CN Isomerization Research Articles

Solvent-dependent selective fluorescence sensing of Al3 + and Zn2 + using a single Schiff base

The selective assay of Al3+ and Zn2+ ions is reported using fluorescence enhancement of a simple Schiff base receptor 1 (1=1-((E)-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-ylimino)methyl)naphthalen-2-ol) in aqueous solvents. The Schiff base receptor 1 showed a turn-on fluorescence in the presence of Al3+ in a mixture of methanol–water, as a result of a restricted CN isomerization mechanism. The ion selectivity of 1 could be switched for Zn2+ by swapping the solvent from methanol–water to DMF–water mixture. In particular, this chemosensor could clearly distinguish Zn2+ from Cd2+. The binding properties of 1 with the metal ions were investigated by UV–vis, fluorescence, electrospray ionization mass spectroscopy and 1H NMR titration.

A single fluorescent chemosensor for multiple target ions: Recognition of Zn2+ in 100% aqueous solution and F− in organic solvent

A structurally simple 2-((1-hydroxynaphthalene-2-yl)methyleneamino)acetic acid (1) was used as a turn-on fluorescent sensor for both Zn2+ and F−. This receptor 1 showed a significant fluorescence enhancement in the presence of zinc ion over most other competitive metal ions in 100% aqueous solution. In particular, this chemosensor could clearly distinguish Zn2+ from Cd2+. The spectroscopic studies suggested that the selective response to zinc ion involved the disruption of the internal charge transfer (ICT), and the inhibition of the CN isomerization and the excited-state proton transfer (ESIPT). In vitro studies with fibroblasts showed fluorescence when sensor 1 and Zn2+ were present. The receptor 1 could also sense F− selectively in DMSO. The fluorescence, UV–vis titration and 1H NMR titration indicated that F− ion enhanced the fluorescence of 1 through deprotonation.

Two highly sensitive Schiff-base fluorescent indicators for the detection of Zn2+

Two novel Schiff-based fluorescent probes 1 and 2 for Zn2+ ion were developed. The spectroscopic behavior toward metal ions has been investigated by the absorption and fluorescence method. Both of the two probes were found to show good selectivity to Zn2+ ion over other metal ions in ethanol–water solution, such as Li+, Na+, Mg2+, Al3+, K+, Ca2+, Ba2+, Cr3+, Mn2+, Cu2+, Co2+, Ni2+, Fe3+, Hg2+, Ag+, Cd2+ and Pb2+. The selectivity mechanism of 1 and 2 to metal ions is based on a combinational effect of proton transfer (ESPT), CN isomerization and chelation-enhanced fluorescence (CHEF). pH response experiments show that the probes can be used to indicate pH values under certain conditions.

Colorimetric and turn-on fluorescence detection of Ag(I) ion

In the present work, we have synthesized a new probe 2,-2-disulfediyl bis-(N-anthracen-9-ylmethylene)ethanamine (Cysan) based on anthracene platform. The probe was characterized using NMR spectroscopy, UV–visible, emission spectroscopy and mass spectrometry. The probe was tested for its sensing behaviour towards metal ions like Mn2+, Fe2+, Ba2+, Co2+, K+, Ni2+, Ca2+, Al3+, Cu2+, Cu+, Cr3+, Zn2+, Pb2+, Cd2+, Na+, Au+, Hg2+ and Ag+ in the ethanol/water mixture buffered with HEPES (pH=7.4) by UV–visible and fluorescent techniques. It shows high selectivity towards sensing Ag+ metal ions. Importantly fluorescence enhancement at 440nm was observed for probe upon complexation with Ag+ ions over other metal ions. This fluorescence enhancement is attributable to the restriction of the CN isomerization and the prevention of photoinduced electron transfer from nitrogen to anthracene fluorophore. These photonic studies indicate that the probe can be adopted as a selective, sensitive and reversible fluorescent chemosensor for Ag+ ions.

