Anthracene-triphenylamine-based platinum(II) metallacages as synthetic light-harvesting assembly

Article abstract of DOI:10.1021/jacs.0c12853

Two trigonal prismatic metallacages 1 and 2 bearing triphenylamine and anthracene moieties are designed and synthesized to fabricate artificial light-harvesting systems (LHSs). These two cages are prepared via the coordination-driven self-assembly of two

Full text of DOI:10.1021/jacs.0c12853

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Anthracene−Triphenylamine-Based Platinum(II) Metallacages as
Synthetic Light-Harvesting Assembly
Yanrong Li,* Sreehari Surendran Rajasree, Ga Young Lee, Jierui Yu, Jian-Hong Tang, Ruidong Ni,
Guigen Li, Kendall. N. Houk, Pravas Deria,* and Peter J. Stang*
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ABSTRACT: Two trigonal prismatic metallacages 1 and 2 bearing
triphenylamine and anthracene moieties are designed and synthesized
to fabricate artificial light-harvesting systems (LHSs). These two cages
are prepared via the coordination-driven self-assembly of two
anthracene−triphenylamine-based tripyridyl ligand 3, three dicarbox-
ylates, and six 90° Pt(II) acceptors. The design of the anthracene−
triphenylamine chromophore makes possible the tunable excited-state
property (like the emissive transition energy and lifetime) as a
function of the solvent polarity, temperature, and concentration. The
synergistic photophysical footprint of these metallacages, defined by
their high absorptivity and emission quantum yield (QY) relative to
the free ligand 3, signifies them as a superior light sensitizer
component in an LHS. In the presence of the fluorescent dye Nile
Red (NR) as an energy acceptor, the metallacages display efficient
(>93%) excited energy transfer to NR through an apparent static quenching mechanism in viscous dimethyl sulfoxide solvent.
agents, and so on.20−24 While tetraphenylethylene (TPE)-
INTRODUCTION
■
based tetragonal prismatic Pt(II) metallacages have been
Light harvesting is a key process for photonic energy
developed as highly emissive SCCs25−29 that can be exploited
conservation. In the natural photosynthetic apparatus, the
as a light-harvesting system,30 we wanted to integrate
antenna pigments are precisely assembled within the ring of
photophysical tunability in these metallacages. For this, we
light-harvesting complexes (LHCs).1 Such arrangement of
adopted ligand design based on a triphenylamine (TPA) as an
chlorophylls (and carotenoids) provides highly synergistic
electron-rich, propeller-shaped C3 symmetric central compo-
excited-state properties like strong photon absorptivity and
nentwith the expectation that the resulting pyridine-
modular emission energy, where various energy-wasting
appended ligand and the corresponding cage-assemblies
excited-state decay processes are systematically suppressed.
could provide external stimuli-responsive photophysics. While
These features enable excited energy transfer to neighboring
TPA-based ligand has been used in the construction of
LHC and eventually to the reaction center (RC) with
hexagonal Pt(II) metallacycles,31 we targeted trigonal prismatic
remarkable efficiency. Inspired by this extraordinary structure
Pt(II) cages given that such organic fluorophores are rare and
and functionality, many artificial light-harvesting systems
can potentially open up further exploration of new cages with
(LHSs) were developed,2−6 among which entirely synthetic
tunable size and photophysical properties for LHSs. With
supramolecular pigment assembly signifies a novel ap-
impressive photophysical properties, we integrated the blue-
proach.7−10 Such techniques can provide easier access to (a)
emitting anthracene component32,33 within our pyridine-
modular interchromophoric positioning to control the photo-
appended TPA-based macromolecular ligand design for
physical properties like high photon absorptivity and sup-
stimuli-responsive fluorescent 3D metallacages.34,35 Herein,
pressed unproductive excited-state recombination and (b)
we describe the preparation of two trigonal prismatic
scalable synthesis for applications.
Coordination-chemistry-driven self-assembly offers a
straightforward synthetic approach for the construction of
diverse supramolecular coordination complexes (SCCs),
Received: December 11, 2020
Published: February 12, 2021
ranging from two-dimensional (2D) metallacycles to three-
dimensional (3D) metallacages.11−19 With precisely controlled
inner cavities, various 3D metallacages have proven to be
useful in guest encapsulation, catalysis, sensors, bioimaging
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Scheme 1. Formation of Metallacages Bearing Triphenylamine and Anthracene Moieties via Coordination-Driven Self-
Assembly
Figure 1. 31P {1H} NMR spectra of cage 1 (a), ligand 6 (b), and cage 2 (c) (121 MHz, acetone-d6, 298 K). Partial 1H NMR spectra of cage 1 (d),
ligand 3 (e), and cage 2 (f) (500 MHz, acetone-d6, 298 K).
