Comparative Supramolecular Chemistry of Coronene and Hexahelicene: Helix Alignment in Crystalline Complexes with Trimesic Acid (=Benzene-1,3,5-tricarboxylic Acid) andπ-Acceptor Compounds

Abstract

The crystallization of the molecular complex (TMA)2⋅COR is reported, and evidence is produced that its structure is built up of planar hexagonal, H-bonded TMA honeycomb sheets, the large pores of which accommodate the almost perfectly complementary COR molecules by means of favorable straight C−H⋅⋅⋅O contacts. The detailed crystal structure of (TMA)2⋅COR could, however, not be unravelled by X-ray means, probably due to sheet stacking disorder. On the other hand, the crystals of the corresponding (racemic) complex (TMA)2⋅HEL, engineered by exploiting the geometric similarities of HEL and COR, are fully ordered and have an analogous structure, although the HEL-filled TMA honeycomb sheets are puckered for improved `helix following' via C−H⋅⋅⋅O contacts. The absence of stacking disorder is ascribed to the non-planarity of the HEL molecules protruding out of the mean plane of the TMA sheets. Further remarkable structural features of (TMA)2⋅HEL comprise: substantial helix flattening of HEL, perfect helix alignment (parallel helix axes of HEL molecules), TMA sheets with homochiral HEL guests, opposite chirality of HEL molecules in neighboring sheets, striking non-natural example of a mutually induced-fit molecular recognition process. In the ordered crystals of the racemic π donor-acceptor complexes HEL⋅TNB (two crystal forms), HEL⋅DNBA, HEL⋅TCNQ, and (NIPA)2⋅HEL, the molecular helices of the HEL molecules are also well aligned, and, except for HEL⋅TNB, arranged in homochiral sheets. In these HEL sheets, some helix flattening is again observed, as well as head-to-tail contacts among the HEL molecules via protrusion into their molecular clefts. The crystal structure of (NIPA)2⋅HEL is particularly amazing and may be characterized as an elegant triply interpenetrating, distorted diamond architecture involving H-bonded NIPA zig-zag chains, which embrace the HEL molecules by means of C−H⋅⋅⋅O contacts with almost perfect `helix following'. Simultaneously, good π donor-acceptor contacts between HEL and NIPA molecules are enabled, upon which the diamondoid model may also be based. Alternatively, the solid-state structure of (NIPA)2⋅HEL can also be viewed as a system of stacked, triply concatenated `molecular fences' of the crossed lath-and-hinge type, with the laths representing the NIPA zig-zag chains and the HEL molecules providing the hinges. Both models account well for the alignment and chirality characteristics of the HEL sheets in (NIPA)2⋅HEL. The crystal structure of the adduct PHEN⋅NIPA is also presented, which has architectural elements in common with (NIPA)2⋅HEL, (TMA)2⋅HEL, and (TMA)2⋅COR. Apparently, PHEN⋅NIPA owes its stability in substantial measure to favorable C−H⋅⋅⋅O contacts (`C−H⋅⋅⋅O assisted π donor-acceptor complex'). Furthermore, the crystal architecture of PHEN⋅NIPA involves remarkably close intermolecular H⋅⋅⋅H contacts between neighboring PHEN molecules, which may be estimated as short as 1.86 A, i.e., more than 0.5 A smaller than the van der Waals diameter of H. Finally, the crystal structures of the complexes COR⋅TNB and COR⋅DNBA are briefly reported and compared to the analogous HEL adducts. The COR molecules are twofold orientationally disordered in these molecular compounds, which involve the usual donor-acceptor π stacks, but with only limited alignment of the COR molecular disks. The complex (NIPA)2⋅COR could be crystallized, but resisted X-ray structure solution efforts, probably again due to substantial disorder phenomena. The corresponding pairs of COR and HEL molecular complexes studied generally agree in their stoichiometries, but differ conspicuously as regards their crystal-structural order, which is significantly perturbed if the perfectly circular, disk-shaped COR molecules are involved. The results are evaluated as regards the following prospects: recovery of COR from PAH mixtures, scanning-probe microscopic applications, extreme chiroptic and non-linear optical (NLO) properties of helix-aligned HEL molecular complexes, extension to crystalline adducts of other (higher) helicenes. Emphasis is laid on the stimulation of future attempts to engineer crystalline helix-aligned molecular complexes of optically active helicenes, which may be expected to bring forth particularly intriguing, useful, and extreme material properties.