Competition between Photoswitching and Fluorescence in Hemiindigos and Hemithioindigos

Intrinsically fluorescent photoswitches (IFPSs) are crucial for super-resolution microscopy techniques. This study explores the impact of substituents on fluorescence and photoswitching efficiency in hemiindigos (HIs) and hemithioindigos. A series of functionalized derivatives  was synthesized by one-step Knoevenagel condensations using a sustainable mechanochemical approach and characterized across various environments. Ortho-OH-substituted hemiindigo derivatives exhibited high fluorescence quantum yields (Φ_FL = 0.65) while retaining significant photoswitching capabilities (Φ_Z→E = 0.29 in THF) and fast recovery. These enhanced properties are attributed to the π-electron-donating characteristics of the OH substituents and the presence of intramolecular hydrogen bonds. The results of CC2 ab initio calculations revealed a detailed photoswitching mechanism: (1) initial population of the fluorescent ππ* singlet excited state minimum, (2) subsequent elongation of the C═O bond leading to depopulation of the ππ* state to the nπ* excited state minimum, (3) twisting of the double bond within the nπ* state, and (4) relaxation to the ground electronic state via a conical intersection. The analysis of the potential-energy surfaces modulated by EDG substituents, both in the ππ* and nπ* excited states, provides insights into the structure-property relationships of IFPSs, guiding future rational design.