Excimer-Monomer Photoluminescence Mechanochromism and Vapochromism of Pentiptycene-Containing Cyclometalated Platinum(II) Complexes

Article abstract of DOI:10.1021/acs.inorgchem.7b00009

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 Pt^II···Pt^II 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 [Pa-NCNPtCl], [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.

Full text of DOI:10.1021/acs.inorgchem.7b00009

Article
pubs.acs.org/IC
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 [Pa-
NCNPtCl], [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 cyclo-
metalated platinum(II) complexes [Br-NCNPtPa], [Pa-
NCNPtCl], 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 square-
planar 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 red-
shifted 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 correspond-
ing 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 V-
shaped “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 π-
stacking interactions and thus improve the photoluminescence
Received: January 3, 2017
© XXXX American Chemical Society
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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
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-to-
excimer emission intensity on external mechanical forces and
chemical vapors and on the number and locations of the
pentiptycene groups is elucidated.
coupling between 2 equiv of 2-pyridylzinc bromide prepared
in situ and 1,3,5-tribromobenzene afforded the bromo-
substituted terdentate N^C^N ligand 1-bromo-3,5-bis(2-
pyridyl)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 [Br-
NCNPtCl] 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 Br-
NCN.
2. RESULTS AND DISCUSSION
Single crystals of [Br-NCNPtCl] (Figure 1) and [Pa-
2.1. Synthesis and Structures. Scheme 1 shows the
synthesis of the target platinum(II) complexes. Negishi
NCNPtCl] (Figure 2) suitable for X-ray crystallography were
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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 pentipty-
cene−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 [Br-
NCNPtCl] (2.5 g cm−3; Tables S1 and S2).
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.
The lack of metallophilic PtII···PtII interactions for both [Br-
NCNPtCl] 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 [Pa-
NCNPtCl] 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] {H-
NCN = 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 [CF3-
NCNPtC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 [Pa-
NCNPtCl] 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 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 square-
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Figure 3. (a) Absorption and (b) normalized emission (phosphorescence) spectra of [Br-NCNPtCl] (blue), [Br-NCNPtPa] (green), [Pa-
NCNPtCl] (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.
ray crystallography for further confirmation. A more detailed
discussion on the solid-state photoluminescence will be
provided in sections 2.2 and 2.3.
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 [Pa-
NCNPtPa]. As shown in Figure S2, the H orbitals of [Br-
NCNPtCl] and [Pa-NCNPtCl] and the H−2 orbitals of [Br-
NCNPtPa] 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 [Pa-
NCNPtPa]. 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], [Br-
NCNPtPa], and [Pa-NCNPtCl], but the band undergoes
both bathochromic and hyperchromic shifts for [Pa-
NCNPtPa], 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) > [Pa-
NCNPtCl] (521 nm) > [Br-NCNPtPa] (512 nm) > [Br-
NCNPtCl] (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
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, nitro-
gen, 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 [Pa-
NCNPtCl], 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 [Br-
NCNPtPa] and [Pa-NCNPtPa] is similar to that in [Br-
NCNPtCl] 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, config-
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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.
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.
Table 1. Photophysical Data of the Cyclometalated
Platinum(II) Complexes in THF
Figure 4 shows the images of photoluminescence of the
water/THF mixed solutions in an open atmosphere, and Figure
λabs (nm) [ε
(×104
kp
c
τp
(×104
a
b
c
compound
[Br-
L mol−1 cm−1)]
λp (nm)
Φp (%)
(μs)
s−1
)
383 [0.7], 428
[0.6]
501 (535)
512 (538)
521 (553)
529
11 (2.4)
33 (1.3)
66 (1.4)
1.6
6.9
7.9
5.7
5.3
NCNPtCl]
[Br-
NCNPtPa]
[Pa-
NCNPtCl]
[Pa-
NCNPtPa]
429 [0.6]
4.2
11.6
10.4
328 [2.4], 430
[0.6]
330 [4.6], 430
[1.1]
55 (1.6)
Figure 4. Photographic images of the emission color for [Br-
NCNPtCl], [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.
a
b
The concentration is (4.0 1.0) × 10−6 M. Shoulders are given in
c
parentheses. Values were obtained in a degassed solution (excitation
at 355 nm), and values in parentheses are for the aerated solution.
