Origin of the highly efficient electroluminescence as studied by Time-resolved EPR

研究了有机染料和π共轭聚合物体系中高效电致发光的机理。采用时间分辨EPR技术，在低温下观察了刚性基体和薄膜样品的极化EPR光谱。由辐照产生的顺磁性物质被分配到它们的激发态，然后在第二激光脉冲的激发下从它们的高激发态反向系统交叉（RISC）。通过双色双激光TREPR实验，确定了几种有机染料的RISC产率。根据自旋相关自由基对机理，得到了由自由基对引起的信号，并进行了分析。研究结果揭示了有机染料和π共轭聚合物的激发态弛豫过程以及在电荷输运过程中起重要作用的瞬态自由基对的电子结构。金属（Ir和Pt）配合物也是研究对象，因为它们在大的自旋轨道耦合（SOC）相互作用下产生的磷光量子产率很高。在低温条件下，在Ir和Pt配合物体系中均未发现TREPR信号。由于大的偶极-偶极相互作用和激发态的有效弛豫，导致三重态EPR谱的观测受到干扰。为了获得这些金属配合物的激发态信息，在室温下进行了EPR测量，观察了离子自由基的种类，这些离子自由基与电化学方法产生的相应激发态具有相似的电子性质。虽然考察了电解样品的最佳实验条件，如溶剂和/或施加电压，但没有观察到明显的信号。

The research was carried out to elucidate the mechanisms of highly efficient electroluminescence in organic dyes and π-conjugated polymers systems. Using by time-resolved EPR technique polarized EPR spectra were observed both in the rigid matrices and film samples at low temperature. The paramagnetic species generated by the irradiation were assigned to their excited triplet states, followed by the reverse intersystem crossing (RISC) from their higher excited triplet state upon excitation by the second laser pulse. The RISC yields were determined for several organic dyes from the two-color two-laser TREPR experiments. The signals due to radical pairs were also obtained and analyzed in terms of the spin correlated radical pair mechanism. The results gave the information on the relaxation processes in excited states of organic dyes and π-conjugated polymers and the electronic structures of the transient radical pair species, playing important roles in the charge transport processes.The metal (Ir and Pt) complexes were also of the subject because of their high quantum yields of the phosphorescence induced by the large spin-orbit coupling (SOC) interactions. No TREPR signals were obtained in the system of Ir and Pt complexes even at cryogenic temperatures. The facts were interpreted by the large SOC leading to disturb the observation of triplet EPR spectra because of large dipole-dipole interactions and also the effective relaxation in their excited states. To obtain the information on the excited triplet states of these metal complexes the EPR measurements were carried out to observe the ion radical species, which were expected to have similar electronic properties with corresponding excited triplet states, generated by the electrochemical method at room temperature. Although the optimum experimental conditions such as solvents and/or applied voltages for electrolysis of the samples were examined, no significant signals were observed.