Probing the structural origins of vapochromism of a triarylboron- functionalized platinum(II) acetylide by optical and multinuclear solid-state NMR spectroscopy

Article abstract of DOI:10.1021/ic102349h

A vapoluminescent triarylboron-functionalized platinum(II) complex that displays a mechanism of vapochromism differing from all previously reported platinum(II) compounds has been synthesized. The luminescence color of 1 switches in response to many volat

Full text of DOI:10.1021/ic102349h

ARTICLE
pubs.acs.org/IC
Probing the Structural Origins of Vapochromism of a
Triarylboron-Functionalized Platinum(II) Acetylide by Optical and
Multinuclear Solid-State NMR Spectroscopy
Zachary M. Hudson,† Christina Sun,† Kristopher J. Harris,‡ Bryan E. G. Lucier,‡ Robert W. Schurko,*,‡ and
Suning Wang*,†
†Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6
‡Department of Chemistry & Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor, Ontario, Canada N9B 3P4
S Supporting Information
b
ABSTRACT: A vapoluminescent triarylboron-functionalized
platinum(II) complex that displays a mechanism of vapochro-
mism differing from all previously reported platinum(II) com-
pounds has been synthesized. The luminescence color of 1
switches in response to many volatile organic compounds in the
solid state, including hexanes, CH2Cl2, benzene, and methanol.
While vapochromism due to changes in PtꢀPt or πꢀπ stacking
interactions has been commonly observed, absorption and
luminescence studies and single-crystal and powder X-ray diffraction data as well as multinuclear solid-state NMR experiments
(
195Pt, 13C, 11B, 2H, and 1H) revealed that the vapochromic response of 1 is instead due to changes in the excited-state energy levels
resulting from local interactions of solvent molecules with the complex. Furthermore, these interactions result in inversion of the
lowest-energy excited states of the complex in some cases, the first observation of this phenomenon in the solid state.
’ INTRODUCTION
optical properties,7 and highly sensitive and selective chemical
sensors for fluoride and cyanide.8 Furthermore, the triarylboron
group has recently been shown to greatly enhance metal-to-ligand
charge-transfer (MLCT) phosphorescence in complexes of Pt(II)
and iridium(III), making this functionality attractive for use in
phosphorescent materials as well.9
There has been considerable interest in recent years in the
development of vapochromic sensor materials for the detection
of volatile organic compounds (VOCs).1ꢀ3 Such materials rever-
sibly show changes in absorption and/or luminescence color
upon exposure to VOCs and are attractive because of their ability
to provide a highly sensitive and rapid response to volatile
contaminants, as well as their potential for use in portable
fiber-optic devices.3c,d,4 Vapochromic materials based on gold(I)
[Au(I)]3 and platinum(II) [Pt(II)]1,2aꢀ2h are the most exten-
sively investigated and usually take advantage of metalꢀmetal or
metallophilic interactions in the solid state. The enhancement or
disruption of this interaction upon adsorption of VOCs can thereby
alter the highest occupied molecular orbital (HOMO)ꢀlowest
unoccupied molecular orbital (LUMO) gap, leading to distinct
absorption or emission color changes. Vapochromic systems
have also been investigated based on the reversible binding of an
analyte such as SO2 to a fifth coordination site of Pt(II).2g Further-
more, flexible coordination polymers based on a Pt(II) scaffold have
proven useful in hostꢀguest sensing applications as well.2i
Herein we report the first example of a vapochromic material
based on triarylboron and Pt(II), which shows a distinct luminescent
response to a wide variety of VOCs (1 in Chart 1). Furthermore,
extensive investigations into the structural origins of vapochromism
in this system using optical and multinuclear solid-state 195Pt, 13C,
2
1
11B, H, and H NMR spectroscopies have indicated that the
mechanism of this vapochromic behavior arises not from differences
in metallophilic or πꢀπ stacking interactions but instead from
modulations of the excited-state energy levels on individual molecules
due to interactions with an adsorbed VOC analyte. We report herein
the first comprehensive solid-state NMR study on a vapochromic
material and demonstrate that the vapochromism of this material
differs in origin from those of all previously reported examples.
Luminescent materials based on triarylboranes have also been
the focus of much recent research activity, although until now toward
entirely different applications. Because of the electron-accepting
empty pπ orbital on the boron center, these compounds have
been developed into a wide range of functional materials.5 These
include highly efficient luminescent and electron-transport materials
for organic light-emitting diodes,6 compounds with nonlinear
’ EXPERIMENTAL SECTION
Reagents and Physical Characterization. All reagents were
purchased from Sigma-Aldrich and used without further purification.
Received: November 24, 2010
Published: March 22, 2011
r
2011 American Chemical Society
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Chart 1
Thin-layer and flash chromatography were performed on silica gel. Solution
1H and 13C NMR spectra were recorded on a Bruker Avance 400, 500, or
600 MHz spectrometer. Deuterated solvents were purchased from
Cambridge Isotopes and used without further drying. Excitation and
emission spectra were recorded using a Photon Technologies Interna-
tional QuantaMaster model 2 spectrometer. UV/visible spectra were
recorded using an Ocean Optics CHEMUSB4 absorbance spectro-
photometer. Cyclic voltammetry (CV) experiments were performed
using a BAS CV-50W analyzer with a scan rate of 0.2ꢀ1.0 V/s using 5
mg of the sample in 3 mL of dry N,N-dimethylformamide (DMF). The
electrochemical cell was a standard three-compartment cell composed of
a Pt working electrode, a Pt auxiliary electrode, and an Ag/AgCl
reference electrode. CV measurements were carried out at room
temperature with 0.1 M tetrabutylammonium hexafluorophosphate as
the supporting electrolyte, with ferrocene/ferrocenium as the internal
standard (E° = 0.55 V). Elemental analyses were performed by Canadian
Microanalytical Service Ltd., Delta, British Columbia, Canada. Pow-
der X-ray diffraction (PXRD) patterns were acquired using a Bruker D8
Discover diffractometer using Cu KR radiation (λ = 1.540 56 Å);
samples of 1 were exposed to the vapor of each solvent for a minimum
of 8 h in a sealed chamber and then packed into glass capillaries,
which were sealed with silicon grease. Photoluminescent quantum
yields were measured using the optically dilute method (A ≈ 0.1)
at room temperature in degassed CH2Cl2 using fac-Ir(ppy)3 in
2-methyltetrahydrofuran (2-MeTHF) as the standard (Φr = 0.97),10
using the equation
(dppp = 1,3-diphenylphosphinopropane),15 and Pt(dbbpy)(CtCC6H5)2
(3)16 have been reported previously.
