Effect Of One-sided Substituent On Formation And Luminescence Of Pyrene π-π Stacking Dimers

Since F?rster and Kasper discovered a pyrene（PY）excimer in cyclohexane solution in 1954,organic aromatic excimers have aroused great interest in many fields,such as chemistry,biology and materials.The formation of a PY excimer depends on the orientation and proximity between two PY molecules,which tends to form strongπ-πstacking.In previous research,it has been reported that the formation of a pyrene excimer in solution is governed by a diffusion-controlled process,that is,the concentrated solution facilitates two PY molecules to be closer than in dilute solution,thus readily forming an excimer.However,whether PY excimer can be formed in rigid solids or not,is rarely reported.Because solid-state materials are extensively used to the practical applications,it is of scientific significance to study the formation of PY excimer in solid state.Therefore,we proposed a strategy thatπ-plane molecule was modified by one-sided substituent,and achieved the discrete anthraceneπ–πdimer stacking in solids with high-efficiency anthracene excimer emission.In this case,excimer formation depends on intermolecularπ–πstacking mode（e.g.,coplanar overlap and distance）between two anthracene groups in a dimer.Different from anthracence,as for PY with two-dimension planar structure,the design strategy of one-sided substituent modification needs a further verification and optimization to obtain discrete PYπ–πdimer stacking,aiming at achieving high-efficiency PY excimer.Thus a systemic study on the substituent architecture（e.g.,size and orientation）is of great importance for the construction of discrete PYπ–πdimer stacking and formation of PY excimer,also providing a way to adjust the luminescence properties of high-efficiency PY excimer.Generally,in fluid media,PY excimer from diverse PY derivatives usually exhibits a similar emission color of sky-blue or cyan（460-485 nm）,probably resulting from almost the same excited-state geometry（i.e.,the excimer geometry of PY units）in unrestricted media.For that in rigid solids,the luminescence properties（e.g.,emission color）of excimer are expected to improve greatly.Based on the above description,we have synthesized a series of PY derivatives with one-sided substituents,studied the luminescence properties of both monomer and dimer,and revealed a basic rule how one-sided substituent affected the formation and luminescence properties of PY excimer.The conclusions obtained in this paper are as follows:1.We reported two high-efficiency crystals with PYπ–πdimer stacking,exhibiting different emission colors.Photoluminescence（PL）efficiency(η_PL)is as high as 77%for crystal-B and 60%for crystal-G,which are among the reported high-efficiency PY excimers.The experimental and theoretical investigations revealed the essential reason that one-sided substituent affected the luminescence properties of PY excimer.In crystal-G,the small twist angle between sunstituent and PY facilitates the larger overlap between two PY planes in the excited state,resulting in more red-shifted excimer emission.In addition,in crystal-G,the existing intermolecular interactions between adjacent PY dimers decrease the exciton localization and increase the nonradiative energy dissipation,resulting in lower luminescence efficiency.To explore the potential applications of PY dimer,organic light-emitting diodes（OLEDs）were fabricated with doped and non-doped device structures.Relative to doped OLEDs,non-doped OLEDs showed the PY dimer emission and exhibited the best performance with an external quantum efficiency（EQE）of 4.46%.2.According to the design principle of constructing discreteπ–πdimers,we further synthesized five pyrene-based donor-acceptor（D-A）derivatives（1-DTPS-PY,1-m-TPH-PY,1-TPE-PY,1-DPA-PY,1-TPA-PY）.Single-crystal X-ray diffraction（XRD）experiment revealed that pyrene derivatives with electron-withdrawing or neutral substituents are easier to form PY dimers while pyrene derivatives with electron-donating substituents are difficult to form PY dimers.Theoretical calculation demonstrated that electron-withdrawing substituents could enhanceπ-πinteractions by decreasing theπ-electron density of PY groups,which was beneficial to the formation of PY dimers,and electron-donating substituents could increaseπ-electron density of PY groups which was negative to the formation of PY dimers.