Ioannis Michaela Fecková

Konstantinos Kalis

Thierry Roisnel

Pascal Le Poul

Oldřich Pytela

Milan Klikar

Françoise Robin Le Guen

Filip Bureš
email: filip.bures@upce.cz

Mihalis Fakis
email: fakis@upatras.gr

Sylvain Achelle
email: sylvain.achelle@univ-rennes1.fr

Ioannis Konstantinos Kalis

Françoise Robin-Le Guen

Photophysics of 9,9-dimethylacridan substituted phenylstyrylpyrimidines exhibiting long lived intramolecular charge transfer fluorescence and aggregation induced emission characteristics

Keywords: Pyrimidine, Dual emission, Intramolecular charge transfer, Aggregation induced emission, 10.1002/chem.202004328

Introduction

During the past few years, there has been a great interest in the design of organic emissive molecules that are indeed the key element of organic light emitting diodes (OLEDs). [1] Both fluorescent and phosphorescent emitters were utilized in the first and second generation OLEDs. [2] Whereas the first generation OLEDs suffer from an internal quantum efficiency (IQE) limited to 25% (the external quantum efficiency (EQE) is often below 5%), [3] the second generation OLEDs (so-called PhOLEDs) overcome this problem and IQE can reach 10.1002/chem.202004328

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Chemistry -A European Journal theoretically 100 %. However, materials for PhOLEDs are mostly based on heavy metal (Ir, Pt, Os…) complexes, which are rather expensive, have limited resources, and exhibit potential toxicity. [4] Thermally activated delayed fluorescence (TADF) is another recently utilized approach towards third generation OLEDs with theoretical IQE of 100% and the real EQE up to 30%. [2,5] This principle utilizes conversion of triplet excitons into luminescent singlet excitons via a reverse intersystem crossing (RISC) leading to increased fluorescent lifetime (up to several microseconds). [2,5] In principle, a TADF emitter should possess a small energy difference between T 1 and S 1 excited states (∆E ST ), [6] which is generally achieved by orthogonal arrangement of the S 1 state with spatially separated HOMO and LUMO. [5a,5b,7] In this context, bulky electron-donating substituents such as carbazol-9-yl, [8] acridan, [9] spiroacridan, [10] phenoxazine [11] and azasiline, [12] with a phenylene linker have been often used.

Dual emission is generally obtained by a combination of luminophores that are linked together. [13] Two emission bands can be also observed in case of coexistence of two forms of the same compound (equilibrium). This is for example what is observed with excimer/exciplex [14] or when excited state intramolecular proton transfer occurs. [15] Recently, single-luminophore presenting dual emission based on localized and charge-transfer excited states or two charge-transfer excited states have been described. [16] Apart from the aforementioned properties, organic chromophores applicable for OLEDs should be also highly emissive in the solid state. However, many push-pull chromophores that are highly luminescent in solution, exhibit aggregation-caused quenching (ACQ) and are therefore non (or weakly) emissive in solid state. [START_REF] Birks | Photophysics of Aromatic Molecules[END_REF] Nevertheless, in specific cases, aggregation induced emission (AIE), a concept proposed by Tang et al in 2001, induces intensive emission in the solid state of non (or slightly) emissive chromophores in solution. [18] In general, a reduction of π-π stacking and a restriction of intramolecular motions have been 10.1002/chem.202004328

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Chemistry -A European Journal recognized as the working mechanisms of AIE. In order to reduce the π-π stacking, molecules suitable for AIE are not planar but adopt a twisted conformation. When both AIE and TADF phenomena are combined, the term of aggregation-induced delayed fluorescence (AIDF) is used. [19] Among numerous series of emitters, push-pull pyrimidine chromophores are wellknown for their luminescence properties. [20] In particular, some of us have described a large library of styrylpyrimidine fluorophores [21] and it is also noteworthy that pyrimidine seems to exhibit all necessary properties towards applications in lighting devices. The pyrimidine ring can be protonated and in specific cases white light emission can be obtained from a mixture of neutral and protonated forms by controlling the amount of the used acid. [22] Also, the pyrimidine core has been used in the structure of TADF chromophores leading to highly efficient blue OLEDs with EQE > 25%. [23] Compared to the widely used triazine, the pyrimidine core is a weaker electron-withdrawing group, and therefore pyrimidine derivatives exhibit increased band gap energy (E g ) and higher S 1 and T 1 energy levels. [24] Finally, AIE was also reported for some 4,6-distyrylpyrimidimine derivatives. [25] Based on the possible matching of the aforementioned OLED requirements and pyrimidine fundamental characteristics, we have designed a series of 4-styryl and 4,6-distyrylpyrimidine chromophores that are linked to an acridan fragment via a phenylene linker in position C2 of the pyrimidine core. In these molecules we aim to combine the AIE properties together with the Intramolecular Charge Transfer (ICT) fluorescence. Their synthesis, photophysical properties, some X-ray structures and quantum-chemical investigations are reported herein. In principle, the charge transfer may occur either from the phenylacridan C2 branch or the C4/C6 styryl branches which are substituted either by methoxy or N,N-diphenylamino electron donors. In case of methoxy substituted styryl branches, this arrangement leads to dual emission with long-lived fluorescence characteristics.

The AIE is also reported in solutions upon addition of a non-solvent together with broad white light dual emission in neat films rendering the herein reported chromophores potentially suitable for application as single emissive layers in WOLEDs.

Results and Discussion

Synthesis

Compounds in the series 3 and 4 were obtained in two steps from commercially available 2-chloro-4-methylpyrimidine and 2-chloro-4,6-dimethylpyrimidine respectively (Scheme 1).

In both cases, the first step consists of a palladium catalyzed Suzuki-Miyaura cross coupling reaction with 4-(9,9-dimethyl-9,10-dihydroacridine)phenylboronic acid pinacol ester leading to intermediates 1 and 2 with moderate yields. As well documented in the chemistry of pyrimidine, the electron-withdrawing character of the pyrimidinyl fragment permits the oxidative addition of palladium into the chlorine-carbon bond without the use of special and expensive ligands. [26] The second step involves Knoevenagel condensations with the corresponding aldehyde in boiling aqueous sodium hydroxide in the presence of Aliquat 336 phase transfer catalyst, according to the conditions initially described by Vanden Eynde [27] and developed by us. [21a-c] It should be noted that the isolated yields of N,N-diphenylamino derivatives c are significantly lower than those of the methoxy ones (a and b) due to complicated purification. All compounds were fully characterized by 1 H and 13 C NMR (Figures S29-S44), as well as HRMS. According to previous observations, [21a-c] the condensation reactions generate only the E-isomer, as shown by the coupling constant of the 1 H NMR spectra (J ≈ 16 Hz).

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Chemistry -A European Journal Scheme 1: Synthesis of the compounds in series 3 and 4.

Thermal properties

The thermal stability of organic chromophores for potential application in OLEDs is a key parameter. Therefore, the thermal behavior of all target fluorophores 3-4 was examined by differential scanning calorimetry (DSC). Figure 1 shows thermograms of representative compounds 3a-c while Table 1 lists the measured melting points (T m ) and temperatures of thermal decompositions (T d ). All DSC records are provided in the SI. The measured melting points and temperature of decomposition range from 199-271 and 370-380 °C, respectively.

The compound 3c was obtained as amorphous solid, this phenomenon is clearly reflected in its different thermal behavior. It follows, that the molecule 3c does not melt but it undergoes a 10.1002/chem.202004328

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Chemistry -A European Journal glass transition at 70 °C. On the other hand, the other compounds 3a,b and 4a-c are crystalline solids, which sharply melted and their liquid phase proved to be stable during heating process up to ca. 375 °C. Upon continued heating, all target fluorophores 3-4 were gradually decomposed (T d exceeded T m by about ca. 110-175 °C). Moreover, the branched compound 4b underwent monotropic solid-solid transition of its metastable crystals at 253 °C.

Similarly, polymorphous enantiotropic transition was detected for compound 4a between 140 and 150 °C. Based on the performed DSC analysis the following outcomes can be deduced:

• Within 4a-c, the melting points are affected by the nature of peripheral methoxy or N,Ndiphenylamino groups.

• Thermal decomposition of all compounds 3-4 does not depend on the peripheral donor (ac) or branching (3 or 4). This is clearly demonstrated by almost identical T d values for 3-4 (Fig. 1, Table 1).

• Their large thermal robustness is rather given by the parent acridan-pyrimidine scaffold.

Variation of the peripheral groups or structural branching influences the thermal stability only negligibly. a T m = melting point (the point of intersection of a baseline and a tangent of thermal effect = onset).

b T d = thermal decomposition (pyrolysis in N 2 atmosphere).

X-ray analysis

Crystals of 3a, 4a and 4c, suitable for x-ray analysis, have been obtained by slow evaporation of their CH 2 Cl 2 /AcOEt solution. In all cases, the measured crystals, colorless or yellow prisms, confirm the proposed molecular structure, including the E stereoisomery of the alkene fragments (Figures 2 andS8, Tables S1-S3). For 3a and 4a a monoclinic crystal system is observed with a P2 1 /c space group whereas a triclinic crystal system with P1 space group has been identified for 4c. In the case of 4a, the presence of a molecule of ethyl acetate in the asymmetric unit of two molecules of 4a has been observed (see Figure S7 in the ESI). In all cases, the dihedral angle between the pyrimidine core and the C2-phenyl fragment is below 17.5°, similar to those observed for other 2-phenylpyrimidine derivatives. [26] The RTG analyses of 3a, 4a and 4c revealed a pre-twisted molecular structure with the vertical torsion angles between the acridan donor and the phenyl bridge of 82.3°, 81.1° and 85.0°. The torsion angles between the central pyrimidine core and the acridan fragment are 87.6, 88.3° and 83.3°, respectively. This structural arrangement would point to an important spatial separation of the frontier molecular orbitals, one of the necessary aspects to afford small ∆E ST values. In contrast to previous styrylpyrimidines, the crystal structures further show that planes of the

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Chemistry -A European Journal phenyl rings of the styryl arms and the pyrimidine core are not coplanar (torsions angle < 22°). [21c,22c,28] For compounds 3a and 4a, bond length alternation (BLA) values of the vinylic linker were calculated to range between 0.1265 to 0.1316 Å. For 4c, a significantly lower BLA value of 0.107 Å has been calculated for one of the two styryl branches. This would suggest a stronger charge transfer in one of the styryl arms for N,N-diphenylamino derivative 4c than for methoxy derivatives 3a and 4a. Thermal ellipsoids are shown at 50% probability level.

Density functional theory calculations

The spatial and electronic properties of the target chromophores were investigated using Gaussian ® 16W software [29] package at the DFT level. Initial geometries of molecules 3a-c

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Chemistry -A European Journal 2. There are no significant differences between the particular values, and, therefore, the HOMO-LUMO gaps ∆E fall within a short range of 2.61-2.80 eV. The gap is almost independent on the appended peripheral methoxy

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and N,N-diphenylamino donors (a-c), which would imply that these are not engaged in the ICT transition that is responsible for the longest wavelength absorption band as will be discussed below. feature, the HOMO always occupies the acridan nitrogen atom, whereas the LUMO is spread over the pyrimidine central ring and the appended olefinic linker(s). On the contrary, the HOMO-1 is localized over the methoxy-or N,N-diphenylamino-terminated styryl branch(es).

In 3a-c, the HOMO-2 occupies again the phenylacridan branch, whereas in 4a-c it remains localized similarly to HOMO-1. The higher unoccupied molecular orbitals LUMO+1 and LUMO+2 are generally localized over the central pyrimidine ring and the adjacent π-system or may be shifted towards acridan moiety.

