Elucidating the reaction kernel and probing the effect of anharmonicity in the ring-closing reaction of fulgide single crystals

Chemistry involves dynamics that transform chemical structures from one form to another. However, among the vast milieu of quantum vibrations in a molecule, it boils down to a few key motions that drive the system across the transition state. It is the anharmonicity at the transition state or barrier-crossing region that couples normal modes, leading to localized motions and reduced dimensionality. The interplay of strongly anharmonic local modes collectively drives the system across the barrier-crossing region, forming a photoproduct. Ultrafast broadband transient absorption spectroscopy has revealed the effect of reduced dimensionality in a prototypical ring-closing reaction in fulgide single crystals. The relatively large anharmonicity at the reactive crossing and the strong reaction forces experienced during the chemical transformation provide a significant driving force for the vibrational modes, revealing a new mechanism of coherent vibrational energy transfer between molecular modes. This effect is observed as a non-impulsive growth of modulation in the amplitude of an 80 cm^-1 mode coupled to the reaction coordinate. Our study sheds light on the lattice-coupled reaction dynamics owing to specific system-bath interactions and provides new insight into utilizing lattice alignment for chemical transformation in a solid-state crystalline environment.