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Towards More Effective, Reversible pH Control by Visible Light Alone: A Thioindigo Photoswitch Undergoing a Strong p K a Modulation by Isomer‐Specific Hydrogen Bonding
Author(s) -
Koeppe Benjamin,
Rühl Steffen,
Römpp Florian
Publication year - 2019
Publication title -
chemphotochem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.13
H-Index - 18
ISSN - 2367-0932
DOI - 10.1002/cptc.201800209
Subject(s) - photoswitch , photostationary state , chemistry , photochromism , hydrogen bond , isomerization , intramolecular force , azobenzene , photochemistry , cis–trans isomerism , isoprene , photodissociation , dissociation (chemistry) , stereochemistry , photoisomerization , organic chemistry , molecule , polymer , copolymer , catalysis
We have synthesized and characterized a thioindigo photoswitch featuring phenolic residues forming an intramolecular hydrogen bond exclusively in the cis isomer. In acetone–water mixtures, wavelength‐dependent photostationary states reached their maximum and minimum values of up to 95 % and down to 11 % cis isomers at circa λ =580 and 480 nm, respectively. Within a few hours, room‐temperature thermal isomerization is negligible in neutral and mildly acidic solutions. Protic media decrease trans → cis but increase cis → trans quantum yields. In CD 3 OD solution, in which light of any wavelength absorbed triggers the latter process almost exclusively, a 19 F NMR titration is used to calculate p K a values of 7.2 ( trans ) and 5.0 ( cis ; both rescaled to water) corresponding to a change in dissociation constant of 2.2 orders of magnitude. Future derivatives featuring solubility and reversible photochromism in water may provide pH modulation of similar amplitude in a range adjustable by the choice of phenolic substituents.