Self-Assembled Fluorescent Pt(II) Metallacycles as Artificial Light-Harvesting Systems

Article abstract of DOI:10.1021/jacs.9b08403

Light-harvesting is one of the key steps in photosynthesis, but developing artificial light-harvesting systems (LHSs) with high energy transfer efficiencies has been a challenging task. Here we report fluorescent hexagonal Pt(II) metallacycles as a new platform to fabricate artificial LHSs. The metallacycles (4 and 5) are easily accessible by coordination-driven self-assembly of a triphenylamine-based ditopic ligand 1 with di-platinum acceptors 2 and 3, respectively. They possess good fluorescence properties both in solution and in the solid state. Notably, the metallacycles show aggregation-induced emission enhancement (AIEE) characteristics in a DMSO-H₂O solvent system. In the presence of the fluorescent dye Eosin Y (ESY), the emission intensities of the metallacycles decrease but the emission intensity of ESY increases. The absorption spectrum of ESY and the emission spectra of the metallacycles show a considerable overlap, suggesting the possibility of energy transfer from the metallacycles to ESY, with an energy transfer efficiency as high as 65% in the 4^a+ESY system.

Full text of DOI:10.1021/jacs.9b08403

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Communication
Self-Assembled Fluorescent Pt(II) Metallacycles
as Artificial Light-Harvesting Systems
Koushik Acharyya, Soumalya Bhattacharyya, Hajar Sepehrpour, Shubhadip Chakraborty,
Shuai Lu, Bingbing Shi, Xiaopeng Li, Partha Sarathi Mukherjee, and Peter J. Stang
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 03 Sep 2019
Downloaded from pubs.acs.org on September 3, 2019
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Self-Assembled Fluorescent Pt(II) Metallacycles as Artificial
Light-Harvesting Systems
Koushik Acharyya,*† Soumalya Bhattacharyya,‡ Hajar Sepehrpour,† Shubhadip Chakraborty,¶
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Shuai Lu,§ Bingbing Shi,† Xiaopeng Li,§ Partha Sarathi Mukherjee,* ‡ and Peter J. Stang*†
†Department of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112, United States
‡Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
¶Institut de Physique de Rennes, UMR CNRS 6251, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex,
France
§Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, United States
Supporting Information Placeholder
should not suffer fluorescence quenching in their closely packed
arrangements.
ABSTRACT: Light-harvesting is one of the key steps in
photosynthesis, while, developing artificial light-harvesting
systems (LHSs) with high energy transfer efficiencies has
been a challenging task. Here we report fluorescent hexagonal
Pt(II) metallacycles as a new platform to fabricate artificial LHSs.
The metallacycles (4 and 5) are easily accessible by coordination
driven self-assembly of a triphenylamine-based ditopic ligand 1
with di-platinum acceptors 2 and 3, respectively. They possess
good fluorescence properties both in solution and in the solid
state. Notably, the metallacycles show aggregation induced
emission enhancement characteristics (AIEE) in a DMSO-H2O
solvent system. In the presence of a fluorescent dye Eosin Y
(ESY), the emission intensities of the metallacycles decrease but
the emission intensity of ESY increases. The absorption spectrum
of ESY and the emission spectra of the metallacycles show a
considerable overlap, suggesting the possibility of energy transfer
from the metallacycles to ESY, with an energy transfer efficiency
as high as 65% in the 4a+ESY system.
The LHSs reported to date could be broadly classified into two
categories: a) covalently bonded networks and b) self-assembled
architectures. The former one has the advantage of providing a
stable platform but it needs laborious synthetic steps. In contrast,
self-assembled architectures could be easily obtained by mixing
appropriate building blocks under suitable conditions. Therefore,
supramolecular coordination complexes (SCCs) with easy
synthetic accessibility, well-defined shapes, sizes, easy solution
processability and tunable photophysical properties could be
interesting materials for light harvesting.