Synthesis of Pt(dbbpy)(CtCC6H4BMes2)2 (1). To a
100 mL Schlenk flask with a stir bar and condenser was added
p-(dimesitylboryl)phenylacetylene (143 mg, 0.408 mmol, 2.2 equiv),
dichloro(dbbpy)platinum(II) (99 mg, 0.185 mmol, 1.0 equiv), CuI (8
mg, 0.041 mmol, 0.1 equiv), and 55 mL of degassed tetrahydrofuran
(THF)/triethylamine (NEt3) (10:1, v/v). The mixture was heated to
reflux under N2 for 16 h, then concentrated in vacuo, and partitioned
between CH2Cl2 and water. The aqueous layer was further extracted
with 2 ꢁ 30 mL of CH2Cl2, dried with MgSO4, filtered, and concen-
trated to 5 mL with the appearance of a brown precipitate. This was then
further purified by column chromatography on silica (benzene) to afford
compound 1 as an orange powder. (188 mg, 87%). 1H NMR (400 MHz,
C6D6): δ 9.60 (d, br, J = 4.7 Hz, 2H, bipy), 7.90 (d, J = 7.9 Hz, 4H,
ꢀPhꢀ), 7.82 (d, J = 8.1 Hz, 4H, ꢀPhꢀ), 7.59 Hz (s, br, 2H, bipy), 6.92
(s, 8H, Mes), 6.61 (d, J = 4.7 Hz, 2H, bipy), 2.32 (s, 12H, Mes), 2.31 (s,
24H, Mes), 1.09 (s, 18H, t-Bu). 13C NMR (100 MHz, C6D6): δ 162.1,
156.7, 150.4, 143.2, 142.5, 141.1, 138.6, 137.1, 134.2, 132.0, 128.8,
128.2, 127.9, 124.2, 119.8, 35.4, 30.1, 23.9, 21.3. Anal. Calcd for
C70H76B2N2Pt: C, 72.35; H, 6.59; N, 2.41. Found: C, 72.38; H, 6.55;
N, 2.25. Single crystals of 1 4CH2Cl2 were dried under a vacuum prior
3
to analysis, although they may contain residual solvent.
Synthesis of Pt(dppp)(CtCC6H4BMes2)2 (2). 2 was prepared
similarly to 1, using p-(dimesitylboryl)phenylacetylene (135 mg, 0.386
mmol, 2.2 equiv), Pt(dppp)Cl2 (119 mg, 0.175 mmol, 1.0 equiv), CuI (7
mg, 0.039 mmol, 0.1 equiv), and 55 mL of degassed THF/NEt3 (10:1, v/v).
The mixture was purified by column chromatography on silica (hexanes
and then CH2Cl2) and then concentrated in vacuo to afford compound
2 as a white powder (129 mg, 56%). 1H NMR (400 MHz, acetone-d6): δ
7.95 (m, br, 8H, dppp), 7.42 (m, 8H, dppp), 7.40 (m, 4H, dppp), 7.14
Hz (d, J = 7.2 Hz, 4H, ꢀPhꢀ), 6.78 (s, 8H, Mes), 6.77 (d, J = 7.2 Hz, 4H,
ꢀPhꢀ), 2.81 (m, 2H, dppp), 2.78 (m, 4H, dppp), 2.24 (s, 12H, Mes),
1.93 (s, 24H, Mes). 13C NMR (100 MHz, acetone-d6): δ 142.9, 142.5,
141.2, 139.1, 136.65, 134.6, 132.9, 132.2, 131.3, 129.7, 129.1, 128.9,
115.1, 110.6, 26.1, 23.6, 21.2, 20.7. 31P NMR (162 MHz, acetone-d6):
δ ꢀ5.26 (t, 2P, JPꢀPt = 1088.9 Hz). Anal. Calcd for C79H78B2P2Pt: C,
72.65; H, 6.02. Found: C, 72.68; H, 6.04.
IsArηs2
Φs ¼ Φr
IrAsηr2
where Φs and Φr are the quantum yields of the sample and reference, I is
the integrated area of the emission band, A is the absorbance at the
wavelength of excitation, and η is the refractive index of the solvent.
Samples for quantum yield measurement were prepared by bubbling dry
N2 through a quartz cuvette fitted with a rubber septum and then sealed
with epoxy and measured immediately. Using this method, no loss of the
emission intensity was observed after 5 min of standing under air, and
<5% intensity loss was observed after 30 min. Vapor-dependent solid-
state emission spectra were obtained by exposing a powdered film of 1 in
a quartz cuvette to a drop of solvent vapor. Films of 1 were prepared by
manually applying small amounts of powdered material to the quartz
surface. Phosphorescent decay lifetimes were below the measurement
limit for our instrument (e2 μs). Molecular orbital (MO) calculations
were performed using the Gaussian 03 program suite using crystal
structures as the starting point for geometry optimizations where possible.
