Unichromophoric platinum-acetylides that contain pentiptycene scaffolds: Torsion-induced dual emission and steric shielding of dynamic quenching

Article abstract of DOI:10.1021/ic4025052

The effect of the rigid bulky pentiptycene scaffolds on the photoluminescence, redox properties, and oxygen sensing behavior of unichromophoric Pt-acetylides is reported. When the pentiptycene groups are near the Pt(PBu₃)₂ center, the Pt-acetylides display both blue fluorescence and green phosphorescence with long phosphorescence lifetimes (90-202 μs) in THF. Their phosphorescence intensity is highly sensitive to molecular oxygen, and the emission color depends on the concentration of not only oxygen but also the complexes, which allows a feasible determination of oxygen in the range of 1-5% air volume. The dynamic quenching rate constants decrease linearly with increasing the number of pentiptycene groups, revealing the steric shielding effect of the peripheral rings of pentiptycene. A dependence of oxidation potential on the number of pentiptycene groups also revealed the steric shielding effect on the electron transfer between the complexes and the electrode. In a PMMA matrix, the dual emissive properties are diminished due to increased phosphorescence and decreased fluorescence intensity, and the phosphorescence lifetimes are significantly increased (up to ～700 μs), leading to an on-off optical response to oxygen concentration. Both the dual emissive properties and long-lived triplet excitons are attributed to diminished spin-orbit couplings caused by twisting and steric shielding of the π-conjugated backbone around the Pt center.

Full text of DOI:10.1021/ic4025052

Article
pubs.acs.org/IC
Unichromophoric Platinum-Acetylides That Contain Pentiptycene
Scaffolds: Torsion-Induced Dual Emission and Steric Shielding of
Dynamic Quenching
Che-Jen Lin, Chih-Yuan Chen, Sandip Kumar Kundu, and Jye-Shane Yang*
Department of Chemistry, National Taiwan University, No 1, Sec 4, Roosevelt Rd, Taipei 10617, Taiwan
S
* Supporting Information
ABSTRACT: The effect of the rigid bulky pentiptycene scaffolds on the
photoluminescence, redox properties, and oxygen sensing behavior of
unichromophoric Pt-acetylides is reported. When the pentiptycene groups are
near the Pt(PBu3)2 center, the Pt-acetylides display both blue fluorescence and
green phosphorescence with long phosphorescence lifetimes (90−202 μs) in
THF. Their phosphorescence intensity is highly sensitive to molecular oxygen,
and the emission color depends on the concentration of not only oxygen but also
the complexes, which allows a feasible determination of oxygen in the range of 1−
5% air volume. The dynamic quenching rate constants decrease linearly with
increasing the number of pentiptycene groups, revealing the steric shielding effect
of the peripheral rings of pentiptycene. A dependence of oxidation potential on
the number of pentiptycene groups also revealed the steric shielding effect on the
electron transfer between the complexes and the electrode. In a PMMA matrix, the dual emissive properties are diminished due
to increased phosphorescence and decreased fluorescence intensity, and the phosphorescence lifetimes are significantly increased
(up to ∼700 μs), leading to an “on−off” optical response to oxygen concentration. Both the dual emissive properties and long-
lived triplet excitons are attributed to diminished spin−orbit couplings caused by twisting and steric shielding of the π-conjugated
backbone around the Pt center.
appropriate attenuation of the ISC efficiency of a known
phosphorescent transition metal complex could lead to dual
emissive systems. Of the various photoluminescent transition
metal complexes, the square-planar d8 Pt(II) complexes are
ideal candidates because of the relatively weaker spin−orbit
coupling exerted by the Pt atom.2,3 A known strategy to
attenuate the ISC efficiency for Pt complexes is to extend the π-
conjugation length of the ligand.3,4,25,26 For example, the Pt-
acetylides [trans-Pt(PBu3)2((CCC6H4)nH)2] display
a decreased triplet quantum yield through the series (n = 1 →
3). While the quantum yield of fluorescence (Φf) is as low as
0.001 at n = 2, it is comparable to that of phosphorescence
(Φp) at n = 3, where Φp ∼ Φf ∼ 0.016.25 Despite the potential
dual emissive properties, Pt-acetylides have not been
investigated as ratiometric oxygen probes.
Pentiptycene is an H-shaped member of the iptycene
family.27 Because of the rigid and nonplanar scaffold,
pentiptycene has been incorporated into π-conjugation
oligomers28 and polymers29,30 to diminish interchain π-stacking
and thus the fluorescence quenching in thin solid films. The
same concept has also been applied to Pt-acetylide polymers to
explore the intra- versus interchain effects in the triplet excited
states.31 Although the torsional constraint between the bulky
pentiptycene groups and the neighboring Pt(PBu3)2 units in
INTRODUCTION
■
Chromophores that display fluorescence−phosphorescence
dual emissions at ambient temperature are attractive in both
fundamental and technological points of view.1−6 The dual
emissive behavior requires a subtle balance of reaction rates
between fluorescence and intersystem crossing (ISC) in the
lowest singlet excited state (S1) and between phosphorescence
and nonradiative decays in the lowest triplet excited state (T1).
