Ytterbium(III) Porpholactones: β‐Lactonization of Porphyrin Ligands Enhances Sensitization Efficiency of Lanthanide Near‐Infrared Luminescence

The near‐infrared (NIR) luminescence efficiency of lanthanide complexes is largely dependent on the electronic and photophysical properties of antenna ligands. Although porphyrin ligands are efficient sensitizers of lanthanide NIR luminescence, non‐pyrrolic porphyrin analogues, which have unusual symmetry and electronic states, have been much less studied. In this work, we used porpholactones, a class of β‐pyrrolic‐modified porphyrins, as ligands and investigated the photophysical properties of lanthanide porpholactones Yb‐1 a – 5 a . Compared with Yb porphyrin complexes, the porpholactone complexes displayed remarkable enhancement of NIR emission (50–120 %). Estimating the triplet‐state levels of porphyrin and porpholactone in Gd complexes revealed that β‐lactonization of porphyrinic ligands lowers the ligand T₁ state and results in a narrow energy gap between this state and the lowest excited state of Yb³⁺. Transient absorption spectra showed that Yb^III porpholactone has a longer transient decay lifetime at the Soret band than the porphyrin analogue (30.8 versus 17.0 μs). Thus, the narrower energy gap and longer lifetime arising from β‐lactonization are assumed to enhance NIR emission of Yb porpholactones. To demonstrate the potential applications of Yb porpholactone, a water‐soluble Yb bioprobe was constructed by conjugating glucose to Yb ‐ 1 a . Interestingly, the NIR emission of this Yb porpholactone could be specifically switched on in the presence of glucose oxidase and then switched off by addition of glucose. This is the first demonstration that non‐pyrrolic porphyrin ligands enhance the sensitization efficiency of lanthanide luminescence and also display switchable NIR emission in the region of biological analytes (800–1400 nm).     

Light‐Harvesting Ytterbium(III)–Porphyrinate–BODIPY Conjugates: Synthesis,Excitation‐Energy Transfer,and Two‐Photon‐Induced Near‐Infrared‐Emission Studies

Based on a donor–acceptor framework, several conjugates have been designed and prepared in which an electron‐donor moiety, ytterbium(III) porphyrinate (YbPor), was linked through an ethynyl bridge to an electron‐acceptor moiety, boron dipyrromethene (BODIPY). Photoluminescence studies demonstrated efficient energy transfer from the BODIPY moiety to the YbPor counterpart. When conjugated with the YbPor moiety, the BODIPY moiety served as an antenna to harvest the lower‐energy visible light, subsequently transferring its energy to the YbPor counterpart, and, consequently, sensitizing the Yb^III emission in the near‐infrared (NIR) region with a quantum efficiency of up to 0.73 % and a lifetime of around 40 μs. Moreover, these conjugates exhibited large two‐photon‐absorption cross‐sections that ranged from 1048–2226 GM and strong two‐photon‐induced NIR emission.     

Luminescent Palladium(II) Complexes with π‐Extended Cyclometalated [RC^N^NR′] and Pentafluorophenylacetylide Ligands: Spectroscopic,Photophysical, and Photochemical Properties

A series of cyclometalated Pd^II complexes that contain π‐extended R? C^N^N? R′ (R? C^N^N? R′=3‐(6′‐aryl‐2′‐pyridinyl)isoquinoline) and chloride/pentafluorophenylacetylide ligands have been synthesized and their photophysical and photochemical properties examined. The complexes with the chloride ligand are emissive only in the solid state and in glassy solutions at 77 K, whereas the ones with the pentafluorophenylacetylide ligand show phosphorescence in the solid state (λ_max=584–632 nm) and in solution (λ_max=533–602 nm) at room temperature. Some of the complexes with the pentafluorophenylacetylide ligand show emission with λ_max at 585–602 nm upon an increase in the complex concentration in solutions. These Pd^II complexes can act as photosensitizers for the light‐induced aerobic oxidation of amines. In the presence of 0.1 mol % Pd^II complex, secondary amines can be oxidized to the corresponding imines with substrate conversions and product yields up to 100 and 99 %, respectively. In the presence of 0.15 mol % Pd^II complex, the oxidative cyanation of tertiary amines could be performed with product yields up to 91 %. The Pd^II complexes have also been used to sensitize photochemical hydrogen production with a three‐component system that comprises the Pd^II complex, [Co(dmgH)₂(py)Cl] (dmgH=dimethylglyoxime; py=pyridine), and triethanolamine, and a maximum turnover of hydrogen production of 175 in 4 h was achieved. The excited‐state electron‐transfer properties of the Pd^II complexes have been examined.     

