Exploring Conformational Regulation of TICT in a D–π–A Fluorophore toward Trace Water Detection

Elucidating excited-state conformational dynamics in D-π-A systems is central to the development of functional molecules. In particular, the emergence of twisted intramolecular charge transfer (TICT) states offers a powerful handle to regulate electronic relaxation pathways through molecular design. Herein, a systematic investigation of two structurally similar D-π-A chromophores (SV27 and SV31) is engineered to either suppress or facilitate excited-state twisting. The control molecule SV27 exhibits limited conformational flexibility and relaxes predominantly through a locally excited state, whereas the strategic incorporation of a sterically demanding donor fragment in SV31 induces substantial excited state torsional reorganization, leading to the stabilization of a TICT state. A combination of steady-state spectroscopy, time-resolved fluorescence measurements, and quantum chemical calculations reveals a strong coupling between molecular architecture, excited-state potential energy surfaces, and charge transfer dynamics. Solvent-dependent studies further demonstrate that higher polarity of the medium selectively stabilizes the twisted charge separated state, which is identified as a crucial element for the accessibility of TICT. Upon leveraging this sensitivity to the local environment, the TICT-active chromophore SV31 is employed as a highly responsive fluorescent probe for trace water detection in organic solvents, achieving trace level sensitivity (30 ppm). Altogether, this study offers a rational framework by correlating molecular architecture with excited-state twisting and charge separation, providing a rationale for activating TICT and laying the foundation for the development of advanced functional molecular architectures for the detection of water.