# Impact of isomerism on the photoproduction of carbon monoxide and singlet oxygen by visible-light absorbing rhenium(I) photoCORMs

Valentine Guilbaud, Prashant Kumar, Alexis Grosjean, Evelyne Delfourne, Martial Boggio-Pasqua, Sonia Mallet-Ladeira, Charles-Louis Serpentini, Eric Benoist, Isabelle Dixon, Suzanne Fery-Forgues

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

## ► To cite this version:

Valentine Guilbaud, Prashant Kumar, Alexis Grosjean, Evelyne Delfourne, Martial Boggio-Pasqua, et al.. Impact of isomerism on the photoproduction of carbon monoxide and singlet oxygen by visible-light absorbing rhenium(I) photoCORMs. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy [1994-..], 2025, 344 (Part I), pp.126647. 10.1016/j.saa.2025.126647. hal-05225272

HAL Id: hal-05225272

https://hal.science/hal-05225272v1

Submitted on 3 Dec 2025

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_2.png" /></figure>
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

Check for
updates

# Impact of isomerism on the photoproduction of carbon monoxide and singlet oxygen by visible-light absorbing rhenium(I) photoCORMs☆

Valentine Guilbaud<sup>a, ID</sup>, Prashant Kumar<sup>b,1 ID</sup>, Alexis Grosjean<sup>b</sup>, Evelyne Delfourne<sup>a ID</sup>, Martial Boggio-Pasqua<sup>b ID</sup>, Sonia Mallet-Ladeira<sup>c ID</sup>, Charles-Louis Serpentini<sup>d ID</sup>, Eric Benoist<sup>a ID</sup>, Isabelle M. Dixon<sup>b, * ID</sup>, Suzanne Fery-Forgues<sup>a, * ID</sup>

## HIGHLIGHTS

* PhotoCORMs are potentially capable of producing both CO and $ ^1O_2 $.

* By adding one nitrogen atom in the ligand, absorption is moved to the visible.

* Isomerism deeply influences the photochemical and spectroscopic properties.

* Theoretical calculations allow to correlate these properties.

## GRAPHICAL ABSTRACT

<figure><img src="image_5.png" /></figure>

## ARTICLE INFO

**Keywords:**
Rhenium
PhotoCORM
Carbon monoxide
Singlet oxygen
Photochemistry
DFT calculations

## ABSTRACT

Photoactive rhenium(I) complexes have high therapeutic potential, particularly in the fight against cancer and antibiotic-resistant bacteria. They have the particularity of combining several modes of action, including photogeneration of carbon monoxide (CO) and singlet oxygen. However, their design is still to be improved. One limitation is that most of them absorb in the UV range, as is the case for **Re-Pyta-TPP**, a very attractive photochemically CO-releasing molecule (PhotoCORM). To shift absorption to the visible, an additional nitrogen atom was introduced into the pyridyltriazole bidentate ligand of this complex. The four resulting isomers **Re-Ln-TPP** (n = 1–4) were thus synthesized and studied from both an experimental and a theoretical point of view. Remarkably, their spectroscopic and photochemical properties strongly depend on the substitution pattern.

<footnote><sup>a</sup> SPCMIB, CNRS UMR 5068, Université de Toulouse, 118 route de Narbonne, 31062 Toulouse cedex 9, France</footnote>

<footnote><sup>b</sup> Laboratoire de Chimie et Physique Quantiques (LCPQ), CNRS UMR 5626, Université de Toulouse, 118 route de Narbonne, 31062 Toulouse cedex 9, France</footnote>

<footnote><sup>c</sup> Service Diffraction des Rayons X, Institut de Chimie de Toulouse, ICT- UAR 2599, Université de Toulouse, 118 route de Narbonne, 31062 Toulouse cedex 9, France</footnote>

<footnote><sup>d</sup> Laboratoire SOFTMAT, CNRS UMR 5623, Université de Toulouse, 118 route de Narbonne, 31062 Toulouse cedex 9, France</footnote>

<footnote>* Electronic Supplementary Information (ESI) available: Experimental details including proton numbering for NMR, NMR spectra, HRMS and FTIR spectra, molecular structures, spectroscopic monitoring of the photoreactivity, crystallographic data, computed absorption and emission maxima, Mulliken spin densities in the $ ^3MLCT $ state, selected parameters for **Re-pyta-TPP** and **Re-L1-TPP** (pdf). Cartesian coordinates are provided separately as a single xyz file.</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: dixon@irsamc.ups-tlse.fr (I.M. Dixon), suzanne.fery-forgues@utoulouse.fr (S. Fery-Forgues).</footnote>

<footnote n="1">¹ Current address: Department of Chemistry, Christopher Ingold Building, University College London, London WC1H 0AJ, United Kingdom.</footnote>
Three out of four complexes can indeed be excited in the visible. The photodecarbonylation yield of the new complexes was lower than that of **Re-Pyta-TPP**. Interesting links were found between absorption and photoluminescence data, and photoreactivity. DFT and TD-DFT calculations provided rational bases to these observations, showing that the quantum yield for CO photorelease can be correlated to the potential energy profile for the complexes in their lowest triplet state. The **Re-Ln-TPP** complexes that absorb and emit at the longest wavelengths have the best singlet oxygen photoproduction. This shows that it is not easy to optimize all the spectroscopic and photochemical properties at once, and that a fair balance must be found between them. This study is another step towards rationally-designed photoCORMs.

# 1. Introduction

In the past few years, light-activated tricarbonylrhenium(I) complexes have emerged as promising anticancer and antimicrobial agents [1–6]. The presence of a heavy metal, which induces a rapid population of excited triplet state, endows them with particular photochemical behaviour. A large number of complexes generate reactive oxygen species (ROS), in particular singlet oxygen $^1O_2$ ($^1Δ_g$), making them attractive photosensitizers for photodynamic therapy [7–18]. A small number are photochemically CO-releasing molecules (PhotoCORMs). They are able to deliver CO in a spatially and temporally controlled way under light irradiation [19–27], CO itself being a molecule with very strong biological activity [28–32]. Their room-temperature phosphorescence facilitates cellular tracking and monitoring of the photoreaction progress by optical methods [2,4,19–24]. PhotoCORMs may also have a direct biological effect in the absence of light [33], as is the case for many tricarbonylrhenium(I) complexes [1–5,12,15,34,35]. They can thus combine multiple modes of action that contribute to the therapeutic effect. Curiously, despite their potential, there is still a lack of information for a rational design of photoCORMs with dual activity, and in particular their capacity to produce both CO and $^1O_2$ has been scarcely investigated [7]. Additionally, for in vivo applications, it would be desirable that these molecules absorb in the visible range, in order to promote the penetration of light into the tissues and improve biocompatibility [36–38]. At the moment, very few Re(I) photoCORMs meet these criteria. The Mascharak team observed that a complex absorbing at 500 nm did not release CO when irradiated at this wavelength [22]. A noticeable exception is the phthalocyanine-based complex reported by Wang et al., which is photochemically active when excited with red light. However, this complex fragments after light absorption, instead of exchanging a CO ligand for a solvent molecule like most photoCORMs do [27]. It should be noted that in these studies $^1O_2$ production has not been reported.

Recently, our team studied new photoCORMs incorporating a bidentate pyridyltriazole (pyta) ligand, with excellent chelation properties [33,39]. The presence of an adamantyl substituent, well known in drug design for its biological affinity [40,41], was associated to the best phototherapeutic effect against a bacterial strain [33]. The triphenylphosphine ancillary ligand promotes photoreactivity, as opposed to photostable chlorido derivatives [42,43]. For instance, the **Re-Pyta-TPP** complex (Fig. 1) showed rapid CO release associated with long-lasting generation of $^1O_2$, mainly due to the decarbonylated photoproduct. This two-speed process seems important for the therapeutic effect, which could possibly be enhanced by the direct biological effect of the decarbonylated complex. However, the limitation of this design is that our molecules absorb in the near-UV range. To face this issue, we propose to take advantage of the versatility of the pyta ligand, and to introduce a second nitrogen atom in the pyridyl ring. As a matter of fact, it has been shown that adding a heteroatom into the aromatic system of photochemically stable chlorido complexes induces a red-shift of their absorption spectra [44,45]. Then arises the problem of diazine isomerism, which so far has not been studied with regard to photoCORMs. However, if considering the parent series of halogenated complexes, the teams of Crowley [46] and Policar [47,48] have shown that moving from 4-(2-pyridyl)-1,2,3-triazole to 1-(2-pyridyl)-1,2,3-triazole ligand

motif strongly influences the luminescence and $CO_2$ reduction properties. Our team also reported dramatic differences in the electronic and spectroscopic properties of Re(I) complexes incorporating the 4-(2-pyridyl)-1,2,3-triazole and 3-(2-pyridyl)-1,2,4-triazole isomers [49,50]. Finally, Comia et al. have recently reported that complexes based on 4-(pyrimidin-2-yl)- and 4-(pyrazin-2-yl)-1,2,3-triazole have very different photophysical and electrocatalytic properties [45]. It can be anticipated that variations on the six-membered cycle can also be important in the field of photoCORMs, and deserve a comprehensive study.

Depending on the position of the second nitrogen atom in the six-membered ring, four possible skeletons emerge. The present work is thus a systematic comparison between four new isomeric photoCORMs (Re-Ln-TPP, n = 1–4, Fig. 1). A parallel is drawn with the parent molecule **Re-Pyta-TPP**. The various opportunities offered by the presence of rhenium(I) (phosphorescence, CO release, singlet oxygen production) were considered. Density Functional Theory (DFT) calculations made it possible to rationalize the CO photorelease properties. It appeared that one additional nitrogen atom in the molecules is sufficient to shift the absorption spectrum to the visible, provided it is inserted in the right position. Above all, it was shown for the first time that the position of this atom has a dramatic influence on both the CO and $^1O_2$ photoproduction, and must be taken into account to make the most of this family of photoCORMs.

# 2. Experimental section

## 2.1. Synthesis

All purchased chemicals were used without further purification. Methyl-pyrimidine-2-carboxylate, methyl-pyridazine-3-carboxylate, pyrimidine-4-carboxylic acid and pyrazine-2-carboxylic acid were purchased from BLD Pharm. Analytical grade solvents were used as

<figure><img src="image_6.png" /><figcaption>**Fig. 1.** Chemical structures of **Re-Pyta-TPP** and the four isomeric photoCORMs **Re-Ln-TPP** (n = 1–4).</figcaption></figure>
received. All reactions were carried out under a nitrogen atmosphere. They were monitored by TLC on silica gel Alugram® Xtra SIL G/UV254. Column chromatography was performed on Macherey-Nagel silica gel or neutral alumina. NMR, mass and infrared spectra were obtained in the relevant 'Services communs de l'Institut de Chimie de Toulouse'. ¹H, ¹³C and ³¹P NMR spectra were recorded on Bruker Avance 300 MHz or 500 MHz spectrometers. Signals are described as follows: s, singlet; d, doublet; t, triplet; m, multiplet; br, broad. Microanalyses were performed using a PerkinElmer 2400 Series II CHNS/O elemental analyzer. Samples came from the medium in which the crystals were grown for XRD analysis. HRMS data were recorded on a Xevo G2 QTOF (Waters) instrument or a GCT Premier (Waters) instrument. Infrared spectra were obtained on a Nexus Thermonicolet apparatus with DTGS as the detector (sh = shoulder).

## 2.2. 2-Pyrimidinecarboxylic acid hydrazide

A mixture of methyl pyrimidine-2-carboxylate (14.5 mmol), obtained from adaptation of a published procedure [51], and hydrazine hydrate (16.3 mL) in ethanol was heated at reflux overnight. After cooling to room temperature, the reaction medium was concentrated under reduced pressure. The crude product was crystallized with methanol. Filtration of the crystalline powder and washing with cold methanol led to the expected product as a beige solid (1.65 g, 83%). ¹H NMR (300 MHz, CDCl₃): $\delta$ (ppm) = 8.86 (d, J = 4.9 Hz, 2H), 7.45 (t, J = 4.9 Hz, 1H). ¹³C NMR (75 MHz, CDCl₃): $\delta$ (ppm) = 163.0 (C₅), 157.7 (2C, C₁, C₃), 157.1 (C₄), 123.0 (C₂). HRMS-ESI: *m/z* = 139.0620 ([M + H]⁺ calcd for C₅H₇N₄O: 139.0618).

