Synthesis of organoplatinum oligomers by employing N-donor bridges with predesigned geometry: Structural and photophysical properties of luminescent cyclometalated platinum(II) macrocycles

Abstract

A series of luminescent di- and trimeric cyclometalated platinum(II) macrocycles, namely, [Pt(C-N)(N-N)]2 (N-N = pyrazolate, pz (2); 7-azaindolate, aza (3); C-N = 2-(2′-thienyl)-pyridyl, Thpy (a); 7,8-benzoquinolate, Bzqn (b); 2-phenylpyridyl, Phpy (c)) and [Pt(C-N)-(N-N)]3 (N-N = benzimidazolate, bzim (4); C-N = Thpy (a) and Bzqn (b)), are synthesized in high yields (70-90%). The rigid, predefined coordination geometry of the pz, aza, and bzim bridging ligands ensure the efficient and selective assembly of the respective cyclic oligomers. The stacking arrangements in the crystal structures of 1a and 4a reveal π-π interactions between 2-(2′-thienyl)pyridyl moieties. The lowest energy absorption band in the UV-vis spectra is assigned to metal-to-ligand charge transfer (MLCT) transitions. The 298 K fluid emissions of complexes 2-4a bearing Thpy ligands are assigned to transitions with mixed MLCT and IL (intraligand) character. In crystalline form, the observed red-shift is attributed to excimeric emission arising from π-stacking in the solid state. In contrast, a number of Bzqn derivatives are nonemissive in CH2Cl2 at 298 K. The energy of the lowest energy UV-vis and emissive bands for Thpy oligomers are red-shifted relative to the Bzqn and Phpy congeners. Significantly, Thpy complexes and especially the μ-pyrazolate species 2a (τo = 15.5 μs, Φo = 0.18) emit with longer lifetimes and higher quantum yields.