D-A Type Organic Luminescent Materials Based On Biscarbazole Substituted Benzonitrile

The organic photoelectric functional materials have drawn enormous attention owing to the advantages such as low density,easy modification and talents of spin-coating Although amazing progress has been made in organic light emitting diodes(OLEDs),organic solar cells(OSCs),and organic field-effect transistors(OFETs),etc.,there are still many challenges remained for us.For instance,how to tailor molecules to achieve particular properties,how to regulate photoelectric properties based on single-molecule and supramolecular structure and how to explain various anomalous photophysical phenomena.In-depth study on these issues can pragmatically bring more high-performance materials,and scientifically promote the development of organic optoelectronics at the same time.The combination of benzonitrile and carbazole is the hotspot in the field of organic optoelectronics,on which a lot of well-known scholars have systematically studied However,its potential has not been fully explored.Therefore,in this thesis,we employed biscarbazole substituted benzonitrile as the core and constructed a series of organic light-emitting materials through reasonable molecular design,inculding High-performance blue electroluminescent materials,mechanochromic luminescent materials,purely organic room temperature phosphorescent materials and single-molecule full-color electroluminescent materials.The main research contents are as followsIn Chapter ?,two blue emissive thermally activated delayed fluorescence(TADF)materials,namely DPAc-DCzBN and DPAc-DtCzBN were developed with benzonitrile(BN),9,9-diphenylacridan(DPAc)and carbazole(Cz)derivatives.More importantly,we proposed a novel molecular design strategy so called D-A-P(donor-acceptor-protector),by which the S1 and T1 states are confined to the region of BN and DPAc,thus the Cz moieties are able to act as protecting units.As a result,the quenching of the excitons caused by short-range charge exchange can be effectively suppressed,so that the electroluminescent devices will possess low efficiency roll-off characteristic.Moreover,the rigid structure of carbazole unit is conducive to form order packing,so the undoped devices constructed by these two materials also perform well.Eventually,the nondoped device of DPAc-DCzBN achieved a high EQEmax(External Quantum Efficiency)value of 20.0%and standard sky-blue emission with CIE(Commission Internationale de L'Eclairage)coordinates of(0.16,0.26).Meanwhile,it also presented vastly suppressed efficiency roll-off that the EQE values remained 19.5,16.1 and 12.6%at 100,500 and 1000 cd m-2,respectively.This device was the best performing nondoped sky blue TADF device among all public reports at the time.The doped device of DPAc-DtCzBN demonstrated a EQEvalue up to 23.1%,which remained 18.3 and 11.5%at 100 and 500 cd m-2,respectively.It was also one of the highest levels of pure blue(CIEy?0.15)TADF doped devices at the timeIn Chapter ?,we conducted two multifunctional materials,namely PXZ-DCzBN and PXZ-DtCzBN,composed of benzonitrile(BN),phenoxazine(PXZ)and carbazole(Cz)or tert-butyl carbazole(tCz)moieties,possessing TADF and mechanochromic luminescence(MCL)characteristics.Intriguingly,PXZ-DtCzBN simply displayed bicolor conversion,but PXZ-DCzBN presented rare multicolor transformation including single-molecule white-light emission under the stimuli of poor solvents,which is really rare.Through in-situ detection of the color switching process of PXZ-DCzBN in poor solvents by fluorescence microscope,we found that this behavior actually is the recrystallization of solid samples in poor solvents.The high-resolution scanning electron microscopy(SEM)photos also confirmed this hypothesis from the morphological changes.Furthermore,combinging the analysis of single crystal structure of target molecules,we proposed that the recrystallization will induce the conformational isomerization.Whereafter,some phenoxazine unit will change from a planar conformation(quasi-equatorial,eq)to a distorted conformation(quasi-axial,ax),resulting in a mixture of PXZeq-DCzBN and PXZax-DCzBN.Since the former is green emissive and the latter is blue emissive,then the coexistence of two conformers will produce white-light emission.Adjusting the degree of recrystallization(by controlling the ultrasound,heating,and processing time,etc.)can effectively control the ratio of the two isomers to manipulate the white light spectrum Through theoretical calculations,powder X-ray diffraction(PXRD)and differential scanning calorimetry(DSC),the interpretation has been proved from both theoretical and experimental levels.In addition,owing to the D-A-P structure,the nondoped device of PXZ-DCzBN exhibited quite high performance.The