platinum(II) cages with anthracene and triphenylamine
moieties. The cages display synergistic, yet tunable photo-
physical properties as a function of solvent polarity, temper-
ature, and concentrations. Notably, these supramolecular
constructs display superior light-harvesting behavior with a
strong light absorptivity and emission quantum yield (QY)
which can efficiently transfer the excited-energy to a suitable
acceptor like Nile Red (NR) in dimethyl sulfoxide solution.
added to the dried residues, followed by the addition of diethyl
ether. The resulting precipitates were collected by centrifuga-
tion, and the desired metallacages were obtained in over 90%
yields. These metallacages exhibit better solubility in acetone
compared to the pristine ligands. The structures of these cages
are established by multinuclear NMR (1H NMR and 31P {1H}
NMR), correlation spectroscopy (COSY) NMR, and electro-
spray ionization time-of-flight mass spectroscopy (ESI-TOF-
MS). While the 31P {1H} NMR spectrum of 90° Pt(II) (6)
195
RESULTS AND DISCUSSION
exhibits a sharp singlet with concomitant
Pt satellites
■
(Figure 1b), the 31P {1H} NMR spectra of cage 1 and cage 2
(Figures 1a and 1c) exhibit two doublets of approximately
equal intensity with concomitant 195Pt satellites at 8.02 and
2.46 ppm for 1 and 7.92 and 2.70 ppm for 2. They are
corresponding to two distinct phosphorus environments,
indicating the heteroligation of the Pt(II) center on 1 or 2
Cage Preparation and Characterization. The organic
ligand 3 was synthesized by a Pd-catalyzed Suzuki−Miyaura
cross-coupling reaction between 4-(10-bromoanthracen-9-yl)-
pyridine and (nitrilotris(benzene-4,1-diyl))triboronic acid
(Schemes S1−S4 in the Supporting Information).36 Metal-
lacages 1 and 2 were prepared via a three-component
coordination-driven self-assembly strategy (Scheme 1).37,38
Ligand 3, one of the dicarboxylates (4 or 5), and 90° Pt(II)
(6) were mixed in a molar ratio of 2:3:6 in a mixture of
acetone and water (4/1, v/v). The mixture was stirred for 12 h
at 35 °C, and then the solvents were removed. Acetone was
1
with one pyridyl and one carboxylate moiety. In the H NMR
spectra of 1, Hα shifted downfield from 8.90 to 9.28 ppm and
Hβ shifted downfield from 7.54 to 7.84 ppm compared to the
free pyridine of ligand 3, indicating the loss of electron density
of the pyridyl unit upon coordination (Figures 1d and 1e).
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Figure 2. Calculated (red) and experimental (blue) ESI-TOF-MS spectra of metallacage 1 [M − 4OTf−]4+ (a, b) and metallacage 2 [M −
4OTf−]4+ (c, d).
Figure 3. Computational modeling and dimensions of (a) cage 1 and (b) cage 2. Structures were optimized with the PM6 semiempirical method.
Similar downfield shifts are also observed for the pyridyl
protons Hα (from 8.90 to 9.26 ppm) and Hβ (from 7.54 to
7.76 ppm) of 2 (Figures 1e and 1f). The assignments of these
protons are further supported by 1H−1H COSY NMR analysis
(Figures S6, S8, and S13). In Figures S11 and S16, DOSY
NMR experiments of 1 and 2 display a single set of signals,
indicating the formation of a single product. The measured
diffusion coefficients for 1 and 2 are 5.61 × 10−10 and 4.34 ×
10−10 m2/s, respectively. Electrospray ionization time-of-flight
mass spectra (ESI-TOF-MS) show isotopically resolved peaks
at m/z = 1347.20 and 1404.16, which correspond to the [M −
4OTf−]4+ species of metallacages 1 and 2, respectively, and
agree well with their calculated theoretical distributions, further
supporting the stoichiometric formation of these discrete
metallacages (Figure 2).39−41 Additionally, the ESI-TOF-MS
results of [M − 3OTf−]3+ signals of the two metallacages also
match well with the calculated values (Figures S10 and S15).
The structures and dimensions of metallacages 1 and 2 were
simulated by using PM6 computations. Both cages show a
triangular prism 3D structure with two planar triphenylamine
faces, three dicarboxylate pillars, and six ∼90° Pt vertices. The
height (N−N distance) and the width (Ph−Ph centroid
distance) of cage 1 are 12.8 and 25.0 Å, respectively (Figure
3a). For cage 2, the height and the width are 16.7 and 25.1 Å,
respectively (Figure 3b). This shows that the cavity heights can
be tuned depending on the length of the dicarboxylate ligands.
In fact, the anthracene moieties are slanted and hinder the
cavities’ opening, making the accessible width of these cavities
smaller and more preorganized.
