PHOTOINDUCED COUPLED PROTON AND ELECTRON TRANSFERS .1. 6-HYDROXYQUINOLINE

Excited-state processes in 6-hydroxyquinoline (6-HQ) were investigated in acidic, basic, and neutral media. When 6-HQ is in the quinolinium form (i.e. with protonated ring nitrogen) in acidic aqueous solutions, the hydroxyl group behaves like a very strong acid in the excited sta te: deprotonation occurs even in a solution of 10 M HClO4! Such a very high photoacidity is explained by the absence of proton back-recombin ation, as shown by time-resolved measurements, rather than by a high r ate constant for deprotonation. The lack of proton recombination, surp rising at first sight in very acidic solutions, is shown to be due to intramolecular electron transfer from the hydroxylate group to the pos itively charged pyridinium ring as soon as the proton is ejected: this leads to an excited tautomer predominantly in a quinonoid form. Deexc itation of this tautomer occurs via reverse electron transfer, the gro und-state form being predominantly zwitterionic; this route of deexcit ation is mainly nonradiative as in the case of betaines. In fact, solv atochromism experiments performed on the parent compound 1-methyl-6-ox yquinolinium confirm the strong analogy with betaines and in particula r with betaine 30 which is known to undergo nonradiative deexcitation, via ultrafast intramolecular electron transfer, toward the ground-sta te zwitterionic form. When 6-HQ is in the phenolate form in basic aque ous solutions, the heterocyclic nitrogen atom behaves as a very strong base in the excited state, and proton uptake is coupled to fast intra molecular electron transfer from the hydroxylate group to the adjacent ring. Finally; the photophysical behavior of 6-HQ in the neutral form , and in particular its very low quantum yield, can be interpreted in terms of double proton transfer coupled to intramolecular electron tra nsfer. Therefore, it is concluded that all the excited-state processes in 6-HQ can be explained along the same line whatever the acidity or basicity of the solution: the cooperativity between the two functional groups -OH an greater than or equal to N in the excited state leads t o apparently enhanced photoacidity and photobasicity of the molecule b ecause of the coupling between proton and electron transfers. The high rate of the latter process in the excited state and in the transfer b ack to the ground state drives the proton transfer which is only limit ed by the ability of the water molecules to behave as proton accepters (in concentrated acidic media) or proton donors (in concentrated basi c media). No excited state equilibrium is ever established. This kinet ic scheme relying on coupled proton and electron transfers is also sho wn to be valid for 5-, 7-, and 8-hydroxyquinolines. Some differences, e.g., in quantum yield of 7-hydroxyquinoline, can be explained in term s of different relative proportions of the zwitterionic and quinonoid forms of the tautomer, depending on the position of the OH group. More over, this scheme may be transposable to other classes of bifunctional molecules undergoing phototautomerization.