2- naphthol fluorescence quantum yield

If pK is greater than 2, a plateau is observed for the relative fluorescence quantum yield of the acidic form and the basic form for pH ranging from pK to pK (Figure 4.10A) because of the absence of diffusional recombination. In fact, Eqs (4.59) and (4.60) which are relevant to this case show that /HA and IA- are constants. A typical example is 2-naphthol (pK = 9.3, pK = 2.8). 

In 1966 Wehry and Rogers [464] examined the pK s of 2-naphthol and several substituted phenols and found ApK, = pKj,(D20) - pK (H20 ranged from 0.48 to 0.70. The same general trend held true for pK s in the lowest excited singlet states of the phenols and for the pK, s in a small set of benzoic acids and naphthoic acids. Stryer [463] found that excited-state proton transfer rates in a set of aromatics were greater in HjO than in D2O As a result, the fluorescence quantum yields were higher in DjO than in H2O in most cases. Jencks and Salvesen [486] found similar effects in a small group of thiol acids, with ApK = 2.0-2.5. 

The crucial requirement of excited-state proton transfer (ESPT) is suggested by the failure of 1-naphthyl methyl ether to undergo self-nitrosation under similar photolysis conditions. The ESPT is further established by quenching of the photonitrosation as well as 1-naphthol fluorescence by general bases, such as water and triethylamine, with comparable quenching rate constants and quantum yield. ESPT shows the significance in relation to the requirement of acid in photolysis of nitrosamines and acid association is a photolabile species. 

For 0-naphtholate at 25 °C. the quantum yield of fluorescence is about 0.2 of solvated electron formation about 0.1. This solvated electron formation is a major factor competing with fluorescence for that part of the original quantum energy reaching the lowest singlet state. As previously stated (37), it may be a major factor leading to fluorescence quenching in polar solvents, and particularly in aqueous solutions. 

The quantum yield of photodissociation in neutral aqueous suspension of vesicles like egg lecithin (EL) and dipalmitoylphosphatidylcholine (DPPC) are significantly smaller than in the aqueous solution [83], INpOH (pKa = 9.2, pK a = 0.4) emits only from its DP form (2) in water. On incorporation into liposome membrane, a substantial increase of the P form (1) fluorescence is seen with concomitant decrease of DP form fluorescence. A similar effect is seen for 2NpOH with membrane incorporation. The biexponential fluorescence decay of the P form in fully incorporated naphthol suggests the presence of two localization sites... 

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