Porphyrin lanthanide complexes for NIR emission

Luminescent lanthanide-based molecular materials: applications in photodynamic therapy

Luminescent lanthanide molecular compounds have recently attracted attention as potential photosensitizers (PSs) for photodynamic therapy (PDT) against malignant cancer tumours because of their predictable systemic toxicity, temporospatial specificity, and minimal invasiveness. A photosensitizer exhibits no toxicity by itself, but in the presence of light and oxygen molecules, it can generate reactive oxygen species (ROS) to cause damage to proteins, nucleic acids, lipids, membranes, and organelles, which can induce cell apoptosis. This review focuses on the latest developments in luminescent lanthanide-based molecular materials as photosensitizers and their applications in photodynamic therapy. These molecular materials include lanthanide coordination complexes, nanoscale lanthanide coordination polymers, and lanthanide-based nanoscale metal-organic frameworks. In the end, the future challenges in the development of robust luminescent lanthanide molecular materials-based photosensitisers are outlined and emphasized to inspire the design of a new generation of phototheranostic agents.

Magnetic Anisotropy of Homo- and Heteronuclear Terbium(III) and Dysprosium(III) Trisphthalocyaninates Derived from Paramagnetic 1H-NMR Investigation

1H-NMR spectroscopy of lanthanide complexes is a powerful tool for deriving spectral-structural correlations, which provide a clear link between the symmetry of the coordination environment of paramagnetic metal centers and their magnetic properties. In this work, we have first synthesized a series of homo- (M = M* = Dy) and heteronuclear (M ≠ M* = Dy/Y and Dy/Tb) triple-decker complexes [(BuO)8Pc]M[(BuO)8Pc]M*[(15C5)4Pc], where BuO- and 15C5- are, respectively, butoxy and 15-crown-5 substituents on phthalocyanine (Pc) ligands. We provide an algorithmic approach to assigning the 1H-NMR spectra of these complexes and extracting the axial component of the magnetic susceptibility tensor, χax. We show how this term is related to the nature of the lanthanide ion and the shape of its coordination polyhedron, providing an experimental basis for further theoretical interpretation of the revealed correlations.

Tailoring the pore size of expanded porphyrinoids for lanthanide selectivity

Despite increase in demand, capacity for the recycling of rare earth elements remains limited, partly due to the inefficiencies with processes currently utilised in the separation of lanthanides. This study highlights the potential use of expanded porphyrinoids in lanthanide separation through selective binding, dependent on the tailored pore size of the macrocycle. Each emerging trend is subjected to multi-factored analysis to decompose the underlying source. Results promote the viability of size-based separation with preferential binding of larger lanthanum(iii) ions to amethyrin and isoamethyrin macrocycles, while smaller macrocycles such as pentaphyrin(0.0.0.0.0) present a preferential binding of lutetium(iii) ions. Additionally, the porphyrin(2.2.2.2) macrocycle shows a selectivity for gadolinium(iii) ions over both larger and smaller ions. An upper limit of applicable pore size is shown to be ≈2.8 Å, beyond which the formed complexes are predicted to be less stable than the corresponding nitrate complexes.

A Novel Near-Infrared Ytterbium Complex [Yb(DPPDA)2](DIPEA) with Φ = 0.46% and τobs = 105 μs

The luminescent performances of near-infrared (NIR) lanthanide (Ln) complexes were restricted greatly by vibration quenching of X-H (X = C, N, O) oscillators, which are usually contained in ligands and solvents. Encapsulating Ln³⁺ into a cavity of coordination atoms is a feasible method of alleviating this quenching effect. In this work, a novel ytterbium complex [Yb(DPPDA)₂](DIPEA) coordinated with 4,7-diphenyl-1,10-phenanthroline-2,9-dicarboxylic acid (DPPDA) was synthesized and characterized by FT-IR, ESI-MS and elemental analysis. Under the excitation of 335 nm light, [Yb(DPPDA)₂](DIPEA) showed two emission peaks at 975 and 1011 nm, respectively, which were assigned to the characteristic ²F_5/2 → ²F_7/2 transition of Yb³⁺. Meanwhile, this ytterbium complex exhibited a plausible absolute quantum yield of 0.46% and a luminescent lifetime of 105 μs in CD₃OD solution. In particular, its intrinsic quantum yield was calculated to be 12.5%, and this considerably high value was attributed to the near-zero solvent molecules bound to Yb³⁺ and the absence of X-H oscillators in the first coordination sphere. Based on experimental results, we further proposed that the sensitized luminescence of [Yb(DPPDA)₂](DIPEA) occurred via an internal redox mechanism instead of an energy transfer process.

