Evolution of Nano‐ to Microsized Spherical Assemblies of a Short Oligo(p‐phenyleneethynylene) into Superstructured Organogels

Abstract

The self-assembly of linear p-conjugated molecules to form nanoarchitectures of different morphologies is important in various fields of nanoscience and technology, particularly in the emerging field of supramolecular electronics. [1,2] In this context, oligo(p-phenyleneethynylene)s (OPEs), a class of conjugated molecules often referred as "molecular wires" has attracted much attention, particularly for the design of molecular electronic devices.[3] Although meta-and paralinked oligo-and poly(p-phenyleneethynylene)s (PPEs) are known to form solvophobically induced assemblies in solvent mixtures of different polarities, [4][5][6] the creation of stable supramolecular selfassemblies of short OPEs with different shapes and sizes from a single solvent of choice continues to be challenging. Herein, we report an unprecedented hierarchical selfassembly of a short OPE into nanoparticles, microspheres, and superstructures of different size and morphology that eventually result in stable blue-light emitting organogels.Alhough the design of organic nanoparticles of small molecules [7, 8] and polymers [9] are known, linearly conjugated oligomers are only rarely exploited for this purpose; however, a few reports have been published recently.[10] p-Extended molecules preferably self-assemble, with a few exceptions, [10a,d,g] to form elongated structures; for example, the self-assembly of oligo(p-phenylenevinylene)s (OPVs) results in the formation of supramolecular tapes and helices. [11,12] We have now targeted OPEs, the dehydrogenated analogues of OPVs in an attempt to understand the self-assembly processes of conjugated systems. For this purpose, OPE1 and OPE2 were synthesized by using multistep synthetic strategies involving palladium-catalyzed Sonogashira-Hagihara crosscoupling reactions (Scheme 1).[13] These molecules were characterized by 1 H NMR and 13 C NMR spectroscopy as well as by MALDI-TOF mass spectrometry. [14] The UV/Vis spectrum of OPE1 in chloroform (1 10 À5 m) at 25 8C showed a strong p-p* absorption band at 377 nm. In decane, the absorption maximum is red-shifted to 385 nm and the formation of an additional band at 419 nm was observed. The emission maximum in chloroform was at 414 nm, which was significantly red-shifted towards 443 nm in decane but with decreased intensity. [14] These observations are attributed to the J-type aggregation of OPE1 in decane. Variabletemperature UV/Vis absorption spectra recorded in decane (1.1 10 À5 m) showed there was a reversible transition of the aggregates to isotropic species between 20 and 70 8C (Figure 1 a). A plot of the fraction of the aggregates (a) versus temperature revealed a sharp transition between 30 and 40 8C as well as a broad transition in the range of 45-55 8C, thus indicating the presence of aggregates of different thermal stabilities (Figure 1 a, inset). Furthermore, the absorption spectrum of OPE1 (1.1 10 À5 m) in decane at room temperature showed a time-dependent decrease in the absorption maximum, which reached a stable state after 4 h (Figure 1 b)....

Bile acid alkylamide derivatives as low molecular weight organogelators: Systematic gelation studies and qualitative structural analysis of the systems

“…In addition, the variability in bands observed in the IR spectra indicated that the polymorphism in the studied systems can be diverse, as reported previously for 1 [85]. Systems, where the gel was formed by vesicles or spherical colloidal aggregates rather than the more common fibrous structures have occasionally been described in the literature [40,78,79,[86][87][88][89]. Intriguingly, bile acid-based organogelation based on colloidal aggregates in the range of hundreds of nanometers have been reported [29].…”

Challenges and breakthroughs in recent research on self-assembly

“…For example, they have reported the formation of micrometer-size supramolecular tapes and helices through self-assembly of oligo (p-phenylenevinylene) derivatives (figure 26A) [189]. Formation of superstructures of micrometer size in a dried assembly obtained from decane was revealed by SEM imaging.…”

Sterol-containing tetraphenylethenes: synthesis, aggregation-induced emission, and organogel formation

“…In the gel state, the luminophores may assemble and align in a onedimensional fashion, which may result in interesting fluorescence phenomena [11][12][13][14][15]. Thus, these fluorescent organogels may find high-technological applications such as in optical devices and sensors.…”