English

• 1. INTRODUCTION

  Square-planar Pt(Ⅱ) complexes have inspired considerable research interest because of their intriguing molecular arrangements and their versatile potentials in the development of high-efficient luminescent materials^[1-3]. In recent years, cyclometalated cationic (N^C^N)Pt (N^C^N = 1,3-bis(2΄-pyridyl)benzene) and (N^N^C)Pt (N^N^C = 6-phenyl-2,2΄-bipyridine) isocyanide complexes with superb planarity have gained increasing attention, and these planar complexes can be assembled into supramolecular aggregates and functional nanostructures driven by intermolecular Pt···Pt and/or π-π stacking interactions^[4-7]. In common, pincer-type (N^C^N)Pt and (N^N^C)Pt isocyanide complexes can be facilely prepared by a simple ligand substitution of precusors (N^C^N)PtCl and (N^N^C)PtCl with arylisocyanide at room temperature, and the plane of the cyclometalated ligand N^C^N or N^N^C is almost coplanar with the ring of arylisocyanide. Upon tuning the cocrystallized solvents, various solvatomorphic crystal structures can be obtained with different molecular arrangements and Pt···Pt distances, resulting in variable solid-state emission colors from blue (monomer emission) to red (multimolecular emission) and interesting semiconducting characteristic^{[8, 9]}. Accordingly, these luminescent cyclometalated cationic Pt(Ⅱ)-isocyanide complexes can be utilized as excellent photodetectors, DNA intercalators and environment-responsive materials^[10-14].

  Pinenes are accessible natural products with rigid lipophilic groups, and can be conveniently introduced in pyridine ligands through Kröhnke-type method^{[15, 16]}. Due to the defined carbon stereocenters and steric bulk fragments derived from pinene sources, the complexes modified with these optically active units are endowed with preferential configurations or significant distortions, leading to potential applications in enantioselective synthesis and stereoselective recognition^[17-21]. For square-planar Pt(Ⅱ) complexes, by incorporation of bulky and chiral pinene groups, the steric hindrance of molecular environments would be increased, therefore hindering intermolecular packings and preventing self-quenching of platinum(Ⅱ) complexes^[22]. In addition, chirality around Pt(Ⅱ) center and helical packing can be induced that are attributed to the distortion of planarity and the staggered stacking of adjacent molecules, respectively, which will give rise to appealing chiroptical properties^[23].

  In previous work, we have prepared a series of pinene-fused NCN and NNC-type Pt(Ⅱ) complexes^[24-28]. When bulk pinene groups with defined carbon stereocenters are attached to NCN and NNC moieties, distorted configuration and intramolecular or intermolecular helical chirality of Pt(Ⅱ) complexes can be induced, resulting in interesting CD and CPL properties. In addition, the electronic structure and energy band gap may be changed as pinene groups are incorporated. A pinene-containing cyclometalated Pt(Ⅱ) complex [Pt((−)-NNC)(Dmpi)]Cl ((–)-1) was prepared (Fig. 1), and its enhanced chiroptical signals and environment-responsive properties have been studied^[26]. Moreover, due to aliphatic pinene groups, complex (–)-1 has good solubility in a large range of solvents, from non-polar solvent dichloromethane to aqueous solution. In this work, we have investigated the structural variance, absorption and luminescent properties of polymorphic solid forms obtained from the evaporation of complex (–)-1 in different solvents. Through DFT calculation, the energy band gaps of different aggregates are explored.

  Figure 1

  

  Figure 1.  Molecular structure of complex (–)-1, and stacking arrangements of form-Y and form-R

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  2. EXPERIMENTAL

  2.1 General methods

  Solid-state UV-Vis absorption spectra were measured on a PerkinElmer Lambda 750s spectrophotometer. The powder XRD patterns were recorded on a Shimadzu XD-3A X-ray diffractometer. Photoluminescence (PL) spectra were measured by the Hitachi F-7000 PL spectrophotometer. Lifetimes were measured on a HORIBA JY system. The thermogravimetric analysis (TGA) characterization was conducted using a thermal analyser (PerKinElmer, USA).

