Phosphorescent Molecules That Resist Concentration Quenching in

Jun 24, 2019 - Figure 8. Schematic representation of the heterochiral, cofacial .... 0.92 (t, J = 7.2 Hz, 6 H, CH3), 1.30–1.46 (m, 8 H), 1.85 (tt, J...
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Article Cite This: Inorg. Chem. 2019, 58, 9076−9084

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Phosphorescent Molecules That Resist Concentration Quenching in the Solution State: Concentration-Driven Emission Enhancement of Vaulted trans-Bis[2-(iminomethyl)imidazolato]platinum(II) Complexes Ngoc Ha-Thu Le,† Ryo Inoue,† Soichiro Kawamorita,† Naruyoshi Komiya,†,‡ and Takeshi Naota*,†

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Department of Chemistry, Graduate School of Engineering Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-8531, Japan ‡ Chemistry Laboratory, The Jikei University School of Medicine, Kokuryo, Chofu, Tokyo 182-8570, Japan S Supporting Information *

ABSTRACT: In this paper, we describe the first phosphorescent molecules that do not exhibit the concentration quenching in the homogeneous solution state throughout the entire range of concentrations. A series of newly designed polymethylene-vaulted trans-bis[2(iminomethyl)imidazolato]platinum(II) complexes (1a, n = 10; 1b, n = 12; 1c, n = 14) was prepared by treating [PtCl2(CH3CN)2] with the corresponding N,N′-bis[(1H-imidazol-2-yl)methylene]-1,ω-alkanediamines. The trans coordination and vaulted structures of 1 have been unequivocally established from X-ray diffraction studies. When the concentration of a clear homogeneous solution of 1a−c in organic solvents increases from the diluted to the saturated state, the emission intensity and quantum efficiency increase continuously without concentration quenching at ambient temperature. This is in contrast to the emission profiles of other analogues 2−4 and typical AIEgens, which show ordinary concentration quenching under the same measurement conditions. The present concentration-driven emission enhancement is observed more intensely in a solution of a racemic mixture of 1 in comparison to that of the optically pure solution. Kinetic studies, 1H NMR, XRD analyses, and DFT calculations revealed that this specifically intense emission enhancement of 1 is attributed to an increase in the contribution of a 3MMLCT to 1GS transition, which is caused by the specific ability for the formation of a cofacial association dimer of 1.

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INTRODUCTION Emission enhancement in condensed states is an important subject, not only for the development of future functional lightemitting materials and devices with high luminance1−3 but also in regard to fundamental research on aggregation-induced emission enhancement (AIEE).4 All of the previous works on fluorescent5−7 and phosphorescent8−10 AIEE were successfully conducted by the controlled fabrication of higher-ordered aggregates such as crystals,5,8 nanoparticles,6,9 and gels7,10 formed under various conditions, where the dispersion of excited energy in a phosphor unit that occurs in the solution state is suppressed by the restriction of molecular mobility upon intermolecular congestions in condensed states.4 Highdensity integration upon an increase in the concentration of the solution is a straightforward concept to enhance the solution-state emission simply because the assumption that increasing phosphor units leads to an increase of the photoexcited species; however, this has long been considered to be impossible in the clear homogeneous solution state due to inevitable concentration quenching caused by selfquenching11 or aggregation-caused quenching (ACQ),4,12 © 2019 American Chemical Society

which are attributable to energy loss through nonradiative pathways with intermolecular connections. Although there are a few reports on slight emission enhancement of fluorescent13 and phosphorescent14,15 compounds with an increase in concentration in an extraordinarily low range of concentration (10−4−10−6 M), these complexes encounter steep emission quenching just over the turning points of the lower concentrations without any exception in the previous reports13,14 and our own confirmation (Figures S1−S4).15 Representative AIEE motifs with high solvophobic responsiveness5b,6a have also been confirmed to exhibit typical concentration quenching profiles in homogeneous solutions upon an increase of the concentration over a very wide range (Figures S5−S7). Thus, it is reasonable that the concentration quenching of luminescence molecules still remains to be ascertained and there is no clear guiding index for molecular design to resist such concentration quenching, despite the long history of research on solution-state emission. Received: March 1, 2019 Published: June 24, 2019 9076

