# Unsymmetrical Anthracene Platforms as Singlet Oxygen Batteries: Effects of Substituents on Photooxygenation and Endoperoxide Thermolysis

Paul de Bonfils, Pierrick Nun, Vincent Coeffard

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Paul de Bonfils, Pierrick Nun, Vincent Coeffard. Unsymmetrical Anthracene Platforms as Singlet Oxygen Batteries: Effects of Substituents on Photooxygenation and Endoperoxide Thermolysis. European Journal of Organic Chemistry, 2024, 27 (14), 10.1002/ejoc.202400099. hal-04730403

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# Unsymmetrical Anthracene Platforms as Singlet Oxygen Batteries: Effects of Substituents on Photooxygenation and Endoperoxide Thermolysis

Paul De Bonfils,*[a] Pierrick Nun,[a] and Vincent Coeffard*[a]

Aromatic architectures able to capture, store and release singlet oxygen have sparked the interest of the scientific community as an attractive strategy to generate singlet oxygen. Among these structures, anthracenes are efficient singlet oxygen batteries with applications in various fields such as chemistry, materials science or medicine. Until now, studies have mainly focused on symmetrical 9,10-disubstituted anthracenes bearing (hetero)aryl

groups or substituted alkyne motifs. In this work, we investigated unsymmetrical anthracene molecules to explore their propensity to trap and to release singlet oxygen. Kinetic studies confirmed the impact of electron-rich and electron-deficient substituents on the photooxygenation step and on the release of singlet oxygen triggered by thermolysis of the endoperoxides.

## Introduction

Singlet oxygen ($^1O_2$), the lowest metastable excited state of molecular oxygen, is a highly reactive oxygen species employed in various fields, ranging from synthetic chemistry to biological and materials science.[1] Among the different strategies to produce singlet oxygen, the use of aromatic endoperoxides (EPOs) has attracted great attention in the scientific community. The endoperoxides are produced by reaction with singlet oxygen, and this process is generally reversible.[2] Depending on the structure, the release of singlet oxygen can be triggered by thermolysis,[3] chemical action[4] or other stimuli.[5] Since the report in 1967 by Wasserman and Scheffer of the production of singlet oxygen through the thermolysis of a 9,10-diphenylanthracene EPO,[6] a wide range of acene-based EPOs have been investigated for singlet oxygen release such as naphthalenes,[4c,e,7] anthracenes[5c,7f,8] or other polyacene derivatives.[4f,9] In particular, substituted anthracenes serve as efficient singlet oxygen batteries and have found valuable applications in materials science,[10] switchable organic cages[11] and medical fields.[12] Several studies have been conducted to explore the effects of substituents on the photooxygenation of anthracenes and thermolysis of the corresponding endoperoxides (Scheme 1). In 2008, Fudickar and Linker investigated the reactivity of 9,10-diarylanthracenes with singlet oxygen and the

<figure><img src="image_3.png" /><figcaption>Scheme 1. Anthracene structures as singlet oxygen batteries.</figcaption></figure>

reverse reaction. The presence of electron-donating groups in *para* substituted 9,10-diarylanthracenes increases the photo-oxygenation rate while *ortho* substituted anthracenes exhibit a lower reactivity towards singlet oxygen due to steric interactions.[8j] Interestingly, the regeneration of singlet oxygen is substantially accelerated by steric hindrance and the cyclo-reversion of *ortho* substituted phenyl anthracenes is faster than 9,10-diphenylanthracene. Replacing the aryl substituents by arylalkynyl groups strongly impacts the endoperoxide stabilities.[8g,h] For endoperoxides derived from arylalkynyl anthracenes, the cycloversion proceeds at lower temperature

<footnote>[a] Dr. P. De Bonfils, Dr. P. Nun, Dr. V. Coeffard
Nantes Université
CNRS, CEISAM, UMR 6230
44000, Nantes, France
E-mail: paul.de-bonfils@univ-nantes.fr
vincent.coeffard@univ-nantes.fr</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202400099</footnote>

