Abstract
Donor-acceptor Stenhouse adducts (DASAs) are molecular photoswitches with applications spanning materials to molecular machines. Although structural modularity has enabled seminal DASA studies, sulfonyl substituents in amino DASAs remain unexplored. Evaluation of sulfonyl substituents reveals a distinct photoswitching pathway between acyclic isomers about the C(3)-C(4) bond, as evidenced by UV-vis spectroscopy and low-temperature NMR spectroscopy. This discovery was achieved by significantly improving DASA structural stability under irradiation, which in turn enables the creation of a compound that sets the record for amino DASA isomer distribution at the photostationary state. Thermal reversion is rapid with little to no fatigue in dichloromethane and solvent versatility is demonstrated with photoswitching and full recovery in toluene, albeit with slower thermal recovery rates. Sulfonyl substituents are also found to modulate molar absorption coefficients and structural charge distribution in a manner that broadly follows Hammett substituent constants with modest correlations, thereby providing qualitative guidance for molecular design. These findings enable the greater DASA photoswitch class to selectively access multiple isomers through structural design, offering opportunities for further developments in responsive materials, soft robotics, and selective reactivity.