Thermoreversible Polymorph Transitions in Supramolecular Polymers of Hydrogen-Bonded Squaramides

Abstract

Hydrogen-bonded squaramide (SQ) supramolecular polymers exhibit uncommon thermoreversible polymorph transitions between particle- and fiber-like nanostructures. SQs 1-3, with different steric bulk, self-assemble in solution into particles (AggI) upon cooling to 298 K, and SQs 1 and 2, with only one dendronic group, show a reversible transformation into fibers (AggII) by further decreasing the temperature to 288 K. Nano-DSC and UV/Vis studies on SQ 1 reveal a concentration-dependent transition temperature and ΔH for the AggI-to-AggII conversion, while the kinetic studies on SQ 2 indicate the on-pathway nature of the polymorph transition. Spectroscopic and theoretical studies reveal that these transitions are triggered by the molecular reorganization of the SQ units changing from slipped to head-to-tail hydrogen bonding patterns. This work unveils the thermodynamic and kinetic aspects of reversible polymorph transitions that are of interest to develop stimuli-responsive systems.

Keywords: Hydrogen Bonding; Squaramides; Stimuli-Responsive Systems; Supramolecular Polymers; Supramolecular Polymorphism.

Figure 1

a) Molecular structures of SQ derivatives 1–3. b) Schematic representation of the self‐assembly behavior of SQs 1–3 into AggI and the thermoreversible AggI‐to‐AggII transition for 1 and 2.

Figure 2

a) VT‐UV absorption spectra of 1 (3×10⁻⁵ M, MCH). Cooling rate: 1 K min⁻¹. b) Aggregation degree (α_agg) vs. temperature extracted from VT‐UV at λ=319 nm at different concentrations along with the corresponding fits to the nucleation‐elongation model using the global fitting approach. c) ϵ vs. temperature (325–278 K) extracted from VT‐UV at λ=319 nm at different concentrations. d) Polymorph diagram for SQ 1 obtained from cooling cycles of the VT‐UV (dots) and nano‐DSC (squares) experiments.

Figure 3

AFM images of a) 2‐AggI and b) 2‐AggII (5×10⁻⁵ M) drop‐casted onto HOPG at 298 and 278 K, respectively. c) SAXS curves of SQ 2 in MCH at 5×10⁻³ M at 286 (blue triangles) and 298 K (orange squares), and at 1×10⁻³ M at 298 K (red circles). The samples at 5×10⁻³ M were best fitted to the flexible cylinder model (orange and blue lines) and the sample at 1×10⁻³ M was fitted to the sphere customized model (red line). d) SLS counts of SQ 2 at 298 K (AggI) as function of the concentration in MCH.

Figure 4

Infrared spectra of 2 (5×10⁻³ M, MCH‐d₁₄ ) at 299 (AggI, red) and 278 K (AggII, blue). Inset shows the magnification of the C=O signals.

Figure 5

Minimum‐energy structures calculated at the GFN2‐xTB level, including solvent effects (n‐hexane), for the AggI (a) and AggII (b) models of SQ 1. Relevant intermolecular distances (d _c and d _HB) are indicated in Å and the rotational angle between vicinal SQ dimers θ in degrees (°). Color coding: C in green, N in blue, O in red and H in white.

Figure 6

a) Procedure to carry out the time‐dependent AggI‐to‐AggII transformation without (top) and with (bottom) addition of seeds. b) Time‐dependent UV/Vis absorption spectra at 278 K of 2 (4×10⁻⁵ M, MCH) after rapid cooling of the solution from 298 to 278 K. c) Normalized absorbance of SQ 2 vs. time plots extracted from the time‐dependent UV/Vis experiments at λ=324 nm at different concentrations (5×10⁻⁵ M (orange), 4×10⁻⁵ M (purple), and 3×10⁻⁵ M (grey)). d) Normalized absorbance vs. time plots (2, 4×10⁻⁵ M, MCH) extracted from the time‐dependent UV/Vis experiments at λ=324 nm, without seeding (violet dots), adding 10 μL of MCH (green triangles), and seeding with a 10 μL solution of 2‐AggII (4×10⁻⁵ M, kept for 2 h at 278 K) (blue squares). The aliquots were added at t=10 min as indicated in the plot with a blue arrow.

Figure 7

a) Nano‐DSC heating (red) and cooling (blue) traces for SQ 1 (5×10⁻⁴ M, MCH). Heating/cooling rate 1 K min⁻¹. MHC: Molar heat capacity. B) Stacked plots of T _I‐II (top), ΔH, and ΔS (bottom) of the AggI‐to‐AggII conversion (obtained from nano‐DSC cooling experiments in MCH) as function of the concentration of SQ 1. Green lines indicate the variation regimes of the parameters with concentration.