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 [Br-
NCNPtCl], a moderate-intensity enhancement for [Br-
NCNPtPa], 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 under-
standing 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 [Br-
NCNPtCl] 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
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 bromine-
containing 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
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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) [Pa-
NCNPtPa] 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.
change for [Pa-NCNPtCl] complicates the comparison, a
moderate emission enhancement is concluded by spectral
analysis. Evidently, the AIEE behavior qualitatively correlates
with the number of pentiptycene units in the molecular
complexes: namely, the more pentiptycene groups in the
complex, the weaker the ACQ and/or the larger the OSE. It
should be noted that in degassed solutions it is ACQ but not
AIEE that is expected, as demonstrated by the case of [Pa-
NCNPtCl] (Figure S4).
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 [Pa-
NCNPtCl]. The same conclusion of excimer formation without
invoking a MMLCT transition has also been made for the
parent [H-NCNPtCl] system at high solution concentra-
tions,4,20 although PtII···PtII interactions have been concluded
to be a key factor for forming excimers of platinum(II)
complexes.41
The green-to-magenta emission color switching for [Pa-
NCNPtCl] upon forming aggregates indicates the presence of
strong intermolecular interactions. Two pieces of evidence
reveal that the red emission band results from a LC excimeric
state rather than from an MMLCT state: First, forming an
MMLCT state generally requires a PtII···PtII distance shorter
than 3.5 Å in the ground state, but this requirement is less likely
to be fulfilled in view of the large PtII···PtII distance of 5.324 Å
in the X-ray crystal structure (Figure 2); in contrast, the short
interplanar NCNPt distance of 3.353 Å allows π−π* dimer or
excimer formation. Second, formation of an MMLCT state
would be associated with a new and highly bathochromic
transition band in the absorption and excitation spectra,39,40
which is not the case (Figure S5). A broadening of the
absorption and emission spectra upon forming aggregates
indicates the presence of ground-state intermolecular inter-
actions or association. Although MMLCT emission could not
be completely ruled out in any case, its contribution to the
observed broad long-wavelength emission should be rather
small. Because excimer emission has been detected for [Pa-
NCNPtCl] at high concentration (Figure S6) and a pure dimer
emission is generally more structured25 and less red-shifted
than an excimer emission (vide infra), we conclude that the
observed broad long-wavelength emission band is from an
excimer (repulsive in the ground state) formed from a
preassociated dimer (attractive in the ground state). We
might call it a “preassociated excimer” state. For the sake of
simplicity, “excimer emission” is adopted here and thereafter in
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 cyclo-
metalated platinum(II) complexes were investigated with two
types of samples. The first type of samples was prepared by
spreading the as-prepared powdered compounds (<1 mg) on a
quartz plate (1 cm × 1 cm). The films were then fumed with
dichloromethane vapor for 10−60 s to remove any potential
mechanical forces (vide infra) employed upon sample spread-
ing. Such dichloromethane (DCM)-fumed powder films are
called “unground samples”. The second type of samples is the
crystalline powders prepared from DCM/hexane mixed
solutions, and we call them “crystalline samples”.
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Figure 6 shows the normalized solid-state emission spectra of
the unground and crystalline samples recorded at ambient
Figure 6. Normalized solid-state emission spectra of unground (solid
curves) and crystalline (dashed curves) samples of [Br-NCNPtCl]
(blue), [Br-NCNPtPa] (green), [Pa-NCNPtCl] (black), and [Pa-
NCNPtPa] (red). The excitation wavelength is 340 nm.
