Demystification of luminescence properties and the near imperceptible thermal quenching of Sm3+-doped tungstate phosphors.

Ultra-high thermally stable Ca2MgWO6:xSm3+ (x = 0.5, 0.75, 1, 1.25, and 1.5 mol%) double perovskite phosphors were synthesized through solid-state reaction method. Product formation was confirmed by comparing the X-ray diffraction (XRD) patterns of the phosphors with the standard reference file. The structural, morphological, thermal, and optical properties of the prepared phosphor were examined in detail using XRD, Fourier transform infrared spectra, scanning electron microscopy, diffused reflectance spectra, thermogravimetric analysis (TGA), photoluminescence emission, and temperature-dependent PLE (TDPL). It was seen that the phosphor exhibited emission in the reddish region for the near-ultraviolet excitation with moderate Colour Rendering Index values and high colour purity. The optimized phosphor (x = 1.25 mol%) was found to possess a direct optical band gap of 3.31 eV. TGA studies showed the astonishing thermal stability of the optimized phosphor. Additionally, near-zero thermal quenching was seen in TDPL due to elevated phonon-assisted radiative transition. Furthermore, the anti-Stokes and Stokes emission peaks were found to be sensitive toward the temperature change and followed a Boltzmann-type distribution. All these marked properties will make the prepared phosphors a suitable candidate for multifield applications and a fascinating material for further development.

Photoluminescence properties of Pb5(PO4)3Br:RE (RE = Dy3+, Eu3+ and Tb3+) phosphor synthesised using solid-state method.

This study describes the luminous properties of Pb5(PO4)3Br doped with RE3+ (RE = Dy3+, Eu3+ and Tb3+) synthesised using the solid-state method. The synthesised phosphor was characterised using Fourier-transform infrared, X-ray diffraction, scanning electron microscopy and photoluminescence measurements. Dy3+-doped Pb5(PO4)3Br phosphor exhibited blue and yellow emissions at 480 and 573 nm, respectively, on excitation at 388 nm. Eu3+-doped Pb5(PO4)3Br phosphor exhibited orange and red emissions at 591 and 614 nm, respectively, on excitation at λex = 396 nm. Pb5(PO4)3Br:Tb3+ phosphor exhibited the strongest green emission at 547 nm on excitation at λex = 380 nm. Additionally, the effect of the concentration of rare-earth ions on the emission intensity of Pb5(PO4)3Br:RE3+ (RE3+ = Dy3+, Eu3+ and Tb3+) phosphors was investigated.

A Water-Soluble Wavy Coordination Polymer of Cu(II) as a Turn-On Luminescent Probe for Histidine and Histidine-Rich Proteins/Peptides.

Histidine plays an essential role in most biological systems. Changes in the homeostasis of histidine and histidine-rich proteins are connected to several diseases. Herein, we report a water-soluble Cu(II) coordination polymer, labeled CuCP, for the fluorimetric detection of histidine and histidine-rich proteins and peptides. Single-crystal structure determination of CuCP revealed a two-dimensional wavy network structure in which a carboxylate group connects the individual Cu(II) dimer unit in a syn-anti conformation. The weakly luminescent and water-soluble CuCP shows turn-on blue emission in the presence of histidine and histidine-rich peptides and proteins. The polymer can also stain histidine-rich proteins via gel electrophoresis. The limits of quantifications for histidine, glycine-histidine, serine-histidine, human serum albumin (HSA), bovine serum albumin, pepsin, trypsin, and lysozyme were found to be 300, 160, 600, 300, 600, 800, 120, and 290 nM, respectively. Utilizing the fluorescence turn-on property of CuCP, we measured HSA quantitatively in the urine samples. We also validated the present urinary HSA measurement assay with existing analytical techniques. Job's plot, 1H NMR, high-resolution mass spectrometry (HRMS), electron paramagnetic resonance (EPR), fluorescence, and UV-vis studies confirmed the ligand displacement from CuCP in the presence of histidine.

A strategy to enhance the water solubility of luminescent ß-diketonate-Europium(III) complexes for time-gated luminescence bioassays.