DOI: 10.1021/acs.inorgchem.9b00608 Inorg. Chem. 2019, 58, 9076−9084

Article

Inorganic Chemistry In the present work, we report the first phosphorescent molecules that resist concentration quenching in the solution state. A planar chiral, polymethylene-bridged trans form of divalent platinum complex 1 exhibits significant emission enhancement in the solution state upon an increase of concentration within the full concentration range without any decreased profile of concentration quenching. The present concentration-driven emission enhancement is unprecedented and was observed more intensely in a solution of a racemic mixture of 1 in comparison to that of the optically pure solution, which is also a rare case of chirality-induced emission enhancement.10b

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RESULTS AND DISCUSSION Synthesis and Structure of Vaulted Platinum Complexes. A series of platinum complexes 1a−c (1a, n = 10; 1b, n = 12; 1c, n = 14, Scheme 1) were prepared by the reaction of Scheme 1. Chemical Structures of 1−4 Figure 1. Molecular structures of (a) (R)-1a, (b) (R)-1b, and (c) (R)-1c as their racemic crystals and (d) 2. Thermal ellipsoids are shown at the 50% probability level. The C(2)−Pt(1)−C(6) angles and N(1)−Pt(1)−N(3)−C(4)/N(4)−Pt(1)−N(6)−C(8) dihedral angles (in parentheses) are given under each structure.

Figure 2. Packing of (±)-1b crystals. S and R units are denoted as blue and red molecules, respectively.

PtCl2(CH3CN)2 with the corresponding N,N′-bis[(1H-imidazol-2-yl)methylene]-1,ω-alkanediamine in boiling dimethyl sulfoxide (DMSO) and toluene (Figures S8−S10 and S12− S14). The optically pure (100% ee) complexes (+)-1a, (−)-1a, (+)-1b, (−)-1b, (+)-1c, and (−)-1c were obtained from the racemic mixture using a recycling preparative HPLC system. The trans coordination and the vaulted structures of 1 have been unequivocally established by single-crystal X-ray diffraction (XRD) analysis of racemic crystals (±)-1a−c (Table S1 in the Supporting Information). ORTEP drawings of vaulted 1a−c and nonvaulted analogue 2 (prepared for comparison; see the Experimental Section and Figures S11 and S15) are presented in Figure 1, in which the C(2)−Pt(1)− C(6) angles and N(1)−Pt(1)−N(3)−C(4)/N(4)−Pt(1)− N(6)−C(8) dihedral angles are listed to express a macrocyclic view of the coordination planarity for the trans-bis[2(iminomethyl)imidazolato]platinum moieties. The planarity of the coordination blades of 1a−c is slightly lower than that of nonvaulted 2. Figure 2 and Figure S16 shows packing in racemic crystal (±)-1b, which indicated that the heterochiral cofacial association dimer units with a Pt−Pt contact of 3.37 Å are aligned regularly in a layer by layer manner in the crystal. Specific Concentration Dependence in Emission Properties. A clear homogeneous solution of complexes 1a−c in organic solvents such as CHCl3, 2-methyltetrahydrofuran, and MeOH exhibits phosphorescence under UV

irradiation at ambient temperature. Upon an increase in the solution concentration from diluted to the saturated state, the emission intensity of these solutions increases continuously without concentration quenching under the same measurement conditions. A diluted solution (0.2 mM) of racemic (±)-1b in CDCl3 typically shows very weak orange emission under UV irradiation at 298 K, while the saturated solution is highly emissive, as shown in Figure 3a. Dynamic light scattering (DLS) analysis indicated that solutions of (±)-1b in CHCl3 do not form any aggregates throughout the entire range of concentrations (0.2−60 mM), while those of complex 2 contrastingly generate nanoparticles with average diameters of 290 nm under the same conditions (Figure S17). The absence of aggregates in the homogeneous solution of (±)-1b can also be visualized by the absence of Tyndall light scattering upon irradiation of the diluted and concentrated solutions with a laser beam (Figure S18). Filtration of a 60 mM solution of (±)-1b in CHCl3 through a microporous membrane of poly(tetrafluoroethylene) (pore size 0.1 μm) also had no effect on the emission intensity (Figure S19). These results indicate that this emission enhancement is not induced by the formation of large aggregates such as microcrystals, particles, and fibers. When the concentration exceeds 62 mM (maximum concentration), highly emissive microcrystals precipitated, 9077

