卟啉钯敏化剂构效性质与三线态-三线态湮灭上转换性能研究

上转换发光是一种将长波长的激发光转化为短波长发射的反斯托克斯发光现象，三线态-三线态湮灭上转换（TTA-UC）能够在较低密度能量下被激发，且上转换量子产率高，因此获得研究者们广泛关注。关于敏化剂分子结构与上转换发光性能相关性的研究一直是TTA-UC研究领域的重要热点，选择两种代表性的卟啉钯光敏剂PdOEP-八乙基卟啉钯(Ⅱ)和PdBrTPP-四溴苯基卟啉钯(Ⅱ)]与蒽衍生物9,10-(4-羟甲基)苯基蒽p-DHMPA发光剂组合上转换体系作为研究模型，通过一系列合成工作获得材料分子后，进一步比较两种敏化剂的光谱性质与体系最终上转换性能之间关系。通过细致研究敏化剂和发光剂的荧光发射和寿命等光谱性质对敏化剂系间窜越，三线态-三线态能量转移及三线态-三线态湮灭等能量传递过程的影响后，发现在532 nm处的摩尔吸光系数PdBrTPP (10.8 cm-1·mmol-1)大于PdOEP (3.0 cm-1·mmol-1)；三线态寿命PdBrTPP (173.13 μs)大于PdOEP (109.21 μs)。但与p-DHMPA配对时光敏剂与发光剂的三线态能级差ΔE_TT，PdOEP (0.140 eV)却高于PdBrTPP (0.062 eV)，通过Stern-Volmer方程得到Stern-Volmer猝灭常数K_SV和双分子猝灭常数k_q值也是PdOEP略高，最终表现出上转换阈值PdOEP/p-DHMPA (22.40 mW·cm-2)小于PdBrTPP/p-DHMPA (29.78 mW·cm-2)，上转换发光效率Φ_UC，PdOEP/p-DHMPA (28.3％)大于PdBrTPP/p-DHMPA (26.8％)。因此，卟啉钯敏化剂的构效对三重态湮灭上转换发光效率影响最为重要的决定因素是敏化剂三线态高低。对于不同的敏化剂，在分子主体结构、摩尔吸光系数与三线态寿命等光谱参数差别不大的情况下，敏化剂的三线态能级越高，就将会具有更大的上转换发光效率。然而如果以总上转换能力指标来评价，PdBrTPP的共轭结构能够提升其在激发波长处吸收更多光子的能力，具有比PdOEP更高的摩尔吸光系数，造成其总上转换能力η比PdOEP高3.4倍。因此从上转换总效能指标来评价，通过敏化剂分子设计调控其在激发光波长处的摩尔吸光系数也不失为一种简单易行的方法。

Study on the Structure/Energy-Level of Palladuim-Porphyrin Sensitizers on the Triplet-Triplet-Annihilation Upconversion Performance

The photon upconversion(UC)can realize the conversion of low-frequency photons to high-frequency photons.UC based on triplet-triplet-annihilation(TTA)has the characteristics of low threshold excitability,high quantum yield,and wide spectral conversion range,which have drawn broad research interested in the world.In this paper,two kinds of porphyrin complexes(named as PdOEP and PdBrTPP,respectively)were chosen as the sensitizer doped with p-DHMPA(as the emitter)to set up the TTA-UC research models.The absorption,fluorescence and phosphorescence spectra,as well as the upconversion spectra,were measured respectively.The relationships between triplet properties and the upconversion emission were discussed.Also,the relationship between the singlet-triplet energy level difference(ΔEST)of the sensitizer and their intersystem cross-section is discussed.And lastly,the relationship between the triplet energy level difference(ΔETT)from sensitizer to the emitter and the triplet-triplet energy transfer(TTET)is discussed.The results show that PdBrTPP has longer triplet lifetime(173.13μs)than PdOEP(109.21μs)and larger molar absorption coefficient(10.8 cm^-1·mmol·L^-1)than PdOEP(3.0 cm^-1·mmol·L^-1).Meanwhile,the quenching constant(kq)of PdOEP/p-DHMPA(1.64×10^-3μmol·L^-1·s^-1)is larger than PdBrTPP/p-DHMPA(6.53×10^-4μmol·L^-1·s^-1);moreover,the threshold excitation intensity(Ith)of PdOEP/p-DHMPA(22.40 mW·cm-2)is smaller than PdBrTPP/p-DHMPA(29.78 mW·cm-2).All of these results in the upconversion efficiency(ΦUC)of PdOEP/p-DHMPA(28.3%)is larger than PdBrTPP/p-DHMPA(26.8%).Therefore,it has been proved that the most important influence on the ΦUC value is depended on the triplet energy level(ET1)of sensitizer.Those porphyrin palladium sensitizers with higher triplet energy-levels can obtain high upconversion efficiency,regardless of difference among the molar absorption coefficient and the triplet lifetime.However,when using the overall upconversion ability(η)to revalue the upconversion processes,PdBrTPP has higher molar absorptivity than PdOEP,which can result in more absorbance.The total upconversion capability(η)of PdBrTPP is 3.4 times larger than PdOEP.Therefore,increasing the molar absorption coefficient of the sensitizer could be a feasible strategy to enhance the TTA-UC performance.