# Using a diphenyl-bi-(1,2,4-triazole) tricarbonylrhenium(i) complex with intramolecular π-π stacking interaction for efficient solid-state luminescence enhancement

Alexandre Poirot, Corinne Vanucci-Bacqué, Béatrice Delavaux-Nicot, Clarisse Meslien, Nathalie Saffon-Merceron, Charles-Louis Serpentini, Florence Bedos-Belval, Eric Benoist, Suzanne Fery-Forgues

► To cite this version:

Alexandre Poirot, Corinne Vanucci-Bacqué, Béatrice Delavaux-Nicot, Clarisse Meslien, Nathalie Saffon-Merceron, et al. Using a diphenyl-bi-(1,2,4-triazole) tricarbonylrhenium(i) complex with intramolecular π-π stacking interaction for efficient solid-state luminescence enhancement. Dalton Transactions, 2023, 10.1039/d2dt03573a. hal-04023657

HAL Id: hal-04023657

https://hal.science/hal-04023657

Submitted on 10 Mar 2023

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>

Cite this: DOI: 10.1039/d2dt03573a

# Using a diphenyl-bi-(1,2,4-triazole) tricarbonylrhenium(i) complex with intramolecular π-π stacking interaction for efficient solid-state luminescence enhancement†

Alexandre Poirot, ID a Corinne Vanucci-Bacqué, ID a Béatrice Delavaux-Nicot, ID b Clarisse Meslien, a Nathalie Saffon-Merceron, c Charles-Louis Serpentini, d Florence Bedos-Belval, ID a Eric Benoist ID a and Suzanne Fery-Forgues ID a*

Since intramolecular π-π stacking interactions can modify the geometry, crystal packing mode, or even the electronic properties of transition metal complexes, they are also likely to influence the solid-state luminescence properties. Following this concept, a new tricarbonylrhenium(i) complex (Re-BPTA) was designed, based on a simple symmetrical 5,5'-dimethyl-4,4'-diphenyl-3,3'-bi-(1,2,4-triazole) organic ligand. The complex was prepared in good yield using a three-step procedure. The crystallographic study revealed that both phenyl rings are located on the same side of the molecule, and twisted by 71° and 62°, respectively, with respect to the bi-(1,2,4-triazole) unit. They overlap significantly, although they are slipped parallel to each other to minimize the intramolecular interaction energy. The π-π stacking interaction was also revealed by ¹H NMR spectroscopy, in good agreement with the results of theoretical calculations. In organic solutions, a peculiar electrochemical signature was observed compared to closely-related pyridyl-triazole (pyta)-based complexes. With regard to the optical properties, the stiffness of the Re-BPTA complex led to the stabilization of the ³MLCT state, and thus to an enhancement of the red phosphorescence emission compared to the more flexible pyta complexes. However, an increased sensitivity to quenching by oxygen appeared. In the microcrystalline phase, the Re-BPTA complex showed strong photoluminescence (PL) emission in the green-yellow wavelength range (λ<sub>PL</sub> = 548 nm, Φ<sub>PL</sub> = 0.52, ⟨τ<sub>PL</sub>⟩ = 713 ns), and thus a dramatic solid-state luminescence enhancement (SLE) effect. These attractive emission properties can be attributed to the fact that the molecule undergoes little distortion between the ground state and the triplet excited state, as well as to a favorable intermolecular arrangement that minimizes detrimental interactions in the crystal lattice. The aggregation-induced phosphorescence emission (AIPE) effect was clear, with a 7-fold increase in emission intensity at 546 nm, although the aggregates formed in aqueous medium were much less emissive than the native microcrystalline powder. In this work, the rigidity of the Re-BPTA complex is reinforced by the intramolecular π-π stacking interaction of the phenyl rings. This original concept provides a rhenium tricarbonyl compound with very good SLE properties, and could be used more widely to successfully develop this area of research.

Received 5th November 2022,
Accepted 13th February 2023

DOI: 10.1039/d2dt03573a

rsc.li/dalton

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

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

<footnote>E-mail: suzanne.fery-forgues@univ-tlse3.fr</footnote>

<footnote><sup>b</sup>Laboratoire de Chimie de Coordination, CNRS (UPR 8241), Université de Toulouse (UPS, INPT), 205 route de Narbonne, 31077 Toulouse Cedex 4, France</footnote>

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

<footnote><sup>d</sup>Laboratoire IMRCP, CNRS UMR 5623, Université de Toulouse III Paul Sabatier, 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, molecular views and crystallographic data, electrochemical experiments, and photoluminescence spectra and decays. CCDC 2212965 and 2212966. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt03573a</footnote>
Introduction

In recent years, transition metal complexes that exhibit aggre-
gation-induced phosphorescence emission (AIPE) or more gen-
erally solid-state luminescence enhancement (SLE),¹ i.e. whose
emission is significantly more intense in the various solid
states than in solution, have become increasingly attractive.²⁻⁵
Their good photoluminescence (PL) efficiency, ease of emis-
sion wavelength tuning, photo- and thermal stabilities, rela-
tively long excited-state lifetime and possibility to generate
active oxygen species make them promising candidates for
various applications such as chemosensors,⁶ smart materials,⁷
emissive materials in optoelectronic devices,⁸ as well as nano-
particles for biological imaging and theranostic systems.⁹⁻¹¹
However, their development is a delicate task because their
optical behavior depends on many factors (i.e. electronic pro-
perties, geometry, packing mode, intermolecular interactions)
that are difficult to disentangle. Intramolecular π-π stacking
could be a powerful tool to act on all these factors. In the early
part of the last decade, this phenomenon has been used to
improve the emission properties of iridium complexes.
Indeed, the stacking between aromatic rings of cyclometalated
ligands blocks nucleophilic attacks on the complex cores, thus
reducing the formation of degradation products capable of
quenching luminescence,¹²⁻¹⁷ and the direct effect on the elec-
tronic levels stabilizes the complexes.¹⁷ The result is an
increased lifetime of light-emitting electrochemical cells
(LECs),¹²⁻¹⁸ which is observed as long as the stacking does not
induce an exaggerated distortion of the complexes.¹⁹,²⁰
Intramolecular π-π stacking has also been shown to improve
the emission efficiency in solution and in polymer films.¹⁸,²¹
Noticeably, it is associated to solid-state luminescent and
piezochromic materials.¹⁷ Recently, the encapsulation of an
iridium complex with intramolecularly stackable and bulky
substituents has been reported. It leads to enhanced photo-
luminescence both in solution and in the crystalline state.²²
For rhenium(i) complexes, intramolecular π-π interactions
between the bipyridine (bpy) ligand and the phenyl rings of
the phosphorous ligand of fac-[Re(bpy)(CO)₃(PR₃)]⁺ (R =
p-MeOPh, p-MePh, Ph, p-FPh, OPh) complexes have been
shown to affect their structure, electrochemical behavior and
some spectroscopic properties in solution. However, no effect
on possible solid-state emission properties has been
reported.²³,²⁴ Practical implementation of the aforementioned
concept is therefore virtually unprecedented in the field of
SLE/AIPE-active transition metal complexes, including for
rhenium(i) complexes.

Over the past few years, our group has developed the syn-
thesis of tricarbonylrhenium(i) complexes that are well suited
to various substitution schemes and, for most of them, exhibit
remarkable solid-state emission properties.²⁵⁻²⁹ The SLE/AIPE
behavior was shown to be related to the presence of an aroma-
tically-substituted 3-(2-pyridyl)-1,2,4-triazole (pyta) ligand.²⁵
More specifically, it is promoted by a large phenyl-pyta angle,²⁵
a significant steric hindrance at the phenyl ring,²⁶,²⁸ and the
presence of a chloride ancillary ligand,²⁹ as illustrated in

Fig. 1. Keeping these results in mind and prioritizing simpli-
city of synthesis, a new **Re-BPTA** complex (Fig. 1) was designed to
investigate the influence of an intramolecular π-π stacking inter-
action on AIPE/SLE properties. The organic pyta ligand is now
replaced by a symmetrical bi-(1,2,4-triazole) ligand,³⁰ i.e. an aro-
matic molecule with six potentially coordinating N atoms, known
to exhibit various geometrical configurations and coordination
modes with transition metal ions such as Mn(u), Co(u), Cu(u), Cd
(u) and Zn(u).³¹⁻³³ In the present case, density functional theory
(DFT) calculations allowed anticipating that the incorporation of
a phenyl ring at the 4 and 4'-positions of this ligand would lead
to a rigid complex where the phenyl rings are close to perpendicu-
lar with respect to the bi-triazole moiety, and therefore, signifi-
cantly stacked (Fig. S9†). Knowing that face-to-face π-π staking of
phenyl rings is unfavorable from an energetic viewpoint,³⁴,³⁵ it
seemed instructive to know what type of structure is actually
obtained and what is the effect on the electronic properties. We
report here the synthesis, and the complete theoretical and
experimental study of the original complex **Re-BPTA**, with particu-
lar emphasis on its SLE and AIPE behavior.

