Electronic states of phosphorescent molecules Ir(ppy)3 and PtOEP used in organic light emitting diodes

Photoluminescence (PL) of phosphorescent molecules fac tris (2-phenylpyridine) iridium (Ir(ppy)3) and platinum octaethyl porphine (PtOEP) has been studied using films of Ir(ppy)3 doped in CBP and PtOEP doped in TPD, respectively. A weak emission band is observed at 396 nm in Ir(ppy)3. Its intensity increases with decreasing temperature from 300 K. This emission is attributed to the singlet emission from 1MLCT state. The singlet emission band of PtOEP is observed at 542 nm, which appears below 150 K. The intersystem crossing conversion is estimated to be 98.7% for Ir(ppy)3. From the excitation spectra for Ir(ppy)3 emission, we suggest energy transfer from the image vibrational state of Ir(ppy)3 to the image state of CBP host.