A simple and effective fluorescent chemosensor for the cascade recognition of Zn2+ and H2PO4− ions in protic media

The chiral Schiff base N-(2-hydroxy-1-naphthyl)methylidene-(S)-α-phenylethylamine (1) was designed and synthesized as a fluorescent chemosensor based on the photoinduced electron transfer (PET) and CN isomerization mechanisms. The receptor exhibited a highly selective switch-on fluorescence response toward Zn2+ and especially avoided the interference of Cd2+ when the detection was conducted in unbuffered methanol solution. Furthermore, the in situ generated 1·ZnII-complex could be employed for the specific recognition of H2PO4− by fluorescence turn-off signaling through a ligand displacement process.

A facile fluorescent probe based on coumarin-derived Schiff base for Al3 + in aqueous media

A facile coumarin-based Schiff base sensor, 7-hydroxy-8-((2-hydroxy-phenylimino)methyl)-coumarin (H2L), has been designed and synthesized for selective detection of Al3+ in methanol–water (1:1, v/v). A fluorescence enhancement of 12-fold at 528nm can be realized upon addition of 5.0 equiv. Al3+ with excitation at 421nm. The mechanism of CN isomerization, coupled with photoinduced energy transfer, is proposed to explain the observed spectral response. Most importantly, this Al3+ sensor has good water-solubility and can be excited with light in the visible range.

A new carbazole-based Schiff-base as fluorescent chemosensor for selective detection of Fe3+ and Cu2+

A new carbazole-based Schiff-base (1) as a multi-functional fluorescent chemosensor was designed, synthesized and characterized, which can selectively recognized Fe3+ and Cu2+ ions over a number of other metal ions. Compound 1 could detect Fe3+ and Cu2+ by UV–Vis method and fluorescence method. The stoichiometry ratio of 1-Fe3+ and 1-Cu2+ are 2:1 and 1:1, respectively, by the method of Job’s plot. Moreover, the detection limits were calculated to be 4.23×10−6mol/L for Fe3+ ion and 5.67×10−6mol/L for Cu2+ ion. In the presence of Fe3+/Cu2+ ions, the fluorescence enhancement was attributed to the inhibited CN isomerization and the obstructed excited state intramolecular proton transfer (ESIPT) of compound 1. At the same time, the interactions of compounds 1 with other ions were also investigated and unobvious UV–Vis absorption and fluorescence spectral changes were observed. Thus a new kind of chemosensor for Fe3+/Cu2+with high sensitivity and selectivity was introduced.

A novel “turn-on” fluorescent probe for Fe3+in aqueous media based on CN isomerization

A new type of fluorescent probe (L) for Fe3+ based on CN isomerization was synthesized. It shows fluorescence enhancement (turn-on) response for Fe3+ in aqueous media (v(CH3CN):v(H2O) = 1:9). The complexation mode and the corresponding fluorescent enhancement mechanism were elucidated by IR spectra, transient spectra, Job plot, and DFT calculations.

A Zn2+-responsive highly sensitive fluorescent probe and 1D coordination polymer based on a coumarin platform

A coumarin-based Schiff base (receptor 1) exhibited fluorescence enhancement selectively with Zn(2+) at a nanomolar level in near-aqueous medium (EtOH-H2O; 1:1, v/v). The response was instantaneous with a detection limit of 3.26 × 10(-9) M. The sensing event is supposed to incorporate a combinational effect of intramolecular charge transfer (ICT), chelation-enhanced fluorescence (CHEF) and C[double bond, length as m-dash]N isomerization mechanisms. Various spectroscopic methods, viz. IR, UV-visible, fluorescence and NMR in association with single crystal XRD studies, were used for thorough investigation of the structure of receptor 1 as well as of the sensing event. The Zn(2+) complex of receptor 1 exhibited a very nice 1D chain coordination polymeric framework in its single crystal XRD.

A turn-on Schiff base fluorescence sensor for zinc ion

A simple Schiff base type fluorescent receptor 1 was prepared and evaluated for its fluorescence response to heavy metal ions. Receptor 1 exhibits an ‘off–on-type’ mode with high selectivity in the presence of Zn2+ ion. The selectivity of 1 for Zn2+ is the consequence of combined effects of chelation-enhanced fluorescence (CHEF), CN isomerization, and inhibition of photoinduced electron transfer (PET).