The theoretical absorption spectrum of 1 was generated using the
Gausssum 2.1.6 software package.11 Calculations were performed at the
B3LYP level of theory using LANL2DZ as the basis set for Pt and 6-31G*
for all other atoms.12 Syntheses of p-(dimesitylboryl)phenylacetylene,13
Pt(dbbpy)Cl2 (dbbpy = 4,40-di-tert-butyl-2,20-bipyridine),14 Pt(dppp)Cl2
Single-Crystal X-ray Diffraction Analysis. Single crystals of
1 4CH2Cl2 were obtained from a CH2Cl2/hexanes solution, while
3
those of 2 toluene were obtained from toluene/hexanes. Data were
3
collected on a Bruker AXS Apex II single-crystal X-ray diffractometer
with graphite-monochromated Mo KR radiation, operating at 50 kV and
30 mA at 180 K. Data were processed on a PC with the aid of the
Bruker SHELXTL software package (version 6.14)17 and corrected
for absorption effects. All structures were solved by direct methods.
The crystal lattice of 1 contains four CH2Cl2 solvent molecules per
molecule of 1, while the lattice of 2 contains three toluene molecules per
molecule of 1. All solvent molecules were modeled and refined successfully.
All non-hydrogen atoms were refined anisotropically. The positions of
the hydrogen atoms were calculated, and their contributions in structure
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ARTICLE
Figure 1. Left: structure of 1 [PtꢀC = 1.939(4) Å, PtꢀN = 2.060(3) Å; CꢀPtꢀN = 175.33(14)°]. Right: structure of 2 [PtꢀP = 2.2898(14) and
2.2900(15) Å; PtꢀC = 2.009(6) and 2.017(6) Å; CꢀPtꢀP = 173.79(16) and 175.96(16)°].
factor calculations were included. The crystal structural data of
method; 12 subspectra were collected 40 kHz apart, using a 10 kHz
spikelet spacing and 6.0 μs 195Pt 180° pulses. All 195Pt NMR spectra
were referenced using the signal from a 1 M aqueous solution of
1 4CH2Cl2 and 2 toluene have been deposited at the Cambridge
3
3
Crystallographic Data Centre as CCDC 797877 and 797878,
respectively.
Na2PtCl6 at δiso(
195Pt) = 0 ppm.31,32 Simulated spectra were
Solid-State NMR. Solid-state NMR spectra were acquired using a
Varian Infinity Plus console and a 9.4 T Oxford magnet at resonance
frequencies of 399.7 MHz for 1H, 128.3 MHz for 11B, 100.5 MHz for
13C, 85 MHz for 195Pt, and 61.4 MHz for 2H. Samples studied include
neat 1 (solid, amorphous samples without adsorption of VOCs) and
samples of 1 exposed to CH2Cl2, benzene, and hexane vapors in a sealed
chamber for a minimum of 8 h. The samples were packed in 4-mm-o.d.
zirconia rotors sealed with airtight Teflon caps. All solid-state NMR
experiments were conducted using a Varian/Chemagnetics 4 mm HX
MAS probe. Bloch-decay magic-angle-spinning (MAS) experiments were
used for 1H NMR spectra and were referenced to the signal from solid
adamantane under MAS at δiso(1H) = 1.85 ppm. 90°ꢀτ1ꢀ180°ꢀτ2ꢀacq
echo experiments using 1.5 μs selective 90° pulse lengths (3.0 μs
refocusing pulses) were used to acquire 11B NMR spectra of samples
under stationary and MAS conditions, and high-power continuous-wave
generated with WSOLIDS,33 for static spectra and SIMPSON34 for
MAS spectra.
’ RESULTS AND DISCUSSION
Syntheses. 1 and 2 can be readily synthesized by a copper
(I)-catalyzed transmetalation reaction using 2 equiv of p-(di-
mesitylboryl)phenylacetylene with the appropriate platinum(II)
dichloride. 3 can be prepared analogously, using previously
reported procedures.16 These complexes can then be purified
by column chromatography and isolated in high yield. 1 is a very
robust molecule with excellent stability under ambient condi-
tions in both the solution and solid state, while 2 decomposes
slowly under air. Both compounds have been fully characterized
by NMR, elemental analyses, and single-crystal X-ray diffraction.
Crystal Structures. Single crystals of 1 4CH2Cl2 and 2 to-
1
(CW) H decoupling was used during the acquisition period (ν2 ≈
3
3
50 kHz); spectra were referenced using the signal of liquid F3B O(C2H5)2
3
luene for X-ray diffraction analyses were obtained by slow evapora-
tion from CH2Cl2/hexanes and toluene/hexanes, respectively,
and the structures are shown in Figure 1. The PtꢀC bonds in
13
at δiso(11B) = 0 ppm. 13C NMR spectra were acquired using 1Hꢀ C
variable-amplitude cross-polarization (VACP) MAS experiments,18ꢀ20
with high-power (ν2 ≈ 50 kHz), two-pulse, phase-modulated 1H
decoupling;21 spectra were referenced to the high-frequency peak of
solid adamantane under MAS at δiso(13C) = 38.56 ppm.22 The 13C
homonuclear double-quantum-filtered spectrum was acquired with the
SR26114 pulse sequence applied at an MAS rate of 9.615 kHz using an
experimentally optimized power level (found to be near the theoretical
62.5 kHz);23,24 VACP preparation was used, and 1H decoupling (ν2 ≈
50 kHz) was applied only during signal acquisition, not during the
dipolar recoupling period.25 2H NMR spectra were acquired using
90°ꢀτ1ꢀ90°ꢀτ2ꢀacq echo experiments with 3.5 or 4.4 μs pulse
1 4CH2Cl2 are significantly shorter than those in 2 because of
3
the greater trans effect of the phosphine chelate. Both com-
pounds form crystal lattices with large voids, as evidenced by the
large amount of solvent incorporated into both structures: four
CH2Cl2 molecules per molecule of 1 and three toluene mol-
ecules per molecule of 2. While many of the previously reported
platinum bipyridine acetylide derivatives form either extended
pseudolinear or dimeric antiparallel stacked structures in the solid
state, with either short Pt Pt distances (<5 Å) or bipyridyl
3 3 3
1
πꢀπ stacking (<4 Å), neither arrangement is observed for
lengths, 40 μs interpulse delays, and high-power CW H decoupling
(ν2 ≈ 50 kHz) during acquisition. 195Pt NMR spectra of the sample of
1 4CH2Cl2. Instead, the Pt(II) square planes of these molecules
3
1 4CH2Cl2 were acquired using the WURST-CPMG protocol;26ꢀ28
are oriented at approximately 75° with respect to each other, with
3
the shortest intermolecular Pt Pt distance being greater than
three subspectra were collected 150 kHz apart, using a 10 kHz spikelet
spacing and 10 μs WURST pulses swept from þ500 to ꢀ500 kHz and
using a maximum radio-frequency power corresponding to a nutation rate
of ca. 75 kHz. For neat 1, C6H6@1, and hexanes@1, 1Hꢀ195Pt CP/CPMG
experiments29,30 were used instead of the direct WURST-CPMG
3 3 3
12 Å and the shortest πꢀπ distances being ∼5.7 Å (Figure 2).