temperature in an open atmosphere. The pentiptycene-free
complex [Br-NCNPtCl] displays only excimer emission with a
maximum at 700 nm for the unground samples and at 720 nm
for the crystalline samples. The absence of monomer emission
reveals strong intermolecular π−π interactions. This is
consistent with the antiparallel molecular packing mode
observed in the X-ray crystal structure (Figure 1) but in great
contrast to its aggregates formed in water/THF solutions
(Figure 5a). For [Pa-NCNPtCl] and [Br-NCNPtPa], which
contain a pentiptycene group at one of the ligands, both
monomer and excimer emissions are present for the unground
samples. The wavelength for the maximum of excimer emission
is in the order [Br-NCNPtCl] (700 nm) > [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 [Pa-
NCNPtPa] 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 (<0.1%) than the
other two complexes [Pa-NCNPtCl] (3.8%) and [Pa-
NCNPtPa] (1.3%) in the solid state (Table S7).
Figure 7. Normalized emission spectra (excitation at 380 nm) and
corresponding luminescent photographs of unground (black) and
ground (red) samples for (a) [Br-NCNPtCl], (b) [Br-NCNPtPa], (c)
[Pa-NCNPtCl], and (d) [Pa-NCNPtPa].
shift of 19 nm for the excimer emission band of [Br-NCNPtCl],
a diminution of the monomer-to-excimer emission intensity for
[Br-NCNPtPa], a blue shift of 14 nm for the excimer emission
along with a small increase of the monomer intensity for [Pa-
NCNPtCl], and formation of excimer emission at 662 nm for
[Pa-NCNPtPa]. The variations in the excimer versus monomer
emission intensity and/or the blue shift of excimer emission
indicate that intermolecular interactions in [Br-NCNPtCl] and
[Pa-NCNPtCl] are weakened by external forces, but the
opposite is true for the cases of [Br-NCNPtPa] and [Pa-
NCNPtPa]. Strong π−π interactions for unground [Br-
NCNPtCl] and [Pa-NCNPtCl] have been supported by their
X-ray crystal structures (Figures 1 and 2). A minor interruption
of such interactions by external forces that increase the
interplanar distance or reduce the interplanar stacking area
could account for the mechanochromic luminescence behavior.
In contrast, as schematically depicted in Figure 8, [Br-
NCNPtPa] and [Pa-NCNPtPa] have weak intermolecular
π−π interactions in the unground form due to a relatively
2.3. Mechanochromic and Vapochromic Lumines-
cence Properties. The solid-state unground samples of the
cyclometalated platinum(II) complexes have been tested with
external stimuli such as mechanical forces and organic solvent
vapors. The mechanical forces were exerted by gentle manual
pressing and grinding with a spatula spoon. Both the unground
and ground samples were tested with aromatic and non-
aromatic solvent vapors.
Figure 7 shows the normalized emission spectra and
photographic images of the unground and ground samples.
The mechanochromic photoluminescence responses are highly
dependent on the complex structure: grinding resulted in a blue
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diffraction (PXRD) patterns of the unground and ground
samples show that grinding converts the phase from crystalline
to amorphous (Figure S7). The broader PXRD patterns for the
unground [Br-NCNPtPa] and [Pa-NCNPtPa] relative to the
other two systems indicate that the Pa ancillary ligand lowers
the crystallinity of the platinum(II) complexes, which might
account for the difficulty of obtaining their single crystals.
The mechanochromism is reversible, and the initial spectra
for unground samples could be restored by DCM fuming. This
could be understood by a solid−vapor “recrystallization”
process, which converts the metastable amorphous phase
back to the crystalline phase. This rational is supported by the
recovery of PXRD signals (Figure S7), albeit in weaker
intensity. The DCM vapor molecules function as a grease
that facilitates motion of the platinum(II) substrate in the solid
state.
Among the four platinum(II) complexes, the ground [Pa-
NCNPtCl] displays a unique response to benzene vapor.
