Synthesis, characterization, and photophysical and supramolecular properties of shape-persistent luminescent binuclear platinum(II) complexes

Abstract

Light-emitting platinum(II) complexes have shown promise in many research fields and applications, owing to the diverse nature of their excited states and interesting emissive properties. This thesis describes the design, synthesis and characterization as well as photophysical and supramolecular properties of new classes of shape-persistent binuclear platinum(II) complexes supported by tridentate oligopyridyl, cyclometalated and tetradentate Schiff base ligands. In Chapter 1, an introduction to inorganic photochemistry and organoplatinum complexes, and a brief review of recent developments in the supramolecular chemistry of platinum(II) complexes are presented. The general experimental procedures are described in Chapter 2. The development of molecular frameworks derived from binuclear platinum(II) Schiff base (salphen) complexes and their supramolecular chemistry has been undertaken. Importantly, the nucleophilic phenoxo groups of Pt-salphen moieties may be amenable for further coordination to guest metal ions. In Chapter 3, a new class of binuclear Pt-salphen complexes, in which the two luminophores are tethered by a rigid backbone (Xan = xanthene, Dib = dibenzofuran, or Bph = biphenylene), have been constructed in anticipation of creating a potentially O4-binding pocket. All complexes are luminescent in solution at room temperature. Their photophysical and solvatochromic properties have been studied, and the emissions are assigned to mixed triplet O(p)/Pt(d) → π*(diimine) excited states. The red-shifted emission of the binuclear complex tethered by xanthene and biphenylene, namely Xan(Pt-salphen)2 and Bph(Pt-salphen)2, can be attributed to intramolecular π-stacking interactions between the two Pt-salphen moieties. In addition, selective colorimetric and luminescent responses to various metal ions are obtained. Using experimental observations, a binding mechanism is proposed based on occupation of the O4-binding cavity accompanied by perturbation of weak intramolecular contacts within the (Pt-salphen)2 moiety. In Chapter 4, the assembly of a more rigid cavity with novel binding properties was pursued. Hence, cyclometalated platinum(II) moieties containing 6-phenyl-2,2'-bipyridines (C^N^N) have been utilized as building blocks for the synthesis of binuclear frameworks in which the rotation of the two Pt(C^N^N) moieties can be impeded to a greater extent, and their photophysical properties have been studied. A π-excimeric fluid emission approaching the near-infrared region (λmax ~730 nm) was observed without ambiguity from excited states involving Pt...Pt interactions. Furthermore, xanthene-tethered binuclear [Pt(C^N^N)]2 complexes incorporating linear alkoxy or ethylene glycol groups have been prepared to further hinder the axial rotation of the [Pt(C^N^N)] units, and comparable π-excimeric emissions have been observed in the near-infrared region. Intriguingly, perturbation of the π-excimeric emission in different solvents and by selected metal ions has been detected. A new binuclear platinum(II) complex was synthesized, in which the two Pt(C^N^N) luminophores are tethered by both xanthene and an ethylene glycol group, but the resultant 'closed' macrocyclic derivative is weak emissive in solution at room temperature. Following the aforementioned observation of low-energy π-excimeric emissions in fluid solutions, a series of binuclear cyclometalated and terpyridyl platinum(II) complexes containing 1,3-di(2-pyridyl)benzene (N^C^N) and terpyridine (N^N^N) respectively were targeted in Chapter 5. Two [Pt(N^C^N)]2 complexes linked by a rigid backbone (Xan or Dib), in which the interaction between the two platinum(II) units are dissimilar due to their contrasting distances and angles, were prepared. An intense, broad red emission in solution at room temperature (λmax 663 nm, Φem = 0.25, τ = 1.7 μs) is observed for Xan[Pt(N^C^N)Cl]2. However, Dib[Pt(N^C^N)Cl]2 displays a highly structured emission (λmax 512 nm, Φem = 0.49, τ = 4.6 μs), which resembles the emission from the mononuclear analogue. The unusual concentration-independent low-energy emission from Xan[Pt(N^C^N)Cl]2 is proposed to arise from the formation of intramolecular π-excimers. The solid-state and 77-K glassy emissions of these binuclear platinum(II) complexes have also been investigated. Furthermore, platinum(II) 2,2':6',2"-terpyridine [Pt(N^N^N)] units have been employed as building blocks to prepare a new series of cationic binuclear platinum(II) complexes. The photophysical properties of Xan-, Dib- and Bph-bridged [Pt(N^N^N)]2 complexes have been studied; Dib[Pt(N^N^N)Cl]2 is non-emissive in fluid solution at 298 K, but the broad, low-energy band for the xanthene congener is ascribed to π-excimeric emission. The construction of a rectangular rigid [Pt]4 framework supported by Pt(N^N^N) and acetylide units was investigated. Finally, a general conclusion for this thesis is provided in Chapter 6.

Date of Award	15 Jul 2011
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Michael Chi Wang CHAN (Supervisor)