Structural Modulations And Optical Properties Of Binary Organic Crystals

A binary organic crystal is composed of two kinds of organic small-molecular-semiconductor materials with a periodically packing style.Compared with traditional inorganic semiconductors,organic semiconductors have the advantages of easy modification,low-temperature preparation,good compatibility with flexible substrates,and large-area preparation.Organic semiconductor is a hot field among semiconductor studying,showing extensive applications in electronic and optoelectronic devices such as organic field effect transistors(OFETs),organic light emitting diodes(OLEDs),and solar cells.Among these devices,multiple active materials are integrated in them so as to enable a variety of electronic and optoelectronic functions.But amorphous or polycrystalline films containing mixture components usually limit the enhancing of device performances due to relatively disordered molecular packing styles.Binary or multicomponent organic crystals have many novel and unique properties.The properties are not simple superpositions of the components but result from the cooperations of them.Among them,binary organic crystals are the simplest and mostly studied.Like single component crystals,binary organic crystals containing donor and acceptor molecules have a close packing pattern,which reduces impurities and defects during self-assembly;binary organic crystals composed of different donor and acceptor molecules tend to form mixed stacks that benefit for the formation of charge transfer(CT)states;by selecting suitable components,binary organic crystals can effectively adjust the energy band structures and change the charge transport modes of materials(p or n type);studying the relationships between different packing styles and properties by experiments and calculations helps to develop high-performance binary organic crystals.Binary organic crystals provide a new strategy for synthesizing multifunctional materials,bring the innovation of molecular cooperation,which have attracted great interests in recent years.Although binary organic crystal owns many merits,it is still at the initial development stage.There are still many key issues to be solved:First,at present,the material systems of binary organic crystals are very scarce.Not any two donor and acceptor molecules can form binary crystals,which brings the binary organic crystals are limited in the combinations of known acceptor molecules(such as Anthracene,TTF.Trans-Bpe)and donor molecules(such as TCNQ,TCNB,C60).Second,recent binary organic crystals are limited in the combinations of planar acceptors and donors.In addition,the prepared binary organic crystals are fixed at particular molar ratios,such as 1:1 and 1:2,and components are difficult to adjust continuously.Therefore,these situations hinder the continuous regulations of electrical.optical,ferroelectric and ferromagnetic properties of binary organic crystals.Also,it is harmful to study the relationships between component variations and physical properties of binary organic crystals.Third,the formation mechanisms of binary organic crystals are not clear.The current researches have proved that the intermolecular weak interactions have great influences on the formation of binary organic crystals,but there is still a lack of deep understanding of the specific physical mechanism behind it.Fourth,the relationships between the structures of the component molecules,the intermolecular weak interactions and the physical properties of binary organic crystals have yet to be established.For example,it is remained to be solved that what are the relationships between molecular symmetries,intermolecular weak interactions and molecular two-photon absorption properties in binary organic cocrystals.Last,the relationships between the properties of binary organic crystals and components themselves are not well studied.For example,what's the relationships between the two-photon absorption properties of binary organic crystals and components themselves?Can binary organic crystals be assembled with two-photon absorption properties by using components without two-photon absorption properties?Or further,can binary crystals with two-photon excited fluorescence properties be obtained by using components that do not have fluorescent properties?In response to the above questions,in this thesis,we prepared several new binary organic crystals that extend the binary organic crystal systems;we proposed a new preparation strategy to prepare binary crystals with arbitrarily adjustable stoichiometries,prepared several new binary organic crystals based on 8-hydroxyquinoline metallic complexes(MQx,M=Al,Ga,Fe,Cu,Zn;Q = 8-hydroxyquinoline)and studied their structures and charge transfer properties;we prepared organic cocrystals based on centrosymmetric molecules and studied the physical mechanisms behind the two-photon excited fluorescence properties;we prepared cocrystals based on non-fluorescent molecules,studied its two-photon excited fluorescence properties and the physical mechanisms of tunable fluorescence properties.The researches of this thesis play important roles in enriching the binary organic crystal systems,arbitrarily regulating the binary crystal components,and deeply understanding the relationships between the molecular structures,the physical properties of the components and the physical properties of the binary crystals,and help to promote the researches and developments of binary organic crystals.The main contents and conclusions of this thesis are as follows:1.Preparations and charge transfer properties of binary crystals based on 8-hydroxyquinoline metallic complexes(1)We proposed a new method for the preparations of binary crystals with arbitrarily adjustable stoichiometries based on tris(8-hydroxyquinoline)metallic complexes that have similar structrues.By selecting combinations such as AlQ3 and GaQ3,GaQ3 and FeQ3 that have similar solubility,we prepared AlxGa1-xQ3 and GaxFe1-xQ3 binary crystals by microemulsion method using chloroform(CHCl3)/cetyltrimethylammonium bromide(CTAB).Scanning electron microscopy(SEM)and X-ray diffraction(XRD)results show that the prepared crystals have good crystallinity.Energy dispersive spectrometer(EDS)results indicate that the component ratios of prepared binary crystals are in accordance with expectations and featured metal element distributions evident the component molecules are uniformly distributed.This strategy can extend to other molecular systems with similar structure and solubility.Strutural analyses show that the Alo.5Ga0.5Q3 crystal belongs to space group P 21in[a =11.115(3)A,b = 13.164(4)A,c = 16.625(5)?,? = 90.00 °,? 