Toward Near-Infrared Emission in Pt(II)-Cyclometallated Compounds: From Excimers’ Formation to Aggregation-Induced Emission

Abstract

Two series of Pt(II)-cyclometallated compounds containing N^C^N tridentate and alkynyl-chromophore ligands have been synthesized and structurally characterized. The N^C^N ligands differ on the presence of R 1 = H or F in the central aromatic ring, while six different chromophores have been introduced to the alkynyl moiety. Single-crystal X-ray structures for some of the compounds reveal the presence of weak intermolecular contacts responsible for the formation of some dimers or aggregates. The photophysical characterization shows the presence of two emission bands in solution assigned to the 3 π−π* transition from the N^C^N ligands mixed with 3 MLCT/ 3 ILCT transitions (higher energy band) in deaerated samples. The formation of excimers has also been identified as a broad band at longer wavelengths [near-infrared (NIR) emission] that becomes the main emission band for compounds containing phenanthrene as the chromophore. NIR emission behavior has also been explored using acetonitrile/water mixtures, and the presence of aggregates that emit at ca. 650 nm has also been detected.

Effect of substituents on the 1O2 production and biological activity of (N^N^N)Pt(py) complexes

“…The origin of these emission bands has been attributed to 3 MMLCT transitions. 24,28,29,[31][32][33][34][35][36] The recorded emission quantum yields are on the same order as those previously recorded in solution while the emission lifetimes are slightly shorter and on the order of hundreds of nanoseconds (0.15-0.69 μs), supporting the triplet origin of the emission (see the ESI †).…”

Supramolecular luminescent Pt(ii) tweezers: aggregation studies and 1O2 production

“…The deep knowledge of these weak interactions arises from complementary studies on different fields of research such as organic and inorganic chemistry, coordination chemistry, theoretical studies of non-covalent interactions, molecular recognition and molecular devices among others. 1–18…”

“…28 In fact, the different degrees of aggregation modulate the emission wavelength (color), intensity, emission lifetime and quantum yields. 13,29 The formation of these aggregates may affect their potential applications in different fields such as molecular recognition (sensing) and optoelectronic devices or even a combination of both by the design of optoelectronic architectures used as sensors where the detection process may depend on the weak interactions of the supramolecular assemblies. 29…”

Luminescent Platinum(II) Complexes with Terdentate N∧C∧C Ligands

“…Therefore, many efficient Pt­(II) emitters are of the type [Pt­(C∧N)­(L∧X)], where L∧X is a monoanionic chelating ligand, often a β-diketonate. , Increasing the number of C-donor moieties is desirable to achieve higher emission efficiencies, but bis-cyclometalated complexes of the type cis -[Pt­(C∧N) 2 ] are generally not emissive, which has been attributed to molecular flexibility and the geometrical distortions caused by the steric hindrance between the C∧N ligands . The development of more efficient Pt­(II) emitters has often been addressed by using terdentate N∧N∧C, − N∧C∧N, − or C∧N∧C − heteroaromatic ligands, which impart rigidity and reduce nonradiative deactivation. However, the N∧C∧N and N∧N∧C ligand classes provide only one C-donor moiety, and C∧N∧C ligands often lead to weakly emissive complexes because the trans arrangement of metalated carbon atoms causes significant distortions in the excited state .…”

“…An aspect of great importance to the development of Pt­(II) emitters is the possibility to modulate their luminescence through the formation of aggregates or molecular assemblies, ,,− ,− which can occur via π interactions between the aromatic systems of the ligands and/or metallophilic Pt···Pt contacts, that usually fall in the range 3.0–3.5 Å. , These interactions, either separately or in combination, can cause significant modifications of frontier orbitals or favor the formation of supramolecular entities, such as aggregates or excimers, which usually produce emissions that are red-shifted with respect to those from the monomeric complexes. Several environmental factors can affect the formation of these entities, such as the solvent or solid matrix, the concentration, and the presence of other molecules that may interact with the complex.…”