Change in Luminescence Induced by Solution-Mediated Phase

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Change in Luminescence Induced by Solution-Mediated PhaseTransition of Cyclometalated Platinum(II) Complex with Isoquinoline Carboxylate Keiji Ohno,† Mami Hasebe,† Akira Nagasawa,† and Takashi Fujihara*,‡ †

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Department of Chemistry, Graduate School of Science and Engineesring, Saitama University, 255 Shimo-Okubo, Sakuraku, Saitama 338-8570, Japan ‡ Comprehensive Analysis Center for Science, Saitama University, 255 Shimo-Okubo, Sakuraku, Saitama 338-8570, Japan S Supporting Information *

ABSTRACT: Changes in luminescence due to solution-mediated phase transition (SMPT) and crystal-to-crystal phase transitions of a novel cyclometalated platinum(II) complex with isoquinoline-1-carboxylate (Iq-1-COO−) [Pt(bzq)(Iq1-COO)] (bzq−: benzoquinolinate) were studied experimentally and theoretically. Recrystallization of the complex allowed three crystal forms depending on the solvent: red polymorph RDMF (λemi = 689 nm), yellow polymorph YDMSO (λemi = 641 nm), and red pseudopolymorph RBCHCl3 (λemi = 721 nm). Crystals of RDMF in the DMF solution at room temperature showed SMPT to yellow crystals YDMF (λemi = 627 nm), in which RDMF first dissolved partly into the solution, and then the dissolved complex crystallized as YDMF. RBCHCl3 exhibited crystal-to-crystal phase transitions to RDMF and YDMF by being heated to 150 °C and stored in atmospheric conditions, respectively. Molecular structures of each of the four crystal forms, analyzed by X-ray crystallography, showed different planarities because of the dihedral angles between the bzq− and Iq planes being 3.15, 4.80, 8.35, and 17.7° for RBCHCl3, RDMF, YDMSO, and YDMF, respectively. The planar complexes in RDMF and RBCHCl3 constructed dimers through intermolecular Pt−Pt and π−π interactions, whereas the distorted complexes in YDMF and YDMSO remained as monomers. Density functional theory (DFT) calculations, which reveal the relation between the molecular structure and thermodynamic stability, suggest that the SMPT is triggered by thermodynamic transformation from the metastable planar structure to the stable distorted structure. The intradimer interactions in RDMF and RBCHCl3 induced red-shifts in the absorption and emission colors from those of YDMF and YDMSO; thus, the photophysical properties of RBCHCl3 and RDMF originate from the MMLIqCT state in contrast with the MLIqCT/LbzqLIqCT character for YDMF and YDMSO. DFT and time-dependent DFT (TDDFT) calculations in the ground and excited states provide insight into the structural, electronic, and optical properties.

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INTRODUCTION Polymorphism of luminescent square-planar platinum(II) complexes with an aromatic ligand (e.g., oligo pyridine or cyclometalating ligand) have attracted much attention because of their multicolor emissions from one chemical species.1 In the solid state, the complexes were assembled to form a dimer and a one-dimensional (1D) chain structure through Pt−Pt and/or π−π interactions.1i,j,2 The Pt−Pt interaction resulted in the appearances of new absorption and emission bands originating from a metal−metal-to-ligand charge transfer (MMLCT) on the low-energy side of the monomeric absorption and emission bands assigned to the metal-to-ligand charge transfer (MLCT). The degrees of the red-shifts are influenced by the strength of the Pt−Pt interaction; thus, the stacking pattern is greatly responsible for the absorption and emission colors.1b,c,3 Recently, emission color changes in PtII complexes in response to external stimuli such as heat,4 mechanical f o r c e , 1 g , i , 4 a − c , 5 a n d vo l a t i l e o r g a n i c c o m p o u n d s (VOCs)1c,g,3e,4a,6 have been developed for application in high sensitivity sensors. These phenomena were based on changes in © 2017 American Chemical Society

the intermolecular interactions during crystal-to-crystal and crystal-to-amorphous phase transitions. In particular, the alternation of the Pt−Pt interaction in the ground state and formation of an excimer in the excited state are known to lead to a dramatic change in the emission color. Solution-mediated phase transition (SMPT) is an interesting behavior because the phase transition allows transformations of molecular and/or crystal structures from the first crystallized kinetically stable crystal form to the thermodynamically stable crystal form via a solution phase.7 The phase transition occurs in three steps. When kinetically stable crystals are put into their solution or into a solvent, the partial dissolution of the crystals causes supersaturation with respect to the stable crystal form. The supersaturation state leads to nucleation and growth of the stable crystal form. The phase transition is a significant and important phenomenon in the development and manufacture of drugs from the viewpoint of their physicochemical properties Received: June 9, 2017 Published: September 25, 2017 12158

