Introduction
Photoacids are a class of compounds that are weak acids in their ground electronic state and much stronger acids in their excited singlet or triplet states.
This class of compounds has been extensively researched for the last 50 years by optical steady-state and time-resolved techniques. (1-17) The ground-state pKa values range from 5 < pKa < 10 and the difference, ΔpKa, between their ground- and excited-state pKa values range between 3 < ΔpKa < 13. When the pKa* of a photoacid is large enough (for example, 2-naphthol with pKa* = 2.7), the excited-state proton-transfer (ESPT) rate constant, kPT, is of the order of 108s–1 and since the radiative rate of the singlet state, krad, is usually >108s–1, the ESPT quantum efficiency, ϕPT, is smaller than 0.5. Therefore, weak photoacids like phenols, with pKa* ≈ 3.4, do not exhibit detectable photoacidity. Photoacidity depends on the solvent.
Water at room temperature is the best solvent known for ESPT processes. For weak photoacids with pKa* > 0.5, ESPT is not observed in methanol or longer-chain alkanols, because kPT in methanol is smaller by about 3 orders of magnitude than its value in water for photoacids with pKa* > 0.5.
In a previous paper we classified the photoacids into four groups. (18) Group I, “the weak photoacids”, with 0.4 < pKa*<3.4 can transfer a proton within the excited-state lifetime to water and D2O, but not to alkanols. Group II, with −4 < pKa*<0.4, can transfer a proton, within the excited-state lifetime, to many protic solvents like DMSO and alkanols.
The more negative the pKa*, the larger the ESPT rate constant. Group III constitutes a unique class of photoacids with pKa* ∼ −5. The ESPT rate constant and the process itself depend on the solvent-reorientation relaxation time, or S(t).
For water ⟨S(t)⟩ is of the order of 1 ps, for methanol it is about 5 ps, and for ethanol, 15 ps. The fourth group of photoacids transfers a proton to the solvent at rates exceeding that of the solvent-reorientation relaxation. Among such photoacids are quinone cyanine 7 (QCy7) and quinone cyanine 9 (QCy9). Up to now, the fastest ESPT rate reported is for QCy9, (19) with kPT ∼ 1013s–1 (τPT ≈ 100 fs).