Results and discussion

Synthesis and characterization

Complex **Re-BPTA** was easily obtained by an efficient three-step synthesis (Scheme 1). The intermediate 5,5'-dimethyl-2,2'-bi-(1,3,4-oxadiazole) (1) was initially prepared from 5-methyl-tetrazole and oxalyl chloride,³⁶ and then it was condensed on aniline in the presence of catalytic *p*-TsOH to obtain 5,5'-dimethyl-4,4'-diphenyl-3,3'-bi-(1,2,4-triazole) (**BPTA**) in 92% yield. The classical complexation conditions (slight excess of [ReCl(CO)₅] in refluxing methanol) did not provide the corresponding complex. The complexation of **BPTA** was then performed at higher temperature in refluxing butan-2-ol. Complex **Re-BPTA**, which precipitates in the medium, was isolated in 73% yield. The overall yield of the synthesis was 46%.

Complex **Re-BPTA** was unambiguously identified by ¹H and ¹³C NMR (Fig. S1–S4, ESI†) and high resolution mass spectrometry (Fig. S5†). Its purity was checked by microanalysis. The Fourier transform infrared (FTIR) spectrum in dichloromethane (DCM) solutions showed stretching bands at 2017, 1914 and 1888 cm⁻¹ as the characteristic signature of the three CO groups in a *fac*-[Re(CO)₃] arrangement. The average value of these bands (1940 cm⁻¹), which reflects the electron density at the metal center,³⁷ was slightly lower than that of 1,2,4-pytabased complexes (1947–1950 cm⁻¹),²⁶,²⁸ suggesting that ligand **BPTA** is a slightly better electron donor than the pyta ligand.

The ¹H NMR spectrum of complex **Re-BPTA** at room temp-
erature (298 K) (Fig. S2,† with proton numbering available in
Fig. S1, ESI†) shows that the aromatic protons H₈ and H₁₂ at
ortho positions of the phenyl ring, on the one hand, and H₉
and H₁₁ at meta positions of the phenyl ring, on the other
hand, are magnetically non-equivalent, meaning that the com-
pound displays a rigid conformation on the NMR time scale.
Indeed, this is confirmed by the variable temperature (VT)

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 1** Chemical structures of the SLE-active tricarbonylrhenium(i) complexes previously studied (ref. 25–29) and of the new Re-BPTA complex.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Scheme 1** Synthesis of the tricarbonylrhenium(i) complex Re-BPTA. (a) Xylenes, N<sub>2</sub>, 2 h, 120 °C, 69%; (b) Xylenes, aniline (3.6 eq.), p-TsOH.H<sub>2</sub>O (0.3 eq.), 18 h, 140 °C, 92%; (c) [ReCl(CO)<sub>5</sub>] (1.3 eq.), butan-2-ol, 100 °C, 18 h, 73%.</figcaption></figure>

NMR performed in deuterated 1,1,2,2-tetrachloroethane (Fig. S6†). At 298 K, the signals of H<sub>8</sub> and H<sub>12</sub> look like two distinct apparent doublets of doublet at 6.85 and 6.77 ppm. When increasing the temperature, these signals progressively coalesce, and give rise to a single apparent large doublet situated at 6.86 ppm at 368 K. Similarly, the signals corresponding to H<sub>9</sub> and H<sub>11</sub>, which appear as a split quartet at 7.19 ppm at room temperature, coalesce to give an apparent triplet centered at 7.23 ppm at 368 K. The whole phenomenon is reversible when coming back to room temperature. These observations indicate that the rotation of the phenyl groups is virtually prevented at room temperature and only allowed when heating, meaning that the intramolecular π-π interaction actively contributes to the stabilization of the complex.

## Crystal structures

X-Ray quality crystals of ligand **BPTA** and complex **Re-BPTA** were grown by slow evaporation of chloroform solutions.

Selected crystallographic data are collected in the experimental section and in Tables S1 and S2.† Ligand **BPTA** shows a symmetrical structure (Fig. 2). The bi-triazole moiety is planar, and the phenyl rings are displayed almost perpendicular to it (~83°), at opposite sides of the molecule. In the network, the molecules exhibit a classical herringbone packing pattern, without significant overlap between the aromatic moieties (Fig. S7†). Regarding complex **Re-BPTA**, the coordination sphere exhibits a slightly distorted octahedral geometry (Fig. 2). The rhenium atom is coordinated to three carbonyl groups in a *fac* configuration, two nitrogen atoms of the organic ligand and one chlorine atom. The bi-triazole moiety is slightly twisted (dihedral angles N3–C4–C7–N6 = 23.6(8)° and N4–C7–C4–N2 = 13.5(5)°). Above all, the two phenyl rings are located on the same side of the molecule. They swing in the same direction, and their planes form an angle close to 71° and 62° with respect to the corresponding triazoles (Fig. 3). Their aromatic systems show a wide overlap. The centroid–centroid distance is 3.5 Å, indicating a strong π–π stacking interaction. By comparing the complex and the ligand structures, it is obvious that complexation is the main driving force that induces the conformational change and makes the phenyl rings closer to each other. The phenyl rings are now slipped parallel to each other to minimize energy. Indeed, among the three stacking patterns currently described for phenyl-phenyl π–π stacking interactions, two (i.e. edge-to-face (T-shaped), and offset face-to-face (parallel-displaced)) are stable in energy, while the third one, i.e. face-to-face (sandwich), is unstable and rarely observed because of the repulsion between the two stacking phenyl rings,<sup>34,35,38</sup> including for metal complexes with aromatic nitrogen-containing ligands.<sup>39</sup> The complex molecules are arranged as herringbone (Fig. S8a†). The network is structured by strong interactions that take place between the N1/N5 triazole nitrogen atom or the chlorine atom of one molecule, and one methyl hydrogen atom of a neighboring molecule (D ~ 2.6 Å and 2.9 Å, respectively) (Fig. S8b and c†). Remarkably, there is virtually no
<figure><img src="image_7.png" /><figcaption>**Fig. 2** Molecular views of ligand **BPTA** and complex **Re-BPTA**. Hydrogen atoms are not represented for the sake of clarity. Displacement ellipsoids are drawn at 30% probability.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 3** Crystallographic arrangement of complex **Re-BPTA** showing the centroid-to-centroid distances between phenyl rings (green ink) and the phenyl-triazole dihedral angles (red ink).</figcaption></figure>

overlap between the aromatic systems of two phenyl rings belonging to neighboring molecules (centroid-to-centroid distance ~4.2 Å) (Fig. 3), and the closest superimposed triazoles are distant by about 4.7 Å (Fig. S8d†).

## Electronic properties

Computational studies were made using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods considering complex **Re-BPTA** in dichloromethane continuum. The composition of the frontier molecular orbitals and the main electronic transitions are described in Tables S3 and S4.† As shown in Fig. 4, the three highest occupied molecular orbitals (HOMO, HOMO − 1 and HOMO − 2) are localized on the rhenium atom, the carbonyl ligands and the chlorine atom, as is commonly the case for tricarbonylrhenium(i) complexes. Regarding the lowest unoccupied molecular orbitals, the LUMO is mainly localized on the bi-triazole moiety, and the LUMO + 1 on both phenyl rings. This feature is characteristic of the **Re-BPTA** complex compared to the pyta-based **Re-Phe** complex, where the phenyl contribution is only significant at the LUMO + 3 level (at −1.03 eV). Lastly, the LUMO + 2 is mainly localized on

the carbonyl ligands, and higher orbitals (LUMO + 3 to LUMO + 5) partly on the phenyl rings. Compared to the pyta-based complexes, the HOMO and HOMO − 1 of **Re-BPTA** are almost at the same energy level (±0.03 eV) than the corresponding orbitals in **Re-Phe** and **Re-PBO**, and a difference of +0.2 eV is observed for the LUMO with respect to **Re-Phe**. Noticeably, the LUMO + 1–LUMO energy difference of +1.32 eV observed for **Re-BPTA** is the largest one among these three compounds.

The lowest energy transition associated with high oscillation strength is a HOMO − 1 → LUMO transition with preponderant metal-to-ligand charge transfer (MLCT) character, and a minor halide-to-ligand and ligand-to-ligand charge transfer (XLCT and LLCT, respectively) character. This transition is predicted at 392.7 nm. High energy transitions, predicted between 290.6 and 249.8 nm mainly involve low-lying HOMO orbitals (i.e. H-6, H-8 and H-9) and the LUMO, and they have mixed MLCT and LLCT character.