Facile Tuning of Luminescent Platinum(II) Schiff Base Complexes from Yellow to Near‐Infrared: Photophysics,Electrochemistry, Electrochemiluminescence and Theoretical Calculations

The photophysical and related properties of platinum(II) Schiff base complexes can be finely and predictably tuned over a wide range of wavelengths by small and easily implemented changes to ligand structure. A series of such complexes, differing only in the number and positioning of methoxy substituents on the phenoxy ring, were synthesised and their photophysical, electrochemical and electrochemiluminescent (ECL) properties investigated. Theoretical calculations were performed in order to gain further insight into the relationship between structure and properties in these materials. By positioning methoxy groups para and/or ortho to either the imine or the oxygen group on the ligand, electron density could be directed selectively toward the LUMO or HOMO as required. This allowed the emission colour (both photoluminescent and electrochemiluminescent) to be tuned over a wide range between 587 and 739 nm. The variation in orbital energies was also manifested in the positions of the absorption bands and the redox properties of the complexes, as well as in the NMR shifts for the uncoordinated ligands. All reported complexes displayed intense electrochemiluminescence (ECL), which could be initiated either by annihilation or co‐reactant pathways. The relationship between the electrochemical and photophysical properties and the efficiency of the ECL is discussed. For two of the complexes solid‐state ECL could be generated from electrodeposited layers of the complex.     

A Promising MgII‐Ion‐Selective Luminescent Probe: Structures and Properties of Dy–Mn Polymers with High Symmetry

Two Dy–Mn polymers, {[Dy(L¹)₃Mn_1.5(H₂O)₃]?3.125 H₂O}_n ( 1 , L¹=pyridine‐2,6‐dicarboxylic acid) and {[Dy(L²)₃Mn_1.5(H₂O)₆]?8.25 H₂O}_n ( 2 , L² = 4‐hydroxylpyridine‐2,6‐dicarboxylic acid), with high symmetry (S₆) have been prepared. Polymer 1 has a nanoporous 3D framework with channel of about 17.6 Å diameter, while 2 has a honeycomb‐type 2D structure with the cavity of approximately 14.4 Å diameter. In the construction of multidimensional porous polymers with 3d–4f mixed metals, it is the first observation that a ligand substituent effect leads to dramatic differences in the structures formed. Luminescent studies reveal that the emission intensities of 1 and 2 increase significantly upon the addition of Mg²⁺, whereas the introduction of other metal ions leaves the intensity unchanged or even weakens it; hence, both of them may serve as good candidates of Mg²⁺ luminescent probes. To our knowledge, complex 1 is also the first example of a 3d–4f metal‐based nanoporous polymer to exhibit luminescent selectivity for Mg²⁺. Magnetic susceptibility measurements reveal a rather rare ferromagnetic interaction in 2 . Thermal gravimetric analyses and powder X‐ray diffraction investigations have also been performed, suggestive of high thermal stability of 1 .     

Luminescent Pincer Platinum(II) Complexes with Emission Quantum Yields up to Almost Unity: Photophysics,Photoreductive CC Bond Formation,and Materials Applications

Luminescent pincer‐type Pt^II complexes supported by C‐deprotonated π‐extended tridentate R? C^N^N? R′ ligands and pentafluorophenylacetylide ligands show emission quantum yields up to almost unity. Femtosecond time‐resolved fluorescence measurements and time‐dependent DFT calculations together reveal the dependence of excited‐state structural distortions of [Pt(R? C^N^N? R′)(C?C‐C₆F₅)] on the positional isomers of the tridentate ligand. Pt complexes [Pt(R‐C^N^N? R′)(C?C‐Ar)] are efficient photocatalysts for visible‐light‐induced reductive C? C bond formation. The [Pt(R‐C^N^N? R′)(C?C‐C₆F₅)] complexes perform strongly as phosphorescent dopants for green‐ and red‐emitting organic light‐emitting diodes (OLEDs) with external quantum efficiency values over 22.1 %. These complexes are also applied in two‐photon cellular imaging when incorporated into mesoporous silica nanoparticles (MSNs).     

