Ligand Control of Supramolecular Chloride Photorelease

Abstract

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 CHCl and a luminescent excited state whose dipole was directed toward, 1, or away, 2, from an associated chloride ion. The dipole orientation had little influence on the ground-state equilibrium constant, K ∼ 4 × 10 M, but induced a profound change in the excited-state equilibrium. Light excitation of [1,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 [2,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.

Ter-Ionic Complex that Forms a Bond Upon Visible Light Absorption

“…Coulombic repulsion will move the iodide in the tmam pocket away from the 3,3′-H atoms of the bipyridine toward the cationic amines. Indeed, prior research has shown full photorelease of chloride anions associated with a ligand where the excited state resides . Coulombic repulsion of the other iodide is expected to be less, as it is not directly associated with the tmam ligand, and may be attracted to the more Lewis acidic Ru III .…”

Halide Photoredox Chemistry

“…The electronic coupling between the ions is expected to be smaller for the solvent-separated ion pairs than for the contact ion pair. Common techniques to investigate the nature of ion pairs, which are encompassed in the broader context of host–guest chemistry, are NMR, ,,,− UV–visible, ,,,,,,, and photoluminescence spectroscopy ,,,,,,, as well as isothermal titration calorimetry (ITC). ,− To quantify ion pair structures without excited-state reactivity, Ward et al utilized chloride as an “innocent” ion, as its large reduction potential precluded excited-state electron transfer . A series of ruthenium complexes, i.e., [Ru­(bpy) 3–x (deeb) x ] 2+ where x = 0, 1, 2 or 3, and chloride was studied by 1 H NMR, UV–vis, and photoluminescence spectroscopy in CH 2 Cl 2 .…”

“…The colored arrow indicates the time dependent spectral shift measured. Reproduced with permission from ref . Copyright 2018 American Chemical Society.…”

“…The demonstration that excited-state localization could induce Coulombic repulsion of an ion-paired halide was clearly evident in a study by Turlington et al that showed chloride photorelease within a supramolecular assembly. 395 Two complexes were synthesized in order to examine the influence of the excited-state dipole on the excited-state equilibrium with the possibility for photorelease of chloride. The two complexes shared a common daea ligand, where daea is 4,4′-(aminoethylamino)ethanolamine-2,2′-bipyridine, that was specifically designed to promote ion-pairing with halides.…”

Recent advances in photorelease complexes for therapeutic applications

“…Follow up work by the same group further studied the halide photorelease concept using RU23 to RU27 with chloride and bromide. 67,68 Fig. 10.…”