Mechanically Coupled Molecular Rotors Built with Bis(triazolo)benzene Scaffolds

Abstract

In this Thesis are described conformationally well-defined synthetic systems named as triple-deckers, which are designed to mimic and better understand the compact three-dimensional (3-D) structures of biological origin. Key structural features of these systems include (i) 1,8-substituted naphthalene motif as a turn motif to minimize the number of rotatable bonds, and (ii) bis(triazolo)benzene and aryl pendant groups to maximize the intramolecular donor–acceptor (D–A) type π–π stacking. A combination of 1D and 2D (COSY and NOESY) 1H NMR, and X-ray crystallographic studies established the presence of strong non-covalent interactions that are reinforced by a parallel arrangement of aromatic π-stacks. Dynamic NMR studies by variable-temperature (VT) measurements on a series of double- and triple-decker π-stacks provided detailed mechanistic understanding of the molecular motions that can be controlled by electronic and steric factors to control the energetics of site exchange. We also found that the light-emitting properties of triple-decker molecules vary significantly as a function of the aryl pendant group, and this property can best be explained by through-space charge-transfer (CT). Moreover, structural folding through π-π stacking responds sensitively to the solvent polarity, and temperature-dependent changes in fluorescence efficiency become more pronounced for systems that undergo large structural changes between folded (= stacked) and unfolded (= unstacked) conformations. From these studies emerges a coherent mechanistic model that relate the structure, dynamics, and photophysical properties of spontaneously folding synthetic molecules.