Ligand and counteranion effects in cyclometalated Pt(II) diphosphine complexes: photophysics, singlet-oxygen generation and photocatalysis.

This work reports the synthesis, characterization and study of the photophysical properties of two families of cyclometalated [Pt(C^N)(P^P)]X [C^N = 2-phenylbenzothiazolate (pbt), 1-phenylisoquinolinate (piq)] complexes. This study establishes structure-property relationships across variations in the C^N ligand (pbt vs. piq), the diphosphine backbone [P^P = 1,2-bis(diphenylphosphino)benzene (dppbz), 2,3-bis(diphenylphosphino)pyrazine (dpppyz), 2,2'-bis(diphenylphosphino)-1,1'-binaphthalene (BINAP) and bis[2-(diphenylphosphino)phenyl]ether (DPEphos), and the counteranion [X = PF6-, NO3-, [Al(ORF)4]-]. Single-crystal X-ray diffraction reveals well-defined trends in coordination geometry, with wide-bite-angle diphosphines inducing pronounced distortions that correlate with weakened emissive behaviour. The photophysical study shows that the nature of the C^N ligand predominantly governs the excited-state energy landscape: pbt complexes emit in the yellow region with high efficiencies, whereas the more conjugated piq derivatives display red-shifted phosphorescence with longer emission lifetimes. In solution, several complexes exhibit dual fluorescence-phosphorescence emission. All complexes (except 5) act as efficient triplet sensitizers. Guided by their increased photophysical stability and higher solubility, two Krossing-type anion [Al(ORF)4]- based salts were selected as catalysts for the photooxidation of p-bromothioanisole under homogeneous conditions. While the pbt derivative shows limited activity, due to catalyst degradation under near-UV irradiation, the piq analogue operates efficiently under mild blue-light excitation (460 nm), achieving 100% conversion with only 1% catalyst loading in 9 hours.