Coumarin based emissive rod shaped new schiff base mesogens and their zinc(II) complexes: synthesis,photophysical, mesomorphism,gelation and DFT studies

New homologous series of coumarin Schiff base derived from 6-aminocoumarin and their zinc(II) complexes have been synthesised. The spectroscopic characterisations, photophysical properties, phase transition temperature, characterisation of phase and gelation behaviour are reported. The ligand is non-mesogenic at lower 4-n-alkoxy chain length (n = 4) and mesogenic for longer chains (n > 4). For small chains (n = 5, 7, 8), ligand displays monotropic nematic or nematic-smectic A phase sequences, whereas longer homologues (n = 12, 14, 16 and 18) display only enantiotropic smectic A phase. The ligands and their Zn(II) complexes are fluorescent in nature. Interestingly, ligands exhibit gelation property only in polar solvents, whereas Zn(II) complexes discourage gelation. The effect of 4-n-alkoxy chain length on the gelation properties was also discussed. Density functional theory calculations show broad agreement with observed molecular conformation, dipole moment, molecular orbitals and polarisability of the coumarin Schiff base molecules and their Zn(II) complexes.     

Two-photon absorption and photoluminescence of europium based emissive probes for bioactive systems

Observation of two-photon excitation (760 nm) and emission of two responsive water soluble europium complexes is reported with cross-sections of up to 2 GM. Two-photon excitation spectra have also been measured, acquisition being achieved by the use of a cavity-dumped mode locked Ti-sapphire laser. Time-gated detection is used to differentiate the ligand fluorescence and metal centred emission in these europium complexes.     

Highly emissive pi-conjugated alkynylpyrene oligomers: their synthesis and photophysical properties

We newly prepared para- and meta-linked alkynylpyrene oligomers and examined their photophysical properties. Oligomerization of monomeric building blocks was performed by CuI-promoted oxidative coupling reaction. The resulting oligomers mainly consist of 2-mer to 6-mer that were assigned on the basis of MALDI-TOF mass spectra, and the 2-mer, 3-mer, and 4-mer were isolated and fully characterized. From their absorption and fluorescence spectra, the para-linked oligomers were found to be somewhat pi-conjugated compared with meta-linked ones, and the fluorescence quantum yields decreased with increasing oligomer length (Phif = 0.79-0.55).     

Design, synthesis and evaluation of ratiometric probes for hydrogencarbonate based on europium emission

A series of cationic, zwitterionic and anionic macrocyclic europium complexes has been prepared incorporating a N or C- linked acridone chromophore that allows sensitisation of Eu emission following excitation at 390-410 nm. Each of these complexes selectively binds bicarbonate at physiological pH and reversible binding is signalled by a change in the form and relative intensity of the Eu emission spectrum. Affinity for bicarbonate is regulated by overall complex charge and falls within the range required for intracellular or extracellular analyses. Monitoring the ratio of the intensity of Eu emission at up to three wavelengths, e.g. 618/588 or 618/702 nm allows the solution concentration of bicarbonate to be deduced in a background of competing anions such as lactate, citrate and phosphate. Preliminary screens reveal the complexes to be non-toxic to NIH-3T3 cells and to be taken inside the cell, encouraging further study.     

Highly emissive dimesitylboryl-substituted 2,1,3-benzothiadiazole derivatives: photophysical properties and efficient fluorescent sensor for fluoride anions

One symmetrical and two unsymmetrical dimesitylboryl-substituted BTD derivatives 1–3 were prepared through Pd(0)-catalyzed Suzuki coupling reaction. All these compounds display intense fluorescence not only in solution but also in the solid state due to steric bulkiness of the boryl group, which is effective to suppress the intermolecular interactions in the solid state. In addition, the boryl-substituted BTD 1 displays prompt fluorescence responses to fluoride ions with high sensitivity through the complexation of the boron center with fluoride, demonstrating its potential utility as fluorescent sensor for fluoride ions.     

