Interplay of Light Antenna and Excitation “Energy Reservoir” Effects in a Bichromophoric System Based on Ruthenium−Polypyridine and Pyrene Units Linked by a Long and Flexible Poly(ethylene glycol) Chain

Steady-state and time-resolved spectroscopic properties of bichromophoric species containing [Ru(bpy)3]2+ and pyrene (pyr) units linked together by flexible poly(ethylene glycol) chains of variable length, [Ru(bpy)2(bpy-pyr)](PF6)2 (1) and [Ru(bpy)2(bpy-O6-pyr)](PF6)2 (2), have been investigated in acetonitrile solvent. The complexes were designed with the aim of examining the intercomponent energy-transfer processes taking place after light absorption at the two chromophores and the influence of the distance separation between them; in the case of complex 2, the linking chain in the extended conformation is as long as 21 Å. Direct excitation of the pyrene unit (λexc = 410 nm) results in singlet-to-singlet energy transfer (an antenna effect) to the Ru-based component, 1pyr → 1MLCT, which we analyze in terms of the Förster mechanism taking place with unit efficiency. Analysis of the time-resolved pyrene fluorescence reveals that the actual center-to-center distance separation (dcc) between the photoactive centers changes according to a Gaussian distribution, with an average dcc = 13.6 Å (distribution width, a = 2.8 Å) and 12 Å (a = 10.2 Å), for 1 and 2, respectively; this is ascribed to folding of the poly(ethylene glycol) linking chain. In O2-free solvent at room temperature, after population of the 1MLCT level (which takes place either because of direct excitation by using λexc > 355 nm or via the “antenna” effect) and subsequent intersystem crossing localized at the Ru center, 1MLCT → 3MLCT, a triplet−triplet thermal equilibration is established which involves the physically separated centers, 3MLCT ↔ 3pyr, with Keq = 11 (the energy gap between the two levels is 480 cm-1, as determined from luminescence data obtained at 77 K). As a consequence of this equilibrium, the 3MLCT luminescence lifetime becomes τRu 9 μs both in 1 and 2, i.e., 1 order of magnitude longer than for the unsubstituted [Ru(bpy)3]2+ luminophore. In air-equilibrated solvent, diffusional quenching by O2 effectively depletes the 3pyr level and only the forward 3MLCT → 3pyr energy transfer step is observed with ken = 4 × 108 and 2 × 108 s-1 for 1 and 2, respectively. As briefly discussed, reasons for the high rate constants observed for the various triplet−triplet steps may be traced back to the folding properties of the linking chains