ABSORPTION-SPECTRA AND LUMINESCENCE PROPERTIES OF A SERIES OF PYRIDINE CARBOXALDEHYDE PHENYLHYDRAZONE LIGANDS AND THEIR PALLADIUM(II) COMPLEXES

A series of pyridine carboxaldehyde phenylhydrazone ligands and their palladium(II) complexes were synthesized and their absorption and emission properties were investigated. A relatively intense absorption band which is present in all compounds in the region 300 - 420 nm is assigned to a π-π* transition with partial charge transfer (intraligand charge transfer (ILCT)) from the iminio moiety to the phenyl ring. All the compounds are luminescent in acetonitrile solution at room temperature and in butyronitrile rigid matrix at 77 K. The emitting level is the lowest-lying singlet ILCT excited state. The values of the emission quantum yields in acetonitrile solution are in the range 10-4 - 10-5; they depend on the excitation wavelength due to competition between radiative decay and Z → E photoisomerization; higher luminescence quantum yields are found at lower excitation energies, where the photoreaction quantum yield is lower. The geometry of the ligand and the presence of an electron-withdrawing substituent on the (acceptor) phenyl ring strongly affect the absorption and photophysical properties. © 1990.