Electrostatic Bubbles and Supramolecular Assistance of Photosensitization by Carboxylated Ru(II) Complexes

Abstract

The paper examines the supramolecular effects at play during photosensitization by carboxylated Ru^II sensitizers, both by experiment and by modeling. Experimentally, twelve Ru^II complexes of pyrazolylpyridine and polypyridine ligands, including two benchmark complexes and two new species, were assessed as photosensitizers by measurement of the kinetics of methyl viologen cation radical (MV^•+) generation through an oxidative, photoinduced electron transfer (PET) to methyl viologen (MV²⁺) under continuous irradiation in the presence of a sacrificial reductant. All complexes, luminescent or not, produced measurable amounts of MV^•+ in CH₃CN. The assessment protocol was found to be useful with sensitizers of widely varying excited-state lifetimes (τ) as well as being easier and faster than conventional approaches. The seven sensitizers bearing peripheral COOH groups were found to be significantly more active than their non-carboxylated analogues, which is consistent with ionization of the COOH groups and electrostatic promotion of PET. Only the luminescent complexes were active in aqueous solvents, where τ appears to be the dominant effector. The benefits are exemplified by the singly carboxylated [Ru(H1)(bpy)₂]²⁺ (H1 is 1-(4-carboxyphenyl)-3-(2-pyridyl)-4,5,6,7-tetrahydroindazole), a weakly luminescent sensitizer that was less active in aqueous solvents than [Ru(bpy)₃]²⁺ (bpy is 2,2‘-bipyridine), but which became the better sensitizer in CH₃CN. Computationally, electrostatic field and dissociation energy calculations demonstrated that even a single peripheral COO^- substituent suffices to provide supramolecular assistance: it defines a spheric “bubble” of electrostatically attractive space that is sufficiently large to allow the supramolecular preassociation of MV²⁺, which provides an entropic advantage to PET that reduces the importance of τ in organic solvent. Calculations also show that the PET is electrostatically favored over its reverse (BET) even with cationic sensitizers because the “bubble” contracts after PET while the bulk medium becomes more repulsive, and favorable cation exchanges can occur to effect post-PET dissociation. Two peripheral COO^- groups can define a two-point binding site for MV²⁺ in an attractive sector of space that contracts to a kidney-shaped “bubble” after PET. This enables unimolecular PET while the reverse reaction remains bimolecular. The resultant benefits are illustrated with [Ru(Na1)₂(bpy)]²⁺, a very weakly luminescent sensitizer that was totally inactive in H₂O but appreciably active in CH₃CN, despite the need to displace Na⁺ in order to derive any electrostatic benefit. The Marcus free energies of activation for PET and BET corroborate the benefits of carboxylation, solvent, and other factors and correlated with the experimental rate constants.