Lanthanide porphyrinoids as molecular theranostics

In recent years, lanthanide (Ln) porphyrinoids have received increasing attention as theranostics. Broadly speaking, the term 'theranostics' refers to agents designed to allow both disease diagnosis and therapeutic intervention. This Review summarises the history and the 'state-of-the-art' development of Ln porphyrinoids as theranostic agents. The emphasis is on the progress made within the past decade. Applications of Ln porphyrinoids in near-infrared (NIR, 650-1700 nm) fluorescence imaging (FL), magnetic resonance imaging (MRI), radiotherapy, and chemotherapy will be discussed. The use of Ln porphyrinoids as photo-activated agents ('phototheranostics') will also be highlighted in the context of three promising strategies for regulation of porphyrinic triplet energy dissipation pathways, namely: regioisomeric effects, metal regulation, and the use of expanded porphyrinoids. The goal of this Review is to showcase some of the ongoing efforts being made to optimise Ln porphyrinoids as theranostics and as phototheranostics, in order to provide a platform for understanding likely future developments in the area, including those associated with structure-based innovations, functional improvements, and emerging biological activation strategies.

Lanthanide-tetrapyrrole complexes: synthesis, redox chemistry, photophysical properties, and photonic applications

Tetrapyrrole derivatives such as porphyrins, phthalocyanines, naphthalocyanines, and porpholactones, are highly stable macrocyclic compounds that play important roles in many phenomena linked to the development of life. Their complexes with lanthanides are known for more than 60 years and present breath-taking properties such as a range of easily accessible redox states leading to photo- and electro-chromism, paramagnetism, large non-linear optical parameters, and remarkable light emission in the visible and near-infrared (NIR) ranges. They are at the centre of many applications with an increasing focus on their ability to generate singlet oxygen for photodynamic therapy coupled with bioimaging and biosensing properties. This review first describes the synthetic paths leading to lanthanide-tetrapyrrole complexes together with their structures. The initial synthetic protocols were plagued by low yields and long reaction times; they have now been replaced with much more efficient and faster routes, thanks to the stunning advances in synthetic organic chemistry, so that quite complex multinuclear edifices are presently routinely obtained. Aspects such as redox properties, sensitization of NIR-emitting lanthanide ions, and non-linear optical properties are then presented. The spectacular improvements in the quantum yield and brightness of Yb^III-containing tetrapyrrole complexes achieved in the past five years are representative of the vitality of the field and open welcome opportunities for the bio-applications described in the last section. Perspectives for the field are vast and exciting as new derivatizations of the macrocycles may lead to sensitization of other Ln^III NIR-emitting ions with luminescence in the NIR-II and NIR-III biological windows, while conjugation with peptides and aptamers opens the way for lanthanide-tetrapyrrole theranostics.

Mono-/Bimetallic Neutral Iridium(III) Complexes Bearing Diketopyrrolopyrrole-Substituted N-Heterocyclic Carbene Ligands: Synthesis and Photophysics

The synthesis and photophysics (UV-vis absorption, emission, and transient absorption) of four neutral heteroleptic cyclometalated iridium(III) complexes (Ir-1-Ir-4) incorporating thiophene/selenophene-diketopyrrolopyrrole (DPP)-substituted N-heterocyclic carbene (NHC) ancillary ligands are reported. The effects of thiophene versus selenophene substitution on DPP and bis- versus monoiridium(III) complexation on the photophysics of these complexes were systematically investigated via spectroscopic techniques and density functional theory calculations. All complexes exhibited strong vibronically resolved absorption in the regions of 500-700 nm and fluorescence at 600-770 nm, and both are predominantly originated from the DPP-NHC ligand. Complexation induced a pronounced red shift of this low-energy absorption band and the fluorescence band with respect to their corresponding ligands due to the improved planarity and extended π-conjugation in the DPP-NHC ligand. Replacing the thiophene units by selenophenes and/or biscomplexation led to the red-shifted absorption and fluorescence spectra, accompanied by the reduced fluorescence lifetime and quantum yield and enhanced population of the triplet excited states, as reflected by the stronger triplet excited-state absorption and singlet oxygen generation.

