From Photoinduced Supramolecular Polymerization to Responsive Organogels

Abstract

Controlling supramolecular polymerization by external stimuli holds great potential toward the development of responsive soft materials and manipulating self-assembly at the nanoscale. Photochemical switching offers the prospect of regulating the structure and properties of systems in a noninvasive and reversible manner with spatial and temporal control. In addition, this approach will enhance our understanding of supramolecular polymerization mechanisms; however, the control of molecular assembly by light remains challenging. Here we present photoresponsive stiff-stilbene-based bis-urea monomers whose trans isomers readily form supramolecular polymers in a wide range of organic solvents, enabling fast light-triggered depolymerization–polymerization and reversible gel formation. Due to the stability of the cis isomers and the high photostationary states (PSS) of the cis – trans isomerization, precise control over supramolecular polymerization and in situ gelation could be achieved with short response times. A detailed study on the temperature-dependent and photoinduced supramolecular polymerization in organic solvents revealed a kinetically controlled nucleation–elongation mechanism. By application of a Volta phase plate to enhance the phase-contrast method in cryo-EM, unprecedented for nonaqueous solutions, uniform nanofibers were observed in organic solvents.

Mechanistic Insight into Supramolecular Polymerization in Water Tunable by Molecular Geometry

“…2a), comparable with the spectrum of the aggregates in toluene. 63 Upon heating, the band at 371 nm decreases in intensity, accompanied by the formation of two new local maxima at 340 nm and 357 nm, resulting in a spectrum that closely resembles the one of monomeric (E)-SA dissolved in an organic solvent (e.g., DMSO). 63 These changes indicate the disassembly of the supramolecular polymer during heating.…”

“…Stiff-stilbene is a five-membered ring analog of stilbene 54 used for various applications, e.g., switchable receptors/ligands, [55][56][57][58] force probes, 59 and aggregation-induced emission. 60 The C-Ph torsion angle of the (E)isomer is nearly zero, 61 imposing a planar π system and facilitating the formation of supramolecular assemblies, 54,[62][63][64] whereas the (Z)-isomer loses this planarity. 65 The huge geometric differences between (E)and (Z)-isomers offer the opportunity to study the influence of π-π interactions on supramolecular polymerization.…”

“…64 Recently, we reported a photo-induced cooperative supramolecular polymerization of stiffstilbene bis-ureas in toluene, taking advantage of the geometric differences and changes in the intramolecular and intermolecular hydrogen-bonding between isomers. 63 In the present study, we investigated the assembly and disassembly of a photoswitchable stiff-stilbene bis-urea amphiphile (SA) in pure water. We compared the experimental thermodynamic parameters of its assembly process in water and toluene, to gain insights into the role of hydrogen bonding, π-stacking, and hydrophobic effects in supramolecular polymerization in water (Fig.…”

Computational Tools to Rationalize and Predict the Self-Assembly Behavior of Supramolecular Gels

“…The researchers rationalized that the presence of the sodium ion could stem from the pH-triggered gelation procedure ( Figure 2 ). Following this study, other groups relied on DFT calculations to suggest a reasonable supramolecular stacking of gelators, from which both structural and electronic information could be retrieved [ 30 , 31 , 32 , 33 , 34 ]. However, the computational requirements of the quantum mechanical DFT calculations limit the size of the system that can be tackled.…”

White-Light-Emitting Supramolecular Polymer Gel Based on β-CD and NDI Host-Guest Inclusion Complex

“…Recognition, combination, and repeated arrays of complementary monomeric building blocks connected by highly directional and reversible non-covalent interactions yield supramolecular polymers [ 1 , 2 , 3 ]. Supramolecular polymers and gels have potential due to their wide applications in various fields such as optoelectronics [ 4 , 5 ], delivery vehicles [ 6 , 7 ], tissue repairing and engineering [ 8 , 9 , 10 , 11 ], and stimuli-responsive materials [ 12 , 13 , 14 ]. Supramolecular polymers are formed by highly directional and reversible non-covalent interactions such as hydrogen bonding, π-π stacking, Van der Waals interaction and hydrophobic interactions [ 15 , 16 , 17 , 18 ].…”