1,2,3-Triazol-5-ylidene-vs. 1,2,3-triazole-based tricarbonylrhenium( I ) complexes: in ﬂ uence of a mesoionic carbene ligand on the electronic and biological properties

The tricarbonylrhenium complexes that incorporate a mesoionic carbene ligand represent an emerging and promising class of molecules, the solid-state optical properties of which have rarely been investigated. The aim of this comprehensive study is to compare three of these complexes with their 1,2,3-tri-azole-based analogues. The Hirshfeld surface analysis of the crystallographic data revealed that the tria-zolylidene derivatives are more prone to π – π interactions than their 1,2,3-triazole-based counterparts. The FT-IR and electrochemical data indicated a stronger electron donor e ﬀ ect from the organic ligand to the rhenium atom for triazolylidene derivatives, which was con ﬁ rmed by DFT calculations. All compounds were phosphorescent in solution, where the 1,2,3-triazole-based complexes showed unusually strong dependence on dissolved oxygen. All compounds also emitted in the solid state, some of them exhibited marked solid-state luminescence enhancement (SLE) e ﬀ ect. The 1,2,3-triazole based complex Re-Phe even displayed astounding photoluminescence e ﬃ ciency with quantum yield up to 0.69, and proved to be an excellent candidate for applications linked to aggregation-induced emission (AIE). Interestingly, one triazolylidene-based complex ( Re-T-BOP ) showed attractive antibacterial activity. This study highlights the potential of these new molecules for applications in the ﬁ elds of photoluminescent and therapeutic materials, and provides the ﬁ rst bases for the design of e ﬃ cient molecules in these research areas


Introduction
][3] Their modular synthesis and impressive sigma electron-donating properties have made them excellent ligands for transition metals.Many of the resulting Ir(III), 4 Au(III), 5 Ru(II) and Os(II) 6 complexes exhibit very attractive physicochemical and biological properties.Curiously, rhenium(I) complexes are completely under-represented in these studies.An exception is the pioneering work of Sarkar and co-workers.By comparing seven tricarbonylrhenium(I) complexes, this group has established through detailed FTIR spectroscopy and electrochemical investigations that triazolylidenes are indeed stronger electron donors than pyridine and triazoles, 7 and that these phosphorescent complexes are excellent electrocatalysts for the reduction of CO 2 to CO. 8 If considering a broader perspective, other types of tricarbonylrhenium(I) N-heterocyclic carbenes (NHC) complexes have proven to be particularly valuable. 9,10][24] According to Massi and co-workers, they exhibit a particular photochemical reactivity 15,16 and can be used as photoinitiators of polymerization upon visible light. 25Regarding biological applications, these complexes are precursors for radiopharmaceuticals and diagnostic agents. 26They have been tested as luminescent markers for aminoacids, 27 imaging probes to detect amyloid plaques of Alzheimer's disease, 28,29 anticancer drugs 30,31 and antibacterial agents. 32In light of these inspiring examples, this research field deserves to be more widely explored, especially when it comes to tricarbonylrhenium(I) complexes incorporating a 1,2,3-triazol-5-ylidene ligand, and to their solid-state photoluminescence (PL) and biological properties.
4][35][36][37][38] The chloride ancillary ligand endows our complexes with good photochemical stability.The bidentate pyridyl-triazole ( pyta) moiety, appended with an aromatic ring, allows the luminescence behavior to be finely tuned.The pyta moiety has excellent chelating ability and exists in various isomeric forms.4][45] These properties seem to be promoted by a large phenyl-pyta dihedral angle.In contrast, complex Re-BOP that incorporates an almost planar organic ligand based on a 4-(2-pyridyl)-1,2,3-triazole moiety is only weakly emissive in the solid state, 39 and there is no information on how other members of this series may behave.Consequently, it seemed instructive to compare a series of original complexes based on pyridyl-triazolylidene ( pytrz), and more exactly pyridyl-1,2,3triazol-5-ylidene, with their poorly-known pyridyl-1,2,3-triazole counterparts, in order to move forwards two steps at once.Variations of the electron-donating capacity of the ligand should impact the electronic properties in solution and biological properties.Changes of the molecular shape and intermolecular arrangement should strongly affect the solid-state PL properties. 38To this end, triazolylidene-based complexes Re-T-BOP and Re-T-Phe were synthesized, as well as complex Re-T-Tol that allows studying the influence of the methyl group in the ortho position of the phenyl ring.In fact, tiny modifications of the organic ligand are enough to change drastically the PL properties of the complexes. 44The triazole-based complexes Re-Phe and Re-Tol were prepared for the sake of comparison, in addition to the data already available about Re-BOP. 39The electronic properties of the new complexes were studied both experimentally and theoretically.Their compari-son brings out unexpected information about the factors that govern the PL properties and highlights the exceptional potential of one of the molecules.To the best of our knowledge, the present study is the first one that considers the influence of mesoionic carbene ligands on the solid-state photoluminescence properties of rhenium(I) complexes.Additionally, some rhenium(I) complexes have been recently reported to be efficient antibacterial agents, 46,47 a capability currently in great demand to address the growing danger raised by antibiotic-resistant bacteria. 48This prompted us to evaluate the antibacterial properties of our complexes, and one triazolylidene-based complex appeared as a promising candidate.

