Enhancement of Chiroptical Responses of trans‐Bis[(β‐iminomethyl)naphthoxy]platinum(II) Complexes with Distorted Square Planar Coordination Geometry

Abstract The relationship between the coordination geometry and photophysical properties of trans‐bis[(β‐iminomethyl)naphthoxy]platinum(II) was investigated both experimentally and theoretically. A series of platinum(II) complexes with differently substituted iminomethyl groups were synthesized, and their photophysical properties were examined in solution, in the crystalline, and in the PMMA film‐dispersed state, respectively (PMMA=poly(methyl methacrylate)). These complexes showed structure‐dependent emission spectra, in which the color of the luminescence in the crystalline state varied over a range of about 40 nm depending on the specific bowl‐shaped molecular structure. The chiral complexes with (R,R)‐ and (S,S)‐configurations were found to have structure‐dependent chiroptical properties both in solution and the PMMA film‐dispersed state such that the intensity of circular dichroism (CD) and circularly polarized luminescence (CPL) were enhanced with bulky cyclic substituents at the nitrogen atoms. A theoretical study using density functional theory (DFT) and time‐dependent (TD)‐DFT calculations revealed that the enhancement of chiroptical responses is due to the amplification of the magnetic dipole moment caused by the distortion of the square planar geometry.


Introduction
Circularly polarized luminescence (CPL), [1][2][3][4][5] which is a differential emission of right-and left-circularly polarized light, has been attracting increasing attention in recent years due to the wide potential for applying 3D optical displays, [6] information storage and processing, [7] asymmetric synthesis, [8,9] and biological probes. [10] In general, one of the most important factors in developing CPL-active materials is the luminescence dissymmetry factor (g lum = 2ΔI/I = 2(I L À I R )/(I L + I R ), in which I L and I R are the intensity of left-and right-circularly polarized luminescence). [11][12][13] In the last decade, many examples of CPLactive organic and organometallic compounds with chiral skeletons of binaphthyls, [14] helicenes, [15] cyclophanes, [16] and coordination chirality [17] have been investigated and much effort has been devoted to elucidate the relationship between their molecular structures and CPL properties (g lum ).
Square planar organometallic platinum(II) complexes have been extensively studied due to their interesting phosphorescent properties. [18] Among them, platinum(II) complexes with CPL activity [19] have attracted particular attention in recent years due to their potential applications in CP-OLEDs. [20] Most of the known examples of chiral CPL-active platinum(II) complexes contain chiral π-systems such as helicenes, [19a,b,f] binaphthyls, [19h,j] and cyclophanes. [19k] There are only a few reports on the use of coordination chirality. [19e] In order to develop novel functional materials based on square planar d 8 transition metal complexes, we have focused on the stereochemistry of trans-bis [(β-iminomethyl)aryloxy]palladium(II) [21] and platinum(II) [22] complexes. Previously, we subdivided 52 crystal structures of trans-bis [(β-iminomethyl)aryloxy]palladium(II) complexes into two categories, namely step and bowl arrangements, based on the Cambridge Crystallographic Database (Figure 1), and found that step complexes with chiral substituents and all the bowl complexes induce chiral distortions in the square planar geometries, resulting in Δ/Λ configuration ( Figure 2). [23] In the present work, we found that a series of trans-bis [(β-iminomethyl)naphthoxy]platinum(II) complexes (1 a-e) with chiral ligands exhibited structure-dependent chiroptical properties, and the unique bowl-shaped structure with Δ/Λ chirality enhanced the chiroptical properties of these complexes (Scheme 1). The emission spectra showed a chromogenic change of approximately 40 nm for solid-state emission based on conformational changes in the crystal. Theoretical calculations revealed a relationship between the structural distortion and photophysical properties. These results provide a new way to design CPLactive square planar transition metal complexes. In this paper we describe the synthesis, structure and photophysical properties of a series of trans-bis [(β-iminomethyl)naphthoxy]platinum(II) complexes with a focus on the mechanistic rationale for the structure-dependence of their chiroptical responses.
Single crystals of platinum complexes 1 a-e were obtained by recrystallization from CH 2 Cl 2 /n-hexane and employed in single-crystal X-ray diffraction (XRD) analysis. Details of the crystal data and the structure refinement are presented in Table S1 (Supporting Information), including analysis of the intermolecular interactions (Figures S11-S17). ORTEP [24] drawings of (S,S)-1 a, (R,R)/(S,S)-1 a, (R,S)-1 a, (S,S)-1 b-d, and (R,R)-1 e are shown in Figures 3 and S10, where the chirality angles of square planar geometry O(1)À N(1)À O(2)À N(2) (ϕ) and bowl angles θ between the mean planes of the naphthalene rings are provided to express the degree of distortion of the coordination planes. [23] The molecular structures of (S,S)-1 a, (R,R)/(S,S)-1 a, and (R,S)-1 a are shown in Figures 3a and S10. (S,S)-1 a has a bowl configuration (θ = 49.6°), and the two methyl groups have a syn-orientation ( Figure 3a). The chirality angle ϕ of (S,S)-1 a in the spiro-chelate rings has a positive value of 7.94°. Hence, (S,S)-1 a has a (S,S,Λ) configuration in the crystal. These structural data correspond well with the previously reported palladium(II) complex. [21c] The chelate rings in (R,R)/(S,S)-1 a have an envelope form with a ϕ angle of À 2.64°( Figure S10a). Therefore, (S,S)-1 a in (R,R)/(S,S)-1 a has a (S,S,Δ) configuration in the crystal, whilst (S,S)-1 a in the enantiopure crystal showed the opposite Λ configuration. (R,S)-1 a adopts a step configuration with C 2 symmetry, and the two methyl groups in (R,S)-1 a take an anti-orientation ( Figure S10b).
The molecular structures of (S,S)-1 b-d and (R,R)-1 e are shown in Figures 3b-e. The structure of (S,S)-1 b adopts the bowl configuration (θ = 19.2°) with anti-orientation between the phenyl groups ( Figure 3b). The chirality of OÀ N-OÀ N in the spiro-chelate rings shows a Δ configuration with a negative value of ϕ = À 2.74°. The methyl groups of (S,S)-1 c are in antiorientation, and the structure can be described as a distorted bowl-configuration with a low bowl angle (θ = 3.56°) (Figure 3c). The OÀ NÀ OÀ N system has a Λ configuration with a positive value of ϕ = 0.91°. Complex (S,S)-1 d has a bowl-configuration with θ = 48.2°as the bowl angle and a Λ configuration with a positive value of ϕ = 1.30° (Figure 3d). The structure of (R,R)-1 e adopts the step configuration (θ = 5.14°) with an anti-orientation between the methyl groups ( Figure 3e). The OÀ NÀ OÀ N group adopts the Δ configuration with a negative value of ϕ = À 0.91°.

