Reversible Spatial Control in Aqueous Media by Visible Light: A Thioindigo Photoswitch that is Soluble and Operates Efficiently in Water

Abstract

Aiming to extend the scope of indigoid photoswitches to polar protic environments, we have synthesized a sulfonated thioindigo derivative highly soluble in water. Studies by UV/Vis absorption, fluorescence, and NMR spectroscopy indicate that, despite aggregation effects at micromolar concentrations, the novel dye offers satisfactory performance in aqueous solution in the absence of solvation aides. Enrichment of the metastable cis isomer by irradiation may exceed 65 % and its half-life at room temperature may exceed several hours. Degradation after 20 yellow and blue light irradiation cycles within 2 hours is less than 2 %. Performance can be expected to further improve in future molecular designs if the tendency towards self-association is further reduced. Crucially, photoisomerization of indigoids is not necessarily inhibited by water and, thus, the superior spatial control offered by this class of molecular switches may be of great benefit also in biological systems.

Towards More Effective, Reversible pH Control by Visible Light Alone: A Thioindigo Photoswitch Undergoing a Strong pKaModulation by Isomer‐Specific Hydrogen Bonding

“…Setting out to conceive a new type of pH modulator, we envisaged an effective system with at least two units modulation amplitude controlled reversibly by visible light alone and robust in the sense of low fatigue and little interplay between pH and spectral and (photo)chemical parameters of the photoswitch. We based our design ( 1 , Scheme ) on the thioindigo motif, the core fragment of a class of negatively photochromic visible light photoswitches that provide excellent spatial control as their rigid skeletons undergo a near 180° rotation around the central double bond during trans ↔ cis isomerization …”

“…The spectroscopic and photochemical characterization of 1 b was done in acetone solutions containing aqueous citrate buffer (pH 4). Acetone, being an exceptionally good solvent for the compound, prevents possible complications from aggregation . Variation of additive concentrations between 0.5 and 8 % by volume serves to elucidate the effects of protic medium constituents on the behavior of the switch; corresponding absorption spectra are shown in Figure a and b, respectively.…”

“…We based our design (1, Scheme 1) on the thioindigo motif, the core fragment of a class of negatively photochromic [15] visible light photoswitches that provide excellent spatial control as their rigid skeletons undergo a near 1808 rotation around the central double bond during trans$cis isomerization. [16] [a] Dr. Scheme 1. Imidazolone-linked phenolic thioindigo derivative 1. a) trans$cis isomerization and the quasi-symmetric OHO hydrogen bond forming in the cis isomer promoting the expulsion of the second OH proton.…”

“…, spatial control enforces the cis specific formation of an intramolecular OHO hydrogen bond between the phenolic residues tightly tethered to the dye via imidazolone type linkers which yet preclude p conjugation (and hence limit electronic interplay) between chromophore and the protic groups. Exploratory computational geometry optimizations of monoanionic 1 (Scheme 1b) indicate that in the cis isomer the phenolic residues readily adopt an OÀO distance in the order of 2.3 Å , i. e. a typical value for very short and strong hydrogen bonds [17,18] such as in (FHF) À without signs of distortion in the molecular backbone. Thus, the scene is set for an efficient hydrogen bond induced acidity modulation [6,9,12,19] whose operation region can be tailored in a broad range including neutrality [20] by modification of the substituents X.Compound 1 b, in which substituents X, R and Z in the structure of Scheme 1a have been replaced by fluorine, an ether chain imparting hydrophilicity, and bromine (optional late stage substitution), respectively, was synthesized as detailed in the Supporting Information.The spectroscopic and photochemical characterization of 1 b was done in acetone solutions containing aqueous citrate buffer (pH 4).…”

“…Acetone, being an exceptionally good solvent for the compound, prevents possible complications from aggregation. [16] Variation of additive concentrations between 0.5 and 8 % by volume serves to elucidate the effects of protic medium constituents on the behavior of the switch; corresponding absorption spectra are shown in Figure 1a and b, respectively. Solid traces represent a selection of spectra of the extreme cases among the isomer mixtures established under irradiation by a Xenon lamp from which spectral ranges were selected by eleven alternative line filters (470 to 600 nm).…”