Tuning Molecular Assembly and Optical Properties via Cocrystallization: A Case Study of Roseolumiflavin in Binary and Ternary Multicomponent Systems

Extending our previous work on cocrystallization of roseolumiflavin ( R ), the rational design approach with various N -heterocyclic coformers has been explored to systematically tune photophysical properties of the chromophore. A series of binary and, for the first time, ternary flavin cocrystals was systematically synthesized and analyzed with particular focus on the modification of spectroscopic behavior. Structural analyses based on obtained single crystal data reveal distinct molecular packing patterns, with binary cocrystals maintaining either face-to-face or slipped stacking motifs, while ternary systems demonstrate acid-mediated protonation effects and modified π-stacking arrangements. The original hydrogen-bonded R dimer is replaced by a donor–acceptor–donor (DAD) motif and, in the case of the ternary systems, further stabilized through hydrogen bonds with acid molecules. The structural modifications directly correlate with the photophysical behavior, as all cocrystals exhibit blue-shifted fluorescence relative to R . The most pronounced influence on the spectral behavior occurs in ternary cocrystals with the emission band shifts of up to 158 nm. This work expands on to this date scarcely available knowledge of flavin cocrystallization and highlights the role of hydrogen bonding, stacking interactions and coformer selection in fine-tuning its optical properties.