Excimer–Monomer Photoluminescence Mechanochromism and

Apr 13, 2017 - Synopsis. Four new photoluminescent N^C^N cyclometalated platinum(II) complexes are reported. The incorporation of the bulky H-shaped p...
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Excimer−Monomer Photoluminescence Mechanochromism and Vapochromism of Pentiptycene-Containing Cyclometalated Platinum(II) Complexes Che-Jen Lin, Yi-Hung Liu, Shie-Ming Peng, Teruo Shinmyozu, and Jye-Shane Yang* Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: The ability of the bulky H-shaped pentiptycene scaffold in promoting the mechanochromic and vapochromic luminescence properties for organometallic materials has been demonstrated with the N^C^N cyclometalated platinum(II) complexes [X-NCNPtY], where X = Br or Pa, the substituent on the terdentate dipyridylbenzene N^C^N ligand, and Y = Cl or Pa, the ancillary ligand, in which Pa = pentiptycene acetylene. Intermolecular interactions between the planar NCNPt cores are described by π−π and d−π interactions with negligible PtII···PtII bonding, corresponding to ligand-centered excimer rather than metal−metal-to-ligand charge-transfer emission, for these platinum(II) complexes in aggregates and in the solid state. Interplay of the relative excimer-to-monomer emission intensity in response to external force and/or vapor stimuli accounts for the luminescence mechanochromism and vapochromism of the pentiptycene-incorporated platinum(II) complexes [PaNCNPtCl], [Br-NCNPtPa], and [Pa-NCNPtPa], whereas the pentiptycene-free counterpart [Br-NCNPtCl] undergoes little or no emission color response. In particular, the complex [Pa-NCNPtCl] displays a distinct response to aromatic versus nonaromatic organic vapors: namely, aromatic vapors such as benzene convert the excimer emission to monomer emission, but the opposite is true with nonaromatic vapors. A two-stage emission color change from red to orange and then to yellow could thus be achieved by grinding and by subsequent benzene-vapor fuming. Another feature associated with [Pa-NCNPtCl] is an aggregation-induced green → magenta luminescence color change in water/tetrahydrofuran mixed solutions. The structure− luminescence property relationship is discussed in relation to intermolecular interactions and packing modes that depend on the number and positions of pentiptycene groups. quantum efficiency in aggregates, powders, and thin solid films; the putative porous structures created by the stacked pentiptycene scaffolds could also facilitate interactions with chemical vapors for photoluminescence quenching.24,28−30 Recently, we demonstrated a new function of pentiptycene, that is, promoting both mechanochromic and vapochromic luminescence for organic π systems toward multicolor and multicomponent responses.31 This finding has prompted us to investigate the corresponding performance of pentiptycene in photoluminescent orgaometallic platinum(II) complexes. We report herein the mechanochromic and vapochromic luminescence properties of pentiptycene-containing cyclometalated platinum(II) complexes [Br-NCNPtPa], [PaNCNPtCl], and [Pa-NCNPtPa], in which NCN stands for the terdentate 1,3-bis(2-pyridyl)benzene N^C^N ligand and Pa the pentiptyceneacetylene (PaH) group (Chart 1). The pentiptycene-free system [Br-NCNPtCl] was also prepared as a reference for the discussion of the pentiptycene effect. Both the X-ray crystallographic and electronic spectroscopic data

1. INTRODUCTION Cyclometalated platinum(II) complexes have attracted much attention because of their intriguing photoluminescent and photoresponsive supramolecular properties.1−7 The squareplanar coordination geometry of the platinum center as well as the flat π ligands allows intermolecular PtII···PtII and π−π interactions, which could drive self-assembly in solution and the solid state and thus switch the monomer emission to redshifted metal−metal-to-ligand charge-transfer (MMLCT) or excimer (excited-state dimer) emission.8,9 When the PtII···PtII or π−π interactions are susceptible to external perturbations such as mechanical stress and chemical vapors, the corresponding systems would display mechanochromic and vapochromic luminescence properties and hold great promise as sensory materials.9−20 Pentiptycene is a rigid H-shaped molecule, in which the central phenylene ring is sterically shielded by the two Vshaped “iptycenyl” substituents.21 Pentiptycene has been incorporated into π-conjugated organic or organometallic systems such as platinum acetylides22−24 and oligo-25−27 and poly(aryleneethynylene)s28−30 to diminish intermolecular πstacking interactions and thus improve the photoluminescence © 2017 American Chemical Society

