# Visible-light Organic Photosensitizers Based on 2-(2-Aminophenyl)benzothiazoles for Photocycloaddition Reactions

Ekaterina Pylova, Benjamin Lasorne, Nathan Mcclenaghan, Gediminas Jonusauskas, Marc Taillefer, Sergey Konchenko, Alexis Prieto, Florian Jaroschik

► To cite this version:

Ekaterina Pylova, Benjamin Lasorne, Nathan Mcclenaghan, Gediminas Jonusauskas, Marc Taillefer, et al.. Visible-light Organic Photosensitizers Based on 2-(2-Aminophenyl)benzothiazoles for Photo-cycloaddition Reactions. Chemistry - A European Journal, 2024, 10.1002/chem.202401851. hal-04728210

HAL Id: hal-04728210

https://hal.science/hal-04728210v1

Submitted on 14 Oct 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

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

Accepted Article

**Title:** Visible-light Organic Photosensitizers Based on 2-(2-Aminophenyl)benzothiazoles for Photocycloaddition Reactions

**Authors:** Ekaterina Pylova, Benjamin Lasorne, Nathan McClenaghan, Gediminas Jonusauskas, Marc Taillefer, Sergey Konchenko, Alexis Prieto, and Florian Jaroschik

This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). The VoR will be published online
in Early View as soon as possible and may be different to this Accepted
Article as a result of editing. Readers should obtain the VoR from the
journal website shown below when it is published to ensure accuracy of
information. The authors are responsible for the content of this Accepted
Article.

To be cited as: *Chem. Eur. J.* **2024**, e202401851

Link to VoR: https://doi.org/10.1002/chem.202401851
RESEARCH ARTICLE

# Visible-light Organic Photosensitizers Based on 2-(2-Aminophenyl)benzothiazoles for Photocycloaddition Reactions

Ekaterina Pylova,[a][b][c] Benjamin Lasorne,[a], Nathan D. McClenaghan,[d] Gediminas Jonusauskas,[e] Marc Taillefer,[a] Sergey N. Konchenko,[b] Alexis Prieto,[*a] Florian Jaroschik*[a]

[a] E. Pylova, Dr. B. Lasorne, Dr. M. Taillefer, Dr. A. Prieto, Dr. F. Jaroschik
ICGM, Univ Montpellier, CNRS, ENSCM
34090 Montpellier, France
E-mail: alexis.prieto@enscm.fr; florian.jaroschik@enscm.fr

[b] E. Pylova, Prof. S. N. Konchenko
Nikolaev Institute of Inorganic Chemistry SB RAS
Prosp. Lavrentieva 3, 630090 Novosibirsk, Russia

[c] E. Pylova
Department of Natural Sciences
National Research University—Novosibirsk State,
630090 Novosibirsk, Russia

[d] Dr. N. D. McClenaghan
Université Bordeaux, CNRS
Bordeaux INP, ISM, UMR 5255
Talence F-33400, France

[e] Dr. G. Jonusauskas
Laboratoire Ondes et Matières d'Aquitaine
Université Bordeaux, CNRS
Bordeaux INP, LOMA, UMR 5255
Talence F-33400, France

Supporting information for this article is given via a link at the end of the document.

**Abstract:** We have studied 2-(2-aminophenyl)benzothiazole and related derivatives for their photophysical properties in view of employing them as new and readily tunable organic photocatalysts. Their triplet energies were estimated by DFT calculations to be in the range of 52-57 kcal·mol⁻¹ suggesting their suitability for the [2+2] photocycloaddition of unsaturated acyl imidazoles with styrene derivatives. Experimental studies have shown that 2-(2-aminophenyl)benzothiazoles comprising alkylamino groups (NHMe, NHiPr) or the native amino group provide the best photocatalytic results in these visible-light mediated [2+2] reactions without the need of any additives yielding a range of cyclobutane derivatives. A combined experimental and theoretical approach has provided insights into the underlying triplet-triplet energy transfer process.

## Introduction

Photocatalysis is a rapidly growing research field in organic chemistry with high potential for constructing a plethora of new synthetic scaffolds *via* unconventional bond breaking and bond forming processes.[1] In this field, three main areas have been explored for the generation of radical species : (i) redox-active photocatalysts (PC) operating *via* electron-transfer (ET) mechanisms;[2] (ii) organic dyes which, after photoexcitation, are capable of direct atom transfer (AT), (mainly HAT);[3] (iii)

photosensitizers (PS) operating *via* triplet-triplet energy transfer (TTEnT).[4] Even though the latter has been known since the 1960s with the development of the first organic photocatalysts,[4a] many new synthetic methodologies have recently emerged with improved photocatalysts, often based on precious transition metals (Ir, Ru). These catalysts work under mild conditions, using visible light, and their properties can be tuned through the ligand environment.[5] However, they suffer from economic and sustainability drawbacks. Most of the cheaper and readily available organic photocatalysts work under UV light activation and are consequently often less efficient in visible-light mediated EnT processes than their metallic counterparts (slower Inter-system crossing (ISC), lower extinction coefficients). In recent years, new classes of organic photocatalysts have appeared for visible-light mediated reactions, including benzils, flavins, cyanoarenes, acridinium salts or xanthene derivatives.[6] In certain cases, catalyst modifications have opened up the opportunity for enantioselective processes.[7]

This work focuses on 2-arylbenzothiazoles, which are easily available and widely employed building blocks. They have been applied as ligands in transition metal complexes[8] and for the design of biologically active molecules (anti-cancer, imaging).[9] They can also act as the light-emitting elements in chemical sensors,[10] often involving Aggregation Induced Emission (AIE) or Excited State Intramolecular Proton Transfer (ESIPT), or in luminescent materials (white light emitting OLEDs).[11] The parent 2-phenylbenzothiazole scaffold can be readily modified on the
RESEARCH ARTICLE

benzothiazole and the aryl part. Importantly, deprotonation of the ortho-position of the aryl group has been exploited for the synthesis of photoactive metal-complexes (Ir, Ru, Pd), which have been used in a large range of photocatalytic processes.[12] Furthermore, the introduction of functional groups, such as -OH, -SH or -NHR in the 2-position of the aryl group has also attracted much interest. This can lead to additional metal-coordination sites,[13] intra- or intermolecular hydrogen bonding and ESIPT.[14]

