Luminescence properties of cyclopalladated complexes with Schiff base ligands

Direct reaction between the hydroxo-complexes [{Pd(μ-OH)(C^N)}2] (C^N = 2-(2-pyridyl)phenyl (Phpy) I; C^N = 7,8-benzoquinolyl (Bzq) II) and N-naphtylsalycilaldimine (N-naphsal) 1 yields new mononuclear cyclometallated palladium(II) complexes [Pd(N-naphsal)(C^N)] (I1, II1). Photophysical properties were investigated together with those of complexes with related ligands N-phenylsalycilaldiminate (N-Phsal) 2, N-p-chlorophenylsalycilaldiminate (N-pClPhsal) 3. All the compounds absorb intensely below 300 nm via 1LC transitions located in Bzq or Phpy ligands, and display additional low energy absorptions of mixed 1MLCT–1LC character. The complexes under study are quite unusual in terms of luminescence behavior, since some of them are emissive in solution at room temperature and all display intense emissions in frozen CHCl3 solution, but also in solid state at 298 and 77 K. Structural characterization by X-ray diffraction of complexes I2, I3 and II2 confirmed the proposed formulae.