Collapse of stiff conjugated polymers with chemical defects into ordered, cylindrical conformations

The optical, electronic and mechanical properties of synthetic and biologic al materials consisting of polymer chains depend sensitively on the conform ation adopted by these chains. The range of conformations available to such systems has accordingly been of intense fundamental(1,2) as well as practi cal(3-6) interest, and distinct conformational classes have been predicted, depending on the stiffness of the polymer chains and the strength of attra ctive interactions between segments within a chain(7-10). For example, flex ible polymers should adopt highly disordered conformations resembling eithe r a random coil or, in the presence of strong intrachain attractions, a so- called 'molten globule'(2,10). Stiff polymers with strong intrachain intera ctions, in contrast, are expected to collapse into conformations with long- range order, in the shape of toroids or rod-like structures(8,9,11). Here w e use computer simulations to show that the anisotropy distribution obtaine d from polarization spectroscopy measurements on individual poly[2-methoxy- 5-(2'-ethylhexyl)oxy-1,4-phenylenevinylene] polymer molecules is consistent with this prototypical stiff conjugated polymer adopting a highly ordered, collapsed conformation that cannot be correlated with ideal toroid or rod structures. We rnd that the presence of so-called 'tetrahedral chemical def ects', where conjugated carbon-carbon links are replaced by tetrahedral lin ks, divides the polymer chain into structurally identifiable quasi-straight segments that allow the molecule to adopt cylindrical conformations. Indee d, highly ordered, cylindrical conformations may be a critical factor in di ctating the extraordinary photophysical properties of conjugated polymers, including highly efficient intramolecular energy transfer and significant l ocal optical anisotropy in thin films.