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

Franco Molina, Michel Broquier, Satchin Soorkia, Gustavo. Pino, Gilles Grégoire

<figure><img src="image_1.png" /></figure>

► To cite this version:

Franco Molina, Michel Broquier, Satchin Soorkia, Gustavo. Pino, Gilles Grégoire. Tuning the Electronic Excitation of Meta Nitrophenolate Anions to the Biological Transparency Window by Amino Substitution. Journal of Physical Chemistry A, 2025, 129 (45), pp.10437-10442. 10.1021/acs.jpca.5c06040 . hal-05486231

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Submitted on 30 Jan 2026

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
Tuning the Electronic Excitation of meta
Nitrophenolate Anions to the Biological
Transparency Window by Amino Substitution

Franco L. Molina,ª,ᵇ,ᶜ,ᵈ Michel Broquier,ᵈ Satchin Soorkia,ᵈ Gustavo. A. Pino,ª,ᵇ,ᶜ* and Gilles Grégoired*

[a] Centro Láser de Ciencias Moleculares, Universidad Nacional de Córdoba. Haya de la Torre y Medina Allende. Pabellón Argentina, Ciudad Universitaria. X5000HUA Córdoba, Argentina.

[b] INFIQC: Instituto de Investigaciones en Físico-Química de Córdoba (CONICET-UNC).
Haya de la Torre y Medina Allende. Pabellón Argentina, Ciudad Universitaria. X5000HUA
Córdoba, Argentina.

[c] Departamento de Fisicoquímica, Fac. de Ciencias Químicas, Universidad Nacional de Córdoba. Haya de la Torre y Medina Allende. Pabellón Argentina, Ciudad Universitaria. X5000HUA Córdoba, Argentina.

[d] Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d'Orsay. F-91405 Orsay, France.

* Corresponding Authors

E-mail: gpino@unc.edu.ar

E-mail: gilles Gregoire@cnrs.fr
ABSTRACT

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, *meta* NP⁻ (*m*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 *m*NP⁻ of methyl and amino electron-donating groups at ortho position relative to NO₂. Unlike the bare *m*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 *m*NP⁻ anions show large red shifts, with the absorption band starting below 800 nm for 4NH₂*m*NP⁻, *i.e.*, 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 ortho position relative to NO₂ perturb the electronic properties of *m*NP⁻ through opposing steric and electrostatic effects.
INTRODUCTION

For therapeutic applications of a nitric oxide (NO) photodonor,¹ an ideal compound requires excitation of the NO photodonor in the biological medium.² A fundamental understanding of optical effects as a function of tissue and penetration depth is necessary, with an emphasis on the absorption process by blood, water, melanin, fat and yellow pigments, as well as the diffusion process in skin and fatty tissues, among others.³ However, there is a range of wavelengths in the near IR in which absorption and diffusion are minimized, the biological transparency window from 650 to 950 nm.⁴

Large nitrobenzene derivatives bearing extended π-electron systems can photorelease NO through photoreaction via laser excitation by absorbing a single UV photon⁵ or two visible photon.⁶ Interestingly, gas phase photodissociation studies on simple nitrophenolate anions corroborate the potential of this important class of molecular anions as novel NO photodonors, as they release NO when excited in the visible. Unlike nitrobenzene species that absorb via an S₀-S₄ transition in the UV,⁷ NP⁻ compounds release NO upon excitation to a low-lying S₁ state, which corresponds to a charge transfer (CT) state with the phenolate oxygen coupled to the aromatic ring as the donor group and the nitro as the acceptor group.⁸,⁹ S. B. Nielsen et al. reported the photodissociation spectroscopy of room temperature NP⁻ isomers. *Ortho* and *para* NP⁻ exhibit maximum absorption at 395 nm while that of the meta isomer is found at 530 nm.

