Access to long-lived room temperature phosphorescence through auration of 2,1,3-benzothiadiazole

A series of 2,1,3-benzothiadiazole-Au(I)-L complexes have been synthesised, structurally characterised and investigated for their photophysical properties. These are the first organometallic Au(I) complexes containing a C-Au bond on the highly electron-deficient benzothiadiazole unit. The complexes exhibit solution-phase phosphorescence at room temperature, assigned to the intrinsic triplet state of the benzothiadiazole unit that is efficently populated through its attachment to gold. Comparison with routinely reported Au(I) complexes, which include intervening alkenyl linkers, suggests that previous assignments of their phosphorescence as 1π → π*(CCR) might be incomplete. Our observations affirm that, in addition to the heavy atom effect, breaking symmetry in the involved aryl motif may be of importance in controlling the luminescence properties.

Synthesis, structures, and photophysical properties of π-extended arsaborins

In this study, various (hetero)arene-fused arsaborins were synthesized. All the synthesized arsaborins were stable under ambient conditions and allowed for the chemical modification of the lone pair of the arsenic atom. Experimental and computational studies revealed that these compounds possessed planar structures and weak anti-aromatic properties. Fluorescence with large Stokes shifts was observed due to drastic structural relaxation at 298 K, whereas intense phosphorescence due to the heavy-atom effect of arsenic was observed at 77 K. Furthermore, a thiophene-fused derivative demonstrated a temperature-dependent emission color change in the solid state, attributable to the gradual alteration in the ratio of monomer fluorescence, excimer fluorescence, and phosphorescence.

Robust and highly emissive copper(I) halide 1D-coordination polymers with triphenylarsine and a series of bridging N-heteroaromatic co-ligands

Various 1D-coordination polymers with dinuclear rhombic {Cu2X2} cores (X = Br, I) were synthesized using a spontaneous evaporation method employing triphenylarsine (AsPh3) and six types of bidentate N-heteroaromatic co-ligands. The coordination polymers exhibited intense emission even at 298 K (quantum yield: up to 0.60), and their emission color was dependent on the N-heteroaromatic co-ligand. The emission efficiencies of these coordination polymers were higher than those of the discrete complexes with AsPh3 and monodentate N-heteroaromatic co-ligands reported in our previous work. In addition, the luminescence of these coordination polymers was more resistant to mechanical stimuli than that of the discrete ones.

Gold Complexes in Anticancer Therapy: From New Design Principles to Particle-Based Delivery Systems

The discovery of the medicinal properties of gold complexes has fuelled the design and synthesis of new anticancer metallodrugs, which have received special attention due to their unique modes of action. Current research in the development of gold compounds with therapeutic properties is predominantly focused on the molecular design of drug leads with superior pharmacological activities, e.g., by introducing targeting features. Moreover, intensive research aims at improving the physicochemical properties of gold compounds, such as chemical stability and solubility in the physiological environment. In this regard, the encapsulation of gold compounds in nanocarriers or their chemical grafting onto targeted delivery vectors could lead to new nanomedicines that eventually reach clinical applications. Herein, we provide an overview of the state-of-the-art progress of gold anticancer compounds, andmore importantly we thoroughly revise the development of nanoparticle-based delivery systems for gold chemotherapeutics.

Systematic Study of Pnictogen-Fused Heterofluorenes

Heteroatom-fused π-conjugated molecules have attracted considerable attention, and various elements for such fusion have been investigated. Herein, we focused on pnictogen-fused heterofluorenes. The structures, reactivity with O₂ and I₂, coordination ability to AuCl, and photophysical properties were systematically studied to better understand the effects of pnictogen atoms on the nature of π-conjugated molecules.