Luminescent ß-diketonate-europium(III) complexes have been found a wide range of applications in time-gated luminescence (TGL) bioassays, but their poor water solubility is a main problem that limits their effective uses. In this work we propose a simple and general strategy to enhance the water solubility of luminescent ß-diketonate-europium(III) complexes that permits facile synthesis and purification. By introducing the fluorinated carboxylic acid group into the structures of ß-diketone ligands, two highly water-soluble and luminescent Eu3+ complexes, PBBHD-Eu3+ and CPBBHD-Eu3+, were designed and synthesized. An excellent solubility exceeding 20 mg/mL for PBBHD-Eu3+ was found in a pure aqueous buffer, while it also displayed strong and long-lived luminescence (quantum yield φ = 26%, lifetime τ = 0.49 ms). After the carboxyl groups of PBBHD-Eu3+ were activated, the PBBHD-Eu3+-labeled streptavidin-bovine serum albumin (SA-BSA) conjugate was prepared, and successfully used for the immunoassay of human α-fetoprotein (AFP) and the imaging of an environmental pathogen Giardia lamblia under TGL mode, which demonstrated the practicability of PBBHD-Eu3+ for highly sensitive TGL bioassays. The carboxyl groups of PBBHD can also be easily derivatized with other reactive chemical groups, which enables PBBHD-Eu3+ to meet diverse requirements of biolabeling technique, to provide new opportunities for developing functional europium(III) complex biolabels serving for TGL bioassays.

Dual-Emissive Iridium(III) Complexes and Their Applications in Biological Sensing and Imaging.

Phosphorescent iridium(III) complexes are widely recognized for their unique properties in the excited triplet state, making them crucial for various applications including biological sensing and imaging. Most of these complexes display single phosphorescence emission from the lowest-lying triplet state after undergoing highly efficient intersystem crossing (ISC) and ultrafast internal conversion (IC) processes. However, in cases where these excited-state processes are restricted, the less common phenomenon of dual emission has been observed. This dual emission phenomenon presents an opportunity for developing biological probes and imaging agents with multiple emission bands of different wavelengths. Compared to intensity-based biosensing, where the existence and concentration of an analyte are indicated by the brightness of the probe, the emission profile response involves modifications in emission color. This enables quantification by utilizing the intensity ratio of different wavelengths, which is self-calibrating and unaffected by the probe concentration and excitation laser power. Moreover, dual-emissive probes have the potential to demonstrate distinct responses to multiple analytes at separate wavelengths, providing orthogonal detection capabilities. In this concept, we focus on iridium(III) complexes displaying fluorescence-phosphorescence or phosphorescence-phosphorescence dual emission, along with their applications as biological probes for sensing and imaging.

Guest-host doped strategy for constructing ultralong-lifetime near-infrared organic phosphorescence materials for bioimaging.

Organic near-infrared room temperature phosphorescence materials have unparalleled advantages in bioimaging due to their excellent penetrability. However, limited by the energy gap law, the near-infrared phosphorescence materials (>650 nm) are very rare, moreover, the phosphorescence lifetimes of these materials are very short. In this work, we have obtained organic room temperature phosphorescence materials with long wavelengths (600/657-681/732 nm) and long lifetimes (102-324 ms) for the first time through the guest-host doped strategy. The guest molecule has sufficient conjugation to reduce the lowest triplet energy level and the host assists the guest in exciton transfer and inhibits the non-radiative transition of guest excitons. These materials exhibit good tissue penetration in bioimaging. Thanks to the characteristic of long lifetime and long wavelength emissive phosphorescence materials, the tumor imaging in living mice with a signal to background ratio value as high as 43 is successfully realized. This work provides a practical solution for the construction of organic phosphorescence materials with both long wavelengths and long lifetimes.

A Dinuclear Osmium(II) Complex Near-Infrared Nanoscopy Probe for Nuclear DNA.

With the aim of developing photostable near-infrared cell imaging probes, a convenient route to the synthesis of heteroleptic OsII complexes containing the Os(TAP)2 fragment is reported. This method was used to synthesize the dinuclear OsII complex, [{Os(TAP)2}2tpphz]4+ (where tpphz = tetrapyrido[3,2-a:2',3'-c:3″,2''-h:2‴,3'''-j]phenazine and TAP = 1,4,5,8- tetraazaphenanthrene). Using a combination of resonance Raman and time-resolved absorption spectroscopy, as well as computational studies, the excited state dynamics of the new complex were dissected. These studies revealed that, although the complex has several close lying excited states, its near-infrared, NIR, emission (λmax = 780 nm) is due to a low-lying Os → TAP based 3MCLT state. Cell-based studies revealed that unlike its RuII analogue, the new complex is neither cytotoxic nor photocytotoxic. However, as it is highly photostable as well as live-cell permeant and displays NIR luminescence within the biological optical window, its properties make it an ideal probe for optical microscopy, demonstrated by its use as a super-resolution NIR STED probe for nuclear DNA.