DOI: 10.1021/acs.inorgchem.9b00608 Inorg. Chem. 2019, 58, 9076−9084

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entire range of concentrations (0.2−62 mM). The same concentration-driven emission enhancement was observed exclusively in CHCl3 solutions of analogues (±)-1a and (±)-1c throughout the entire range of concentrations (0.2−60 mM for (±)-1a, 0.2−65 mM for (±)-1c), while other analogues such as nonvaulted 2 (Figure 3b) and vaulted β(iminomethyl)pyrrolato and β-(iminomethyl)pyrazolato complexes (±)-3 and (±)-4 (Scheme 1) exhibited typical concentration quenching. Such a contrasting concentration dependence is presented by the emission spectra of complexes 1−4 in CHCl3 under UV excitation at 298 K, as shown in Figure 4. Complexes 1a−c exhibit significant consecutive increases in emission intensity with an increase in the concentration from extremely low (0.2 mM) to their maxima (60−65 mM) (Figure 4a−f). This is in contrast to the profiles of analogues 2−4 bearing nonvaulted imidazolato, vaulted pyrrolato, and vaulted pyrazolato platforms, which exhibit typical continuous decreases in emission intensity with an increase in concentration (Figure 4g−j). The emission intensities of racemic (±)-1b are notably enhanced much more intensely in comparison to those of optically pure (−)-1b (Figure 4c,d). Similar chirality-induced emission

Figure 3. Contrast of the concentration dependence in emission properties for (a) concentration-enhancing (±)-1b and (b) concentration-quenchable 2 in CDCl3 at 298 K under UV illumination (λex 302 nm).

which indicates that complex (±)-1b does not exhibit concentration quenching in the solution state throughout the

Figure 4. Concentration dependence of the emission spectra for (a) (±)-1a, (b) (−)-1a, (c) (±)-1b, (d) (−)-1b, (e) (±)-1c, (f) (−)-1c, (g) (±)-3, (h) (−)-3, (i) (±)-4 and (j) 2 in CHCl3 at 298 K (concentration 0.2−60 mM (saturated solution) for (±)-1a and (−)-1a, 0.2−62 mM (saturated solution) for (±)-1b, 0.2−65 mM (saturated solution) for (−)-1b, (±)-1c, and (−)-1c, 0.2−60 mM for 2−4; λex 430 nm (1a−c, 2, and 4), 420 nm (3)). The insets show plots of concentration versus intensity of the emission maxima ((a−f) 549−586 nm, (g, h) 550 nm, (i) 537 nm, (j) 550−565 nm). Emission spectra of all samples were obtained under the same measurement conditions. 9078

DOI: 10.1021/acs.inorgchem.9b00608 Inorg. Chem. 2019, 58, 9076−9084

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(Figures S22−S26) fit well with a single-exponential relationship of I = Aexp(−t/τ) + I0 throughout the lower range of concentration. The concentration dependence of the observed lifetimes (τobs) for (±)-1b, (−)-1b, 2, (±)-3, and (±)-4 in CHCl3 are shown in Figure 6 and Figures S27−S29. It is

enhancements were observed for 1a (Figure 4a,b) and 1c (Figure 4e,f), although the chirality effect in 1a and 1c is much weaker than that in 1b. Typical AIEE motifs such as tetraphenylethene5b and hexaphenylsilole6a in tetrahydrofuran are entirely nonemissive over a wide range of concentration (Figure S5)5b or exhibit typical concentration quenching profiles with very weak emission (Figures S6 and S7).6a Highly diluted solutions of 1a−c (0.2 mM) exhibit sharp emission bands (λmax 549 nm) with typical vibrational structures, which can be attributed to 3MLCT (metal to ligand charge transfer)/3ILCT (intraligand charge transfer) transition, discussed later. The emission intensities of the 3 MLCT/3ILCT bands for 1a−c increased with an increase in the concentration in the lower range of 0.2−2.5 mM, after which the intensities of another broad band (λmax 586 nm), attributable to the 3MMLCT (metal−metal to ligand charge transfer) transition of the cofacial association dimer (Figure 2), increased continuously without quenching until the maximum concentration (Figure 4a−f). Complexes 2−4 exhibit typical concentration quenching properties where the intensity of the 3 MLCT/3ILCT band is simply decreased with an increase of the concentration and no significant incline of the dimer 3 MMLCT band (Figure 4g−j). UV−vis spectra of complexes 1−4 show their lower energy absorption bands at around 420 nm (Figure S20), which could be assigned to GS-1MLCT/1ILCT transitions in the isolated molecules. Concentration-dependent changes in quantum yields for (±)-1b, (−)-1b, 2, and (±)-4 in CHCl3 at 298 K (Φ298 K) are shown in Figure 5 and Figure S21, which clearly verifies the

