We examined exciton quenching mechanisms in organic electrophosphorescent emitters. As an example we have performed detailed steady-state (dc) and alternating (ac) electric-field-modulated (photo)luminescence (EML) studies of the green phosphorescent iridium (III) complex, $\left[\mathrm{Ir}{\left(ppy\right)}_3\right]$, doped into a hole-transporting diamine derivative (TPD) blended with polycarbonate, varying electrode contacts from the hole-injecting indium-tin-oxide (ITO) and Au (dc EML) to the weakly electron-injecting Al on both sides (ac EML) of a sandwich type formed thin film samples. Analysis of ac EML results in terms of the three-dimensional (3D)-Onsager theory of geminate recombination shows that both the EML for the $\mathrm{Ir}{\left(ppy\right)}_3$ phosphorescence and TPD fluorescence components of the emission spectrum from the emitters is underlain by the dissociation of $\mathrm{Ir}{\left(ppy\right)}_3$ triplet and TPD singlet exciton precursors composed of an electron and a hole separated by a distance $r_0=2.1\pm 0.1\mathrm{nm}$. We find that for samples with the ITO/Au electrode combination, the dissociation is accompanied by exciton annihilation on injected charge. The annihilation proceeds by injected holes with the second order rate constant of ${\gamma }_{\mathrm{Sq}}=(2\pm 1)\times {10}^{-11}{\mathrm{cm}}^3∕\mathrm{s}$ characterizing the diffusivity of TPD singlets, and ${\gamma }_{\mathrm{Tq}}=(1\pm 0.5)\times {10}^{-14}{\mathrm{cm}}^3∕\mathrm{s}$ indicative of the relative diffusion motion of the $\mathrm{Ir}{\left(ppy\right)}_3$ triplet and a charge carrier. It is shown that the annihilation to dissociation efficiency has a minimum at an electric field of $F_{\min }\cong {10}^6\mathrm{V}∕\mathrm{cm}$ for the present system but the derived theoretical expression allows us to predict the evolution of $F_{\min }$ with various material and sample parameters, and by this to establish criteria to assist in the optimized design of quenching effects in organic electrophosphorescent devices.

Bacteriochlorophylls (BChls) c , d , and e are the main light-harvesting pigments of green photosynthetic bacteria that self-assemble into nanostructures within the chlorosomes forming the most efficient antennas of photosynthetic organisms. All previous models of the chlorosomal antennae, which are quite controversially discussed because no single crystals could be grown so far from these organelles, involve a strong hydrogen-bonding interaction between the 3 ¹ hydroxyl group and the 13 ¹ carbonyl group. We have synthesized different self-assemblies of BChl c mimics having the same functional groups as the natural counterparts, that is, a hydroxyethyl substituent, a carbonyl group and a divalent metal atom ligated by a tetrapyrrole. These artificial BChl mimics have been shown by single crystal x-ray diffraction to form extended stacks that are packed by hydrophobic interactions and in the absence of hydrogen bonding. Time-resolved photoluminescence proves the ordered nature of the self-assembled stacks. FT-IR spectra show that on self-assembly the carbonyl frequency is shifted by ≈30 cm ⁻¹ to lower wavenumbers. From the FT-IR data we can infer the proximal interactions between the BChls in the chlorosomes consistent with a single crystal x-ray structure that shows a weak electrostatic interaction between carbonyl groups and the central zinc atom.

Results based on the electric-field dependence of electro-modulated fluorescence and steady-state photoconduction in films of tris-(8-hydroxyquinolinato) aluminum (III) $({\mathrm{Alq}}_3)$ are consistent with a model where singlet excitons dissociate into electron–hole pairs through charge transfer state. Dissociation into free carriers or predominantly geminate recombination results from this intermediate state. The quantum efficiency of charge separation is in agreement with predictions based on a model using the three-dimensional Onsager theory of geminate recombination combined with volume (bimolecular) recombination of the photogenerated space charge. This model is used to account for the decrease in the electroluminescence quantum efficiency of ${\mathrm{Alq}}_3$ -based organic light-emitting diodes at high electric fields.