<footnote>© 2024 The Authors. European Journal of Organic Chemistry published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_4.png" /></figure>

(25°C) than for aryl anthracene EPOs (90°C).[13] While many studies have been reported on symmetrical anthracenes as singlet oxygen batteries, a very limited number of investigations has been carried out on unsymmetrical 9,10-disubstituted anthracenes.[8g,12c, 14] In 2012, Fudickar and Linker studied the ability of an unsymmetrical anthracene bearing a phenyl and a phenylethynyl substituent to trap singlet oxygen through an endoperoxide structure, and to further decompose by cycloreversion.[8g] The kinetic data of thermolyses revealed a stability comprised between 9,10-di(phenylethynyl)anthracene EPO and 9,10-diphenylanthracene EPO. A similar observation was made later by the group of Li who successfully applied an anthracene bearing a tolyl and a phenylethynyl substituent as a singlet oxygen reservoir for antitumor phototherapy.[14] However, these studies remain limited to a single derivative in each report, and the impact of the substituents on the photo-oxygenation and the cycloreversion has never been explored

with unsymmetrical 9,10-disubstituted anthracenes. As part of our ongoing research into the development of singlet oxygen batteries,[15] we detail the synthesis and the properties towards singlet oxygen of a selection of 9,10-disubstituted anthracenes bearing (hetero)arylalkyne and (hetero)aromatic substituents with different electronic properties (Figure 1). Notably, the photooxygenation generates relevant aromatic endoperoxides for applications as singlet oxygen sources.

## Results and Discussion

### Synthesis

The anthracene derivatives **1–7** were prepared following the synthetic routes shown in Scheme 2. From the commercially available bromo anthracene **8a**, compounds **1, 5** and **9** were obtained in good yields (69–89%) by Sonogashira cross-coupling reactions. Switching from triethylamine to diisopropyl-amine (DIPA) increased the reaction conversion from 63% to 91% for **5** with an isolated yield of 89%. The unsymmetrical anthracenes **6** and **7** were prepared *via* 9-ethynyl-10-phenyl-anthracene **10**. The protected alkyne **9** underwent a deprotection step under basic conditions with sodium hydride leading to **10** in 99% yield. The intermediate **10** can be cross-coupled with 4-bromobenzonitrile or 2-bromopyridine to provide **6** and **7** with high yields of 93% and 89%, respectively. The pyridine derivative **8b** is also amenable to Sonogashira cross-coupling reaction affording **4** in 75% yield. Another synthetic route was employed for the preparation of **2** and **3**. The readily available compound **11** was efficiently converted to the alkyne derivative **12** *via* a double elimination reaction in the

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Unsymmetrical anthracenes **1–7** investigated in this work.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Scheme 2.** Synthetic routes toward anthracene derivatives **1–7**. Reaction conditions: a) alkyne (1.2 equiv.), Cul (8 mol%), Pd(PPh$_3$)$_2$Cl$_2$ (5 mol%), THF/Et$_3$N, reflux, Ar, 16–48 h. For **5**, THF/DIPA was used. b) NaNH (1.5 equiv.), THF, reflux, Ar, 20 h. c) benzyl phenyl sulfone (1.2 equiv.), DECP (1.2 equiv.), LiHMDS (5 equiv.), THF, r.t., Ar, 36 h. d) Aryl boronic acid (1.2 equiv.), Na$_2$CO$_3$ (5 equiv.), Pd(PPh$_3$)$_4$ (5 mol%), PhCH$_3$/H$_2$O/tBuOH, reflux, Ar, 24–48 h.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

