The Yaron Research Group

Photophysics of disordered organic systems

Schematic representation of exciton
					model used to study effects of disorder.

Schematic representation of exciton model used to study effects of disorder.

Overview

Most organic semiconductors share a structural motif in which carbon-carbon single bonds alternate with multiple bonds or aromatic rings. This motif leads to extensive Π-electron delocalization and desirable semiconducting or photophysical properties. This structural motif also imparts flexibility to the system, due to relatively facile rotation about the single C-C bonds. We have been exploring a number of aspects of these low-frequency torsions, including especially effects of disorder in the torsional degrees of freedom on the photophysics.

Spectral lineshapes: Role of torsional entropy

Disorder in the torsional angles of a conjugated polymer plays an important role in establishing the spectral lineshape. The excitation created on absorption of a photon delocalizes over the polymer, due to couplings between aromatic rings, β(θ), that vary with the angle between adjacent rings. The result is a delocalized excitation whose energy, and thus spectral position, is influenced by the torsional angles between the rings over which it is delocalized.

We have shown that an excitation delocalized over N torsional angles has an energy that is primarily a function of the average cosine of these angles:

Equation1

This reduction to a single dimension introduces a configurational entropy (shown below) that can be understood as follows. For an oligomer with two rings, there is a single angle, θ, and all values of Θo are equally probable. For the two angles of an oligomer with three rings, Θo=45 is much more probable than 0 or 90. (This is analogous to the roll of two die producing an intermediate value of 7 more often than the extreme values of 2 or 12). For large N, the law of large numbers leads to a Gaussian peaked at 45. The contribution of torsional motion to the line shape in solution can then be obtained from a thermal distribution on a free energy surface that combines the torsional barrier with the configurational entropy. This provides a simple and intuitive model of the lineshapes of conjugated polymer systems. In addition to this torsional entropy effect, disorder also localizes the excitation. We are currently working on a simple but accurate description that includes the effects of both localization and torsional entropy.

Probability Density and Free Energy Diagrams

Figure 1: Probability of finding an average angle Θo for an oligomer with N rings (left) and the contribution of the resulting torsional entropy to the free energy (right).

Environmental disorder

Environmental, or outer-sphere disorder, arises from the amorphous nature of the material surrounding the conjugated polymer. In collaboration with Linda Peteanu's group, we have been studying the effects of disorder through a combined experimental/theoretical approach. The electroabsorption spectra measured in the Peteanu group yields the difference in dipole moment, Δμ, and polarizability, Δα, between the ground and first-excited electronic state. For a nominally centro-symmetry polymer, Δμ should be zero. The measured Δμ arises from symmetry breaking and therefore provides a direct handle on the degree of disorder in the sample. Our results show that outer-sphere disorder is at least as important as disorder in the chromophore structure (inner-sphere disorder) in establishing this symmetry breaking.

Effects of aggregation

A new project, in collaboration with the Peteanu group, is a combined experimental/theoretical study of the effects of aggregation on the photophysics of conjugated polymers.