## 2.3. General procedure for the synthesis of oxadiazole derivatives

p-Toluenesulfonic acid (pTSA) (0.1 eq.) was added to a solution of the hydrazide derivative (1 eq.) in triethylorthoformate, and the mixture was heated at 130 °C for 6 h under argon atmosphere. After cooling to room temperature and stirring overnight, the precipitate was collected by filtration.

## 2.4. 2-(1,3,4-Oxadiazol-2-yl)pyrimidine

General procedure involving pTSA (138 mg, 0.72 mmol), 2-pyrimidinecarboxylic acid hydrazide (1 g, 7.24 mmol) and triethylorthoformate (4 mL). The expected compound was obtained as a beige solid (0.78 g, 78%). ¹H NMR (300 MHz, CDCl₃): $\delta$ (ppm) = 8.99 (d, J = 4.9 Hz, 2H), 8.63 (s, 1H), 7.51 (t, J = 4.9 Hz, 1H). ¹³C NMR (75 MHz, CDCl₃): $\delta$ (ppm) = 162.9, 158.3 (2C), 154.1, 153.3122.7. HRMS-ESI⁺: *m/z* = 149.0461 ([M + H]⁺ calcd for C₆H₅N₄O: 149.0463).

## 2.5. 3-(1,3,4-Oxadiazol-2-yl)pyridazine

General procedure involving pTSA (138 mg, 0.72 mmol), 3-pyridazinecarboxylic acid hydrazide [52] (1 g, 7.24 mmol) and triethylorthoformate (4 mL). The expected compound was obtained as a grey solid (0.45 g, 42%). ¹H NMR (300 MHz, CD₃OD): $\delta$ (ppm) = 9.39 (dd, J = 5.1, 1.6 Hz, 1H), 9.23 (s, 1H), 8.50 (dd, J = 8.6, 1.7 Hz, 1H), 7.97 (dd, J = 8.6, 5.1 Hz, 1H). ¹³C NMR (75 MHz, CDCl₃): $\delta$ (ppm) = 154.4 (2C), 152.9, 147.5, 127.3, 125.7. HRMS-DCI-CH₄: *m/z* = 149.0459 ([M + H]⁺ calcd for C₆H₅N₄O: 149.0463).

## 2.6. 4-(1,3,4-Oxadiazol-2-yl)pyrimidine

General procedure involving pTSA (138 mg, 0.72 mmol), 4-pyrimidinecarboxylic acid hydrazide [53] (1 g, 7.24 mmol) and triethyl orthoformate (4 mL). The compound was obtained as a pale-yellow solid (0.68 g, 66%). ¹H NMR (300 MHz, CD₃OD): $\delta$ (ppm) = 9.38 (d, J = 1.4 Hz, 1H), 9.22 (s, 1H), 9.07 (d, J = 5.2 Hz, 1H), 8.26 (dd, J = 5.2, 1.4 Hz, 1H). ¹³C NMR (75 MHz, CDCl₃): $\delta$ (ppm) = 162.7, 159.7, 159.0, 154.3,

150.0, 119.4. HRMS-ESI⁺: *m/z* = 149.0464 ([M + H]⁺ calcd for C₆H₅N₄O: 149.0463)

## 2.7. General procedure for ligand synthesis

To a solution of oxadiazole derivative (200 mg, 1.35 mmol, 1 eq.) in xylenes (4 mL) were added 4-(1-adamantyl)aniline [54] (308 mg, 1.35 mmol, 1 eq.) and pTSA (26 mg, 0.14 mmol, 0.1 eq.). The reaction mixture was stirred under nitrogen atmosphere at 140 °C for 24 h. After cooling to room temperature, DCM was added. The organic layer was separated and washed with 1 M aqueous NaOH solution (10 mL), dried over Na₂SO₄, and concentrated under reduced pressure. Column chromatography afforded the pure expected compound Ln.

## 2.8. 3-[4-(4-Tricyclo[3.3.1.1¹,⁷]dec-1-ylphenyl)-4H-1,2,4-triazol-3-yl] pyridazine (L1)

General procedure involving 3-(1,3,4-oxadiazol-2-yl)pyridazine. Ligand L1 was obtained after column chromatography (silica gel, DCM/ MeOH 98:2 v/v) as a white solid (193 mg, 54%). ¹H NMR (300 MHz, CD₃OD): $\delta$ (ppm) = 9.19 (dd, J = 5.1, 1.6 Hz, 1H), 8.85 (s, 1H), 8.30 (dd, J = 8.6, 1.6 Hz, 1H), 7.87 (dd, J = 8.6, 5.1 Hz, 1H), 7.55-7.45 (m, 2H), 7.39-7.30 (m, 2H), 2.15-2.07 (m, 3H), 1.98-1.94 (m, 6H), 1.92-1.77 (m, 6H). ¹³C NMR (75 MHz, CDCl₃): $\delta$ (ppm) = 152.9, 151.4, 150.7, 150.3, 132.3, 127.0, 126.9, 126.1 (2C), 125.8 (2C), 43.2 (3C), 36.8, 36.5 (3C), 29.0 (3C). HRMS-ESI⁺: *m/z* = 358.2031 ([M + H]⁺ calcd for C₂₂H₂₄N₅: 358.2032).

## 2.9. 4-[4-(4-Tricyclo[3.3.1.1³,⁷]dec-1-ylphenyl)-4H-1,2,4-triazol-3-yl] pyrimidine (L2)

General procedure involving 4-(1,3,4-oxadiazol-2-yl)pyrimidine. Ligand L2 was obtained after column chromatography (silica gel, DCM/ EtOAc 5:5 v/v) as a white solid (195 mg, 54%). ¹H NMR (300 MHz, CDCl₃): $\delta$ (ppm) = 8.99 (d, J = 1.4 Hz, 1H), 8.86 (d, J = 5.2 Hz, 1H), 8.37 (s, 1H), 8.18 (dd, J = 5.2, 1.4 Hz, 1H), 7.50-7.44 (m, 2H), 7.25-7.20 (m, 2H), 2.20-2.13 (m, 3H), 1.99-1.93 (m, 6H), 1.90-1.75 (m, 6H). ¹³C NMR (75 MHz, CDCl₃): $\delta$ (ppm) = 158.5, 158.0, 153.9, 153.1, 150.5, 146.8, 132.2, 126.0 (2C), 125.6 (2C), 120.0, 43.2 (3C), 36.7, 36.5 (3C), 28.9 (3C). HRMS-ESI⁺: *m/z* = 358.2035 ([M + H]⁺ calcd for C₂₂H₂₄N₅: 358.2032).

## 2.10. 2-[4-(4-Tricyclo[3.3.1.1³,⁷]dec-1-ylphenyl)-4H-1,2,4-triazol-3-yl]pyrazine (L3)

General procedure involving 2-(1,3,4-oxadiazol-2-yl)pyrazine [55]. The expected ligand L3 was obtained after flash chromatography (silica gel, DCM/MeOH 99:1 v/v) as a pale yellow solid (89 mg, 33%). ¹H NMR (300 MHz, CDCl₃): $\delta$ (ppm) = 9.37 (d, J = 1.5 Hz, 1H), 8.55 (d, J = 2.5 Hz, 1H), 8.39-8.34 (m, 2H), 7.48-7.40 (m, 2H), 7.24-7.19 (m, 2H), 2.17-2.09 (m, 3H), 1.98-1.90 (m, 6H), 1.89-1.75 (m, 6H). ¹³C NMR (75 MHz, CDCl₃): $\delta$ (ppm) = 153.0, 150.2, 146.3, 145.1, 144.9, 143.5, 142.9, 132.0, 126.1 (2C), 125.6 (2C), 43.1 (3C), 36.7, 36.4 (3C), 28.9 (3C). HRMS-DCI-CH₄: *m/z* = 358.2040 ([M + H]⁺ calcd for C₂₂H₂₄N₅: 358.2032).

## 2.11. 2-[4-(4-Tricyclo[3.3.1.1³,⁷]dec-1-ylphenyl)-4H-1,2,4-triazol-3-yl]pyrimidine (L4)

General procedure involving 2-(1,3,4-oxadiazol-2-yl)pyrimidine. Ligand L4 was obtained after column chromatography (silica gel, DCM/ MeOH 99:1) as a white solid (180 mg, 50%). ¹H NMR (300 MHz, CDCl₃): $\delta$ (ppm) = 8.73 (d, J = 4.9 Hz, 2H), 8.35 (s, 1H), 7.46-7.40 (m, 2H), 7.30-7.26 (m, 1H), 7.24-7.19 (m, 2H), 2.16-2.09 (m, 3H), 1.94-1.92 (m, 6H), 1.86-1.71 (m, 6H). ¹³C NMR (126 MHz, CDCl₃): $\delta$ (ppm) = 157.5 (2C), 156.2, 152.7, 151.5, 145.9, 132.5, 126.0 (2C), 125.5 (2C),
121.0, 43.2 (3C), 36.7, 36.5 (3C), 28.9 (3C). HRMS-ESI⁺: *m/z* = 358.2027 ([M + H]⁺ calcd for C₂₂H₂₄N₅: 358.2032).

## 2.12. General procedure for complexation

A mixture of ligand **Ln** (200 mg, 0.56 mmol, 1 eq.) and [Re(CO)₅Cl] (226 mg, 0.59 mmol, 1.05 eq.) in methanol (6 mL) was stirred for 16 h at 65 °C. The reaction mixture was cooled to room temperature and the precipitate was collected by filtration, washed with methanol, and dried under vacuum. The crude product was purified by column chromatography (DCM/MeOH 99:1 v/v) to give the expected complex **Re-Ln-Cl**.

## 2.13. [Re(CO)₃(L1)Cl] (Re-L1-Cl)

General procedure involving ligand **L1**. The expected complex was obtained as a yellow solid (340 mg, 85%). ¹H NMR (300 MHz, CDCl₃): *δ* (ppm) = 9.16 (dd, *J* = 5.1, 1.8 Hz, 1H), 8.41 (s, 1H), 7.73–7.67 (m, 2H), 7.61 (dd, *J* = 8.7, 5.1 Hz, 1H), 7.51 (d, *J* = 8.3 Hz, 2H), 7.26 (dd, *J* = 8.7, 1.8 Hz, 1H), 2.20–2.17 (m, 3H), 2.05–1.98 (m, 6H), 1.90–1.75 (m, 6H). ¹³C NMR (126 MHz, CDCl₃): *δ* (ppm) = 196.3, 187.3, 156.7, 152.7, 149.4, 147.1, 128.8 (2C), 128.0 (2C), 127.8, 126.1 (3C), 43.2 (3C), 37.0, 36.6 (3C), 28.8 (3C). HRMS-ESI⁺: *m/z* = 681.1379 ([M + NH₄]⁺ calcd for C₂₅H₂₇N₆O₃Cl¹⁸⁷Re: 681.1391). FT-IR (ATR): *ν*(CO) = 2024, 1913, 1884 cm⁻¹.

## 2.14. [Re(CO)₃(L2)Cl] (Re-L2-Cl)

General procedure involving ligand **L2**. The expected complex was obtained as an orange solid (360 mg, 90%). ¹H NMR (300 MHz, CDCl₃): *δ* (ppm) = 9.70 (d, *J* = 1.3 Hz, 1H), 8.84 (d, *J* = 5.2 Hz, 1H), 8.39 (s, 1H), 7.71 (d, *J* = 8.7 Hz, 2H), 7.47 (d, *J* = 8.7 Hz, 2H), 6.98 (dd, *J* = 5.2, 1.4 Hz, 1H), 2.21–2.18 (m, 3H), 2.01–1.99 (m, 6H), 1.90–1.77 (m, 6H). ¹³C NMR (126 MHz, CDCl₃): *δ* (ppm) = 196.4, 194.9, 187.2, 162.2, 159.8, 156.8, 153.6, 151.4, 147.6, 128.8 (2C), 128.1 (3C), 117.5, 43.2 (3C), 37.1, 36.6 (3C), 28.8 (3C). HRMS-ESI⁺: *m/z* = 628.1366 ([M – Cl]⁺ calcd for C₂₅H₂₃N₅O₃¹⁸⁷Re: 628.1358). FT-IR (ATR): *ν*(CO) = 2024, 1915 (sh), 1904, 1890 cm⁻¹.