## Electrochemical properties

The electrochemical behavior of the **BPTA** ligand and **Re-BPTA** complex was studied by cyclic voltammetry (CV) and

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 4** Isodensity plots (isovalue = 0.03 e bohr⁻³) and energy levels of the first frontier molecular orbitals, for complex **Re-BPTA** in dichloromethane, according to DFT calculations at the PBE0/LANL2DZ level of theory.</figcaption></figure>

Ostyeryoung square wave voltammetry (OSWV) measurements in DCM at room temperature (Fig. 5 and Fig. S12–S17†). In the OSWV anodic part, the first oxidation potential of the complex situated at 1.37 V can be mainly assigned to an irreversible Re(I)/Re(II) oxidation process.⁴⁰,⁴¹ This oxidation potential is

around 90 mV lower than that of the **Re-Phe** complex and its derivatives, thus indicating a better electron donating capability of the new species. In contrast, in the cathodic region, the first reduction potential observed at −1.51 V is around 0.2 V more negative than that of the first characteristic reduction potential of the 3-(2-pyridyl)-1,2,4-triazole complexes generally detected at ≈−1.3 V (Table S6†). This reduction is ligand-centered and probably first occurs on both triazole moieties. A second reduction process of minor intensity is also observed as a shoulder at −1.66 V. Regarding the CV experiments, the latter two processes correspond to a complex and broad reduction wave of the whole ligand (Fig. 5). Contrary to our previous pyta derivatives, no quasi-reversible reduction process was detected, even by thorough examination at different reduction potentials and different scan rates. In addition, the intensity of this whole reduction process is not in a 1/1 ratio when compared to the first one-oxidation process, as only observed for **Re-Phe(I)**. Furthermore, the first reduction potential of **Re-BPTA** is close to that of dichloromethane presenting a first reduction process of very weak intensity at −1.60 V under the same conditions, which limits the electrochemical analysis. A particular electrochemical behavior, such as a decomposition process or reaction with the solvent could be envisaged, thus influencing the above observations related to

<figure><img src="image_11.png" /><figcaption>**Fig. 5** Cyclic voltammograms of **Re-BPTA** (green line) and **Re-Phe** (orange line) on a Pt working electrode in CH₂Cl₂ + 0.1 M n[Bu₄N][BF₄] at room temperature and at a scan rate of 0.2 V s⁻¹ toward anodic potentials.</figcaption></figure>
the broad reduction wave in CV. However, careful examination of the reduction wave after accumulation of several successive scans under different conditions does not really support this hypothesis since no significant changes were detected. All of these observations in the cathodic region are probably mainly related to the different intrinsic nature of the LUMO-LUMO + 3 frontier molecular orbitals of the **Re-BPTA** compound (see above) leading to this peculiar electrochemical signature. Coordination of the **BPTA** ligand induces a substantial decrease of the value of its reduction potential, as usually observed for the substituted triazole ring.⁴²,⁴³ The resulting new bi-(phenyltriazole) complex formed is more prone to oxidation and more difficult to reduce than the pyta counterpart **Re-Phe**. However, the value of its electrochemical gap (2.70 eV) is very close to that of **Re-Phe** (2.65 eV) (Table S7†) and its derivatives.²⁸ Finally, these electrochemical data are in good agreement with the theoretical study and the UV-visible absorption data.

<figure><img src="image_12.png" /><figcaption>**Fig. 6** Absorption (broken lines) and normalized emission spectra (full lines) of complex **Re-BPTA** in three organic solvents and in the water/acetonitrile 90 : 10 v/v mixture. $\lambda_{ex} = 360$ nm.</figcaption></figure>

UV-visible absorption and emission properties

The optical properties were studied on freshly prepared compound. In fact, it was noticed that ageing of the samples for several months leads to a significant decrease of the emission efficiency, especially for solid-state samples, where energy transfers are particularly strong,⁴⁴ making them very sensitive to the presence of quenching impurities. The UV-visible absorption spectra of complex **Re-BPTA** were recorded in methanol, acetonitrile and DCM (Fig. 6 and Table 1). They showed intense bands below 300 nm and a moderately intense band at long wavelengths, the maximum of which varies significantly between 340 and 358 nm. The experimental spectrum in DCM was situated at higher energy than that predicted by the calculation (Fig. S10†). Such a discrepancy ~0.30 eV, at the limit of what is acceptable when using TD-DFT,⁴⁵ is attributable to intramolecular motions that are not considered by calculations.

The emission spectra of **Re-BPTA** in the three organic solvents were quite close. They displayed only one unresolved band, with a maximum between 586 and 592 nm. The emission quantum yields were low in every case, especially when the polarity and proticity of the solvent increase. A striking difference was found between air-equilibrated and deaerated solutions. After bubbling with argon, the quantum yields were increased by a factor 1.6, 1.9 and 3.2 in methanol, dichloromethane and acetonitrile, respectively, in the same order than the amount of oxygen dissolved by the solvent,⁴⁶,⁴⁷ and this effect was perfectly reversible. The luminescence decays measured in solutions were monoexponential, varying from 71 to 154 ns, which confirms that the emission detected is mainly phosphorescence. A further argument in favor of this attribution is that the experimental emission maximum in DCM is in rather good agreement with that calculated for luminescence arising from the lowest excited triplet state (614 nm). It must be noticed that our theoretical calculations consider that the

<table><caption>**Table 1** Spectroscopic data of complex **Re-BPTA** in organic solutions and comparison with **Re-Phe** and **Re-PBO** in dichloromethane. Maximum absorption wavelength ($\lambda_{abs}$), molar extinction coefficient ($\varepsilon$), maximum wavelength of phosphorescence emission ($\lambda_{p}$), emission quantum yield ($\Phi_{p}$), lifetime ($\tau$) and chi square ($\chi^{2}$) value, radiative ($k_{r}$) and non-radiative ($k_{nr}$) deactivation constants. Concentrations for **Re-BPTA**: ~8 × 10⁻⁵ M for absorption, ~1.0 × 10⁻⁵ M to 3.5 × 10⁻⁵ M for emission</caption><thead><tr><th>Complex</th><th>Solvent</th><th>λ<sub>abs</sub> [nm]</th><th>ε [M<sup>-1</sup> cm<sup>-1</sup>]</th><th>λ<sub>p</sub> [nm]</th><th>Φ<sub>p</sub></th><th>Φ<sub>p</sub><sup>a</sup></th><th>τ [ns]</th><th>χ<sup>2</sup></th><th>k<sub>r</sub> [s<sup>-1</sup>]</th><th>k<sub>nr</sub> [s<sup>-1</sup>]</th></tr></thead><tbody><tr><td rowspan="6">Re-BPTA<sup>b</sup></td><td rowspan="3">Methanol</td><td>238</td><td>11 700</td><td rowspan="3">590</td><td rowspan="3">0.018</td><td rowspan="3">0.029</td><td rowspan="3">71<sup>c</sup></td><td rowspan="3">1.20</td><td rowspan="3">2.5 × 10<sup>5</sup></td><td rowspan="3">1.4 × 10<sup>7</sup></td></tr><tr><td>264</td><td>12 900</td></tr><tr><td>342</td><td>5900</td></tr><tr><td rowspan="3">Acetonitrile</td><td>236</td><td>12 700</td><td rowspan="3">586</td><td rowspan="3">0.024</td><td rowspan="3">0.078</td><td rowspan="3">97<sup>c</sup></td><td rowspan="3">1.06</td><td rowspan="3">2.5 × 10<sup>5</sup></td><td rowspan="3">1.0 × 10<sup>7</sup></td></tr><tr><td>262</td><td>11 700</td></tr><tr><td>342</td><td>4600</td></tr><tr><td rowspan="3">Dichloromethane</td><td>242</td><td>13 800</td><td rowspan="3">592</td><td rowspan="3">0.042</td><td rowspan="3">0.080</td><td rowspan="3">154<sup>d</sup></td><td rowspan="3">1.06</td><td rowspan="3">2.7 × 10<sup>5</sup></td><td rowspan="3">6.2 × 10<sup>6</sup></td></tr><tr><td>264</td><td>13 200</td></tr><tr><td>358</td><td>5700</td></tr><tr><td rowspan="3">Re-Phe<sup>e</sup></td><td rowspan="3">Dichloromethane</td><td>240</td><td>15 100</td><td rowspan="3">626</td><td rowspan="3">0.020</td><td rowspan="3">0.029</td><td rowspan="3">74.7</td><td rowspan="3">1.29</td><td rowspan="3">2.7 × 10<sup>5</sup></td><td rowspan="3">1.3 × 10<sup>7</sup></td></tr><tr><td>282</td><td>11 600</td></tr><tr><td>382</td><td>4000</td></tr><tr><td rowspan="2">Re-PBO<sup>f</sup></td><td rowspan="2">Dichloromethane</td><td>310</td><td>29 600</td><td rowspan="2">632</td><td rowspan="2">0.012</td><td rowspan="2">0.015</td><td rowspan="2">80</td><td rowspan="2">1.19</td><td rowspan="2">1.5 × 10<sup>5</sup></td><td rowspan="2">1.2 × 10<sup>7</sup></td></tr><tr><td>388</td><td>5100</td></tr></tbody></table>

When not specified, the medium was air-equilibrated. <sup>a</sup> Argon-bubbled medium. <sup>b</sup> $\lambda_{ex} = 360$ nm for steady-state emission and $\lambda_{ex} = 371$ nm for decay measurements. <sup>c</sup> $\lambda_{em} = 600$ nm. <sup>d</sup> $\lambda_{em} = 570$ nm. <sup>e</sup> From ref. 28. <sup>f</sup> From ref. 26.