Photophysical Studies. Excited-State Properties. The
photophysical properties of two metallacages 1 and 2, as well
as ligand 3, were investigated by UV−vis absorption and
fluorescence spectroscopy (Figure 4). The UV/vis absorption
spectra of ligand 3, cage 1, and cage 2 collected in acetone
solvent show characteristic π−π*-dominated transitions
peaking at 355, 378, and 395 nm (Figure 4a).42 Nevertheless,
the metallacages manifested higher absorptivity than the ligand
3 (×2) without any significant spectral shift. (A nominal red-
shift in the order of a few nanometers can be discerned for 1
and 2 relative to 3.) These data suggest that such C3 symmetric
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Figure 4. UV/vis absorption spectra of 3, 1 and 2 (a) in acetone and the normalized fluorescence spectra of ligand 3 (b, c = 2.0 × 10−6 M),
metallacage 1 (c, c = 2.0 × 10−6 M), and metallacage 2 (d, c = 2.0 × 10−6 M) in different solvents (λex = 395 nm). Photographs of metallacages 1
(e) and 2 (f) in different solvents on excitation at 365 nm with an ultraviolet lamp at 298 K (c = 2.0 × 10 −6 M). Tol, toluene; EA, ethyl acetate;
THF, tetrahydrofuran; Ace, acetone; DMSO, dimethyl sulfoxide.
chromophores, arranged in a trigonal prismatic cage (fixed at
12−16 Å apart), may not produce a strongly coupled system in
the absorptive excited state. Ligand 3 shows essentially
unchanged absorptive spectral envelope in common solvents
of varying polarities, such as toluene, chloroform, ethyl acetate,
tetrahydrofuran, and dimethyl sulfoxide (Figure S18),
suggesting that the excited state accessed through the vertical
electronic excitation is similar to the ground state, featuring a
nonpolar aromatic structure. A slight band broadening
(particularly at the low-energy side of the lowest energy
transition at 395 nm) can be observed for cages 1 and 2,
especially in less polar solvents (Figures S19 and S20). This
solvent-dependent change in the absorption spectral envelope,
at the red side of the lowest energy transition, may not be
entirely stemming from common aggregation-induced process
known for such macromolecular compounds.43 Instead, this
could be stemming from weak low-energy transitions involving
the ligand and 90° Pt(II) node in the cage assembly.44
Interestingly, the emission spectral envelop for ligand 3 show a
substantial solvatochromism: with an increase in solvent
polarity, the emissive spectral peak gradually shifts to the
lower energy (by ∼3600 cm−1 going from toluene to DMSO
solvents). As shown in Figure 4b, ligand 3 displays an emission
maximum (λem,max) at 454 nm in toluene, which gradually shifts
to 543 nm in dimethyl sulfoxide. Such a wide bathochromic
shift can be displayed by a change in the emission colors,
varying from dark blue to yellow, under a 365 nm ultraviolet
lamp (Figure S21). This solvatochromic behavior is mostly
maintained in cages 1 and 2 (Figures 4c and 4d), where the
degree of the bathochromic shift is smaller than that of 3 with
only a blue to yellow color change (Figures 4e and 4f). This is
because the emission spectra of the cages are already red-
shifted (compared to ligand 3) to start with in less polar
solvent, possibly due to a bit restricted anthracene rotation.
Nature of the Excited States. On the basis of these solvent-
dependent absorption and emission spectroscopic data, we
believe that the excited state accessed via a direct electronic
excitation features an aromatic conformation consisting of a
nonplanar amine moiety and anthracene units with a wide
range of rotational populations thermally accessible at room
temperature. However, this nonpolar aromatic excited state is
coupled with a polar quinoidal state, which is facilitated by the
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Figure 5. Potential energy surface of an aromatic and quinoidal excited state of 3.
Figure 6. Transient emissive decay profiles for 3, 1, and 2 in DMSO (λprobe = 540 nm) and CHCl3 (λprobe = 470, 485, and 510 nm, respectively)
solvents. Experimental conditions: c = 2.0 × 10−6 M, λex = 403 nm, RT.
9,10-connected anthracene moiety resulting in two complete
benzene leading to a planar conformation (Figure 5). The
energy of this polar quinoidal state depends on solvent
polarity. These two excited states are strongly coupled with
efficient intersystem crossing (k ≳ 1011 s−1; as no rise event
was seen in transient emission profile, Figure 6). This
assignment can be supported by thermally arresting the
rotation of the anthracene moiety in nonpolar solvent
encouraging it to adopt a planar conformer: a systematic
red-shift in the emissive peak was observed upon lowering the
temperature of 3 (as well as 1 and 2) in nonpolar chloroform
solvent (Figure S28).
Table 1 summarizes emission QY and lifetime data. Relative
to the ligand 3, cages 1 and 2 show comparable emission QY,
particularly in nonpolar CHCl3 solvent. Interestingly, emission
from the polar quinoidal state, stabilized in high dielectric
DMSO solvent, did not lead to any significant loss in QY as
can be expected from the bandgap law. This planar state is
structurally different (Figure 5) than the initially prepared
aromatic excited state; thus, the radiative decay from this polar
excited state in DMSO is slow (τ = 16 ns) compared to that
observed from an aromatic excited state in nonpolar chloro-
form solvent (3 ns). Overall, with improved (or comparable)
absorptivity and emission QY, along with a prolonged excited-
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state lifetime, these cage compounds signify good photo-
sensitizers.
