Unveiling the Complex Evolution in Mixed Br–Cl Perovskite Precursors for High‐Efficiency Deep‐Blue Light‐Emitting Diodes

Perovskite light‐emitting diodes (PeLEDs) based on 3D mixed Br‐Cl compositions have shown good color stability and efficiency in the sky‐blue color range. However, when the Cl/Br ratio increases to reach deep‐blue emission, the performance of the PeLEDs decreases drastically, with the external quantum efficiency (EQE) typically below 1%. Such performance decay is due largely to the poor morphology of the emissive layer. Herein, theoretical and experimental approach is combined to investigate the evolution of solvated lead complexes in the precursor solution of mixed Br–Cl perovskites. It is found that the energetically favorable exchange of halide ions in the lead complexes drives the precipitation of CsCl, leading to extremely low precursor solubility at high Cl/Br ratios and consequently poor perovskite film coverage. Based on the findings, a metastable dissolution strategy for perovskite film preparation is proposed and deep‐blue PeLEDs with a record high EQE of 4% in 3D‐based PeLEDs and excellent spectral stability is achieved.

instability in 3D perovskites has been found to be highly related to the initial halide homogeneity [18][19][20] and recently addressed through a vapor-assisted crystallization (VAC) treatment. [19] Compared to the high efficiency (EQE > 10%) achieved in the sky-blue color range, 3D perovskite-based deep-blue LEDs exhibit inferior performances with a peak EQE of 1% or less and poor spectral stability. [18,19] A major cause for such performance decay is that as the Cl/Br ratio increases, which is required to enhance the bandgap of the perovskite for deep-blue emission, the solubility of the precursor mixture reduces drastically, resulting in extremely poor film coverage. [21,22] Besides, the photoluminescence quantum yield (PLQY) is also significantly reduced if the perovskite film is deposited with low-concentration precursor. [21] It is noteworthy that individually CsBr, PbBr 2 , and PbCl 2 all have reasonable solubility in the common solvent dimethyl sulfoxide (DMSO); but when mixed together in DMSO, the combination exhibits extremely low solubility. [18,19] Clearly, there exists certain complex reaction in the solution, which has altered the solute species and creates new precipitates. Therefore, understanding the evolution of complexes and precipitation-solubility equilibria in the precursor solution of the Br-Cl system will have important implication on the development of new precursor preparation strategy to high-quality perovskite film preparation.
In this work, we investigate the evolution of solvated lead complexes in the precursor solution of mixed Br-Cl perovskites. Specifically, we studied the complex formation processes of PbBr 2 -CsBr, PbCl 2 -CsCl, PbCl 2 -CsBr, and PbBr 2 -CsCl mixtures in DMSO solvent by combining photoluminescence (PL) spectroscopy with density functional theory (DFT) calculations. It is found that partial substitution of Cl À by Br À from the lead-chloro complex to form mixed halide lead complexes is energetically favorable in the PbCl 2 -CsBr mixture, resulting in the formation of least soluble CsCl and subsequent CsCl precipitation. Such solubility-precipitation equilibrium governs the overall solubility of the precursor solution and results in poor perovskite film morphology. Based on the findings, we proposed a metastable dissolution strategy to maximize the amount of Cl À bound in the lead halide complex and thus increase the overall precursor concentration. The resultant film shows significantly improved film coverage and reduced defect density. Based on the perovskite films, deep-blue 3D based PeLEDs over a spectra range of 455-463 nm, meeting the Rec. 2020 standard, are fabricated with a record peak EQE of 4% and excellent spectral stability.