Received: January 3, 2017 Published: April 13, 2017 4978

DOI: 10.1021/acs.inorgchem.7b00009 Inorg. Chem. 2017, 56, 4978−4989

Article

Inorganic Chemistry Chart 1. Structures of PaH and the Target N^C^N Cyclometalated Platinum(II) Complexes

Scheme 1. Synthetic Scheme for the Target Cyclometalated Platinum(II) Complexes

coupling between 2 equiv of 2-pyridylzinc bromide prepared in situ and 1,3,5-tribromobenzene afforded the bromosubstituted terdentate N^C^N ligand 1-bromo-3,5-bis(2pyridyl)benzene (Br-NCN).32 The cycloplantinated complex [Br-NCNPtCl] was obtained by treatment of Br-NCN with K2PtCl4.33 Under basic conditions, the reaction between [BrNCNPtCl] and the known compound25 PaH (Chart 1) resulted in [Br-NCNPtPa]. The same methodology was applied to the synthesis of [Pa-NCNPtCl] and [Pa-NCNPtPa] simply by replacing Br-NCN with Pa-NCN, which was, in turn, synthesized by the Sonogashira coupling between PaH and BrNCN. Single crystals of [Br-NCNPtCl] (Figure 1) and [PaNCNPtCl] (Figure 2) suitable for X-ray crystallography were

show that, unlike the majority of stimuli-responsive platinum(II) complexes that are dictated by the on−off switching of PtII···PtII interactions and thus the MMLCT emission,9−19 intermolecular interactions between the flat NCNPt core of these pentiptycene-incorporated platinum(II) systems are dominated by π−π and d−π interactions for ligand-centered (LC) excimer formation. The dependence of the monomer-toexcimer emission intensity on external mechanical forces and chemical vapors and on the number and locations of the pentiptycene groups is elucidated.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Structures. Scheme 1 shows the synthesis of the target platinum(II) complexes. Negishi 4979