Ratiometric Luminescence Detection of Copper(I) by a Resonant System Comprising Two Antenna/Lanthanide Pairs.

Selective and sensitive detection of Cu(I) is an ongoing challenge due to its important role in biological systems, for example. Herein, we describe a photoluminescent molecular chemosensor integrating two lanthanide ions (Tb3+ and Eu3+) and respective tryptophan and naphthalene antennas onto a polypeptide backbone. The latter was structurally inspired from copper-regulating biomacromolecules in Gram-negative bacteria and was found to bind Cu+ effectively under pseudobiological conditions (log KCu+ = 9.7 ± 0.2). Ion regulated modulation of lanthanide luminescence in terms of intensity and long, millisecond lifetime offers perspectives in terms of ratiometric and time-gated detection of Cu+. The role of the bound ion in determining the photophysical properties is discussed with the aid of additional model compounds.

Mannosylation of pH-sensitive liposomes promoted cytoplasmic delivery of protein to macrophages: green fluorescent protein (GFP) performed as an endosomal escape tracer.

Conventional non-pH-sensitive liposomes for cytoplasmic delivery of protein suffer from poor efficiency. Here we investigated mannosylated pH-sensitive liposomes (MAN-PSL) for cytoplasmic delivery of protein to macrophages RAW 264.7 using PSL and non-pH-sensitive liposomes for comparison. We characterised the pH-dependent fluorescence of green fluorescent protein (GFP) and encapsulated it in liposomes as an intracellular trafficking tracer. GFP showed a reversed 'S'-shaped pH-fluorescence curve with a dramatic signal loss at acidic pH. GFP stored at 4 °C with light protection showed a half-life of 10 days (pH 5-8). The entrapment efficiency of GFP was dominated by the volume ratio of intraliposomal core to external medium for thin-film hydration. Mannosylation did not affect the pH-responsiveness of PSL. Confocal microscopy elucidated that mannosylation promoted the cellular uptake of PSL. For both these liposomes, the strongest, homogeneously distributed GFP fluorescence in the cytoplasm was found at 3 h, confirming efficient endosomal escape of GFP. Conversely, internalisation of non-pH-sensitive liposomes was slow (peaked at 12 h) and both Nile Red and GFP signals remained weak and punctuated in the cytosol. In conclusion, GFP performed as a probe for endosome escape of liposomal cargo. Mannosylation facilitated the internalisation of PSL without compromising their endosomal escape ability.

Tyrosine-Templated Dual-Component Silver Nanomaterials Exhibit Photoluminescence and Versatile Antimicrobial Properties through ROS Generation.

The role of small molecules in the preparation of metal nanomaterials generates considerable interest in the fields from materials science to interdisciplinary sciences. In this study, a small amino acid, l-tyrosine (Tyr), has been used as a ligand precursor for the preparation of silver nanomaterials (AgNMs) comprising a dual system: smaller silver nanoclusters (responsible exclusively for the photophysical properties) and larger silver nanoparticles (responsible exclusively for the antimicrobial properties). The luminescent properties of this AgNM system substantiate the role played by Tyr as a capping and a reducing agent outside the protein environment. An interesting feature of this report is the promising antimicrobial properties of the AgNMs against Saccharomyces cerevisiae, Candida albicans, Escherichia coli, and Bacillus cereus cell lines. The importance of this work is that this investigation demonstrates the combating ability of our AgNM system against pathogenic strains (C. albicans and B. cereus) as well. Moreover, the mechanistic aspects of the antimicrobial activity of the AgNMs were elucidated using various methods, such as propidium iodide staining, monitoring reactive oxygen species generation, leakage of proteins, DNA cleavage, etc. We propose that AgNM-mediated cytotoxicity in S. cerevisiae stems from the generation of singlet oxygen (1O2) species that create oxidative stress, disrupting the cell membrane and thereby resulting in leakage of proteins from the cells. This study can pave the way toward elucidating the role of a small molecule, Tyr, in the formation of NMs and describes the use of new NMs in potential antimicrobial applications.

