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  1. Article ; Online: Fluorescence quenching of the N-methylquinolinium cation by pairs of water or alcohol molecules.

    Rodríguez-Prieto, Flor / Corbelle, Carlos Costa / Fernández, Berta / Pedro, Jorge A / Ríos Rodríguez, M Carmen / Mosquera, Manuel

    Physical chemistry chemical physics : PCCP

    2017  Volume 20, Issue 1, Page(s) 307–316

    Abstract: N-Methylquinolinium cation (MQ+) in its first-excited singlet state is a strong oxidant commonly used as a photosensitizer, whose fluorescence is therefore quenched by electron donors. Interestingly, the fluorescence of MQ+ is also quenched by hydroxy ... ...

    Abstract N-Methylquinolinium cation (MQ+) in its first-excited singlet state is a strong oxidant commonly used as a photosensitizer, whose fluorescence is therefore quenched by electron donors. Interestingly, the fluorescence of MQ+ is also quenched by hydroxy compounds such as water and alcohols, more difficult to oxidize. We investigated the quenching mechanism of MQ+ fluorescence by small amounts of water and alcohols in acetonitrile solution. The fluorescence intensities and lifetimes exhibited a nonlinear dependence on the quencher concentration. We found evidence that emissive exciplexes MQ+*-ROH are formed between the excited quinolinium and the hydroxy compounds. An accurate quantitative description of the results was obtained with a model in which the exciplex reacts with a second molecule of the hydroxy compound, which quenches the fluorescence. The rate constant of this process increased as the quencher ionization energy decreased. We showed also that a low basicity of the hydroxy compound inhibits the quenching process. These results are consistent with the existence of a concerted photoinduced proton-coupled electron transfer (PCET) involving an intermediate complex of the excited quinolinium with a H-bonded molecular pair of the hydroxy compounds. In these pairs, a water or an alcohol molecule is able to donate an electron to the photoexcited quinolinium cation and a proton to the second H-bonded hydroxy molecule, showing an enhanced reducing power in comparison with the isolated molecule. The structure of the intermediate complex was investigated using high-level quantum mechanical calculations. At high water concentrations in acetonitrile/water mixtures, the quenching process is slowed down, indicating that higher water aggregates are less effective for a PCET process. The results obtained may be relevant to the study of water oxidation and electron transfer in biological systems.
    Language English
    Publishing date 2017-12-20
    Publishing country England
    Document type Journal Article
    ZDB-ID 1476244-4
    ISSN 1463-9084 ; 1463-9076
    ISSN (online) 1463-9084
    ISSN 1463-9076
    DOI 10.1039/c7cp07057h
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  2. Article ; Online: Excited-state proton and charge transfer in protonated amino and methylated derivatives of 2-(2'-hydroxyphenyl)benzimidazole.

    Ríos Vázquez, Sonia / Pérez Lustres, J Luis / Rodríguez-Prieto, Flor / Mosquera, Manuel / Ríos Rodríguez, M Carmen

    The journal of physical chemistry. B

    2015  Volume 119, Issue 6, Page(s) 2475–2489

    Abstract: We studied the excited-state behavior of a family of mono- and diprotonated derivatives of 2-phenylbenzimidazole in different solvents, using steady-state and time-resolved fluorescence spectroscopy. The species investigated were 2-(4'-amino-2'- ... ...

