Supramolecular assembly of multicomponent photoactive systems via cooperatively coupled equilibria

Abstract

Here, we show that the synergistic interplay between two binding equilibria, acting at different sites of a (Zn)phthalocyanine-amidine molecule (Pc1), enables the dissociation of the photoinactive phthalocyanine dimer (Pc1)2 into a three-component system, in which a sequence of light harvesting, charge separation, and charge shift is successfully proven. The aforementioned dimer is assembled by dual amidine-Zn(II) coordination between neighboring Pc1 molecules and gives rise to high association constants (KD ≈ 1011 M–1). Such extraordinary stability hampers the individual binding of either carboxylic acid ligands through the amidine group or pyridine-type ligands through the Zn(II) metal atom to (Pc1)2. However, the combined addition of both ligands, which cooperatively bind to different sites of Pc1 through distinct noncovalent interactions, efficiently shifts the overall equilibrium toward a photoactive tricomponent species. In particular, when a fullerene-carboxylic acid (C60A) and either a dimethylamino-pyridine (DMAP) or a phenothiazine-pyridine ligand (PTZP) are simultaneously present, the photoactivity is turned on and evidence is given for an electron transfer from photoexcited Pc1 to the electron-accepting C60A that affords the DMAP-Pc1•+-C60A•– or PTZP-Pc1•+-C60A•– radical ion pair states. Only in the latter case does a cascade of photoinduced electron transfer processes afford the PTZP•+-Pc1-C60A•– radical ion pair state. The latter is formed via a thermodynamically driven charge shift evolving from PTZP-Pc1•+-C60A•– and exhibits lifetimes that are notably longer than those of DMAP-Pc1•+-C60A•–.