The electronic absorption spectra were calculated at TD-DFT level employing two functionals: convenient CAM-B3LYP/6-311++G(2d,p) as well as B3LYP/ 6-311++G(3d,f,2p). The spectra are shown in Figures S11 andS12; the calculated 10.1002/chem.202004328

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Chemistry -A European Journal positions of the longest-wavelength absorption maxima are listed in Table 2. Compared to the experimental values, the B3LYP slightly overestimates λ max values (∆λ max = 10-42 nm), while the CAM-B3LYP behaves conversely (∆λ max = 12-49 nm). A dependency of both calculated values revealed tighter correlation of the B3LYP values (Figure S13) and, therefore, we consider the B3LYP more appropriate for describing electronic properties of 3 and 4. This is in agreement with the recent observation on heterocyclic push-pull molecules made by Tang et al. [30] Compared to the experimental spectra measured in CH 2 Cl 2 , the calculated spectra possess the same features and bands (Figure S11). An inspection of the particular transitions and oscillator strengths (Table S4) revealed that the longest-wavelength absorption band is exclusively generated by the HOMO-1→LUMO transition. Pure HOMO→LUMO transition, a typical feature of push-pull molecules, [31] has been found within a range of 495-529 nm but possess zero oscillator strength. Considering the HOMO-1 localization over the methoxy and N,N-diphenylamino peripheral donors, their D-A interaction with the central pyrimidine acceptor (LUMO) accounts for the intramolecular charge transfer (ICT) and also explains the observed bathochromic shift of the longest-wavelength absorption maxima when going from 3a to 3c or 4a to 4c. It is also indicative that, compared to methoxy group, the N,N-diphenylamino donor is strongly engaged in the ICT. Correlation of the λ max DFT values (Table 2) against the experimental λ max (Table 3) clearly demonstrate very tight dependence of these two quantities. On the contrary, there is no correlation between the HOMO-LUMO gap and the experimentally obtained optical gap (see Figure S14). Apart from the longest wavelength HOMO-1→LUMO transition, the remaining minor transitions in the DFT calculated absorption spectra (Figure S11) mostly involve HOMO-1→LUMO+1 (3a-c) and also HOMO-2→LUMO (4a-c) contributions (Table S4).

An inspection of geometries of the calculated singlet and triplet states of model 3a revealed very small spatial differences of both states. Whereas the acridan moiety remained

Photophysical properties

Photophysical properties in solutions. The UV/Vis and photoluminescence (PL) properties of compounds 3 and 4 were initially measured in CH 2 Cl 2 and the results are presented in the SI (Table S5). These compounds show absorption properties similar to other pyrimidines without C2-substituents, [21a-b] but surprisingly almost no emission was detected for the methoxy derivatives 3a-b and 4a-b. On the other hand, the N,N-diphenylamino derivatives 3c (PL λ max = 531 nm, Φ F = 0.75) and 4c (PL λ max = 552 nm, Φ F = 0.94) are highly luminescent in CH 2 Cl 2 . Measurements have been performed also in the nonpolar n-heptane, the results are presented in Table 3. As an example, the normalized spectra of the compounds 3 are shown in Figure 5 (the corresponding spectra of chromophores 4 are shown in Figure S15). Compared to methoxy derivatives 3a and 4a, the N,N-diphenylamino derivatives 3c and 4c exhibited significantly red-shifted absorption and emission. The replacement of the phenyl linker by naphthyl one (3b vs. 3a or 4b vs. 4a) caused a red shift of the absorption maxima (∆λ max = 30/12 nm), whereas the emission spectra are almost identical. In n-heptane, a dramatic increase of the emission intensity was observed for methoxy derivatives suggesting AIE a All spectra were recorded at room temperature at c = 0.9 × 10 -5 M to 1.02 × 10 -5 . b Fluorescence quantum yield (±10%) determined relative to 9,10-bisphenylethynylanthracene in cyclohexane (Φ F = 1.00). [32] c Deoxygenated solutions were prepared by bubbling N 2 through the solutions.

10.1002/chem.202004328

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Chemistry -A European Journal As well documented for other donor-substituted styrylpyrimidine derivatives, [21] N,N-diphenylamino derivatives 3c and 4c exhibit large positive emission solvatochromism (Figures 6 and S16-S17). This phenomenon, characteristic of push-pull derivatives is explained by the stabilization of the highly polar emitting excited state by polar solvents. [33] λ [nm] Normalized absorbance/emission intensity

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Chemistry -A European Journal However, the methoxy derivatives 3a, 3b and 4a, 4b do not exhibit typical solvatochromic behavior as observed for 3c and 4c. The fluorescence spectra of 3a become red-shifted and broader upon increasing the solvent polarity from n-heptane to toluene and to THF, which is indicative of emission originating from an excited state with ICT characteristics. However, when increasing further the polarity to acetone, a dual emission is observed (Figure 7A). It consists of a broad red-shifted band, emitted from a state with ICT or Twisted-ICT (TICT) nature, that exhibits expected solvatochromic behavior and a blue-shifted band caused by 10.1002/chem.202004328

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Chemistry -A European Journal fluorescence of the locally excited (LE) state, which became significant due to a very low intensity of the ICT/TICT emission. For 4a, the dual emission is observable even in THF with the LE state exhibiting a high-energy fluorescence shoulder (Figure 7B). The LE emission is dominant in acetone. Chromophores 3b and 4b exhibit a simultaneous dual emission from the LE and ICT/TICT states in THF, while in acetone, the emission from the solvated LE band prevails (Figure S18). The above mentioned solvatochromic behavior of chromophores 3 and 4 is indicative of significant differences in the fundamental photophysics of N,N-diphenylamino and methoxy derivatives which is based on the fact that these electron donating groups participate in ICT (and not the acridine) as revealed by the DFT calculations.

The differences in photophysics will be further analyzed by investigating the excited state dynamics.

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The effect of bubbling with N 2 on the fluorescence spectra was investigated in THF as shown in Figures 8 andS19. For 3a-b and 4a-b it is obvious that the intensity of the low energy broad emission peak increases almost twice upon bubbling, which suggests that this emission band is quenched by oxygen (Figure 8). [34] On the other hand, the high energy peak experiences almost negligible changes upon bubbling. In general, oxygen acts as a fluorescence quencher for both LE and ICT states but affects more dramatically the longerlived states, this being a first evidence that in the methoxy derivatives, the ICT state is highly stabilized with long lifetime. The fluorescence spectra of N,N-diphenylamino derivatives, which as mentioned before showed single peak independently on the solvent polarity (Figures 6 andS16), exhibited negligible changes upon bubbling (Figure S19) which is again indication for the differences in the nature of the emitting ICT states between the methoxy and N,N-diphenylamino derivatives.

Excited state dynamics were investigated at both emission bands of 3 and 4 in THF; the results before and after bubbling with N 2 are shown in Figures 9 andS20. Compared to methoxy derivatives, the decays of the N,N-diphenylamino compounds 3c and 4c (Figure S20 B and D) exhibited again a different behavior. Both 3c and 4c possess 2.45 ns average decay lifetimes, which is typical for compounds emitting from an ICT state. Furthermore, the lifetimes barely depend on bubbling with N 2 (the lifetimes of both 3c and 4c increased to 2.75 ns after bubbling). The decays of the methoxy compounds are complex and cannot be explained by a typical ICT emitting state. The decays also differ for the high-and low-energy bands. For example, for 4b, at the high-energy band, the dynamics are very fast (Figure 9) with lifetimes within the IRF of our system (< 80 ps). Such decay dynamics is expected from a molecule having very small quantum yield in solvents of medium polarity. However, the decays at the low-energy band exhibit a very fast component (which is again < 80 ps, ~ 90% amplitude), followed by a longer one (within the order of 10 ns, ~ 10% amplitude). The long 10.1002/chem.202004328

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Chemistry -A European Journal component becomes even more retarded after bubbling with N 2 (within the order of 30 ns, ~10% amplitude). After exposure to air, the lifetime of the long component again decreases.

These observations imply that methoxy chromophores have two contributions in emission with different origin. The high-energy emission band originates from fluorescence of the LE state, has very small quantum yield and is not affected by bubbling. The low-energy emission band contains LE fluorescence as well as long-lived fluorescence from a highly stabilized ICT state , which is strongly dependent on bubbling due to its long lifetime. [35] 0 The contribution of the long-lived part of the fluorescence depends on the solvent used. In nheptane solutions, where a single emission band is observed, the dynamics is only very weakly dependent on bubbling (Figure S21). This means that in this low polarity solvent, the emission is mainly due to a singlet LE fluorescence without contribution from ICT states. In n-heptane, in which the excited state faces a very weak solvation reaction field, the ICT/TICT

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Chemistry -A European Journal formation is endergonic , while in THF which is a solvent of medium polarity the energy of the ICT state is reduced, aided by means of solvation, thus ICT population is energy efficient.

As written before, AIE is an important characteristic for chromophores with potential applications in lighting devices. In order to study potential AIE properties of compounds 3 and 4, their emission spectra were recorded in THF-water mixtures of different ratio. Figure 10 shows representative examples for 3b and 3c, the remaining spectra are provided in the SI (Figures S22-S25). In the case of the methoxy derivative 3b, the dual emission behavior observed in THF disappears upon adding 10% of water and only the emission of the LE state remains, which slightly shifts to the red for larger amounts of water due to the increase of solvent polarity. No significant increase of the emission intensity is observed with water fraction below 80%. A progressive increase of the emission intensity accompanied by a significant red-shift of the emission band is observed for higher portion of water. However, further increasing the water content from 80% to 97% caused a blue shift. For 3c, increasing the water fraction up to 60%, induces a progressive decrease of the emission intensity with a slight red shift of the emission maxima. This phenomenon is attributed to the increased polarity of the media. A slight rebound of the emission accompanied by a blue-shift of the maxima was detected when increasing the water content to 80%. In the solutions with more than 80% of water, the emission intensity decreases again. As a further investigation on the AIE properties the excited state dynamics were examined upon increasing the amount of the poor solvent. Since in AIE chromophores, aggregation is expected to suppress non-radiative processes, the lifetime in such systems is expected to increase and this will be a valuable tool in order to assess the potential of these molecules for solid state lighting applications. In the herein studied acridan molecules, the out-of-plane rotation of the acridan moiety is expected to play the role of an energy consuming process reducing the luminescence intensity. Aggregation is considered to hinder this process and this motion. This hypothesis is confirmed by the intense emission band observed for 3a at 402 nm in glycerol, a viscous solvent hindering molecular motion (Figure S26). Corresponding results for the dynamics of compounds 3b and 4b in THF-water mixtures, are shown in Figures 11 and S27 respectively. Indeed, the time resolved emission results depict an increase of the average lifetime for 3b and 4b that takes place upon increasing the amount of water.

Specifically, for 3b, the amplitude average lifetime increases from < 0.050 ns for 20% water to 2.7 ns for 95% water while for 4b, the lifetime increases from < 0.050 ns to 3.6 ns (Tables S6 andS7). Photophysical properties in solid state. The solid state (powder) emission spectra of compounds in series 3 and 4 were also measured. The emission maxima are summarized in Table 3 while Figure 12 shows their normalized spectra and emission color. The methoxy derivatives 3a-b and 4a-b exhibited blue emission whereas the N,N-diphenylamino coumpounds 3c and 4c showed green emission.

10.1002/chem.202004328

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Chemistry -A European Journal The emission spectra of thin films, prepared by spin-coating, provided further important results on the differences between the methoxy and diphenylamino chromophores (Figure 13).

The methoxy compounds a and b clearly exhibited dual emission behavior with the emission spectra covering almost the whole visible region. The dual emission is more pronounced for 3a and 3b where two peaks are clearly identified. The emission spectra of 4a and 4b feature a high-energy shoulder instead of two separate peaks. The corresponding chromaticity coordinates, that are not far from pure white light, are provided in Table 4. On the contrary, the N,N-diphenylamino compounds c showed a single emission peak attributed to the emission from their S 1 state. The two emission bands of the methoxy compounds in neat film clearly have different origin, similarly to their THF solutions. The low-energy fluorescence band involves long-lived ICT state as revealed by the emission decays with long dynamics (for 3a and 3b see Figure S28). The broad white light spectrum of their neat films render 3a 3b 3c 4a 4b 4c

Normalized emission intensity

λ [nm]

Conclusions

We have designed multipodal D-π-A(-π-D) n (n = 1-2) molecules based on pyrimidine central acceptor and two/three peripheral donors of different electronic and spatial nature. Whereas the acridan moiety linked via 1,4-phenylene linker at C2 of the pyrimidine ring possesses outof-plain arrangement, the peripheral methoxy or N,N-diphenylamino groups interacts directly with the pyrimidine acceptor. The quantum-chemical calculations revealed that despite the HOMO is localized on the acridan moiety, it is completely isolated and does not affect the fundamental absorption properties. These are fully affected by the peripheral methoxy and N,N-diphenylamino groups bearing HOMO-1. The N,N-diphenylamino strong donor in 10.1002/chem.202004328

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Chemistry -A European Journal compounds 3c and 4c caused formation of an intense single band emission in both solution and solid state. It showed positive solvatochromism attributed to the ICT from the NPh 2 group to the pyrimidine core (HOMO-1→LUMO transition). On the other hand, the methoxy derivatives are poorly emissive in moderately polar solvents such as CH 2 Cl 2 but exhibit intense AIE. Dual emission has been observed in THF as well as in neat thin film and is attributed to the LE and ICT/TICT excited states with the lowest energy emission band having long lifetime. The dual fluorescence observed for 3a-b and 4a-b thin films makes these materials highly promising for the development of WOLEDs.

