Study of phosphorescent organic EL materials on single-molecule level

单分子光谱学已被用来表征磷光Ir络合物的光学性质。对于最典型的Ir(Ppy)_3络合物，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。这些特征可以解释为配合物的光学性质改变的结果，要么是由于不均匀的环境，要么是由于个别配体的部分光反应。为了阐明这一点，我们制备了一种单配体修饰的Ir络合物Irfppy(Ppy)_2，并以同样的方式进行了研究。结果表明，仅通过醛修饰配体发光的Irfppy(Ppy)_2表现出相同的宽发射强度分布，而Ir(Ppy)_3由于周围基质的扭曲导致激发和发射局域化在单一配体上。…在此基础上，利用皮秒激光二极管和时间相关的单光子计数技术，在系综和单分子水平上研究了Ir(Ppy)_3的激发态弛豫。当集合样品发射随预期的1.2μS寿命衰减时，样品浓度的降低伴随着长1.2μS分量旁边出现3 ns的短分量。激光重复频率依赖关系表明，随着脉冲间隔的增加，短分量的相对比例增大，表明短分量的起源是基态吸收。此外，短分量依赖于激发激光强度的二次幂，而对于长分量，强度依赖于线性。这一事实表明，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488 nm激发的图像在370-450 nm之间被检测到，与通常的单光子激发图像非常相似。双光子吸收是以π-π*配体为中心的跃迁。激发后，能量通过内部转换转移到发出短寿命荧光的^1MLCT态。较少

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