2-Naphthol excited state acidity

Tsutsumi, K., Shizuka, H., "Proton Transfer and Acidity Constant in the Excited State of Naphthols by Dynamic Analyses," Z. Phys. Chem., 1980, 122,129. 

The observation of a fast time decay for the n = 2 mass channel excited at the cluster origin is surprising. Isotopic substitution experiments show that the decay is due to proton tunneling, yet naphthol, which is a stronger excited state acid, does not exhibit proton transfer until it is clustered with three ammonia... 

With the above reservations in mind, we summarize below the different approaches that attempt to elucidate the excited-state acidity of 1- and 2-naphthol by analyzing the structure of their electronic spectra. As already pointed out, there is a considerable difference between the photoacidity of 1- and 2-naphthol (about 3 pX a units). In contrast, the two naphthol isomers exhibit almost identical ground-state acidities, the difference between the pX a of the two isomers being less than 0.2 pffa units (pK = 9.4 and 9.5 for 1-and 2-naphthol, respectively). This simple observation suggests, although does not prove, that the two isomers differ mainly in their electronic structure in the excited state. Direct comparison between the electronic spectra of the two isomers has provided, arguably, the... 

Interestingly, Forster-cycle calculations of the pAla in methanol (Table 2) seem to confirm the substituent effect on the polarity of the emitting state of 1-naphthol as discussed above the less polar the emitting state of the acid compared to the emitting state of its conjugate base, the larger the Forster-cycle acidity of the photoacid. The calculated Forster-cycle difference between the ground-state and excited-state acidities in methanol was 12.3, 11.3, 10.9, 9.3 and 8.8 for the 2-substituted, 3-substituted, unsubstituted 4- and... 

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.

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. 

Acid/base equilibria can be very different in ground and excited states. A well-known example is (I-naphthol, which becomes highly acidic in the excited state and transfers a proton to the surrounding water solvent acting as base within the excited... 

Using 2NpOH and 2-naphthol-3,6-disulfonate [121] as excited-state proton emitters, a transient high proton concentration is achieved on the membrane surface. With bromocresol green dye adsorbed on the membrane serving as a pH indicator, it has been found that the protons first react with the acidic ionized moieties on the surface and then reach the strongest base on the surface by rapid exchange. 

It is well known that the acidity of many hydroxy-aromatic compounds is strongly affected by electronic excitation and that these molecules become significantly more acidic in the excited states than in the ground state. For example, phenol and 1-naphthol... 

In view of the clear relationship between pX-changes and absorption spectra, a study of the influences of substituents and other consitutional changes upon such spectra has a very direct bearing upon the field of acid-base properties in excited states. For example, the —OH and —0 groups function as different substituents at the 2-position in naphthalene. Any theory which accounts for their different effects upon the naphthalene transitions therefore automatically also explains the change in the naphthol-naphtholate equilibrium upon excitation. The search for linear free energy relationships in electronic spectra will therefore continue to impinge upon this field. 

The kinetic isotope effects shown in Fig. 11 (Forster, 1972) resemble those reported for 2-naphthol by Stryer (1966). Like 2-naphthylamine, 2-naphthol shows an increased quantum yield and protonation of Sj occurs at lower acidities in D20 than in H20. For 2-naphthol, p-KJSj )-values of 3 0 in H20 and 3-4 in D20 are calculated from the measured excited state rate constants in H20 k.j = 5-29 x 107 s 1 and k2 = 5-5 x 101 0 dm3 mole-1 s-1, while in D20 k1 = 1 3 x 107 s-1 and k2 = 3-5 x 1010 dm3 mole-1 s-1. These results confirmed the earlier p/ (S )-values calculated by Wehry and Rogers (1966) using the Forster cycle (Table 9), which show incidentally that the pK-values are closer by about 0 1 unit in the Sj state. 

The discovery of photoacidity was made by Forster more then 50 years ago . Forster correctly explained the unusual large Stokes shift found in the fluorescence of several classes of aromatic dyes, including 1- and 2-naphthol derivatives as an indication of excited state proton-transfer reaction which results in the formation of the molecular anion still in the excited state. Thus, it become clear that excited-state proton transfer may compete with other radiative and non-radiative decay routes of the photoacid. The main modern-day importance of photoacids lies in their ability to initiate and then to follow acid-base reactions so they may be regarded as optical probes for the study of general proton-transfer reactions. 

This sort of argument demonstrates the need for defining a photoacidity scale which is independent of whether or not the photoacid is able to dissociate within the excited-state lifetime. The five photoacid derivatives of 1-naphthol discussed above do not dissociate in methanol. The order of their acidity in methanol extracted from Forster-cycle calculations awaits further confirmation. It should be conducted by some other method which would... 

Before such an endeavor is carried out one must rely on circumstantial evidence. Doing so, it appears as if polar substituents affect photoacidity not just by processes identified in ground-state acids, such as the inductive and resonance effects, but, in the case of 1-naphthol, also by systematically affecting the character of its electronic excited state. 

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