A Bio-inspired Approach for Chromophore Communication: Ligand-to-Ligand and Host-to-Guest Energy Transfer in Hybrid Crystalline Scaffolds

Article abstract of DOI:10.1002/anie.201507400

Efficient multiple-chromophore coupling in a crystalline metal-organic scaffold was achieved by mimicking a protein system possessing 100 % energy-transfer (ET) efficiency between a green fluorescent protein variant and cytochrome b₅₆₂. The two

Full text of DOI:10.1002/anie.201507400

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507400
German Edition: DOI: 10.1002/ange.201507400
Metal–Organic Frameworks
A Bio-inspired Approach for Chromophore Communication: Ligand-
to-Ligand and Host-to-Guest Energy Transfer in Hybrid Crystalline
Scaffolds
Ekaterina A. Dolgopolova, Derek E. Williams, Andrew B. Greytak, Allison M. Rice,
Mark D. Smith, Jeanette A. Krause, and Natalia B. Shustova*
Abstract: Efficient multiple-chromophore coupling in a crys-
talline metal–organic scaffold was achieved by mimicking
a protein system possessing 100% energy-transfer (ET)
efficiency between a green fluorescent protein variant and
cytochrome b562. The two approaches developed for ET relied
on the construction of coordination assemblies and host–guest
coupling. Based on time-resolved photoluminescence meas-
urements in combination with calculations of the spectral
overlap function and Fçrster radius, we demonstrated that both
approaches resulted in a very high ET efficiency. In particular,
the observed ligand-to-ligand ET efficiency value was the
highest reported so far for two distinct ligands in a metal–
organic framework. These studies provide important insights
for the rational design of crystalline hybrid scaffolds consisting
of a large ensemble of chromophore molecules with the
capability of directional ET.
conditions.[21–43] Previously, the synthetic and structural ver-
satility of hybrid scaffolds was successfully deployed to design
and study Dexter and Fçrster ET mechanisms.[1] However,
development of the next generation of artificial systems
possessing enhanced and directional ET still requires new
structural insights.[1,12,44]
In designing the system presented here, we were inspired
by the high ETefficiency achieved in a protein system (a “bio-
inspired approach”): a didomain protein made from a green
fluorescent protein variant (EGFP) and a heme-binding
protein, cytochrome b562 (cyt b562, Scheme 1).[19] Through
modulation of chromophore coupling, Jones and co-workers
showed that a rational design of the protein scaffold could
lead to nearly 100% ET efficiency.[19] Herein, we focused on
replication of the efficient multiple intermolecular chromo-
E
fficient energy utilization that could significantly affect the
current energy landscape involves a number of challenges,
including achieving energy transfer (ET) in a predesigned
pathway. For example, to mimic the natural photosystem,
possessing high efficiency of directional ET, an artificial
system should rely on the cooperative work of hundreds of
chromophores. Owing to the complexity of the hierarchical
chromophore organization, self-assembly typically becomes
a key strategy to design ensembles with efficient ET,
mimicking the natural analogues. Coordination polymers
possessing well-defined rigid structures (for example, metal–
organic frameworks (MOFs)) could potentially address the
existing challenges in the modeling of long- and short-range
ET processes.[1–18] A great advantage of these crystalline
scaffolds is that the distances and angles between chromo-
phores, as well as their molecular conformations, can be
determined from single-crystal X-ray studies and controlled
through the ligand design[20] or variation of the experimental
[*] E. A. Dolgopolova, D. E. Williams, Prof. Dr. A. B. Greytak, A. M. Rice,
Dr. M. D. Smith, Prof. Dr. N. B. Shustova
Department of Chemistry and Biochemistry,
University of South Carolina
Scheme 1. (top) Representation of ET between the two coupled chro-
631 Sumter street, Columbia, SC 29208 (USA)
E-mail: shustova@sc.edu
mophore cores of a green fluorescent protein variant (EGFP) and the
[19]
electron-transfer protein, cytb562
.
(bottom) Approaches I and II
Dr. J. A. Krause
Department of Chemistry, University of Cincinnati
Cincinnati, OH 45221 (USA)
involved incorporation of chromophores with HBI- and porphyrin-
based cores inside the rigid scaffold. Approach I focused on coordina-
tive immobilization of both chromophores in crystalline scaffolds
1 and 1’ while Approach II is based on inclusion of the BI donor in the
porphyrin-based framework 2.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507400.
