The Excited State Properties of 6-Azaindole
Date Issued
2015
Date
2015
Author(s)
Tu, Ting-Hsun
Abstract
In 1969, Taylor discovered the hydrogen-bond dimer of 7-azaindole and its associated excited-state double proton transfer reaction. We discover that 6-azaindole in nonpolar solvent (e.g. cyclohexane) would self-assemble cyclic hydrogen-bonded trimer. The steric hindrance prohibits simultaneous dual N(1)-H---N(6) H-bonded dimer formation. Instead, self-assembly via N(1)-H---N(6) H-bond takes place in non-polar solvents, forming the H-bonded trimer with an association constant of 6.4 ×106 M-2 in cyclohexane. When UV excitation (310 nm) the H-bonded trimer, it would produce normal emission at 325 nm and undergo ESTPT to result in a tautomer emission at 435 nm .Computational approach further affirms the cyclic H-bond trimer formation and its energetically favorable ESTPT reaction. Otherwise, we also study the properties and photophysical characteristics of 6-azaindole in water and ethanol. In ethanol, 6-azaindole would proceed solvent reorganization to form 1:3 cyclic structure and couple intrinsic proton transfer in the excited state which like the property of 7-hydroxyquinoline in ethanol. In water, the pKa of 6-azaindole protonated form is drastically increased from 8.0 (ground state) to 14.37 in the excited state. When the pH is greater than 10, neutral (normal) is the dominant form in the ground state which has distinct properties of excited state. Following we take two different pH as example. Firstly, at pH = 10.9, 6-azaindole can form the 1:3 cyclic structure through solvent reorganization then proceed excited state proton transfer. The pKa* of the 6-azaindole protonated form is greater than this pH so the cationic emission can also be observed. Secondly, at pH = 12.7, 6-azaindole can undergo proton transfer to form the tautomer as well. In this pH is still smaller than the pKa* of 6-azaindole protonated form so cation would also be produced. Besides, the pKa* (~11.16) of the 6-azaindole tautomer protonated form is smaller than the environment. Therefore, the N1-H may dissociate and end up with the tautomer form after a redistribution of the electron density. This phenomenon can be considered as two-step proton-transfer mechanism. The cationic form is just an intermediate specie in the tautomerization process.
Subjects
Excited State Proton Transfer
6-Azaindole
trimer
Type
thesis
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