Impact of a Carboxyl Group on a Cyclometalated Ligand: Hydrogen-Bond- and Coordination-Driven Self-Assembly of a Luminescent Platinum(II) Complex

Controlled crystallisation of porous crystals of luminescent platinum(II) complexes by electronic tuning of ancillary ligands

Porous molecular crystals (PMCs) have gained significant importance as next-generation functional porous materials. However, the selective crystallisation of the PMC phase remains a challenge. Herein, we have systematically controlled the stability of the luminescent PMC phase prepared using the luminescent Pt(II) complex [Pt(pbim)(N^O)] (pbim = 2-phenylbenzimidazolate, N^O = N-heteroaryl carboxylate) with Pt⋯Pt electronic interactions. The PMC phase formation varied significantly among the complexes depending on the heteroaryl group of the ancillary N^O ligand; the oxazolyl-bearing complex did not form a PMC phase, whereas the pyrazyl- and 5-fluoropyridyl-bearing complexes spontaneously formed a porous structure. This difference was rationalised by the π-stacking capability of the heteroaryl group of the ancillary ligand. Furthermore, owing to the presence of the one-dimensional Pt⋯Pt chains in this PMC phase, the photophysical properties of PMCs resulting from the Pt⋯Pt interactions were also significantly changed by the ancillary ligands.

Boosting the Methanol Oxidation Reaction Activity of Pt-Ru Clusters via Resonance Energy Transfer

The sluggish kinetics of the methanol oxidation reaction (MOR) with PtRu electrocatalyst severely hinder the commercialization of direct methanol fuel cells (DMFCs). The electronic structure of Pt is of significant importance for its catalytic activity. Herein, it is reported that low-cost fluorescent carbon dots (CDs) can regulate the behavior of the D-band center of Pt in PtRu clusters through resonance energy transfer (RET), resulting in a significant increase in the catalytic activity of the catalyst participating in methanol electrooxidation. For the first time, the bifunction of RET is used to provide unique strategy for fabrication of PtRu electrocatalysts, not only tunes the electronic structure of metals, but also provides an important role in anchoring metal clusters. Density functional theory calculations further prove that charge transfer between CDs and Pt promotes the dehydrogenation of methanol on PtRu catalysts and reduces the free energy barrier of the reaction associated with the oxidation of CO* to CO2 . This helps to improve the catalytic activity of the systems participating in MOR. The performance of the best sample is 2.76 times higher than that of commercial PtRu/C (213.0 vs 76.99 mW cm - 2 mg Pt - 1 ${\rm{mW\ cm}}^{ - 2}{\rm{\ mg}}_{{\rm{Pt}}}^{ - 1}$ ). The fabricated system can be potentially used for the efficient fabrication of DMFCs.

Elastic and bright assembly-induced luminescent crystals of platinum(II) complexes with near-unity emission quantum yield

Molecular crystals of Pt(II) complexes with metallophilic interactions can provide bright assembly-induced luminescence with colour tunability. However, the brittleness of many of these crystals makes their application in flexible optical materials difficult. Herein, we have achieved the elastic deformation of crystals of polyhalogenated Pt(II) complexes exhibiting bright assembly-induced luminescence. A crystal of [Pt(bpic)(dFppy)] (Hbpic = 5-bromopicolinic acid, HdFppy = 2-(2,4-difluorophenyl)pyridine) and a co-crystal of [Pt(bpic)(dFppy)] and [Pt(bpic)(ppy)] (Hppy = 2-phenylpyridine) were found to exhibit significant elastic deformation due to their highly anisotropic interaction topologies. While the crystal of [Pt(bpic)(dFppy)] exhibited monomer-based ligand-centred ³ππ* emission with an emission quantum yield of 0.40, the co-crystal exhibited bright, triplet metal-metal-to-ligand charge transfer (³MMLCT) emission owing to Pt⋯Pt interactions, thereby achieving a significantly higher emission quantum yield of 0.94.

Polymorphism and Mechanochromism in 2-Phenylbenzothiazole Cyclometalated PtII Complexes with Chelating N∧O Ligands