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Previous applications of pbt derivatives (above) and as photosensitizers in this work (below).</figcaption></figure>

Some of us have recently started to explore the coordination chemistry and photophysical properties of 2-(2-aminophenyl)benzothiazole (2-NH₂-pbt) derivatives.[15] Even though these compounds are well-studied for their ESIPT processes,[14] to date no application in photocatalysis has been reported. The planarity of the structure of 2-NHR-pbt derivatives, induced by the intramolecular H-bonding between the nitrogen atoms as shown by X-ray diffraction,[16] leads to an extended π-system, analogous to many organic photosensitizers. This should provide the possibility of coordination to unsaturated systems via π-bonding, in addition to potential intermolecular H-bonding from the amino group. A close contact between photosensitizer and photoactivable substrate is essential for efficient EnT. From these data, we assumed that 2-NHR-pbt compounds should have the necessary structural requirements for photosensitization processes. We herein report the full experimental characterization of 2-NH₂-pbt and several other amino-pbt derivatives for their photophysical properties and the evaluation of their triplet energies. Furthermore, a first application of this new family of organic photosensitizers in the [2+2] photocycloaddition of acyl imidazoles and styrenes will be described, together with experimental and theoretical mechanistic insights.

Results and Discussion

Photophysical properties of pbt derivatives.

Table 1 summarizes the electronic absorption and luminescence (fluorescence) data of various pbt derivatives measured in DCM.[17] Furthermore, the energies related to the vertical transition S₁ ← S₀ and the S₁ and T₁ values were obtained theoretically using quantum-chemical calculations (for computational details see SI). The substituents on the aryl ring significantly influence the absorption and emission properties. Whereas most pbt derivatives absorb in the UV range, the 2-NHR-pbt (R = alkyl) derivatives show a significant red-shift close to the visible wavelengths. The molar extinction coefficient was determined for 2-NH₂-pbt in DCM (excitation at 368 nm) to be 1.2 x 10⁴ M⁻¹·cm⁻¹ similar to other pbt derivatives.[17e] For most compounds the emission can be found in the violet-blue region (400-450 nm), except for molecules with a dominant ESIPT, leading to emission around 550 nm.

<table><caption>**Table 1.** Experimental absorption (nm), emission (nm) and calculated singlet and triplet energies (kcal·mol⁻¹) of various pbt derivatives.</caption><thead><tr><th>R</th><th>λ<sub>max</sub><br>(abs)</th><th>λ<sub>max</sub><br>(em)</th><th>S<sub>1</sub> ← S<sub>0</sub></th><th>S<sub>1</sub></th><th>T<sub>1</sub></th></tr></thead><tbody><tr><td>H</td><td>298<sup>[17a]</sup></td><td>363<sup>[17a]</sup></td><td>94.1</td><td>80.2</td><td>58.8</td></tr><tr><td>2-OH</td><td>337<sup>[17b]</sup></td><td>517<sup>[a17b]</sup></td><td>87.2</td><td>76.3</td><td>57.6</td></tr><tr><td>2-OMe</td><td>308<sup>[17c]</sup></td><td>367<sup>[17c]</sup></td><td>92.9</td><td>77.7</td><td>57.6</td></tr><tr><td>2-NH<sub>2</sub></td><td>365</td><td>430</td><td>79.0</td><td>70.4</td><td>55.1</td></tr><tr><td>3-NH<sub>2</sub></td><td>298</td><td>415</td><td>82.8</td><td>71.7</td><td>54.9</td></tr><tr><td>4-NH<sub>2</sub></td><td>332</td><td>395</td><td>85.5</td><td>73.8</td><td>54.5</td></tr><tr><td>2-NHMe</td><td>392<sup>[17d]</sup></td><td>458<sup>[17d]</sup></td><td>77.5</td><td>66.7</td><td>52.9</td></tr><tr><td>2-NH'Pr</td><td>395</td><td>450</td><td>74.4</td><td>66.6</td><td>52.7</td></tr><tr><td>2-NHBn</td><td>378<sup>[b17e]</sup></td><td>446<sup>[b17e]</sup></td><td>76.0</td><td>68.1</td><td>53.9</td></tr><tr><td>2-NMe<sub>2</sub></td><td>300</td><td>550</td><td>76.6</td><td>59.7</td><td>52.3</td></tr><tr><td>2-NHAc</td><td>335<sup>[17d]</sup></td><td>565<sup>[a17d]</sup></td><td>85.3</td><td>74.4</td><td>56.9</td></tr><tr><td>2-NHTos</td><td>330<sup>[17d]</sup></td><td>555<sup>[a17d]</sup></td><td>91.6</td><td>76.6</td><td>57.8</td></tr><tr><td>2-NHBoc</td><td>340</td><td>540<sup>a</sup></td><td>84.4</td><td>74.1</td><td>56.7</td></tr></tbody></table>

[a] emission band corresponds to ESIPT-process, [b] in MeOH.

Experimental triplet energies for the enol-form of pbt derivatives obtained by phosphorescence studies are scarce.[18] Indirect measurements using Gd complexes have been reported.[19] We have attempted to record low temperature (77 K) emission spectra to estimate triplet energies, but no long-lived phosphorescence emission could be observed for the parent 2-NH₂-pbt or the 2-NHMe-pbt. It should be noted that even for the well-studied 2-OH-pbt the phosphorescence spectrum of the enol form could only be obtained under special conditions.[18] The reported value for the spectroscopically obtained triplet energy of
RESEARCH ARTICLE

2-OH-pbt of 20700 cm⁻¹ (59.3 kcal·mol⁻¹) is in good agreement with our calculated value (20145 cm⁻¹, 57.6 kcal·mol⁻¹).

The calculated triplet energies for the various pbt derivatives are close to the value for benzil (53.7 kcal·mol⁻¹), a well-known photosensitizer, in non-polar solvents.[⁴a] As, in addition, the molar extinction coefficient was found to be significantly higher for 2-NH₂-pbt than that for benzil (63 M⁻¹·cm⁻¹ with excitation at 351 nm),[²⁰] we then turned our interest to the application of 2-NH₂-pbt derivatives as catalysts in [2+2] photocycloaddition reactions.