Table 1. Emission QY and Lifetime for the 3, 1, and 2 in
Two Different Solvents
Ligand 3, cage 1, and cage 2 also display a concentration-
dependent emissive behavior where the transition energy and
QY change as a function of concentration. Emission spectra
collected in acetone solvent at different concentrations (7.8 ×
10−7−4.8 × 10−5 M) are summarized in Figures S22, S24, and
S26. For example, after reaching 2.4 × 10−5 M, 3 displays a
decrease in QY with a further increase in concentration. This is
due to the increased nonradiative decay of the excited states
through aromatic−aromatic interactions at high concentra-
tions.45 A similar phenomenon is also observed in the
emissions of 1 and 2. However, the emission bands of 1 and
comp (solvent)
QY/%
τ/ns (amplitude)
a
a
a
3 (DMSO)
1 (DMSO)
2 (DMSO)
3 (CHCl3)
1 (CHCl3)
2 (CHCl3)
71
60
65
15.8 (100)
16.2 (100)
16.0 (100)
3.3 (100)
3.3 (86), 6.4 (14)
2.4 (55), 5.4 (45)
b
b
b
79
72
68
b
67.8
a
b
3 μM. 0.3 μM.
Figure 7. Excitation−emission mapping spectra highlighting EnT process in DMSO solvent involving 1 (3 μM) at two different NR
concentrations: (a) 3 μM NR and (b) 45 μM NR. Mapping spectra for pristine (c, d) cage 1 of 3 μM and (e, f) NR of 3 and 45 μM solutions in
DMSO solvent are provided for comparison (the vertical dashed lines are to guide the NR emission wavelength). The red arrows in panels a and b
highlight enhanced emission intensity of NR at 400 nm due to 1* → NR EnT compared to its pristine state as shown by the purple arrows in
panels e and f.
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2 are red-shifted (Δv ≈ 1600 cm−1) after they reach a
concentration of 3.0 × 10−6 and 6.0 × 10−6 M, respectively.
This shift is not observed in 3, suggesting that metallacages are
quicker to aggregate than the ligand 3.
Excited-State Energy Transfer. With several advantageous
photophysical properties, we wanted to test whether the cage
compound can transfer its excited state energy to an acceptor
and, if so, how the EnT matrices will fare in compounds 1 and
̈
2. In singlet manifold, the Forster resonance energy transfer
(FRET) rate constant (kFRET = 2πJ2Θ/ℏ) depends on the
overlap integral Θ (eV−1) between the area-normalized donor
fluorescence and acceptor absorption bands and donor−
acceptor interchromophoric electronic coupling J (eV−1).46−50
The requirement for high Θ stems from Fermi’s golden rule to
conserve energy and may not involve absorption of an emitted
photon by the acceptor, and J can be modulated by the
donor−acceptor distance and orientation (i.e., the relative
position of the chromophores).46−50 With these criteria in
mind, we chose Nile Red (NR) with maximum spectral overlap
(the transition maximum: λabs(NR) ≈ λem(cage 1 or 2) ∼ 554 nm) as
a candidate acceptor to study EnT in DMSO solvent.51
Excitation−emission mapping spectra of 3 μM cage 1 in the
presence of 3 and 45 μM NR are shown in Figure 7a,b (see
Figure S29 for the corresponding experiment with cage 2). The
data highlight, for the equal mixture, an increase in NR
emission at 400 nm excitation where it originally does not emit
well (Figure 7e). It is also clear that a significant amount of
cage 1 emission exists for this equal mixture. The NR emission
at 400 nm excitation can be more clearly observed in the
excitation−emission mapping spectra for the 1:15 (i.e., in the
presence of large excess of NR) mixture (Figure 7b; shown by
the red arrows) as emission from cage 1 is nearly quenched.
Excitation−emission mapping spectra collected over a wider
excitation wavelength range (Figure S30) suggests that the
components (cage and NR) can also be independently excited
in their mixtures. To ensure that the enhanced NR emission
intensity is stemming from the 1* → NR EnT process, we
analyzed the excitation profiles: Figure 8a shows that the
emission probed at 635 nm (at λmax for NR emission) has a
contribution from the cage 1 excitation at around the 400 nm
region. Given that the tail of the cage 1 emission also extends
to the 635 nm region (Figure 7c,d), it is obvious that emission
probed at 635 nm should have some contribution from cage 1
excitation. To clarify this point, we extracted the emission
profiles contribution of the NR component (using Voigt
deconvolution profile; vide infra); data presented in Figure 8b
highlight a more than 30-fold increase in NR emission
intensity in an equal mixture with the cage 1 compared to
the pristine (3 μM) NR due to the underlying 1* → NR EnT
process at 400 nm excitation (see Figure S31 for the
corresponding experiment with cage 2).
