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

Abstract

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.