Naphthalenediimide (NDI)-Conjugated Foldable Polyurethanes: Impact of Chromophoric Location on Hierarchical Supramolecular Assembly and Conductivity

Abstract

The manuscript reports supramolecular assembly and photophysical properties of two polyurethanes (P1, P2) with appended naphthalene-diimide (NDI) chromophore which serves as a n-type semiconductor. P1 and P2 are constitutional isomers and they differ in the location of the NDI moiety. In P1, NDI is a part of the polymer backbone (placed between the two urethane groups in each repeating unit) while in P2, it remains as a pendant group. In a non-polar solvent like tetra-chloroethylene (TCE), both adopt a pleated structure driven by intra-chain H-bonding among the urethane groups. Subsequently the secondary structure of P2 assembles to a meta-stable spherical structure (similar to polymersome) which after 70–75 h transforms to nanotubes with the folded polymer chains constructing the wall of the tube. In sharp contrast, the intra-chain folded conformation of P1, owing to its enhanced rigidity contributed by the synergistic π-stacking and H-bonding, does not get trapped in the spherical intermediate, but spontaneously grows to fibrils leading to macroscopic gelation within 30 min. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements together with transient absorption spectroscopy studies revealed remarkably high charge carrier mobility (3×10−3 cm2 V−1 s−1) which can be attributed to the long range stacking order of the NDI chromophores in the fibrillar assembly of P1 while for the nanotubular structure of P2, the value was almost an order of magnitude less reflecting importance of the mesoscopic structure on generation of percolated pathway for efficient charge transport.