Bernard Wenger

Abstract:

Electron transfers (ET) between a donor (D) and an acceptor (A) through a molecular bridge (B) are of great importance in biological systems, molecular electronics and molecular based light-energy conversion systems. Here, the back and the forward electron transfer rates have been measured by femtosecond and nanosecond spectroscopy in a heterogeneous donor- bridge-acceptor (D-B-A) system, where D = ruthenium terpyridine complex, B = p-oligo(phenylene) and A = TiO2. The forward ET rate (from 0.85 to 3.7 ps^-1) is faster than the nonequilibrium vibrations relaxation rate of the hot ³MLCT (metal-to-ligand charge transfer) state of the donor (12 ps^-1 in solution). The back ET occurs on the microsecond time scale. Regarding the distance dependence behaviour, damping factors 0.16 and 0.47 Å^-1 of the forward and the back ET respectively are obtained. These results confirm that the damping factor is not only linked to the nature of the molecular bridge but to the full D-B-A system. This unusual low damping factor observed for the forward ET is attributed to a decrease of the tunnelling energy gap ΔE, which is induced by the nonequilibrium vibrations at the donor-bridge interface. This enhanced electron transmission is briefly discussed within the concept of a nonequilibrium polaron relaxation towards the dissipative acceptor. In this case, the dissipation of the excess vibrational energy and the electron transfer occur in a synchronized cooperative way.

Abstract:

Here, the use of metal oxide layers both for charge transport and injection into an emissive semiconducting polymer and also for the control of the in-plane waveguided optical modes in light-emitting diodes (LEDs) is reported. The high refractive index of zinc oxide is used to confine these modes away from the absorbing electrodes, and include a nano-imprinted grating in the polymer layer to introduce distributed feedback and enhance optical out-coupling. These structures show a large increase in the luminescence efficiency over conventional devices, with photoluminescence efficiency increased by up to 45%. Furthermore, optically-pumped lasing in hybrid oxide polymer LEDs is demonstrated. A tuneable lasing emission is also obtained in a single device structure by employing a graduated thickness of a zinc oxide inter-layer. This demonstrates the scope for using such architectures to improve the external efficiency of organic semiconductor LEDs, and opens new possibilities for the realization of polymer injection lasers. ©2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.