Because of a much longer lifetime for T1 versus S1, the intensity
of phosphorescence is much more sensitive than that of
fluorescence to molecular oxygen. Therefore, these dual
emissive chromophores are potential ratiometric luminescent
probes for the detection of oxygen in biological and
environmental systems.2,4−7 Ratiometric luminescence detec-
tion is a self-referencing method and superior to the
luminescence quenching or amplification approach. Many
ratiometric oxygen sensors have been designed by integrating
fluorescent and phosphorescent chromophores.2,8−10 One
potential problem of these bi- or multichromophoric systems
is the chromophore-dependent photobleaching activity, which
challenges the self-referencing reliability. While dual emissive
unichromophores can circumvent this problem, examples of
unichromophore-based ratiometric oxygen probes are rather
limited.2,4−7,11
Photoluminescent transition metal complexes are an
important class of phosphorescent materials,12−15 which have
been extensively studied as oxygen probes.16−24 In principle, an
Received: July 1, 2013
Published: January 3, 2014
© 2014 American Chemical Society
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Chart 1
Scheme 1
the pentiptycene-derived Pt-acetylide polymer was noticed,
such a conformational effect on the photophysical properties
has not been fully addressed.
trimethylsilylacetylide to pentiptycene quinone followed by
aromatization with SnCl2 and then by removal of the
trimethylsilyl (TMS) group under basic condition. Ligands
L3H and L4H were synthesized through the Sonogashira
reactions with octylarylacetylene and trimethylsilyl bromoaryl
acetylene followed by removal of the TMS group (Supporting
Information Scheme S1). Whereas L3H and L4H react readily
with [trans-PtCl2(PBu3)2] to form C3 and C4, respectively, the
corresponding reactions with L1H and L2H only led to
monosubstituted intermediates [trans-PtCl(Ln)(PBu3)2] (n = 1
and 2). The poor reactivity of the intermediates with L1H and
L2H reveals the pentiptycene effect on this metalation reaction.
Instead, C1 and C2 were prepared with the more reactive [cis-
PtCl2(PBu3)2] substrate followed by cis-to-trans isomerization.
The isomerization occurred efficiently even at room temper-
We report herein the effect of pentiptycene scaffolds on the
photophysical properties of Pt-acetylides (Chart 1). We
observed that decreasing the torsional flexibility of the arene
near the Pt center of Pt-acetylides could attenuate the ISC
efficiency and thus create dual emission. This is manifested by
the Pt-acetylides C1−C4, in which C1 and C2 are dual
emissive but C3 and C4 display essentially only phosphor-
escence. The key structural feature in the former two species is
the bulky pentiptycene groups in neighbor of the PtP2C2 unit.
As a result of the long phosphorescence lifetimes (90−202 μs),
C1 and C2 in dilute THF solutions display nearly complete
phosphorescence quenching at low oxygen conditions, resulting
in luminescence color change from green to blue. The oxygen
quenching dynamics of C1−C4 also reveals an intriguing steric
shielding effect exerted by the pentiptycene scaffolds. The solid-
state photophysical properties and oxygen sensing behavior of
C1−C4 in a PMMA matrix were also investigated for
comparison.
1
ature, indicating a low energy barrier. The H, 13C, and 31P
NMR spectra of C1−C4 are provided as Supporting
Information (Figures S1−S12). The trans-configuration of
C1−C4 is confirmed by 31P NMR spectroscopy; the chemical
shifts of the PBu3 groups, which are in the range 3.33−4.29
ppm, and the coupling constants for Pt−P, which are in the
range 2342−2350 Hz, are similar to those observed for [trans-
Pt(PBu3)2(CCC6H5)2].26 On the basis of the order of
chemical shifts C1 (4.29 ppm) > C2 (4.19 ppm) > C3 (3.53
ppm) > C4 (3.33 ppm), a magnetic deshielding of the PBu3
groups by the neighboring pentiptycene groups is evidenced.
Photophysical Properties. The absorption spectra of C1−
C4 in THF are shown in Figure 1a. The absorption maxima are
in the small range 352−357 nm for all four complexes (Table
1). However, they can be divided into two groups in terms of
the spectral features. The group consisting of C1 and C2
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of C1−C4 adopted Hagihara’s
method32 using the corresponding ligands L1H−L4H and the
Pt substrates [cis-PtCl2(PBu3)2] (for C1 and C2) and [trans-
PtCl2(PBu3)2] (for C3 and C4). The two synthetic protocols
were illustrated in Scheme 1, as represented by the cases of C2
and C3, respectively. The ligand L1H was previously
reported,33 and L2H was synthesized through the same
protocol: sequential nucleophilic addition of arylacetylide and
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Figure 2. Schematic drawings of the Ln−Pt−Ln backbone
conformation for (a) twisted and (b) coplanar geometry between
the PtP2C2 plane (green) and the arene of ligands (yellow).
The emission properties of C1−C4 were investigated in
dilute THF solutions at room temperature. The two groups
defined above for absorption also differ from one another in the
emission properties. For C1 and C2 (group I), the emission
spectra have dual bands with maxima at ∼400 and ∼510 nm,
and the latter band is more intense and structured (Figure 1b).
