Vallan Lorenzo, Urriolabeitia Esteban P, Ruipérez Fernando, Matxain Jon M, Canton-Vitoria Ruben, Tagmatarchis Nikos, Benito Ana M, Maser Wolfgang K
Instituto de Carboquímica (ICB-CSIC) , C/Miguel Luesma Castán 4 , E-50018 Zaragoza , Spain.
Instituto de Síntesis Quı́mica y Catálisis Homogénea (ISQCH-CSIC Universidad de Zaragoza) , C/Pedro Cerbuna 12 , E-50009 Zaragoza , Spain.
J Am Chem Soc. 2018 Oct 10;140(40):12862-12869. doi: 10.1021/jacs.8b06051. Epub 2018 Sep 27.
The emission of a bright blue fluorescence is a unique feature common to the vast variety of polymer carbon dots (CDs) prepared from carboxylic acid and amine precursors. However, the difficulty to assign a precise chemical structure to this class of CDs yet hampers the comprehension of their underlying luminescence principle. In this work, we show that highly blue fluorescent model types of CDs can be prepared from citric acid and ethylenediamine through low temperature synthesis routes. Facilitating controlled polycondensation processes, the CDs reveal sizes of 1-1.5 nm formed by a compact network of short polyamide chains of about 10 monomer units. Density functional theory calculations of these model CDs uncover the existence of a spatially separated highest occupied molecular orbital and a lowest unoccupied molecular orbital located at the amide and carboxylic groups, respectively. Photoinduced charge transfer between these groups thus constitutes the origin of the strong blue fluorescence emission. Hydrogen-bond-mediated supramolecular interactions between the polyamide chains enabling a rigid network structure further contribute to the enhancement of the radiative process. Moreover, the photoinduced charge transfer processes in the polyamide network structure easily explain the performance of CDs in applications as revealed in studies on metal ion sensing. These findings thus are of general importance to the further development of polymer CDs with tailored properties as well as for the design of technological applications.
发出亮蓝色荧光是由羧酸和胺前驱体制备的各种聚合物碳点（CDs）共有的独特特征。然而，难以给这类CDs确定精确的化学结构，这仍然阻碍了对其潜在发光原理的理解。在这项工作中，我们表明可以通过低温合成路线由柠檬酸和乙二胺制备出高蓝色荧光的模型类型的CDs。促进可控的缩聚过程，这些CDs显示出由约10个单体单元的短聚酰胺链紧密网络形成的1 - 1.5纳米尺寸。对这些模型CDs的密度泛函理论计算揭示了分别位于酰胺和羧基处的空间分离的最高占据分子轨道和最低未占据分子轨道的存在。这些基团之间的光致电荷转移因此构成了强蓝色荧光发射的起源。聚酰胺链之间通过氢键介导的超分子相互作用形成刚性网络结构，进一步有助于增强辐射过程。此外，聚酰胺网络结构中的光致电荷转移过程很容易解释在金属离子传感研究中所揭示的CDs在应用中的性能。因此，这些发现对于进一步开发具有定制特性的聚合物CDs以及技术应用的设计具有普遍重要性。