<table><caption>**Table 1.** Kinetic data of photooxygenation and HOMO energy level of anthracene derivatives 1–7.</caption><thead><tr><th>Entry</th><th>Anthracenes</th><th>R<sup>1</sup></th><th>R<sup>2</sup></th><th>k<sub>obs</sub> (min<sup>-1</sup>)×10<sup>-2</sup>(a)</th><th>HOMO (eV)<sup>(b)</sup></th></tr></thead><tbody><tr><td>1</td><td>1</td><td>C<sub>6</sub>H<sub>5</sub></td><td>C<sub>6</sub>H<sub>5</sub></td><td>1.59±0.13</td><td>-7.12</td></tr><tr><td>2</td><td>2</td><td>C<sub>6</sub>H<sub>5</sub></td><td>4-MeOC<sub>6</sub>H<sub>4</sub></td><td>1.75±0.15</td><td>-7.08</td></tr><tr><td>3</td><td>3</td><td>C<sub>6</sub>H<sub>5</sub></td><td>4-NCC<sub>6</sub>H<sub>4</sub></td><td>0.70±0.07</td><td>-7.26</td></tr><tr><td>4</td><td>4</td><td>C<sub>6</sub>H<sub>5</sub></td><td>o-pyridyl</td><td>1.40±0.26</td><td>-7.11</td></tr><tr><td>5</td><td>5</td><td>4-MeOC<sub>6</sub>H<sub>4</sub></td><td>C<sub>6</sub>H<sub>5</sub></td><td>1.84±0.09</td><td>-6.98</td></tr><tr><td>6</td><td>6</td><td>4-NCC<sub>6</sub>H<sub>4</sub></td><td>C<sub>6</sub>H<sub>5</sub></td><td>0.65±0.15</td><td>-7.28</td></tr><tr><td>7</td><td>7</td><td>o-pyridyl</td><td>C<sub>6</sub>H<sub>5</sub></td><td>0.93±0.02</td><td>-7.17</td></tr><tr><td>8</td><td>9,10-diphenylanthracene</td><td>-</td><td>-</td><td>7.57±0.02</td><td>nd<sup>(c)</sup></td></tr></tbody></table>

[a] For experimental procedure: 1–7 (7.5×10⁻⁵ mol·L⁻¹), I (1.75×10⁻⁶ mol·L⁻¹), PhCH₃, green LED (525 nm, 10 mW·cm⁻²). The kinetics were performed three times for each anthracene. Observed rate constants (k<sub>obs</sub>) were determined by UV-visible spectroscopy (See Supporting Information for further details). [b] HOMO levels calculated at the PCM(Toluene)-RωB97X–D/aug-cc-pVTZ//PCM(Toluene)-RωB97X–D/aug-cc-pVDZ level. [c] not determined.

presence of benzyl phenyl sulfone, diethyl chlorophosphate (DECP) and LiHMDS.[16] Starting from 12, the compounds 2 and 3 were then obtained in good-to-excellent yields by a Suzuki-Miyaura cross-coupling reaction.

## Photooxygenation

Our studies began through investigation of the reactivity of anthracenes 1–7 towards singlet oxygen under photooxygenation conditions. These experiments were performed under green light irradiation (525 nm, 10 mW·cm⁻²) in the presence of a catalytic amount of the photosensitizer diiodo-Bodipy (I) which exhibits a high singlet oxygen quantum yield in organic solvents (Table 1).[17] The reactions were carried out at 23 °C in toluene to ensure solubilization of anthracene compounds 1–7 and to enable high temperatures to be reached for thermolysis. The kinetic studies were carried out by UV-visible spectroscopy by monitoring the disappearance of absorption bands of anthracenes 1–7 between 360–460 nm (Figure 2). The observed rate constants (k<sub>obs</sub>) of the photooxygenation of anthracenes 1–7 were determined spectroscopically, and a moderate substituent effect was observed for the endoperoxide formation (Table 1). The lowest rate constants were observed for 3 and 6 (3, k<sub>obs</sub> = 0.70 × 10⁻² min⁻¹ and 6, k<sub>obs</sub> = 0.65 × 10⁻² min⁻¹) bearing a cyano group, and the highest rate constants were found for 2 and 5 (2, k<sub>obs</sub> = 1.75×10⁻²min⁻¹ and 5, k<sub>obs</sub> = 1.84×10⁻²min⁻¹) bearing a methoxy group (entries 2–3 and 5–6). Therefore, an increase of the photooxygenation rate was observed when replacing an electron-withdrawing group by an