The two emission bands have distinct decay times, which is
<4.0 ns for the 400-nm band and ≥90 μs for the 510-nm band
in degassed solutions, assignable to fluorescence and
phosphorescence, respectively. The total luminescence quan-
tum yield is near 0.07 with the Φp/Φf ratio near 5 (Table 1). In
contrast, complexes C3 and C4 (group II) display only a single
phosphorescence band with negligible fluorescence (Φf <
0.001). The phosphorescence properties are nearly the same for
both complexes (Table 1), which indicates that the terminal
arenes being either phenylene or pentiptycene is unimportant.
Compared to group I, group II displays larger phosphorescence
quantum yields (Φp ∼ 0.20) at longer wavelength (λp = 528
nm). The phosphorescence lifetime (τp ∼160 μs) is longer than
that of C1 (90 μs) but shorter than that of C2 (202 μs). It is
also noted that the τp is much larger for C4 than the parent Pt-
acetylide [trans-Pt(PBu3)2(CCC6H4CC
C6H5)2] (∼42 μs in benzene).25 If the prolonged lifetime
observed for C4 versus the parent system is not caused by
different degassing or solvent conditions, it shows a significant
alkoxy substituent effect. Long-lived triplet states of analogous
Pt-acetylides containing alkyl or alkoxy substituents on the
phenylene rings have been reported.35
The influence of pentiptycene groups on the phosphor-
escence quantum yield and lifetime of Pt-acetylides C1−C4
deserves further analysis. The magnitude of spin−orbit
coupling and that of S1−T1 energy gap (ΔEST) and the
electronic nature of T1 are three important parameters for
consideration.36,37 Large spin−orbit couplings would enhance
the triplet yield and thus decrease the Φf. A small ΔEST
facilitates not only the formation of T1 but also the increase
of Φp due to enhanced phosphorescence rate (i.e., shorter τp).
For the electronic character of T1, the τp is increased with
increasing the LC character. It has been shown that the T1 state
of Pt-acetylides is less delocalized than the S1 state and is
confined in the Pt−Ln segment.38,39 As the structural difference
Figure 1. (a) Absorption and (b) normalized emission spectra of C1−
C4 in THF.
(group I) displays a narrower, more structured, and less tailing
long-wavelength absorption band than the other group
containing C3 and C4 (group II). The spectral differences
between the two groups could be attributed to the difference in
steric interactions between the PtP2C2 unit and the neighboring
arenes, in which an increased conformational constraint toward
“twisted” geometries is expected for group I relative to group II
(Figure 2). The narrower and blue-shifted absorption onset for
group I versus II could then be attributed to a diminishment of
rotational heterogeneity along the Ln−Pt−Ln backbone by
reducing the population of the coplanar rotamers. It is well-
documented that the more planar the rotamer of phenyl-
eneethynylene oligomers is, the lower energy it absorbs.33,34
A
similar argument has been proposed by Schanze and co-
workers for a pentiptycene-derived Pt-acetylide polymer.31 It is
also possible that the Pt-pentiptycene twisting might weaken
the contribution of metal-to-ligand charge transfer (MLCT) to
the S1 state. A MLCT absorption band is generally broader and
less structured than an LC band. However, according to the
structured phosphorescence spectra (see below), the MLCT
character for the T1 state should be minor for both groups. For
the short-wavelength absorption bands in 250−280 nm, they
can be attributed to the π → π* transition of the peripheral
nonconjugated phenylene rings of pentiptycene, as the intensity
is proportional to the number of pentiptycene moieties in the
complexes.
a
Table 1. Photophysical Data for C1−C4 in THF
b
c
compd λabs (nm) ε (L mol−1 cm−1
)
λf (nm)
Φf
τf (ns)
λp (nm)
Φp
τp (μs)
KSV (bar−1
)
kq (μs−1 bar−1
)
C1
C2
C3
C4
354
352
357
353
90 000
402
402
398
398
0.012
0.011
<10−3
<10−3
0.39 (97%) 3.61 (3%)
506 (548)
512 (553)
528 (560)
528 (560)
0.047
0.064
0.20
90
430 60
5
115 000
111 000
116 000
1.07 (92%) 2.74 (8%)
202
159
160
2600 180
2300 200
3600 90
13
15
23
<0.2
<0.2
0.20
a
Wavelength of maximum for absorption (λabs), fluorescence (λf), and phosphorescence (λp), extinction coefficients (ε) at λabs, quantum yields of
fluorescence (Φf) and phosphorescene (Φp), lifetimes of fluorescence (τf) and phosphorescence (τp), and Stern−Volmer constants (KSV) and rate
b
c
constants (kq) for the phosphorescence quenching by O2. The percentage of each component is shown in parentheses. The second vibronic bands
are given in parentheses.
