Systematic study of the dynamics of triplet exciton transfer between conjugated host polymers and phosphorescent iridium (III) guest emitters

摘要

A systematic study on the triplet exciton transfer between phosphorescent guest emitters and conjugated host polymers has been performed by means of fac-tris[2-(2-pyridinyl-kN)(5-(3,4-bis(2-methylpropyloxy)phenyl) phenyl)-kC]-iridium(III) [Ir-SC4] blended into three conjugated host polymers featuring higher, lower, and resonate triplet levels compared to the phosphorescent emitter, respectively. Here, the guest emission decay was measured over seven orders of magnitude in intensity after both optical and electrical excitation using nanosecond gated spectroscopy. Strongly dispersive decays are observed if the guest triplet energy level exceeds that of the host, which we discuss in terms of a guest host distance distribution. Hereby design rules for the synthesis of guest emitters emerge. Considering the reverse situation, i.e., the guest triplet level is lower than that of the host, almost no lifetime quenching of the guest emission is observed, but there is an additional delayed emission contribution of order 10~(-5). Several experimental findings, such as electric field quenching experiments and decay kinetics, indicate a charge transfer state between the guest metal and the host polymer rather than the late arriving of host polymer triplet excitons at the dopant sites as origin of this kind of delayed luminescence.