Similarly, in the crystal lattice of 2 toluene (Figure 3), these
3
planes are arranged nearly perpendicularly to one another, and
the shortest Pt Pt distance is ∼7.5 Å.
3 3 3
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Electronic and Photophysical Properties. 1 is brightly phos-
phorescent in the solid state and solution (Φ = 0.73 in CH2Cl2).
The absorption spectrum of 1 shows a low-energy band at
407 nm in CH2Cl2, accompanied by a much more intense absorp-
tion at 357 nm. These transitions can be unambiguously assigned
as a MLCT to the bipyridine group and a ligand-centered (LC)
transition on the boron ligand, respectively. This is supported by
the absorption spectra of control compounds 2 and 3, which
possess only the LC and MLCT states, respectively (Figure 4).
CV experiments further establish that the LUMO of 1 is centered
on the bipyridine chelate because the first reduction peak of this
compound is very similar to that of 3. In contrast, 2 undergoes
reduction at a much more negative potential typical of triarylbor-
anes (Table 1). Density functional theory (DFT) calculations
further establish that the HOMOs for 1ꢀ3 are dominated by the
acetylene π and Pt d orbitals, while the LUMO is located on the
bipyridine chelate in 1 and 3 and the boron ligands in 2, with the
orbitals from the boron ligands comprising the LUMOþ3 and
LUMOþ4 in 1 [see the Supporting Information (SI), Figure S7].
Therefore, 1 can be characterized as having two main electronic
excited states: the lower-energy (MLCT) state centered on the
bipyridine chelate and a higher-energy (LC) state arising from
the boron ligands.
In CH2Cl2, 1 displays a bright and featureless yellow phosphor-
escence peak at 541 nm, which is similar to that of 3 and attributable
to MLCT phosphorescence. In contrast, 2 displays a weak pho-
sphorescence band with well-resolved vibrational features at
498 nm, due to emission from the LC state on the triarylboron
acetylide ligands (Figure 4). The absorption and emission spectra of
1 both show negative solvatochromism typical of the polar ground
state in Pt(II) complexes, although the blue shift in the absorption
spectrum is much greater for the low-energy MLCT band (50 nm)
than the intense LC band (10 nm) from hexanes to CH2Cl2.
Similarly, the emission spectrum shows a 25 nm blue shift from
hexanes to CH2Cl2. In contrast, the emission spectrum of 2 shows
only a small blue shift with increasing solvent polarity (9 nm from
hexane to CH2Cl2; see the SI). In both cases, the shape of the
emission band does not change significantly with the solvent.
Hence, we can conclude that the origin of the electronic transitions
in solution remains MLCT for 1 and LC for 2 for all of the solvents
that we investigated; however, a remarkably different phenomenon
was observed in the solid state.
Figure 2. Unit cell packing diagram of 1 4CH2Cl2 showing the
3
locations of the CH2Cl2 solvent molecules.
Figure 3. Unit cell packing diagram of 2 toluene. Toluene solvent
molecules are shown in yellow. Hydrogen atoms are omitted for clarity.
Figure 4. Absorption and normalized emission spectra of 1ꢀ3 at 10ꢀ5
3
M CH2Cl2 at 298 K.
Table 1. Photophysical Properties of 1ꢀ3
solutiona/solid emission, 298 K
E1/2red (V)c
a
b
compound
absorption, λmax, nm (ε, 104 cmꢀ1 Mꢀ1
)
λmax (nm) of CH2Cl2/solid
ΦP
1
2
3
248 (6.10), 357 (9.55), 407 (2.00)
236 (6.22), 360 (8.87)
541/559
496/496
548/535
0.73
0.002
0.51
ꢀ1.78
ꢀ2.26
ꢀ1.80
256 (4.81), 286 (4.73), 386 (0.89)
0/þ
a Measured in CH2Cl2 at 1 ꢁ 10ꢀ5 M. b In CH2Cl2 solution: relative to Ir(ppy)3 (Φ = 0.97 in 2-MeTHF),10 ( 10%. c In DMF relative to FeCp2
.
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Figure 5. Response of films of 1 under UV irradiation to various organic vapors.
Vapochromism. While 1 and 2 are both capable of acting as
selective chemical sensors for fluoride and cyanide anions in solution
in the same manner as that of previously reported triarylboranes
(see the SI, Figure S3), 1 alone shows reversible vapochromism
upon exposure to organic vapors. Both 1 and 2 are luminescent in
the solid state under UV irradiation, with emission energies similar
to those in a CH2Cl2 solution (Table 1). When solid or neat films
of 1 are exposed to VOCs, however, the absorption and emission
colors of the sample are observed to shift according to the nature
of the VOC, producing a response in a matter of seconds (Figure 5).
These luminescent changes are reversible either by application of
a vacuum or by dissolution and recasting of the film and are persistent
in the presence of the detected VOC. The original color of the
sample can also be restored upon extended standing under air or
mild heating to remove solvent vapor.
