Joining a host-guest platform and a light-emission motif: Pyrazinamide-calixarene hybrids

Abstract

Calixarenes are macrocyclic polyphenols widely used as hosts in supramolecular chemistry approaches. Recently, aminopyrazine has emerged as a high-efficiency blue light emitting moiety. Inspired by this knowledge, here we prepared two pyrazinamide-calixarene hybrids, in order to join in a single molecular platform both host-guest and light-emission properties. Both compounds were decorated with pyrazinamide at the upper rim, on two phenyl rings alternated into the four-units of the calixarene macrocycle. Their difference was the t‑butyl substitution on the other two alternate phenyl rings. The t‑butyl substituted derivative crystallized in two crystal forms, a non-solvated and a DMF mono-solvate phase, both with just one crystallographically independent hybrid molecule, while the compound without t‑butyl moieties crystallized with three hybrid conformers and four DMF molecules. The molecular substitution pattern has also impacted on the crown conformation, being related to more inclined pyrazinamide-phenyl average planes in the absence of the t‑butyl substituents. The coplanarity pattern between pyrazinamide and its bonded phenyl ring and the conformational profile of this substituent changed among the three conformers of derivative without the t‑butyl group and between those assembling the two crystals forms of the other compound, which was also found in our DFT calculations. Under an excitation wavelength of 388 nm, a broad band emission between 400 nm and 550 nm was found for the three crystal forms, with maxima at ca. 500 nm, with low quantum yields of up to 0.4%. TD-DFT absorption spectra were overlaid to the experimental excitation spectra, while the low efficiency was rationalized both as an effect of the DMF guest and the tail-to-tail π…π stacking interactions between coplanar pyrazinamide-phenyl moieties of the centrosymmetric related molecules.