The Excited-State Triple Proton Transfer Reaction of 2,6-Diazaindoles and 2,6-Diazatryptophan in Aqueous Solution

3-Me-2,6-diazaindole
((2,6-aza)­Ind) was strategically
designed and synthesized to probe water molecule catalyzed excited-state
proton transfer in aqueous solution. Upon electronic excitation (λmax ∼ 300 nm), (2,6-aza)­Ind undergoes N(1)–H
to N(6) long-distance proton transfer in neutral H2O, resulting
in normal (340 nm) and proton-transfer tautomer (480 nm) emissions
with an overall quantum yield of 0.25. The rate of the water-catalyzed
proton transfer shows a prominent H/D kinetic isotope effect, which
is determined to be 8.3 × 108 s–1 and 4.7 × 108 s–1 in H2O and D2O, respectively. Proton inventory experiments
indicate the involvement of two water molecules and three protons,
which undergo a relay type of excited-state triple proton transfer
(ESTPT) in a concerted, asynchronous manner. The results demonstrate
for the first time the fundamental of triple proton transfer in pure
water for azaindoles as well as pave a new avenue for 2,6-diazatryptophan,
an analogue of tryptophan exhibiting a similar ESTPT property with (2,6-aza)­Ind, to probe biowaters in proteins.