Precursor Composition for Deep-Blue Emission
To reach the deep-blue (<465 nm) emission range, we adopt a perovskite composition based on the general formula of Rb 0.1 FA 0.2 Cs 1.2 PbCl 1.75 Br 1.75 . (Note that for LEDs the perovskite composition typically employs a considerable amount of excess alkali metal halides for grain surface defect passivation. [23,24] ) A small amount of Rb þ and formamidinium (FA þ ) ions are incorporated in the Cs-based perovskite system to stabilize the lattice structure and reduce defect formation, [24][25][26] while the high Cl ratio is used to ensure a sufficiently wide bandgap for deep-blue emission. A diamine passivation molecule 4,7,10-trioxa-1,13tridecanediamine (TTDDA) is introduced into the perovskite precursor for defect passivation of the perovskite films. [19,24] It is noted that as the Cl to Br ratio reaches 1:1 (Table S1, Supporting Information), the precursor solubility becomes extremely low, with the maximum concentration limited to only %0.08 M, which is significantly lower than the typical concentration required for fabricating high-efficiency PeLEDs. [19,22,27,28] As the result, the perovskite film shows extremely poor coverage and optical properties, as will be discussed in the later section.
In order to understand the origin of such low solubility of the precursor mixtures, we first investigated the solubility of individual precursors and their mixtures, as shown in Table 1. (Here to simplify the solution model we eliminate Rb þ and FA þ from the precursor solution as they only take up a small ratio and are not the limiting factors for precursor solubility.) Interestingly, we observed two opposite trends for pure halide and mixed halide systems: 1) For pure halides, both the CsPbBr 3 and CsPbCl 3 precursor mixtures exhibit higher solubility than the corresponding CsBr and CsCl components, i.e., adding PbBr 2 (or PbCl 2 ) to CsBr (or CsCl) increases the overall concentration of the precursors. 2) For mixed halides, on the other hand, mixing CsBr, PbBr 2 , and PbCl 2 drastically reduces the maximum precursor concentration from 0.26 M (for CsBr), 2 M (for PbBr 2 ), and 1.3 M (for PbCl 2 ) to 0.12 M (for CsPbBr 1.5 Cl 1.5 ) and 0.1 M (for CsPbBr 1 Cl 2 ). Such opposite trends clearly suggest that the occurrence of complex reactions breaks the original precipitation-solubility equilibrium in individual precursor solution. (Note that here we only focus on the CsBr-PbBr 2 -PbCl 2 combination since CsCl has extremely low solubility in DMSO and is not suitable for solution deposition.)

PL Characterizations on Complex Evolution in Precursor Solution
To understand the opposite trends in the solubility variation of pure halide and mixed halide systems, we monitored the complex formation and reaction processes by applying PL spectroscopy to track the characteristic fluorescence peaks of different complex species. It has been previously found that in the lead halide solution in DMSO or DMF, the Pb 2þ ions typically form six-coordinated complexes with the solvent molecules and halide ions, which act as ligands. [29] Due to the ligand-to-metal charge transfer (LMCT) process, [30] some complexes with specific Pb-X (X: Cl or Br) coordination combination will exhibit a characteristic PL peak.Accordingly, in the following, we will use these optical features to analyze the complex evolution in 1) pure-halide precursor systems, i.e., PbBr 2 -CsBr and PbCl 2 -CsCl mixtures and 2) mixed-halide precursor systems, i.e., PbCl 2 -CsBr and PbBr 2 -CsCl mixtures.  [29,31] Therefore, PL peaks centering at 560 nm ( Figure 1a) and 518 nm (Figure 1b) 3 ] À coexist, [31] here we only observe one emissive complex specie; the reason for such solvent-dependent coordination will be discussed further in the later section.) As shown in Figure 1a,b, when the PbX 2 :CsX ratio decreases from 1:0.5 to 1:2 (keeping Pb 2þ at the same concentration), the increasing PL peak intensity indicates the [PbX 2 (DMSO) 4 ] complex gradually transforms to [PbX 4 (DMSO) 2 ] 2À . The increase of the coordination number of the halide ions in the lead complexes may explain the increased solubility of the PbX 2 -CsX mixtures (i.e., CsPbX 3 in Table 1) as compared to that of pure CsX solution.

Complex Evolution in Mixed-Halide Systems
We follow the similar procedure to study the mixed halide system and monitor the PL spectrum of the PbCl 2 solution with gradually added CsBr. We start with a low concentration of 0.05 M to ensure the precursors are fully dissolved in the solvent. As shown in Figure    To further examine the precipitation-solubility equilibrium in the mixed-halide precursors, we gradually increased the concentration of the CsBr-PbCl 2 mixtures (at a molar ratio of 1:1) from the initial concentration of 0.05 M. When the concentration of the mixture reaches 0.12 M, which is still below the CsBr solubility limit, we already observed formation of precipitates in the solution and a slight red-shift of the PL peak. Further increasing the concentration to 0.3 M leads to significant red-shift of the PL peak ( Figure S2a, Supporting Information) and more precipitates. Energy Dispersive X-ray Spectroscopy (EDX) measurements on the precipitates ( Figure S3 and Table S2, Supporting Information) suggest their composition is CsCl, again confirming exchange of Cl À from the complex by Br À . It is important to note that this process is temperature-dependent for the supersaturated mixtures ( Figure S2b, Supporting Information): more Cl À are replaced from the complex at higher stirring temperature as evidenced by the slightly enhanced and red-shifted PL peak.