Emission Switching in the Near-Infrared by Reversible Trans-Cis Photoisomerization of Styrylbenzene-Conjugated Osmium Terpyridine Complexes

A new array of homoleptic osmium(II) complexes based on styrylbenzene-conjugated terpyridine ligands (tpy-pvp-X) were synthesized and their photophysical, electrochemical, and photoisomerization behaviors thoroughly investigated in this work. Both electron-donating and -withdrawing substituents were incorporated onto a tpy-pvp-X (X = H, Me, Cl, NO₂, and Ph) moiety to tune the optical properties and also the rate of photoisomerization behaviors in the complexes. All complexes display strong spin-allowed singlet metal-to-ligand charge-transfer bands in the visible (495-506 nm) and weak singlet ground state to triplet metal-to-ligand charge-transfer (³MLCT) broad bands within the 600-700 nm range. The complexes also exhibit strong phosphorescence emission from their ³MLCT state in the near-infrared domain (737-752 nm) at room temperature with excited-state lifetimes spanning between 107 and 165 ns. Two styrylbenzene units promote reversible trans-trans to trans-cis/cis-cis isomerization induced by light. The rate constants and quantum yields of photoisomerization were found to vary linearly with the Hammett σ_p parameters of the substituents. The rate and quantum yields were also found to decrease with increasing polarity of the solvents. Considerable modulation of the optical behavior along with luminescence switching in the complexes has been achieved upon photoisomerization. Moreover, the optical outputs as a function of two photonic stimuli inputs were used to demonstrate the binary function of a two-input IMPLICATION logic gate. In conjunction with the experimental study, computational investigations were also carried out in all three conformations of the complexes (trans-trans, trans-cis, and cis-cis) to have a perception of their electronic structures and for correct assignment of their absorption and emission spectral bands.

Recent Advances of Near-Infrared (NIR) Emissive Metal Complexes Bridged by Ligands with N- and/or O-Donor Sites

Near-infrared (NIR) emissive metal complexes have shown potential applications in optical communication, chemosensors, bioimaging, and laser and organic light-emitting diodes (OLEDs) due to their structural tunability and luminescence stability. Among them, complexes with bridging ligands that exhibit unique emission behavior have attracted extensive interests in recent years. The target performance can be easily achieved by NIR light-emitting metal complexes with bridging ligands through molecular structure design. In this review, the luminescence mechanism and design strategies of NIR luminescent metal complexes with bridging ligands are described firstly, and then summarize the recent advance of NIR luminescent metal complexes with bridging ligands in the fields of electroluminescence and biosensing/bioimaging. Finally, the development trend of NIR luminescent metal complexes with bridging ligands are proposed, which shows an attractive prospect in the field of photophysical and photochemical materials.

The effect of terminal N-donor aromatic ligands on the sensitization and emission of lanthanide ions in Zn2Ln (Ln = Eu, Tb) complexes with 4-biphenylcarboxylate anions

The systematic series of trinuclear carboxylate complexes [Zn₂Ln(NO₃)(phbz)₆(L)₂] (Ln = Eu, Gd, and Tb, where phbz is the anion of 4-biphenylcarboxylic acid, and L is pyridine, 2,3-lutidine or 2,2′-bipyridine) were synthesized. Luminescence properties were investigated in detail.

Porphyrin metalation catalyzed by DNAzymes and nanozymes

In this review, DNA and nanomaterial based catalysts for porphyrin metalation reactions are summarized, including the selection of DNAzymes, choice of nanomaterials, their catalytic mechanisms, and applications of the reactions.

