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

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 (<b>P1</b>) with a predictable degree of polymerization and end-capping by chiral units. In aliphatic hydrocarbons, such as methylcyclohexane (MCH), <b>P1</b> 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. <b>P1</b> 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

Donor/Acceptor Segregated π‑Stacking Arrays by Use of Shish-Kebab-Type Polymeric Backbones: Highly Conductive Discotic Blends of Phthalocyaninatopolysiloxanes and Perylenediimides

Construction of large-area electron donor–acceptor (D–A) interfaces and hole/electron pathways is important for photoconducting and photovoltaic functions. Although blends of D- and A-type discotic π-systems have a possibility to realize one-dimensional charge carrier pathways as well as heterointerfaces, D–A segregated structures are difficult to develop by self-assembly because they are entropically unfavored structures. Here we report the use of shish-kebab-type hole-transporting discotic columns fixed by a self-threading polysiloxane chain and approach to such segregated nanostructures. Electron-donor/acceptor blends of soluble phthalocyaninato­polysiloxanes (Poly-SiPcs) and perylenedicarboximides (PDIs) were prepared, and their photoconductive property was investigated. Although <b>Poly-SiPc1</b> shows a photoinduced charge separation with <b>PDI1</b> analogous to the corresponding monomeric phthalocyanines (<b>SiPc1</b> and <b>H</b>_<b>2</b><b>Pc1</b>), the <b>Poly-SiPc1</b>/<b>PDI1</b> system displays a remarkably larger photoconductivity than <b>SiPc1/PDI1</b> and <b>H</b>_<b>2</b><b>Pc1</b>/<b>PDI1</b>, which mostly results from the presence of hole-transporting pathways with the mobility μ_h,1D ∼ 0.1 cm² V^–1 s^–1 in <b>Poly-SiPc1</b> along the polysiloxane covalent bonds even upon mixing with <b>PDI1</b>. When π-stackable <b>PDI2</b> is used instead of <b>PDI1</b>, X-ray diffraction analysis disclosed obvious signs of π-stacking periodicities for both Pc and PDI planes in the mixture, indicating the presence of donor–acceptor segregated domains of columnar structures. As a result, photoexcitation of <b>Poly-SiPc1</b>/<b>PDI2</b> generates highly mobile holes and electrons, leading to the observation of a much larger conductivity