Unraveling the Driving Forces in the Self-Assembly of Monodisperse Naphthalenediimide-Oligodimethylsiloxane Block Molecules

Abstract

Block molecules belong to a rapidly growing research field in materials chemistry in which discrete macromolecular architectures bridge the gap between block copolymers (BCP) and liquid crystals (LCs). The merging of characteristics from both BCP and LCs is expected to result in exciting breakthroughs, such as the discovery of unexpected morphologies or significant shrinking of domain spacings in materials that possess the high definition of organic molecules and the processability of polymers. Here we report the bulk self-assembly of two families of monodisperse block molecules comprised of naphthalenediimides (NDIs) and oligodimethylsiloxanes (ODMS). These materials are characterized by waxy texture, strong long-range order, and very low mobility, typical properties of conformationally disordered crystals. Our investigation unambiguously reveals that thermodynamic immiscibility and crystallization direct the self-assembly of ODMS-based block molecules. We show that a synergy of high incompatibility between the blocks and crystallization of the NDIs causes nanophase separation, giving access to hexagonally packed columnar (Colh) and lamellar (LAM) morphologies with sub-10 nm periodicities. The domain spacings can be tuned by mixing molecules with different ODMS lengths and the same number of NDIs, introducing an additional layer of control. X-ray scattering experiments reveal macrophase separation whenever this constitutional bias is not observed. Finally, we highlight our “ingredient approach” to obtain perfect order in sub-10 nm structured materials with a simple strategy built on a crystalline “hard” moiety and an incompatible “soft” ODMS partner. Following this simple rule, our recipe can be extended to a number of systems.

Photoswitchable Nanomaterials Based on Hierarchically Organized Siloxane Oligomers

“…This result is consistent with previous studies, which suggest that phase segregation and end group crystallization effects synergize to create such layered nanomaterials. [30,31] In accordance with the photoisomerization of azobenzene from the thermodynamically favored trans-isomer to the cis-isomer with UV irradiation, absorbance spectroscopy of our trans-4-MAZOSi16 dissolved in chloroform indeed shows high absorbance between 300 and 400 nm (λ max = 359 nm) corresponding to the π−π* transition ( Figure 3A). More interestingly, solvent-free trans-4-MAZOSi16 films exhibit a similar spectrum but with fine structure appearing at λ max = 345, 358, and 377 nm ( Figure 3B).…”

“…Small angle X-ray scattering (SAXS) confirmed the existence of lamellar morphology with 5.1 nm periodicity (Figure 2A). Since previous studies suggest that the length of the ODMS16 spacer is ≈2.5 nm, [30,31] this periodicity suggests that the azobenzene domain consists of a single tilted trans-4-methoxyazobenzene monolayer. Sharp scattering peaks in the wide angle region (4.97 nm −1 < q < 27.8 nm −1 ) further suggests that trans-4-methoxyazobenzene end groups are highly organized, with the peak at 3.8 Å possibly corresponding to the stacking distance between H-aggregated planar trans-azobenzene.…”

“…Thus, to design photoswitchable materials with well-organized molecular arrangements, we created a series of oligodimethylsiloxanes end-functionalized with azobenzene motifs (Scheme 1). Perfectly monodisperse ODMS with 8 or 16 Si atoms were synthesized using a stepwise procedure described previously by our group [30][31][32] and olefin-terminated AZO motifs were subsequently attached via platinum-catalyzed hydrosilylation.…”

“…[30,31] Here, we utilize these self-assembly principles to direct azobenzene motifs into spatially ordered domains as well as promote a high degree of molecular organization. The resulting nanomaterials can exhibit rapid and reversible solid-liquid athermal phase transitions with UV and blue light, thus showing photoswitching between adhesive properties due to crystallization of the azobenzene and lubricant properties due to the oligodimethylsiloxane (ODMS) spacers.…”

(De)bonding on Demand with Optically Switchable Adhesives

“…Meijer and co‐workers created an LC system based on telechelic dimethylsiloxane oligomers of defined length, which were end‐functionalized with azobenzenes (Azo‐ODMS; Figure c) . Monodisperse Azo‐ODMS containing 8 or 16 siloxane repeat units were prepared in a stepwise procedure utilizing chlorosilane and silanol building blocks . ODMS16 functionalized with trans‐ 4‐methoxyazobenzene end groups was shown to assemble into a lamellar microstructure, in which crystalline azobenzene monolayers are separated by amorphous ODMS.…”