Double helical π-aggregate nanoarchitectonics for amplified circularly polarized luminescence

Abstract

The canonical double helical π-stacked array of base pairs within DNA interior has inspired the interest in supramolecular double helical architectures with advanced electronic, magnetic and optical functions. Here, we report a selective-recognized and chirality-matched co-assembly strategy for the fabrication of fluorescent π-amino acids into double helical π-aggregates, which show exceptional strong circularly polarized luminescence (CPL). The single crystal structure of the optimal combination of co-assemblies shows that the double-stranded helical organization of these π-amino acids is cooperatively assisted by both CH-π and hydrogen-bond arrays with chirality match. The well-defined spatial arrangement of the π-chromophores could effectively suppress the non-radiative decay pathways and facilitate chiral exciton couplings, leading to superior CPL with a strong figure of merit (g_lum = 0.14 and QY = 0.76). Our findings might open a new door for developing DNA-inspired chiroptical materials with prominent properties by enantioselective co-assembly initiated double helical π-aggregation.

Fig. 1. Schematic representation of double helical π-aggregation through enantioselective co-assembly.

a Molecular structures of histidine amphiphile L-TPEHis and Fmoc-protected alanine amphiphile enantiomers, L-FmocAla and D-FmocAla, respectively. b L-TPEHis molecules self-assemble into single helix TPE-aggregates. c Enantioselective co-assembly of L-TPEHis and L-FmocAla results in the formation of double helical π-aggregates, showing synchronously amplified g_lum and FLQY values. d Random co-assembly of L-TPEHis and D-FmocAla leads to unordered aggregates (0-D nanospheres) and quenched CPL emission. P and M in (b, c) represent right-handed and left-handed helicity of the helical aggregates, respectively.

Fig. 2. Self-assembly of TPEHis.

a UV–Vis absorption spectra of monomeric and aggregated L-TPEHis. b The relative fluorescence intensity (I/I_max) of TPEHis versus f_w (I is the fluorescence intensity of TPEHis under different f_w; I_max is the FL intensity at f_w = 90%; the inserted image is the photograph of TPEHis aggregates with different f_w values under UV light irradiation, λ_ex = 365 nm). c f_w–dependent CPL spectra of L-TPEHis. d CPL spectra of L-TPEHis and D-TPEHis aggregates at f_w = 90%. e, f SEM images of L-TPEHis assemblies at different f_w values, the inserted image in (f) is the TEM image of an isolated TPEHis nanotube. Unless otherwise noted, [TPEHis] = 10 mM for assemblies. For fluorescence and CPL measurements, λ_ex is 320 nm.

Fig. 3. Enantioselective co-assembly of L-TPEHis with Fmoc amino acids.

a Co-assembly of L-TPEHis with 20 Fmoc-protected essential amino acids led to selective recognition of L-Fmoc-alanine by CPL. b Fluorescence spectra of L-TPE, L-TPE/L-Fmoc, and L-TPE/D-Fmoc assemblies, the inserted figure is sample vials under UV light irradiation. c CD and d CPL spectra of TPE/Fmoc co-assemblies. L/L, L/D, D/D, D/L represent different chirality combinations of the two enantiomeric amino acids. e The emission decay curves of L-TPE/L-Fmoc, L-TPE/D-Fmoc, and L-TPE assemblies. f, g SEM images of L-TPE/L-Fmoc and L-TPE/D-Fmoc assemblies, respectively, the inserted figures at the bottom left corners are cartoon representations of the nanostructures. h Comparison of g_lum and QY values of L-TPE/L-Fmoc, L-TPE/D-Fmoc, and L-TPE assemblies. All the spectroscopic measurements are conducted by transferring the suspensions into cuvettes. Possible contributions from linear dichroism (LD) and linear birefringence (LB) caused by macroscopic anisotropy during the CD and CPL measurements were eliminated (Supplementary Fig. 16). TPEHis and FmocAla are abbreviated as TPE and Fmoc for clarity in Figs. 3–6.

Fig. 4. Single-crystal structures revealing the formation of single and double helical π-aggregates.

a–c The single-crystal structure of L-TPE and its structural features. d The rotational configurations of TPE chromophore in L-TPE (left) and L-TPE/L-Fmoc (right) crystals. e The unit cell of L-TPE/L-Fmoc co-crystal. f–h The co-crystal structure of L-TPE/L-Fmoc and its structural features, g consists of three parallel stacked unit cells and the middle one was covered with a double-helix cartoon figure. i, j The average dihedral angel (α) of benzene ring/ethene plane (left figures) and the geometry relations of three nearest neighboring TPE chromophores (right figures) in L-TPE (upper) and L-TPE/L-Fmoc (bottom) crystals. The blue arrows indicate the direction of TPE transition moment. k Illustration of S₀-S₁ electric transition moment (μ) of TPEHis calculated by TD-DFT at B3LYP 6-311G** level and visualized by VMD program, the z axis points outside the plane.

Fig. 5. FT-IR and XRD measurements.

a FT-IR spectra of L-TPE, L-TPE/L-Fmoc, L-TPE/D-Fmoc, and L-FmocAla assemblies. b Experimental XRD of L-TPE/L-Fmoc assemblies (red line) and simulated XRD of L-TPE/L-Fmoc co-crystal (black line), the inset indicates the bilayer width in the co-crystal.

Fig. 6. Proposed self-assembly mechanism and computational simulation.

a Chemical structures of possible packing mode of L-TPEHis. b PM6-D3H4 method optimized structure of L-TPE aggregates showing twisted topology. c The cross-section of L-TPE nanobute. r, l, and d represent the radius, wall thickness, and a bilayer width, respectively. d–e Proposed formation mechanism of nanotubes. f Chemical structures of possible packing mode of L-TPEHis/L-FmocAla co-assemblies. g The PM6-D3H4 method and h molecular dynamic (MD) simulation optimized structures of L-TPE/L-Fmoc co-assemblies show a flat multi-lamellar geometry, which further assembles into microplates (i).