Many polar solvents, such as CH2Cl2, CHCl3, CH3CN, acetone,
THF, and EtOH, induce an emission color shift from yellow
(λmax = 559 nm) to green (λmax = 490ꢀ500 nm). This is highly
unusual because application of a variety of very different solvent
molecules induces the same response. In contrast, application of
benzene, cyclohexane, or 1,4-dioxane vapors switches the emis-
sion color to red, with the appearance of a new broad emission
peak at λmax = 580ꢀ620 nm. Because dioxane contains oxygen
donor atoms and yet has a dielectric constant similar to that of
benzene, the observation of a red shift suggests that the solvent
polarity is the key factor in determining the emission shift. Inter-
estingly, exposure to linear hydrocarbons or methanol quenches
the emission of the sample entirely. Toluene was also observed to
produce green emission, although the vapors were found to readily
wet powder films of 1, making solid-state studies difficult. It
should be noted, however, that a similar green emission peak was
present in the predominantly red emission spectra of benzene or
cyclohexane-treated films of 1. When 1 is placed in the presence
of multiple solvents simultaneously, the effect of polar solvents
such as CH2Cl2 that turn the sample green appears to be favored.
This switch to green upon exposure to polar solvent vapors
may be explained as a solvent-induced inversion of the lowest-
energy excited state, from an MLCT transition to a LC transition.
Upon exposure to vapors such as CH2Cl2, the emission energy
and band shape of 1 shifts, closely resembling that of 2, which
emits from a 3LC state with or without VOCs (Figure 6). This
suggests that the emissions from neat 2 and 1 after exposure to
polar solvents have a common origin. This can be attributed
primarily to a change in the MLCT level of 1, as evidenced by
solid-state absorption spectra of powdered films taken before and
Figure 6. Impact of excited-state level modulation on the emission
colors of 1, using CH2Cl2 and benzene as representative examples.
after exposure to VOCs. The absorption spectrum of a neat film
of 1 closely resembles that observed in solution, but once VOCs
such as CH2Cl2 are added, the MLCT band shifts to higher
energy until obscured by the intense LC band.
Inversion of the charge-transfer (CT) and LC excited-state
energy levels due to solvent polarity has recently been demonstrated
in solution-based systems by Castellano and co-workers for
platinum(II) bipyridine diacetylide complexes with energetically
proximate triplet CT (3CT) and 3LC states.35,36 They have shown
that the energy of the 3CT state can be modulated by the solvent
polarity, leaving the 3LC energy relatively unaffected. Pt(II) com-
plexes are well-known to display negative solvatochromism because
of large ground-state dipole moments, giving CT excited states that
increase in energy with the solvent polarity.1,2 By variation of the
polarity of the solvent, it is thus possible to alter the energy gap
between the 3CT and 3LC states by modulation of the 3CT energy
level, thereby altering the degree of mixing between the two excited
states. In this way, the lowest-energy excited state from which the
complex emits can be “inverted”, from 3CT in nonpolar media to
3LC in polar media. However, the same “excited-state inversion”
phenomenon has not been observed in the solid state previously.
Our observations indicate that the vapochromic response of 1 to
polar VOCs such as CH2Cl2 is due to such excited-state inversion
and represents the first observation of this phenomenon in solid-
state systems. Conversely, nonpolar solvents such as benzene and
cyclohexane appear to reduce the 3CT energy, shifting the emission
color to red. Highly nonpolar solvents such as linear hydrocarbons
reduce this energy further still, giving a 3CT state so low in energy
that vibronic quenching becomes dominant and rendering the
sample nonemissive. These processes are outlined in Figure 6.
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Figure 7. Comparison of the emission spectral change of 1 upon
exposure to CH2Cl2 in the solid state with that of 2. The solution
spectra for both compounds in CH2Cl2 are also shown.
Interestingly, a similar response is observed for methanol vapor.
Fluorescence quenching by methanol in various systems was
known previously.37 In the case of 1, the quenching of lumines-
cence by methanol in the solid state may be due to the introduction
of many closely spaced energy levels, which readily facilitate
vibronic relaxation. Nonetheless, the fact that a similar excited-
state inversion was not observed for 1in solution remains peculiar. It
is likely that the localized dipoleꢀdipole interactions between a
molecule of 1 and guest solvent molecules occupying a specific
region of free volume in the solid state differ substantially from
the interactions between 1 and the continuously changing dipoles of
the bulk solution.
Figure 8. PXRD patterns measured after exposure of 1 to selected
solvent vapors.
reduction in long-range ordering may explain our inability to
obtain X-ray-quality single crystals from other solvents despite
repeated attempts. It is possible that the combination of low
conformational flexibility and the complicated molecular shape
of 1 prevents stacking without void spaces, unless secondary
molecules of suitable size and properties are present; these void
spaces would, of course, provide a mechanism for reducing long-
range order. The most important conclusion that can be drawn
from these data is that there is only a minimal structural re-
organization accompanying the vapochromism of 1. Changes in
metallophilic contacts can, therefore, be ruled out as the source of
the vapochromic behavior in this system because the crystal
PXRD and Solid-State NMR Studies. In most previously
reported Pt(II)-based vapochromic systems, the change in the
luminescence energy is a direct result of differences in the degree
of intermolecular stacking involving the Pt(II) centers or, more
rarely, πꢀπ interactions.1,2 However, the crystal structure of
structure of 1 4CH2Cl2 would require a significant reorganiza-
1 4CH2Cl2 contains no close contacts between the square-
3
3
tion to bring the Pt(II) moieties together; furthermore, this conclu-
sion is entirely consistent with the SSNMR data presented below.
SSNMR seems an obvious choice to characterize such solid
systems but, to our knowledge, has only been applied in two
related studies: the tetracyanoplatinum and tetracyanopalladium
double salts of platinum tetraarylisonitriles with limited data.1j,k
The results presented below demonstrate the utility of multi-
nuclear SSNMR for investigating vapochromic materials.
