Circularly polarized luminescence of pinene-modified tetradentate platinum(II) enantiomers containing fused 5/6/6 metallocycles

In this study, a couple of tetradentate Pt(II) enantiomers ((−)-1 and (+)-1) and a couple of tetradentate Pt(IV) enantiomers ((−)-2 and (+)-2) containing fused 5/6/6 metallocycles have been synthesized by controlling reaction conditions. Two valence forms could transform into each other through mild chemical oxidants and reductants. Single-crystal X-ray diffraction confirms the structures of (−)-1 and (−)-2. The coordination sphere of the Pt(II) cation in (−)-1 displays a distorted square-planar geometry and a platinum centroid helix chirality. In contrast, the structure of (−)-2 reveals a distorted octahedral geometry. The solution and the solid of (−)-1 are highly luminescent. Complex (−)-1 shows a prominent aggregation-induced emission enhancement (AIEE) behavior in DMSO/water solution with emission quantum yield (Φem) up to 73.2%. Furthermore, highly phosphorescent Pt(II) enantiomers exhibit significant circularly polarized luminescence (CPL) with a dissymmetry factor (glum) of order 10−3 in CH2Cl2 solutions at room temperature. Symmetrically appreciable CPL signals are observed for the enantiomers (−)-1 and (+)-1.


General methods
All reagents were purchased from commercial suppliers and used as received. High-resolution ESI (HR-ESI) mass spectrometry spectra were acquired on a Thermo Scientific Q Exactive Mass Spectrometer. Elemental analysis was performed on a PerkinElmer 240C analyzer. The samples were dried under vacuum at 50 C for 24 h to remove solvated molecules prior to elemental analysis. The 1 H and 13 C NMR spectra were obtained on a Bruker DRX-400 spectrometer. Coupling constants are given in hertz. UV-vis spectra were measured on a UV-3600 spectrophotometer. Photoluminescence (PL) spectra were measured with the Hitachi F-4600 PL spectrophotometer (λ ex ¼ 420 nm). Circular dichroism (CD) spectra in CH 2 Cl 2 solution were recorded on a Jasco J-810 spectropolarimeter at a scan rate of 100 nm min À1 and a resolution of 1 nm at room temperature. CPL spectra were recorded using a circularly polarizer on a Jasco CPL-300 spectrophotometer at a scan rate of 100 nm min À1 and a resolution of 1 nm at room temperature.

Synthetic procedures
Synthesis of (¡)-L. A 15 mL DMSO (pretreated with molecular sieve) solution of 394 mg (À)-4,5-pinene-3-bromophenyl-pyridine (1.2 mmol, 1.2 eq) and 187 mg 3-(pyridin-2-yloxy)phenol (1.0 mmol, 1.0 eq) was added into a 25 mL Schlenk flask containing anhydrous potassium phosphate (2.1 eq), CuI (0.10 eq) and 2-picolinic acid (0.20 eq). The solution was refluxed at 120 C under dark in an argon atmosphere for three days. The solution was cooled to room temperature and extracted Scheme 1. Synthesis route of (À)-1 and (À)-2. with CH 2 Cl 2 3 times. The organic layers were combined and dried over Na 2 SO 4 . The crude product was purified by silica gel flash chromatography with an eluent of PE: EA Synthesis of (¡)-1. A 50 mL eggplant-shaped bottle was charged with (À)-L (434 mg, 1 mmol), K 2 PtCl 4 (415 mg, 1 mmol) and tetrabutylammonium bromide (644 mg, 2 mmol) in AcOH (25 mL) at 77 K and was degassed and refilled with argon three times. The reaction mixture was refluxed at 115 C for three days under argon. After the reaction finished, the residue was filtered and washed with water. The crude product was purified by silica gel flash chromatography with an eluent of CH 2 Cl 2 . A green-yellow powder was obtained finally. 1  Synthesis of (¡)-2. A mixture of (À)-L (434 mg, 1 mmol), K 2 PtCl 4 (415 mg, 1 mmol) and tetrabutylammonium bromide (644 mg, 2 mmol) in AcOH (25 mL) was allowed to reflux at 115 C for three days under air. After the reaction finished, the residue was filtered and washed with water. The crude product was purified by silica gel flash chromatography with a CH 2 Cl 2 eluent. A pale-yellow powder was obtained. 1  Transformation from (¡)-2 to (¡)-1. Complex (À)-2 can be successfully transformed to (À)-1 by a heterogeneous reduction with Zn 0 . Under an argon atmosphere, a mixture of (À)-2 (698 mg, 1 mmol), activated Zn 0 dust (130 mg, 2 mmol) and 15 mL anhydrous CH 2 Cl 2 in a 25 mL Schlenk flask was heated to 40 C for 2 days. After cooling down, the solution was filtered through Celite and the filtrate was concentrated by rotary evaporation. The crude product was purified by silica gel flash chromatography eluting with CH 2 Cl 2 to give (À)-1 as a green-yellow solid.

