Photophysical properties of endohedral amine-functionalized bis(phosphine) Pt(II) complexes as models for emissive metallacycles

Article abstract of DOI:10.1021/ic400491q

The photophysical properties of bis(phosphine) Pt(II) complexes constructed from 2,6-bis(pyrid-3-ylethynyl) aniline and 2,6-bis(pyrid-4-ylethynyl) aniline vary significantly, even though the complexes differ only in the position of the coordinating nitrog

Full text of DOI:10.1021/ic400491q

Article
pubs.acs.org/IC
Photophysical Properties of Endohedral Amine-Functionalized
Bis(phosphine) Pt(II) Complexes as Models for Emissive Metallacycles
J. Bryant Pollock,*,† Timothy R. Cook,† Gregory L. Schneider,† Daniel A. Lutterman,‡ Andrew S. Davies,†
and Peter J. Stang*,†
†Department of Chemistry, University of Utah, 315 South 1400 East, RM 2020, Salt Lake City, Utah 84412, United States
‡Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
S
* Supporting Information
ABSTRACT: The photophysical properties of bis(phosphine) Pt(II) complexes constructed from 2,6-bis(pyrid-3-ylethynyl)
aniline and 2,6-bis(pyrid-4-ylethynyl) aniline vary significantly, even though the complexes differ only in the position of the
coordinating nitrogen. By capping the ligands with an aryl bis(phosphine) Pt(II) metal acceptor, the photophysical properties of
the two isomeric systems were directly compared, revealing that the low-energy absorption and emission bands of the two
systems were separated by 30 nm (1804 cm−1) and 39 nm (1692 cm−1), respectively. From the analysis of time-dependent
density functional (TD-DFT) calculations and excited-state lifetime measurements, it was determined that the nature of the Pt−
N bond in the HOMO and the sums of the radiative (krad) and nonradiative (knr) rate constants were significantly different in the
two systems. As the dominant nonradiative decay pathway in aniline systems is relaxation from the triplet state through
intersystem crossing (ISC), the difference in knr can be ascribed to changes in ISC between isomers of the bis(phosphine) Pt(II)-
capped 2,6-bis(pyrid-3-ylethynyl) aniline system. It was also determined that the photophysical properties of these capped
systems can be altered by functionalizing the aryl capping ligand on the bis(phosphine) Pt(II) metal center, which perturbs the
molecular orbitals involved in the observed optical transitions. In addition, an isoelectronic bis(phosphine) Pd(II)-capped system
was prepared for comparison with the bis(phosphine) Pt(II) suite of complexes. The Pd(II) system showed significant changes
in its low-energy absorption band, but preserved the characteristic emissive properties of its Pt(II) analogue with an even higher
quantum yield.
The well-established photophysical properties of mono- and
multinuclear bis(phosphine)platinum coordination complexes
(e.g., low-energy and long-lived excited states, facile tunability,
and high quantum yields) have set the foundation for studying
SCCs constructed using bis(phosphine)platinum metal centers
that can preserve these useful absorption and emission
characteristics.4 Recently, a D2h [2 + 2] endohedral amine-
functionalized rhomboid (8, Scheme 1) constructed from 2,6-
bis(pyrid-4-ylethynyl) aniline (1, Scheme 1) and 2,9-bis[trans-
Pt(PEt3)2NO3] phenanthrene5 that displayed a low-energy
absorption band in the visible region and emitted above 500
nm was reported.6 The emissive properties were attributed to
ligand-centered transitions involving π-type molecular orbitals
with modest contributions from metal-based atomic orbitals.
INTRODUCTION
■
Transition-metal-mediated coordination-driven self-assembly
has blossomed over the past two decades, leading to
increasingly complex systems with multiple functionalizations
and various shapes and volumes for host−guest encapsulation,
catalysis, and biomedical applications.1 The Stang group has
pioneered a strategy called the directional-bonding approach
whose archetypal construction method utilizes bis(phosphine)
Pt(II) metal nodes and pyridyl-based organic spacers; each
component has encoded directionality and angularity that
dictates the architectural outcome.2 Fundamental studies on the
photophysical properties of Pt-based supramolecular coordina-
tion complexes (SCCs), such as polygons, are rare and warrant
attention if these systems are to realize their promising
potential for applications in photon-emitting devices or
bioimaging.3
Received: February 25, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Systems for investigating the effects of structural isomerism
Further investigations of these systems are critical for designing
SCCs that exhibit the attractive photophysical properties
established for mono- and multinuclear bis(phosphine)
platinum coordination complexes.
ligand. Given that the photophysical properties of the
aforementioned rhomboidal SCC containing 1 were attributed
to ligand-centered transitions, it was anticipated that the
emissive behavior of such SCCs could be largely preserved in
model systems wherein a ligand is capped by two Pt centers,
truncating a metallacycle to a single Pt−ligand−Pt fragment. As
models of the Pt−ligand−Pt fragment of SCCs, 4 and 5a reveal
the effects of structural isomerism for ligands commonly used
in metallacycle formation (Scheme 1).3a Although 5a can exist
as two additional conformers, the configuration shown in
Scheme 1 is the energetic minimum as indicated by density
functional theory (DFT) calculations (vide infra). Interestingly,
the nature of the molecular orbitals involved with the low-
energy optical transition for 4 are significantly different from
that of the previously reported system.
Investigations probing the effects of size and shape on the
photophysical properties of Pt-based SCCs are relatively rare
when compared to synthetic and structural studies.3a For
instance, although it is clear that the position of the
coordinating nitrogen dictates the architectural outcome of a
self-assembly, the photophysical ramifications of structural
isomerism are not well understood. Here, we employ two
isomeric ligands: the previously mentioned 120° 2,6-bis(pyrid-
4-ylethynyl) aniline5 donor (1), which has the coordinating
nitrogen para to the ethynyl moiety, and 2,6-bis(pyrid-3-
ylethynyl) aniline7 (2), which, with its nitrogen meta to the
ethynyl moiety, represents a cliplike 0° donor (Scheme 1).
Because the different angularities of donors 1 and 2 necessarily
obviate the ability to form structurally analogous SCCs, a
platinum acceptor (3) was synthesized to effectively cap each
It has been established that the molecular orbitals involved
with the observed optical transitions from endodedral amine-
functionalized systems arise from mainly ligand-centered
transitions, with only modest contributions from the metal
B
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Systems for probing the effects of the metal-containing fragments
Figure 1. 31P{1H} NMR spectra of 14 (left, top) and 15 (left, bottom) and ESI mass spectrum of 13 (right).
center; despite the lack of metal character in the orbitals
involved in allowed electronic transitions, complexation of the
dipyridyl ligands gives marked spectral shifts, as high as 100 nm
(4540 cm−1) when 1 coordinates to Pt.6 As such, a
complementary suite of compounds was synthesized to probe
the photophysical influence stemming from the metal-
containing fragment of these model systems. To accomplish
this, the trans aryl group on the bis(phosphine) Pt(II) metal
center was functionalized with either a methoxy group (13,
Scheme 2) or a nitro group (14, Scheme 2). These molecules
were investigated using computational methods to determine
the origins of any observed spectral changes. Also, an
isoelectronic bis(phosphine) Pd(II) analogue of 4 (15, Scheme
2) was prepared to answer the following questions: (i) Can ISC
be attenuated using a metal less prone to exhibit spin−orbit
coupling, thus circumventing the main nonradiative decay
pathway of these aniline compounds? (ii) Are desirable
photophysical properties (visible wavelength emission, tunable
bands, etc.) retained when using a second-row d8 metal center
versus a third-row d8 Pt(II) metal center? Herein, the synthesis
and steady-state absorption and emission and excited-state
lifetime measurements of these SCC model systems are
described with the intention of establishing the effects of
structural isomers and effects originating from the perturbations
of the electronics of the metal nodes by incorporating auxiliary
functional groups on the bis(phosphine) aryl Pt(II) capping
unit and substituting Pt(II) for Pd(II) metal centers. These
experiments were augmented by DFT and time-dependent
DFT (TD-DFT) calculations to probe the nature of the
observed optical transitions.