**Visible-light mediated [2+2] photocycloaddition catalysed by 2-NHR-pbt derivatives.**

Among the many [2+2] photocycloaddition reactions studied in the literature,[²¹] the photocatalysed reaction of activated cinnamyl derivatives with styrenes or dienes has attracted considerable interest as complex cyclobutane products can be obtained, even in an enantioselective fashion.[²²] These reactions have been described under various conditions, including (chiral) Lewis or Brønsted acid catalysis. Meggers and Yoon have also shown that this reaction proceeds with (sub)stoichiometric amounts of benzil under blue light in acetone.[²²a,b] A TTEnT process has been proposed for this reaction, which proceeds *via* the excitation of the photosensitizer to the singlet state and after relaxation and intersystem crossing the triplet energy is transferred onto the cinnamyl substrate to form a diradical species. The latter subsequently reacts with the styrene to the corresponding cyclobutane product (Scheme 1). As the triplet energy of the cinnamyl substrate **1a** was previously calculated to be 48 kcal·mol⁻¹, this reaction seemed suitable to study with the pbt derivatives shown in Table 1 to evaluate their potential as organic photosensitizers.

<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Proposed mechanism of [2+2] photocycloaddition *via* TTEnT.[⁴]</figcaption></figure>

**Screening of photosensitizers.**

We set out in our investigations using the commercially available 2-(2-aminophenyl)-benzothiazole (2-NH₂-pbt) as photosensitizer (20 mol%) in the reaction of cinnamyl imidazole **1a** with a large excess of styrene **2a** (10 equiv.), necessary to avoid the homocoupling of **1a**,[²²c] in DCM under blue LED irradiation (455 nm, 30 W) in a dry nitrogen atmosphere (Scheme 2). Pleasingly, after 18 h at room temperature, 89% conversion of the starting compound **1a** was observed. Formation of the desired cyclobutane derivative **3a** was obtained in 60% NMR yield as a 2:1 mixture of diastereoisomers, a ratio comparable to recently reported results.[²²c] It should be noted that in the absence of 2-NH₂-pbt a 15% background reaction was observed. From this starting point, we investigated a range of 2-phenyl-pbt derivatives as shown in Scheme 2. The position of the amino group was first varied, leading to significantly lower yields for 3-NH₂-pbt and 4-NH₂-pbt derivatives (26% and 27%, respectively). Replacement of the 2-NH₂ group by 2-OH or 2-OMe groups also afforded only poor yields of **3a** with 15 and 30%, respectively, as did the parent pbt without NH₂, which only provided background reaction (yield 17%). The doubly substituted N-derivative 2-NMe₂-pbt was studied next, yielding the product in 44% yield. As mentioned above, ESIPT processes are common in pbt derivatives, especially with electron-withdrawing groups on the amino group. Consequently, the 2-NHR-pbt molecules bearing acetyl, Tos or Boc groups were employed subsequently in the reaction of **1a** with **2a**. Significantly lower yields in the range of 30-45% were observed. On the other hand, the introduction of alkyl groups (Me, 'Pr) in 2-NHR-pbt compounds led to a slight improvement of the reaction outcome (63-70%), whereas the benzyl group gave a lower yield. Analysis of the crude NMR reaction mixtures after the reaction showed in most cases that the pbt derivatives were still present. Significant degradation was observed only in the cases of 2-NH₂-pbt and 2-BnNH-pbt

From these data, we can conclude that even though all the pbt derivatives have the potential triplet energy to enable the visible-light [2+2] photocycloaddition reaction of substrate **1a** with styrene, the best photocatalysts are those with the lowest vertical transition S₁ ← S₀ gaps (ca. 3.2 – 3.4 eV) and the most bathochromic absorptions. Other parameters, such as ESIPT, may also play a potential role in lowering the catalyst efficiency.
RESEARCH ARTICLE

<figure><img src="image_6.png" /><figcaption>**Scheme 2.** Screening of pbt-based photosensitizers in [2+2] photocycloaddition reaction and correlation with their photophysical properties.</figcaption></figure>

**Scope exploration with 2-NH₂-pbt as photosensitizer.**

Even though 2-NH<sup>Pr</sup>-pbt provided the highest yields in the PS screening studies thus far, the synthetic routes to this derivative provided only very poor yields (10%) and hampered the use of this compound for studying the reaction scope. Instead, the commercially available 2-NH<sub>2</sub>-pbt was employed for further optimization and exploration of the scope. The best reaction conditions were found to be the reaction of **1a** (1.0 equiv.) and **2a** (10 equiv.) with 20 mol% 2-NH<sub>2</sub>-pbt under blue light irradiation for 21 h in a 0.1 M solution of DCM (see SI for optimization) (Scheme 3). This provided compound **3aa** in good 60% isolated yield in a 2.4:1 mixture of diastereoisomers. Subsequently, the electronic and steric factors of substituents on the aryl ring in the cinnamyl substrate were investigated. A *p*-tolyl group in **1b** led to a lower yield of **3ba** (44%). With electron-withdrawing groups in the *para*-position, such as *p*-NO<sub>2</sub> (**1c**), *p*-F (**1d**), *p*-CN (**1e**), the reaction with styrene provided products **3ca-3ea** in good isolated yields. Introduction of a methoxy group in the *ortho* (**1f**), *meta* (**1g**) or *para* (**1h**) position gave moderate yields of the corresponding cyclobutanes **3fa-3ha**, showing no significant steric influence. When the phenyl group was replaced with a 2-thienyl group (**1i**), the reaction proceeded well to give **3ia** in 52% yield. Next, a variety of styrene derivatives was examined in this reaction. Bromine-bearing styrenes **2b-2d** gave moderate to good yields in the [2+2] cycloaddition reaction with **1a** depending on the *ortho*, *meta* or *para* substitution. A lower yield was obtained with *p*-Cl-substituted styrene **2e**. In contrast, a *p*-NO<sub>2</sub> group (**2f**) provided efficient reactivity to afford **3af** in 69% isolated yield. Most noteworthy, also with respect to previous studies,[22] is the successful use of pyridinyl-styrene **2g** to afford **3ag** in a 45% NMR yield (isolated 32% yield of the major diastereoisomer). This can be related to the absence of any acid or metal in our approach. Subsequently, dienes were investigated as coupling partners. With 2,3-dimethylbutadiene **2h** the cyclobutane **3ah** was obtained in 67% as a 5.0:1 mixture of diastereoisomers. With isoprene **2i**, a 3.3:1 mixture of product **3ai** was isolated in 67% yield. Finally, variation of the imidazole groups bearing a *n*-butyl chain (**1j**) or a phenyl group (**1k**) on the nitrogen in the reaction with styrene led to increased yields of 64% and 79% of cyclobutane derivatives

