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 A, Direct excitation of the pyrene unit (lambda(exc) = 410 nm) results in singlet-to-singlet energy transfer (an antenna effect) to the Ru-based component, (1)pyr --> (MLCT)-M-1, which we analyze in terms of the Forster mechanism taking place with unit efficiency. Analysis of the time-resolved pyrene fluorescence reveals that the actual center-to-center distance separation (d(cc)) between the photoactive centers changes according to a Gaussian distribution, with an average d(cc) = 13.6 Angstrom (distribution width, a = 2.8 Angstrom) and 12 Angstrom (a = 10.2 Angstrom), for 1 and 2, respectively; this is ascribed to folding of the poly(ethylene glycol) linking chain. In O-2-free solvent at room temperature, after population of the (MLCT)-M-1 level (which takes place either because of direct excitation by using lambda(exc) > 355 nm or via the antenna effect) and subsequent intersystem crossing localized at the Ru center, (MLCT)-M-1 --> (MLCT)-M-3, a triplet-triplet thermal equilibration is established which involves the physically separated centers, (MLCT)-M-3 <----> (3)pyr, with K-eq = 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 tau(Ru) similar to 9 mus 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 O-2 effectively depletes the (3)pyr level and only the forward (MLCT)-M-3 --> (3)pyr energy transfer stop is observed with k(en) = 4 x 10(8) and 2 x 10(8) 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.