New cyclometalated Pt^II complexes with 2-phenylbenzothiazole (pbt) and two different picolinate ligands [Pt(pbt)(R-pic-κN,O)] (R = H (1), OH (2)) were prepared. In contrast to 1, the OH substituent group on 2 allows modulation of the packing in the solid state through donor-acceptor H-bonding interactions with the CH₂Cl₂ solvent. Thus, three pseudopolymorphs of 2 with different aggregation degrees were isolated, including yellow 2-Y, orange-red 2-R (2·0.5CH₂Cl₂) and black 2-B (2·0.75CH₂Cl₂) with emissions at 540, 656, and 740 nm, respectively, in the solid state at 298 K. 2-R and 2-B can be transformed to the pristine solid 2. Studies of their crystal structures show that 1 and 2-Y stack in columns with only π···π stacking interactions, whereas 2-R displays strong aggregated 1D infinite chains based on Pt···Pt and π···π stacking interactions, consistent with the colors and the photophysical properties, measured in several media. Interestingly, 1 and 2 exhibit reversible mechanochromic behavior with high contrast in the color and color emission upon mechanical grinding due to a phase transition between a crystalline and an amorphous state, as confirmed by powder X-ray diffraction (PXRD) studies. Theoretical calculations indicate that Pt···Pt contacts are more relevant in the trimers and tetramers than in the dimers, particularly in their T₁ states, associated with a change from a ³IL/³MLCT transition in the monomer to ³MM(L+L')CT in the oligomers. Noncovalent interaction (NCI) theoretical studies indicate that the π···π stacking among chelates also exerts a strong influence in the metal-metal-to-ligand charge transfer transition character.

Cyclometalated Platinum(II) Complexes in a Cis-N,N Configuration: Photophysical Properties and Isomerization to Trans Isomers

Synthesis, isomerization, and photophysical properties of novel cis-N,N-cyclometalated complexes [Pt(C^∧N)(l-Phe)] (C^∧N = benzoquinolinate (1), phenylpyridinate (2), 2-(p-tolyl)pyridinate (3), and 1-phenylpyrazolate (4); l-Phe^- = l-phenylalaninate) are reported herein. In solution, the cis forms of the complexes isomerize to their trans forms via an associative mechanism. This cis/trans isomerism barely influences the absorption and luminescence properties of the complexes in solution, except for a characteristic absorption at approximately 340 nm in the absorption spectra of the cis complexes that is not observed for the trans complexes. Interestingly, the cis complexes are spontaneously assembled in a crystalline phase and show bathochromic absorption and emission colors compared with those of the corresponding trans isomers, which are aggregated in an amorphous phase. cis-1 and cis-2 demonstrate hypsochromic luminescence mechanochromism. The influence of the geometrical isomerism on the photophysical properties and the isomerization mechanism are supported by density functional theory calculations.

Emissive Supramolecular Systems Based on Reversible Bond Formation and Noncovalent Interactions

Noncovalent interactions and reversible bond formations are widely seen in natural systems for the construction of sophisticated molecular systems that perform various biological processes. Inspired by the natural systems, luminescent supramolecular systems constructed by coordination-driven self-assembly and homometallic metal-metal interations have been studied increasingly. These supramolecular systems show fascinating luminescent behaviors that are not observed from single components. This review summarizes our progress in the development of two types of unique luminescent supramolecular systems. The mononuclear Pt(II) complex units can sandwich coinage metal ions to form heteropolynuclear complexes involving heterometallic metal-metal interactions. A close proximity of the two or three different metal ions by the noncovalent forces lead to orbital overlapping among the coinage metal ions and the Pt(II) complex units, showing emission color change accompanied with structural transformation and reversible metal binding behaviors. Emissive host-guest systems consisting of mononuclear metal complexes and a hydrogen-bonded capsule are also developed, that show a unique encapsulation-induced emission enhancement (EIEE) behavior.

Effect of the Fluoro-Substituent Position on the Crystal Structure and Photoluminescence of Microcrystals of Platinum β-Diketonate Complexes

Molecular packing has an important effect on the photophysical properties of crystalline materials. We demonstrate in this work the modulation of molecular packing and emission properties of microcrystals by minor molecular structural variations. Four platinum β-diketonate complexes, with two fluoro substituents (1) or one fluoro atom substituted on different positions of the auxiliary phenylpyridine ligand (2-4) have been synthesized. These complexes were used to prepare one-dimensional microcrystals with well-defined shapes and uniform sizes. Although 1-4 display similar emission spectra in the solution state, the corresponding microcrystals display different emission colors from green to yellow and orange. In addition, different temperature-responsive (80-298 K) emission spectral changes have been observed from these microcrystals, including the intensity variation of the locally excited (LE) emission without obvious wavelength shifts, competition between the LE and metal-metal-to-ligand charge-transfer emissions, and the sole wavelength shift of the π-π excimer emissions. These differences in emission properties are rationalized by different molecular packings of these materials, as revealed by single-crystal X-ray analyses.

