Photochemistry and Photophysics of Coordination Compounds

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224 M.T. Indelli et al. The absorption spectrum of 10 (Fig. 2) shows LC transitions of the bpy and ppy ligands in the 240–310 nm range. In addition, a prominent band is present at 366 nm, which is attributed to MLCT transitions [63, 64]. The presence of MLCT transitions at relatively low energy, a feature completely absent in analogous polypyridine complexes, is the result of the strongly electron donating character of the cyclometalating ligand, which makes the formally Rh(III) center relatively easy to oxidize (irreversible wave observed at ca. + 1.1 V vs. NHE) [63]. The complex, which is only very weakly emissive at room temperature, exhibits an intense, long-lived (τ, 170 µs), structured emission at 77 K (Fig. 2). The emission has been attributed to ppy-based LC phosphorescence, although various degrees of mixing between LC and with MLCT triplet states have been invoked on the basis of absence of dual emissions [64], lifetime considerations [63], and high-resolution spectroscopy [65, 66]. In fact, because of the strong covalency of the carbon–metal bonds, the HOMO in this class of complexes has a mixed character, being delocalized on the central metal and on the cyclometalating ligand. This has been clearly shown by a number of recent TD/DFT calculations performed on Rh(III) [67] and related Ir(III) [68, 69] cyclometalated complexes. Therefore, a classifica- Fig. 2 Absorption spectrum (dashed line) and emission spectrum (room-temperature, continuous line; 77 K, dotted line) of Rh(ppy)2(bpy) + in 4/1 EtOH/MeOH (adapted from [64])
Photochemistry and Photophysics of Coordination Compounds: Rhodium 225 tion of the excited states in classical, localized terms (MC, LC, MLCT) is not generally applicable for cyclometalated complexes. The general behavior of other Rh(N^C)2(N^N) + complexesissimilar to that described above for Rh(ppy)2(bpy) + , with some easily understandable specific differences. For instance, the main effect of substituting thpy (8) for ppy is an enhancement of emission intensity and lifetime in roomtemperature solutions [63]. This is likely a result of the lower energy of the emission and the consequent lower efficiency of the thermally activated decay [70] via higher MC states. The remarkably long-lived (4.4–6.6 µs) emission observed in fluid solution for an analog of 10 with aldehyde substituents on the phenyl ring of the cyclometalating ligand [71] is probably justified by the same type of argument. On the other hand, the photophysical behavior of 10 is only slightly affected by substitution of bpy with similar N^N ligands, such as, e.g., 1,10-phenanthroline, 2,2 ′ -biquinoline [72], or 4-amino-3,5-bis(2-pyridyl)-4H-1,2,4-triazole) (11) [73]. However, when strong π-deficient ligands such as 1,4,5,8-tetraazaphenanthrene (TAP, 12) or 1,4,5,8,9,12-hexaazatriphenylene (HAT, 13) are used as N^N ligand [74], a definite switch in behavior is observed, with the presence of broad structureless 77 K emissions that have a clear charge transfer character. Indeed, for this type of complexes, TD/DFT calculations show that the HOMO involves the metal and the cyclometalating ligand-carbon bonds, but the LUMO is now exclusively localized on the non-cyclometalating ligand (Fig. 3) [67]. Thus, in this case the emission is best considered as having a mixed LLCT/MLCT character.
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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Topics in Current Chemistry Also Av
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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