Design,synthesis, characterization,photophysical and pH chemosensor studies of novel 2,4,6-trisubstituted pyridines

For the application in organic electronics as well as pH meters, a series of novel 2,4,6-trisubstituted pyridines have been synthesized by the reaction of substituted aromatic aldehydes and substituted 3-acetylcoumarin with ammonium acetate. The structures of all the new compounds were characterized by IR, NMR, and GC–MS analysis. The important photo physical prerequisites for organoelectronic such as optical absorption and thermal stability were determined for the synthesized molecules. Optical properties were studied by UV–visible absorption and fluorescence spectroscopy. Optical band gaps of the 2,4,6-trisubstituted pyridines were found to be around 3.01–3.06?eV as calculated from their onset absorption edge. The pH-dependent changes in the fluorescence intensity suggest that 2,4,6-trisubstituted pyridines are useful applicants in intracellular pH meters.     

Synthesis, photophysical and thermal studies of symmetrical and unsymmetrical zinc phthalocyanines

The novel zinc phthalocyanine (3) with malonylester and chloro groups on each benzo unit was synthesized from 4-diethoxymalonyl-5-chloro-phthalonitrile (1). The unsymmetrically substituted zinc phthalocyanine (5), carrying hexylthio, malonylester and chloro groups at the periphery, was obtained from 4-diethoxymalonyl-5-chloro-phthalonitrile (1) and 4,5-bis-hexylsulfanyl-phthalonitrile (2) by a statistical condensation method as an A₃B type unsymmetrical phthalocyanine compound. Transesterification of the malonyl esters of the new symmetrical and unsymmetrical phthalocyanines occurred during the cyclotetramerization of dinitriles with Zn(CH₃COO)₂ in 1-pentanol in the presence of DBU. Octa-hexylthio-substituted zinc phthalocyanine (4) was prepared according to the literature. The photophysical and thermal properties of all the phthalocyanine complexes are described for the first time. These novel symmetrical and unsymmetrical phthalocyanine macrocycles have been characterized by a series of spectroscopic methods including ¹H NMR, electronic absorption, IR and mass spectroscopy, in addition to elemental analysis. Their narrow long wavelength absorption band shows that the bulky substituents on the periphery prevent aggregation. The unsymmetrically substituted phthalocyanine (5) gave a greater fluorescence quantum yield in chloroform than the symmetrical analogues (3 and 4).     

Design, synthesis, and photophysical characterization of water-soluble chlorins

The use of chlorins as photosensitizers or fluorophores in a range of biological applications requires facile provisions for imparting high water solubility. Two free base chlorins have been prepared wherein each chlorin bears a geminal dimethyl group in the reduced ring and a water-solubilizing unit at the chlorin 10-position. In one design (FbC1-PO3H2), the water-solubilizing unit is a 1,5-diphosphonopent-3-yl ("swallowtail") unit, which has previously been used to good effect with porphyrins. In the other design (FbC2-PO3H2), the water-solubilizing unit is a 2,6-bis(phosphonomethoxy)phenyl unit. Two complementary routes were developed for preparing FbC2-PO3H2 that entail introduction of the protected phosphonate moieties either in the Eastern-half precursor to the chlorin or by derivatization of an intact chlorin. Water-solubilization is achieved in the last step of each synthesis upon removal of the phosphonate protecting groups. The chlorins FbC1-PO3H2 and FbC2-PO3H2 are highly water-soluble (>10 mM) as shown by 1H NMR spectroscopy (D2O) and UV-vis absorption spectroscopy. The photophysical properties of the water-soluble chlorins in phosphate-buffered saline solution (pH 7.4) at room temperature were investigated using static and time-resolved absorption and fluorescence spectroscopic techniques. Each chlorin exhibits dominant absorption bands in the blue and the red region (lambda = 398, 626 nm), a modest fluorescence yield (Phi f approximately 0.11), a long singlet excited-state lifetime (tau = 7.5 ns), and a high yield of intersystem crossing to give the triplet state (Phi isc = 0.9). The properties of the water-soluble chlorins in aqueous media are comparable to those of hydrophobic chlorins in toluene. The high aqueous solubility combined with the attractive photophysical properties make these compounds suitable for a wide range of biomedical applications.     

Engineering emissive europium and terbium complexes for molecular imaging and sensing

Emissive f-block coordination complexes constitute an important class of optical probes, with applications ranging from sensing of bioactive species, high throughput assays and screening protocols in vitro, to time-resolved imaging studies in cellulo or in vivo. The key chemistry issues to be addressed in complex design and characterisation are defined, with an emphasis on the use of emissive europium and terbium complexes and their conjugates in molecular imaging. Both luminescent 'tags' useful in energy transfer studies and 'responsive' systems for sensing are discussed.     