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among C1−C4 is the location and number of pentiptycene
groups, the major pentiptycene effect is believed to constrain
the planarization relaxation of the conjugated backbone in the
excited states. Regarding the fact that the degree of steric
congestion is C1 > C2 > C3 ∼ C4 and the relative λp is in the
opposite order C1 (506 nm) < C2 (512 nm) < C3 = C4 (528
nm), the observed λp might simply reflect the relative backbone
planarity in T1. Accordingly, the smaller Φp and larger Φf
observed for group I versus II indicate that twisting the Pt-
acetylides π-backbone could attenuate the spin−orbit couplings
and/or increase the ΔEST. While a larger τp for C2 relative to
group II conforms to the scenario of a larger ΔEST, the smaller
τp for C1 might indicate an increased rate for nonradiative
decays. These results appear to suggest that twisting of the Pt
center from the Ln planes slows down the S1 → T1 ISC and
that twisting within the Ln moiety increases the rate of
nonradiative decay for T1. Further studies are required to
confirm this hypothesis.
(CV) and differential pulse voltammetry (DPV) in CH2Cl2
solution. Figure 4 shows the CV and DPV profiles of C1−C4
DFT Calculation. To strengthen the argument of the
pentiptycene steric effect, DFT calculations (B3LYP40−42
functional with 6-31G (d) for C, H, O, and P and SDD43
basis set for Pt) were performed on C2 and C4, in which the
terminal −OC8H17 groups were replaced with the −OCH3 to
reduce the cost of calculation. The DFT-optimized structures
show a coplanar π-conjugated backbone in the L2 and L4
moieties, and the internal phenylene rings are nearly
perpendicular (90 3°) to the central PtP2C2 plane in both
C2 and C4. According to the well-documented torsional
studies on phenylene-ethynylene oligomers,44 torsions about
the Csp2−Csp single bonds in Ln should encounter a negligible
barrier (<1.0 kcal mol−1). Whereas constraining one of the Ln
planes coplanar to the central PtP2C2 plane raises the energy by
only 0.71 kcal mol−1 for C4, it costs as much as 10.6 kcal mol−1
in the case of C2. The presence of strong Pt-pentiptycene steric
interactions in the planarized form of C2 is also evidenced by
the distorted and bended geometry around Pt (Supporting
Information Figure S13 and Table S1). The larger torsional
freedom for C4 versus C2 is consistent with the broader and
red-shifted absorption band.
Figure 4. Cyclic (black) and differential pulse (red) voltammograms of
C1−C4 in CH2Cl2 with supporting electrolyte 0.01 M Bu4NPF6 at a
scan rate of 50 mV s−1.
with the redox potential relative to that of the ferrocene/
ferrocenium (Fc/Fc+) couple. For all four cases, two anodic
waves are present, and no cathodic processes occur within the
solvent window ( 1.2 V). The first oxidation potential (Eox1) is
in the order C1 (0.82 V) > C3 (0.72 V) ∼ C2 (0.71 V) > C4
(0.66 V). That the Eox1 of C2 is similar to that of C3 rather than
C1 clearly indicates that the Pt-pentiptycene torsional
constraint in C1 and C2 but not in C3 and C4 plays a
minor role in determining the value of Eox1. It appears that the
number of pentiptycene groups, which is 4−2−2−0 through
the compound series C1 → C4, is more important, as C2 and
C3 have the same number of pentiptycene groups and
meanwhile similar value of Eox1. This might suggest that the
heterogeneous electron transfer (HET) between electrode and
the substrates is significantly affected by the bulkiness of
substrates due to the pentiptycene groups. The phenomenon of
HET retarding by bulky substituents has been reported for the
reduction of nitroalkane and diketone.45,46 The relationship
between Eox1 and number of pentiptycene groups also indicates
a delocalized nature of the redox state, which is consistent with
the HOMO character (Figure 3) and the assignment for related
Pt complexes.47 For the second oxidation potential (Eox2), it is
larger than the Eox1 by 0.26 0.02 V for all four cases (Figure
4). This again indicates that the Pt-pentiptycene torsional
constraint has little effect on the electronic communication
along the Ln−Pt−Ln backbone in the first oxidation state. In
contrast, the oxidation potentials for the free ligands are in the
The DFT-derived frontier molecular orbitals HOMO and
LUMO of C2 and C4 are shown in Figure 3. Whereas the
Figure 3. Frontier molecular orbitals of C2 and C4.
HOMO is located at the Ln−Pt−Ln backbone with the
contribution of the Pt atom, the LUMO is mainly on the
ligands. The HOMO → LUMO transition is expected to be
mainly ligand-centered in nature with a minor MLCT character.
The delocalized HOMO and LUMO also conform to a
unichromophoric character of C1−C4 (Supporting Informa-
tion Figure S14).