Experimental section

General Conditions. Reagents and solvents were reagent-grade and were purchased from Sigma Aldrich or TCI Europe and used as received. 4-(9,9-Dimethyl-9,10dihydroacridine)phenylboronic acid pinacol ester was prepared according to literature procedure. [36] NMR spectra were acquired at room temperature at 400/100 MHz with a Bruker AVANCE III or 500/125 MHz with Bruker Ascend TM . Chemical shifts are given in parts per million relative to TMS ( 1 H, 0.0 ppm) and CDCl 3 ( 13 C, 77.0 ppm). Acidic impurities in CDCl 3 were removed by treatment with anhydrous K 2 CO 3 . High resolution MALDI MS spectra were measured on a MALDI mass spectrometer equipped with nitrogen UV laser (337 nm, 60 Hz) and quadrupole analyser (positive-ion mode over a normal mass range (m/z 50-2000) with resolution 100 000 at m/z = 400). Trans-2- [3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was used as a matrix. Mass spectra were averaged over the whole MS record for all measured samples.

Photophysical characterization:

The absorption spectra of the samples in solutions and films were detected with a Jasco V-650 while the emission spectra were detected by a Horiba Fluoromax spectrophotometer respectively. UV-vis and fluorescence spectra were recorded 10.1002/chem.202004328

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Chemistry -A European Journal using standard 1 cm quartz cells. Compounds were excited at their absorption maxima (band of lowest energy) to record the emission spectra. The Φ F values were calculated using a wellknown procedure with 9,10-diphenylethynylanthracene in cyclohexane as standard (Φ F = 1). X-Ray Structural Analysis: Single-crystal X-ray-diffraction data were obtained from a D8 VENTURE Bruker AXS difractometer equipped with a (CMOS) PHOTON 100 detector, using Mo-Kα radiation (λ = 0.71073 Å, multilayer monochromator) at T = 150 K. Structures of the crystals were solved by dual-space algorithm using the SHELXT program, [37] and then refined with full-matrix least-squares methods based on F 2 (SHELXL program). [38] All nonhydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions and treated as riding on their parent atom with constrained thermal parameters. Drawings were produced using ORTEP-3. CCDC-200148 (3a), CCDC-200147 (4a) and CCDC-200149 (4c) contain supplementary crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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Chemistry -A European Journal General procedure for Suzuki-Miyaura cross-coupling reaction: 4-(9,9-Dimethyl-9,10dihydroacridine)phenylboronic acid pinacol ester (1 equiv) and 2-chloro-4-methylpyrimidine or 2-chloro-4,6-dimethylpyrimidine (1 equiv) were dissolved in THF (20 mL), and nitrogen was bubbled trough the solution for 10 min. Pd(PPh 3 ) 4 (0.05 equiv) and 20% aqueous Na 2 CO 3 (5 mL) were added, and then the reaction was stirred under nitrogen atmosphere at 65 °C for 15h. The reaction mixture was cooled to room temperature and CH 2 Cl 2 /aqueous NH 4 Cl (1:1, 50 mL) was added. The organic layer was separated, and the aqueous layer was extracted with additional CH 2 Cl 2 (2 × 25 mL). The combined organic extracts were dried over anhydrous Na 2 SO 4 and solvent was evaporated under reduced pressure. The residue was purified by column chromatography (SiO 2 ; EtOAc/hexane 9:1).

General procedure for Knoevenagel condensation: Aldehyde (1 equiv) and 1 (1 equiv) or 2 (0.5 equiv) were added in 5 M aqueous NaOH (20 mL) containing aliquat 336 (0.1 equiv).

The suspension was stirred at 110 °C for 3h and then cooled to room temperature. The precipitate was filtered off, washed with water, dried under reduced pressure, and purified by column chromatography (SiO 2 ; indicated solvents) and/or by recrystallization from CH 2 Cl 2 /hexane.

2- [4-(9,9-Dimethyl-9,10-dihydroacridine)phenyl]-4-methylpyrimidine (1): Synthesized from 2-chloro-4-methylpyrimidine (83 mg, 0.64 mmol) and boronic acid pinacol ester (265 mg, 0.64 mmol) following the general procedure for Suzuki-Miyaura cross-coupling reaction.

Yield 138 mg (57 %); pale yellow solid; R f = 0.2 (SiO 2 ; hexane/EtOAc 9:1); 1 H NMR (400 MHz, CDCl 3 ) δ = 8.71 (d,3 J = 5 Hz,1H),2H),4H),7.13 (d,3 J = 5 Hz,1H),4H),2H), 2.64 (s, 3H), 1.71 (s, 6H) ppm. 13 C NMR (125 MHz, CDCl 3 ) δ = 167. 69, 164.00, 157.13, 143.48, 140.82, 137.95, 131.63, 130.92, 130.11, 126.50, 125.40, 120.74, 119.04, 114.24, 36.11, 31.46, 24.62

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Chemistry -A European Journal 2- [4-(9,9-Dimethyl-9,10-dihydroacridine)phenyl]-4,6-methylpyrimidine (2): Synthesized from 2-chloro-4,6-dimethylpyrimidine (111 mg, 0.78 mmol) and boronic acid pinacol ester (321 mg, 0.78 mmol) following the general procedure for Suzuki-Miyaura cross-coupling reaction.

Yield 166 mg (54 %); pale yellow solid; R f = 0.3 (SiO 2 ; hexane/EtOAc 9:1); 1 H NMR (400 MHz, CDCl 3 ) δ = 8.73-8.71 (m, 2H), 7.54-7.51 (m, 4H), 7.07 (s, 1H), 7.04-6.97 (m, 4H), 6.42-6.40 (m, 2H), 2.66 (s, 6H), 1.78 (s, 6H) ppm. 13 C NMR (125 MHz, CDCl 3 ) δ = 167. 21, 163.87, 143.22, 131.58, 130.99, 130.04, 126.49, 125.40, 120.69, 118.42, 114.26, 36.11, 31.52, 24.36 (E)-2- [4-(9,9-Dimethyl-9,10-dihydroacridine)phenyl]-4-[ (4methoxyphenyl)ethenyl]pyrimidine (3a): Synthesized from compound 1 (138 mg, 0.36 mmol) and 4-methoxybenzaldehyde (50 mg, 0.36 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization. Yield 126 mg (70 %); yellowish solid; R f = 0.5 (SiO 2 ; CH 2 Cl 2 ); mp: 204°C. 1 H NMR (500 MHz, CDCl 3 ) δ = 8.78-8.75 (m, 3H), 8.02 (d,3 J = 16 Hz,1H),2H),4H),1H),7.04 (d,3 J = 16 Hz,1H),6H),2H), 3.87 (s, 3H), 1.72 (s, 6H) ppm. 13 C NMR (125 MHz, CDCl 3 ) δ = 163. 95, 163.09, 160.89, 157.81, 143.49, 140.85, 138.12, 137.14, 131.60, 130.99, 130.12, 129.41, 128.57, 126.53, 125.40, 123.79, 120.74, 116.53, 114.49, 114.27, 55.54, 36.13, 31.46 (E)-2- [4-(9,9-Dimethyl-9,10-dihydroacridine)phenyl]- 4-[(6-methoxynaphtalen-2yl)ethenyl]pyrimidine (3b): Synthesized from compound 1 (102 mg, 0.27 mmol) and 6methoxy-2-naphtalenaldehyde (51 mg, 0.27 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by column chromatography (hexane/EtOAc 7:3). Yield 109 mg (74 %); yellowish solid; R f = 0.5 (SiO 2 ; hexane/EtOAc

Accepted Manuscript

Chemistry -A European Journal 7:3); mp: 199 °C. 1 H NMR (400 MHz, CDCl 3 ) δ 3H),8.20 (d,3 J = 16 Hz,1H),7.99 (br s,1H),3H),4H),4H),4H),2H),3.95 (s,3H), 1.73 (s, 6H) ppm. 13 C NMR (100 MHz, CDCl 3 ) δ = 164. 02, 162.96, 158.72, 157.92, 143.60, 140.89, 138.11, 137.76, 135.40, 131.61, 131.25, 131.03, 130.18, 130.13, 129.13, 129.05, 127.62, 126.54, 125.40, 125.26, 124.42, 120.77, 119.53, 116.69, 114.31, 106.16, 55.53, 36.15, 31.47 (E)-2- [4-(9,9-Dimethyl-9,10-dihydroacridine)phenyl]-4-[ (4diphenylaminophenyl)ethenyl]pyrimidine (3c): Synthesized from compound 1 (228 mg, 0.60 mmol) and 4-diphenylaminobenzaldehyde (164 mg, 0.60 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by column chromatography (CH 2 Cl 2 ). Yield 182 mg (48 %); yellow solid; R f = 0.8 (SiO 2 ; CH 2 Cl 2 ); mp: 375 °C (dec.). 1 H NMR (400 MHz, CDCl 3 ) δ = 8.87-8.84 (m, 3H), 8.09 (d,3 J = 16 Hz,1H),6H),18H),2H), 1.81 (s, 6H) ppm. 13 C NMR (100 MHz, CDCl 3 ) δ = 163. 92, 163.08, 157.70, 149.22, 147.21, 143.50, 140.85, 138.13, 137.04, 131.53, 130.97, 130.13, 129.56, 129.16, 128.90, 126.51, 125.35, 123.86, 123.76, 122.38, 120.74, 116.43, 114.28, 36.10, 31.44

Accepted Manuscript

Chemistry -A European Journal 6H) ppm. 13 C NMR (125 MHz, CDCl 3 ) δ = 163. 69, 163.45, 160.74, 143.27, 140.90, 138.60, 136.50, 131.49, 131.17, 130.07, 129.33, 128.80, 126.53, 125.40, 124.20, 120.70, 114.46, 114.33, 114.17, 55.52, 36.13, 31.51 (E,E)-2- [4-(9,9-Dimethyl-9,10-dihydroacridine)phenyl]- 4,6-bis[(6-methoxynaphtalen-2yl) 7.54-7.49 (m, 4H), 7.27-7.24 (m, 3H), 7.19-7.15 (m, 4H), 7.02-6.94 (m, 4H), 6.44-6.42 (m, 2H), 3.94 (S, 6H), 1.74 (s, 6H) ppm. 13 C NMR (125 MHz, CDCl 3 ) δ = 163. 74, 163.39, 158.60, 143.35, 140.92, 138.54, 137.18, 135.29, 131.52, 131.42, 131.22, 130.12, 130.10, 129.04, 127.57, 126.56, 125.59, 125.43, 124.44, 120.73, 119.48, 114.58, 114.34, 106.10, 55.52, 36.15, 31.53 (E,E)-2- [4-(9,9-Dimethyl-9,10-dihydroacridine) 8.84 (m,2H),8.02 (d,3 J = 16 Hz,2H),8H),8H),9H),10H),4H),2H), 1.74 (s, 6H) ppm. 13 C NMR (125 MHz, CDCl 3 ) δ = 163. 74, 163.44, 149.05, 147.81, 147.29, 143.25, 140.90, 138.66, 10.1002/chem.202004328

Accepted Manuscript

Chemistry -A European Journal 136.40, 131.49, 131.17, 130.07, 129.56, 129.47, 128.84, 126.52, 125.38, 125.26, 124.23, 123.93, 123.80, 122.51, 120.69, 114.34, 114.11, 36.13, 31.50

Figure 1 .

