Cooperative Supramolecular Block Copolymerization for the Synthesis of Functional Axial Organic Heterostructures

Supramolecular
block copolymerzation with optically or electronically
complementary monomers provides an attractive bottom-up approach for
the non-covalent synthesis of nascent axial organic heterostructures,
which promises to deliver useful applications in energy conversion,
optoelectronics, and catalysis. However, the synthesis of supramolecular
block copolymers (BCPs) constitutes a significant challenge due to
the exchange dynamics of non-covalently bound monomers and hence requires
fine microstructure control. Furthermore, temporal stability of the
segmented microstructure is a prerequisite to explore the applications
of functional supramolecular BCPs. Herein, we report the cooperative
supramolecular block copolymerization of fluorescent monomers in solution
under thermodynamic control for the synthesis of axial organic heterostructures
with light-harvesting properties. The fluorescent nature of the core-substituted
naphthalene diimide (cNDI) monomers enables a detailed spectroscopic
probing during the supramolecular block copolymerization process to
unravel a nucleation–growth mechanism, similar to that of chain
copolymerization for covalent block copolymers. Structured illumination
microscopy (SIM) imaging of BCP chains characterizes the segmented
microstructure and also allows size distribution analysis to reveal
the narrow polydispersity (polydispersity index (PDI) ≈ 1.1)
for the individual block segments. Spectrally resolved fluorescence
microscopy on single block copolymerized organic heterostructures
shows energy migration and light-harvesting across the interfaces
of linearly connected segments. Molecular dynamics and metadynamics
simulations provide useful mechanistic insights into the free energy
of interaction between the monomers as well as into monomer exchange
mechanisms and dynamics, which have a crucial impact on determining
the copolymer microstructure. Our comprehensive spectroscopic, microscopic,
and computational analyses provide an unambiguous structural, dynamic,
and functional characterization of the supramolecular BCPs. The strategy
presented here is expected to pave the way for the synthesis of multi-component
organic heterostructures for various functions.