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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

Morales, Angeles Farrán, Accorsi, Gianluca, Armaroli, Nicola, Barigelletti, Francesco, Pope, Simon J. A. and Ward, Michael D. 2002. 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. Inorganic Chemistry 41 (25) , pp. 6711-6719. 10.1021/ic025811d

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Abstract

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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Publisher: American Chemical Society
ISSN: 0020-1669
Last Modified: 04 Jun 2017 04:37
URI: http://orca.cf.ac.uk/id/eprint/41648

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