Zinc-Specific Fluorescent Response of Tris(isoquinolylmethyl)amines (isoTQAs)

Isoquinoline-based tetradentate ligands with <i>C</i>₃-symmetry, tris­(1- or 3-isoquinolylmethyl)­amine (1- or 3-isoTQA), have been prepared and their zinc-induced fluorescence enhancement was investigated. Upon excitation at 324 nm, 1-isoTQA shows very weak fluorescence (ϕ = ∼0.003) in DMF/H₂O (1/1) solution. In the presence of zinc ion, 1-isoTQA exhibits fluorescence increase (ϕ = 0.041) at 359 and 470 nm. This fluorescence enhancement at 470 nm is specific for zinc. However, 3-isoTQA exhibited a smaller fluorescence enhancement upon zinc complexation (ϕ = 0.017, λ_em = 360 and 464 nm) compared with 1-isoTQA. Crystal structures of zinc complexes of isoTQAs demonstrate the diminished steric crowding and shorter Zn–N_aromatic distances compared with isoTQENs (<i>N,N,N′,N′</i>-tetrakis­(isoquinolylmethyl)­ethylenediamines) leads to a higher fluorescent response toward zinc relative to cadmium

Zinc-Specific Fluorescent Response of Tris(isoquinolylmethyl)amines (isoTQAs)

Isoquinoline-based tetradentate ligands with <i>C</i>₃-symmetry, tris­(1- or 3-isoquinolylmethyl)­amine (1- or 3-isoTQA), have been prepared and their zinc-induced fluorescence enhancement was investigated. Upon excitation at 324 nm, 1-isoTQA shows very weak fluorescence (ϕ = ∼0.003) in DMF/H₂O (1/1) solution. In the presence of zinc ion, 1-isoTQA exhibits fluorescence increase (ϕ = 0.041) at 359 and 470 nm. This fluorescence enhancement at 470 nm is specific for zinc. However, 3-isoTQA exhibited a smaller fluorescence enhancement upon zinc complexation (ϕ = 0.017, λ_em = 360 and 464 nm) compared with 1-isoTQA. Crystal structures of zinc complexes of isoTQAs demonstrate the diminished steric crowding and shorter Zn–N_aromatic distances compared with isoTQENs (<i>N,N,N′,N′</i>-tetrakis­(isoquinolylmethyl)­ethylenediamines) leads to a higher fluorescent response toward zinc relative to cadmium

Fluorescent Detection of Phosphate Ion via a Tetranuclear Zinc Complex Supported by a Tetrakisquinoline Ligand and μ₄‑PO₄ Core

The tetrakisquinoline ligand HT­(6-MeO8Q)­HPN (<i>N,N,N</i>′<i>,N</i>′-tetrakis­(6-methoxy-8-quinolylmethyl)-2-hydroxy-1,3-propanediamine) exhibited Zn²⁺-induced fluorescence enhancement with high specificity and sensitivity (<i>I</i>_Zn/<i>I</i>₀ = 57 and <i>I</i>_Cd/<i>I</i>_Zn = 6% in the presence of 2 equiv of Zn²⁺; LOD (limit of detection) = 15 nM). This ligand also exhibited fluorescence enhancement specific to inorganic phosphate (PO₄^3–) in DMF–HEPES buffer (50 mM HEPES, 100 mM KCl, pH = 7.5) (1:1) in the presence of 2 equiv of Zn²⁺. The structure of the unprecedented tetranuclear zinc complex with a μ₄-PO₄ bridge was elucidated by X-ray crystallography as the key species responsible for fluorescence enhancement

Quantitative Fluorescent Detection of Pyrophosphate with Quinoline-Ligated Dinuclear Zinc Complexes

Dinuclear zinc complex [Zn₂(TQHPN)­(AcO)]²⁺ exhibits characteristic fluorescence response (λ_ex = 317 nm and λ_em = 455 nm) toward pyrophosphate (PPi) with maximum fluorescence upon 1:1 Zn₂(TQHPN)–PPi complex formation. The crystallographic investigation utilizing P¹P²–Ph₂PPi revealed that the fluorescent response mechanism is due to intramolecular excimer formation of two quinoline rings

Quantitative Fluorescent Detection of Pyrophosphate with Quinoline-Ligated Dinuclear Zinc Complexes

Dinuclear zinc complex [Zn₂(TQHPN)­(AcO)]²⁺ exhibits characteristic fluorescence response (λ_ex = 317 nm and λ_em = 455 nm) toward pyrophosphate (PPi) with maximum fluorescence upon 1:1 Zn₂(TQHPN)–PPi complex formation. The crystallographic investigation utilizing P¹P²–Ph₂PPi revealed that the fluorescent response mechanism is due to intramolecular excimer formation of two quinoline rings

Pyrophosphate-Induced Intramolecular Excimer Formation in Dinuclear Zinc(II) Complexes with Tetrakisquinoline Ligands

