Vibrational-Gated Intersystem Crossing in Phenolphthalein Dianion: A TDDFT Insight into the Fluorescence Quenching Mechanism

The structural rigidity of molecular frameworks plays a pivotal role in modulating intersystem crossings (ISCs). Here, we systematically investigate the distinct fluorescence behaviors of the phenolphthalein dianion (PHP2–) and the fluorescein dianion (FLU2–)─two structurally analogous xanthene derivatives. By employing time-dependent density functional theory (TDDFT), our results reveal that, although both dianions possess nearly degenerate 1ππ* and 3nπ* states with comparable spin–orbital couplings (∼37 cm–1) in the Franck–Condon region, the subsequent relaxation pathways are dictated by distinct vibrational modes. In PHP2–, the twisting of a specific phenol ring serves as the critical vibrational gate that funnels the initially populated 1ππ* state to the 3nπ* state. This is followed by a barrierless internal conversion from 3nπ* to the lowest 3ππ* state, ultimately leading to complete fluorescence quenching. In stark contrast, in FLU2–, the coplanar xanthene core rigidified by an oxygen bridge effectively suppresses this twisting motion. The available torsional modes drive the system toward its 1ππ* minimum, thereby precluding ISC and allowing efficient radiative decay. These findings establish a clear vibrational-mode-dependent ISC mechanism, demonstrating that molecular rigidity does not simply alter energy gaps but rather selectively eliminates the specific nuclear motion required to activate ISC.