Ultrastable Tb-Organic Framework as a Selective Sensor of Phenylglyoxylic Acid in Urine.

Industrial pollution and harmful chemicals seriously affect environment and human health. Styrene is a common air toxicant with widespread exposure sources, including smoking, automobile exhaust, and plastic pollutants. Phenylglyoxylic acid (PGA) is a typical biomarker for exposed styrene. Therefore, it is crucial to quickly identify and quantitatively detect PGA. Herein, an ultrastable terbium metal-organic framework (Tb-MOF 1) was developed, and the luminescence film (1/PLA) consisting of polylactic acid (PLA) and 1 was fabricated as a sensor for rapid detection of PGA. The sensor possesses the advantages of efficient detection [limit of detection (LOD) is 1.05 × 10-4 mg/mL] and rapid response speed (less than 10 s) for PGA in urine. Furthermore, this sensor exhibits high stability, outstanding anti-interference ability, and excellent recyclability. Based on this film technology, a paper-based probe was then developed for portable and convenient detection. The probe could easily distinguish different concentrations of PGA under the naked eye toward practical sensing applications. Meanwhile, photoinduced electron transfer was demonstrated to be responsible for the luminescence sensing. Hence, this study indicates that Tb-MOF is a promising material to detect PGA for evaluating the effect of styrene on the body.

Seeking Illumination: The Path to Chemiluminescent 1,2-Dioxetanes for Quantitative Measurements and In Vivo Imaging.

Chemiluminescence is a fascinating phenomenon that evolved in nature and has been harnessed by chemists in diverse ways to improve life. This Account tells the story of our research group's efforts to formulate and manifest spiroadamantane 1,2-dioxetanes with triggerable chemiluminescence for imaging and monitoring important reactive analytes in living cells, animals, and human clinical samples. Analytes like reactive sulfur, oxygen and nitrogen species, as well as pH and hypoxia can be indicators of cellular function or dysfunction and are often implicated in the causes and effects of disease. We begin with a foundation in binding-based and activity-based fluorescence imaging that has provided transformative tools for understanding biological systems. The intense light sources required for fluorescence excitation, however, introduce autofluorescence and light scattering that reduces sensitivity and complicates in vivo imaging. Our work and the work of our collaborators were the first to demonstrate that spiroadamantane 1,2-dioxetanes had sufficient brightness and biological compatibility for in vivo imaging of enzyme activity and reactive analytes like hydrogen sulfide (H2S) inside of living mice. This launched an era of renewed interest in 1,2-dioxetanes that has resulted in a plethora of new chemiluminescence imaging agents developed by groups around the world. Our own research group focused its efforts on reactive sulfur, oxygen, and nitrogen species, pH, and hypoxia, resulting in a large family of bright chemiluminescent 1,2-dioxetanes validated for cell monitoring and in vivo imaging. These chemiluminescent probes feature low background and high sensitivity that have been proven quite useful for studying signaling, for example, the generation of peroxynitrite (ONOO-) in cellular models of immune function and phagocytosis. This high sensitivity has also enabled real-time quantitative reporting of oxygen-dependent enzyme activity and hypoxia in living cells and tumor xenograft models. We reported some of the first ratiometric chemiluminescent 1,2-dioxetane systems for imaging pH and have introduced a powerful kinetics-based approach for quantification of reactive species like azanone (nitroxyl, HNO) and enzyme activity in living cells. These tools have been applied to untangle complex signaling pathways of peroxynitrite production in radiation therapy and as substrates in a split esterase system to provide an enzyme/substrate pair to rival luciferase/luciferin. Furthermore, we have pushed chemiluminescence toward commercialization and clinical translation by demonstrating the ability to monitor airway hydrogen peroxide in the exhaled breath of asthma patients using transiently produced chemiluminescent 1,2-dioxetanedione intermediates. This body of work shows the powerful possibilities that can emerge when working at the interface of light and chemistry, and we hope that it will inspire future scientists to seek out ever brighter and more illuminating ideas.

Targeted pH switched europium complexes monitoring receptor internalisation in living cells.

