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Controlled Orientation of Polyconjugated Guest Molecules in Tunable Host Cavities

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posted on 20.10.2010, 00:00 by Airon C. Soegiarto, Angiolina Comotti, Michael D. Ward
Linear conjugated guest molecules with high aspect ratios form inclusion compounds with guanidinium organodisulfonate (GDS) host frameworks in which organodisulfonate “pillars” connect opposing GS sheets to generate lamellar architectures that reflect templating by the guest. Through judicious selection of pillars having adjustable lengths (lS−S, as measured by the separation between distal sulfur atoms) and guests of various lengths (lg), the framework architecture can be controlled systematically in a manner that enables regulation of the guest orientation and aggregation in the host framework. Inclusion compounds for which lg/lS−S ≤ 0.9 exhibit a bilayer architecture with 1-D channels containing guests oriented parallel to the long axis of the pillar. Guests with values of lg comparable to lS−S, however, promote the formation of a brick architecture in which the guests and the pillar are arranged in a herringbone motif. Surprisingly, longer guests (lg = 1.25lS−S) favor the formation of the bilayer architecture despite their larger volume because the guests are forced to align end-to-end as single-file arrays due to the vertical constraints of the 1-D channels. Bithiophene and biphenyl guests (lg < lS−S) are exceptional, promoting bilayer structures in which turnstile rotations of the pillars afford an unusual motif in which the guests are isolated from one another. The ability to synthesize a large family of compounds based on a common supramolecular building block (the GS sheet) permits construction of a structural “phase diagram” based on two simple molecular parameters, lg and lS−S, that can be used to sort the inclusion compounds according to their framework architectures and enable prediction of crystal structures for new host−guest combinations. The effects of these different framework architectures and packing motifs is manifested as bathochromic shifts in the absorption and emission spectra of the guests compared with their spectra in methanol solutions. This behavior is supported by ab initio TDDFT calculations that reproduce the bathochromic shifts associated with the effects of guest−guest and guest−host interactions, combined with conformational constraints imposed on the guest molecules by the rigid host framework.

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