<figure><img src="image_8.png" /><figcaption>**Figure 2.** Absorbance spectra in toluene (c = 7×10⁻⁵ mol·L⁻¹) of the photo-oxygenation of 6 to form 6-O₂ under green LED irradiation (525 nm) in toluene showing the disappearance of absorption bands between 360–460 nm.</figcaption></figure>

electron-donating group and this result is in agreement with other studies about the photooxygenation of 9,10-disubstituted anthracenes.[8g] The impact was also important when the phenyl substituent of the alkyne was replaced by a pyridyl heterocycle (entries 1 and 7). The photooxygenation rates of anthracenes 1–7 are lower than that of 9,10-diphenylanthracene which is consistent with data reported in the literature (entry 8).[8g]

HOMO calculations were calculated by density functional theory using the Self Consistent Reaction Field (SCRF) approach with the Polarizable Continuum Model (PCM) method using toluene as a solvent. As expected, the methoxy group raised
<figure><img src="image_9.png" /></figure>

the HOMO energy while lower energy levels were calculated for anthracenes bearing cyano substituents. A good correlation was noticed between the HOMO levels and the observed rate constants (Figure 3).

Based on experimental and theoretical data, different mechanisms have been suggested for the endoperoxide formation depending on the anthracene structure and the reaction conditions (gas phase versus solvent).[9a] For instance, a stepwise addition involving zwitterionic intermediates has been suggested for the reaction of singlet oxygen with 9,10-diaryl-anthracene while a radical pathway was ruled out based on control experiments.[8] A concerted cycloaddition was proposed for the reaction of 9,10-diarylalkynylanthracenes with singlet oxygen and therefore, switching from aryl to alkynyl substituents would change the reaction mechanism.[8g] The reaction of singlet oxygen with 9,10-diarylanthracenes was investigated by Density Functional Theory (DFT) calculations by Fudickar and Linker in gas phase and in solvents.[18] Interestingly, the authors concluded that taking the solvent into account changed the mechanism and therefore, the reaction of singlet oxygen with 9,10-diarylanthracenes would proceed through a concerted transition state when solvents are considered. As there is not a single mechanism, DFT calculations were carried out to get more information about the reaction mechanism of 1–7 with singlet oxygen (Figure 4). Calculations involving singlet oxygen are challenging owing to its multireference character. Following the method reported by the group of Mikkelsen,[19] theoretical investigations were performed by running calculations on the triplet state of molecular oxygen, and then adding the experimentally energy difference (22.5 kcal.mol⁻¹) between triplet O₂ (²Δg) and singlet O₂ (¹Δg) to obtain the computed energies of the starting materials. Equilibrium structures and transition state geometries were optimized at PCM(Toluene)-RwB97X–D/aug-cc-pVDZ and energies were refined by means of single point calculations using aug-cc-pVTZ for all atoms. This level is designated PCM(Toluene)-RωB97X–D/aug-cc-pVTZ//PCM(Toluene)-RωB97X–D/aug-cc-pVDZ (See Supporting Information for computational details). The reaction of 9-phenyl-10-(phenylethynyl)anthracene 1 with singlet oxygen was chosen as a model reaction. The concerted transition state was located at 9.8 kcal.mol⁻¹ above the starting materials whereas the transition states and intermediates for the stepwise

mechanism were significantly higher in energy. For the stepwise pathway, the C–O bond formation occurred at the C-9 position (TS1) leading to an intermediate (I1) which subsequently produced the endoperoxide 1-O₂ via TS2. It is worthwhile noting that no transition state for the stepwise reaction of singlet oxygen at the C-10 position of anthracene 1 was located. Therefore, the reaction of singlet oxygen with anthracenes 1–7 would proceed through a concerted mechanism, and the enthalpies of activation were then determined for all substrates. The results showed that anthracenes 2, 4 and 5 would react faster than 9-phenyl-10-(phenylethynyl)anthracene 1 with singlet oxygen.