Theoretical Calculation on Complex Formation and Reaction
To help understand complex evolution in pure halide and mixed halide mixtures, we applied solvation model based on density (SMD) to calculate the Gibbs free energies of different structures of the complexes (details could be found in the Computational section). Noting that [PbX 4 (Sol) 2 ] 2À (X: Cl or Br, solvent is abbreviated as Sol)) and [PbX 3 (Sol) 3 ] À are both commonly formed complexes in perovskite precursors, [31] we first analyze which complex structure is more favorable in DMSO. The complexes possess different stereoisomers, with mer-[PbCl 3 (DMSO) 3 ] À and cis-[PbCl 2 Br 2 (DMSO) 2 ] 2À possessing lower electronic energy being and thus more stable (Table S3, S4, Supporting Information). We therefore use these two stereoisomers (Figure 2a-c) for calculation of the reaction energy (details for stereoisomer calculations are shown in Figure S4, S5, Supporting Information).

Complex Evolution in Pure-Halide Systems
We first calculated the heat of reactions in terms of Gibbs free energy difference (ΔG) in pure halide systems and chose X as Br to evaluate the stability of different [PbX n (Sol) (6Àn) ] (2Àn) (n = 2, 3, 4), as described by Equation (1) and (2) ½PbBr 2 ðDMSOÞ 4 þ 2Br À ¼½PbBr 4 ðDMSOÞ 2 2À þ 2DMSO (1) The ΔG of Equation (1) is À3.00 kcal mol À1 while that of Equation (2) (Figure 1a) in DMSO. The complexation reactions in DMF are also discussed in (Equation (S1) and (S2), Supporting Information ), and the result is again consistent with the previous report on the presence of two characteristic emission peaks in the precursor solution in DMF. [31] According to the calculation results, the phase evolution in pure halide mixtures is illustrated in Figure 2d. As the ratio of CsBr to PbBr 2 increases, the [PbBr 2 (DMSO) 4 ] component gradually transforms to [PbBr 4 (DMSO) 2 ] 2À . This process consumes free halide ions (Br À ) in the solution and helps dissolve more CsBr by shifting the precipitation-solubility equilibrium toward the solvation side. This is why the pure halide mixture corresponding to CsPbBr 3 exhibits higher solubility than the single components CsBr and PbBr 2 ( Table 1).

Complex Evolution in Mixed-Halide Systems
After the analysis of the pure halide systems, we perform the calculations on complex formation in mixed-halide precursors. Experimentally, we have already confirmed that both the PbCl 2 -CsBr and PbBr 2 -CsCl combinations (Figure 1c,d) have the same complex form of [PbCl 4Àx Br x (DMSO) 2 ] 2À in the precursor solution. Therefore, in the following discussion, we just focus   (Figure 2c) as a model system to calculate the energy of complex phase evolution, as shown in Equation (3) ½PbCl 4 ðDMSOÞ 2 2À þ 2CsBr ⇆ ½PbCl 2 Br 2 ðDMSOÞ 2 2À þ 2CsCl At low precursor concentrations where all reactants and products are soluble, the reaction of Equation (3) is reversible with a small ΔG of %1.00 kcal mol À1 and could shift rightward when adding more CsBr into the PbCl 2 /DMSO solution. As the precursor concentration gradually increases, Cs þ (aq) and Cl À (aq) are likely to form clusters and even precipitate as CsCl(s) due to the large lattice energy of CsCl(s). [35] Accordingly, we replace the solvated CsX in Equation (3) with either Cs n X n clusters (n = 2 or 4) or solid-state CsX (denoted as CsX(s)). The free energies of different species are listed in Table S4, Supporting Information. The ΔG of Equation (3) decreases to À0.68 and À3.78 kcal mol À1 , respectively if the reaction product is the n = 2 and 4 clusters, and further to À12.60 kcal mol À1 for solid-state CsX. Note that although here we use the cluster calculation for both CsBr and CsCl, in practice CsBr can be well solvated, instead of forming Cs n Br n clusters or CsBr(s), at the concentration of CsCl precipitation (Table 1). Therefore, the real ΔG of Equation (3) can be even more negative, indicating the rightward reaction direction of Equation (3) is dominant at relatively high precursor concentration.
One schematic model of complex transformation in the mixed halide system is illustrated in Figure 2e. At very low concentration (stage I), all reactants and products are solvated and the phase transition in Equation (3) is reversable. As the precursor concentration increases, Cs n X n clusters start to form (stage II), driving the reaction toward increased formation of mixed halide complex; at this stage, the Cl À in [PbCl 4 (DMSO) 2 ] 2À are gradually substituted by Br À and released to the solution, as evidenced by the redshifted characteristic PL spectrum (Figure 1c). At a relatively high concentration, CsCl(s) starts to precipitate (stage III), further driving the equilibrium of Equation (3) rightward and limiting the maximum concentration of the target mixed halide composition. The model is consistent with our experimental result that the CsPbBrCl 2corresponding precursor mixture (i.e., PbCl 2 :CsBr = 1:1) has much lower solubility than single component CsBr and PbCl 2 ( Table 1).