DOI: 10.1021/acs.inorgchem.7b01466 Inorg. Chem. 2017, 56, 12158−12168

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Inorganic Chemistry

Figure 1. Structural transformation scheme. Dimers in RDMF and RBCHCl3 and packing structures in YDMF and YDMSO with thermal ellipsoids at the 50% probability level. The H atoms in the four crystal forms and the lattice solvent molecules in RBCHCl3 were omitted for clarity.

such as stability and solubility, depending on their structures.8 Although many investigations into the SMPT of organic compounds have been accomplished,7,8 to the best of our knowledge, research of SMPT with luminescent transitionmetal complexes currently remains undeveloped. Herein, we present the first example of luminescence change by SMPT of a novel mononuclear PtII complex [Pt(bzq)(Iq-1COO)] (bzq−: benzoquinolinate; Iq-1-COO−: isoquinoline-1carboxylate; Figure 1). The complex crystallizes into three crystal forms, RDMF, RBCHCl3, and YDMSO, depending on the recrystallization solvent, and RDMF in the DMF solution shows SMPT to YDMF. The crystals were characterized by X-ray crystallography and spectroscopy, and theoretical calculations of their molecular and packing structures were performed. The relations between the structures, their thermodynamic stability, and luminescent properties are presented and discussed herein, and the results exhibit important information for understanding the luminescence change by SMPT. Furthermore, emission color changes by crystal-to-crystal phase transitions upon heating and storing in atmospheric conditions are also reported.

Figure 2. UV−vis absorption (solid black line) and emission (λexi = 365 nm, dashed black line) spectra of [Pt(bzq)(Iq-1-COO)] in DMSO. The calculated stick spectrum is drawn according to the TDDFT.

RESULTS AND DISCUSSION Synthesis and Characterization. A red pristine powder of the complex [Pt(bzq)(Iq-1-COO)] was obtained by the reaction of the precursor PtII dimer [{PtII(bzq)}2(μ-Cl)2] and Iq-1-COOH in the presence of Na2CO3 in 2-ethoxyethanol. The 1H NMR in DMSO-d6 showed the existence of a 1:1 ratio of bzq− to Iq-1-COO− from the integral intensities of the signals. The UV−vis absorption spectrum in DMSO (a clear yellow solution) is presented in Figure 2. Intense absorption bands at high energy region (λ < 350 nm) are derived from ligandcentered π−π* transitions. For the absorption bands in the range of 360−500 nm, which were not observed in the ligand precursors, the absorptions at 373 and 439 nm are assigned to 1 (MLbzq)LIqCT and 1(MLbzq)LbzqCT (metal−ligand-to-ligand charge transfer) mixed transitions and to 1MLIqCT and 3 MLIqCT mixed transitions, respectively.9 These assignments are also supported by a time-dependent density functional theory (TD-DFT) study (vide infra). The solution with excitation at 365 nm showed yellow luminescence, and the emission spectrum displayed a vibronic

structured emission band with emission maxima at 540 and 581 nm (Figure 2). The quantum yield was 0.007. The luminescence may be derived from 3(MLbzq)LIqCT transition.9 DFT Calculation. A TD-DFT calculation based on the optimized geometrical parameters of [Pt(bzq)(Iq-1-COO)] in the ground state (S0) was performed, in which a polarized continuum model (PCM) for DMSO was used during the optimized process and TD-DFT calculation.10 The Cartesian coordinates of the geometrically optimized complex are given in Table S1. The energy levels and their relative compositions are listed in Table 1, and the selected dominant frontier molecular orbitals (MOs) are depicted in Figure 3A. The optimized structure showed a distorted form with the dihedral angle between bzq− and Iq planes being 20.23°, and the distortion may be caused by a steric repulsion between the bzq− and Iq-1-COO− ligands. The coordination bonds (Pt− Nbzq: 2.056 Å; Pt−Cbzq: 2.025 Å; Pt−NIq: 2.069 Å; Pt−O: 2.135 Å) are comparable to the corresponding bonds observed by X-ray crystallography (Table 4). The C−Pt−Nbzq and O− Pt−NIq chelate planes are nearly coplanar with the dihedral angle of 7.22°, in which the NIq atom lay at 0.26 Å out of

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HOMO−2 were localized mainly on the dZ2 orbital of the PtII center. The lowest unoccupied molecular orbital (LUMO) and the LUMO+1 mainly lie on the Iq-1-COO− and bzq− ligands, respectively. The HOMO and LUMO showed similar distributions to those of the previously reported cyclometalated complexes with picolinates as auxiliary ligand, and the complexes displayed the HOMO and LUMO distributions on the cyclometalating and auxiliary ligands, respectively.9 The calculated spectral pattern accords with the experimental spectrum (Figure 2), and the calculated electronic transitions and their energies are summarized in Table 2. The calculated

Table 1. Calculated Energy Level and Their Relative Compositions (Pt:Iq-1-COO−:bzq−) of the Optimized Structure in the S0 State orbital

energy level (eV)

relative compositions (Pt:Iq-1COO−:bzq−)