Furthermore, the anthracenes bearing electron-withdrawing groups such as 3 and 6, and the *ortho*-pyridyl derivative 7 would be less reactive towards singlet oxygen. These trends follow the reactivity order observed for the photooxygenation of anthracenes (Table 1 and Figure 3).

<figure><img src="image_10.png" /><figcaption>**Figure 4.** Reaction energy profiles (concerted versus stepwise) for the formation of 1-O₂ at PCM(Toluene)-RωB97X–D/aug-cc-pVTZ//PCM(Toluene)-RωB97X–D/aug-cc-pVDZ. Enthalpy of activation for the reaction of anthracenes 1–7 with ¹O₂ through a concerted reaction.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 3.** Correlation of calculated HOMO energy levels with the photo-oxygenation observed rate constants ($k_{obs}$).</figcaption></figure>
Thermolysis

To investigate the cycloreversion, pure endoperoxides [1-7]-O₂ were synthesized under the conditions described in Scheme 3. The polar photosensitizer Rose Bengal (II) was used instead of diiodo-Bodipy (I) to avoid the co-elution of the desired EPO with I during purification by column chromatography. The endoperoxides were isolated as solids with yields ranging from 83% to 92% after a short column on neutral alumina gel to avoid their decomposition. The structures were confirmed by ¹H and ¹³C NMR as well as mass spectrometry, and the endoperoxides can be stored at -20 °C for several weeks depending on the substitution pattern.

The thermolysis of endoperoxides [1-7]-O₂ was carried out at different temperatures (50 °C, 70 °C and 90 °C) in toluene under dark conditions (Table 2 and Figure 5a). The reappearance of the parent anthracene was monitored by UV-visible spectroscopy upon heating. These data gave access to half-lives

of endoperoxides (t₁/₂) calculated from the cycloreversion rate (k, s⁻¹) through the equation t₁/₂ = ln 2/k (Figure 5b). The kinetic data obtained at different temperatures were used to calculate the enthalpies (ΔH⁺) according to the Eyring-Polanyi equation (See Supporting Information and Figure 5c). Before starting thermolysis kinetic studies, the formation of anthracenes 1-7 from the corresponding endoperoxides was confirmed by ¹H NMR spectroscopy at the highest temperature investigated, i.e. 90 °C (Figure 5a). Thermolyses of endoperoxides [1-7]-O₂ at 90 °C for 30 min afforded quantitatively parent anthracenes 1-7 without any formation of side products. Half-lives of endoperoxides [1-3]-O₂ and [5-6]-O₂ are only slightly affected by the electronic properties of the substituents even with an electron-donating group (i.e. methoxy) or an electron-withdrawing group (i.e. cyano). For instance, endoperoxides 5-O₂ and 6-O₂ with electron-rich or electron-poor substituents on the phenyl ring borne by the alkyne decomposed only slightly faster than 2-O₂ and 3-O₂, respectively. Surprisingly, we observed a higher stability of 4-O₂ compared to other endoperoxides, which is in accordance with the higher ΔH⁺ required for the decomposition of this endoperoxide (Figure 4c). 2-(10-(phenylethynyl)anthracen-9-yl)pyridine EPO 4-O₂ underwent cycloreversion with a rate constant an order of magnitude smaller than 1-O₂ at 50 °C. Therefore, replacing a pyridyl group (4-O₂) by a phenyl group (1-O₂) destabilizes the endoperoxide. The decrease in endoperoxide lifetime could be explained by an increase in steric hindrance due to the substitution of a nitrogen atom with a C-H bond, which would weaken the C-O bond.[8e] While half-lives of endoperoxides [1-3]-O₂ and [5-7]-O₂ range from 0.9 h to 2.6 h at 50 °C, the value reaches 13.8 h for 4-O₂. These results demonstrate that mixed aryl- and (hetero)arylethynyl-substituted anthracene EPOs have stabilities between 9,10-diarylanthracene EPOs (slow thermolysis rate) and 9,10-diarylethynylanthracene EPOs (fast thermolysis rate).[8e,g,h,j,13]