Luminescent Quantum Clusters of Gold in Bulk by Albumin‐Induced Core Etching of Nanoparticles: Metal Ion Sensing,Metal‐Enhanced Luminescence,and Biolabeling

The synthesis of a luminescent quantum cluster (QC) of gold with a quantum yield of ～4 % is reported. It was synthesized in gram quantities by the core etching of mercaptosuccinic acid protected gold nanoparticles by bovine serum albumin (BSA), abbreviated as Au_QC@BSA. The cluster was characterized and a core of Au₃₈ was assigned tentatively from mass spectrometric analysis. Luminescence of the QC is exploited as a “turn‐off” sensor for Cu²⁺ ions and a “turn‐on” sensor for glutathione detection. Metal‐enhanced luminescence (MEL) of this QC in the presence of silver nanoparticles is demonstrated and a ninefold maximum enhancement is seen. This is the first report of the observation of MEL from QCs. Folic acid conjugated Au_QC@BSA was found to be internalized to a significant extent by oral carcinoma KB cells through folic acid mediated endocytosis. The inherent luminescence of the internalized Au_QC@BSA was used in cell imaging.     

Optical Activity in the Near‐IR Region: The λ=980 nm Multiplet of Chiral Yb3+ Complexes

We used a very simplified electrostatic model based on charge and polarizability of atoms and groups on an organic ligand around a lanthanide ion to predict the near‐infrared electronic circular dichroism (NIR ECD) spectra of Yb³⁺ (a monoelectronic ion). We tuned our method by using two widely different complexes. The first was the heterobimetallic species CsYb(hfbc)₄ [hfbc=(?)‐3‐heptafluorobutyrylcamphorate], in which the ligand is a diketonate and, as such, is endowed with a chromophore with strong UV absorption (π–π^*). Its oxygen atoms define a square antiprism, which provides a symmetric coordination polyhedron. The second system was Yb DOTMA [DOTMA=(1R,4R,7R,10R)‐α,α′,α′′,α′′′‐tetramethyl‐1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid], a chiral Yb analogue of Gd DOTA (DOTA=1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid), in which the ligand lacks relevant electronic transitions and provides a dissymmetric cage. The relative weights of dynamic (ligand polarization) and static contributions to Yb NIR ECD were evaluated, and the spectra appear to have been well predicted by theory through the introduction of a heuristic weight factor. To validate the approach and to confirm the value of the weight factor, we applied it to two other compounds, namely, Na₃Yb(BINOLate)₃ and Yb(BINOLAM)₃ [BINOLate=2,2′‐dihydroxy‐1,1′‐binaphthyl; BINOLAM=3,3′‐bis(diethylaminomethyl)‐1‐1′‐bi‐2‐naphthol].     

Strongly Luminescent Tungsten Emitters with Emission Quantum Yields of up to 84 %: TADF and High‐Efficiency Molecular Tungsten OLEDs

Metal‐TADF (thermally activated delayed fluorescence) emitters hold promise in the development of next generation light‐emitting materials for display and lighting applications, examples of which are, however, largely confined to Cu^I and recently Au^I, Ag^I, and Au^III emitters. Herein is described the design strategy for an unprecedented type of metal‐TADF emitter based on inexpensive tungsten metal chelated with Schiff base ligand that exhibit high emission quantum yields of up to 56 % in solutions and 84 % in thin‐film (5 wt % in 1,3‐bis(N‐carbazolyl)benzene, mCP) at room temperature. Femtosecond time‐resolved emission (fs‐TRE) spectroscopy and DFT calculations were undertaken to decipher the TADF properties. Solution‐processed OLEDs fabricated with the W‐TADF emitter demonstrated external quantum efficiency (EQE) and luminance of up to 15.6 % and 16890 cd m^?2, respectively.     

