Tuning the Electronic Excitation of Meta Nitrophenolate Anions to the Biological Transparency Window by Amino Substitution

Nitrophenolate anions are molecules that release nitric oxide (NO) after electronic excitation. This class of molecules is particularly interesting for potential applications in skin cancer treatment, which requires the use of wavelengths in the biological transparency window. Among the various isomers, <i>meta</i> NP^- (<i>m</i>NP^-) has the lowest excitation energy with a band origin at 625 nm. Here, using cryogenic photodissociation spectroscopy, we studied the effect on the electronic spectroscopy of <i>m</i>NP^- of methyl and amino electron-donating groups at the ortho position relative to NO₂. Unlike the bare <i>m</i>NP^- anion, all compounds exhibit broad photodissociation spectra, revealing significant geometric changes between the S₀ and S₁ (ππ_NO2*) states. While methyl substitution causes a blue shift of the absorption band maximum, amino-substituted mNP^- anions show large red shifts, with the absorption band starting below 800 nm for 4NH₂<i>m</i>NP^-, <i>i.e</i>., in the biological transparency window, while still leading to NO photorelease. The spectral shifts induced by the substituents are interpreted by comparison with coupled-cluster calculations performed in the ground state and the first excited state. These electron-donating groups in the ortho position relative to NO₂ perturb the electronic properties of <i>m</i>NP^- through opposing steric and electrostatic effects.