Synthesis And Characterization Of Bipyridinium-based Thermochromic Materials And Exploration On Their Structure-property Relationship

Compete the idea of charge transfer(CT)based supramolecular assemblies to thermochromism is a topic that full of creativity and research significant.Until now,however,only limited examples of thermochromic donor-acceptor CT-based supramolecular materials have been reported and facile processing of these materials remains a challenge.In this thesis,we have firstly developed a charge transfer interaction induced selfassembly systems to construct supramolecular thermochromic materials.This system used bis-bipyridinium amphiphile 5 as acceptor structures and halogen counterions,?-electron-rich unites 6-12 as donor.This novel class of functional bis-bipyridiniumbased supramolecular materials,which can easily be synthesized in a multi-gram scale and processed from water via a reversible sol-gel transition.UV-Vis spectroscopy,NMR spectroscopy,rheology measurement,PXRD,SEM and methods were used to investigate the properties of these self-assembled complexes.Self-assembled complexes of amphiphile 5·4Br with donors 6-12 resulted in the formation of long helical and entangled nanofibers in water.Thin films and aerogels processed from these solutions exhibited dramatic reversible colour changes upon reaching the thermochromic transition temperature.Furthermore,stamped and ink-jet printed thermochromic patterns can be prepared from their aqueous solutions which also showed favorable compatibility with commercial inks.The thermochromic transition temperature of this series can be tailored within the range of 45 to 105 °C by varying the donors.The color of this system also can be tuned by change the counterion of amphiphile 5 from bromide to iodide.In this system,the tetracationic amphiphile,5·4Br,serves as a charge acceptor while ?-electron rich molecular templates and halogen counterions participate in the thermochromic mechanism as competing CT donor units.The strength of the competing CT interactions can be actuated based on the hydration state of the supramolecular assemblies.The removal of coordinating water molecules at elevated temperatures favours ion-pair formation between the bis-bipyridinium groups and their halide counterions,leading to a dehydration induced structural change.Under these conditions,the colour of the thermochromic material is principally derived from n ? ?* CT interactions between the ion-pair on account of the strong intermolecular polarization which decreases the ability of the bipyridinium groups to form additional CT complexes with neutral ?-electron rich donors.Secondly,according to the previous work,we hypothesis that the dimension of the bridge unit connecting bis-bipyridinium have largely influenced the properties of donor-acceptor self-assemblies.So,we designed and synthesized another five electrondeficient bis-bipyridinium derivatives 13-17,then together with 5,overall six amphiphiles which have different bridge units with their halogen counterions Br/Cl were combined to donor 10.A series of experiments were processed to investigate the relationship between the structure of amphiphiles and their ability to formation supramolecular thermochromic materials.The dimension of the bridge unit connecting bis-bipyridinium have largely influenced the properties of donor-acceptor selfassemblies.Acceptors with a long bridge moiety favors the supramolecular polymerization between the donor and the acceptor,leading to the formation of supramolecular fibers and hydrogels composed of fibrous network.On the contrary,no hydrogels can be observed for acceptors with shorter bridge moieties.The strength of the hydrogels also influenced by the structure of amphiphile acceptors.While different counterions did not change the hydrogel-forming abilities of donor-acceptor assemblies,but can tune the color of the thermochromic materials,and the hydrogels formed with chloride salt showed a lower melting point than their bromide counterparts due to weaker donor-acceptor interactions.Moreover,the supramolecular assemblies capable of forming hydrogels also exhibited reversible thermochromic property.These materials can be easily inkjet-printed into thermochromic patterns and texts with different color transition temperatures.These results broaden the spectrum of viologen based supramolecular thermochromic materials and provide a new protocol for screening the structure of thermochromic supramolecular materials.