Breast Cancer Treatment: The Case of Gold(I)-Based Compounds as a Promising Class of Bioactive Molecules

Breast cancers (BCs) may present dramatic diagnoses, both for ineffective therapies and for the limited outcomes in terms of lifespan. For these types of tumors, the search for new drugs is a primary necessity. It is widely recognized that gold compounds are highly active and extremely potent as anticancer agents against many cancer cell lines. The presence of the metal plays an essential role in the activation of the cytotoxicity of these coordination compounds, whose activity, if restricted to the ligands alone, would be non-existent. On the other hand, gold exhibits a complex biochemistry, substantially variable depending on the chemical environments around the central metal. In this review, the scientific findings of the last 6–7 years on two classes of gold(I) compounds, containing phosphane or carbene ligands, are reviewed. In addition to this class of Au(I) compounds, the recent developments in the application of Auranofin in regards to BCs are reported. Auranofin is a triethylphosphine-thiosugar compound that, being a drug approved by the FDA—therefore extensively studied—is an interesting lead gold compound and a good comparison to understand the activities of structurally related Au(I) compounds.

Real time optical detection of gold in living cells through genetically-encoded probe

To study the efflux of gold (Au) in living cells, a genetically encoded fluorescence resonance energy transfer (FRET)-based sensor has been developed. The gold-sensing domain GolB from Salmonella typhimurium has been fused to the N- and C-termini of the FRET pair enhanced cyan fluorescent protein (ECFP) and Venus respectively. In living cells, this probe is highly selective and sensitive to gold and it can withstand changes in variable pH ranges. GolSeN-25, the most efficient sensor variant, binds gold with an affinity (K_d) of 0.3 × 10⁻⁶ M, covering gold concentrations of nM to μM, and can be used for non-invasive real-time in vivo gold measurement in living cells. A simple and sensitive FRET probe was designed for the detection of gold with high selectivity and can be applied to the analysis of real samples.

To study the efflux of gold (Au) in living cells, a genetically encoded fluorescence resonance energy transfer (FRET)-based sensor has been developed.

Zn Coordination and the Identity of the Halide Ancillary Ligand Dramatically Influence the Excited-State Dynamics and Bimolecular Reactions of 2,3-Di(pyridin-2-yl)benzo[g]quinoxaline

The optical properties of coordination complexes with ligands containing nitrogen heterocycles have been extensively studied for decades. One subclass of these materials, metal complexes utilizing substituted pyrazines and quinoxalines as ligands, has been employed in a variety of photochemical applications ranging from photodynamic therapy to organic light-emitting diodes. A vast majority of this work focuses on characterization of the metal-to-ligand charge-transfer states in these metal complexes; however, literature reports rarely investigate the photophysics of the parent pyrazine or quinoxaline ligand or perform control experiments utilizing metal complexes that lack low-lying charge-transfer (CT) states in order to determine how metal-atom coordination influences the photophysical properties of the ligand. With this in mind, we examined the steady-state and time-resolved photophysics of 2,3-di(pyridin-2-yl)benzo[g]quinoxaline (dpb) and explored how the coordination of ZnX₂ (X = Cl^-, Br^-, I^-) affects the photophysical properties of dpb. In dpb, we find that the dominant mode of deactivation from the singlet excited state is intersystem crossing (ISC). Coordination of ZnX₂ perturbs the relative energies of the ππ* and nπ* excited states of dpb, leading to drastically different rates of ISC as well as radiative and nonradiative decay in the [Zn(dpb)X₂] complexes compared to dpb. These differences in the rates change the dominant singlet-excited-state decay pathway from ISC in dpb to a mixture of ISC and fluorescence in [Zn(dpb)Cl₂] and [Zn(dpb)Br₂] and to nonradiative decay in [Zn(dpb)I₂]. Coordination of ZnX₂ and the choice of the halide ligand also have profound effects on the rate constants for excited-state bimolecular reactions, including triplet-triplet annihilation and oxygen quenching. These results demonstrate that metal coordination, even in complexes lacking low-lying CT states, and the choice of the ancillary ligand can dramatically alter the photophysical properties of chromophores containing nitrogen heterocycles.