Synthesis and characterization
The 2-pyridyl-1,2,3-triazole ligands were obtained via condensation of an azide derivative with 2-ethynylpyridine via a copper(I) catalyzed alkyne-azide cycloaddition using classical click reaction conditions.Specifically, 2-(1-phenyl-1H-1,2,3-triazol-4-yl)pyridine L-Phe was prepared following a described one-pot procedure, in which azidobenzene was generated in situ from iodobenzene. 49The o-tolyl derivative L-Tol was obtained in 77.5% yield starting from commercially available azido-2-methylbenzene.The benzoxazole derivative L-BOP was prepared as previously described. 39Subsequent reaction of the ligands L-Phe, L-PBO and L-Tol in refluxing methanol in the presence of [Re(CO) 5 Cl] afforded respectively the previously described tricarbonylrhenium complexes Re-Phe 50 and Re-BOP, 39 as well as the new complex Re-Tol in 86% yield (Scheme 1).
Concerning the methylated triazolylidene precursors, the triazolium salt L-T-Phe was selectively synthesized from L-Phe with good efficiency, using a 3-step described procedure that involves the protection of the pyridine nitrogen as an N-oxide, followed by a methylation using Meerwein's salt, and subsequent N-O bond cleavage in the presence of Mo(CO) 6 . 51ollowing a similar procedure, the triazolium salt L-T-Tol was obtained from L-Tol in 82.2% yield.However, this protocol failed when applied to the phenylbenzoxazole pyridyl-1,2,3-triazole ligand L-T-BOP, as the methylation step only afforded an unusable mixture of compounds.Thus, direct methylation of the ligand in the presence of methyl trifluoromethane sulfonate (MeOTf ) was performed, leading to a mixture of the expected product L-T-BOP in 25% yield, along with the undesired product N-MP-BOP resulting from the methylation of the pyridine nitrogen, which occurred in 43% yield (Scheme 2).The syntheses of the tricarbonylrhenium complexes from the triazolium salts 7 were finally accomplished in the presence of an excess of NEt 3 in refluxing toluene for more than two days, leading to Re-T-Phe, Re-T-Tol and Re-T-BOP in good yields (76%, 65.9% and 69%, respectively).
The new compounds were characterized by 1 H and 13 C NMR spectroscopy (Fig. S1-8 †), mass spectrometry (Fig. S9-12 †), single crystal X-ray diffraction analysis (vide infra) and FT-IR spectroscopy (Table S1 and Fig. S13, 14 †).It is noteworthy that the IR stretching bands of all complexes were split when using the attenuated total reflectance (ATR) technique on microcrystalline powders, which is not the case in solution.This effect, already noticed by us for closely related complexes, was attributed to crystallinity. 41,43ccording to Sarkar's group, the average of the ν CvO values measured in solution accurately reflects the electron density on the metal center. 7In the present case, these values were found to be very close for the compounds of the same series, showing the weak effect of substitution.In contrast, they were significantly lower for the triazolylidene-based complexes (ν CvO ∼ 1939 cm −1 ) than for their triazole-based counterparts (ν CvO ∼ 1951 cm −1 ), indicating that the introduction of the mesoionic carbene leads to an increase in the overall donor capacity of the organic ligand, in good agreement with the literature. 7The purity of the complexes was checked by elemental analysis.The coordination sphere of all complexes exhibits a quasioctahedral geometry, as confirmed by the calculation of the distortion parameters (Table S5 †).In every case, the rhenium atom is coordinated to three carbonyl groups in a fac configuration, one chlorine atom and the nitrogen atom of the pyridyl ring.It is also coordinated to the triazolylidene C(4) atom for the Re-T-Phe and Re-T-Tol complexes, and to one of the triazole nitrogen atoms for Re-Phe and Re-Tol.Distances and angles of the coordination sphere are quite close for the various compounds, and the pyridyl-triazolylidene and pyridyltriazole moieties are almost planar.In contrast, significant differences appear when considering the molecular geometry.The most striking indicator is probably the dihedral angle α between the phenyl ring and the triazolylidene or triazole moiety.For instance, Re-T-Phe is a moderately twisted structure, whose α value is 42.8°.The α angle widens till 78.6°for Re-T-Tol due to steric hindrance of the methyl group borne by the phenyl with the triazolylidene ring, on the one hand, and the carbonyl ligand, on the other hand (Fig. 2).Triazole-based complexes, where the phenyl ring is far from the coordination sphere, are comparatively less twisted.Complex Re-Phe exhibits an angle of 34.6°, which increases to 54.2°for Re-Tol.It is instructive to compare the effect of the benzoxazole substituent.The X-ray structure of Re-T-BOP could not be obtained, but DFT calculations (vide infra) indicated that the angle in di-chloromethane (DCM) solution should be around 55°( Fig. S31 †).In contrast, in the triazole-based series, the presence of benzoxazole leads to planarization of the whole electron conjugated system for Re-BOP (α = 9°). 39Some substituents therefore play a very different role on the molecular geometry in both series.