 1 Figure 1. Representative DSC curves of compounds 3a-c determined with a scanning rate of 3 °C/min under N 2 atmosphere.

Figure 2 :

 2 Figure 2: ORTEP drawings of chromophore 3a (A) and 4a (B) obtained at 150 K (R = 0.05).

Figure 3 :

 3 Figure 3: X-ray (A) and DFT-optimized (B) geometries of chromophores 3a and 4a.

Figure 4 :

 4 Figure 4: HOMO-1 (red) and LUMO (blue) localizations in chromophores 3 and 4.

Overall, chromophores 3

 3 and 4 possess the HOMO isolated on the out-of-plane acridan and the initial absorption properties are rather controlled by peripheral methoxy and N,N-diphenylamino donors bearing the HOMO-1. Whereas the N,N-diphenylamino group in chromophores c brings molecular unsymmetry and strong D-A interaction, the methoxy group is weaker donor and chromophores a and b possess C s and C 2v group of symmetry, respectively.

Figure 5 :

 5 Figure 5: Normalized absorption (solid lines) and emission spectra (dashed lines) of compounds 3a (black), 3b (green) and 3c (blue) in n-heptane solution.

Figure 6 :Figure 7 :

 67 Figure 6: A) Normalized emission spectra of 4c in different aprotic solvents, B) Fluorescence color changes experienced by 4c in various solvents. Picture was taken in the dark upon irradiation with a handheld UV lamp (λ em = 254 nm).

Figure 8 :

 8 Figure 8: Emission spectra of 3a, 3b and 4a, 4b in THF before and after bubbling with N 2 .

Figure 9 :

 9 Figure 9: Emission dynamics of 3b and 4b in THF before and after bubbling with N 2 detected at both the high-and low-energy emission bands. The instrument's response function is also shown.

Figure 10 :

 10 Figure 10: Fluorescence emission spectra of 3b (A) and 3c (B) (c = 1 × 10 -5 M) in THF/water mixture. Integration of emission band versus volume percent of water in mixtures of THF/water of 3a and 3c. Insert pictures were taken in the dark upon irradiation with a handheld UV lamp (λ em = 254 nm).

Figure 11 .

 11 Figure 11. Emission dynamics of 3b (c = 1 × 10 -5 M) in THF/water mixtures (a) 20-70 % and (b) 80-95 %.

Figure 12 :

 12 Figure 12: A) Normalized emission spectra of solid state compounds 3a-c and 4a-c. B) Fluorescence color of compounds 3 and 4 (powder). Picture was taken in the dark upon irradiation with a handheld UV lamp (λ em = 254 nm).

Figure 13 :

 13 Figure 13: Emission spectra of neat thin films of flurophores 3 and 4.

  Stokes shifts were calculated by considering the lowest energetic absorption band. The detection of the dynamics has been carried out via a Time Correlated Single Photon Counting (TCSPC) technique (Fluotime 200 spectrometer Picoquant). The excitation source was a diode laser at 400 nm emitting 60 ps pulses. The resolution was ~80 ps however shorter lifetimes can be calculated by fitting the experimental results with multi-exponential functions convoluted with the instrument's response function. DSC: Thermal properties of molecules 3 and 4 were investigated by DSC measurements with a Mettler-Toledo STARe System DSC 2/700 equipped with FRS 6 ceramic sensor and cooling system HUBER TC100-MT RC 23. Thermal behavior of the target chromophores were measured in open aluminous crucibles under N 2 inert atmosphere. DSC curves were determined with a scanning rate of 3 °C/min within the range 25-500 °C.

  ppm. HR-MALDI-MS (DCTB) m/z calcd for C 25 H 20 N 3 [M-CH 3 + ] 362.1652, found 362.1640.

  ppm. HR-MALDI-MS (DCTB) m/z calcd for C 33 H 26 N 3 O [M-CH 3 + ] 480.2070, found 480.2058.

  ppm. HR-MALDI-MS (DCTB) m/z calcd for C 37 H 28 N 3 O [M-CH 3 + ] 530.2227, found 530.2221.

  ppm. HR-MALDI-MS (DCTB) m/z calcd for C 44 H 33 N 4 [M-CH 3 + ] 617.2700, found 617.2689.(E,E)-2-[4-(9,9-Dimethyl-9,10-dihydroacridine)phenyl]-4,6-bis[(4methoxyphenyl)ethenyl]pyrimidine (4a): Synthesized from compound 2 (158 mg, 0.40 mmol) and 4-methoxybenzaldehyde (110 mg, 0.81 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization. Yield 186 mg (73 %); yellow solid; R f = 0.4 (SiO 2 ; CH 2 Cl 2 ); mp: 201 °C. 1 H NMR (500 MHz, CDCl 3 ) δ = 8.85-8.83 (m, 2H), 8.04 (d, 3 J = 16 Hz, 2H), 7.63-7.62 (m, 4H), 7.52-7.48 (m, 4H), 7.19 (s, 1H), 7.06 (d, 3 J = 16 Hz, 2H), 7.00-6.93 (m, 8H), 6.41-6.39 (m, 2H), 3.87 (s, 6H), 1.73 (s, 10.1002/chem.202004328

  ppm. HR-MALDI-MS (DCTB) m/z calcd for C 50 H 38 N 3

Table 1 . Thermal properties of 3-4. Comp. Tm a (°C) T d

 1

	10.1002/chem.202004328

Table 2 :

 2 DFT-calculated fundamental parameters of 3 and 4.

	10.1002/chem.202004328

a Calculated at DFT B3LYP/6-311++G(3d,f,2p) level in vacuum; b HOMO-LUMO/HOMO-1-LUMO gaps; c TD-DFT calculated (nstates = 8) using B3LYP/6-311++G(3d,f,2p) or CAM-B3LYP/6-311++G(2d,p) in vacuum; d Group of Symmetry.

10.1002/chem.202004328

  arranged in both excited states, a slight enlargement of the C=C bond of the styryl moiety occupied by the LUMO was observed (1.328 and 1.388 A for the ground and triplet states). The remaining frontier molecular orbitals are positioned analogously to the ground state. This imply a relatively small barrier between both singlet and triplet states.

	Chemistry -A European Journal	10.1002/chem.202004328
		Accepted Manuscript

perpendicularly

  , aggregation of such choromophores being plausible in this solvent even at low concentration. In contrast, the quantum yields of chromophores 3c and 4c in n-heptane are significantly reduced compared to the values in CH 2 Cl 2 solution. Deoxygenating the solutions slightly increases the emission quantum yields.

	Chemistry -A European Journal	10.1002/chem.202004328
		Accepted Manuscript

phenomena

Table 3 :

 3 UV/Vis and PL data in n-heptane.

		UV/vis λ max , nm	PL	Solidst.			Stokes shift
	Compd a	(ε, mM -1 •cm -1 )	λ max , nm	λ max , nm	Φ F b	Φ F (deox.) c	cm -1
	3a	337 (29.4)	432	471	0.11	0.15	6525
	3b	368 (24.5)	429	471	0.09	0.13	3864
	3c	396 (30.7)	444	521	0.37	0.40	2730
	4a	383 (24.3)	426	474	0.06	0.08	2635
	4b	395 (48.8)	435	508	0.22	0.29	2328
	4c	432 (53.6)	460	535	0.63	0.74	1409

Table 4 :

 4 CIE chromaticity coordinates (x, y) for compounds 3 and 4.

	3a	3b	3c	4a	4b	4c
	(0.28, 0.40) (0.35, 0.41) (0.27, 0.54) (0.29, 0.42) (0.35, 0.45) (0.32, 0.60)

  ppm. HR-MALDI-MS (DCTB) m/z calcd for C 26 H 22 N 3 [M-CH 3

	+ ] 376.1808, found
	376.1796.

  ppm. HR-MALDI-MS (DCTB) m/z calcd for C 42 H 34 N 3 O 2 [M-CH 3 + ] 612.2646, found 612.2637.

  ethenyl]pyrimidine (4b): Synthesized from compound 2 (100 mg, 0.26 mmol) and 6methoxy-2-naphtalenaldehyde (96 mg, 0.51 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by recrystallization. Yield 133 mg (72 %); yellow solid; R f = 0.3 (SiO 2 ; hexane/EtOAc 8:2); mp: 271 °C.1 H NMR (500 MHz, CDCl 3 ) δ 2H), 8.22 (d, 3 J = 16 Hz, 2H), 7.99 (s, 2H), 6H),

  phenyl]-4,6-bis[(4- diphenylaminophenyl)ethenyl]pyrimidine (4c): Synthesized from compound 2 (196 mg, 0.50 mmol) and 4-diphenylaminobenzaldehyde (274 mg, 1.00 mmol) following the general procedure for Knoevenagel condensation. The crude product was purified by column chromatography (hexane/CH 2 Cl 2 1:1) and recrystallization. Yield 110 mg (22 %); yellow solid; R f = 0.6 (SiO 2 ; hexane/CH 2 Cl 2 1:1); mp: 233°C.1 H NMR (500 MHz, CDCl 3 ) δ = 8.85-

  ppm. HR-MALDI-MS (DCTB) m/z calcd for C 64 H 48 N 5 [M-CH 3 + ] 886.3904, found 886.3892. Aknowledgements M. F. Thanks the Région Bretagne, France, for founding her Ph D. F. B. and M.K. are indebted to the Ministry of Education, Youth and Sports of the Czech Republic (LTAIN19101).


Jianyu Wei

J.-F Halet

Samia Kahlal

Jean-Yves Saillard

Alvaro Munoz-Castro

Toward the Formation of N-Heterocyclic-Carbene-Protected Gold Clusters of Various Nuclearities. A Comparison with Their Phosphine-Protected Analogues from Density Functional Theory Calculations

published or not. The documents may come   L'archive ouverte pluridisciplinaire

[ †] Univ Rennes, CNRS, Institut des Sciences Chimiques de Rennes (ISCR) -UMR 6226, F-35000 Rennes, France.

[ §] Grupo de Química Inorgánica y Materiales Moleculares, Facultad de Ingenieria, Universidad Autonoma de Chile, El Llano Subercaseaux 2801, Santiago, Chile KEYWORDS: Gold, superatoms, NHC, phosphines A c c e p t e d M a n u s c r i p t

Introduction

Ligated atom-precise gold nanoclusters (AuNCs) are currently the subject of considerable interest. [START_REF] Qian | Controlling nanoparticles with atomic precision: The case of Au 144 (SCH 2 CH 2 Ph) 60[END_REF][START_REF] Levi-Kalisman | Synthesis and characterization of Au 102 (p-MBA) 44 nanoparticles[END_REF][START_REF] Qian | Quantum sized gold nanoclusters with atomic precision[END_REF][START_REF] Tsukuda | Optical properties of ultra-small gold nanostructures[END_REF][START_REF] Kang | Au 25 (SR) 18 : the captain of the great nanocluster ship[END_REF][START_REF] Tang | Insights into interfaces, stability, electronic properties, and catalytic activities of atomically precise metal nanoclusters from first principles[END_REF][START_REF] Pei | Growth-rule-guided structural exploration of thiolateprotected gold nanoclusters[END_REF] On one side, they represent the lowest size limit of gold nanoparticles (AuNPs), with, in addition, ideal mono-dispersity. They constitute thus interesting models for fundamentally understanding the numerous properties of potential technological interest of larger AuNPs. [START_REF] Qian | Controlling nanoparticles with atomic precision: The case of Au 144 (SCH 2 CH 2 Ph) 60[END_REF][START_REF] Levi-Kalisman | Synthesis and characterization of Au 102 (p-MBA) 44 nanoparticles[END_REF][START_REF] Qian | Quantum sized gold nanoclusters with atomic precision[END_REF][START_REF] Tsukuda | Optical properties of ultra-small gold nanostructures[END_REF][START_REF] Kang | Au 25 (SR) 18 : the captain of the great nanocluster ship[END_REF][START_REF] Tang | Insights into interfaces, stability, electronic properties, and catalytic activities of atomically precise metal nanoclusters from first principles[END_REF][START_REF] Pei | Growth-rule-guided structural exploration of thiolateprotected gold nanoclusters[END_REF][START_REF] Daniel | Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology[END_REF][START_REF] Tsukuda | Aerobic oxidations catalyzed by colloidal nanogold[END_REF][START_REF] Zhu | Catalysis opportunities of atomically precise gold nanoclusters[END_REF][START_REF] Kwak | Selective determination of dopamine using quantumsized gold nanoparticles protected with charge selective ligands[END_REF][START_REF] Sakai | Photovoltaic properties of glutathione-rotected gold clusters adsorbed on TiO 2 electrodes[END_REF][13][14][15][16] On the other side, AuNCs have also their own intrinsic interest, due to their specific properties associated with their delocalized 6s electrons. The field of their various chemical, biochemical, physicochemical and physical properties is as large as their extremely rich structural chemistry. 1- 7 Numerous theoretical and experimental research efforts have contributed to the rationalization of their structure and stability towards the design of novel noble-metal nanostructures. 17 -31 It appears obvious that the role played by the AuNC ligand shell is decisive in the stabilization of a given structure for a given electron count. Ligands do not only play the role of passivating the metal core surface, but can also have significant influence on the AuNC structure and properties. Thus, introducing new ligands in the synthetic chemistry of AuNCs is of prime interest, in particular for tuning their optical properties.

