Luminescent Pt(II) Complexes Using Unsymmetrical Bis(2-pyridylimino)isoindolate Analogues

Abstract

A series of ligands based upon a 1,3-diimino-isoindoline framework have been synthesized and investigated as pincer-type (N^∧N^∧N) chelates for Pt(II). The synthetic route allows different combinations of heterocyclic moieties (including pyridyl, thiazole, and isoquinoline) to yield new unsymmetrical ligands. Pt(L^1-6)Cl complexes were obtained and characterized using a range of spectroscopic and analytical techniques: ¹H and ¹³C NMR, IR, UV-vis and luminescence spectroscopies, elemental analyses, high-resolution mass spectrometry, electrochemistry, and one example via X-ray crystallography which showed a distorted square planar environment at Pt(II). Cyclic voltammetry on the complexes showed one irreversible oxidation between +0.75 and +1 V (attributed to Pt^2+/3+ couple) and a number of ligand-based reductions; in four complexes, two fully reversible reductions were noted between -1.4 and -1.9 V. Photophysical studies showed that Pt(L^1-6)Cl absorbs efficiently in the visible region through a combination of ligand-based bands and metal-to-ligand charge-transfer features at 400-550 nm, with assignments supported by DFT calculations. Excitation at 500 nm led to luminescence (studied in both solutions and solid state) in all cases with different combinations of the heterocyclic donors providing tuning of the emission wavelength around 550-678 nm.

Figure 1

Molecular structure of bis(2-pyridylimino)isoindoline (BPI).

Figure 2

Relevant examples of Pt(II) complexes of BPI-related ligands demonstrate strategies for tuning emission. Left-to-right: site-specific variation in conjugation; changes in the auxiliary ligand (L); and use of substituents on isoindolate.

Scheme 1. Synthetic Routes to the Family of Isoindoline-1,3-Diimine Ligands; Reagents: (i) 1 Equiv 2-Aminopyridine, NaH, THF; (ii) 1 Equiv 4-Ethyl-2-Aminopyridine, n-BuOH; (iii) 1 Equiv 2-Aminothiazole, n-BuOH; (iv) 1 Equiv 3-Aminoisoquinoline, n-BuOH; (v) 1-Aminoisoquinoline, n-BuOH; (vi) 2 Equiv 4-Ethyl-2-Aminopyridine, n-BuOH, CaCl₂; and (vii) 2 Equiv 2-Aminothiazole, n-BuOH, CaCl₂

Scheme 2. Structures of the Synthesized Pt(II) Complexes

Figure 3

Example of the HRMS data for Pt(L¹)Cl. Theoretical isotopic distributions (top) for [M + H]⁺ and [M – Cl + MeCN]⁺, with experimental data shown below.

Figure 4

Structural representation obtained from single-crystal diffraction studies of Pt(L⁵)Cl showing one of the independent molecules of the asymmetric unit (Z′ = 2). Ellipsoids are drawn at 50%, and hydrogen atoms are not displayed.

Figure 5

Examples of cyclic voltammograms of Pt(L^1–3)Cl and Pt(L⁵)Cl. Measurements were recorded in degassed CH₂Cl₂, 293 K and 0.25 M [n-Bu₄N][PF₆] at a scan rate of 150 mV/s, relative to Fc/Fc⁺ (0 V).,

Figure 6

UV–vis spectra of the free ligands, HL^1–6 (10^–5 M in CHCl₃).

Figure 7

UV–vis spectra of the complexes Pt(L^1–6)Cl (10^–5 M in CHCl₃).

Figure 8

Pictorial representation of the Frontier orbitals for Pt(L³)Cl (top) and Pt(L⁴)Cl (bottom) and the DFT-calculated absorption spectra.

Figure 9

Normalized emission spectra for the family of Pt(II) complexes obtained in solution (left, 293 K, aerated CHCl₃, 10^–5 M; right, solid state).