Study of phosphorescent organic EL materials on single-molecule level

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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