Luminescent Liquid-Crystalline J-Aggregate Based on a Columnar Axial Coassembly

Abstract

Controlling the self-assembly of dyes is essential for designing functional materials with tailored optical, electronic, and mechanical properties. However, achieving precise structures from two distinct chromophores remains a major challenge in the field, requiring sophisticated strategies to direct their organization at the molecular level. In the present work, we report a novel approach to engineer complex liquid-crystalline (LC) columnar nanostructures through the precise coassembly of two bis-dendronized chromophores: a tris(p-phenyleneethynylene) (TPE) dicarboxylic acid (1) and a tris(p-phenylenevinylene) (TPV) bis(pyridine) (2). TPE 1 forms an unconventional four-stranded orthogonal columnar LC phase via hydrogen bonding between carboxylic acid groups, while TPV 2 adopts a lamellar soft-crystalline phase. Remarkably, their equimolar mixture (1·2) gives rise to an unprecedented two-component columnar liquid crystal. This coassembly is grounded on the complementary hydrogen bonding between pyridine and carboxylic acid groups that leads to the formation of 1D strands composed of alternating molecules of 1 and 2. These strands hierarchically organize by π-π interactions into eight-stranded columnar structures in which the 1/2 molecules are oriented with the transition dipole moments parallel to the columnar axis. This configuration promotes slipped π-π interactions and J-type coupling of the TPE and TPV components, resulting in a fluorescent LC material. This work paves the way for the design of precision multicomponent assemblies, opening exciting avenues for advanced optoelectronic and photonic materials.

1

(a) Schematic representation of a conventional columnar LC assembly based on a disc-like liquid crystal. (b) Schematic representation of nonconventional simple (left) and core–shell (right) columnar LC assemblies with the chromophores oriented parallel to the columnar axis. The orientation of the transition dipole moments (μ_ag) is illustrated with purple arrows, and the direction of the H-bonds is illustrated with green arrows. (c) Molecular structures of TPE 1 and TPV 2. (d) Illustration of the molecular assembly of 1 (left), 2 (right), and 1·2 (middle) into columnar liquid crystals (1 and 1·2) and a crystalline lamellar phase (2). The columnar assemblies of 1 and 1·2 consists of orthogonal orientation of the molecules with μ_ag (purple arrows) parallel to the columnar axis, based on four (1) and eight (1·2) strands, respectively. H-bonding and slipped π–π interactions of the assemblies are shown as magnifications. Insets show pictures of the thin films of each compound under 360 nm UV irradiation and the emission quantum yield (ϕ).

2

POM images of compounds (a) 1, (d) 2, and (g) 1·2 at 100, 60, and 100 °C, respectively. DSC first cooling (blue) and second heating (red) curves for (b) 1, (e) 2, and (h) 1·2. Heating/cooling rate 10 °C min^–1. X-ray patterns of (c) 1, (f) 2, and (i) 1·2 at 100, 25, and 100 °C, respectively. Miller indices, layer lines (L), phases, and lattice parameters are indicated in the inset.

3

(a) Schematic representation of the shearing induced alignment of liquid crystals 1 and 1·2 and the proposed orientation of the molecules in the columnar assemblies. (b) Illustration of the setup and sample alignment for the GiWAXS experiments. (c) POM images at 30 °C of an aligned sample of 1·2 with the shearing direction (right) parallel to the analyzer and (left) rotated by 45° to the analyzer. The sample was aligned by mechanical shearing at 125 °C. GiWAXS patterns of aligned samples of (d) 1 (100 °C) and (e) 1·2 (100 °C) on silicon plates. The incidence of the X-ray beam and the relative alignment of the sample are shown in Figure b. (f) Polarized UV/vis absorption spectra of an aligned thin film of 1·2 on a quartz plate. The spectra were recorded with the polarizer parallel (0°, blue line) and perpendicular (90°, red line) to the shearing direction. (g) Polarized FT-IR spectra of an aligned sample of 1·2 on a KBr plate, with the polarizer parallel (0°, blue line) and perpendicular (90°, red line) to the direction of the alignment.

4

UV/vis (solid lines) and emission (dashed lines) spectra of (a) 1 and (b) 1·2 in solid-state films. Dashed vertical lines indicate the absorption maxima of the monomers of 1 (1 Mon) and 2 (2 Mon) in CHCl₃. (c) Fluorescence decay curves for the solid-state films of 1 (red line), 2 (black line), and 1·2 (green line). Fittings are shown in the Supporting Information (Figures S28–S30). (d) FLIM contact angle experiments on annealed samples of 1 and 2 measured on a quartz plate at 25 °C.

5

Optimized geometries (RI-BP86-D4/def2-TZVP) of (a) the H-bonded homodimer of 1 and (b) the H-bonded heterodimer of 1·2. Magnifications show the H-boning patterns. (c) Optimized geometry of the H-bonded and π-stacked homotetramer of 1. (d) Optimized geometry of the H-bonded and π-stacked heterotetramer of 1·2. Magnifications show a single slipped π–π interaction for each assembly. (e) On-top and side views of the GFN2-xTB optimized geometry of the assembly model of 1 composed of 12 molecules (3504 atoms). (f) On-top and side views of the assembly model of 1·2 composed of 16 molecules of each coformer (9280 atoms). This was generated by replicating half of this model that was fully optimized at the GFN2-xTB semiempirical level.