Synthesis And Characterization Of Cyanostilbene-Based Organic Solid State Luminescent Materials

Mechanofluorochromic or mechanochromic fluorescent materials are a class of "smart" materials with fluorescent properties that change in response to external force stimuli.These materials are widely used in sensors,memory chips,and security inks.Compared to conventional chemical structural change,physical structural change is belieyed to be easier to apply in realizing the dynamic control of highly efficient and reversible solid-state fluorescence.In 2007,Araki et al.reported the first pure organic mechanofluorochromic compound pyrene-based dye.When pressed with a spatula,the luminescence color of the dye change from blue to green.The blue luminescence of the pressed sample could be fully recovered by heating at above 112?.However,organic mechanofluorochromic materials that are dependent on changes in physical molecular packing modes are extremely rare.This rarity may be attributed to lacking of full understanding of the mechanochromic mechanisms at the molecular-level.Thus,It was of great significance and challenge to investgate the molecular packing modes and intermolecular interactions for obtaining the practical stimuli-responsive material.A series of novel organic solid state luminescent materials containing cyanostilbene were desighed and synthesized.Then the relationship between external factor(heating,pressure and solvent vapor),molecular packing modes and luminescent properties was systematically investigated.Firstly,a novel cyanostilbene-based fluorescent emitter PH-CN has been synthesized and characterized.The optical properties in solution and in the film state were investigated via UV-visible spectrum and photoluminescence spectrum(PL).The PL emission peak of PH-CN in film state were found in 497 nm,belong to a green emitter.The thermal stability and electrochemical property of as-prepared samples were studied by differential scanning calorimetry and cyclic voltammetry respectively.The glass transition temperature of PH-CN was measured at 213?,and the estimated highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)energy levels were-5.7 eV and-3.5 eV,respectively.Based on properties discussed above,PH-CN droped multi-layer OLEDs with the structures of ITO/PEDOT:PSS(40 nm)/NPB(30 nm)/CBP:5%PH-CN(20 nm)/TPBI(40 nm)/LiF(0.5 nm)/Al(100 nm)was fabricated.The device exhibited an external quantum efficiency of 1.02%with color coordinates of(0.20,0.40),which means the potential applications on the display.Then,a new type of stimuli-responsive dye DiCF3,which consists of the twisted diphenylacrylonitrile as acceptor moiety and the bulky triphenylamine as donor moiety was desighed and synthesized.The dye exhibited aggregation-induced emission activity and mechanochromism behaviors.The recrystallization powder showed strong yellow luminescence under 365 nm ultraviolet irradiation.When pressed with a spatula,the luminescence color transformed into strong orange red luminescence,and the emission could be recovered by exposure to organic vapor.X-ray diffraction patterns revealed that the yellow dye was crystalline and the orange red was amorphous,indicating that grinding converted the phases from crystalline to amorphous.Finally,we report a new type of stimuli-responsive fluorophore pCN-TPA with obvious solvatochromic effect and a high fluorescence efficiency,which consists of the twisted diphenylacrylonitrile as acceptor moiety and the bulky triphenylamine as donor moiety.The solid-state fluorophore reveals four different luminescence color of green(507 nm),yellow(535 nm),orange(608 nm)and red(628 nm)with a huge ??MCF of 121 nm under only mechanical grinding.The mechanochromic behavior can be attributed to the phase transition from crystalline state to amorphous state,accompanied by the excited states alteration between LE-state and CT-state.pCN-TPA exhibited HLCT properties upongrinding which make emitter show huge ??MCF and high fluorescence efficiency simultaneously.Thus,employing ICT-type luminophor,which reveals different excited states,may be a novel design strategy for exploring multi-colored MCF materials.