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For applications as diverse as molecular electronics and nonlinear optics, organic molecular systems have long been vaunted as ideal candidates due to their low cost and the tailorability of their physical properties through well explored organic chemistry. Indeed, examples of porphyrins and systematically structured oligomers show how the properties of these systems can be finely tuned leading to simple structure-property relationships. In most cases, however, these finely tuned properties are lost in the solid state, and even at moderate concentrations in solution, as illustrated for the liquid crystal forming materials, hexabenzocoronenes. Not only are structure property relationships lost, but the optical properties, in particular fluorescence efficiency, are greatly suppressed. In polymers, which are structurally less defined, such aggregration effects can be both interchain and intrachain and have been seen to depend on the backbone isomerism. Raman spectroscopy has been shown to be a valuable probe of the purity of the backbone isomerisation and thus of the inhibition of interchain interaction. The use of nanospacers has been also explored for the local inhibition of intermolecular interaction. In particular, carbon nanotubes are shown to selectively interact with organic polymers in order to reduce the polymer-polymer interaction and so enhance fluorescence in the solid state. Minimisation of the interaction between molecules in order to preserve the as designed properties is a further challenge to molecular science. In fullerenes in the solid state, however, it appears that interaction between excited state species enhances both the electronic and optical properties of the material. The insulating, weakly optically active crystalline material becomes highly fluorescent and highly conducting under high intensity illumination. The origin of the nonlinear process is proposed to be an optically induced insulator to metal transition as a result of the increased interaction of neighbouring excited states. It is proposed that while in many molecular systems intermolecular can be detrimental, they do potentially offer a further design parameter to the molecular scientist.
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