Dinuclear Zn²⁺ complexes with HTQHPN (<i>N,N,N</i>′<i>,N</i>′-tetrakis­(2-quinolylmethyl)-2-hydroxy-1,3-propanediamine) derivatives have been prepared, and their pyrophosphate (PPi, P₂O₇^4–) sensing properties were examined. The ligand library includes six HTQHPN derivatives with electron-donating/withdrawing substituents, an extended aromatic ring, and six-membered chelates upon zinc binding. Complexation of ligand with 2 equiv of Zn²⁺ promotes small to moderate fluorescence enhancement around 380 nm, but in the cases of HTQHPN, HT­(6-FQ)­HPN (<i>N,N,N</i>′<i>,N</i>′-tetrakis­(6-fluoro-2-quinolylmethyl)-2-hydroxy-1,3-propanediamine), and HT­(8Q)­HPN (<i>N,N,N</i>′<i>,N</i>′-tetrakis­(8-quinolylmethyl)-2-hydroxy-1,3-propanediamine), subsequent addition of PPi induced a significant fluorescence increase around 450 nm. This fluorescence enhancement in the long-wavelength region is attributed to the conformational change of the bis-(quinolylmethyl)­amine moiety which promotes intramolecular excimer formation between adjacent quinolines upon binding with PPi. The structures of PPi- and phosphate-bound dizinc complexes were revealed by X-ray crystallography utilizing phenyl-substituted analogues. The zinc complex with HT­(8Q)­HPN exhibits the highest signal enhancement (<i>I</i>_PPi/<i>I</i>₀ = 12.5) and selectivity toward PPi sensing (<i>I</i>_ATP/<i>I</i>_PPi = 20% and <i>I</i>_ADP/<i>I</i>_PPi = 25%). The fluorescence enhancement turned to decrease gradually after the addition of more than 1 equiv of PPi due to the removal of zinc ion from the ligand–zinc–PPi ternary complex, allowing the accurate determination of PPi concentrations at the fluorescence maximum composition. The practical application of the present method was demonstrated monitoring the enzymatic activity of pyrophosphatase

Cd²⁺-Specific Fluorescence Response of Methoxy-Substituted <i>N</i>,<i>N</i>‑Bis(2-quinolylmethyl)-2-methoxyaniline Derivatives

The N3O1 tetradentate ligand, TriMeOBQMOA (N,N-bis(5,6,7-trimethoxy-2-quinolylmethyl)-2-methoxyaniline), was developed as a Cd2+-specific fluorescent sensor. The structure of TriMeOBQMOA is half of TriMeOBAPTQ (N,N,N′,N′-tetrakis(5,6,7-trimethoxy-2-quinolylmethyl)-1,2-bis(2-aminophenoxy)ethane), which is a tetrakisquinoline derivative of the well-known calcium chelator BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid). The fluorescent Cd2+ selectivity of TriMeOBAPTQ (IZn/ICd = 5.3% in the presence of 3 equiv of metal ions in MeOH–HEPES buffer (9:1)) comes from the formation of fluorescent dinuclear cadmium (M2L) and nonfluorescent OH-bridged dizinc ((μ-OH)M2L) complexes. TriMeOBQMOA also exhibits excellent Cd2+ specificity in fluorescence enhancement (IZn/ICd = 2.3% in the presence of 5 equiv of metal ions in DMF–HEPES buffer (1:1, HEPES 50 mM, KCl 0.1 M, pH = 7.5)) via substantial formation of a highly fluorescent bis(μ-chloro)dinuclear cadmium complex ([Cd2(μ-Cl)2L2]2+), which is in equilibrium with the mononuclear Cd2+ complex ([CdLCl]+), and extremely poor stability of the TriMeOBQMOA-Zn2+ complex. The all-nitrogen derivatives of BQMOA and BAPTQ, namely, N,N-BQDMPHEN (N,N-bis(2-quinolylmethyl)-N′,N′-dimethyl-1,2-phenylenediamine) and BPDTQ (N,N,N′,N′-tetrakis(2-quinolylmethyl)-2,2′-(N,N′-dimethylethylenediamino)dianiline), respectively, and their methoxy-substituted derivatives were also prepared, and the fluorescent metal ion sensing properties are discussed

Formation of η²‑Coordinated Dihydropyridine–Ruthenium(II) Complexes by Hydride Transfer from Ruthenium(II) to Pyridinium Cations

Reactions between various pyridinium cations with and without a −CF₃ substituent at the 3-position and [Ru­(tpy)­(bpy)­H]⁺ (tpy = 2,2′:6′,2″-terpyridine and bpy = 2,2′-bipyridine) were investigated in detail. The corresponding 1,4-dihydropyridines coordinating to a Ru­(II) complex in η² mode through a CC bond were quantitatively formed at the initial stage. The only exception observed was in the case of the 1-benzylpyridinium cation, where a mixture of two adducts with 1,4-dihydropyridine and 1,2-dihydropyridine was formed in the ratio 96:4. Cleavage of the Ru–(CC) bond proceeded at a slower rate in all reactions, giving the corresponding dihydropyridine and [Ru­(tpy)­(bpy)­(NCCH₃)]²⁺ when acetonitrile was used as a solvent. Kinetic activation parameters for the adduct formation indicated that the 1,4-regioselectivities were induced by formation of sterically constrained structures