Tunable Dynamic Excimer Formation in Bisphenalenyl Derivatives through Molecular Packing

Dynamic excimer formation in solution-phase π-conjugated systems presents a promising route toward tunable photophysical properties, yet precise control over these transient species remains limited. Herein, a series of bisphenalenyl derivatives is shown to exhibit excimer emission that is modulated through strategic tailoring of side chains (ethylphenyl, n-butylphenyl, and n-hexyl). Two phenyl-substituted derivatives exhibit reversible, concentration-dependent excimer emission consistent with excited-state dimerization. In contrast, an aliphatically substituted bisphenalenyl moiety displays exclusively monomeric emission. Steady-state and time-resolved spectroscopy, time-dependent density functional theory, and diffusion-ordered NMR spectroscopy are employed to confirm that excimer formation arises due to excited-state encounters, with no evidence of ground-state aggregation in acetonitrile. However, diffusion-ordered NMR spectroscopy data reveal dimer formation in tetrachloroethane. Notably, the introduction of substoichiometric molar ratios of HBF₄ induces excimer emission at even lower concentrations of the bisphenalenyl moiety, demonstrating a route to stimulus-responsive control. These results provide a structure-environment framework for modulating dynamic excimer formation in charged π-systems and inform the rational design of responsive fluorescent materials.