Dual-Emissive Bis(diphenylphosphino)amine Platinum Complexes: Structural, Reactivity, Photophysical, and Theoretical Investigations

Abstract

By the reactions of bis(diphenylphosphino)amine (dppa) with equimolar amounts of cycloplatinated(II) complexes 1 with two neutral (SMe2 or DMSO) and anionic (Cl- or CF3CO2-) labile ligands the bis-chelate complexes 2 with a chelating mode of dppa were synthesized that have one of the smallest bite angles in a diphosphine. As the usual strategy for synthesizing the binuclear complexes, the reactions of 0.5 equiv of dppa with cycloplatinated(II) complexes 1 with a Cl- ligand were successful in making symmetrical binuclear complexes. Also, the chelating dppa has a strong angle strain that makes it a susceptible precursor for biphosphine ring opening. Furthermore, this new strategy was successful for designing symmetrical and unsymmetrical binuclear complexes with Cl- ligands. Interestingly, all efforts to make binuclear complexes with bridging dppa from precursors that contain CF3CO2- were unsuccessful by both strategies. The effects of dimerization via a change in the coordination mode of dppa, from a chelate mode in complexes 2 with green emission to a bridge mode in complexes 3, with dual-phosphorescence red emission were investigated in the powder and poly(methyl methacrylate) (PMMA) phases at 77 and 298 K. The emission decay curves of all complexes in the solid state show long and short lifetime values as characteristics of the triplet excited states. The reaction pathway, reactivity, and structural aspects of the complexes were compared with those of other similar dppm analogues and the mechanism of conversion via possible intermediates was considered by density functional theory (DFT) calculations. For interpretation of the emission spectra, time-dependent DFT (TD-DFT) calculations were carried out on the isolated and stacked forms.