Unlike DCM fuming, which removes the effect of grinding, the
benzene fuming of the ground [Pa-NCNPtCl] samples follows
the effect of grinding to further enhance the monomer-to-
excimer intensity ratio but meanwhile shifts the excimer
emission band back to the position for the unground form
(i.e., from 681 to 695 nm; Figure 9a). This suggests that
benzene vapors not only recrystallize the amorphous ground
samples but also enlarge the interplanar NCNPt−NCNPt
distance by intercalation. The recrystallization effect of benzene
vapors is indeed evidenced by strong PXRD signals of the
benzene-fumed samples (Figure S7), and the 1H NMR
spectrum for the benzene-fumed samples (Figure S8) indicates
intercalation and/or adsorption of benzene with a ratio of
benzene:[Pa-NCNPtCl] = 1.5:1. For comparison, the effects of
benzene vapors on the unground [Pa-NCNPtCl] samples were
also investigated. The results shown in Figure 9b reveal the
same behavior of benzene intercalation that converts excimer
into monomer emission. The process of intercalation in
reaching equilibrium is fast (9−15 min) for both the unground
and ground samples, although the ground one is slightly faster,
attributable to weaker π−π and d−π interactions between the
NCNPt cores and/or more vapor diffusion channels in the
amorphous versus crystalline phase.
Figure 8. Schematic drawings for the mechanochromic luminescence
behavior of (a) [Br-NCNPtPa] and (b) [Pa-NCNPtPa] and for (c)
variable pentiptycene−pentiptycene packing modes.
large pentiptycene steric shielding effect, and a breakdown of
the shielding by external forces toward short contacts of the
NCNPt cores is viable. Note that, despite the changes in the
emission spectra, the grinding processes did not alter the
emission quantum yields of the complexes in the solid state
(Table S7). In addition, the monomer emissions have shorter
lifetimes than excimer emissions, and the values are little
affected by grinding (Table S8).
Regarding the photoluminescence color, it is essentially
unchanged for the pentiptycene-free system [Br-NCNPtCl] in
spite of a 19 nm blue shift of excimer emission. In contrast, the
other three pentiptycene derivatives display emission color
changes, which are from yellowish brown to dark orange for
[Br-NCNPtPa], from red to orange for [Pa-NCNPtCl], and
from green to yellow for [Pa-NCNPtPa]. In the aspect of
spectral change, the main difference between [Pa-NCNPtCl]
and [Br-NCNPtCl] is the involvement of monomer emission
for the former but not for the latter. The other two
pentiptycene-containing platinum(II) complexes also involve
both monomer and excimer emission. Therefore, we might
conclude that interplay of the monomer versus excimer dual
emission is key for the observed mechanochromism. The
interplay of the monomer−excimer intensity might be
associated with the potential polymorphism of pentiptycene
derivatives because pentiptycene−pentiptycene interactions
could lead to a variety of possible packing modes (e.g., the
UU, UV, and VV packings in Figure 8c) of similar energy. This
structural variability is believed to play an important role in the
observed mechanochromism. Indeed, the powder X-ray
Further investigation on the response of [Pa-NCNPtCl] to
other chemical vapors shows that the vapor intercalation
occurred only with aromatic vapors, indicating the important
role of solvent−substrate π−π interactions. In addition, the
vapochromism is slightly enhanced with the electron-donating
N,N-dimethylaniline (DMA) and slightly reduced with the
electron-accepting benzonitrile (BN) compared with benzene
Figure 9. Benzene-vapor-induced photoluminescence responses for (a) ground and (b) unground [Pa-NCNPtCl] samples. The excitation
wavelength is 390 nm.
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(Figure S9), consistent with the expected size of substrate−
vapor π−π interactions for an electron-accepting NCNPt core.
However, unlike the organic counterpart that forms exciplex
between the substrate and the aniline vapor molecules,31 the
NCNPt−aniline π−π interactions drive the excimer to
monomer rather than exciplex emission. Therefore, formation
of triplet-state exciplexes between the NCNPt core and
aromatic electron donors or acceptors is unfavorable, even
though the T1 state of [Pa-NCNPtCl] is of significant LC π−π*
character.
