The binuclear [NC-Ru(bpy)2-NC-Cr(CN)5]2- and trinuclear [(CN)5Cr-CN-Ru(bpy)2-NC-Cr(CN)5]4- bimetallic complexes have been synthesized and their photophysical behavior has been studied. Visible light absorption by the Ru(bpy)22+ chromophore leads to phosphorescence from the Cr(CN)63- luminophore. The results demonstrate the occurrence of a fast (τ < 10 ns), efficient (η = 1) intramolecular exchange energy transfer process from the MLCT triplet of the Ru(II) fragment to the doublet state of the Cr(CN)63- fragment. Distinctive features of these chromophore-luminophore complexes with respect to the behavior of the isolated luminophore are as follows: (i) large light-harvesting efficiency (antenna effect); (ii) response to visible light (spectral sensitization); (iii) 100% efficient population of the emitting state; (iv) photostability. The excited-state absorption (ESA) spectrum of both bimetallic complexes exhibits a peculiar visible band not shown by free Cr(CN)63-. This band corresponds to intervalence-transfer transitions from Ru(II) to excited Cr(III). Contrary to the behavior of free Cr(CN)63-, the bimetallic complexes also undergo a distinct bimolecular doublet-doublet annihilation process (rate constants k of the order of 107-108M-1 s-1). The mechanism is thought to involve oxidation of Ru(II) and reduction of Cr(III). Intramolecular processes of the same type are probably responsible for the failure to observe doubly excited species upon two-photon excitation of the trinuclear complex.
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Bignozzi et al. (1989) studied this question.