Host-guest chemistry using an oriented mesoporous host: Alignment and isolation of a semiconducting polymer in the nanopores of are ordered silica matrix

Oriented polymer/silica composites have been synthesized by incorporating the conjugated semiconducting polymer MEH-PPV into the pores of an aligned, hexagonally ordered mesoporous silica. The ordered silica framework is synthesized by a silica/surfactant coorganization process that proceeds via a silicate/surfactant liquid crystalline intermediate. Magnetic fields are used to orient this liquid crystalline intermediate, followed by chemical cross-linking of the silica framework and removal of the surfactant. Hexagonally ordered (p6mm) aligned silica samples are treated with organic chlorosilanes to optimize interactions between the polymer and the silica surface. Polymers are incorporated from solution; thermal cycling is used to drive the polymers into the pores. The resultant polymer incorporation is monitored by polarized photoluminescence spectroscopy. Polymer chains which are oriented in the aligned nanoporous silica show strong polarization anisotropy in their photoluminescence (I-VV/I-VH = 4.4, I-HH/I-HV = 0.68 for vertically oriented pores). Spectroscopic results are compared to the results of a geometric model for transition dipole orientations and used to conclude that as much as 0.8 of the incorporated polymer is isolated within the nanopores of the silica matrix, while as little as 0.2 is located in the macroporous regions formed between grains of the silica host. Intentional oxidation can be used to degrade the polymer not isolated within the porous silica. After oxidation, essentially all of the photoactive polymer appears to be contained within the oriented silica matrix (I-VV/I-VH = 5.7, I-HH/I-HV = 0.71). The results prove that a semiconducting polymer can be isolated within the pores of an ordered silica host and that this isolation can be used to control the optical properties of the guest molecules (in this case the polarization). Further, the ability to simply characterize the degree of polymer incorporation using optical techniques allows us to learn about the effects of processing variables such as the role of surface chemistry, pore size, and thermal cycling on the final degree of polymer incorporation.