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 and hemithioindigos. A series of functionalized derivatives were 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 (ΦFL = 0.65) while retaining significant photoswitching capabilities (ΦZ→E = 0.29 in THF) and fast recovery. These enhanced properties are attributed to the -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, (2) subsequent elongation of the C=O bond leading to depopulation of the ππ* state to the nπ* excited state, (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 potential-energy surfaces in the excited states provides insights into the structure-property relationships of IFPSs, guiding future rational design.