¹⁰ Furthermore, they proposed an approach to adjust the coupling between donor-acceptor groups, using a π-spacer between two fragments.¹⁰,¹¹ The π-spacer was introduced into the pNP⁻ isomer which has the strongest coupling of the donor and acceptor groups leading to high excitation energy. The π-spacer induces the expansion of the delocalization of the electronic density involved in the electronic transition (HOMO-LUMO), which decreases its excitation energy in the visible region. An additional effect was observed in larger pNP⁻ derivatives
containing a π-spacer, which enhanced the CT character of the π–π* transition by decoupling
the donor-acceptor groups due to the increased distance between the two groups.

Among the three nitro phenolate isomers, the *meta*-NP⁻ anion has the lowest excitation energy. A recent cold ion photodissociation experiment has shown that this species has a band origin at 625 nm along with a progression of well-resolved vibronic transitions.¹² Hydration of mNP⁻ slightly shifts the excitation energy by + 0.2 eV compared to the bare anion, which reflects the strong ionic hydrogen bond between the negatively charged oxygen of the phenolate and the water.¹³ It is interesting to note that NO loss was still observed in cold monohydrate mNP⁻ compounds, whereas it was not detected for nitrobenzene in solution.¹⁴

The first excited state of mNP⁻ has a CT (ππ<sub>NO2</sub>*) character since an electron is transferred
from the π orbital of the phenolate ring to the π* orbital of NO₂ (electron withdrawal). This CT
is a typical feature of organic π-systems containing electron-accepting and electron-donating
groups, commonly referred to as push-pull systems. ¹⁵ Several approaches have been proposed
to adjust the photoexcitation properties of push-pull systems. For instance, adding a second
donor or acceptor group modifies the electronic energy of the low-lying electronic
transition.¹⁶,¹⁷

2 and 4 substituted mNP⁻

<figure><img src="image_3.png" /></figure>

Scheme 1. Molecular structures and atom labelling of the 4 substituted (methyl and amino) mNP⁻ in position 2 and 4.
We aim at adjusting the electronic excitation of *m*NP⁻ in the biological transparency window while maintaining the NO loss efficiency. Therefore, by taking advantage of the electronic excitation of *m*NP⁻ in the visible region, it is proposed to add an additional substituent with electron-donating properties in the *ortho* position relative to the NO₂ group, at positions 2 and 4 (see scheme 1 for atom labelling). The substituents considered here are NH₂ and CH₃ groups, whose electron-donating properties are classified as resonant and inductive effects, respectively. The electron-donating effect can be explained as follows: the resonant effect is due to the electrons of the lone pair of the nitrogen atom participating in π conjugation, and the inductive effect is related to the change in electron density through the σ bond. Electron donation by resonance, as observed for NH₂, generally dominates the inductive effect characteristic of CH₃ substitution.

The series of four compounds studied here are plotted in scheme 1 and correspond to the deprotonated forms of 2-methyl- or 4-methyl-3-Nitrophenol and 2-amino- or 4-amino-3-Nitrophenol, here after called 2 or 4CH₃*m*NP⁻ and 2 or 4NH₂*m*NP⁻, respectively. We have studied their electronic properties using cryogenic ion photodissociation spectroscopy and have interpreted the results with the help of *ab initio* coupled-cluster calculations performed at the ground state and the first excited (¹ππ*) state.

## METHODS

The setup has already been described in detail.¹⁸,¹⁹ Substituted *m*NP⁻ anions are produced in an electrospray source (100 µM in methanol solution), first stored in an octopole ion trap for 100 ms and then extracted and accelerated at 160 V by a pulsed exit electrode and finally transferred to the cold 3D-QIT (Jordan Tof Inc.). The latter is biased at 160 V to ensure efficient trapping and to avoid collision-induced-dissociation of the incoming anions with He buffer gas injected by a pulsed valve 1-2 ms before. The 3D-QIT is housed in a copper box directly mounted on a cold head of a compressed helium cryostat (CH-204 S, Sumitomo) that maintains
the temperature around 15 K. A quadrupole mass filter located before the entrance of the Paul trap ensures mass selection of the parent anion. The photodissociation laser is triggered 60 ms after the trapping of the anions when thermalization is achieved and the helium buffer gas is mostly pumped. All ionic fragments and parent molecules are then extracted and accelerated for mass-analysis in a linear time-of-flight mass spectrometer and detected by MCP (dual MCP, Jordan Tof Inc.). All reported photodissociation spectra are obtained after normalizing the NO loss fragmentation signal by the parent signal and laser power.