LUMO+2 LUMO+1 LUMO HOMO HOMO−1 HOMO−2 HOMO−3 HOMO−4

−1.215 −1.791 −2.256 −6.076 −6.675 −6.710 −6.610 −7.181

4:11:85 4:12:84 3:85:12 36:7:57 38:24:38 87:6:7 10:60:30 38:15:47

Table 2. Experimental and Calculated Absorption Maxima, Transition Energy, Oscillator Strength, and Major Contribution of Transitions for the Optimized Complex in the S0 State λexp (nm)

λcalcd (nm)

E (eV)

fcalcd

major contribution

439

420

2.95

0.0408

373

374

3.32

0.0429

351

3.52

0.171

302

4.10

0.218

HOMO → LUMO (68%) HOMO−4 → LUMO (10%) HOMO → LUMO (9%) HOMO → LUMO+1 (64%) HOMO−1 → LUMO (64%) HOMO−3 → LUMO (13%) HOMO−1 → LUMO+1 (47%)

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relative intense absorptions at 420, 374, 351, and 302 nm are derived from the principal excitation of HOMO → LUMO ((MLbzq)LIqCT), HOMO → LUMO+1 ((MLbzq)LbzqCT), HOMO−1 → LUMO ((MLIq)LIqCT and LbzqLIqCT), and HOMO−1 → LUMO+1 ((MLIq)LbzqCT and ILbzq), respectively. A DFT calculation of the complex in the lowest excited triplet state (T1) using the PCM for DMSO was carried out, in which the Cartesian coordinates of the geometrically optimized complex are listed in Table S2. The optimized structure in the T1 state showed a planarity lower than that in the S0 state because the dihedral angles between C−Pt−Nbzq and O−Pt− NIq planes and between bzq− and Iq planes in the T1 sate were 25.79 and 32.18°, respectively. Furthermore, departure of the coordinated O atom being 0.89 Å from PtCNbzqO coordination plane indicates a significant distorted square-planar geometry around the PtII center. The Pt−O bond being 2.036 Å and other coordination bonds (Pt−Nbzq: 2.092 Å; Pt−Cbzq: 1.991 Å; Pt−NIq: 2.028 Å) exhibited short and similar distances, respectively, compared to those of the corresponding bonds in the S0 state. The electrons of LUMO distributed mainly on the Iq-1-COO− ligand (91%) and those of HOMO lie mainly on the PtII center (46%) and bzq− ligand (38%), as drawn in Figure 3B. The spin density was distributed throughout the complex, as seen in Figure 3C, suggesting the luminescence originates from 3(MLbzq)LIqCT state. Crystallizations and SMPT. Cooling of a hot DMF solution of the red pristine powder to room temperature led to the recrystallization of red crystals RDMF with red luminescence suitable for X-ray crystallography. Solvent vapor diffusion into the complex solution produced red-brown crystals RBCHCl3 (emission color: red) and yellow crystals YDMSO (yellow); hexane diffused into the CHCl3 solution for RBCHCl3, and acetone diffused into the DMSO solution for YDMSO. RDMF in DMF at room temperature allowed SMPT to yellow crystals YDMF with yellow luminescence, and the photographs

Figure 3. Structures and MOs of the optimized [Pt(bzq)(Iq-1-COO)] (A) in the S0 state and (B) in the T1 state using the PCM for DMSO. (C) Spin density distribution of the optimized complex in the T1 state.

PtCNbzqO coordination plane. These dihedral angles are degrees similar to those of the complex in the crystal YDMF (vide infra). The highest occupied molecular orbital (HOMO) and HOMO−4 comprised dπ(Pt) and π(bzq−) orbitals with some contribution of π orbitals on the COO− moiety of the Iq-1COO− ligand. The electrons of HOMO−1 and HOMO−3 were distributed throughout the complex, and those of 12160