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planar geometry.4,33 The molecular packing adopts an antiparallel slipped stacking mode with an intermolecular π−π distance of 3.394 and 3.451 Å. The intermolecular PtII··· PtII distances are 4.667 and 4.391 Å, indicating negligible PtII··· PtII interactions (the van der Waals radius of platinum is 1.75 Å). In the case of [Pa-NCNPtCl], the platinum center also adopts a distorted square-planar geometry. The acetylene group is essentially linear, with the two C−CC bond angles being 177.0° and 177.1°, and the central phenylene ring of pentiptycene is twisted from the platinum square plane by ∼49°. The molecules are arranged in a “like-interact-with-like” packing mode: namely, the flat NCNPt core interacts with neighboring NCNPt cores and the bulky pentiptycene group interacts with an adjacent pentiptycene. Consequently, the NCNPt cores are in a pairwise packing mode with an interplanar distance of 3.353 Å, revealing the presence of π−π and d−π interactions, and the PtII···PtII distance of 5.324 Å again indicates negligible PtII···PtII interactions. The pentiptycene−pentiptycene interactions are characterized by π−π as well as C−H−π interactions between the V-shaped iptycenyl substituents with short contacts of 2.761−2.868 Å. The terminal octyloxy chains lie in the U-shaped cavity of an adjacent pentiptycene, showing the presence of C−H−π interactions. The imbalanced size for the flat NCNPt versus the bulky pentiptycene group results in void volumes, in which toluene solvate molecules are situated, between the pairwise NCNPt cores. The pentiptycene effect on preventing molecules from close packing is also evidenced by the significantly lower crystal density for [Pa-NCNPtCl] (1.5 g cm−3) relative to [BrNCNPtCl] (2.5 g cm−3; Tables S1 and S2). The lack of metallophilic PtII···PtII interactions for both [BrNCNPtCl] and [Pa-NCNPtCl] can be attributed to the antiparallel molecular packing of the NCNPt cores. Unlike a parallel molecular packing that allows both PtII···PtII and π−π/ d−π interactions, an antiparallel molecular packing for the NCNPt core can have only one of the two types of interactions.5,34 While the antiparallel packing for [PaNCNPtCl] could be, in part, attributed to the presence of a bulky pentiptycene group, the choice of an offset antiparallel packing for the pentiptycene-free [Br-NCNPtCl] reflects the importance of π−π/d−π interactions in this system. Indeed, the packing mode for N^C^N cyclometalated platinum(II) complexes is very sensitive to the substituents on the NCN ligand, the ancillary ligand, and the solvate molecule.5−7,14 The parent systems [H-NCNPtCl] and [H-NCNPtCC-Ph] {HNCN = 1,3-di(2-pyridyl)benzene} also adopted an antiparallel packing mode that lacks PtII···PtII interactions, but examples of cofacial or staggered parallel packing that features both PtII··· PtII and π−π/d−π interactions have been reported for [CF3NCNPtCC-Ph] and [CF3-NCNPtCC-C6H4-NMe2].5,33 Potential polymorphism of the pentiptycene-incorporated platinum(II) complexes is noted with an observation of distinct emission color from crystals of [Pa-NCNPtCl]. The optical microscopy images of crystals grown in the same batch of [PaNCNPtCl] solution show both granule- and needle-shaped crystals having orange and green emission, respectively (Figure S1). The crystal structure shown in Figure 2 corresponds to the granular form. The orange emission for the granular form is consistent with the presence of NCNPt−NCNPt π−π/d−π interactions. Because [Pa-NCNPtCl] displays green emission in dilute tetrahydrofuran (THF) solutions (vide infra), molecules in needle form are expected to have negligible intermolecular interactions. Unfortunately, the needles are not suitable for X-

Figure 1. Single-crystal X-ray structure of [Br-NCNPtCl] showing (a) the two crystallographically independent conformers, (b) the π−π and PtII···PtII distances, and (c) the antiparallel slipped stacking mode. Thermal ellipsoids were drawn at the 50% probability level. Hydrogen atoms were omitted for clarity.

Figure 2. Single-crystal X-ray structure of [Pa-NCNPtCl] showing (a) the pairwise π−π and d−π interactions of the NCNPt cores, (b) the herringbone-like packing mode and the octylpentiptycene C−H−π interactions, (c) the pentiptycene−pentiptycene π−π and C−H−π interactions, and (d) the enclathration of toluene solvate molecules. Thermal ellipsoids were drawn at the 50% probability level. Hydrogen atoms were omitted for clarity.

obtained by the slow diffusion of toluene into the chloroform solution containing the substrate in a vial. Crystallographic data and selective structural parameters are given in Tables S1−S3. For [Br-NCNPtCl], there exist two crystallographically independent molecules in the crystal with similar bond lengths and bond angles. The platinum center is planar, as evidenced by the sum of the four bond angles being 360°: namely, it is ∼80° for both C−Pt−N bond angles and ∼100° for both N−Pt−Cl bond angles. Deviation of the bond angles about the platinum atom from 90° is typical for terdentate cyclometalated platinum(II) systems and corresponds to a distorted square4980

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Figure 3. (a) Absorption and (b) normalized emission (phosphorescence) spectra of [Br-NCNPtCl] (blue), [Br-NCNPtPa] (green), [PaNCNPtCl] (black), and [Pa-NCNPtPa] (red) in THF at a concentration of (4.0 ± 1.0) × 10−6 M. The excitation wavelength for the emission spectra is 330 nm.