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phore coupling achieved in the protein system through
integration of chromophores with 4-hydroxybenzylidene
imidazolinone (HBI) and porphyrin cores into an artificial
rigid framework (Scheme 1). Recently, we showed that
a porous MOF could be utilized as a mimic of the GFP b-
barrel to maintain emission of HBI-based chromophores, and,
therefore, replicate the photophysical properties of natural
GFP-like systems.[46] In the present study, we designed
multiple-chromophore hybrid scaffolds for modeling ET
processes. The choice of chromophores with HBI and
porphyrin cores was dictated by the necessary overlap of
the emission spectrum of the donor (HBI-based derivative)
with the absorption spectrum of the acceptor (porphyrin-
based chromophore), which is required to accomplish effi-
cient resonance energy transfer (RET). With this in mind, we
prepared donor–acceptor pairs and developed two distinct
approaches to achieve chromophore coupling through their
incorporation into
a rigid hybrid matrix (Scheme 1).
Approach I involved coordinative immobilization of methyl-
2-(4-(2,5-di(pyridin-4-yl)benzylidene)-2-methyl-5-oxo-4,5-
dihydro-1H-imidazol-1-yl)acetate (DPB-BI, donor; Support-
ing Information, Figure S1) and tetrakis(4-carboxyphenyl)-
porphyrin (H4TCPP, acceptor), which resulted in formation of
the crystalline MOFs. Approach II focused on non-coordina-
tive inclusion of benzylidene imidazolinone (BI; Supporting
Information, Figure S2) molecules (donor) inside a three-
dimensional (3D) porphyrin-based host (acceptor). The guest
size, host aperture, and chromophore photoluminescence
(PL) responses were the main selection criteria in
Approach II to achieve efficient chromophore coupling. To
the best of our knowledge, the ligand-to-ligand ET efficiency
(65%), calculated based on the experimental time-resolved
PL data, is the highest value achieved so far between two
distinct linkers in a MOF matrix.
Figure 1. The X-ray structures of Zn2(ZnTCPP),[45] DPB-BI, 1, and 1’.
Increase of the interlayer distance occurred, owing to coordinative
immobilization of DPB-BI.
In Approach I, incorporation of DPB-BI and H4TCPP
ligands into a rigid scaffold was achieved using a stepwise
procedure. The four-step synthesis and molecular structure of
the novel DPB-BI ligand utilized in scaffold preparation is
described in the Supporting Information (Scheme S1, Figur-
es S1,S3,S4). The first step of Approach I was preparation of
a two-dimensional Zn2(ZnTCPP) framework (Figure 1).[29]
Afterwards, coordinative immobilization of DPB-BI was
carried out through immersion of the Zn2(ZnTCPP) crystals
into N,N-dimethylformamide (DMF) or N,N-diethylforma-
mide (DEF) solutions of DPB-BI (Supporting Information).
Depending on the solvent choice in the second step,
formation of two MOFs, [Zn2(ZnTCPP)(DPB-BI)0.86
-
(DMF)1.14]·(DMF)8.86(H2O)20 (1) and [Zn2(ZnTCPP)(DPB-
BI)0.64(DEF)0.36]·(DEF)6.94·(H2O)12.55 (1’), were achieved
(Figure 1). In Approach II (Scheme 1), solvothermally pre-
pared Pb2(TCPP)·4DMF (2)[47] (Figure 2; Supporting Infor-
mation, Figure S7) was soaked in the BI solution for 3 days,
which resulted in BI inclusion and formation of BI@2. The
frameworks 1, 1’, and BI@2 underwent comprehensive
characterization by single-crystal and powder X-ray crystal-
lography, elemental and thermogravimetric analyses, and FT-
IR spectroscopy (Figures 1,2; Supporting Information, Fig-
ures S5–S14). Furthermore, the digested 1, 1’, and BI@2
samples (destroyed in the presence of acid) were analyzed by
Figure 2. PXRD patterns of 2 and BI@2. The inset shows the single-
crystal X-ray structure of 2. The grey arrow indicates the 1D channels
suitable for BI incorporation. H atoms and guest solvent molecules
were omitted for clarity.
mass-spectrometry and 1H NMR spectroscopy (Supporting
Information, Figures S15–S17).
The structural analysis of 1 and 1’ is shown in Figure 1
(Supporting Information, Figures S5,S6). The Zn2(ZnTCPP)
framework consists of two-dimensional (2D) layers in which
ZnTCPP4À is coordinated to paddle-wheel Zn2(O2C–)4 sec-
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ondary building units (SBUs, Figure 1). Immersion of the 2D
framework into a DPB-BI solution resulted in coordination of
the second ligand by the replacement of the apical solvent
molecules in the SBUs, and, thereby, formation of the
crystalline scaffolds 1 and 1’. The choice of solvent (DMF
[1] versus DEF [1’]) used for DPB-BI immobilization affected
the stacking of the 2D layers (Figure 1). In the case of 1, DPB-
BI connects pairs of 2D layers, while in 1’ DPB-BI connects all
layers along the c axis. In both 1 and 1’, DPB-BI insertion
increased the interlayer distance by 2.8 Š (O···O distance in
Zn2(ZnTCPP)) to 11.5 Š (11.4 Š) (N···N distance in 1 (1’)),
which is consistent with the DPB-BI length determined from
its molecular structure (11.45 Š, Figure 1; Supporting Infor-
mation, Figure S1). The powder X-ray diffraction (PXRD)
studies of 1 and 1’ confirmed the preservation of crystallinity
during the two-step chromophore immobilization procedure
(Supporting Information, Figures S8,S9).