Over the past few decades several SCCs like one dimensional
(1D) helices, two dimensional (2D) polygons and three-
dimensional (3D) polyhedrons have been developed utilizing the
directional coordination bonding approach.9 Such SSCs are found
to be attractive materials owing to their widespread applications in
catalysis,10 sensing,11 biomedical science,12 proton conduction,13
and so on. Although several luminescent platinum based
metallacycles and cages have been reported,14 their application
towards light harvesting is in its infancy.15 We report here two
new fluorescent hexagonal platinum (II) metallacycles 4 and 5
(Figure 1), along with their light-harvesting properties.
Photosynthesis is the main energy source for the survival of the
living world. The natural photosynthesis process that takes place
in the green plants and other organisms starts with the absorption
of light energy (sunlight) by a large number of closely packed
green pigments chlorophyll, embedded in a complex known as
antenna protein or light-harvesting complex.1 Subsequently, the
absorbed energy funnels through chlorophylls to the reaction
center where light energy transforms into chemical energy
(carbohydrates, like sugar). Inspired by nature chemists have
developed several artificial light-harvesting systems (LHSs) to
understand the key aspects of the process, intricacies of the
mechanism along with their potential applications in
The desired metallacycles were designed and synthesized from
ditopic organic donor 1 and di-platinum (II) acceptors 2 and 3
(Figure 1). Ligand 1 has a triphenylamine core with a
cyanostilbene moiety decorated with an alkyl chain bearing a
phenyl group, which could facilitate supramolecular assembly
formation through non-covalent interactions.16 The cyanostilbene
moiety has interesting optical properties of aggregation induced
emission enhancement (AIEE), as a result of their restricted
rotation in the aggregate state.17 Moreover, the cyanostilbene
moieties promote self-assembly to form various nano-
architectures. Accordingly, the metallacycles may easily form
supramolecular assemblies, and in their closely packed state will
not suffer fluorescence quenching and thus, could serve as an
excellent platform for light harvesting.
photosynthesis, photocatalysis, and photovoltaics. In this context,
2
wide varieties of scaffolds such as dendrimers,
organo or
hydrogels, vesicles,4 micelles, host-guest assemblies,6 metal-
3
5
organic frameworks, organic polymers8 and so on have been
7
utilized in the last few years.
The hexagonal [3+3] metallacycle 4 was readily accessible by
stirring an equal molar mixture of 1 and Pt-acceptor 2 in a mixed
solvent system of dichloromethane-methanol (1:1; v/v) at
There are two prime factors that one should take into account to
achieve high energy collection efficiency in LHSs: I) the system
should possess multiple donors per acceptor and II) the donors
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Figure 1. Metallacycles 4, 5 and the building blocks 1, 2, and 3.
room temperature for 6 h; a [6+6] metallacycle 5 was obtained
from 1 and Pt-acceptor 3, under similar reaction conditions
(Schemes S2 and S3). The metallacycles were obtained as yellow
solids upon addition of diethyl ether onto the reaction mixtures,
and were characterized by various spectroscopic techniques like
1
1
1H NMR, 31P{1H} NMR, H-1H COSY, 2D diffusion-ordered H
NMR spectroscopy (DOSY), and electrospray ionization time-of-
flight mass spectrometry (ESI-TOF-MS) analysis (Figures S1-
S14).
In the 1H NMR spectrum of 4 characteristic NMR signals
corresponding to 1 and the Pt acceptor 2 observed at around 8.57
ppm and 7.12-7.19 ppm respectively with a 1:1 intensity ratio
indicated the formation of [3+3] assembly (Figures 2a and S4).
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Similarly, metallacycle 5 formation was indicated by H NMR
signals with a 1:1 intensity ratio corresponding to the ligand 1 and
Pt-acceptor 3 at around 8.56 ppm and 7.19 ppm respectively
(Figure S8). The 31P{1H} NMR of both metallacycles displayed
sharp singlet peaks (Figures 2b and S9) with concomitant 195Pt
satellites (δ= 16.06 ppm and 16.30 ppm for 4 and 5 respectively),
which indicated the presence of only one type of phosphorus in
the assemblies, confirming the formation of highly symmetric
structures. An upfield shift of ~ 6 ppm was observed in the
31P{1H} NMR of the metallacycles with respect to their
corresponding Pt-building blocks, attributed to the metal-ligand
coordination. The stoichiometry of the formation of metallacycles
4
and 5 was confirmed by ESI-TOF-MS analysis. For
metallacycle 4, isotopic patterns of the peaks at m/z 1053.24 Da
and 852.84 Da for the [M-5OTf]5+ and [M-6OTf]6+ respectively
matches well with their calculated isotopic distribution patterns
(Figure 2c). In contrast, the larger metallacycle 5 shows a peak at
m/z 1186.82 Da for [M-9OTf]9+, however, the isotopic
distribution pattern suggests that the peak is actually due to two
fragments X3+ and Y6+(Figures S11 and S12) resulting from [M-
9OTf]9+.
