Competition between Photoswitching and Fluorescence in Hemiindigos and Hemithioindigos

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Abstract
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 (ΦFL = 0.65) while retaining significant photoswitching capabilities (ΦZ→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.
Funding Information
  • H2020 European Research Council (H2020‐MSCA‐ITN‐2017‐765297)
  • Narodowe Centrum Nauki (OPUS 2021/41/B/ST4/01650)
  • Academic Computer Centre Cyfronet, AGH University of Science and Technology (PLG/2025/018173)

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