Fluorescence Amplification of Unsaturated Oxazolones Using Palladium: Photophysical and Computational Studies

Weakly fluorescent (Z)-4-arylidene-5-(4H)-oxazolones (1), ΦPL < 0.1%, containing a variety of conjugated aromatic fragments and/or charged arylidene moieties, have been orthopalladated by reaction with Pd(OAc)2. The resulting dinuclear complexes (2) have the oxazolone ligands bonded as a C^N-chelate, restricting intramolecular motions involving the oxazolone. From 2, a variety of mononuclear derivatives, such as [Pd(C^N-oxazolone)(O2CCF3)(py)] (3), [Pd(C^N-oxazolone)(py)2](ClO4) (4), [Pd(C^N-oxazolone)(Cl)(py)] (5), and [Pd(C^N-oxazolone)(X)(NHC)] (6, 7), have been prepared and fully characterized. Most of complexes 3–6 are strongly fluorescent in solution in the range of wavelengths from green to yellow, with values of ΦPL up to 28% (4h), which are among the highest values of quantum yield ever reported for organometallic Pd complexes with bidentate ligands. This means that the introduction of the Pd in the oxazolone scaffold produces in some cases an amplification of the fluorescence of several orders of magnitude from the free ligand 1 to complexes 3–6. Systematic variations of the substituents of the oxazolones and the ancillary ligands show that the wavelength of emission is tuned by the nature of the oxazolone, while the quantum yield is deeply influenced by the change of ligands. TD-DFT studies of complexes 3–6 show a direct correlation between the participation of the Pd orbitals in the HOMO and the loss of emission through non-radiative pathways. This model allows the understanding of the amplification of the fluorescence and the future rational design of new organopalladium systems with improved properties.


■ INTRODUCTION
The synthesis of new luminescent compounds and the study of their photophysical properties are research areas undergoing nowadays a strong development due to the deep social impact of their applications. The awareness of a more rational use of energetic resources and a simultaneous daily increase of the energy demand prompt this development. In this respect, luminescent-based technologies (chemoluminescence and fluorescent sensors) have been declared as "one of the top ten emerging technologies" in 2021 and 2022 by the IUPAC. 1 Despite the increasing number of reported luminescent compounds, their "a priori" rational design searching for tunable properties is not a trivial task due to the high number of variables involved: charge separation, conformational restrictions, planarity, electronic delocalization, presence (or not) of metals, among others. 2 Due to these facts, it is still not possible to make accurate predictions about luminescence and, in general, modular approaches are used. 3 Therefore, gathering additional information for the building of new luminescent systems is highly desirable. Following models found in nature, we focused our attention on two intensely fluorescent proteins, green fluorescent protein (GFP) and Kaede protein, whose chromophores are small molecules containing the (Z)-4arylidene-5(4H)-imidazolone skeleton ( Figure 1).
The emissive properties of these chromophores have been studied in depth, 4 and it has been determined that the rigid environment present in the inner part of the protein�the βbarrel, where the imidazolone is located�is the main responsible of the bright fluorescence observed. In the absence of a tight local environment, the isolated chromophores relax through different deactivation channels, such as internal motions (hula twist), excited-state proton transfer (ESPT), and/or isomerization, and the fluorescence in solution is almost completely lost. 5 Typical values of luminescence quantum yields for such species are in the range 0.1−0.01%, depending on the substituents. Different solutions have been proposed to mitigate this loss. The immobilization of the imidazolone core in different confined media (MOF, COF, hosting proteins) allows a partial recovery of the fluorescence. 6 Substrate engineering based on the choice of substituent positions, for instance the meta-ef fect, has also achieved success, though limited. 7 The most promising strategies share the necessity of an intramolecular lock between the imidazolone and the arylidene fragments to increase the rigidity and decrease the number of degrees of freedom. As far as we know, only two strategies, namely, (i) the intramolecular constraint 8 and (ii) the building of BODIPY-like structures using BF 2 as bridge, 9 have been successfully implemented as general methods for the recovery and amplification of the fluorescence in these molecules .
However, these achievements are strictly limited to arylidene-imidazolones. (Z)-4-Aryliden-5(4H)-oxazolones are structurally related to imidazolones and show high fluorescence quantum yields (Φ) in the solid state. Despite this, their use as luminescent sources is scarcely represented due to the low quantum yields exhibited in solution (Φ < 0.1% in general). 10a−d In clear contrast to the chemistry reported for imidazolones, 9 there are no reports in the literature about the use of intramolecular locks to amplify the fluorescence of oxazolones in solution, except those coming from our group using Pd (Figure 1d). 10e−g Our choice for the use of Pd as intramolecular lock is due to the promising previous results obtained, with increments of the quantum yield of one order of magnitude after Pd incorporation in some cases, despite the fact that Pd is not a metal often used for the building of luminescent compounds. 11 In this respect, there are no orthometallated oxazolones with other transition metals typically used in luminescent complexes (Pt, Au, Ir, Ru, Re, for instance), 11a which remains an open field to discover. However, the increase of the fluorescence by Pd incorporation is not a general trend, showing that the intramolecular lock was not enough to promote by itself the amplification of the fluorescence. Due to these facts, and taking advantage of our knowledge about the orthopalladation of oxazolones, we aim to find a general method to amplify the luminescence of oxazolones in solution, using Pd as the intramolecular bridge between the oxazolone and arylidene rings. To achieve this task, we need to fully understand the reasons of the puzzling behavior shown by the Pd complexes, as well as the role of the Pd center and the ancillary ligands in the fluorescence.
