A Pyrene-Embedded Molecular Nanotube with Donor-Acceptor Properties

Shape-persistent pyrene-embedded macrocycles remain scarce, and tubular conjugated molecules with intrinsic donor-acceptor (D-A) character have only recently emerged, largely within [n]-cyclo-para-phenylene-based systems. Here we introduce a structurally distinct pyrene-containing nanotube (Pyr ₄ ), synthesized through a templated approach using an n-pentyl resorcin[4]-arene, in which pyrene units and quinoxaline walls form an integrated D-A framework. Upon light excitation, Pyr ₄ displays behavior consistent with internal charge-transfer (ICT) between the pyrene donor (D) and the quinoxaline acceptor (A). The analogue with tolyl groups replacing pyrene (Tol ₄ ) shows markedly weaker D-A behavior. This assignment is supported by electrochemical studies, frontier molecular orbital analysis, and fluorescence solvatochromism, where emission shifts bathochromically by roughly 100 nm from 468 nm in hexane to 564 nm in DMSO. The fluorescence quantum yield (ϕ _f) falls with increasing solvent polarity, consistent with a stabilized charge-transfer state. In DCM, Pyr ₄ emits at 528 nm (ϕ _f = 0.2), red-shifted relative to previously reported pyrene-containing conjugated macrocycles.