uration interactions, and electronic character of the low-lying singlet excited states (S1, S2, and/or S3) and the lowest triplet excited state (T1). These excited states are dominated by the configurations highest occupied molecular orbital (HOMO, H) → lowest unoccupied molecular orbital (LUMO, L) and H → L+1 for [Br-NCNPtCl] and [Pa-NCNPtCl] and by H → L+1, H−2 → L, and H−2 → L+1 for [Br-NCNPtPa] and [PaNCNPtPa]. As shown in Figure S2, the H orbitals of [BrNCNPtCl] and [Pa-NCNPtCl] and the H−2 orbitals of [BrNCNPtPa] and [Pa-NCNPtPa] are localized on the molecular long-axis π backbone, i.e., Br−Ph−Pt−Cl for [Br-NCNPtCl], Ph−CC−Ph−Pt−Cl for [Pa-NCNPtCl], Br−Ph−Pt−CC for [Br-NCNPtPa], and Ph−CC−Ph−Pt−CC for [PaNCNPtPa]. The Pa ancillary ligand perpendicularly twisted from the above long-axis π backbone is the location of the H orbitals for the latter two complexes. For the unoccupied molecular orbitals, the L is mainly localized on the terdentate NCN ligand, and the L+1 is concentrated in the NCNPt core for all four complexes. Table S5 shows the data of the relative contribution of structural components to these frontier orbitals. Accordingly, the low-lying excited states have intraligand charge-transfer (ILCT) and/or ligand-to-ligand charge-transfer (LLCT) character, and there exists a minor contribution from the metal-to-ligand charge-transfer (MLCT) transition for some states. The absorption spectra and normalized emission spectra of the cyclometalated platinum(II) complexes in THF at room temperature are shown in Figure 3. The absorption spectra display an intense band (ε > 10000 L mol−1 cm−1) at 300−330 nm and a broad shoulder (ε ≈ 6000−10000 L mol−1 cm−1) at 370−450 nm. These two bands might be assigned as a LC π → π* transition and mixed ILCT, LLCT, and MLCT transitions, respectively, on the basis of the above TDDFT calculations. The position, shape, and molar absorptivity (ε) for the broad low-energy band are similar for [Br-NCNPtCl], [BrNCNPtPa], and [Pa-NCNPtCl], but the band undergoes both bathochromic and hyperchromic shifts for [PaNCNPtPa], attributable to an increased ILCT character according to the TDDFT calculation. The emission spectra are located in the range 480−650 nm for all four cases with varied degrees of vibrational structure, revealing significant LC character for the T1 state. The maximum of the 0−0 emission band (λp) is on the order [Pa-NCNPtPa] (529 nm) > [PaNCNPtCl] (521 nm) > [Br-NCNPtPa] (512 nm) > [BrNCNPtCl] (501 nm), indicating that the Pa substituent on NCN plays a more important role than the Pa ancillary ligand upon lowering of the T1 state energy. This is consistent with

ray crystallography for further confirmation. A more detailed discussion on the solid-state photoluminescence will be provided in sections 2.2 and 2.3. In the absence of crystal structures, structural information about [Br-NCNPtPa] and [Pa-NCNPtPa] were obtained from density functional theory (DFT) calculations with the M062X35/GenECP [6-31G(d,p) for carbon, hydrogen, nitrogen, oxygen, chlorine, and bromine atoms and Stuttgart− Dresden (SDD)36 for a platinum atom] method. To justify the calculation, the corresponding calculations on [Br-NCNPtCl] and [Pa-NCNPtCl] were also conducted and compared with the X-ray crystal structures. Selected bond lengths, bond angles, and torsion angles for all four complexes are provided in Table S3. The DFT-calculated bond angles about the platinum atom for [Br-NCNPtCl] and [Pa-NCNPtCl] agree well with those in the X-ray crystal structures of [Br-NCNPtCl] and [PaNCNPtCl], but the calculated Pt−N and Pt−Cl bond lengths are slightly larger by ∼0.05 Å than those observed in the crystals. A major discrepancy between the calculated and crystal structures for [Pa-NCNPtCl] is the dihedral angle between the pentiptycene central ring and the NCNPt plane: whereas the dihedral angle is as large as ∼49° in the crystal, the calculated angle is nearly zero (0.1°). Such a discrepancy could be attributed to the presence of intermolecular interactions in the crystals but not in the gas phase for calculations. The calculated square-planar geometry for the platinum center in [BrNCNPtPa] and [Pa-NCNPtPa] is similar to that in [BrNCNPtCl] and [Pa-NCNPtCl], respectively. The calculated pentiptycene−NCNPt dihedral angle for the Pa-NCN moiety in [Pa-NCNPtPa] is 11.5°, a value slightly larger than that (0.1°) in [Pa-NCNPtCl]. However, when the Pa group directly coordinates to the platinum, the pentiptycene central ring is perpendicularly twisted from the rest of the molecular π moiety by 89.4° in [Br-NCNPtPa] and by 83.7° in [Pa-NCNPtPa], indicating negligible conjugation interactions. 2.2. Photophysical Properties in Different Degrees of Intermolecular Interactions. The photophysical properties of the four platinum(II) complexes have been investigated in different conditions, including the gas phase, dilute solutions, aggregates, and the solid state, such that the effect of intermolecular interactions on photoluminescence could be addressed. The electronic structures of the platinum(II) complexes in the gas phase were obtained by time-dependent DFT (TDDFT) calculations using SDD36 as the basis set for platinum and M062X(d,p)35 for the other atoms. Table S4 lists the TDDFT-derived state energy, oscillator strength, config4981