Electrochemical Properties. To investigate the effect of
Pt-pentiptycene steric interactions on the ground-state
electronic properties of C1−C4, the redox potentials of C1−
C4 and L1H−L4H were determined with cyclic voltammetry
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Figure 5. Dependence of emission spectra of (a) C1 and (b) C2 (3 × 10−6 M) in THF on the concentration of O2 at 1−5%, (c) the color of
emission of C2 in the concentration of (i) 3 × 10−6 M, (ii) 9 × 10−5 M, (iii) 1.8 × 10−4 M, and (iv) 3.6 × 10−4 M at different volumetric ratio of
molecular oxygen (O2) in the mixture of O2 and N2, and (d) the Stern−Volmer plots of the phosphorescence intensity against the pressure of O2 for
C1−C4 (3 × 10−6 M). The insets in parts a and b are the enlarged spectra, and in part d it is the correlation between number of pentiptycene and kq.
order L1H (1.18 V) = L3H (1.18 V) > L2H (1.05 V) ∼ L4H
(1.02 V) (Supporting Information Figure S15). This might
suggest that the octyloxy-substituted arene, which is the
unsubstituted phenylene ring in L2H and L4H but the bulky
pentiptycene group in L1H and L3H, plays a critical role in the
redox process. In other words, the electron-donating octyloxy
group directs the redox process for the free ligands so that the
arene with the octyloxy substituent is important in determining
the oxidation potential; however, in the Pt complexes C1−C4,
both arenes of the ligands are important in determining the
redox potential. The oxidation potential is raised whenever the
directing arenes are pentiptycene relative to phenylene rings.
Oxygen Sensing. The features of dual emissions with
relatively stronger phosphorescence intensity and long
phosphorescence lifetimes render C1 and C2 ideal candidates
as ratiometric oxygen probes. In this context, their ratiometric
oxygen sensing behavior was investigated in THF solutions (3
× 10−6 M). As shown in Figure 5a,b, the phosphorescence but
not the fluorescence intensity of C1 and C2 is highly sensitive
to oxygen in the surroundings. The phosphorescence is
essentially quenched at 1% of oxygen in the air volume. This
is accompanied with a color change from green to blue in the
luminescence (panel i in Figure 5c). Such a high sensitivity of
the phosphorescence to oxygen prompted us to investigate the
concentration effect of the complex C2. It is expected that
increasing the concentration of C2 would need a higher
concentration of oxygen to reach a full phosphorescence
quenching. Therefore, the detection range of C2 as a
ratiometric oxygen probe can be extended to an oxygen
concentration larger than 1%. Indeed, as shown in panels ii−iv
of Figure 5c, a semiquantitative visual detection of oxygen can
be extended to 5% in the air volume. The Stern−Volmer plots
in Figure 5d show a linear relation between oxygen pressure
and intensity of phosphorescence, indicating a simple dynamic
quenching mechanism.18,48 The Stern−Volmer constants
(KSV), namely, the slopes of the plots, are significantly larger
(430−2600 bar−1) than previously reported systems.2,4−6,11 As
KSV is a product of the phosphorescence lifetime (τp) and the
bimolecular quenching constant (kq), the high efficiency of
phosphorescence quenching by oxygen is mainly due to the
large τp values (90−202 μs).
For the purpose of mechanistic elucidation, the phosphor-
escence quenching of C3 and C4 was also investigated under
the same conditions (Table 1 and Figure 5d). The KSV values of
C3 and C4 are comparable or even larger than that of C2,
although their τp values are somewhat lower (∼160 μs).
Accordingly, the kq values (in unit of bar−1μs−1) are in the order
C1 (5) < C2 (13) < C3 (15) < C4 (23). The kq values are
inversely proportional to the number of pentiptycene groups in
complexes, which is 4−2−2−0 through the compound series
C1 → C4. Regarding the high probability of quenching at any
collision between oxygen and the exciton,49 the Ln-dependent
kq values indicate that the lateral nonconjugated phenylene
groups of pentiptycene shield the triplet exciton from
interacting with oxygen. This is reminiscent of the application
of iptycenes as steric protecting group for stabilizing a reactive
center such as carbenes.50
Solid-State Behavior. To compare the photoluminescence
behavior of C1−C4 in the solid state versus in solution phase,
the photophysical properties of C1−C4 in a poly-
(methylmethacrylate) (PMMA) matrix under pure nitrogen
atmosphere have been determined (Figure 6), and pertinent
data are summarized in Table 2. Both the absorption and
phosphorescence maxima undergo small shifts (Δλ = 0−3 nm)
on going from THF solutions to the thin solid films, which
indicates the absence of significant intermolecular interactions
in the PMMA films. The shoulder in the region above 400 nm
of absorption spectra could be attributed to light scattering due
to nonhomogeneity of the films, although the contribution of
changes in molecular conformations toward more planar
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Figure 7 shows the solid-state emission spectra and images of
C2 in a PMMA thin film under atmosphere of varied oxygen/
Figure 6. Normalized (a) absorption and (b) emission spectra of C1−
C4 in a PMMA matrix at room temperature under nitrogen
atmosphere.