    Abstract We studied the excited-state behavior of a family of mono- and diprotonated derivatives of 2-phenylbenzimidazole in different solvents, using steady-state and time-resolved fluorescence spectroscopy. The species investigated were 2-(4'-amino-2'-hydroxyphenyl)benzimidazole (1), the diethylamino analogue 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (2) and its N-methylated derivative 1-methyl-2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (3). The O-methoxy derivatives of 2 and 3 (2-OMe and 3-OMe), and the simpler models 2-phenylbenzimidazole (4) and its 4'-amino (5) and 4'-dimethylamino (6) derivatives were also studied. We found that the dications of 1, 2, and 3 (protonated at the benzimidazole N3 and at the amino group) were strong photoacids, which were deprotonated at the hydroxyl group upon excitation in aqueous solution (totally for 2 and 3) to give a tautomer of the ground-state monocation. In contrast, no photodissociation was observed for the monocations of these species. Instead, some of the monocations studied behaved as molecular rotors, for which electronic excitation led to a twisted intramolecular charge transfer (TICT) state. The monocations of 2, 3, 2-OMe, 3-OMe, and 6, protonated at the benzimidazole N3, experienced a polarity- and viscosity-dependent radiationless deactivation associated with a large-amplitude rotational motion. We propose that this process is connected to an intramolecular charge transfer from the dimethylaminophenyl or diethylaminophenyl moiety (donor) to the protonated benzimidazole group (acceptor) of the excited monocation, which yields a twisted charge-transfer species. No fluorescence from this species was detected except for 3 and 3-OMe in low-viscosity solvents.
    MeSH term(s) Absorption, Physicochemical ; Benzimidazoles/chemistry ; Electron Transport ; Hydrogen-Ion Concentration ; Methylation ; Models, Molecular ; Molecular Conformation ; Protons ; Solvents/chemistry ; Water/chemistry
    Chemical Substances 2-(2'-hydroxyphenyl)benzimidazole ; Benzimidazoles ; Protons ; Solvents ; Water (059QF0KO0R)
    Language English
    Publishing date 2015-02-12
    Publishing country United States
    Document type Journal Article ; Research Support, Non-U.S. Gov't
    ISSN 1520-5207
    ISSN (online) 1520-5207
    DOI 10.1021/jp507917u
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  3. Article ; Online: Towards ratiometric sensing of amyloid fibrils in vitro.

    Freire, Sonia / Rodríguez-Prieto, Flor / Ríos Rodríguez, M Carmen / Penedo, J Carlos / Al-Soufi, Wajih / Novo, Mercedes

    Chemistry (Weinheim an der Bergstrasse, Germany)

    2015  Volume 21, Issue 8, Page(s) 3425–3434

    Abstract: The aggregation of amyloid-β peptide and its accumulation in the human brain has an important role in the etiology of Alzheimer's disease. Thioflavin T has been widely used as a fluorescent marker for these amyloid aggregates. Nevertheless, its complex ... ...

    Abstract The aggregation of amyloid-β peptide and its accumulation in the human brain has an important role in the etiology of Alzheimer's disease. Thioflavin T has been widely used as a fluorescent marker for these amyloid aggregates. Nevertheless, its complex photophysical behavior, with strong wavelength dependencies of all its fluorescence properties, requires searching for new fluorescent probes. The use of 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine (HPIP), which shows two emission bands and a rich excited-state behavior due to the existence of excited-state intramolecular processes of proton transfer and charge transfer, is proposed. These properties result in a high sensitivity of HPIP fluorescence to its microenvironment and cause a large differential fluorescence enhancement of the two bands upon binding to aggregates of the amyloid-β peptide. Based on this behavior, a very sensitive ratiometric method is established for the detection and quantification of amyloid fibrils, which can be combined with the monitoring of fluorescence anisotropy. The binding selectivity of HPIP is discussed on the basis of the apparent binding equilibrium constants of this probe to amyloid-β (1-42) fibrils and to the nonfibrillar protein bovine serum albumin. Finally, an exhaustive comparison between HPIP and thioflavin T is presented to discuss the sensitivity and specificity of these probes to amyloid aggregates and the significant advantages of the HPIP dye for quantitative determinations.
    MeSH term(s) Alzheimer Disease/metabolism ; Amyloid beta-Peptides/chemistry ; Animals ; Binding Sites ; Cattle ; Electrochemistry ; Fluorescence ; Fluorescent Dyes/chemistry ; Humans ; Imidazoles/chemistry ; Imidazoles/metabolism ; Imidazoles/pharmacology ; Protein Binding ; Pyridines/chemistry ; Pyridines/metabolism ; Pyridines/pharmacology ; Serum Albumin, Bovine/chemistry ; Serum Albumin, Bovine/metabolism ; Thiazoles/chemistry ; Thiazoles/pharmacology
    Chemical Substances 2-(2'-hydroxyphenyl)imidazo(4,5-b)pyridine ; Amyloid beta-Peptides ; Fluorescent Dyes ; Imidazoles ; Pyridines ; Thiazoles ; thioflavin T (2390-54-7) ; Serum Albumin, Bovine (27432CM55Q)
    Language English
    Publishing date 2015-02-16
    Publishing country Germany
    Document type Journal Article ; Research Support, Non-U.S. Gov't
    ZDB-ID 1478547-x
    ISSN 1521-3765 ; 0947-6539
    ISSN (online) 1521-3765
    ISSN 0947-6539
    DOI 10.1002/chem.201406110
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  4. Article ; Online: Moderately Strong Photoacid Dissociates in Alcohols with High Transient Concentration of the Proton-Transfer Contact Pair.