We report the design and evaluation of pH responsive luminescent europium(iii) probes that allow conjugation to targeting vectors to monitor receptor internalisation in cells. The approach adopted here can be used to tag proteins selectively and to monitor uptake into more acidic organelles, thereby enhancing the performance of time-resolved internalisation assays that require pH monitoring in real time.

Construction of Zn(II)/Cd(II)-Yb(III) Schiff Base Complexes for the NIR Luminescent Sensing of Fluoroquinolone Antibiotics.

Two NIR luminescent Zn(II)/Cd(II)-Yb(III) complexes were obtained by the use of a Schiff base ligand with a binaphthyl backbone. Cd(II)-Yb(III) complex 2 has a triangular structure and exhibits interesting luminescent sensing activity to antibiotics, in particular to ciprofloxacin (CPFX) and norfloxacin (NFX) due to the inner filter effect. The limits of the detection of 2 to CPFX and NFX are 0.18 and 0.36 µM, respectively, and the fluorescence sensitivity is not changed with the existence of other antibiotics tested in this study.

A bioluminescent probe for in vivo imaging of pyroglutamate aminopeptidase in a mouse model of inflammation.

Pyroglutamate aminopeptidase (PGP) specifically cleaves the peptide bond of pyroglutamic acid linked to the N-terminal end of a polypeptide or protein. Previous studies showed that PGP was associated with several physiological processes and diseases especially those involving inflammation. Utilizing a 'caging' strategy, we designed and synthesized a bioluminescence probe (PBL) with a limit-of-detection of 3.7 * 10-4 mU/mL. In vivo imaging in a mouse model of inflammatory liver disease revealed that the probe has excellent sensitivity and selectivity and provides a powerful tool for studying the physiological and pathological processes involving PGP.

KLa(0.95-x)GdxF4:Eu3+ hexagonal phase nanoparticles as luminescent probes for in vitro Huh-7 cancer cell imaging.

A facile chemical route is reported for synthesizing red-emitting photoluminescent/MRI multi-functional KLa(0.95-x)GdxF4:Eu3+ (x = 0 to 0.4) bio-compatible nanomaterials for targeted in vitro tumor imaging. Hexagonal phase pure nanoparticles show a significant and systematic change in morphology with enhanced photoluminescence due to the substitution of La3+ with Gd3+ ions. Single phase ß-KLa(0.95-x)GdxF4:Eu3+ exhibits multifunctional properties, both intense red emission and strong paramagnetism for high-contrast bioimaging applications. These silica capped magnetic/luminescent nanoparticles show long-term colloidal stability, optical transparency in water, strong red emission, and low cytotoxicity. The cellular uptake of coated nanoparticles was investigated in liver cancer cell line Huh-7. Our findings suggest that these nanoparticles can serve as highly luminescent imaging probes for in vitro applications with potential for in vivo and live cell imaging applications.

Narrow-band red-emitting phosphor with negligible concentration quenching for hybrid white LEDs and plant growth applications.

Narrow-band red-emitters are the key to solving problems encountered by the current white LED technology. In this context, a series of new red-emitting Li3BaSrLa3(MoO4)8:Eu3+ phosphors were synthesized and characterized through various spectroscopic methods. All phosphor compositions were crystallized in the monoclinic phase, with space group C2/c. A broad charge transfer (O2-→ Mo6+) extended up to the blue region along with strong 7F0→5L6, 5D3 absorption, making them looked-for materials for warm white LED applications. The concentration quenching study reveals that there was no concentration-quenching occuring and the quantum yield of this non-concentration-quenching Li3BaSrLa0.3Eu2.7(MoO4)8 phosphor reaches 92.6%. The Li3BaSrLa0.3Eu2.7(MoO4)8 retain >80% of its emission intensity at 150 °C. The best red-emitting composition was integrated with near UV LED and obtained bright red emission with CIE x = 0.6647, y = 0.3357. White LED was fabricated by integrating the blue LED with yellow dye + red phosphor and white LED showed bright white light with CCT (5546 K), CIE (0.331, 0.385), and CRI (81%). In addition, the red LED spectrum is well-matched with the phytochrome (Pr) absorption spectrum and is useful for plant growth applications.

Carboxymethyl chitosan‒promoted luminescence of lanthanide metallogel and its application in assay of multiple metal ions.