The visible photodissociation laser is the output of an optical parametric amplifier (EKSPLA-PG411) pumped by the third harmonic (355 nm) of a mode-locked picosecond Nd-YAG laser (EKSPLA-SL300) operated at 10 Hz which provides tunable visible light in the 680-420 nm region and near IR light from 740 nm to 840 nm. The spectral resolution is in the order of 8-10 cm⁻¹. Typical laser pulse energies are in the order of 200 µJ. For wavelengths between 680 nm and 740 nm, we used the output of a dye laser (TDL 90, Quantel) pumped by a second harmonic (532 nm) of a ns Nd-YAG laser (Quantel).

Ab initio calculations have been performed with the TURBOMOLE program package²⁰ (v7.1) making use of the resolution-of-the-identity (RI) approximation for the evaluation of the electron-repulsion integrals.²¹,²² Equilibrium geometries in the electronic ground state (S0) and first excited state (S₁) were evaluated at the CC2 level with the correlation-consistent polarized valence double-ζ aug-cc-pVDZ basis set augmented with diffuse functions.²³ In particular, the spin-component scaled approach (SCS) is applied since it improves the accuracy of CC2 calculations for the charge transfer states.²⁴ The vibrational modes of the ground state and the first excited state have been calculated at the same level in order to obtain the adiabatic excitation energy corrected for the difference in zero-point energy between the two states.
RESULTS

<figure><img src="image_4.png" /><figcaption>**Figure 1** Visible photodissociation spectra (Vis PD) of the four substituted *m*NP⁻ anions. The band origin of *m*NP⁻ (16 006 cm⁻¹) is plotted as a red dotted line.¹³</figcaption></figure>

The cold ion spectroscopy of the four substituted *m*NP⁻ compounds was recorded in the visible range from 12 000 to 23 800 cm⁻¹ and is shown in Figure 1. For all compounds, NO loss is the unique fragmentation channel, as it can be readily seen in Figure S1. Unlike *m*NP⁻, which exhibits a well-resolved vibrational spectrum with a band origin at 16 006 cm⁻¹ (red dotted line in Figure 1) and a vibronic progression extending to 17 500 cm⁻¹,¹²,¹³ the visible photodissociation spectra (Vis PD) of all substituted compounds show broad absorption bands. These bell-shaped photodissociation spectra suggest a significant geometric change between the ground and excited states for these substituted *m*NP⁻. For the two methylated isomers, the band starts around 18 000 cm⁻¹ with a maximum at 21 000 cm⁻¹. The maximum of the absorption band is clearly shifted toward the blue relative to *m*NP⁻. Furthermore, no band origin is
observed. For amino-substituted compounds, as for methyl compounds, the Vis PD spectra are broad, with the onset of the absorption band red-shifted relative to the band origin of *m*NP⁻. For 2NH₂*m*NP⁻, the broad band begins at 14 630 cm⁻¹ and exhibits a vibronic progression of low frequency modes of about 30 cm⁻¹ extending to 14 900 cm⁻¹ (Figure S2). No other vibronic transitions are observed at higher excitation energy, and the maximum of the broad absorption band is at 16 260 cm⁻¹ (615 nm). For 4NH₂*m*NP⁻, the onset of the broad absorption band is shifted even further to the red, below 12 500 cm⁻¹ (800 nm), with a maximum at 16 900 cm⁻¹ (590 nm). It should be noted that the photodissociation signal of 4NH₂*m*NP⁻ increases so smoothly in the low energy edge that the onset of the band could not be determined with accuracy. Finally, the power-dependence of the fragmentation yield of 2NH₂*m*NP⁻ at the band origin reported in Figure S3 shows a linear evolution, suggesting that a single visible photon is required to induce fragmentation.

