Picosecond Fluorescence and Two-Step LIF Studies of the Excited-State Proton Transfer in Methanol Solutions of 7-Hydroxyquinoline and Methyl-Substituted 7-Hydroxyquinolines

Nakagawa, Takako; Kohtani, Shigeru; Itoh, Michiya

doi:10.1021/ja00135a013

Cited by 73 publications

(119 citation statements)

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“…[8][9][10][11][12][13][14][15][16][17][18] Four prototropic species of 7-HQ are equilibrated in aqueous solution: a normal molecule (7-HQ(N)), an imine-protonated cation (7-HQ(C)), an enol-deprotonated anion (7-HQ(A)), and an enol-deprotonated imine-protonated tautomer (7-HQ(T)). 8 The pK a of the phenolic moiety of 7-HQ has been reported to be 9.0.…”

Section: Introductionmentioning

confidence: 99%

“…Several studies have focused on the ground singlet states and excited single and triplet states of 7-HQ and some derivatives. 9,[11][12][13] The mechanisms of the proton tautomerization of 7-hydroxuquinoline and the participation of solvent molecules have been explored in nonaqueous protic solvents 8,10,11,14 and in aqueous solution. 13,[15][16][17] It is conceivable that BHQ-OAc, as a derivative of 7-HQ, may have similar properties but the influence of its substituent groups on these properties is not yet clear.…”

Section: Introductionmentioning

confidence: 99%

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Resonance Raman Characterization of Different Forms of Ground-State 8-Bromo-7-hydroxyquinoline Caged Acetate in Aqueous Solutions

Nganga

et al. 2009

J. Phys. Chem. A

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The 8-bromo-7-hydroxyquinolinyl group (BHQ) is a derivative of 7-hydroxyquinoline (7-HQ) and BHQ molecules coexisting as different forms in aqueous solution. Absorption and resonance Raman spectroscopic methods were used to examine 8-bromo-7-hydroxyquinoline protected acetate (BHQ-OAc) in acetonitrile (MeCN), H(2)O/MeCN (60:40, v/v, pH 6 approximately 7), and NaOH-H(2)O/MeCN (60:40, v/v, pH 11 approximately 12) to obtain a better characterization of the forms of the ground-state species of BHQ-OAc in aqueous solutions and to examine their properties. The absorption spectra of BHQ-OAc in water show no absorption bands of the tautomeric species unlike the strong band at about 400 nm observed for the tautomeric form in 7-HQ aqueous solution. The resonance Raman spectra in conjunction with Raman spectra predicted from density functional theory (DFT) calculations reveal the observation of a double Raman band system characteristic of the neutral form (the nominal C=C ring stretching, C-N stretching, and O-H bending modes at 1564 and 1607 cm(-1)) and a single Raman band diagnostic of the enol-deprotonated anionic form (the nominal C=C ring, C-N, and C-O(-) stretching modes in the 1593 cm(-1) region). These results suggest that the neutral form of BHQ-OAc is the major species in neutral aqueous solution. There is a modest increase in the amount of the anionic form and a big decrease in the amount of the tautomeric form of the molecules for BHQ-OAc compared to 7-HQ in neutral aqueous solution. The presence of the 8-bromo group and/or competitive hydrogen bonding that hinder the formation and transfer process of a BHQ-OAc-water cyclic complex may be responsible for this large substituent effect.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Resonance Raman Characterization of Different Forms of Ground-State 8-Bromo-7-hydroxyquinoline Caged Acetate in Aqueous Solutions

Nganga

et al. 2009

J. Phys. Chem. A

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“…6Me7HQ and 8Me7HQ were synthesized according to the conventional Skraup reaction following the procedure previously reported, 25 and subsequently purified twice by column chromatography. 7HQ (99%) was purchased from Across and used without further purification.…”

Section: Methodsmentioning

confidence: 99%

Water-wire catalysis in photoinduced acid–base reactions

Kwon

Mohammed

2012

Phys. Chem. Chem. Phys.