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /></figure>

lowest energy transition is a LUMO → HOMO, but a LUMO → HOMO – 1 transition of very close energy is also very likely to occur. The molecule in its lowest MLCT triplet excited state is shown in Fig. 7 and Fig. S11.† The spin density distribution is almost exclusively localized on the coordination sphere, as is also the case for pyta-based complexes. This suggests that, while the phenyl rings contribute to the overall rigidity of complex **Re-BPTA**, they are not directly involved in the emissive phosphorescence process.

At first glance, the spectroscopic features of **Re-PBO**, **Re-Phe** and **Re-BPTA** in DCM are relatively close (Table 1). All of them absorb in the UV range and emit orange-red light. The shorter absorption and emission wavelengths of **Re-BPTA** can be mainly attributed to the change in the chelating unit. However, a closer examination reveals that **Re-BPTA** exhibits better luminescence efficiency and a significantly longer lifetime than the pyta-based complexes. The calculation of the photophysical rate constants shows that the non-radiative constant $k_{nr}$ of **Re-BPTA** is divided by two with respect to the pyta complexes (Table 1). An explanation could be that, in the rigid coordination sphere of **Re-BPTA**, vibrations and distortions responsible for the waste of excitation energy are limited, making the emitting ³MLCT state more stable than for the pyta-based complexes. The other side of the coin is that complex **Re-BPTA** is more sensitive to the effect of oxygen because of its longer lifetime.

The solid-state spectroscopic data are gathered in Table 2 and Table S8.† Two microcrystalline samples of complex **Re-**

<figure><img src="image_15.png" /><figcaption>**Fig. 7** Spin density distribution for the lowest MLCT triplet excited state of **Re-BPTA** (isovalue = 0.03 e bohr⁻³) according to DFT calculations at the PBE0/LANL2DZ level of theory.</figcaption></figure>

**BPTA** were investigated. One is the powder obtained by precipitation in butan-2-ol. The second one results from recrystallization in chloroform, and corresponds to the crystals analyzed by XRD. Both powders emitted bright greenish-yellow light when illuminated by a hand-held UV lamp (365 nm) (an example is given in Fig. 8a, inset). Their excitation spectra revealed the presence of a strong band around 374 nm, and a weaker one around 470 nm, tailing till 500 nm. It was checked that the powders were emissive when excited in the blue region of the visible spectrum. The emission spectrum of the powder obtained from butan-2-ol showed one unresolved band peaking at 548 nm (Fig. 8a). The photoluminescence quantum yield (PLQY) was 0.52. For the compound recrystallized from chloroform, the emission spectrum was shifted to 554 nm, with a slightly different shape (Fig. 8a), and the PLQY was only 0.27. Such an influence of the crystallization process on the PL properties has already been noticed for **Re-Phe(I)**.²⁹ In fact, the PL properties are highly dependent on molecular geometry, intermolecular interactions and crystal defects.¹ Reabsorption phenomena related to the crystal size can also strongly alter the shape and position of the PL spectra.⁴⁸ These parameters may vary more or less depending on the crystallization method. For both samples, a very strong SLE effect was observed with respect to solutions.

The PL decay was measured on the powder precipitated from butan-2-ol. It was found to be multiexponential, although it essentially consists of one long lifetime with a fraction of intensity of 97%. Minor contributions associated to short lifetimes probably arise from molecules located in different microenvironments. The mean lifetime was around 713 ns, drastically longer than for solutions. The spectroscopic differences, i.e. marked blue shift, increased PLQY and longer lifetime, observed for the microcrystalline powders with respect to solutions, stem from environmental effects on the molecules. On the one hand, intermolecular interactions that take place in the solid state influence the energy of the ground and excited states. On the other hand, intramolecular motions are much more limited in crystals than in solution, which prevents vibrational deactivation of the excited states. The crystal packing mode, which does not show any detrimental intermolecular interaction, favors efficient light emission. Besides, the access of oxygen is limited in the crystal phase. All these

<table><caption>**Table 2** Spectroscopic data of complex **Re-BPTA** in the solid state (microcrystalline powder crystallized from butan-2-ol), as suspension in the water/acetonitrile 90 : 10 v/v mixture (AIPE experiment), and after dispersion of the powder in water by sonication. Comparison with the pristine microcrystalline powders of pyta-based complexes **Re-Phe**, **Re-Phe-Ada** and **Re-PBO**. Photoluminescence emission maximum ($\lambda_{PL}$) and quantum yield ($\Phi_{PL}$), lifetime ($\tau$) with fractions of intensity (fi), weighted average lifetimes ($\langle \tau \rangle$), and chi square ($\chi^2$) values</caption><thead><tr><th>Compound</th><th></th><th>λ<sub>PL</sub> [nm]<sup>a</sup></th><th>Φ<sub>PL</sub><sup>a</sup></th><th>τ<sub>PL</sub> [ns]</th><th>⟨τ<sub>PL</sub>⟩ [ns]</th><th>χ<sup>2</sup></th></tr></thead><tbody><tr><td rowspan="3"><b>Re-BPTA</b></td><td>Microcrystalline powder</td><td>546</td><td>0.52</td><td>60 (0.02), 730 (0.97)<sup>b</sup></td><td>713</td><td>1.11</td></tr><tr><td>Suspension in water/acetonitrile 90 : 10 v/v<sup>c</sup></td><td>548</td><td>0.09</td><td>58 (0.05), 395 (0.95)<sup>b</sup></td><td>377</td><td>1.09</td></tr><tr><td>Dispersion in water<sup>c</sup></td><td>538</td><td>—</td><td>49 (0.01), 799 (0.99)<sup>b</sup></td><td>791</td><td>1.05</td></tr><tr><td><b>Re-Phe</b></td><td>Microcrystalline powder</td><td>550<sup>d</sup></td><td>0.42<sup>d</sup></td><td>60 (0.02), 639 (0.98)<sup>b</sup></td><td>624</td><td>1.07</td></tr><tr><td><b>Re-Phe-Ada</b></td><td>Microcrystalline powder</td><td>550<sup>d</sup></td><td>0.59<sup>d</sup></td><td>60 (0.02), 646 (0.97)<sup>b</sup></td><td>631</td><td>1.07</td></tr><tr><td><b>Re-PBO<sup>e</sup></b></td><td>Microcrystalline powder</td><td>542</td><td>0.55</td><td>93 (0.12), 563 (0.85)</td><td>490</td><td>—</td></tr></tbody></table>

<footnote><sup>a</sup> $\lambda_{ex} \sim 382$ nm. <sup>b</sup> This work. $\lambda_{ex} = 370$ nm and $\lambda_{em} = 550$ nm. <sup>c</sup> Air-equilibrated. <sup>d</sup> From ref. 28. <sup>e</sup> From ref. 27.</footnote>
<figure><img src="image_16.png" /><figcaption>**Fig. 8** (a) Normalized PL spectra of Re-BPTA as powders obtained from butan-2-ol (blue line) and chloroform (red line), λ<sub>ex</sub> = 380 nm. Inset: powder obtained from butan-2-ol observed in daylight (top) and under illumination at 365 nm (bottom). (b) Samples of Re-BPTA at 3.3 × 10<sup>-5</sup> M in acetonitrile solutions containing from 0 to 95% water (from left to right), illuminated at 365 nm. (c) Corresponding emission spectra, λ<sub>ex</sub> = 350 nm. (d) Suspension with f<sub>w</sub> = 80% observed under the fluorescence microscope. (e) Evolution of the PL intensity at 546 nm as a function of the water/acetonitrile ratio. All samples for AIPE measurements were observed 2 h after preparation.</figcaption></figure>

parameters contribute to the good solid-state emission of **Re-BPTA**.

