Design And Characterization Of Organic Ferroelectric Materials With Photocontrolled Polarization Switch

The ferroelectric materials is a kind of materials with spontaneous polarization and its polarization direction can be switched by applying an external electric filed.It has a very wide application in modern industry,such as the preparation of ultra-fast switches,low-cost room temperature magnetic field detectors,electric thermal coolers,phased array radar,dynamic random access memory,three-dimensional grooves,effectors,couplers,medical guidance wires,and intelligent devices,and so on.Since the first ferroelectric was discovered by Valasek in 1920,more than a hundred years ago,ferroelectric materials have experienced several vigorous developments under the joint efforts of people in the industry.Today,ferroelectric materials are numerous,rich in types and functions,and have excellent properties,such as ceramic ferroelectric,ferroelectric polymer,ferroelectric liquid crystal,molecular ferroelectric,and so on.Among ferroelectric materials,a class of ferroelectrics can respond to light and can be controlled by light.Light is contact-free,non-destructive,and remotely controllable relative to stress or electric field,but organic molecular ferroelectric materials with photoresponsive properties are still not fully developed.In this context,we took to photoisomerized photosensitive groups,namely salicylaldehyde Schiff base,and diarylethene,as the basic functional structure,and carried out the following work under the guidance of"ferroelectrochemistry"molecular ferroelectric oriented design theory.First,we present two new organic single-component homochiral photochromic multiferroics,（R）-and（S）-N-3,5-di-tert-butylsalicylidene-1-4-bromophenylethylamine（SA-Ph-Br（R）and SA-Ph-Br（S））,which show a full ferroelectric/ferroelastic phase transition of 222F2 type at 336 K.Under photoirradiation,their spontaneous polarization/strain can be switched quickly within seconds and reversibly between two ferroelectric/ferroelastic phases with the respective enol and trans-keto forms triggered by structural photoisomerizations.In addition,they possess superior acoustic impedance characteristics with the value of～2.42×10⁶ kg·s^-1·m^-2,lower than that of polyvinylidene fluoride(PVDF,3.69-4.25×10⁶ kg·s^-1·m^-2),which can better match human tissues.This work realizes for the first time that multiple ferroic orders in single-component organic crystals with ultralow acoustic impedance can be simultaneously controlled and coupled by three physical channels（electric,stress,light fields）,suggesting their great potential in multi-channel data storage,optoelectronics,and related applications compatible with all-organic electronics and human issues.Second,we present a pair of single-component organic enantiomorphic ferroelectric/ferroelastic crystals,（R）-and（S）-N-3,5-di-tert-butylsalicylidene-1-（1-naphthyl）ethylamine（SA-NPh-（R）and SA-NPh-（S））.It is notable that not only can their ferroic domain patterns disappear and reappear during reversible thermodynamic phase transformations,but they can also disappear and reappear during reversible light-driven phase transformations induced by enol–keto photoisomerization,both of which are from P1 to P2₁ polar space groups.Most importantly,the domain patterns are the same in the initial and final states,demonstrating the existence of a memory effect for the ferroic domains in SA-NPh-（R）and（S）.To our knowledge,this is the first time that the domain memory effect triggered by both thermodynamic and light-driven ferroelectric/ferroelastic phase transformations is realized in ferroic materials.Thermal and optical control of domain memory effect would open up a whole new research field for smart ferroic materials.Third,we present a pair of enantiomorphic diarylethene derivative ferroelectric crystals,4,4’-（perfluorocyclopent-1-ene-1,2-diyl）bis（5-methyl-N-（（R/S）-1-phenylethyl）thiophene-2-carboxamide）,showing a light-driven phase transition triggered by photoisomerization between the open and closed forms.Under visible light,they exhibit a bi-nary-domain state in the open form with white color and a bandgap of3.26 e V,while they show a single-domain state in the closed form with blue color and a bandgap of 1.68 e V after UV irradiation of 254/365 nm.In addition to writing and erasing ferroelectric domains with light,we can also use light to read their color to determine the polarization state of domains.Moreover,diarylethene derivatives have better thermal stability,higher photoexcited conversion efficiency,and larger changes in the absorption wavelength between two isomers than those in salicylideneaniline derivatives.This work not only discovers the first diarylethene-based ferroelectric crystals but also successfully realizes completely con-tactless manipulation of Write–Read–Erase data storage in organic ferroelectric semiconductors.These works open up a new way to realize multi-channel control of multi-ferroicity of organic crystals,provide an effective material system for the development of optically controlled polarization switch ferroic devices,and encourage people to further explore a new generation of optically controlled ferroelectric/ferroelastic devices.Combined with the excellent tunable light absorption characteristics,it provides a bright prospect for optically controlled ferroelectric appliances.