Supramolecular materials: Molecular packing of tetranitrotetrapropoxycalix[4]arene in highly stable films with second-order nonlinear optical properties

Highly stable films of tetranitrotetrapropoxycalix[4]arene (9) with second-order nonlinear optical (NLO) properties and a noncentrosymmetric structure were obtained by a novel crystallization process at 130 - 140°C in a dc electric field. The packing of 9 in these films was elucidated by a combination of X-ray diffraction, angle-dependent second-harmonic generation, and scanning force microscopy (SFM). The experimental results agree well with solid-state molecular dynamics calculations for these films. No crystalline phase was observed for nitrocalix[4]arene derivatives with longer or branched alkyl chains; this explains the limited NLO stability of films of these calixarenes. Scanning force microscopy on the aligned films of 9 showed two distinct surface lattice structures: a rectangular lattice (a = 9.3, b = 11.7 Å) and a pseudohexagonal lattice (d ≃ 11.4 Å). The combination of these data with the interlayer distance of 8.9 Å (X-ray diffraction) allowed the packing of molecules of 9 in these structures to be fully elucidated at the molecular level.