Wavelength-Dependent Energy and Charge Transfer in MOF: A Step toward Artificial Porous Light-Harvesting System

Article abstract of DOI:10.1021/jacs.9b08078

Chromophore assemblies within well-defined porous coordination polymers, such as metal-organic frameworks (MOFs), can emulate the functionality of the antenna rings of chlorophylls in light-harvesting complexes (LHCs). The chemical, electronic, and structural diversities define MOFs as a promising platform where photogenerated excitons can be displaced to redox catalysts similar to the reaction center of the LHC. The precise positioning of the pigments and complementary redox units enables us to understand the charge/energy-transfer process within these crystalline solid compositions. In this study, we postsynthetically anchored tetraphenylporphyrinato zinc(II) (TPPZn)-derived complementary pigment within the 1D pores of 1,3,6,8-tetrakis(p-benzoicacid)pyrene (H₄TBAPy)-derived NU-1000 MOF to form a high-density donor-acceptor system. The ground- and excited-state redox potentials of the donor and acceptor were chosen to facilitate an energy transfer (EnT) from the excited MOF (i.e., NU-1000*) to TPPZn and a charge transfer (CT) from excited porphyrin (i.e., TPPZn*). Thus, the processes depend on the excitation wavelength. The energy transfer process was spectroscopically probed by excitation-emission mapping: MOF emission was completely quenched at 460 nm, where the pyrene-centered emission was expected. Instead, the excited MOF efficiently transfers the energy to manifest a TPPZn-centered emission at 670 nm (k_EnT ≈ 4.7 × 10¹¹ s^-1). The excited TPPZn pigment, with a neighboring TBAPy linker, forms an artificial "special-pair"-like system driving the charge-separation process (k_CT = 1.2 × 10¹⁰ s^-1). The findings demonstrate a synthetic MOF-based artificial LHC system where their well-defined structure will open up new possibilities as the separated charge can hop along the 1D pore channel for further mechanistic understanding and future developments.

Full text of DOI:10.1021/jacs.9b08078

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Article
Wavelength-Dependent Energy and Charge Transfer in MOF:
A Step towards Artificial Porous Light-Harvesting System
Xinlin Li, Jierui Yu, David J Gosztola, Harry Christopher Fry, and Pravas Deria
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b08078 • Publication Date (Web): 30 Sep 2019
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Wavelength-Dependent Energy and Charge Transfer in MOF:
A Step towards Artificial Porous Light-Harvesting System
Xinlin Li,a Jierui Yu,a David J. Gosztola,b H. Christopher Fry,b and Pravas Deria*,a
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aDepartment of Chemistry and Biochemistry, Southern Illinois University, 1245 Lincoln Drive, Carbondale, Illinois
62901, United State.
bCenter for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States
KEYWORDS. Pyrene; porphyrin; porous coordination polymer, energy transfer; charge transfer; artificial LHC
ABSTRACT: Chromophore assemblies within well-defined porous coordination polymers, such as metal-organic
frameworks (MOFs), can emulate functionality of the antenna rings of chlorophylls in light-harvesting complexes
(LHCs). The chemical, electronic, and structural diversities define MOFs as a promising platform where exciton can be
displaced to redox catalysts similar to the reaction center of the LHC. The precise positioning of the pigments and
complementary redox units enable us to understand the charge/energy-transfer process within these crystalline solid
compositions. In this study, we post-synthetically anchored tetraphenylporphyrinato zinc(II) (TPPZn) derived
complementary pigment within the 1D pores of 1,3,6,8-tetrakis(p-benzoicacid)pyrene (H4TBAPy) derived NU-1000 MOF
to form a high-density donor-acceptor system. The ground and excited-state redox potentials of donor and acceptor
were chosen to facilitate an energy transfer (EnT) from the excited MOF (i.e. NU-1000*) to TPPZn and a charge
transfer (CT) from excited porphyrin (i.e. TPPZn*). Thus, the processes depend on the excitation wavelength. The
energy transfer process was spectroscopically probed by excitation-emission mapping: MOF emission was completely
quenched at 460 nm –where the pyrene-centered emission was expected. Instead, the excited MOF efficiently transfers
the energy to manifest a TPPZn –centered emission at 670 nm (kEnT ≈ 4.7 ×1011 s-1). The excited TPPZn pigment, with
a neighboring TBAPy linker, forms an artificial ‘special-pair’ –like system driving charge separation process (kCT = 1.2
×1010 s-1). The findings demonstrate a synthetic MOF-based artificial LHC system where their well-defined structure will
open up new possibilities as the separated charge can hop along 1D pore channel for further mechanistic
understanding
and
future
developments.