Preparation of Metastable Precursor Solution
After identifying the release of Cl À from the lead-chloro complex as the origin of the low solubility of the deep-blue perovskite precursors, we seek an alternative procedure to prepare the precursor solution. To maximize the amount of solvated Cl À and avoid precipitation of CsCl, we propose a metastable dissolution strategy. As shown in Figure 3a, in the new process, we prepare separate CsBr and PbCl 2 solution so that the reaction described in Equation (3) is disabled and the amount of Cl À ions bound to the soluble Pb-complexes are maximized; the two solutions are only proportionally mixed (without heating and stirring) before deposition of the perovskite films. This process allows us to achieve a relatively high concentration of the mixed solution while keeping it metastable (i.e., no precipitation) for several hours, providing sufficient time for film deposition. It is noteworthy that CsCl would quickly precipitate at high dissolution temperature. With this procedure, a 0.15 M mixture (abbreviated as "0.15 mixture" in Figure 3) can be obtained (experimental details are provided in the Experimental section), which provides sufficient materials to ensure a decent coverage of the perovskite film on the substrate. For fair comparison with the standard procedure (directly dissolving all chemicals in one solution), which allows for only 0.08 M maximum precursor concentration (abbreviated as "0.08" in Figure 3), we used the identical stoichiometric precursor ratio in both procedures.
Affected by the difference in the nucleation speed, [18,19] the as-deposited (before annealing) mixed Br-Cl perovskites often contain segregated Br-rich and Cl-rich phases. Such inhomogeneous phase distribution is much more severe in the metastable solution procedure, as evidenced by the broad PL spectrum ( Figure S6a, Supporting Information) and distinct short wavelength absorption peak ( Figure S6b, Supporting Information) of the as-deposited 0.15 mixture produced perovskite films. To address this issue, the as-deposited perovskite films are treated by the dimethylformamide (DMF) vapor to facilitate redistribution of halide ions and increase grain sizes with high compositional homogeneity. [19] As shown in Figure 4a, the shape of the PL spectra of perovskite films after VAC treatment is significantly narrowed, suggesting improved phase homogeneity.
After optimizing the film preparation process, we evaluated the morphology of the perovskite films fabricated through different precursor preparation methods. The scanning electron microscope (SEM) images in Figure 3b,c show that the metastable precursor solution (corresponding to the 0.15 mixture) results in significantly enhanced film coverage as compared to the standard precursor solution (corresponding to the 0.08 mixture), raising the coverage ratio from 22% to 50%. We have also measured the elemental distribution of the perovskite film, and the results ( Figure S7, Supporting Information) confirm that the increased grains are not CsCl precipitates. Atomic force microscope (AFM) characterization ( Figure S8, Supporting Information) shows that the 0.08 and 0.15 mixture perovskite films have comparable thickness, but that the latter has a slightly smoother surface. As revealed by the X-ray diffraction (XRD) results (Figure 3d), the crystallinity of the 0.15 mixture perovskite film also shows some enhancement. The optical properties of the 0.08 and 0.15 mixture perovskite films were systematically evaluated in Figure 4. As shown in Figure 4a, the 0.15 mixture perovskite film shows significantly enhanced PL intensity with the full-width at half-maximum (FWHM) of the PL peak reduced to %20 nm. As shown in Figure 4b, the absorbance of the 0.15 mixture perovskite film is greatly enlarged due to the improved film coverage. The absorbance edge (Figure 4b) of the 0.15 mixture perovskite film is also steeper, which could be ascribed to better crystallinity [36] and less defect density. [37] We further carried out excitation-intensity-dependent PLQY and time-resolved PL measurements on the films, which help to exclude the effect of film coverage on the PL properties. As shown in Figure 4c, the PLQY of the 0.15 mixture perovskite film is around 15%, which is approximately three times the PLQY of the 0.08 perovskite film; the PL decay time of the former is also longer than the latter (Figure 4d). These results suggest that the two-step prepared precursor solution yield mixed halide perovskite films with reduced defect density. We note that the PLQY of our perovskite films is still significantly lower than that of their sky-blue counterparts, which could be associated with the increased defect density at high Cl/Br ratio. [18,21,38,39]