X-Ray-quality crystals of
For all complexes, two enantiomers, which differ by the position of the organic ligand with respect to the chlorine atom, coexist in the crystal cell in identical proportions.The intermolecular arrangement varies from one complex to another (Fig. 3), as described in detail in the ESI (Tables S6-8 and Fig. S15 S6 †).Specifically, intermolecular hydrogen bonds between the CH group of the phenyl ring and the oxygen atom of the equatorial carbonyl group induce the neighboring molecules of Re-T-Phe to assemble into an infinite one-dimensional chain, which expands along the c axis (Fig. S15 †).For Re-T-Tol, the same type of chain results from interactions between the CH group of the pyridine ring and the chloride ligand of an adjacent molecule (Fig. S17 †).Additionally, for both compounds, the packing arrangement promotes intermolecular C-H⋯π and π-π stacking interactions (Tables S7 and 8 †), because the phenyl group of one molecule partly overlaps the pyridyl ring of a neighboring one (Cp-Cp < 3.68 Å) (Fig. 3, S16 and S18 †).
Considering the triazole-based complexes Re-Phe and Re-Tol, the pyta moieties are situated on rather close planes, with their long axis oriented along four distinct directions.The neighboring molecules of Re-Tol interact with each other via C-H⋯Cl hydrogen bonds between the triazole and pyridine rings, and form a one-dimensional infinite chain along the c axis (Fig. S22 †).Numerous C-H⋯O contacts induce the formation of a two-dimensional network in the ab plane (Fig. S23 †).Re-Tol is further stabilized by π⋯π interactions between the pyridyl ring of one molecule and the triazole ring of a neighboring one (Cp-Cp = 3.78 Å) (Fig. S24 †).Regarding Re-Phe, intermolecular C-H⋯O hydrogen bonds between one hydrogen atom of the pyridyl ring and one carbonyl oxygen atom connect the neighboring molecules, which form a onedimensional infinite zig-zag chain along the b axis (Fig. S19 †).The presence of three basic nitrogen atoms within a five-membered 1,2,3-triazole ring makes the C-H bond highly acidic, strongly enhancing its involvement in hydrogen bonding with chloride acceptors.The bifurcated hydrogen bond between the chloride ligand, on the one hand, and the C-H groups of the 1,2,3-triazole ring and pyridine ring of neighboring molecules, on the other hand, contributes to the formation of a twodimensional network in the ac plane (Fig. S20 †).Crystals of Re-Phe are further stabilized by C-H⋯π interactions (Fig. S21 and Table S7 †), but they differ from those of the other analogues by the total absence of π⋯π interactions.
The Hirshfeld surface (HS) analysis 52 was performed to visualize and quantify the intermolecular interactions in the crystal lattice of the four complexes.In the example given in Fig. 4a, HS were mapped over d norm for Re-Phe and Re-T-Phe.The red spots mainly correspond to C-H⋯O and C-H⋯Cl short interatomic contacts.The white zones refer to intermolecular distances close to the sum of the van der Waals radii of the atoms considered; they indicate H⋯H interactions.The blue areas illustrate the domains where neighboring atoms are too far away to interact with each other.The comparison of the four molecules shows that the distribution of short contacts is indeed different, but the number and type of H-mediated interactions remain relatively close (Fig. S26 †).In contrast, the overall molecular shape is very different.Re-Phe is the most angular molecule and the least able to offer flat surfaces where π-π stacking can take place.The shape index of the Hirshfeld surface (Fig. 4b) is a tool to visualize π-π stacking interactions by the presence of adjacent red and blue triangles.Blue triangles represent the convex region, which is formed due to the carbon atoms present in the molecule inside the surface, while red triangles represent concave regions due to the carbon atoms of the π-stacked molecule above it.Such adjacent triangles were not observed for Re-Phe, which confirms the absence of π-π stacking interactions, while this  pattern can be found at least twice for the other complexes.The relative contributions of the various interactions to the total HS are shown in the 2D fingerprint plots given in Fig. S27-30.† The reciprocal C-O⋯H/H⋯O-C interactions give two sharp symmetrical spikes, with a contribution to the HS reaching 27.8% (for Re-T-Phe).The C⋯H/H⋯C interactions, which correspond to C-H⋯π interactions, appear as two symmetrical broad wings and contribute up to 23.4% (for Re-Phe).The H⋯H interactions appear in the middle of the scattered points, with an overall contribution varying between 15.1% and 23.5% (for Re-Phe and Re-T-Tol, respectively).The C-H⋯Cl interactions also generate two sharp spikes, and they contribute for ∼10%.Other interactions (N⋯H, C⋯N, C⋯O, O⋯O, C⋯C) have minor contributions.For the sake of comparison, the relative contributions of the various interactions to the HS are collected in Fig. 4c.Globally, triazole-based complexes show slightly more C⋯H interactions, and less C-H⋯Cl and O⋯H interactions, than their triazolylidene-based counterparts.
Noticeably, π⋯π stacking interactions (C⋯C/C⋯C) are only 2.5% for Re-Phe, and between 4.2% and 5.8% for the other three complexes.

Electronic properties
Computational studies were made using the time-dependent density functional theory (TD-DFT) method considering the complexes in vacuum and in dichloromethane continuum (Tables S9-30 and Fig. S31-41 †).Only the latter ones will be commented.The tolyl derivatives Re-T-Tol and Re-Tol were considered assuming that their properties are very close to those of the unsubstituted analogues.Complex Re-T-BOP was also studied to understand the influence of the BOP fragment; the results were compared to those previously acquired for Re-BOP. 39For the three new complexes, the calculated bond lengths and angles were in excellent agreement with the experimental data (Tables S9-11 †).It is also the case for the FT-IR spectra (Fig. S41 †).Regarding the composition of the frontier molecular orbitals (Tables S13-18, S25 and Fig. 5, S32-38 †), the three highest occupied molecular orbitals (HOMO, HOMO−1 and HOMO−2) are localized on the rhenium atom and carbonyl ligands, and on the chlorine atom for the first two, as is commonly the case for tricarbonylrhenium(I) complexes.For Re-T-Tol and Re-Tol, the LUMO, L+1 and L+2 are centered on the pyta and pytrz moieties, respectively.Some differences appear for lower and higher orbitals.For example, if considering the orbitals centered on the organic ligand, the H−4 and H−5 orbitals of Re-T-Tol are concentrated on the phenyl ring, while electrons are more evenly distributed between the pyta moiety and the phenyl ring for the H−3, H−4 and H−5 orbitals of Re-Tol.Regarding Re-T-BOP, an important contribution of the PBO moiety is noticed for orbitals H−2, H−5 and H−6, as well as for L+1 and L+5 (H−2, H−4 and H−5, as well as L+1, L+2, L+3 and L+5 for Re-BOP 39 ).
For all compounds, the lowest energy transitions correspond to a shift of the electronic density from the coordination sphere to the organic ligand (Tables S19-24 and S26 †).They are therefore of metal-to-ligand charge transfer (MLCT), halogen-to-ligand charge transfer (XLCT) and ligand-to-ligand charge transfer (LLCT) type.At low energy, the significantlyactive transitions are H−1 → LUMO, expected at 366.2, 344.5, 364.8 and 347.4 nm for Re-T-Tol, Re-Tol, Re-T-BOP and Re-BOP, respectively.Higher-energy transitions are of various natures.Re-T-Tol is characterized by a strong MLCT transition around 244.9 nm, while Re-Tol exhibits a strong ILCT band around 236.7 nm.The transitions with the highest oscillator factor have intraligand charge transfer (ILCT) character for Re-T-BOP and Re-BOP.
Calculations allow to highlight differences in the electronic system of the two series of complexes in the ground state.For instance, according to the natural population analysis (NPA) (Table S29 †) that indicates the charge transferred between the donor and acceptor moieties, the charge on the rhenium atoms is ∼−1.0(e) in complexes Re-Tol and Re-BOP, vs. ∼−1.2(e) in complexes Re-T-Tol and Re-T-BOP.The pyridyl nitrogen atom donates as much as 0.40 (e) to the Re atom in the first series of complexes, and 0.38 (e) in the second series.In triazole-based complexes, the triazole nitrogen atom also donates ∼0.19 (e), while the C(4) atom of the triazolylidenebased complexes is enriched with a charge of ∼0.12 (e).Consequently, there is better electron density delocalization from the organic ligand to the metal center for triazolylidenebased complexes.In every complex, the positively-charged carbon atom of the carbonyl ligands is found to accept as much as ∼0.76 (e) from the Re atom, while the chlorine atom Cl(1) gives it ∼0.47 (e).
The HOMO-LUMO gap, which reflects the chemical reactivity or stability of the molecules, was smaller for the triazolylidene-based complexes than for their triazole-based counterparts (∼4.12 eV for Re-T-Tol and Re-T-BOP vs. 4.43 eV for Re-Tol and 4.38 eV for Re-BOP 39) due to the increase of the HOMO energy level in the first case.The examination of the frontier molecular orbital descriptors (Table S30 †), i.e. ionization potential, electron affinity, hardness, electronegativity, chemical potential, softness and electrophilicity index, allows to conclude that the triazolylidene-based complexes Re-T-Tol and Re-T-BOP are less kinetically stable and more chemically reactive than their triazole-based counterparts Re-Tol and Re-BOP.