Photophysical Properties of the Platinum Complexes 1 a-e
Circular dichroism (CD) spectra of (R,R)-and (S,S)-1 a-e were recorded in solution and in the PMMA (PMMA = poly(methyl methacrylate) film-dispersed state at room temperature (Figures 4a, S18, and S19). Complexes (S,S)-1 a-d exhibited a strong negative Cotton effect at approximately 420 nm, which can be assigned to a mixture of intra-ligand charge transfer ( 1 ILCT) and metal-to-ligand charge transfer ( 1 MLCT) from the UV/Vis spectra showing a mixture of 1 ILCT and 1 MLCT bands around 370-500 nm observed under the same measurement conditions ( Figure 4b). On the other hand, complex (S,S)-1 e showed a comparably weak negative Cotton effect in the same region, although it has almost the same UV/Vis spectrum as (S,S)-1 a-d. The g abs (= Δɛ/ɛ) values around the absorption maxima in the low energy region are À 0.0026 (422 nm) for (S,S)-1 a, À 0.0023 (422 nm) for (S,S)-1 b, À 0.0030 (417 nm) for (S,S)-1 c, À 0.0025 (418 nm) for (S,S)-1 d, and À 0.00088 (410 nm) for (S,S)-1 e, respectively. These results indicate that the bulkiness of the substituents at the nitrogen atoms exert a strong influence on the chiral properties of 1 even in the diluted solution state.
All the complexes exhibited orange to red luminescence under UV excitation at room temperature in solution, crystalline, and PMMA film-dispersed state, respectively ( Figures S21-S23). The photophysical data of complexes 1 a-e are presented in Table 1. The emission spectra in dilute CH 2 Cl 2 solution at 298 K are shown in Figure 5a. A clear hypsochromic shift was observed for (S,S)-1 d (λ max = 589 nm) with the bulkiest 1naphthyl substituent at the nitrogen atoms compared to those of (S,S)-1 a-c (λ max = 602 nm for (S,S)-1 a, 601 nm for (S,S)-1 b, and 602 nm for (S,S)-1 c, respectively). In contrast, a bathochromic shift was observed for (S,S)-1 e (λ max = 608 nm) with the less bulky alkyl substituents at the nitrogen atoms. These results indicate that the bulkiness of the substituents at the nitrogen atoms affects the d-π conjugation of the coordination platform in dilute solution.
The change in the emission color of (S,S)-1 a-e was more remarkable in the crystalline state. Figure 5b shows the  emission spectra of (S,S)-1 a-e in the crystalline state, where the emission peak maxima clearly changes depending on the substituent at the nitrogen atoms (λ max = 596, 620 nm for (S,S)-1 a, 623 nm for (S,S)-1 b, 605, 628 nm for (S,S)-1 c, 605 nm for (S,S)-1 d, and 632 nm for (S,S)-1 e).
It is noteworthy that the crystals of (R,R)/(S,S)-1 a and (R,S)-1 a showed bathochromic emission patterns that were different from that of enantiopure crystals (S,S)-1 a ( Figure S25). XRD analysis showed that the maximum emission wavelength (λ max ) in the crystalline state of complexes 1 was negatively correlated with the bowl angle ( Figure S26). Therefore, we assume that bowl-shaped structures exert a strong influence on the d-π conjugation of 1, resulting in a change in the emission color in the crystalline state.
The circularly polarized luminescence (ΔI) and total luminescence (I) spectra measured for (R,R)-and (S,S)-1 a-e in the PMMA film-dispersed state at 298 K are shown in Figures 6 and S27. The CPL spectra of (R,R)-and (S,S)-1 a-d exhibit mirrorimage positive and negative signals around 570-590 nm, which are corresponding to the emission maxima (λ max ) observed in their total emission spectra taken under the same measurement conditions ( Figure 6). The g lum values around the maximum emission wavelength are À 0.0013 (574 nm) for (S,S)-1 a, À 0.0011 (575 nm) for (S,S)-1 b, À 0.0036 (590 nm) for (S,S)-1 c, and À 0.0018 (575 nm) for (S,S)-1 d, respectively. It should be noted that (R,R)-and (S,S)-1 e did not exhibit detectable CPL signals ( Figure S27), which indicates that circularly polarized luminescence was induced by introduction of bulky substituents at the nitrogen atoms.