C
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Photophysical Data of Compounds 4, 5a, and 6−9
a
b
b
compound
absorption bands λmax (nm) [ε × 10−3 (cm−1 M−1)]
λexc (nm) λemiss (nm)
Φ
(%) τ (10−9 s) krad (108 s−1
)
knr (108 s−1
)
c
1
282 [28.6], 300 sh [20.7], 373 [13.7]
300 sh [17.7], 362 [12.2]
356
365
423
393
402
425
430
422
422
408
500
461
466
510
522
505
65
2.55
2.59
1.37
2.58
2.55
1.16
2.34
0.74
1.37
2.70
4.96
3.14
2
30
32
19
12
34
28
15
4
318 [44], 423 [26]
5a
6
292 [35], 393 [16]
293 [16], 402 [10]
7
321 [36], 425 [20]
c
8
c
258 [135], 267 sh [130], 288 [85.2], 317 [112], 430 [39.9]
9
284 sh [95.2], 318 [139], 422 [87.6]
a
b
c
Quinine sulfate at 365 nm was used for quantum yield determination. krad = Φτ−1, knr = τ−1 − krad. Absorption and quantum yield data obtained
from ref 6.
RESULTS
■
Synthesis. The Pt-capped dipyridyl aniline compounds in
Scheme 1 were obtained under synthetic conditions similar to
those used for SCC formation. A CH2Cl2 solution of 1 or 2
with 3, reflecting the 2:1 stoichiometry of acceptor to donor,
furnished the Pt−ligand−Pt triads after 3 h of stirring at room
temperature and precipitation by diethyl ether. The combina-
tion of 2 and a linear diplatinum donor8 in a 1:1 ratio extends
the Pt−ligand−Pt motif to a closed metallacycle, 6, which was
used to validate 5a as an appropriate model for larger SCCs.
Similarly, the combination of 1 with the linear donor furnished
hexagonal metallacycle 96 whose properties were useful in
comparison to model system 4. These systems were
characterized through electrospray ionization mass spectrom-
etry (ESI-MS) and 31P{1H} and 1H NMR spectrometry
(Figures S1−S23, Supporting Information).
Figure 2. Absorption (dashed) and emission (solid) spectra of 4
The M−ligand−M triads in Scheme 2 (13−15) were
synthesized by stirring 1 with 10,9 11,10 or 1211 in 1:2
(green), 5a (blue), 6 (black), and 7 (red). The spectra were collected
in aerated CH2Cl2 at room temperature.
stoichiometry with 2 equiv of silver triflate in CH2Cl2. After 24
h of stirring at room temperature in the dark, the solutions were
filtered to remove insoluble silver halide. Diethyl ether was then
The single emission bands for this series of compounds are
centered at 500 nm for 4, 461 nm for 5a, 466 nm for 6, and 510
added to the homogeneous solution to precipitate the product.
nm for 7 with corresponding quantum yields (Φ) of 32%, 19%,
The solids were then dried overnight in vacuo and redissolved
12%, and 34%, respectively.
in CD2Cl2 for characterization (Figures S24−S39, Supporting
Excited-state lifetime measurements were performed using an
Information). Figure 1 displays the 31P{1H} NMR spectra of 14
800 ps pulse width excitation at 336 nm. The decay of the
(top left) and 15 (bottom left), where the loss of 195Pt satellites
excited state was monitored by the loss of emission intensity at
for 15 is evident. The electrospray ionization mass spectrum of
the wavelength maximum for each sample, as determined from
the doubly charged [M − 2OTf]2+ ion for 13 is shown in
steady-state measurements. The decay profiles were fit to
Figure 1 (right) and is in good agreement with the theoretical
isotopic distribution pattern (shown in red). In each case, a
characteristic downfield shift of the pyridyl protons was
observed upon coordination to the metal centers.
biexponential functions where τ1 corresponded to the intact
platinated species of 4 (1.37 ns) and 5a (2.58 ns), with 83.6%
and 97.7% contributions, respectively. All excited-state lifetime
traces can be found in Figure S39 (Supporting Information).
Photophysical Properties. As reported in Table 1, ligands
Figure 3 displays the absorption and emission spectra of 13−
1 and 2 were determined to have low-energy absorption bands
15, with the relevant metrics summarized in Table 2. Each
centered at 373 and 362 nm, respectively, with similar molar
compound is characterized by a high-energy band between 306
absorption coefficients (1, 13700 cm−1 M−1; 2, 12200 cm−1
and 325 nm, with the largest molar absorption coefficient being
M−1). A single emission band was observed for 1 and 2,
observed for 14. In addition, all species contain a less intense
centered at 422 and 408 nm, respectively. The quantum yield of
band at longer wavelengths of ∼400−425 nm. The molar
1 (Φ = 65%) was roughly twice that of 2 (Φ = 30%).
Figure 2 displays the absorption and emission spectra of 4−
7, with the relevant metrics summarized in Table 1. Each
compound is characterized by a high-energy absorption band
between 300 and 350 nm, with the largest molar absorption
coefficient being observed for 4. In addition, all four species
exhibited a less intense band at longer wavelengths of ∼400−
450 nm. The molar absorption coefficients of the second bands
(∼10000−25000 cm−1 M−1) are roughly half those of the
higher-energy bands (∼15000−45000 cm−1 M−1).
absorption coefficient of the higher-energy band (∼35000−
65000 cm−1 M−1) is roughly twice that of the low-energy band
(∼20000−23000 cm−1 M−1).
The single emission bands for this series of compounds were
centered at 500 nm for 13, 508 nm for 14, and 494 nm for 15
with quantum yields (Φ) of 20%, 13%, and 41%, respectively.
The effect of excitation wavelength on the lifetimes of 13−15
was investigated by employing both 336 and 458 nm light in
the measurements to determine whether the lifetime was
independent of wavelength. The decay profiles were fit to
D
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the red-shifted low-energy absorption and emission band
maxima of 1 (373 and 422 nm, respectively) when compared
to those of 2 (362 and 408 nm, respectively). The quantum
yield of 1 (Φ = 65%) was determined to be more than twice
that of 2 (Φ = 30%). It should be noted that previous reports
have determined that the main nonradiative decay pathway
from the S1 excited state of aniline-based compounds is through
ISC to a nonradiative triplet state.12
Interestingly, upon platination, ligands 1 and 2 have marked
shifts in the low-energy absorption and emission bands.
Comparing ligand 1 to complex 4, the low-energy absorption
band maximum is red-shifted by 50 nm (3169 cm−1), and the
emission band maximum is red-shifted by 78 nm (3697 cm−1).
Similarly, but to a lesser degree, the low-energy absorption and
emission band maxima of 2 are red-shifted when compared to
5a by 31 nm (2179 cm−1) and 53 nm (2818 cm−1),
respectively. An overall loss of quantum yield was also observed
upon platination with 4 (Φ = 32%) being 33% lower than that
of 1, whereas 5a (Φ = 19%) is 11% lower than that of 2.
The apparent differences in the photophysical properties that
arise from the position of the pyridyl nitrogen atom in ligands 1
and 2 are preserved when comparing 4 and 5a, as evidenced by
the blue shift of the low-energy absorption and emission peak
maxima of 5a (30 and 39 nm, or 1804 and 1692 cm−1,
respectively) when compared to those of 4. In addition, the
molar absorption coefficient for the low-energy absorption
band was 16000 cm−1 M−1 (393 nm) for 5a but was less than
that of 4 at 26000 cm−1 M−1 (423 nm).
Figure 3. Absorption (dashed) and emission (solid) spectra of 13
(black), 14 (red), and 15 (blue). The spectra were collected in aerated
CH2Cl2 at room temperature.
Table 2. Photophysical Data of Compounds 13−15
absorption
bands λmax
b
b
(nm) [ε ×
10−3 (cm−1
M−1)]
τ
(10−9
s)
krad
knr
a
λexc
λem
Φ
(108
)
(108
compound
(nm) (nm) (%)
s−1
s−1
)
13
317[35], 420
[21]
420
429
500
508
20
13
2.20
0.91
3.64
14
15
325 [65], 377
sh [22], 429
[20]
2.62
0.50
3.32
The photophysical differences between isomers 4 and 5a
were further manifested in the quantum yields, as that of 5a (Φ
= 19%) is lower than that of 4 (Φ = 32%). To better
understand these differences, excited-state lifetime measure-
ments were employed to probe the radiative and nonradiative
rate constants of the excited state.
306 [44], 404
[23]
404
494
41
3.26
1.26
1.81
a
Quinine sulfate at 365 nm was used for quantum yield determination.
b
krad = Φτ−1, knr = τ−1 − krad
.
The excited-state lifetimes (τ1) of 1 (2.55 ns), 2 (2.59 ns), 4
(1.37 ns), and 5a (2.58 ns) are all similar in magnitude, as are
biexponential functions where the average of the τ1 value
corresponded to the intact platinated species of 13 (2.20 ns)
and 14 (2.62 ns) and the palladium-bound 15 (3.26 ns) with
95.8%, 95.5%, and 88.5% contributions, respectively, at 336 nm.
