Photocontrolled Isomerization Pathways of Tolylsulfonyl- and Pyrene-Linked Norbornadiene Systems Driven by Absorption...

Norbornadiene (NBD) photoswitches are promising for molecular solar energy and information storage, as they isomerize to quadricyclane (QC) after absorbing irradiation energy. Here, we report pyrene-linked tolylsulfonyl-substituted NBD hybrids that enable wavelength-selective back-isomerization from QC to NBD. Using femtosecond-to-microsecond transient absorption spectroscopy in the deep-UV range, direct observation of QC formation could resolve the temporal windows of competing mechanisms, which confirm that the wavelength-dependent isomerization proceeds through two ultrafast charge-transfer (CT) pathways and longer-lived radical ion pair intermediates. Furthermore, chromophore coupling converts charge recombination from a loss channel into a productive process by enabling triplet energy transfer, leading to comparatively high quantum yields in both the switching directions. In a symmetric bis-NBD architecture, early CT productivity is reduced, and the mechanism shifts toward triplet-dominated isomerization, demonstrating how molecular symmetry redistributes pathway branching. Selective excitation of the QC unit or the pyrene chromophore biases singlet versus triplet channels, providing wavelength-dependent control over reaction efficiency. These results establish pathway engineering across singlet and triplet manifolds as a strategy for designing NBD systems for solar energy storage and optically addressable molecular information technologies.