Modulation of supramolecular chirality by stepwise axial coordination in a nano-size trizinc(ii)porphyrin trimer

Abstract

Herein, we report a chiral guest's triggered spring-like contraction and extension motions coupled with unidirectional twisting in a novel flexible and 'nano-size' achiral trizinc(ii)porphyrin trimer host upon step-wise formation of 1 : 1, 1 : 2, and 1 : 4 host-guest supramolecular complexes based on the stoichiometry of the diamine guests for the first time. During these processes, porphyrin CD responses have been induced, inverted, and amplified, and reduced, respectively, in a single molecular framework due to the change in the interporphyrin interactions and helicity. Also, the sign of the CD couplets is just the opposite between R and S substrates which suggests that the chirality is dictated solely by the stereographic projection of the chiral center. Interestingly, the long-range electronic communications between the three porphyrin rings generate trisignate CD signals that provide further information about molecular structures.

Scheme 1. (A) Synthetic outline of the trizinc(ii)porphyrin trimer host (1) and (B) host–guest complexation between 1 and chiral guests (L).

Fig. 1. (A) Molecular structure of 1. Bond lengths (Å) Zn1⋯Zn2/Zn2⋯Zn3, 8.40 and Zn1⋯Zn3, 16.81. (B) Molecular structure of 1·(CHDA_(R,R))₂. Bond lengths (Å) and dihedral angles (deg) Zn1⋯Zn2, 6.46; Zn2⋯Zn3, 5.88; Zn1⋯Zn3, 12.34; Zn1–N1L, 2.156(4); Zn2–N2L, 3.521(4); Zn2–N3L, 2.193(4); Zn3–N4L, 2.191(4); Zn1–C33–C83–Zn2, 44.09; Zn2–C83A–C33A–Zn3, −42.91. (C) Molecular structure of 1·(CHDA_(R,R))₄. Bond lengths (Å) and dihedral angles (deg): Zn1⋯Zn2/Zn2⋯Zn3, 8.63; Zn1⋯Zn3, 17.25; Zn1–N1L/Zn3–N7L, 2.183(12); Zn2–N3L/Zn2–N5L, 2.397(14); Zn1–C33–C83–Zn2, 74.69; Zn2–C83A–C33A–Zn3, −74.69.

Fig. 2. (A) Schematic representation of the formation of 1 : 1, 1 : 2 and 1 : 4 host–guest complexes between 1 and CHDA_(R,R). CD and UV spectral changes (in CH₂Cl₂ at 295 K) of 1 (at 3 × 10⁻⁶ M) upon addition of CHDA_(R,R) as the host–guest molar ratio changes from (B) 1 : 0 to 1 : 50, (C) 1 : 50 to 1 : 500 and (D) 1 : 500 to 1 : 4000. Calculated (cal) and observed (obs) CD spectra (E) of 1·CHDA_(R,R) (brown, cal), 1·CHDA_(S,S) (green, cal), 1·CHDA_(R,R) (blue, obs) and 1·CHDA_(S,S) (red, obs); (F) 1·(CHDA_(R,R))₂ (brown, cal), 1·(CHDA_(S,S))₂ (green, cal), 1·(CHDA_(R,R))₂ (blue, obs) and 1·(CHDA_(S,S))₂ (red, obs); and (G) 1·(CHDA_(R,R))₄ (brown, cal), 1·(CHDA_(S,S))₄ (green, cal), 1·(CHDA_(R,R))₄ (blue, obs) and 1·(CHDA_(S,S))₄ (red, obs).

Fig. 3. (A) DFT-optimized structure of 1·(CHDA_(R,R))₂; bent arrows represent the direction of interporphyrin twisting. (B) TDDFT-calculated (blue line) and experimental (red line) CD spectra of 1·(CHDA_(R,R))₂.