## 2.15. [Re(CO)₃(L3)Cl] (Re-L3-Cl)

General procedure involving ligand **L3**. The expected complex was obtained as an orange solid (220 mg, 57%). ¹H NMR (300 MHz, CDCl₃): *δ* (ppm) = 9.08 (dd, *J* = 1.4, 2.9 Hz, 1H), 8.75 (d, *J* = 2.9 Hz, 1H), 8.40 (d, *J* = 1.4 Hz, 1H), 8.37 (s, 1H), 7.71 (d, *J* = 8.7 Hz, 2H), 7.50 (d, *J* = 8.7 Hz, 2H), 2.21–2.17 (m, 3H), 2.01–1.98 (m, 6H), 1.89–1.78 (m, 6H). ¹³C NMR: (126 MHz, CDCl₃) *δ* (ppm) = 196.5, 195.0, 186.9, 179.1, 156.7, 153.4, 148.5 (2C), 147.7 (2C), 143.3 (2C), 140.7, 129.0, 128.1, 43.1 (3C), 37.0, 36.6 (3C), 28.8 (3C). HRMS-ESI⁺: *m/z* = 708.1035 ([M + HCOO]⁻ calcd for C₂₆H₂₄N₅O₅Cl¹⁸⁷ Re: 708.1023). FT-IR (ATR): *ν*(CO) = 2036, 1911, 1905 (sh), 1875 cm⁻¹.

## 2.16. [Re(CO)₃(L4)Cl] (Re-L4-Cl)

General procedure involving ligand **L4**. The expected complex was obtained as a yellow solid (331 mg, 83%). ¹H NMR (500 MHz, CDCl₃): *δ* (ppm) = 9.22 (dd, *J* = 5.6, 2.2 Hz, 1H, H₁), 8.76 (dd, *J* = 4.9, 2.2 Hz, 1H), 8.36 (s, 1H), 7.59–7.55 (m, 2H), 7.47 (dd, *J* = 5.6, 4.9 Hz, 1H), 7.45–7.40 (m, 2H), 2.20–2.12 (m, 3H), 2.05–1.92 (m, 6H), 1.87–1.76 (m, 6H). ¹³C NMR (126 MHz, CDCl₃) *δ* (ppm) = 196.7, 194.9, 187.6, 160.8, 158.2, 156.1, 154.9, 152.0, 147.5, 129.6, 126.5 (2C), 125.9 (2C), 122.5, 43.2, 36.8, 36.7, 28.9. HRMS-ESI⁺: *m/z* = 681.1380 ([M + NH₄]⁺ calcd for C₂₅H₂₇N₆O₃Cl¹⁸⁷Re: 681.1391). FT-IR (ATR): *ν*(CO) = 2034, 1907, 1889 (sh) cm⁻¹.

## 2.17. General procedure for ancillary ligand substitution

A mixture of complex [Re(CO)₃(Ln)Cl] (70 mg, 0.11 mmol, 1 eq.) and silver trifluoroacetate (0.28 mmol, 2.5 eq.) in ethanol (30 mL), carefully protected from light, was heated at 80 °C for 5 h under argon atmosphere. After cooling to room temperature, the mixture was filtered using a PTFE Millipore (0.2 µm) filter. Then, triphenylphosphine (277 mg, 1.1 mmol, 10 eq.) was added to the filtrate, and the solution was heated overnight at 80 °C. After cooling at room temperature, the solution was filtered. The filtrate was concentrated under reduced pressure. Purification of the crude product by chromatography on alumina column (DCM/MeOH 99:1 v/v) protected from light gave the expected compound after solvent evaporation under vacuum. A microcrystalline powder was obtained by dissolving the compound in a minimum amount of methanol, covered by 10 times its volume of diethylether.

## 2.18. [Re(CO)₃(L1)PPh₃][SO₃CF₃] (Re-L1-TPP)

General procedure involving complex [Re(CO)₃(L1)Cl]. The expected complex was obtained as dark yellow solid (84 mg, 77%). ¹H NMR (500 MHz, CDCl₃): *δ* (ppm) = 9.14 (dd, *J* = 5.1, 1.7 Hz, 1H), 8.34 (s, 1H), 8.11 (dd, *J* = 5.1, 8.8 Hz, 1H), 7.80–7.66 (m, 2H), 7.50–7.42 (m, 3H), 7.42–7.32 (m, 8H), 7.25–7.19 (m, 7H), 2.24–2.12 (m, 3H), 2.01–1.95 (m, 6H), 1.90–1.75 (m, 6H). ¹³C NMR (126 MHz, CDCl₃): *δ* (ppm) = 193.8, 193.6, 184.9, 156.8, 154.4, 152.4, 148.7, 147.7, 133.3 (d, *J* = 11 Hz, 6C), 131.4 (d, *J* = 2.5 Hz, 3C), 130.9, 129.1 (d, *J* = 10.8 Hz, 6C), 128.9 (d, *J* = 46.5 Hz, 3C), 128.1 (2C), 127.7, 127.5 (2C), 125.6, 42.9 (3C), 37.0, 36.6 (3C), 28.9 (3C). ³¹P NMR (121 MHz, CDCl₃): *δ* (ppm) = 16.81. HRMS-ESI⁺: *m/z* = 890.2268 ([M]⁺ calcd for C₄₃H₃₈N₅O₃¹⁸⁷Re: 890.2270). FT-IR (ATR): *ν*(CO) = 2042, 1966, 1951 cm⁻¹. Anal. calcd (%) for C₄₃.₃H₃₇.₃Cl₀.₆F₃N₅O₆PrES (Re-L1-TPP + 0.3 CHCl₃): C 48.98, H 3.54, N 6.60; found: C 49.14, H 3.18, N 6.47.

## 2.19. [Re(CO)₃(L2)PPh₃][SO₃CF₃] (Re-L2-TPP)

General procedure involving complex [Re(CO)₃(L2)Cl]. The expected complex was obtained as a yellow/orange solid (92 mg, 84%). ¹H NMR (500 MHz, CDCl₃): *δ* (ppm) = 9.16 (d, *J* = 1.2 Hz, 1H), 8.86 (dd, *J* = 5.3, 0.8 Hz, 1H), 8.37 (s, 1H), 7.66 (d, *J* = 8.5 Hz, 2H), 7.48–7.43 (m, 3H), 7.39–7.35 (m, 8H), 7.27–7.17 (m, 6H), 6.99 (dd, *J* = 5.2, 1.3 Hz, 1H), 2.20–2.09 (m, 3H), 2.00–1.90 (m, 6H), 1.90–1.76 (m, 6H). ¹³C NMR (126 MHz, CDCl₃): *δ* (ppm) = 193.6 (d, *J* = 8.2 Hz), 192.1 (d, *J* = 9.5 Hz), 182.8 (d, *J* = 54.2 Hz), 160.5 (C₁), 160.1, 155.7, 152.6, 149.2, 147.5, 133.6 (d, *J* = 10.1 Hz, 6C), 130.6 (d, *J* = 2.3 Hz, 3C), 128.6 (d, *J* = 9.9 Hz, 6C), 127.9 (d, *J* = 46.6 Hz, 3C), 127.1 (2C), 121.2 (2C), 118.6, 118.4, 43.0 (3C), 37.0, 36.7 (3C), 28.9 (3C). ³¹P NMR (121 MHz, CDCl₃): *δ* (ppm) = 17.27. HRMS-ESI⁺: *m/z* = 890.2274 ([M]⁺ calcd for C₄₃H₃₈N₅O₃¹⁸⁷Re: 890.2270). FT-IR (ATR): *ν*(CO) = 2041, 1972, 1942 cm⁻¹. Anal. calcd (%) for C₄₄H₃₈.₂Cl₀.₆F₃N₅O₆PrES (Re-L2-TPP + 0.2 CHCl₃): C 49.95, H 3.62, N 6.59; found: C 49.82, H 3.07, N 6.55.

## 2.20. [Re(CO)₃(L3)PPh₃][SO₃CF₃] (Re-L3-TPP)

General procedure involving complex [Re(CO)₃(L3)Cl]. The complex was obtained as a yellow/orange solid (78 mg, 71 %). ¹H NMR (500 MHz, CDCl₃): *δ* (ppm) = 8.68–8.65 (m, 1H), 8.60 (d, *J* = 3.1 Hz, 1H), 8.39 (s, 1H), 8.05 (d, *J* = 1.3 Hz, 1H), 7.65–7.57 (m, 2H), 7.47–7.40 (m, 3H), 7.35 (td, *J* = 7.8, 2.6 Hz, 7H), 7.25–7.19 (m, 7H), 2.25–2.05 (m, 3H), 2.05–1.93 (m, 6H), 1.91–1.75 (m, 6H). ¹³C NMR (126 MHz, CDCl₃): *δ* (ppm) = 194.4 (d, *J* = 7.7 Hz), 193.1 (d, *J* = 8.7 Hz), 183.4 (d, *J* = 59.3 Hz), 156.4, 153.4, 148.9, 147.8, 147.7, 143.9, 140.0, 133.2 (d, *J* = 10.9 Hz, 6C), 131.4 (d, *J* = 2.3 Hz, 3C), 129.2 (d, *J* = 10.0 Hz, 6C), 128.9 (d, *J* = 46.6 Hz, 3C), 128.5 (2C), 122.0, 119.4 (2C), 43.1 (3C), 36.9, 36.7 (3C), 28.9 (3C). ³¹P NMR (121 MHz, CDCl₃): *δ* (ppm) = 16.63. HRMS-ESI⁺: *m/z* = 890.2277 ([M]⁺ calcd for C₄₃H₃₈N₅O₃¹⁸⁷Re: 890.2270). FT-IR (ATR): *ν*(CO) = 2047, 1965, 1929 cm⁻¹. Anal. Calcd for
C₄₄H₃₈.₂Cl₀.₆F₃N₅O₆PReS (Re-L3-TPP + 0.2 CHCl₃): C 49.95, H 3.62, N 6.59; Found: C 49.84, H 3.75, N 6.36.

## 2.21. [Re(CO)₃(L4)PPh₃][SO₃CF₃] (Re-L4-TPP)

General procedure involving complex [Re(CO)₃(L4)Cl]. The expected complex was obtained as a green/yellow solid (87 mg, 79%). ¹H NMR (500 MHz, CDCl₃): δ (ppm) = 8.87 (m, 2H), 8.33 (s, 1H), 7.81 (dd, J = 5.7, 4.9 Hz, 1H), 7.55-7.49 (m, 2H), 7.49-7.41 (m, 3H), 7.41-7.34 (m, 6H), 7.26-7.18 (m, 6H), 7.17-7.11 (m, 2H), 2.20-2.09 (m, 3H), 1.99-1.91 (m, 6H), 1.86-1.74 (m, 6H). ¹³C NMR (126 MHz, CDCl₃): δ (ppm) = 193.8 (d, J = 7.5 Hz), 192.8 (d, J = 8.4 Hz), 184.7 (d, J = 59.88 Hz), 161.0, 160.1, 155.2, 154.0, 151.6, 148.0, 133.2 (d, J = 10.9 Hz, 6C), 131.6 (d, J = 2.3 Hz, 3C), 129.4 (d, J = 9.9 Hz, 6C), 128.7 (d, J = 46.5 Hz, 3C), 128.7, 126.6 (2C), 125.6 (2C), 125.2, 43.09 (3C), 36.8, 36.7 (3C), 28.9 (3C). ³¹P NMR (121 MHz, CDCl₃): δ (ppm) = 16.75. HRMS-ESI⁺: m/z = 890.2264 ([M]⁺ calcd for C₄₃H₃₈N₅O₃P¹⁸⁷Re: 890.2270). FT-IR (ATR): ν(CO) = 2040, 1954, 1922 cm⁻¹. Anal. calcd (%) for C₄₈H₄₈F₃N₅O₇PReS (Re-L4-TPP + C₄H₁₀O): C 51.79, H 4.35, N 6.29; found: C 51.67, H 3.86, N 6.29.

## 2.22. Crystallography

Crystallographic data were collected at low temperature (193(2) K), with an Oxford Cryosystems Cryostream 700+ Series device, on a Bruker APEX II Quazar diffractometer equipped with a 30 W air-cooled microfocus source using MoKα radiation (wavelength = 0.71073 Å) for Re-L4-TPP, and on a Bruker D8-Venture diffractometer equipped with a Photon III-C14 detector and a microfocus source with CuKα radiation (wavelength = 1.54178 Å) for Re-L1-TPP, Re-L2-TPP and Re-L3-TPP. Phi- and omega-scans were used. The space group was determined on the basis of systematic absences and intensity statistics. An empirical absorption correction was employed [56]. The structures were solved using an intrinsic phasing method (ShelXT) [57]. All non-hydrogen atoms were refined anisotropically using the least-square method on F² [58]. Hydrogen atoms were refined isotropically at calculated positions using a riding model. For Re-L3-TPP, the SQUEEZE [59] function of PLATON was used to remove the electron density contribution of the highly disordered solvent molecules from the model. Supplementary crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/.

ure/s. Selected crystallographic data are collected in Table 1.