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reflections from that of RDMF appeared (10 min), and a S/N ratio of the reflection increased gradually with the passage of time (10 min∼). The newly appeared reflection pattern accords with the simulated pattern obtained by X-ray crystallography of YDMF. Reflection other than RDMF and YDMF was not observed, and an increase in the reflection intensity of YDMF showed a different rate from a decrease in that of RDMF, suggesting immediate dissolution of RDMF followed by gradual crystallization of YDMF, i.e., indirect crystal-to-crystal phase transition from RDMF to YDMF. Unfortunately, the solubility of RDMF in the solution of DMF could not be detected by the absorbance measurement of the saturated solution at various temperatures. X-ray Crystallography. The complexes of YDMF, YDMSO, RDMF, and RBCHCl3 crystallized in monoclinic P21/c, orthorhombic Pbca, monoclinic C2/c, and orthorhombic P21212 space groups, respectively. RDMF, YDMF, and YDMSO comprised only complex molecules, indicating polymorphs, whereas RBCHCl3 included two lattice solvents, CHCl3, per one complex, indicating a pseudopolymorph. The crystallographic data are summarized in Table 3, and the selected bond lengths for the complexes in the four crystal forms and hydrogen bonding parameters in RBCHCl3 are listed in Tables 4 and 5, respectively. The complexes of four crystal forms showed the same neutral composition (Figure 1), but the complexes in YDMF and YDMSO exhibited more distorted structures compared with those of RDMF and RBCHCl3, as presented in Figure 5. The PtII centers in all the complexes lay in a square-planar coordination geometry coordinated by C1N2O1 in the trans-(N,N)-configuration. The dihedral angles between C−Pt−Nbzq and O−Pt−NIq planes and between bzq− and Iq planes were 3.06 and 4.80° in RDMF, 3.26 and 3.15° in RBCHCl3, 2.60 and 8.35° in YDMSO, and 7.69 and 17.7° in YDMF, respectively. The dihedral angles of YDMF have degrees similar to those of the optimized structure in the S0 state calculated by DFT method with PCM solvation (vide supra), suggesting that the distorted structure in YDMF is a stable form compared to the others. For packing structures, the planar complexes in both RDMF and RBCHCl3 formed the dimers with adjacent complexes through Pt−Pt and π−π interactions (Figure 1); however, their fashions were different from each other (i.e., face-to-face arrangements of each of the bzq− ligands or Iq-1-COO− ligands in RDMF and bzq− and Iq-1-COO− ligands in RBCHCl3). The Pt···Pt separations of 3.21 and 3.25 Å inside the dimers in RDMF and RBCHCl3, respectively, are shorter than twice the van der Waals radius of Pt (3.50 Å), indicating effective metallophilic interactions. For the dimer of RDMF, the separations of 3.48 or 3.54 Å between each of the bzq− moieties or Iq moieties, respectively, suggest the presence of two types of π−π interactions between them. The dimer in RBCHCl3 showed π−π interaction between the bzq− and Iq moieties with the interplanar distance being 3.38 Å. In the crystal of RDMF, the dimers interacted with the adjacent four dimers through π−π interactions between the bzq− and Iq moieties, where the interplanar distance between them was 3.39 Å, as depicted in Figure 6A. The dimers in RBCHCl3 were stacked in a 1D column through Pt−π interaction (Figure 6B).11 The separation between the PtII center and the phenyl ring in bzq− of the adjacent dimer was 3.57 Å, proposing a weak but significant Pt−π interaction. These results suggest that the PtII centers interacted inside and

for the changes in crystal color and morphology from RDMF to YDMF in DMF at room temperature are displayed in Figure 4A.

Figure 4. (A) Photographs of SMPT from RDMF to YDMF in the saturated DMF solution at room temperature. (B) Changes in PXRD patterns during SMPT induced by dropping the saturated DMF solution over the crystalline powder of RDMF.

After crystallization of RDMF in the DMF solution (vide supra) or addition of RDMF into DMF, setting aside the solution, including the crystals at room temperature, led to the partial dissolution of RDMF and the subsequent crystallization and growth of YDMF. After 1 h, there was only YDMF in the solution. Below 0 °C, the phase transition was not detected for a few days because of the lack of dissolution of RDMF. These observations suggest that the phenomenon is first triggered by the dissolution of RDMF, and then the supersaturation state induces nucleation and growth of YDMF. The powder X-ray diffraction (PXRD) analysis of RDMF in the saturated DMF solution exhibited the crystal-to-crystal phase transition from RDMF to YDMF via solution state. The powder sample of RDMF (0 min in Figure 4B) showed a reflection pattern similar to the simulated pattern obtained by X-ray crystallography of RDMF. Upon dropping of the saturated DMF solution over the powder sample, a signal-to-noise (S/N) ratio of the reflections decreased immediately (3 min), indicating a disappearance of the RDMF. Subsequently, distinct 12161


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Inorganic Chemistry Table 3. Summary of Crystallographic Data for YDMF, YDMSO, RDMF, and RBCHCl3 deposit number formula formula weight temperature (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρ (cal.) (g cm−3) μ (mm−1) F(000) reflections collected independent reflections R1 [I > 2σ(I)] wR2 (all) GOF

YDMF

YDMSO

RDMF

RBCHCl3

1555031 C23H14N2O2Pt 545.45 200(2) monoclinic P21/c 13.1564(12) 8.8638(8) 15.2038(13) 90.00 110.1770(10) 90.00 1664.2(3) 4 2.177 8.455 1040 18072 3671 [R(int) = 0.0191] 0.0142 0.0349 1.038

1555033 C23H14N2O2Pt 545.45 200(2) orthorhombic Pbca 14.853(3) 8.4207(15) 27.275(5) 90.00 90.00 90.00 3411.3(11) 8 2.124 8.250 2080 35823 3753 [R(int) = 0.0332] 0.0217 0.0414 1.033

1555032 C23H14N2O2Pt 545.45 200(2) monoclinic C2/c 14.0419(13) 16.0765(15) 15.8554(14) 90.00 107.4730(10) 90.00 3414.1(5) 8 2.122 8.243 2080 18713 3766 [R(int) = 0.0203] 0.0154 0.0361 1.067

