Study On Two-photon Excited Fluorescence From Electron Donor-Acceptor Based Exciplex

Organic fluorescent materials play an increasingly prominent role in the applications field such as light-emitting devices,solar cells,biomedical imaging and sensors for their advantages on excellent photoelectric performance and biocompatibility.In addition,organic fluorescent materials also have the characteristics of designable structure and easy modification,and can be designed and synthesized according to different application requirements to produce molecular structures with novel structures and excellent performance.The advantages of organic fluorescent materials make researches on the design,synthesis,theoretical mechanism,and practical application of these materials more and more abundant.In evaluating the performance of organic fluorescent materials,the emission wavelength,quantum efficiency,and fluorescence lifetime are three important evaluation parameters.In recent years,a thermally activated delayed fluorescence(TADF)material based on electron donor/acceptor(D/A)exciplex has flourished in the field of organic electroluminescence.This fluorescent material converts a non-radiative triplet state into a radiative singlet state through an effective reverse intersystem crossing(RISC)process to maximize quantum efficiency and obtain long-lived delayed fluorescence.At the same time,the emission wavelength depends on the simple D/A combination realization,which avoids the complicated design and synthesis steps of single molecules.These advantages make the exciplex material a star material.Among the organic fluorescent materials,there is a type of fluorescent materials that can be excited by a near-infrared femtosecond laser and emit in the visible light band,that is,two-photon excited fluorescence(TPEF)materials.Because two-photon absorption(TPA)has three-dimensional spatial selectivity and strong penetrating ability,TPEF materials have a broad application space in the fields of life sciences and medicine.With the continuous update of application requirements and the continuous progress of material synthesis technology,many materials with excellent TPEF performance have been designed and synthesized,and have shown good value in practical applications.Through a large number of experimental synthesis,people have also explored the relationship between structure and performance,making the structure of TPEF materials more and more complex and more and more outstanding.The complex with ?-conjugation as the skeleton and strong electron push/pull electron as the generating chromophore has excellent TPA performance,however,it often causes rapid energy loss due to the formation of non-radiative or low-radiation excited states such as twisted intramolecular charge transfer(TICT)states,which greatly reduces the luminous efficiency of TPEF materials.Therefore,the development of new TPEF material design strategies has become an urgent research taskBy analyzing the structure of the donor/acceptor molecules that make up the exciplex,we found that these structures are very similar to TPA materials,and the RISC process in the system after the formation of the exciplex can achieve effective energy harvesting.Focusing on the purpose of achieving the TPA process and the energy harvesting process in the same material system,this thesis has carried out the following four aspects of innovative work1.We choose m-MTDATA/3TPYMB as the exciplex system,and use a Ti:Sapphire femtosecond pulsed laser(pulse frequency:80 MHz,pulse width 100 fs)as the near-infrared excitation light source.The amorphous film samples prepared by the drop coating method were successfully excited to emit strong upconversion fluorescence.Compared with the donor m-MTDATA sample,the fluorescence of the exciplex m-MTDATA/3TPYMB is significantly enhanced.Further one-photon and two-photon excited spectroscopy studies have shown that the enhanced up-conversion fluorescence emitted by exciplexes combines a series of excited state processes,including the two-photon absorption(TPA)process of the donor m-MTDATA and the TADF process of D/A exciplexes.The energy harvesting mechanism in the exciplex TADF material greatly promotes up-conversion fluorescence.The results have explained the formation mechanism of the excited exciplex state after the donor part of the D/A blend being excited by two-photon,and pointed out the potential of exciplex materials in nonlinear optical applications.In addition,the maximum D/A distance during the formation of the two-photon excited exciplex is about 6.9 nm.2.We choose TAPC/3TPYMB as the exciplex system,and use a Ti:Sapphire femtosecond pulse amplification system(pulse frequency:1000 Hz,pulse width 25 fs)as the near-infrared excitation light source.The amorphous film samples prepared by the drop coating method were successfully excited to emit strong upconversion fluorescence.Compared with the pure donor TAPC and pure acceptor 3TPYMB film samples,the exciplex TAPC/3TPYMB film samples exhibited enhancements of?129%and?365%in upconversion fluorescence,respectively.The results of spectroscopic studies explained the formation mechanism of the excited exciplex state after the donor part and the acceptor part of the TAPC/3TPYMB blend were simultaneously excited by two-photon,and the enhancement effect of the RISC process on the excited fluorescence.3.We choose TAPC/3TPYMB as the blue exciplex host material,and TTPA and DCM as the green and orange guest fluorescent dopant materials to form a host-guest system.A Ti:Sapphire femtosecond pulse amplification system(pulse frequency:1000 Hz,pulse width 25 fs)is used as the near-infrared excitation light source.Spectral studies were performed on amorphous film samples prepared by the drop coating method.The research results show that after the fluorescent emitter is incorporated into the D/A exciplex,the measured photoluminescence quantum yield(PLQY)is as high as 98.65%,and greatly enhanced up-conversion fluorescence is observed.Our results reveal that the fast Forster resonance energy transfer(FRET)process in the host-guest doping system of the D/A exciplex doped with fluorescent emitters realizes the collection of excited state energy after two-photon excitation,and greatly enhance the two-photon excited fluorescence.4.By choosing m-MTDATA/3TPYMB as the exciplex system,and a triblock polymer PEG-b-PPG-b-PEG as the wrapper,we successfully prepared water-soluble exciplex nanoparticles.The aqueous solution of the nanoparticles present a transparent and uniform state,and show the same emission and absorption properties as the m-MTDATA/3TPYMB film.Excited by femtosecond pulsed laser,nanoparticles in aqueous solution successfully emitted two-photon excited fluorescence.Such water-soluble nanoparticles will have a wide range of applications in the field of two-photon fluorescence imaging.