From this view point, it is worth noting that N-heterocyclic carbenes (NHC) have been very recently introduced as new protecting ligands of gold clusters. [START_REF] Tang | Insights into interfaces, stability, electronic properties, and catalytic activities of atomically precise metal nanoclusters from first principles[END_REF][START_REF] Ube | A carbon-centered hexagold (I) cluster supported by Nheterocyclic carbene ligands[END_REF][START_REF] Narouz | Robust, highly luminescent Au 13 superatoms protected by N-heterocyclic carbenes[END_REF][START_REF] Narouz | Nheterocyclic carbene-functionalized magic-number gold nanoclusters[END_REF][START_REF] Tang | Comprehensive view of the ligand-gold interface from first principles[END_REF][START_REF] Shen | Highly robust but surface-active: An N-heterocyclic carbene-stabilized Au 25 nanocluster[END_REF][START_REF] Jiang | N-heterocyclic carbenes on close-packed coinage metal surfaces: bis-carbene metal adatom bonding scheme of monolayer films on Au[END_REF] In particular, Crudden and coworkers synthezised NHC-protected Au 13 superatoms that exhibit large photoluminescent quantum yields. [START_REF] Narouz | Robust, highly luminescent Au 13 superatoms protected by N-heterocyclic carbenes[END_REF][START_REF] Narouz | Nheterocyclic carbene-functionalized magic-number gold nanoclusters[END_REF] It turns out that one of us recently published a Density Functional Theory (DFT) investigation on the effect of substituting the diphosphine ligands by NHCs in the wellknown [Au 13 (dppe) 5 Cl 2 ] 3+ cluster (dppe = diphenylphosphinethane). [START_REF] Muñoz-Castro | Potential of N-heterocyclic carbene derivatives from Au 13 (dppe) 5 Cl 2 gold superatoms. Evaluation of electronic, optical and chiroptical properties from relativistic DFT[END_REF] It was found that the substitution does not substantially modify the AuNC stability, but significantly affects the optical A c c e p t e d M a n u s c r i p t properties. Thus, NHCs appear to be an interesting alternative to their phosphine isolobal analogs, allowing increasing cluster versatility, owing to the electronic variability of NHCs, which range from strong to weak donor ligands. [START_REF] Tolman | Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis[END_REF][START_REF] Munz | Pushing Electrons-which carbene ligand for which application?[END_REF][START_REF] Gusev | The Tolman electronic parameter (TEP) and the metal-metal electronic communication in ditopic NHC complexes[END_REF][START_REF] Arduengo | Carbenes introduction[END_REF][START_REF] Nelson | Quantifying and understanding the electronic properties of Nheterocyclic carbenes[END_REF] As a continuation of our first work, [START_REF] Muñoz-Castro | Potential of N-heterocyclic carbene derivatives from Au 13 (dppe) 5 Cl 2 gold superatoms. Evaluation of electronic, optical and chiroptical properties from relativistic DFT[END_REF] we report here a comprehensive DFT investigation on the substitution of phosphines by NHCs in a series of small and medium size phosphine-protected AuNCs. The analysis is mainly focused on the substitution-induced changes on the structural arrangement, electronic structure and optical (including emissive) properties. The chosen AuNCs phosphine reference series (see Table 1) is composed of the experimentally characterized [Au 6 (dppp) 4 ] 2+ [START_REF] Van Der Velden | Gold clusters[END_REF]45 3+ 52 (Au 13 Cl 2 -P) (dppp = diphenylphosphinepropane). These compounds were chosen because their offer a reasonable range of nuclearity (from 6 to 13), structure types and superatom electron counts (from 4 to 8 -5d electrons are not considered). Some of them have diphosphines, whereas others have simple phosphines. Some bear chloride ligands whereas the others are homoleptic. Moreover, photoluminescence data are available for most of them. 46,47,52,[START_REF] Konishi | Phosphine-coordinated pure-gold clusters: diverse geometrical structures and unique optical properties/responses[END_REF] The hypothetical [Au 13 (PPh 3 ) 9 Cl 3 ] 2+ (Au 13 Cl 3 -P) compound, which is the phosphine counterpart of several recently characterized NHC derivatives [Au 13 (NHC) 9 Cl 3 ] 2+ was added to this series. [START_REF] Narouz | Robust, highly luminescent Au 13 superatoms protected by N-heterocyclic carbenes[END_REF] Thus, we believe that the variety of these compounds makes this series a good reference sampling for investigating the changes occurring upon substituting the phosphine ligands by NHC homologs. These homologs were designed by substituting the mono-phosphines by the mono-carbene N,N'-diisopropylimidazolidene ligands (noted NHC' in the followings, see Scheme 1), taken from the litterature. [START_REF] Ube | A carbon-centered hexagold (I) cluster supported by Nheterocyclic carbene ligands[END_REF] The dppe and dppp diphosphine ligands were substituted by their NHC analogs, namely 1,3-bis{N-isopropylimidazolidenyl}ethane (di-NHC'e) and 1,3-

Computational Details

Geometry optimizations were carried out at the Density Functional Theory (DFT) level [START_REF] Parr | Density-Functional Theory of Atoms and Molecules[END_REF] with the addition of Grimme's D3 empirical corrections [START_REF] Grimme | Semiempirical GGA-type density functional constructed with a long-range dispersion correction[END_REF] in order to take into account dispersion effects. The Amsterdam Density Functional code [START_REF] Velde | Chemistry with ADF[END_REF] (ADF2017), incorporating the relativistic scalar corrections via the ZORA Hamiltonian was used. [START_REF] Van Lenthe | Relativistic total energy using regular approximations[END_REF] The all-electron triple-ξ Slater basis set plus a polarization function (STO-TZP), [START_REF] Lenthe | Optimized Slater-type basis sets for the elements 1-118[END_REF] were used, together with the Becke-Perdew (BP86) [START_REF] Becke | Density-functional exchange-energy approximation with correct asymptotic behavior[END_REF]60 exchange-correlation functional. This functional was chosen for its good performance in reproducing cluster ground state geometries and its reasonable CPU time demand. 19 An energy convergence criterion of 5 × 10 -5 Hartree and a radial convergence criterion of 5 × 10 -3 Å were employed. All the optimized structures were confirmed as true minima on their potential energy surface by analytical vibration frequency calculations. All the computed species have a closedshell ground state. The Gaussian16 package 61 was used to calculate natural atomic orbital (NAO) populations and Wiberg bond indices (WBI) using the ADF-optimized geometries with its NBO6.0 module, [START_REF] Glendening | NBO 6.0[END_REF] with the Def2-SVP [START_REF] Weigend | Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy[END_REF] basis set and the BP86 functional. In order to reduce computational efforts, the Gaussian16 package was also used for calculating the UV-Vis optical transitions by time-dependent density functional theory [START_REF] Runge | Density-functional theory for time-dependent systems[END_REF] (TD-DFT) calculations, using the Def2-SVP [START_REF] Weigend | Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy[END_REF] basis set (which includes effective core potentials accounting for scalar relativistic effects) and the B3LYP functional. [START_REF] Stephens | Ab Initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields[END_REF] The choice of this functional was made after having tested several of them, namely B3LYP, 65 BP86, [START_REF] Becke | Density-functional exchange-energy approximation with correct asymptotic behavior[END_REF]60 and LB94, [START_REF] Van Leeuwen | Exchange-correlation potential with correct asymptotic behavior[END_REF] with respect to the experimental spectra of the phosphine-protected AuNCs listed in Table 1. Similar trends were found for the three functionals, but the B3LYP-computed optical transitions were found to be in the closest quantitative agreement with experiment. The UV-Vis spectra were simulated from the computed A c c e p t e d M a n u s c r i p t TD-DFT transition energies and their oscillator strength by using the SWizard program, [START_REF] Gorelsky | SWizard program[END_REF] each transition being associated with a Gaussian function of half-height width equal to 3000 cm -1 , a value that best reproduces experimental spectra. To calculate the fluorescence emission wavelengths, the geometry of the excited singlet states was optimized at the TD-DFT/B3LYP level with the Gaussian16 program. To compute the phosphorescence emission wavelengths, the geometry of the triplet state was calculated at the DFT/BP86 level with ADF2017 and its energy and that of the ground state were recalculated for this geometry as single points at the B3LYP level with Gaussian16. To compute the energy differences between the singlet and triplet exited states, the ADF optimized triplet exited geometries were re-calculated via single point calculations using Gaussian 16.

Results and Discussion

Structures and Bonding Modes

The geometries of the phosphine-protected AuNCs (Au n -P) and their NHC homologs (Au n -C)

were optimized and compared. The former are found to be in good agreement with their corresponding X-ray structures. [START_REF] Narouz | Robust, highly luminescent Au 13 superatoms protected by N-heterocyclic carbenes[END_REF][START_REF] Van Der Velden | Gold clusters[END_REF]46,47,49,51,52 The core structure of Au 6 -P consists of an edge-bridged tetrahedron (Figure 1). Whereas its Xray structure is of S 4 symmetry, its DFT-optimized geometry was found to be slightly distorted into D 2 symmetry. In a first approximation, the computed data given in Table 2 are consistent with a view of Au 6 -P made of two R 3 P-Au(0)-Au(0)-PR 3 sub-systems (each containing a 2 electron/2-center bond) and two R 3 P-Au(I)-PR 3 units. In both fragment types, the 14-electron gold centers adopt the (approximately) linear 'ML 2 ' coordination mode expected to favor its stability for this particular electron count. [START_REF] Albright | Orbital Interactions in Chemistry[END_REF] This is confirmed by the nature of the HOMO (b 1 )

and HOMO-1 (a) of Au 6 -P, which can be described as the out-of-phase and in-phase combinations, respectively, of the σ(Au-Au) bonding orbitals of the two R 3 P-Au(0)-Au(0)-PR 3 moieties (Figure S7). These orbitals, of large 6s/6p character, are moderately delocalized on the R 3 P-Au(I)-PR 3 fragments, suggesting that the 4 electrons remain principally located on the two 2). It thus can be concluded that these longer Au-Au contacts are largely the result of metallophilic interactions. The LUMO (a) is also of important 6s/6p character and has significant localization on the two bridging Au atoms. Au 6 -C differs from Au 6 -P in the fact that each of its two L-Au(0)-Au(0)-L and two L-Au(I)-L fragments are even less interacting with the others (see data in Table 2), so that they can be considered as The core structure of Au 8 -P consists of three edge-sharing linearly arranged tetrahedra (Figure 1). Its ideal symmetry is S 4 , whereas after relaxation its symmetry is lowered to C 2 . It can be described as made of four quasi-linear L-Au-Au-L units packed perpendicularly along a common pseudo-S 4 axis in a staggered fashion. The shortest Au-Au distances are those within these L-Au-Au-L fragments. The favored electron count for an Au 4 tetrahedron is 2, 69 so it is expected that the 6 electrons of Au 8 -P are delocalized over the three tetrahedra. This is supported by the Au-Au WBIs given in Table 3 that are all significant, contrarily to the case of Au 6 -P (Table 2). A more detailed analysis of their disparity and examination of the NAO charges tend to favor the view of a 2-electron central tetrahedron [Au 4 (PR 3 ) 4 ] 2+ interacting in a moderate but significant A c c e p t e d M a n u s c r i p t The structural arrangement of the Au 8 Cl 2 -P cluster differs from that of Au 8 -P (Figure 1). It exhibits a core made of two edge-sharing tetrahedra capped on both sides by two nearly linear R 3 P-Au b -Cl units. Its ideal symmetry is D 2h , but DFT computations found it slightly distorted into C 1 after relaxation. This is a 4-electron species, and the NAO charge distribution (Table 4) The 6-electron Au 9 -P cluster that we investigated (Figure 1) is generally described as a bicapped centered chair, 47,48 that is, a centered Au 6 chair-like ring with two opposite vertices the HOMO-1 (b 1 ) are the P x and P y orbitals, whereas the LUMO+1 is the P z counterpart (Figure S7). The LUMO is the D x2-y2 orbital. The electronic structure of Au 9 -C is the same as that of Au 9 -P, except the LUMO and LUMO+1, which are inverted in energy (Figure S8). Again, a small strengthening of the Au-Au bonding is noted when going from Au 9 -P to Au 9 -C.

