Full-cycle photoswitching dynamics in PdII acylthiourea complexes

Abstract The reversible control of molecular structure by light offers a powerful route toward adaptive and out-of-equilibrium chemical systems, yet detailed mechanistic insight across all relevant time scales remains rare. Here, we investigate the complete photoswitching cycle of square-planar Pd II acylthiourea complexes using time-resolved spectroscopic techniques to resolve photodynamics from femtoseconds to hours. Upon excitation of the thermodynamically stable cis isomer, rapid intersystem crossing is followed by structural reorganization into a disphenoidal intermediate that evolves into a semi-dissociated species with transient Pd–O bond cleavage. This intermediate enables ligand flexibility and governs product formation, i.e., allowing access to the metastable trans isomer. The system subsequently undergoes a thermal back-reaction to regenerate the cis ground state, completing a reversible switching cycle on the hour timescale. Both forward and reverse processes are highly sensitive to ligand structure and solvent environment, highlighting the role of coordinating solvents in modulating the pathway. By resolving the full reaction cascade over more than fifteen orders of magnitude in time, this work provides a mechanistic framework for reversible photoswitching in transition metal coordination systems and establishes design principles based on coordination lability and solvent effects.