Shen Chia-An, Bialas David, Hecht Markus, Stepanenko Vladimir, Sugiyasu Kazunori, Würthner Frank
Institut für Organische Chemie, Universität Würzburg, Am Hubland, 97074, Würzburg, Germany.
Center for Nanosystems Chemistry (CNC) and Bavarian Polymer Institute (BPI), Universität Würzburg, Theodor-Boveri-Weg, 97074, Würzburg, Germany.
Angew Chem Int Ed Engl. 2021 May 17;60(21):11949-11958. doi: 10.1002/anie.202102183. Epub 2021 May 2.
A bis(squaraine) dye equipped with alkyl and oligoethyleneglycol chains was synthesized by connecting two dicyanomethylene substituted squaraine dyes with a phenylene spacer unit. The aggregation behavior of this bis(squaraine) was investigated in non-polar toluene/tetrachloroethane (98:2) solvent mixture, which revealed competing cooperative self-assembly pathways into two supramolecular polymorphs with entirely different packing structures and UV/Vis/NIR absorption properties. The self-assembly pathway can be controlled by the cooling rate from a heated solution of the monomers. For both polymorphs, quasi-equilibrium conditions between monomers and the respective aggregates can be established to derive thermodynamic parameters and insights into the self-assembly mechanisms. AFM measurements revealed a nanosheet structure with a height of 2 nm for the thermodynamically more stable polymorph and a tubular nanorod structure with a helical pitch of 13 nm and a diameter of 5 nm for the kinetically favored polymorph. Together with wide angle X-ray scattering measurements, packing models were derived: the thermodynamic polymorph consists of brick-work type nanosheets that exhibit red-shifted absorption bands as typical for J-aggregates, while the nanorod polymorph consists of eight supramolecular polymer strands of the bis(squaraine) intertwined to form a chimney-type tubular structure. The absorption of this aggregate covers a large spectral range from 550 to 875 nm, which cannot be rationalized by the conventional exciton theory. By applying the Essential States Model and considering intermolecular charge transfer, the aggregate spectrum was adequately reproduced, revealing that the broad absorption spectrum is due to pronounced donor-acceptor overlap within the bis(squaraine) nanorods. The latter is also responsible for the pronounced bathochromic shift observed for the nanosheet structure as a result of the slip-stacked arranged squaraine chromophores.
通过用亚苯基间隔单元连接两个二氰基亚甲基取代的方酸菁染料，合成了一种带有烷基和低聚乙二醇链的双（方酸菁）染料。在非极性甲苯/四氯乙烷（98:2）溶剂混合物中研究了这种双（方酸菁）的聚集行为，结果表明其存在相互竞争的协同自组装途径，形成两种具有完全不同堆积结构和紫外/可见/近红外吸收特性的超分子多晶型物。自组装途径可以通过从单体的加热溶液中冷却的速率来控制。对于这两种多晶型物，都可以建立单体与各自聚集体之间的准平衡条件，以推导热力学参数并深入了解自组装机制。原子力显微镜测量显示，热力学上更稳定的多晶型物具有高度为2 nm的纳米片结构，动力学上更有利的多晶型物具有螺旋间距为13 nm、直径为5 nm的管状纳米棒结构。结合广角X射线散射测量，得出了堆积模型：热力学多晶型物由砖式纳米片组成，这些纳米片表现出J聚集体典型的红移吸收带，而纳米棒多晶型物由八条双（方酸菁）的超分子聚合物链相互缠绕形成烟囱型管状结构。这种聚集体的吸收覆盖了从550到875 nm的大光谱范围，这无法用传统的激子理论来解释。通过应用基本态模型并考虑分子间电荷转移，聚集体光谱得到了充分再现，表明宽吸收光谱是由于双（方酸菁）纳米棒内明显的供体 - 受体重叠。后者也是由于方酸菁发色团的滑移堆积排列而导致纳米片结构出现明显红移的原因。