Let us now compare the performance of **Re-BPTA** (precipitated from butan-2-ol) with that of the pyta-based complexes as microcrystalline powders. The PLQY of **Re-BPTA** is slightly below that of the most emissive complexes, i.e. **Re-Phe-Ada** and **Re-PBO**, and much better than that of **Re-Phe**, whose structure is the most closely related. Its lifetime is also much longer than that of all pyta-based complexes investigated so far. The lowest triplet excited state, responsible for the phosphorescence of **Re-BPTA**, therefore seems particularly well stabilized in the crystalline phase. Probably, the main reason is that the overall geometry of the molecules does not vary much between the ground state and the emitting triplet state, according to the results of theoretical calculations (Fig. S9 and Table S5†). Besides, these calculated geometries are close to that of the molecule in the crystal, experimentally determined by XRD analysis (Fig. 3 and Table S2†). When the molecules reach the ³MLCT state, both phenyl rings undergo a little rotation in the same direction and the dihedral angle that they form with the triazole unit is reduced by about 27°. But, the

π-π stacking interaction appears to be maintained with a dis-
tance of 3.82 Å between the centroids of the both phenyl rings.
Therefore, in the excited state, even though they are not
directly involved in the emission process, the two phenyl rings
help to minimize the molecular deformations in the coordi-
nation sphere, thus allowing the emissive part of the molecule
to be effective.

Subsequently, mechanoresponsive luminescence (MRL)
properties were investigated (Table S8†). For the powder crys-
tallized from butan-2-ol, the emission wavelength was only
red-shifted by 2 nm upon grinding the pristine powder with a
pestle and a mortar, and the quantum yield was decreased
(0.38). The spectral characteristics of the pristine powder were
fully recovered after fuming with tetrahydrofuran for 48 h. The
sample crystallized in chloroform did not show any particular
spectral change upon grinding. The insignificant MRL behav-
ior of Re-BPTA is therefore very different from that of the pyta-
based complexes investigated so far, and particularly Re-PBO
whose emission spectrum is dramatically red-shifted by 58 nm
after mechanical stimulus.²⁷ This could be explained by the
stiffness of Re-BPTA: the relaxation of the packing constraints
due to a more or less advanced amorphization does not lead
to major geometrical changes in the molecule.

As the comparison between the solution and solid-state
emission quantum yields shows a clear solid-state lumine-
scence enhancement (SLE) effect, the effect of aggregation in
aqueous medium was also investigated. The popular procedure
that consists in increasing the water proportion in an organic
solution of the complex was used. Fig. 8b-e illustrates this
effect starting from pure acetonitrile, which was chosen for
solubility reasons although the complex is more emissive in
this solvent than in methanol. The photoluminescence inten-
sity was increased steadily when the water proportion in aceto-
nitrile passed from 0 to 90%, and then the intensity abruptly
decreased for 95% water. This intensity change was
accompanied by a significant (~40 nm) blue shift of the spec-
trum, which peaked at around 546-550 nm, much closer to
the microcrystalline powder than to solutions (Fig. 6). The
mean emission lifetime of the sample containing 90% water
was 377 ns, almost four times longer than in acetonitrile. It is
characteristic of molecules in a solid phase. Besides, the obser-
vation of the suspensions with the fluorescence microscope
showed the presence of tiny microcrystals that strongly emit in
the yellow (Fig. 8d), and a diffuse background of small, weakly
emissive particles. Microcrystals seemed to be fewer in the
sample that contains the largest amount of water, where amor-
phous particles may form very quickly, which could explain
the weaker PL signal. A slight increase of the emission inten-
sity of the water-containing samples, and an evolution of their
respective intensities, were also observed after 24 h, probably
related to slow growth of the particles (Fig. S18†). At 546 nm,
the intensity of the PL signal was increased more than seven
times after 2 h, and ten times after 24 h. However, the AIPE
experiment does not reflect the strong difference observed
between the PLQY of acetonitrile solution and microcrystalline
powders of **Re-BPTA**. The comparison between air-equilibrated
and argon-bubbled samples showed that the effect of dissolved
oxygen does not explain this discrepancy: for the sample con-
taining 90% water, the PLQY only varied from 0.09 to 0.11. It
can then be imagined that the microcrystals formed are sensi-
tive to quenching by water. To test this hypothesis, a small
amount of microcrystalline **Re-BPTA** was dispersed by soni-
cation (5 min) in water, where it is insoluble. The sample was
strongly emissive under the UV lamp, the shape of the emis-
sion spectrum and the long lifetime (i.e. 791 ns) resembled
those of the native powder, showing that the aqueous sur-
rounding has a weak effect on microcrystal emission. The com-
parison with the suspension in H₂O/CH₃CN 90:10 v/v
(Fig. S19†) confirms that molecules in the solid state are the
emitting species in both samples, but their nature is probably
different. The lower quantum yield and shorter lifetime of the
suspensions formed during the AIPE experiment could be
attributed to the predominant formation of a solid, i.e. amor-
phous aggregates, hydrates, or very small crystals containing a
lot of surface defects, much less emissive than the microcrys-
tals prepared from an organic solution. Finally, it must be
added that a phosphorescence decrease was observed after

several days for the water-containing samples, suggesting a
slow degradation of **Re-BPTA** in these conditions, but this
phenomenon was not noticeable given the duration of the
AIPE experiment.

Conclusions

Although this approach has been known for a long time, until
now limited efforts have been devoted to the design and syn-
thesis of transition metal complexes with intramolecular π-π
stacking for solid-state luminescence enhancement. The fact
that π-π stacking intramolecular interactions improve the
molecular rigidity, and thus lead to increased PL, has already
been reported in the literature e.g. for heteroleptic mono- and
di-nuclear Cu(II) complexes⁴⁹ and for Ir(I) complexes with very
elaborated structures.²² In the present work, the intra-
molecular interaction does not require very bulky
ligands¹²⁻¹⁸,²¹,²² or polyphenyl units,²³,²⁴ specially designed to
form a protecting cage around the metal ion. Instead, we
designed a minimalist ligand. A symmetrical bi-(1,2,4-triazole)
unit (less popular than the bi-(1,2,3-triazole) unit that is easily
obtained by click chemistry⁵⁰⁻⁵²), provided with two appended
phenyl rings, was efficiently prepared. The resulting ligand, *i.e.*
**BPTA**, exhibits excellent chelating properties and leads to
complex **Re-BPTA**, the rigidity of which is reinforced by the π-π
stacking interaction between the phenyl rings. One conse-
quence of the molecular stiffness is that, for molecules in solu-
tion, the emissive ³MLCT state is stabilized with respect to our
previously studied pyta-based complexes in which the phenyl
ring rotates more freely. The phosphorescence quantum yield
and lifetime are thus improved, but this effect is partly coun-
terbalanced by an increased sensitivity to dissolved oxygen.
Consequently, the overall emission efficiency remains weak. In
the microcrystalline phase, **Re-BPTA** shows high PLQY and
competes well with the most emissive pyta-based complexes.
The particularly long emission lifetime reflects the good stabi-
lity of the ³MLCT state. In addition, evidence was given for
clear AIPE properties, most of the emission enhancement
being due to the formation of microcrystals.

To the best of our knowledge, our work presents the first
example of Re(I) complex exploiting the concept of intra-
molecular π-π stacking for solid state luminescent enhance-
ment. Moreover, this simple **Re-BPTA** complex can be seen as a
new starting block for the preparation of valuable families of
SLE-active photoluminescent materials. Indeed, preliminary
experiments have shown that the **Re-BPTA** complex can be sub-
stituted with a linear alkyl chain in the *para*-position of each
phenyl rings, while retaining almost the same optical pro-
perties. The advantage of this symmetrical molecule is that it
can easily carry a double recognition unit, thus increasing its
affinity towards a given target, potentially making the substi-
tuted complex a better candidate than the parent compound
for AIPE-based biological applications. More generally, substi-
tution on the phenyl rings with different groups could also
allow the molecular framework to be modified. All these possi-
bilities offer the means to better understand the role of intra-molecular π-π stacking interactions on the SLE properties of Re(i) complexes, and to modulate them through an original approach.

