Nitrated Fluorophore Formation upon Two-Photon Excitation of an Azide with Extended Conjugation

The transformation of an aromatic azide into a highly fluorescent species through a nonlinear optical process was studied. The azide system was designed to undergo N₂ release and nitrene to nitro conversion upon two-photon electronic excitation. The formation of the nitro form of the compound through reactions with O₂ and its high radiative quantum yield implies that the azide can be used as a biphotonic activatable fluorogen. The electronic state in which the azide to nitrene transformation takes place can be accessed nonlinearly with near-infrared light which allows for photoactivation with commonly available lasers. Furthermore, the system was built with a sulfonate functionality which allows for the molecule to be adsorbed at surfaces like that of cadmium sulfide nanocrystals which further improves the nonlinear optical absorption properties in the composite, through an energy transfer mechanism. The yield of the process as a function of the excitation photon energy together with computational studies indicate that the N₂ release in this azide is due to a reactive channel in the second singlet excited state of the molecule. This feature implies that the system is intrinsically photostable for excitation below and above a certain wavelength and that the system can be phototriggered selectively by the nonlinear optical process.