Heli(aza)cene: A Helical Molecular Tweezer with Tunable Intra‐ and Intermolecular Charge Transfer

Abstract

Non-planar fluorophores offer unique avenues of intra- and intermolecular energy transfer not available in their planar counterparts. We have rationally designed a molecular tweezer based on the pyridine-2,6-dicarboxamide framework having two structurally similar arms with extended π-surface. We termed this molecular tweezer as Heli(aza)cene (HAC) due to its spontaneous adoption of helical conformation stabilized by the amide and imine moieties present in it. In the helical conformation, the two arms of HAC are twisted unequally. This asymmetry confers dissimilar electronic character to the two arms and results in intramolecular charge transfer interactions in HAC. Homochiral stacking of the P- and the M- helices in crystal, and profound redshifting of the emission at higher concentrations of HAC was attributed to intermolecular charge-transfer interactions in aggregated/crystal state. Exposure of HAC, in solution as well as in the solid state, to Lewis/Brønsted acids results in rapid and vibrant color changes. This is the first example of a π-layered helical molecule exhibiting tunable intra-/intermolecular charge-transfer characteristics.

Dynamic Coordination between a Triphenylamine-Functionalized Salicylaldehyde Schiff Base and a Copper(II) Ion

“…The crystal TPA-Py belongs to triclinic space group P1̅ . As shown in Figure 7, the fragment containing the biphenyl amine units as arms was crossed in front of the bridging pyridyl unit, and the threedimensional twist molecules form a helical conformation, 45 similar to our previous report. 29 In addition, two groups form weak intermolecular interactions, namely an O−H…”

A turn-off fluorescent probe for the detection of Cu2+ based on a tetraphenylethylene-functionalized salicylaldehyde Schiff-base

“…As shown in Fig. 1, the structure of the fluorescence dye TPE-An-Py possesses a bridging pyridyl unit that adopts a twisted molecular conformation with a twist angle of 29.81, and the two terminal TPE units which overlap with each other and formed a helical conformation; 28 several weak O-HÁ Á ÁN interactions (O2-H2AÁ Á ÁN4 = 1.882 Å, O3-H3Á Á ÁN5 = 1.858 Å) and N-HÁ Á ÁO interactions (N1-H1Á Á ÁO2 = 2.344 Å, N23-H2Á Á ÁO3 = 2.394 Å) were observed. Moreover, a crown-like ether ring with a diameter of B4.5 Å was formed by the self-assembly of the bridge unit and two TPE moieties (Fig.…”

Macrocyclic enforcement of twist in a secondary amide: reactivity and influence on photoisomerisation

“…The constriction of PDC-based helical scaffolds on lowering temperature due to stronger IHB has been reported previously. 21 We believe that a similar constriction occurs in cDP2, forcing one of the amide groups to twist out of the plane of the macrocycle to a sufficient extent to make it experimentally visible even in solution phase NMR studies.…”

“…PDC-based scaffolds have been previously designed for preorganizing molecules into a helical architecture due to strong IHB. [19][20][21] Further, diazobenzene (DAB) as the electron-withdrawing residue was incorporated into the macrocycle to effect destabilization of the ground state of the amide residue attached to the adjacent carbon. These design features were successfully implemented in the penta-aza macrocycle (cDP2) shown in Fig.…”