1
Figure 2. (a) H NMR (400 MHz, CD2Cl2, 298 K) and (b)
31P{1H} NMR (121.4 MHz, CD2Cl2, 298 K) spectra of the
metallacycle 4 and its building blocks, (c) experimental (red) and
calculated (blue) ESI-TOF-MS spectra of 4.
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Moreover, single diffusion bands at 7.6× 10-9 m2/sec and 3.6×
Figure 3. TEM images of (a) 4, (b) 5, SEM images of (c) 4, (d) 5,
and DLS profiles of (e) 4, (f) 5 in H2O-DMSO (4:1; v/v). Insets:
TEM image of a single spherical particle.
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10-9 m2/sec in the H DOSY NMR spectra (Figures S13 and S14)
of 4 and 5 respectively are in agreement with the formation of a
single product in the reaction. To gain further insight into the
structural characteristics of the metallacycles PM6 semi-empirical
calculations were carried out, where the ethyl groups of the
phosphine ligands were modeled as hydrogen atoms. The
computational results indicate that the metallacycles have an
almost planar hexagonal backbone, with internal cavity sizes of
around 3.2 nm and 6.6 nm for 4 and 5 respectively (Figures S15
and S16).
The morphologies of the metallacycles in the aggregated state
were investigated by scanning electronic microscopy (SEM) and
transmission electron microscopy (TEM). The TEM analysis
(Figures 3a and 3b) shows the formation of spherical particles,
while the SEM analysis suggests an agglomeration of these
spherical particles (Figure 3c and 3d). Dynamic light scattering
(DLS) experiments (Figures 3e and 3f) provided average
hydrodynamic diameters of the aggregates of around 250 nm and
174 nm for 4 and 5 respectively in H2O-DMSO (4:1; v/v).
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The photophysical properties of the metallacycles were
investigated in DMSO. The UV-Vis spectrum of 4 exhibits two
absorption bands at around 300 nm and 400 nm, whereas, 5 has
absorption bands at around 340 nm and 400 nm (Figure S17 and
S18). Upon excitation at 410 nm both metallacycles display
similar emission pattern with an emission maximum at 521 nm
(Figure S19). The absorption and emission of the metallacycles
were also investigated in various solvents, which did not indicate
any significant solvatochromic characteristics (Figure S20). With
increasing water content in the DMSO solution of the
metallacycle (4 or 5), a substantial increase in fluorescence
intensity is observed (Figures S21 and S22), as expected due to
the presence of the cyanostilbene moieties that facilitate AIEE.
The highest emission intensity was observed in 80% H2O/DMSO,
while further increase in water fraction leads to a decrease in
emission intensity, presumably due to the precipitation of the
metallacycles. This AIEE behavior is also supported by a sharp
increase in fluorescence quantum yields (φ) of the metallacycles
in the solid-state and in the aggregates (4a and 5a). For instance,
fluorescence quantum yield of 4 was found to be 28 % in the
solid-state and 17 % in a H2O-DMSO (4:1; v/v) solvent system,
which are substantially higher than the quantum yield of the
metallacycle in the DMSO solution (φ= 9 %). A similar trend was
observed in metallacycle 5, which has fluorescence quantum
yields of 33 %, 28 % and 10 % in the solid-state, H2O-DMSO
solvent system (4:1; v/v) and in a DMSO solution respectively
(Table S1).18
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Figure 4. Fluorescence spectra of the metallacycles (a) 4 and (b)
5 with gradual addition of ESY (λex= 410 nm) in H2O-DMSO
(4:1; v/v). Change in the fluorescence decay profiles of the
metallacycles (c) 4 and (d) 5 in presence of ESY (metallacycle:
ESY
= 20:1) in H2O-DMSO (4:1; v/v). (e) A cartoon
representation of light harvesting system generated from 4 and
ESY, with visible color change of the solution under 365 nm UV
light in H2O-DMSO (4:1; v/v).