Figure 8. (a) Excitation spectra of 1, NR, and their 1:1 mixture
highlighting the excitation contribution of 1 for the NR emission
probed at 635 nm and (b) deconvoluted emission spectra of NR*
(blue) showing its improved intensity in the presence of the
equivalent amount of cage 1 at λex = 400 nm (for clarity, the NR
emission is extracted by using the Voigt deconvolution of the
experimental data containing 1; see text and Figure S30).
Experimental conditions: [1] = 3 μM; [NR] = 3 μM; DMSO solvent.
Figure 9. Transient emission decay profiles for the pristine 1 (3 μM;
green) and in the presence of 3 μM NR (blue) and 45 μM NR (red)
probed at 540 nm in DMSO solvent (λex = 403 nm).
To gain insight into the 1* → NR EnT mechanism, we
collected transient emission profiles for the cage compounds in
the presence of different concentrations of NR. Figure 9
highlights the transient decay profiles of 1 in pristine and in the
presence of NR (see Figure S32 for cage 2), and the fitting
data suggest the emission decay time for 1* remains
unchanged even with a large excess of NR (relative to its
pristine solution). Given that the presence of NR only
manifests a diminished QY without impacting the emissive
lifetime, we anticipate that the EnT event occurs through a
static quenching. Such a mechanism does not involve collision-
mediated quenching of 1* with the NR but instead through
the formation of a possible nonemissive (ground-state)
complex between the cages and NR.
To glean into the nature of these cage−NR complexations,
we performed a series of experiments. First, emission spectra of
the cages in the presence of a different concentration of NR
were collected in an emissive titration experiment. The
emissive contributions of the cage and NR were deconvoluted
through a Voigt profile analysis and used for a Stern−Volmer
type analyses (Figure 10; see Figure S33):
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Figure 10. (a) Emissive titration for 3 μM 1 with varying amounts of NR (0−45 μM, λex = 400 nm; see Figure S34 for similar titration of 2) (b)
The corresponding SV plot of 1 and 2 in the presence of different concentration ratios of NR.
I0
I
I0
IeV[Q]
τ0
τ
= 1 + Ks[NR]
= 1 + KD[NR] =
( = 1 in our case and
(1)
maintained for all [NR] studied)
(2)
where I0 and I are the emission intensity of cage 1* in the
absence and presence of NR, and the S−V slope KSV is
represented by Ks, the association constant of the complex-
ation. Because there is no change in the fluorescence time
constant, I0/I ≠ τ0/τ (the τ0/τ = 1 in our experiment). Figure
10 highlights that both the cages manifest an identical Ks value
of ∼0.3 μM−1, meaning ∼3.3 μM NR is required to quench
half of the emission of 1 μM cage. This is consistent with the
1H NMR spectra of the NR:1 (or 2) mixture that display
essentially nonshifted proton peaks compared to the pristine
NR and cages components (Figure S17). Such a weak
association suggests that the cage and NR may not have
formed any complex in that senserather, the NR present
within a static “effective sphere” at the moment of cage
excitation, which led to a quick quenching of the excited cage
before they diffuse apart (Figure 11).52,53
The parameter V contains the volume information about the
active sphere, and KD is the dynamic S−V constant (different
than the Ks discussed in eq 1). Solving for V produces a linear
plot with an intercept of one and zero slopes (i.e., no dynamic
quenching in place; Figure S37). An active sphere with a radius
in the order of tens of nanometers (Note S1 in the Supporting
Information) can be extracted from this data. Although a bit
overestimated, this large active sphere is clearly referring to a
highly diffusion-limited volume in a viscous DMSO solvent (η
= 1.99 cP) where an NR, present adjacent to a cage molecule,
cannot diffuse away before quenching (Figure 11). Thus, we
can fairly sayeven though a dynamic quenching is not
observed in this highly viscous mediumthe quenching is
highly effective, with a rate constant higher than the estimated
diffusion-controlled bimolecular rate of ca. 3.7 × 109 M−1 s−1
(assuming the radius of cage 1 ∼ 14 Å and NR ∼ 7 Å; Note S2
in Supporting Information). Note that this rate constant
accounts for the large size of the cage in highly viscous DMSO
solvent, and thus, it is about one order of magnitude slower
than common diffusion-controlled bimolecular rates for
moderate size fluorophores. What it did not account for is a
more realistic picture of much slower diffusion-controlled
bimolecular kinetics involving a hexa-cationic cage and its large
solvation sphere in a high dielectric solvent; thus, the energy
transfer rate ≳1 × 109 s−1 can be a good expectation (i.e.,
corresponding time scale ∼0.2−1 ns). It is important to note
that the radiative rate of these cage compounds is much slower
(6.25 × 107 s−1), and thus, any NR present adjacent to the
cage (within the active sphere)54 will have ample time to
efficiently quench its excited state. The underlying EnT that
quenches the excited cage also forms NR*, which undergoes
radiative recombination.55 Given that no dynamic quenching is
involved, it is difficult to estimate the EnT efficiency; however,
since the NR species adjacent to the cage molecule near-
quantitatively quenches the excited cage, we can estimate a
>93% EnT efficiency.56 To further probe that improving
thermal diffusion will lower the EnT efficiency, we performed
the titration at 45 °C, which resulted in a Ks of 0.18 μM−1
Figure 11. Schematic diagram showing EnT based on an apparent
static quenching of 1* by NR in viscous DMSO medium.