    Brenlla, Alfonso / Veiga Gutiérrez, Manoel / Ríos Rodríguez, M Carmen / Rodríguez-Prieto, Flor / Mosquera, Manuel / Pérez Lustres, J Luis

    The journal of physical chemistry letters

    2014  Volume 5, Issue 6, Page(s) 989–994

    Abstract: Proton transfer from strong photoacids to hydroxylic solvents is much under debate. Experimentally, the main issue stems from relaxation and diffusion processes that are concomitant with ultrafast proton transfer and blur population dynamics. To overcome ...

    Abstract Proton transfer from strong photoacids to hydroxylic solvents is much under debate. Experimentally, the main issue stems from relaxation and diffusion processes that are concomitant with ultrafast proton transfer and blur population dynamics. To overcome this, we propose a fast photodissociation reaction that, however, proceeds slower than solvent relaxation. Fluorescence spectroscopy of the cationic photoacid 2-(1'-hydroxy-2'-naphtyl)benzimidazolium reveals a two-stage mechanism: (a) reversible elementary proton transfer inside the solvent shell and (b) irreversible contact-pair splitting. The time evolution of the fluorescence signal is complex, yet this is explained quantitatively by simultaneous, spectrally overlapping emission of the acid, the conjugate base, and the contact proton-transfer pair. The latter attains high transient concentration in linear alcohols. Microscopic rate constants of dissociation are determined.
    Language English
    Publishing date 2014-03-20
    Publishing country United States
    Document type Journal Article
    ISSN 1948-7185
    ISSN (online) 1948-7185
    DOI 10.1021/jz5001648
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  5. Article ; Online: Photoinduced proton and charge transfer in 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine.

    Brenlla, Alfonso / Veiga, Manoel / Pérez Lustres, J Luis / Ríos Rodríguez, M Carmen / Rodríguez-Prieto, Flor / Mosquera, Manuel

    The journal of physical chemistry. B

    2013  Volume 117, Issue 3, Page(s) 884–896

    Abstract: This paper deals with the interplay between solvent properties and isomerism of 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine (1), and the proton and charge-transfer processes that the different isomers undergo in the first-excited singlet state. We ... ...