distance to reach appended electrode or redox catalysts
for charge-separation and their subsequent utilization.
INTRODUCTION
Natural photosynthetic systems (e.g. PSII) have
efficiently integrated antenna pigments within light-
harvesting complexes (LHCs) consisting of chlorophylls
rings and carotenoids. This arrangement maintains
strong photon absorptivity, manifests highly dispersed
excited-states or excitons, and ensures their vectorial
migration to the special-pair reaction center for
subsequent charge splitting. Thereon, directional charge-
transfer chains drive chemical reactions in separate
sites, i.e. - CO2 reduction and water oxidation1-3 to store
the solar energy in chemical bonds. This extraordinary
structure and functionalities have inspired chemists to
develop artificial and semi-artificial photosynthetic
systems to harness solar energy.4-7 Molecular
photophysics and photochemistry, established in solution
Solid 3D coordination- polymers, especially the
porous metal-organic frameworks (MOFs),10-11 can
organize a wide range of pigments12-17 and/or redox-
active18-26 molecules in the form of multitopic organic
linkers/struts interconnected with inorganic nodes. These
solid-state platforms order and fix these building blocks
around distinct pores geometry in high concentration. A
sub-molar concentration pigment assembly around 10-
30 Å pores provides high photon absorptivity and unique
exciton interaction and dynamics that are different from
those observed in typical π-stacked organic crystals and
aggregates. The pores, besides ensuring a directional
substrate and electrolyte delivery, can also be decorated
with complimentary photo-/redox- active units to enrich
possibilities of unique photo-induced processes. Thus,
MOFs can serve as scaffolds27-30 to develop artificial
LHC system hosting both energy transfer (EnT) and
charge transfer (CT) among intentionally placed photo-
/redox- active units.31-32
phase,
often
display
non-productive
excitons
recombination dynamics8-9 when cast into solid
aggregates or monolayer based working compositions.
Delineating solid-state molecular compositions with LHC
like functionality requires strategies that should facilitate
(i) high photon absorptivity (ii) producing delocalized
and/or weakly bound excitons, and (iii) displacing the
photo-generated excitons directionally over long-
In this direction, we have recently discovered with a
series of pyrene (TBAPy)33 and porphyrin34–derived
MOFs that the optically active excited states can be
highly delocalized over multiple pigment/linker unis,
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which can be directionally moved based on their
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and dark excited-states, which may play a role in the
excitons dynamics.31 Among various metal-nodes (also
known as the secondary building units, SBUs), well
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orientation (i.e., by manipulating the interchromophoric
coupling) determined by their topological network.31
Furthermore, depending on the topological positioning of
the pigments, excitonic interaction may generate polar
established ZrIV −based MOFs28, 35-40 have been popular
as they do not alter the photophysics of the
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Figure 1. Schematic representation of TPPZn@NU-1000 realized by anchoring TPPZn at the Zr-SBUs via SALI
functionalization (bottom). (Atom color code: Zr –green; C –grey; O –red; H –omitted for clarity).
