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Water-catalyzed excited-state proton-transfer reactions in 7-azaindole and its analogues

Journal of Physical Chemistry B, ISSN: 1520-5207, Vol: 119, Issue: 6, Page: 2302-2309
2015
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Article Description

The mechanism of the water-catalyzed excited-state proton-transfer (ESPT) reaction for 7-azaindole (7AI) has long been investigated, but there are some controversial viewpoints. Recently, owing to the superiority of sensing biowaters in proteins by a 7AI analogue, 2,7-diazatryptophan, it is timely to reinvestigate water-catalyzed ESPT in 7AI and its analogues in an attempt to unify the mechanism. Herein, a series of 7AI analogues and their methylated derivatives were synthesized to carry out a systematic study on pK, pK∗, and the associated fluorescence spectroscopy and dynamics. The results conclude that all 7AI derivatives undergo water-catalyzed ESPT in neutral water. However, for those derivatives with -H (7AI) 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 tautomer species, for which pK∗ of the cationic form, that is, the N(7)-CHN(1)-H 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 7AI, the previously missing T∗ emission is clearly resolved with a peak wavelength at 530 nm in the pH interval of 13.0-14.3 (H 14.2).

Bibliographic Details

Wu, Yu-Sin; Huang, Huai-Ching; Shen, Jiun-Yi; Tseng, Huan-Wei; Ho, Jr-Wei; Chen, You-Hua; Chou, Pi-Tai

American Chemical Society (ACS)

Chemistry; Materials Science

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