Photoreactive helical nanoaggregates exhibiting morphology transition on thermal reconstruction

Abstract

The supramolecular design of photochromic molecules has produced various smart molecular assemblies that can switch their structures and/or functions in response to light stimuli. However, most of these assemblies require large structural changes of the photochromic molecules for an efficient conversion of assembled states, which often suppresses the photoreactivity within the self-assemblies. Here we report molecular assemblies, based on a photo-cross-linkable chromophoric dyad, in which a small amount of ultraviolet-generated photochemical product can guide the entire system into different assembly processes. In apolar solution, the intact dyad self-assembles into right-handed superhelical fibrils. On ultraviolet-irradiation of these fibrils, an effective photoreaction affords a sole photo-cross-linked product. When right-handed helical fibrils, containing a minor amount of the photoproduct, are thermally reconstructed, the intact molecule and the photoproduct undergo a co-assembly process that furnishes superhelical fibrils with different molecular packing structures. This molecular design principle should afford new paradigms for smart molecular assemblies.

Figure 1. Molecular structures and assembly processes of 1 and 1cyc.

(a,b) Molecular structures of the stilbene dyad 1 (a) and its corresponding [2+2] photocycloaddition product 1cyc (b). (c) Proposed self-assembly process of 1 and co-assembly process of 1 and 1cyc.

Figure 2. Ultraviolet (UV) absorption and CD spectroscopic, together with morphogical studies of 1.

(a,b) Change of the (a) UV absorption and (b) CD spectra of 1 ([1]=1.0 × 10⁻⁴ M) in MCH upon cooling the solution from 353 to 293 K (interval: 5 K, cooling-rate: 1 K min⁻¹). Since no spectral change was observed between 353 and 333 K, only the spectra for 333 K (red) and 293 K (blue) are shown for clarity. (c) Normalized CD intensity (φ_n) at 365 nm as a function of the cooling temperature. Using a cooperative model, the red and black lines correspond to the simulated curves in the nucleation and elongation regimes, respectively. (d–f) AFM height images of (P)-helical ribbons of 1_agg at 323 K. Scale bar, 50 nm. (g,h) AFM cross-sectional analysis between the red dots in images d and e. (i) Schematic representation of a (P)-superhelical ribbon. (j,k) TEM images of (P)-superhelical fibrils of 1_agg found at 293 K. Scale bar, 300 nm. (l,o) AFM height images of (P)-superhelical fibrils of 1_agg found at 293 K. Scale bar, 300 nm. (m,n) AFM cross-sectional analysis between the blue dots in image l. Samples were prepared by spin-coating MCH solutions of 1 ([1]=1.0 × 10⁻⁴ M) at appropriate temperatures onto HOPG (AFM) or carbon-coated copper grid (TEM). (p) Schematic representation of a (P)-superhelical fibril.

Figure 3. Analysis of the photoproducts formed by exposing 1_agg to ultraviolet (UV)-irradiation.

(a) UV absorption spectra of a MCH solution of 1 ([1]=1.0 × 10⁻⁴ M) at 293 K upon UV-irradiation (365 nm, 90 min). The photoconversion yield for 1cyc (80%) was determined by ¹H NMR analysis. (b) AFM height image of aggregates (1:1cyc=20:80) spin-coated from a MCH solution onto HOPG. Scale bar, 500 nm. (c,d) FT/IR spectra of 1_agg (blue) and aggregates (1:1cyc=20:80; black). Film samples for infreared spectroscopy were prepared by drop-casting MCH solutions (before and after UV-irradiation for 90 min) onto substrates. (e) ESI-MS spectra of 1 (blue) and 20:80 mixture of 1 and 1cyc (black). (f) Possible intramolecular photo-cross-linking reactions of 1 with assignment of the corresponding ¹H NMR signals. (g) ¹H NMR spectrum of 1 in CDCl₃. (h–j) ¹H NMR spectra of 1cyc in CDCl₃ after UV-irradiation in MCH. Pure 1cyc was prepared by UV-irradiation of a MCH solution ([1]=1.0 × 10⁻³ M) at 333 K for 1 h. (k) ¹H NMR spectrum of a mixture of photoproducts in CDCl₃ after UV-irradiation in CHCl₃.

Figure 4. CD studies for the reconstruction processes of co-aggregates consisting of 1 and 1cyc.

(a) Schematic representation of the conversion procedure from 1_agg to (1:1cyc_f)_recon. (b,c) CD spectra of mixtures of 1 and 1cyc in MCH (total concentration=1.0 × 10⁻⁴ M) at 293 K (b) before [(1:1cyc_f)_agg] and (c) after thermal annealing [(1:1cyc_f)_recon]. (d) Normalized CD intensity at 370 nm (φ_n) as a function of the temperature in the reconstruction processes of (1:1cyc_f)_recon from 353 to 293 K (cooling rate: 1 K min⁻¹). The arched arrow indicates the transition of the cooling-curves of 1 on increasing the fraction of 1cyc. The inset shows magnified plots in the nucleation regimes close to T_e, and the arrow indicates the transition of the cooling curves of 1 on increasing the fraction of 1cyc. (e,f) Isodesmic model fitting of the cooling curves of (1:1cyc_f=0.6)_recon and (1:1cyc_f=0.8)_recon. (g,h) Cooperative model fitting of the cooling curves of (1:1cyc_f=0.1)_recon and (1:1cyc_f=0.25)_recon.

Figure 5. Morphologies of co-aggregates consisting of 1 and 1cyc.

(a,b) AFM height images, (c) AFM cross-sectional analysis between the red dots in image a, (d) cartoon representation of the observed nanostructure of (1:1cyc_f=0.8)_recon spin-coated from a MCH solution (total concentration=1.0 × 10⁻⁴ M) onto HOPG. Scale bar, 300 nm. (e,f) AFM height images, (g) AFM cross-sectional analysis between the red dots in image f, (h) cartoon representation of the observed nanostructure of (1:1cyc_f=0.6)_recon spin-coated from a MCH solution (total concentration=1.0 × 10⁻⁴ M) onto HOPG. Scale bar, 300 nm. (i) AFM height images of the observed nanostructure of (1:1cyc_f=0.25)_recon spin-coated from a MCH solution (total concentration=1.0 × 10⁻⁴ M) onto HOPG. Scale bar, 300 nm. (j,k) AFM height images, and (l) cartoon representation of the observed nanostructure of (1:1cyc_f=0.1)_recon spin-coated from a MCH solution (total concentration=1.0 × 10⁻⁴ M) onto HOPG. Scale bar, 300 nm.

Figure 6. Comparison of the cooperative assembly processes of 1_agg and (1:1cyc_f=0.1)_recon.

(a,d) AFM height images of 1_nuc and (1:1cyc_f=0.1)_nuc, respectively. Scale bar, 150 nm. (b,e) Magnified AFM images of typical nanostructures in (a,d), respectively. Scale bar, 150 nm. Samples were prepared by spin-coating MCH solutions (total concentration=1.0 × 10⁻⁴ M) at 328 K (T_e) onto HOPG. (c,f) DLS-derived size distribution of aggregates of 1_nuc and (1:1cyc_f=0.1)_nuc in MCH, respectively. (g,h) Schematic representation of the proposed cooperative assembly processes of 1_agg and (1:1cyc_f=0.1)_recon, respectively.