Examples of benzene-vapor-induced luminescence color
change for platinum(II) complexes are rare. Mann and co-
workers reported two cis-diisocyanidedicyanoplatinum(II)
systems, [Pt(CN-p-(C2H5)C6H4)2(CN)2] and [Pt(CN-i-
C3H7)2(CN)2],42,43 that display blue-shifted emission upon
exposure to volatile aromatic hydrocarbons such as benzene
and toluene. In contrast, Zhang and co-workers reported a red
shift of the emission spectra for the platinum(II) acetylide
[Pt(TMSCCPhenCCTMS)(CCC6H4-p-Cl)2] upon
fuming with benzene vapor.44 All of these cases are associated
with changes in the PtII···PtII distance and interactions. Our
results of [Pa-NCNPtCl] provide a new example of benzene-
vapor-responsive platinum(II) complexes with the less
common mechanism of monomer−excimer intensity ratio
changes.
expected. However, unlike the case of [Pa-NCNPtCl], the
benzene intercalation effect on promoting the monomer
emission follows the same trend of DCM fuming for both
[Br-NCNPtPa] and [Pa-NCNPtPa]. Consequently, the inter-
play of monomer−excimer dual emission in response to
benzene vapors is the same as that to DCM vapors. In
summary, the unique vapochromic response of [Pa-NCNPtCl]
to benzene vapors originates from a molecular packing that
contains void volume and weak NCNPt−NCNPt π−π
interactions.
For comparison, a preliminary test of the effect of heating on
the photoluminescence of the unground and ground samples
has been conducted. As shown in Figure S12, the effect of
heating is similar to that of grinding: namely, intermolecular
π−π interactions are diminished for [Br-NCNPtCl] and [Pa-
NCNPtCl] but enhanced for [Br-NCNPtPa] and [Pa-
NCNPtPa]. The heating effect is more significant for [Br-
NCNPtCl] than the others by causing not only a blue shift of
the excimer emission but also growth of the monomer
emission, indicating a larger perturbation than grinding on
molecular packing.
3. CONCLUSION
Unlike the majority of stimuli-responsive platinum(II)
complexes that rely on modification of the metallophilic Pt···
Pt interactions and thus the nature of the MMLCT transition, it
is the interplay between LC monomer and excimer emission
that accounts for the mechanochromic and vapochromic
luminescence properties of the cyclometalated platinum(II)
complexes [Pa-NCNPtCl], [Br-NCNPtPa], and [Pa-
NCNPtPa]. The unique features of benzene-induced vapo-
chromism and aggregation-induced emission color change for
[Pa-NCNPtCl] are associated with the pairwise π−π/d−π
interactions between the NCNPt cores. The critical role of the
pentiptycene scaffold in promoting the mechanochromism and
vapochromism is manifested by a comparison with the
pentiptycene-free system [Br-NCNPtCl]. The distinct behavior
among the three pentiptycene-containing systems also reveals
the importance of the locations and number for the
pentiptycene group in the molecular design. This work
confirms the utility of the H-shaped pentiptycene scaffold in
creating new mechanochromic and vapochromic luminescent
materials, which applies not only to fluorescent organic systems
but also to phosphorescent organometallic systems.
Figure 10 summarizes the mechanochromic and vapochro-
mic luminescence properties of [Pa-NCNPtCl]. The red,
Figure 10. Mechanochromic and vapochromic luminescence proper-
ties of [Pa-NCNPtCl]. The red, blue, and black arrows represent
stimuli of grinding, benzene fuming, and DCM fuming, respectively.
orange, and yellow emission colors of the thin solid films could
be generated by DCM fuming, grinding, and benzene fuming,
respectively. A schematic representation of possible molecular
alignments in accounting for the difference of emission color is
provided in Figure S10.