Mechanochemical synthesis of mononuclear gold(I) halide complexes of diphosphine ligands with tuneable luminescent properties

A mechanochemical method is reported for the synthesis of Au(diphos)X complexes of diphosphine (diphos = XantPhos and N-XantPhos) ligands and halide ions (X = Cl and I). The Au(XantPhos)X (1: X = Cl; 2: X = I) and Au(N-XantPhos)Cl (3) complexes exhibited either yellowish green (1) or bluish green (2) emission, whereas 3 was seemingly non-emissive in the solid state at room temperature. Blue- (2B) and bluish green (2G) luminescent concomitant solvates of 2 were obtained by recrystallization. Luminescent colour changes from blue (2B) or bluish green (2G) to yellow were observed when these forms were subjected to mechanical stimulus, while the original emission colour can be recovered in the presence of solvent vapours. Moreover, the luminescence of 2B can be reversibly altered between blue and yellow by heating/cooling-cycles. These results demonstrate the power of mechanochemistry in the rapid (4 min reaction time), efficient (up to 98% yield) and greener synthesis of luminescent and stimuli-responsive gold(I) complexes.

Dinuclear gold(I) complexes with N-phosphanyl, N-heterocyclic carbene ligands: synthetic strategies, luminescence properties and anticancer activity

A small library of dinuclear gold(I) complexes with the title ligands has been prepared, encompassing neutral, mono- and dicationic complexes. The luminescence properties of the complexes in the solid state have been evaluated, and it turns out that neutral and monocationic complexes not presenting a rigid metallamacrocyclic structure can exhibit rather strong emissions that extend towards the red region of the visible spectrum. The in vitro anticancer activity of the complexes has been also preliminarly evaluated; cytotoxicity seems to correlate with complex lipophilicity, whereas selectivity towards cancer cells can be apparently enhanced upon a judicious choice of the ligands.

Unprecedented [d9]Cu[d10]Au coinage bonding interactions in {Cu(NH3)4[Au(CN)2]}+[Au(CN)2]- salt

The X-ray structure of the {Cu(NH₃)₄[Au(CN)₂]}⁺[Au(CN)₂]^- salt is reported showing an unprecedented [d⁹]Cu[d¹⁰]Au coinage bond. The physical nature of the interaction has been studied using DFT calculations, including the quantum theory of atoms-in-molecules, the noncovalent interaction plot and the natural bond orbital analysis, revealing the nucleophilic role of the [d¹⁰]Au metal and the electrophilic role of [d⁹]Cu metal.

Fluorination Effects in XPhos Gold(I) Fluorothiolates

Gold phosphine derivatives such as thiolates have been recently proposed as catalysts or catalyst precursors. The relevance of the supramolecular environment on the fine-tuning of the catalytical activity on these compounds incentivizes the use of tools that are convenient to characterize in detail the non-covalent landscape of the systems. Herein, we show the molecular and supramolecular diversity caused by the changes in the fluorination pattern in a family of new XPhos goldfluorothiolate derivatives. Furthermore, we studied the supramolecular interactions around the Au centers using quantum chemical topology tools, in particular the quantum theory of atoms in molecules (QTAIM) and the non-covalent interaction index. Our results give detailed insights into the fluorination effects on the strength of intramolecular and intermolecular interactions in these systems. We have also used QTAIM delocalization indexes to define a novel hapticity indicator. Finally, we assessed the trans influence of the fluorothiolates on the phosphine in terms of the change in the δ 31P-NMR. These results show the feasibility of the use of fluorination in the modulation of the electronic properties of Buchwald phosphine gold(I) compounds, and thereby its potential catalytic activity.

Homo- and hetero-metallophilicity-driven synthesis of highly emissive and stimuli-responsive Au(i)–Cu(i) double salts

Discrete complex salts having Au···Au and Au···Cu interactions were obtained as three crystalline polymorphs exhibiting various emission colors with high efficiency. Solvent vapor caused crystal-to-crystal transition, changing the emission color.