Figure 6. Concentration dependence of emission lifetime for (a) (±)-1b and (b) 2 in CHCl3 at 298 K (λex 355 nm). The lifetimes were estimated from single-exponential fitting analyses (0.05−15 mM for (±)-1b, 0.05−3 mM for 2) of the time-dependent emission decays monitored at (a) 590 and (b) 550 nm (Figures S22 and S24).

noteworthy that the estimated lifetimes for (±)-1b (Figure 6a) and (−)-1b (Figure S27) increase continuously with the concentration, while those for 2 (Figure 6b), (±)-3 (Figure S28), and (±)-4 (Figure S29) exhibit typical patterns of concentration quenching with diffusion-controlled quenching constants kq of 2.0 × 107, 2.1 × 108, and 2.3 × 107 M−1 s−1, respectively (Figures S30−S32). Concentration-dependent variations of observed radiative kr(obs) and nonradiative knr(obs) rate constants for (±)-1b, (−)-1b, 2, and (±)-4 at 298 K were estimated from Φ298 K and τobs in the range of lower concentrations (Figure 7 and Figures S33 and S34 and Tables S2−S5). Complexes 2 and (±)-4 exhibit typical patterns of concentration quenching where knr(obs) constantly increases with the concentration (Figure 7b and Figure S34a), while kr(obs) remains unchanged (Figure 7d and Figure S34b). This typical quenching phenomenon is rationally explained by the increase in intermolecular interactions, which leads to acceleration of the nonradiative pathway by dispersion of the photoexcited energy. In sharp contrast, knr(obs) for complexes (±)-1b and (−)-1b decrease steeply with a slight increase of kr(obs) with an increase of the concentration (Figure 7a,c and Figure S33). Given the concentration-dependent variation of emission patterns of 1b (Figures 4c,d), we can be reasonably certain that the unprecedented concentration-dependent variation in Φ298 K (Figure 5a), τ (Figure 6a), and k(obs) values (Figure 7a,c) for 1b is not due to a single transition but to a variation in the component ratio of a 3MLCT-1GS (ground state) transition from the monomer and a 3MMLCT-1GS transition from an association dimer, although the time-dependent decays can be analyzed by single-exponential fitting (Figures S22 and S23), probably due to the proximity of the component parameters. Mechanistic Rationale. In order to obtain insight into the mechanism of the present concentration-driven emission enhancement of 1, association properties of 1−4 in the solution state were examined by means of 1H nuclear magnetic resonance (NMR) spectroscopic analyses in CDCl3. The

Figure 5. Concentration dependence of quantum yield for (a) (±)-1b (red) and (−)-1b (blue) and (b) 2 (black) in CHCl3 at 298 K (0.2− 60 mM, λex 430 nm). The inset in (a) shows an enlarged view from 0.2 to 5 mM.

foregoing points observed from emission spectra (Figure 4), including (1) specific emission enhancement of 1 with an increase of the concentration, (2) contrasting but typical concentration quenching of other analogues, and (3) stronger enhancement in racemic 1 than in optically pure 1. It is noteworthy that the present concentration-driven emission enhancements in the intensities and quantum efficiencies are observed constantly throughout the entire range of concentration (Figures 4a−f and 5a). This is in contrast to the reported AIEE phenomenon that can be observed suddenly when higher-ordered aggregates are formed with an increase in the solvophobicity of the solutions.4 Kinetic data for the time-dependent emission decay of (±)-1b, (−)-1b, 2, (±)-3, and (±)-4 in CHCl3 at 298 K 9079

