Enantioselective Recognition of Helicenes by a Tailored Chiral Benzo[ghi]perylene Trisimide π-Scaffold

Abstract

Enantioselective molecular recognition of chiral molecules that lack specific interaction sites for hydrogen bonding or Lewis acid-base interactions remains challenging. Here we introduce the concept of tailored chiral π-surfaces toward the maximization of shape complementarity. As we demonstrate for helicenes it is indeed possible by pure van-der-Waals interactions (π-π interactions and CH-π interactions) to accomplish enantioselective binding. This is shown for a novel benzo[ghi]perylene trisimide (BPTI) receptor whose π-scaffold is contorted into a chiral plane by functionalization with 1,1'-bi-2-naphthol (BINOL). Complexation experiments of enantiopure (P)-BPTI with (P)- and (M)-[6]helicene afforded binding constants of 10 700 M^-1 and 550 M^-1 , respectively, thereby demonstrating the pronounced enantiodifferentiation by the homochiral π-scaffold of the BPTI host. The enantioselective recognition is even observable by the naked eye due to a specific exciplex-type emission originating from the interacting homochiral π-scaffolds of electron-rich [6]helicene and electron-poor BPTI.

Keywords: Binding Studies; Chirality; Dyes/Pigments; Enantioselectivity; Molecular Recognition.

Scheme 1

Synthesis of BPTI 7 from perylene dianhydride 1 with separation of the enantiomers on a chiral stationary phase.

Figure 1

Molecular structure of (P)‐BPTI 7 according to single‐crystal X‐ray analysis of a racemic mixture. a) Top view (28.3° as dihedral angle of bay position) and b) side view. In addition, an enlarged excerpt of the bay area is shown to illustrate the chiral helical twist (angles of 13.4° and 11.2° show the rotation of the successive π‐planes to each other). The ellipsoids are set to 50 % probability (C: gray, O: red, N: blue, H: white). Disorder of imides as well as solvent molecules and hydrogen atoms are omitted for clarity. For the packing pattern of (P)‐ and (M)‐enantiomers, see the Supporting Information (Figure S9).

Figure 2

a) CD (c≈10⁻⁵ M), b) CPL (c≈10⁻⁷ M), c) UV/Vis absorption (c≈10⁻⁵ M) and d) fluorescence (c≈10⁻⁷ M) spectra of (M)‐BPTI 7 (red line; λ _ex=400 nm) and (P)‐BPTI 7 (blue line; λ _ex=400 nm) in MCH at 293 K.

Figure 3

a) DFT‐optimized geometry for the 1 : 1 complex of (P)‐BPTI 7 and (P)‐[6]helicene at the wb97xd/6‐31 g(d) level of theory. b), c) UV/Vis and fluorescence titration experiments for (P)‐BPTI 7 host (blue line) upon addition of 55 equiv (P)‐[6]helicene guests (red line) in MCH at 293 K (black dashed line: calculated spectrum for the pure 1 : 1 complex (see the Supporting Information)). d), e) Data points measured at 502 nm for the titration experiment of (P)‐BPTI 7 upon addition of (P)‐[6]helicene and (M)‐[6]helicene, respectively, with the nonlinear curve fit according to the 1 : 1 binding model. The insets show photographs taken under UV light of the free (P)‐BPTI 7 solution (left) and (P)‐BPTI 7 after addition of the respective (P)‐ and (M)‐[6]helicene (right). f) Plot for the experimental Gibbs free energies for the BPTI‐helicene complexes versus the number of π‐electrons of the helicene guest molecules.