Study of phosphorescent organic EL materials on single-molecule level

采用单分子光谱法对磷发光铱配合物的光学性质进行了表征。对于最典型的Ir(ppy)_3配合物，单分子发射强度的分布呈现出向高强度方向不对称的分布，光漂白时间与单分子发射强度呈非线性关系。这些特性可以解释为由于不均匀环境或由于单个配体的部分光反应而修饰的配合物的光学性质的结果。为了阐明这一点，用同样的方法制备了一种单配体修饰的铱配合物Irfppy(ppy)_2。结果表明，仅通过醛修饰配体发光的Irfppy(ppy)_2具有同样宽的发射强度分布，而在Ir(ppy)_3由于周围基质的分子畸变导致激发和发射定位于单个配体的情况下。此外，还利用皮秒激光二极管和时间相关单光子计数技术研究了Ir(ppy)_3在系综和单分子水平上的激发态弛豫。样品发射随预期寿命1.2 μs衰减，样品浓度降低时，在1.2 μs长组分旁边出现了一个3 ns短组分。激光重复频率依赖性表明，随着脉冲间隔的增加，短分量的相对比例增加，表明基态吸收是其来源。此外，短分量依赖于激发激光强度的第二功率，而长分量的强度依赖是线性的。这一事实指出双光子吸收可能是短分量的来源。单光子和双光子激发的单分子成像证实了这一点，其中在488nm激发和在370和450 nm之间检测到的图像与通常的单光子激发图像非常相似。双光子吸收是π-π*配体中心跃迁。激发后，能量通过内部转换转移到^1MLCT状态，从那里发出短暂的荧光。少

Single-molecule spectroscopy has been used to characterize optical properties of phosphorescent iridium complexes. For the most typical complex of Ir(ppy)_3, distributions of single molecule emission intensities show profiles asymmetrical towards higher intensities and photobleaching times depend non-linearly on the single molecule emission intensities. These characteristics can be explained as a result of modified optical properties of the complexes, either due to the inhomogeneous environment or due to partial photoreaction of individual ligands. To clarify this point, an iridium complex with one ligand modified, Irfppy(ppy)_2 has been prepared and studied in the same manner. The results indicate that Irfppy(ppy)_2 which emits light via only the aldehyde-modified ligand shows the same wide distribution of emission intensities and that in the case of Ir(ppy)_3 molecular distortion due to the surrounding matrix causes localization of the excitation on and emission from a single ligand. … More Further, picosecond laser diode and time-correlated single-photon counting have been used to study excited state relaxation of Ir(ppy)_3 on ensemble and single molecule level. While the ensemble sample emission decays with the expected 1.2 μs lifetime, decreasing sample concentration is accompanied by the appearance of a short component of 3 ns next to the long 1.2 μs component. Laser repetition frequency dependence reveals that with increasing pulse separation the relative ratio of the short component increases, pointing to the ground-state absorption as its origin. Further, the short component depends on the second power of the excitation laser intensity while for the long component the intensity dependence is linear. This fact points to two-photon absorption as the probable origin of the short component. This is confirmed by one-photon and two-photon excited single-molecule imaging, where the image excited at 488 nm and detected between 370 and 450 nm is very similar to the usual one-photon excited image. The two-photon absorption is a π-π* ligand-centered transition. After excitation, the energy is transferred via internal conversion to the ^1MLCT state from where the short-lived fluorescence is emitted. Less