1555030 C25H16Cl6N2O2Pt 784.19 200(2) orthorhombic P21212 26.973(4) 7.9391(10) 11.8943(15) 90.00 90.00 90.00 2547.1(6) 4 2.045 6.167 1504 28658 5836 [R(int) = 0.0304] 0.0223 0.0494 1.054

Table 4. Selected Bond Lengths (Å) for YDMF, YDMSO, RDMF, and RBCHCl3 C1−O1 C1−O2 C21−Pt1 N1−Pt1 N2−Pt1 O1−Pt1 Pt1−Pt1 a

YDMF

YDMSO

RDMF

RBCHCl3

1.292(3) 1.221(3) 2.006(3) 2.028(2) 2.018(2) 2.0756(17)

1.286(3) 1.221(3) 2.018(3) 2.031(2) 2.007(2) 2.061(2)

1.287(3) 1.219(3) 2.021(3) 2.025(2) 2.004(2) 2.0770(19) 3.2106(4)a

1.277(7) 1.240(7) 2.012(5) 2.036(4) 2.007(4) 2.087(4) 3.2532(5)b

2 − x, y, 0.5 − z. b−x, −y, +z. Figure 5. Superimposition of the crystallographic geometry for RBCHCl3 (red), YDMSO (green), and YDMF (blue) compared with RDMF (orange).

Table 5. Hydrogen Bonding Parameters for RBCHCl3 D−H···A (Å)

D−H (Å)

H···A (Å)

D···A (Å)

D−H−A (deg)

C(25)−H(25)···O(1) C(25)−H(25)···O(2) C(24)−H(24)···O(2)a

1.00 1.00 1.00

2.41 2.40 2.36

3.382(7) 3.208(7) 3.336(8)

165.3 137.3 165.6

a

Iq moieties were 3.39 or 3.31 Å, respectively, suggesting the presence of π−π interactions. Structures and Their Stability. To reveal the relation between the planarity of the complex and the thermodynamically stability, DFT calculations for the complex molecules (monomeric complexes) among the four crystal forms and the dimers in RDMF and RBCHCl3 were performed.10 Their structural parameters were extracted from the corresponding X-ray crystallographic data, and the positions of the hydrogen atoms were optimized, while the positions of all other atoms were fixed to the original positions. The Cartesian coordinates of the geometrically optimized structures are given in Tables S3−S8. The dominant frontier MOs of the four monomeric complexes exhibited electron distributions similar to those of the optimized complex in the S0 state using the PCM solvation, as seen in Figure 7. The energy levels and their relative compositions are summarized in Table 6. The dimers in RDMF and RBCHCl3 displayed the MOs for the dz2 σ* Pt−Pt

x, y + 1, z.

outside of the dimer through Pt−Pt and Pt−π interactions, respectively. The lattice solvent molecules interacted with the COO− moieties of the complexes through hydrogen bonding (Table 5). Crystal structures of YDMSO and YDMF exhibited 1D column and double-decker arrangement, respectively, formed through π−π interactions but no Pt−Pt interaction, as shown in Figures 6C and D, respectively. The complexes in YDMSO were stacked perpendicularly to the complex plane through π−π interactions between the Iq and bzq− moieties with an interplanar distance of 3.41 Å. The complexes in YDMF were aligned horizontally in an alternately overturned manner in a head-to-tail fashion, and the interplanar distances between each of the bzq− moieties or 12162


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antibonding orbital on HOMO and dz2 σ Pt−Pt bonding orbital on HOMO−15, which were split by the overlap of dz2occupied orbitals between the adjacent two complexes, indicating the formation of dimers through Pt−Pt interaction. Both HOMO−1 and HOMO−2 of the dimers comprised dπ(Pt) and π(bzq−) orbitals with some contribution of π orbitals on the COO− moieties of the Iq-1-COO− ligands, and both LUMO and LUMO+1 were mainly localized on the Iq-1COO− ligands. The relation between the monomeric structure and thermodynamic stability reveals that lowering the planarity throughout the complex enhances the thermodynamic stability because their calculated sum of electronic and thermal free energies were in the following sequence: YDMF (−3 315 953.11 kJ mol−1) < Y DMSO (−3 315 937.98 kJ mol−1 ) < R DMF (−3 315 926.05 kJ mol−1) < RBCHCl3 (−3 315 923.84 kJ mol−1). The plot of the energy difference (Δ, in kJ mol−1) from RDMF versus the dihedral angle between intramolecular bzq− and Iq planes displays a decrease in the energy as an increase in the dihedral angle (Figure 8). The relation indicates that the low energies of the complexes in both YDMF and YDMSO are caused by a distortion between the bzq− and Iq planes, whereas the high planarity structures in both RDMF and RBCHCl3 may be retained by stabilization originating from the formation of the dimer through Pt−Pt interactions. Furthermore, the results suggest that the SMPT is induced by structural transformation from the thermodynamically unstable planar

Figure 6. Packing structures for (A) RDMF, (B) RBCHCl3, (C) YDMSO, and (D) YDMF.