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is the number of nuclei for aggregation. With the same substrate concentration, the more nuclei for aggregation, the lower the average number of substrate molecules per aggregate particle and thus the smaller the size for the resulting aggregates. When the water fraction is increased, the substrate solubility is decreased, which would tend to form more nuclei during the mixing of the THF solutions with water. Figure 4 shows the images of photoluminescence of the water/THF mixed solutions in an open atmosphere, and Figure

the DFT-predicted conformations, in which the pentiptycene group in the Pa ancillary ligand is perpendicularly twisted (i.e., deconjugated) from the rest of the molecular π system. The absorption maximum (λabs), molar absorptivity (ε), and λp are listed in Table 1. Table 1. Photophysical Data of the Cyclometalated Platinum(II) Complexes in THF compound

a

[BrNCNPtCl] [BrNCNPtPa] [PaNCNPtCl] [PaNCNPtPa]

λabs (nm) [ε (×104 L mol−1 cm−1)]

kp (×104 −1 s )

λp (nm)

Φpc

(%)

τpc (μs)

383 [0.7], 428 [0.6] 429 [0.6]

501 (535)

11 (2.4)

1.6

6.9

512 (538)

33 (1.3)

4.2

7.9

328 [2.4], 430 [0.6] 330 [4.6], 430 [1.1]

521 (553)

66 (1.4)

11.6

5.7

529

55 (1.6)

10.4

5.3

b

The concentration is (4.0 ± 1.0) × 10−6 M. bShoulders are given in parentheses. cValues were obtained in a degassed solution (excitation at 355 nm), and values in parentheses are for the aerated solution.

Figure 4. Photographic images of the emission color for [BrNCNPtCl], [Br-NCNPtPa], [Pa-NCNPtCl], and [Pa-NCNPtPa] in mixed water/THF (v/v) from 0% to 90% water fraction at a 10% interval in an open atmosphere. The excitation wavelength is 365 nm.

