Chalcogen substitution co-tunes photochromism and hydrogen bonding in semicarbazone photoswitches

Replacing even a single atom can profoundly alter the performance of photoswitches. Yet, using this strategy to co-tune light-responsiveness and supramolecular function in photoswitches remains unexplored. We synthesized two series of semicarbazone photoswitches, varying the C[double bond, length as m-dash]X unit (X = O, S, Se) and the substituent on the imine moiety (phenyl vs. methoxy-pyridyl). UV-vis spectroscopy and DFT analysis reveal a red-shift in absorption towards the visible region as the π_HOMO-π_LUMO gap narrows from O to Se. In parallel, heavier chalcogens increase the E → Z photoisomerization quantum yield. Beyond these optical effects, chalcogen substitution reshapes hydrogen-bonding pathways. In the phenyl series, it amplifies supramolecular self-association, yielding more stable π-π stacked, hydrogen-bonded aggregates. In the pyridyl series, it reinforces intramolecular hydrogen bonding, locking the sulfur and selenium analogue in the Z-isomer, whereas the oxygen derivative remains exclusively in the E-form. In mixtures of O-, S-, and Se-derivatives, we achieve wavelength-selective, stepwise deactivation of supramolecular aggregates, switching off the strongest associating species first. Overall, swapping a single chalcogen atom provides control over where these photoswitches absorb, how they isomerize, and how they self-associate. More broadly, this atom-level modification offers a strategy to modify both photophysics and supramolecular organization across carbonyl-containing photoswitch families.