Synthesis,photophysical and photochemical studies of water soluble cationic zinc phthalocyanine derivatives

Peripherally and non-peripherally 2-diethylaminoethanethiol tetra-substituted zinc phthalocyanine (5a and 6a) and their quaternized derivatives (5b and 6b) have been synthesized and characterized. The quaternized derivatives (5b and 6b) show excellent solubility in aqueous medium. The photophysical and photochemical properties of the 2-diethylaminoethanethiol appended zinc phthalocyanine in dimethylsulfoxide (DMSO) for the non-ionic (5a and 6a) and in both DMSO and aqueous medium (phosphate buffered saline solution PBS, pH 7.4) (in the presence and absence of cremophore EL (CEL)) for the quaternized (5b and 6b) derivatives were studied and compared with that of the peripherally octa-substituted derivatives (7a and 7b). The complexes have intense absorption in the visible/near-IR region though the quaternized forms (5b, 6b and 7b) were slightly blue shifted and highly aggregate in aqueous solution. The triplet state quantum yields (_ΦT${\Phi }_{\text{T}}$) and the triplet lifetimes (_τT${\tau }_{\text{T}}$) were found to be higher in DMSO (_ΦT${\Phi }_{\text{T}}$ values ranged from 0.57 to 0.75 while _τT${\tau }_{\text{T}}$ values ranged from 190 to 220 μs in DMSO for all complexes) compared to aqueous medium (_ΦT${\Phi }_{\text{T}}$ values ranged from 0.15 to 0.17 while _τT${\tau }_{\text{T}}$ values ranged from 20 to 70 μs in pH 7.4 buffer). Addition of cremophore EL in aqueous solution resulted in induced disaggregation leading to increased _ΦT${\Phi }_{\text{T}}$ and _τT${\tau }_{\text{T}}$.     

Porphyrin-appended europium(III) bis(phthalocyaninato) complexes: synthesis, characterization, and photophysical properties

Mixed cyclization of 3-mono-, 4-mono-, or 4,5-di(porphyrinated) phthalonitrile compounds 2, 3, or 6 and unsubstituted phthalonitrile with the half-sandwich complex [EuIII(acac)(Pc)] (Pc=phthalocyaninate, acac=acetylacetonate) as the template in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in n-pentanol afforded novel porphyrin-appended europium(III) bis(phthalocyaninato) complexes 7-9 in 30-40% yield. These mixed tetrapyrrole triads and tetrad were spectroscopically and electrochemically characterized and their photophysical properties were also investigated with steady-state and transient spectroscopic methods. It has been found that the fluorescence of the porphyrin moiety is quenched effectively by the double-decker unit through an intramolecular photoinduced electron-transfer process, which takes place in several hundred femtoseconds, while the recombination of the charge-separated state occurs in several picoseconds. By using different phthalocyanines containing different numbers of porphyrin substituents at the peripheral or nonperipheral position(s) of the ligand, while the other unsubstituted phthalocyanine remains unchanged in these double-deckers, the effects of the number and the position of the porphyrin substituents on these photophysical processes were also examined.     

Luminescent triarylborane-functionalized polystyrene: synthesis, photophysical characterization, and anion-binding studies

A new class of highly fluorescent triarylborane polymers has been prepared from trimethylsilyl-substituted polystyrene via a modular approach that involves selective polymer modification reactions with organometallic reagents. The photophysical properties, environmental stability, and the Lewis acidity of the boron sites have been tailored through modifications in the substitution pattern on boron. The photophysical properties are indicative of electronic communication between the chromophores attached to polystyrene, which has been exploited for the efficient probing of fluoride and cyanide in the micromolar concentration range.     

