The Effect of Aldehyde and Carboxylic Acid Substitution on the Isomerization of Hemithioindigo Photoswitches

Abstract

Hemithioindigo (HTI) photoswitches exhibit robust photoisomerization under visible light and relatively high thermal bistability. In this work, we report various modifications of the HTI core, namely the introduction of aldehydes and carboxylic acids at the para position of the stilbene fragment with different oxidation states of the sulfur center, and the incorporation of a Schiff base moiety. These modifications allowed tuning of the absorption properties, quantum yields of isomerization, and thermal stability of the metastable E-isomers. Notably, the formyl- and carboxyl-substituted HTI switches achieved high yields of isomerization under visible light in various solvents, while sulfur oxidation enhanced quantum yields but reduced photochromism. Schiff base formation led to red-shifted absorption and increased thermal stability. Finally, by leveraging the carboxyl substituents, we incorporated an HTI chromophore into the NU-1000 metal-organic framework (MOF), and demonstrated solid-state photoisomerization. These findings highlight key structural modifications that expand the applicability of HTI photoswitches for molecular switching in solution and solid-state environments.

Keywords: hemithioindigo; isomerization; metal‐organic frameworks; photochromism; photoswitches.

Figure 1

a) Structure of hemithioindigo (HTI) and its Z/E‐isomerization; b) The first HTI‐based molecular motor and a visible‐light‐responsive tubulin inhibitor; c) Hemithioinidigo photoswitches investigated in this study.

Figure 2

Synthetic procedure for HTI switches 1–7. (i) piperidine (cat.), ethanol or toluene, room temperature or reflux, 3–5 hours; (ii) 0.5 N HCl; (iii) sodium triacetoxyborohydride (STAB, 1.0 eq.), 1,2‐dichloroethane (DCE), room temperature, 2.5 hours; (iv) molecular sieves, ethanol, 40 °C, overnight.

Figure 3

Photochemical properties of HTIs 1, 2, 3, and 5 analyzed by UV‐Vis and ¹H NMR spectroscopy at 293 K. a) E/Z‐1 in MeCN; b) E/Z‐2 in methanol; c) E/Z‐3 in toluene and; d) E/Z‐5 in toluene. The upper panel presents the experimentally determined molar absorption coefficients (ε) of the pure Z‐isomer (red) and the coefficients for the pure E‐isomer (blue) retrieved using the photostationary distribution obtained by ¹H NMR. The lower panel shows the aromatic region of the ¹H NMR spectra before (top) and after (bottom) irradiation.

Figure 4

Isomerization of 7. Bottom left: molar absorption coefficients (ε) of 7 (toluene, 293 K); bottom right: ¹H NMR spectra of 7 before (top) and after irradiation with 455 nm (middle) and after subsequent irradiation with 505 nm (bottom) in toluene‐d₈.

Figure 5

(a) PXRD pattern of the free NU‐1000 (blue) and 2‐MOF (orange). (b) Raman spectra (λ_exc 785 nm) of 2‐MOF before (black), after irradiation with 455 nm (blue), and after subsequent irradiation with 505 nm (green).