**3ja** and **3ka**, respectively. Attempts to carry out the reaction of **1a** with styrene at a ten times larger reaction scale afforded a drop in isolated yield of **3aa** to 20 – 30%. Some coupling partners did not afford the expected products under our optimized conditions, such as indene, methylenecyclohexane, thianaphthene, 1,1-diphenylethylene, 2-bromo-3,3,3-trifluoro-1-propene, ethyl acrylate, 1,4-naphthoquinone or 2-vinylnaphthalene. Furthermore, other cinnamyl substrates led either to no conversion (methylcinnamate, 4-phenyl-3-buten-2-one) or to very poor yields (E-chalcone) of cyclobutane product after reaction with styrene under standard conditions.

<figure><img src="image_7.png" /><figcaption>**Scheme 3.** Scope exploration.[a][b]</figcaption></figure>

[a] reaction conditions: **1** (0.15 mmol, 1 equiv.), 2-NH₂-pbt (0.03 mmol, 0.2 equiv.), **2** (1.5 mmol, 10 equiv.) in DCM (1.5 mL) under dry nitrogen irradiated by a blue LED lamp (450 nm, 30W) at room temperature for 21 h; [b] isolated
RESEARCH ARTICLE

yields, *dr* values of isolated compounds; [c] *dr* of crude reaction mixture, only major diastereomer could be isolated.

**Theoretical and experimental insights into triplet-triplet energy transfer.**

In order to study the mode of action of the pbt derivatives in this [2+2] photocycloaddition we have undertaken further experimental and theoretical studies. Photophysical experiments were conducted to gain new insights into possible intervention of triplet states of pbt derivatives and excited-state quenching with substrate **1a** consistent with photosensitization. Taking representative 2-NHMe-pbt, exciting into the lowest-energy absorption band, where the photocatalyst absorbs strongly (λ<sub>em</sub> = 355 nm), yielded short-lived emission (λ<sub>em max</sub> = 465 nm; τ = 310 ps, see Fig. S4). Transient absorption spectroscopy was undertaken in order to detect the possible population of longer-lived non-emissive dark states, following excitation. Several spectral features are observed in the map shown in Fig. 2. A negative band created at zero delay and centred at 475 nm corresponds to the blue emission band. This is ascribed to stimulated emission from a singlet (S<sub>1</sub>) excited state. A long-living positive signal centred at 440 nm starts to grow immediately after excitation with the time constant of 2.5 ns (estimated from the experimental kinetics integrated over 425-525 nm). This absorption band (see spectrum in Fig. S4) may be ascribed to a relatively-short lived triplet state upon comparison with triplet state transient spectrum observed for related pbt molecules,<sup>[23]</sup> showing that it is indeed populated. However, the rise of the band does not directly correlate with the singlet decay suggesting the intervention of a further non-emitting state. It should be noted that no spectral signatures for radical cation or anion species are discerned in the transient absorption spectrum, which would affirm the occurrence of a competitive photoredox process (see SI for electrochemical study of 2-NH<sub>2</sub>-pbt).

<figure><img src="image_8.png" /><figcaption>**Figure 2.** Transient absorption map (time delay vs wavelength, colours represent intensity (ΔO.D.) according to scale) of MeNHpbt in DCM (λ<sub>ex</sub> = 355 nm).</figcaption></figure>

In order to get further insights into the potential electronic energy transfer between the organic photocatalyst and substrates, we have carried out quenching experiments of 2-NHMe-pbt with **1a**. After excitation of an equimolar mixture (0.07 M) in DCM at 430 nm in a dry nitrogen atmosphere, which is close to the catalytic conditions, a faster triplet decay was observed compared to 2-NHMe-pbt in the absence of the substrate. This observation is shown in Fig. 3 and shows the introduction of an additional deexcitation pathway in the presence of the substrate, suggesting electronic energy transfer from the photocatalyst to the substrate.

<figure><img src="image_9.png" /><figcaption>**Figure 3.** Kinetics of triplet decay of 2-MeNH-pbt alone (black) and in the presence of substrate **1a** (red) in DCM.</figcaption></figure>

We then investigated the interaction between substrate and photocatalyst in solution. In previous cases in the literature, the coordination of the substrate to the PC was evidenced.[22] For efficient triplet-triplet energy transfer to occur, the distance between photocatalyst and substrate should typically be under 10 Å, this would be the case if ground-state association were present (static quenching) or in a dynamic collision regime. UV-vis and NMR spectroscopy studies were carried out under experimental conditions (see Fig. S8, S9, S11). Whereas the UV-vis studies did not show significant changes upon combination of substrate **1a** and **2-NH<sub>2</sub>-pbt**, some substrate-catalyst interaction was observed by ¹H NMR spectroscopy with a certain shift in signals of both the photosensitizer and the substrate.

This was further complemented by quantum-chemical calculations, which were performed using density functional theory (DFT) as implemented in the Gaussian 16 program package.[24] To modulate the non-covalent interaction of substrate with photocatalyst we used the Conformer-Rotamer Ensemble Sampling Tool (CREST)[25] as a pre-optimized step, further the “substrate-PC” adducts were optimized using the PBE0-D3BJ/cc-pVTZ level of theory considering the solvent effect.[26] These calculations show π-π-stacking between the aromatic parts of substrate and various PC (Fig. 4 and Fig. S13-S14). Depending on the position and nature of the substituents in the aryl part the π-π stacking arrangements could vary, however, no significant H-bonding was observed in any case (distance between oxygen of keto-group and nitrogen of amino group =
RESEARCH ARTICLE

3.5Å). Orbital interactions between the substrate and the photocatalyst are negligible (see Fig. S12).