First, 1H and 13C NMR spectra are used to demonstrate that
the solvent molecules are incorporated in each material. Variable-
planar Pt(II) units (all PtꢀPt distances >12 Å). Furthermore,
molecular modeling using two molecules of 1 (from the
1 4CH2Cl2 crystal structure) suggests that the steric bulk of
3
the tert-butyl and dimesitylboron groups is inconsistent with
close intermolecular contacts between the platinum atoms or the
bipyridine rings, particularly because 1 has little conformational
flexibility. To further investigate the origin of vapochromism in 1,
we examined this system using multinuclear solid-state NMR
spectroscopy (SSNMR) and PXRD. Neat 1 as well as samples of
1 exposed to the vapors of CH2Cl2, C6H6, and hexanes (termed
solvent@1 below) were studied as representative of systems
featuring the yellow, green, red, and quenched luminescent
states, respectively (Figure 7).
2
temperature H NMR spectra of deuterated solvent molecules
are then used to comment on their motion within the material,
which is important for a complete description of how their
presence induces the color changes. Next, changes in the host
molecules of 1 upon absorption of each vapor are studied directly
via the use of 195Pt and 11B NMR.
PXRD patterns collected from neat 1 and from each VOC
treatment are shown in Figure 8. It is immediately apparent that
the structure of CH2Cl2@1 matches that determined in the
SSNMR of Guest Solvent Molecules. Displayed in Figure 9 are
1H and 13C MAS SSNMR spectra of the four preparations of 1.
The changes in the 13C NMR spectral resolution with sample
preparation mimic those of the PXRD patterns and are consis-
above single-crystal diffraction study of 1 4CH2Cl2, and we will
3
therefore identify this material as 1 4CH2Cl2 below. Apart from
3
the lower resolution, the diffraction patterns of neat 1, C6H6@1,
and hexanes@1 are very similar to that of 1 4CH2Cl2, indicating
3
tent with a higher degree of crystallinity in 1 4CH2Cl2 than in
that the structures of all four materials must be similar. The broadened
3
the other forms. Aside from resolution changes, the vapor absorp-
tion causes only minor changes in the isotropic chemical shifts of
diffraction peaks in these latter three materials are indicative of
reduced long-range ordering, as compared to 1 4CH2Cl2. This
3
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Figure 9. 1H and 13C MAS SSNMR spectra of samples of 1 exposed to selected VOC vapors. Spinning sidebands are marked with gray boxes, and NMR
peaks from the incorporated solvents are each marked with a dagger. The spectra were acquired at MAS rates of 7.5 kHz for 1 4CH2Cl2, 10 kHz for neat
3
1, 5.5 kHz for C6H6@1, and 11 kHz (1H NMR spectrum)/8 kHz (13C NMR spectrum) for hexanes@1.
the host material 1, consistent with the behavior displayed in 13
MAS NMR spectra of the vapochromic double salts of
C
þ40 to ꢀ120 °C (Figure 10). Given the low melting point of
CH2Cl2, it is not surprising that the spectra of 1 4CH2Cl2 from
3
[Pt(arylisocyanide)4][Pd(CN)4] and [Pt(arylisocyanide)4]
ambient temperature to ꢀ40 °C are dominated by a narrow peak,
indicative of a rapid isotropic motion of the solvent molecules.
Furthermore, the spectra demonstrate that solvent motion is not
completely stopped at ꢀ120 °C because the line shape expected
from a distribution of stationary molecules with the known
quadrupolar parameters is not observed.43
1
[Pt(CN)4].1j,k Both H and 13C isotropic chemical shifts are
similar to those reported above from the solution NMR mea-
surements of 1. NMR peaks from the absorbed VOCs are readily
observed in both 1H and 13C NMR spectra (a second experiment
using 13C-labeled benzene, not shown, was necessary to distin-
guish the solvent peak from the crowded aromatic region in
C6H6@1). Because the intensity of the 1H solvent peaks are of
the same order as those from 1, these spectra demonstrate con-
clusively that the vapochromic behavior of 1 is accompanied by
absorption of the VOCs into the matrix, as opposed to them
simply acting as agents of structural change. Interestingly, the
distinct changes observed in the 13C NMR spectrum after exposure
to CH2Cl2 show that the sample is completely converted to the
Numerous publications have featured studies of deuterated
benzene as a guest molecule in various types of porous materials,
where it has been shown that the 2H NMR spectrum of benzene
undergoing rapid jumps about its C6 rotational axis is character-
2
ized by an approximate 70 kHz splitting.44ꢀ52 The H NMR
spectrum of C6H6@1 at ꢀ140 °C displays a quadrupole-per-
turbed powder pattern with a splitting of 66 kHz, which is
assigned to benzene molecules undergoing the aforementioned
rapid in-plane rotations. As the temperature is raised, an increas-
ing fraction of the benzene molecules are undergoing rapid
isotropic motion, contributing to the narrow peak in the center
of the spectrum. Even at the highest temperature investigated,
þ40 °C, the presence of a quadrupole-perturbed powder pattern is
observed, indicating that a portion of the benzene molecules are
solvate form (1 4CH2Cl2) in this case.
3
An attempt was made to probe the proximity of the various
13C nuclei in 1 to guest 13C-labeled benzene molecules using the
SR26411 homonuclear correlation experiment;23,24 however, the
spectrum (not shown) did not contain correlation peaks more
intense than the background of natural-abundance 13C spin pairs.
It is possible that the large chemical shift anisotropies of the
aromatic carbon atoms for which recoupling was attempted and/
or the motion of the benzene molecules (vide infra) serve to
drastically reduce the efficiency of the dipolar recoupling.38
2H NMR experiments conducted over a range of temperatures
can be utilized to gain information on the nature of these dynamics
by modeling the spectra using well-known techniques.39ꢀ42
Thus, we conducted quadrupolar echo experiments (i.e., 90°ꢀ
still
under-
going restricted motion; i.e., these molecules do not undergo
isotropic motion that is rapid on the NMR time scale (1/CQ ≈
5 μs).46 Notably, the transition from isotropic to anisotropic
motional regimes occurs over a broad temperature range, and
none of the spectra is indicative of a mixture of the two types of
motion; such behavior is typical of disordered materials in which
a range of activation energies are present47,53 and is, therefore,
consistent with the PXRD and other NMR measurements of
C6H6@1 (cf. the behavior46,52 in uniform, crystalline materials).
