The photodynamic approach to the molecular-level origin of metal-guided photochromism and ultrafast absorption spectroscopy.

Metal-guided photochromic material (photochromic complex) is one of the latest versions of photo-responsive materials due to their smart behaviour and promising real-world applications. The present work explores the molecular-level origin of metal-guided photochromism using a photodynamic approach and ultrafast absorption spectroscopy, to address all existing lacunas. Here, rhodamine B (RhB) dye containing the Schiff base zinc complex is considered a representative photochromic complex for both theoretical treatment and experimental observations. Detailed theoretical studies, including geometry optimization, frontier molecular orbital (FMO) analysis, transition state (TS) identification, and natural bond orbital (NBO) analysis, along with spectral studies, are employed to investigate the photodynamic equilibrium (enol-form keto-form). This equilibrium is regulated by the interplay of intrinsic factors (push-pull effect) and extrinsic factors (such as UV-light, the phenolic-OH group, metal ions, and solvents). The potential energy surface (PES) of the photo-conversion (enol →enol*→keto*→ meta-stable keto) is evaluated. While, the PES of the reversion (meta-stable keto →enol) is constructed based on the studies of thermo-reversion and photo-reversion. Finally, the theoretical findings related to the photodynamic equilibrium are validated by direct experimental evidence obtained through femtosecond transient absorption (fs-TA) spectroscopy.