Crystal-Phase Transitions and Photocatalysis in Supramolecular Scaffolds

Abstract

The energy landscape of a supramolecular material can include different molecular packing configurations that differ in stability and function. We report here on a thermally driven crystalline order transition in the landscape of supramolecular nanostructures formed by charged chromophore amphiphiles in salt-containing aqueous solutions. An irreversible transition was observed from a metastable to a stable crystal phase within the nanostructures. In the stable crystalline phase, the molecules end up organized in a short scroll morphology at high ionic strengths and as long helical ribbons at lower salt content. This is interpreted as the result of the competition between electrostatic repulsive forces and attractive molecular interactions. Only the stable phase forms charge-transfer excitons upon exposure to visible light as indicated by absorbance and fluorescence features, second-order harmonic generation microscopy, and femtosecond transient absorbance spectroscopy. Interestingly, the supramolecular reconfiguration to the stable crystalline phase nanostructures enhances photosensitization of a proton reduction catalyst for hydrogen production.

Figure 1

(A) Molecular structure of T3-PMI chromophore amphiphile. (B) Photographs of 7.25 mM T3-PMI solution with 50 mM NaCl before (left) and after (right) annealing. (C) Cryo-TEM of 7.25 mM T3-PMI with 50 mM NaCl shows ribbons that are 15 ± 4 nm in width. (D) Cryo-TEM of annealed T3-PMI solution reveals scrolls of 90 ± 13 nm in width. (E) WAXS patterns of freshly dissolved, 50 mM NaCl, and annealed solution of T3-PMI. Freshly dissolved pattern shows no peaks, 50 mM NaCl (α-phase) pattern shows peaks at 8.3, 11.2, 14.9, 16.5, and 19.3 nm^–1, and annealed (β-phase) pattern shows peaks at 15.7 and 18.6 nm^–1. (F) Absorbance spectroscopy of freshly dissolved T3-PMI shows an absorbance maximum at 490 nm and a shoulder at 558.5 nm, 50 mM NaCl (α-phase) shows a blue-shifted absorbance maximum at 420 nm, and the annealed (β-phase) shows an absorbance maximum at 523 nm with a new feature at 565 nm.

Figure 2

Schematic representation of (A) α-phase and (B) (β-phase) nanostructures including side and top views of the molecular basis that occupies one quadrant of the oblique unit cell.

Figure 3

(A) VT X-ray scattering of 7.25 mM T3-PMI solution with 50 mM NaCl. Samples were heated at 1 °C/min and data collected every minute. (B) Normalized intensity vs temperature plots for the 8.3 and 16 nm^–1 peaks in (A). (C) VT absorbance spectroscopy of 50 mM NaCl T3-PMI solution in a 50 μm demountable cuvette monitored at 565 nm. Temperature was ramped at 1 °C/min.

Figure 4

(A) Isothermal heating curves 7.25 mM T3-PMI solution with 50 mM NaCl monitored at 565 nm. (B) Plot to determine activation energy using eq 1 in the SI. (C) DSC of 7.25 mM T3-PMI solution with 50 mM NaCl heated at 1.5 °C/min. (D) Energy landscape diagram of T3-PMI phase transition at 50 mM NaCl (not drawn to scale).

Figure 5

(A) Schematic diagram describing the two pathways for annealing T3-PMI solution. Confocal microscopy of annealed aggregates formed under path 1 (B) and path 2 (C). Cryo-TEM (D) and AFM (E) of helical aggregates formed in path 2.

Figure 6

(A) Variable-temperature absorbance spectroscopy (left axis) and DLS (right axis) of cooling path 2 solution from 80 °C at 1 °C/min. (B) Rheology of CaCl₂ gels of annealed aggregates formed in both paths.

Figure 7

(A) Hydrogen production experiments with T3-PMI and Mo₃S₁₃^2– proton reduction catalyst (inset, top). Continuous H₂ production was carried out over 100+ hours (inset, bottom). (B) Hydrogen production with β-phase helicies and scrolls compared to α-phase structures in the presence of the Mo₃S₁₃^2– catalyst. All solutions were gelled with 20 μL of 5 wt % PDDA before sample preparation. (Inset) Photographs of β-phase (left) and α-phase (right) PDDA gels. (C) Schematic of photoinduced electron transfer from T3 helical ribbons to catalytic clusters.

Figure 8

SHG microscopy of (A) β-phase helixes (B) and α-phase nanostructures.

Figure 9

Femtosecond transient absorption spectra of β-phase helices (A) and α-phase nanostructure (B). β-Phase was excited at 565 nm, while the α-phase was excited at 414 nm.