2
τ1ꢀ90°ꢀτ2ꢀacq) to record H NMR spectra of CD2Cl2 and
C6D6 molecules incorporated into 1, in the temperature range
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Figure 10. Variable-temperature 2H NMR spectra of 1 4CH2Cl2-d2 and C6H6-d6@1.
3
For approximately 50% of the benzene molecules at ambient
temperature, the isotropic mode of motion is slow on the NMR
time scale and is, therefore, also slow on the time scale of the
optical spectroscopy (the lifetimes of the electronic excited states
were measured to be e2 μs). The two-component line shape
2
apparent in the ambient-temperature H NMR spectrum of
1 4CH2Cl2 demonstrates that a significant fraction of the
3
CH2Cl2 molecules are also undergoing restricted motion (a
quantitative measure of this fraction would require extensive experi-
mentation given that the longitudinal and transverse relaxation
rate constants are likely different in the two motional regimes,
and each may, furthermore, mirror the structural heterogeneity
of the sample). It is therefore possible that the vapor-induced
color changes in 1 are accompanied by directional interactions
between the guest VOCs and host molecules of 1.
SSNMR of the Host Material. Because the platinum atom is
central to the phosphorescent emission of 1, the effects of VOC
absorption were also studied using 195Pt SSNMR. Before the
experimental results are discussed, a brief description is given on
the well-known but often underexploited relationship between
optically excited transitions and the nuclear magnetic shielding
(NMS) measured using NMR; this relationship also extends to
the chemical shift (CS) because this is simply the difference in
NMS from a reference compound (the relationships between the
Figure 11. 195Pt SSNMR static spectra of samples of 1 exposed to
vapors of selected VOCs.
NMS and CS have been discussed lucidly elsewhere).32,54 The
NMS of a nucleus is due to a small local magnetic field induced by
a large external magnetic field and can be expressed using the
familiar expressions of time-independent perturbation theory, as
was first done by Ramsey.55ꢀ57 Only the paramagnetic
shielding term is described herein because it is responsible
for nearly all of the differences in magnetic shielding between
chemical environments. External-field-perturbed MOs,
which are similar to the zero-field MOs except that they
contain some admixture of virtual (unoccupied) orbitals, are
the source of the response magnetic field. The amount that a
particular virtual orbital mixes into a particular occupied
orbital, and therefore the NMS contribution from that occu-
pied/virtual MO pair, is inversely dependent on the energy gap
between the two MOs. In addition, each MO pair’s influence on
the magnetic shielding depends on its degree of localization near
the nucleus of interest, as well as on its shape, determined by its
constituent atomic orbitals.57ꢀ59 The NMS is therefore depen-
dent on a large array of occupied/virtual MO energy spacings in
the valence level, while optical spectroscopy probes some of
these same energy spacings one at a time. Because the color
changes observed in 1 must arise from solvent-induced energy
shifts in the real and/or virtual MO involved in a particular
optical absorption, the Pt CS tensor should change if the MO that
shifts is localized near Pt.
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Table 2. 195Pt CS Tensors Measured from Samples of 1 Exposed to the Vapors of Selected VOCsa
material
δ11
δ22
δ33
δiso
Ω
κ
1 4CH2Cl2
ꢀ1100(100)
ꢀ1150(100)
ꢀ1000(100)
ꢀ1000(100)
ꢀ4550(50)
ꢀ4450(200)
ꢀ4600(200)
ꢀ4550(200)
ꢀ5450(100)
ꢀ5300(200)
ꢀ5350(200)
ꢀ5350(200)
ꢀ3700(100)
ꢀ3600(200)
ꢀ3650(200)
ꢀ3600(200)
4350(200)
4150(300)
4350(300)
4350(300)
ꢀ0.59(3)
ꢀ0.59(10)
ꢀ0.66(10)
ꢀ0.63(10)
3
neat 1
C6H6@1
hexanes@1
a Values for the CS tensor elements are derived using line-shape simulations and are given here using the convention δ11 g δ22 g δ33.
An alternate representation of the CS tensor is also included for convenience: isotropic shift, δiso = 1/3(δ11 þ δ22 þ δ33); span, Ω =
δ11 ꢀ δ33; skew, κ = 3(δ22 ꢀ δiso)/Ω (ꢀ1 e κ e 1).66 Spectral simulations are consistent with the experimental line shapes within the
ranges given in brackets for the principal components.
The 195Pt SSNMR spectra of neat 1 and each vapor-treated
sample are displayed in Figure 11. Under the conditions used
(stationary samples, high-power 1H decoupling, etc.), the pow-
der pattern depends only on the Pt CS tensors, which are given in
Table 2. It should be noted that the signal-to-noise ratios of these
spectra were increased by acquiring a train of echoes, Fourier
transformations of which yield the “spikelet” spectra shown, as in
a CPMG-type experiment.29,30 These spikelet peaks trace out the
classic CSA-dominated patterns (Figure 11). The 400 kHz broad
VOC CH2Cl2 increases the energy of the MLCT by stabilizing
the polar ground state of the molecule to the point that the LC
transition (localized on the triarylboron moiety) becomes the
lowest-energy excited state and the source of the observed
phosphorescence. The ground state of 1, as in 3, has a strong
electric dipole moment, and because the MLCT transition is
opposite to the orientation of the ground-state dipole moment35
and smaller in magnitude, more polar VOCs cause the MLCT
tranisition to shift to higher energy. This inverse solvatochromic
effect induces excited-state switching, as demonstrated by Castella-
no and others for similar dual-chromophore compounds in liquid
solutions.35
spectrum from 1 4CH2Cl2 was acquired using only three
3
subspectra by making use of the broad excitation bandwidth of
frequency-swept WURST pulses. The transverse relaxation of
the 195Pt nuclei was found to be slightly more efficient in the
To further rule out chromophoreꢀchromophore contacts as the
1
other three samples, so cross-polarization (CP) from H was
cause of the unique vapochromic effect observed, 11B SSNMR
used instead of direct excitation. The lower bandwidth of the CP
method required the acquisition of many more subspectra (12 vs
3), but this was balanced by the signal enhancement of CP,
making the total acquisition time ca. 24 h for all of the spectra in
Figure 11.