almost
indicates
Comparable HOMO-LUMO gaps are computed for both species. The 8-electron Au 11 -P cluster (Figure 1) can be viewed as resulting from the bridging of the 2P y level (Figure S7). Au 11 -C has comparable geometry (Table 6) and electronic structure (Figure S8). Finally, the last three phosphine clusters of the series (Table 1), namely Au 11 Cl 2 -P, Au 13 Cl 2 -P (already theoretically studied previously) [START_REF] Weerawardene | Luminescence and electron dynamics in atomically precise nanoclusters with eight superatomic electrons[END_REF] and Au 13 Cl 3 -P (Figure 1), are more compact, exhibiting a centered icosahedral core (incomplete in the case of Au 11 Cl 2 -P). The somewhat distorted icosahedra display rather homogeneous Au c -Au p distances (Table 7). They are all regular 8-electron pseudo-spherical superatoms with 1S 2 1P 6 configuration. The HOMOs and LUMOs are of 1P and 1D nature, respectively (Figure S7). Their NHC counterparts, Au 11 Cl 2 -C, Au 13 Cl 2 -C, and Au 13 Cl 3 -C exhibit quite similar features (Table 7 and Figure S8).

A c c e p t e d M a n u s c r i p t

Hence, besides the already characterized NHC-protected as protecting ligands for Au 6 and Au 13 clusters, [START_REF] Ube | A carbon-centered hexagold (I) cluster supported by Nheterocyclic carbene ligands[END_REF][START_REF] Narouz | Robust, highly luminescent Au 13 superatoms protected by N-heterocyclic carbenes[END_REF] carbene ligands appear to be able to stabilize gold clusters of any size. Thus, size-dependent properties related to cluster core nuclearity can be explored, in addition to the versatility shown by NHC's characteristics ranging from strong to weak donor ligands. [START_REF] Munz | Pushing Electrons-which carbene ligand for which application?[END_REF] A c c e p t e d M a n u s c r i p t

Bonding Energies

A more quantitative comparison of the thermodynamic stability of the investigated phosphine and NHC species can be performed from the comparison of the energies of interaction between the ligands and the metal core, but also from the evaluation of the energy cost associated with the distortion afforded by the metal core upon complexation. To this end, we optimized the geometries of the bare [Au n ] x+ core of the investigated species, obtained after removal of the neutral phosphine (or NHC) and anionic chloride (if any) ligands. Interestingly, these optimized bare ionic metal cores maintain the structure that they adopt in the Au n -P, Au n Cl m -P experimental structures (see Figure S17). Apart from the icosahedral [Au 13 ] 5+ core, [START_REF] Gam | Coinage Metal Superatomic Cores: Insights into Their Intrinsic Stability and Optical Properties from Relativistic DFT Calculations[END_REF] there is no certainty that such configurations are global minima, 73 but at least they are local minima, as ascertained by vibrational frequency analyses. Consistently, they all present a significant HOMO-LUMO gap (> 1 eV), except that of [Au 8 ] 2+ (0.39 eV), perhaps due to its low compacity. -corrected in the case of Au 6 -X couple), except for the Au 13 Cl 3 -X system where it is a bit higher (10 kcal/mol).

Optical properties

The UV-Vis absorption spectra of the computed clusters were simulated (see Computational Details above). They are gathered in Figure 3 and the major computed electronic transition energies associated with are given in Table 9, together with corresponding experimental λ max values taken from the literature where available. [START_REF] Narouz | Robust, highly luminescent Au 13 superatoms protected by N-heterocyclic carbenes[END_REF][START_REF] Van Der Velden | Gold clusters[END_REF][45][46][47][48][49][50][51][52][START_REF] Konishi | Phosphine-coordinated pure-gold clusters: diverse geometrical structures and unique optical properties/responses[END_REF] Although spin orbit corrections were not considered in our calculations, a good match was obtained between the computed excitation energies and the available experimental λ max values, in particular for the low-energy transitions.

This agreement provides confidence in the reliability of our results, retaining small deviations (0.07 to 0.32 eV) within the method error. 19 Going from L = phosphine to L = NHC does not change importantly the nature and the energies of the optical transitions of a given cluster and consequently the shape of its simulated spectrum. A general blue-shift tendency is observed when going from L = phosphine to L = NHC, except for the low-energy absorption band of Au 9 -X (Figure 3) in agreement with the smaller HOMO-LUMO gaps computed for the later over all.

Indeed, in the Au 6 -P to Au 11 -P series, the lowest energy band is associated with a HOMO→LUMO transition of metal-to-metal charge transfer (MMCT) nature (Table 9). A transition involving orbitals of the same nature is found for the homologous Au 6 -C to Au 11 -C series, although in the cases of Au 6 -C and Au 9 -C it is a HOMO→LUMO+2 transition, which is mainly responsible. In the pseudo-spherical Au 11 Cl 2 -X to Au 13 Cl 3 -X (X = P, C) series, the lowenergy band is associated to MMCT 1P→1D transitions.

In the two Au 6 -X and Au 11 -X (X = P, C) series the major transitions of higher energy are mainly of 5d(Au)→ligand character, sometimes mixed with some 5d(Au)→6s/6p(Au) character. a X = P except for Au 13 Cl 3 -X, for which X = C. [START_REF] Narouz | Robust, highly luminescent Au 13 superatoms protected by N-heterocyclic carbenes[END_REF] In order to get an idea of the emission wavelength change when going from phosphine-to NHC-protected species, we have computed the corresponding transitions assuming both fluorescence and phosphorescence behavior. The emission energies were calculated by TD-DFT as vertical transitions between the optimized singlet excited state and the ground state (see Computational Details). They are given in Table 10, together with the experimental data available for the Au n -P series 46,47,52,53 and Au 13 Cl 3 -C. [START_REF] Narouz | Robust, highly luminescent Au 13 superatoms protected by N-heterocyclic carbenes[END_REF] A reasonably good agreement is found between the experimental and TD-DFT values, giving us confidence in the computed data. When comparing the two Au n -P and Au n -C series, similar values are obtained, except for the Au 6 -X homologues for which the large difference in their emission wavelength can be related to the A c c e p t e d M a n u s c r i p t significant difference in their structure (see below). In any case, no systematic tendency (blue-or red-shifting) is found when going from Au n -P to Au n -C, which is attributable to size-dependent and core-composition effects exhibited by the respective cluster core along the series. The phosphorescence emission energies were computed as vertical transitions between the triplet state in its equilibrium geometry and the singlet state in the same geometry. The results are given in Table 11. To our knowledge, no experimental evidence of phosphorescence has been reported for the known compounds. As in the case of fluorescence, the results do not show any Finally, we would like to make some comments about the energy difference between the first excited singlet and triplet states (Table S1). These gaps range 1.9-3.3 eV and 0.

Conclusions

The hypothetical NHC-protected gold clusters we have calculated exhibit similar structural chemistry as their experimentally characterized phosphine-protected analogs. The similarities in bond distances and Wiberg bond indices are indicative of analogous thermodynamic stability. This is confirmed by the evaluation of the gold-P and gold-C bonding energies in these species, the latter being even slightly stronger than the former by a few kcal/mol. This is in phase with what is known from the few NHC-protected gold clusters known so far. [START_REF] Ube | A carbon-centered hexagold (I) cluster supported by Nheterocyclic carbene ligands[END_REF][START_REF] Narouz | Robust, highly luminescent Au 13 superatoms protected by N-heterocyclic carbenes[END_REF][START_REF] Narouz | Nheterocyclic carbene-functionalized magic-number gold nanoclusters[END_REF][START_REF] Shen | Highly robust but surface-active: An N-heterocyclic carbene-stabilized Au 25 nanocluster[END_REF] The comparable HOMO-LUMO gaps are also indicative of comparable kinetic stability. Their optical properties are also likely to be quite similar, the NHC derivatives being however more prone to exhibit phosphorescence behavior. Our results show the feasibility to achieve variable-sized stable NHC-protected gold clusters with properties related to their parent phosphine-protected species.

This allows further evaluation of size-dependent properties, which in addition, takes advantage (PDF).

(13) Wu, Z.; Wang, M.; Yang, J.; Zheng, X.; Cai, W.; Meng, G.; Qian, H.; Wang, H.; Jin, R.

Well-defined nanoclusters as fluorescent nanosensors: A case study on Au 25 (SG) 18 . Small. 2012,

  Towards the Formation of N-Heterocyclic Carbene-Protected Gold Clusters of Various Nuclearities. A Comparison with their Phosphine-Protected Analogs from DFT Calculations Jianyu Wei, † Jean-François Halet, † Samia Kahlal, † Jean-Yves Saillard* , † and Alvaro Muñoz-Castro* , §

A c c e

  Scheme 1. NHC Ligands Considered in the Calculations

A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p tFigure 1 .

 1 Figure 1. Optimized geometries of the computed Au n -X clusters (X = P, C).

A c c e p t e d M a n u s c r i p t

Figure 2 .

 2 Figure 2. Density of states of Au 6 -X (top) and Au 13 Cl 2 -X (bottom) (X = P, C). Gaussian full

A c c e p t e d M a n u s c r i p t

  short Au-Au bonds of the Au 4 tetrahedron, as also confirmed by the much smaller Wiberg bond indices of the other Au-Au contacts (Table

  independent from each other. Nevertheless, the HOMO (b 1 ) and HOMO-1 (a) of Au 6 -C are similar to their Au 6 -P counterparts (FigureS8). The LUMO and LUMO+1 are ligand-based levels. Although localized in part on the bridging Au atoms, the LUMO is different from the LUMO of Au 6 -P. The LUMO+2 (a), mainly localized on the outer Au atoms, is the counterpart of the Au 6 -P LUMO.

A c c e p t e d M a n u s c r i p t

A c c e

  p t e d M a n u s c r i p t covalent way with two linear R 3 P-Au(0)-Au(0)-PR 3 units. The HOMO (a) and HOMO-1 (b), of large 6s/6p character, can be associated with the two outer Au(0)-Au(0) bonding pairs, partly delocalized on the central tetrahedron (FigureS7). It was not possible to identify a unique orbital associated with the third bonding pair, because of orbital mixing, but several 5d-type levels present some non-negligible 6s/6p bonding character within the central tetrahedron. The LUMO and LUMO+1, both of b symmetry, are 6s/6p Au-Au antibonding orbitals.As for the Au 6 systems described above, substituting the phosphines in Au 8 -P by NHC ligands tends to reinforce the Au-Au bonding in the four R 3 P-Au-Au-PR 3 units and weakens somewhat the other Au-Au contacts, but the effect is not as strong as in the Au 6 systems. The HOMO and HOMO-1 of Au 8 -C are the counterparts of those of Au 8 -P. The LUMO+2 and LUMO of Au 8 -C are the counterparts of the LUMO and LUMO+1 of Au 8 -P (FigureS8). As for the Au 6 systems, a slightly smaller HOMO-LUMO gap is computed when phosphines are substituted by NHC ligands.

  that the two bridging atoms can be considered as Au b (I) centers. The 4 electrons can thus be associated with the bi-tetrahedron, where they are predominantly located on the R 3 P-Au m -Au m -PR 3 and R 3 P-Au s -Au s -PR 3 bonds, as shown by the corresponding WBIs. This is in line with the composition and shape of the HOMO and HOMO-1 (FigureS7) orbitals of Au 8 Cl 2 -P, which are associated with these 4 electrons. The LUMO and LUMO+1 are 6s/6p in composition associated with the bridging Au b atoms and the bi-tetrahedron, respectively (FigureS7).