94.204(7)°,?= 90.00°]and the molecules stack by?-? styles.(2)For components that are less soluble in a solvent and are not suitable for microemulsion methods(such as InQ3),we prepared Alo.5Ino.5Q3 crystal by solution evaporation method.It belongs to space group P 21/n[a = 11.0193(6)A,b = 13.1394(8)?,c = 16.8585(9)?,a = 90.00°,B = 96.504(2)°,??= 90.00°].There are not solvate molecules included in the Al0.5In0.5Q3 crysal and the molecules stack by ?-? and CH-?styles.(3)For components that can form stable solution phase structure in DMSO by heating,we successfully prepared the ZnxCu1-xQ2 binary crystals by reprecipitation method.We also optimized preparation conditions and studied molecular packing styles of Zn0.5Cu0.5Q2 crystal.EDS results indicate component ratios of Zn0.5Cu0.5Q2 crystal accord with our expectations,demonstrating the feasibility of preparation of binary crystal with the help of solution phase structure.The results of FTIR and Raman spectra show that the molecular packing styles in the Zn0.5Cu0.5Q2 crystal are similar with that in pure ZnQ2 crystal.(4)We studied the charge transfer properties of binary crystals based on tris(8-hydroxyquinoline)metal complexes by experiments and calculations.Experimental and calcualted results show that,in Al0.5Ga0.5Q3 crystal,the highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)of GaQ3 and AIQ3 are staggered,so CT states can occur.While in Al0.5In0.5Q3 crystal,the energy levels of InQ3 and AIQ3 are so close that Al0.5In0.5Q3 is unable to form CT states.These results indicate that the energy level matching of components plays a key role in the preparation of CT binary crystals.Although the molecular structures of ZnQ2 and CuQ2 favor the formation of CT states,the CT interactions of Zno.5Cuo.5Q2 crystal are not observed in the experiments.Experiments and calculations show that the overlap decreasing of ZnQ2 and CuQ2 results in the loss of CT states due to the presence of water molecules.2.Two-photon excited fluorescence properties of CT cocrystal based on centrosymmetric moleculesWe synthesized organic TSB-TCNB(TTC)cocrystal by solution self-assembly method with trans-stilbene(TSB)and 1,2,4,5-tetracyanobenzene(TCNB)that are both centrosymmetric molecules.The TTC cocrystal belongs to space group P-1[a =7.1741(6)A,b = 7.5257(8)A,c = 12.5910(12)A,? = 99.826(3)°,? = 92.613(4)°,? =93.981(4)°].The molar ratio of TSB to TCNB is 1:2 and D-A molecules alternately stack along the[-4 2 1]direction with the intermolecular spacing 3.392 A and 3.405 A,forming long-range ordered CT states in the cocrystal.The photoluminescence quantum yield(PLQY)of TTC was calculated to be 13.05%permitting the observation of fluorescence in the two-photon excited fluorescence(TPEF)measurement.Compared with single component,the TPEF properties were enhanced with a wide two-photon excitation window,and no second harmonic generation(SHG)signals were detected.The intermolecular CT interactions contribute to the improvement of third-order nonlinear coefficient and radiative recombination rates.Experiments reveal the importance of intermolecular CT interactions in achieving high-performance nonlinear optical(NLO)cocrystals and may overcome the limitation of non-symmetric molecular structure required by the intramolecular CT process.These results will fundamentally affect the material designs and selections of nonlinear cocrystal materials without complex molecules prepared by complicated synthetic procedures.3.Two-photon excited fluorescence and tunable fluorescence properties of cocrystal based on non-fluorescent componentWe prepared NBA-TCNB(NTC)cocrystal with non-fluorescent trans-N-benzylideneaniline(NBA)and TCNB by a simple mechanical grinding method.XRD analyses evidence that the NBA:TCNB = 1:2 is a reasonable ratio in cocrystal.The melting point of the NBA is about 55 ? determined by thermogravimetric analysis/differential scanning calorimetry(TGA/DSC)experiments.The molecular diffusion between components,sharp grinding and the appearance of NBA droplets contribute to the formation of NTC cocrystal.The intermolecular CT interactions in NTC cocrystal enhance the PLQY of NTC.Compared with pure NBA(PLQY? 0.01%),the PLQY of NTC increases more than twenty times,which extends the sources of cocrystal to non-fluorescent organic materials.The increased PLQY enables the measurements of photoluminescence(PL)and TPEF.The intermolecular CT interactions of NBA and TCNB lead to the TPEF property of NTC.It is the first time that TPEF occurs in a cocrystal containing non-fluorescent component,which is important for further understanding of the NLO mechanism in organic cocrystals.The NTC shows a wide two-photon absorption window from 700 nm to 850 nm.The formation of NTC cocrystal promotes the hydrolysis reaction of NBA.The aniline formed by hydrolysis reaction of NBA forms a new TCNB-aniline complex with TCNB,which realizes a wide modulation of PL from blue-green to orange-red(176 nm).The preparation of NTC cocrystal may not only help us to understand the mechanochemical mechanism but also promote its applications in electronics and optoelectronics.