Spectroscopic study of photophysical change in collapsed coils of conjugated polymers: Effects of solvent and temperature

A relationship between conformation and photophysics of poly [2-methoxy-5-(2'-ethylhexoxy)-p-phenylenevinylene] (MEH-PPV) in dilute solution was investigated by utilizing UV/vis absorption, excitation, and emission spectroscopy. By tuning polymer-solvent interactions, a control of conjugation length which relates to state of chain collapse is achieved. Position of absorption and emission spectra can be systematically moved within 60 nm by using a series of alcohols and aromatic solvents as well as mixed solvents. In addition to a decrease of conjugation length, the collapsed chain exhibit optical characteristics different from that of the extended counterpart. While a single type of emitter is present in the extended chain, separate emission from multiple emitters with various conjugation lengths is detected from the collapsed coils. Fluorescence decay measurements support the presence of multiple emitters. Studies of UV/vis absorption and photoluminescence upon increasing temperature detect a blue shift of transition energies and a decrease of absorption and emission efficiency. In addition, rate of the shift is found to decrease with increasing magnitude of chain collapse.