Photophysics of disordered systems

Although electronic structure calculations on conjugated polymers typically assume perfectly ordered systems, the amorphous nature of the materials is known to play an important role in the photophysics. Theoretical descriptions of the effects of disorder have typically used phenomenological site models. Such models assume that energy or charge can exist at various sites in the system, and considers hopping between these sites. But consider a disordered polymer sample as a plate of spaghetti. What is the nature of the "sites" at which energy or charge can exist? This is the question we are exploring with our energy landscape model.

Our methodology couples the well-tested Hamiltonians of semi-empirical quantum chemistry with a new computational approach that mimics the effective particle language commonly used in discussing the photophysics of these materials. This approach allows us to use quantum chemistry to study the dynamics of effective particles in complex, disordered systems. The resulting computational savings make quantum chemical (INDO) calculations on large systems computationally feasible. We also obtain intermediate results, such as energy landscapes and effective masses, that aid in interpreting the large amount of information produced by such calculations. For instance, the figure at the bottom shows the energy landscape for an optical excitation (exciton) as it moves along a polyene. The end of the polyene models a break in conjugation, a common type of defect. It is somewhat surprising that the energy of the exciton only rises by about 0.2eV as it approaches the end of the chain (this is to be compared to its kinetic energy of about 3eV). The fact that the energy does rise much until the exciton hits the very end of the chain supports the hard-wall potential implicit in particle-in-a-box models of such defects.

We are currently extending this model along two lines. One is to describe chemical defects, such as the carbonyl defects in PPV that are known to quench fluorescence. The other is to study conformational disorder, since the energy landscape for a distorted polymer will give insight into the microscopic nature of the "sites" in phenomenological models.