Molecular Self-Assemblies in Co-Crystals: Engineering Chemical Reactivity and Organic Semiconductors - Molecular Self-Assembly: Advances and Applications - page 233

7.5 Stacking of Organic Semiconductors via

Finite Assemblies in Co-Crystals

Although CCFs based on res were employed to direct chemical
reactivity, subsequent work by our group has revealed the ability
of res to assemble organic semiconductors in the solid state in
positionsappropriateforefficientmovementofcharge.Organic
semiconductors are of much interest owing to a promise to make
availablelow-costflexibleelectronics(e.g.,RFIDtags,displays,
electronicpaper)[27].Avarietyoforganicoligomersandpolymers
have been studied as semiconductor materials. A prerequisite for a
molecule to function as an organic semiconductor is the presence
ofanextendedπ-conjugatedsurface.Moreover,computationaland
structure studies have shown that face-to-face, or co-facial, stacking
of semiconductor molecules can lead to higher mobilities owing
to increased orbital-orbital overlap of the neighboring molecules
[8].Theπ-surfacesofmostcommonorganicsemiconductors(i.e.,
pentacene, oligothiophenes), however, tend to crystallize in an edge-
to-face, or herringbone, structure motif [10]. Edge-to-face packing is
not an ideal geometry to achieve maximum performance of organic
semiconductor materials owing to poor orbital overlap. It is, thus,
of great value to establish the control of

π

-orbital overlap within

semiconductor solids.
In particular, we have described how face-to-face stacking of
semiconductor molecules can be achieved using a CCF in two-
componentco-crystals[4].Similartotemplatedreactivity,themethod
involves co-crystallizing a semiconductor with a res-based CCF that
enforces face-to-face stacking via hydrogen bonds. We employed a res
to enforce co-facial stacking of acenes and thiophenes functionalized
withpyridylethynylgroups.Morespecifically,weused5-iodo-res
(5-I-res)and5-methyl-res(5-Me-res)toenforce

π π

stacking of
ananthraceneandthiopheneinco-crystalsof2(5-I-res)∙2(9,10-
bis(4-pyridylethynyl)anthracene) and 2(5-Me-res)∙2(2,5-bis(4-
pyridylethynyl)thiophene),respectively(Fig.7.5).Theaceneand
thiophene were forced into face-to-face stacked geometries with
separation distances of 3.44 and 3.56 Å, respectively. The hydrogen-
bonded complexes self-assembled into face-to-face arrangements,
whichenabledtheformationofextendedπ-stackedstructures.
Efortsarenowfocusedtoexpandthecomponentsoftheco-crystal

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