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an extended network, properties of a solid with a structure based
onafiniteassemblycanbeexpectedtostemfromthelocalized
arrangement of the molecules within the assembly rather than as a
resultoflong-rangepacking.Althoughitcanbedifficulttoidentify
moleculargeometriesandintermolecularforcesthatafordfinite
versus extended assemblies in both the solid state and solution,
advancesinthefieldsofmolecularrecognitionandself-assembly
[3], when coupled with an increasing understanding of structural
consequences of intermolecular forces in solids [2], can provide
fertilegroundstoexplorehowfiniteassembliesofmoleculescanbe
designedandinfluencethepropertiesofsolid-statematerials.
It is with these ideas in mind that we describe here our work that
wehaveperformedinourlaboratorythatemploysfinitemolecular
assemblies to control chemical reactivity [4] and charge mobility
in organic solids [5]. We show how two-component solids in the
form of co-crystals [6] can be used to direct packing of molecular
substratestoachieveprerequisitegeometriesthatafordanability
todirectintermolecular[2+2]photodimerizationsofolefins[7]and
achieve face-to-face stacking of organic semiconductor molecules
[8].Wewillbeginbyaddressinggeneralmotivationstodesignfinite
molecular assemblies in the solid state and then outline our work
tocontrolreactivityandafectchargemobility.Structuralcriteriato
achieve the targeted properties will be described and pathways to
more complex assemblies will be addressed.
7.2
Finite Molecular Assemblies and Co-Crystals
Inaminimalistcase,afiniteassemblyofmoleculeswillconsistoftwo
identical(i.e.,homodimer)ordiferent(i.e.,heterodimer)molecules
that interact via noncovalent forces [3]. The interactions propagate in
aconvergentfashiontogiveadiscreteaggregateofmolecules(Scheme
7.1).Amajorimpetustodesignandconstructafiniteassemblyisto
create a function not realized by the individual components [3]. In
the liquid phase, a discrete assembly will be in equilibrium with its
parts, as well as possible undesired complexes [3a]. The presence of
suchmultipleequilibriawillhaveanefectofreducingthestructural
integrity of the assembly and may require stronger intermolecular
forces to hold the components together. It has been suggested that
the sensitivity of multiple equilibria to subtle environmental factors
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