spectra of the stationary and MAS samples of 1 4CH2Cl2 and
3
C6H6@1 were acquired. Because the boron atom is a distinct
moiety central to the LC transition, looking for changes in the CS
and electric-field-gradient tensors is another way to examine
possible chromophoreꢀchromophore interactions in the solid
state induced by solvent uptake into the void space of 1. Both the
CS and EFG tensors, determined using line-shape simulations,
are found to be similar to those of trimesitylborane (see S8 of the
Supporting Information for spectra and further details).38 The
Isotropic Pt CSs are known to span a range of ca. 15000 ppm,
and the present results are at the lower end of the þ5000 to
ꢀ5000 ppm range typical for Pt(II) compounds.60,61 The CS
tensors observed for neat 1, 1 4CH2Cl2, C6H6@1, and hex-
3
anes@1 are characterized by large spans (Ω > 4000 ppm), as is
typical for square-planar Pt(II) compounds; for example, Ω =
3472 ppm in (cod)PtI2,62 and the largest span published to date
is for K2PtCl4, where Ω = 10500 ppm.61,63ꢀ65 The CS tensors are
each characterized by a skew value near ꢀ1 and likely have an
orientation similar to that proposed for a series of (η2-1,5-
cyclooctadiene)PtX2 (X = I or Cl) and other related Pt(II)
materials, with δ11 perpendicular to the rectangular plane.62 The
most notable feature apparent in the data of Figure 11 and
Table 2 is that the 195Pt CS tensor of neat 1 is not measurably
different from those of the three solvent-exposed forms. From
this evidence and the above discussion of the relationship
between MOs and the CS tensor, we conclude that the vapo-
chromic behavior of 1 does not involve changes in the MOs
localized on Pt. Clearly, there can be no PtꢀPt stacking in any of
the preparations of 1 because the metalꢀmetal interactions
would lead to Pt-localized MOs significantly different from those
11B NMR spectra of 1 4CH2Cl2 and C6H6@1 are completely
3
indistinguishable, demonstrating that the boron environment is
unaltered by absorption of these VOCs. While the boron atom in
1 is shown above to be accessible by small anions, the steric
crowding at this site ensures that there is no close contact with
the much larger VOC molecules. Small amounts of impurities
from repeated cycling in a liquid solvent over a long period of
time are seen in each spectrum; however, these impurities could
not be removed because of the small amount of sample remain-
ing for these final experiments. While these impurities hinder a
complete determination of the NMR parameters, they do not
affect the conclusions noted above.
’ CONCLUSIONS
In summary, the first example of a triarylboron-functionalized
platinum(II) acetylide compound that displays distinct lumines-
cent responses to a variety of VOCs in the solid state has been
presented. 1H and 13C SSNMR spectroscopies were was used to
directly observe adsorption of solvent into 1, and 195Pt SSNMR
results discount PtꢀPt metallophilic interactions as being re-
sponsible for the response to VOCs. On the basis of optical and
NMR spectroscopic data, we conclude that (1) the vapochromic
response of 1 toward polar solvents such as CH2Cl2 is caused by
inversion of the lowest-energy excited state from 3MLCT to 3LC
in 1 4CH2Cl2. The solid-state NMR data therefore indicate, in
3
agreement with the PXRD measurements and molecular model-
ing, that changes in PtꢀPt interactions are not responsible for the
vapochromic behavior of 1.
The invariant 195Pt NMR spectra of 1 as well as PXRD data
rule out modulations in the structure that involve PtꢀPt inter-
actions. The nonpolar VOC benzene reduces the MLCT energy,
allowing this optical pathway to dominate. In contrast, the polar
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ARTICLE
in the solid state because of an increase in the MLCT level, (2) a
decrease in the MLCT level is responsible for the shift in the
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3
emission color to red upon exposure to benzene/cyclohexanes,
and (3) the quenching response obtained by the treatment of 1
with linear hydrocarbons or methanol is likely due to the introduction
of many closely spaced energy levels, which readily facilitate vibronic
relaxation. Because 11B and 195Pt NMR spectra show no changes
upon exposure to VOCs, we postulate that interactions between
adsorbed solvent molecules and the bipyridine chelate are likely
responsible for changes in the MLCT level, though it remains
unclear why a similar effect is not observed in solution. This study
demonstrates that metalꢀmetal or πꢀπ stacking interactions
between molecules of the sensor are not necessary for a vapochro-
mic response and that interactions between the sensor and VOC
analyte can indeed be sufficient.
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’ ASSOCIATED CONTENT
S
Supporting Information. Solution NMR spectra for 1
b
and 2, additional absorption/emission spectra, CV diagrams,
DFT results, Fꢀ and CNꢀ titration data for 1 and 2, 11B SSNMR
spectra of 1, and crystal structural data for 1 and 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: suning.wang@chem.queensu.ca (S.W.), rschurko@
uwindsor.ca (R.W.S.).
(7) (a) Collings, J. C.; Poon, S. Y.; Droumaguet, C. L.; Charlot, M.;
Katan, C.; Palsson, L. O.; Beeby, A.; Mosely, J. A.; Kaiser, H. M.;
’ ACKNOWLEDGMENT
ꢂ
The authors gratefully acknowledge the Natural Sciences and
Engineering Research Council of Canada for financial support.
R.W.S. also acknowledges the Canadian Foundation for Innova-
tion, the Ontario Innovation Trust, and the University of Wind-
sor for funding the solid-state NMR facility and the Ontario
Ministry of Research and Innovation for an Early Researcher Award.
Kaufmann, D.; Wong, W.-Y.; Blanchard-Desce, M.; Marder, T. B. Chem.—
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