Electrochemical properties
The electrochemical behavior of the new complexes Re-Tol, Re-T-Tol and Re-T-BOP was studied by cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV) measurements in DCM at room temperature.The results are summarized in Tables 1 and S31, † and Fig. 6, and selected curves are given in Fig. S42-50.† The new compound Re-Tol exhibited electrochemical characteristics close to those of Re-BOP. 39hus, the first one-electronic oxidation process at 1.48 V was mainly assigned to the Re(I) oxidation, while the first reduction process around −1.60 V was ligand centered on the pyta moiety.These data also indicate that the pyridyl-1,2,3-triazole ligand is not very sensitive to the nature of its phenyl substituent.However, it is noteworthy that the first reduction process

Dalton Transactions Paper
presented more reversibility at high scan rate for the Re-Tol compound than for Re-BOP (see below).
The two new triazolylidene-based complexes Re-T-Tol and Re-T-BOP displayed distinct electrochemical properties.Indeed, in CV at 0.2 V s −1 , two irreversible processes were detected at around 1.26 V in oxidation, and −1.52 V in reduction.41]44 The first oneelectron oxidation process occurred now at lower anodic potential (around −200 mV) when compared with those of their pyridyl-1,2,3-triazole-based counterparts.This is due to the increased electron density on the rhenium center, which is supported by theoretical calculations.These results also suggest that the triazolylidene ligands are likely better electron donors than the corresponding pyridyl-1,2,3-triazole ligands.The Re-T-BOP complex also presented a second reduction process around −1.9 V that was attributed to the reduction of the PBO moiety, thus confirming that the latter is not involved in the complexation process. 39egarding now the literature data, the value of the first oxidation potential of the Re-T-Tol and Re-T-BOP complexes was close to that of the mesoionic carbene complexes synthesized by Suntrup et al.: 0.74-0.78V range vs. FcH + /FcH. 8This value is not very sensitive to the nature of the R group grafted on the triazole ring, and the comparison with 1,2,3-triazole-based complexes shows that it correlates well with the overall donor strength of the corresponding organic ligands.The first reduction potential can be used as an approximation to gauge the π-acceptor capacity of our ligands, the triazolylidene ligand being a better π acceptor than the 1,2,3-triazole ligand. 7The first reduction potentials of the rhenium complexes reported in Table 1 maintain the same π acceptor character than their respective ligands.
The experimental data of Table 1 were also in good agreement with the TD-DFT calculations.In particular, the latter predict that (i) for Re-Tol and Re-BOP, the HOMO are Re-centered and very close in energy; (ii) for Re-T-Tol and Re-T-BOP, the (Re centered) HOMO and (triazolylidene-centered) LUMO present respectively the same energy level; (iii) the calculated HOMO-LUMO energy gap value E calc for the first series of compounds is superior to that of the second one (around 0.3 eV) as observed experimentally.
Considering further the Re-BOP and Re-T-BOP complexes, a different electrochemical behavior regarding the first reduction process was noted in CV when increasing the scan rate.This one became more reversible only for Re-T-BOP.This property is probably related to the different nature of the two compounds.Indeed, for Re-BOP, the LUMO and LUMO+1 energy levels only differ by 0.07 eV.Consequently, the first reduction potential detected at −1.60 V may originate from the contribution of both the LUMO and LUMO+1 energy levels involving respectively the π*( pyta) and π*(pyta) + π*(PBO) orbitals.In contrast, for Re-T-BOP, the energy difference between these levels is greater (0.28 eV) and allows an easier electrochemical assignment of this reduction process exclusively resulting from its π*( pytrz) orbital.This is reminiscent of the electrochemical first reduction behavior of Re-[1,2,4]-BOP, which presents a LUMO-LUMO+1 energy difference of 0.4 eV and a LUMO of exclusively π*( pyta) nature. 39In the latter case, the first reduction process becomes quasi-reversible at 1 V s −1 .Concerning the Re-Tol and Re-T-Tol complexes, the LUMO-LUMO+1 energy difference is at least of 0.6 eV, and their d Other oxidation processes at 1.69, 1.78, and 1.90 V. e More reversible process at 50 V s −1 (Fig. S44 †).f More reversible process at 5 V s −1 (Fig. S46 and S47 †).g More reversible process at 10 V s −1 (Fig. S50 †).LUMO is respectively of pure π*( pyta) or π*( pytrz) nature, suggesting that some reversibility can occur at high scan rate in CV for each compound, which is actually observed.
Finally, the values of the electrochemical HOMO-LUMO gap (E el g ) found for Re-Tol and the triazolylidene-based complexes, i.e. 3.0 and 2.7 eV, respectively, also fit very well with the calculated E calc gap values 3.0 and 2.8 eV, highlighting good correlations with the theoretical study (Table S31 †).

