Harnessing Excited-State Intramolecular Proton-Transfer
Reaction via a Series of Amino-Type Hydrogen-Bonding Molecules
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Abstract
A series of new amino (NH)-type hydrogen-bonding
(H-bonding) compounds
comprising 2-(2′-aminophenyl)benzothiazole and its extensive
derivatives were designed and synthesized. Unlike in the hydroxyl
(OH)-type H-bonding systems, one of the amino hydrogens can be replaced
with electron-donating/withdrawing groups. This, together with a versatile
capability for modifying the parent moiety, makes feasible the comprehensive
spectroscopy and dynamics studies of amino-type excited-state intramolecular
proton transfer (ESIPT), which was previously inaccessible in the
hydroxyl-type ESIPT systems. Empirical correlations were observed
among the hydrogen-bonding strength (the N–H bond distances
and proton acidity), ESIPT kinetics, and thermodynamics, demonstrating
a trend that the stronger N–H···N hydrogen bond
leads to a faster ESIPT, as experimentally observed, and a more exergonic
reaction thermodynamics. Accordingly, ESIPT reaction can be harnessed
for the first time from a highly endergonic type (i.e., prohibition)
toward equilibrium with a measurable ESIPT rate and then to the highly
exergonic, ultrafast ESIPT reaction within the same series of amino-type
intramolecular H-bond system