The emission quantum yield (Φp) and lifetime (τp) for the cyclometalated platinum(II) complexes in degassed THF solutions are also reported in Table 1. The microsecond time scale for the emission decay is consistent with the character of phosphorescence expected for platinum(II) complexes. Both [Pa-NCNPtCl] (0.66) and [Pa-NCNPtPa] (0.55) have high Φp similar to the parent system [H-NCNPtCl] (0.60).4,37 However, in the presence of the bromine substituent on NCN, the Φp values of [Br-NCNPtCl] and [Br-NCNPtPa] are decreased by 2−5-fold. Assuming that the quantum efficiency of S1 → T1 is unity for the platinum(II) complexes, the phosphorescence rate constant (kp = Φp/τp) is similar for all four cases. Therefore, the decrease in Φp for both brominecontaining systems is a consequence of increased nonradiative decay rates, presumably due to the heavy bromine atom that facilitates the nonradiative T1 → S0 intersystem crossing process. Besides the monomeric behavior in a dilute THF solution, we also investigated the spectroscopic properties of the platinum(II) complexes in aggregated form. The aggregates were formed by adding poor solvent water to the dilute THF solutions. The mixed water/THF solvents are denoted as n%, which represents the percentage of water in volume and runs from 10% to 90% with an interval of 10%, and the concentration of the platinum(II) substrate was fixed at 1.0 × 10−5 M in the mixed water/THF solvents. Aggregates were formed at 50−90% for [Pa-NCNPtPa] but at 70−90% for the other three complexes. The dynamic size of the aggregates determined by dynamic light scattering (DLS; see Figure S3 and Table S6) depends on the water fraction as well as on the ligand of the platinum(II) complex. In general, the size of aggregate particles decreases with increasing water fraction. More specifically, the diameters are 1870, 970, 140, and 140 nm for the aggregates of [Pa-NCNPtPa] in 50%, 60%, 70%, and 80%, respectively, and are 130−230, 60−190, and 60−80 nm for the other three platinum(II) complexes in 70%, 80%, and 90%, respectively. The only exception to the trend is that for [Pa-NCNPtPa] in 90%, which was determined to have a diameter of 470 nm. A possible explanation for the dependence of the aggregate size on the water fraction of the mixed solvents

5 shows the corresponding emission spectra. The weak phosphorescence for all four systems in open air indicates an effective quenching by molecular oxygen. Prior to aggregate formation, the emission intensity was slightly increased (1−3%) with an increase in the water fraction for all four cases, attributable to the decreased solubility of molecular oxygen in water versus THF (2.29 × 10−5 vs 8.16 × 10−4 in molar fraction).38 However, upon the formation of aggregates, the luminescence behavior of the four platinum(II) complexes becomes very different: a large-intensity diminishment for [BrNCNPtCl], a moderate-intensity enhancement for [BrNCNPtPa], growth of a new broad emission band at longer wavelength at the expense of the monomer emission with an overall moderate emission enhancement for [Pa-NCNPtCl], and a large-intensity enhancement for [Pa-NCNPtPa]. The Φp value of [Pa-NCNPtPa] in the aggregate form determined in deaerated solutions provides a clue to understanding the effect of aggregation on the luminescence behavior of the platinum(II) complexes. The degassed 90% solution of [Pa-NCNPtPa] has a Φp of 0.23, which is significantly lower than the Φp value of 0.55 in a degassed THF solution. Evidently, forming aggregates have two opposite effects on Φp: one is to diminish the inherent emission quantum yield, a phenomenon of static quenching called aggregation-caused quenching (ACQ) as a result of intermolecular π−π interactions, and the other is to block the molecular oxygen from dynamic quenching, an oxygen shielding effect (OSE). The OSE of pentiptycene has recently been reported for pentiptycene-incorporated platinum(II) acetylides.24 A weak ACQ and/or an effective OSE could account for the phenomenon of aggregation-induced emission enhancement (AIEE) for [Pa-NCNPtPa] in aerated solutions. Because the emission in 60% and 90% is relatively stronger than that in 50%, 70%, and 80% (Figure 5), it appears that the aggregate size in the range 500−1000 nm is the optimal one for reducing ACQ and/or invoking OSE. In contrast, the emission diminishment for the pentiptycene-free complex [BrNCNPtCl] could be attributed to significant ACQ and poor OSE. The moderate emission enhancement for [Br-NCNPtPa] reveals a condition between the cases of [Pa-NCNPtPa] and [Br-NCNPtCl]. Although the green-to-magenta emission color

a

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Figure 5. Emission spectra (the excitation wavelength is 330 nm) of (a) [Br-NCNPtCl], (b) [Br-NCNPtPa], (c) [Pa-NCNPtCl], and (d) [PaNCNPtPa] in mixed water/THF (v/v) from 0% to 90% water fraction at a 10% interval in an open atmosphere and (e) plots of the relative intensity of the maxima for monomer emission (line charts) and the aggregate size for complexes (empty symbols) against the water fraction. The legend in part a applies to parts b−d.