Theoretical Investigations
In order to clarify the structure-dependence of the photophysical properties of the series of platinum complexes 1 a-e, the optimized structures of (S,S)-1 a-e in an isolated system were estimated using DFT (B3LYP/6-31G*, LanL2DZ) calculations on the basis of the X-ray structures. The optimized structures of (S,S)-1 a-e are shown in Figure 7, where the chirality angles of square planar geometry O(1)À N(1)À O(2)À N(2) (ϕ) are 0.90°for 1 a, 0.78°for 1 b, 0.29°for 1 c, 1.39°for 1 d, and 0.20°for 1 e, indicating that the substituents on the nitrogen atoms distort the coordination geometry which induces Λ chirality. In addition, (S,S)-1 a-d have a bowl-configuration with similar bowl angles θ, while (S,S)-1 e has a step-configuration with a small θ value. These structural differences are expected to affect the chiroptical properties in solution and in the PMMA filmdispersed state.
The frontier orbitals of (S,S)-1 a-e and their eigenvalues were estimated by using DFT (B3LYP/6-31G*, LanL2DZ) calculations on the basis of the optimized structure in the ground state. The HOMOs of all complexes are principally Pt(d zx )-ligand (π) hybrids, whereas the LUMOs are in the ligand (π*) (Figure 8).
The energy levels and electronic configurations of the singlet and triplet states of these complexes were estimated    (Table S2). The major consequence of the electronic configuration of the S 1 (T 1 ) states is the HOMO-to-LUMO transition, which implies that the present emission is principally attributable to a mixture of MLCT/ILCT. The S 0 -T 1 transition energies were calculated to be 2.17 eV (570 nm) for 1 a, 2.18 eV (568 nm) for 1 b, 2.19 eV (567 nm) for 1 c, 2.21 eV (561 nm) for 1 d, and 2.14 eV (579 nm) for 1 e, which is consistent with the  hypsochromic and bathochromic shift of the emission peak maxima for 1 d and 1 e in CH 2 Cl 2 ( Figure 5a). We gained further insight into the origin of the structuredependent chiroptical responses observed for (S,S)-1 a-e using TD-DFT calculations (B3LYP/6-31*G, LanL2DZ). The validity of the calculation was confirmed by comparison between theoretical and experimental CD spectra of (S,S)-1 a-e (Figures 4a and  S28), where the substituent-dependent change in the experimental CD spectra was well reproduced by our theoretical simulations. The dissymmetry factors g are calculated with the equation g = 4(j μ e j j μ m j cosθ e,m )/(j μ e j 2 + j μ m j 2 ), where j μ e j , j μ m j , and θ e,m are the electric transition dipole moments, magnetic transition dipole moments, and the angles between two vectors μ e and μ m , respectively. [12,25] The values of j μ e j , j μ m j , and θ e,m of (S,S)-1 a-e were estimated in order to understand the origin of the structure-dependence of S 0 !S 1 transitions. Figure 9 shows the electric and magnetic dipole moments j μ e j and j μ m j , calculated for the S 0 !S 1 transition of (S,S)-1 a-e. The scalar value j μ m j of (S,S)-1 e is smaller than that of (S,S)-1 a-d, whereas the scalar values j μ e j are similar for all complexes, indicating that specific bowl-shaped structures cause amplification of the magnetic transition dipole moment which results in an enhancement of the g factor.