For such a scenario the absorption spectroscopic profiles of
the cage should not be altered by the NR: the UV−vis spectra
(Figure S35) collected for cage 1 in the presence of 1 and 15
equiv of NR did not display any discernible spectral change
(this is also true for the NR component). Furthermore, with
the increase in NR concentration, the possibility for NR
present within a static effective sphere of a cage molecule will
increase and the corresponding I0/I vs [NR] Stern−Volmer
plot will deviate upward (Figure S36). This apparent static
mechanism can be described with a modified S−V equation:
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compared to ∼0.3 μM−1 at room temperature (Figure S38).
This value indicates that more NR would be required (5.5 μM
compared to ∼3.3 μM at room temperature) to quench half of
the emission of the 1 μM cage. Experiment at low viscous
medium such as CHCl3 (η = 0.56 cP) can be performed;
however, it requires a tedious selection of acceptor
chromophore with high spectral overlap and good solubility
(for example, our attempt with FITC did not go well due to its
poor solubility).
were collected in a quartz capillary tube (ID = 3 mm) in a home-built
Dewar cell in the front-face configuration. The absolute quantum
yields were measured by using a 150 mm integrating sphere, where
respective background spectra were collected by using a cuvette filled
with the corresponding solvent. The QY values were calculated with
EI F980 software that accounts for the diminished intensity (counts)
of the incident beam over the increased intensity (counts) of
fluorescence, based on the manually selected wavelength range.
Recorded spectra were corrected by using the instrumental correction
functions for the excitation light source as well as detector response.
Fluorescence lifetime emission decay profiles were recorded by using
an Edinburgh Lifespec II picosecond time-correlated single photon
counting spectrophotometer equipped with a Hamamatsu H10720-01
detector and a 405 nm picosecond pulsed diode laser as TCSPC
source (IRF ≈ 150 ps). An iterative reconvolution procedure with
exponential fitting used the EI F980 software to extract lifetime data.
Synthesis of Ligand 3. A 100 mL flask was charged with 4-(10-
bromoanthracen-9-yl)pyridine (1.00 g, 3 mmol), (nitrilotris(benzene-
4,1-diyl))triboronic acid (226 mg, 0.6 mmol), Pd(PPh3)4 (69.3 mg,
0.06 mmol), Cs2CO3 (978 mg, 3 mmol), and toluene/methanol (45
mL/15 mL). The reaction mixture was then heated at 80 °C under
nitrogen for 48 h with stirring. After cooling to room temperature, the
solvent was removed under reduced pressure. The resulting mixture
was extracted with ethyl acetate (3 × 50 mL). The combined organic
layers were washed with brine (100 mL), dried over MgSO4, and
evaporated to dryness under vacuum. The residue was purified by
silica gel chromatography (methanol/ethyl acetate, 10/1) to afford
397 mg of 4-(anthracen-9-yl)pyridine (3) as a pale-yellow solid in
58% yield. 1H NMR (500 MHz, CDCl3, 298 K) δ: 8.90 (d, J = 5.2 Hz,
6H), 7.98 (d, J = 8.9 Hz, 6H), 7.70 (d, J = 8.3 Hz, 6H), 7.64 (d, J =
8.7 Hz, 6H), 7.55 (d, J = 8.2 Hz, 6H), 7.49 (d, J = 5.0 Hz, 6H), 7.46
(d, J = 8.5 Hz, 6H), 7.44−7.40 (m, 6H). 13C NMR (126 MHz,
CDCl3, 298 K) δ: 150.03, 147.79, 147.15, 137.93, 133.66, 133.21,
132.43, 129.99, 129.17, 127.27, 126.61, 126.14, 125.78, 125.30,
CONCLUSION
■
In summary, two trigonal prismatic Pt(II) metallacages 1 and 2
were designed and synthesized by using two anthracene−
triphenylamine-based tripyridyl ligands as the faces, three
dicarboxylates as the pillars, and six 90° Pt(II) acceptors as the
linkers. The excited state of the anthracene−triphenylamine-
based ligand possesses a stable planar polar quinoidal
conformation, which is strongly coupled with the initially
generated aromatic excited state. The energy of the polar
quinoidal conformer can be tuned by the solvent polarity. Such
photophysical behaviors are maintained in their respective cage
assemblies which manifest good photon absorptivity (≳2× of
the linker) without significant spectral shift and good emission
QY with unchanged lifetime. The polar excited state, accessed
and stabilized in DMSO solvent, radiatively decays with a
longer lifetime (16 ns) than the initially generated aromatic
excited state which can be stabilized in CHCl3 solvent (3 ns).
