TEMPERATURE-DEPENDENT BEHAVIOR OF CONJUGATED POLYMERS IN SOLUTION

We use Monte Carlo simulation to study the effect of temperature on structural and electronic properties of conjugated polymers in solution, using a Hamiltonian which has been designed to describe the complex pi-electron dynamics within a simple two-parameter model. The changes in optical absorption spectra as a function of temperature or the length of the side group are quantitatively similar to those observed experimentally. They can be understood in terms of the shift in the energy gap from decreasing pi-electron conjugation as rotational fluctuations grow with rising temperature. The values of the model Hamiltonian parameters required to achieve this agreement suggest the importance of both sigma-pi overlap and steric hindrance. Although conjugation and persistence lengths decrease with growing thermal disorder, the predicted change in the radius of gyration during rod-to-coil transition is neither as abrupt nor as large as those observed in long (2000 monomer) polydiacetylene chains. However, our results on the longest chains practical for the calculation (of length 64) are consistent with the gradual changes observed in chains of comparable length (N almost-equal-to 50).