EQEvalues remained 13.6,12.8 and 10.0%at 100,1000 and 10000 cd m-2,respectively.Such low efficiency roll-off indicates that this is one of the best orange undoped TADF devicesIn Chapter ?,based on benzonitrile carbazole derivatives and diphenylamine derivatives,we designed four purely organic room temperature phosphorescence(RTP)materials with MCL property.The target molecules were prepared by commercially available carbazoles and laboratory synthesized carbazoles,respectively,to investigate the effects of impurities in commercial carbazoles on RTP characteristics.Among them,the products of the former are referred to as molecule 1-4,and the products of the latter are referred to as molecule 1'-4'.In short,both types of matreials demonstrate very different photophysical properties while their completely identical characterization results of 1H NMR spectroscopy,mass spectrometry,elemental analysis and even single crystal analyses For instance,the single crystal of molecule 1 presents typical white light emission.The short-wave emission is conventional fluorescence,and the long-wave emission is room temperature phosphorescence.The phosphorescence efficiency(?p)is 16.5%,and the phosphorescent lifetime(?p)is 0.11 s.The single crystal of molecule 1'also exhibits dual emission characteristics,but its spectrum is completely different from that of molecule 1 Its room temperature phosphorescent emission takes a higher proportion,with ?P=27.3%and ?p=78.60 ?s.In terms of material properties alone,solid samples of molecules 1-4 achieve highly efficient RTP emission with adjustable lifetimes in the ?s to s range,and solid samples of molecule 1'-4' achieve high performance RTP in ?s magnitude.We carefully studied the solution and solid photophysical properties and constants of these two series of materials,combined with DSC,XRD,single crystal analysis,and theoretical calculations to further investigate the RTP characteristic,and the following important conclusions were reached:(i)pure carbazole based materials can also achieve high efficiency RTP emission;(ii)the introduction of impurities in commercial carbazoles will prolong the lifetimes of RTP,leading to the generation of persistent afterglow;(iii)we speculated that the interactions among impurities and target molecules depend on ?-?stacking,so the reasonable introduction of t-butyl groups can effectively regulate the phosphorescence in the commercial carbazole based materials;(iv)the design of RTP materials requires careful introduction of tert-butyl,as it will enhance the molecular vibration which is not conducive to RTP characteristics.In summary,this work provides a variety of high performance room temperature phosphorescent materials on the one hand,which may have important applications in biological imaging,sensing and solid-state lighting.On the other hand,we systematically studied the effects of impurities in commercial carbazoles on the properties of RTP,proposed novel strategies for regulating the phosphorescent property,and made some contributions to future researchIn Chapter ?,a highly symmetrical and rigid TADF molecule was developed through the coupling of biscarbazole benzonitrile units.The four rigid carbazole groups of 4CzDBN are prone to form strong intermolecular ?-? interactions,providing a channel for controlling the light-emitting properties by adjusting the supramolecular structure.The emission peak of 4CzDBN in toluene solution is 492 nm,that of the powder and crystal is 544 and 585 nm,respectively.At the same time,with the increase of the doping concentration,the emission peak of thin film also red-shifted from 494 to 554 nm,which means that the aggregate structure of 4CzDBN negligibly manipulates its luminescent characteristics.The single crystal structure shows that the carbazole units in the monomer forms uninterrupted strong ?-? interaction with the adjacent ones,which penetrates the and is responsible for the red shift.DFT calculations found that as the degree of molecular aggregation increases,intermolecular charge transfers will gradually occur,further leading to a red shift in the emission.Finally,based on 4CzDBN,we successfully built the first full-color OLEDs with a single emitter.With the increase of the doping concentration,the corresponding electroluminescence peak red-shifted from 480 to 580 nm,covering the range from sky blue to orange-red.At 5 wt%doping concentration,the device was sky blue(0.19,0.36)emission,with EQEmax=18.3%.When the doping concentration increased to 15 and 30 wt%,the emission spectra were blue-green(0.24,0.47)and green(0.31,0.50),respectively,with EQEmax=16.2 and 8.2%.As the doping concentration increased to 60 and 100 wt%,the spectra were yellow(0.40,0.54)and orange-red(0.48,0.51),respectively,with EQEax=2.3%.