  2.2 Synthesis

  Complex (–)-1 [Pt((−)-NNC)(Dmpi)]Cl was facilely prepared by an aqueous/organic phasetransfer substitution reaction of the corresponding cyclometalated platinum(Ⅱ) chloride precursor with 2,6-dimethylphenylisocyanide according to the reported procedure. The product was obtained by separating the aqueous phase and evaporating under vacuum.

  2.3 Computational details

  The calculations were carried out by density functional theory (DFT) method at the B3LYP-D3 level with Gaussian09 program^{[29, 30]}. The CAMB3LYP functional was used, SVP basis set for C, H, N and P, and MWB60 pseudopotential basis set for Pt. The crystal structures of Form-Y and Form-R were used as the starting geometries of monomer and dimer, respectively. The geometries of trimer and tetramer were built on the basis of Form-R through adding one and two molecules on the side, respectively. Meanwhile, the Pt···Pt distance and torsion angle between adjacent molecules were stayed the same with the Form-R.

  3. RESULTS AND DISCUSSION

  3.1 Structural variance in solid states

  Polymorphic structures of cyclometalated cationic Pt(Ⅱ)-isocyanide complexes with different packing modes have been reported. The solution of complex (–)-1 exhibits distinct aggregate states and spectroscopic properties in different solvents^[26]. And two polymorphs (Form-Y and Form-R) of complex (–)-1 can be isolated in acetonitrile/dichloromethane (v/v = 1:1) and methanol solution, respectively, showing different stacking arrangements and different intermolecular Pt···Pt distances (Fig. 1). Therefore, we envisaged that various packing modes, Pt···Pt interactions and luminescent properties could be observed in polymorphic solid forms, and recrystallization of (–)-1 has been performed in a series of solvents to obtain different solid-state forms. As shown in Fig. 2, form Ⅰ is yellow, while forms Ⅶ and Ⅷ are orange, and forms Ⅱ to Ⅵ are red. According to previous studies, isolated molecules are revealed in form Ⅰ, and strong Pt···Pt interactions may be found in Ⅱ to Ⅵ. Orange forms Ⅶ and Ⅷ are envisioned to be an intermediate state between the red and yellow forms that very weak Pt···Pt and/or π-π interactions may be involved. The phenomenon agrees with the previous studies that yellow needles could be grown slowly in acetonitrile/dichloromethane (v/v = 1:1) solution. Therefore, dichloromethane, chloroform and acetonitrile facilitate the formation of solids with discrete molecules^{[25, 26]}.

  Figure 2

  

  Figure 2.  Photograph of different solid-state forms (Ⅰ: evaporating in dichloromethane slowly; Ⅱ: evaporating in water; Ⅲ: evaporating in mixed acetone/methanol (v/v = 5:1) solution; Ⅳ: evaporating in methanol solution; Ⅴ: evaporating in mixed ethyl acetate/methanol (v/v = 5:1) solution; Ⅵ: evaporating in ethanol solution; Ⅶ: evaporating in mixed acetonitrile/methanol (v/v = 5:1) solution; Ⅷ: evaporating in mixed chloroform/methanol (v/v = 5:1) solution)

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  The XRD patterns and thermogravimetric properties of all forms have been investigated (Fig. 3). Forms Ⅰ, Ⅶ and Ⅷ exhibit similar profile shape with intense and narrow peaks, while comparatively weak and broad peaks are observed in Ⅱ to Ⅵ. The difference of XRD patterns is consistent with the observed difference of color due to various packing arrangements^[9]. Correspondingly, due to more solvent molecules in pores, more weight has been lost below 100 ℃ for red forms Ⅱ to Ⅵ (Fig. 4).