Single crystal X-ray structure determination
Single-crystal X-ray diffraction measurements were performed on a Bruker SMART APEX CCD and Xcalibur Atlas Gemini ultra. Intensities were collected with graphite monochromatized Mo Kα radiation (λ ¼ 0.71073 Å) or Ga Kα radiation (λ ¼ 1.34139 Å) operating at 50 kV and 30 mA, using ω/2θ scan mode. The data reduction was made with the Bruker SAINT package [56]. Absorption corrections were performed using the SADABS program [57]. The structures were solved by direct methods and refined on F 2 by full-matrix least-squares using SHELXL-2018/3 (Sheldrick, 2018) with anisotropic displacement parameters for all non-hydrogen atoms in the two structures. Hydrogen atoms bonded to carbon atoms were placed in calculated positions and refined as riding mode, with C-H ¼ 0.93 Å (methane) or 0.96 Å (methyl) and Uiso(H) ¼ 1.2 Ueq (C methane ) or Uiso(H) ¼ 1.5 Ueq (C methyl ). All computations were carried out using the SHELXL-2018/3 program package [58]. CCDC numbers 2206659-2206660 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif.

Calculation methods
The crystal structures of (À)-1 and (À)-2 were used as starting geometries, and calculations were performed with the Gaussian 09 program [59]. Geometry optimizations of ground states were simulated with density functional theory (DFT) at the hybrid functional PBE1PBE-D3/LANL2DZ (Pt) and PBE1PBE-D3/6-31g(d,p) (H, C, N, Cl) levels using CH 2 Cl 2 as solvent. The solvent effect is based on the polarizable continuum model (PCM). The optimized structures of (À)-1 and (À)-2 were used to calculate the lowest singlet electronic transition using the time-dependent density functional theory (TDDFT) method. The geometry of the first triplet state (T 1 ) was optimized and the analysis of the natural transition orbital (NTO) was carried out for the excitation of S 0 → T 1 [40,[44][45][46]. Mulliken population analysis (MPA) was utilized to obtain the electron density distribution of each atom in the specific molecular orbital of the Pt(II) complexes using the Multiwfn program [60].
The CH 2 Cl 2 solution of complex (À)-1 is highly emissive, dominated by a green phosphorescence in the 475-650 nm (Figure 3). The emission maximum is 501 nm, accompanied by a shoulder peak at 530 nm at low  1 (a, b), and crystal structure of (À)-2. concentrations (<5000 μM). According to the reported studies [40,41,42,43,44,45,46,47,48], the emission arises mostly from a 3 LC ( 3 π,π) state mixed with some 3 MLCT characters. The peak at 501 nm mainly derives from the 3 LC state ( 3 π,π) state, and the peak at 530 nm involves some 3 MLCT characters. Further increasing the concentration, the shoulder peak evolves to the emission maximum. However, a lower energy emission corresponding to 3 MMLCT or excimer emissive state could not be detected at high concentrations. By examining the crystal structure, none of the effective intermolecular Pt⋯Pt and πÀπ interactions are involved in (À)-1, and any molecular aggregates could not form. Therefore, the emission is almost concentration-independent. Upon increasing concentration, we tentatively figured out that molecular conformations would vary in concentrated solutions. The geometry would change to maximize the conjugation of 5/6/6 metallocycles, leading to more charge-transfer (CT) character involvement in the emissive state [64]. Accordingly, the emission maximum is bathochromically shifted to 530 nm at high concentrations. Solvent effects on the absorption and emission of (À)-1 are also examined ( Figure S8). The lowest-energy absorption band is hypsochromically shifted with the increased polarity of the solvents, from 381 nm in toluene to 360 nm in methanol, verifying the character of a CT transition [65,66,67]. However, only minimal change (Δλ ¼ 5 nm) of emission maximum can be perceived upon variation of the solvent polarity. The insignificant shift of emission spectra supports the origin of the lowest triplet state from a predominant 3 LC state with limited 3 MLCT characters [40,41,42,43]. In addition, the solid-state emission spectrum of (À)-1 shows the maximum at 537 nm and two shoulders at 509 and 569 nm ( Figure S9).