In this contribution, we have prepared a collection of highly luminescent Pd complexes from orthometallated oxazolones  Inorganic Chemistry pubs.acs.org/IC Article following synthetically simple methods, avoiding the use of sophisticated ligands of high denticity. The coordination sphere of the Pd is completed with apparently innocent ancillary ligands, such as pyridine (py), chloride (Cl), Nheterocyclic carbene (NHC), and/or trifluoroacetate, providing a versatile system for fine-tuning of the photophysical properties. Despite its simplicity, the resulting complexes have structural environments, which result in intense emissive properties, achieving fluorescent quantum yields as high as 28% in solution at room temperature, which are similar to the highest values reported up to now in the bibliography for chelating systems. 11 In addition, we have studied by TD-DFT these simple models, we have been able to determine the origin of the fluorescence and the orbitals involved, and, more importantly, we have established a correlation between the participation of the Pd orbitals in the absorption−emission process and the fluorescence intensity, fully explaining previous results and allowing us to make predictions for the design of improved systems. We present here the obtained results.
■ RESULTS AND DISCUSSION

Synthesis and Characterization of Oxazolones and Orthopalladated
Derivatives. The oxazolones 1a−1m shown in Figure 2 have been prepared by the classical Erlenmeyer−Plochl method, 12 while the hippuric acids have been prepared by the Schotten−Baumann method. 13 Oxazolones 1a−1d and 1f−1h appear on SciFinder, 14 while 1e and 1i−1m are new species.
We have selected a variety of 4-arylidene fragments, taking into account facts directly involved on the luminescent properties of the oxazolones, which could increase their quantum yields, 10,15 such as a highly conjugated electronic nature (1a, 1b, 1c, 1i) or a push−pull character (1d−1f, 1h, 1j, 1l, 1m), aiming to increase intraligand (IL) charge transfer effects. The group in position 2 of the oxazolone either contains strong electroattracting moieties (1e, 1f) or is able to extend the delocalization of the charge density (1g−1m). The molecular structures of 1a and 1e have been determined by Xray diffraction methods (SI, Figures S1 and S2).
The orthopalladation of all oxazolones 1a−1m has been attempted. The reaction works under standard conditions in all cases, 10,15,16 except for 1l, probably due to the steric hindrance of the methoxy group in 5-position. The corresponding dinuclear derivatives with carboxylate bridges 2a−2k and 2m were obtained as shown in Figure 3.
The yields of isolated compounds 2a−2k and 2m are in most of the cases higher than 90%, showing that the orthopalladation of oxazolones through C−H bond activation is a general process. Their characterization shows that they are obtained as dimers�as it is evident from the HRMS spectra� and that the relative arrangement of the orthometallated moieties is transoid, as inferred from the observance of a single peak in the 19 F NMR spectra. Only in the case of 2b were additional 19 F signals due to the cisoid isomer observed. The selectivity in the position of the C−H activation is also remarkable, because only one position of the arylidene ring is activated in cases where more than one C−H bond is able to be activated (1a−1c, 1i, 1m).