pigment assemblies. This feature has been in dispute for
quite some time with the notion that a possible ligand-to-
metal CT (LMCT) or electronic mixing of the ligand//Zr-
oxo is the reason behind various unusual photophysical
behaviors of the MOFs. Our recent works17, 31, as well as
some of the theoretical and experimental reports,41-42
clarify that unlike titanium(IV), zirconium(IV) oxo
nodes/SBUs will have significantly large electronic band-
gap to mend with any typical organic linker. Typical Zr-
SBUs are defined by an octahedral cluster with a
chemical formula of [Zr6(3-O)4(3-OH)4]12⁺ which can
form a 12-connected network, alternatively an 8-
connected [Zr6(3-O)4(3-OH)4(–OH)4(–OH2)4]8⁺ node is
also widely known where four terminal hydroxyl ligands
balance the charge. The strong ZrIV-carboxylate bond
makes these Zr-MOFs extremely robust, even in
aqueous acid, which is crucial for photo and
electrocatalytic energy conversion processes.18, 23, 43-44
carboxylate ligands by replacing a pair of terminal
hydroxyl and aqua ligands (Figure 1). Thus,
a
mesoporous ZrIV-oxo MOF possessing 1D pore-channel
that can be functionalized with complementary redox
partner can potentially define an artificial LHC like
system. Recently, we have reported a prototype system
that was constructed from a deprotonated 1,3,6,8-
tetrakis(p-benzoicacid)pyrene (H4TBAPy) –based Zr-
MOF, NU-1000, which was decorated with ferrocene-
carboxylate within its mesoporous channel.47 The
resulting Fc@NU-1000 displayed systematic quenching
of the photoexcited MOF via a CT process with ca 10 ps
charge separation (ΔG ~0.8 eV) and a relatively long
(450 ps) charge recombination dynamics.47 Note that
ferrocene moiety has poor photon absorptivity,
nevertheless, the charge on the resulting ferrocenium
species can migrate through a self-exchange type redox
hopping along the 1D channel.48-50
Here, we have delineated a donor-acceptor dyad
Since the octahedral Zr6-oxo cluster can bind to a
maximum of 12 carboxylates, 8-connected Zr-MOFs,
therefore, can be decorated with four carboxylate-
system
defined
by
the
SALI-anchored
tetraphenylporphyrinato zinc(II) (TPPZn) within the 1D
pores of NU-1000 (Figure 1). The TBAPy assembly in
the resulting TPPZn@NU-1000 serves as the antenna
pigment assemblies with significantly delocalized S1
excited state31 and remarkable excitons hopping
terminated
counterparts (per each node). This post-synthesis
approach, known as solvent-assisted ligand
incorporation (SALI),35, 45-46 accommodates the incoming
complimentary
pigments
or
redox
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efficiency (~180 chromophores are visited within the were packed in quartz capillary tube for measurement.
emissive lifetime)47 to efficiently deliver the energy to the
complementary partner (i.e. TPPZn unit; kEnT = 3.7× 1011
s-1). The electronic and redox properties of these two
complementary chromophores are such that the
resulting excited TPPZn* can facilitate charge separation
(CS) with sizable efficiency (kCT = 1.2 ×1010 s-1). A direct
TPPZn excitation also results in a similar charge
separation, albeit no energy transfer process to
commence. We believe that the TPPZn@NU-1000
defines the first MOF-based dyad that displays various
LHC like functionalities to operate in a wide solar
spectral window.
Fluorescence lifetimes were determined by fitting the
respective emission decay profiles recorded on an
Edinburgh Lifespec II picosecond time-correlated single-
photon counting (TCSPC) spectrophotometer equipped
with Hamamatsu H10720-01 detector and a 403 nm or
507 nm picosecond pulsed diode laser as TCSPC
source (IRF ~180 ps).
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Transient absorption spectroscopic data in the
femtosecond-picosecond time domain were collected
through pump-probe experiments (Ultrafast Systems,
HELIOS, Sarasota, FL, USA). The details of the fs
transient absorption (fs-TA) spectrometer and the data
analysis procedures are described in the Supporting
Information.52-53 The TPPZn@NU-1000 sample was
examined in α,α,α-trifluorotoluene (CF3Tol) solvent as a
suspension. Solid MOF powder (~1 mg/5 ml) were taken
in solvent and degassed via 5-cycle of freeze-pump-thaw
and transferred in a 2 mm Schlenk quartz cuvette to
mount at the cell holder. The sample suspension was
constantly stirred using a magnetic palette to ensure
transmittance of the probe light with minimal excitation
light scattering. A 500 nm long-pass filter (ThorLabs,
Inc.) was used to further remove scattered photon from
excitation beam.