Although not studied here, one of two ligands in these cage
assemblies can be expected to attain the planar quinoidal
structure where the other ligand remains in its higher energy
aromatic confirmation and therefore can efficiently tunnel its
excited energy to the planar component: in other words, two
ligands in the cage can technically work in tandem to emit
from the planar component. These features make such cage
compounds to be exploited as a superior photosensitizer. With
appropriate spectral characteristics, the energy transfer with
Nile Red was studied. In a viscous DMSO medium, the
diffusion of both the photosensitizer and acceptor is restricted
which led to an EnT through an apparent static quenching
mechanism, where NR adjacent to the cage (i.e., within an
active sphere) efficiently quenches the excited cage. This study
opens new design criteria for the fabrication of highly efficient
LHSs in the future.
1
124.26. To compare with the metallacages, H NMR in acetone-d6 is
1
also shown. H NMR (500 MHz, acetone-d6, 298 K) δ: 8.90 (d, J =
5.8 Hz, 6H), 7.97 (d, J = 8.0 Hz, 6H), 7.79 (d, J = 8.5 Hz, 6H), 7.63
(d, J = 7.8 Hz, 6H), 7.61 (d, J = 8.3 Hz, 6H), 7.54 (d, J = 5.9 Hz, 6H),
7.50−7.47 (m, 12H). HRMS (ESI, m/z): calcd for C75H49N4 (M +
H)+: 1005.3952; found: 1005.3949.
Self-Assembly of 1. Ligand 3 (2.52 mg, 2.5 μmol), dicarboxylate
ligand 4 (0.79 mg, 3.75 μmol), and cis-(PEt3)2Pt(OTf)2 6 (5.48 mg,
7.5 μmol) were mixed in a 2:3:6 molar ratio and dissolved in acetone/
water (8.0 mL, 8:2, v/v). The mixture was stirred for 12 h at 35 °C
and then cooled to room temperature. The solvent was removed by
nitrogen flow. Acetone (6.0 mL) was added to the mixture, and the
solution was stirred for 12 h at 35 °C. After cooling to room
temperature, the solution was filtered to remove insoluble materials
and the solvent was again removed by nitrogen flow. The solid was
dissolved in acetone (1.0 mL) and followed by the addition of 7 mL
of diethyl ether. The resulting precipitate was collected by
centrifugation to give 7.5 mg of 1 as a pale-yellow solid in 92%
yield. 1H NMR (500 MHz, acetone-d6, 298 K) δ: 9.28 (d, J = 5.1 Hz,
12H), 7.97−7.95 (m, 24H), 7.84 (d, J = 5.1 Hz, 12H), 7.69−7.67 (m,
12H), 7.59 (d, J = 8.6 Hz, 24H), 7.43 (d, J = 8.3 Hz, 24H), 2.23−2.17
(m, 72H), 1.46−1.39 (m, 108H). 31P {1H} NMR (acetone-d6, 121
MHz, 298 K) δ (ppm): 8.02 (d, 2JP−P = 21.4 Hz, 195Pt satellites, 1JPt−P
EXPERIMENTAL SECTION
■
Materials and Methods. All reagents were commercially
available and used as supplied without further purification. Deuterated
solvents were purchased from Cambridge Isotope Laboratory
(Andover, MA). 4-(10-Bromoanthracen-9-yl)pyridine,57 (nitrilotris-
(benzene-4,1-diyl))triboronic acid,34 and 658 were prepared according
to the literature procedures. Carboxylate ligands 4 and 5 were
prepared by neutralization of terephthalic acid with 2 equiv of NaOH.
NMR spectra were recorded on a Varian Unity 300, 500, or 600 MHz
spectrometers. 1H and 13C NMR chemical shifts were reported
relative to residual solvent signals, and 31P NMR chemical shifts are
referenced to an external unlocked sample of 85% H3PO4 (δ = 0.0).
Mass spectra were recorded on a Micromass Quattro II triple-
quadrupole mass spectrometer using electrospray ionization with a
MassLynx operating system. The UV−vis experiments were
conducted on a Hitachi U-4100 spectrophotometer (absorption).
Steady-state emission and excitation−emission mapping spectra
and emission QYs were recorded by using an Edinburgh Instruments
FS5 spectrofluorometer measured with corresponding solutions in a 1
× 1 cm2 quartz cuvette. Temperature-dependent emission spectra
2
1
= 3263 Hz), and 2.46 (d, JP−P = 21.5 Hz, 195Pt satellites, JPt−P
=
3400 Hz). ESI-TOF-MS: m/z 1347.15 ([M − 4OTf−]4+); found:
1347.20. m/z 1846.86 ([M − 3OTf−]3+); found: 1846.80.