UV-visible absorption and emission properties
Whatever the complex, no photochemical instability that could interfere with the measurements was detected during the spectroscopic study.The dilute DCM solutions of all complexes were almost colorless in daylight.The absorption spectra were in very good agreement with the calculated ones (Fig. S41 †).All of them (Fig. 7a and Table 2) showed an intense highenergy band, attributed to a combination of ILCT and MLCT transitions, as well as a low-energy band of moderate intensity with MLCT character, almost not visible for Re-BOP.The absorption maxima of the triazolylidene-based complexes were slightly red-shifted with respect to triazole-based complexes.The absorptivity values of the high-energy band were markedly increased for both BOP derivatives, due to the extention of the conjugated electron system.
Regarding the emission properties in DCM solutions, the complexes can clearly be divided into two groups.When illuminated by UV light (365 nm), the triazolylidene-based complexes emitted very weak orange-red light, while triazole-based complexes emitted green light.All emission spectra showed a single unresolved band, peaking between around 616 nm for triazolylidene complexes and 546 nm for triazole-based complexes.For Re-T-Tol, Re-T-BOP and Re-Tol, the experimental emission maxima were close to those calculated by DFT and TD-DFT (Table S27 †).Since calculations consider the involvement of the lowest 3 MLCT state, the comparison of the experimental and theoretical values confirms that emission is indeed phosphorescence in every case.For triazolylidenebased complexes in aerated solutions, the quantum yields were quite low, around 1 × 10 −2 .The emission decays (Fig. S51 †) were monoexponential for Re-T-Phe and Re-T-Tol, with lifetimes below 50 nanoseconds.For Re-T-BOP, a second lifetime at 217 ns with contribution around 29% could arise from the presence of conformers in solution.For the triazolebased complexes, the quantum yields were 1.5 fold higher, and the lifetimes were longer, between 145 and 192 ns, than for their triazolylidene-based analogues.Considering the whole set of compounds, the radiative rate constants were slightly higher, and the non-radiative rate constants were markedly higher for triazolylidene-based complexes.This suggests that these complexes are intrinsically a little more emissive than their triazole-based counterparts, but this trend is compensated by the promotion of thermal de-excitation pathways , as can be expected from the energy gap law. 53t is striking to see that the three triazolylidene derivatives, on the one hand, and the three 1,2,3-triazole derivatives, on the other hand, show great similarity in their emission pro-  perties in solution.This is explained by the fact that, within the same of compounds, the first excited triplet state is almost the same for each derivative because the electron density is centered on the rhenium coordination sphere and does not involve the organic substituents borne by the pytrz or pyta moiety (Fig. S39 †).
The effect of dissolved oxygen on the emission efficiency was investigated.Bubbling the complex solutions with argon for 4 min multiplies the emission intensity by 1.6, 2.9 and 1.3 for Re-T-Phe, Re-T-Tol and Re-T-BOP, respectively, like for the previously studied 1,2,4-triazole-based complexes.The short emission lifetimes of triazolylidene derivatives may explain the low sensitivity to oxygen quenching.In contrast, the emission was dramatically enhanced by 8.3, 12.1 and 13.4 for Re-Phe, Re-Tol and Re-BOP in the same conditions, so that the quantum yield changes by one order of magnitude.The longer lifetimes are one of the factors contributing to this effect, making the 1,2,3-triazole derivatives the most oxygen-sensitive compounds among all rhenium(I) complexes examined to date in the group.
All compounds were emissive in the solid state.Unlike solutions, where the spectra were identical by class of compounds, dramatic differences appeared between all compounds.The PL spectra covered a large range of wavelengths (Fig. 8a), with very different intensities (Fig. 8b).The microcrystalline pristine powders of the triazolylidene-based complexes emitted yellow to orange-yellow light, with moderate to low quantum yields (between 0.16 for Re-T-Phe to 0.04 for Re-T-BOP) (Table 2).Triazole-based complexes emitted green light.The emission of Re-Phe was exceptionally strong, with quantum yield around 0.69.These superior solid-state PL properties can be explained jointly by the absence of π-π staking in the crystal network and by the prevention of quenching by dissolved oxygen, to which the compound is very sensitive when in solution.Again, the BOP derivative was the weakest luminophore of the series.Curiously, pristine Re-T-Tol was virtually not emissive, but emission was induced by grinding the powder with a pestle in a mortar, which generally leads to partial amorphization of the sample, suggesting that the crystal packing mode is detrimental to light emission.The influence of mechanical stimulus was investigated by grinding the pristine powders of the other five complexes, but no significant differences were observed, ruling out any mechanoresponsive luminescence behavior for these compounds.
All decays were found to be multiexponential (Fig. S52 †).The preponderant contribution was associated to lifetimes between 414 and 1164 ns.Minor contributions with lifetimes of many tens of nanoseconds can be attributed to molecules experiencing a different environment.Very weak contributions associated to lifetimes in the nanosecond range are probably due to fluorescent impurities.The mean lifetimes were much longer than for the corresponding complexes in solution, as currently observed in the team for Re(I) complexes in the solidstate.
It was interesting to see if our molecules can be used as AIE-gens in aqueous medium, knowing that the formation of aggregates in this medium is not always compatible with good light emission.The AIE behavior was investigated using Re-T-Phe and Re-Phe that exhibit the best solid-sate emission properties in their respective series of complexes.The complexes were dissolved in acetonitrile, where they are very stable chemically and photochemically (no change in the UV-vis spectra was detected for 3 h upon irradiation with a 385 nm emitting diode).By increasing the water proportion in acetonitrile solutions of Re-T-Phe, the weak orange-red emission centered at 612 nm progressively became a strong green-yellow emission with maximum at 536 nm, and then the emission intensity decreased again (Fig. 9a and b).Under the same conditions, the green-yellow emission of Re-Phe at 544 nm was shifted to 508 nm with dramatic increase of intensity for water content ( f w ) above 70% (Fig. 9c and d).The intensity of the PL signal at the maximum wavelength of the most intense band was multiplied by 33 and 72, respectively.It is noteworthy that these emission maxima are similar to those of the compounds.In fact, these spectroscopic changes were accompanied by the appearance of small particles.The examination under the fluorescence microscope showed for Re-T-Phe the presence of rare, strongly-emissive rod-like microcrystals, which were about 200-150 µm × 5 µm (Fig. 9e).Suspensions of Re-Phe contained a large number of particles visible to the naked eye, which turned out to be agglomerates of very thin microfibers measuring 10-20 µm long (Fig. 9f ).The AIE effect was therefore clear in both cases, but the superior emission properties of Re-Phe (Fig. 9g) make this compound the best candidate for AIE-related applications.

