Semiconducting Nanotubes by Intrachain Folding Following Macroscopic Assembly of a Naphthalene–Diimide (NDI) Appended Polyurethane

Abstract

This article describes a well-designed supramolecular assembly of a classical polyurethane scaffold containing pendant n-type semiconducting naphthalene–diimide (NDI) chromophores and consequences on excited state dynamics and charge carrier mobilities. A polycondensation reaction between hexamethylene–diisocyanate and a NDI-containing diol in the presence of a chiral “mono-functional impurity” produced the desired polymer (P1) with a predictable degree of polymerization and end-capping by chiral units. In aliphatic hydrocarbons, such as methylcyclohexane (MCH), P1 adopts a folded conformation with appreciably high thermal stability by intrachain H-bonding among the urethane groups as established by solvent, concentration and temperature-dependent FT-IR and 1H NMR spectroscopy and small angle XRD studies. Folded structure can be further ascertained by the pronounced Cotton effect in MCH owing to the chiral induction by the so-called “sergeant and soldiers” principle from the asymmetric units located only at the chain ends. Intrachain folding facilitates spatial organization of the pendant groups leading to π–π interaction among the neighboring NDI chromophores attached to the same polymer chain resulting in intense green emission in MCH in sharp contrast to the blue-emitting unfolded polymer in benign solvents such as CHCl3 or THF. P1 in the folded state resembles the organization of classical bolaamphiphile and thus adopts a polymersome-like spherical structure. Upon aging macroscopic gelation can be observed owing to the fusion of these discrete spherical assemblies generating micrometer long multiwall nanotubes as noticed in HRTEM, AFM and fluorescence microscopy images. Transient absorption spectroscopy studies indicate formation of NDI radical anions in the excited state both in unfolded and folded conformation which contribute to their intrinsic electron transporting (n-type) property, as revealed by flash-photolysis time-resolved microwave conductivity (FP-TRMC). Significantly larger electron mobility and longer lifetime of charge carriers were observed for the folded tubular assembly than those for unfolded polymer, likely due to a better delocalization of the charge-carriers in the integrated tubular assembly consisting of stacked NDI arrays inside the multilayer wall.