Effect of the aromatic substituent on the para-position of pyridine-bis(oxazoline) sensitizers on the emission efficiency of their EuIII and TbIII complexes

Two efficient lanthanide ion sensitizers 2,6-bis(oxazoline)-4-phenyl-pyridine (PyboxPh, 1) and 2,6-bis(oxazoline)-4-thiophen-2-yl-pyridine (Pybox2Th, 2) were synthesized. 1 crystallizes in the monoclinic space group P21/c with cell parameters a = 16.3794(4) Å, b = 7.2856(2) Å, c = 11.7073(3) Å, β = 97.229(1)° and V = 1385.97(6) Å3. 2 crystallizes in the monoclinic space group P21/n with cell parameters a = 5.9472(2), b = 16.0747(6), c = 14.3716(5) Å, β = 93.503(1)° and V = 1371.35(8) Å3. Photophysical characterization of 1 shows that its triplet state energy is located at 22 250 cm-1 and efficient energy transfer is observed for EuIII and TbIII. Solutions of [Ln(PyboxPh)3]3+ in dichloromethane display an emission efficiency of 37.2% for Ln[double bond, length as m-dash]Eu and 24.0% for Ln[double bond, length as m-dash]Tb. The excited state lifetimes for EuIII and TbIII are 2.227 ms and 723 μs, respectively. The triplet state energy of 2 is located at 19 280 cm-1 and is therefore too low to efficiently sensitize TbIII emission. However, the sensitization of EuIII is effective, with an emission quantum yield of 14.5% and an excited state lifetime of 714 μs. This shows that the derivatization of the chelator is strongly influenced by the aromatic substituents on the para-position of the pyridine ring. New isostructural 1 : 1 complexes of PyboxPh with EuIII (3) and TbIII (4) were also isolated and crystallize in the triclinic space group P1[combining macron] with cell parameters a = 9.1845(2) Å, b = 10.3327(2) Å, c = 11.9654(2) Å, α = 98.419(1)°, β = 108.109(1)°, γ = 91.791(1)°, V = 1064.08(4) Å3 and a = 7.8052(1) Å, b = 11.8910(1) Å, c = 14.2668(2) Å, α = 72.557(1)°, β = 86.355(1)°, γ = 77.223(1)°, V = 1231.95(3) Å3, respectively.

Energy transfer in supramolecular [Crypt-RE]-[W6I14] solids

Photophysical properties of tungsten iodides with the [W6I14]2- cluster core have been described with respect to phosphorescence and phosphorescence quenching by molecular oxygen. This process involves energy transfer from excited triplet states of the cluster onto molecular oxygen. In the present study we investigate deactivation channels of exited triplet states of the [W6I14]2- cluster towards rare earth ions. For this purpose, we synthesized several supramolecular assemblies made of [W6I14]2- clusters and metal cryptates and investigated their crystal structures and photophysical properties. UV/Vis photoexcitation of solid [Crypt-A]-[W6I14] (A = alkaline metal) and [Crypt-RE]-[W6I14] revealed phosphorescence of the cluster, respectively of the photophysically active rare earth metal (RE) center. A cluster to cryptate energy transfer is proven with a photophysically active rare earth ion by the emission of Yb3+ at 977 nm (2F5/2-2F7/2) and Nd3+ 1072 nm (4F3/2-4I11/2). These results show that an effective excitation of near-infrared-emitting rare earth ions is possible under excitation up to 550 nm with [Crypt-RE]-[W6I14] assemblies.

Metal ion adaptive self-assembly of photoactive lanthanide-based supramolecular hosts

Metal-adaptive self-assembly and post-assembly transmetallation modification of functional lanthanide-porphyrin hosts were presented.