geometry cannot be completely excluded. The phenomenon of
dual emission for C1 and C2 observed in THF solutions is
significantly diminished in the PMMA films, because the
fluorescence is decreased and the phosphorescence is enhanced,
indicating an increased spin−orbit coupling in the latter. Under
the oxygen-free condition, the Φp/Φf ratio is as high as 55−100
in the thin films, which makes the ratiometric optical detection
less practical (vide infra). By following the concept of torsion-
induced attenuation of spin−orbit coupling discussed above, a
reduced Pt-pentiptycene torsion angles and/or torsional
flexibility might be responsible for the changes in dual emissive
properties for the complexes in the PMMA matrix. In contrast
to the increase of Φp by ∼50% for C1 and C2 on going from
THF solutions to thin solid films, the Φp is decreased by 50% in
the cases of C3 and C4. Evidently, the Pt-pentiptycene steric
effect on phosphorescence is more pronounced in the thin
films. The τp is also largely affected by the medium, and it is
generally increased in the thin films versus the THF solutions,
although a minor shorter-lived component is present for C1−
C3 in the former condition. The phenomenon of an increased
τp in a thin film versus dilute solution is intriguing in view of the
fact that the inevitable impurities and/or energy traps in a thin
film generally reduce the lifetime of the doped excitons.29
Provided that the different degassing condition is not
responsible for the difference in τp, the larger τp values in
PMMA could be attributed to more effective shielding of the
triplet exciton to quenchers. The current results show that the
photophysical properties of C1−C4 are highly sensitive to the
environments.
Figure 7. (a) Emission spectra, (b) the Stern−Volmer plot, and (c)
emission images for C2 in a PMMA matrix at varied O2 concentration
in O2/N2 (v/v) mixed gas. The small dark region on the left-hand side
of the images corresponds to an intentionally prepared nonemissive
region with a triplet quencher (a drop of 10% benzophenone in
dichloromethane) as an internal reference.
nitrogen concentration. Like the case in THF solutions, the
phosphorescence but not the fluorescence intensity is sensitive
to the oxygen concentration. However, compared to the case in
THF solutions, the sensitivity of phosphorescence in response
to oxygen is much lower so that phosphorescence remains to
dominate the emission even under the open air; a full
quenching of the phosphorescence requires the condition of
100% oxygen atmosphere. The Stern−Volmer constant (KSV
=
33 0.5 bar−1) is ∼80-fold lower than that in THF solutions
(Figure 7b). By using the pre-exponential weighted mean
lifetime (∼630 μs),18 the quenching rate constant kq is 0.05
μs−1 bar−1, which is 260-fold lower than that in THF (Table 1).
This could be attributed to limited oxygen permeability of the
a
Table 2. Photophysical Properties of C1−C4 in a PMMA Matrix
compd
λabs (nm)
λf (nm)
λp (nm)
Φf
Φp (air)
Φp (N2)
τp (air) (μs)
τp (N2) (μs)
C1
C2
C3
C4
356
354
360
353
395
391
505 (545)
511 (553)
528 (561)
526 (560)
0.001
0.002
<10−3
<10−3
0.01
0.01
0.02
0.02
0.10
0.11
0.10
0.09
37
123 (26%) 596 (74%)
46 (10%) 699 (90%)
63 (36%) 245 (64%)
215
8 (12%) 40 (88%)
9 (15%) 48 (85%)
50
b
na
b
na
a
b
Notations are the same as in Table 1. Not available because of extremely weak intensity.
742
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PMMA films.51−53 Nevertheless, the Stern−Volmer plot of the
whole data is linear with a better fit than the plot with data of
low oxygen concentrations (pO2 < 0.2 bar), indicating that the
dynamic quenching is in operation and the films have good
homogeneity. Because of the weak fluorescence, the response of
photoluminescence of the films is more like an “on−off” type
(Figure 7c) rather than the ratiometric color change as seen in
the THF solutions (Figure 5c). A solution of this limitation
might rely on further engineering of thin solid films in different
matrix or solid support and on a more sophisticated setup and
readout system such as fiberoptic sensory devices.54
The relative Φp and τp for C1−C4 under the pure nitrogen
and air conditions (Table 2) also deserve a comment. For C1
and C2, the Φp is lowered by 10 times on going from pure
nitrogen to air, but it is lowered by only 5 times for C3 and C4.
Such a relationship is roughly proportional to the correspond-
ing changes in τp. Thus, the data again show that the magnitude
of phosphorescence lifetime plays a key role in the sensitivity of
oxygen sensing.
concentrations were obtained from the corresponding emission
spectra relative to the spectra in air.
The cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) were performed at a sweep rate of 50 mV s−1 on a CHI 612B
electrochemical analyzer, and the electrochemical cells adopted a
glassy carbon as the working electrode, a Pt wire as the counter
electrode, an Ag wire as the reference electrode, and 0.01 M Bu4NPF6
as electrolyte. The substrates are ∼0.1 mM in CH2Cl2, and the
solutions were outgassed by N2 before measurement. All oxidation
potentials were determined by DPV and calibrated with the ferrocene/
ferrocenium (Fc/Fc+) redox couple. The 1H, 13C, and 31P NMR
spectra were collected using Varian Mercury A-400 or Bruker DPX
400-MHz spectrometer. The 31P NMR chemical shifts determined for
the Pt complexes are referenced to 85% H3PO4(aq) in CDCl3. IR
spectra were recorded by Varian 640-IR. High-resolution mass spectra
were collected by JEOL JMS-700 at National Central University.
Elemental analysis was carried out on Heraeus VarioEL-III analyzer at
National Taiwan University.