Aiming to understand the role of each ligand in the luminescence of the complexes, we have studied the effect of the change of the oxazolone and the ancillary ligands on the luminescent properties of the resulting complexes. On the one hand, we have first screened different oxazolones 1a−1m while keeping two simple ancillary ligands (pyridine and trifluoroacetate), completing the coordination sphere of the Pd. As shown in Figure 3, the dinuclear derivatives 2a−2k and 2m react in CH 2 Cl 2 with pyridine (1:2 molar ratio) to give the highly soluble monomeric complexes 3a−3k and 3m, which can be isolated in good yields and show an intense luminescence. On the other hand, we have also examined the effect of the change of the ancillary ligands around the Pd center for the most emissive palladated oxazolone fragments (3d and 3h). To prepare these new complexes, we have selected trifluoroacetate, chloride, pyridine, and NHC as ligands, aiming to cover a wide range of different trans influences. Treatment of 2h with LiCl (1:4 molar ratio) in MeOH affords the known dinuclear chloride bridging complex shown in Figure 3, 10f which in turn reacts with pyridine (1:2 molar ratio) in CH 2 Cl 2 to give mononuclear 5h, or with AgClO 4 and pyridine (1:2:4 molar ratio) in CH 2 Cl 2 /acetone to give the cationic bis-pyridine derivative 4h. Complex 4d has been prepared from 2d following the same synthetic Inorganic Chemistry pubs.acs.org/IC Article procedure. In addition, 2h reacts with the bis(mesityl)imidazolium salt in the presence of K 2 CO 3 in MeOH, as shown in Figure 3, to give the corresponding NHC-derivative 6h. Complexes 4d, 4h, 5h, and 6h show similar or higher emissive quantum yields than 3d and 3h and allow emission energy tuning. The characterization of all prepared complexes is straightforward from their NMR and HRMS data, as shown in SI. In general, the orthopalladation takes place selectively in one position and the ligand arrangement follows the antisymbiotic effect shown by the soft Pd(II) center, 17 with the pyridine ligand cis to the Pd−C bond. 18 The X-ray crystal structures of complexes 3c, 3d, 3g, and 7h have been determined by diffraction methods and are discussed in detail in the SI ( Figures S3−S7). Crystals of compound 7h were obtained when complex 6h was left to crystallize in CHCl 3 , probably due to the presence of residual HCl in the chlorinated solvent.
2.-Photophysical Studies of Orthopalladated Complexes 3−6. Photophysical data of 1h, 3a−3k, 3m, 4d, 4h, 5h, and 6h including their absorption, emission maxima, life times, and emission quantum yields (ϕ em ) are summarized in Table  1. Most oxazolones have the strongest absorption maxima in the UV region of the spectrum, corresponding to a π− π* charge transfer from the 4-arylidene ring to the oxazolone heterocyclic moiety. 10a,b In the reported orthopalladated derivatives, the maxima are red-shifted to the blue-green region with respect to free oxazolones. 10e The complexes reported in this study follow this trend. Their absorption spectra in CH 2 Cl 2 solutions resemble each other and display an intense absorption in the blue-green region (424−516 nm) accompanied by a less intense band in the UV region. For complexes 3g, 3k, and 3m, the absorption maxima at 386 or 371 nm show a similar intensity than those observed in the visible region, at λ > 440 nm (see Table 1). It is worth noting that the complexes with the styryl fragment in the R 2 position have the most intense absorption band more shifted to a higher wavelength compared to those with R 2 = Ph (Figure 1), which could be attributable to an extended π conjugation. This red shift is also observed upon increasing aromaticity from 3b to 3c at R 1 . The bands with the highest absorbance are assigned to IL π → π* transition of the oxazolone fragment, as explained above. This assignment is supported by similar absorption bands present in the free oxazolones and by computational MO calculations (see below). The most intense absorptions are observed for those complexes with R 2 = styryl and R 1 = 2,4-OMe-C 6 H 4 (3h, 4h, 5h, and 6h) (Figure 4a).
Palladacycles are usually fraught with low-efficiency emissions, only detected at low temperature, due to the thermal deactivation of the π → π* excited state to the lowlying metal-centered orbitals (MC; d−d*), which experience non-radiative decay. 19 In this work, the emissive properties of the orthometallated complexes 3−6 have been studied. These complexes have been built in order to carry out a systematic study of the influence of R 1 , R 2 , L 1 , and L 2 ( Figure 1) in the emissive properties. Their excitation spectra closely match the corresponding absorption spectra (Figure 4b and Figure SI), and their emission maxima in dichloromethane solution (from 485 to 630 nm) almost cover the whole visible spectrum ( Table 1). This point is illustrated in Figure 5, which shows the CIE 1931 coordinates.
Emission lifetimes (τ) of the cyclometallated complexes range from 0.2 to 1.7 ns. An IL-dominated 1 ILCT π → π* emissive state origin of the emission is strongly suggested on the basis on the observed lifetime values, which resemble that reported for the free oxazolone 1h (τ = 0.2 ns) and to the fact that the emission properties remain unchanged in the presence of air, as was proved for the most emissive sample 4h.
