6-Azaindole的激發態特性

Abstract

1969年由Taylor發現7-azaindole會自身聚集產生Hydrogen-bonded dimer並且進行excited state double proton transfer (ESDPT) 。我們發現其衍生物6-azaindole於非極性環境下 (例如:環己烷) ，由於立體障礙的關係會阻止6-azaindole自身聚集成dimer，取而代之的是形成hydrogen-bonded trimer，其平衡常數大約為6.4 × 106 M-2。當光激發trimer吸收的位置 (310 nm) 時，除了會產生normal - 325 nm放光之外還會產生進行excited state triple proton transfer (ESTPT) 後的tautomer - 435 nm放光，我們也透過理論計算的結果說明trimer在能量上是可以傾向形成的。 除此之外，我們也研究6-azaindole於乙醇和水中的性質與光物理現象。6-azaindole於乙醇中從基態被激發到激發態後可以透過溶劑重組成1:3的環狀結構後再進行激發態質子轉移，其性質類似於7-Hydroxyquinoline於乙醇中﹔而6-azaindole在水中被質子化的pKa = 8和pKa* = 14.37，所以在pH > 10的環境中，基態都是以中性分子存在時，6-azaindole會有不同的激發態特性，像是在pH = 10.9的時候，除了可進行溶劑重組成1:3環狀結構再進行激發態質子轉移產生tautomer之外，也因為在此pH值下小於6-azaindole被質子化的pKa*所以仍可觀察到陽離子的放光﹔而在pH = 12.7的時候，除了會進行與上條件相同的激發態質子轉移，還會產生陽離子因為於此pH值下依舊小於6-azaindole被質子化的pKa*，但在此pH值下比6-azaindole tautomer被質子化的pKa* ~ 11.1大，所以產生的陽離子經共振後會將N1-H解離產生tautomer，此過程近似於兩步驟的質子轉移，而陽離子於過程中只是扮演中間體的角色。

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.