Vapochromic crystals: understanding vapochromism from the perspective of crystal engineering

Vapochromic materials, which undergo colour and/or emission changes upon exposure to certain vapours or gases, have received increasing attention recently because of their wide range of applications in, e.g., chemical sensors, light-emitting diodes, and environmental monitors. Vapochromic crystals, as a specific kind of vapochromic materials, can be investigated from the perspective of crystal engineering to understand the mechanism of vapochromism. Moreover, understanding the vapochromism mechanism will be beneficial to design and prepare task-specific vapochromic crystals as one kind of low-cost 'electronic nose' to detect toxic gases or volatile organic compounds. This review provides important information in a broad scientific context to develop new vapochromic materials, which covers organometallic or coordination complexes and organic crystals, as well as the different mechanisms of the related vapochromic behaviour. In addition, recent examples of supramolecular vapochromic crystals and metal-organic-framework (MOFs) vapochromic crystals are introduced.

Crystal structure of (pyridine-2-carboxylato-κ2N,O)-[2-(2-pyridyl)phenyl-κ2N,C1]palladium(II), C17H12N2O2Pd

C17H12N2O2Pd, orthorhombic, Pna21 (no. 33), a = 20.6775(12) Å, b = 11.7539(7) Å, c = 5.4868(3) Å, V = 1333.52(13) Å3, Z = 4, Rgt(F) = 0.0154, wRref(F2) = 0.0373, T = 223(2) K.

Photokinetic study on remarkable excimer phosphorescence from heteroleptic cyclometalated platinum(ii) complexes bearing a benzoylated 2-phenylpyridinate ligand

Novel heteroleptic cyclometalated platinum(ii) complexes consisting of 5'-benzoylated 2-phenylpyridinate (ppy) cyclometalated and acetylacetonate ancillary ligands were synthesized, and their photoluminescence (PL) properties were investigated. The 5'-benzoylated complex without any other substituents exhibited phosphorescence-based monomer emission at 479 nm in dichloromethane (10 μM, rt) with a PL quantum yield of 0.28. On the other hand, in poly(methyl methacrylate) (PMMA) film, remarkable excimer emission additionally emerged at ca. 600 nm with a relatively high PL quantum yield of 0.47 as the doping level increased to 0.20 mmmol g^-1, which was comparably intense in comparison with the monomer emission. In the case of the complexes with unsubstituted, 4'-benzoylated, and 5'-fluorinated ppy cyclometalated ligands, excimer emission was modestly generated at the same doping level, and thus the introduction of a benzoyl group to the 5'-position is effective to obtain remarkable excimer emission. The combination of benzoyl and fluoro groups was more effective at inducing excimer emission, and the intensity of excimer emission of the 2-(5-benzoyl-4,6-difluorophenyl)pyridinate-based complex was 3.5 times larger than that of monomer emission at a doping level of 0.20 mmmol g^-1 in PMMA. From the analysis of PL lifetimes at varying concentrations, photokinetic profiles were fully analyzed according to the model system for the irreversible excimer formation, and the excimer formation rate constant of the 5'-benzoylated complex was determined in dichloromethane as 2.2 × 10⁹ M^-1 s^-1, which is 4.4 times larger than that of the unsubstituted complex. We also fabricated an organic light-emitting diode using the 2-(5-benzoyl-4,6-difluorophenyl)pyridinate-based complex as a single emitter. The device exhibited pseudo-white EL with the Commission internationale de l'éclairage chromaticity coordinates of (0.42, 0.42).

Design of a flexible organometallic tecton: host–guest chemistry with picric acid and self-assembly of platinum macrocycles

A new “flexible” and ditopic Pt(ii) organometallic compound is a tecton for the self-assembly of neutral metallacycles. It also exhibits significant binding affinity for picric acid.

Novel columnar metallomesogens based on cationic platinum(ii) complexes without long peripheral chains

Phosphorescent cationic platinum(ii) complexes without any peripheral flexible chains around the platinum(ii) center show thermotropic columnar liquid crystal properties.

Luminescence of tartrate bridged dinuclear 2,2′-bipyridine platinum(ii) complexes: emission color controlled by intra- and inter-molecular interactions in the solid state

Mono- and dinuclear Pt^II complexes with tartrate (tartH₂²⁻) [Pt(bpy)(tartH₂)] and [{Pt(bpy)}₂(μ-tart)], respectively, in crystals exhibited an emission color controlled by intra- and inter-molecular Pt–Pt and/or π–π interactions.

New phosphorescent platinum(ii) complexes: lamellar mesophase and mechanochromism

Several new square planar platinum(ii) complexes based on modified 2-phenylpyridine derivatives as the main ligand and picolinic acid as the auxiliary ligand were synthesized and their photophysical properties, and mechanochromic and liquid crystalline behavior were investigated.

Mechanochromism in the luminescence of novel cyclometalated platinum(ii) complexes with α-aminocarboxylates

Cyclometalated platinum(ii) complexes with α-aminocarboxylato ligands [Pt^II(ppy)L] (ppy = 2-phenylpyridinato, L⁻ = Gly, Ala, Leu, Ile, Phe, Sar) in the solid state exhibited reversible luminescent mechanochromism.