Quantum yield values range from 3 to 7% for complexes 3a, 3b, 3d, 3e, 3f, and 3i and from 10 to 28% for 3h, 4d, 4h, 5h, and 6h, while 3c, 3g, 3j, 3k, and 3m are slightly emissive (Φ < 1%). Figure 6 shows a pseudo-2D distribution of the complexes, taking into account the measured quantum yield (X-axis) and the observed emission wavelength (Y-axis) for each compound and may lead to the following trends. R 1 Substituent. From Figure 6, it can be inferred that the presence of emissive groups in the starting oxazolone (pyrenyl 3c, naphthyl 3b, fluorenyl 3a or 3i) does not guarantee the obtention of a fluorescent complex. Figure 6 also shows that the presence of methoxy groups at 3-and 5-positions of the R 1 ring (that is, in meta to the Pd−C bond) is almost mandatory to achieve high quantum yields. These positions, and not others, gave the best results. If we consider the series of complexes 3j [3,4-(OMe) 2 ], 3m [4,5-(OMe) 2 ], and 3h [3,5-(OMe) 2 ], the observed quantum yields range from <1% for 3j and 3m to 18% for 3h ( Figure 6). The presence of other substituents such as fluorine in 3,5 positions do not cause an increase of the quantum yield, as observed in previous studies where the intensity of emission of the acetylacetonate complex was very close to that obtained for the free oxazolone. 10f R 2 Substituent. Once the best R 1 group (2,4-OMe-C 6 H 4 ) was found, the influence of the R 2 fragment may be studied. An attempt was made to have larger charge distributions in the oxazolone fragment using more pronounced push−pull systems 3f (R 2 = C 6 H 4 -NO 2 ) and 3e (R 2 = C 6 F 5 ). This is a valid strategy used to improve the emissive properties of free oxazolones. 10c,d However, for the studied complexes, that was not the case and complex 3d with R 2 = C 6 H 5 displays a higher quantum yield compared with those with R 2 = C 6 H 4 -NO 2 or R 2 = C 6 F 5 . When the R 2 = C 6 H 5 fragment was replaced by a

Inorganic Chemistry
pubs.acs.org/IC Article styryl fragment, a marked increase in the quantum yield was found for 3d (7%) vs 3h (18%) or for 4d (10%) vs 4h (28%). Ancillary Ligands (L 1 and L 2 ). Aiming to increase the σdonor ability of the auxiliary ligands, a bulky NHC 20 was used instead of pyridine in complex 6h. While this complex is one of the most strongly luminescent (Φ = 15%) complexes presented in this work, it shows lower emission efficiency with respect to pyridine derivatives 3h or 4h, which is especially important upon comparison with 4h. Substitution of the CF 3 CO 2 ligand in 3h by a chloride is detrimental, and the corresponding complex 5h shows a lower value of quantum yield (Φ = 12%). Better results were obtained changing the trifluoroacetate ligand by a neutral ligand such as another pyridine, from 3h (Φ = 18%) to 4h (Φ = 28%). The same trend has been observed from 3d (Φ = 7%) to 4d (Φ = 10%), although the increase is lower. In this case, it must be taken into account the change in the complex charge, from neutral to cationic.
The quantum yield of 4h (Φ = 28%) is one of the highest values found for an organometallic palladium complex. 11 This result is remarkable not only from the point of view of the absolute value but mostly because it arises from a ligand, which is almost non-luminescent in free state in solution. 10f That is, we have produced an amplification of the luminescence of several orders of magnitude, which is a very scarce phenomenon despite the fact that intensely luminescent Pd complexes are known. 11 In fact, this is the opposite behavior to what is commonly found in Pd complexes, because in most reported cases, the incorporation of the palladium causes a big decrease or almost complete loss of luminescence. 10a,11,21 Aiming to gain more insight about the intimate reasons for this amplification, we have tried to correlate the molecular parameters with the observed fluorescence. Following the crystallographic analysis performed in complexes 3c, 3d, 3g, and 6h, detailed in the Supporting Information, it is possible to observe that molecules with similar distortions (for instance, 3c and 3d) show very different values of quantum yield (<1 and 7%, respectively) or that highly distorted molecules as 5h (by similarity with 6h) show high values of quantum yield (15% for 5h). These facts suggest that Φ does not correlate in a straight way with the steric factors, and that is probably more related with electronic factors. Therefore, we have studied these systems by molecular modeling using DFT/TDDFT.
3.-Computational Simulations. The absorption and emission properties of the whole set of compounds 3−6 have been calculated using the M06-2X functional, as described in the Computational Details section. The obtained results are summarized in Tables 2 and 3, which contain information regarding the ground state, vertical absorption properties from the ground-state optimized geometries, and emission properties from the optimized geometries of S 1 excited states.
In general, all studied compounds show a similar characteristic excited states behavior, summarized in Figure 7, even if they show different experimental quantum yield values. According to this Jablonski diagram, the absorption properties correspond to the excitation from the ground state (S 0 ) to the first excited singlet state (S 1 ), which may be seen as HOMO to LUMO transition. In between these singlet states, several triplet excited states are located. These calculated triplet states and the calculated transition probabilities to higher singlet excited states are collected in Tables S4 and S5 of the Supporting Information. The S 2 excited states lie much higher in energy and have much lower oscillator strengths. In addition, the calculated wavelengths for the S 1 state compare rather well with the experimental ones. According to these facts, the emission properties would be determined by the evolution of the S 1 electronic state.