Calculations. Density functional theory (DFT) calculations were
performed using Guassian 09 program56 package at the B3LYP/6-
31G(d,p) level40−42 for the ligands. For the complexes, the Stuttgart−
Dresden (SDD)43 basis set was used with a relativistic effective core
potential for Pt, and all ligand atoms (C, H, P, and O) were described
by the B3LYP 6-31G(d) basis set. The optimized ground-state
geometries were investigated by varying dihedral angle between every
four atoms except for hydrogen atoms. The coplanar geometry was
obtained by fixing the dihedral angle between the Pt−P bond and the
neighboring arene C−C bond (i.e., ψC3−C4−Pt‑P in Supporting
Information Table S1) to be 0° followed by the same protocol of
ground-state geometry optimization.
Oxygen Sensing. Flow rate of N2 and O2 was controlled by single
tube flowmeter (AALBORG), and the volumetric flow rate was
monitored by bubble flowmeter. For the solution-phase sensing, the
THF solutions were saturated with N2 and O2 at different air volume
ratio by bubbling (flow rate = 30 mL min−1; time = 15 min). For the
solid-phase sensing, the same drop-cast film of C2 used for the
measurements of electronic spectra was used. The emission images
were taken with spin-cast films on quartz plates prepared with the
same CHCl3 solution used for the drop-cast films.
Materials. Solvents for organic synthesis were reagent grade or
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, mass spectrometry, and/or
element analysis. The synthetic procedure and characterization data
for C1−C4 are provided in the following, and those for the ligands
LnH and the intermediates are provided as Supporting Information.
General Procedure for Synthesis of C1−C4. To a mixture of
the ligand LnH (0.10 mmol), CuI (0.003 g, 0.02 mmol), and cis-
PtCl2(PBu3)2 (for C1 and C2) or trans-PtCl2(PBu3)2 (for C3 and C4)
(0.03 g, 0.05 mmol) in a 50-mL Schlenk flask was added triethylamine
(5.0 mL) and CH2Cl2 (10.0 mL), and then the mixture was stirred
under nitrogen at ambient temperature for 12 h. The solvent was
removed under reduced pressure, and the residue was purified by
column chromatography with CH2Cl2/hexane (1:3, v/v) as eluent
afforded the desired compounds C1−C4.
CONCLUSION
■
This work uncovers the iptycene substituent effect on the
electronic properties of Pt-acetylides [trans-Pt(PBu3)2(C
CArCCAr′OC8H17)2] (Ar and Ar′ = phenylene or
pentiptycene) and on the electronic interactions between the Pt
complexes and the environments (oxygen and electrode). The
results show that the Ln−Pt−Ln backbone conformation plays
an important role in determining the excited-state decay
behavior of Pt-acetylides. Steric torsion and shielding of the
backbone result in dual emission of fluorescence and
phosphorescence and long phosphorescence lifetime. This in
turn leads to a highly sensitive phosphorescence response to
oxygen. In addition, the pentiptycene scaffolds impose a steric
shielding not only to the diffusion of oxygen to the triplet
exciton but also to the electron transfer to the electrode. The
structure−property relationship revealed herein might prove
values for developing new Pt-acetylides as long-lived triplet
sensitizers and ratiometric probes.
EXPERIMENTAL SECTION
■
General Methods. Electronic spectra were recorded at room
temperature (23
1 °C). UV−vis 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 N2-
outgassed (5 min) solution of anthracene (λex = 338 nm, Φf = 0.27 in
hexane)55 was used as standard for emission quantum yields
determination, corrected with solvent refractive index, of compounds
in a solution under three freeze−pump−thaw cycles. The optical
density of all solutions was about 0.1 at the wavelength of excitation,
and an error of 10% is estimated for emission quantum yields.
Phosphorescence decays were measured by means of the Edinburgh
FLS920 spectrometer apparatus with a μF900, a pulsed Xenon
flashlamp, with a R928 detector, and range of measured decay is from
1.5 μs to 10 s. The goodness of 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 solid-state electronic spectra and lifetime measurements
were conducted with drop-cast films formed inside a quartz cuvette
prepared with a CHCl3 solution (0.5 mL) containing the complex (1.0
mg) and PMMA (10.0 mg). The absolute quantum yields for samples
in air were determined using an integrating sphere (150 mm diameter,
BaSO4 coating) of Edinburgh Instruments by the Edinburgh FLS920
spectrometer. The quantum yields for samples in other oxygen
Complex C1. White solids: yield 45%; mp >300 °C (with
decomposition). 1H NMR (400 MHz, CDCl3) δ: 0.97−1.05 (m,
24H), 1.42−1.64 (m, 28H), 1.72−1.73 (m, 4H), 1.96−2.10 (m, 16H),
2.47 (m, 12H), 4.06 (t, J = 6.8 Hz, 4H), 5.81 (s, 4H), 6.23 (s, 4H),
6.30 (s, 4H), 6.31 (s, 4H), 7.02−7.04 (m, 32H), 7.40−7.44 (m, 16H),
7.56−7.58 (m, 16H). 13C NMR (100 MHz, CDCl3) δ: 14.4, 22.9, 24.6,
26.6, 27.1, 29.6, 29.8, 30.8, 32.1, 35.0, 48.5, 52.5, 53.0, 69.7, 76.3, 85.1,
91.0, 91.3, 95.4, 101.3, 104.2, 111.7, 123.4, 123.6, 123.8, 125.2, 125.4,
131.4, 135.5, 142.5, 143.1, 145.0, 145.1, 145.6, 145.8. 31P NMR (162
MHz, CDCl3) δ: 4.29 (1JP−Pt = 2348 Hz). IR (KBr): 2085 (CC)
cm−1. HRMS calcd for C184H168O2P2Pt−H+: 2666.2219. Found:
2666.2229.