Figure 7. Dominant frontier MOs of the monomeric complexes in YDMF, YDMSO, RDMF, and RBCHCl3 and the dimers in RBCHCl3 and RDMF. 12163


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Table 6. Calculated Energy Level (eV) and Their Relative Compositions (Pt:Iq-1-COO−:bzq−) of Monomeric Complexes in YDMF, YDMSO, RBCHCl3, and RDMF and Dimers in RBCHCl3 and RDMF monomeric complex orbital LUMO + 2 LUMO + 1 LUMO HOMO HOMO − 1 HOMO − 2 HOMO − 3 HOMO − 4

dimer

YDMF

YDMSO

RBCHCl3

RDMF

RBCHCl3

RDMF

−1.099 3:48:49 −1.695 4:14:82 −2.164 3:83:14 −5.798 40:11:49 −6.512 44:18:38 −6.530 84:8:8 −6.788 15:74:11 −6.849 20:53:27

−1.064 2:48:50 −1.666 4:12:84 −2.186 3:69:28 −5.833 41:11:48 −6.492 43:13:44 −6.545 86:7:7 −6.789 16:71:13 −6.838 22:52:26

−1.088 3:14:83 −1.668 4:12:84 −2.192 3:84:13 −5.869 40:10:50 −6.515 43:14:43 −6.566 75:9:16 −6.705 23:73:4 −6.842 19:53:28

−1.062 2:68:31 −1.667 4:17:79 −2.157 3:80:17 −5.863 41:48:11 −6.498 46:12:42 −6.562 85:6:9 −6.806 18:77:5 −6.866 20:50:30

−1.516 9:15:76 −1.986 6:89:5 −2.220 5:75:20 −5.510 87:4:9 −5.594 48:10:41 −5.810 40:12:48 −6.346 36:14:50 −6.413 54:12:34 −7.326 91:2:7

−1.636 13:20:67 −1.803 3:86:11 −2.139 5:73:22 −5.405 87:4:9 −5.687 48:11:41 −5.756 42:11:47 −6.307 52:19:29 −6.416 46:15:39 −7.301 89:3:8

HOMO − 15

Figure 8. Plot of the energy difference (E − ERDMF, kJ mol−1) versus the corresponding dihedral angle (°) between the bzq− and Iq planes.

structure in RDMF to the thermodynamically stable distorted structure in YDMF. Photophysical Properties. The UV−vis absorption spectra of YDMF, YDMSO, RDMF, and RBCHCl3 reveal the presences of slight intermolecular interactions in YDMF and YDMSO and significant metallophilic interactions in RDMF and RBCHCl3, as shown in Figure 9A. The spectrum of YDMF displayed a pattern similar to that of YDMSO, and a comparison of these spectra with that of the complex in solution state suggests a slight influence of intermolecular interactions in YDMF and YDMSO on the electronic state in the ground state. Absorption bands of RDMF and RBCHCl3 appeared in the low-energy region compared with YDMF and YDMSO, and the red-shifts originate from the formation of dimers through Pt−Pt interactions. The lowenergy absorption of RBCHCl3 compared to that of RDMF may be derived from the Pt−π interaction. These results suggest that the absorption bands in the low-energy region can be attributed to (MLbzq)LIqCT transition for YDMF and YDMSO and to MMLIqCT transition for RDMF and RBCHCl3.9

Figure 9. (A) UV−vis absorption and (B) emission (λexi = 345 nm) spectra for the crystals of YDMF, YDMSO, RDMF, and RBCHCl3.

The luminescent properties of the four crystal forms depend on the molecular arrangement and/or hydrogen bonding between complex and lattice solvents. The emission spectra with excitation at 345 nm displayed broad bands centered at 627, 641, 689, and 721 nm for YDMF, YDMSO, RDMF, and RBCHCl3, respectively, (Figure 9B). The trend of the red-shifts in emission bands is coincident with that in the absorption properties, indicating that the luminescence is controlled by the 12164


