On the nature of the photochemical reaction of polypyridyl Ru(ii) complexes leading to sunlight-to-chemical energy conversion: density functional analysis

Abstract

We investigated the nature of the visible-light-driven photochemical reactions of the Ru(II) polypyridyl complexes [Ru(II)(L)(bpy)₂](PF₆)_n (bpy = 2,2′-bipyridine; L = phbn, n = 1; L = pad, n = 1; L = pbn, n = 2) with Et₃N as a sacrificial reductant on the basis of density functional calculations. It is estimated that the reduction of the photo-excited species [Ru*(II)(L)(bpy)₂]ⁿ⁺ to [Ru(II)(L⁻) (bpy)₂]⁽ⁿ⁻¹⁾⁺ is moderately endothermic, the protonation of the latter to [Ru(II)(LH)(bpy)₂]ⁿ⁺ substantially exothermic, and the bimolecular hydrogen-transfer interaction of the latter to [Ru(II)(LHH)(bpy)₂]ⁿ⁺ and [Ru(II)(L)(bpy)₂]ⁿ⁺ weakly exothermic. The amount of visible-light energy that the Ru(II) polypyridyl complexes 1–3 can store by forming one N–H and one C–H bond is of the order of ∼20–30 kcal mol⁻¹. The visible-light absorption of [Ru(II)(L)(bpy)₂]ⁿ⁺ forming the photo-excited species [Ru*(II)(L)(bpy)₂]ⁿ⁺ induces electron loss in the ligand L, showing that the character of the ligand-to-metal charge transfer is stronger than that of the metal-to-ligand charge transfer.