Keep it tight: a crucial role of bridging phosphine ligands in the design and optical properties of multinuclear coinage metal complexes

Copper subgroup metal ions in the +1 oxidation state are classical candidates for aggregation via non-covalent metal-metal interactions, which are supported by a number of bridging ligands. The bridging phosphines, soft donors with a relatively labile coordination to coinage metals, serve as convenient and essential components of the ligand environment that allow for efficient self-assembly of discrete polynuclear aggregates. Simultaneously, accessible and rich modification of the organic spacer of such P-donors has been used to generate many fascinating structures with attractive photoluminescent behavior. In this work we consider the development of di- and polynuclear complexes of M(i) (M = Cu, Ag, Au) and their photophysical properties, focusing on the effect of phosphine bridging ligands, their flexibility and denticity.

A Macrocyclic Gold(I)-Biphenylene Complex: Triangular Molecular Structure with Twisted Au2 (diphosphine) Corners and Reductive Elimination of [6]Cycloparaphenylene

The digold(I) complex [Au₂ Cl₂ (Cy₂ PCH₂ PCy₂ )] reacts with 4,4'-diphenylene diboronic acid to form a triangular macrocyclic complex with twisted Au-P-C-P-Au groups at the three corners. The synthesis of the complex and its chemical oxidation produced [6]cycloparaphenylene ([6]CPP) in 59 % overall yield.

New selective thiolate gold(i) complexes inhibit the proliferation of different human cancer cells and induce apoptosis in primary cultures of mouse colon tumors

New thiolate gold(i) complexes with P(NMe₂)₃ (HMPT) as phosphane group have been developed as proapoptotic and selective anticancer drugs.

Harvesting greenish blue luminescence in gold(i) complexes and their application as promising bioactive molecules and cellular bioimaging agents

This work presents new gold materials as promising antitumoral and antibacterial agents and their potential use as luminescent probes for cellular bioimaging.

Coordination-Driven Self-Assembly of Silver(I) and Gold(I) Rings: Synthesis, Characterization, and Photophysical Studies

In this work, we investigate the self-assembly between Ag(I) and Au(I) centers and pyridyl donors to form hexagonal metallacycles and related linear complexes. The precipitation of hexagonal metallacycles upon assembly in chloroform/methanol mixtures results in high solid-state photo-stability. Whereas, the Ag(I) species have fast kinetics and high formation constants in acetone, this solvent interferes in the formation of the analogous Au(I) complexes. The photophysical properties of this suite of metallacycles was investigated including steady-state absorption, emission, and time-resolved lifetime measurements. All ligands and hexagons exhibited ligand-centered singlet emissions with ground-state absorption and emission perturbed upon coordination. The ligand-based fluorescent photoluminescence was affected by the heavy-atom effect when halide or metals are present, attenuating quantum yields as evidenced by increases in the experimentally measured non-radiative rate constants. The formation of group 11 metallacycles is motivated by their potential applications in mixed-matrix materials wherein metal ions can interact with substrate to facilitate separations chemistry with reduced energy requirements, in particular the isolation of ethylene and light olefins. Existing processes involve cryogenic distillation, an energy intensive and inefficient method.

Chiral AuI - and AuIII -Isothiourea Complexes: Synthesis, Characterization and Application

During an investigation into the potential union of Lewis basic isothiourea organocatalysis and gold catalysis, the formation of gold-isothiourea complexes was observed. These novel gold complexes were formed in high yield and were found to be air- and moisture stable. A series of neutral and cationic chiral gold(I) and gold(III) complexes bearing enantiopure isothiourea ligands was therefore synthesized and fully characterized. The steric and electronic properties of the isothiourea ligands was assessed through calculation of their percent buried volume and the synthesis and analysis of novel iridium(I)-isothiourea carbonyl complexes. The novel gold(I)- and gold(III)-isothiourea complexes have been applied in preliminary catalytic and biological studies, and display promising preliminary levels of catalytic activity and potency towards cancerous cell lines and clinically relevant enzymes.

Fluorescent functionalised naphthalimides and their Au(i)–NHC complexes for potential use in cellular bioimaging

A series of fluorescent gold(i)–NHC complexes have been developed and investigated as cell imaging agents.