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Inorganic Chemistry molecular arrangement in the ground state. In fact, the intradimer Pt−Pt interactions in RDMF and RBCHCl3 triggered the low-energy emissions compared with YDMF and YDMSO, and the Pt−π interaction in RBCHCl3 led to a red-shift from the emission of RDMF. The emission lifetimes in the range of 0.18− 0.40 μs indicate a phosphorescent character for these emissions. These results suggest that the luminescence is attributed to the 3 (MLbzq)LIqCT transition for YDMF and YDMSO and the 3 MMLIqCT transition for RDMF and RBCHCl3.2k,3 High quantum yield of RDMF (ΦPL = 0.24) compared with YDMF (0.04) and YDMSO (0.02) may originate from an inhibition of vibrational quenching by the formation of the rigid dimer. Although the complexes in RBCHCl3 constructed the dimer, the crystals showed a low quantum yield (ΦPL = 0.02) compared with that of RDMF probably because of vibrational quenching by hydrogen bonding between the dimer and lattice solvent molecules. Crystal-to-Crystal Phase Transition. The crystalline powders of RBCHCl3 exhibited irreversible crystal-to-crystal phase transitions by heating or storing in atmospheric conditions to crystalline powders of RDMF or YDMF, respectively. Heating of RBCHCl3 at 150 °C changed the absorption and emission colors to bright red with an increase in emission intensity, whereas storing of RBCHCl3 in atmospheric conditions at room temperature for two months varied the colors to yellow. The PXRD analyses of the heated and stored samples presented almost identical patterns to the simulated reflection patterns obtained by the X-ray crystallography of RDMF or YDMF, respectively, as shown in Figures 10A and B, respectively. These results indicated that the crystal-to-crystal phase transitions were induced by desorption of the lattice solvent molecules. Vapor diffusion of CHCl3 to the crystalline powders of RDMF and YDMF exhibited no color changes. However, solvent evaporation from the CHCl3 solution of the complex under reduced pressure provided red-brown powders, and the PXRD pattern of the powder is coincident with the simulated pattern obtained by the X-ray crystallography of RBCHCl3 (Figure 10C), suggesting the crystallization of RBCHCl3. Thermogravimetric analysis of the recovered RBCHCl3 crystalline powders showed a weight loss of 28% until 130 °C (Figure 11), and the decrease is derived from sequential loss of about two CHCl3 molecules per one dinuclear complex. The strong hydrogen bonding between the complex and CHCl3 prevents desorption of the lattice CHCl3 molecules in RBCHCl3 at room temperature for two months.

Figure 10. Experimental PXRD patterns (bottom) and simulated reflection patterns obtained by X-ray crystallography (top). (A) Bottom: RDMF obtained by heating of RBCHCl3; top: the single crystals of RDMF. (B) Bottom: YDMF obtained by storing of RBCHCl3 under atmospheric conditions; top: the single crystals of YDMF. (C) Bottom: RBCHCl3 obtained by evaporation of CHCl3 solution of YDMF; top: the single crystal of RBCHCl3.

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CONCLUSION We presented four crystal forms of a novel cyclometalated PtII complex with isoquinoline carboxylate [Pt(bzq)(Iq-1-COO)] and found that one of the crystal forms shows luminescence change by SMPT. Cooling the hot DMF solution of the complex led to crystallization of the kinetically stable RDMF with red luminescence, and storing RDMF in the DMF solution induced changes in its color and morphology by SMPT to the yellow luminescent thermodynamically stable polymorph YDMF. Vapor diffusion of a poor solvent into the DMSO or CHCl3 solutions of the complex rendered the yellow luminescent polymorphs YDMSO and the red luminescent pseudopolymorphs RBCHCl3, respectively. Irreversible crystal-to-crystal phase

Figure 11. Thermogravimetric analysis of the recovered RBCHCl3 crystalline powder.

transitions from RBCHCl3 to RDMF or YDMF were triggered by desorption of the lattice solvent molecules. X-ray crystallography of the four crystal forms and our theoretical calculations reveal that thermodynamically unstable planar complexes in RBCHCl3 and RDMF constructed the dimers through Pt−Pt and 12165


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Inorganic Chemistry π−π interactions, whereas thermodynamically stable distorted complexes in YDMF and YDMSO existed as monomers. Thus, SMPT is triggered by structural transformation from thermodynamically unstable structure in RDMF to thermodynamically stable structure in YDMF via the solution state. The absorption and emission colors of four crystal forms depend on the molecular and crystal structures, and the metallophilic interactions in RBCHCl3 and RDMF led to appearances of the absorption and emission bands in low-energy side of the monomeric bands of YDMF and YDMSO.

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The frontier MOs and spin density distributions of the optimized structures were visualized using GaussView 5.0. Synthesis. The platinum(II) dichlorido-bridged dimer [{PtII(bzq)}2(μ-Cl)2] (0.12 g, 0.15 mmol), isoquinoline-1-carboxylic acid (0.065 g, 0.38 mmol), and Na2CO3 (0.040 g, 0.38 mmol) in 2ethoxyethanol (10 mL) were heated to 70 °C for 10 h. After cooling to room temperature, H2O was added into the reaction solution. The precipitated red powder was filtered out, and then the powder was washed with water and diethyl ether and dried in a vacuum. Yield: 0.11 g (66%). 1H NMR (400 MHz, DMSO-d6): δ 9.92 (d, J = 8.6 Hz, 1H), 9.31 (d, J = 6.5 Hz, 1H), 9.17 (d, J = 5.3 Hz, 1H), 8.69 (d, J = 7.7 Hz, 1H), 8.37 (d, J = 6.5 Hz, 1H), 8.22 (d, J = 8.1 Hz, 1H), 8.04 (t, J = 7.7 Hz, 1H), 7.93−7.87 (m, 3H), 7.82−7.74 (m, 3H), 7.65 (t, J = 7.4 Hz, 1H). UV−vis (DMSO, 1.4 × 10−4 mol/L) at room temperature, λmax nm (ε/M−1cm−1): 270 (19 000), 308 (15 000), 373 (10 000), 439 (1700). Anal. Calcd for C23H14N2O2Pt: C, 50.65; H, 2.59; N, 5.14. Found: C, 50.26; H, 2.47; N, 4.84. Crystallization. Polymorph RDMF. The red pristine powder obtained by the synthesis of the complex was dissolved in hot DMF, and the yellow DMF solution was cooled to room temperature. The precipitated red crystals with intense red luminescence were collected. Polymorph YDMF. After recrystallization to obtain the single crystals of RDMF, setting aside the solution including the crystals at room temperature led to the crystallization of the yellow single crystals of YDMF (emission color: yellow) by solution-mediated phase transition. Pseudopolymorph RBCHCl3. The solvent vapor diffusion of hexane into the yellow CHCl3 solution of the complex rendered the red brown single crystals of RBCHCl3 with red luminescence. Polymorph YDMSO. The solvent vapor diffusion of acetone into the yellow DMSO solution of the complex allowed a crystallization of the yellow single crystals of YDMSO with yellow luminescence.

