Prolonging the Excited State Lifetime

Harriman et al. [187] subsequently described a series of photoactive dyads bearing pyrene and metal (M = Ru or Os ) tris(2,2 -bipyridine) terminals bridged by an ethynylene group (Fig. 2.24). For the Os -based dyads, the triplet state re- 

Castellano et al. [188] have described the photophysical properties of ruthe-nium(ii) tris(l,10-phenanthroline) derivatives having either one or three pyrene units attached at the 5-position, complex 50 (Fig. 2.25). These latter systems show phosphorescence only from the MLCT state of the metal complex. The triplet lifetimes are much enhanced relative to the parent complex, with the dyad and triad, respectively, displaying phosphorescence lifetimes of 24 and 148 ps in deoxy-genated acetonitrile at room temperature. At the time, this was the longest lifetime to be assigned to the MLCT triplet state of a ruthenium(II) poly(pyridine) complex. 

The long triplet lifetimes are attributed to reversible triplet energy transfer with the appended pyrene moieties and, using ultrafast transient absorption spectroscopy, the rates of formation of the equilibrium distribution of triplet states were determined [188]. 

Other systems have been constructed that contain more than one pyrene unit attached to the metal fragment for example, see complex 51 in Fig. 2.25 [189]. Thus, the excited triplet state lifetimes of a series of nine Ru-bipy derivatives have been found to increase linearly with the number of appended pyrene residues [190]. 

The lifetimes range from 0.8 to 18.1 ps. A supramolecular structure has been assembled around a central zinc(ii) cation in such a way that the metal complex is held close to a pyrene unit [191]. Here, the triplet lifetime of the metal complex is somewhat enhanced because of reversible triplet energy transfer between the two chromophores. The pyrene unit can be replaced with other chromophores provided the triplet energy levels can be balanced. Thus, Castellano et al. [192] have reported prolonged triplet lifetimes for complexes 52 and 53. 

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