## 2.23. Spectroscopy

All spectroscopic measurements in solutions were performed at 25 °C on undegassed solutions using temperature-controlled cells of 1 cm optical pathway. UV-visible absorption spectra were measured on Analytik Jena SPECORD S600 spectrophotometer and emission spectra were measured with a Xenius SAFAS spectrofluorometer. All emission spectra were corrected. The emission quantum yields (Φ<sub>em</sub>) in acetonitrile were determined using the classical formula:

$$ \Phi_{em\,s} = (A_r \times I_s \times n_s^2 \times \Phi_{em\,r}) / (A_s \times I_r \times n_r^2) \quad (1) $$

where subscripts *r* and *s* refer to the reference and to the sample of unknown quantum yield, respectively. Coumarin 153 (Φ<sub>em</sub> = 0.53) in ethanol was used as the standard reference [60]. *A* is the absorbance at the excitation wavelength, *I* the integrated emission intensity and *n* the refractive index of the solvent. The absorbance of the solutions was equal to or less than 0.04 and 0.06 for coumarin and complexes, respectively, at the excitation wavelength. The quantum yields are given with an error of 10 %, which includes the primary inner-filter effect that is low at the absorbances used [61]. There is no reabsorption with phosphorescence emission.

Emission decay curves were recorded using the time-correlated single photon counting method (TCSPC) on a Horiba Fluorolog 3-2 iHR320 spectrofluorometer equipped with a Nanoled-370 (λ<sub>ex</sub> = 371 nm). Emitted photons were detected at 90° by means of a Hamamatsu R928 photomultiplier. Emission was recorded near the maximum with a bandpass of 4 nm. The instrumental response was recorded directly before each decay curve, using the scattering of the sample at 370 nm. All analyses were recorded using the Datastation v2.7 software from Horiba. The decay curves were analyzed with reconstitution and global non-linear least-squares minimization method using DAS6 v6.8 software. The absorbance of the solutions at λ<sub>ex</sub> was lower than 0.1.

The photophysical rate constants were calculated as:

$$ k_r = \Phi_{em}/\tau \text{ and } k_{nr} = (1-\Phi_{em})/\tau \quad (2) $$

## 2.24. Photodecarbonylation

The photochemical reactivity was measured at 25 °C on magnetically-stirred undegassed solutions placed in 1 cm × 1 cm quartz

<table><caption>**Table 1**
Selected crystallographic data for complexes Re-Ln-TPP (n = 1-4).</caption><thead><tr><th></th><th>Re-L1-TPP</th><th>Re-L2-TPP</th><th>Re-L3-TPP</th><th>Re-L4-TPP</th></tr></thead><tbody><tr><td>Empirical formula</td><td>C<sub>43</sub>H<sub>38</sub>N<sub>5</sub>O<sub>3</sub>P Re, CF<sub>3</sub>O<sub>3</sub>S</td><td>C<sub>43</sub>H<sub>38</sub>N<sub>5</sub>O<sub>3</sub>P Re, CF<sub>3</sub>O<sub>3</sub>S</td><td>C<sub>43</sub>H<sub>38</sub>N<sub>5</sub>O<sub>3</sub>PRe, CF<sub>3</sub>O<sub>3</sub>S</td><td>C<sub>43</sub>H<sub>38</sub>N<sub>5</sub>O<sub>3</sub>P Re, CF<sub>3</sub>O<sub>3</sub>S</td></tr><tr><td>Formula weight</td><td>• 2 CH<sub>3</sub></td><td>• 2.5 CH<sub>3</sub></td><td>1039.03</td><td>• C<sub>4</sub>H<sub>10</sub>O</td></tr><tr><td>Crystal system</td><td>Triclinic</td><td>Triclinic</td><td>Monoclinic</td><td>Triclinic</td></tr><tr><td>Space group</td><td>P<span style="text-decoration: overline">1</span></td><td>P<span style="text-decoration: overline">1</span></td><td>P 2/n</td><td>P<span style="text-decoration: overline">1</span></td></tr><tr><td>Unit cell dimensions:</td><td></td><td></td><td></td><td></td></tr><tr><td>a (Å)</td><td>13.8783(5)</td><td>13.9958(8)</td><td>14.0973(8)</td><td>12.0573(4)</td></tr><tr><td>b (Å)</td><td>15.0184(6)</td><td>14.7388(8)</td><td>16.9156(9)</td><td>13.9636(4)</td></tr><tr><td>c (Å)</td><td>15.1331(6)</td><td>15.1196(8)</td><td>20.6834(11)</td><td>15.1424(6)</td></tr><tr><td>α (°)</td><td>70.845(2)</td><td>106.479(2)</td><td>90</td><td>86.9460(10)</td></tr><tr><td>β (°)</td><td>63.579(2)</td><td>90.098(2)</td><td>97.104(2)</td><td>74.2740(10)</td></tr><tr><td>γ (°)</td><td>67.367(2)</td><td>114.268(2)</td><td>90</td><td>69.3810(10)</td></tr><tr><td>Volume (Å<sup>3</sup>)</td><td>2559.80(18)</td><td>2701.4(3)</td><td>4894.4(5)</td><td>2294.28(14)</td></tr><tr><td>Z</td><td>2</td><td>2</td><td>4</td><td>2</td></tr><tr><td>Density (calculated) (Mg/m<sup>3</sup>)</td><td>1.658</td><td>1.644</td><td>1.410</td><td>1.611</td></tr><tr><td>Crystal size (mm<sup>3</sup>)</td><td>0.100 × 0.080 × 0.040</td><td>0.200 × 0.200 × 0.060</td><td>0.280 × 0.200 × 0.180</td><td>0.100 × 0.080 × 0.080</td></tr><tr><td>Reflections collected</td><td>49010</td><td>93519</td><td>65656</td><td>40116</td></tr><tr><td>Independent reflections</td><td>10123 [R(int) = 0.0672]</td><td>10699 [R(int) = 0.0540]</td><td>9687 [R(int) = 0.0503]</td><td>11390 [R(int) = 0.0263]</td></tr><tr><td>Restraints/parameters</td><td>348/705</td><td>838/887</td><td>0/551</td><td>0/597</td></tr><tr><td>Final R1 index I > 2σ(I)</td><td>0.0351</td><td>0.0472</td><td>0.0242</td><td>0.0225</td></tr><tr><td>wR2 (all data)</td><td>0.0871</td><td>0.1253</td><td>0.0635</td><td>0.0535</td></tr><tr><td>Largest diff. Peak and hole (e Å<sup>-3</sup>)</td><td>1.442 and -1.324</td><td>1.978 and -0.877</td><td>0.442 and -1.256</td><td>0.903 and -0.813</td></tr><tr><td>CCDC</td><td>2422941</td><td>2433942</td><td>2433943</td><td>2433944</td></tr></tbody></table>
fluorescence cells, using the Analytik Jena SPECTORD S600 absorption spectrophotometer equipped with a 385 nm LED for continuous irradiation. CO photorelease was considered to be complete when no further change in the UV–vis spectra was observed. The quantum yields of CO photoproduction ($\Phi_{CO}$) were determined with solutions where all photons were absorbed ($A \ge 2.5$) using the equations:

$$A_0 = C_0 \times \varepsilon_{\text{complex}} \qquad (3)$$

$$A_f = C_0 \times \varepsilon_{\text{photoproduct}} \qquad (4)$$

$$A_t = (C_0 - x) \varepsilon_{\text{complex}} + x \times \varepsilon_{\text{photoproduct}} \qquad (5)$$

$$N_x = C_0 \times (A_{t-} A_0) / (A_{f-} A_0) \times N_a \times \text{Dil} \qquad (6)$$

$$\Phi_{CO} = N_x / N_{abs} \qquad (7)$$

where $A_0$, $A_t$, $A_f$, are respectively the absorbance values at the maximum absorption wavelength of the MLCT band ($\lambda_{max}$ $^{MLCT}$) before, during (<10 % of the reaction), and after irradiation; $C_0$ is the initial concentration in complex, $\varepsilon_{complex}$ and $\varepsilon_{photoproduct}$ are the molar extinction coefficients at $\lambda_{max}$ $^{MLCT}$of the complex and photoproduct, x refers to the concentration of the photoproduct at time t; $N_x$ is the number of photolyzed molecules; $N_a$ is Avogadro number; Dil refers to the dilution of the measured solution; $N_{abs}$ is the number of absorbed photons. The number of photons received by the samples was measured to be $1.6 \times 10^{16}$ photons.s$^{-1}$ by actinometry of ferrioxalate ions [62]. The $k_{CO}$ values were calculated from $\ln(A_0/A)$ versus time (t) plots.

## 2.25. Singlet oxygen photoproduction

$N$, $N$-Dimethyl-4-nitrosoaniline (RNO) was purchased from Sigma Aldrich. Imidazole was purchased from Alfa Aesar. Stock solutions of RNO (12 mM) and imidazole (16.5 mM) were prepared separately. The absorbance at 385 nm of the compound of interest was adjusted between 0.3 and 0.5. After mixing, the final concentrations were ~ $7 \times 10^{-5}$ M for the Re(I) complex, $2 \times 10^{-5}$ M for RNO and $5.5 \times 10^{-3}$ M for imidazole. In situ irradiation at 385 nm was performed as described above, and the decrease in the absorption of RNO at 414 nm as a function of time was monitored by UV-visible spectroscopy. The experiment was performed in triplicate for each complex. The measurements were corrected by taking into account the same experiment conducted without RNO where only the degradation of the rhenium complex can be observed. The quantum yields were then calculated using the following equation:

$$\Phi_{\Delta s} = \Phi_{\Delta r} \times A_r / A_s \times S_s / S_r \qquad (8)$$

where $\Phi_{\Delta}$ is the singlet oxygen quantum yield, S is the corrected slope of the absorbance variation recorded at 414 nm, and A is the absorbance at the irradiation wavelength of 385 nm. Subscripts s and r refer to the sample and the reference, respectively. The reference compound was Phenalenone (Perinaphthenone purchased from Alfa Aesar) in acetonitrile ($\Phi_{\Delta} = 0.98$) [63].

## 2.26. Computational details

Geometry optimizations were performed with Gaussian 16 [64] using the B3LYP hybrid functional, def2-SVP basis set, GD3BJ dispersion correction [65,66], a 60-electron relativistic pseudopotential on rhenium and CPCM(acetonitrile) as implicit solvent model. Tight optimization criteria were used for all states but $^{3}$MC-loose for Re-Pyta-TPP. Stationary points were confirmed by frequency calculations at the same level of theory. Minimum energy crossing point (MECP) optimizations were performed using Orca 5 [67]. At convergence, single point energy calculations were run with Gaussian 16 to verify singlet-triplet degeneracy and to obtain a comparable electronic energy for the MECP.

Optimization of a transition state connecting $^{3}$MLCT and $^{3}$MC-tight bearing an elongated Re–C distance turned out to be extremely challenging and repeatedly failed due to the presence of another transition state in the neighbourhood. This TS is related to another type of $^{3}$MC minimum bearing elongated Re–N and Re–P distances, which is not expected to be involved in the CO photorelease mechanism.

TD-DFT modelling of the absorption properties consisted in computing 30 singlet states with Gaussian 16 at the same level of theory. DFT estimation of the emission wavelength was performed by running singlet closed-shell single point energy calculations at the relaxed $^{3}$MLCT geometry, with Gaussian 16, at the same level of theory.

