Semiconducting Nanotubes by Intrachain Folding Following
Macroscopic Assembly of a Naphthalene–Diimide (NDI) Appended
Polyurethane
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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 (<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 <sup>1</sup>H 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 CHCl<sub>3</sub> 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