# Experimental section

## General methods

All purchased chemicals were of the highest purity commercially available and used without further purification. Unless otherwise noted, all experiments were carried out under a nitrogen atmosphere. All reactions were monitored by TLC on silica gel Alugram® Xtra SIL G/UV254. Column chromatography was performed on Machery-Nagel silica gel. NMR spectra were recorded with Bruker Avance 300, 400 and 500 instruments. Chemical shifts are given in ppm and are referenced by using the residual signals of the solvent as internal standard. Signals are described as follow: s, singlet; d, doublet; t, triplet; m, multiplet. HRMS data were recorded on a Xero G2 QTOF (Waters) instrument. Infrared spectra were obtained on a Nexus Thermonicolet apparatus with DTGS as the detector. The microanalysis was made in the “Service d’analyse” of LCC using a PerkinElmer 2400 series II analyzer.

### 5,5'-Dimethyl-2,2'-bi(1,3,4-oxadiazole) (1)

Oxalyl chloride (200 µL, 2.33 mmol) was slowly introduced into a solution of 5-methyltetrazole (567 mg, 6.74 mmol) in anhydrous xylenes (10 mL) under nitrogen atmosphere. The mixture was heated for 2 h at 120 °C. The solvent was removed by evaporation under vacuum and the resulting solid was purified on a silica gel column chromatography (DCM/MeOH from 100:0 to 98:2 v/v) to yield compound 1 as a white solid (267 mg, 69%).³⁶

¹H NMR (300 MHz, CDCl₃): δ (ppm) = 2.71 (s, 6H, H₆). ¹³C NMR (75 MHz, CDCl₃): δ (ppm) = 165.9 (C₂), 153.5 (C₅), 11.2 (C₆).

### 5,5'-Dimethyl-4,4'-diphenyl-4H,4'H-3,3'-bi(1,2,4-triazole) (BPTA)

A mixture of compound 1 (148 mg, 0.9 mmol), p-toluenesulfonic acid hydrate (51 mg, 0.27 mmol) and aniline (0.3 mL, 3.2 mmol) in xylenes (8 mL) was heated at 140 °C under nitrogen atmosphere overnight. After cooling to room temperature, the solvent was removed under vacuum and the residue was purified on a silica gel column chromatography (DCM/MeOH 97:3) to yield compound BPTA as a white solid (261 mg, 92%).

¹H NMR (300 MHz, CDCl₃): δ (ppm) = 7.56-7.34 (m, 6H, H₉,10,11), 7.14-6.99 (m, 4H, H₈,12), 2.28 (s, 6H, H₆). ¹³C NMR (75 MHz, CDCl₃): δ (ppm) = 153.1 (C₂), 143.6 (C₅), 133.6 (C₇), 129.91 (C₁₀), 129.86 (C₉,11), 127.0 (C₈,12), 11.3 (C₆). HRMS (ESI⁺): m/z = 317.1515 ([M + H]⁺ calcd for C₁₈H₁₇N₆: 317.1506).

**Complex [ReCl(CO)₃(BPTA)] (Re-BPTA).** A mixture of ligand BPTA (50 mg, 0.158 mmol) and [ReCl(CO)₅] (74 mg, 0.205 mmol) in butan-2-ol (5 mL) was stirred overnight at

100 °C. The reaction mixture was cooled to room temperature. The yellow precipitate was collected by filtration, washed with methanol and dried in vacuo, yielding complex Re-BPTA (72 mg, 73%) as a bright yellow solid, which was pure enough to be used without further purification.

¹H NMR (500 MHz, CDCl₃): δ (ppm) = 7.35 (tt, J = 7.6, 1.1 Hz, 2H, H₁₀), 7.20 (m, 4H, H₉,11), 6.90 (d, J = 7.9 Hz, 2H, H₈), 6.86 (d, J = 8.0 Hz, 2H, H₁₂), 2.20 (s, 6H, H₆). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) = 196.3 (CO₃), 188.1 (CO₆), 156.2 (C₂), 147.2 (C₅), 132.3 (C₇), 131.2 (C₁₀), 130.9 (C₁₁), 130.6 (C₉), 127.0 (C₁₂), 126.4 (C₈), 11.2 (C₆). HRMS (ESI⁺): m/z = 585.0820 ([M - Cl]⁺ calcd for C₂₁H₁₆N₆O₃¹⁸⁵Re: 585.0813). IR (ATR): ν(CO) = 2017, 1914, 1888 cm⁻¹. Anal. Calcd (%) for C₂₁H₁₆N₆O₃ReCl: C 40.55, H 2.59, N, 13.51; found C 40.04, H 2.24, N, 13.28.

## Crystallography

Crystal data were collected at 193K using MoKα radiation (wavelength = 0.71073 Å) on a Bruker AXS D8-Venture diffractometer equipped with a Mo Kα sealed tube (λ = 0.71073 Å), a multilayer TRIUMPH X-ray mirror, a Photon III-C14 detector and an Oxford Instruments Cryostream 700+ Series low-temperature device. Phi- and omega-scans were used. The space group was determined on the basis of systematic absences and intensity statistics. Semi-empirical absorption correction was employed.⁵³ The structures were solved using an intrinsic phasing method (ShelXT).⁵⁴ All non-hydrogen atoms were refined anisotropically using the least-square method on F².⁵⁵ Hydrogen atoms were refined isotropically at calculated positions using a riding model with their isotropic displacement parameters constrained to be equal to 1.5 times the equivalent isotropic displacement parameters of their pivot atoms for terminal sp³ carbon and 1.2 times for all other carbon atoms. CCDC 2212965 (BPTA) and CCDC 2212966 (Re-BPTA)† contain the supplementary crystallographic data for this paper. Selected crystallographic data are collected in Table 3.

## Spectroscopy

Spectroscopic measurements in solutions were conducted at 20 °C in a temperature-controlled cell. UV-visible absorption spectra and emission spectra in solutions were measured with a Xenius SAFAS spectrofluorometer using cells of 1 cm optical pathway. All emission spectra were corrected. The emission quantum yields in solution (Φ) were determined using the classical formula:

$$ \Phi_x = (A_s \times I_x \times n_x^2 \times \Phi_s)/(A_x \times I_s \times n_s^2) \quad (1) $$

where A is the absorbance at the excitation wavelength, I the integrated emission intensity and n the refractive index. Subscripts s and x refer to the standard and to the sample of unknown quantum yield, respectively. Coumarin 153 (Φs = 0.53) in ethanol was used as the standard.⁵⁶ The absorbance of the solutions was equal or below 0.06 at the excitation wavelength. The error on the quantum yield values is estimated to be about 10% for solutions and 20% for suspensions.

<figure><img src="image_17.png" /></figure>
<table><caption>**Table 3** Selected crystallographic data of ligand BPTA and complex Re-BPTA</caption><thead><tr><th></th><th>BPTA</th><th>Re-BPTA</th></tr></thead><tbody><tr><td>Empirical formula</td><td>C<sub>18</sub>H<sub>16</sub>N<sub>6</sub></td><td>C<sub>21</sub>H<sub>16</sub>N<sub>6</sub>O<sub>3</sub>ClRe</td></tr><tr><td>Formula weight</td><td>316.37</td><td>622.05</td></tr><tr><td>Crystal system</td><td>Monoclinic</td><td>Monoclinic</td></tr><tr><td>Space group</td><td>P2<sub>1</sub>/c</td><td>P2<sub>1</sub>/c</td></tr><tr><td><strong>Unit cell dimensions</strong></td><td></td><td></td></tr><tr><td>a (Å)</td><td>9.5185(5)</td><td>8.3018(5)</td></tr><tr><td>b (Å)</td><td>7.0893(4)</td><td>22.7231(10)</td></tr><tr><td>c (Å)</td><td>11.7987(6)</td><td>11.6873(7)</td></tr><tr><td>α (°)</td><td>90</td><td>90</td></tr><tr><td>β (°)</td><td>98.989(2)</td><td>99.998(2)</td></tr><tr><td>γ (°)</td><td>90</td><td>90</td></tr><tr><td>Volume (Å<sup>3</sup>)</td><td>786.39(7)</td><td>2171.2(2)</td></tr><tr><td>Z</td><td>2</td><td>4</td></tr><tr><td>Density (calculated) (Mg m<sup>-3</sup>)</td><td>1.336</td><td>1.903</td></tr><tr><td>Crystal size (mm<sup>3</sup>)</td><td>0.300 × 0.150 × 0.100</td><td>0.200 × 0.080 × 0.060</td></tr><tr><td>Reflections collected</td><td>32 207</td><td>69 475</td></tr><tr><td>Independent reflections</td><td>2389 [R<sub>(int)</sub> = 0.0310]</td><td>5397 [R<sub>(int)</sub> = 0.0637]</td></tr><tr><td>Restraints/parameters</td><td>0/110</td><td>0/291</td></tr><tr><td>Final R<sub>1</sub> index I > 2σ (I)</td><td>0.0476</td><td>0.0306</td></tr><tr><td>wR<sub>2</sub> (all data)</td><td>0.1303</td><td>0.0648</td></tr><tr><td>Largest diff. peak and hole (e Å<sup>-3</sup>)</td><td>0.409 and -0.211</td><td>1.768 and -1.581</td></tr><tr><td>CCDC</td><td>2212965†</td><td>2212966†</td></tr></tbody></table>

For AIPE measurements, a small volume (30 µL) of a concentrated solution of Re-BPTA in acetonitrile was injected in 2.97 mL of various acetonitrile/water mixtures. The samples were left to stand under stirring in the dark, and then they were sonicated for 5 min before optical measurement, so that they were as homogeneous as possible. Absorbance variations due to scattering by microparticles, and in particular the baseline deviation, were taken into account for measuring the extinction coefficient value, the fluorescence quantum yields of the suspension in the water/acetonitrile 90 : 10 v/v mixture, and the magnitude of the AIPE effect.

