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

Abstract

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(OR^F)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(OR^F)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.