<figure><img src="image_10.png" /><figcaption>**Figure 4.** DFT calculations showing π-π-stacking interactions between substrate and photocatalyst.</figcaption></figure>

Conclusion

We have shown through a combined experimental, spectroscopic and theoretical approach that 2-(2-aminophenyl)benzothiazole derivatives are suitable photocatalysts in [2+2] photocycloaddition reactions proceeding through triplet-triplet energy transfer. The substituent on the pbt molecule has a significant influence on the reaction outcome which can be related to variations in the vertical transition gap S₁ ← S₀, interaction with the substrate and deactivation *via* ESIPT. The metal and additive free [2+2] process provides a range of cyclobutane products in moderate to good yields with good functional group tolerance. Further applications of various pbt derivatives in light-mediated processes are currently explored.

Supporting Information

The authors have cited additional references within the Supporting Information.[27-51]

Acknowledgements

E. P. thanks the French Embassy for a Vernadski PhD scholarship. The Centre National de la Recherche Scientifique (CNRS), the Ecole Nationale Supérieure de Chimie de Montpellier (ENSCM), the Université de Montpellier are acknowledged for financial support. The authors also thank the Balard Platform Analysis and Characterization (PAC Balard) facilities for technical support. Authors thanks Novosibirsk State University for providing the computational resources.

**Keywords:** photocatalysis • benzothiazole • [2+2] cycloaddition • energy transfer • mechanism

[1] a) J. J. Douglas, M. J. Sevrin, C. R. J. Stephenson, *Org. Process Res. Dev.* **2016**, *20*, 1134-1147; b) B. König, *Eur. J. Org. Chem.* **2017**, 2017, 1979-1981; c) L. Marzo, S. K. Pagire, O. Reiser, B. König, *Angew. Chem. Int. Ed.* **2018**, *57*, 10034-10072; d) S. G. E. Amos, M. Garreau, L. Buzzetti, J. Waser, *Beilstein J. Org. Chem.* **2020**, *16*, 1163-1187; e) S. Reischauer, B. Pieber, *iScience 2021*, *24*, 102209; f) T. Noël, E. Zysman-Colman, *Chem Catalysis 2022*, *2*, 468-476; g) L. Geniller, M. Taillefer, F. Jaroschik, A. Prieto, *ChemCatChem* **2023**, *15*, e202300808.

[2] M. H. Shaw, J. Twilton, D. W. C. MacMillan, *J. Org. Chem.* **2016**, *81*, 6898-6926.

[3] L. Capaldo, D. Ravelli, M. Fagnoni, *Chem. Rev.* **2022**, *122*, 1875-1924.

[4] a) W. G. Herkstroeter, A. A. Lamola, G. S. Hammond, *J. Am. Chem. Soc.* **1964**, *86*, 4537-4540; b) Q.-Q. Zhou, Y.-Q. Zou, L.-Q. Lu, W.-J. Xiao, *Angew. Chem. Int. Ed.* **2019**, *58*, 1586-1604; c) F. Strieth-Kalthoff, F. Glorius, *Chem* **2020**, *6*, 1888-1903; d) D. S. Lee, V. K. Soni, E. J. Cho, *Acc. Chem. Res.* **2022**, *55*, 2526-2541; e) S. Dutta, J. E. Erchinger, F. Strieth-Kalthoff, R. Kleinmans, F. Glorius, *Chem. Soc. Rev.* **2024**, *53*, 1068-1089.

[5] a) N. Hu, H. Jung, Y. Zheng, J. Lee, L. Zhang, Z. Ullah, X. Xie, K. Harms, M.-H. Baik, E. Meggers, *Angew. Chem. Int. Ed.* **2018**, *57*, 6242-6246; *Angew. Chem.* **2018**, *130*, 6350-6354; b) L. Zhang, E. Meggers, *Acc. Chem. Res.* **2017**, *50*, 320-330.

[6] a) N. A. Romero, D. A. Nicewicz, *Chem. Rev.* **2016**, *116*, 10075-10166; b) T.-Y. Shang, L.-H. Lu, Z. Cao, Y. Liu, W.-M. He, B. Yu, *Chem. Commun.* **2019**, *55*, 5408-5419; c) M. A. Bryden, E. Zysman-Colman, *Chem. Soc. Rev.* **2021**, *50*, 7587-7680; d) M. V. Bobo, J. J. Kuchta, A. K. Vannucci, *Org. Biomol. Chem.* **2021**, *19*, 4816-4834; e) P. P. Singh, V. Srivastava, *Org. Biomol. Chem.* **2021**, *19*, 313-321; f) A. Tlili, S. Lakhdar, *Angew. Chem. Int. Ed.* **2021**, *60*, 19526-19549; g) M. A. Bryden, F. Millward, T. Matulaitis, D. Chen, M. Villa, A. Fermi, S. Cetin, P. Ceroni, E. Zysman-Colman, *J. Org. Chem.* **2023**, *88*, 6364-6373; h) C.-H. Hu, Y. Li, *J. Org. Chem.* **2023**, *88*, 6401-6406; i) T. S. Mayer, T. Taeufer, S. Brandt, J. Rabeah, J. Pospech, *J. Org. Chem.* **2023**, *88*, 6347-6353.

[7] M. J. Genzink, J. B. Kidd, W. B. Swords, T. P. Yoon, *Chem. Rev.* **2022**, *122*, 1654-1716.

[8] a) J. Ma, X. Zhang, X. Huang, S. Luo, E. Meggers, *Nat. Protoc.* **2018**, *13*, 605-632; b) Y. Grell, N. Demirel, K. Harms, E. Meggers, *Organometallics* **2019**, *38*, 3852-3859; c) V. A. Ilchev, T. V. Balashova, S. K. Polyakova, A. F. Rogozhin, D. S. Kolybalov, D. A. Bashirov, S. N. Konchenko, A. N. Yablonskiy, R. V. Rumyantcev, G. K. Fukin, M. N. Bochkarev, *Russ. Chem. Bull.* **2022**, *71*, 298-305; d) O. A. Mironova, A. A. Ryadun, T. S. Sukhikh, N. A. Pushkarevsky, S. N. Konchenko, *New. J. Chem.* **2023**, *47*, 3406-3416; e) T. S. Sukhikh, R. M. Khisamov, S. N. Konchenko, *Symmetry* **2023**, *15*, 157; f) M. Y. Afonin, P. A. Martynenko, D. S. Kolybalov, R. M. Khisamov, S. N. Konchenko, T. S. Sukhikh, *Inorg. Chem.* **2024**, *63*, 369-380; g) S. Ibrahim, N. Naik, C. Shivamallu, H. L. Raghavendra, A. A. Shati, M. Y. Alfaifi, S. E. I. Elbehairi, R. G. Amachawadi, S. P. Kollur, *Inorg. Chim. Acta* **2024**, *559*, 121792; h) M.-H. Wang, M.-Y. Tsai, Y.-C. Su, S.-T. Chiu, P.-H. Lin, J. Long, *Cryst. Growth Des.* **2024**, *24*, 422-431.

