Hydrogen-/halogen-bond switching enables exciton switching for color and singlet-triplet dynamics tuning in pyridine N-oxide cocrystals.

Regulating solid-state emission without altering molecular backbones remains challenging because excited-state pathways are highly packing-dependent. Here we show that supramolecular cocrystallization can reprogram the dominant excitation character of a pyridine N-oxide luminophore (NyPO) by selectively engaging hydrogen bonding versus halogen bonding at the same acceptor site. Under identical 365 nm excitation, the hydrogen-bonded NyPO·H2O crystal exhibits blue emission at approximately 460 nm, whereas the halogen-bonded NyPO·TITFB·DCM cocrystal shows red-shifted green emission at approximately 539 nm, accompanied by accelerated prompt singlet decay (τavg decreased from 8.69 to 1.20 ns) and the emergence of a minor long-lived delayed component (801 μs, 6.87%). Natural transition orbitals and charge density difference analyses suggest an exciton-type switch from a homomolecular intermolecular charge-transfer (homo-CT) excitation to a donor-acceptor intermolecular charge-transfer (CT) excitation from NyPO to TITFB (hetero-CT), with a markedly stabilized low-energy S1 (the calculated S1 energy decreased from 3.296 to 2.494 eV) and reduced oscillator strength. Spin-orbit coupling calculations further suggest intersystem crossing pathway reorganization in the cocrystal. These results demonstrate cocrystallization as a practical strategy to regulate emission color and excited-state dynamics via interaction-programmed exciton switching.