Series of Highly Luminescent Macrocyclic Sm(III) Complexes: Functional Group Modifications Together with Luminescence Performances in Solid-State, Solution, and Doped Poly(methylmethacrylate) Film

Here, we report our trials to regulate the luminescence performance of the macrocyclic samarium(III) complex and prepare four excellent luminescent Sm(III) complex-doped poly(methylmethacrylate) (PMMA) composites. Four 23-membered [1 + 1] Schiff-base macrocyclic mononuclear Sm(III) complexes, Sm-2 _a -Sm-2 _d , originating from dialdehydes with different pendant arms and 1,2-bis(2-aminoethoxy)ethane, have been constructed by the template method. Crystal structures reveal that every Sm(III) ion with the coordination geometry of a distorted bicapped square antiprism is capsulated by the macrocyclic cavity environment forming the "lasso-type" protection. Relative photophysical properties of macrocyclic Sm(III) complexes are carefully investigated in solid-state, methanol solution, and doped PMMA film, and all these show characteristic emissions of the Sm(III) ion associated with satisfactory lifetimes and quantum yields in all media, which could be comparable to reported outstanding examples. Especially, the luminescence performance for this type of Sm(III) complex could be regulated in the solid state by the use of different functional groups in the pendant arm while it is not achieved in solution and the doped PMMA composite. High emitting and air-stable plastic materials could be obtained when these Sm(III) complexes are doped in PMMA with 0.1 wt % mixing ratio, and the corresponding maximum lifetime and quantum yield are 61.2 μs and 0.63% in the case of complex Sm-2 _a , respectively. We believe that these highly luminescent "lasso-type" Sm(III) complexes and doped PMMA composites are valuable references in the design of luminescent lanthanide(III) hybrid materials.

Lanthanide-Oligomeric Brush Films: From Luminescence Properties to Structure Resolution

Lanthanide (Ln) based luminescent materials are experiencing an increasing interest in their applications in several fields. In this study, we report a series of new lanthanide-oligomeric brush films, supported on quartz substrates and prepared using a layer-by-layer method (LbL). Oligomeric brush films are composed of small oligomers from our previously reported coordination polymers [x-EuL] and [x-TbL] (with x = 1, 3, and 5 generations of Ln complexes), which are grown perpendicularly from a carboxylate self-assembled monolayer. Oligomers composed of our previously described helical lanthanide complex LnL (Ln: Eu and Tb) as a luminescent moiety and benzene-1,4-dicarboxylate acid (bdc) used as a linker. Mixed films having the fifth-generation Ln complexes composed of equimolar mixture of Eu and Tb ions were prepared. Oligomeric brush films are highly transparent and exhibited a colored emission under UV irradiation. Pure Ln (Eu or Tb) films showed a strong luminescence from the Ln ions. Their luminescent properties depended on the number of lanthanide layers in the films composed of the first to third generations of lanthanide complexes. Then, the increase of the complex layers induced no difference in the luminescent properties. An energy transfer from Tb to Eu ions in the mixed films indicated a short distance between lanthanide ions of a fifth layer. The structural analysis together with the observed luminescent properties and some previous studies allowed to clarify the disposition of the oligomers in the films.

Impressive near-infrared brightness and singlet oxygen generation from strategic lanthanide-porphyrin double-decker complexes in aqueous solution

Although lanthanide double-decker complexes with hetero-macrocyclic ligands as functional luminescent and magnetic materials have promising properties, their inferior water solubility has negated their biomedical applications. Herein, four water-soluble homoleptic lanthanide (Ln = Gd, Er, Yb and La) sandwiches with diethylene-glycol-disubstituted porphyrins (DD) are reported, with their structures proven by both quantum chemical calculations and scanning tunneling microscopy. Our findings demonstrate that the near-infrared emission intensity and singlet oxygen (¹O₂) quantum yields of YbDD and GdDD in aqueous media are higher than those of the reported capped lanthanide monoporphyrinato analogues, YbN and GdN; the brightness and luminescence lifetime in water of YbDD are greater than those of YbN. This work provides a new dimension for the future design and development of molecular theranostics-based water-soluble double-decker lanthanide bisporphyrinates.

