Zhao Tonghan, Busko Dmitry, Richards Bryce S, Howard Ian A
Institute of Microstructure Technology, Karlsruhe Institute of Technology, Karlsruhe, Germany.
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology (NCNST), Beijing, China.
Front Chem. 2022 Oct 31;10:1010857. doi: 10.3389/fchem.2022.1010857. eCollection 2022.
The effect of triplet-triplet annihilation (TTA) on the room-temperature phosphorescence (RTP) in metal-organic frameworks (MOFs) is studied in benchmark RTP MOFs based on Zn metal centers and isophthalic or terephthalic acid linkers (ZnIPA and ZnTPA). The ratio of RTP to singlet fluorescence is observed to decrease with increasing excitation power density. Explicitly, in ZnIPA the ratio of the RTP to fluorescence is 0.58 at 1.04 mW cm, but only 0.42 at (the still modest) 52.6 mW cm. The decrease in ratio is due to the reduction of RTP efficiency at higher excitation due to TTA. The density of triplet states increases at higher excitation power densities, allowing triplets to diffuse far enough during their long lifetime to meet another triplet and annihilate. On the other hand, the shorter-lived singlet species can never meet an annihilate. Therefore, the singlet fluorescence scales linearly with excitation power density whereas the RTP scales sub-linearly. Equivalently, the efficiency of fluorescence is unaffected by excitation power density but the efficiency of RTP is significantly reduced at higher excitation power density due to TTA. Interestingly, in time-resolved measurements, the fraction of fast decay increases but the lifetime of long tail of the RTP remains unaffected by excitation power density. This may be due to the confinement of triplets to individual grains, leading decay to be faster until there is only one triplet per grain left. Subsequently, the remaining "lone triplets" decay with the unchanging rate expressed by the long tail. These results increase the understanding of RTP in MOFs by explicitly showing the importance of TTA in determining the (excitation power density dependent) efficiency of RTP. Also, for applications in optical sensing, these results suggest that a method based on long tail lifetime of the RTP is preferable to a ratiometric approach as the former will not be affected by variation in excitation power density whereas the latter will be.
在基于锌金属中心以及间苯二甲酸或对苯二甲酸连接体（ZnIPA和ZnTPA）的基准室温磷光（RTP）金属有机框架（MOF）中，研究了三重态-三重态湮灭（TTA）对室温磷光的影响。随着激发功率密度的增加，观察到RTP与单线态荧光的比率降低。具体而言，在ZnIPA中，RTP与荧光的比率在1.04 mW/cm²时为0.58，但在（仍然适中的）52.6 mW/cm²时仅为0.42。比率的降低是由于在较高激发下TTA导致RTP效率降低。在较高的激发功率密度下，三重态的密度增加，使得三重态在其较长寿命期间能够扩散得足够远，从而与另一个三重态相遇并湮灭。另一方面，寿命较短的单线态物种永远不会相遇并湮灭。因此，单线态荧光与激发功率密度呈线性关系，而RTP呈亚线性关系。等效地，荧光效率不受激发功率密度的影响，但由于TTA，在较高激发功率密度下RTP效率会显著降低。有趣的是，在时间分辨测量中，快速衰减的部分增加，但RTP长尾巴的寿命不受激发功率密度的影响。这可能是由于三重态被限制在单个晶粒中，导致衰减更快，直到每个晶粒只剩下一个三重态。随后，剩余的“孤立三重态”以长尾巴表示的不变速率衰减。这些结果通过明确显示TTA在确定（依赖于激发功率密度的）RTP效率方面的重要性，增加了对MOF中RTP的理解。此外，对于光学传感应用，这些结果表明基于RTP长尾巴寿命的方法比比率法更可取，因为前者不会受到激发功率密度变化的影响，而后者会受到影响。