[9] a) M. Sagnou, B. Mavroidi, A. Shegani, M. Paravatou-Petsotas, C. Raptopoulou, V. Psycharis, I. Pirmettis, M. S. Papadopoulos, M. Pelecanou, *J. Med. Chem.* **2019**, *62*, 2638-2650; b) E. N. Djuidje, S. Sciabica, R. Buzzi, V. Dissette, J. Balzarini, S. Liekens, E. Serra, E. Andreotti, S. Manfredini, S. Vertuani, A. Baldisserotto, *Bioorg. Chem.*
RESEARCH ARTICLE

**2020, 101**, 103960; c) S. Mokesch, K. Cseh, H. Geisler, M. Hejl, M. H. M. Klose, A. Roller, S. M. Meier-Menches, M. A. Jakupec, W. Kandioller, B. K. Keppler, *Front. Chem.* **2020**, *8*, 209; d) M. J. Ramaiah, D. Karthikeyan, S. Mathavan, R. B. R. D. Yamajala, S. Ramachandran, P. J. Vasavi, N. V. Chandana, *Environ. Toxicol. Pharmacol.* **2020**, *79*, 103415; e) Z. Zhan, Z. Su, L. Chai, C. Li, R. Liu, Y. Lv, *Anal. Chem.* **2020**, *92*, 8285-8291; f) N. C. Kuznik, V. Solozobova, N. Jung, S. Gräßle, Q. Lei, E. M. Lewandowski, R. Munuganti, A. Zoubeidi, Y. Chen, S. Bräse, A. C. B. Cato, *ACS Chem. Biol.* **2021**, *16*, 2103-2108; g) Q. Sun, Y. Wang, Q. Fu, A. Ouyang, S. Liu, Z. Wang, Z. Su, J. Song, Q. Zhang, P. Zhang, D. Lu, *Angew. Chem. Int. Ed.* **2021**, *60*, 4841-4848; *Angew. Chem.* **2021**, *133*, 4891-4898; h) Z.-H. Xu, H. Gao, N. Zhang, W. Zhao, Y.-X. Cheng, J.-J. Xu, H.-Y. Chen, *Anal. Chem.* **2021**, *93*, 1686-1692.

[10] a) Y. Liu, B. Feng, X. Cao, G. Tang, H. Liu, F. Chen, M. Liu, Q. Chen, K. Yuan, Y. Gu, X. Feng, W. Zeng, *J. Anal. Chem.* **2019**, *144*, 5136-5142;
b) O. Sadek, L. A. Galán, F. Gendron, B. Baguenard, S. Guy, A. Bensalah-Ledoux, B. Le Guennic, O. Maury, D. M. Perrin, E. Gras, *J. Org. Chem.* **2021**, *86*, 11482-11491; c) P. He, Y. Chen, X.-N. Li, Y.-Y. Yan, C. Liu, *Dalton Trans.* **2023**, *52*, 128-135; d) P. He, Y. Chen, X.-N. Li, Y.-Y. Yan, C. Liu, *Chemosensors* **2023**, *11*, 177.

[11] a) Y.-X. Hu, X. Xia, W.-Z. He, H.-J. Chi, Y. Dong, G.-Y. Xiao, Y.-L. Lv, X. Li, D.-Y. Zhang, *Dyes Pigment.* **2019**, *166*, 254-259; b) D. Liu, Q. Ding, Y. Fu, Z. Song, Y. Peng, *Org. Lett.* **2019**, *21*, 2523-2527; c) R. Paul, S. Chandra Shit, H. Mandal, J. Rabeah, S. S. Kashyap, Y. Nailwal, D. B. Shinde, Z. Lai, J. Mondal, *ACS Appl. Nano Mater.* **2021**, *4*, 11732-11742;
d) Y. Song, L. Hu, Q. Cheng, Z. Chen, H. Su, H. Liu, R. Liu, S. Zhu, H. Zhu, *J. Mater. Chem.* **2022**, *10*, 6392-6401.

[12] a) C. Yang, F. Mehmood, T. L. Lam, S. L.-F. Chan, Y. Wu, C.-S. Yeung, X. Guan, K. Li, C. Y.-S. Chung, C.-Y. Zhou, T. Zou, C.-M. Che, *Chem. Sci.* **2016**, *7*, 3123-3136; b) Z.-G. Niu, J. Chen, P. Tan, W. Sun, Y.-X. Zheng, G.-N. Li, J.-L. Zuo, *Dalton Trans.* **2018**, *47*, 8032-8040; c) Z.-J. Yao, N. Lin, X.-C. Qiao, J.-W. Zhu, W. Deng, *Organometallics* **2018**, *37*, 3883-3892; d) X.-C. Qiao, D.-C. Qiu, N. Lin, J.-W. Zhu, W. Deng, Z.-J. Yao, *Appl. Organomet. Chem.* **2019**, *33*, e4875; e) J. Yang, F. Zhang, X. Dong, B. Yu, M. Zhang, D. Wei, Z. Li, B. Wei, C. Zhang, S. Li, G. Cao, B. Zhai, *New. J. Chem.* **2019**, *43*, 4272-4281; f) V. Mdluli, S. Diluzio, J. Lewis, J. F. Kowalewski, T. U. Connell, D. Yaron, T. Kowalewski, S. Bernhard, *ACS Catal.* **2020**, *10*, 6977-6987; g) D. Gómez de Segura, R. Lara, M. Martínez-Junquera, E. Lalinde, M. T. Moreno, *Dalton Trans.* **2022**, *51*, 274-285; h) T. Kitamura, K. Yamanishi, S. Inoue, Y.-N. Yan, N. Yano, Y. Kataoka, M. Handa, T. Kawamoto, *Eur. J. Inorg. Chem.* **2022**, *2022*, e202200259; i) H. Zhang, C. Liu, J. Zhang, C.-x. Du, B. Zhang, *Organometallics* **2022**, *41*, 1381-1390.