DISCUSSION

SCS-CC2 calculations were performed on the ground state and first excited state of the four substituted *m*NP⁻ compounds, which are compared with the results of bare *m*NP⁻. We first present the results obtained at the optimized geometry of the ground electronic state for which vertical excitation energies were calculated. The four substituted *m*NP⁻ compounds exhibit broad absorption bands, whose maxima must therefore be compared to the vertical excitation energies calculated for the S₁ state.

The main structural parameters of the two electronic states are presented in Table 1. S₀ geometry optimizations lead to radically different structures for the substituted compounds (Figure 2). In particular, the NO₂ group twists by 41° and 33° for the 2CH₃ and 4CH₃ isomers, respectively, while for the two amino substituted compounds, the system mainly retains the Cs symmetry as for *m*NP⁻ with a slight pyramidalization of the amino group (butterfly mode).
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Optimized geometries in S₀ and S₁ states of the methyl and amino *m*NP⁻ anions. For 4CH₃*m*NP⁻, geometry optimization of the S₁ state led to internal conversion with S₀, at which point the optimization is stopped.</figcaption></figure>

<table><caption>**Table 1.** Structural parameters for the *m*NP⁻ compounds in the S₀ and S₁ states. Distances in Å, dihedral angles in degree.</caption><thead><tr><th></th><th>C-N</th><th>N-O<sub>(8)</sub></th><th>N-O<sub>(9)</sub></th><th>CNO<sub>(8)</sub>O<sub>(9)</sub></th><th>C<sub>(2)</sub>C<sub>(3)</sub>NO<sub>(8)</sub></th><th>C<sub>(2)</sub>C<sub>(3)</sub>NO<sub>(9)</sub></th></tr></thead><tbody><tr><td><strong>mNP<sup>-</sup></strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>S<sub>0</sub></td><td>1.481</td><td>1.257</td><td>1.26</td><td>180</td><td>0</td><td>180</td></tr><tr><td>S<sub>1</sub></td><td>1.402</td><td>1.318</td><td>1.326</td><td>180</td><td>0</td><td>180</td></tr><tr><td><strong>2CH<sub>3</sub> mNP<sup>-</sup></strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>S<sub>0</sub></td><td>1.477</td><td>1.258</td><td>1.260</td><td>178.9</td><td>-41</td><td>140</td></tr><tr><td>S<sub>1</sub></td><td>1.414</td><td>1.329</td><td>1.335</td><td>165.1</td><td>-30.2</td><td>163.9</td></tr><tr><td><strong>4CH<sub>3</sub> mNP<sup>-</sup></strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>S<sub>0</sub></td><td>1.476</td><td>1.258</td><td>1.261</td><td>179.5</td><td>33.1</td><td>-146.4</td></tr><tr><td>S<sub>1</sub></td><td>1.462</td><td>1.374</td><td>1.413</td><td>120.3</td><td>11.5</td><td>-114.6</td></tr><tr><td><strong>2NH<sub>2</sub> mNP<sup>-</sup></strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>S<sub>0</sub></td><td>1.44</td><td>1.274</td><td>1.264</td><td>179.8</td><td>-2.56</td><td>177.7</td></tr><tr><td>S<sub>1</sub></td><td>1.480</td><td>1.362</td><td>1.326</td><td>-138.2</td><td>50.3</td><td>-166.9</td></tr><tr><td><strong>4NH<sub>2</sub> mNP<sup>-</sup></strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>S<sub>0</sub></td><td>1.461</td><td>1.258</td><td>1.272</td><td>179.5</td><td>3.8</td><td>-175.7</td></tr><tr><td>S<sub>1</sub></td><td>1.48</td><td>1.322</td><td>1.360</td><td>141.5</td><td>21.1</td><td>-123.1</td></tr></tbody></table>
For all compounds, the S₁ state has a ππ<sub>NO2</sub>* nature, with the HOMO orbital localized on the aromatic ring and oxygen phenolate, while the electron density of the anti-bonding orbital is centered on the NO₂ fragment (see Figure 3). This electronic configuration reveals the strong CT character of the first excited state in all *mNP⁻* compound, as for bare *mNP⁻*. Because the CH₃ and NH₂ substituents are electron-donating groups by induction and π resonance, respectively, these substituents will disrupt the π-ππ<sub>NO2</sub>* transition, altering the π conjugation and the coupling between the donor, O⁻, and the acceptor, NO₂, groups. Since the amino substituent is a better electron donor than the methyl substituent, we would expect a greater decrease in the excitation energy of the amino substituted *mNP⁻* compounds than for the methyl substituted compounds. It turns out that their Vis PD spectra change with the substituent. Substitution by CH₃ and NH₂ induces a blue shift and a red shift, respectively, of the maximum of the excitation band. As we will explain below, it is not only pure electron-donating effects, but also geometry changes in the S₀ and S₁ states that are responsible for the observed shifts of the band maxima.