Self-Assembly of 2. Ligand 3 (2.52 mg, 2.5 μmol), dicarboxylate
ligand 5 (1.08 mg, 3.75 μmol), and cis-(PEt3)2Pt(OTf)2 6 (5.48 mg,
7.5 μmol) were mixed in a 2:3:6 molar ratio and dissolved in acetone/
water (8.0 mL, 8:2, v/v). The mixture was stirred for 12 h at 35 °C
and then cooled to room temperature. The solvent was removed by
nitrogen flow. Acetone (6.0 mL) was added to the mixture, and the
solution was stirred for 12 h at 35 °C. After cooling to room
temperature, the solution was filtered to remove insoluble materials,
and the solvent was again removed by nitrogen flow. The solid was
dissolved in acetone (1.0 mL) and followed by the addition of 7 mL
2916
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J. Am. Chem. Soc. 2021, 143, 2908−2919

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
of diethyl ether. The resulting precipitate was collected by
centrifugation to give 7.8 mg of 2 as a pale-yellow solid in 90%
yield. 1H NMR (500 MHz, acetone-d6) δ: 9.26 (s, 12H), 8.05 (d, J =
8.3 Hz, 12H), 7.96−7.91 (m, 12H), 7.79−7.73 (m, 36H), 7.65−7.58
(m, 24H), 7.49−7.34 (m, 24H), 2.24−2.19 (m, 72H), 1.49−1.40 (m,
108H). 31P {1H} NMR (121 MHz, acetone-d6) δ (ppm): 7.92 (d,
Kendall. N. Houk − Department of Chemistry and
Biochemistry, University of California, Los Angeles, Los
Angeles, California 90095, United States; orcid.org/
0000-0002-8387-5261
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c12853
2JP−P = 21.7 Hz, 195Pt satellites, 1JPt−P = 3251 Hz), and 2.70 (d, 2JP−P
=
1
21.3 Hz, 195Pt satellites, JPt−P = 3427 Hz). ESI-TOF-MS: m/z
1404.17 ([M − 4OTf−]4+); found: 1404.16. m/z 1921.88 ([M −
3OTf−]3+); found: 1921.81.
Author Contributions
Y.L. and S.S.R. contributed equally to this work.
Computational Methods. To find the ground-state structures of
the host 1 and 2, the lowest energy conformers of each metallacage
components, 3 and 4, were found by using Spartan 1659 conforma-
tional search with a MMFF60 force field. Then, the lowest energy
conformers were combined to build metallacages 1 and 2 and further
optimized by using the PM661 method in Gaussian09.62
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
G.L. thanks the Robert A. Welch Foundation (D-1361, USA)
and the National Natural Science Foundation of China
(22071102 and 91956110). P.D. acknowledges funding from
the U.S. National Science Foundation (NSF CAREER CHE-
1944903). Y.L. thanks Z. Yang, Q. Zhang, X. Wang, and B. Shi
for discussions and checking the data.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge via the
Internet at. The Supporting Information is available free of
charge at https://pubs.acs.org/doi/10.1021/jacs.0c12853.
Syntheses and characterization data (NMR, ESI-TOF-
MS, fluorescence spectra), including Figures S1−S40;
calculation data (PDF)
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AUTHOR INFORMATION
■
Corresponding Authors
Yanrong Li − Institute of Chemistry and BioMedical Sciences,
School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210093, China; Department of
Chemistry, University of Utah, Salt Lake City, Utah 84112,
United States; orcid.org/0000-0001-5180-1965;
Email: dg1524042@smail.nju.edu.cn
Pravas Deria − Department of Chemistry and Biochemistry,
Southern Illinois University, Carbondale, Illinois 62901,
United States; orcid.org/0000-0001-7998-4492;
Email: pderia@siu.edu
Peter J. Stang − Department of Chemistry, University of Utah,
Salt Lake City, Utah 84112, United States; orcid.org/
0000-0002-2307-0576; Email: stang@chem.utah.edu
Authors
Sreehari Surendran Rajasree − Department of Chemistry and
Biochemistry, Southern Illinois University, Carbondale,
Illinois 62901, United States
Ga Young Lee − Department of Chemistry and Biochemistry,
University of California, Los Angeles, Los Angeles, California
90095, United States
Jierui Yu − Department of Chemistry and Biochemistry,
Southern Illinois University, Carbondale, Illinois 62901,
United States; orcid.org/0000-0001-8422-3583
Jian-Hong Tang − Department of Chemistry, University of
Utah, Salt Lake City, Utah 84112, United States;
orcid.org/0000-0002-3299-5167
Ruidong Ni − Department of Chemistry, University of South
Florida, Tampa, Florida 33620, United States
Guigen Li − Institute of Chemistry and BioMedical Sciences,
School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210093, China; Department of
Chemistry and Biochemistry, Texas Tech University,
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