Efficient Fluoride-Selective Fluorescent Host: Experiment and Theory

Photophysical Studies of Anion-Induced Colorimetric Response and Amplified Fluorescence Quenching in Dipyrrolylquinoxaline-Containing Conjugated Polymers

The dipyrrolylquinoxaline (DPQ)-containing monomer and polymers were synthesized and employed as chromogenic and fluorescent chemosensors for inorganic anions. We have found that in the presence of fluoride or pyrophosphate, the receptors do not form hydrogen bonds between the pyrrole protons and anions. The colorimetric responses and fluorescence quenching in these chemosensors are indeed the result of deprotonation of the N-H proton. The anion selectivity is primarily determined by the relative basicity of anions. The sensitivity of DPQ-based chemosensor was found to display a 34-fold enhancement by incorporation into the conjugated polymer. The anion-induced deprotonation generates low-energy, non-fluorescent trapping sites and is responsible for the signal amplification where the quenching of the excited state occurs from the deprotonated DPQ site in the network by rapid exciton migration along the polymeric backbone.

Conformations and Electronic Structures of Axially Coordinated Fullerene−Porphyrin−Fullerene Triad (C60CHCOO)2−Sn(IV) Porphyrin

The conformational (cis and trans) stability and electronic structures of (C(60)CHCOO)(2)-Sn(IV) porphyrin, recently synthesized as a novel fullerene-porphyrin-fullerene triad linked by metal axial coordination, have been studied by ab initio calculations. The cis conformer was found to be slightly more stable than the trans by 1.38 kcal/mol in the neutral compound. Upon the addition of an electron to the triad, the relative stability of the cis conformer was found to be higher (3.29 kcal/mol) than that in the neutral one. From the investigation of frontier molecular orbitals, for the cis conformer, it was found that the electrons are localized in HOMO of the porphyrin, while the electrons are localized in LUMO of the syn-fullerene. For the trans conformer, it was found that the electrons are localized in HOMO of the porphyrin, while the electrons are localized in LUMO of one of the two fullerene moieties, and the electrons are localized in LUMO2 of the other fullerene moiety, but the LUMO and LUMO2 have the same orbital energy. Thus, the PET may take place unidirectionally in the cis conformer from the porphyrin to the syn-fullerene, while it is bidirectional from the porphyrin to both of the fullerene moieties.

Host–guest complexation of a fluorescent and electrochemical chemsensor for fluoride anion

Host-guest complexation of a ferrocenenylphalene dyad 1, 1,1'-diaceylferrocenyl-3-hydroxyl-2-naphthoylhydrazone, as a fluorescent and electrochemical chemosensor for fluoride anion, was investigated. Crystal structure analysis revealed that the two naphthyl arms of compound 1 positioned in the same side about the ferrocene moiety and interacted through pi-pi stacking interactions. The intermolecular pi-pi stacking interactions and the C-H...pi interactions between the ferrocene moieties and the naphthalene rings linked the molecules together featuring a two-dimensional layered structure. Fluorescence titrations of compound 1 indicated that in the presence of F- and H2PO4-, the emission intensities enhanced significantly. Electrochemical titrations revealed that compound 1 sensed the F- anion in high selectivity with a cathodic shift of 120 mV, and had no sense in recognizing H2PO4- anion. 1H NMR titrations demonstrated that while compound 1 hydrogen-bonded to H2PO4- forming simple 1 : 1 host-guest complex, further addition of F- induced the deprotonation of compound 1.

Heteroditopic ferrocene-based ureas as receptors for anions and cations

The synthesis of structurally new types of ferrocene-based ureas, in which the ferrocene moiety is simultaneously attached to two urea groups, have been prepared directly from 1,1-bis(isocyanato)ferrocene 1. Receptors 2a, 2b, and 2d show spectral and electrochemical anion-sensing capability for hydrogenphosphate and fluoride anions, in addition 2d also shows a sensing capability for acetate anion. Heteroditopic receptor 2a, bearing a pyridine binding site, does not complex Cu2+ cations but instead an intramolecular redox reaction takes place to give the oxidized form 2a+. The results obtained by using the heteroditopic receptor 2b, containing two crown ethers units, demonstrate that K+ cations can only be electrochemically detected in the presence of hydrogenphosphate anion. Whereas the new structural motif 2d, in which the two urea groups are part of the macrocycle framework, is a selective electrochemical sensor for Li+ cation in the presence of alkali metal ions. The preferred binding modes and their extent are proposed for the most representative complexes by means of DFT-based theoretical calculations.

Highly Selective Two-Photon Chemosensors for Fluoride Derived from Organic Boranes

[reaction: see text] Three organoboron compounds are shown to be two-photon fluorescent sensors for fluoride anion with high sensitivity and selectivity. The recognition mechanism is attributed to the unique steric structure of the bulky dimesitylboryl group and the Lewis acid-base interaction between trivalent boron atom and fluoride anion.

