Structures of Bifunctional Molecules Containing Two Very Different Supramolecular Synthons: Carboxylic Acid and Strong π•••π Stacking 1,8-Naphthalimide Ring

A series of molecules containing a carboxylic acid and a 1,8-naphthalimide group joined by different linkers (HLC1 = CH2; HLC2 = CH2CH2; HLC3 CH2CH2CH2; HLophen = ortho-C6H4; HLC4 = para-C6H4; HLala, = S-CHCH3) have been prepared and structurally characterized. The structures of HLC1, HLC3, and HLala are similar, with alternating hydrogen bonding of the carboxylic acids and pi center dot center dot center dot pi stacking interactions of the naphthalimide groups assembling the molecules into parallel chains that are linked into sheets by a second set of pi center dot center dot center dot pi stacking interactions. Hydrogen bonding and pi center dot center dot center dot pi stacking interactions of the naphthalimide groups also assemble HLC2 into chains, but the chains are alternately oriented at nearly right angles causing the interchain pi center dot center dot center dot pi stacking interaction to organize the chains in an open three-dimensional structure. Three of these open structural units interpenetrate forming a unique three-dimensional network. The rigid ortho-arene linker in HLophen directs the orientation of the pi center dot center dot center dot pi stacking interaction of the naphthalimide rings to be at 60 degrees; when combined with the hydrogen bonding interactions helical one-dimensional chains form that pack into a unique rhombohedral architecture. In the structure of HLC4 center dot DMF, each acid group is hydrogen bonded with the dimethylformamide (DMF) molecule; the structure contains one-dimensional ribbons supported only by the pi center dot center dot center dot pi stacking interactions from the 1,8-naphthalimide groups. All six molecules show fluorescence in the 432-449 nm region. Overall these structural studies show that the 1,8-naphthalimide supramolecular synthon is extremely versatile because it can simultaneously enter into multiple noncovalent interactions.