  Figure 3

  

  Figure 3.  XRD patterns of all solid-state forms

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  Figure 4

  

  Figure 4.  TGA curves of all solid-state forms

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  3.2 Spectroscopic properties

  As depicted in Fig. 5, solid-state powders of yellow form Ⅰ show low-energy absorption bands at 400~550, originating mainly from ¹LLCT and ¹MLCT transitions. Compared to yellow powders, the low-energy absorption bands of red forms Ⅱ to Ⅵ show distinct redshifts and occur in the 450~600 nm range, and the absorptions extend to a longer wavelength, which can be attributed to the effective Pt···Pt interactions, along with the corresponding ¹MMLCT emission state^[25]. Among these powders, the absorption of form Ⅱ obtained from aqueous solution reaches the longest wavelength, and the result is consistent with previous studies that 1D infinite chains with extended and strong Pt···Pt interactions appear in the solid state and aqueous solution^[5]. Weak Pt···Pt and/or π-π interactions may be involved in orange forms Ⅶ and Ⅷ, and the absorptions locate between yellow and red forms. The difference of low-energy absorption is correlated with the variance of solid-state Pt···Pt interactions.

  Figure 5

  

  Figure 5.  Absorption spectra of all solid-state forms

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  Solid-state emission of cyclometated Pt(Ⅱ) complexes could be significantly affected by the molecular arrangement with variable Pt···Pt separations. Solid-state powders of yellow form Ⅰ exhibit brightly green-yellow light under UV radiation at room temperature, and a structured emission profile is observed with λ_max at 562 nm, a shoulder at 529 nm and a broad band spreading from 600 to 800 nm (Fig. 6), which is designated as a mixture of triplet metal-to-ligand charge-transfer and triplet intraligand chargetransfer (³MLCT/³ILCT) excited states^{[9, 25]}. At 77 K, the emission spectrum of form Ⅰ evolves into a narrow and more structured one with maximum wavelength at 540 nm (Fig. 7). The intensity of the broad emission band between 600 and 800 nm decreases distinctly. At both room temperature and 77 K, the red forms of Ⅱ to Ⅵ display a broad and structureless emission with maximum wavelength at 650~675 nm. The emission shapes of red forms also become narrow at 77 K, and according to previous studies, the emission states are originated from ³MMLCT^[25]. The intermediate form Ⅷ shows a broad and structureless emission, while the form Ⅶ exhibits a structured emission similar to form Ⅰ, indicating that the emission of two intermediate forms may come from different excited states.

  Figure 6

  

  Figure 6.  Emission spectra of all solid-state forms at room temperature (λ_ex = 450 nm)

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  Figure 7

  

  Figure 7.  Emission spectra of all solid-state forms at 77 K (λ_ex = 450 nm)

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  The excited state lifetimes of λ_max have been investigated in the order red form < orange form < yellow form (Table 1, Fig. 8). Among all powders, the lifetime value of form Ⅰ is the largest, 643 ns. With the formation of aggregates, lifetime takes a decrease, which is consistent with the previous studies^[31].

  Table 1

  

  Table 1.  Excited State Lifetime of λ_max

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  Form	Ⅰ	Ⅱ	Ⅲ	Ⅳ	Ⅴ	Ⅵ	Ⅶ	Ⅷ
  Lifetime/ns	643	202	227	184	206	174	533	405

  Figure 8

  

  Figure 8.  Time-resolved emission decay curves of all solid-state forms at room temperature (λ_ex = 450 nm)