the manuscript for the description of such an emission. The excimer structure is expected to have an antiparallel molecular orientation on the basis of the crystal structure of [PaNCNPtCl]. The same conclusion of excimer formation without invoking a MMLCT transition has also been made for the parent [H-NCNPtCl] system at high solution concentrations,4,20 although PtII···PtII interactions have been concluded to be a key factor for forming excimers of platinum(II) complexes.41 The emission spectra for [Br-NCNPtPa] and [Pa-NCNPtPa] in 90% also display excimer-like emission, albeit much weaker than that observed for [Pa-NCNPtCl]. Regarding the structural similarity to [Pa-NCNPtCl], the complex [Br-NCNPtPa] might also adopt an antiparallel packing between the NCNPt cores. The much lower excimer emission for [Br-NCNPtPa] versus [Pa-NCNPtCl] might reflect a larger steric effect when the pentiptycene is closer to the platinum atom. The even weaker but still noticeable excimer emission for [Pa-NCNPtPa] is consistent with the largest pentiptycene steric effect among the four platinum complexes. In contrast, the π−π interactions in the pentiptycene-free system [Br-NCNPtCl] resulted in neither excimer nor MMLCT emission but only emission quenching (i.e., ACQ). Evidently, the bulky pentiptycene groups gate the π−π interactions of the NCNPt moieties in a way that ACQ is largely reduced in favor of excimer emission. The solid-state photoluminescence properties of the cyclometalated platinum(II) complexes were investigated with two types of samples. The first type of samples was prepared by spreading the as-prepared powdered compounds ( [Pa-NCNPtCl] (695 nm) > [Br-NCNPtPa] (675 nm), and the opposite order is true for the relative intensity of the monomer emission. The trend might simply reflect the pentiptycene steric effect on the degree of π−π interactions for the NCNPt core in the solid state. Along this line, the lack of excimer emission for [PaNCNPtPa] in both unground and crystalline forms is not unexpected. The difference between the unground and crystalline samples of [Br-NCNPtPa] is informative. Unlike the broad emission band at 675 nm for the unground samples, the long-wavelength band at 575 nm for the crystalline samples is more structured but much less red-shifted relative to the monomer emission. This indicates that the 675 nm band has more excimer character but the 575 nm band more dimer character. The inherently lower emissive bromine-substituted complexes [Br-NCNPtCl] and [Br-NCNPtPa] remain to have relatively lower emission quantum yields (261 °C (dec). IR (KBr): 3041 (Ar C−H stretch), 1605, 1470 (ring CC stretch) cm−1. 1H NMR (400 MHz, CDCl3): δ 7.31 (m, 2H), 7.50 (s, 2H), 7.64 (d, J = 7.8 Hz, 2H), 8.00 (td, J = 7.8 and 1.5 Hz, 2H), 9.29 (d, J = 5.5 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 115.6, 119.6, 123.8, 126.7, 139.2, 142.5, 152.3, 159.7, 166.0. HRMS (FAB). Calcd for C16H10BrClN2Pt: m/z 538.9364. Found: m/z 538.9371. Synthesis of [Br-NCNPtPa]. A mixture of PaH (5.6 mg, 0.096 mmol) in 1.0 mL of THF and sodium hydroxide (3.8 mg, 0.096 mmol) in 0.5 mL of MeOH was stirred at room temperature for 30 min. To the mixture was added [Br-NCNPtCl] (0.030 g, 0.055 mmol), and the resulting mixture was stirred for another 24 h. After removal of the volatile solvent under reduced pressure, the residual was extracted by CH2Cl2 and water. The organic layer was dried over anhydrous magnesium sulfate and concentrated in vacuo. The crude product was recrystallized using CH2C2/MeOH to afford [Br-NCNPtPa] as an orange solid in a yield of 54%. Mp: >300 °C. IR (KBr): 3066 (Ar C− H stretch), 2925 (−C−H stretch), 2087 (CC stretch), 1604, 1457 (ring CC stretch) cm−1. 1H NMR (400 MHz, CDCl3): δ 0.96 (t, J = 6.7 Hz, 3H), 1.42−1.57 (m, 8H), 1.68 (m, 2H), 2.03 (m, 2H), 3.96 (t, J = 6.7 Hz, 2H), 5.69 (s, 2H), 6.27 (s, 2H), 6.91−6.93 (m, 8H), 7.30− 7.38 (m, 10H), 7.71−7.73 (m, 4H), 8.00 (t, J = 7.6 Hz, 2H), 9.73 (d, J = 5.4 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 14.2, 22.7, 26.5, 29.4, 29.6, 30.6, 32.0, 48.5, 52.5, 76.1, 105.8, 116.1, 116.5, 119.9, 123.3, 123.9, 124.1, 124.8, 125.2, 126.7, 134.4, 138.9, 141.7, 144.4, 144.9, 145.7, 146.2, 147.4, 155.8, 168.7, 177.2. HRMS (FAB). Calcd for C60H47BrN2OPt: m/z 1085.2520. Found: m/z 1085.2529.

