Study On One-/Two-Photon Excited Fluorescence From Exciplexes And Their Enhancement Mechanisms

Organic molecular materials possess advantages including low cost,high efficiency,fast response,wide color gamut,good flexibility,tailorable structure,simple molecular modification and accurate property modulation,so they exhibit important application potentials in the fields of organic light-emitting diodes（OLEDs）,organic solar cells,light sensors and organic lasers.In the direction of organic luminescence,the singlet-triplet energy difference（ΔEST）of thermal activated delayed fluorescence（TADF）materials is extremely small,which makes them breakthrough the theoretical internal quantum efficiency limit of 25%to achieve high fluorescence efficiency.In contrast to monomolecular TADF materials which require complex molecular synthesis,exciplexes naturally have separated frontier molecular orbitals.In exciplexes,small ΔEST can be achieved through simple combinations of commercialized donor（D）and acceptor（A）materials.As a result,exciplexes have intrinsic advantages of simple preparation and excellent optical properties,and are expected to be applied on a large scale in the near future.Organic materials also have charming potentials in nonlinear optics such as two-photon excited fluorescence（TPEF）.Because of their unique optical properties,two-photon materials have vital applications in the fields of biomedical imaging,photodynamic therapy,three-dimensional microfabrication and optical storage.For the design of new organic two-photon materials,both large two-photon absorption cross sections and high fluorescence efficiencies are required to achieve larger two-photon action cross sections and better nonlinear optical performance.However,the improvement of two-photon absorption cross sections generally depends on the complexity of the molecular conjugate structures,which usually brings about not only increased synthesis routes and cost but also generally decreased fluorescence efficiencies.Therefore,it is very necessary to explore novel design strategies and develop new organic two-photon materials with simple preparation technology and excellent properties.Due to the conjugated structure of D-A molecules,exciplexes have the potential of two-photon absorption（TPA）.At the same time,the TADF effect in exciplexes is conducive to the efficient TPEF emission.As a result,it is possible for exciplexes to match both the two-photon absorption cross sections and fluorescence efficiencies without complex synthesis,which brings exciplexes infinite application possibilities for not only electroluminescence and linear optics but also nonlinear optics.In this thesis,we successfully prepared a series of exciplexes with excellent one-and two-photon excited fluorescence properties through appropriate material design.In order to improve the fluorescence properties of exciplexes,several strategies on TPEF enhancement were proposed or developed:1)constructing an exciplex-host-fluorescence-guest system;2)constructing a ternary exciplex with double RISC channels;3)utilizing long range coupled exciplex by inserting spacer molecules to modulate the exciton distance.By using these novel strategies,the one-and two-photon excited fluorescence properties of exciplex-based systems were successfully improved.And based on theoretical calculations and experimental results,these photophysical mechanisms were systematically analyzed and discussed.The results of this thesis are expected to promote the applications of exciplexes in OLED and nonlinear optics.The research work of this thesis is summarized as follows:1)We prepared two exciplexes（m-MTDATA:TPBi and TAPC:TPBi）with TPEF properties.Under femtosecond laser excitation,significant TPEF emission was successfully observed for the first time.In particular,a two-photon absorption cross section of～387 GM was achieved in m-MTDATA:TPBi.By analysing the TPEF mechanisms of donor and acceptor materials,it is found that the TPA ability of the m-MTDATA:TPBi exciplex only comes from m-MTDATA molecules in the test excitation band,while the TPA ability of the TAPC:TPBi exciplex comes from both TAPC and TPBi molecules.Based on experimental results and theoretical analysis,a photophysical mechanism of the complete TPEF process in exciplexes was proposed.2)Exciplex-host-fluorescence-guest systems（TAPC:PBD:TTPA/rubrene/DCJTB）were first prepared by using TAPC:PBD as the exciplex host,and TTPA,rubrene and DCJTB as green,orange and red fluorescent guests,respectively.It was found that the fast F?rster resonance energy transfer（FRET）introduced by fluorescent guests greatly improved the air PLQY（from 13.7%to 98.4%,59.2%,85.5%for TTPA,rubrene and DCJTB,respectively）.According to the analysis of TPEF and TPA,it was found that the system can effectively improve fluorescence efficiencies while maintaining an efficient light absorption process（the two-photon absorption cross section can be maintained at～110 GM）.Combined with experimental results and theoretical analysis,the photophysical mechanism of one-and two-photon excited fluorescence,the fluorescence enhancement mechanism of FRET and the TPA mechanism in this host-guest system were systematically discussed.3)A TPEF enhancement strategy by utilizing dual-or multi-RISC channels was proposed and an A-D-A type ternary exciplex（3TPYMB:TAPC:T2T）with TPEF properties was first prepared.Linear and nonlinear fluorescence efficiency improvement caused by synergy effect was found.In detail,the RISC rate was significantly increased from 1.8×10⁶ s^-1 to 3.2×10⁶ s^-1,and an air PLQY up to 77.8%was observed,which is not only larger than that of pure binary exciplexes（39.6%and 3.7%）,but also greatly larger than their sum（43.3%）.Then,the TPEF performance of termary and binary exciplexes under the same excitation conditions were studied and an obvious enhancement on TPEF induced by the synergy effect with dual RISC channels was found,and the mechanism was also investigated.As controls,two ternary exciplexes without synergy effect were also designed and prepared.Finally,a tentative requirement for the occurrence of synergy effect in ternary exciplexes was proposed.4)A strategy for the TPEF enhancement by using long-range coupled charge-transfer excitons in exciplex-based materials was proposed.A bulk mixed D-S-A exciplex（TAPC:mCP:T2T）was first prepared by using TAPC:T2T as the exciplex and mCP as the spacer.It is found that the long-range coupled charge-transfer states induced by the spacer effectively decreased the ΔEST of the exciplex,and leading to a significant increase of RISC rate from 1.1×10⁶ s^-1 to 3.3×16 s-1,a decrease of the non-radiation rate from 2.8×10⁶ s^-1 to 0.4×10⁶ s^-1 and a great improvement on air PLQY from 39.6%to 66.0%.Bright TPEF emission with a wide excitation window（700-900 nm）was also observed.The excited state dynamics characterized by transient absorption measurements revealed the effect of the spacer in the formation process of the exciplex.The mechanisms of one-and two-photon excited fluorescence and their enhancement mechanism in long-range coupled exciplexes were systematically discussed.