Photoinduced Electron Transfer in Supramolecular Assemblies Composed of Dialkoxybenzene-Tethered Ruthenium(II) Trisbipyridine and Bipyridinium Salts

Ruthenium(II) tris{4, 4ʹ-bis[(methyleneoxy)tris(ethyleneoxy)(4-methoxybenzene)]-2, 2ʹ-bipyridine} (1), forms supramolecular complexes with N, N'-dimethyl-4, 4ʹ-bipyridinium, MV²⁺ (2), and with cyclo [bis(N, N'-p-xylylene-4, 4ʹ-bipyridinium)], BXV⁴⁺ (3). The association constants of the complexes generated between the dialkoxybenzene-tethered units of 1 and the electron acceptors 2 and 3 correspond to K_a = 28 ± 1 M^-1 for MV²⁺ and K_a = 1200 ± 100 M^-1 for BXV⁴⁺. Time-resolved laser flash photolysis and steady-state emission studies show that electron transfer from excited 1 proceeds by an intramolecular pathway to the electron acceptors organized in the supramolecular assemblies, k_iq = 1.7 X 10⁸ s^-1 for MV²⁺ and k_iq = 2.9 X 10⁸ s^-1 for BXV⁴⁺, and by a diffusional route from free-uncomplexed 1, k_dq = 1.9 X 10⁸ M^-1 s^-1 for MV²⁺ and k_dq = 3.4 X 10⁸ M^-1 s^-1 for BXV⁴⁺. Formation of the supramolecular assemblies between 1 and 2 or 3 is supported by the intramolecular electron-transfer quenching of 1 and by the fact that the assemblies dissociate upon addition of β-cyclodextrin.