Fluorescence Spectroscopic Studies of Solvent- and Temperature-Induced Conformational Transition in Segmented Poly[2-methoxy-5-(2'-ethylhexyl)oxy-1,4-phenylenevinylene] (MEHPPV)

Segmented poly[2-methoxy-5-(2'-ethylhexyl)oxy-1,4-phenylenevinylene] (MEHPPV-x, where x is the mole percent of conjugated segments) represent unique polymeric systems in which a large number of chromophores of different molecular conjugation lengths (different excitation/emission energies) are strung together in a single polymer chain, which thereby forces them to occupy a relatively small volume that is determined by the hydrodynamic size of the macromolecule. Changing the hydrodynamic volume by varying either solvent composition or temperature, therefore, provides a straightforward approach to modulate the interaction between the chromophores. Fluorescence spectroscopic studies of very dilute solutions (in dichloromethane/1,2-dichloroethane) of segmented MEHPPVs with increasing amounts of a nonsolvent (methanol/cyclohexane/ethanol) reveal an interesting inverted S-shaped variation in the emission yield with increasing nonsolvent composition, which is accompanied by a red shift of the emission maxima. Both these changes are indications of a conformational collapse of individual polymer chains, resulting in enhanced energy transfer from highly emissive short conjugation length segments to weakly emissive longer ones and/or to the formation of weakly emissive interchromophore excitons. The solvent composition at the onset of the steep decline in the S-shaped curve, termed the chain-collapse point, was found to vary with the extent of conjugation in a manner which suggests that the polymers bearing longer chromophores undergo collapse earlier than those bearing shorter ones. In samples with very low levels of conjugation, such as in MEHPPV-10, a rather unexpected observation of an initial increase in the emission yield prior to the sudden decline was seen. This is consistent with the unusual conjugation length dependence of the fluorescence quantum yield of oligoPPVs (OPV) reported by earlier workers. Fluorescence spectral variation as a function of temperature also reveals several interesting features of these systems, the most interesting of which is the observation that either an increase or a decrease of emission yields with increase in temperature could be observed, depending on the initial conformation of the polymer chain. When the chain is in a highly collapsed (or a highly solvated) state, an increase in temperature causes the expected decrease in emission yield, while in an intermediate partially collapsed conformation, a significant increase in the emission yield with temperature is observed. The latter observation is ascribed to polymer coil expansion that results in depletion of aggregated species and/or a reduction in energy transfer to weakly emitting chromophoric segments