# 3. Results and discussion

## 3.1. Synthesis of the complexes

The Re-Pyta-TPP complex was prepared as previously described [33]. The four new complexes were synthesized through a four-step sequence starting from the hydrazides (Scheme 1). 3-Pyridazinecarboxylic acid hydrazide [52], 2-pyridinecarboxylic acid hydrazide [55] and 4-pyrimidinecarboxylic acid hydrazide [53] were obtained according to reported procedures, whereas 2-pyrimidinecarboxylic acid hydrazide was prepared in two steps from 2-pyrimidinecarboxylic acid by esterification with methanol and concentrated sulfuric acid, and then action of hydrazine hydrate in ethanol. 2-(1,3,4-Oxadiazol-2-yl)pyrazine was prepared as reported in the literature [55], the other oxadiazole derivatives resulted from the cyclization of the corresponding hydrazides in the presence of triethylorthoformate and p-toluene sulfonic acid. Condensation of oxadiazoles with 4-(1-adamantyl)aniline [54] gave the ligands Ln (n = 1–4), which were subsequently reacted with [Re(CO)5Cl] to yield the corresponding tricarbonylrhenium(I) complexes with a chloride ligand as ancillary ligand (Re-Ln-Cl). Finally, substitution of this ancillary ligand by a triphenylphosphine provided the positively charged complexes Re-Ln-TPP. All new complexes were characterized by $^{1}$H, $^{13}$C, and $^{31}$P NMR, and by high-resolution mass spectrometry. The fac-[Re(CO)3] arrangement structure was confirmed by FT-IR, through the presence of the three stretching bands characteristic of carbonyl groups. All spectra are given in Fig. S1–S32. The crystal structures of the compounds obtained by XRD analysis (vide infra) are also a proof of structure. The purity of the complexes was checked by elemental microanalysis.

## 3.2. Crystal structures

Crystallization of Re-L1-TPP, Re-L2-TPP and Re-L3-TPP in chloroform/pentane and that of Re-L4-TPP in methanol/diethyl ether allowed growing crystals suitable for XRD analysis (Fig. 2). As expected, the coordination spheres of the four complexes exhibit a slightly distorted octahedral geometry. The rhenium atom is coordinated to three carbonyl groups in a fac configuration, two nitrogen atoms N1 and N2 of the organic ligand, and one phosphorus atom. The axial Re–C1 bond is systematically longer than the equatorial Re–C2 and Re–C3 bonds, because the TPP moiety directly competes for the rhenium electrons with the carbonyl ligand trans to the phosphine, which results in an elongation of the Re–C1 bond. While the bidentate ligand is almost perfectly flat in Re-Pyta-TPP, a small angle between 6° and 12.3° is noted between the diazine and triazole moieties of the Re-Ln-TPP complexes. The phenyl-triazole angle is comprised between 47.6° and 64.8°. Except for Re-L4-TPP, one of the TPP phenyl rings is positioned more or less parallel to the bidentate ligand, resulting in weak intramolecular $\pi$-$\pi$ stacking (Table S1).

## 3.3. Spectroscopy

The spectroscopic properties of the four new complexes were investigated in acetonitrile, a coordinating solvent commonly employed
<figure><img src="image_7.png" /><figcaption>**Scheme 1.** Synthesis of Re-Ln-TPP (n = 1–4) complexes. Conditions and reagents: (A) pTSA, triethylorthoformate, 130 °C, 6 h; (B) pTSA, xylenes, 140 °C, 24 h; (C) [Re(CO)₅Cl], MeOH, 65 °C, 16 h; (D) 1) Silver triflate, ethanol, 80 °C, 5 h, N₂; 2) Triphenylphosphine, 80 °C, overnight.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 2.** Molecular views of the asymmetric unit of complexes Re-Ln-TPP (n = 1–4). Displacement ellipsoids are drawn at 50 % probability and hydrogen atoms are omitted for the sake of clarity.</figcaption></figure>

in photochemistry, in which our compounds were readily soluble. As for **Re-Pyta-TPP**, the UV-vis absorption spectra showed intense bands below 300 nm and a band of moderate intensity between 350 and 400 nm. TD-DFT calculations allowed the high-energy bands to be attributed

to a combination of intraligand charge transfer (ILCT) and metal-to-ligand charge transfer (MLCT) transitions, while the low-energy band has strong MLCT character. The shape and maximum wavelength of the experimental spectra were close to the calculated ones (mean absolute
error MAE = 0.167 eV, Table S2). Compared to the reference complex **Re-Pyta-TPP**, the **Re-Ln-TPP** complexes display higher molar absorption coefficients. The maximum wavelength of their ¹MLCT bands is redshifted by 7 to 30 nm (Table 2). The additional nitrogen atom brought by the 2-pyrimidine moiety (complex **Re-L4-TPP**) only has a moderate effect on the absorption spectrum. However, when pyridine is replaced by 2-pyridazine, 4-pyrimidine and 2-pyrazine (complexes **Re-L1-TPP** to **Re-L3-TPP**), the additional nitrogen atom causes part of the MLCT band to expand into the visible range (Fig. 3a).

The presence and position of the second nitrogen atom also impact significantly the luminescence properties. When illuminated by a hand-held UV lamp, solutions of the **Re-Ln-TPP** complexes emitted light, the colour of which spanned from yellow to orange, while **Re-Pyta-TPP** emitted in the green. This observation was confirmed by a spectral shift of 74 nm of the emission spectra when going from **Re-pyta-TPP** ($\lambda_{em}$ = 536 nm) to **Re-L2-TPP** ($\lambda_{em}$ = 610 nm) (Fig. 3b). The computed emission wavelengths match both the absolute experimental data (MAE = 0.035 eV, Table S2) and the relative ordering, which is the same as for absorption: **Re-Pyta-TPP** < **Re-L4-TPP** < **Re-L1-TPP** < **Re-L3-TPP** < **Re-L2-TPP**. Judging from the Mulliken spin density on the metal, which is lower than 1 as it would be for a pure MLCT state, the lowest triplet state has various amounts of ligand centered (LC) character depending on the nature of the bidentate ligand (Table S3). The particle is always purely located on that ligand, and the hole has mixed metal/ligand character. The ³LC character varies as **Re-L1-TPP** < **Re-L2-TPP** < **Re-L4-TPP** < **Re-L3-TPP** < **Re-pyta-TPP**. The luminescence quantum yields were less than or equal to 10 % in aerated acetonitrile, and they decreased as the emission wavelength shifted to the red. In argon-flushed solvent, these values were increased by a factor 1.3 to 1.9. The decays were mono-exponential, with lifetimes between 315 and 618 ns, confirming that the emitted light results indeed from phosphorescence (Fig. S33). The analysis of the rate constants shows that **Re-L2-TPP** is the one that deactivates most effectively on a non-radiative mode, according to the energy gap law [68], but no correlation with the emission wavelength could be found for the other complexes. Similarly, no clear trend appears between the rate constants and the ³LC character. All complexes emit weakly and the imprecision of quantum yield measurements can affect the reliability of rate constants. As for chlorido complexes, the common feature of TPP complexes is their tendency to dissipate excitation energy through vibrations of the bidentate pincer and CO ligands, interactions with surrounding water molecules [69–71], and possible rotation of the phenyl groups around single bonds [72]. Of course, additional photochemical pathways that open up for TPP derivatives, may also contribute to the dissipation of excitation energy.

### 3.4. CO-Photorelease

In the dark, all the complexes were stable in acetonitrile solution for 2 weeks. However, irradiation at 385 nm quickly induced an irreversible

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 3.** Absorption (a) and normalized emission spectra (b) of **Re-Ln-TPP** (n = 1–4) and **Re-Pyta-TPP** complexes in acetonitrile. Complex concentrations for absorption: ~1 × 10⁻⁴ M. Complex concentrations for emission: ~4 × 10⁻⁵ M, $\lambda_{ex}$ = 370 nm.</figcaption></figure>

modification of the absorption spectrum: the ¹MLCT band progressively vanished while a new band situated at longer wavelength appeared (Fig. 4 and Fig. S34). The presence of clear isosbestic points suggested the formation of only one photoproduct. Interestingly, as shown in Fig. 5, the photoproducts were almost not emissive. They were analyzed by mass spectrometry, which clearly showed the disappearance of the starting complex, and the appearance of a signal attributable to a new species, i.e. [M + CH₃CN – CO], and by FT-IR spectroscopy where the bands corresponding to the three carbonyl groups vanished in favor of a two-peak signal (Fig. S35–38). The photoproducts were therefore identified as decarbonylated photoCORMs, where one carbonyl ligand has been replaced by an acetonitrile molecule. Qualitatively, the photoreaction was similar to that already observed for **Re-Pyta-TPP**. However, it was much slower, because it was completed in less than 5

<table><caption>**Table 2** Spectroscopic and photochemical data for the five complexes in acetonitrile. Calculated ($\lambda_{abs-calc}$) and measured ($\lambda_{abs}$) maximum absorption wavelength for the low energy MLCT band. Molar extinction coefficient ($\varepsilon$). Calculated ($\lambda_{em-calc}$) and measured maximum emission wavelength ($\lambda_{em}$), phosphorescence quantum yield ($\Phi_{em}$), emission lifetime ($\tau$) and chi square ($\chi^2$) values, photophysical radiative ($k_r$) and non-radiative ($k_{nr}$) rate constants.</caption><thead><tr><th></th><th>λ<sub>abs-calc</sub><br>(nm)</th><th>λ<sub>abs</sub><br>(nm)</th><th>ε<br>(M<sup>-1</sup> cm<sup>-1</sup>)</th><th>λ<sub>em-calc</sub><br>(nm)</th><th>λ<sub>em</sub><br>(nm)</th><th>Φ<sub>em</sub></th><th>τ<br>(ns)</th><th>χ<sup>2</sup></th><th>k<sub>r</sub><br>(10<sup>5</sup> s<sup>-1</sup>)</th><th>k<sub>nr</sub><br>(10<sup>5</sup> s<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="2">Re-L1-TPP</td><td>-</td><td>236</td><td>26200</td><td rowspan="2">605</td><td rowspan="2">598</td><td rowspan="2">0.07</td><td rowspan="2">566</td><td rowspan="2">1.43</td><td rowspan="2">1.24</td><td rowspan="2">16.43</td></tr><tr><td>382</td><td>365</td><td>6150</td></tr><tr><td rowspan="2">Re-L2-TPP</td><td>-</td><td>236</td><td>25800</td><td rowspan="2">610</td><td rowspan="2">610</td><td rowspan="2">0.03</td><td rowspan="2">315</td><td rowspan="2">1.21</td><td rowspan="2">0.95</td><td rowspan="2">30.79</td></tr><tr><td>387</td><td>370</td><td>5040</td></tr><tr><td rowspan="2">Re-L3-TPP</td><td>-</td><td>236</td><td>25100</td><td rowspan="2">607</td><td rowspan="2">604</td><td rowspan="2">0.10</td><td rowspan="2">678</td><td rowspan="2">1.42</td><td rowspan="2">1.47</td><td rowspan="2">13.27</td></tr><tr><td>384</td><td>370</td><td>4930</td></tr><tr><td rowspan="2">Re-L4-TPP</td><td>-</td><td>236</td><td>27200</td><td rowspan="2">593</td><td rowspan="2">570</td><td rowspan="2">0.06</td><td rowspan="2">352</td><td rowspan="2">1.21</td><td rowspan="2">1.70</td><td rowspan="2">26.70</td></tr><tr><td>373</td><td>347</td><td>4830</td></tr><tr><td rowspan="2">Re-Pyta-TPP</td><td>-</td><td>236</td><td>20200</td><td rowspan="2">550</td><td rowspan="2">536</td><td rowspan="2">0.06</td><td rowspan="2">583</td><td rowspan="2">1.29</td><td rowspan="2">1.03</td><td rowspan="2">16.12</td></tr><tr><td>356</td><td>340</td><td>3800</td></tr></tbody></table>
<figure><img src="image_11.png" /><figcaption>**Fig. 4.** Evolution of the UV–vis absorption spectrum of Re-L1-TPP (~ 1 × 10⁻⁴ M) in acetonitrile upon irradiation at 385 nm. Inset: Variation of absorbance at 438 nm as a function of time.</figcaption></figure>

min for Re-Pyta-TPP, and in 30 min to 3 h for the Re-Ln-TPP complexes.

The rates of CO photorelease ($k_{CO}$) were determined from the UV-visible absorption measurements (Table 3). These measurements are highly dependent on solution concentrations and irradiation equipment. However, after correction for absorbance at excitation wavelength ($k_{CO}^{cor}$), they can be used to compare the five complexes irradiated in the same conditions. It can be seen that $k_{CO}^{cor}$ varies by more than one order of magnitude between Re-Pyta-TPP and the least photoreactive Re-L2-TPP. The comparison of the CO release quantum yields $\Phi_{CO}$, which should be independent of the experimental conditions, confirmed this observation. It appeared that the complexes release CO with very different efficiencies, decreasing in the order: Re-Pyta-TPP >> Re-L4-TPP > Re-L1-TPP ≈ Re-L3-TPP >> Re-L2-TPP. The position of the additional nitrogen atom is therefore crucial for the photochemical reactivity. Interestingly, it can be noted that the CO-photorelease ability decreases as the ¹MLCT absorption band and the emission spectra shift towards longer wavelengths, as illustrated in Fig. 5. This observation suggests a relationship between the energy level of the absorbing ¹MLCT state, that of the emitting ³MLCT state, and the efficiency of the CO-photoreleasing process.

