Photochemical, photophysical and redox properties of novel fulgimide derivatives with attached 2,2′-bipyridine (bpy) and [M(bpy)3]2+ (M = Ru and Os) moieties

Abstract

Fulgimides monosubstituted with [M(bpy)₃]²⁺ (M = Ru, Os; bpy = 2,2′-bipyridine) chromophore units and with a single bpy group were synthesized and investigated as components of conceivable dinuclear photochromic switches of luminescence. The E-, Z- and closed-ring (C) photoisomer forms of the bpy-bound fulgimide were successfully separated by semi-preparative HPLC. The same procedure failed, however, in the case of the [M(bpy)₃]²⁺-substituted fulgimides. Energy transfer from the excited photochromic unit to the metal-bpy centre competes with the fulgimide cyclization, reducing the photocyclization quantum yields by approximately one order of magnitude compared to the non-complexed fulgimide-bpy ligand (φ_EC = 0.17, φ_EZ = 0.071, φ_ZE = 0.15 at λ_exc = 334 nm). The cycloreversion of the fulgimide-bpy ligand is less efficient (φ_CE = 0.047 at λ_exc = 520 nm). The intensity of the ³MLCT-based luminescence of the metal-bpy chromophore (in MeCN, φ_deaer = 6.6 × 10⁻² and τ_deaer = 1.09 μs for Ru; φ_deaer = 6.7 × 10⁻³ and τ_deaer = 62 ns for Os) is not affected by the fulgimide photoconversion. These results and supporting spectro-electrochemical data reveal that the lowest triplet excited states of the photochromic fulgimide moiety in all its E-, Z- and closed-ring forms lie above the lowest ³MLCT levels of the attached ruthenium and osmium chromophores. The actual components are therefore unlikely to form a triad acting as functional switch of energy transfer from [Ru(bpy)₃]²⁺ to [Os(bpy)₃]²⁺ through the photochromic fulgimide bridge.