Water-Catalyzed
Excited-State Proton-Transfer Reactions
in 7‑Azaindole and Its Analogues
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Abstract
The
mechanism of the water-catalyzed excited-state proton-transfer
(ESPT) reaction for 7-azaindole (<b>7AI</b>) has long been investigated,
but there are some controversial viewpoints. Recently, owing to the
superiority of sensing biowaters in proteins by a <b>7AI</b> analogue, 2,7-diazatryptophan, it is timely to reinvestigate water-catalyzed
ESPT in <b>7AI</b> and its analogues in an attempt to unify
the mechanism. Herein, a series of <b>7AI</b> analogues and
their methylated derivatives were synthesized to carry out a systematic
study on p<i>K</i><sub>a</sub>, p<i>K</i><sub>a</sub>*, and the associated fluorescence spectroscopy and dynamics.
The results conclude that all <b>7AI</b> derivatives undergo
water-catalyzed ESPT in neutral water. However, for those derivatives
with −H (<b>7AI</b>) and a electron-donating substituent
at C(3), they follow water-catalyzed ESPT to form an excited N(7)–H
proton-transfer tautomer, T*. T* is rapidly protonated to generate
an excited cationic (TC*) species. TC* then undergoes a fast deactivation
to the N(1)–H normal species in the ground state. Conversely,
protonation in T* is prohibited for those derivatives with an electron-withdrawing
group at the C(2) or C(3) or with the C(2) atom replaced by an electron-withdrawing
nitrogen atom (N(2) in, e.g., 2,7-diazatryptophan), giving a prominent
green T* emission. Additional support is given by the synthesis of
the corresponding N(7)–CH<sub>3</sub> tautomer species, for
which p<i>K</i><sub>a</sub>* of the cationic form, that
is, the N(7)–CH<sub>3</sub>N(1)–H<sup>+</sup> species,
is measured to be much greater than 7.0 for those with electron-donating
C(3) substituents, whereas it is lower than 7.0 upon anchoring electron-withdrawing
groups. For <b>7AI</b>, the previously missing T* emission is
clearly resolved with a peak wavelength at 530 nm in the pH interval
of 13.0–14.3 (<i>H</i><sub>–</sub> 14.2)