To shed light on the relationship between ligand isomer and properties, we embarked on exploring the topology of the lowest triplet potential energy surface (³PES) for three representative behaviours

<caption>**Table 3**</caption>

Photochemical data of the complexes in acetonitrile irradiated at 385 nm. Photodecarbonylation time ($t_{CO}$), quantum yield ($\Phi_{CO}$), rate constant ($k_{CO}$) and rate constant corrected for absorbance variations between samples at the excitation wavelength ($k_{CO}^{cor}$); Quantum yield of singlet oxygen photoproduction ($\Phi_{\Delta}$).

<table><thead><tr><th></th><th>t<sub>CO</sub><br>(min)</th><th>k<sub>CO</sub><br>(min<sup>-1</sup>)</th><th>k<sub>CO</sub><sup>cor</sup><br>(min<sup>-1</sup>)</th><th>Φ<sub>CO</sub></th><th>Φ<sub>Δ</sub></th></tr></thead><tbody><tr><td>Re-L1-TPP</td><td>≈ 30</td><td>2.8 ×<br>10<sup>-2</sup></td><td>2.5 ×<br>10<sup>-2</sup></td><td>0.0041 ± 0.0009</td><td>0.06 ± 0.04</td></tr><tr><td>Re-L2-TPP</td><td>≈ 180</td><td>4.5 ×<br>10<sup>-3</sup></td><td>4.2 ×<br>10<sup>-3</sup></td><td>0.0015 ± 0.0004</td><td>0.11 ± 0.04</td></tr><tr><td>Re-L3-TPP</td><td>≈ 60</td><td>1.9 ×<br>10<sup>-2</sup></td><td>1.8 ×<br>10<sup>-2</sup></td><td>0.0038 ± 0.0011</td><td>0.05 ± 0.03</td></tr><tr><td>Re-L4-TPP</td><td>≈ 30</td><td>4.4 ×<br>10<sup>-2</sup></td><td>2.8 ×<br>10<sup>-2</sup></td><td>0.0204 ± 0.0003</td><td>0.03 ± 0.02</td></tr><tr><td>Re-Pyta-<br>TPP</td><td>&lt;5</td><td>1.5 ×<br>10<sup>-1</sup></td><td>4.4 ×<br>10<sup>-2</sup></td><td>0.0603 ± 0.0011</td><td>0.04 ± 0.02</td></tr></tbody></table>

observed experimentally, i.e. Re-Pyta-TPP, the most CO-photoreleasing case ($\Phi_{CO} = 6\%$); Re-L1-TPP, one of the least CO-photoreleasing cases ($\Phi_{CO} = 0.41\%$); and Re-L4-TPP, the intermediate case ($\Phi_{CO} = 2\%$).

Fig. 6 schematically shows three channels by which the ³MLCT state, populated after light absorption and intersystem crossing, can deactivate: phosphorescence, ¹O₂ sensitization, or CO loss via population of metal-centered (MC) excited states. For each complex, we therefore started by optimizing ¹GS, ³MLCT, ³MC and the minimum energy crossing point shown as ³/₁MECP in Fig. 6.

Inspection of the selected parameters characterizing the minima (Table 4 for Re-L4-TPP, Table S4 for Re-Pyta-TPP and Table S5 for Re-

<figure><img src="image_12.png" /><figcaption>**Fig. 6.** Schematic illustration of the three channels for ³MLCT state depopulation.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 5.** Summary of the CO-photoreleasing activities of Re-Ln-TPP complexes in acetonitrile and images of the solutions illuminated by a UV lamp (365 nm) before (a) and after (b) irradiation at 385 nm (Ad = adamantyl).</figcaption></figure>
L1-TPP), particularly the Re–C distance, led us to search for another $ ^{3}MC $ minimum in the vicinity of the MECP. Indeed, a local minimum was found, which we called $ ^{3}MC- $ loose to distinguish it from $ ^{3}MC-tight $ bearing a much shorter Re–C distance. The other noteworthy difference between $ ^{3}MC-tight $ and $ ^{3}MC- $ loose is the shift in position and orientation of the CO ligand, as seen in the selected angles (Table 4) and in Fig. 7.

During CO photorelease, the expected sequence is $ ^{3}MLCT-^{3}MC-tight-^{3}MC- $ loose-MECP–$ ^{1}PES $ followed by solvent capture on the $ ^{1}PES $ [73,74], even though the MECP appears before $ ^{3}MC- $ loose in Fig. 6 due to its shorter Re–C1 distance (the reaction coordinate is multidimensional and changes at each step, but is shown as Re–C1 for simplicity). The triplet potential energy profiles are shown in Fig. 8 for comparison of the selected complexes. The reference complex, Re-Pyta-TPP, also being the most photoreleasing, displays the flatter surface, in particular the smallest $ ^{3}MLCT-^{3}MC-tight $ energy gap (0.22 eV) and lowest-lying MECP (0.33 eV). On the other hand, the least photoreleasing complex of the three, Re-L1-TPP, displays the highest-lying MECP, which rationalizes their relative photoreactivities.

### 3.5. Photosensitization

Finally, the photoproduction of singlet oxygen was measured for all complexes in acetonitrile (Table 3). A variant of the popular method where histidine is oxidized by singlet oxygen and then bleaches N,N-dimethyl-4-nitrosoaniline (RNO) [75] was used, using imidazole instead of histidine for solubility reasons [18]. The decrease of the RNO absorption at 414 nm as a function of time was monitored by UV–visible spectroscopy. $ ^{1}O_{2} $ photoproduction was found to increase in the order: Re-L4-TPP < Re-Pyta-TPP ≈ Re-L3-TPP ≈ Re-L1-TPP < Re-L2-TPP, with quantum yields varying between 3% for Re-L4-TPP and 11% for Re-L2-TPP. The latter complex produced almost two to three times more singlet oxygen as Re-Pyta-TPP, Re-L1-TPP or Re-L3-TPP ($ \Phi_{\Delta} = 4–6 $ %).

As shown in Fig. 6, singlet oxygen is generated by energy transfer between the excited complex and triplet oxygen [16]. The energy required (94.3 kJ.mol$^{-1}$) to achieve $ ^{3}O_{2} $ → $ ^{1}O_{2} $ excitation, corresponding to a transition of 1270 nm/0.98 eV, is lower than that of the $ ^{3}MLCT $ state of all complexes (ca. 2 eV, as estimated from the maxima of phosphorescence emission). From an energy point of view, all our complexes are therefore capable of producing singlet oxygen, but some are more efficient than others. This bimolecular reaction should be favoured by a long triplet state lifetime [7,17,76]. In the present case, no direct link was found between $ ^{1}O_{2} $ production efficiency and the emission lifetimes $ \tau $, which reflect the triplet state lifetimes. The difference between the $ \tau $ values may be too small to have a significant influence on $ ^{1}O_{2} $ production. Many good Re(I) photosensitizers have emission lifetimes in the microsecond range [7,11]. The rather short lifetimes of our complexes can explain that they are moderately efficient. A literature review shows that Re(I) complexes with lifetimes between 100 and 400 ns have a modest photosensitizing capacity [7,76], but some are much better [77]. It is clear that many other parameters come into play [16]. In the relationship that defines the quantum yield of $ ^{1}O_{2} $ production ($ \Phi_{\Delta} $):

$$ \Phi_{\Delta} = \Phi_{T} k_{en}[O_{2}]/(k_{r} + k_{nr} + k_{q}[O_{2}]) \quad (9) $$

$ \Phi_{T} $ is the quantum yield of triplet formation, $ k_{en} $ is the rate constant of energy transfer, $ k_{r} $ and $ k_{nr} $ are the radiative and non-radiative rate

<table><caption>**Table 4**
Selected parameters for complex Re-L4-TPP: Mulliken spin density on rhenium (SD), distance between metal and CO to be lost, and selected angles.</caption><thead><tr><th></th><th><sup>1</sup>GS</th><th><sup>3</sup>MLCT</th><th><sup>3</sup>MC-tight</th><th>MECP</th><th><sup>3</sup>MC-loose</th></tr></thead><tbody><tr><td>SD</td><td>0</td><td>0.61</td><td>1.05</td><td>1.27</td><td>1.19</td></tr><tr><td>Re-C1 (Å)</td><td>1.966</td><td>2.028</td><td>2.550</td><td>3.189</td><td>3.729</td></tr><tr><td>P-Re-C1 (°)</td><td>177.7</td><td>174.6</td><td>170.0</td><td>155.2</td><td>149.5</td></tr><tr><td>Re-C1-O1 (°)</td><td>179.1</td><td>179.2</td><td>137.6</td><td>123.7</td><td>112.7</td></tr></tbody></table>

<figure><img src="image_14.png" /><figcaption>**Fig. 7.** Topology of the lowest $ ^{3}PES $ for complex Re-L4-TPP, focusing on the core of the complex. Energy gaps relative to the $ ^{3}MLCT $ minimum in eV.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 8.** Comparison of the triplet potential energy profiles for Re-Pyta-TPP (most photoreleasing; grey dashed line), Re-L4-TPP (intermediate; blue solid line), and Re-L1-TPP (least photoreleasing; green dotted line). Energy gaps relative to the $ ^{3}MLCT $ minimum in eV. $ ^{3}MLCT $ energies are all set to zero for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

constants, respectively, and $ k_{q} $ is the sum of all rate constants for the quenching of the excited triplet state T by $ O_{2} $ (including formation of $ ^{1}O_{2} $ by energy transfer, formation of superoxide anion $ O_{2}^{-} $, and deactivation) [78]. However, this relationship cannot be applied as it is in our case. In fact, when decarbonylation is efficient, the decarbonylated photoproduct contributes significantly to $ ^{1}O_{2} $ generation, but its contribution is not taken into account by the photophysical constants $ k_{r} $ and $ k_{nr} $ that are exclusively measured on the parent photoCORM. Similarly, the $ \Phi_{\Delta} $ value reflects the photoproduction of $ ^{1}O_{2} $ by two different species, the proportion of which vary over time. The entanglement of multiple deactivation processes impedes the rationalization of the behaviour of this set of complexes. Nevertheless, our study clearly shows that in the Re-Ln-TPP series, the photosensitizing capacity increases when the emission is shifted to long wavelengths. It is envisaged that future work will examine a larger number of complexes and explore in depth the $ ^{1}O_{2} $ generation process.

### 4. Conclusion

This joint experimental and theoretical approach showed that the presence of an additional nitrogen atom in the pyridyl ring of our photoCORMs is sufficient in most cases to extend the absorption spectrum into the visible. The substitution pattern plays a major role on the spectroscopic and photochemical properties in solution, and there is a direct relationship between these properties. The efficiency of singlet oxygen production increases for long-wavelength absorbing and emitting compounds. Indeed, it has been shown theoretically that the absorption of Re(I) complexes can be redshifted while retaining good sensitizing properties [14], and this has been confirmed experimentally
[8,9,11,16]. On the other hand, there is a limit beyond which the energy content of the ³MLCT state is insufficient to cause photo-decarbonylation. Besides, photosensitization is generally favoured in complexes with long triplet lifetimes, which should not be the case for CO photorelease. Consequently, it is not easy to play on both sides of the coin, and the right balance must be found in view of the desired properties. This study shows that, from a practical point of view, the emission wavelength maximum can be used as indicator of the photo-decarbonylation and photosensitization efficiency in this series of compounds. It also reveals that by playing on fine parameters such as the insertion of a single nitrogen atom in a given position, it is possible to govern the CO¹O₂ photoproduction balance, in view of modulating the therapeutic efficiency of rhenium(I) photoCORMs.

## Funding sources

Agence Nationale de la Recherche (ANR) (#ANR-22-29-0008CE).

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

ANR is gratefully acknowledged for funding (PANTHERA project # ANR-22-29CE-0008). CALMIP is kindly acknowledged for providing HPC resources under project number p23013.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.saa.2025.126647.

## Data availability

Data will be made available on request.

