Spin-orbit coupling effects on energy transfer channel in organic semiconductors

Abstract

Spin-orbit coupling (SOC) effects on energy transfer channel in polymer-phosphorescent dopant system have been investigated. The blue-emitting host polymer, poly [9, 9-di-n-hexyl-fluorenyl-2, 7-diyl] (PFO), was doped into a variety of concentrations between 0.1 and 5% with a red emissive phosphorescent dye, bis (2-[2′-benzothienyl)-pyridinato-N, C3′] iridium (acetyl-acetonate) (BtpIr). The mechanism for the SOC effects on energy transfer channel between the host and the dopant was studied using both photoluminescence (PL) and electroluminescence (EL) techniques. The SOC effect enhances enormously the intersystem crossing (ISC) rate arising by the strong overlap of the delocalized π orbitals of the PFO and BtPIr. The ISC energy transfer channel competes effectively with Fӧrster energy transfer channel and Dexter energy transfer channel. The energy transfer from S1 (PFO) state to T1 (PFO and BtPIr) state is increased by SOC. Consequently, the population of the S1 (PFO) state decreased. So the Förster energy transfer intensity is deceased. SOC leads to such an indirect decrease for the Förster energy transfer intensity. In other hand, the intensity of T1 (PFO) state increased. So the Dexter energy transfer channel intensity is increased. It allows more PFO triplets to the Dexter energy transfer channel to BtPIr triplets. The increased population of the triplet state brings about a greater number of detectable photons that are emitted from the BtPIr molecules. EL spectra implied that only a few singlets formed on the PFO contribute to the emission, most of them are transformed into triplet state by ISC channel.