DOI: 10.1021/acs.inorgchem.9b00608 Inorg. Chem. 2019, 58, 9076−9084

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10 M−1, respectively, by curve-fitting analysis17 of the concentration dependence of the H4 chemical shifts (Figure S41). The heterochiral association constant KD(hetero) at 298 K was estimated to be 1.1 × 102 M−1, on the basis of the relationship between KD(racemic) and the homo- and heterochiral association constants: K D(racemic) = 1/2K D(homo) + 1/ 4KD(hetero).18 Similarly, equilibrium constants for dimer formation for vaulted 1a,c at 298 K were determined to be KD(hetero) = 9.1 × 10 M−1 and KD(homo) = 1.8 × 10 M−1 for 1a (Figure S42) and KD(hetero) = 6.3 × 10 M−1 and KD(homo) = 2.0 × 10 M−1 for 1c (Figure S43), while those of 2 (KD = 7.5 M−1, Figure S41), (±)-3 (no association properties, Figure S38), and (±)-4 (KD(racemic) = 1.7 M−1, Figures S44) were much smaller than those of 1 (Figures S41−S43). The concentration-dependent variation in the dimer component ratio [(1)2]/[1]0 in CDCl3 was estimated from the KD values, as shown in Figure S45, which indicated that the dimer ratio increases with the concentration, where the observed inclination is in the order of (±)-1b > (±)-1a > (±)-1c > (−)-1b. This is highly consistent with the structural dependence in the concentration-driven emission enhancement of 1 (Figures 4a−f and 5a).

Figure 7. Concentration dependence of (a, b) observed nonradiative and (c, d) radiative rate constants for (a, c) (±)-1b (red) and (b, d) 2 (black) in CHCl3 at 298 K (0.05−3 mM).

formation of the heterochiral association dimer of (±)-1b (Figure 2) in the solution state was rationalized by (1) a significant upfield shift of H3 and H4 signals with an increase in concentration (Figure 8a and Figure S35a) and (2)

To elucidate the emission properties of 1, the molecular orbitals of monomer 1b and the heterochiral association dimer [(+)-1b·(−)-1b] were estimated using spin−orbit coupling density function theory (SOC-DFT) 19a calculations (B3LYP19b,c/DZP19d,e) on the basis of optimized geometries in the triplet state (Figure S46 and Tables S6 and S7), determined by unrestricted DFT (UDFT) calculations (UMN12L19f/6-31+G(d),19g−i LanL2DZ19j,k). The HOMO of monomer 1b is a mixture of Pt(dxz) and ligand π, and the LUMO is ligand π* (Figure 9a), while the HOMO of dimer [(+)-1b·(−)-1b] is Pt(dz2) and the LUMO is principally ligand π* (Figure 9b). The major electronic configurations of the T1 state for the monomer and dimer were determined to be HOMO to LUMO by SOC time-dependent DFT calculations19l−n (Table S8) on the basis of optimized geometries in the triplet state. These results indicate that the monomer and dimer emissions can be attributed to 3MLCT-1GS and 3 MMLCT-1GS transitions, respectively. This is consistent with the fact that crystal (±)-1b bearing Pt−Pt interactions in the cofacial heterochiral association dimers (Figure 2) exhibits an intense orange emission at 298 K with an emission maximum of 592 nm and quantum yield of 0.35 (Table S2 and Figure S47). Given the results including (1) increasing profiles of concentration dependence, in both the dimer component ratio (Figure S45) and the photophysical data (Figures 5a and 6a), (2) the proximity of emission properties between condensed solution and crystalline states (Figure 4c and Figure S47), (3) evidence of the formation of heterochiral cofacial association dimers in the solution and crystalline states obtained from 1H NMR (Figure 8 and Figure S40) and XRD

Figure 8. Schematic representation of the heterochiral, cofacial association of the vaulted complexes observed based on (a, side view) an 1H NMR shielding effect and (b, upper view) NOESY remote correlations for (±)-1. The vaulted polymethylene linkers located on the upper and lower sides of the coordination planes in (b) are omitted for clarity. The spectroscopic data are given in Figures S35− S37 and S40.

intermolecular correlation between H3 and H4 in 2D nuclear Overhauser effect spectroscopy (NOESY) experiments (Figure 8b and Figure S40). On the basis of a monomer−dimer equilibrium model (eqs 1 and 2),16 the equilibrium constants for the dimerization of (±)- and (−)-1b in CDCl3 (KD(racemic), KD(homo)) at 298 K were determined to be 3.8 × 10 and 1.7 × 9080

DOI: 10.1021/acs.inorgchem.9b00608 Inorg. Chem. 2019, 58, 9076−9084

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in the contribution of a 3MMLCT to 1GS transition, which is caused by the specific ability for the formation of a cofacial association dimer of 1.

