Secondary nucleation guided noncovalent synthesis of dendritic homochiral superstructures via growth on and from surface

Abstract

Secondary nucleation is an emerging approach for synthesizing higher-order supramolecular polymers with exciting topologies. However, a detailed understanding of growth processes and the synthesis of homochiral superstructures is yet to be demonstrated. Here, we report the non-covalent synthesis of dendritic homochiral superstructures using NIR triimide dyes as building blocks via a secondary nucleation elongation process. Detailed analysis of kinetics and temporal evolution of morphology indicates that the formation of dendritic homochiral superstructures proceeds via growth on the surface and growth from the surface of the seeds. The combination of these two processes leads to the formation of elegant homochiral superstructures with a size of ~0.4 mm², having a superhelix at the center and helical fibres as branches. Moreover, these dendritic homochiral superstructures exhibit significantly high chiro-optical photoresponse with the magnitude of g_factor reaching a value as high as 0.55 - 0.6. Thus, our results provide insights into the growth process of homochiral superstructures with dendritic topology, which can be critically important for the design and optimization of chiral-selective optoelectronic devices leveraging controlled self-assembly.

Fig. 1. Molecular structures and summary of the formation of dendritic homochiral superstructures (DHS).

a Molecular structures of NIR triimides having chiral (S-G and R-G) and achiral alkyl (A-G) side chains. b schematic illustration of the formation of dendritic homochiral superstructures via ‘growth on the surface’ and ‘growth from the surface’ secondary nucleation-elongation process. The schematic has been made using Blender 3.5 software.

Fig. 2. Probing the temporal evolution of supramolecular polymers.

a Time-dependent absorption spectra of 30 μM solution of S-G in 15% CHCl₃ in IPA over a period of 48 min. representing its temporal growth. b Time-dependent CD spectra of a 30 μM solution of S-G and R-G in 15% CHCl₃ in 85% IPA. c Photographs of solution of S-G (30 μM) in 15% CHCl₃ in IPA at different time intervals. d Time-dependent dynamic light scattering (DLS) data of S-G (30 μM) in 15% CHCl₃ in IPA.

Fig. 3. Probing the evolution of supramolecular polymers into dendritic homochiral superstructures (DHS) using FE-SEM.

Field-emission scanning electron microscopy (FE-SEM) images of S-G (30 μM) in 15% CHCl₃ in IPA spin-coated on a silicon wafer at various time intervals. a At 0 min., freshly prepared aggregates are smaller in size, consisting of single and double helices, white arrows show primary nucleation fibers, and circled areas indicate branching (inset: transmission electron microscopy (TEM) image showing a double helix formation). b After 15 min., intertwined helices grow bigger in length and width with increased branching. c After 30 min., the intertwined helices start forming a superhelix of widths between 190 nm to 220 nm and with further increased branching. d After 45 min., we observed dendritic homochiral superstructures (DHS) having a superhelix of width 430 nm with extensive branching. e A magnified FE-SEM image of the central superhelix from which smaller helices are originating shows that the superhelix is composed of multiple helices. f A 3D image of DHS showing a superhelix core with a height of ~305 nm.

Fig. 4. Importance of bay substituted phenyl ring and kinetic analysis of self-assembly.

a The radial distribution functions (RDF), g(r), between geometric center of phenyl rings in S-G stack. The phenyl rings are observed to engage in three distinct types of interactions within the S-G stack: first, π-π interactions between neighboring phenyl rings, corresponding to the shoulder at 4.3 Å in the g(r); second, CH-π interactions between adjacent phenyl rings, leading to the peak at 5.2 Å; and third, CH-π interactions between the branched methyl group of one molecule and the phenyl ring of a neighboring molecule, resulting in the peak at 8.2 Å. b Double logarithmic plot of concentration and half-time of S-G at concentrations of 15 μM, 20 μM, 25 μM, 30 μM and 35 μM showing a linear fit. The slope gives the value of scaling exponent (γ) to be −2.264. c Kinetic profiles of aggregation of 15 μM, 20 μM, 25 μM, 30 μM and 35 μM solutions in 15% CHCl₃ in IPA and corresponding fit in secondary nucleation dominated-unseeded, shows increasing lag time with decreasing concentration of S-G. Here α @650 nm represents the degree of supramolecular polymerization monitored at 650 nm.

Fig. 5. Schematic illustration of DHS formation.

Schematic illustration of the formation of dendritic homochiral superstructures (DHS) via ‘growth on the surface’ and ‘growth from the surface’ of the seeds via secondary nucleation-elongation mechanism. The schematic has been made using Blender 3.5 software.

Fig. 6. Effect of solvent composition on dendritic homochiral superstructures (DHS) formation.

a Time-dependent variation in the absorbance of S-G (30 μM) at various volume % of IPA in CHCl₃ monitored at 650 nm. Field-emission scanning electron microscopy (FE-SEM) images of a 30 μM solution of S-G (b) in 15% CHCl₃, (c) 10% CHCl₃ and (d) 5% CHCl₃ in IPA, spin-coated onto a silicon wafer. The inset of d shows the enlarged FE-SEM image of superhelices with very little branching.

Fig. 7. Chiro-optical response of dendritic homochiral superstructures (DHS) in solution.

a Schematic representation of device structure utilized for the time-dependent I–V measurements in solution. The schematic has been made using Blender 3.5 software. b I–V characteristics of S-G in 15% CHCl₃ in IPA mixture at 0 min. (pristine) and after 50 min. (DHS). c Temporal evolution of I_max for the solution of S-G. d Photocurrent generated upon illumination of S-G with left circularly polarized (LCP) and right circularly polarized (RCP) light. The inset shows the variation of g_factor with voltage. e Estimation of photoresponsivity upon illumination of S-G solution with LCP and RCP.