Effect of cyclopalladation on the spectroscopic properties of a Coumarin dye

The cyclopalladated Pd(C6)Cl]₂ and Pd(C6)En]PF₆ complexes based on Coumarin 6 are studied by ¹H NMR spectroscopy and electronic absorption and emission spectroscopy. It is shown that cyclopalladation leads both to a long-wavelength shift of the spin-allowed intraligand absorption bands of coumarin and to the formation of a characteristic metal-ligand absorption band. It is found that, in comparison with coumarin, the complexes are characterized by a bathochromic shift and by a decrease in the fluorescence quantum yield due to more efficient intersystem crossing to the lower excited triplet state responsible for the low-temperature (77 K) phosphorescence of the complexes.