Organic Microcrystal Lasers:Design,Synthesis And Device Applications

Organic photonic materials have attracted great attention in optoelectronic devices,due to their chemically tunable optoelectronic properties,self-assembly amenability to low-cost fabrication and flexible devices.Nevertheless,electrically pumped organic lasers remain a grand challenge.In this doctoral thesis,organic micro-nano lasers have been extensively investigated by modulating the excited-state dynamics and reducing the gain loss brought by triplets.A number of high-performance organic gain materials are designed and synthesized,and high-quality optical microcavities are constructed through self-assembly methods.In brief,low-threshold organic micro-lasers,thermally activated delayed fluorescence（TADF）micro-lasers,and exciton-polariton micro-lasers have been developed.1.The compound of 1,4-bis（（E）-2,4-dimethylstyryl）-2,5-dimethylbenzene（6M-DSB）was designed and synthesized,which can self-assemble into single crystal micro-belts through the solution reprecipitation method.Crystal data and spectral characterization confirmed that 6M-DSB molecules were stacked in brickwork arrangements along the molecular short-axis direction and stacked in face-to-face parallel modes along the molecular long-axis direction,forming Hj-type aggregates（strong H aggregation and realtively weak J aggregation）.Compared with monomers,Hj-type aggregates exhibit an enhanced radiative decay rate,so that the micro-belts exhibits a photoluminescence quantum yield near to 100%,therefore minimizing the probability of intersystem crossing（ISC）and reducing the gain loss caused by the generation of triplet states.The two lateral sides of 6M-DSB single crystal micro-belts act as mirrors,forming a high-quality FP resonant cavity along the width direction.Furthermore,a method of preparing micro-nano laser arrays assisted by periodic PDMS templates has been developed.The lasing thresholds,wavelengths and micro-cavity mode spacing of each sub-unit in the micro-nano laser array are almost the same,providing a coupled coherent light source for the integration of organic photonics.2.The sulfur-substituted difluoroboron derivative SBF₂BTF was designed and synthesized,which exhibits thermally activiated delayed fluorescence（TADF）.The microcrystals prepared based on the molecular self-assembly successfully realized TADF stimulated emission.The results of spectroscopic measuremens and theoretical calculations show that the lowest singlet state S₁（¹HLECT）of the SBF₂BTF molecule and the high-lying triplet state T₂（³HLECT）have a smallΔE_S1-T2 value（0.11 e V）and a large spin-orbital coupling（SOC）constantδ（S₁,T₂）=21.53 cm^-1 and a small reorganization energy（86 me V）,thereby increasing the ISC rate k _ISC between S₁ and T₂ and the reversed ISC（RISC）rate k _RISC,which are faster than those of traditional TADF compounds by 1-2 orders of magnitude.The SBF₂BTF microbelt obtained by solution self-assembly forms a high-quality FP microcavity（quality factor Q up to2160）,and realizes a high-performance multi-mode TADF laser,which provides a new design basis for the further development of TADF gain materials.3.The DPAVBi microbelt with controllable size was synthesized by the solution self-assembly method,and the exciton polariton characteristics in the microcavity of DPAVBi microbelt were investigated.The waveguide modes were simulated by a coupled oscillator model,confirming that strong exciton-photon coupling occurs in the DPAVBi microbelt microcavity at room temperature.Thanks to the high exciton binding energy of the Frenkel exciton,the exciton-photon coupling energy in the DPAVBi micro-belt microcavity is as high as hundreds of me V,and increases as the micro-belt size decreases.The lasing modes of a single micro-belt observed above the threshold are in good agreement with the resonances observed below the threshold.The high group refractive index calculated from the the optical resonances suggests that the lasing behavior above the threshold originates from the exciton polaritons.