Synthesis and characterisation of fluorescent aminophosphines and their coordination to gold(i)

Three novel fluorescent aminophosphine ligands have been synthesised that incorporate napthyl (L1), pyrenyl (L2) and anthraquinone (L3) chromophores into their structures. The ligands react with [AuCl(tht)] (tht = tetrahydrothiophene) to give neutral complexes of the form [AuCl(L1-3)]. Solid state, X-ray crystallographic data was obtained for the anthraquinone derivative, [AuCl(L3)], and showed a distorted linear coordination geometry at Au(i). The packing structure also revealed a number of intermolecular π-π interactions that involve the anthraquinone and phenyl units of the aminophosphine ligand. 31P NMR spectroscopic data revealed δP values of +42.2 (L1), +42.1 (L2) and +26.1 (L3) ppm, which shifted downfield upon coordination to Au(i) to +64.6, +64.7, and +55.8 ppm, respectively. Supporting TD-DFT studies were able to reproduce the structure and 31P NMR chemical shifts of [AuCl(L3)] as well as rationalise the HOMO-LUMO compositions. Photophysical studies showed that the appended fluorophore dominates the absorption and emission properties for the ligands and complexes, with the anthraquinone derivatives showing visible emission at ca. 570 nm which was attributed to the intramolecular charge transfer character of the phosphinoaminoanthraquinone fragment.

Achieving yellow emission by varying the donor/acceptor units in rod-shaped fluorenyl-alkynyl based π-conjugated oligomers and their binuclear gold(i) alkynyl complexes

Fluorenyl-alkynyl based π-conjugated rod-shaped oligomers bearing different central aromatic moieties and functionalizable di-alkynyl termini, such as H-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-H (OH1), H-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Btz-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-H (OH2) and H-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Btd-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-H (OH3) where Fl = 9,9-dioctylfluorene, Btz = N-hexylbenzotriazole, and Btd = benzothiadiazole, were successfully synthesized by a Pd(0) catalyzed Stille coupling protocol. Electron withdrawing benzothiadiazole and benzotriazole as strong to moderate acceptors and fluorene as the donor have been incorporated to adjust the Donor-Acceptor (D-A) strength for fine-tuning the bandgap (E_g) as well as the emission wavelength. The corresponding digold(i) σ-complexes (PPh₃)Au-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Au(PPh₃) (OM1), (PPh₃)Au-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Btz-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Au(PPh₃) (OM2) and (PPh₃)Au-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Btd-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Au(PPh₃) (OM3) have also been prepared by a reaction of Au(PPh₃)Cl and methanolic NaOMe in DCM with the corresponding alkynyl functionalized oligomers to take advantage of the heavy-atom effect on their emissive properties. The synthesized rod-shaped π-conjugated fluorene based oligomers and their binuclear Au(i) σ-complexes have been unambiguously characterized by various spectroscopic tools such as FTIR and multinuclear NMR as well as MALDI-TOF and CHN analyses. The absorption and emission spectral studies exhibited a progressive red shift with increasing the electron withdrawing character of the central aromatic unit. The rod-like oligomers having alkynyl termini and the corresponding digold(i) complexes are found to be blue, cyan and yellow emissive, demonstrating the fine-tuning of the emission wavelength. Most importantly, the fluorene based π-conjugated yellow light emitters OH3 and OM3 are successfully achieved by varying the donor/acceptor moiety to the fluorenyl-alkynyl backbone. The digold(i) diacetylide organometallic wires exhibit phosphorescence at 77 K in degassed CH₂Cl₂ due to the efficient intersystem crossing from the S₁ to the T₁ excited state as induced by heavy atoms.

Aurophilicity vs. thiophilicity: directing the crystalline supramolecular arrangement in luminescent gold compounds

The fluorination of thiolate ligands is applied as a tool that allows control of the prevalence of thiophilic (Au–S) interactions over the expected aurophilic contacts in luminescent gold compounds.

Recent development of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents, anticancer drugs, and antibacterial agents

This Perspective summarizes recent advances in the biological applications of luminescent rhenium(i) tricarbonyl polypyridine complexes.

Polypyridyl-functionalizated alkynyl gold(i) metallaligands supported by tri- and tetradentate phosphanes

The synthesis of photophysical active polytopic alkynyl gold(i) metallaligands and preliminary studies of their interaction with metal cations are presented.