These results indicated that a H2O-DMSO (4:1; v/v) solvent
system would be the best medium to investigate the light-
harvesting property of the metallacycles, as under such
aggregated state the metallacycles showed the highest
fluorescence emission without any self-quenching and
precipitation from the solution. To prepare the LHSs we used the
fluorescent dye Eosin Y (ESY) as a fluorescence energy acceptor.
It is worth noting that for efficient energy transfer the absorption
band of the acceptor should overlap with the emission band of the
donor. The UV-Vis spectrum of ESY in H2O-DMSO has an
absorption band from 430 nm to 550 nm with an absorption
maximum at 520 nm. As shown in Figure S23 the absorption
spectrum of ESY and the emission spectra of the metallacycles
overlap well, suggesting the possibility of energy transfer from
the metallacycles to ESY. When a 1 μM solution (H2O-DMSO,
4:1, v/v) of ESY was gradually added to a 3 mL 1 μM solution
(H2O-DMSO, 4:1, v/v) of a metallacycle (4 or 5), a sharp decrease
in emission intensity of the metallacycle was observed (Figures 4a
and 4b), and the fluorescence intensity corresponding to ESY was
found to increase upon excitation at 410 nm. In contrast, under
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(2) (a) Ziessel, R.; Ulrich, G.; Haefele, A.; Harriman, A. An Artificial
similar condition, ESY alone was found to be non-emissive upon
excitation at 410 nm (Figure S24), ruling out the possibility of
direct excitation of ESY. Moreover, a substantial increase in the
quantum yields of the metallacycles was observed in the presence
of ESY, which suggests that ESY accepts and emits most of the
excitation energy. The energy transfer efficiencies are calculated
as 65 % and 52 % in the 4a+ESYand 5a+ESY systems
respectively at a 10:1 donor/acceptor ratio, which are comparable
with some of the recently reported single molecule based
supramolecular light-harvesting systems.15,19
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9
The best way to understand the energy transfer process is to
measure the fluorescence lifetime of the donor in the presence of
the acceptor. Therefore, fluorescence decay experiments were
carried out to support the occurrence of the light-harvesting
process. The fluorescence lifetime of the metallacycle 4 (τ1= 0.75
ns and τ2 = 2.94 ns) was found to be higher than the fluorescence
lifetime of 4 in the presence of ESY (τ1= 0.51 ns and τ2 = 2.65 ns)
(Figure 4c). Similarly, the fluorescence lifetime of the assembly
of 5 and ESY (τ1= 0.57 ns and τ2 = 2.86 ns) was found to be
shorter (Figure 4d) than the fluorescence lifetime of 5 (τ1= 0.75 ns
and τ2 = 2.97 ns). These results indicate that the energy transfer
takes place from the metallacycle to the ESY (Figure 4e).
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5) were devised to construct artificial light-harvesting systems.
The metallacycles were found to have good fluorescence
properties and AIEE characteristics. Most importantly, simple
mixing of the acceptor (Eosin Y) and a metallacycle, results in an
efficient Förster resonance energy transfer (FRET) process
between the metallacycle and ESY, leading to the in-situ
formation of an efficient artificial LHS. These artificial LHSs
exhibit good energy transfer efficiencies, as high as 65% in the
4a+ESY system at a 10:1 donor/acceptor ratio. We anticipate that
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AUTHOR INFORMATION
Corresponding Authors
koushikchemistry@gmail.com
psm@iisc.ac.in
stang@chem.utah.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
P.J.S. thanks the U.S. National Institutes of Health (Grant R01
CA215157) for financial support. P.S.M. thanks the SERB (India)
for financial support (Grant No. CRG/2018/000315). S.B. thanks
CSIR India for research fellowship. K. A. is thankful to Heng
Wang and Ruidong Ni for their help in ESI-TOF-MS analysis.
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