In order to explain the observed emitted light, geometry optimizations of the S 1 state have been carried out. Calculated emission wavelengths match pretty well with the experimental ones, and hence, we can conclude that the fluorescent emission  Inorganic Chemistry pubs.acs.org/IC Article occurs from the S 1 to S 0 states, as one might expect. In addition to this, comparing the oscillator strengths for absorption and emission, given in Tables 2 and 3, one may observe that in both cases they are high, indicating that these transitions are very probable. Despite these facts, the quantum yields in solution are in general small for many studied compounds and we wonder about the reasons of this behavior. A more detailed analysis of Figure 7 provides strong clues to understanding our orthopalladated systems. We can observe that in the S 1 optimization, this electronic excited state crosses several triplet states (see Table S6 of SI for more details). Hence, the evolution of the S 1 electronic state could follow an intersystem crossing (ISC) to a triplet excited state because of the presence of a heavy atom such as Pd. In this case, eventually the system would end up in the lowest triplet state Figure 6. Representation of the wavelength (Y-axis) and quantum yield (X-axis) for the studied palladium complexes showing the oxazolone R 1 and R 2 building blocks and the ancillary ligands L 1 and L 2 .  .This near-degeneracy would imply a non-radiative decay from T 1 to S 0 via vibrational states, which would be responsible for decreasing the quantum yields. In summary, S 1 evolution may follow two different paths, which would end up in fluorescent radiative or non-radiative decays. The probability of both paths occurring would be determined by the spin−orbit coupling between S 1 and triplet states. These calculations are very time consuming; moreover, we must keep in mind that for different oxazolone-Pd compounds, this crossing may occur with different triplet states. Therefore, the calculation of the spin−orbit couplings for these compounds is out of the scope of this work. Nevertheless, it is known that transition metals increase this coupling and, hence, the presence of the Pd center in these compounds is the main responsible for this second alternative non-radiative path.
An alternative and more approximate way to estimate the S 1 −T i crossing is by the Pd participation in the HOMO orbital of the ground state. 22 Since the S 0 −S 1 transition is predominantly a HOMO−LUMO transition, the higher metal participation in the HOMO orbital would be connected with a higher spin−orbit coupling between S 1 and a triplet state, enhancing the transition to the triplet state. In Figure 8, the participation of the Pd atom in the HOMO orbital versus the observed experimental quantum yield is depicted. Notice that the nature of the oxazolones used in these compounds is very different, and, despite this fact, a clear trend may be observed in Figure 8. In general, larger participation of the metal in the HOMO orbital relates to smaller quantum yields, as expected. Recall that large metal participation would relate to more probable non-radiative decay, and hence low metal participation would decrease the probability for the nonradiative decay process. Unfortunately, although the observed trend is clear, it is not possible to quantitatively connect the calculated metal participation in the HOMO with the experimentally measured quantum yield, since the nature of the oxazolone compounds is rather different.
According to the experimental data, one could conclude that there are three main factors to obtain fluorescent Pd-oxazolone complexes. The most important one would be the substituents used and their position in the aromatic ring, and then, the extension of the π-system of the oxazolone and the nature of the ligands attached to the Pd atom. In order to rationalize this experimental evidence with the metal participation in the HOMO orbital, we have focused on six different oxazolones with different fluorescence activities, aiming to cover all representative situations. In Figure 9, four of these oxazolone complexes are collected, from left to right, with increasing quantum yields of 1, 7, 18, and 28%, corresponding to complexes 3k, 3d, 3h, and 4h, respectively. 3k shows no fluorescence activity, like the free oxazolone. This compound has F substituents in 3-and 5-positions. 3k is representative of a set of oxazolone complexes where no amplification is produced, despite the π-extension and/or the presence of py/ trifluoroacetate ligands (3g, or 3i, or any of the GFP derivatives: 3a, 3b, 3c, and so on). Complex 3d is representative of oxazolone complexes with amplification without π-extension and with the "standard" ligands py/ trifluoroacetate. It has OMe substituents in 2-and 4-positions and can be considered as the simplest complex among those experiencing amplification. Notice that this change in substituents increases the quantum yield from <1 to 7%. Interestingly, the metal participation in the HOMO orbital decreases significantly from 21.7 to 7.7%. Introduction of πextension in 3h further increases the quantum yield to 18%, while the metal participation in the HOMO orbital decreases even more to 4.5%.
Finally, in 4h, one of the ligands has been substituted and we observe the same small metal participation in agreement with the observed high quantum yield. However, the differences in the observed quantum yields for 3h and 4h cannot be explained only with the calculated metal participation in the HOMO. In Figure 9, the participation of the C atom belonging to the phenyl ring of the oxazolone is also included. In complexes with observed fluorescence activity, the participation of both atoms decreases significantly. This may indicate a decrease of the electronic density around the Pd atom, which would be the reason to decrease the Pd participation in the HOMO and hence the increase of the fluorescence quantum yield.