<table><caption>**Table 2.** Vertical (E<sub>v</sub>) and adiabatic (E<sub>ad</sub> + ΔZPE) excitation energies for the S<sub>1</sub> state of the *mNP⁻* compounds, energy gap between the S<sub>1</sub> state and S<sub>0</sub> state (S<sub>1</sub>/S<sub>0</sub>) at the S<sub>1</sub> optimized geometry and oscillator strength of S<sub>0</sub>(ππ)→S<sub>1</sub>(ππ<sub>NO2</sub>*). For the methyl compounds, vertical excitation energies with planar NO<sub>2</sub> structure are given in parentheses. All energies in cm<sup>-1</sup>.</caption><thead><tr><th>Molecules</th><th>E<sub>v</sub></th><th>E<sub>ad</sub> + ΔZPE</th><th>S<sub>1</sub>/S<sub>0</sub> energy gap at the S<sub>1</sub> optimized geometry</th><th>Osc. strength</th></tr></thead><tbody><tr><td>mNP</td><td>19 920</td><td>16 202</td><td>13 588</td><td>0.034</td></tr><tr><td>2CH<sub>3</sub>mNP<sup>-</sup></td><td>21 298 (19 968)</td><td>16 545</td><td>13 037</td><td>0.031</td></tr><tr><td>4CH<sub>3</sub>mNP<sup>-</sup></td><td>20 344 (19 480)</td><td>IC *</td><td>IC *</td><td>0.033</td></tr><tr><td>2NH<sub>2</sub>mNP<sup>-</sup></td><td>18 735</td><td>14 850</td><td>4 079</td><td>0.061</td></tr><tr><td>4NH<sub>2</sub>mNP<sup>-</sup></td><td>15 598</td><td>11 057</td><td>2 230</td><td>0.059</td></tr></tbody></table>

<footnote>* not converged due to internal conversion (IC) to S₀</footnote>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Frontier Molecular Orbitals (MOs) of the S<sub>1</sub> (ππ<sub>NO2</sub>*) state at the S<sub>0</sub> optimized structure for the methyl and amino substituted *m*NP<sup>-</sup> compounds.</figcaption></figure>

In order to compare the effect of the electron-donating group on *m*NP<sup>-</sup> without the contribution of structural rearrangements, in particular the rotation of the NO<sub>2</sub> group, we first evaluated the vertical excitation energy of the substituted *m*NP<sup>-</sup> with NO<sub>2</sub> in the plane of the phenolate ring, as found for *m*NP<sup>-</sup>. This structure corresponds to a saddle point of the ground state potential energy surface of the methyl compounds with one imaginary frequency of i62 cm<sup>-1</sup> and i45 cm<sup>-1</sup> for 2 and 4 CH<sub>3</sub>*m*NP<sup>-</sup>. For amino compounds, NO<sub>2</sub> lies in the plane of the aromatic ring within less than 5°, so the optimized S<sub>0</sub> structures are considered planar. For these structure with NO<sub>2</sub> in the plane of the aromatic ring, the vertical excitation energies of the two methyl isomers are similar to that calculated for *m*NP<sup>-</sup>, with 4CH<sub>3</sub>*m*NP<sup>-</sup> slightly lowered by 500 cm<sup>-1</sup>, as shown in Table 2. This indicates that the inductive effect of CH<sub>3</sub> on the π orbital of the phenolate ring is weak. For amino compounds, however, the decrease in vertical excitation energy is greater than 1000 cm<sup>-1</sup> and 4000 cm<sup>-1</sup> in 2NH<sub>2</sub>*m*NP<sup>-</sup> and 4NH<sub>2</sub>*m*NP<sup>-</sup>, respectively.
These shifts can be explained by examining the MOs energies of the S<sub>1</sub> state for the different *m*NP<sup>-</sup> compounds, shown in Figure 4. For planar NO<sub>2</sub> structures, the energy of the antibonding orbital on NO<sub>2</sub> does not change significantly and oscillates around 4.5 eV. However, the energy of the phenolate *π* orbital gradually increases with methyl and amino substitution, which is related to the stronger donor character of the NH<sub>2</sub> group, which increases *π* conjugation.