Formation, Photophysics, and Photochemistry of Anionic Lanthanide(III) Mono- and Bisporphyrins

Since water-soluble porphyrin complexes of lanthanides(III) have proved to be promising for medical applications (e.g., luminescence imaging, photodynamic therapy, and theranostics), the investigation of the formation, photophysical, and photochemical properties of such coordination compounds provides useful pieces of information for their potential usage. Steady-state and time-resolved fluorometry, UV-Vis absorption spectroscopy, and continuous-wave photolysis were utilized for this purpose. 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin formed mono- and bisporphyrin complexes with samarium(III), europium(III), and gadolinium(III) as representatives in the middle of the lanthanide series. The special photoinduced behavior of these compounds was mostly determined by the position of the metal center, which was located out of the ligand plane, thus distorting it. Besides, the photochemical and, especially, photophysical features of the corresponding mono- and bisporphyrin complexes were similar because, in the latter species, two monoporphyrins were connected by a weak metal bridge between the peripheral sulfonato substituents (tail-to-tail dimerization). The formation of these coordination compounds and the transformation reactions between the mono- and bisporphyrins were rather slow in the dark at room temperature. These processes were accelerated by visible irradiation. However, dissociation and, especially, redox degradation were the main photoreactions in these systems, although with low quantum yields. Additionally, depending on the excitation wavelength, new types of photoproducts were also detected.

Stimuli-Responsive Near-Infrared Emissive Os(II)-Terpyridine Complexes with a Sense of Logic

Two bis-tridentate Os(II) compounds based on a heteroditopic terpyridine-bipyridine-type ligand were synthesized, and their photophysical properties were thoroughly studied. The compounds exhibit strong spin-allowed ¹MLCT bands in the visible domain (489-521 nm) as well as weak ¹GS to ³MLCT bands within the 668-815 nm domain. The compounds display strong luminescence from the ³MLCT state in the near-infrared domain (728-780 nm) at room temperature having lifetimes in the range of 20.0-171.0 ns. After coordination of [Os(tpy-PhCH₃/H₂pbbzim)₂]²⁺ unit to the terpyridine site of tpy-Hbzim-dipy, the complexes offer vacant pyridine-imidazole motifs for interacting with cationic and anionic guests. Consequently, photophysical properties of the compounds were tuned to a great extent upon interaction with selected cations, anions, pH, as well as protons. Anion-induced alteration of the ground- and excited-state properties of the compound lead to recognition of specific anions in solution. Significant change in the optical spectral behaviors as well as switching of emission spectral properties of the compounds was done in the NIR region upon treating with anions, cations, protons, and solvents (dichloromethane, acetonitrile, methanol, dimethylsulfoxide, and water). Moreover, the optical outputs in response to external stimuli were used to demonstrate binary functions of two-input IMPLICATION, NOR, and XNOR logic gates.

Synthesis and structural, redox and photophysical properties of tris-(2,5-di(2-pyridyl)pyrrolide) lanthanide complexes

A first series of lanthanide complexes of tris(dipyridyl)pyrrolide ligands has been prepared. The [Ln(dppR1,R2)3] complexes (Ln = La(iii), Sm(iii), Eu(iii), Gd(iii) and Yb(iii); and dppR1,R2 = 2,5-di(2-pyridyl-3-(R1)-4-(R2)) pyrrolide) have been isolated and their structures and photophysical and redox properties characterised, both in the solid-state and in solution. In the complexes, the three dpp- ligands form a distorted tricapped trigonal prismatic coordination geometry about the lanthanide ions, with the antiparallel isomer observed in the solid state for non-symmetric (dppCO2Me,Me)-. However, 1H NMR spectroscopy of the diamagnetic and paramagnetic [Ln(dppR1,R2)3] complexes in d6-benzene solution reveal evidence for a statistical distribution of all possible isomers. Time-resolved luminescence studies suggest that the dpp- ligand (with triplet excited state T1 energy at 18 622 cm-1) sensitises red emission from [Eu(dppCO2Me,Me)3] and near-infrared emission from [Yb(dppCO2Me,Me)3] through the antenna effect. Cyclic voltammetry reveals three consecutive, reversible, one-electron oxidation processes for each [Ln(dppR1,R2)3] complex, corresponding to oxidations of each dpp- ligand between 0.3-0.8 V vs. E1/2 (Fc+/0), and for [Eu(dppCO2Me,Me)3] the EuIII/II couple was -2.099 V vs. E1/2 (Fc+/0).