EXPERIMENTAL SECTION

General Procedures. The precursor PtII dichloridao-bridged dimer [{PtII(bzq)}2(μ-Cl)2] and the complex were synthesized according to previous literature.12 The DMSO solvent to measure the emission properties was freshly distilled and deaerated by bubbling argon for at least 15 min. Other commercially available reagents were purchased and used without further purification. The 1H NMR spectrum was recorded on a Bruker Avance-400 spectrometer with an internal reference of (CH3)4Si (δ = 0) in DMSO-d6. The UV−vis and emission spectra were recorded using Jasco V-530 and FP-6600 spectrometers, respectively. The luminescence quantum efficiency was measured using a Hamamatsu C9920-02 absolute photoluminescence quantum yield measurement system equipped with integrating sphere apparatus and a 150 W CW xenon light source. The luminescence lifetimes were recorded using a Hamamatsu C11367-04 fluorescence lifetime measurement system with an LED laser at 365 nm excitation. The powder X-ray diffractions (PXRD) were obtained using a Bruker AXS D2 Phaser and D8 Advance. The thermogravimetric analysis was performed with SII Exstar TG/DTA 6200. Elemental analysis was performed using Fisons EA 1112 and EA 1108 instruments at the Comprehensive Analysis Center for Science, Saitama University, Japan. X-ray Crystallography. Single crystal X-ray data were collected using MoK α (λ = 0.71073 Å) radiation on a Bruker Apex II Ultra diffractometer equipped with a CCD area detector. Indexing was performed using APEX2.13 Data integration and reduction were performed using SAINT.13 Absorption corrections were performed by a multiscan method implemented in SADABS.13 Space groups were determined using XPREP implemented in APEX2. All structures were solved using the direct method and refined using SHELXL-2016 (fullmatrix least-squares on F2) in APEX2.14 All nonhydrogen atoms for all complexes were refined anisotropically. Hydrogen atoms were refined using a riding model for all the complexes. Molecular graphics were performed with ORTEP-3 for Windows15 and Mercury CSD 3.9.16 Crystallographic information files are also available from the Cambridge Crystallographic Data Centre (CCDC nos.: 1555031 (YDMF), 1555033 (YDMSO), 1555032 (RDMF), and 1555030 (RBCHCl3)). Theoretical Calculation Study. Quantum mechanical calculations were performed using a Gaussian 09W program suit.10 Geometry optimizations and TD-DFT calculations with the PCM solvation model for the complex in the ground state (S0) and the lowest excited triplet state (T1) and geometry optimizations and of single point energy calculations in gas phase for the monomeric and/or dimeric structure(s) in each of four crystal forms were conducted using a DFT method with RM06 and unrestricted M06 (UM06) hybrid exchange correlation function implemented in the Gaussian suite of the program.17 For calculations in the gas phase, the molecular structures were extracted from the corresponding crystallographic data, and the positions of the hydrogen atoms were optimized; whereas, the remaining atoms were retained in the original positions. These calculations were followed by frequency calculations to verify that the obtained stationary points were the true energy minima. The 6-31G* basis set was used for all atoms except for the Pt atom,18 which was treated with a Stuttgart−Dresden basis set comprising the effective core potentials.19 The PCM20 was used to mimic the influence of the DMSO solvent for the calculation of complex in the solution state.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01466. Cartesian coordinates for the optimized geometries (PDF) Accession Codes

CCDC 1555030−1555033 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Takashi Fujihara: 0000-0003-2404-609X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Masa-aki Haga and Dr. Hiroaki Ozawa (Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University) for measurement of emission lifetime. The authors express their heartfelt thanks to Profs. Takuji Hirose and Koichi Kodama and Dr. Hiroaki Shitara (Department of Applied Chemistry, Saitama University) for measurement of UV−vis absorption spectra in the solid state. 12166


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