Light emitting organic materials are utilized in a large number of applications, including photonics, biotechnology and optoelectronics. In particular, blends of different materials can be effectively used to tailor the spectral properties of the resulting composite, enabling for example the realization of white organic light emitting diodes (OLEDs), efficient solar cells and low threshold laser devices. In these composite materials, the energy transfer by dipole-dipole coupling (Förster transfer) occurs, provided that the emission spectrum of the donor (D) component overlaps with the absorption spectrum of the acceptor (A), and the dipoles are sufficiently close to each other. Thus, the transfer rate (kFRET) depends on the overlap between the D emission and the A absorption spectra, the rate of photoluminescence emission of D (kD) and on the mutual distance, R, between the D and A particles. The Förster theory for dipole-dipole interaction predicts an energy transfer decay rate scaling as: R^-6

where R0, known as Förster radius, is a characteristic dipole-dipole distance depending on the overlap between the donor emission spectra and the acceptor absorbance. We carried out extensive studies on blends of conjugated polymers with the objectives of (i) understanding the basic nonraditive energy transfer processes that occur in blends and hybrid organic-inorganic materials and (ii) exploiting the composites as active layer in waveguides and laser devices [1-3]. By analysing polymer blends we found, for instance, that the optical gain properties of polymeric composite can be effectively tailored by tuning the donor/acceptor relative content (Fig. 1). In these composites amplified spontaneous emission (ASE) can be observed from the donor or acceptor component depending on the relative concentration. These results show that polymeric composite materials have unique optical properties, relevant for the realization of tuneable, highly polarized lasers and low loss polymer active waveguides.

Fig. 1.  ASE emission of a polymer blend as a function of the acceptor/donor ratio.