Strong Fluorescent Lanthanide Salen Complexes: Photophysical Properties, Excited-State Dynamics, and Bioimaging

The synthesis, excited-state dynamics, and biological application of luminescent lanthanide salen complexes (Ln = Lu, Gd, Eu, Yb, salen = N, N'-bis(salicylidene)ethylenediamine-based ligands) with sandwich structures are described. Among them, Lu(III) complexes show unusually strong ligand-centered fluorescence with quantum yields up to 62%, although the metal center is close to a chromophore ligand. The excited-state dynamic studies including ultrafast spectroscopy for Ln-salen complexes revealed that their excited states are solely dependent on the salen ligands and the ISC rates are slow (10⁸-10⁹ s^-1). Importantly, time-dependent density functional theory calculations attribute the low energy transfer efficiency to the weak spin-orbital coupling (SOC) between the singlet and triplet excited states. More importantly, Lu-salen has been applied as a molecular platform to construct fluorescence probes with organelle specificity in living cell imaging, which demonstrates the advantages of the sandwich structures as being capable of preventing intramolecular metal-ligand interactions and behaviors different from those of the previously reported Zn-salens. Most importantly, the preliminary study for in vivo imaging using a mouse model demonstrated the potential application of Ln coordination complexes in therapeutic and diagnostic bioimaging beyond living cells or in vitro.

Design of Near-Infrared Luminescent Lanthanide Complexes Sensitive to Environmental Stimulus through Rationally Tuning the Secondary Coordination Sphere

The design of near-infrared (NIR) emissive lanthanide (Ln) complexes sensitive to external stimulus is fundamentally important for the practical application of Ln materials. Because NIR emission from Ln is extremely sensitive to X-H (X = C, N and O) bond vibration, we herein report to harness the secondary coordination sphere to design NIR luminescent lanthanide sensors. Toward this goal, we designed and synthesized two isomeric [(η⁵-C₅H₅)Co{(D₃CO)₂P = O}₃]-Yb(III)-7,8,12,13,17,18-hexafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porpholactol NIR emitters, Yb-up and Yb-down, based on the stereoisomerism of porphyrin peripheral β-hydroxyl group. Yb-up, in which β-OH is at the same side of Yb(III) center, can form an intramolecular hydrogen bond with the axial Kläui ligand, whereas Yb-down cannot because its β-OH is opposite to Yb(III) center. X-ray crystal structures and photophysical studies suggested that the intramolecular hydrogen bond plays important roles on the NIR luminescence of ytterbium(III), which shortens the distance between β-OH and Yb(III) and facilitates the nonradiative deactivation of Ln excited state. Importantly, Yb-up/down were demonstrated to be highly sensitive toward temperature and viscosity. The PMMA polymer using Yb-up as the dopant NIR emitter showed thermosensitivity up to 6.0% °C^-1 in the wide temperature range of 77-400 K, higher than that of Yb-down (3.8% °C^-1). These complexes were also explored as the first NIR viscosity sensor, revealing their potential applications as optical sensors without visible light interference. This work demonstrates the importance of secondary coordination sphere on designing NIR Ln luminescent functional materials.

Lutetium(iii) porphyrinoids as effective triplet photosensitizers for photon upconversion based on triplet–triplet annihilation (TTA)

We described the first application of lanthanide porphyrinoids, exemplified by lutetium, as efficient photosensitizers in photon upconversion based on TTA.

The peculiarities of complex formation and energy transfer processes in lanthanide complexes with 2-(tosylamino)-benzylidene-N-benzoylhydrazone

The understanding of the peculiarities of the energy transfer processes in europium complexes allowed to obtain the luminescence thermometer materials with the highest sensitivity value at above the liquid-nitrogen temperatures.

