Bambusuril Small-Molecule Ionic Isolation Lattices for Exciton Coupled Dimers and Dicationic Fluorophores

Abstract

Ionic self-assembly of molecular fluorophores is a promising avenue to program properties into optical materials. Controllable optical energies, lifetimes, exciton hopping and energy transfer have been engineered in small-molecule, ionic isolation lattices (SMILES) by using preferred binding stoichiometries that pair singly charged complexes (−1) with singly charged fluorophores (+1) to isolate them on lattice sites. We hypothesize that these rules can be generalized if charge balance can be expanded from one anion and one cation to pair of each ion, i.e., from 1:1 to 2:2. If true, opportunities emerge to instead bring two cations together as discrete isolated dimers potentially turning on new optical states. We tested these ideas by exchanging cyanostar receptors for bambusuril receptors known to host two chloride anions. When charge matched with two monocationic cyanines in a 2:2 ratio, the crystal structure reveals π-stacked dimers form into isolated emitters that pack charge-by-charge with (bambusuril)·(chloride)₂ complexes. New red-shifted emission at 615 nm from cyanine–cyanine dimers and DFT calculations agree with Kasha’s theory of exciton coupling as offset H-dimers. The bambusuril can also host single anions leading to isolation of single cyanines with monomer emission at 507 nm. In addition, we also discovered a SMILES material using dicationic fluorophores (+2) in the form of styryldipyrylium and a new packing stoichiometry with bambusuril. Stoichiometry, anion identity and charge define crystal engineering rules of bambusuril SMILES. Thus, we add valence control to the rules for isolation of discrete dye dimers and inclusion of discrete dicationic dyes in emissive SMILES materials.