Conclusion
In summary, we experimentally and theoretically investigated the structure-dependent characteristics of photophysical properties in a series of chiral platinum(II) complexes. Complexes 1 a-e showed structure-dependent emission properties, where specifically the bowl-shaped structure influences d-π conjugation of the coordination platform resulting in a change of emission color over a range of approximately 40 nm in the crystalline state. More importantly, chiroptical responses (CD and CPL) of these complexes were induced by the specific bowl-shaped structures in diluted solution and in the PMMA film-dispersed state. DFT and TD-DFT calculations of the structures and electronic configurations of (S,S)-1 a-e revealed that the enhancement of the chiroptical responses is due to an increase in the magnetic transition dipole moment based on the distortion of the square planar geometry.

Experimental Section
General: Melting points were measured by a ATM-01 melting temperature measurement device (AS ONE Corporation). IR spectra were acquired with a JASCO FT/IR4100ST spectrometer. Highresolution mass spectrometry was recorded on ThermoQuest Finnigan TSQ 7000, Finnigan MAT 95, Agilent Q-TOF 6540 UHD, or Bruker micrOTOF II spectra. 1 H and 13 C NMR spectra were recorded on Bruker Avance 400 and Bruker Avance III 500 spectrometers, TMS was used as an internal standard. Elemental analyses were performed on a YANAKO MT-5 at A-Rabbit-Science Japan Co., Ltd. UV/Vis absorption spectra were obtained on a SHIMADZU UV-1900i spectrophotometer. CD spectra were recorded on a Jasco J-720 spectropolarimeter. Emission spectra were obtained on a FP-6500 spectrometer. CPL spectra were obtained at room temperature using a JASCO CPL-300 spectrofluoropolarimeter. Optical rotation was measured on a Jasco DIP-370 digital polarimeter.

X-Ray Crystallography:
Crystals suitable for XRD studies were analyzed using a Rigaku RAXIS-RAPID imaging plate diffractometer using Mo-K α radiation (graphite monochromated, λ = 0.71073 Å, fine focus tube, ω-scan) and a Rigaku XtaLAB mini2 benchtop X-ray crystallography system equipped with a Mo rotating-anode X-ray generator with monochromated Mo-K α radiation (λ = 0.71073 Å). The molecular structures and packings in crystals (S,S)-1 a-d, (R,R)-1 e, (R,R)/(S,S)-1 a, and (R,S)-1 a were solved by direct methods and refined using the full-matrix least-squares method. In subsequent refinements, the function Σω(F o 2 À F c 2 ) 2 was minimized, where F o and F c are the observed and calculated structure factor amplitudes, respectively. The positions of non-hydrogen atoms were found from difference Fourier electron density maps and refined anisotropically. All calculations were performed using the Crystal Structure crystallographic or CrysAlisPro program software package, and illustrations were drawn by using ORTEP.
Computational Methods: All calculations were carried out based on DFT with the B3LYP exchange-correlation functional, [27] using the Gaussian 16 W program package. [28] The basis set used was the effective core potential (LanL2DZ) for iodine and platinum atoms [29] and 6-31G* for the remaining atoms. [30] Molecular orbitals and their eigenvalues for 1 a-e were estimated using the optimized geometries determined by the DFT calculations using initial geometries obtained from XRD analysis. The singlet-singlet (E(S n )) and singlettriplet (E(T n )) transition energies were estimated by time-dependent (TD) DFT calculation (B3LYP/6-31G*, LanL2DZ). [31]