Fabrication of Deep-Blue PeLEDs
Finally, we evaluate the performances of deep-blue PeLEDs based on the mixed halide perovskite films. The devices consist of a stacked structure of ITO/NiO x /poly(9-vinylcarbazole) (PVK)/ polyvinylpyridine (PVP)/perovskite/1,3,5-tris(1-phenyl-1H-benzimidazol-2-l) benzene (TPBi)/LiF/Al. As shown in Figure S9a, Supporting Information, the EQE of the PeLEDs produced from the 0.08 precursor solution rises slowly with current density and reaches a maximum of %0.8% at 16 mA cm À2 . In contrast, the EQE of the 0.15 mixture-based PeLED rises quickly and peaks at 4.1% at a low current density of 0.2 mA cm À2 (Figure 5a), indicating a relatively low density of defects in the perovskite film. Moreover, the devices fabricated with 0.15 mixture solution show reasonable reproducibility with an average peak EQE of 3.4% ( Figure S10, Supporting Information). We note that the difference in the EQE of the two types of devices is slightly larger than that of the PLQY of the corresponding perovskite films, which could be related to the improved charge injection and photon outcoupling due to better film morphology.
The current-voltage and luminescence-voltage characteristics of the 0.15 mixture-based PeLED are shown in Figure 5b. After the device turns on, the luminance increases rapidly and reaches 150 cd m À2 at 5.5 V. The EL spectra of the device exhibit a stable emission peak at 459 nm when the driving voltage increases from 2.5 to 5.5 V (Figure 5c), and the CIE coordinates show negligible variation (Figure 5d). We also tested the device operating stability at a constant current density of 1 mA cm À2 . As shown in Figure S11, Supporting Information, the EL spectrum only shows negligible shift during operation, indicating the perovskite film produced by the metastable precursor solution has reasonable phase stability. The lifetime of the devices is relatively short compared to that of the red and green PeLEDs, which is likely caused by ion migration under electrical bias. By modulating the Br/Cl ratio in the lead halide precursor, the emission wavelength of the device can be tuned in the deep-blue region. Here PeLEDs with the emission wavelength locating at 463 nm (CIE: 0.136,0.046) and 455 nm (CIE: 0.149,0.028) ( Figure S12, Supporting Information) are also fabricated and demonstrated a maximum EQE of 4.1% and 3.7% (Figure 5e), respectively. These efficiencies set the records in the <465 nm deep-blue spectral region in three-dimensional-based PeLEDs and are about two    times higher than those achieved in the literatures, as illustrated in Figure 5f and Table S5, Supporting Information. The turn-on voltage is quite low and only shows slight increase from 2.6 V for 463 nm emission to 2.9 V for 455 nm emission ( Figure S12b,e, Supporting Information), suggesting an effective carrier injection process. Importantly, all devices show excellent spectral stability ( Figure S12c,f, Supporting Information) and meet the requirement of Rec. 2020 (Table S5, Supporting Information).