In order to analyze the influence of the substituent positions, we summarize the metal and C 6 participation of 3j, 3h, and 3m complexes in Figure 10. The only difference is the position of  Inorganic Chemistry pubs.acs.org/IC Article the OMe groups in these compounds. It may be seen that the presence of the OMe groups in 2-and 4-positions favors the decrease of participation of these atoms in the HOMO orbital, in agreement with previous results. Aiming to shed more light on this, we have also calculated the Natural Charges by means of NBO calculations 23 of some selected complexes, including those with the highest quantum yields. All data are collected in Table 4.
The calculated charge of C6, which is linked to the Pd atom, is significantly decreased in the compounds with observed fluorescent activity. This is in agreement with the decrease of the participation of this atom in the HOMO orbital. Noticeably, the charge of the C1 atom, which is linked with the rest of the oxazolone ligand, increases in these cases. This is an indication that the electronic density is moved from those atoms of the oxazolone ring to the rest of the ligand. The large

■ CONCLUSIONS
The amplification of the fluorescence of 4-aryliden-5(4H)oxazolones through intramolecular lock in Pd(II) complexes has been achieved. Starting from non-luminescent ligands (Φ < 0.01%), values of the quantum yield up to 28% can be obtained in the corresponding Pd derivatives. The amplification is quite sensible to the nature of substituents on the 4-arylidene ring, with the presence of two OMe groups in meta to the Pd atom being the case where the highest increases are produced. As a general trend, the wavelength of the emission is tuned by the nature of the oxazolone for identical ancillary ligands, while the quantum yield is modulated by the PdL 2 fragment in complexes with the same oxazolone. DFT and TD-DFT modeling of the prepared complexes show that the emission band has a clear IL character, with a small yet critical participation of the Pd orbitals, and that there is a clear inverse relationship between the quantum yield measured and the participation of the Pd in the HOMO. The main non-radiative deactivation pathway has been determined also by TD-DFT and is due to the ISC of the S 1 state with lower-energy triplets (S 1 → T 2 or S 1 → T 1 ) and the final non radiative process from T 1 to the ground state (T 1 → S 0 ). The probability of ISC increases as the contribution of the Pd increases, due to the spin−orbit coupling constant of the Pd. Therefore, this mechanism explains the role of the Pd in the fluorescence loss and allows to make predictions for the design of new Pd complexes with improved fluorescent properties.

■ EXPERIMENTAL SECTION
General Methods. Solvents were obtained from commercial sources and were used without further purification. All reactions were performed without special precautions against air and moisture. Electrospray ionization (ESI + ) mass spectra were recorded using Bruker Esquire 3000 Plus or Amazon Speed ion-trap mass spectrometers equipped with standard ESI sources. High-resolution mass spectra-ESI (HRMS-ESI) were recorded using either a Bruker micrOTOF-Q system equipped with an API-ESI source and a QToF mass analyzer, or a TIMS-TOF system, both allowing a maximum error in the measurement of 5 ppm. Acetonitrile was used as solvent. For all types of MS measurements, samples were introduced in a continuous flow of 0.2 mL/min and nitrogen served as both the nebulizer gas and the dry gas. The 1 H, 13 C, and 19 F NMR spectra of the isolated products were recorded in CDCl 3 , CD 2 Cl 2 , and dmso-d 6 solutions at 25°C (other conditions were specified) on Bruker AV300, AV400, or Bruker AV500 spectrometers (δ in ppm, J in Hz) at 1 H operating frequencies of 300.13, 400.13, and 500.13 MHz, respectively. The 1 H and 13 C NMR spectra were referenced using the solvent signal as internal standard, while 19 F NMR spectra were referenced to CFCl 3 . The assignment of 1 H NMR peaks has been performed through standard 2D 1 H COSY (2 K points in t 2 using a spectral width of 10 ppm; 128 t 1 experiments were recorded and zerofilled to 1 K; for each t 1 value, four scans were signal-averaged using a recycle delay of 1 s) and selective 1D 1 H-SELNOE experiments. Typical mixing times in the case of selective 1D-SELNOE experiments were in the range 0.8−2 s, as a function of the irradiated signal. These values of optimized mixing times were set equal to the longitudinal relaxation time T 1 , determined using the inversion− recovery sequence. The 13 C NMR peaks were identified using standard 1 H− 13 C edited-HSQC and 1 H− 13 C HMBC 2D experiments. In both cases, 4 K points in t 2 using spectral widths of 10 ppm ( 1 H) and 200 ppm ( 13 C) were used, with averaged values of the coupling constants 1 J CH = 145 Hz and long-range n J CH = 10 Hz. Typically, 128 t 1 experiments were recorded and zero-filled to 2 K. For each t 1 value, 8 (HSQC) or 32 (HMBC) scans were signal-averaged using a recycle delay of 1 s. Absorption spectra were measured on a Thermo Scientific Evolution 600 spectrophotometer. The steady-state excitation−emission spectra were measured on a Jobin Yvon Horiba FluoroLog FL3-11 spectrofluorometer. All measurements were carried out at room temperature on solutions of 10 −5 M concentration using quartz cuvettes of 1 cm path length. Lifetime measurements were carried out in a FluoTime 300 (PicoQuant) fluorescence spectrometer, using excitation LEDs of 450 nm. The measurement of the quantum yield values (Φ PL ) was carried out using the absolute method on a Quantaurus-QY C11347 spectrometer. Two different CH 2 Cl 2 solutions of each compound (10 −5 M) were measured in order to check data reproducibility. Relative uncertainty for the absolute method has been determined as less than 6%, although studies were carried out for different substances using both the absolute method and the comparative one. 24 In addition, one solution of compounds 4d and 3h as representative examples was deoxygenated by passing argon through it, and the value of the QY was redetermined to check the influence of O 2 in the intensity of the luminescence. The oxazolones 1a−1m were prepared using the Erlenmeyer−Plochl method, by reaction of the corresponding hippuric acids and aldehydes in acetic anhydride. 