Synthesis and Luminescence of Cyclometalated Compounds with Nitrile and Isocyanide Ligands

Mononuclear cationic cyclometalated palladium complexes [Pd(C boolean AND N)(NCMe)(2)]ClO4 [C boolean AND N = benzoquinolinate (bzq) 1, 2-phenylpyridinate (ppy) 2], analogous to the previously described platinum complexes [Pt(C boolean AND N)(NCMe)(2)]ClO4 [C boolean AND N = bzq 3, ppy 4], and the isocyanide platinum benzoquinolinate [Pt(bzq)(CNR)(2)]X (R = tert-butyl (t-Bu, 5), 2,6-dimethylphenyl (Xyl, 6), 2-naphthyl (2-Np, 7); X = ClO4- a, PF6- b) have been prepared and characterized. The solid-state structures of the cation [Pt(C boolean AND N)(CN-Xyl)(2)](+) with different counteranions (6a and 6b) were found to be different in terms of packing, although in both cases they were dominated by;r-g intermolecular interactions. The influence of the counteranion in the UV-vis spectra, both in solution and in the solid state of 5-7, is negligible. Time-dependent density-functional theory calculations on cation [Pt(C boolean AND N)(CN-Xyl)(2)](+) (6(+)) have been performed, suggesting that the lowest absorption is (IL)-I-1 in nature mixed with some (MLCT)-M-1 character. Acetonitrile platinum complexes (3, 4) are photoluminescent at low temperature (77 K) and at room temperature, whereas analogous palladium complexes (1, 2) are emissive only at 77 K (solid-state and glassy acetonitrile). Isocyanide derivatives 5-7 are intensely luminescent in all media. The emissions are assigned to ligand-centered fluorescence, to mixed (LC)-L-3/(MLCT)-M-3 phosphorescence, or to excimeric (or ground-state) (3)pi pi* or (MMLCT)-M-3 [(sigma*(M)->pi*(C boolean AND N)] transitions depending on the medium and the excitation wavelength. The effect of the counteranion in governing the degree of aggregation and the extent of the interactions seem to be relatively important, especially in a rigid medium, with the smaller ClO4- inducing a more excimeric character. The tendency to form pi-pi excimers and/or Pt center dot center dot center dot Pt oligomerization follows the order CN-2-Np > CN-t-Bu > CN-Xyl and ClO4- > PF6-.