Scheme 1. Synthesis of Fused Tetracyclics
Figure 2. Crystal packing for 2a,c and 3. Views parallel (top)
and perpendicular (bottom) to molecular planes. Arrows indicate
lateral orientation of fluorinated “heads”.
procedures (Scheme 1). At 110 °C, the Stille coupling gave
poor conversions even after 3 days. When conducted at 150
°C in a sealed vessel, good yields were realized within 12
h. Proper conditions6 for the cyclizations such as those that
follow depend on the substitution pattern: Mallory-type
photocyclization7 was effective for 2, while chemical oxida-
tion with FeCl3 resulted in extensive side reactions including
polymerization.
The synthesis of 3 via benzyne intermediates has been
reported with minimal characterization.8 We prepared a
tetrafluoro-o-terphenyl analogously to compounds 1, but it
resisted fusion by either of the methods described above.
Target 3 could be prepared, however, in one pot via double-
nucleophilic attack of 2,2′-dilithiobiphenyl on hexafluo-
robenzene. Little product is observed by GC-MS after 12
h in ether, but starting material is consumed rapidly after
addition of DME.9
Crystalline packing of compounds 2a,c and 3 is shown in
Figure 2. A typical herringbone-like motif is observed for
2c, similar to that of nonfluorinated triphenylene.10 However,
both 2a and 3 form face-to-face columnar stacks with disk
planes orthogonal to the stacking axis. This arrangement
could prove crucial to performance as organic semiconduc-
tors.11 Compound 3 is significantly distorted from planarity,
as are perhalogenated triphenylenes;12 however, this does not
alter the packing motif.
While C6H6:C6F6 cocrystals consist of alternating stacks
with essentially eclipsed rings, other Ar:ArF cocrystals
display variable lateral offsets between successive disks,
correlating to MEP’s or quadrupole moments.13 Despite
differing electronic structures, 2a and 3 stack with nearly
identical offsets (Figure 2, bottom) with the fluorinated rings
bisected by C-C bonds of successive molecules. Their
π-overlap is much increased compared to 2c.
Compound 2b is our first attempt at a thermotropic liquid
crystal exploiting this design. At room temperature, the alkyl
chains of 2b interdigitate the stacks, limiting π-overlap
between every second and third disk to just one peripheral
bond. Within dimer pairs, face-to-face stacking is similar to
2a, but with lateral offsets approaching that of ABAB/
hexagonal crystalline graphite (see Supp Info).
Future studies will involve (opto)electronic characterization
of these materials and various substituted derivatives as well
as analogues with more fused rings.
Acknowledgment. We thank the University of Kentucky
Research Foundation for funding this research.
Supporting Information Available: Experimental details
and spectroscopic and crystallographic data (CIF). This
material is available free of charge via the Internet at
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