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  Substituent groups on the cyclometalated ligand, counter anions, concentration and temperature may affect the aggregation state and the corresponding spectroscopic properties of cationic Pt(Ⅱ)-isocyanide complexes. In addition, the polarity of solvent has significant effect on the absorption and emission spectra. Commonly, monomeric molecules are always present in nonpolar solvents, and the absorption and emission mainly come from ¹MLCT and ³MLCT, respectively. Aggregation states with appreciable Pt···Pt interactions are involved in polar solvents, so the absorption and emission originating from ¹MMLCT and ³MMLCT will be red-shifted^{[5, 26]}. As shown in Figs. 6 and 7, solid-state absorption and emission conform to each other. Monomeric state has been formed in nonpolar solvent, such as dichloromethane and chloroform. However, in methanol solution, head-to-tail dimers have been found, and the aqueous solution would facilitate the aggregation of molecules and formation of infinite Pt···Pt chains, leading to the most remarkable redshift of absorption and emission.

  3.3 DFT calculation

  Different electronic configurations and excited states would be present due to the variable molecular packings. Although steric hindrance is present in pinene groups, effective Pt···Pt interactions are observed in the optimized configurations of dimer, trimer and tetramer (Fig. 9). The calculated bond lengths and bond angles around Pt(Ⅱ) ions of monomer, dimer, trimer and tetramer are consistent with those in crystal structures (Table 2)^{[25, 26]}. The Pt–N1 bond length is about 2.15 Å, and Pt–N2 and Pt–C1 bond lengths are both approximately 2.00 Å. Due to the strong coordination ability of isocyanide, a much shorter bond length (ca. 1.91 Å) of Pt–C2 is present. The bite angle (C2−Pt−N2) resides in the range of 175º~179º, however, owing to the chelating strain effect, the angle (C1−Pt−N1) deviates significantly from linear geometry with a bond angle of ca. 159º. In addition, the calculated energies and dipole moments of four aggregates have been compared, and a lower energy is exhibited with the increase of molecules in the calculated system (Table 2). As plotted in Fig. 10, the calculations of frontier molecular orbitals of monomer, dimer, trimer and tetramer have been performed. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of monomer, dimer, trimer and tetramer are calculated to be –8.864 and –5.053, –10.788 and –7.048, –12.120 and –8.620, and –13.246 and –10.018 eV, respectively (Fig. 10). For all aggregation states, the HOMO is mainly localized on the benzene and central pyridine rings of N^N^C ligand, while LUMO is largely composed of Pt nucleus (5d_z²) orbitals and some components of central pyridine ring. Due to the increase of intermolecular Pt···Pt interactions, HOMO-LUMO gap takes a decrease in the order of monomer > dimer > trimer > tetramer, which accounts for variable colors (yellow, orange or red) and luminescent performances of solid powders obtained from the evaporation of different solvents.

  Figure 9

  

  Figure 9.  Optimized configurations of monomer, dimer, trimer and tetramer

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  Table 2

  

  Table 2.  Structural Parameters of Optimized Configurations of Monomer, Dimer, Trimer and Tetramer

  DownLoad: CSV

  	Monomer	Dimer	Trimer	Tetramer
  Pt–N(1) bond length (Å)	2.16	2.15, 2.15	2.15, 2.14, 2.15,	2.15, 2.15, 2.14, 2.15,
  Pt–N(2) bond length (Å)	2.01	2.00, 2.00	2.01, 2.00, 2.00	2.01, 2.00, 2.00 2.01
  Pt–C1 bond length (Å)	2.00	2.00, 2.00	2.00, 2.00, 2.00	2.00, 2.00, 2.00, 2.00
  Pt–C(2) bond length (Å)	1.91	1.91, 1.91	1.92, 1.91, 1.91	1.92, 1.91, 1.91, 1.92
  N(1)–Pt–C1 angle (°)	159.37	159.64, 159.49	159.51, 159.70, 159.46	159.43, 159.64, 159.68, 159.44
  N(2)–Pt–C(2) angle (°)	178.52	178.32, 175.82	174.96, 175.23, 178.64	175.94, 177.83, 177.28, 175.49
  Energy (eV)	–41336.34	–82672.86	–124008.15	–165342.44
  Dipole moments (Debye)	3.36	1.47	3.70	1.57

