Role of conformational disorder in the electronic structure of conjugated polymers: Substituted polydiacetylenes

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Abstract
We consider a novel type of disorder relevant to the electronic structure of certain conjugated polymers: disorder induced by conformational changes in the chain geometry. A prototypical system in which strong disorder of this variety can be present is a substituted polydiacetylene. Both in the solid phase and in solution, these polymers undergo conformational transitions accompanied by color changes as one raises the temperature. We first present a simplified model for this transition which defines a probability distribution for conformational disorder. Real-space renormalization-group methods are then used to calculate the density of states and localization length for the model. Our results indicate that conformational disorder can cause qualitative changes in electronic structure in a fashion consistent with experiments. We also conclude that since such disorder is correlated, more than one length scale is needed to characterize an electronic state. In particular, ‘‘conjugation length’’ does not suffice.