Similar lifetimes were observed at 458 nm (Table S2,
Supporting Information), indicating that the excited state
leading to radiative decay is accessed through internal
conversion and is thus independent of the wavelength of
excitation.
the rate constants for the radiative decay pathway of 1 (krad
=
2.55 × 108 s−1) and 4 (krad = 2.34 × 108 s−1), which were
calculated using the quantum yields and excited-state lifetimes.
However, the rate constant for the nonradiative decay pathway
for 4 (knr = 4.96 × 108 s−1) is more than twice that of 1 (knr =
1.37 × 108 s−1). Because knr is the sum of the rate constants for
all nonemissive processes, this indicates that 4 either has access
to more nonradiative decay pathways or has better overlap with
a nonradiative excited state; moreover, the inclusion of
platinum in these systems enhances the rate of spin-forbidden
processes, coined the “heavy-atom effect”. From the TD-DFT
Photophysical Discussion. It is readily apparent that the
position of the pyridyl nitrogen atom has an effect on the
resulting photophysical properties of 1 and 2, as evidenced by
Figure 4. Jablonski diagrams for 1 (left) and 4 (right).
E
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
calculations (vide infra), it was determined that 4 utilizes the
frontier orbitals during the lowest-energy electronic transition.
Therefore, although it is advantageous in some Pt-based
systems to increase ISC,4d,13 the decrease in the quantum yield
of 4 (Φ = 32%) when compared to that of 1 (Φ = 65%) could
be attributed to better overlap between the nonradiative triplet
and S1 excited states (shown in Figure 4); no phosphorescence
was observed in the platinated species in degassed, oxygen-free
solvent. Interestingly, the rate constants for the radiative decay
pathways for 2 (krad = 1.16 × 108 s−1) and 5a (krad = 0.74 × 108
s−1) are different, indicating that platination not only induces
ISC, as was seen in the case of 1 versus 4, but also affects the
radiative decay pathway.
The rate constants for the radiative and nonradiative decay
pathways of 4 (krad = 2.34 × 108 s, knr = 4.96 × 108 s−1) are
higher than those of 5a (krad = 0.74 × 108 s−1, knr = 3.14 × 108
s−1), which implies that the higher quantum yield of 4 is due to
ligand 1 (Φ = 65%) when compared to 2 (Φ = 30%). Also, the
quantum yield of 4 (Φ = 32%) is one-half that of 1 (Φ = 65%),
whereas the quantum yield of 5a (Φ = 19%) is two-thirds that
of 2 (Φ = 30%). Understanding these differences requires an
analysis of the molecular orbitals involved in the relevant
optical transitions (vide infra).
Platinum complexes 4 and 5a were then compared to
metallacyclic SCC analogues to evaluate the use of capped M−
L−M fragments as models for larger systems. Because
computational investigations implicate ligand-centered highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) in the low-energy optical transitions
of 4 and 5a (vide infra), it was expected that these truncated
fragments were valid models.
Comparing 5a to its hexagonal SCC counterpart, 6, the low-
energy absorption and emission bands of 5a are blue-shifted by
9 nm (569 cm−1) and 5 nm (233 cm−1), respectively. The
quantum yield of 5a (Φ = 19%) is higher than that of 6 (Φ =
12%). The absorption and emission profiles are in good
agreement for 5a versus 6, and the quantum yield difference
can be explained by the inclusion of more Pt(II) metal centers,
which will enhance the heavy-atom effect. Therefore, 5a is a
good model for 6.
low-energy absorption and emission peak maxima that were
red-shifted when compared to those of 4 by 2 nm (112 cm−1)
and 10 nm (392 cm−1), respectively. This result is interesting
because the counterions of an SCC are often thought to be
outer-sphere and typically are not ascribed any photophysical
relevance. Also, the low-energy absorption and emission peak
maxima of 7 are blue-shifted by 5 nm (273 cm−1) and 12 nm
(451 cm−1), respectively, when compared to those of 8, which
indicates that these systems are sensitive to the nature of the
aryl group trans to the Pt−N.
To better understand and probe this effect, the aryl group
trans to the coordinating nitrogen was functionalized at the
para position to give systems with an electron-donating
methoxy group (13, Scheme 2) and an electron-withdrawing
nitro group (14, Scheme 2). Previous studies hypothesized that
π back-bonding from the bis(phosphine) Pt(II) metal center to
the coordinated pyridyl nitrogen accounted for the observed
optical shifts between free ligands and their coordinated
counterparts.6,14,15 These two compounds allowed for a direct
probe into the effects of perturbing the π system of the metal
fragments. An isoelectronic bis(phosphine) Pd(II) metal
complex (15, Scheme 2) was also utilized to attenuate spin−
orbit coupling, thereby decreasing the ISC. Because ISC is a
known nonradiative decay pathway for aniline-based com-
pounds, this should result in a higher quantum yield.
The NO2-aryl Pt system (14) displayed the lowest-energy
absorption (429 nm) and emission (508 nm) band maxima,
which were red-shifted when compared to both the non-
functionalized aryl system, 4 (λabs = 423 nm, λem = 500 nm) and
OMe-aryl Pt system 13 (λabs = 420 nm, λem = 500 nm). The
quantum yield, however, of system 14 (Φ = 13%) is lower than
those of 13 (Φ = 20%) and 4 (Φ = 32%), whereas the excited-
state lifetime of 14 (τ = 2.62 ns) is higher than those of 13 (τ =
2.20 ns) and 4 (τ = 1.37 ns). From these results, it was
calculated that the rate constants for the nonradiative (krad) and
radiative (knr) decay pathways for 14 (krad = 0.50 × 108 s−1, knr
= 3.32 × 108 s−1) were the lowest as compared to those of 13
(krad = 0.91 × 108 s−1, knr = 3.64 × 108 s−1) and of 4 (krad = 2.34
× 108 s−1, knr = 4.96 × 108 s−1).
Pd-based 15 displays a blue shift in both the absorption and
emission bands as compared to its isoelectronic Pt-based
system, 4. The low-energy absorption band of 15 was blue-
shifted by 19 nm (1111 cm−1), and the emission band
maximum was blue-shifted by 6 nm (243 cm−1) when
compared to those of 4. The excited-state lifetime, however,
was much higher for 15 (τ = 3.26 ns) than for 4 (τ = 1.37 ns),
which could account for the overall 9% higher quantum yield of
the former over the latter. For 15, we calculated krad = 1.26 ×
108 s−1 and knr = 1.81 × 108 s−1, and the values calculated for 4
were krad = 2.34 × 108 s−1 and knr = 4.96 × 108 s−1. The
difference in krad value implies that a different radiative pathway
(i.e., the nature of the molecular orbitals involved in the
electronic transition) might exist. Therefore, to better under-
stand why the observed optical transitions were higher in
energy for 15 when compared to 4, time-dependent density
functional theory (TD-DFT) calculation were employed (vide
infra).
The previously reported endohedral amine-functionalized
D2h [2 + 2] rhomboid (8, Scheme 1) synthesized from ligand 1
and a 60° phenanthrene diplatinum nitrate acceptor (Scheme
1) has photophysical properties that show relatively greater
discrepancies from those of 4. The low-energy absorption and
emission peak maxima of 8 are both blue-shifted by 7 nm (385
cm−1) and 22 nm (843 cm−1), respectively. However, when
compared to the endohedral amine-functionalized [6 + 6]
hexagon6 (9, Scheme 1), which contains ligand 1 and a 180°
benzene-based diplatinum triflate acceptor, the low-energy
absorption bands differs by only 1 nm (56 cm−1), and the
emission band of 9 is red-shifted by 5 nm (198 cm−1) relative
to that of 4; the shifts in the observed optical transitions are
thought to arise from the differences in the aromatic scaffolds
(i.e., 3 has a benzene core scaffold much like the 180° benzene-
based diplatinum acceptor, whereas the rhomboid is synthe-
sized from a 60° phenanthrene-based diplatinum acceptor). As
such, 4 appropriately models the photophysical properties of its
analogous hexagonal SCC, but not its rhomboidal counterpart.
To probe the differences in the photophysical properties of 4
and 8, the counterion and the aromatic group trans to the
coordinating nitrogen were investigated. The nitrate counterion
analogue of 4 (7, Scheme 1) was prepared and foundto exhibit
DFT and TD-DFT General Information. Geometry
optimization calculations were performed using a split basis
set where the Becke three-parameter hybrid exchange and the
Lee−Yang−Parr correlation functionals (B3LYP)16 and 6-
31G** basis set17 were used for C, H, N, and P atoms, whereas
the Los Alamos National Laboratories 2 Double Zeta
F
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 5. Two additional conformers of 5a that are possible.