4. EXPERIMENTAL SECTION
4.1. General Methods. The 1H and 13C NMR spectra were
collected using a Bruker AVIII 400 MHz spectrometer. Mass spectra
were collected by using fast-atom-bombardment (FAB) ionization
with a JEOL JMS-700 spectrometer or by using matrix-assisted laser
desorption/ionization (MALDI) with a Bruker MALDI-TOF/TOF
(TOF = time-of-flight) spectrometer. Electronic spectra were recorded
at room temperature (23 1 °C). UV/visible spectra were recorded
using a Cary300 double-beam spectrophotometer. Emission spectra
were recorded using an Edinburgh FLS920 spectrometer at ambient
temperature and corrected for the response of the detector. A solution
of quinine bisulfate [Φf = 0.546 in 1 N H2SO4(aq)]45 was used as the
standard for the phosphorescence quantum yield determination,
corrected with a solvent refractive index, of compounds in a solution
under three times freeze−pump−thaw cycles. The optical density of all
solutions was about 0.1 at the wavelength of excitation, and an error of
5% was estimated for emission quantum yields. Phosphorescence
decays were measured by means of an Edinburgh FLS920
spectrometer apparatus with a μF900, a pulsed xenon flash lamp,
The lack of new spectroscopic responses for the other three
platinum(II) complexes with benzene versus DCM fuming
deserves a discussion (Figure S11). For [Br-NCNPtCl], this
could be understood by the fact that the unground form has a
close packing mode with strong π−π and d−π interactions
(Figure 1), which would not allow diffusion or intercalation of
benzene molecules. Such a situation remains for the ground
form because the intermolecular π−π and d−π interactions are
only slightly weakened on going from the unground to ground
form. Consequently, benzene vapors only assist “recrystalliza-
tion” of the metastable ground form back to the stable
unground form like the case of nonaromatic DCM vapors. For
[Br-NCNPtPa] and [Pa-NCNPtPa], the void volume created
by the pentiptycene group(s) like the case of [Pa-NCNPtCl] is
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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 (<1.2 in all
cases), the randomness of the residuals, and the autocorrelation
function. The absolute quantum yields for powders in air were
determined using an integrating sphere (150 mm diameter, BaSO4
coating) of Edinburgh Instruments by the Edinburgh FLS920
spectrometer. Photographs were taken using an Olympus E-M1
camera or an Olympus IX73 inverted microscope equipped with a
DP73 color camera. The light source of the microscope was a halogen
lamp equipped with an Olympus U-FUW filter, which passed light in
the range of 340−390 nm. Single crystals were determined using a
Rigaku Oxford Diffraction diffractometer (measurement device type:
Xcalibur, Atlas, Gemini). The PXRD patterns were collected on a
Bruker D2 Phaser diffractometer using nickel-filtered Cu Kα radiation
(λ = 1.5413 Å) at a voltage of 30 kV and a current of 10 mA. Data
generated with a step size of 0.02° and with a step time of 30 s over a
2θ range of 5−20°. Data were processed in the DIFFRAC.EVA
program. The particle size was measured by a Malvern Nano ZS90
laser particle analyzer at room temperature.
and the mixture was stirred for 2 h at room temperature. To the
mixture was added a solution of 1,3,5-tribromobenzene (1.57 g, 5.0
mmol) and Pd(PPh3)4 (0.35 g, 0.3 mmol) in 10 mL of THF, and the
mixture was stirred for another 24 h at room temperature. After
removal of the volatile solvent under reduced pressure, the residual
was purified by column chromatography with ethyl acetate/hexane
(1:3, v/v) as the eluent, affording yellow powder Br-NCN in a yield of
46%. Mp: 159.1−159.8 °C. 1H NMR (400 MHz, CDCl3): δ 7.24−7.28
(m, 2H), 7.74−7.81 (m, 4H), 8.22 (d, J = 1.5 Hz, 2H), 8.56 (s, 1H),
8.71 (d, J = 4.5 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 120.6,
122.7, 123.5, 123.9, 130.2, 136.8, 141.5, 149.7, 155.5. IR (KBr): 3051
(Ar C−H stretch), 1585, 1560, 1472, 1415 (ring CC stretch) cm−1.
HRMS (FAB). Calcd for C16H11BrN2: m/z 310.0106. Found: m/z
310.0105.
