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. 2020 Apr 8;142(14):6538-6547.
doi: 10.1021/jacs.9b10430. Epub 2020 Mar 24.

Computational Design and Synthesis of a Deeply Red-Shifted and Bistable Azobenzene

David B Konrad  1   2   3 , Gökcen Savasci  2   4 , Lars Allmendinger  1 , Dirk Trauner  2   5 , Christian Ochsenfeld  2   4 , Ahmed M Ali  1   2   6
Affiliations

Computational Design and Synthesis of a Deeply Red-Shifted and Bistable Azobenzene

David B Konrad et al. J Am Chem Soc. .

Abstract

We computationally dissected the electronic and geometrical influences of ortho-chlorinated azobenzenes on their photophysical properties. X-ray analysis provided the insight that trans-tetra-ortho-chloro azobenzene is conformationally flexible and thus subject to molecular motions. This allows the photoswitch to adopt a range of red-shifted geometries, which account for the extended n → π* band tails. On the basis of our results, we designed the di-ortho-fluoro di-ortho-chloro (dfdc) azobenzene and provided computational evidence for the superiority of this substitution pattern to tetra-ortho-chloro azobenzene. Thereafter, we synthesized dfdc azobenzene by ortho-chlorination via 2-fold C-H activation and experimentally confirmed its structural and photophysical properties through UV-vis, NMR, and X-ray analyses. The advantages include near-bistable isomers and an increased separation of the n → π* bands between the trans- and cis-conformations, which allows for the generation of unusually high levels of the cis-isomer by irradiation with green/yellow light as well as red light within the biooptical window.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Structures of red-AzCA-4, azobenzene (1), and tetra-ortho-chloro azobenzene (2).
Figure 2
Figure 2
Numerical descriptors for the dihedral angles.
Figure 3
Figure 3
Mono-, di-, and trichlorinated azobenzenes.
Figure 4
Figure 4
Computed orbital energies and n-MO densities of the optimized 2 conformation (left) and the conformation of 1 imposed on 2 (right).
Figure 5
Figure 5
Influence of the C–C–N–N dihedral angle ΦS on the excitation energy: optimized structure (PBE0-D3/def2-TZVP) with the corresponding vertical excitation energies (TD-PBE0/def2-SVP) of the photoswitches 1 and 2.
Figure 6
Figure 6
X-ray structures of trans-2 and cis-2.
Figure 7
Figure 7
Structures of di-ortho-fluoro di-ortho-chloro (dfdc, 8), tetra-ortho-fluoro (9) azobenzene, as well as the dfdc derivatives 10 and 11.
Figure 8
Figure 8
Synthesis of the tetra-ortho-hydrids 8 and 10 using a palladium-catalyzed C–H chlorination.
Figure 9
Figure 9
(a) Photoswitching of tetra-ortho-chloro azobenzene (2). UV–vis spectra of 1 and 2: (b) 50 μM in DMSO is used to show the full spectrum and (c) 500 μM in DMSO is used to visualize the extent of the n → π* band tails.
Figure 10
Figure 10
(a) Photoswitching of di-ortho-fluoro di-ortho-chloro azobenzene (8). UV–vis spectra of 8: (b) 50 μM in DMSO is used to show the full spectrum; (c) 500 μM in DMSO and 9:1 DMSO:H2O are used to visualize the extent of the n → π* band tails; and (d) X-ray structure of trans-8x and its calculated vertical excitation energy (TD-PBE0/def2-TZVP).
Figure 11
Figure 11
(a) Photoswitching of the electron-poor di-ortho-fluoro di-ortho-chloro azobenzene 10. UV–vis spectra of 8 and 10: (b) 50 μM in DMSO is used to show the full spectrum; and (c) 500 μM in DMSO is used to visualize the extent of the n → π* band tails.
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