(LANL2DZ)18 basis set and pseudopotential were used for Pt
and Pd. To minimize computational costs, P(CH3)3 ligands
were utilized instead of PEt3; therefore, the model used to
approximate compound X is abbreviated as X−P(CH3)3, where
X is the compound number. A frequency analysis was also
performed to determine whether any imaginary states existed
below the energy minimum. The vertical singlet transition
energies of the complexes were computed by time-dependent
density functional theory within Gaussian 0919 using the
ground-state optimized structure. For the structures that were
calculated, 1000 or more total molecular orbitals were
observed, and each molecular orbital number that is listed is
real and in its absolute energetic order. The nomenclature
utilized to discuss these molecular orbitals is relative to the
HOMO and LUMO (i.e., HOMO−1 is the molecular orbital
directly below the HOMO).
that are very closely related, configuration 5a−P(CH3)3 was
used in subsequent TD-DFT calculations.
TD-DFT Results and Discussion. From the TD-DFT
calculations of 4−P(CH3)3 and 5a−P(CH3)3 (see Table 4), the
low-energy optical transitions were determined to originate
from a HOMO-to-LUMO electronic transition that involves
ligand-centered molecular orbitals with π-type symmetry
(Figure 6). The predicted observed low-energy optical
transitions occur at 451 nm for 4−P(CH3)3 and 424 nm for
5a−P(CH3)3. In the low-energy transition for both 4−
P(CH3)3and 5a−P(CH3)3, the HOMO has the nitrogen p
orbital and ethynyl π system in phase and bonding, whereas the
LUMO has little electron density on the aniline nitrogen p
orbital and an antibonding ethynyl π system; however, despite
sharing similar characteristics, 4−P(CH3)3 also has a significant
amount of charge-transfer character. After performing a
population analysis, it was determined that, in the HOMO
for 4−P(CH3)3, 8% of the electron density is on the Pt-
phosphine metal center and 15% of the electron density is on
benzyl group, whereas 5% and 0.1% of the electron density lie
on the Pt-phosphine and benzyl group, respectively, in the
LUMO. The ligand gains an appreciable amount of electron
density during the HOMO- (77%) to-LUMO (95%) transition,
which leads to the charge transfer being described as a mixture
of metal−ligand (MLCT) and ligand−ligand (LLCT) charge
transfer; the charge-transfer character of the low-energy
transition could explain why the emission of 4 is sensitive to
the nature of the aromatic group trans to the coordinating N
atom of the pyridyl group. Also, for the HOMO of 5a−
P(CH3)3 (Figure 6, left), the coordinating nitrogen and
As mentioned previously, compound 5a can exist as one of
three conformers as shown in Figure 5. DFT optimizations of
the three compounds determined that 5a−P(CH3)3 is the
lowest in energy (Table 3) by 3.70 kcal/mol when compared to
Table 3. DFT Optimization Energies for 5a−P(CH3)3−5c−
P(CH3)3
compound
energy (hartrees)
Δ (kcal/mol)
5a−P(CH3)3
5b−P(CH3)3
5c−P(CH3)3
−3480.00293429
−3479.99704283
−3480.00052333
0.0000
3.6970
1.5129
5b−P(CH3)3 and 1.51 kcal/mol when compared to 5c−
P(CH3)3; therefore, even though the conformers have energies
Table 4. Electronic Transitions Predicted for 4−P(CH3)3 and 5a−P(CH3)3 with
M
> 0.2
compound
wavelength (nm)
451
orbital transitions
orbital transitions
oscillator strength, M
description
4−P(CH3)3
219 → 220
HOMO → LUMO
0.925
MLCT
LLCT
1
1π → π*
343
424
329
214 → 221
219 → 220
HOMO−5 → LUMO + 1
HOMO → LUMO
1.200
0.715
0.746
ILCT
1
1π → π*
5a−P(CH3)3
ILCT
1π → π*
1
214 → 221
216 → 221
HOMO−5 → LUMO + 1
HOMO−3 → LUMO + 1
LLCT
LMCT
1π → π*
1
G
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. HOMOs and LUMOs of 4−P(CH3)3 (right) and 5a−P(CH3)3 (left).
Table 5. Electronic Transitions Predicted for 13−P(CH3)3, 14−P(CH3)3, and 15−P(CH3)3 with
M
> 0.2
compound
wavelength (nm)
447
orbital transitions
orbital transitions
oscillator strength, M
description
ILCT
13−P(CH3)3
233 → 236
HOMO−2 → LUMO
0.839
1
1π → π*
343
336
455
349
230 → 237
HOMO−5 → LUMO + 1
1.316
0.253
0.932
1.069
ILCT
1
1π → π*
230 → 236
233 → 237
241 → 242
HOMO−5 → LUMO
HOMO−2 → LUMO + 1
HOMO → LUMO
ILCT
1π → π*
1
14−P(CH3)3
15−P(CH3)3
ILCT
1π → π*
1
238 → 243
239 → 242
240 → 243
HOMO−3 → LUMO + 1
LLCT
MLCT
1π → π*
1
HOMO−2 → LUMO
HOMO−1 → LUMO + 1
HOMO → LUMO
446
342
219 → 220
0.872
1.166
ILCT
1π → π*
1
216 → 221
218 → 221
HOMO−3 → LUMO + 1
LLCT
MLCT
1π → π*
1
HOMO−1 → LUMO + 1
HOMO−3 → LUMO
HOMO−1 → HOMO + 1
335
216 → 220
219 → 221
0.248
ILCT
1π → π*
1
platinum metal centers are nonbonding versus antibonding in
4−P(CH3)3 (Figure 6, right), which could account for the
apparent differences in the measured quantum yields and
calculated rate constants between 4 and 5a. The LUMOs in
both systems have antibonding character between the
coordinating nitrogen and platinum metal center. This
modulation of the energies of the HOMO and LUMO, with
5a−P(CH3)3 (0.125 eV) having a larger energy gap than 4−
P(CH3)3 (0.119 eV), is in agreement with 4 having a lower-
energy absorption band. It should be noted that the oscillator
strengths of the lowest-energy predicted transitions from the
calculations of 4 (0.925) and 5a (0.715) reflect the trend
observed in the experimental molar absorption coefficients of
the low-energy optical transitions.
The predicted high-energy electronic transitions for 4−
P(CH3)3 and 5a−P(CH3)3 were determined to originate from
lower-lying occupied molecular orbitals [e.g., HOMO−5 for 4−
P(CH3)3 and HOMO−5 and HOMO−3 for 5a−P(CH3)3] to
unoccupied destination molecular orbitals that are higher than
the LUMO [e.g., LUMO + 1 for 4−P(CH3)3 and 5a−
P(CH3)3]. The observed optical transitions arising from the
electronic transitions are predicted to occur at 343 nm for 4−
P(CH3)3 and 329 nm for 5a−P(CH3)3. The molecular orbitals
involved in the optical transition at 343 nm for 4−P(CH3)3
show that the transition arises from the HOMO having the
nitrogen p orbital and ethynyl π system in-phase and bonding,
whereas the LUMO has little electron density on the aniline
nitrogen p orbital and an antibonding ethynyl π system.
Population analysis on the predicted high-energy electronic
transition at 329 nm for 5a−P(CH3)3 determined that the
singlet transition arises from charge transfer from the Pt-aryl
group to the ligand (>80% → <1%). Also, ca. 12% of the
HOMO electron density lies on the Pt metal center for 5a−
P(CH3)3. In contrast, for the LUMO, more than 5% of the
electron density is localized on the Pt metal centers; therefore,
despite the mild ligand-to-metal charge-transfer (LMCT)
character, the HOMO → LUMO transition appears to be
more appropriately described as a ligand-to-ligand charge
transfer (LLCT).
The TD-DFT calculations predict three electronic transitions
to occur with oscillator strengths above 0.2 for 13−P(CH3)3
(see Table 5). The predicted low-energy transition arises from
an electron being promoted from the HOMO−2 to the
LUMO, wherein the orbitals are largely ligand-centered (>95%
electron density) and of π-type symmetry. The same electronic
transition has a modest change in electron density on the
H
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
platinum metal center (1.5% → 4.7%). A similar electronic
transition was predicted at 455 nm for 14−P(CH3)3, wherein
the HOMO and LUMO are involved. The HOMO-to-LUMO
transition involves π-type molecular orbitals with over 94%
electron density being centered on the ligand, and it is
accompanied by an increase in electron density on the platinum
metal node (1.8% → 4.8%). This result is consistent with the
previously discussed population analysis performed for 4−
P(CH3)3, wherein the ethynyl π system is weakened (1π →
1π*) during the transition, with a loss of electron density on the
aniline nitrogen.