Highly near-IR emissive ytterbium(iii) complexes with unprecedented quantum yields

The design of highly near-infrared (NIR) emissive lanthanide (Ln) complexes is challenging, owing to the lack of molecular systems with a high sensitization efficiency and the difficulty of achieving a large intrinsic quantum yield. Previous studies have reported success in optimizing individual factors and achieving high overall quantum yields, with the best yield being 12% for Yb(iii). Herein we report a series of highly NIR emissive Yb complexes, in which the Yb is sandwiched between an octafluorinated porphyrinate antenna ligand and a deuterated Kläui ligand, which allowed optimization of two factors in the same system, and one of the complexes had an unprecedented quantum yield of 63% (estimated uncertainty 15%) in CD2Cl2 with a long lifetime (τobs) of 714 μs. Systematic analysis of the structure-photophysical properties relationship suggested that porphyrinates are effective antenna ligands with a sensitization efficiency up to ca. 100% and that replacement of the high-energy C-H oscillators in porphyrinate and Kläui ligands significantly improves the intrinsic quantum yield up to 75% (τobs/τrad), both of which contribute to enhancing the NIR emission intensity of Yb(iii) up to 25-fold. Besides the high luminescence efficiency, these Yb complexes have other attractive features such as excitation in the visible range and large extinction coefficients which make these Yb(iii) complexes outstanding optical materials in the NIR region.

Influence of lanthanide ion energy levels on luminescence of corresponding metalloporphyrins

Lanthanide (Ln) porphyrins exhibit diverse luminescence properties that have not been fully explained yet. A series of Ln ions (Ln ions = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, and Lu³⁺) were coordinated with hematoporphyrin monomethyl ether (HMME), and their luminescence properties and related differences were studied. Spectral analysis indicated that all Ln-HMMEs exhibit fluorescence emission. Gd- and Lu-HMMEs were the only lanthanide-HMMEs displaying strong molecular π-π room-temperature phosphorescence (RTP) with quantum yield Φ_P > 10^-3. Tb³⁺ can also induce RTP from HMME but Φ_P of Tb-HMME is much smaller (Φ_P ∼ 10^-4). The observed luminescence property differences were analyzed in detail, focusing on the 4f energy levels of Ln ions. These levels mostly lie below the lowest singlet (S₁) and triplet (T₁) excited states of HMME, resulting in energy transfer from the T₁ state in HMME to Ln ions and, therefore, in the absence of RTP from the corresponding metalloporphyrins. Gd³⁺ and Lu³⁺ are the only lanthanide ions not possessing such 4f energy levels, avoiding T₁ quenching in Gd- and Lu-HMMEs. Although Tb³⁺ has low-lying 4f energy levels, the corresponding transition from the ground state is partly forbidden, resulting in weak energy transfer from HMME to Tb³⁺ that accounts for the low RTP quantum yield of the corresponding complex. Thus, our results indicate that the luminescence property differences of lanthanide porphyrins are due to the disparate energy levels of the Ln ions.

Donor–acceptor type A2B2porphyrins: synthesis, energy transfer, computational and electrochemical studies

Synthesis, photophysical, electrochemical and DFT studies of donor–acceptor type A₂B₂porphyrins and their Zn(ii) and Pd(ii) complexes have been described.

A redox active switch for lanthanide luminescence in phenolate complexes

The reversible oxidation of coordinated phenolates into phenoxyl radicals results in a dramatic quenching (>95%) of the luminescence of the f metal ion.

N-Substituted Azacalixphyrins: Synthesis, Properties, and Self-Assembly

Pre- and postintroduction of substituents with respect to the macrocyclization step leads to previously unknown N-substituted azacalixphyrins. The stepwise synthetic approach has been studied in detail to highlight the key role of the N-substituents of the precursors and/or intermediates in terms of reactivity. Based on a combined experimental and theoretical investigation, the relationship between the properties of the macrocycles and their degree of substitution is rationalized. Depending on the nature of the N-substituents, the formation of supramolecular ribbon-like structures could also be observed, as demonstrated by combined TEM, SEM, AFM, and FTIR experiments.

Synthesis and characterisation of lanthanide-hydroporphyrin dyads

Hydroporphyrin-linked lanthanide complexes with variations in tetrapyrrole structure, linker length and mode of attachment are reported.

Lanthanide complexes with 2-(tosylamino)benzylidene-N-benzoylhydrazone, which exhibit high NIR emission

New NIR emitting materials were found among the lanthanide complexes with 2-(tosylamino)benzylidene-N-benzoylhydrazone.