For relaxed ground state geometries, the rotation of the NO₂ group in methyl isomers induces a large increase in the vertical excitation energy, now calculated at 21 298 cm⁻¹ and 20 344 cm⁻¹ for the 2CH₃ and 4CH₃ isomers, respectively. This value is higher than the vertical excitation of *m*NP⁻. The observed blue shift of the band maxima of the methyl compounds relative to the bare *m*NP⁻ is in fact due to the NO₂ twist and the steric hindrance of the methyl group which destabilizes the π*NO₂* orbital.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Energy (eV) of Molecular Orbitals (MOs) of the S<sub>1</sub> state at the optimized S<sub>0</sub> geometry of *m*NP<sup>-</sup> anions containing methyl and amino groups in position 2 and 4. In black, MOs energies for methyl compounds with a planar NO<sub>2</sub> group.</figcaption></figure>

The geometry optimization of the S<sub>1</sub> state resulted in a significant distortion of NO<sub>2</sub> for all methyl and amino compounds. It should be noted that for 4CH<sub>3</sub>*m*NP<sup>-</sup>, geometry optimization of the S<sub>1</sub> state actually led to internal conversion with S<sub>0</sub>, at which point the optimization is stopped. For all compounds, the two NO bonds were considerably elongated compared to the ground state. This is an effect of the π*NO<sub>2</sub>* orbital involved in the S<sub>1</sub> state, which destabilizes
the NO bond. Then, the CNO(8)O(9) dihedral angle deviates significantly from the initial
structure in S₀ (180 ± 1°) by approximately 40° in both amino compounds and by 15° in the
2CH₃mNP⁻ isomer. For the 4CH₃mNP⁻, the distortion of the NO₂ group is even more
pronounced. In these structures where NO₂ is distorted, the NO bond in anti-position relative to
the substituent is mainly parallel to the plane of the aromatic ring while the other NO bond
pointing toward the substituent is oriented out of the plane. This can be readily verified by
examining the CCNO(8) and CCNO(9) dihedral angles shown in Table 1. Furthermore, NO₂ is
no longer symmetric, the distance of the NO bond pointing outwards from the plane of the
aromatic ring is ~0.04 Å longer than the distance of the NO bond remaining in the plane. Finally,
at the optimized geometry of the S₁ state, the S₁/S₀ energy gap becomes very small for both
amino compounds, less than 0.5 eV (Table 2), suggesting that internal conversion (IC) to the
ground state is likely to occur. Indeed, as already mentioned above for 4CH₃mNP⁻, the geometry
optimization of the S₁ state did not converged due to IC to S₀. The strongly distorted NO₂
geometry in S₁ causes a large destabilization of the ground state energy, ultimately leading to
IC, whereas the excited state energy decreases compared to the vertical excitation energy by an
amount roughly identical to that of mNP⁻. The S₁ geometry optimization of the 2CH₃mNP⁻
isomer leads to the same general trend as for the other isomers but with less pronounced effects.
The nitro group is slightly distorted with the CNO(8)O(9) dihedral angle at 165°, and it returns
back to the phenolate plane with a CCNO(8) dihedral angle of 30° instead of 41° in the ground
state. The two NO bonds are similar, with an average interatomic distance of 1.332 ± 0.003 Å,
as calculated for mNP⁻ (1.332 ± 0.004 Å). The adiabatic excitation energy, corrected by the
difference in zero-point energy (ΔZPE) between the ground and excited states, is predicted at
16 545 cm⁻¹, i.e., +300 cm⁻¹ relative to the band origin of mNP⁻. At the optimized geometry of
the S₁ state, the S₁/S₀ energy gap remains quite large, around 13 000 cm⁻¹, as for mNP⁻.
Finally, it is worth noting that all these nitrophenolate species underwent the NO loss in the form of a single photoproduct, even for amino compounds whose excitation energy is shifted to the near IR. For 4NH₂mNP⁻, we cannot rule out the possibility that the low fragmentation signal around 800 nm is not limited by a kinetic shift occurring close to the dissociation energy threshold. This raises the question of the possible mechanisms responsible for the NO release with such low excess energy²⁵ if dissociation occurs in the ground state by a roaming mechanism, and/or by access to the triplet state manifold as suggested for the photorelease of nitric and nitro fragments in neutral nitrobenzene.²⁶⁻²⁸ Such studies would require a pump-probe spectroscopy scheme,²⁹ ultimately revealing long-lived excited states potentially involved in the deactivation of the locally excited ππₙO₂* state, as we have already shown in protonated amino pyridine³⁰,³¹ and DNA bases.³²,³³ However, for anionic species, the absorption of a probe photon would likely exceed the electron photodetachment threshold,³⁴ so that photodetachment would compete with photofragmentation. We are currently setting up a velocity map imaging device to detect and analyze the kinetic energy of the photoelectrons and study the excited state dynamics of the ππₙO₂* state of nitrophenolate compounds.³⁵