Stimuli‐Responsive Dual‐Color Photon Upconversion: A Singlet‐to‐Triplet Absorption Sensitizer in a Soft Luminescent Cyclophane

Reversible emission color switching of triplet–triplet annihilation‐based photon upconversion (TTA‐UC) is achieved by employing an Os complex sensitizer with singlet‐to‐triplet (S‐T) absorption and an asymmetric luminescent cyclophane with switchable emission characteristics. The cyclophane contains the 9,10‐bis(phenylethynyl)anthracene unit as an emitter and can assemble into two different structures, a stable crystalline phase and a metastable supercooled nematic phase. The two structures exhibit green and yellow fluorescence, respectively, and can be accessed by distinct heating/cooling sequences. The hybridization of the cyclophane with the Os complex allows near‐infrared‐to‐visible TTA‐UC. The large anti‐Stokes shift is possible by the direct S‐T excitation, which dispenses with the use of a conventional sequence of singlet–singlet absorption and intersystem crossing. The TTA‐UC emission color is successfully switched between green and yellow by thermal stimulation.     

Tunable Trimers: Using Temperature and Pressure to Control Luminescent Emission in Gold(I) Pyrazolate‐Based Trimers

A systematic investigation into the relationship between the solid‐state luminescence and the intermolecular Au???Au interactions in a series of pyrazolate‐based gold(I) trimers; tris(μ₂‐pyrazolato‐N,N′)‐tri‐gold(I) ( 1 ), tris(μ₂‐3,4,5‐ trimethylpyrazolato‐N,N′)‐tri‐gold(I) ( 2 ), tris(μ₂‐3‐methyl‐5‐phenylpyrazolato‐N,N′)‐tri‐gold(I) ( 3 ) and tris(μ₂‐3,5‐diphenylpyrazolato‐N,N′)‐tri‐gold(I) ( 4 ) has been carried out using variable temperature and high pressure X‐ray crystallography, solid‐state emission spectroscopy, Raman spectroscopy and computational techniques. Single‐crystal X‐ray studies show that there is a significant reduction in the intertrimer Au???Au distances both with decreasing temperature and increasing pressure. In the four complexes, the reduction in temperature from 293 to 100 K is accompanied by a reduction in the shortest intermolecular Au???Au contacts of between 0.04 and 0.08 Å. The solid‐state luminescent emission spectra of 1 and 2 display a red shift with decreasing temperature or increasing pressure. Compound 3 does not emit under ambient conditions but displays increasingly red‐shifted luminescence upon cooling or compression. Compound 4 remains emissionless, consistent with the absence of intermolecular Au???Au interactions. The largest pressure induced shift in emission is observed in 2 with a red shift of approximately 630 cm^?1 per GPa between ambient and 3.80 GPa. The shifts in all the complexes can be correlated with changes in Au???Au distance observed by diffraction.     

Exploring Excited‐State Tunability in Luminescent Tris‐cyclometalated Platinum(IV) Complexes: Synthesis of Heteroleptic Derivatives and Computational Calculations

The synthesis, structure, electrochemistry, and photophysical properties of a series of heteroleptic tris‐ cyclometalated Pt^IV complexes are reported. The complexes mer‐[Pt(C^N)₂(C′^N′)]OTf, with C^N=C‐deprotonated 2‐(2,4‐difluorophenyl)pyridine (dfppy) or 2‐phenylpyridine (ppy), and C′^N′=C‐deprotonated 2‐(2‐thienyl)pyridine (thpy) or 1‐phenylisoquinoline (piq), were obtained by reacting bis‐ cyclometalated precursors [Pt(C^N)₂Cl₂] with AgOTf (2 equiv) and an excess of the N′^C′H pro‐ligand. The complex mer‐[Pt(dfppy)₂(ppy)]OTf was obtained analogously and photoisomerized to its fac counterpart. The new complexes display long‐lived luminescence at room temperature in the blue to orange color range. The emitting states involve electronic transitions almost exclusively localized on the ligand with the lowest π–π* energy gap and have very little metal character. DFT and time‐dependent DFT (TD‐DFT) calculations on mer‐[Pt(ppy)₂(C′^N′)]⁺ (C′^N′=thpy, piq) and mer/fac‐[Pt(ppy)₃]⁺ support this assignment and provide a basis for the understanding of the luminescence of tris‐cyclometalated Pt^IV complexes. Excited states of LMCT character may become thermally accessible from the emitting state in the mer isomers containing dfppy or ppy as chromophoric ligands, leading to strong nonradiative deactivation. This effect does not operate in the fac isomers or the mer complexes containing thpy or piq, for which nonradiative deactivation originates mainly from vibrational coupling to the ground state.