New soluble methylendioxy-phenoxy-substituted zinc phthalocyanine derivatives: Synthesis, photophysical and photochemical studies

The syntheses of new three phthalonitriles (1, 2 and 3), together with photophysical and photochemical properties of the resulting peripherally and non-peripherally tetrakis- and octakis 3,4-(methylendioxy)-phenoxy-substituted zinc phthalocyanines (4, 5 and 6) are described for the first time. Complexes 4, 5 and 6 have been synthesized and characterized by elemental analysis, IR, ¹H NMR spectroscopy, electronic spectroscopy and mass spectra. Complexes 4, 5 and 6 have good solubility in organic solvents such as CHCl₃, DCM, DMSO, DMF, THF and toluene and are mainly not aggregated (except for complex 6 in DMSO) within a wide concentration range. General trends are described for singlet oxygen, photodegradation, fluorescence quantum yields, triplet quantum yields and triplet life times of these complexes in DMSO and toluene. Complex 4 has higher singlet oxygen quantum yields, fluorescence quantum yields, triplet quantum yields and triplet life times than complexes 5 and 6. The effect of the solvents on the photophysical and photochemical parameters of the zinc(II) phthalocyanines (4, 5 and 6) are also reported.     

Design, synthesis and photophysical studies of simple fluorescent anion PET sensors using charge neutral thiourea receptors

The synthesis of four fluorescent photoinduced electron transfer (PET) chemosensors 1-4 for anions is described. These are all based on a simple design employing charge neutral aliphatic or aromatic thiourea anion receptors connected to an anthracene fluorophore via a methylene spacer. Here the anion recognition occurred through 1 : 1 hydrogen bonding between the thiourea protons and the anion, as demonstrated by observing the changes in the (1)H NMR in DMSO-d(6) where the two thiourea protons were shifted downfield upon addition of anions. Whereas 1-3 were designed for the detection of anions such as fluoride, acetate or phosphate, 4 was made for the recognition of N-protected amino acids. All the sensors showed 'ideal' behaviour where only the fluorescence emission was quenched upon anion recognition, due to enhanced efficiency of electron transfer quenching from the receptor to the excited state of the fluorophore. By simply varying the nature of the thiourea substituent it was possible to modulate, or tune, the acidity of the thiourea receptor moiety, altering the sensitivity of the anion recognition. For, the anion selectivity and the degree of the fluorescence quenching were in the order of F(-) > AcO(-) > H(2)PO(4)(-), with Cl(-) or Br(-) not being detected.     

A bis(rhodamine 6G)-based fluorescent sensor for Hg2+: microwave-assisted synthesis,photophysical properties,and computational studies

A fluorescent and colorimetric sensor based on rhodamine 6 g (RD6g) was designed, synthesized, and characterized using microwave irradiation. The sensing behavior of this compound was studied by UV–visible and fluorescence spectroscopy. Sensor RD6g exhibits a high selectivity and an excellent sensitivity and is a dual-responsive colorimetric and fluorescent Hg²⁺-specific sensor in aqueous buffer solution. Mercury ions give rise to the development of a very fluorescent ring-open amide spirolactam system. The detection limit for Hg²⁺ was found to be 1.2?×?10^?8 M. The binding ratio of RD6g-Hg²⁺ complex was determined to be 1:1 according to the Job’s plot. The reversibility of RD6g?Hg²⁺ complex has been achieved with CN^? anions. The test strip based on RD6g was developed, which could be used as a suitable and methodical Hg²⁺ test kit.

     

Design and synthesis of an enzyme-cleavable sensor molecule for phosphodiesterase activity based on fluorescence resonance energy transfer

Ratiometric measurement is a technique that can provide precise data and even quantitative detection. To carry out ratiometric measurements, it is necessary that the sensor molecule exhibits a large shift in its emission or excitation spectrum after reaction with the target molecule. Fluorescence resonance energy transfer (FRET) is one mechanism used to obtain a large spectral shift. In this study, our aim was to develop a ratiometric fluorescent sensor molecule for phosphodiesterase activity based on FRET. We designed and synthesized CPF4 with a coumarin donor, a fluorescein acceptor, and two phenyl linkers having the phosphodiester moiety interposed between them. In the emission spectrum of CPF4 in aqueous buffer excited at 370 nm, the emission of the coumarin donor was strongly quenched and the emission of the fluorescein acceptor was observed. This emission spectrum demonstrates that energy transfer from the coumarin donor to the fluorescein acceptor proceeds efficiently. Addition of a phosphodiesterase to an aqueous solution of CPF4 resulted in an increase in the donor fluorescence and a decrease in the acceptor fluorescence. CPF4 exhibited a large shift in its emission spectrum after the hydrolysis of the phosphodiester group by the enzyme. This large shift of the emission spectrum indicates that ratiometric measurements can be made by using CPF4. The method described in this paper for designing enzyme-cleavable sensor molecules based on FRET should be readily applicable to other hydrolytic enzymes.     