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EXPERIMENTAL SECTION

General Considerations. Melting points were measured in a glass capillary using a Büchi B-545 melting point apparatus. IR spectra were recorded using a Bruker Equinox 55 spectrometer. 1H and 13C NMR spectra were recorded using a Varian Unity-Inova 500 spectrometer. Mass spectra were obtained using a Jeol JMS-DX 303 instrument. UV−vis spectra of solutions were obtained using a Shimadzu MultiSpec-1500 spectrometer. Optical rotations were measured on a Jasco DIP-370 digital polarimeter. DLS measurements were performed on an Otsuka Electronics nanoSAQLA spectrophotometer. Emission spectra were obtained using a Jasco FP-6500 spectrometer. Quantum yields were measured by the absolute method using a Jasco FP-6500 spectrometer equipped with a Jasco INK-533 integrating sphere. Emission lifetime measurements were conducted using a system including a Contnuum PowerPrecision 9010 Nd:YAG laser, a Princeton Instruments Acton SP 2300 monochromator, a Hamamatsu R928 photomultiplier tube, and a Tektronix TDS220 digital oscilloscope. Materials for the synthesis of platinum complexes are described in the Supporting Information. Synthesis of Platinum Complexes. A series of platinum complexes 1 and 2 was prepared by stirring a mixture of PtCl2(CH3CN)2 (1.50 mmol) and N,N′-bis[(1H-imidazol-2-yl)methylene]-1,ω-alkanediamine (1.50 mmol for 1) or N-[(1Himidazol-2-yl)methylene]pentanamine (3.00 mmol for 2) in DMSO (75 mL) and toluene (300 mL) at 135 °C (reaction time: 2 h for 1a, 20 h for 1b,c and 2). The resulting mixture was diluted with EtOAc (200 mL), washed with brine (200 mL × 3), and dried over Na2SO4. After removal of the solvent under reduced pressure, the resulting crude solid was subjected to column chromatography (Fuji Silysia Chromatorex NH-DM1020, EtOAc/MeOH (95/5)) affording 1a−c or 2 as an orange solid. The optically pure complexes of (+)-1a, (−)-1a, (+)-1b, (−)-1b, (+)-1c, and (−)-1c (100% ee) were obtained from the racemic mixture using a recycling preparative HPLC system (Japan Analytical Industry, LC-9201) with a Daicel Chiralpak IA column (CHCl3/MeOH (95/5)). The H2 and H3 protons on the imidazole ring (Scheme 1) were assigned by NOESY spectra (Figure S49). Complexes 3 and 4 were prepared by a similar method (Supporting Information). Compound 1a: orange solid (16%). (±)-1a: mp 213 °C dec; IR (KBr) 2926, 2852, 1596, 1457, 1409, 1346, 1153, 937, 761 cm−1; 1H NMR (CDCl3, 500 MHz) δ 0.94−1.05 (m, 2 H), 1.20−1.35 (m, 8 H), 1.48−1.65 (m, 4 H), 2.20−2.31 (m, 2 H), 3.36 (ddd, J = 12.5, 12.5, 2.4 Hz, 2 H), 4.19 (ddd, J = 12.5, 2.4, 2.4 Hz, 2 H), 7.07 (s, 2 H, H3), 7.39 (s, 2 H, H2), 7.85 ((s, 1.4 H) and (d, J195Pt,H = 40.0 Hz, 0.6 H), H4); 13C NMR (CDCl3, 125 MHz) δ 161.2 (C4), 155.8 (C1), 133.7 (C2), 130.3 (C3), 61.1 (C9/C18), 27.2, 26.8, 25.3, 22.6; HRMS (FAB) m/z calcd for C18H27N6195Pt 522.1945, found 522.1928 [M + H]+. Anal. Calcd for C18H26N6Pt: C, 41.45; H, 5.03; N, 16.11. Found: C, 41.23; H, 4.71; N, 15.73. (−)-1a: mp 212 °C dec; [α]D26 = −523 ± 2 (c 0.052, CHCl3). (+)-1a: [α]D26 = +523 ± 1 (c 0.052, CHCl3). Compound 1b: orange solid (47%). (±)-1b: mp 253 °C dec; IR (KBr) 2982, 2927, 2854, 1599, 1500, 1460, 1434, 1409, 1345, 1152, 935, 755 cm−1; 1H NMR (CDCl3, 500 MHz) δ 0.91−1.01 (m, 2 H), 1.05−1.41 (m, 14 H), 1.45−1.55 (m, 2 H), 2.01−2.11 (m, 2 H), 3.22 (ddd, J = 12.7, 12.7, 2.8 Hz, 2 H), 4.00 (ddd, J = 12.7, 2.8, 2.8 Hz, 2 H), 7.04 (s, 2 H, H3), 7.38 (s, 2 H, H2), 7.77 ((s, 1.5 H) and (d, J195Pt,H = 38.5 Hz, 0.5 H), H4); 13C NMR (CDCl3, 125 MHz) δ 161.3 (C4), 155.7 (C1), 133.4 (C2), 130.3 (C3), 61.0 (C9/C20), 28.1, 26.8, 26.7, 26.6, 22.6; HRMS (FAB) m/z calcd for C20H31N6195Pt 550.2258, found 550.2255 [M + H]+. Anal. Calcd for C20H30N6Pt: C, 43.71; H, 5.50; N, 15.29. Found: C, 43.52; H, 5.26; N, 15.14. (−)-1b: mp 233 °C dec; [α]D25 = −495 ± 2 (c 0.055, CHCl3). (+)-1b: [α]D25 = +496 ± 2 (c 0.055, CHCl3). Compound 1c: orange solid (58%). (±)-1c: mp 229 °C dec; IR (KBr) 2926, 2854, 1600, 1460, 1435, 1411, 1348, 1155, 939 cm−1; 1H

