Photochemical funnel in stiff conjugated polymers: interplay between defect mediated polymer conformations, side chain interactions and resonance energy transfer

Abstract

Observed optical properties of conjugated polymers pose interesting and challenging problems that are yet to be understood quantitatively. The problem is complicated due to the competition between energy migration among chromophores influenced by defects, conformations dictated by attractions among monomers and differing fluorescence rates of the chromophores of different conjugation lengths. In a conjugated polymer in the presence of defects, these chromophores are, to a good approximation, nearly straight planar chain segments with unbroken conjugation. Defects limit the length of a conjugated segment. Single molecules spectroscopic studies have shown that conformation of PPV-derivative polymers is not spherically symmetric as was commonly described using Flory theory; instead they are better represented as defect cylinders. As the defects are randomly distributed along the segments, they render the lengths of the segments random, thus introducing a disorder that does not allow easy formation of ordered structures like rods and toroids. Interactions among side chains of monomers in a conjugated polymer (as in MEH–PPV) can also determine the shape and the arrangement. The said non-spherical shape or conformation can have tremendous implication in optical properties of these polymers. While energy migration among the chromophores may be modelled as a random walk problem, the transfer rates and fluorescence properties of each chromophores need to be obtained quantum mechanically. The rate of hopping between different chromophores can be treated as a resonance energy transfer (RET) process, although Forster's well-known expression might not be reliable because in many cases the chromophores are spatially quite close to each other. Another issue revolves around the coherent or incoherent nature of transport. We discuss a recent theoretical approach that includes some of the features of energy transport discussed above. We discuss how a photochemical funnel can develop in such a system where the longest conjugated segment can form the funnel.