  Figure 10

  

  Figure 10.  Contour plots of the HOMO and LUMO of monomer, dimer, trimer and tetramer

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  4. CONCLUSION

  Thus the structural properties, absorption and luminescence of a series of solid-state powders of complex (–)-1 have been investigated. As the aggregates with effective intermolecular Pt···Pt interactions have been formed in solid state, the color changes from yellow to red, and the luminescence is red-shifted (Δλ = ca. 100 nm) with excited states changing from ³MLCT/³LLCT to ³MMLCT. In addition, excited state lifetimes of λ_max give an attenuation with the formation of aggregates. Despite the steric hindrance of pinene groups, effective Pt···Pt interactions (3.3~3.5 Å) are involved in the optimized configurations of dimer, trimer and tetramer, and HOMO-LUMO gap takes a decrease in the order monomer > dimer > trimer > tetramer.

  
  
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• 

  Figure 1  Molecular structure of complex (–)-1, and stacking arrangements of form-Y and form-R

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  Figure 2  Photograph of different solid-state forms (Ⅰ: evaporating in dichloromethane slowly; Ⅱ: evaporating in water; Ⅲ: evaporating in mixed acetone/methanol (v/v = 5:1) solution; Ⅳ: evaporating in methanol solution; Ⅴ: evaporating in mixed ethyl acetate/methanol (v/v = 5:1) solution; Ⅵ: evaporating in ethanol solution; Ⅶ: evaporating in mixed acetonitrile/methanol (v/v = 5:1) solution; Ⅷ: evaporating in mixed chloroform/methanol (v/v = 5:1) solution)

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  Figure 3  XRD patterns of all solid-state forms

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  Figure 4  TGA curves of all solid-state forms

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  Figure 5  Absorption spectra of all solid-state forms

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  Figure 6  Emission spectra of all solid-state forms at room temperature (λ_ex = 450 nm)

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  Figure 7  Emission spectra of all solid-state forms at 77 K (λ_ex = 450 nm)

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  Figure 8  Time-resolved emission decay curves of all solid-state forms at room temperature (λ_ex = 450 nm)

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  Figure 9  Optimized configurations of monomer, dimer, trimer and tetramer

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  Figure 10  Contour plots of the HOMO and LUMO of monomer, dimer, trimer and tetramer

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  Table 1.  Excited State Lifetime of λ_max

  Form	Ⅰ	Ⅱ	Ⅲ	Ⅳ	Ⅴ	Ⅵ	Ⅶ	Ⅷ
  Lifetime/ns	643	202	227	184	206	174	533	405

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  Table 2.  Structural Parameters of Optimized Configurations of Monomer, Dimer, Trimer and Tetramer

  	Monomer	Dimer	Trimer	Tetramer
  Pt–N(1) bond length (Å)	2.16	2.15, 2.15	2.15, 2.14, 2.15,	2.15, 2.15, 2.14, 2.15,
  Pt–N(2) bond length (Å)	2.01	2.00, 2.00	2.01, 2.00, 2.00	2.01, 2.00, 2.00 2.01
  Pt–C1 bond length (Å)	2.00	2.00, 2.00	2.00, 2.00, 2.00	2.00, 2.00, 2.00, 2.00
  Pt–C(2) bond length (Å)	1.91	1.91, 1.91	1.92, 1.91, 1.91	1.92, 1.91, 1.91, 1.92
  N(1)–Pt–C1 angle (°)	159.37	159.64, 159.49	159.51, 159.70, 159.46	159.43, 159.64, 159.68, 159.44
  N(2)–Pt–C(2) angle (°)	178.52	178.32, 175.82	174.96, 175.23, 178.64	175.94, 177.83, 177.28, 175.49
  Energy (eV)	–41336.34	–82672.86	–124008.15	–165342.44
  Dipole moments (Debye)	3.36	1.47	3.70	1.57

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