Ligand Control of Supramolecular Chloride Photorelease

Supramolecular assembly is shown to provide control over excited-state chloride release. Two dicationic chromophores were designed with a ligand that recognizes halide ions in CH₂Cl₂ and a luminescent excited state whose dipole was directed toward, 1²⁺, or away, 2²⁺, from an associated chloride ion. The dipole orientation had little influence on the ground-state equilibrium constant, K_eq ∼ 4 × 10⁶ M^-1, but induced a profound change in the excited-state equilibrium. Light excitation of [1²⁺,Cl^-]⁺ resulted in time-dependent shifts in the photoluminescence spectra with the appearance of biexponential kinetics consistent with the photorelease of Cl^-. Remarkably, the excited-state equilibrium constant was lowered by a factor of 20 and resulted in nearly 45% dissociation of chloride. In contrast, light excitation of [2²⁺,Cl^-]⁺ revealed a 45-fold increase in the excited-state equilibrium constant. The data show that rational design and supramolecular assembly enables the detection and photorelease of chloride ions with the potential for future applications in biology and chemistry.