Mukherjee Anurag, Sakurai Tsuneaki, Seki Shu, Ghosh Suhrit
School of Applied and Interdisciplinary Sciences, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Kolkata 700032, India.
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan.
Langmuir. 2020 Nov 3;36(43):13096-13103. doi: 10.1021/acs.langmuir.0c02604. Epub 2020 Oct 25.
Two-dimensional (2D)-supramolecular assemblies of π-conjugated chromophores are relatively less common compared to a large number of recent examples on their low dimensional (0D or 1D) assemblies or 3D architectures. This article reports a rational design for the 2D supramolecular assembly of an amphiphilic core-substituted naphthalene-diimide derivative (cNDI-1). The building block contains a naphthalene-diimide (NDI) chromophore, symmetrically substituted with two dodecyl chains from the aromatic core while the imide positions are functionalized with two hydrophilic wedges containing oligo-oxyethylene chains. In water, it exhibits entropically favorable self-assembly with a critical aggregation concentration of 1.5 × 10 M and a lower critical solution temperature of 55 °C. The UV/vis absorption spectrum in water shows bathochromically shifted absorption bands compared to that of the monomeric dye in THF, indicating offset π-stacking among the NDI chromophores. C-H symmetric and asymmetric stretching frequencies in the FT-IR spectrum support the presence of organized hydrocarbon chains in conformation in the self-assembled state, similar to that in the crystalline -alkanes, which is further supported by studying the general polarization (GP) values of a noncovalently entrapped Laurdan dye. The atomic force microscopy (AFM) image shows the formation of ultrathin (height < 2.0 nm) ribbons for the spontaneously assembled sample which eventually produces a large-area 2D nanosheet by the lateral organization. The powder X-ray diffraction pattern of the drop-casted film, prepared from the preformed aggregates, reveals sharp peaks that indicate a crystalline lamellar packing along the direction of the 2D growth. Differential scanning calorimetry trace shows the melting of the crystalline alkyl chain domain at > 75 °C, which destroys the 2D assembly. Local-scale photoconductivity of the ordered 2D assembly, studied by the flash-photolysis time-resolved microwave conductivity (FP-TRMC) technique, reveals an anisotropic conductivity with ∼3 times larger conductivity along the parallel direction compared to that along the perpendicular one.
与近期大量关于低维（0D或1D）组装体或3D结构的例子相比，π共轭发色团的二维（2D）超分子组装体相对较少见。本文报道了一种两亲性核心取代萘二亚胺衍生物（cNDI-1）的二维超分子组装的合理设计。该结构单元包含一个萘二亚胺（NDI）发色团，其从芳香核对称地被两条十二烷基链取代，而酰亚胺位置用两条含低聚氧乙烯链的亲水性楔形物官能化。在水中，它表现出熵有利的自组装，临界聚集浓度为1.5×10 M，低临界溶液温度为55°C。与在四氢呋喃中的单体染料相比，水中的紫外/可见吸收光谱显示吸收带发生红移，表明NDI发色团之间存在错位π堆积。傅里叶变换红外光谱中的C-H对称和不对称伸缩频率支持在自组装状态下存在呈构象的有序烃链，类似于结晶烷烃中的情况，通过研究非共价包封的劳丹染料的一般极化（GP）值进一步得到证实。原子力显微镜（AFM）图像显示自发组装样品形成了超薄（高度<2.0 nm）的条带，最终通过横向组织形成大面积的二维纳米片。由预先形成的聚集体制备的滴铸膜的粉末X射线衍射图谱显示出尖锐的峰，表明沿二维生长方向存在结晶层状堆积。差示扫描量热曲线显示结晶烷基链域在>75°C时熔化，这破坏了二维组装体。通过闪光光解时间分辨微波电导率（FP-TRMC）技术研究的有序二维组装体的局部尺度光电导率显示出各向异性电导率，平行方向的电导率比垂直方向大~3倍。
