Supramolecular block copolymers by kinetically controlled co-self-assembly of planar and core-twisted perylene bisimides

Abstract

New synthetic methodologies for the formation of block copolymers have revolutionized polymer science within the last two decades. However, the formation of supramolecular block copolymers composed of alternating sequences of larger block segments has not been realized yet. Here we show by transmission electron microscopy (TEM), 2D NMR and optical spectroscopy that two different perylene bisimide dyes bearing either a flat (A) or a twisted (B) core self-assemble in water into supramolecular block copolymers with an alternating sequence of (AmBB)n. The highly defined ultralong nanowire structure of these supramolecular copolymers is entirely different from those formed upon self-assembly of the individual counterparts, that is, stiff nanorods (A) and irregular nanoworms (B), respectively. Our studies further reveal that the as-formed supramolecular block copolymer constitutes a kinetic self-assembly product that transforms into thermodynamically more stable self-sorted homopolymers upon heating.

Figure 1. Molecular design and optical properties of PBIs 1 and 2.

(a) Chemical structures and space-filling (CPK) model of PBIs 1 and 2. (b) Ultraviolet–visible absorption spectra (solid lines) and corresponding fluorescence spectra (dashed lines) of PBI 1 (blue) and PBI 2 (red) in THF; concentration, ∼1 × 10⁻⁵ M of each sample. Excitation wavelength is 470 nm. Inset, photograph of PBI 1 and PBI 2 solutions in THF under ultraviolet light.

Figure 2. Morphology of unimolecular PBI self-assemblies.

(a) TEM image of PBI 1 aggregates in water, [PBI 1]=0.05 mM (0.077 mg ml⁻¹) and schematic space-filling models illustrating the self-assembly of PBI 1. Scale bar, 100 nm. Inset, magnified TEM image of the same sample. (b) TEM image of PBI 2 aggregates in water, [PBI 2]=0.03 mM (0.077 mg ml⁻¹) and schematic space-filling models illustrating the self-assembly of PBI 2. Scale bar, 50 nm. Inset, magnified TEM image of the same sample.

Figure 3. Optical properties of unimolecular PBI self-assemblies.

Ultraviolet–visible absorption spectra (solid lines) and corresponding normalized fluorescence spectra (dashed lines) of PBI 1 (blue) and PBI 2 (red) in water; concentration, ∼1 × 10⁻⁵ M of each sample. Excitation wavelength is 470 nm. Magic-angle setup was applied.

Figure 4. Co-assembly of planar and core-twisted PBIs.

(a) TEM images of segmented nanowires formed by co-assembly of PBI 1/PBI 2 in water. Scale bar, 50 nm. Inset, magnified TEM images of the single-molecule-thin nanowires. Scale bars, 2 nm. [PBI 1]:[PBI 2]=2:1 in molar ratio, [PBI 1]=3.2 × 10⁻⁴ M (0.5 mg ml⁻¹). (b) ¹H, ¹H-ROESY spectroscopy for PBI 1/PBI 2 co-assemblies in D₂O/[D₈]THF (400/600 μl). [PBI 1]:[PBI 2]=2:1, [PBI 1]=4.9 × 10⁻³ M. (c) Schematic illustration based on space-filling model for PBI 1/PBI 2 co-assembly. For clarity, only representative protons are shown in the structural model of one co-assembling segment from semiempirical AM1 calculations.

Figure 5. Optical properties of PBI 1/PBI 2 nanowires.

Normalized absorption spectrum (red solid) and corresponding fluorescence spectrum (red dashed line, λ_exc=480 nm, magic-angle setup) of nanowires formed by co-assembly of PBI 1/PBI 2 in a molar ratio of 2:1 ([PBI]=7.5 × 10⁻⁵ M in water) and calculated absorption spectrum (black solid) for a 2:1 mixture of PBI 1 and PBI 2 based on the linear superposition of normalized absorption spectra of PBI 1 and PBI 2 (see Fig. 3), respectively, in water. Inset, photograph of a solution of nanowires in water under ultraviolet light.

Figure 6. LCST behaviour of co-assembled PBI 1/PBI 2 nanowires and unimolecular PBI 1 self-assemblies in water.

Determination of LCSTs for nanowires (open symbols) formed by co-assembly of PBI 1 and PBI 2 in a molar ratio of 2:1 ([PBI 1]=5 × 10⁻⁵ M in water) and for unimolecular PBI 1 aggregates (closed symbols, [PBI 1]=5 × 10⁻⁵ M). Transmission was monitored at 800 nm. Heating rate was 0.1 °C min⁻¹.