Controlling Optical Properties and Interchain Interactions in Semiconducting Polymers by Encapsulation in Periodic Nanoporous Silicas with Different Pore Sizes

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Abstract
The photophysics of MEH-PPV incorporated into the pores of periodic silica hosts has been investigated in an effort to understand the role played by interchain aggregation and chain morphology in polaron production. In this work, guest/host interactions were used to incorporate MEH-PPV into the straight, homogeneous pores of hexagonal surfactant- or polymer-templated mesoporous silicas of varying pore diameters. Polarized photoluminescence and photoluminescence excitation spectroscopy were then used to investigate the polymers' environment within the silica pores. Experiments exploiting luminescence peak shifts and depolarization indicate that depending on the pore size and preparation conditions, the alignment and packing of the polymer chains within the pores could be controlled. Samples could be produced with isolated chains, interacting straight chains, and coiled interacting chains. The sub-bandgap absorption by polarons was then measured with photoinduced absorption as a function of pore size. Small-diameter pores that allowed single polymer chains to reside within the pore showed little evidence of interchain contact and had a low polaron yield. Increasing the number of polymer chains within the pore increased the polaron yield. Finally, when the pores were large enough that the chains could coil, strong polaron absorption was observed, indicative of a further increase in polaron yield or an increase in polaron lifetime. The polaron absorption spectra also sharpen and red shift with increasing pore diameter, suggesting that excitons may migrate to lower energy polymer segments in samples where polymer chains are both coiled and interacting.