<figure><img src="image_12.png" /><figcaption>Scheme 3. Synthesis of endoperoxides [1-7]-O₂.</figcaption></figure>

<table><caption>Table 2. Thermolysis of endoperoxides [1-7]-O₂.[a]</caption><thead><tr><th rowspan="2">EPOs</th><th>50 °C</th><th rowspan="2">t<sub>1/2</sub> (h)</th><th>70 °C</th><th rowspan="2">t<sub>1/2</sub> (min)</th><th>90 °C</th><th rowspan="2">t<sub>1/2</sub> (min)</th></tr><tr><th>k (s<sup>-1</sup>)</th><th>k (s<sup>-1</sup>)</th><th>k (s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>1-O<sub>2</sub></td><td>1.23×10<sup>-4</sup></td><td>1.6</td><td>1.15×10<sup>-3</sup></td><td>10.0</td><td>8.70×10<sup>-3</sup></td><td>1.3</td></tr><tr><td>2-O<sub>2</sub></td><td>7.50×10<sup>-5</sup></td><td>2.6</td><td>9.91×10<sup>-4</sup></td><td>11.7</td><td>1.14×10<sup>-2</sup></td><td>1.0</td></tr><tr><td>3-O<sub>2</sub></td><td>8.10×10<sup>-5</sup></td><td>2.4</td><td>9.45×10<sup>-4</sup></td><td>12.2</td><td>1.11×10<sup>-2</sup></td><td>1.0</td></tr><tr><td>4-O<sub>2</sub></td><td>1.40×10<sup>-5</sup></td><td>13.8</td><td>2.25×10<sup>-4</sup></td><td>51.3</td><td>2.90×10<sup>-3</sup></td><td>4.0</td></tr><tr><td>5-O<sub>2</sub></td><td>1.52×10<sup>-4</sup></td><td>1.3</td><td>1.47×10<sup>-3</sup></td><td>7.9</td><td>9.36×10<sup>-3</sup></td><td>1.2</td></tr><tr><td>6-O<sub>2</sub></td><td>2.03×10<sup>-4</sup></td><td>0.9</td><td>1.46×10<sup>-3</sup></td><td>7.9</td><td>1.11×10<sup>-2</sup></td><td>1.0</td></tr><tr><td>7-O<sub>2</sub></td><td>1.31×10<sup>-4</sup></td><td>1.5</td><td>1.27×10<sup>-3</sup></td><td>9.1</td><td>9.09×10<sup>-3</sup></td><td>1.3</td></tr></tbody></table>

[a] For experimental procedure: [1-7]-O₂ (7.5×10⁻⁵ mol.L⁻¹), PhCH₃, Δ (50 °C, 70 °C and 90 °C), in the dark. Cycloreversion rate constants (k) were determined by UV-visible spectroscopy (See Supporting Information for further details).
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 5.** a) Thermal cycloreversion of 6-O₂ monitored by ¹H NMR at 90 °C in deuterated toluene in the dark showing the clean reconversion of 6 after 30 minutes (see Supporting Information for all other compounds). b) Plot of ln((A<sub>end</sub>-A)/[(A<sub>end</sub>-A₀)] versus time (sec) for the thermolysis of [1-7]-O₂ at 90 °C in toluene in the dark. The experiments were followed by UV-vis spectroscopy. c) Correlation between half-lives of endoperoxides at 50 °C (t₁/₂ / hours) and activation enthalpies (ΔH° / kcal.mol⁻¹) for [1-7]-O₂.</figcaption></figure>