12 The hippuric acids were prepared by the Schotten−Baumann method. 13 X-ray Crystallography. Single crystals of 1a, 1e, 3c, 3d, 3g, and 7h CHCl 3 of suitable quality for X-ray diffraction measurements were grown by slow diffusion of n-pentane into CH 2 Cl 2 or CHCl 3 solutions of the crude products at −18°C for several weeks, except for 7h. Crystals of 7h were obtained when 6h was left to crystallize in CHCl 3 , due to the presence of residual HCl in the chlorinated solvent. One selected single crystal of each compound was mounted at the end of a quartz fiber in a random orientation, covered with perfluorinated oil (magic oil) and placed under a cold stream of N 2 gas. Crystallographic measurements were carried out at 100 K on a Bruker APEX D8 Venture CCD diffractometer, using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). A hemisphere of data was collected in each case based on ω-scan or φ-scan runs. The diffraction frames were integrated using the program SAINT, 25 and the integrated intensities were corrected for absorption with SADABS. 26 The structures were solved by direct methods with SHELXT-2014. 27 All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were placed at idealized positions and treated as riding atoms. Each hydrogen atom was assigned an isotropic displacement parameter equal to 1.2−1.5 times the equivalent isotropic displacement parameter of its parent atom. For structure solving and refinement, the SHELXL-2016 28 program in the WINGX Package was used. 29 The structures were refined to F o 2 , and all reflections were used in least-square calculations. CCDC-2122909 (1a), CCDC-2122910 (1e), CCDC-2122911 (3c), CCDC-2122912 (3d), CCDC-2122913 (3g), and CCDC-2145279 (7h) contain supplementary crystallographic data, which can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Computational Details. All calculations were carried out within the density functional theory (DFT), 30 using the Gaussian 16 program package. 31 First, in order to characterize the ground electronic state of the selected complexes, geometry optimizations and harmonic frequency calculations were performed by using the wB97XD 32 and M06-2X 33 exchange-correlation functionals, combined with the 6-31+G (d,p) basis set for the non-metal atom, 34 and the ECP10MDF Sttutgart−Cologne relativistic core potentials along with the aug-cc-pVDZ-PP basis set for the Pd atom. 35 Solvent effects (dichloromethane) were taken into account by means of the integral equation formalism of the polarized continuum model (IEFPCM). 36

Inorganic Chemistry pubs.acs.org/IC Article
Finally, in order to study the photoabsorption and photoemission processes, time-dependent density functional theory (TDDFT) 37 was used, at the same levels of theory used in the characterization of the ground states. Vertical transitions were calculated for absorption properties, and geometry optimizations and frequency calculations were carried out for the first singlet and triplet excited states in order to account for emission properties. Both ωB97XD and M06-2X provide similar qualitative and quantitative values for both absorption and emission (see Tables S1−S6 of the Supporting Information). In order to choose one functional, the absorption wavelengths have been calculated and compared to the experimental values for a group of selected complexes. The obtained results are collected in Table 1. It may be observed that both functionals provide similar results for all complexes. Nevertheless, comparing the differences with respect to the experimental value (D1 for ωB97XD and D2 for M06-2X), it may be seen that systematically M06-2X is closer to the experimental value. The calculated average D1 (54 nm) and D2 (48 nm) values confirm that indeed M06-2X is slightly closer than ωB97XD from the experimental value by 6 nm. Hence, for the sake of clarity hereafter, only the M06-2X values will be considered. The molecular orbitals representations were generated by PyMOL 38 using Paton Research Group openly accessible display settings. 39 General Synthesis and Characterization of Oxazolones 1a− 1m. The oxazolones 1a, 14a 1b, 14b 1c, 14c 1d, 14d 1f, 14e 1g, 14f and 1h 10f appear on SciFinder as previously reported. They were characterized by comparison of their NMR data with those previously published. The oxazolones 1e and 1i−1m have not been previously reported, or they appear on SciFinder without references associated, so they have been fully characterized here. The oxazolones were prepared following the Erlenmeyer−Plochl method, which is exemplified here with the detailed synthesis of 1a. The synthesis and characterization of all the other oxazolones appear as the Supporting Information.
