N–H···N Hydrogen-Bonded Helices to Halogen···Halogen and Cation···π Interactions in Aprotic, Monoprotic, and Biprotic Halogen-Substituted Lophines

Abstract

2,4,5-triphenyl-1H-imidazole (TPI), also known as lophine, has been widely studied for its luminescence properties. With both hydrogen bond donor (N–H) and acceptor (N) sites, lophine molecules self-assemble to form 1D helices via N–H···N hydrogen bonding. Herein, crystal engineering studies of halogen-substituted 2,4,5-triphenyl-1H-imidazole (TPI) derivatives are explored to understand their supramolecular self-assembly and photophysical properties. Three classes of TPI derivatives are synthesized and studied: monoprotic (1–4), N-methylated aprotic (1M–3M), and biprotic salts with nitrate anions (1H–3H). Monoprotic derivatives form helices through N–H···N hydrogen bonding, while aprotic derivatives exhibit corrugated layers stabilized by halogen···π and halogen···halogen interactions. Biprotic salts form cation···π stacks that assemble into two-dimensional layers via hydrogen bonding with nitrate ions. Notably, compounds within and across classes demonstrate isostructural features and distinct intermolecular interactions that are influenced by halogen substitutions. Photophysical studies reveal absorption bands in the 243–352 nm range in solid state and 292–313 nm in solution, with fluorescence emissions spanning 377–416 and 450–470 nm upon excitation at 300 and 340 nm, respectively. These findings highlight the role of noncovalent interactions in directing crystal packing and their impact on the photophysical properties of TPI derivatives.