Synthesis of Pa-NCN. A mixture of Br-NCN (0.16 g, 0.51 mmol),
PaH (0.36 g, 0.62 mmol), Pd(PPh3)4 (0.030 g, 0.06 mmol), and
diisopropylamine (1.0 mL) in dry benzene (3.0 mL) was heated to 80
°C for 24 h under a nitrogen/hydrogen mixed gas. The mixture was
cooled to room temperature and diluted with CH2Cl2. The
undissolved portion was filtered off, and the filtrate was concentrated
under reduced pressure. The residual was purified by column
chromatography with ethyl acetate/hexane (1:2, v/v) as the eluent
to afford Pa-NCN as a yellow solid in a yield of 81%. Mp: 71.1−72.4
°C. IR (KBr): 3067 (Ar C−H stretch), 2925 (−C−H stretch), 2205
(CC stretch), 1583, 1566, 1459 (ring CC stretch) cm−1. 1H
NMR (400 MHz, CDCl3): δ 0.99 (t, J = 6.6 Hz, 3H), 1.42−1.57 (m,
8H), 1.71 (quintet, J = 7.5 Hz, 2H), 2.05 (quintet, J = 7.2 Hz, 2H),
4.01 (t, J = 6.7 Hz, 2H), 5.74 (s, 2H), 5.99 (s, 2H), 6.96−7.01 (m,
8H), 7.34−7.37 (m, 6H), 7.46−7.48 (m, 4H), 7.88 (td, J = 7.7 and 1.5
Hz, 2H), 8.00 (d, J = 8.0 Hz, 2H), 8.49 (d, J = 1.4 Hz, 2H), 8.73 (s,
1H), 8.86 (d, J = 4.2 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ14.1,
22.7, 26.4, 29.4, 29.6, 30.5, 31.9, 48.3, 52.4, 76.1, 85.4, 94.6, 110.9,
120.9, 122.7, 123.4, 124.0, 124.9, 125.2, 125.7, 130.6, 135.4, 137.0,
140.5, 145.0, 145.2, 146.0, 149.9, 156.6. HRMS (FAB). Calcd for
C60H48N2O: m/z 812.3767. Found: m/z 812.3771.
4.2. Sample Preparations. Preparation of Aggregates. For each
water/THF solution, a 3 mL solution sample of 1.0 × 10−5
M
concentration was prepared. The designated amount of substrate was
first dissolved in the designated amount of THF, to which the
designated amount of water was then added to make a total volume of
3 mL. The resulting solution was shaken vigorously for 10 s, and then
the solution was intermediately subjected to DLS measurements on
the particle size to determine the emission spectra.
Preparation of Crystalline Samples. The substrate (0.3 mg) was
dissolved in dichloromethane (2 mL) in an open 7 mL vial, which was
put into a closed 50 mL vial containing 10 mL of hexane for the
growth of crystals. The crystalline powders were collected by a glass
pipet and dried with warm air followed by determination of the
emission spectra.
Preparation of Unground Samples. The as-prepared powdered
substrate (<1 mg) was spread on a quartz plate (1 cm × 1 cm). The
plates were fumed with DCM vapor for 10−60 s until the
photoluminescence color of the film was homogeneous. Such DCM-
fumed films are called “unground samples”.
Preparation of Ground Samples. The ground samples were
prepared by gentle hand pressing and grinding of the unground
samples with a spatula spoon until the photoluminescence color was
homogeneous.
Synthesis of [Br-NCNPtCl]. A mixture of Br-NCN (0.16 g, 0.50
mmol) and K2PtCl4 (0.45 g, 0.50 mmol) in 8.0 mL of glacial acetic
acid after three freeze−pump−thaw cycles was heated at 130 °C for 3
d. After cooling to ambient temperature, the resulting orange
precipitate was filtered off and washed sequentially with water,
methanol (MeOH), and diethyl ether. The precipitate was recrystal-
lized using CH2Cl2/MeOH to afford [Br-NCNPtCl] as an orange
solid in a yield of 38%. Mp: >261 °C (dec). IR (KBr): 3041 (Ar C−H
Preparation of Fumed Samples. The unground and/or ground
samples were put in a 7 mL vial containing cotton that soaked 0.5 mL
of the target solvents or compounds for 10−60 s until no further
change in the photoluminescence.
Preparation of Annealed Samples. The quartz plates were put on
a hot plate of 130 °C for 30 s.
