Singlet oxygen state reaction, efficiency

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

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