Antibacterial properties
Finally, the antibacterial activities of the complexes were tested.One Gram-positive strain, i.e.Staphylococcus aureus (S. aureus), and three Gram-negative strains, Pseudomonas aeruginosa (P.aeruginosa), Acinetobacter baumannii (A.baumannii) and Escherichia coli (E.coli), were selected as common causative agents of the most dangerous nosocomial infections.Both antibiotic susceptible (S) and multidrug-resistant (R) strains were used.The three triazolylidene complexes and their 1,2,3triazole-based counterparts were studied, together with Re-[1,2,4]-Phe and Re-[1,2,4]-BOP for the sake of comparison.Given their low solubility in water, the complexes were previously dissolved in DMSO, before introduction and dilution into aqueous medium.The final concentration of complexes in each well ranged from 0.125 to 128 µM, and the highest concentration of DMSO was below 1.25%.The bacteria suspended in their culture medium were then put in contact with the complexes and incubated for 18 h in the dark.The minimum inhibitory concentration (MIC), i.e. the lowest concentration of a compound at which bacterial growth is completely inhibited, was measured.Results are collected in Table 3. Remarkably, antibacterial activity was observed only against S. aureus, whatever the strain is susceptible or resistant to conventional antibiotics.Among the active complexes, the triazolylidene-based derivative Re-T-BOP exhibited the stronger antibacterial activity, with a MIC value around 4 µM.This complex is the most lipophilic of all, but probably not enough to explain the difference in antibacterial activity.A connection between the antibacterial activity of all complexes and common drug likeliness parameters (i.e.topological polar surface area (TPSA), lipophilicity (miLogP) and molecular weight (MW)) 54,55 was sought.7][58] In particular, the lipophilicity may facilitate drug penetration and cellular uptakes through interaction with bacterial membranes. 59Likewise, TPSA has been associated with bioavailability, and it is suggested that compounds with a TPSA greater than 140 tend to be poor at permeating cell membranes. 56In the present case, if considering the whole set of data, no obvious correlation was found with the drug likeliness parameters considered.The presence of the benzoxazole substituent in Re-T-BOP certainly brings a benefit, because this fragment is considered to be one of the most relevant biologically-active heterocycles, often incorporated in ligands to target a variety of receptors and enzymes in medicinal chemistry studies. 60,61However, the presence of benzoxazole in Re-BOP and Re-[1,2,4]-BOP is not sufficient to endow these compounds with an antibacterial activity.Besides, it is noteworthy that the presence of the methyl group in Re-Tol and Re-T-Tol modifies the antibacterial activity in comparison with their analogues Re-Phe and Re-T-Phe.Therefore, the activity could be related to the nature of the coordination sphere, and more precisely to the molecular geometry in space or flexibility.The examination of new complexes substituted by a methyl group in the meta and para positions could be instructive.It would allow validat- ing our hypothesis by the analysis of a greater of data.Of a deeper insight into the involved mechanism will also be necessary.
In comparison with control antibiotics gentamicin and ampicillin, Re-T-Phe, Re-T-BOP and Re-Tol showed lower or comparable MIC values against multidrug resistant S. aureus ATCC 43300 strain demonstrating their antibacterial potentials.No compound was active against Gram-negative bacteria.Indeed, these bacteria have additional outer membrane, as well as an abundance of efflux pumps and highly selective porins, which make internalization of active compounds particularly difficult. 62inally, a photochemical effect was investigated.Our complexes are not prone to photodecarbonylation.However, the presence of rhenium as a heavy atom induces fast population of the first excited triplet state, and the photogeneration of singlet oxygen could take place via energy transfer.In the present case, irradiation of the samples in the near UV does not lead to different results in most of the cases.Exceptions are Re-BOP and Re-T-Tol that show enhanced activity towards the antibiotic-susceptible strain of S. aureus (Table S32 †).