    Abstract This paper deals with the interplay between solvent properties and isomerism of 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine (1), and the proton and charge-transfer processes that the different isomers undergo in the first-excited singlet state. We demonstrate the strong influence of these processes on the fluorescence properties of 1. We studied the behavior of 1 in several neutral and acidified solvents, by UV-vis absorption spectroscopy and by steady-state and time-resolved fluorescence spectroscopy. The fluorescence of 1 showed a strong sensitivity to the environment. This behavior is the result of conformational and isomeric equilibria and the completely different excited-state behavior of the isomers. For both neutral and cationic 1, isomers with intramolecular hydrogen bond between the hydroxyl group and the benzimidazole N undergo an ultrafast excited-state intramolecular proton transfer (ESIPT), yielding tautomeric species with very large Stokes shift. For both neutral and cationic 1, isomers with the OH group hydrogen-bonded to the solvent behave as strong photoacids, dissociating in the excited state in solvents with basic character. The pyridine nitrogen exhibits photobase character, protonating in the excited state even in some neutral solvents. An efficient radiationless deactivation channel of several species was detected, which we attributed to a twisted intramolecular charge-transfer (TICT) process, facilitated by deprotonation of the hydroxyl group and protonation of the pyridine nitrogen.
    MeSH term(s) Benzimidazoles/chemistry ; Hydrogen Bonding ; Isomerism ; Protons ; Pyridines/chemistry ; Quantum Theory ; Solvents/chemistry ; Spectrometry, Fluorescence
    Chemical Substances Benzimidazoles ; Protons ; Pyridines ; Solvents ; benzimidazole (E24GX49LD8) ; pyridine (NH9L3PP67S)
    Language English
    Publishing date 2013-01-24
    Publishing country United States
    Document type Journal Article ; Research Support, Non-U.S. Gov't
    ISSN 1520-5207
    ISSN (online) 1520-5207
    DOI 10.1021/jp311709c
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  6. Article ; Online: Solvent-modulated ground-state rotamerism and tautomerism and excited-state proton-transfer processes in o-hydroxynaphthylbenzimidazoles.

    Brenlla, Alfonso / Rodríguez-Prieto, Flor / Mosquera, Manuel / Ríos, Miguel A / Ríos Rodríguez, M Carmen

    The journal of physical chemistry. A

    2009  Volume 113, Issue 1, Page(s) 56–67

    Abstract: The ground-state rotamerism and tautomerism and the excited-state proton-transfer processes of 2-(1'-hydroxy-2'-naphthyl)benzimidazole (1) and 2-(3'-hydroxy-2'-naphthyl)benzimidazole (2) have been investigated in various solvents by means of UV-vis ... ...

    Abstract The ground-state rotamerism and tautomerism and the excited-state proton-transfer processes of 2-(1'-hydroxy-2'-naphthyl)benzimidazole (1) and 2-(3'-hydroxy-2'-naphthyl)benzimidazole (2) have been investigated in various solvents by means of UV-vis absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and quantum-mechanical ab initio calculations. For both compounds, a solvent-modulated rotameric equilibrium, and also tautomeric for 1, was observed in the ground state. In apolar solvents, both 1 and 2 exist as planar syn normal forms, with the hydroxyl group hydrogen bonded to the benzimidazole N3. In acetonitrile and ethanol, a rotameric equilibrium is established between the syn form and its planar anti rotamer, with the phenyl ring rotated 180 degrees about the C2-C2' bond. In ethylene glycol, glycerol, and aqueous solution with 40% ethanol, a tautomeric equilibrium was detected for 1 between the syn and anti normal forms and the tautomer form, with the hydroxyl proton transferred to the benzimidazole N3. In all of the solvents studied, the syn normal form of 1 and 2 undergoes an ultrafast excited-state intramolecular proton transfer (ESIPT) to yield the excited tautomer. The anti normal forms of 1 and 2, unable to experience ESIPT, give normal form fluorescence. In addition, the anti normal conformer of 2 partly deprotonates at the hydroxyl group in aqueous solution with 40% ethanol, giving the excited anion. The monocations of 1 and 2, protonated at the benzimidazole N3, are strong photoacids that deprotonate completely in aqueous solution with 40% ethanol and to a great extent in ethanol, giving the excited tautomer.
    MeSH term(s) Benzimidazoles/chemistry ; Hydrogen-Ion Concentration ; Molecular Structure ; Protons ; Quantum Theory ; Solvents/chemistry ; Spectrometry, Fluorescence ; Stereoisomerism ; Thermodynamics
    Chemical Substances 2-(1'-hydroxy-2'-naphthyl)benzimidazole ; 2-(3'-hydroxy-2'-naphthyl)benzimidazole ; Benzimidazoles ; Protons ; Solvents
    Language English
    Publishing date 2009-01-08
    Publishing country United States
    Document type Journal Article ; Research Support, Non-U.S. Gov't
    ISSN 1520-5215
    ISSN (online) 1520-5215
    DOI 10.1021/jp8076003
    Database MEDical Literature Analysis and Retrieval System OnLINE

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