[13] a) M. A. Katkova, A. P. Pushkarev, T. V. Balashova, A. N. Konev, G. K. Fukin, S. Y. Ketkov, M. N. Bochkarev, *J. Mater. Chem.* **2011**, *21*, 16611-16620; b) T. V. Balashova, M. E. Burin, V. A. Ilichev, A. A. Starikova, A. V. Marugin, R. V. Rumyantcev, G. K. Fukin, A. N. Yablonskiy, B. A. Andreev, M. N. Bochkarev, *Molecules* **2019**, *24*, 2376.

[14] a) P. Yang, J. Zhao, W. Wu, X. Yu, Y. Liu, *J. Org. Chem.* **2012**, *77*, 6166-6178; b) O. A. Mukhina, A. G. Kutateladze, *J. Am. Chem. Soc.* **2016**, *138*, 2110-2113; c) C.-L. Chen, Y.-T. Chen, A. P. Demchenko, P.-T. Chou, *Nat. Rev. Chem.* **2018**, *2*, 131-143.

[15] a) T. S. Sukhikh, D. S. Kolybalov, E. K. Pylova, S. N. Konchenko, *Inorganics*, **2022**, *10*, 138; b) D. K. Sinitsa, E. K. Pylova, O. A. Mironova, D. A. Bashirov, A. A. Ryadun, T. S. Sukhikh, S. N. Konchenko, *Dalton Trans.* **2024**, *53*, 2181-2192.

[16] CCDC 1483661: S. Palanisamy, W.-Z. Lee, Y.-M. Wang, *CSD Communication*, **2017**.

[17] a) E. A. Katlenok, E. V. Ivanova, M. V. Puzyk, K. P. Balashev, *Opt. Spectrosc.* **2012**, *113*, 279-283; b) J. Ma, J. Zhao, P. Yang, D. Huang, C. Zhang, Q. Li, *Chem. Commun.* **2012**, *48*, 9720-9722; c) L. Chen, C. Yang, S. Li, J. Qin, *Spectrochim. Acta A: Mol. Biomol. Spectrosc.* **2007**, *68*, 317-322; d) H.-W. Tseng, J.-Q. Liu, Y.-A. Chen, C.-M. Chao, K.-M. Liu, C.-L. Chen, T.-C. Lin, C.-H. Hung, Y.-L. Chou, T.-C. Lin, T.-L. Wang, P.-T. Chou, *J. Phys. Chem. Lett.* **2015**, *6*, 1477-1486; e) G. Dhaka, J. Singh, N. Kaur, *Inorg. Chim. Acta* **2017**, *462*, 152-157.

[18] P.-T. Chou, S. L. Studer, M. L. Martinez, *Chem. Phys. Lett.* **1991**, *178*, 393-398.

[19] V. A. Ilichev, A. V. Rozhkov, R. V. Rumyantcev, G. K. Fukin, I. D. Grishin, A. V. Dmitriev, D. A. Lypenko, E. I. Maltsev, A. N. Yablonskiy, B. A. Andreev, M. N. Bochkarev, *Dalton Trans.* **2017**, *46*, 3041-3050.

[20] E. Frick, C. Schweigert, B. B. Noble, H. A. Ernst, A. Lauer, Y. Liang, D. Voll, M. L. Coote, A.-N. Unterreiner, C. Barner-Kowollik, *Macromolecules* **2016**, *49*, 80-89.

[21] a) S. Poplata, A. Tröster, Y.-Q. Zou, T. Bach, *Chem. Rev.* **2016**, *116*, 9748-9815; b) T. Lei, C. Zhou, M.-Y. Huang, L.-M. Zhao, B. Yang, C. Ye, H. Xiao, Q.-Y. Meng, V. Ramamurthy, C.-H. Tung, L.-Z. Wu, *Angew. Chem. Int. Ed.* **2017**, *56*, 15407-15410; *Angew. Chem.* **2017**, *129*, 15609-15612; c) M. Sicignano, R. I. Rodríguez, J. Alemán, *Eur. J. Org. Chem.* **2021**, *2021*, 3303-3321; d) S. J. Chapman, W. B. Swords, C. M. Le, I. A. Guzei, F. D. Toste, T. P. Yoon, *J. Am. Chem. Soc.* **2022**, *144*, 4206-4213; e) Z. C. Girvin, L. F. Cotter, H. Yoon, S. J. Chapman, J. M. Mayer, T. P. Yoon, S. J. Miller, *J. Am. Chem. Soc.* **2022**, *144*, 20109-20117; f) D. Tian, X. Sun, S. Cao, E.-M. Wang, Y. Yin, X. Zhao, Z. Jiang, *Chin. J. Catal.* **2022**, *43*, 2732-2742.

[22] a) X. Huang, T. R. Quinn, K. Harms, R. D. Webster, L. Zhang, O. Wiest, E. Meggers, *J. Am. Chem. Soc.* **2017**, *139*, 9120-9123; b) M. E. Daub, H. Jung, B. J. Lee, J. Won, M.-H. Baik, T. P. Yoon, *J. Am. Chem. Soc.* **2019**, *141*, 9543-9547; c) E. M. Sherbrook, H. Jung, D. Cho, M.-H. Baik, T. P. Yoon, *Chem. Sci.* **2020**, *11*, 856-861; d) E. M. Sherbrook, M. J. Genzink, B. Park, I. A. Guzei, M.-H. Baik, T. P. Yoon, *Nat. Commun.* **2021**, *12*, 5735.

[23] M. Ikegami, T. Arai, *J. Chem. Soc., Perkin Trans. 2*, **2002**, 1296-1301.

[24] Gaussian 16, Revision C.01: M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseeman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidì, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, D. J. Fox, *Gaussian Inc.*, Wallingford CT, **2019**.