Theoretical investigation
Time-dependent density functional theory (TD-DFT) calculations have been performed to interpret the nature of the transitions [35,40,44,45,46]. The calculated S 0 geometry of (À)-1 agrees well with the one determined by the X-ray crystallography, showing a significant distortion from square planarity with a large dihedral angle (37.09 ) between the terminal pyridine planes ( Figure S10). The optimized S 0 geometry of (À)-2 exhibits a distorted octahedral geometry, and the tetradentate ligand is curved, which is also comparable to the results in the crystal structure ( Table 1).
As depicted in Table S2, the lowest-energy absorption of (À)-1 mainly derives from the S 0 →S 1 transition involving HOMO→LUMO (93.0%) and HOMO-2→LUMO (4.5%). According to molecular orbital (MO) patterns and orbital composition analysis (Figure 4 and Table S2), the HOMO is delocalized on the benzene ring of phenyl-pyridine (Ring D), benzene ring of phenoxyl-pyridine (Ring E), and bridging O atom between phenyl-pyridine and phenoxyl-pyridine, and the rest on central Pt atom (24.02%). In contrast, the LUMO is spread over phenyl-pyridine (Ring A and Ring D), the pyridine ring of phenoxyl-pyridine (Ring B), and the Pt nucleus (6.11%).

AIEE properties
Because of solvatochromic behaviors of tetradentate phosphorescent Pt(II) complexes in the solid-state, MacLachlan et al. investigated solventand temperature-induced change of emissions in mixed DMSO/water solvents [62]. The continuous addition of water into the DMSO solution of tetradentate phosphorescent Pt(II) complexes caused a transformation from monomeric green emission to excimeric orange emission. Herein, we imagine whether this emission variation exists in pinene-modified tetradentate Pt(II) complex (À)-1. However, we failed to observe the change of luminescence from green to orange due to the impossible Pt⋯Pt interactions between neighboring molecules.
Notably, the luminescence of the DMSO solution of (À)-1 is negligible with Φ em below 1% ( Figure 6 and Figure S11). Improving the water content (f w ) from 0 to 40%, we find that the emission intensity has a progressive rise with Φ em attaining 15.92% at f w ¼ 40%, concomitantly the emission outline stays unchanged with λ em at 501 nm and a shoulder peak at 530 nm, emitting light in green color. Once the proportion of water in DMSO attained 50%, the shoulder peak at 530 nm in pure DMSO solution evolves to the maximum. The solution luminescence appears yellow under UV radiation. Significantly, the emission is intensified sharply, with Φ em reaching the maximum (73.2 %) at f w ¼ 70%, demonstrating a distinct aggregation-induced emission enhancement (AIEE) behavior.
As a six-membered metallacycle is more flexible than a five-membered metallacycle, the aromatic planes forming the six-membered metallacycle undergo a vibration up and down supported by the big dihedral angles in the crystal structure (Table 1), which would favor non-radiative decay, inducing relatively weak emissions [64]. In pure DMSO solution, the non-radiative vibration is continual, leading to an extremely low Φ em . Continuous addition of water to the DMSO solution causes the formation of aggregates. On the basis of the RIR and RIV mechanisms, the intramolecular vibration and rotation can be efficiently suppressed in aggregates, restraining non-radiative decay and promoting radiative decay processes [68,69,70]. When the water content increases to a critical value of 50%, many aggregates form in a mixed solution, resulting in a dramatic increase in emission intensity. Concurrently, the geometrical change to maximize conjugation of 5/6/6 metallocycles occurs in the aggregates, leading to more 3 MLCT transitions in the emissive state [64,71]. Consequently, the shoulder peak at 530 nm develops to the emission maximum.

Author contribution statement
Hua-Hong Zhang: Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.
Jing Jing: Performed the experiments; Contributed reagents, materials, analysis tools or data.
Guo Xu: Performed the experiments; Analyzed and interpreted the data.
Yi-Xin Song: Performed the experiments. Shui-Xing Wu, Zai-Feng Shi: Analyzed and interpreted the data. Xing-Han Chen, Da-Shuai Zhang: Contributed reagents, materials, analysis tools or data.
Xiao-Peng Zhang: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper.

Funding statement
This work is supported by the National Natural Science Foundation of China (No. 21961009) and the Natural Science Foundation of Hainan Province (No. 220RC591).

Data availability statement
Data included in article/supplementary material/referenced in article.

Declaration of interests statement
The authors declare no conflict of interest.

Additional information
Supplementary content related to this article has been published online at https://doi.org/10.1016/j.heliyon.2022.e11358.