Solid state spectra were recorded on the same Xenius SAFAS spectrofluorometer using an integrating sphere. They were corrected using a home-made correction curve. Solid samples were deposited on a metal support. The absolute photoluminescence quantum yield values ($\Phi_P$) were determined by a method based on the one developed by De Mello et al.,⁵⁷ as described elsewhere.²⁸ The error was estimated to be about 20%.

The 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 ($\lambda_{ex}$ = 371 nm). The absorbance of solutions and suspensions at $\lambda_{ex}$ was lower than 0.1. The solid state sample was deposited on a quartz holder. Photons were detected at 90° through a monochromator by means of a Hamamatsu R928 photomultiplier. Emission was recorded near the maximum with a bandpass of 10–15 nm. The instrumental response was recorded at 371 nm. All analyses were recorded using the Datastation v2.7 software. The decay curves (Fig. S20–S27†) were analyzed with reconvolution and global non-linear least-squares minimization method using DAS6 v6.8 software.

The rate constants for radiative ($k_r$) and nonradiative ($k_{nr}$) deactivations were calculated using the following equation:

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

with $\Phi$ the emission quantum yield and $\tau$ the luminescence lifetime in solution.

Fluorescence microscopy was performed with a Leitz Laborlux D fluorescence microscope equipped with an Andor Luca camera ($\lambda_{ex}$ ~ 450–490 nm, $\lambda_{em}$ > 500 nm).

## Computational details

The ORCA software was employed for all calculations (the geometry optimization, the ground-state and excited-state electronic structures, and optical spectra) with the aid of the Gabedit visualization program.⁵⁸ Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed with the PBE0 functional.⁵⁹ The ground state ($S_0$) and the lowest triplet state ($T_1$) geometries were fully optimized with the DFT method using the Perdew-Burke-Ernzerhof PBE0 functional without symmetry constraints.⁶⁰ The “double-ζ” quality basis set LANL2DZ with Hay and Wadt’s relative effective core potential ECP (outer-core [(5s²5p⁶)]) electrons and the (5d⁶) valence electrons⁶¹,⁶² was employed for the Re⁺ cation. The solvent effect (DCM, $\epsilon$ = 9.08) was simulated using the Conductor-like Polarizable Continuum Model (CPCM).⁶³,⁶⁴ The vibrational frequency calculations were performed using the optimized structural parameters of the complex, to confirm that the optimized structure represents a local minimum on the potential energy surface. On the basis of the optimized ground state geometry, the absorption properties were calculated by the TD-DFT method at the PBE0/LANL2DZ level. The emission has been calculated by DFT considering

<figure><img src="image_18.png" /></figure>
the difference of energy between the optimized triplet state and the singlet state at the same geometry. Orbital compositions have been calculated using Multifwn software⁶⁵ with the orbital composition analysis (Mulliken partition) function.⁶⁶

## Electrochemistry

The electrochemical properties of **BPTA** and **Re-BPTA** were determined by cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV) in DCM. The solutions used during the electrochemical studies were typically $1 \times 10^{-3}$ M in complex, and 0.1 M in supporting electrolyte. The supporting electrolyte (nBu₄N)(BF₄) (Fluka, 99% electrochemical grade) was used as received and simply degassed under Ar. DCM was dried using an MB SPS-800 solvent purification system just prior to use. The measurements were carried out with an Autolab PGSTAT100 potentiostat controlled by GPES 4.09 software. Experiments were performed at room temperature (r.t.) in a homemade airtight three-electrode cell connected to a vacuum/Ar line. The reference electrode consisted of a saturated calomel electrode (SCE) separated from the solution by a bridge compartment. The counter electrode was a Pt wire of *ca.* 1 cm² apparent surface. The working electrode was a Pt microdisk (0.5 mm diameter). Before each measurement, the solutions were degassed by bubbling Ar and the working electrode was polished with a polishing machine (Presi P230). Under these experimental conditions, Fc+/Fc is observed at +0.55 ± 0.01 V vs. SCE. OSWVs were obtained using an amplitude of 20 mV, a frequency of 20 Hz, and a step potential of 5 mV.

## Conflicts of interest

There are no conflicts to declare.

## Acknowledgements

ANR is gratefully acknowledged for funding (PANTHERA project # ANR-22-CE29-0008-1). We thank Dr Alix Sourià-Saquet and Mr Alain Moreau (LCC) for their help in electrochemical measurements and Mr Anthony Augé for repreparing the **Re-BPTA** complex.

## References

1. J. Gierschner, J. Shi, B. Milián-Medina, D. Roca-Sanjuán, S. Varghese and S. Y. Park, *Adv. Opt. Mater.*, 2021, 2002251.

2. P. Alam, C. Climent, P. Alemany and I. R. Laskar, *J. Photochem. Photobiol., C*, 2019, 41, 100317.

3. L. Ravotto and P. Ceroni, *Coord. Chem. Rev.*, 2017, **346**, 62–76.

4. V. Sathish, A. Ramdass, P. Thanasekaran, K.-L. Lu and S. Rajagopal, *J. Photochem. Photobiol., C*, 2015, **23**, 25–44.

5. M. Mauro and C. Cebrián, *Isr. J. Chem.*, 2018, **58**, 901–914.

6. A. Ramdass, V. Sathish and P. Thanasekaran, *Results Chem.*, 2022, **4**, 100337.

7. Z. Song, R. Liu, H. Zhu, Y. Lu, X. Li and H. Zhu, *Sens. Actuators, B*, 2019, **279**, 385–392.

8. H. Yang, H. Li, L. Yue, X. Chen, D. Song, X. Yang, Y. Sun, G. Zhou and Z. Wu, *J. Mater. Chem. C*, 2021, **9**, 2334–2349.

9. L. Ma, Y. Wang, X. Wang, Q. Zhu, Y. Wang, L. Li, H.-B. Cheng, J. Zhang and X.-J. Liang, *Coord. Chem. Rev.*, 2022, **473**, 214822.

10. H. Shen, C. Xu, F. Sun, M. Zhao, Q. Wu, J. Zhang, S. Li, J. Zhang, J. W. Y. Lam and B. Z. Tang, *ChemMedChem*, 2022, **17**, e202100578.

11. X. Lin, W. Li, Y. Wen, L. Su and X. Zhang, *Biomaterials*, 2022, **287**, 121603.

12. S. Graber, K. Doyle, M. Neuburger, C. E. Housecroft, E. C. Constable, R. D. Costa, E. Ortí, D. Repetto and H. J. Bolink, *J. Am. Chem. Soc.*, 2008, **130**, 14944–14945.

13. R. D. Costa, E. Ortí, H. J. Bolink, S. Graber, C. E. Housecroft and E. C. Constable, *Chem. Commun.*, 2011, **47**, 3207–3209.

14. R. D. Costa, E. Ortí, H. J. Bolink, S. Graber, C. E. Housecroft and E. C. Constable, *J. Am. Chem. Soc.*, 2010, **132**, 5978–5980.

15. R. D. Costa, E. Ortí, H. J. Bolink, S. Graber, C. E. Housecroft and E. C. Constable, *Adv. Funct. Mater.*, 2010, **20**, 1511–1520.

16. H. J. Bolink, E. Coronado, R. D. Costa, E. Ortí, M. Sessolo, S. Graber, K. Doyle, M. Neuburger, C. E. Housecroft and E. C. Constable, *Adv. Mater.*, 2008, **20**, 3910–3913.

17. G.-G. Shan, H.-B. Li, D.-X. Zhu, Z.-M. Su and Y. Liao, *J. Mater. Chem.*, 2012, **22**, 12736–12744.

18. L. He, L. Duan, J. Qiao, D. Q. Zhang, L. D. Wang and Y. Qiu, *Chem. Commun.*, 2011, **47**, 6467–6469.

19. R. D. Costa, E. Ortí, H. J. Bolink, S. Graber, C. E. Housecroft, M. Neuburger, S. Schaffner and E. C. Constable, *Chem. Commun.*, 2009, 2029–2031.

