Conformation-Dependent Photostability among and within Single Conjugated Polymers

Abstract

The relationship between photostability and conformation of 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) conjugated polymers was studied via excitation polarization modulation depth (<i>M</i>) measurements. Upon partial photobleaching, <i>M</i> distributions of collapsed, highly ordered MEH-PPV molecules shifted toward lower values. Conversely, <i>M</i> distributions of MEH-PPV molecules with random coil conformations moved toward higher values after partial photobleaching. Monte Carlo simulations of randomly distributed dipole moments along polymer chains subjected to partial photobleaching revealed that a statistical effect leads to an increase in peak <i>M</i> value. Decreases in <i>M</i> values seen experimentally in the population of MEH-PPV molecules with high <i>M</i> values, however, are due to conformation-dependent photostability within single MEH-PPV polymers. We show that, while folded MEH-PPV molecules are relatively more photostable than extended MEH-PPV molecules in an ensemble, extended portions of particular molecules are more photostable than folded domains within single MEH-PPV molecules