1
stretch), 1605, 1470 (ring CC stretch) cm−1. H 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.
4.3. DFT Calculations. DFT calculations were performed using
the Gaussian 09 program46 package. The SDD36 basis set was used
with a relativistic effective core potential for platinum, and all ligand
atoms (carbon, hydrogen, nitrogen, oxygen, and chlorine) were
described by the M062X 6-31G(d,p) basis set.35 The optimized
ground-state geometries were investigated by varying the dihedral
angle between every four atoms except for the hydrogen atoms.
TDDFT calculation was conducted based on optimized structures
using the same method. Molecular orbital contributions were analyzed
by using GaussSum 3.0.47
4.4. Materials. Solvents for organic synthesis were reagent- or
high-performance liquid chromatography (HPLC)-grade, but for
spectra and quantum yield measurement, they were all HPLC-grade.
All new compounds were characterized with 1H and 13C NMR and IR
spectroscopy and mass spectrometry. Compound PaH was synthesized
according to the literature procedure.25
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.
Synthesis of Br-NCN. To a solution of 2-bromopyridine (1.14 mL,
12.0 mmol) in 40 mL of THF in a 250 mL Schlenk flask was added a
solution of 1.6 M n-butyllithium (8.25 mL, 13.2 mmol) at −80 °C.
The mixture was stirred at −80 °C for another 2 h. A solution of
ZnCl2 (1.64 g, 12.0 mmol) in 15 mL of THF was added to the flask,
J
DOI: 10.1021/acs.inorgchem.7b00009
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Inorganic Chemistry
Article
Synthesis of [Pa-NCNPtC]. The compound was prepared according
to the procedure described for complex [Br-NCNPtCl], except that
Pa-NCN (0.30 g, 0.37 mmol) was used in place of Br-NCN, to afford a
yellow solid [Pa-NCNPtCl] in a yield of 58%. Mp: >300 °C. IR (KBr):
3067 (Ar C−H stretch), 2924 (−C−H stretch), 2202 (CC stretch),
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.
1
1606, 1460 (ring CC stretch) cm−1. H NMR (400 MHz, CDCl3):
δ 0.97 (t, J = 6.8 Hz, 3H), 1.42−1.68 (m, 8H), 1.70 (quintet, J = 7.4
Hz, 2H), 2.05 (quintet, J = 7.2 Hz, 2H), 4.00 (t, J = 6.8 Hz, 2H), 5.73
(s, 2H), 5.93 (s, 2H), 6.96−7.02 (m, 8H), 7.17 (t, J = 5.9 Hz, 2H),
7.35−7.37 (m, 4H), 7.46−7.48 (m, 4H), 7.71 (s, 2H), 7.80 (d, J = 7.6
Hz, 2H), 8.01 (dt, J = 7.6 and 1.4 Hz, 2H), 9.33 (d, J = 5.0 Hz, 2H).
13C NMR (100 MHz, CDCl3): δ 14.2, 22.7, 26.4, 29.4, 29.6, 30.6, 32.0,
48.3, 52.5, 76.2, 77.3, 84.0, 95.7, 111.1, 117.9, 119.7, 123.6, 124.0,
125.26, 125.33, 127.1, 135.6, 139.1, 141.1, 145.1, 145.2, 145.7, 149.8,
152.2, 162.6, 166.4. HRMS (FAB). Calcd for C60H47ClN2OPt: m/z
1041.3025. Found: m/z 1041.3021.
Synthesis of [Pa-NCNPtPa]. The compound was prepared
according to the procedure described for complex [Br-NCNPtPa],
except that [Pa-NCNPtCl] (0.050 g, 0.048 mmol) was used in place of
[Br-NCNPtCl], to afford [Pa-NCNPtPa] as a yellow solid in a yield of
95%. Mp: >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.
1H 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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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00009.
■
S
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 [Pa-
NCNPtCl] in CIF format (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: jsyang@ntu.edu.tw.
ORCID
Jye-Shane Yang: 0000-0003-4022-2989
Notes
The authors declare no competing financial interest.
■
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