Figure 9. Frontier orbitals of (a) (+)-1b and (b) [(+)-1b·(−)-1b] estimated from SOC-DFT calculations (B3LYP/DZP) based on the optimized structures determined by UDFT calculations (UMN12L/631+G(d), LanL2DZ).

analyses (Figure 2 and Figure S16), and (4) support from DFT calculations (Figure 9 and Figure S46 and Tables S6−S8), we can be reasonably certain that the present concentration-driven emission enhancement of 1 is caused by an increase in the contribution of 3MMLCT to 1GS transition with the increasing concentration of the cofacial association dimer. From the variation in the observed rate constants, the concentrationdriven emission enhancement can be mainly attributed to a significantly slower nonradiative process from the 3MMLCT state. The characteristic nonradiative process would be achieved by inhibition of energy migration from the excitons due to (1) increasing conformational rigidity and (2) molecular barrier function against infinite stacking aggregation (Figure S48) that leads to self-quenching. The ordinary concentration quenching of analogues 3 and 4 can be also attributed to their negligible association properties in the solution state (Figures S38 and S44). This can be rationalized by assuming the similar cofacial association models, which are not likely because the nitrogen atoms on the β-(iminomethyl)pyrrolato and β-(iminomethyl)pyrazolato ligands would generate significant electrostatic repulsions between the cofacial units.

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CONCLUSIONS We have synthesized a series of chiral trans-bis[2(iminomethyl]imidazolato]platinum(II) complexes 1 bearing polymethylene bridges that exhibit specific resistance properties toward ordinary concentration quenching in the solution state. The unprecedented consecutive increases in emission intensity and efficiency were observed upon an increase in the concentration of the solution throughout an entire range of concentrations, which can never be achieved in the conventional phosphorescent molecules. A mechanistic investigation based on the NMR, XRD analyses, kinetic studies, and DFT calculations revealed that the present specific concentrationdriven emission enhancement of 1 is attributable to an increase 9081