Complex C2. White solids: yield 57%; mp >300 °C (with
decomposition). 1H NMR (400 MHz, CDCl3) δ: 0.87−0.93 (m,
24H), 1.34−1.54 (m, 32H), 1.85−1.89 (m, 16H), 2.36 (s, br, 12H),
743
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Inorganic Chemistry
Article
4.08 (t, J = 6.8 Hz, 4H), 5.88 (s, 4H), 6.09 (s, 4H), 6.93−6.95 (m,
16H), 7.04 (d, J = 8.8 Hz, 4H), 7.32−7.40 (m, 16H), 7.71 (d, J = 8.8
Hz, 4H). 13C NMR (100 MHz, CDCl3) δ: 14.0, 14.1, 22.7, 24.3, 24.3,
24.4, 26.1, 26.9, 29.3, 29.4, 31.9, 52.3, 52.5, 68.3, 84.4, 95.4, 111.7,
114.8, 116.0, 120.3, 123.3, 123.8, 123.9, 125.0, 133.1, 142.4, 143.1,
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=
2345 Hz). IR (KBr): 2091 (CC) cm−1. HRMS calcd for
C128H136O2P2Pt: 1961.9663. Found: 1961.9677.
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Complex C3. Pale yellow solids: yield 60%; mp 280 °C (with
decomposition). 1H NMR (400 MHz, CDCl3) δ: 0.93−1.04 (m,
24H), 1.37−1.42 (m, 16H), 1.47−1.58 (m, 16H), 1.62−1.71 (m,
12H), 1.98−2.17 (m, 4H), 2.20−2.24 (m,12H), 3.94 (t, J = 6.8 Hz,
4H), 5.67 (s, 4H), 5.85 (s, 4H), 6.90−6.95 (m, 16H), 7.29−7.31 (m,
8H), 7.33−7.40 (m, 12H), 7.61 (d, J = 8.0 Hz, 4H). 13C NMR (100
MHz, CDCl3) δ: 14.1, 14.3, 22.9, 24.1, 24.3, 24.6, 24.6, 24.7, 26.5,
26.6, 29.5, 29.7, 30.7, 32.1, 48.3, 52.5, 76.1, 94.3, 119.7, 123.3, 123.8,
125.0, 130.7, 131.2, 135.2, 144.9, 145.1, 145.5, 149.4. 31P NMR (162
MHz, CDCl3) δ: 3.53 (1JP−Pt = 2342 Hz). IR (KBr): 2098 (CC)
cm−1. HRMS calcd for C128H136O2P2Pt: 1961.9663. Found:
1961.9669.
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Complex C4. Pale yellow solids: yield, 65%; mp, 124 °C. 1H NMR
(400 MHz, CDCl3) δ: 0.86−0.93 (m, 24H), 1.28−1.31 (m, 16H),
1.39−1.48 (m, 16H), 1.52−1.59 (m, 12H), 1.75−1.79 (m, 4H), 2.09−
2.16 (m, 12H), 3.95 (t, J = 6.4 Hz, 4H), 6.83 (d, J = 7.6 Hz, 4H), 7.19
(d, J = 7.2 Hz, 4H), 7.32 (d, J = 7.6 Hz, 4H), 7.41 (d, J = 8.8 Hz, 4H).
13C NMR (100 MHz, CDCl3) δ: 14.0, 14.3, 22.8, 23.9, 24.0, 24.2,
24.46, 24.52, 24.59, 26.1, 26.5, 29.31, 29.34, 29.5, 31.9, 68.1, 88.5,
114.4, 115.3, 130.5, 130.9, 132.7, 158.8. 31P NMR (162 MHz, CDCl3)
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calcd for C72H104O2P2Pt: 1257.7159. Found: 1257.7158.
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed synthetic schemes and procedures; characterization
data of new compounds; 1H, 13C, and 31P NMR spectra of C1−
C4; CV and DPV profiles of the ligands; DFT-optimized
structures and data of C2 and C4; and the Cartesian
coordinates of optimized structures for C1−C4. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jsyang@ntu.edu.tw.
Notes
(30) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107,
The authors declare no competing financial interest.
1339−1386.
(31) Zhao, X.; Cardolaccia, T.; Farley, R. T.; Abboud, K. A.; Schanze,
K. S. Inorg. Chem. 2005, 44, 2619−2627.
ACKNOWLEDGMENTS
■
We thank the National Science Council of Taiwan, ROC, and
National Taiwan University (Excellence Research Project
102R891303) for financial support. We appreciate Mr. Chen-
Yi Kao for the support of photography. The computing time
granted by the National Center of High-Performance
Computing and the Computing Center of NTU is also
acknowledged.
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