The clean thermolysis experiments along with relevant time
scale and temperature prompted us to consider the endoper-
oxides [1-7]-O₂ as singlet oxygen sources. Dark photooxygena-
tion of 2,3-dimethylbut-2-ene 13 was chosen to trap the
released singlet oxygen (Scheme 4). For this purpose, EPO 6-O₂
was selected due its ease to release singlet oxygen at 50°C

(Figure 4c). Thermolysis of 6-O₂ in deuterated toluene at 50°C in the dark afforded the hydroperoxide 14 in 47%, which demonstrated a moderate generation of singlet oxygen. However, the parent anthracene was fully recovered, reflecting a great capacity for possible trapping and release cycles.

Conclusions

In summary, unsymmetrical 9,10-disubstituted anthracene derivatives were prepared for applications as singlet oxygen batteries. These molecules were mainly prepared by cross-coupling reactions to install (hetero)aryl and (hetero)arylalkynyl substituents on the anthracene framework. The ability of these compounds to capture singlet oxygen was evaluated and kinetic studies were realized using UV-vis spectroscopy. For this purpose, photooxygenation of anthracene derivatives were performed with diiodo-Bodipy as a photosensitizer under green LED irradiation. In agreement with previous works, the presence of electron-rich substituents increased the reactivity of the anthracene framework towards singlet oxygen, whereas electron-poor groups induced the opposite effect. Despite the relative unstability of the endoperoxide, we were able to isolate and characterize them. Endoperoxide thermolysis was also monitored by UV-vis spectroscopy at different temperatures in toluene in the dark. Half-lives of the endoperoxides spans from

<figure><img src="image_15.png" /><figcaption>**Scheme 4.** Singlet oxygen-mediated formation of hydroperoxide 14. NMR yields were determined using methyl phenyl sulfone as an internal standard.</figcaption></figure>
<figure><img src="image_16.png" /></figure>

0.9 h to 13.6 hours at 50°C in toluene making these species relevant singlet oxygen carriers. The release of singlet oxygen was confirmed by thermolysis in the presence of 2,3-dimethylbut-2-ene at 50°C in deuterated toluene to form the corresponding hydroperoxide, which validated these compounds as singlet oxygen batteries. Aspects regarding the modification of these structures for applications in materials science or biological systems would be reported in due course.

# Experimental Section

## General Remarks

¹H and ¹³C NMR spectra were recorded on a Pulse 300 MHz BRUKER (300 MHz for ¹H and 75 MHz for ¹³C) or a BRUKER Avance III 400 MHz (400 MHz for ¹H and 100 MHz for ¹³C) at room temperature on samples dissolved in CDCl₃ or C₆D₅CD₃ and referenced to residual solvent peaks. Chemical shifts (δ) are given in parts per million and coupling constants are given as absolute values expressed in Hertz. Multiplicities are indicated by s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet) and app (apparent). UV-Visible spectra were recorded on an Analytik Jena specord 210. High-resolution mass spectrometry (HRMS) analyses were recorded on a XEVOG2 XSQTOF (Waters). Infrared spectra were recorded with a Bruker Tensor 27 FT ATR spectrometer. The photooxygenation were performed with the HepatoChem EvoluChem™ PhotoRedOx Box as a photoreactor with EvoluChem™ LED spotlight (525 nm, 10 mW.cm⁻²). Melting points were determined on a Stuart melting point SMP3 apparatus. Thin-layer chromatography (TLC) was carried out on aluminum sheets precoated with silica gel 60 F₂₅₄ (Merck) or Alugram™ Alox N F₂₅₄ (MACHEREY-NAGEL). Column chromatography separations were performed using Merck Kieselgel 60 M (0.040–0.063 mm) or Acros Organics Aluminum oxide, neutral, 60 A (40-300 µm). The chemicals and solvents were purchased from commercial suppliers without further purification. The benzyl phenyl sulfone[20] and diiodo-BODIPY photosensitizer (I)[21] were prepared according to literature procedures.