New advances in fluorogenic anion chemosensors

The development of anion chemosensors is an area of recent interest. We make here a comprehensive review of new advances on anion chemosensing, reported in the literature during the year 2004. The review follows a classification of the sensing systems based on design principles. It comprises: the binding site-signalling subunit approach, the displacement approach and the use of fluoro-chemodosimeters. The first two approximations are based on the use of a suitable anion coordination site coupled with a signalling unit which signals the anion coordination process via changes in its fluorescence behaviour. The two basic subunits are covalently linked in the binding site-signalling subunit approach and not in the displacement approach. In both approaches the fluorescence variation is reversible. The third way to the development of fluorogenic chemosensors is the use of chemodosimeters (also called reagents or reactands) that work usually through irreversible chemical reactions coupled with drastic changes in the fluorescence emission behaviour.

Visible Colorimetric Fluoride Ion Sensors

[structure: see text] Five new urea derivative naphthalene compounds were synthesized by a reaction of 1,8-diaminonaphthalene and the corresponding isocyanates and showed a distinct color change only when treated with fluoride ions.

New Fluorescent Chemosensor Based on Exciplex Signaling Mechanism

[reaction: see text]. A novel fluorescent chemosensor (compound 1) containing aminonaphthol, which selectively recognizes fluoride anion with high sensitivity, was synthesized. The fluorescence of compound 1 was quenched rapidly by fluoride ion, and a new peak at a longer wavelength emerged concurrently, which constituted the signature for fluoride detection. The mechanism of exciplex formation was proposed for the interesting observation.

Stacking Effect of Polyfluorene on the Chemical Shift and Electron Transport

We investigate the structures, NMR chemical shifts, absorption spectra, frontier molecular orbitals, and transition density matrices of pi-stacked polyfluorenes by ab initio calculations. For F1-F4, we consider two different conformations, syn and anti. The simulated 1H NMR chemical shifts are in good agreement with the previous experiment, and the significantly upfielded chemical shifts explain that the fluorene moieties are stacked on each other. It is found that the relative stability for syn and anti conformers is almost equivalent in B3LYP calculations; however, the syn conformer becomes much more stable than the anti conformer in MP2 calculations, which is consistent with the experimental finding that only the syn conformers are relevant. The vertical detachment energy, which is linearly proportional to the ionization potential, shows the same size dependence as the previous experiment. The electron attachment energy decreases exponentially as the size increases, which implies that the electron transport would be possible even for long chains such as F3 and F4. This was evident from the frontier molecular orbitals (HOMO and LUMO). Also, it is found that the syn conformers are very favorable for electron transport through the pi-stacked fluorene moieties.

Conformational Analysis and Rotational Barriers of Alkyl- and Phenyl-Substituted Urea Derivatives

Potential energy surfaces (PES) for rotation about the N-C(sp(3)) or N-C(aryl) bond and energies of stationary points on PES for rotation about the C(sp(2))-N bond are reported for methylurea, ethylurea, isopropylurea, tert-butylurea, and phenylurea, using the B3LYP/DZVP2 and MP2/aug-cc-pVDZ methods. The analysis of alkylureas reveals cis and (less stable) trans isomers that adopt anti geometries, whereas syn geometries do not correspond to stationary points. In contrast, the analysis of phenylurea reveals that the lowest energy form at the MP2 level is a trans isomer in a syn geometry. The fully optimized geometries are in good agreement with crystal structure data, and PESs are consistent with the experimental dihedral angle distribution. Rotation about the C(sp(2))-N bond in alkylureas and phenylurea is slightly more hindered (8.6-9.4 kcal/mol) than the analogous motion in the unsubstituted molecule (8.2 kcal/mol). At the MP2 level of theory, the maximum barriers to rotation for the methyl, ethyl, isopropyl, tert-butyl, and phenyl groups are predicted to be 0.9, 6.2, 6.0, 4.6, and 2.4 kcal/mol, respectively. The results are used to benchmark the performance of the MMFF94 force field. Systematic discrepancies between MMFF94 and MP2 results were improved by modification of several torsional parameters.

Detection of anions using a fluorescent alizarin–phenylboronic acid ensemble

Selective anion-induced organization of phenylboronic acids and alizarin results in a new TURN-ON fluorescent sensor for anions in MeOH.

An Excimer-Based, Binuclear, On−Off Switchable Calix[4]crown Chemosensor

A new fluorescent chemosensor with two different types of cation binding sites on the lower rims of a 1,3-alternate calix[4]arene (1) is synthesized. Two pyrene moieties linked to a cation recognition unit composed of two amide groups form a strong excimer in solution. For 1, the excimer fluorescence is quenched by Pb2+, but revived by addition of K+ to the Pb2+ ligand complex. Thus, metal ion exchange produces an on-off switchable, fluorescent chemosensor. Computational results show that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of the two pyrene moieties interact under UV irradiation of 1 and its K+ complex, while such HOMO-LUMO interactions are absent in the Pb2+ complex.

Unique Hydrogen Bonds between 9-Anthracenyl Hydrogen and Anions

Unique hydrogen bonds of the 9-H of anthracene moieties in hosts 1 and 2 with fluoride and pyrophosphate ions were observed on the basis of the (1)H NMR experiments. Furthermore, hosts 1 and 2 act as a colorimetric sensor and a fluorescent chemosensor for the recognition of fluoride ion, respectively.