DOI: 10.1021/acs.inorgchem.9b00608 Inorg. Chem. 2019, 58, 9076−9084

Inorganic Chemistry

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NMR (CDCl3, 500 MHz) δ 1.00−1.40 (m, 20 H), 1.60−1.69 (m, 2 H), 1.91−2.11 (m, 2 H), 3.28 (ddd, J = 12.6, 12.6, 3.7 Hz, 2 H), 4.01 (ddd, J = 12.6, 3.7, 3.7 Hz, 2 H), 7.03 (s, 2 H, H3), 7.38 (s, 2 H, H2), 7.76 ((s, 1.4 H) and (d, J195Pt,H = 38.0 Hz, 0.4 H), H4); 13C NMR (CDCl3, 125 MHz) δ 161.4 (C4), 155.7 (C1), 133.4 (C2), 130.1 (C3), 61.1 (C9/C22), 29.3, 27.6, 27.3, 27.04, 27.01, 25.0; HRMS (FAB) m/z calcd for C22H35N6195Pt 578.2571, found 578.2571 [M + H]+. (−)-1c: mp 221 °C dec; [α]D25 = −421 ± 2 (c 0.058, CHCl3). (+)-1c: [α]D25 = +421 ± 2 (c 0.058, CHCl3). Compound 2: yellow solid (63%); mp 178 °C dec; IR (KBr) 2950, 2924, 2857, 1607, 1500, 1463, 1454, 1430, 1410, 1342, 1291, 1156, 1081, 937 cm−1; 1H NMR (CDCl3, 500 MHz) δ 0.92 (t, J = 7.2 Hz, 6 H, CH3), 1.30−1.46 (m, 8 H), 1.85 (tt, J = 7.2, 7.2 Hz, 4 H, N− CH2−CH2−), 3.83 (t, J = 7.2 Hz, 4 H, N−CH2−), 7.08 (s, 2 H, H3), 7.38 (s, 2 H, H2), 7.85 ((s, 1.4 H) and (d, J195Pt,H = 39.5 Hz, 0.6 H), H4); 13C NMR (CDCl3, 125 MHz) δ 161.5 (C4), 155.7 (C1), 133.4 (C2), 129.8 (C3), 60.8 (N−CH2−), 30.2 (N−CH2−CH2−), 28.4, 22.3, 13.9 (CH3); HRMS (FAB) m/z calcd for C18H29N6195Pt 524.2101, found 524.2106 [M + H]+. X-ray Structure Determination. Crystals employed for X-ray diffraction studies were obtained by recrystallization from EtOAc for (±)-1a, CHCl3/n-hexane for (±)-1b, and CHCl3/1,2-dichlorobenzene for (±)-1c and analyzed using a Rigaku XtaLAB P-200 diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71075 Å). The structures of 1a−c and 2 were solved by direct methods and refined using the full-matrix least-squares method. In a subsequent refinement, the function ∑ω(Fo2 − Fc2)2 was minimized, where Fo and Fc 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 software package, and illustrations were produced with ORTEP.20 CCDC 1869956 (for (±)-1a), 1870012 (for (±)-1b), 1869960 (for (±)-1c), and 1869961 (for 2) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre. Computational Methods. DFT calculations were carried out by using Gaussian 0921 program package, with the LanL2DZ19j,k basis set for Pt atoms and 6-31+G(d)19g−i for C, H, and N atoms. Optimized geometries for monomer 1b and heterochiral association dimer [(+)-1b·(−)-1b] in the lowest triplet states were determined by unrestricted DFT calculations, with the MN12L19f functional. The molecular orbitals, energy levels, and electronic configurations in the lowest triplet excited states were estimated from SOC-TD-DFT19a,l−n (B3LYP19b,c) calculation with relativistic ZORA Hamiltonian and the double-ζ19d,e basis sets implemented in the ADF22 program package using the optimized structures in the lowest triplet states.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.N.: [email protected]. ORCID

Soichiro Kawamorita: 0000-0002-2093-5175 Naruyoshi Komiya: 0000-0001-8400-3062 Takeshi Naota: 0000-0001-5348-3390 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grant Numbers JP16J02206 (N.H.-T.L.), JP16J02191 (R.I.), JP15K05476 (N.K.), and JP16H06516 (T.N.)). We gratefully acknowledge Assistant Prof. Yasushi Oshikane (Osaka University) and Mr. Junya Adachi (Osaka University) for assistance with lifetime measurement and elemental analysis, respectively.

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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.9b00608. Additional figures and tables as described in the text (PDF) Accession Codes

CCDC 1869956, 1869960−1869961, and 1870012 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, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 9082

DOI: 10.1021/acs.inorgchem.9b00608 Inorg. Chem. 2019, 58, 9076−9084

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