A c c e

  p t e d M a n u s c r i p t Compound Au 8 Cl 2 -C has similar arrangement and electronic structure as its phosphine homolog (Figure S8). When going from Au 8 Cl 2 -P to Au 8 Cl 2 -C, a strengthening of the R 3 PAu-AuPR 3 bond and some weakening of the other Au-Au bonds is observed, a tendency already noted in the previous clusters, but less pronounced in the present systems. In contrast with the previous Au 6 and Au 8 systems, a larger HOMO-LUMO gap is computed when phosphine ligands are substituted by NHC ligands.

A c c e p t e d M a n u s c r i p t capped by two

  supplementary gold atoms. It has an isomer exhibiting a centered crown,[START_REF] Briant | Structural characterisation of two crystalline modifications of [Au 9 {P(C 6 H 4 OMe-p) 3 } 8 ](NO 3 ) 3 : The first example of skeletal isomerism in metal cluster chemistry[END_REF] which is not discussed here. The whole structure of Au 9 -P is of ideal D 2 symmetry. Analysis of the data in Table5and its Kohn-Sham orbitals allows us to describe Au 9 -P as an oblate-distorted superatom, with corresponding 1S 2 1P 4 closed-shell electron configuration. The HOMO (b 2 ) and

Au 9 -

 9 P metal core on two opposite sides (along the y axis) by two supplementary Au(PPh 3 ) 2 units. It has two more electrons than Au 9 -P that are located in a kind of 1D y 2 HOMO, resulting in the prolate 1S 2 1P 4 1D 2 closed-shell configuration. Its LUMO can be tentatively described as a A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t

  Interestingly, the two optimized [Au 11 ] 3+ structures differ by a substantial energy difference of 1.07 eV at the expense of the less compact one. The energy cost afforded by the bare metal cores to prepare their structure to bonding with ligands (E Prep ) is given in Table8. It is computed as the difference between the energy of the [Au n ] x+ core as it is in the optimized Au n -X or Au n Cl x -X species and that of the free [Au n ] x+ . Most of these values are small, consistently with the moderate change of the metal configuration upon complexation. The exception of Au 6 -C can be explained by its structural difference with its Au 6 -P counterpart (see above). That of the two [Au 11 ] 3+ cores constitute also an exception, but they show similar E Prep values for both phosphine and NHC clusters. Thus, except for the Au 6 -X (X = P, C) clusters, the E Prep. values of the phosphine and NHC couples are very similar.

A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t

  In the pseudo-spherical Au 11 Cl 2 -X and Au 13 Cl 3 -X (X = P, C) series, the high-energy transitions are mainly of 5d(Au)→1D character. Note that similar results were theoretically obtained for Au 13 Cl 2 -P.[START_REF] Weerawardene | Luminescence and electron dynamics in atomically precise nanoclusters with eight superatomic electrons[END_REF]

Figure 3 .

 3 Figure 3. Simulated UV-Vis absorption spectra for computed Au n -P (solid line) and Au n -C

A c c e p t e d M a n u s c r i p t general trend when

  going from phosphine-to NHC-protected clusters. Most of the computed transition energies are within the near-IR and IR range, which probably explains why phosphorescence is disfavored with respect to thermal relaxation, with perhaps the exception of Au 13 Cl 2 -X, Au 13 Cl 3 -X and Au 8 Cl 2 -X (X = P, C).

A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t of the versatility

  in the electronic properties of the NHC's family, ranging from strong to weak donor ligands. Details regarding the calculated density of states and MO diagrams for the cluster series investigated by the paper, the plotted frontier orbitals, the computed energy difference between the first excited singlet and triplet states and structures of the investigated [Au n ] x+ bare systems.

A c c e p t e d M a n u s c r i p t

Cl 2 -P),

  (Au 6 -P), [Au 8 (dppp) 4 ] 2+ 46 (Au 8 -P), [Au 8 (dppp) 4 Cl 2 ] 2+ 46 (Au 8 [Au 9 (PPh 3 ) 8 ] 3+ 47,48 (Au 9 -P), [Au 11 (dppe) 6 ] 3+ 49 (Au 11 -P), [Au 11 (PPh 3 ) 8 Cl 2 ] + 50,51 (Au 11 Cl 2 -P), and [Au 13 (dppe) 5 Cl 2

Table

. Structurally Characterized Phosphine-protected AuNCs from which Models Were Considered in our Calculations

	Compound	Abbreviation Number of 6s	Structure of the Au n core	Ref.
			(Au) electrons
	[Au 6 (dppp) 4 ] 2+	Au 6 -P	4	edge-bridged tetrahedron	44, 45
	[Au 8 (dppp) 4 ] 2+	Au 8 -P	6	edge-sharing tri-tetrahedron	46
	[Au 8 (dppp) 4 Cl 2 ] 2+	Au 8 Cl 2 -P	4	bicapped edge-sharing bi-	46
				tetrahedron
	[Au 9 (PPh 3 ) 8 ] 3+	Au 9 -P	6	bicapped centered chair	47, 48
	[Au 11 (dppe) 6 ] 3+	Au 11 -P	8	doubly-bridged bicapped	49
				centered chair
	[Au 11 (PPh 3 ) 8 Cl 2 ] +	Au 11 Cl 2 -P	8	incomplete centered icosahedron 50,51
	[Au 13 (dppe) 5 Cl 2 ] 3+	Au 13 Cl 2 -P	8	centered icosahedron	52
	[Au 13 (PPh 3 ) 9 Cl 3 ] 2+	Au 13 Cl 3 -P a	8	centered icosahedron	33

(a)

Structure modelized from that of Crudden's NHC derivative [Au 13 (NHC) 9 Cl 3 ] 2+ .

[START_REF] Narouz | Robust, highly luminescent Au 13 superatoms protected by N-heterocyclic carbenes[END_REF]

Together with those of their NHC homologs (Au n - C), they are shown in Figure 1. Their corresponding MO diagrams are represented in the form of

	density-of-states (DOS) plots in Figures S1-S6 (see also Figures S9-S16). They all exhibit
	similar features, with the 5d(Au) states contributing mainly to the large band below the HOMO
	and the 6s/6p(Au) to the lowest unoccupied states. As examples, those of the smallest

Au 6 -X and the largest Au 13 Cl 2 -X species

	(X = P, C) are shown in Figure 2. The HOMOs and LUMOs of the
	computed Au n -P and Au n -C species are pictured in Figures S7 and S8, respectively.

Table 2 . Relevant Computed Data for the [Au 6 L 4 ] 2+ Clusters (L = dppp (Au 6 -P) and L = di- NHC'p (Au 6 -C)). Au t and Au b Designate the Atoms of the Central Tetrahedron and the Bridging Atoms, Respectively. Experimental Interatomic Distances of Au 6 -P Are Provided for Comparison

 2

			Au 6 -P exp 44 (S 4 )	Au 6 -P (D 2 )	Au 6 -C (D 2 )
			2.630 × 2	2.630 [0.302] × 2	2.561 [0.462] × 2
			2.923 × 2	2.861 [0.032] × 2	3.178 [0.019] × 2
		Au t -Au t
	Distance (Å)		2.923 × 2	2.972 [0.023] × 2	3.390 [0.011] × 2
	[WBI] a		av. 2.825	av. 2.821 [0.119]	av. 3.043 [0.164]
		Au b -Au t	2.798 × 4	2.790 [0.090] × 4	3.601 [0.013] × 4
		Au-L	av. 2.378	av. 2.332 [0.391]	av. 2.069 [0.441]
	HOMO-LUMO gap (eV)		1.58	1.53
		Au t		-0.02 × 4	-0.04 × 4
	NAO b charge	Au b		+0.27 × 2	+0.42 × 2
		L		+0.39 × 4	+0.33 × 4

a Wiberg bond index. b Natural atomic orbital.

Table 3 . Relevant Computed Data for [Au 8 L 4 ] 2+ Clusters (L = dppp (Au 8 -P) and L = di- NHC'p (Au 8 -C)). Experimental Interatomic Distances of Au 8 -P Are Provided for Comparison

 3

			Au 8 -P exp 46 (S 4 )	Au 8 -P (C 2 )	Au 8 -C (C 2 )
			min. 2.630	min. 2.687 [0.213]	min. 2.649 [0.268]
		Au t -Au t
			max. 2.896	max. 2.887 [0.086]	max. 3.083 [0.074]
		(center)
			av. 2.785	av. 2.790 [0.135]	av. 2.842 [0.151]
	Distance (Å)
			min. 2.607	min. 2.646 [0.302]	min. 2.640 [0.331]
	[WBI]	Au t -Au t
			max. 2.854	max. 2.866 [0.064]	max. 3.096 [0.038]
		(outer)
			av. 2.795	av. 2.812 [0.108]	av. 2.864 [0.115]
		Au-L	av. 2.304	av. 2.314 [0.381]	av. 2.067 [0.399]
	HOMO-LUMO gap (eV)		1.79	1.67
		Au (center)		+0.10 x 4	+0.17 x 4
	NAO charge
		Au (outer)		-0.01 x 4	+0.06 x 4
	(av.)
		L		+0.41 x 4	+0.27 x 4

Table 4 . Relevant Computed Data for [Au 8 Cl 2 L 4 ] 2+ Clusters (L = dppp (Au 8 Cl 2 -P) and L = di-NHC'p (Au 8 Cl 2 -C)). Experimental Interatomic Distances of Au 8 Cl 2 -P Are Provided for Comparison. Au m and Au s Designate the Middle and Side Atoms of the Bi-tetrahedron, Respectively, and Au b the Bridging Atoms Au 8 Cl 2 -P exp

 4

	46 (D 2h )	Au 8 Cl 2 -P (C 1 )	Au 8 Cl 2 -C (C 1 )

Table 5 . Relevant Computed Data for [Au 9 L 8 ] 3+ Clusters (L = PPh 3 (Au 9 -P) and L = NHC' (Au 9 -C)). Experimental Interatomic Distances of Au 9 -P Are Provided for Comparison. Au c and Au p Designate the Unique Central and the Eight Peripheral Atoms, Respectively

 5

			Au 9 -P exp 47 (D 2 )	Au 9 -P (D 2 )	Au 9 -C (D 2 )
	Distance (Å)	Au c -Au p	av. 2.692	av. 2.717 [0.132] av. 2.729 [0.144]
	[WBI]	Au p -Au p	av. 2.834	av. 2.842 [0.096] av. 2.888 [0.099]
		Au-L	av. 2.281	av. 2.294 [0.441] av. 2.043 [0.460]
	HOMO-LUMO gap (eV)		1.54	1.50
	NAO charge	Au c		-0.05	-0.05
	(av.)	Au p		+0.08	+0.16
		L		+0.30	+0.22

Table 6 . Relevant Computed Data for [Au 11 L 6 ] 3+ Clusters (L = dppe (Au 11 -P) and L = di- NHC'e (Au 11 -C)). Experimental Interatomic Distances of Au 11 -P Are Provided for Comparison. Au c , Au p and Au b Designate the Unique Central, the Eight Peripheral Atoms and the Two Bridging Atoms, Respectively

 6

			Au 11 -P exp 49	Au 11 -P (C 2 )	Au 11 -C (C 2 )
			(C 2 )
	Distance (Å)	Au c -Au p	av. 2.746	av. 2.745 [0.101] av. 2.828 [0.110]
	[WBI]	Au p -Au p	av. 2.859	av. 2.878 [0.104] av. 2.973 [0.109]
		Au b -Au p	av. 2.801	av. 2.773 [0.126] av. 2.814 [0.158]
		Au-L	av. 2.319	av. 2.317 [0.420] av. 2.087 [0.388]
	HOMO-LUMO gap (eV)		1.40	1.49
	NAO charge	Au c		-0.16	-0.18
	(av.)	Au p		+0.03	+0.12
		Au b		+0.29	+0.48
		L		+0.39	+0.21