In this work, the luminescence of lanthanide supramolecular metallogel formed by the self-assembly of 5,5',5″-(1,3,5-triazine-2,4,6-triyl)tris(azanediyl)triisophthalate (H6L) and Tb3+ was efficiently promoted by carboxymethyl chitosan (CMCS). The total quantum yield of the resultant metallogel (denoted as H6L/Tb3+/CMCS gel) was 9 times higher than the gel without CMCS. The average lifetime of H6L/Tb3+/CMCS gel increased from 0.51 ms to 1.20 ms. More importantly, the aqueous dispersion of H6L/Tb3+/CMCS xerogels showed a stable and pH-dependent luminescence. Based on the selective affinity of CMCS to different metal ions as well as with the aid of principal component analysis, H6L/Tb3+ /CMCS can be used as a sensor array to distinguish 11 metal ions (P < 0.05). This work provides a new strategy for the design and development of bio-based functional luminescent lanthanide supramolecular metallogels.

Luminescent cyclometalated platinum(ii) complexes with acyclic diaminocarbene ligands: structural, photophysical and biological properties.

Four new cyclometalated Pt(ii) complexes bearing acyclic diaminocarbene (ADC) ligands, [Pt(C^N)Cl{C(NHXyl)(NHR)}] [C^N = 2,6-difluorophenylpyridine (dfppy), phenylquinoline (pq); R = Pr 3a, 4a, CH2Ph 3b, 4b], were prepared by the nucleophilic attack on the isocyanide [Pt(C^N)Cl(CNXyl)] (C^N = dfppy 1, pq 2) by the corresponding amine RNH2 (R = Pr, CH2Ph). Complexes 3 show in their 1H NMR spectra in CDCl3 a notable concentration dependence, with a clear variation of the δH (NHXyl) signal, suggesting an assembling process implying donor-acceptor NHXylCl bonding, also supported by 1D-PGSE (Pulse Field Gradient Spin Echo) and 2D-DOSY (Diffusion Ordered Spectroscopy) NMR experiments in solution and X-ray diffraction studies. The intermolecular interactions in compounds 3a and 3b were studied by using Hirshfeld surface analysis and Non-Covalent Interaction (NCI) methods on their X-ray structures. Their photophysical properties were investigated by absorption and emission spectroscopies and also by TD-DFT calculations performed on 3a and 4b. These complexes show green (3) or orange (4) phosphorescence, attributed to a mixed 3IL/3MLCT excited state. The carbene ligand does not affect the emission maxima but it produces an increase of the quantum yields in relation to the isocyanide in the precursors. In fluid solutions, the emission is not concentration-dependent, but the complexes may show aggregation induced emission as detailed for complexes 3a and 4a. In addition, cytotoxicity studies in the human cell lines A549 (lung carcinoma) and HeLa (cervix carcinoma) showed good activity for these complexes and 3a, 3b and 4a exhibit a strong effect on DNA electrophoretic mobility. To the best of our knowledge, compounds 3 and 4 represent the first examples of cycloplatinated complexes bearing acyclic diamino carbenes with antiproliferative properties.

Tetraphenylpyrazine-Based Luminescent Metal-Organic Framework for Chemical Sensing of Carcinoids Biomarkers.

A new non-interpenetrated three-dimensional (3D) pillared-layered TPP-based LMOF [Zn3(TPyTPP)0.5(BDC)3]·8DMF (denoted as Zn-MOF 1) was successfully prepared (TPyTPP = tetrakis(4-(pyridin-4-yl)phenyl)pyrazine and H2BDC = 1,4-benzenedicarboxylic acid). Zn-MOF 1 was characterized by single-crystal X-ray diffraction, PXRD, IR, N2 adsorption, thermogravimetric analysis, and luminescent spectrum. Impressively, luminescent sensing studies reveal that activated Zn-MOF 1 not only displays excellent luminescence-quenching efficiency with the values of high Ksv and low LODs toward 5-hydroxytryptamine (5-HT) and 5-hydroxyindole-3-acetic acid (5-HIAA), respectively, but also possesses outstanding sensing characteristics in terms of fast response, high sensitivity, and specific selectivity. Zn-MOF 1 performs as efficient sensing of carcinoid biomarkers to provide a fresh detection platform for the diagnosis of carcinoids. In addition, the sensing mechanism was also explored on the basis of ultraviolet-visible (UV-vis) absorption, DFT calculations, and structural analysis.