Abstract
Square planar platinum(ii) complexes have been known for 150 years and pincer complexes, supported by a tridentate chelating ligand such as terpyridyl, have been known for more than 70 years. The development of cyclometallated platinum(ii) pincer complexes, in which the tridentate ligand forms one or more platinum-carbon bonds, has been much more recent. Particularly, in terms of their solution and solid-state luminescence these cyclometallated complexes show substantial advantages over their terpyridyl analogues. This tutorial review introduces the reader to the area of platinum(ii) cyclometallated pincer chemistry and shows the advantage of having an alkynyl group in the fourth coordination site on the metal. The basic design principles for the preparation of highly luminescent platinum(ii) cyclometallated pincer complexes are outlined and the strategy to improve the luminescence further by chemical manipulation of the pincer ligand and of the auxiliary ligand in the fourth coordination site are illustrated with recent examples from the literature. Recent applications of these cyclometallated pincer complexes in the area of opto-electronics is described, with emphasis on their use in OLEDs, OFETs and as NLO materials as well as demonstrating their potential use as triplet photosensitizers and as metal ion sensors. The aim of this review is to show the recent advances in this rapidly developing research field and to highlight the future promise of these materials.
Highlights
General featuresRigidity and planarity of a Pt(II) square planar complex can promote high luminescence levels by reducing the non-radiative decay pathways caused by structural distortions, and the rigid tridentate pincer ligands are effective at achieving this
Introduction a Department ofChemistry, College of Science, University of Hail, Kingdom of Saudi Arabia b Department of Applied Biology & Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P
The aim of this review is to show the recent advances in this rapidly developing research field and to highlight the future promise of these materials
Summary
Rigidity and planarity of a Pt(II) square planar complex can promote high luminescence levels by reducing the non-radiative decay pathways caused by structural distortions, and the rigid tridentate pincer ligands are effective at achieving this. Cyclometallated Pt(II) complexes bearing N^C^N ligands produce stronger luminescence (with higher F and t) than the isoelectronic tridentate (C^N^N and C^N^C) and related bidentate ligands.[27] As discussed previously, the relatively high performance is due to the shortening of Pt–C (central anionic phenyl ring of the coordinating ligand) bond To counterbalance this the Pt–C (alkynyl ligand) bond is elongated compared to its counterparts.[28] The shortening of the Pt–C bond raises the energy of the d–d state and decreases the knr values.[29] Yersin et al.[30] proved that the Huang–Rhys S-parameter, which quantifies the level of molecular distortion in the excited state compared to the ground state, for a tridentate Pt(II) complex is half (S E 0.1) that of a bidentate complex (S E 0.2) (Fig. 4). The presence of metallophilic interaction led to the formation of 1D chain structure with alternating PdÁ Á ÁPd contacts and pÁ Á Áp interactions
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