A higher-energy optical transition with an oscillator strength
of 1.316 that utilizes HOMO−5 and LUMO + 1 is predicted to
occur at 343 nm for 13−P(CH3)3. This is very similar in nature
to the predicted lowest-energy transition where the molecular
orbitals involved are of π-type symmetry and are ligand-
centered with modest contributions from the metal center.
HOMO−5 has 92% electron density on the ligand and 7.3% on
the metal center. LUMO + 1 has 94% of its electron density
centered on the ligand and 5.9% on the metal center.
The highest-energy predicted electronic transition for 13−
P(CH3)3 at 336 nm has an oscillator strength of 0.253. The
electronic transition utilizes low-lying occupied (HOMO−2
and HOMO−5) and low-lying unoccupied (LUMO and
LUMO + 1) molecular orbitals that were discussed in the
previous predicted electronic transitions for 13−P(CH3)3.
The high-energy electronic transition for 14−P(CH3)3 is
predicted to occur at 349 nm with an oscillator strength of
1.069 and, interestingly, can be characterized as a mixture of
ligand−ligand (LLCT) and metal−ligand charge-transfer
(MLCT) bands. For the occupied molecular orbitals predicted
to be involved (HOMO−3, HOMO−2, and HOMO−1), a
population analysis was performed on the Pt-phosphine metal
center (22%), aryl ligand (46%), and ligand 1 (31%); the
percentages represent the averages of the contributions for the
molecular fragments. The destination molecular orbitals have
94.0% of the electron density on the ligand and 5.5% on the
metal center. This predicted transition, however, is significantly
different from those in 13−P(CH3)3 and previously studied
systems.
CONCLUSIONS
■
The photophysical properties of endohedral amine-function-
alized bis(phosphine) Pt(II) SCCs can be tuned by using
isomeric species. This is important because assembly reactions
using isomeric donor ligands often do not require synthetic
redesigns and therefore offer a way to alter the absorption and
emission profiles of discrete SCCs without losing atom
efficiency or changing synthetic conditions. Given the attractive
photophysical properties observed for SCCs constructed from
aniline-based donor ligands, developing a chemistry to tune
these properties while maintaining the important aniline core is
desirable. It was determined that the position of the
coordinating nitrogen can greatly affect the photophysical
properties of 2,6-diethynyl aniline-based ligands by using M−
ligand−M fragments as suitable models for metallacyclic SCCs.
The difference in the low-energy absorption bands was
attributed to the different Pt−N coordination bonding modes
in the HOMO. The M−ligand−M systems employed also
allowed for this study to be extended to probing the metal
fragment. Studies using functionalized aryl groups on the metal-
based acceptor fragments showed no significant effects on the
overall observed photophysical properties of the M−ligand−M
systems despite having more significant charge-transfer
mechanisms. Also, a Pd analogue, 15, displayed an emission
profile similar to that of its Pt counterpart, which suggests that
less expensive systems employing bis(phosphine) Pd(II) nodes
can be used in the synthesis of systems for photon-emitting
devices without suffering a large penalty in the observed optical
properties.
EXPERIMENTAL SECTION
■
Materials and Methods. 2,6-Bis(pyrid-4-ylethynyl) aniline6 (1),
2,6-bis(pyrid-3-ylethynyl) aniline7 (2), bromophenylbis-
(triethylphosphine)platinum,20 μ-1,4-phenylenetetrakis-
(triethylphosphine)bis(1,1,1-trifluoromethanesulfonato-κO)-
diplatinum,8 bromo(4-methoxy)bis(triethylphosphine)-, (SP-4-3)plat-
inum (10),9 iodo(4-nitrophenyl)bis(triethylphosphine)-, (SP-4-3)
platinum (11),10 and bromophenylbis(triethylphosphine) palladium
(12)11 were prepared using known procedures. All compounds were
used as received from Sigma-Aldrich, Oakwood Chemicals, and TCI
America, whereas deuterated solvents were purchased from Cambridge
Isotope Laboratory (Andover, MA). H and 31P{1H} NMR spectra
1
were recorded on a Varian 300 spectrometer, and mass spectra were
recorded on a Micromass LCT Premier XE time-of-flight (ToF) mass
spectrometer using electrospray ionization and analyzed using the
MassLynx software suite. The ESI-MS samples were dissolved in
methylene chloride and then diluted with acetone unless otherwise
noted. All 31P{1H} NMR spectra were referenced using a 10% H3PO4
(aq) solution. Elemental analysis was performed by Atlantic Microlab,
Inc.
Pd system 15−P(CH3)3 is predicted to have three electronic
transitions with oscillator strengths over 0.20, with the lowest-
energy state corresponding to an excitation at 446 nm. This
electronic transition involves the HOMO and LUMO and
consists of π-type molecular orbitals with >96% of the electron
density centered on the ligand. A loss of electron density on the
aniline nitrogen is accompanied by weakening of the ethynyl π
(1,1,1-Trifluoromethanesulfonato-κO)phenylbis-
(triethylphosphine)platinum (3). Bromophenylbis-
(triethylphosphine)platinum (99.5 mg, 169 μmol) and silver triflate
(49.0 mg, 57.9 μmol) were weighed into a Schlenk flask, evacuated by
reduced pressure, placed under N2 atmosphere, and covered with
aluminum foil. In another Schlenk flask, 10 mL of methylene chloride
was degassed by the freeze−pump−thaw method. The methylene
chloride was then transferred by cannula to the Schlenk flask
containing the starting materials. The reaction mixture was allowed
to stir at room temperature in the dark for 3 h. The resulting mixture
was then filtered using air-free techniques. A clear, colorless solution
was obtained, and after the methylene chloride had been removed by
reduced pressure, an off-white solid was afforded; 88 mg (79% yield).
1H NMR (CDCl3, 300 MHz): δ 7.23 (d, 2H, ArHα, J = 12.6 Hz), 6.85
1
system (1π → π*) during the transition. A similar transition
was observed for the highest-energy predicted transition at 335
nm.
The transition at 342 nm for 15−P(CH3)3 involves
HOMO−3 and HOMO−1 as the occupied molecular orbitals,
with the destination unoccupied molecular orbital being
LUMO + 1. During this transition, HOMO−1 has 73% of
the electron density centered on the aryl group and 26.6%
centered on Pd-phosphine. LUMO + 1 has 0.8% electron
density on the aryl group and 5.4% on Pd-phosphine. This is a
significant transfer of charge during the transition, unlike what
is observed for the 4−P(CH3)3 system, and is described as a
mixture of LLCT and MLCT.
(m, 3H, ArH), 1.61 (bs, 12H, PCH2), 1.11 (m, 18H, PCH3). 31P{1H}
NMR (CD2Cl2, 121.4 MHz): δ 20.79 (bs; 195Pt satellites, 1JPt−P = 2847
I
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Hz). ESI-MS (C19H35F3O3P2PtS) m/z: [M − OTf], 508.19; [M −
OTf + acetone], 566.23. Anal. Calcd. for C19H35F3O3P2PtS: C, 34.70;
H, 5.36. Found: C, 34.85; H, 5.17.
(d, 2H, ArH, J = 9 Hz), 7.31−7.34 (d, 4H, PtArHα, J = 12 Hz, JArH−Pt =
24 Hz), 7.04−7.09 (t, 4H, PtArHβ, J = 15 Hz), 6.95−6.97 (t, 2H,
PtArHγ, J = 6 Hz), 6.66−6.71 (t, 1H, ArH, J = 15 Hz), 5.83 (bs, 2H,
ArNH2), 1.31−1.35 (m, 24H, PCH2), 1.07−1.17 (m, 36H, PCH3).
31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 12.88 (bs; 195Pt satellites,
1JPt−P = 2696 Hz). ESI-MS (C56H83N5O6P4Pt2) m/z: [M − ONO2]+,
1373.47; [M − 2ONO2]2+, 655.74. Anal. Calcd. for C56H83N5O6P4Pt2:
C, 46.83; H, 5.82; N, 4.88. Found: C, 47.08; H, 5.79; N, 5.26.
