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

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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.