Intermolecular Interaction and a Concentration-Quenching Mechanism of Phosphorescent Ir(III) Complexes in a Solid Film

Solid-state self-quenching processes of highly efficient Ir(III) phosphorescent emitters are investigated by the measurement of thin film photoluminescence quantum efficiency and transient lifetime as a function of doping concentration in a host matrix. The radiative decay rate constant is found to be independent from the average distance between dopant molecules ($R$), and the concentration-quenching rate constant is shown to be dependent on $R^{-6}$. The quenching dependence on $R$ strongly suggests that luminescent concentration quenching in a phosphorescent Ir(III) complex:host film is controlled by dipole-dipole deactivating interactions as described by the Förster energy transfer model.

Figure 1

Dependence of radiative (open circle) and concentration-quenching (cross-hair) rate constants, $k_{\mathrm{ph}}$ and $k_{\mathrm{CQ}}$, on average distance ($R$) between two Ir(III) complexes for (a) $\mathrm{Ir}(\mathrm{ppy}{)}_3:\mathrm{CBP}$, (b) FIrpic:mCP, and (c) ${\mathrm{Btp}}_2\mathrm{Ir}(\mathrm{acac}):\mathrm{CBP}$. Insets show the $R$ dependence of PL quantum efficiency ${\eta }_{\mathrm{PL}}$ (filled squares) and emission lifetime ${\tau }_{\mathrm{PL}}$ (open squares).

Figure 2

Dependence of concentration-quenching rate constants ($k_{\mathrm{CQ}}$) on average distance ($R$) plotted by double logarithmic axis for three complexes. The Förster radii ($R_0$) obtained by fitting the $k_{\mathrm{CQ}}\mathrm{\text{−}}R$ relations are shown. Inset also shows $R$ vs $k_{\mathrm{CQ}}$ by linear ($R$)-logarithmic ($k_{\mathrm{CQ}}$) axes.

Figure 3

Absorption (broken line) and PL spectra (solid line) of (a) $\mathrm{Ir}(\mathrm{ppy}{)}_3$, (b) FIrpic, and (c) ${\mathrm{Btp}}_2\mathrm{Ir}(\mathrm{acac})$ in methylenechloride ($2\times {10}^{-5}\mathrm{M}$). The dotted lines show the absorption value expanded 10 times for the $\mathrm{Ir}(\mathrm{ppy}{)}_3$ and FIrpic and 20 times for the ${\mathrm{Btp}}_2\mathrm{Ir}(\mathrm{acac})$. The Förster radii ($R_0$) obtained by the spectral overlap in Eq. (3) are shown.

Figure 4

Concentration dependences of PL spectra of (a) $\mathrm{Ir}(\mathrm{ppy}{)}_3$, (b) FIrpic, and (c) ${\mathrm{Btp}}_2(\mathrm{acac})$ in a CBP host.