Study of phosphorescent organic EL materials on single-molecule level

单分子光谱已用于表征磷光铱配合物的光学性质。对于最典型的 Ir(ppy)_3 配合物，单分子发射强度的分布显示出向更高强度不对称的分布，并且光漂白时间非线性地依赖于单分子发射强度。这些特征可以解释为复合物光学性质改变的结果，这要么是由于不均匀的环境，要么是由于单个配体的部分光反应。为了澄清这一点，以相同的方式制备并研究了具有一个配体修饰的铱络合物，Irfppy(ppy)_2。结果表明，仅通过醛修饰配体发光的 Irfppy(ppy)_2 显示出相同的宽发射强度分布，而在 Ir(ppy)_3 情况下，由于周围基质导致分子畸变，导致单个配体的激发和发射局部化。 … 更多 此外，皮秒激光二极管和时间相关单光子计数已用于研究 Ir(ppy)_3 在整体和单分子水平上的激发态弛豫。虽然整体样本发射会随着预期的 1.2 μs 寿命而衰减，但样本浓度的降低伴随着在 1.2 μs 长分量旁边出现 3 ns 的短分量。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，表明基态吸收是其起源。此外，短分量取决于激发激光强度的二次幂，而对于长分量，强度依赖性是线性的。这一事实表明双光子吸收是短分量的可能来源。这通过一光子和二光子激发的单分子成像得到证实，其中在 488 nm 激发并在 370 至 450 nm 之间检测到的图像与通常的一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 ^1MLCT 状态，并从该状态发射短寿命荧光。较少的

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