Tuning the fluorescence emission in mononuclear heteroleptic trigonal silver(i) complexes

Mononuclear heteroleptic trigonal planar silver(i) complexes have been prepared and characterized, both in solution and in the solid state. Their luminescent behavior was investigated, showing a tunable emission maxima according to the electronic properties of the phosphorous ligand, in the solid state.

The interplay between fluorescence and phosphorescence with luminescent gold(i) and gold(iii) complexes bearing heterocyclic arylacetylide ligands

The photophysical properties of a series of gold(i) [LAu(C 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 CR)] (L = PCy₃ (1a-4a), RNC (5a), NHC (6a)) and gold(iii) complexes [Au(C^N^C)(CCR)] (1b-4b) bearing heterocyclic arylacetylide ligands with narrow band-gap are compared. The luminescence of both series are derived from an intraligand transition localized on the arylacetylide ligand (ππ*(CCR)) but 1a-3a displayed prompt fluorescence (τ_PF = 2.7-12.0 ns) while 1b-3b showed mainly phosphorescence (τ_Ph = 104-205 μs). The experimentally determined intersystem crossing (ISC) rate constants (k_ISC) are on the order of 10⁶ to 10⁸ s^-1 for the gold(i) series (1a-3a) but 10¹⁰ to 10¹¹ s^-1 for the gold(iii) analogues (1b-3b). DFT/TDDFT calculations have been performed to help understand the difference in the k_ISC between the two series of complexes. Owing to the different oxidation states of the gold ion, the Au(i) complexes have linear coordination geometry while the Au(iii) complexes are square planar. It was found from DFT/TDDFT calculations that due to this difference in coordination geometries, the energy gap between the singlet and triplet excited states (ΔE_ST) with effective spin-orbit coupling (SOC) for Au(i) systems is much larger than that for the Au(iii) counterparts, thus resulting in the poor ISC efficiency for the former. Time-resolved spectroscopies revealed a minor contribution (<2.9%) of a long-lived delayed fluorescence (DF) (τ_DF = 4.6-12.5 μs) to the total fluorescence in 1a-3a. Attempts have been made to elucidate the mechanism for the origins of the DF: the dependence of the DF intensity with the power of excitation light reveals that triplet-triplet annihilation (TTA) is the most probable mechanism for the DF of 1a while germinate electron-hole pair (GP) recombination accounts for the DF of 2a in 77 K glassy solution (MeOH/EtOH = 4 : 1). Both 4a and 4b contain a BODIPY moiety at the acetylide ligand and display only ¹IL(ππ*) fluorescence with negligible phosphorescence being observed. Computational analyses attributed this observation to the lack of low-lying triplet excited states that could have effective SOC with the S₁ excited state.

Assembly of a Highly Stable Luminescent Zn5 Cluster and Application to Bio-Imaging

The assembly sequence of the coordination cluster [Zn5 (H2 L(n) )6 ](NO3 )4 ]⋅8 H2 O⋅2 CH3 OH (Zn5 , H3 L(n) =(1,2-bis(benzo[d]imidazol-2-yl)-ethenol) involves in situ dehydration of 1,2-bis(benzo[d]imidazol-2-yl)-1,2-ethanediol (H4 L) through the formation of the [Zn(H3 L)2 ](+) monomer, dimerization to [Zn2 (H3 L)2 ](+) , dehydration of the ligand to [Zn2 (H2 L(n) )2 ](+) , and the final formation of the pentanuclear cluster. The cluster has the following special characteristics: 1) high stability in both refluxing 37 % HCl and 27 % NH3 , 2) low cytotoxicity, and 3) pH-sensitive fluorescence in the visible-to-near-infrared (Vis/NIR) region in the solid state and in solution. We have applied it as a fluorescent probe both in vivo and in vitro. Its H-bonding ability is the key to its affinity and selectivity for imaging lysosomes in HeLa cells and tumors in male BALB/C mice. It provides a new type of sensitive and biocompatible fluorescent probe for detecting small tumors (13.5 mm(3) ).