## CONCLUSION

This study demonstrates that the electronic excitation of mNP⁻ anions can be effectively tuned through strategic substitution with electron-donating groups, particularly NH₂ at position 2 and 4 of the aromatic ring. CC2 calculations corroborate the experimental observations, attributing the spectral shifts to changes in π-conjugation and donor-acceptor coupling. Furthermore, significant structural reorganization between the ground and excited states, particularly the torsion of the nitro group, explains the spectral broadening and the absence of vibronic transitions. A rational explanation has been presented for the introduction of NH₂ and CH₃ as substituents, in which their electron-donating effect modulates the electronic transition by altering the π-πₙO₂* energy gap. In the case of the NH₂ group, the energy of the π orbital of
the phenolate increases significantly, resulting in a red shift of the absorption band into the
biological transparency window (below 800 nm). Conversely, the CH₃ group in ortho position
relative to the nitro group induces its torsion due to steric effect and a blue shift of the excitation
energy. In summary, the substitution of *m*NP⁻ by NH₂ or CH₃ groups at strategic positions is
an effective means of tuning its absorption properties while promoting efficient NO release.
These gas phase results provide fundamental knowledge for understanding the molecular
mechanism of NO phot release without interference from environmental effects, which could
eventually prove important for the design of photoactive NO donors for potential biological and
phototherapeutic applications.

ACKNOWLEDGMENT

This work was conducted within the International Associated Laboratory LEMIR
(CNRS/CONICET) and was supported in part by CNRS, CONICET, FONCyT, SeCyT-UNC,
the ‘‘ADI 2019’’ project funded by the IDEX Paris-Saclay, ANR-11- IDEX-0003-02 and
ECOS Sud - MinCyT (A22U01) cooperation project. The computations were performed at the
HPC resources from the computing center of Centrale Supelec and Ecole Normale Supérieure
Paris-Saclay supported by CNRS and Région Ile-de-France (https://mesocentre.centrale
supelec.fr/) and the HPC resources MAGI from University Sorbonne Paris Nord.

ASSOCIATED CONTENT

Photodissociation mass spectra of the four substituted *m*NP<sup>-</sup> anions, the band origin of Vis
PD spectrum of 2NH<sub>2</sub>*m*NP<sup>-</sup> exhibiting a vibronic progression of 30 cm<sup>-1</sup> and the power-
dependence of the fragmentation yield of 2NH<sub>2</sub>*m*NP<sup>-</sup> The Supporting Information is available
free of charge at https://pubs.acs.org/

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