Synthesis and photophysical properties of silica nanoparticles based on europium(iii) complexes

Monodisperse 280?C480 nm silica particles doped with europium complexes with thenoyltrifluoroacetylacetone and adducts with some phosphine oxides as luminophores were prepared for the first time by the optimized Stober procedure using surfactants. The size and the polydispersity of the obtained particles depend on the luminophore incorporated in the polymer template. The photophysical properties of aqueous dispersions of the europium(iii)-doped silica particles were studied by the luminescence method.     

Fluorescent zinc sensor with minimized proton-induced interferences: photophysical mechanism for fluorescence turn-on response and detection of endogenous free zinc ions

A new fluorescent zinc sensor (HNBO-DPA) consisting of 2-(2'-hydroxy-3'-naphthyl)benzoxazole (HNBO) chromophore and a di(2-picolyl)amine (DPA) metal chelator has been prepared and examined for zinc bioimaging. The probe exhibits zinc-induced fluorescence turn-on without any spectral shifts. Its crystal structure reveals that HNBO-DPA binds a zinc ion in a pentacoordinative fashion through the DPA and HNBO moieties. Steady-state photophysical studies establish zinc-induced deprotonation of the HNBO group. Nanosecond and femtosecond laser flash photolysis and electrochemical measurements provide evidence for zinc-induced modulation of photoinduced electron transfer (PeT) from DPA to HNBO. Thus, the zinc-responsive fluorescence turn-on is attributed to suppression of PeT exerted by deprotonation of HNBO and occupation of the electron pair of DPA, a conclusion that is further supported by density functional theory and time-dependent density functional theory (DFT/TD-DFT) calculations. Under physiological conditions (pH 7.0), the probe displays a 44-fold fluorescence turn-on in response to zinc ions with a K(d) value of 12 pM. The fluorescent response of the probe to zinc ions is conserved over a broad pH range with its excellent selectivity for zinc ions among biologically relevant metal ions. In particular, its sensing ability is not altered by divalent transition metal ions such as Fe(II), Cu(II), Cd(II), and Hg(II). Cell experiments using HNBO-DPA show its suitability for monitoring intracellular zinc ions. We have also demonstrated applicability of the probe to visualize intact zinc ions released from cells that undergo apoptosis. More interestingly, zinc-rich pools in zebrafish embryos are traced with HNBO-DPA during early developmental stages. The results obtained from the in vitro and in vivo imaging studies demonstrate the practical usefulness of the probe to detect zinc ions.     

Design,synthesis, photophysical,and electrochemical properties of DCM‐based conjugated polymers for light‐emitting devices

A series of conjugated hyperbranched polymers, hyperbranched copolymers, and linear polymers containing 2‐pyran‐4‐ylidenemalononitrile (acceptor) and triphenylamine/fluorene (donor) units were synthesized and characterized by FTIR, ¹H NMR, thermogravimetric analyses, differential scanning calorimetry, gel permeation chromatography, UV–visible, photoluminescence, and cyclic voltammetry measurements. All the polymers show red‐light emission in the range of 566–656 nm both in solution and in solid state. The quantum efficiency of the polymers was in the range of 56–82%. Among the six polymers synthesized, only polymers containing fluorene units show T_g and polymers based on triphenylamine not exhibit T_g. The band gap of these polymers were found to be reasonably low; hyperbranched copolymer containing fluorene unit shows lowest band gap of 2.18 eV due to the stabilization of LUMO energy level by the electron withdrawing ? CN groups. The thermal and solubility behavior of the polymers were found to be good. All the EL spectra of the devices (indium‐tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)/polymer/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline/tris(8‐hydroxyquinoline)aluminum)/LiF/Al) show red‐light emission, and the device fabricated with P3 and P4 shows maximum luminance and luminous efficiency of 4104 cd m^?2 and 0.55 cd Å^?1 and 3696 cd m^?2 and 0.47 cd Å^?1, respectively, indicates that they had the best carrier balance. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012