitches for Solar/Thermal Energy Storage and ConversionPhotoisomerization of photoswitches has been proposed as a potential solution for solar energy storage and conversion,and great attention has recently been paid to the usage of azobenzene-based molecular materials.However,their application potential remains to be severely limited by low energy/power density,fast self-discharging effect,and low temperature level of the released heat.Through tailoring the molecular structure,we develop several small-molecule pzAzo ethers as energy materials that can undergo photochemical crystal(?)liquid transitions(PCLTs),i.e.,traps-crystal(?)cis-liquid transitions.Compared with traditional azobenzenes,pzAzo ethers showed high isomerization enthalpy and long cis-isomer half-life that allow high energy density,long-term stable energy storage,and their(near-)quantitative two-way photoconversions and high photoisomerization quantum yields favor high-efficiency energy harvest and release.We find that these photoswitches can co-harvest large amounts of solar energy and ambient heat during traps-crystal?cis-liquid transition and release them on demand as high-temperature heat by the reverse transition.We further fabricate flexible solar thermal battery film devices,which achieve high energy densities of 0.3-0.4 MJ/kg(approaching those of commercial lithium-ion batteries)and a high power density of 2.7 kW/kg(comparable with those of typical supercapacitors)and produce a temperature increase of more than 20? in ambient.They can be readily recharged and show rubust performance during multiple charging/discharging cycles.Thermodynamics analysis reveals that PCLTs-based energy storage is not a simple combination of photoisomerization-based photo energy storage and phase-transition-based thermal energy storage.The two streams of energy collaborate with each other to go beyond the energetics of conventional solar energy storage and enables the upgrading of thermal energy that cannot be achieved by phase-change materials.Our work demonstrates a new regime of renewable energy utilization:combining solar energy and low-grade heat for forming higher-grade heat.3.Azobenzene-Based Functional Molecules for Molecular Monolayer Devices:Design,Synthesis,and Isomerization PropertiesSelf-assembled monolayers(SAMs)of azobenzenes can be used as the active materials for photo-driven functional molecular devices.However,the photoswitch properties of azobenzenes are often depressed in SAMs because of intermolecular steric hindrance and substrate-molecule coupling interactions.In this work,we have rationally designed the anchoring and linker groups for high-performance azobenzene SAMs.Triazatriangulene(TATA)platform is employed as anchoring group because of it large footprint which can provide sufficient spatial freedom for individual azobenzenes,and 6 linker groups(corresponding to TATA-Azo 1?6)with various lengths and electronic structures are designed to modulate the coupling strength between azobenzene groups and substrate.Systematic experiments and calculations are performed to investigate the absorption spectra,electronic structures,and isomerization properties of TATA-Azo 1?6,and we find that TATA-Azo 3 and 6 show relative high photoisomerization yields and long cis isomer half-lives benefitted from the independence of azobenzene units.Based on these results,two kinds of linker structures for effective decoupling are proposed:(1)orthogonal biphenyl structure with dihedral angle of 90° formed by introducing steric hindrance at 2,2',6,6'-positions,which can cut the conjugation between two ?-systems,(2)1,2,3-triazole generated by azide-alkyne click chemistry,which leads to cross conjugation between two substituents.Rational designed azobenzene molecules according to this strategy can lead to high-performance SAMs,which lays a good foundation for our forthcoming work on optoelectronic switches and photomechanics.Since all the molecules for functional SAMs are formulated by anchor-linker-functionality,our molecular design and decoupling strategy are generally applicable for developing high-performance SAMs-based molecular devices.