Synthesis and conformation of substituted chiral binaphthyl-azobenzene cyclic dyads with chiroptical switching capabilities

Abstract

Optically active binaphthyl-azobenezene cyclic dyads were synthesized to develop a photochromic switching molecule. Azobenezene moieties were cis-trans isomerized by photoirradiation. As a reflection of the structural change, the specific optical rotation and circular dichroism underwent significant shifts. Under certain conditions, the positive-negative and zero-positive (or zero-negative) signals were reversed. Optical rotation may potentially be applied in noise-cancelling nondestructive photoswiches. The conformations were studied by experimental and theoretical methods. The results revealed that the helical chirality, (P) or (M), of the cis-azobenzene moiety was induced by intramolecular axial chirality. The twist direction depended on the axial chirality as well as the azobenzene linkage position to the binaphthyls, but was independent of the identity of substituted groups. 2,2'-Linked-(R)-binaphthyl was found to induce cis-(P)-azobenzene, whereas symmetrically 7,7'-linked-(R)-binaphthyl was found to induce cis-(M)-azobenzene.

Figure 1

Photoisomerization of binaphthyl-azobenzene dyads 1.

Figure 2

Helicity of the cis-azobenzenes.

Figure 3

Opically active binaphthyl-azobenzene cyclic dyads 2–8.

Scheme 1

Synthetic route to (R)-2-8.

Figure 4

(a, b) CD spectra of (R)-2. (c, d) Absorption spectra of (R)-2. After 365-nm irradiation (blue line), after 436-nm irradiation (red line). Conditions: 1,4-dioxane (1.0 × 10⁻⁵ M), 20 °C, light path length = 10 mm, irradiation wavelength = 365 nm (10 mW/cm², 100 s) and 436 nm (10 mW/cm², 100 s).

Figure 5

(a, b) CD spectra of (R)-7. (c, d) Absorption spectra of (R)-7. After 365-nm irradiation (blue line), after 436-nm irradiation (red line). Conditions: 1,4-dioxane (1.0 × 10⁻⁵ M), 20 °C, light path length = 10 mm, irradiation wavelength = 365 nm (10 mW/cm², 100 s) and 436 nm (10 mW/cm², 100 s).

Figure 6

¹H-NMR spectra of the benzyl protons of (R)-2 after (a) 365 nm-irradiation, (b) 436 nm-irradiation. Conditions: CDCl₃, 1 × 10⁻³ M, 300 MHz, 22 °C, Irradiation: 365 nm (10 mW/cm², 500 s), 436 nm (10 mW/cm², 500 s) in an NMR test tube (ϕ = 5 mm).

Figure 7

Absorption spectra of concentrated (R)-2 after photo-irradiation; after 365 nm-irradiation (blue line), after 436 nm-irradiation (red line). Conditions: CHCl₃, 0.0126 M (0.01 g/dL), light path length = 10 mm, 20 °C, Irradiation wavelength: 365 nm (10 mW/cm², 500 s), 436 nm (10 mW/cm², 500 s).

Figure 8

First-order plots for cis to trans thermal isomerization of (R)-2-8. (a) 35 °C, (b) 45 °C, (c) 55 °C, (d) 65 °C.Conditions: 1,4-dioxane, 1.0 × 10⁻⁵ M.

Figure 9

Eyring plots for cis to trans thermal isomerization of (R)-2-8.

Figure 10

Structures of cis-(P)-azobenzene and cis-(M)-azobenzenelinked to a 1,1’-binaphthyl.

Figure 11

(a) Optimized structures of (R)-cis-(P)-2 and (R)-cis-(M)-2 obtained by the DFT calculations at the B3LYP/6-31G(d) level under C₂ symmetry. (b) CD calculated by the TD-DFT method with the B3LYP/6-31G(d) level of (R)-cis-(P)-2 (red bar), (R)-cis-(M)-2 (yellow bar), and experimental CD of (R)-2 after 365-nm irradiation (blue line, 1 × 10⁻⁵ M in 1,4-dioxane, 20 °C).

Figure 12

(a) Optimized structures of (R)-cis-(P)-7 and (R)-cis-(M)-7 obtained by the DFT calculations at the B3LYP/6-31G(d) level. (b) CD calculated by the TD-DFT method with the B3LYP/6-31G(d) level of (R)-cis-(P)-7 (red bar), (R)-cis-(M)-7 (yellow bar), and experimental CD of (R)-7 after 365-nm irradiation (blue line, 1 × 10⁻⁵ M in 1,4-dioxane, 20 °C).

Figure 13

(a) Optimized structures of (R)-cis-(P)-8 and (R)-cis-(M)-8 obtained by the DFT calculations at the B3LYP/6-31G(d) level under C₂ symmetry. (b) CD calculated by the TD-DFT method with the B3LYP/6-31G(d) level of (R)-cis-(P)-8 (red bar), (R)-cis-(M)-7 (yellow bar), and experimental CD of (R)-8 after 365-nm irradiation (blue line, 1 × 10⁻⁵ M in 1,4-dioxane, 20 °C).

Figure 14

(a) Structures of cis-(P)-13 and cis-(M)-13. (b) CD calculated by the TD-DFT method with the B3LYP/6-31G(d) level of cis-(P)-13 (red bar), cis-(M)-13 (yellow bar).

Figure 15

Twisting pattern of cis-azobenzenes induced by axial chirality of binaphthyls.