General Procedure for Synthesis of 13−15. Into a 2 dram vial
was weighed 2,6-bis(pyrid-4-ylethynyl) aniline (1; 1 mg, 3.4 μmol)
with bromo(4-methoxy)bis(triethylphosphine)-, (SP-4-3)platinum
(10; 4.2 mg, 6.8 μmol), iodo(4-nitrophenyl)bis(triethylphosphine)-,
(SP-4-3)platinum (11; 4.6 mg, 6.8 μmol), or bromophenylbis-
(triethylphosphine) palladium (12; 3.4 mg, 6.8 μmol) in a 1:2
stoichiometric ratio. Two equivalents of AgOTf (1.7 mg, 6.8 μmol)
was then added to the vial. CH2Cl2 (1 mL) was added, and the mixture
was allowed to stir for 24 h in the dark at room temperature. The
solution was then filtered and purified by precipitation upon addition
of diethyl ether to the solution. The mixture was centrifuged, and the
supernatant was decanted. The solid was redissolved in CD2Cl2 and
characterized.
General Procedure for the Synthesis of Pt-Aryl OTf-Capped
Ligands. To a 2 dram vial was added 2,6-bis(pyrid-4-ylethynyl)
aniline (1; 1.0 mg, 3.4 μmol) or 2,6-bis(pyrid-3-ylethynyl) aniline (2;
1.0 mg, 3.4 μmol) with (1,1,1-trifluoromethanesulfonato-κO)-
phenylbis(triethylphosphine)platinum (3; 4.5 mg, 6.8 μmol) in a 1:2
stoichiometric ratio. Deuterated methylene chloride (1 mL) was then
added, and the mixture was allowed to stir for 3 h. The compound was
then purified by precipitation upon addition of diethyl ether to the
homogeneous solution. The mixture was centrifuged, and the
supernatant was decanted.
Diphenyl{μ-[4,4′-(1-amino-2,6-ethynediylbenzene)]bis-
(pyridine-κN)}tetrakis(triethylphosphine)diplatinum (4). 1H
NMR (CDCl3, 300 MHz): δ 8.56−8.60 (d, 4H, PyHα, J = 12 Hz),
7.85−7.87 (d, 4H, PyHβ, J = 3 Hz), 7.49−7.52 (d, 2H, ArH, J = 9 Hz),
7.32−7.34 (d, 4H, PtArHα, J = 6 Hz, JArH−Pt = 51 Hz), 7.05−7.09 (t,
4H, PtArHβ, J = 12 Hz), 6.95−6.97 (t, 2H, PtArHγ, J = 6 Hz), 6.69−
6.74 (t, 1H, ArH, J = 15 Hz), 5.83 (bs, 2H, ArNH2), 1.31−1.35 (m,
24H, PCH2), 1.07−1.17 (m, 36H, PCH3). 31P{1H} NMR (CD2Cl2,
1
121.4 MHz): δ 10.84 (bs; 195Pt satellites, JPt−P = 2695 Hz). ESI-MS
Platinum Methoxybenzene OTf-Capped Ligand (13). 1H
NMR (CDCl3, 300 MHz): δ 8.56−8.58 (d, 4H, PyHα, J = 6 Hz),
7.84−7.86 (d, 4H, PyHβ, J = 6 Hz), 7.49−7.51 (d, 2H, ArH, J = 6 Hz),
7.17−7.20 (d, 4H, PtArHα, J = 9 Hz, JArH−Pt = 51 Hz), 6.71−6.74 (m,
5H, PtArHβ, ArH), 5.83 (bs, 2H, ArNH2), 3.76 (s, 6H, OCH3), 1.25−
1.40 (m, 24H, PCH2), 1.06−1.16 (m, 36H, PCH3). 31P{1H} NMR
(CD2Cl2, 121.4 MHz): δ 13.39 (bs; 195Pt satellites, JPt−P = 2688 Hz).
ESI-MS (C60H87F6N3O8P4Pt2S2) m/z: [M − OTf]+, 1520.46; [M −
2OTf]2+, 685.75. Anal. Calcd. for C60H87F6N3O8P4Pt2S2 [com-
plex]·CH2Cl2·Et2O: C, 42.67; H, 5.45; N, 2.30. Found: C, 42.58; H,
5.47; N, 2.48.
(C58H83F6N3O6P4Pt2S2) m/z: [M − OTf]+, 1460.44; [M − 2OTf]2+,
655.74. Anal. Calcd. for C58H83F6N3O6P4Pt2S2: C, 43.26; H, 5.19; N,
2.61. Found: C, 43.29; H, 5.23; N, 2.55.
Diphenyl{μ-[3,3′-(1-amino-2,6-ethynediylbenzene)]bis-
(pyridine-κN)}tetrakis(triethylphosphine)diplatinum (5a). 1H
NMR (CDCl3, 300 MHz): δ 8.81 (s, 2H, PyHα), 8.56−8.58 (d, 2H,
PyHα′, J = 6 Hz), 8.30−8.32 (d, 2H, PyHγ, J = 6 Hz), 7.68−7.73 (m,
2H, PyHβ), 7.48−7.50 (d, 2H, ArH, J = 6 Hz), 7.37−7.40 (d, 4H,
PtArHβ, J = 9 Hz), 7.08 (m, 4H, PtArHα), 6.98−7.01 (m, 2H, PtArHγ),
6.69−6.75 (t, 1H, ArH, J = 18 Hz), 5.73 (bs, 2H, ArNH2), 1.34 (m,
24H, PCH2), 1.08−1.178 (m, 36H, PCH31). 31P{1H} NMR (CD2Cl2,
121.4 MHz): δ 10.84 (bs; 195Pt satellites, JPt−P = 2695 Hz). ESI-MS
(C58H83F6N3O6P4Pt2S2) m/z: [M − 2OTf]2+, 655.74. Anal. Calcd for
C58H83F6N3O6P4Pt2S2 [Complex]·CH2Cl2·Et2O: C, 42.76; H, 5.41; N,
2.37. Found: C, 42.81; H, 5.22; N, 2.51.
1
Platinum Nitrobenzene OTf-Capped Ligand (14). H NMR
(CDCl3, 300 MHz): δ 8.58−8.60 (d, 4H, PyHα, J = 6 Hz), 7.91−7.66
(m, 8H, PyHβ, Pt−ArHβ), 7.61−7.63 (d, 4H, Pt−ArHα, J = 6 Hz),
7.50−7.53 (d, 2H, ArH, J = 9 Hz), 6.69−6.75 (t, 1H, ArH, J = 18 Hz),
5.83 (bs, 2H, ArNH2), 1.28−1.39 (m, 24H, PCH2), 1.06−1.19 (m,
36H, PCH3). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 11.98 (bs; 195Pt
satellites, JPt−P = 2597 Hz). ESI-MS (C58H81F6N5O10P4Pt2S2) m/z: [M
− OTf]+, 1550.36; [M − 2OTf]2+, 700.69. Anal. Calcd. for
C58H81F6N5O10P4Pt2S2: C, 40.97; H, 4.80; N, 4.12. Found: C, 41.14;
H, 4.92; N, 4.03.
Self-Assembly (6). In separate 2 dram vials, 1.0 mg (3.4 μmol) of
2,6-bis(pyrid-3-ylethynyl) aniline (2) and 3.4 mg (2.7 μmol) of μ-1,4-
phenylenetetrakis(triethylphosphine)bis(1,1,1-trifluoromethanesulfo-
nato-κO)diplatinum were added. Each compound was then dissolved
in 0.5 mL of deuterated methylene chloride. The μ-1,4-
phenylenetetrakis(triethylphosphine)bis(1,1,1-trifluoromethanesulfo-
nato-κO)diplatinum solution was then added dropwise to a stirring
solution of 2,6-bis(pyrid-3-ylethynyl) aniline, and the mixture was
heated to 30 °C and allowed to stir for 48 h. After this step, the
chartreuse-colored solution was filtered to remove any insoluble
kinetic byproducts (i.e., polymers). The compound was then purified
by precipitation upon addition of diethyl ether, centrifugation, and
Palladium Benzene OTf-Capped Ligand (15). 1H NMR
(CDCl3, 300 MHz): δ 8.52−8.54 (d, 4H, PyHα, J = 6 Hz), 7.85−
7.87 (d, 4H, PyHβ, J = 6 Hz), 7.48−7.50 (d, 2H, ArH, J = 6 Hz), 7.30−
7.32 (d, 4H, Pd−ArHβ, J = 6 Hz), 7.11−7.15 (t, 4H, Pd−ArHα, J = 12
Hz), 6.99−7.02 (t, 2H, Pd−ArHγ, J = 9 Hz), 6.69−6.73 (t, 1H, ArH, J
= 12 Hz), 5.78 (bs, 2H, ArNH2), 1.28−1.39 (m, 24H, PCH2), 1.06−
1.19 (m, 36H, PCH3). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 6.78.
