Photoluminescence of d-transition metal enolates

The transition d-metal enolate complexes generally exhibit no luminescence, and as a consequence there are few studies reported in the literature about the photoluminescence properties of these compounds. Indeed, the d-metal -diketonate complexes are 

FIGURE 13. Typical energy level diagram of the main excited states of a d-metal complex. Reproduced with permission from Reference 192, Copyright 1997 Elsevier 

Kunkely and Vogler reported on the excited state properties of [Cu(hfa)bta], with the absorption at 324 nm assigned to a spin-allowed Cu+ Jt bta MLCT transition. The solid state complex shows a broad emission band at 601 nm arising from MLCT. Besides, the photochemical behavior of this complex is in accordance with MLCT instead of LMCT. 

Recently Zhao and coworkers investigated a series of new Ir complexes based on quinoline derivatives and different S-diketonate ligands. It has been observed that the PL efficiency is improved on replacing acac with pmip. This behavior has also been observed for the electrophosphorescence properties. The explanation is that the T state of pmip ca 23000 cm ) is higher than the one for acac ca 22200 cm ). Consequently, for Ir complexes containing the pmip ligand the emission is dominated by MLCT d r(Ir)- 7r (CAN), where (CaN) represents the quinoline derivatives. 

Rho and Ha ° synthesized a series of Ir complexes with a variety of imidazole-based ligands (L = dpi, tmspi). The [Ir(acac)(L)2] complexes do not exhibit emission. DFT calculations show that the LUMO energy levels of dpi and tmspi are similar to that of acac in these complexes, leading to an ET process involving the dpi and tmspi ligands and landing on a non-radiative channel that quenches the emission. 

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