General Synthesis and Characterization of Orthopalladated Dimers with Trifluoroacetate Bridges 2. The orthopalladated dimers 2d 10g and 2h 10f have been previously described. They were characterized by comparison of their NMR data with those previously published. Complexes 2a−2k and 2m have been obtained following the same synthetic procedure, which is detailed here for the synthesis of 2a. The synthesis and characterization of all the other compounds type 2 is given as SI. In all cases, and despite the use of long accumulation times, signals due to the 13 C nuclei of the CF 3 COO ligand and the C 6 F 5 group were not observed in the 13 C NMR spectra, due to multiple 13 C− 19 F couplings and to the low solubility of the compounds.
Synthesis of Orthopalladated 2a. Pd(OAc) 2 (200.0 mg, 0.89 mmol) was added to a solution of 1a (300.0 mg, 0.89 mmol) in CF 3 CO 2 H (5 mL). The resulting mixture was heated in an oil bath to the reflux temperature of the solvent (72.4°C) for 3 h. After the reaction time, the resulting mixture was cooled to room temperature and distilled water (10 mL) was added. The resulting precipitate was filtered off, washed with more distilled water (3 × 10 mL) until the characteristic smell of trifluoroacetic acid disappeared, dried under vacuum, and identified as 2a (orange solid). Obtained: 455.0 mg (92% yield). 1  C 6 H 5 ), 130.6 (C o , C 6 H 5 ), 129.8 (C 3 , C 13 H 8 ), 128.8 (CH, C 13 H 8 ), 128.4 (C m , C 6 H 5 ), 128.0 (C i , C 6 H 5 ), 127.5 (CH, C 13 H 8 ), 125.6 (CH, C 13 H 8 ), 125.0 (C 5 , C 13 H 8 ), 122.5 (C 2 , C 13 H 8 ), 122.4 (C q , C-Pd), 121.2 (CH, C 13 H 8 ), 36.1 (CH 2 ,C 13 H 8 3d, 3f−3k, 3m, 4d, 4h, 5h, 6h). The synthesis of derivatives 3 containing one pyridine ligand has been carried out in all cases using the same procedure, detailed here for the synthesis of 3a. The synthesis and characterization of all compounds 3 is given as SI. For all prepared complexes, signals assigned to the quaternary carbons of the CF 3 COO ligand were not found in the 13 C NMR spectra. This is due to a dynamic fast coordination release of this ligand, which takes place at room temperature. In the case of 3e, as representative, the 13 C NMR spectrum was measure at low temperature (233 K) and all signals were observed.
Synthesis of Orthopalladated 3a. Pyridine (14.5 μL, 0.185 mmol) was added to a stirred suspension of 2a (100.0 mg, 0.090 mmol) in CH 2 Cl 2 (10 mL) at room temperature. The starting suspension gradually dissolved, and a yellow solution was obtained after a few minutes. The mixture was further stirred at room temperature for 30 min. At this point, any remaining insoluble residue was removed by filtration. The clear yellow solution was evaporated to dryness, and the obtained yellow solid of 3a was dried under vacuum. Obtained: 96.7 mg (86% yield). 1 ,56.84;H,2.86;N,4.42. Found: C,56.74;H,3.16;N,4.70.
Complete experimental section; copies of 1 H and 13 C NMR spectra for all new compounds; copies of the absorption (UV−vis) and excitation/emission spectra of complexes 3−6; decay curves and fitting data for the determination of the half-life time of complexes 3−6; Cartesian coordinates of all new optimized complexes 3−6 in the ground state using the wB97XD and M06-2X functionals; Cartesian coordinates of S1 optimized geometries using the wB97XD and M06-2X functionals; tables of calculated orbital energies (Hartree) and metal participation, along with the HOMO−LUMO gap (eV) using the wB97XD and M06-2X functionals for the ground state of 3−6; tables with the calculated vertical transitions for the lowest T 1 , T 2 , T 3 , S 1 , and S 2 excited states of 3−6 with the wB97XD and M06-2X functionals; tables with the calculated emission properties from the S1 optimized geometry for the lowest T 1 , T 2 , T 3 , S 1 , and S 2 excited states of 3−6 with the wB97XD and M06-2X functionals; crystallographic tables of compounds 1a, 1e, 3c, 3d, 3g and 7h; and discussion of the structures of compounds 1a, 1e, 3c, 3d, 3g, and 7h (PDF) ■ REFERENCES