20. R. D. Costa, F. Monti, G. Accorsi, A. Barbieri, H. J. Bolink, E. Ortí and N. Armaroli, *Inorg. Chem.*, 2011, **50**, 7229–7238.

21. L. He, D. Ma, L. Duan, Y. Wei, J. Qiao, D. Zhang, G. Dong, L. Wang and Y. Qiu, *Inorg. Chem.*, 2012, **51**, 4502–4510.

22. K. Traskovskis, V. Kokars, S. Belyakov, N. Lesina, I. Mihailovs and A. Vembris, *Inorg. Chem.*, 2019, **58**, 4214–4222.

23. H. Tsubaki, S. Tohyama, K. Koike, H. Saitoh and O. Ishitani, *Dalton Trans.*, 2005, 385–395.

24. H. Tsubaki, A. Sekine, Y. Ohashi, K. Koike, H. Takeda and O. Ishitani, *J. Am. Chem. Soc.*, 2005, **127**, 15544–15555.

25. J. Wang, B. Delavaux-Nicot, M. Wolff, S. Mallet-Ladeira, R. Métivier, E. Benoist and S. Fery-Forgues, *Dalton Trans.*, 2018, **47**, 8087–8099.

26. J. Wang, A. Poirot, B. Delavaux-Nicot, M. Wolff, S. Mallet-Ladeira, J. P. Calupitan, C. Allain, E. Benoist and S. Fery-Forgues, *Dalton Trans.*, 2019, **48**, 15906–15916.

27. J. P. Calupitan, A. Poirot, J. Wang, B. Delavaux-Nicot, M. Wolff, M. Jaworska, R. Métivier, E. Benoist, C. Allain and S. Fery-Forgues, *Chem. – Eur. J.*, 2021, **27**, 4191–4196.

<figure><img src="image_19.png" /></figure>
28 A. Poirot, C. Vanucci-Bacqué, B. Delavaux-Nicot, N. Leygue, N. Saffon-Merceron, F. Alary, F. Bedos-Belval, E. Benoist and S. Fery-Forgues, *Dalton Trans.*, 2021, **50**, 13686–13698.

29 A. Poirot, C. Vanucci-Bacqué, B. Delavaux-Nicot, N. Saffon-Merceron, C.-L. Serpentini, N. Leygue, F. Bedos-Belval, E. Benoist and S. Fery-Forgues, *Photochem. Photobiol. Sci.*, 2023, **22**, 1110–1119.

30 K. M. Dawood, B. F. Abdel-Wahab and M. A. Raslanc, *Arkivoc*, 2018, **part i**, 179–215.

31 L.-Y. Du, H. Wang, G. Liu, D. Xie, F.-S. Guo, L. Hou and Y.-Y. Wang, *Dalton Trans.*, 2015, **44**, 1110–1119.

32 L.-H. Jia, A.-C. Liu, B.-W. Wang, Z.-M. Wang and S. Gao, *Polyhedron*, 2011, **30**, 3112–3115.

33 E. V. Govor, A. B. Lysenko and K. V. Domasevitch, *Acta Crystallogr.*, 2008, **C64**, m201–m204.

34 C. A. Hunter and J. K. M. Sanders, *J. Am. Chem. Soc.*, 1990, **112**, 5525–5534.

35 M. O. Sinnokrot and C. D. Sherrill, *J. Phys. Chem. A*, 2006, **110**, 10656–10668.

36 A. V. Kuttatheyil, M. Handke, J. Bergmann, D. Lässig, J. Lincke, J. Haase, M. Bertmer and H. Krautscheid, *Eur. J. Chem.*, 2015, **21**, 1118–1124.

37 L. Suntrup, S. Klenk, J. Klein, S. Sobottka and B. Sarkar, *Inorg. Chem.*, 2017, **56**, 5771–5783.

38 G. R. Desiraju and A. Gavezzotti, *Acta Crystallogr.*, 1989, **B45**, 473–482.

39 C. Janiak, *J. Chem. Soc., Dalton Trans.*, 2000, 3885–3896.

40 R. Eychenne, S. Guizani, J.-H. Wang, C. Picard, N. Malek, P.-L. Fabre, M. Wolff, B. Machura, N. Saffon, N. Lepareur and E. Benoist, *Eur. J. Inorg. Chem.*, 2017, 69–81.

41 T. Y. Kim, A. B. S. Elliott, K. J. Shaffer, C. J. McAdam, K. C. Gordon and J. D. Crowley, *Polyhedron*, 2013, **52**, 1391–1398.

42 W. K. C. Lo, G. S. Huff, J. R. Cubanski, A. D. W. Kennedy, C. J. McAdam, D. A. McMorran, K. C. Gordon and J. D. Crowley, *Inorg. Chem.*, 2015, **54**, 1572–1587.

43 P. Datta, D. Sarkar, A. P. Mukhopadhyay, E. Lopez-Torrez, C. J. Pastor and C. Sinha, *J. Organomet. Chem.*, 2011, **696**, 488–495.

44 J. Su, T. Fukaminato, J.-P. Placial, T. Onodera, R. Suzuki, H. Oikawa, A. Brosseau, F. Brisset, R. Pansu, K. Nakatani and R. Métivier, *Angew. Chem., Int. Ed.*, 2016, **55**, 3662–3666.

45 A. D. Laurent and D. Jacquemin, *Int. J. Quantum Chem.*, 2013, **113**, 2019–2039.

46 T. Sato, Y. Hamada, M. Sumikawa, S. Araki and H. Yamamoto, *Ind. Eng. Chem. Res.*, 2014, **53**, 19331–19337.

47 C. Franco and J. Olmsted 3rd, *Talanta*, 1990, **37**, 905–909.

48 C. Carayon, A. Ghobane, N. Leygue, J. Wang, N. Saffon-Merceron, R. Brown and S. Fery-Forgues, *ChemPhotoChem*, 2019, **3**, 545–553.

49 E. Leoni, J. Mohanraj, M. Holler, M. Mohankumar, I. Nierengarten, F. Monti, A. Sournia-Saquet, B. Delavaux-Nicot, J.-F. Nierengarten and N. Armaroli, *Inorg. Chem.*, 2018, **57**, 15537–15549.

50 S. Hohloch, L. Suntrup and B. Sarkar, *Organometallics*, 2013, **32**, 7376–7385.

51 C. E. Welby, S. Grkinic, A. Zahid, B. S. Uppal, E. A. Gibson, C. R. Rice and P. I. P. Elliott, *Dalton Trans.*, 2012, **41**, 7637–7646.

52 A. Mattiuzzi, I. Jabin, C. Moucheron and A. Kirsch-De Mesmaeker, *Dalton Trans.*, 2011, **40**, 7395–7402.

53 SADABS, Program for data correction, Bruker-AXS.

54 G. M. Sheldrick, *Acta Crystallogr., Sect. A: Found. Adv.*, 2015, **71**, 3–8.

55 G. M. Sheldrick, *Acta Crystallogr., Sect. C: Struct. Chem.*, 2015, **71**, 3–8.

56 K. Suzuki, A. Kobayashi, S. Kaneko, K. Takehira, T. Yoshihara, H. Ishida, Y. Shiina, S. Oishi and S. Tobita, *Phys. Chem. Chem. Phys.*, 2009, **11**, 9850–9860.

57 J. C. De Mello, H. F. Wittmann and R. H. Friend, *Adv. Mater.*, 1997, **9**, 230–232.

58 A.-R. Allouche, *J. Comput. Chem.*, 2011, **32**, 174–182.

59 C. Adamo and V. Barone, *J. Chem. Phys.*, 1999, **110**, 6158–6170.

60 J. P. Perdew, K. Burke and M. Ernzerhof, *Phys. Rev. Lett.*, 1996, **77**, 3865–3868.

61 P. J. Hay and W. R. Wadt, *J. Chem. Phys.*, 1985, **82**, 270–283.

62 P. J. Hay and W. R. Wadt, *J. Chem. Phys.*, 1985, **82**, 299–310.

63 B. Mennucci and J. Tomasi, *J. Chem. Phys.*, 1997, **106**, 5151–2158.

64 M. Cossi, V. Barone, B. Mennucci and J. Tomasi, *Chem. Phys. Lett.*, 1998, **286**, 253–260.

65 T. Lu and F. Chen, *J. Comput. Chem.*, 2012, **33**, 580–592.

66 T. Lu and F. Chen, *Acta Chim. Sin.*, 2011, **69**, 2393–2406 (https://sioc-journal.cn/jwk_hxxb/CN/abstract/abstract340458.shtml).