Measurements of the photoluminescence (PL) quantum yield of high refractive index samples, such as thin films of polymeric semiconductors, are problematic owing to difficulties in determining the angular distribution of the emission, reflectivity, and absorbance. In the context of conjugated polymers, there is great interest in establishing the absolute quantum efficiency for PL as this is considered to determine the limits to the efficiency of electroluminescent diodes. A quantitative measurement of external PL efficiency is useful for a number of other reasons. For instance, in conjunction with time-resolved PL measurements, it provides a means of determining the radiative and nonradiative decay constants.'']The external radiative quantum efficiency, q, is defined by Equation 1. number of photons emitted ' = number of photons absorbed PL efficiency measurements in solution are relatively simple becausc it is usually appropriate to assume an isotropic angular distribution for the emission. However, this is not true for thin solid films. For molecular and polymeric materials, anisotropy in the distribution of chromophores leads to an anisotropy in the emission dipole-moment. In addition, wave-guiding effects also modify the angular dis-
Evolution of lowest singlet and triplet excited states with number of thienyl rings in platinum poly-ynesWe present a joint experimental and theoretical investigation of the electronic excitations in transition metal-containing phenylene ethynylenes. The influence of the metal on the nature of the lowest singlet and triplet excited states is characterized. We find that conjugation occurs through the metal sites, which deeply modifies the optical properties of the conjugated chains. We also analyze the chain-length dependence of the singlet-singlet, S 0 →S 1 , singlet-triplet, S 0 →T 1 , and triplet-triplet, T 1 →T n , transition energies; both experimental data and theoretical results indicate that the lowest triplet exciton, T 1 , is strongly localized on a single phenylene ring while the S 1 and T n states extend over a few repeating units. Finally, we estimate the geometric relaxation phenomena occurring in the lowest excited states and perform a Huang-Rhys analysis of the triplet emission spectrum in model systems.
We present a comparative study of two isostructural platinum and palladium poly-ynes, poly [trans(bis(tributylphosphine»M(1,4) phenylenediethynylene], where M represents either platinum or palladium. Optical excitations are associated with the 7T-molecular orbitals, and with strong spin-orbit coupling provided by the transition metal, both singlet and triplet excitations can be measured. The platinum and palladium polymers show similar low-temperature photoluminescence and photoinduced absorption spectra, with a blue-shift for the palladium polymer of around 5% for electronic transitions with respect to the platinum polymer. For Pt (Pd) we see strong singlet absorption peaks at 3.26 eV (3.49 eV), and weaker triplet absorption peaks at 2.43 eV (':"2.5 eV) at room temperature. Strong photoluminescence from triplet to ground state peaks at 2.38 eV (2.5 eV),'·and weak photoluminescence from the excited singlet peaks at 3.06 eV (3.2 eV) at low temperatures: Photoinduced absorption that peaks at 1.49 eV (1.72 eV) is due to transitions within the triplet manifold. Triplet lifetimes at low temperatures of 30 and 200-600 /Ls are found for the platinum and palladium polymers, respectively.
We have developed modified poly(p-phenylene vinylene) (PPV) to improve its performance in organic electroluminescent devices. Encapsulated single-layer devices with this PPV on indium–tin oxide and a calcium cathode have been operated in air for >7000 h at 20 °C and >1100 h at 80 °C without noticeable degradation. Devices fabricated with this PPV, a conducting polymer layer as anode, and a sputtered low work-function alloy as cathode have efficiencies of over 2 lm/W, brightnesses of >600 Cd/m2 at 4 V, and lifetimes of >1400 h driven at ∼100 Cd/m2 initial brightness.
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