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The pronounced ability of water to form a hyperdense hydrogen (H)-bond network among itself is at the heart of its exceptional properties. Due to the unique H-bonding capability and amphoteric nature, water is not only a passive medium, but also behaves as an active participant in many chemical and biological reactions. Here, we reveal the catalytic role of a short water wire, composed of two (or three) water molecules, in model aqueous acid-base reactions synthesizing 7-hydroxyquinoline derivatives. Utilizing femtosecond-resolved fluorescence spectroscopy, we tracked the trajectories of excited-state proton transfer and discovered that proton hopping along the water wire accomplishes the reaction more efficiently compared to the transfer occurring with bulk water clusters. Our finding suggests that the directionality of the proton movements along the charge-gradient H-bond network may be a key element for long-distance proton translocation in biological systems, as the H-bond networks wiring acidic and basic sites distal to each other can provide a shortcut for a proton in searching a global minimum on a complex energy landscape to its destination.

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“…The two tautomers of 7-hydroxyquinoline as well as the anion 7Q À and the cation 7H 2 Q þ have been extensively characterized by absorption and fluorescence spectroscopy in non-polar, polar and protic solvents [59][60][61][62][63][64][65][66][67][68][69][70]. 7HQ emits in the 370-390 nm range, depending on the solvent, while 7KQ emits in the 525-580 nm range, strongly Stokes shifted relative to 7HQ.…”

Section: Techniques Usedmentioning

confidence: 99%

Controlling Excited‐State H‐Atom Transfer along Hydrogen‐Bonded Wires

Manca

Tanner

Leutwyler

2010

Hydrogen Bonding and Transfer in the Excited State

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Hydrogen-bonded wires are involved in a large variety of chemical and biological processes [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The main reaction associated with hydrogen bonds is proton or H-atom transfer, in which a charge accompanies the transferring proton. A topic of specific interest is proton transport through transmembrane ion channels ('proton wires'), because many transmembrane proteins create, control and use the proton gradient across biological membranes. Hydrogen-bonded wires of water molecules have been identified in numerous membrane-spanning proteins, such as bacteriorhodopsin [17][18][19][20][21][22], the bacterial photosynthetic reaction centre of Rhodobacter sphaeroides [23], the transmembrane channel formed by gramicidin [24][25][26], cytochrome oxidase [27] and other voltage-gated proton channels [28]. The enzymes carbonic anhydrase [29,30] and alcohol dehydrogenase [30] also contain proton relays along chains of water molecules embedded in the interior of the protein. Recently, hydrogen-bonded ammonia wires have been detected in ammonia/ammonium transporter (Amt) transmembrane proteins, which are essential for nitrogen metabolism in all species [31,32]. The translocation mechanisms along these 'proton wires' have become fields of intense theoretical study [5-8, 24-26, 29, 30, 33, 34]. Most of the time, the assumed mechanism is Grotthuss type, after the inventor (1806) of the process. The modern version of the Grotthuss mechanism consists of successive transfers of an excess proton along a hydrogen-bonded wire. These transfers require sequences of reorientation of the wire molecules, which constitutes the rate-limiting step of the mechanism [35]. The excess proton diffuses through a hydrogen bond network via the formation/breaking of covalent bonds, as shown schematically in

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Picosecond Fluorescence and Two-Step LIF Studies of the Excited-State Proton Transfer in Methanol Solutions of 7-Hydroxyquinoline and Methyl-Substituted 7-Hydroxyquinolines

Cited by 73 publications

References 0 publications

Resonance Raman Characterization of Different Forms of Ground-State 8-Bromo-7-hydroxyquinoline Caged Acetate in Aqueous Solutions

Resonance Raman Characterization of Different Forms of Ground-State 8-Bromo-7-hydroxyquinoline Caged Acetate in Aqueous Solutions

Water-wire catalysis in photoinduced acid–base reactions

Controlling Excited‐State H‐Atom Transfer along Hydrogen‐Bonded Wires

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