Conclusions
In this comprehensive study, the properties of three new triazolylidene-based complexes were compared with those of their 1,2,3-triazole-based analogues, for which very little information was available.From an electronic point of view, the superior electron-donating properties of the mesoionic carbene ligand resulted in an increase in the charge density on the rhenium atom.The crystallographic data showed that small structural variations strongly influence the molecular conformation and intermolecular interactions in the solid state, and the Hirshfeld surface analysis allowed the latter to be finely quantified.The compounds of the two series were emissive in solution and in the solid state.Complex Re-Phe showed exceptional emission efficiency in the form of powder, and was a remarkable AIE-gen in aqueous medium.Although less efficient, our triazolylidene-based complexes proved to be photochemically stable, unlike the Re(I)-NHC complexes reported in the literature, 15,16 and are therefore well suited for light-related applications.Until now, only 1,2,4-triazole-based complexes had shown attractive spectroscopic properties in the solid state.This work shows that some triazolylidene-and 1,2,3-triazole-based complexes must also be considered for potential applications in the fields of photoluminescent materials and AIE-gens.However, the mechanoresponsive emission behavior remains the prerogative of the 1,2,4-triazole-based series.18][19][20][21][22][23][24][63][64][65][66][67][68] Finally, one triazolylidene-based complex (Re-T-BOP) showed clear superiority over all other compounds as regards the antibacterial properties.It would be instructive to test the cytotoxicity of this molecule on eukaryotic cells to see if it could be used in therapy.A thorough examination of the mechanism of action would facilitate the further development of more effective molecules.This result confirms that triazolylidene-based Re(I) complexes must be considered as potential candidates in the highly demanding field of therapeutic chemistry.

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 Table 3 Drug likeliness parameters (topological polar surface area (TPSA), lipophilicity (miLogP), molecular weight (MW)) of complexes, and their minimum inhibitory concentration (MIC) towards antibiotic-susceptible (S) and multidrug-resistant (R) bacteria, without and with irradiation by UV light.Comparison with conventional antibiotics

Dalton Transactions Paper
nitrogen atmosphere.All reactions were monitored by TLC on silica gel Xtra SIL G/UV254.chromatography was performed on Machery-Nagel silica gel.NMR spectra were recorded with a Bruker Avance 300 instrument.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 microanalyses were made in the 'Service d'analyse' of LCC using a PerkinElmer 2400 series II analyzer.

Synthesis
The syntheses of ligand L-T-Phe, 51 and complexes Re-Phe 50 and Re-BOP 39 have been previously described.To a suspension of ligand L-T-BOP (32 0.0636 mmol) and [Re(CO) 5 Cl] (24 mg, 0.0663 mmol) in toluene (6 mL) was added an excess of Et 3 N (0.22 mL).The reaction mixture was refluxed for 48 h.After cooling to room temperature, the solvent was removed under reduced pressure.Silica gel column chromatography (CH 2 Cl 2 /MeOH 9.5 : 0.5 v/v) afforded pure compound (29 mg, 69%) as a yellow solid. 1

Crystallography
Crystallographic data were collected at low temperature (193 K) with an Oxford Instruments Cryostream 700+ Series device using MoK α radiation (wavelength = 0.71073 Å) on a Bruker APEX II Quazar diffractometer equipped with a 30 W aircooled microfocus (Re-T-Phe and Re-Phe) and on a Bruker AXS D8-Venture diffractometer equipped with a multilayer TRIUMPH X-ray mirror and a Photon III-C14 detector (Re-T-Tol and Re-Tol).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. 69The structures were solved using an intrinsic phasing method (ShelXT). 70All non-hydrogen atoms were refined anisotropically using the least-square method on F 2 . 71ydrogen atoms were refined isotropically at calculated positions using a riding model.Supplementary crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/ structures/. .3D Hirshfeld surfaces (HS) and 2D fingerprint plots (FP) 72 were generated with high resolution based on the crystallographic information file (CIF) using Crystal Explorer 17.5 software. 73HS were mapped over d norm , shape-index and curvedness.The normalized contact distance (d norm ) is defined by d i and d e , which are the distances from the surface to the nearest atom interior and exterior to the surface, respectively.The vdW radii of atoms is defined according to eqn (1).
Spectroscopy UV-visible absorption spectra and emission spectra in solutions were measured at 20 °C 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: 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 (Φ s = 0.53) in ethanol was used as the standard. 74The 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% and 20% for suspensions.The rate constants for radiative (k r ) and nonradiative (k nr ) decay were calculated using the following equation: with Φ the emission quantum yield and τ the luminescence lifetime in solution.For AIE measurements, a small volume (30 µL) of a concentrated solution of complex in acetonitrile was injected in 2.97 mL of various acetonitrile/water mixtures.

Dalton Transactions Paper
After stirring for 3 h in the dark, the samples were sonicated for 5 min before optical Solid state spectra recorded on a HORIBA Fluorolog 3-2iHR320 spectrofluorometer and were corrected.The absolute photoluminescence quantum yield values (Φ PL ) were determined using the Xenius SAFAS spectrofluorometer provided with an integrating sphere, by a method based on the one developed by De Mello et al., 75 as described elsewhere. 41The error was estimated to be about 20%.
Emission decay curves were recorded using the time-correlated single photon counting method (TCSPC) on the Horiba Fluorolog 3-2 iHR320 spectrofluorometer equipped with a Nanoled-370 (λ ex = 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 371 nm.All analyses were recorded using the Datastation v2.7 software from Horiba.The decay curves were analyzed with reconvolution and global non-linear least-squares minimization method using DAS6 v6.8 software.The absorbance of solutions at λ ex was lower than 0.1.Solid state samples were deposited on a quartz holder.