[25] a) P. Pracht, F. Bohle, S. Grimme, *Phys. Chem. Chem. Phys.* **2020**, *22*, 7169-7192; b) P. Pracht, S. Grimme, C. Bannwarth, F. Bohle, S. Ehlert, G. Feldmann, J. Gorges, M. Müller, T. Neudecker, C. Plett, S. Spicher, P. Steinbach, P. A. Wesołowski, F. Zeller, *J. Chem. Phys.* **2024**, *160*, 114110.
RESEARCH ARTICLE

[26] a) T. H. Dunning, Jr., *J. Chem. Phys.* **1989**, *90*, 1007-1023; b) R. A. Kendall, T. H. Dunning, Jr., R. J. Harrison, *J. Chem. Phys.* **1992**, *96*, 6796-6806; c) D. E. Woon, T. H. Dunning, Jr., J. Chem. Phys. **1993**, *98*, 1358-1371; d) K. A. Peterson, D. E. Woon, T. H. Dunning, Jr., *J. Chem. Phys.* **1994**, *100*, 7410-7415; e) J. P. Perdew, K. Burke, M. Ernzerhof, *Phys. Rev. Lett.* **1996**, *77*, 3865-3868; f) S. Grimme, S. Ehrlich, L. Goerigk, *J. Comput. Chem.* **2011**, *32*, 1456-1465.

[27] J. Zhang, W. Guo, *Chem. Comm.* **2014**, *50*, 4214-4217.

[28] P. R. Sultane, T. B. Mete, R. G. Bhat, *Org. Biomol. Chem.* **2014**, *12*, 261-264.

[29] S. W. Youn, Y. H. Kim, *Org. Lett.* **2016**, *18*, 6140-6143.

[30] J.-R. Deng, W.-C. Chan, N. Chun-Him Lai, B. Yang, C.-S. Tsang, B. Chi-Bun Ko, S. Lai-Fung Chan, M.-K. Wong, *Chem. Sci.* **2017**, *8*, 7537-7544.

[31] T. K. Venkatachalam, G. K. Pierens, P. V Bernhardt, D. C. Reutens, *Mag. Res. Chem.* **2015**, *53*, 448-453.

[32] R. K. Kawade, D. B. Huple, R.-J. Lin, R.-S. Liu, *Chem. Comm.* **2015**, *51*, 6625-6628.

[33] T. K. Venkatachalam, D. H. R. Stimson, R. Bhalla, G. K. Pierens, D. C. Reutens, *J. Labelled Compd. Radiopharm.* **2014**, *57*, 566-573.

[34] X.-L. Yang, J.-D. Guo, T. Lei, B. Chen, C.-H. Tung, L.-Z. Wu, *Org. Lett.* **2018**, *20*, 2916-2920.

[35] R. Pothikumar, V. T. Bhat, K. Namitharan, *Chem. Comm.* **2020**, *56*, 13607-13610.

[36] S. Liu, L. Zhang, X. Liu, *New J. Chem.* **2013**, *37*, 821-826.

[37] A. Rostami, A. Yari, *J. Iran. Chem. Soc.* **2012**, *9*, 489-493.

[38] X. Yu, Z. Zhang, R. Song, L. Gou, G. Wang, *Heterocycl. Commun.* **2020**, *26*, 1-5.

[39] J. S. Willemse, R. P. Megens, G. Roelfes, J. C. M. van Hest, F. P. J. T. Rutjes, *Eur. J. Org. Chem.* **2014**, *2892-2898*.

[40] T. Tanaka, T. Tanaka, T. Tsuji, R. Yazaki, T. Ohshima, *Org. Lett.* **2018**, *20*, 3541-3544.

[41] J. Wang, E. Benedetti, L. Bethge, S. Vonhoff, S. Klussmann, J.-J. Vasseur, J. Cossy, M. Smietana, S. Arseniyadis, *Angew. Chem. Int. Ed.* **2013**, *52*, 11546-11549; *Angew. Chem.* **2013**, *125*, 11760-11763.

[42] H. Zhao, K. Shen, *RSC Adv.* **2014**, *4*, 54051-54059.

[43] N. Duchemin, M. Cattoen, O. Gayraud, S. Anselmi, B. Siddiq, R. Buccafusca, M. Daumas, V. Ferey, M. Smietana, S. Arseniyadis, *Org. Lett.* **2020**, *22*, 5995-6000.

[44] S. Drissi-Amraoui, M. S. T. Morin, C. Crévisy, O. Baslé, R. Marcia de Figueiredo, M. Mauduit, J.-M. Campagne, *Angew. Chem. Int. Ed.* **2015**, *54*, 11830-11834; *Angew. Chem.* **2015**, *127*, 11996-12000.

[45] M. Zhang, T. Zhang, D. Zhang, W. Hu, *Adv. Synth. Catal.* **2020**, *362*, 4662-4667.

[46] B. M. Trost, T. M. Lam, *J. Am. Chem. Soc.* **2012**, *134*, 11319-11321.

[47] (a) G. J. Kavarnos, "Energetics of photoinduced electron transfer", in *Fundamentals of Photoinduced Electron Transfer*. (VCH Publishers, 1993), pp 29-37; (b) Y. Wu, D. Kim, T. S. Teets, *Synlett* **2022**, *33*, 1154-1179.

[48] I. N. Jovanović, R. Vianello, D. Jadreško, L. Racané, M. Hranjec, *J. Electroanal. Chem.* **2023**, *947*, 117787.

[49] K. Tanaka, Y. Iwama, M. Kishimoto, N. Ohtsuka, Y. Hoshino, K. Honda, *Org. Lett.* **2020**, *22*, 5207-5211.

[50] H. O. House, L. E. Huber, M. J. Umen, *J. Am. Chem. Soc.* **1972**, *94*, 8471-8475.

[51] a) M. J. G. Peach, D. J. Tozer, *J. Phys. Chem. A* **2012**, *116*, 9783-9789; b) F. Trani, G. Scalmani, G. Zheng, I. Carnimeo, M. J. Frisch, V. Barone, *J. Chem. Theory Comput.* **2011**, *7*, 3304-3313.
Entry for the Table of Contents

<figure><img src="image_11.png" /></figure>

A series of 2-(2-aminophenyl)benzothiazole derivatives have been characterized for their photophysical properties and their triplet energies were determined by DFT calculations. Subsequent application of this new class of photosensitizer in visible-light [2+2] photocycloaddition reactions revealed a substituent effect on the catalytic efficiency. Mechanistic insights were obtained experimentally and by DFT calculations.

Institute and/or researcher Twitter usernames: ((optional))