## References

[1] L.C.-C. Lee, K.-K. Leung, K.K.-W. Lo, Recent development of luminescent rhenium (I) tricarbonyl polypyridine complexes as cellular imaging reagents, anticancer drugs, and antibacterial agents, Dalton Trans. 46 (2017) 16357–16380, https://doi.org/10.1039/c7dt03465b.

[2] S. Hostachy, C. Policar, N. Delsuc, Re(I) carbonyl complexes: multimodal platforms for inorganic chemical biology, Coord. Chem. Rev. 351 (2017) 172–188, https://doi.org/10.1016/j.ccr.2017.05.004.

[3] Q. Qi, Q. Wang, Y. Li, D.Z. Silva, M.E.L. Ruiz, R. Ouyang, B. Liu, Y. Miao, Recent development of rhenium-based materials in the application of diagnosis and tumor therapy, Molecules 28 (2023) 2733, https://doi.org/10.3390/molecules28062733.

[4] K. Schindler, F. Zobi, Anticancer and antibiotic rhenium tri- and Dicarbonyl complexes: current research and future perspectives, Molecules 27 (2022) 539, https://doi.org/10.3390/molecules27020539.

[5] A. Frei, M. Amado, M.A. Cooper, M.A.T. Blaskovich, Light-activated rhenium complexes with dual mode of action against Bacteria, Chem. Eur. J. 26 (2020) 2852–2858, https://doi.org/10.1002/chem.201904689.

[6] M.N. Pinto, P.K. Mascharak, Light-assisted and remote delivery of carbon monoxide to malignant cells and tissues: Photochemotherapy in the spotlight, J. Photochem. Photobiol. C 42 (2020) 100341, https://doi.org/10.1016/j.jphotochemrev.2020.100341.

[7] S.C. Marker, S.N. MacMillan, W.R. Zipfel, Z. Li, P.C. Ford, J.J. Wilson, Photoactivated in vitro anticancer activity of rhenium(I) Tricarbonyl complexes bearing water-soluble phosphines, Inorg. Chem. 57 (2018) 1311–1331, https://doi.org/10.1021/acs.inorgchem.7b02747.

[8] A. Kastl, S. Dieckmann, K. Wähler, T. Völker, L. Kastl, A.L. Merkel, A. Vultur, B. Shannan, K. Harms, M. Ocker, W.J. Parak, M. Herlyn, E. Meggers, Rhenium complexes with visible-light-induced anticancer activity, ChemMedChem 8 (2013) 924–927, https://doi.org/10.1002/cmdc.201300060.

[9] K. Wähler, A. Ludewig, P. Szabo, K. Harms, E. Meggers, Rhenium complexes with red-light-induced anticancer activity, Eur. J. Inorg. Chem. (2014) 807–811, https://doi.org/10.1002/ejic.201301474.

[10] H.S. Liew, C.-W. Mai, M. Zulkefeli, T. Madheswaran, L.V. Kiew, N. Delsuc, M. L. Low, Recent emergence of rhenium(I) Tricarbonyl complexes as Photosensitizers for Cancer therapy, Molecules 25 (2020) 4176, https://doi.org/10.3390/molecules25184176.

[11] N. Manav, P.E. Kesavan, M. Ishida, S. Mori, Y. Yasutake, S. Fukatsu, H. Furuta, I. Gupta, Phosphorescent rhenium-dipyrrinates: efficient photosensitizers for singlet oxygen generation, Dalton Trans. 48 (2019) 2467–2478, https://doi.org/10.1039/c8dt04540b.

[12] X. Su, W.J. Wang, Q. Cao, H. Zhang, B. Liu, Y. Ling, X. Zhou, Z.W. Mao, A carbonic anhydrase IX (CAIX)-anchored rhenium(I) photosensitizer evokes Pyroptosis for enhanced anti-tumor immunity, Angew. Chem. Int. Ed. 61 (2022) e202115800, https://doi.org/10.1002/anie.202115800.

[13] X. Yi, J. Zhao, J. Sun, S. Guo, H. Zhang, Visible light-absorbing rhenium(I) tricarbonyl complexes as triplet photosensitizers in photooxidation and triplet-triplet annihilation upconversion, Dalton Trans. 42 (2013) 2062–2074, https://doi.org/10.1039/c2dt32420b.

[14] D. Álvarez, M.I. Menéndez, R. López, Computational Design of Rhenium(I) carbonyl complexes for anticancer photodynamic therapy, Inorg. Chem. 61 (2022) 439–455, https://doi.org/10.1021/acs.inorgchem.1c03130.

[15] Z.-Y. Pan, D.-H. Cai, L. He, Dinuclear phosphorescent rhenium(I) complexes as potential anticancer and photodynamic therapy agents, Dalton Trans. 49 (2020) 11583–11590, https://doi.org/10.1039/d0dt02424d.

[16] E. Wolcan, Photosensitized generation of singlet oxygen from rhenium(I) complexes: a review, Inorg. Chim. Acta 509 (2020) 119650, https://doi.org/10.1016/j.ica.2020.119650.

[17] L.D. Ramos, H. Moreira da Cruz, K.P. Morelli Frin, Photophysical properties of rhenium(I) complexes and photosensitized generation of singlet oxygen, Photochem. Photobiol. Sci. 16 (2017) 459–466, https://doi.org/10.1039/c6pp00364h.

[18] A. Leonidova, V. Pierroz, R. Rubbiani, J. Heier, S. Ferrari, G. Gasser, Towards cancer cell-specific phototoxic organometallic rhenium(I) complexes, Dalton Trans. 43 (2014) 4287–4294, https://doi.org/10.1039/c3dt51817e.

[19] I. Chakraborty, J. Jimenez, W.M.C. Sameera, M. Kato, P.K. Mascharak, Luminescent re(I) carbonyl complexes as trackable PhotoCORMs for CO delivery to cellular targets, Inorg. Chem. 56 (2017) 2863–2873, https://doi.org/10.1021/acs.inorgchem.6b02999.

[20] I. Chakraborty, S.J. Carrington, G. Roseman, P.K. Mascharak, Synthesis, structures, and CO release capacity of a family of water-soluble PhotoCORMs: assessment of the biocompatibility and their Phototoxicity toward human breast Cancer cells, Inorg. Chem. 56 (2017) 1534–1545, https://doi.org/10.1021/acs.inorgchem.6b02623.

[21] I. Chakraborty, S.J. Carrington, P.K. Mascharak, Design strategies to improve the sensitivity of photoactive metal carbonyl complexes (photoCORMs) to visible light and their potential as CO-donors to biological targets, Acc. Chem. Res. 47 (2014) 2603–2611, https://doi.org/10.1021/ar500172f.

[22] I. Chakraborty, S.J. Carrington, P.K. Mascharak, Photodelivery of CO by designed PhotoCORMs: correlation between absorption in the visible region and metal–CO bond Labilization in carbonyl complexes, ChemMedChem 9 (2014) 1266–1274, https://doi.org/10.1002/cmdc.201402007.

[23] S.J. Carrington, I. Chakraborty, J.M.L. Bernard, P.K. Mascharak, A Theranostic two-tone luminescent PhotoCORM derived from re(I) and (2-Pyridyl)-benzothiazole: trackable CO delivery to malignant cells, Inorg. Chem. 55 (2016) 7852–7858, https://doi.org/10.1021/acs.inorgchem.6b00511.

[24] T. Soboleva, L.M. Berreau, Tracking CO release in cells via the luminescence of donor molecules and/or their by-products, Isr. J. Chem. 59 (2019) 339–350, https://doi.org/10.1002/ijch.201800172.

[25] T.M. Kirse, I. Maisuls, M.V. Cappellari, E. Niehaves, J. Kösters, A. Hepp, U. Karst, E. Wolcan, C.A. Strassert, Neutral and cationic re(I) complexes with Pnictogen-based Coligands and tunable functionality: from phosphorescence to Photoinduced CO release, Inorg. Chem. 63 (2024) 4132–4151, https://doi.org/10.1021/acs.inorgchem.3c03886.

[26] A.E. Pierri, A. Pallaoro, G. Wu, P.C. Ford, A luminescent and biocompatible PhotoCORM, J. Am. Chem. Soc. 134 (2012) 18197–18200, https://doi.org/10.1021/ja3084434.

[27] M. Wang, K. Murata, Y. Koike, G. Jonusauskas, A. Furet, D.M. Bassani, D. Saito, M. Kato, Y. Shimoda, K. Miyata, K. Onda, K. Ishii, A red-light-driven CO-releasing complex: Photoreactivities and excited-state dynamics of highly distorted Tricarbonyl rhenium Phthalocyanines, Chem. Eur. J. 28 (2022) e20220716, https://doi.org/10.1002/chem.20220716.

[28] R. Motterlini, L.E. Otterbein, The therapeutic potential of carbon monoxide, Natl. Rev. 9 (2010) 729743, https://doi.org/10.1038/ndr3228.

[29] X. Yang, W. Lu, C.P. Hopper, B. Ke, B. Wang, Nature's marvels endowed in gaseous molecules I: carbon monoxide and its physiological and therapeutic roles, Acta Pharm. Sin. B 11 (2021) 1434e1445, https://doi.org/10.1016/j.apsb.2020.10.010.

[30] H.-I. Choi, A. Zeb, M.-S. Kim, I. Rana, N. Khan, O.S. Qureshi, C.-W. Lim, J.-S. Park, Z. Gao, H.-J. Maeng, J.-K. Kim, Controlled therapeutic delivery of CO from carbon monoxide-releasing molecules (CORMs), J. Control. Release 350 (2022) 652–667, https://doi.org/10.1016/j.jconrel.2022.08.055.

[31] B.Y. Chin, L.E. Otterbein, Carbon monoxide is a poison to microbes! CO as a bactericidal molecule, Curr. Opin. Pharmacol. 9 (2009) 490–500, https://doi.org/10.1016/j.coph.2009.06.025.

[32] K. Ling, F. Men, W.-C. Wang, Y.-Q. Zhou, H.-W. Zhang, D.-W. Ye, Carbon monoxide and its controlled release: therapeutic application, detection, and development of carbon monoxide releasing molecules (CORMs), J. Med. Chem. 61 (2018) 2611–2635, https://doi.org/10.1021/acs.jmedchem.6b01153.
[33] V. Guilbaud, M. Goizet, N. Leygue, A. Poirot, S. Mallet-Ladeira, C.-L. Serpentini, T.-S. Ouk, G.M.A. Ndong Ntoutoume, V. Sol, E. Benoist, S. Fery-Forgues, Development of photoactivatable CO-releasing tricarbonylrhenium(I) complexes and their integration into cellulose nanocrystals in view of antibacterial activity, J. Photochem. Photobiol. A 453 (2024) 115600, https://doi.org/10.1016/j.jphotochem.2024.115600.

[34] K. Schindler, Y. Cortat, M. Nedyalkova, A. Crochet, M. Lattuada, A. Pavic, F. Zobi, Antimicrobial activity of rhenium Di- and Tricarbonyl Diimine complexes: insights on membrane-Bound S. Aureus protein binding, Pharmaceuticals 15 (2022) 1107, https://doi.org/10.3390/ph15091107.

[35] P. Collery, D. Desmaele, V. Vijaykumar, Design of Rhenium Compounds in targeted anticancer therapeutics, Curr. Pharm. Des. 25 (2019) 3306-3322, https://doi.org/10.2174/1381612825666190902161400.

[36] E. Kottelat, F. Zobi, Visible light-activated PhotoCORMs, Inorganics 5 (2017) 24, https://doi.org/10.3390/inorganics5020024.

[37] M.A. Wright, J.A. Wright, PhotoCORMs: CO release moves into the visible, Dalton Trans. 45 (2016) 6801-6811, https://doi.org/10.1039/c5dt04849d.

[38] U. Schatzschneider, PhotoCORMs: light-triggered release of carbon monoxide from the coordination sphere of transition metal complexes for biological applications, Inorg. Chim. Acta 374 (2011) 19-23, https://doi.org/10.1016/j.ica.2011.02.068.

[39] Á.D. Hernández Mejías, A. Poirot, M. Rmili, N. Leygue, M. Wolff, N. Saffon-Merceron, E. Benoist, S. Fery-Forgues, Efficient photoreaction of carbon monoxide from a luminescent tricarbonyl rhenium(I) complex incorporating pyridyl-1,2,4-triazole and phosphine ligands, Dalton Trans. 50 (2021) 1313-1323, https://doi.org/10.1039/D0DT03577G.

