Synthesis, characterisation and optical properties of symmetrical and unsymmetrical Pt(II) and Pd(II) bis-acetylides. Crystal structure of trans-[Pt(PPh3)2(CC–C6H5)(CC–C6H4NO2)]

Symmetric trans-[Pt(PPh3)2(CCR)2] and unsymmetrical trans-[Pt(PPh3)2Cl(CCR)], (R=C6H4–pNO2, C6H4–pOCH3), Pt(II) acetylides were prepared and characterised, as well as unsymmetrical Pt(II) bis-acetylides, trans-[Pt(PPh3)2(CCR)(CCR′)], (R=C6H4–pNO2, R′=C6H5; R=C6H4–pOCH3, R′=C6H5; R=C6H4–pOCH3, R′=C6H4–pNO2; R=C6H4–pNO2, R′=[(η5-C5H4)Fe(η5-C5H5)]). Also symmetric Pd(II) bis-acetylides, trans-[Pd(PPh3)2(CCR)2] (R=C6H4–pNO2, C6H5) were synthesised and characterised by IR, NMR, and UV–vis spectroscopies. The optical properties of the new complexes were compared within a series of known Pt(II) complexes containing different phosphines and cis–trans configuration. The absorption and photoluminescence spectra indicate that the emission is found in the range 355–600 nm, depending on the nature of the acetylide ligand bound to Pt. Measurements of SHG for the unsymmetrical bis-acetylides dispersed in polymethylmethacrylate (PMMA) show that the second-order nonlinear optical (NLO) properties depend on the strength of the donor–acceptor substituent of the acetylide ligand. Third-order NLO properties (in particular the nonlinear absorption coefficient α2) were measured by the Z-scan technique; strong push–push and pull–pull substituents on the Pt(II) symmetric bis-acetylides induce electronic delocalisation and tuning of the NLO properties. The single crystal X-ray structure of trans-[Pt(PPh3)2(CC–C6H5)(CC–C6H4NO2)] shows that the unsymmetrical molecule crystallised in the Pbca centric space groups.