Journal of the American Chemical Society
Article
through orthogonal self-assembly based on metal−ligand
coordination and macrocycle-based host−guest interactions.
Within this supramolecular system M1⊂L3, anthracene,
coumarin, and BODIPY moieties were precisely distributed,
resulting in two-step energy transfer from anthracene to
coumarin and then to BODIPY (Scheme 2). As expected,
compared with the corresponding one-step FRET system and a
model supramolecular system containing only the terminal
acceptor moiety (BODIPY), the two-step FRET system
M1⊂L3 showed higher photosensitization efficiency and
photooxidation activity. To the best of our knowledge, this
study presents the first successful example of the fabrication of
a two-step FRET system bearing specific numbers of
anthracene, coumarin, and BODIPY moieties at precise
distances and locations through an efficient and controllable
orthogonal self-assembly approach based on metal−ligand
coordination and host−guest interactions. This research not
only offers an efficient approach for the preparation of two-step
FRET systems with precise arrangements and definite numbers
of fluorophores but also pushes multistep FRET systems
toward the application of photochemical detoxification of a
sulfur mustard simulant.
alkyl bromide, were designed and synthesized (Schemes S8−
According to the design principles of coordination-driven
self-assembly, reactions between 120° donor ligands and 120°
acceptor ligands can result in the formation of a discrete
hexagonal metallacycle.14 With these building blocks in hand,
the key hexagonal metallacycle M1, modified with both
anthracene and coumarin, was constructed through coordina-
tion-driven self-assembly of the 120° dipyridyl building block
L1 and the 120° diplatinum(II) building block L2 in a
quantitative yield (Scheme 2 and Scheme S13). Furthermore,
the related model hexagonal metallacycles M2−M5, modified
with anthracene or coumarin, were fabricated via coordination-
1
NMR (1H, 13C, 31P, 2-D DOSY, and H−1H COSY) analysis
of assemblies M1−M5 revealed the formation of discrete
metallacycles with highly symmetric hexagonal scaffolds
1
the assemblies, the α-hydrogen and β-hydrogen nuclei of the
pyridine rings displayed downfield shifts resulting from the loss
of electron density upon coordination of the pyridine-N atom
to the Pt(II) metal center. As an example, the α- and β-
hydrogen atoms of the M1 pyridine rings shifted 0.38 and 0.40
ppm downfield, respectively, which was attributed to the
formation of platinum−nitrogen (Pt−N) bonds (Figure S47).
All 31P{1H} NMR spectra of metallacycles M1−M5 showed a
sharp singlet with a significant upfield shift from the
corresponding starting platinum acceptor singlet. For example,
metallacycle M1 exhibited a sharp singlet at 17.13 ppm, which
was shifted upfield from the starting platinum acceptor L2
singlet by approximately 4.33 ppm (Figure S48). This change
and a decrease in the coupling of the flanking 195Pt satellites
(e.g., ca. ΔJP−Pt = −151.5 Hz for M1) were consistent with
back-donation from the platinum atoms. Moreover, for all five
metallacycles M1−M5, only one set of signals was observed in
the 2-D DOSY spectra for each metallacycle, indicating the
isolated species.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The 120° dipyridyl
building block L1, labeled with anthracene as a donor
fluorophore, was easily synthesized through an esterification
reaction between 2-anthracenecarboxylic acid and 3,5-bis(4-
(Scheme S1). The 120° coumarin-containing diplatinum(II)
building block L2, decorated with a long-chain alkyl bromide,
was prepared through amine alkylation, Vilsmeier−Haack,
Knoevenagel condensation, and esterification reactions in a
difluoride (BODIPY)-containing pillar[5]arene building block
L3 was also synthesized through an esterification reaction as
presented in Scheme S7. It should be noted that anthracene
and BODIPY were introduced as the initial energy donor and
terminal energy acceptor, respectively. Coumarin was incorpo-
rated as a crucial bridge to transfer the energy from anthracene
to BODIPY to realize sequential two-step FRET because the
absorption spectrum and emission spectrum of coumarin have
substantial overlaps with the emission spectrum of anthracene
and the absorption spectrum of BODIPY, respectively.
Previous studies have shown that appropriately sized pillar[5]-
arene derivatives could bind long-chain alkyl bromide via
host−guest interaction that is mainly driven by dipole−dipole
forces and C−H···π and C−H···O hydrogen bonding;13 thus, a
long-chain alkyl bromide and a pillar[5]arene were introduced
into building blocks L2 and L3, respectively. Notably, two
different kinds of interaction sites were encoded in building
block L2, in which the di-Pt(II) sites could be involved in
metal−ligand coordination interaction with the corresponding
dipyridyl building block L1 and the long-chain alkyl bromide
moiety could complex with the pillar[5]arene unit through
host−guest interaction. Moreover, to confirm the FRET
processes and investigate the energy transfer efficiencies, a
series of model building blocks, L4−L8, i.e., the 120°
diplatinum(II) building blocks L4 and L5 without coumarin
units, the 120° dipyridyl building block L6 without anthracene,
the pillar[5]arene building block L7 without BODIPY, and the
120° coumarin-platinum−iodine complex L8 with long chain
All attempts to grow X-ray-quality single crystals of
hexagonal metallacycles M1−M5 proved unsuccessful. The
GFN2-xTB (geometry, frequency, noncovalent, extended tight
binding) semiempirical tight-binding method was utilized to
acquire further insight into the structural characteristics of
these metallacycles. Molecular simulation indicated that all
metallacycles M1−M5 exhibited a very similar and roughly
planar hexagonal ring with an internal diameter of approx-
imately 3.0 nm (Figure S49). Moreover, the anthracene units
and coumarin units in metallacycle M1 were close to each
other, at a distance of approximately 1.9 nm, which fully meets
the requirements of FRET for the distance between the donor
and the receptor fluorophores.
Investigation of the First-Step FRET Process from
Anthracene to Coumarin. With the aim of investigating the
energy transfer from anthracene to coumarin and then to
BODIPY, the normalized absorption and emission spectra of
building blocks L1, L2, and L3 were measured. As shown in
Figure S50, the absorption spectrum and emission spectrum of
L2 exhibited obvious overlap with the emission spectrum of L1
and absorption spectrum of L3, respectively, which is favorable
for two-step FRET. The energy transfer from anthracene to
coumarin in metallacycle M1, driven by coordination
interactions, was first monitored in real time (Figure 1a).
Dilute solutions of building blocks L1 (30 μM) and L2 (30
μM) in acetone were mixed at room temperature, and the
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J. Am. Chem. Soc. 2021, 143, 399−408