with a R928 detector, and the range of measured decay was from 1.5 μs to 10 s. The goodness of the nonlinear least-squares fit for phosphorescence was judged by the reduced χ2 value (300 °C. IR (KBr): 3067 (Ar C−H stretch), 2926 (−C−H stretch), 2082 (CC stretch), 1606, 1459 (ring CC stretch) cm−1. 1 H NMR (400 MHz, CDCl3): δ 0.98 (t, J = 6.6 Hz, 6H), 1.43−1.58 (m, 16H), 1.71 (quintet, J = 7.2 Hz, 4H), 2.03−2.07 (m, 4H), 3.98− 4.03 (m, 4H), 5.73 (s, 2H), 5.75 (s, 2H), 5.99 (s, 2H), 6.34 (s, 2H), 6.96−7.03 (m, 16H), 7.35−7.39 (m, 10H), 7.43−7.49 (m, 8H), 7.96 (d, J = 7.7 Hz, 2H), 7.99 (s, 2H), 8.08 (td, J = 7.7 and 1.5 Hz, 2H), 9.85 (d, J = 5.7 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 14.2, 22.7, 26.4, 26.5, 29.40, 29.43, 29.60, 29.65, 30.6, 31.95, 31.97, 48.3, 48.5, 52.50, 52.53, 76.1,77.2, 83.7, 95.8, 106.2, 111.2, 116.6, 118.2, 120.0, 123.3, 123.5, 123.9, 123.96. 124.03, 124.8, 125.2, 125.3, 127.1, 134.4, 135.5, 139.0, 142.0, 143.5, 144.5, 145.1, 145.3, 145.7, 145.8, 146.3, 147.5, 149.7, 155.8, 169.2, 180.4. MALDI-TOF. Calcd for C104H84N2NaO2Pt+: m/z 1610.608. Found: m/z 1610.691.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology (MOST 104-2113-M-002-004-MY3), Taiwan, and National Taiwan University (NTU 105R891303) for financial support. We are grateful to Computer and Information Networking Center, NTU, for support of high-performance computing facilities. We thank Sung-Fu Hung for assistance in PXRD experiments.

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REFERENCES

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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.7b00009. DLS diagram and size of the aggregates in THF/water mixed solvents, absorption, excitation, and emission spectra of [Pa-NCNPtCl] in 0% and 70%, degassed emission spectra of [Pa-NCNPtCl] in a concentrated THF solution and in 0%, 30%, 50%, 70%, and 90% water/THF solutions, PXRD patterns of unground, ground, DCM-fumed, and benzene-fumed samples, quantum yields and lifetimes in the solid state, emission spectra of [Pa-NCNPtCl] before and after fuming with benzene, DMA, and BN vapors, emission spectra before and after heating, DFT-derived frontier molecular orbitals and Cartesian coordinates, selected bond lengths, bond angles, and dihedral angles in the X-ray crystal and in DFT-optimized structures, and TDDFT-calculated photophysical data (PDF) X-ray crystallographic data of [Br-NCNPtCl] and [PaNCNPtCl] in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jye-Shane Yang: 0000-0003-4022-2989 Notes

The authors declare no competing financial interest. 4988

DOI: 10.1021/acs.inorgchem.7b00009 Inorg. Chem. 2017, 56, 4978−4989

Article

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