[40] J. Liu, D. Obando, V. Liao, T. Lifa, R. Codd, The many faces of the adamantyl group in drug design, Eur. J. Med. Chem. 46 (2011) 1949-1963, https://doi.org/10.1016/j.ejmech.2011.01.047.

[41] K. Spilovska, F. Zemek, J. Korabecny, E. Nepovilova, O. Soukup, M. Windisch, K. Kuca, Adamantane – a lead structure for drugs in clinical practice, Curr. Med. Chem. 23 (2016) 3245-3266, https://doi.org/10.2174/092967323666160525114026.

[42] K. Koike, N. Okoshi, H. Hori, K. Takeuchi, O. Ishitani, H. Tsubaki, I.P. Clark, M. W. George, F.P.A. Johnson, J.J. Turner, Mechanism of the photochemical ligand substitution reactions of fac-[re(bpy)(CO₃)(PR₃)]⁺ complexes and the properties of their triplet ligand-field excited states, J. Am. Chem. Soc. 124 (2002) 11448-11455, https://doi.org/10.1021/ja017032m.

[43] K. Saita, Y. Harabuchi, T. Taketsugu, O. Ishitani, S. Maeda, Theoretical study on mechanism of the photochemical ligand substitution of fac-[Re'(bpy)(CO₃)(PR₃)]⁺ complex, Phys. Chem. Chem. Phys. 18 (2016) 17557-17564, https://doi.org/10.1039/c6cp02314b.

[44] T. Auvray, A.K. Pal, G.S. Hanan, Electronic properties of rhenium(I) carbonyl complexes bearing strongly donating Hexahydro-Pyrimidopyrimidine based ligands, Eur. J. Inorg. Chem. (2021) 2257-2570, https://doi.org/10.1002/ejic.202100028.

[45] A. Comia, L. Charalambou, S.A.E. Omar, P.A. Scattergood, P.I.P. Elliott, A. Sinopoli, Photophysical and Electrochemical properties of rhenium(I) Triazole-based complexes, Inorganics 8 (2020) 22, https://doi.org/10.3390/inorganics8030022.

[46] W.K.C. Lo, G.S. Huff, J.R. Cubanski, A.D.W. Kennedy, C.J. McAdam, D. A. McMorran, K.C. Gordon, J.D. Crowley, Comparison of inverse and regular 2-Pyridyl-1,2,3-triazole “click” complexes: structures, stability, electrochemical, and Photophysical properties, Inorg. Chem. 54 (2015) 1572-1587, https://doi.org/10.1021/ic502557w.

[47] H. Bertrand, S. Clède, R. Guillot, F. Lambert, C. Policar, Luminescence modulations of rhenium tricarbonyl complexes induced by structural variations, Inorg. Chem. (2014) 536204-553623, https://doi.org/10.1021/ic5007007.

[48] H.Y.V. Ching, X. Wang, M. He, N. Perujo Holland, R. Guillot, C. Slim, S. Grèveau, H. C. Bertrand, C. Policar, F. Bedioui, M. Fontecave, Rhenium complexes based on 2-Pyridyl-1,2,3-triazole ligands: a new class of CO₂ reduction catalysts, Inorg. Chem. 56 (2017) 2966-2976, https://doi.org/10.1021/acs.inorgchem.6b03078.

[49] J. Wang, B. Delavaux-Nicot, M. Wolff, S. Mallet-Ladeira, R. Métivier, E. Benoist, S. Fery-Forgues, The unsuspected influence of the pyridyl-triazole ligand isomerism upon the electronic properties of tricarbonyl rhenium complexes: an experimental and theoretical insight, Dalton Trans. 47 (2018) 8087-8099, https://doi.org/10.1039/C8DT01120F.

[50] C. Vanucci-Bacqué, M. Wolff, B. Delavaux-Nicot, A.M. Abdallah, S. Mallet-Ladeira, C.-L. Serpentini, F. Bedos-Belval, K.W. Fong, X.Y. Ng, M.L. Low, E. Benoist, S. Fery-Forgues, *1,2,3-triazol-5-ylidene* vs *1,2,3-triazole-based tricarbonylrhenium(I) complexes: influence of a *mesoionic carbene* ligand on the electronic and biological properties, Dalton Trans. 53 (2024) 11276-11294, https://doi.org/10.1039/D4DT00922C.

[51] T. Okawa, Y. Aramaki, M. Yamamoto, T. Kobayashi, S. Fukumoto, Y. Toyoda, T. Henta, A. Hata, S. Ikeda, M. Kaneko, I.D. Hoffman, B.-C. Sang, H. Zou, T. Kawamoto, Design, synthesis, and evaluation of the highly selective and potent G-protein-coupled receptor kinase 2 (GRK2) inhibitor for the potential treatment of heart failure, J. Med. Chem. 60 (2017) 6942-6990, https://doi.org/10.1021/acs.jmedchem.7b00443.

[52] S. Rodríguez-Jimenez, H.L.C. Feltham, S. Brooker, Non-porous Iron(II)-based sensor: crystallographic insights into a cycle of colorful guest-induced Topotactic transformations, Angew. Chem. Int. Ed. 55 (2016) 15067-15071, https://doi.org/10.1002/anie.201608813.

[53] S. Rodríguez-Jimenez, A.S. Barltrop, N.G. White, H.L.C. Feltham, S. Brooker, Solvent polarity predictably Tunes spin crossover T1/2 in isomeric Iron(II)

pyrimidine Triazoles, Inorg. Chem. 57 (2018) 6266-6282, https://doi.org/10.1021/acs.inorgchem.8b00128.

[54] I.A. Novakov, B.S. Orlinson, D.V. Zavyalov, V.I. Porkhun, E.N. Savelyev, E. A. Potaenkova, O.V. Vostrikova, M.A. Nakhod, A.V. Kireeva, A.M. Pichugin, Adamantylation of N-aryl and N-arylalkyl acetamides in trifluoroacetic acid, Russ. Chem. Bull. 69 (2020) 1096-1101, https://doi.org/10.1007/s11172-020-2873-9.

[55] G.A. Gamova, M.N. Zavalishina, V.V. Aleksandriyskii, V.A. Sharnin, Pyrazine-2-carbohydrazone of pyridoxal 5'-phosphate: synthesis, stability, formation kinetics, and interaction with DNA, Russ. J. Gen. Chem. 89 (2019) 230-235, https://doi.org/10.1134/S1070363219020105.

[56] SADABS, Program for data correction, Bruker-AXS, 2025.

[57] G.M. Sheldrick, SHELXT-integrated space-group and crystal-structure determination, Acta Cryst. A 71 (2015) 3-8, https://doi.org/10.1107/S2053273341026370.

[58] G.M. Sheldrick, Crystal structure refinement with SHELXL, Acta Cryst. C 71 (2015) 3-8, https://doi.org/10.1107/S2053229614024218.

[59] A.L. Spek, PLATON SQUEEZE: a tool for the calculation of the disordered solvent contribution to the calculated structure factors, Acta Cryst. C 71 (2015) 9-18, https://doi.org/10.1107/S2053229614024929.

[60] K. Suzuki, A. Kobayashi, S. Kaneko, K. Takehira, T. Yoshihara, H. Ishida, Y. Shiina, S. Oishi, S. Tobita, Reevaluation of absolute luminescence quantum yields of standard solutions using a spectrometer with an integrating sphere and a back-thinned CCD detector, Phys. Chem. Chem. Phys. 11 (2009) 9850-9860, https://doi.org/10.1039/B912178A.

[61] A.V. Fonin, A.I. Sulatskaya, I.M. Kuznetsova, K.K. Turoverov, Fluorescence of dyes in solutions with high absorbance. Inner filter effect correction, PLoS One 9 (2014) e103878, https://doi.org/10.1371/journal.pone.0103878.

[62] C.G. Hatchard, C.A. Parker, A new sensitive chemical actinometer - II. Potassium ferrioxalate as a standard chemical actinometer, Proc. R. Soc. Lond. A 235 (1956) 518-536, https://doi.org/10.1098/rspa.1956.0102.

[63] R. Schmidt, C. Tanielian, R. Dunsbach, C. Wolff, Phenalenone, a universal reference compound for the determination of quantum yields of singlet oxygen $O_2^{1\Delta}$ sensitization, J. Photochem. Photobiol. A 79 (1994) 11-17, https://doi.org/10.1016/10-10-6030(93)03746-4.

[64] M.J. Frisch, et al., Gaussian 16, Revision B.01, Gaussian, Inc., Wallingford CT, 2016.

[65] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu, J. Chem. Phys. 132 (2010) 154104, https://doi.org/10.1063/1.3382344.

[66] S. Grimme, S. Ehrlich, L. Goerigk, Effect of the damping function in dispersion corrected density functional theory, J. Comput. Chem. 32 (2011) 1456-1465, https://doi.org/10.1002/jcc.21759.

[67] F. Neese, F. Wennmohs, U. Becker, C. Riplinger, The ORCA Quantum chemistry program package, J. Chem. Phys. 152 (2020) 224108, https://doi.org/10.1063/5.0004608.

[68] J.S. Wilson, N. Chawdhury, M.R.A. Al-Mandhary, M. Younus, M.S. Khan, P. R. Raithby, A. Kolhler, R.H. Friend, The energy gap law for triplet states in Pt-containing conjugated polymers and monomers, J. Am. Chem. Soc. 123 (2001) 9412-9417, https://doi.org/10.1021/ja010986s.

[69] J.A. Baiano, R.J. Kessler, R.S. Lumpkin, M.J. Munlev, W.R. Murphy, Nonradiative decay in rhenium(I) monometallic complexes of 2,3-Di(2-pyridyl)pyrazine and 2,3-Di(2-pyridyl)quinoxaline, J. Phys. Chem. 99 (1995) 17680-17690, https://doi.org/10.1021/j100050a004.

[70] L.A. Worl, R. Duesing, P. Chen, L. Della Ciana, T.J. Meyer, Photophysical properties of polypyridyl carbonyl complexes of rhenium(I), J. Chem. Soc. Dalton Trans. (1991) 849-858, https://doi.org/10.1039/DT991000849.

[71] J.K. Hino, L. Della Ciana, W.J. Dressick, B.P. Sullivan, Substituent constant correlations as predictors of spectroscopic, electrochemical, and Photophysical properties in ring-substituted 2,2'-Bipyridine complexes of rhenium(I), Inorg. Chem. 31 (1992) 1072-1080, https://doi.org/10.1021/ic00032a029.

[72] Z. Zhao, H. Zhang, J.W.Y. Lam, B.Z. Tang, Aggregation-induced emission: new vistas at the aggregate level, Angew. Chem. Int. Ed. 59 (2020) 9888-9907, https://doi.org/10.1002/anie.201916729.

[73] A. Soupart, F. Alary, J.-L. Heelly, P.I.P. Elliott, I.M. Dixon, Recent progress in ligand photorelease reaction mechanisms: theoretical insights focusing on Ru(II) $^3$MC states, Coord. Chem. Rev. 408 (2020) 213184, https://doi.org/10.1016/j.ccr.2020.213184.

[74] I.M. Dixon, S. Bonnet, F. Alary, J. Cuny, Photoinduced ligand exchange dynamics of a Polypyridyl ruthenium complex in aqueous solution, J. Phys. Chem. Lett. 12 (2021) 7278-7284, https://doi.org/10.1021/acs.jpclett.1c01424.

[75] I. Kraljić, S. El Mohsni, A new method for the detection of singlet oxygen in aqueous solutions, Photochem. Photobiol. 28 (1978) 577-581, https://doi.org/10.1111/j.1751-1097.1978.tb06972.x.

[76] M. Saldías, N. Guzmán, F. Palominos, C. Sandoval-Altamirano, G. Günther, N. Pizarro, A. Vega, Electronic and Photophysical properties of Re'(CO₃)Br complexes modulated by Pyrazolyl-Pyridazine ligands, ACS Omega 4 (2019) 4679-4690, https://doi.org/10.1021/acs.inorgchem.1c03130.

[77] F. Ragone, H.H. Martinez Saavedra, P.M. David Gara, G.T. Ruiz, E. Wolcan, Photosensitized generation of singlet oxygen from re(I) complexes: a Photophysical study using LIOAS and luminescence techniques, J. Phys. Chem. A 117 (2013) 4428-4435, https://doi.org/10.1021/jp402550g.

[78] M.C. DeRosa, R.J. Crutchley, Photosensitized singlet oxygen and its applications, Coord. Chem. Rev. 233-234 (2002) 351-371, https://doi.org/10.1016/S0010-8545(02)00034-6.