Fluorescence of Hydroxyphenyl-Substituted “Click” Triazoles

The
structural and optical properties of hydroxyphenyl-substituted-1,2,3-triazole
molecules (“click” triazoles) are described. “Click”
triazoles are prepared from the copper­(I)-catalyzed azide–alkyne
cycloaddition reactions. The alkyne-derived C4 substituent of a “click”
triazole engages in electronic conjugation more effectively with the
triazolyl core than the azide-derived N1 substituent. Furthermore,
triazolyl group exerts a stronger electron-withdrawing effect on the
N1 than the C4 substituent. Therefore, the placement of an electron-donating
group at either C4 or N1 position and the presence or the absence
of an intramolecular hydrogen bond (HB) have profound influences on
the optical properties of these compounds. The reported “click”
triazoles have fluorescence quantum yields in the range of 0.1–0.3
and large apparent Stokes shifts (8000–13 000 cm–1) in all tested solvents. Deprotonation of “click”
triazoles with a C4 hydroxyphenyl group increases their Stokes shifts;
while the opposite (or quenching) occurs to the triazoles with an
N1 hydroxyphenyl substituent. For the triazoles that contain intramolecular
HBs, neither experimental nor computational results support a model
of excited state intramolecular proton transfer (ESIPT). Rather, the
excited state internal (or intramolecular) charge transfer (ICT) mechanism
is more suitable to explain the fluorescence properties of the hydroxyphenyl-substituted “click”
triazoles; specifically, the large Stokes shifts of these compounds.

本描述了羟基苯基取代的1,2,3-三唑（“点击”三唑）分子的结构及光学性质。所谓“点击”三唑，系通过铜（I）催化的叠氮化物-炔烃环加成反应制备而成。在“点击”三唑中，由炔烃衍生而来的C4取代基与三唑基核心之间的电子共轭作用，相较于由叠氮化物衍生而来的N1取代基，更为显著。此外，三唑基团对N1原子的电子吸引效应亦强于C4取代基。因此，在C4或N1位置引入供电子基团，以及分子内氢键（HB）的存在与否，均对这类化合物的光学性质产生深远影响。所报道的“点击”三唑在所有测试溶剂中均展现出0.1–0.3的荧光量子产率和较大的表观斯托克斯位移（8000–13 000 cm–1）。使用C4羟基苯基对“点击”三唑进行去质子化，能够增加其斯托克斯位移；而对于含有N1羟基苯基取代基的三唑，则发生相反的（或猝灭）现象。对于含有分子内氢键的三唑，实验和计算结果均不支持激发态分子内质子转移（ESIPT）模型。相反，激发态内部（或分子内）电荷转移（ICT）机制更适合解释羟基苯基取代的“点击”三唑的荧光性质，特别是这些化合物的较大斯托克斯位移。