Structural analysis of 2 (Figure 2; Supporting Information,
Table S1) revealed that the 3D framework utilized as a host
for the BI molecules consists of TCPP4À linkers connected to
carboxylate-bridged Pb2+ chains.[47] More importantly, 2
contains 1D 8 ” 11 Š channels suitable for BI incorporation
(Figure 2). The PXRD analysis showed that inclusion of BI
molecules does not affect the host crystallinity, and spectro-
scopic studies of digested BI@2 revealed that the framework
contains one guest molecule per two TCPP4À units (Figure 2;
Supporting Information, Figure S17).
To test whether ET can occur in the designed scaffolds,
photophysical properties of donor/acceptor molecules as well
as 1, 1’, and BI@2 were studied by diffuse reflectance (DR),
fluorescence, and time-resolved PL spectroscopies. For effec-
tive RET, the emission spectrum of the HBI-based donor
should overlap with the absorption spectrum of the porphy-
rin-based acceptor. The absorption spectrum in the solid state
was evaluated by DR (Figure 3a,b), and indicated the DPB-
BI (donor) used for preparation of 1 is emissive in the range
of 400–550 nm with lmax = 440 nm (lex = 365 nm). The BI
molecule used in Approach II exhibits a similar PL profile to
DPB-BI and emits in the same 400–550 nm range with lmax =
440 nm (lex = 365 nm, Figure 3c). Notably, the EGFP origi-
nally used in the didomain protein system (see above)
fluoresces in the same range as BI and DPB-BI but with
a slightly red-shifted emission maximum.[19] Therefore, the PL
profiles of both BI and DPB-BI replicate the fluorescence
response of the EGFP that was initially used as a model for
the HBI-based chromophore design. Based on the DR data,
both acceptors, Zn2(ZnTCPP) and framework 2, absorb light
up to 650 nm (Figure 3a,c), which provides the necessary
spectral overlap of their absorption profiles with the donor
emission responses. Coordinative immobilization of both
donor and acceptor moieties in rigid 1 and 1’ resulted in
complete disappearance of donor emission (Figure 3, Sup-
porting Information, Figure S18), which could be attributed
to efficient ET.[7,48]
To quantitatively describe possible ET processes occur-
ring in 1, 1’, and BI@2, time-resolved fluorescence decay
measurements were carried out (Supporting Information,
Figures S19–S21). The ET efficiency (FET) was determined
based on donor lifetimes in the presence and absence of the
acceptor molecules.[48] We investigated the PL decays within
the donor emission range to exclude the PL response of
porphyrin-based acceptors. Time-resolved decays for coordi-
natively immobilized DPB-BI (DPB-BI-1 [or DPB-BI-1’], in
the presence of the acceptor) and DPB-BI coordinated to
Zn2+ (in the absence of the acceptor) demonstrated more
rapid decay than free DPB-BI (Supporting Information,
Figures S19,S20). Analysis of the curves with a reconvolution
fit supported a triexponential decay model in each case and
revealed a shortening of the amplitude-weighted average
lifetimes from 1.09 (DPB-BI) to 0.38 and 0.51 ns in the
presence of the acceptor molecules in 1 and 1’, respectively
(Table 1).
The estimated values of the corresponding FET and ET
rate constant (kET) of 1 were found to be 65% and 1.71 ”
1010 sÀ1, respectively. Interestingly, slightly smaller FET (53%)
was observed for 1’ (Table 1), which could be attributed to the
different topology of 1’, that is, the difference in stacking of
2D layers as shown in Figure 1. Notably, the excitation
spectrum of 1, obtained by scanning from 380 to 540 nm with
fixed emission at 680 nm (PL from Zn2(ZnTCPP)) is different
from the excitation spectrum of Zn2(ZnTCPP) (without
DPB-BI immobilization; Supporting Information, Fig-
ure S22). Thus, the PL studies confirm that efficient ET
from DPB-BI to Zn2(ZnTCPP) takes place in the prepared
Table 1: The amplitude-weighted average lifetimes (htavi), ET rate con-
stants (kET), Fçrster critical radii (Ro), ET efficiency (FET), and spectral
overlap functions (J) for DPB-BI, DPB-BI-1, DPB-BI-1’, BI, and BI@2
samples.
