Chromophore-luminophore complexes

A number of Ru(II)-Cr(IIl) polynuclear complexes, most of which can be considered as chromophore-luminophore complexes, have been designed, synthesized, and studied [69,72,86-89]. Some examples are discussed in the following sections. 

The example discussed in this section is the trinuclear complex [72] 

The sharply different properties of the two types of unit make the detection of energy transfer particularly easy. Upon excitation of the trinuclear complex with visible light (which is only absorbed by the 

The photophysical pathway leading from chromophore absorption to luminophore emission is a-e. Processes f and g correspond to the intervalence transfer transitions detected in flash photolysis. 

The behavior of this Ru(II)-Cr(III) chromophore-Iuminophore complex (as well as of some related ones [69]) provides an example of how the properties of a luminophore can be improved by attachment to a suitable chromophoric component (spectral sensitization, antenna effect, avoidance of possible quartet photoprocesses). When very specific light absorption and light emission characteristics are required (e.g., in the design of luminescent labels for biochemical applications), the use of chromophore-Iuminophore systems may represent a convenient strategy. In particular this strategy permits separate optimization of absorption and emission properties, a possibility which is precluded in simple molecular species. 

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Ru(bpy)2 - chromophoric component) the following observations are made (i) the MLCT emission characteristic of the chromophoric unit is completely quenched (ii) the MC phosphorescence characteristic of the Cr(cyclam)(CN)2 luminophoric units is obtained with high efficiency. This demonstrates the occurrence of very efficient chromophore -> luminophore energy transfer. The behavior of the chromophore-luminophore complex is schematized on the energy level diagram of Fig. 16 [72]. 

As seen in Sections 4.1 and 4.2, a Ru(II) -> Cr(III) intervalence state is present in all the chromophore-luminophore systems. On the other hand, the study of the photophysics originating from this state is precluded by the presence of more intense, overlapping MLCT absorption bands of the chromophore. A system designed to allow selective intervalence transfer excitation is the trinuclear complex shown in Fig. 17 [88]. In this system, the Ru(II) center has similar oxidation potential as in the previously studied cases (and thus similar intervalence transfer transitions are expected), but the Ru(II)-based component is not chromophoric as it lacks of the polypyridine ligands responsible for MLCT absorption. Actually, a single prominent band, of intervalence transfer type, is present at 338 nm (aqueous... 

Taking the imaginative, though somewhat futuristic approach out-lined in Section 1.4, some of the polynuclear complexes discussed in this article can be viewed as very simple photochemical molecular devices. Examples are the Ru(II)-Cr(III) chromophore-luminophore systems of Section 4, which perform the function of spectral sensitization (Fig 7a). The coupling of the systems for photoinduced electron transfer and charge shift described in Section 3 could lead to triads for photoinduced charge separation (Fig 7b). The trichromophoric systems described in section 5.2 can be viewed as very simple examples of the antenna effect (Fig. 7a), while the longer chain-like systems of Section 5.2 could be considered as "molecular optical fibers" suitable for remote photosensitization (Fig. 7a) and other related functions. The system described in Section 5.3, on the other hand, couples antenna effect and photoinduced electron transfer into an antenna-sensitizer function. 

A number of cyano-bridged complexes are included here even though they strictly do not fall in the general family-type defined for the section. The syntheses and photophysical properties of [(NC)(bpy)2Ru(/r-NC)Cr(CN)5] and [(NC)5Cr(Ai-CI Ru(bpy)2(M-NC)Cr(CN)5] have been described. Absorption of visible light by the Ru(bpy)2 unit results in phosphorescence from the Cr(CN)g luminophore, and the results evidence fast intramolecular exchange energy transfer from the MLCT state of the Ru(bpy)2 chromophore to the doublet state of the Cr -based unit. Time-resolved resonance Raman and transient UV-vis absorption spectroscopies have been employed to investigate the MLCT excited states of [(NC)(bpy)2Ru(//-CN)Ru (bpy)2(CN)], [(NC)(bpy)2Ru(//-CN)Ru(phen)2(CN)]+, [(NC)(phen)2Ru(//-CN)Ru (bpy)2(CN)]+, [(NC)(bpy)2... 

The spectroscopic and redox properties of Ru°(bpy)3 have allowed this metal complex to be used for combined optical and electrochemical sensing of anions without the need for additional chromophores or luminophores. 

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