Table 7 . Relevant Computed Data for [Au 11 Cl 2 L 8 ] + Clusters (L = PPh 3 (Au 11 Cl 2 -P) and L = NHC' (Au 11 Cl 2 -C)); [Au 13 Cl 2 L 5 ] 3+ Clusters (L = dppe (Au 13 Cl 2 -P) and L = di-NHC'e (Au 13 Cl 2 -C)); [Au 13 Cl 3 L 9 ] 2+ Clusters (L = PPh 3 (Au 13 Cl 3 -P) and L = NHC' (Au 13 Cl 3 -C)). Experimental Interatomic Distances Are Provided for Comparison. Au c and Au p Designate the Central and Peripheral Atoms, Respectively Au 11 Cl 2

 7

		-X (C 1 )	Au 13 Cl 2 -X (C 1 )	Au 13 Cl 3 -X (C 1 )
	Au 11 Cl 2 -P exp 51	Au

11 Cl 2 -P Au 11 Cl 2 -C Au 13 Cl 2 -P exp 52 Au 13 Cl 2 -P Au 13 Cl 2 -C Au 13 Cl 3 -C exp 33 Au 13 Cl 3 -P Au 13 Cl 3 -C

		Au c -Au p av. 2.687	av. 2.702 [0.140]	av. 2.718 [0.155]	av. 2.764 av. 2.773 [0.121]	av. 2.787 [0.135]	av. 2.768	av. 2.801 [0.119]	av. 2.800 [0.131]
	Distance (Å)	Au p -Au p av. 2.954	av. 2.877 [0.072]	av. 3.019 [0.069]	av. 2.906 av. 2.916 [0.063]	av. 2.930 [0.063]	av. 2.911	av. 2.946 [0.065]	av. 2.945 [0.066]
	[WBI]	Au p -Cl	av. 2.357	av. 2.388 [0.322]	av. 2.422 [0.285]	av. 2.351 av. 2.348 [0.381]	av. 2.355 [0.384]	av. 2.352	av. 2.381 [0.324]	av. 2.394 [0.316]
		Au-L	av. 2.289	av. 2.288	av. 2.042	av. 2.289 av. 2.289	av. 2.036	av. 2.051	av. 2.313	av. 2.047
				[0.411]	[0.429]	[0.664]	[0.448]		[0.440]	[0.442]
	HOMO-LUMO gap		1.89	1.91	1.74	2.02		1.90	2.13
	(eV)
		Au c		-0.22	-0.24	-0.32	-0.33		-0.27	-0.27
	NAO charge	Au p		+0.10	+0.16	+0.15	+0.21		+0.15	+0.21
	(av.)	Cl		-0.60	-0.63	-0.55	-0.57		-0.57	-0.61
		L		+0.17	+0.12	+0.54	+0.39		+0.24	+0.17

Table 8 . Energetic Data Associated with Cluster Bonding (all Values in eV). E Prep. = Energy Cost Afforded by the Bare [Au n ] x+ Metal Core to Go From its Equilibrium Structure to That It Adopts in the Au n -X or Au n Cl m -X Cluster. E bond. = E(Au n -X) -E([Au n ] x+ ) -E(neutral ligand sphere) or E bond. = E(Au n Cl m -X) -E([Au n Cl m ] ( x-m)+ ) -E(Neutral Ligand Sphere). Neutral Ligands = Phosphines or NHCs. All Fragment Structures Taken From the Au n -X or Au n Cl m -X Optimized Geometries

 8

a See Figure S17. b Relative energies between [Au 11 ] 3+ isomers. The bonding energy (E Bond. ) of the neutral phosphine or NHC ligand sphere to the [Au n ] x+ or [Au n Cl m ] (x-m)+ fragments in the Au

n -X or Au n Cl m

  It is clear from Table8that the NHCs are more strongly bonded than their phosphine analogues in all the investigated systems. The difference is not very large, but significant (a few eVs). Even taking into account the E Prep. destabilizing component, the difference remains in favor of stronger NHC bonding. When this difference is normalized to one phosphine/carbene function, it is found to lie within a range of 4-7 kcal/mol (E Prep.

	Compound Au 6 -X Au 8 -X Au 8 Cl 2 -X Au 9 -X Au 11 -X Au 11 Cl 2 -X [Au 11 ] 3+ (C 2 , ΔE = 0.00 b ) Corresponding optimized metal core a [Au 6 ] 2+ (D 2 ) [Au 8 ] 2+ (C 2 ) [Au 8 ] 4+ (D 2h ) [Au 9 ] 3+ (D 2h ) [Au 11 ] 3+ (C 2 , ΔE = 1.07 b ) Au 13 Cl 2 -X [Au 13 ] 5+ (I h ) Au 13 Cl 3 -X	X = P E Prep. (metal core) 0.19 0.38 0.21 0.09 1.09 1.02 0.49 0.38 M a n u s c r i p t X = C E Prep. (metal core) X = P E Bond. X = C E Bond. 2.85 -24.97 -29.25 0.32 -26.88 -28.70 0.23 -29.59 -31.36 0.15 -33.78 -35.86 1.24 -39.68 -41.53 1.00 -25.01 -27.23 0.40 -40.04 -42.58 0.33 -33.67 -37.32
	A c c e p t e d

-X, respectively, was computed according to

E bond. = E(Au n -X) -E([Au n ] x+ ) -E(neutral ligand sphere) or E bond. = E(Au n Cl m -X) -E([Au n Cl m ] (x-m)+

) -E(neutral ligand sphere), respectively. In these equations, all the considered (frozen) fragment structures were taken from the Au n -X or Au n Cl m -X optimized geometries.

Table 9 . Major TD-DFT-Computed UV-Vis Excitation Energies. Bold Values Are Associated With the More Intense Absorption Bands in the Simulated Spectra Shown in Figure 3. Available Experimental λ max Values Are Reported for Comparison

 9

		Absorption λ (nm)		Major character of
	Compound	Exp. a	X = P	X = C	the low-energy band (X = P, C)
	Au 6 -X	587, 432, 326 45	564, 417, 299, 272	517, 363, 321, 262	Au t →Au b
	Au 8 -X	590 (sh), 520, 308 46	516, 390, 304	538, 322, 277	Au outer →Au center
	Au 8 Cl 2 -X	508, 390, 322 46	494, 327, 281	460, 298, 277	Au m+s →Au b
	Au 9 -X	443, 375 (Sh), 352 (sh), 314 47	404, 332	488, 347, 262	1P x →1D x2-y2
	Au 11 -X	663, 471 (sh), 390, 316 49	625, 445, 370	573, 472, 326	1D y2 →2P y
	Au 11 Cl 2 -X	416, 312, 240 51	426, 353	416, 298	1P→1D
	Au 13 Cl 2 -X	493, 359, 304 (sh) 52	505, 390, 340	470, 408, 354, 307	1P→1D
	Au 13 Cl 3 -X	490, 420, 330 33	466, 391, 329	434, 376, 304	1P→1D

Table 10 . TD-DFT computed hypothetical fluorescence emission energies.

 10

	Compound	E emission	Compound	E emission
	Au 6 -P	2.24 eV (552 nm)	Au 6 -C	1.25 eV (992 nm)
		1.81 eV (683 nm)
	Au 8 -P	Non-luminescent 53	Au 8 -C	1.75 eV (707 nm)
		2.08 eV (595 nm)
	Au 8 Cl 2 -P	Exp. 2.07 eV (597 nm) 46	Au 8 Cl 2 -C	2.62 eV (473 nm)
		1.39 eV (891 nm)
	Au 9 -P	Exp. 2.14 eV, 1.45 eV	Au 9 -C	1.52 eV (814 nm)
		(579 nm, 853 nm) 47
	Au 11 -P	1.85 eV (670 nm)	Au 11 -C	1.82 eV (683 nm)
		1.62 eV (764 nm)
	Au 11 Cl 2 -P	Non-luminescent 53	Au 11 Cl 2 -C	1.86 eV (667 nm)
		1.75 eV (709 nm)
	Au 13 Cl 2 -P	Exp. 1.62 eV (766 nm) 52	Au 13 Cl 2 -C	1.72 eV (720 nm)
				1.71 eV (723 nm)
	Au 13 Cl 3 -P	1.76 eV (705 nm)	Au 13 Cl 3 -C	Exp. 1.59 -1.70 eV
				(730-780 nm) 33

Table 11 . DFT-Computed Hypothetical Phosphorescence Emission

 11

	Compound	E emission	Compound	E emission
	Au 6 -P	1.46 eV (849 nm)	Au 6 -C	1.13 eV (1097 nm)
	Au 8 -P	0.68 eV (1823 nm)	Au 8 -C	0.85 eV (1458 nm)
	Au 8 Cl 2 -P	1.70 eV (729 nm)	Au 8 Cl 2 -C	1.82 eV (681 nm)
	Au 9 -P	1.09 eV (1137 nm)	Au 9 -C	0.40 eV (3099 nm)
	Au 11 -P	1.37 eV (904 nm)	Au 11 -C	1.02 eV (1215 nm)
	Au 11 Cl 2 -P	1.01 eV (1227 nm)	Au 11 Cl 2 -C 0.81 eV (1530 nm)
	Au 13 Cl 2 -P	1.40 eV (885 nm)	Au 13 Cl 2 -C	1.41 eV (879 nm)
	Au 13 Cl 3 -P	1.45 eV (855 nm)	Au 13 Cl 3 -C	1.50 eV (826 nm)
	Going from Au

6 -P to Au 6 -C,

a red shift in the fluorescence emission wavelength is found, which is also observed to a lesser extend in the Au

8 -P/Au 8 -C counterparts

  . Interestingly, a blue shift is found in the case of the chlorine-derivatives Au

8 Cl 2 -P/Au 8 Cl 2

-C, which is also observed for the Au

11 -P/ Au 11 -C and Au 11 Cl 2 -P/ Au 11 Cl 2 -C pairs

  . A blue shift in the emission is also calculated for the Au 9 -P/Au 9 -C pair, depicting strong differences with the Au 6 , Au 8 , and Au 11

	cores. Hence, different cluster cores retain different behavior when NHC's ligands are included,
	increasing the versatility of the properties.

  7-1.8 eV for theAu n -P and Au n -C series, respectively. Although these values are not accurate due to the way they were calculated (see Computational Details), two general trends can be noted. The first one is that within each series, this gap increases with the nuclearity. The second one is that it is much larger for Au n -P than for their Au n -C counterparts, whatever n. The particularly large gap computed for the Au n -P derivatives disfavors the intersystem crossing, also in agreement with the non-observation of phosphorescence in this experimentally characterized series. From this point of view, the Au

n -C series appears more capable for enabling intersystem crossing and possible phosphorescence.

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ACKNOWLEDGMENTS

The authors are grateful to the Chilean-French ECOS-CONYCYT program (project C18E04) and to the French-Chilean International Associated Laboratory for "Multifunctional Molecules and Materials" (LIA-CNRS N°1027). The GENCI (Grand Equipment National de Calcul Intensif) is acknowledged for HCP support (project a0010807367). J. W. thanks the China Scholarship Council for a Ph.D. grant. A.M.-C. acknowledges support from Fondecyt 1180683. Dr. R. Marchal is acknowledged for technical support.

AUTHOR INFORMATION Corresponding Authors

Jean-Yves Saillard, Email: jean-yves.saillard@univ-rennes1.fr Alvaro Muñoz-Castro, Email: alvaro.munoz@uautonoma.cl ORCID Jean-François Halet: 0000-0002-2315-4200 Jean-Yves Saillard: 0000-0003-4469-7922 Alvaro Muñoz-Castro: 0000-0001-5949-9449

Notes

The authors declare no conflict of interest.

Table of Contents Graphics and Synopsis

The structure and bonding of phosphine-and NHC-protected gold clusters are compared by means of DFT calculations