Conclusions
In summary, we systematically investigated the complex formation and reaction processes in the precursor solution of PbBr 2 -CsBr, PbCl 2 -CsCl, PbCl 2 -CsBr, and PbBr 2 -CsCl mixtures, with the aim of understanding the origin of poor solution processibility of deep-blue 3D perovskite emitters. It is found that while in pure halides systems, the formation of [PbX 4 (DMSO) 2 ] 2À type complex could help the solvation process of precursors, e.g., in the case of PbCl 2 -CsCl and PbBr 2 -CsBr; in the mixed halide system PbCl 2 -CsBr (i.e., the precursor combination used for making deep-blue 3D perovskites), the energetically favorable formation of [PbCl 4Àx Br x (DMSO) 2 ] 2À drives the exchange of Cl À from the complex by Br À and results in the precipitation of the least soluble CsCl, leading to extremely low precursor concentration at high Cl/Br ratios and consequently poor perovskite film coverage. Based on the findings, we proposed a metastable dissolution strategy to minimize the ion exchange process. The produced films show significantly improved film coverage and reduced defect density. Based on the new precursor preparation methods, we achieved deep-blue PeLEDs with record high EQEs in three-dimensional-based PeLEDs in the wavelength range of 455-463 nm and good spectral stability, fulfilling the Rec. 2020 standard. Our study highlights the importance of engineering the complex reactions in the precursor solutions of mixed halide perovskites and may shed light on the development of new solution processing methods for 3D deep-blue perovskite emitters. The metastable dissolution strategy can also be used in combination with additives or post-deposition treatment methods (e.g., hot pressing [42] or encapsulation growth method [43] ) to further improve the morphology of the emissive layer.

Experimental Section
Materials: PbBr 2 was bought from TCI. FABr was purchased from Greatcell Solar. CsBr and PbCl 2 were purchased from Alfa Aesar. TPBi was purchased from Luminescence Technology Corp. Other chemicals were purchased from Sigma-Aldrich. NiO nanocrystals were prepared based on published procedure. [11] Solution Device Fabrication: The clean substrates were treated by UV-OZONE for 15 min. The NiO layer was deposited at 3000 rpm for 35 s and then annealed at 320°C for 30 min in an oven. After cooling the substrates to room temperature, the NiO-coated substrates were treated by UV-OZONE for 7 min and then transferred to a nitrogen-filled glovebox. A PVK layer (4 mg mL À1 in chlorobenzene) was spin-coated at 3000 rpm for 30 s and annealed at 150°C for 10 min. The PVP layer (2 mg mL À1 in IPA) was deposited at 3000 rpm for 30 s and annealed at 150°C for 10 min. The perovskite layer was deposited at 3000 rpm for 33 s. Before annealing, the perovskite films were treated by DMF vapor for 1 h to exchange halides and improve compositional heterogeneity (details could be found in published work [19] ). The electron-transporting layer TPBi (50 nm) and top electrode LiF/Al (0.5/100 nm) were evaporated in sequence, and the device area was defined as 2.25 mm 2 .
LED Characterization: The device was driven by a source meter (Keithley 2400) with a scan rate of 0.5 V s À1 , and the forward emitting photons were collected by an integration sphere. QE-Pro spectrometer (Ocean Optics) was used to analyze photons and produce the counts-wavelength spectrum. The system was calibrated using a HL-3P-CAL Vis-NIR light source (Ocean Optics).
Material Characterization: The PL spectra were collected by an Avantes spectrometer (ULS2048 Â 64), and samples were excited by a 405 nm laser. The SEM measurement was carried out by Quanta 400. The PL emission spectroscopy of solution was measured using SpectraMax M2 and M2e Multimode Microplate Readers by loading the solutions in quartz cuvettes.
Excitation-Intensity-Dependent PLQY: PLQY measurement is based on the published method. [44] The laser (405 nm) passes through a step beam attenuator to adjust excitation intensity.

Computational Section
Computational Details: Geometry optimization of all the structures was carried out at the M06-2X [45] level of DFT, with the double-ζ basis set def2SVP for describing each element. [46] Solvent effects were considered using SMD implicit solvent model, [47] and explicit coordinative molecules that are necessary to investigate for the particular chemical interactions were also added in. The Gibbs free energies of formation of the reactants, products, and transition states were calculated by adding the thermal correction up to 298.15 K to the single-point energy of the optimized structures. After geometry optimization, some structures retain a single small imaginary frequency due to very low-energy torsion motion of the cluster, which were neglected in the thermal correction. All calculations were performed using Gaussian 09. [48] Likewise, the visualization of the molecules and the geometry analysis were made with Gaussian View 6. [49] Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Foundation of China (62022004). The computational study made use of