Rotary biomolecular machines rely on highly symmetric supramolecular structures with rotating units that operate within a densely packed frame of reference, stator, embedded within relatively rigid membranes. The most notable examples are the enzyme FoF1 ATP synthase and the bacterial flagellum, which undergo rotation in steps determined by the symmetries of their rotators and rotating units. Speculating that a precise control of rotational dynamics in rigid environments will be essential for the development of artificial molecular machines, we analyzed the relation between rotational symmetry order and equilibrium rotational dynamics in a set of crystalline molecular gyroscopes with rotators having axial symmetry that ranges from two-to fivefold. The site exchange frequency for these molecules in their closely related crystals at ambient temperature varies by several orders of magnitude, up to ca. 4.46 × 10 8 s −1 .solid-state NMR | line shape analysis | spin-lattice relaxation | molecular design | crystal engineering

A series of 10 (2-11) new chiral (salicylaldiminato)tin Schiff base complexes obtained from the Schiff base condensation of 4-(diethylamino)salicylaldehyde and various amino acids, in the presence of diphenyltin oxide, is reported together with the parent achiral (1) derivative. Compounds 2-7 crystallize in the P2 1 2 1 2 1 orthorhombic space group, and their quadratic nonlinear optical properties are investigated. At the molecular level, the derivatives possess similar electronic spectra (λ max ≈ 395 nm), and hence molecular hyperpolarizabilities (β), in relation to the same π-conjugated "push-pull" electronic structure. In the solid state, they exhibit efficiencies in second-harmonic generation (SHG) up to 8 times that of urea. The SHG intensities of chromophores 2-6 appear largely correlated with a simple "degree of chirality" (d χ ) parameter, defined from the molecular geometries. The intriguing issue of a possible quantification of chirality by means of SHG measurement is addressed.

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