Polymorphism and Optoelectronic Properties in Crystalline Supramolecular Polymers

Supramolecular
polymers can emulate some of the physical properties
of covalent polymers but offer new opportunities given the possibility
of designing monomers that will form highly ordered assemblies with
defined shapes. Internally ordered supramolecular polymers formed
through nucleation–elongation self-assembly are well-known
but highly crystalline examples which exhibit important properties
such as light harvesting, charge transport, and ferroelectricity are
not common. We report here on a detailed study of supramolecular polymers
formed in water by carboxylated naphtho-p-quinodimethane
amphiphiles. We found that supramolecular polymerization of these
amphiphiles in aqueous media yields crystalline assemblies with morphologies
that included ribbons, helically rolled ribbons, and twisted filaments.
This polymorphism was found to be controlled exclusively by repulsive
electrostatic interactions controlled by the degree of protonation
of the carboxylic head groups which also dictates the nature of supramolecular
packing. Substoichiometric amounts of base lead to highly crystalline
ribbons due to a decreased surface charge density and less electrostatic
repulsion. Increasing deprotonation results in helically rolled ribbons
with a different polymorph crystal lattice, whereas excessive deprotonation
leads to twisted filaments with maximum surface charge density. Ribbons,
helical rolled ribbons, and twisted filaments revealed an increasing
red shift in their visible absorption maxima. These crystalline assemblies
could be potential candidates for solar energy materials and photocatalytic
systems.