Fluorescent sensors based on controllable conformational change for discrimination of Zn2+ over Cd2+

A new coumarin-derived imine IC1 was designed for highly selective sensing of Zn2+ over Cd2+ ions in aqueous solution based on a controllable CN isomerization mechanism. Methyl as a fine controllable unit was incorporated into IC1 to tune the conformational change of imine and thus significantly improve its selectivity to Zn2+ compared with methyl-free imine IC2. Sensor IC1 was also utilized to image intracellular Zn2+ ions in HepG2 cells with a good performance.

Synthesis, characterization and metal ion-sensing properties of two Schiff base derivatives

Two new Schiff base derivatives {2,2′-diphenyl-N,N′-bis(2-pyridylmethylene)biphenyl-4,4′-diamine} (1) and {2,2′-diphenyl-N,N′-bis(salicylidene)biphenyl-4,4′-diamine} (2) were synthesized and characterized by means of elemental analysis, 1H NMR, FT-IR, and standard spectroscopy techniques. The molecular structure of 2 has been determined by X-ray single crystal analysis. The analyses of fluorescence properties of the compounds revealed that 1 and 2 are both poorly fluorescent and display sensitive fluorescence responses to a panel of 24 monovalent, divalent, and trivalent metal ions in CH3CN–DMSO (9:1, v/v). Results with imine 1 showed that Fe3+, Cu2+, Zn2+, Cd2+, Mn2+, Zr4+, Hg2+, Cr2+, Pb2+, Sn2+, Bi2+, Al3+, Ce3+, La3+, Sm3+, Gd3+, Nd3+, Eu3+ and Dy3+ yields red shifts in emission and increases in intensity. And the greatest spectral changes for imine 2 include enhancements in emission intensity coupled with red shifts (Zr4+, Sn2+, Al3+ and Zn2+) and strong quenching (Fe3+). The fluorescence enhancement mechanism of 1 and 2 for metal ions is based on: (i) CN isomerization; (ii) chelation-enhanced fluorescence (CHEF) effect; and (iii) excited-state intra/intermolecular proton transfer (ESPT).

ChemInform Abstract: New Sensing Mechanisms for Design of Fluorescent Chemosensors Emerging in Recent Years

AbstractReview: 94 refs.

A “turn-on” fluorescent probe for hypochlorous acid: convenient synthesis, good sensing performance, and a new design strategy by the removal of CN isomerization

A new design strategy for the development of fluorescent turn-on chemodosimeters toward OCl(-) was proposed by the removal, but not the general inhibition, of the C=N isomerization. Accordingly, Flu-1 was prepared, which, as shown in the fluorescent picture, exhibited the off-on response for OCl(-), sensitively and selectively.

Fluorescence and phosphoescence

A dual-function probe NAHH based on naphthalene was synthesized and characterized. Based on the combination effects derived from the inhabitation of photo-induced electron transfer (PET) and CN isomerization, probe NAHH achieved in the recognition of Zn2+ and Al3+ both through obvious fluorescence enhancement and color changes detected by naked eye, respectively. Probe NAHH showed high sensitivity with the limit of detection as low as 3.02 × 10−7 M for Zn2+ and 7.55 × 10−8 M for Al3+, indicated the capability of probe NAHH in trace detection for Zn2+ and Al3+. The binding ratio of NAHH with Zn2+ and Al3+ were all 1:1 determined by Job plot, and the corresponding association constant was calculated as 8.48 × 104 M−1 and 4.45 × 105 M−1, respectively. The mechanism was further confirmed by FT-IR, 1H NMR titration and ESI-MS analysis. Furthermore, probe NAHH was successfully applied in logic gate construction and the detection of Zn2+ and Al3+ in Songhua River and test stripe. Fluorescence imaging experiments confirmed that NAHH could be used to monitor Zn2+ in plant root.