| Since F?rster and Kasper firstly found the excimer fluorescence of aromatic pyrene in cyclohexane solution in 1954,organic aromatic excimer has been always the research focus in the field of chemistry,biology,materials,etc.Excimer is a class of dimeric excited-state species which are formed by associating an excited-state and a same ground-state molecule,and usually exhibits the red-shifted,structureless and broadened emission spectrum,etc,relative to that of monomer.Excimer species present the potentials in the field of organic optoelectronic devices,organic lasers,chemical and biological sensors,cell imaging,stimulation response and so on.In organic light-emitting field,one of the most important problems existing in excimer is low-efficiency luminescence,and thus in most cases,excimer formation is generally avoided in the application of organic light-emitting materials.Therefore,it is of great significance to develop the high-efficiency excimer system,to explore the new mechanism of high-efficiency excimer and to expand the potential application of excimer.Generally,the origins of weak luminescence from excimer are ascribed to two aspects as follows:one is that charge-transfer interaction and symmetry forbidden state of excimer result in the greatly decreased radiative transition rate（kr）;the other is that the dynamic character,complicacy,potential energy surface intersection,energy transfer etc of excimer structure greatly increase the nonradiative transition rate(knr).These two factors lead to the serious luminescence quenching together.In this thesis,we center on a series of unilateral anthracene（AN）compounds and systematic study of the molecular design,luminescent properties and excited-state geometry of discrete AN dimer.Ultimately,we obtained the strong excimer luminescence and revealed the intrinsic mechanism of high-efficiency excimer.The results are obtained in this thesis as follows:1.We firstly found a high-efficiency AN excimer crystal 2-TA-AN based on AN-thianthrene structure,exhibiting the luminous efficiency of as high as 80%which is one of the highest efficiencies of excimer.Single-crystal X-ray diffraction（XRD）experiment demonstrates a discrete dimeric AN stacking in 2-TA-AN crystal.Theoretical study shows that discreteπ-πdimer stacking is the structural nature of high-efficiency excimer luminescence.On the one hand,constringent interplanarπ-πdistance of dimer excited state reinforces the geometric rigidity and significantly decreases the knr;on the other hand,discrete dimericπ-πstacks results in the pure emissive excited state to avoid the formation of energy-trapping dark state,which effectively suppresses the nonradiative energy transfer pathway.2.We testified in experimental method that AN dimer in solid state was capable of inducing the high-efficiency excimer emission.A compound was that 9-position AN was modified by meta-position bromobenzene,and its two types of crystals had been grown:crystal B（blue-green）and crystal G（green）,respectively.From crystal B,crystal G to powder,dimer concentration is gradually increased with the increased fluorescence efficiency,demonstrating that high-or low-efficiency excimer emission will be determined by the dimer concentration in solids.Furthermore,we put forward the molecular design of discrete dimer stacking,i.e.,AN luminophor is unilaterally substituted,and the substituent orientates on the one side of AN plane to play a role in isolating the AN dimer when two molecules stack in antiparallel mode.We have verified the molecular design strategy through a series of designed AN-olefin derivatives.3.We firstly observed the special fluorescence behaviors of crystal with discrete AN dimer stacking under high pressure,i.e.,emission wavelength is independent of pressure before 1.70 GPa.High-pressure synchrotron powder XRD experiment determined the the equilibrium excimer geometry with the interplanar distance of 3.33?at the beginning pressure point 1.70 GPa of emission wavelength change,providing a novel method to determine theπ-πdistance in excited state. |
