2- naphthol excited state proton dissociation

Figure 2. Correlation between rate constant of proton dissociation and pK of acids. ( ) 8-hydroxypyrene-1,3,6-trisulfonate, excited state ( ) 2-naphthol-3,6-disulfonate, excited state (A) 2-naphthol-6-sulfonate, excited state (V) 2-naphthol, excited state (O) Bromo Cresol Green ( ) Bromo Cresol Purple ( ) Bromo Fymole Blue (A) 8-Hydroxypyrene 1,3,6 trisulfonate, ground state ( ) 2-naphthol 3,6 disulfonate, ground state ( ) 2-naphthol, ground state.

ABSTRACT. Kinetics of proton transfer photoreactions in simple model systems is analyzed from the point of view of reaction kinetics in microphases. Protolytic photodissociation of some hydroxyaromatic compounds ArOH ( 1- and 2-na-phthol, chlorosubstituted naphthols ) was studied in micellar solutions and phospholipid vesicles by fluorescence spectra and kinetics. Experimental results give evidence of at least two localization sites of naphthols in the microphase of these systems. In lipid bilayer membranes of vesicles there are two comparable fractions of ArOH molecules, one of which undergo photodissociation, but another do not dissociate. In micelles only minor fraction ( few per cent ) of ArOH molecules do not take part in excited-state proton transfer reaction. These phenomena reflect heterogeneous structure and dynamic properties of lipid bilayer membranes and micelles. A correlation between proton transfer rate constants and equilibrium constants in microphases similar to that in homogeneous solutions is observed. Microphase approach give a possibility to discuss reactions in dynamical organized molecular systems in terms of classical chemical kinetics. 

The rate constants for protonation of the excited singlet states of several compounds were determined by Weller (1961). Although the measurement of excited state equilibrium constants has become more common, there have been relatively few determinations of the rate constants involved. Trieff and Sundheim (1965) investigated the effects of solvent changes on the rates of protonation and deprotonation of 2-naphthol in the S) state. The dissociation rate constant decreased progressively with the addition of methanol or glycerol to the aqueous solution but the protonation rate constant varied in a more complex manner. As mentioned above, Stryer (1966) found both rate constants smaller in D20 than in H20. 

Proton transfer processes are specially important excited state properties, and several detailed time resolved studies have been reported. Time resolved fluorescence studies of excited l-naphthol-3,6-disulphonate shows there is geminate recombination by proton transfer. Another detailed study is the examination of proton transfer and solvent polarization dynamics in 3-hydroxyflavone . The dynamics of proton transfer using a geminate dissociation and recombination model has also been investigated with 8-hydroxypyrene-l,3,6-trisulphonate 5 and also with... 

We shall describe the various steps in the evolution of the methods and knowledge of proton diffusion on the membrane surface. The methods used for our studies all emanate from one basic technique—the laser-induced proton pulse (I). The common step of the various forms of this method is a pulse excitation of aromatic alcohols (OH), such as naphthols, sulfono naphthols, or pyranine (8-hydroxypyrene- 1,3,6-trisulfonate), to their first excited electronic singlet state (OH ). In this state the compound is very acidic and the hydroxyl proton dissociates in subnanosecond dynamics. 

The exit rate constants of the excited anions after the photoprotolytic dissociation of l,4-dichloro-2-naphthol within decylsulfate, dedecylsulfate, and cetylsulfate micelles were measured with a fluorescence quencher hardly penetrating the micelles, - the nitrate ion [121]. The addition of nitrate into the solution quenched the fluorescence of those anions which escape from the micelles within the lifetime of the excited state only. The exit rate constant of the naphtholate anion increases with increasing length of the hydrocarbon radical in the micelle-forming surfactant. The exit rate is thus controlled by the lowering of the micelle polarity (i.e. by the free energy of the exit process) rather than by the micelle size or the distance that the anion must diffuse. Perhaps one can establish a kind of correlation between the rate constant of this process and its free energy as was done for photochemical electron transfer [126] and proton transfer [156,157]. 

The NaCl effect upon the proton-transfer reactions in the excited state of 2-naphthol has been studied by Harris and Selinger [41]. They have reported that the enhancement of the fluorescence of 2-naphthol is due either to a disruption of the water structure by the high concentration of Na and Cl ions or to an increase in the activity coefficient of the excited 2-naphthol. In previous works [53,106,107], it is shown that NaCl is a very weak quencher there is a weak quenching ability for CT, but no ability for Na. The NaCl effect on the proton dissociation reactions in water at 300 K has been studied by means of nanosecond and picosecond spectroscopy with fluorimetry [108]. The proton dissociation rate constant k decreases with an increase of [NaCl] according to the equation... 

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