DPB-BI DPB-BI-1 DPB-BI-1’
BI
BI@2
Figure 3. a) The DR spectrum of Zn2(ZnTCPP) (dashed grey line) and
emission spectra of DPB-BI (solid black line) and Zn2(ZnTCPP) (solid
grey line). b) The emission spectrum of 1. The inset shows an
epifluorescence microscopy image of a crystal of 1 (lex =510 nm).
c) The DR spectrum of 2 (dashed grey line) and emission spectra of BI
(solid black line) and 2 (solid grey line). d) The emission spectrum of
BI@2. An excitation wavelength of 365 nm was used to acquire all PL
spectra.
htavi [ns][a]
kET [”1010 sÀ1
Ro [Š]
1.09
0.38
1.71
23
65
6.25
0.51
1.04
23
53
6.25
1.89
0.53
1.36
21
72
4.57
]
–
–
–
–
–
–
–
–
FET [%]
J [”10À14 cm3 mÀ1
]
[a] Detailed information relative to fitting of PL decays can be found in
the Supporting Information.
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scaffolds. Furthermore, to address the possibility of RET in
1 (1’), the Fçrster critical radius (Ro) was obtained through
calculation of the spectral overlap function (J) and was found
to be J = 6.25 ” 10À14 cm3 MÀ1 (Table 1; Supporting Informa-
tion, Figure S23). On the basis of the calculated spectral
overlap function, we estimated Ro to be 23 Š (Supporting
Information, Figure S23), which is far beyond the donor–
acceptor distance approximated from the structural data (the
distance between Zn in the ZnTCPP unit and the corre-
sponding N···N centroid of DPB-BI is roughly 7.7 Š). Thus,
based on a combination of PL measurements and calculations
of J and Ro, we attribute the observed changes in donor
emission profile after its coordinative immobilization to RET.
A similar tendency was observed in the case of time-
resolved studies of BI@2. The time-resolved PL decay of
BI@2 decreases more rapidly in comparison with the one
acquired from a non-incorporated BI molecule (in the
absence of acceptor 2; Supporting Information, Figure S21).
Similar to Approach I, the amplitude-weighted average life-
time in the presence of the acceptor (2) was found to be
0.53 ns, which is 3.5 times shorter in comparison to that
determined for the non-incorporated BI chromophore
(1.89 ns). Estimated FET in BI@2 was found to be 72%.
Thus, high FET values were achieved in both developed
approaches. The estimated J for BI@2 and the corresponding
Ro were found to be J = 4.57 ” 10À14 cm3 MÀ1 and 21 Š,
respectively (Supporting Information, Figure S23). Taking
into account the dimensions of the 1D channels in 2 and the
size of the incorporated BI molecules, the estimated Ro is far
beyond the guest–host distance. Therefore, similar to 1 and 1’,
we attribute the changes in the time-resolved PL decays
observed for the BI@2 scaffold to RET.
To summarize, we have developed two distinct
approaches focused on mimicking a protein system that
possesses high ET efficiency. Moreover, both selected HBI-
based chromophores, BI and DPB-BI, were emissive in the
same range as the EGFP used in the targeted protein system.
Combination of time-resolved PL studies with spectral over-
lap function calculations revealed high FET was achieved by
coordinative immobilization of the donor/acceptor chromo-
phores and non-coordinative inclusion of the donor molecules
inside the acceptor scaffold. Furthermore, the experimental
FET obtained through a rational design of 1, based on the
time-resolved PL data, is the highest value for ligand-to-
ligand ET efficiency reported so far for MOFs. Thus, the bio-
inspired approach demonstrated herein could foreshadow the
utilization of hybrid scaffolds to direct chromophore behavior
in large light-harvesting ensembles and, therefore, achieve
directional energy transfer in a predesigned pathway.
Acknowledgements
This work was partially supported by the ASPIRE-I and III
awards granted by the USC Office of the Vice President for
Research. Crystallographic data were collected through the
Service Crystallography at Advanced Light Source Program
at beamline 11.3.1 at the Advanced Light Source (ALS),
Lawrence Berkeley National Laboratory. The ALS is sup-
ported by the U.S. Department of Energy, Office of Basic
Energy Sciences, Materials Sciences Division, under Contract
DE-AC02-05CH11231.
Keywords: coordination polymers · energy transfer ·
fluorescence · green fluorescent protein · metal–
organic frameworks
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www.angewandte.org 13643