Organometallic Platinum(II) Photosensitisers that Demonstrate Ligand‐Modulated Triplet‐Triplet Annihilation Energy Upconversion Efficiencies

Abstract

A series of 2-phenylquinoxaline ligands have been synthesised that introduce either CF 3 or OCF 3 electron-withdrawing groups at different positions of the phenyl ring. These ligands were investigated as cyclometalating reagents for platinum(II) to give neutral complexes of the form [Pt(C^N)(acac)] (in which C^N = cyclometalating ligand; acac = acetyl acetonate). X-ray crystallographic studies on three examples showed that the complexes adopt an approximate square planar geometry. All examples revealed strong PtÀ Pt linear contacts of 3.2041(6), 3.2199(3) and 3.2586(2) Å. The highly coloured complexes display efficient visible absorption at 400-500 nm (ɛ � 5000 M À 1 cm À 1 ) and orange red photoluminescent characteristics (λ em = 603-620 nm; Φ em � 37 %), which were subtly tuned by the ligand. Triplet emitting character was confirmed by microsecond luminescence lifetimes and the photogeneration of singlet oxygen with quantum efficiencies up to 57 %. Each complex was investigated as a photosensitiser for triplet-triplet annihilation energy upconversion using 9,10-diphenylanthracene as the annihilator species: a range of good upconversion efficiencies (Φ UC 5.9-14.1 %) were observed and shown to be strongly influenced by the ligand structure in each case.

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

“…In general, the study of photoactive transition-metal complexes is an expanding area of chemistry and has important applications in many areas, such as light harvesting, − sensing, , energy upconversion, − bioimaging, and theranostics. − In all instances, an ability to tune the emission characteristics of the metal complexes, typically via modifications of the ligand architecture, is absolutely essential. It is in this context that we have revisited the use of BPI-type ligands for the development of luminescent Pt­(II) complexes and present a new and simple synthetic methodology for obtaining unsymmetrical BPI analogues that allows different heterocyclic donors to be integrated into the ligand framework and thus has the potential for numerous iterations.…”

Tuning Excited State Character in Iridium(III) Photosensitizers and Its Influence on TTA-UC

“…Ru­(II), Re­(I), and Ir­(III) complexes have been successfully utilized to produce TTA-UC efficiencies up to 39.3% . Pt­(II) species have also been investigated in TTA-UC, including organometallic complexes and Schiff base ligand complexes and the well-known Pt­(II) octaethylporphyrin, which combines very efficient visible light absorption with a microsecond duration lifetime of the triplet state. , Interestingly, a phosphorescent Cr­(III) complex has recently been reported in TTA-UC, demonstrating an alternative sensitizing mechanism utilizing Cr-centered excited states …”

Alkynylphosphonium Pt(II) Complexes: Synthesis, Characterization, and Features of Photophysical Properties in Solution and in the Solid State

“…8,9 Moreover, the ability to form aggregates can lead to stimuliresponsive properties, such as vapochromism, thermochromism, and mechanochromism. 10,11 These properties make Pt(II) coordination compounds applicable in anticounterfeiting, 12 pressure sensing, 13 and memory devices 14 in addition to traditional applications of organometallic luminophores such as bioimaging, 15 organic light-emitting diodes, 16,17 and lightemitting cell fabrication, 18 as well as chemosensing, 19 upconversion, 20 and nonlinear optics. 21 The phosphorescence of Pt(II) complexes is usually attributed to the radiative decay of metal-to-ligand charge transfer d-π* ( 3 MLCT) states, ligand-centered π−π* ( 3 LC) states, or their combination.…”

“…Platinum­(II) complexes [Pt­(N^N)­(CCR) 2 ] with diimine and alkynyl ligands have been the object of intense attention and extensively studied for the last decades, − and scientific interest is promoted by a rich variety of photophysical properties that these compounds possess. Tuning the coordination environment of Pt­(II) complexes, one can achieve high versatility of emission colors, high quantum yields, and demonstration of nonlinear optical properties. , Moreover, the ability to form aggregates can lead to stimuli-responsive properties, such as vapochromism, thermochromism, and mechanochromism. , These properties make Pt­(II) coordination compounds applicable in anticounterfeiting, pressure sensing, and memory devices in addition to traditional applications of organometallic luminophores such as bioimaging, organic light-emitting diodes, , and light-emitting cell fabrication, as well as chemosensing, upconversion, and nonlinear optics …”