Computational details
The GAUSSIAN16 program package 76 was employed for all calculations (the geometry optimization, the ground-state and excited-state electronic structures, and optical spectra) with the aid of the ChemCraft visualization program. 77The ground state (S 0 ), the first excited state (S 1 ) and the lowest triplet state (T 1 ) geometries of Re-Tol, Re-T-Tol and Re-T-BOP, were fully optimized with the restricted and unrestricted density functional theory (R-DFT and U-DFT) methods using the Perdew-Burke-Ernzerhof PBE1PBE functional with no symmetry constraints. 78In all calculations, the "double-ζ" quality basis set LANL2DZ with Hay and Wadt's relative effective core potential ECP (outer-core [(5s 2 5p 6 )] electrons and (5d 6 ) valence electrons) 79,80 was employed for the Re atom.The 6-311+G** basis set for H, C, N and O atoms was used. 813][84] The vibrational frequencies calculations were performed using the optimized structural parameters of the complexes, to confirm that each optimized structure represents a local minimum on the potential energy surface and all eigenvalues are non-negative.To obtain a good agreement with the experimental vibrational frequencies in solid state, the theoretically calculated vibrational frequencies were scaled with a scaling factor of 0.9485.The optimized Cartesian coordinates of Re-Tol, Re-T-Tol and Re-T-BOP are included in the ESI part (see Tables S33-47 †).On the basis of the optimized ground and excited state geometries, the absorption and emission properties were calculated by the time dependent density functional theory (TD-DFT) method at the PBE1PBE/LANL2DZ/6-31+G** level.These methods have already shown good agreement with experimental studies for different rhenium(I) complexes. 85

Electrochemistry
The electrochemical properties of the complexes were determined by cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV) in DCM.The solutions used during the electrochemical studies were typically 1 × 10 −3 M in complex, and 0.1 M in supporting electrolyte.The supporting electrolyte [nBu 4 N][BF 4 ] (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 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 cm 2 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.
taken and resuspended into 1 mL of sterile normal saline (0.9% NaCl) to bacteria suspension before checking for Then, 200 µL the optical density-adjusted bacteria suspension was added to 19.8 mL of 0.9% NaCl.Lastly, 100 μL of this bacteria suspension was added to each well containing 100 μL of the compounds and plates were incubated for 18 hours at 37 °C.To study the photodynamic therapy activities of the complexes, plates were incubated for 15 minutes after addition of compounds and irradiated for 10 minutes using LED light (365 nm) at a light density of 1.57 mW before incubation overnight.The light intensity was measured with a calibrated power meter through adjustment of distance between the LED light source and the 96-well plates.All assays were performed as at least duplicated experiments.Results were observed visually and tabulated after addition of resazurin.
Re-Phe, Re-T-Phe, Re-Tol and Re-T-Tol were grown from CH 3 CN, CH 3 Cl/pentane, CH 2 Cl 2 and CH 2 Cl 2 / CH 3 CN, respectively.Selected crystallographic data are collected in Tables S2-4.† -25 †).Briefly, the molecules of the triazolylidene derivatives are displayed in two separate planes, at an angle of ∼40°for Re-T-Phe, and close to perpendicular for Re-T-Tol.The crystal structures are stabilized by C-H⋯O and C-H⋯Cl interactions (Table

Fig. 3
Fig. 3 Crystal cells of Re-T-Phe, Re-T-Tol, Re-Phe and Re-Tol.The distances between centroids are indicated in green ink.

Fig. 4
Fig. 4 View of the Hirshfeld surface of Re-Phe and Re-T-Phe plotted over d norm (a) and shape index (b).The adjacent red and blue triangles that reveal π⋯π stacking interactions are highlighted by black circles.(c) Percentage contributions of various interactions to the Hirshfeld surface for the four complexes.

Fig. 5
Fig. 5 Isodensity plots (isovalue = 0.03 e bohr −3 ) and energy levels of the first frontier molecular orbitals, for complexes Re-T-Tol, Re-Tol and Re-T-BOP in DCM, according to DFT calculations at the PBE1PBE/LANL2DZ level of theory.

Fig. 6
Fig. 6 Cyclic voltammograms of the first oxidation and reduction processes of complexes Re-Tol (red dotted line), Re-T-Tol (red line), and Re-T-BOP (green line) on a Pt working electrode in CH 2 Cl 2 + 0.1 M [nBu 4 N][BF 4 ] at room temperature and at a scan rate of 0.2 V s −1 toward cathodic potentials.

Fig. 8
Fig. 8 (a) Normalized emission spectra of complexes Re-T-Phe (blue solid line), Re-T-Tol (red solid line), Re-T-BOP (green solid line), Re-Phe (blue dotted line), Re-Tol (red dotted line) and Re-BOP (green dotted line) in the solid state ( pristine microcrystalline powders, except for Re-Tol (ground powder)).(b) Same spectra with intensity at the maximum emission wavelength proportional to the quantum yield.λ ex = 370 nm.(c) Images of the powders examined in daylight (Vis) and illuminated at 365 nm (UV).

Fig. 9
Fig. 9 Samples of Re-T-Phe (a) and Re-Phe (c) at 3.3 × 10 −5 M in acetonitrile solutions containing from 0 to 95% water (from left to right), illuminated at 365 nm.(b) and (d) Corresponding emission spectra, λ ex = 350 nm and 340 nm, respectively.Suspensions of Re-T-Phe (e) and Re-Phe (f) with f w = 80% observed under the fluorescence microscope.(g) Comparison of the PL intensity increase at 536 nm (Re-T-Phe) and 508 nm (Re-Phe) as a function of the water/acetonitrile ratio f w , after normalization with respect to emission quantum yield in acetonitrile solutions.All samples were observed 3 h after preparation.

Table 1
Selected electrochemical data of complexes Re-Tol, Re-BOP, Re-T-Tol, and Re-T-BOP, [10 −3 M].Values determined by OSWV on a Pt working electrode in CH 2 Cl 2 + 0.1 M [nBu 4 N][BF 4 ] at room temperature.a,b Ferrocene was used as internal reference a OSWVs were obtained using a sweep width of 20 mV, a frequency of 20 Hz, and a step potential of 5 mV.b Potential values in volts vs. SCE (Fc + /Fc is observed at 0.55 V ± 0.01 V vs. SCE).cData from ref.39.