## Syntheses and Methods

**General procedure for the synthesis of endoperoxides [1-7]-O₂.**
The anthracene substrate 1-7 (80 µmol, 1 equiv.) and Rose bengal (2.04 mg, 2 µmol, 2.5 mol%) were introduced into a vial. A mixture of CHCl₃/MeOH (2 mL, v/v, 80/20) was then added. The reaction medium was gently bubbled with oxygen for 5 minutes and was left under oxygen atmosphere throughout the reaction time. The reaction mixture was stirred and irradiated with a green LED (HepatoChem EvoluChem™ PhotoRedOx Box, EvoluChem™ LED spotlight (525 nm, 10 mW.cm⁻²)). After the reaction completion monitored by TLC, the crude was purified by column chromatography on neutral alumina gel (n-pentane/Et₂O). The combined fractions were concentrated at 20 °C to afford the corresponding endoperoxide as a solid, which was then dried under vacuum. EPOs were stored at -20 °C.

**Typical procedure for the photooxygenation experiments monitored by UV-visible spectroscopy.** A stock solution was prepared in toluene (10 mL) with a concentration of 7×10⁻⁵ mol.L⁻¹ for 1 (1 equiv.) and 1.75 x 10⁻⁶ mol.L⁻¹ for I (2.5 mol%). From this stock solution, 3 mL was introduced in a UV cell and irradiated with a green LED (HepatoChem EvoluChem™ PhotoRedOx Box, EvoluChem™ LED spotlight (525 nm, 10 mW.cm⁻²)). The consumption was followed by UV-visible spectroscopy. Periodically, the absorp-

tion spectra of the reaction mixture were recorded. The absorption maxima Aᵢ were taken to monitor the disappearance of the substrate at λ=408 nm. The relative pseudo first order rate constants were obtained from plotting the logarithm (Aᵢ/A₀) against time. The experiments were carried out three times.

**Typical procedure for the thermolysis experiments.** 1-O₂ (2.71 mg, 7.01 µmol) was introduced in toluene (2 mL) to prepare a 3.5×10⁻³ mol.L⁻¹ endoperoxide solution. From this solution, 0.4 mL was introduced in 19.6 mL of toluene heated at 90 °C in a tube to obtain an endoperoxide concentration of 7×10⁻⁵ mol.L⁻¹ in the reaction mixture. The tube was closed with a screw cap and the reaction medium was left to stir at 90 °C. Periodically, 1 mL was taken from the reaction mixture, putted in a vial and placed into an ice bath to stop thermolysis. The sample was then introduced into a UV cell and the formation of the parent compound was monitored by UV-visible spectroscopy. The absorption maxima Aᵢ were taken to monitor the increase of absorption intensity of the parent compound 1 at λ=408 nm. The first order rates constants were obtained from plotting the logarithm of [(Aᵢₐ⁻Aᵢ)/(Aᵢₐ⁻A₀)] against time. The half-lives were calculated as follows: t₁/₂ = ln2/k.

# Supporting Information

The authors have cited additional references within the Supporting Information.[22-27]

## Acknowledgements

This work benefited from the support of the project DELIVRO2 (ANR-22-CE07-0026-03) from the French National Research Agency (ANR). In particular, P. D. B. thanks ANR for a postdoctoral fellow. We also thank Nantes Université and CNRS for financial support. The authors greatly acknowledge AMaCC platform team (CEISAM UMR CNRS 6230, Nantes Université) for their contribution in mass spectrometry to this work. This work includes NMR experiments performed on the CEISAM NMR platform.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords:** endoperoxide · anthracene · photochemistry · photooxygenation · singlet oxygen

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