Investigation On Near-Infrared Organic Solid-State Lasers Based On Excited-State Intramolecular Proton Transfer

Organic gain materials are extremely attractive for lasing due to their chemical tunability and large stimulated emission cross-sections.Organic lasers based on excitedstate intramolecular proton transfer(ESIPT)active materials,especially recently developed micro/nanolasers,have been drawing great attention due to their unique photophysical properties,such as ultralow threshold and high-quality value,as well as near-infrared(NIR)and wavelength tunable emission.In addition,ESIPT-active organic compounds can self-assemble into micro/nano crystals through the solution process,and serve as gain medium and resonant cavity simultaneously,providing conditions for the construction of miniaturized solid-state lasers.Although some works have established a real four-level system through the ESIPT process,the factors that affect the gain performance of ESIPT-active materials have not been studied deeply.In addition,the wavelengths of most organic semiconductor lasers are concentrated in the visible regions,and it is still difficult to realize NIR organic semiconductor lasers exceeding 800 nm.Therefore,this thesis focuses on the realization of NIR organic solid-state lasers based on ESIPT,from the perspective of optimizing the excited state process of ESIPT-active organic laser molecules and promoting the redshift of laser wavelength.The specific research contents are as follows:(1)First,starting from a deep understanding of the factors affecting the gain performance of ESIPT-active organic molecules,we investigated the effect of resonanceassisted intramolecular hydrogen bonds(RAHBs)on ESIPT-active materials.Here,we demonstrated that RAHBs can activate ESIPT-active materials by suppressing nonradiative decays.A new ESIPT-active material 1,5-dihydroxy-2,6diphenyllanthracene anthraquinone(DP-DHAQ)containing two centrosymmetric RAHBs was developed,which exhibited about 100-fold higher radiative decay rate(kr=1.1 × 1010 s-1)in doped polystyrene film than that of 1-hydroxy-5-methoxy-2,6diphenyllanthracene anthraquinone(DP-HMAQ)and 1,5-dimethoxy-2,6diphenyllanthracene anthraquinone(DP-DMAQ),in which with one and two RAHBs were broken respectively by introducing methyl groups.Doping DP-DHAQ into polystyrene(PS)microspheres can display deep red lasing at 710 nm.Neither DP-HMAQ nor DP-DMAQ-doped PS microspheres exhibited laser signals under our test conditions.In addition,the diamond-shaped microsheets self-assembled by DP-DHAQ from the solution can be pumped NIR laser under the nanosecond pulsed laser light of 532 nm.Therefore,our study shows that RAHBs can activate ESIPT lasing by confining nonradiative decays,which provides a new idea for the design of organic laser-active molecules.(2)In this work,we proposed that the combination of ESIPT and ICT process can help organic compounds construct efficient four-level systems and narrower emission bandgap in the NIR region,thereby enabling NIR lasing above 900 nm.Specifically,1,5dihydroxyanthracene-2,6-bis(4-(dimethylamino)phenyl)-anthraquinone(AP-DHAQ)was designed and synthesized as a model for ESIPT-coupled ICT molecular.According to density functional theory(DFT)calculations and ultrafast kinetic analysis,the model molecule undergoes ESIPT and ICT processes,while 1,5-dihydroxy-2,6-bis(4isopropylphenyl)anthracene-anthraquinone(IP-DHAQ)can only undergo the ESIPT process.NIR lasing emission at 810 nm can be observed in AP-DHAQ-doped PS microspheres,while IP-DHAQ only has a deep red laser signal at 720 nm.Furthermore,the AP-DHAQ microrods can be used as a Fabry-Perot(F-P)resonant cavity and exhibit NIR single-mode lasing emission with a threshold of 28.2 μJ cm-2 at 940 nm.This work achieved the longest lasing wavelength in organic microcrystals so far and demonstrates that ESIPT-coupled ICT active compounds are ideal gain media for fabricating organic solid-state lasers beyond 900 nm.(3)Extend the laser wavelength beyond 800 nm by extending the conjugation degree of ESIPT-active organic molecules.1,5-dihydroxy-2,6-bis((E)-styryl)anthraceneanthraquinone(DHS-DHAQ)and 1,5-dihydroxy anthracene-(E)-2-(4-(dimethyl amino)styryl)-anthraquinone(MDS-DHAQ),and the two compounds were doped in PS microspheres respectively.Under the excitation of a 532 nm nanosecond pulsed laser,DHS-DHAQ-doped PS microspheres achieved NIR lasing at 812 nm,with a threshold of 27.6 μJ cm-2.However,the lasing wavelength of the MDS-DHAQ that only extends the degree of unilateral conjugation and introduces an electron-donating group is extended to 890 nm,and the threshold is significantly reduced to only 15.0 μJ cm-2,which will promote the understanding of NIR organic semiconductor gain materials,and promote the development of NIR organic solid-state lasers.