Boosted stepwise NO photorelease with overall fluorescent reporting in a molecular dyad for antimicrobial photoinactivation

We have designed and synthesized a molecular dyad in which an N-nitroso-nitroaniline-based nitric oxide (NO) photodonor (NOP), capable of releasing two NO molecules through a stepwise mechanism, is covalently linked to an anthracene (An) unit via a methylene spacer. The dyad exhibits absorption and emission features that are not merely the sum of its individual components, but rather the result of strong electronic interactions in both the ground and excited states. Excitation of the dyad with blue light significantly enhances the sequential NO photogeneration efficiency compared to the isolated NOP. This effect is likely encouraged by photoinduced electron and energy transfer processes, with An acting as the donor and NOP as the acceptor, providing additional activation pathways to the direct NOP excitation. Furthermore, these intramolecular processes cause a dramatic quenching of the typical anthracene fluorescence, which is fully restored only upon the release of the second NO molecule, thereby enabling the system to act as a fluorescent self-reporter for the overall NO release. Antibacterial tests on Gram-positive methicillin-resistant Staphylococcus aureus and Gram-negative Pseudomonas aeruginosa demonstrate that photoexcitation of the dyad induces remarkable antibacterial activity against both planktonic bacterial strains and the eradication of Pseudomonas aeruginosa biofilms due to the photodynamic effect of NO.