ESI-MS (C58H83F6N3O6P4Pd2S2) m/z: [M − 2OTf]2+, 567.68. Anal.
Calcd. for C58H83F6N3O6P4Pd2S2: C, 48.61; H, 5.84; N, 2.93. Found:
C, 48.50; H, 5.92; N, 2.95.
Steady-State Absorption and Emission Spectroscopy and
Quantum Yield Determination. Absorption and fluorescence
spectra were recorded on Hitachi U-4100 and Hitachi F-7000
spectrophotometers, respectively, with aerated spectrophotometric-
grade methylene chloride (Sigma-Aldrich) at room temperature. The
cells used in the experiments were 1-cm quartz cuvettes from Starna
Cells, Inc. All samples were freshly prepared for each measurement.
The molar absorption coefficients were determined by preparing four
samples ranging in absorption from 0.01 to 1.0 with concentrations of
0.3−95 μM. The molar absorption coeffcient for each solution was
then calculated using Beer’s law, and the four values were averaged.
Subsequent samples were then prepared to confirm the molar
absorption coefficients. Quantum yields were determined by, first,
cross-calibrating the instrument with quinine sulfate in 0.1 M H2SO4
and anthracene in ethanol. Quinine sulfate was then used to determine
the experimental quantum yields at an excitation wavelength of 365
nm with Φ = 0.55. The quantum yield measurements were performed
in multiplicates with values that were within 10% error being averaged.
1
decantation of the supernatant (>85% yield). H NMR (CDCl3, 300
MHz): δ 9.10 (s, 4H, PyHα), 8.58−8.60 (d, 4H, PyHα′, J = 6 Hz),
8.07−8.09 (d, 4H, PyHγ, J = 6 Hz), 7.63−7.68 (m, 4H, PyHβ), 7.47−
7.50 (d, 4H, ArH, J = 9 Hz), 7.21 (s, 4H, PtArH), 7.01 (s, 4H, PtArH),
6.68−6.73 (t, 2H, ArH, J = 15 Hz), 5.99 (bs, 4H, ArNH2), 1.38 (m,
48H, PCH2), 1.12−1.17 (m, 72H, PCH3). 31P{1H} NMR (CD2Cl2,
1
121.4 MHz): δ 11.29 (bs; 195Pt satellites, JPt−P = 2730 Hz). ESI-MS
(C104H154F12N6O12P8Pt4S4) m/z: [M − 2OTf]2+, 1382.39; [M −
3OTf]3+, 871.94. Anal. Calcd. for C104H154F12N6O12P8Pt4S4 [com-
plex]: C, 40.76; H, 5.06; N, 2.74. Found: C, 41.09; H, 5.27; N, 3.07.
Diphenyl{μ-[4,4′-(1-amino-2,6-ethynediylbenzene)]bis-
(pyridine-κN)}tetrakis(triethylphosphine)diplatinum (7). 2,6-
Bis(pyrid-4-ylethynyl) aniline (1; 1.0 mg, 3.4 μmol) was weighed
into a 2 dram vial. Two equivalents of each bromophenylbis-
(triethylphosphine)platinum (4.0 mg, 6.8 μmol) and AgNO3 (1.2
mg, 6.8 μmol) was also added to the same vial. CD2Cl2 (2 mL) was
then added, and the mixture was stirred at room temperature for 48 h
in the dark. The solution was then filtered twice with glass microfiber
filters to remove insoluble AgBr; a green-colored solution of 7 was
1
afforded (96% yield). H NMR (CDCl3, 300 MHz): δ 8.58−8.60 (d,
4H, PyHα, J = 6 Hz), 7.90−7.92 (d, 4H, PyHβ, J = 6 Hz), 7.48−7.51
J
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Excited-State Lifetime Measurement. Time-correlated single-
photon-counting (TCSPC) experiments were performed on an IBH
(Jobin Yvon Horiba) model 5000F instrument equipped with single
monochromators on both the excitation and emission sides of the
instrument. The excitation light source was a NanoLED with a short
800 ps pulse width at 336 nm (458 nm was also used for 13−15).
Emission signals were collected on a picosecond photon detection
module (TBX-04) at an angle perpendicular to excitation for samples
and blanks. Data were collected at the sample’s peak maxima as
determined by steady-state experiments and averaged (30000 counts)
to obtain the decay profile. Decay analysis and curve fitting routines to
determine the sample’s lifetimes were performed by the software
(DAS6) provided by the manufacturer (IBH). The instrument
response for the setup used was determined to be 1 ns, and the
error in the measurements was determined to be 350 ps after
deconvolution of the signal. The biexponential function that was
D.A.L. was sponsored by the Division of Chemical Sciences,
Geosciences and Biosciences, Office of Basic Energy Sciences,
U.S. Department of Energy.
REFERENCES
■
(1) (a) Caulder, D. L.; Bruckner, C.; Powers, R. E.; Konig, S.; Parac,
̈
̈
T. N.; Leary, J. A.; Raymond, K. N. J. Am. Chem. Soc. 2001, 123, 8923.
(b) Caulder, D. L.; Raymond, K. N. Acc. Chem. Res. 1999, 32, 975.
(c) Gianneschi, N. C.; Masar, M. S.; Mirkin, C. A. Acc. Chem. Res.
2005, 38, 825. (d) Farha, O. K.; Hupp, J. T. Acc. Chem. Res. 2010, 43,
1166. (e) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S.
T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38. (f) Fujita, M. Chem. Soc. Rev.
1998, 27, 417. (g) Fujita, M.; Tominaga, M.; Hori, A.; Therrien, B.
Acc. Chem. Res. 2005, 38, 369. (h) Cotton, F. A.; Lin, C.; Murillo, C. A.
Acc. Chem. Res. 2001, 34, 759. (i) Cotton, F. A.; Lin, C.; Murillo, C. A.
Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4810. (j) Chakrabarty, R.;
Mukherjee, P. S.; Stang, P. J. Chem. Rev. 2011, 111, 6810. (k) Ghosh,
S.; Mukherjee, P. S. Organometallics 2008, 27, 316. (l) Vajpayee, V.;
Kim, H.; Mishra, A.; Mukherjee, P. S.; Stang, P. J.; Lee, M. H.; Kim, H.
K.; Chi, K.-W. Dalton Trans. 2011, 40, 3112. (m) De, S.; Mahata, K.;
Schmittel, M. Chem. Soc. Rev. 2010, 39. (n) Safont-Sempere, M. M.;
utilized to calculate the excited-state lifetimes was A(t) = A1e−k t
+
1
1
A2e−k t
.
2
2
Molecular Modeling. All calculations were performed using the
Gaussian 09 program package, revision B.01,19 with the Becke three-
parameter hybrid exchange and the Lee−Yang−Parr correlation
functionals (B3LYP).16 The 6-31G** basis set17 was used for H, C,
N, and P atoms, whereas the Los Alamos National Laboratories
(LANL2DZ)18 basis set and pseudopotential was used for Pt and Pd.
All geometry optimizations were performed without a solvent field
with C1 symmetry; the results are for the gas phase. To minimize
computational cost, the PEt3 ligands on Pt and Pd were modeled as
P(CH3)3 ligands. Orbitals were visualized using Chem3D and
GaussView 5.0 with an isovalue of 0.02.
Fernandez, G.; Wurthner, F. Chem. Rev. 2011, 111, 5784.
́
̈
(2) (a) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502.
(b) Barry, N. P. E.; Edafe, F.; Dyson, P. J.; Therrien, B. Dalton Trans.
2010, 39, 2816. (c) Barry, N. P. E.; Edafe, F.; Therrien, B. Dalton
Trans. 2011, 40, 7172. (d) Barry, N. P. E.; Therrien, B. Eur. J. Inorg.
Chem. 2009, 2009, 4695. (e) Barry, N. P. E.; Zava, O.; Furrer, J.;
Dyson, P. J.; Therrien, B. Dalton Trans. 2010, 39, 5272. (f) Cook, T.
R.; Zheng, Y.-R.; Stang, P. J. Chem. Rev. 2012, 113, 734. (g) Leininger,
S.; Olenyuk, B.; Stang, P. J. Chem. Rev. 2000, 100, 853.
