Molecular tectonics.: Porous hydrogen-bonded networks built from derivatives of 2,2′,7,7'-Tetraphenyl-9,9′-spirobi[9H-fluorene]

The cruciform shape of spirobifluorene disfavors close molecular packing, and more complex derivatives with multiple sites of hydrogen bonding are known to associate to form highly porous networks with significant space for the inclusion of guests. In principle, the porosity can be increased by introducing spacers between the spirobifluorene core and the peripheral sites of association. To test this strategy, compounds 2-3 with multiple diaminotriazine groups attached to a tetraphenylspirobifluorene core were synthesized, and their behavior was compared with that of a model (4) lacking the phenyl spacers. As expected, extended spirobifluorenes 2-3 crystallized to produce open networks held together by hydrogen bonding of diaminotriazine groups; however, the porosities of these networks were lower (53% and 44%, respectively) than that of the network built from model 4 (60%). The decreased porosity arises largely because the added phenyl spacers change the relative contributions of hydrogen bonding and aromatic interactions to the overall lattice energy of the crystals. It becomes advantageous to optimize aromatic interactions at the expense of hydrogen bonds, and crystallization therefore favors networks that permit closer molecular packing.