| Abstract |
Synthetic and natural hydroxyflavylium salts are super-photoacids, exhibiting values of the rate constant for proton transfer to water in the excited state as high as 1.5 x 10(11) s(-1). The synthetic flavylium salt 4-carboxy-7-hydroxy-4 -methoxyflavylium chloride (CHMF) has an additional carboxyl group at the 4-position of the flavylium cation that deprotonates in the ground state at a lower pH (pK(al) = 0.73; AH(2)(+) -> Z) than the 7-hydroxy group (pK(a2) = 4.84; Z -> A(-)). Ground-state deprotonation of the carboxyl group of the acid (AH(2)(+)) to form the zwitterion (Z) is too fast to be detected by nanosecond laser flash perturbation of the ground-state equilibrium, while deprotonation of the hydroxyl group of Z to form the anionic base (A(-)) occurs in the microsecond time range (k(d2) = 0.6 x 10(6) s(-1) and k(p2) = 4.2 x 10(10) M-1.s(-1)). In the excited state, the cationic form (AH(2)(+)*) deprotonates in similar to 9 ps, resulting in the excited neutral base form (AH*), which is unstable in the ground state. Deprotonation of Z* occurs in 30 ps (k(d2)* = 2.9 x 10(10) s(-1)), to form excited A(-)*, which either reprotonates (k(p3)* = 3.7 x 10(10) M-1.s(-1)) or decays in 149 ps, and shows an important contribution from geminate recombination to give the excited neutral base (AH*). Predominant reprotonation of A(-)* at the carboxylate group reflects both the presence of the negative charge on the carboxylate and the increase in the excited-state pK(a) of the carboxyl group. Thus, while the hydroxyl pK(a) decreases by similar to 5 units upon going from the ground state (pK(a) = 4.84) to the excited state (pK(a)* = -0.2), that of the carboxyl group increases by at least this much. Consequently, the excited state of the Z* form of CHMF acts as a molecular proton transporter in the picosecond time range. |