The percentages of platinum, palladium, phosphine, platinum-aryl,
and ligand character in the occupied (canonical) molecular orbitals
(MOs) and virtual orbitals discussed for the previous complexes were
calculated from a full population analysis, using the equation
(3) (a) Flynn, D. C.; Ramakrishna, G.; Yang, H.-B.; Northrop, B. H.;
Stang, P. J.; Goodson, T. J. Am. Chem. Soc. 2010, 132, 1348. (b) Zhao,
G.-J.; Yu, F.; Zhang, M.-X.; Northrop, B. H.; Yang, H.; Han, K.-L.;
Stang, P. J. J. Phys. Chem. A 2011, 115, 6390. (c) Zhao, G.-J.;
Northrop, B. H.; Han, K.-L.; Stang, P. J. J. Phys. Chem. A 2010, 114,
9007. (d) Zhao, G.-J.; Northrop, B. H.; Stang, P. J.; Han, K.-L. J. Phys.
Chem. A 2010, 114, 3418. (e) Chen, J.-S.; Zhao, G.-J.; Cook, T. R.;
Sun, X.-F.; Yang, S.-Q.; Zhang, M.-X.; Han, K.-L.; Stang, P. J. J. Phys.
Chem. A 2012, 116, 9911. (f) Chen, J.-S.; Zhao, G.-J.; Cook, T. R.;
Han, K.-L.; Stang, P. J. J. Am. Chem. Soc. 2013, 135, 6694.
orbital character(Pt or Pd,Pt−Ar,phosphine,lig) (%)
2
∑ φ
(Pt or Pd,Pt−Ar,phosphine,lig)
=
× 100%
2
∑ φ
(all)
where ∑φi (i = Pd, Pt, Pt−Ar, phosphine, lig, or all) is the sum of the
squares of the eigenvalues associated with the atomic orbital (AO) of
interest or all of the AOs in a particular MO. The vertical singlet
transition energies of the complexes were computed by time-
dependent density functional theory using Gaussian 09 with the
ground-state optimized structure.
(4) (a) Wong, K. M.-C.; Yam, V. W.-W. Coord. Chem. Rev. 2007, 251,
2477. (b) Eryazici, I.; Moorefield, C. N.; Newkome, G. R. Chem. Rev.
2008, 108, 1834. (c) Fleischauer, P. D.; Fleischauer, P. Chem. Rev.
1970, 70, 199. (d) Kindahl, T.; Ellingsen, P. G.; Lopes, C.; Brannlund,
̈
C.; Lindgren, M.; Eliasson, B. J. Phys. Chem. A 2012, 116, 11519.
(e) Chan, K. H.-Y.; Chow, H.-S.; Wong, K. M.-C.; Yeung, M. C.-L.;
Yam, V. W.-W. Chem. Sci. 2010, 1, 477. (f) Hui, C.-K.; Chu, B. W.-K.;
Zhu, N.; Yam, V. W.-W. Inorg. Chem. 2002, 41, 6178. (g) Wong, K.
M.-C.; Yam, V. W.-W. Acc. Chem. Res. 2011, 44, 424. (h) Adamson, A.
W.; Waltz, W. L.; Zinato, E.; Watts, D. W.; Fleischauer, P. D.;
Lindholm, R. D. Chem. Rev. 1968, 68, 541. (i) Brooks, J.; Babayan, Y.;
Lamansky, S.; Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E.
Inorg. Chem. 2002, 41, 3055. (j) Caspar, J. V. J. Am. Chem. Soc. 1985,
107, 6718. (k) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.;
Cheung, K.-K.; Zhu, N. Chem.Eur. J. 2001, 7, 4180. (l) Pomest-
chenko, I. E.; Luman, C. R.; Hissler, M.; Ziessel, R.; Castellano, F. N.
Inorg. Chem. 2003, 42, 1394. (m) Danilov, E. O.; Rachford, A. A.;
Goeb, S. b.; Castellano, F. N. J. Phys. Chem. A 2009, 113, 5763.
(n) Goeb, S.; Prusakova, V.; Wang, X.; Vezinat, A.; Salle, M.;
ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 31P{1H} NMR spectra, steady-state absorption and
emission spectra of 2, excited-state lifetime decay profiles,
electrospray ionization mass spectrometry spectra, atomic
coordinates from DFT, and all electronic transitions from
TD-DFT with nonzero oscillator strengths. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: bryant@chem.utah.edu (J.B.P.), Stang@chem.utah.
edu (P.J.S.).
■
́
Castellano, F. N. Chem. Commun. 2011, 47, 4397. (o) Fornies, J.;
Notes
Fuertes, S.; Martín, A.; Sicilia, V.; Lalinde, E.; Moreno, M. T. Chem.
Eur. J. 2006, 12, 8253. (p) Keller, J. M.; Glusac, K. D.; Danilov, E. O.;
McIlroy, S.; Sreearuothai, P.; R. Cook, A.; Jiang, H.; Miller, J. R.;
Schanze, K. S. J. Am. Chem. Soc. 2011, 133, 11289. (q) Keller, J. M.;
Schanze, K. S. Organometallics 2009, 28, 4210. (r) Liao, C.; Yarnell, J.
E.; Glusac, K. D.; Schanze, K. S. J. Phys. Chem. B 2010, 114, 14763.
(s) Rogers, J. E.; Slagle, J. E.; Krein, D. M.; Burke, A. R.; Hall, B. C.;
Fratini, A.; McLean, D. G.; Fleitz, P. A.; Cooper, T. M.; Drobizhev, M.;
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.J.S. thanks the NSF (NSF-CHE 0820955) for financial
support. We also thank the Center for High Performance
Computing (CHPC) at the University of Utah for access and
time allotment to perform the DFT and TD-DFT calculations.
K
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Makarov, N. S.; Rebane, A.; Kim, K.-Y.; Farley, R.; Schanze, K. S. Inorg.
Chem. 2007, 46, 6483.
(5) Kryschenko, Y. K.; Seidel, S. R.; Arif, A. M.; Stang, P. J. J. Am.
Chem. Soc. 2003, 125, 5193.
(6) Pollock, J. B.; Cook, T. R.; Stang, P. J. J. Am. Chem. Soc. 2012,
134, 10607.
(7) Johnson, A. M.; Moshe, O.; Gamboa, A. S.; Langloss, B. W.;
Limtiaco, J. F. K.; Larive, C. K.; Hooley, R. J. Inorg. Chem. 2011, 50,
9430.
(8) Manna, J.; Kuehl, C. J.; Whiteford, J. A.; Stang, P. J.; Muddiman,
D. C.; Hofstadler, S. A.; Smith, R. D. J. Am. Chem. Soc. 1997, 119,
11611.
(9) Arnold, D. P.; Bennett, M. A. Inorg. Chem. 1984, 23, 2117.
(10) Tam, W.; Calabrese, J. C. Chem. Phys. Let. 1988, 144, 79.
(11) Calvin, G.; Coates, G. E. J. Chem. Soc. 1960, 0, 2008.
(12) (a) Yang, J.-S. In PATAI’S Chemistry of Functional Groups; John
Wiley & Sons: New York, 2009, 10.1002/9780470682531.pat0396.
́
(b) Hou, X.-J.; Quan, P.; Holtzl, T.; Veszpremi, T.; Nguyen, M. T. J.
̈
Phys. Chem. A 2005, 109, 10396. (c) Lewis, F. D.; Hougland, J. L.;
Markarian, S. A. J. Phys. Chem. A 2000, 104, 3261. (d) Oosterbaan, W.
D.; Koeberg, M.; Piris, J.; Havenith, R. W. A.; van Walree, C. A.;
Wegewijs, B. R.; Jenneskens, L. W.; Verhoeven, J. W. J. Phys. Chem. A
2001, 105, 5984. (e) Ruckert, I.; Demeter, A.; Morawski, O.; Kuhnle,
̈
̈
W.; Tauer, E.; Zachariasse, K. A. J. Phys. Chem. A 1999, 103, 1958.
(13) (a) Kohler, A.; Wilson, J. S.; Friend, R. H.; Al-Suti, M. K.; Khan,
M. S.; Gerhard, A.; Bassler, H. J. Chem. Phys. 2002, 116, 9457.
(b) Ricciardi, L.; Puoci, F.; Cirillo, G.; La Deda, M. Dalton Trans.
2012, 41, 10923.
(14) Weil, T. A.; Schmidt, P. J.; Rycheck, M.; Orchin, M. Inorg. Chem.
1969, 8, 1002.
(15) Vrieze, K.; Volger, H. C.; Gronert, M.; Praat, A. P. J. Organomet.
Chem. 1969, 16, P19.
(16) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(17) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257.
(18) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
̈
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
revision C.01; Gaussian, Inc.: Wallingford, CT, 2010.
(20) Rieke, R. D.; Kavaliunas, A. V. J. Org. Chem. 1979, 44, 3069.
L
dx.doi.org/10.1021/ic400491q | Inorg. Chem. XXXX, XXX, XXX−XXX