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EXPERIMENTAL SECTION
Materials. Spectroscopic grade solvents were
purchased from Sigma Aldrich and used as received.
Phase-pure spectroscopic grade MOF samples were
synthesized51 and activated35 according to literature
reported procedures.
Instrumentation. Scanning Electron Microscopy
(SEM) images and Energy Dispersive Spectroscopic
(EDS) data were collected on a Quanta FEG 450
Scanning Electron Microscope (SEM) equipped with an
Oxford INCA-EDS system. Powder X-ray diffraction
(PXRD) patterns were recorded on a Rigaku Ultima IV
diffractometer (measurements made over a range of 1.5
°< 2θ < 30° in a 0.05° step width with a 2 deg/min
scanning speed). Nitrogen isotherms were recorded on
Micromeritics ASAP 2050 analyzer at 77 K; the samples
were thermally outgassed at 100 ℃ for 12 h prior to the
adsorption experiments. Thermogravimetric analysis (TGA)
was performed on TGA Q50 at a heating rate 10 °C/min
under nitrogen atmosphere. UV−vis electronic absorptive
transitions were measured using JASCO V-670
UV−vis−NIR spectrophotometer to determine the
TPPZn-COO loading relative to the TBAPy (i.e. loading
per SBU). For this, the solid MOF samples (ca 5 mg)
were digested in a few drops of concentrated sulfuric
acid and subsequently diluted in DMF (5 mL) to afford a
solution of H4TBAPy and TPPH2-COOH.
Computational Methods. To determine the relative
position of the complementary chromophores, a model
was constructed where one TPPZn moiety was placed in
between two adjacent TBAPy units defining a portion of
the large hexagonal pore wall. The structure was then
optimized via
a
density functional theory (Cam-
B3LYP/LanL2dz) computation. During this restricted
optimization, the Zr-node and the carboxylate moieties of
the TBAPy units were kept fixed at the crystallographic
positions (see SI).
Synthesis. Meso-tetraphenylporphyrin (TPPH2)54
and
2-bromo-5,10,15,20-tetraphenylporphyrin
(Br-
TPPH2)55 were synthesized according to literature
reported procedures.
2-((4
’
-Ethoxycarbonyl)phenyl)-5,10,15,20-
(TPPH2-COOEt): 4-phenyl-2-
tetraphenylporphyrin
Electrochemical redox potentials for NU-1000 and
carboxylic acid was attached to TPPH2 through Suzuki-
coupling reaction. For this, Br-TPPH2 (40 mg; 0.046
TPPZn
were
recorded
by
measuring
cyclic
voltammograms (CV) with Autolab 128N potentiostat
using a standard three-electrode cell –a platinum counter
electrode, an Ag/AgCl (3 M KCl) reference electrode,
and a working electrode. NU-1000 electrophoretically
deposited on conductive fluorine-doped tin oxide (FTO)
surface define as working electrode (See SI);
alternatively, a glassy carbon electrode was employed
for molecular solution (e.g., TPPZn-COOEt; 1 mM in
mmol)
was
reacted
with
(4-
(ethoxycarbonyl)phenyl)boronic acid (37 mg; 0.19 mmol)
in presence of tetrakis(triphenylphosphine)palladium (10
mg; 0.0087 mmol) and tri-tert-butylphosphonium
tetrafluoroborate (5 mg; 0.017 mmol) catalyst in basic
(potassium carbonate; 51mg; 0.37 mmol) dioxane (15
mL) medium under argon environment. The reaction was
carried out at 110 °C. The reaction was followed by thin-
layered chromatography and stopped after 24 hours.
The reaction mixture was cooled to room temperature,
treated with brine, and extracted with DCM. The crude
dichloromethane
solvent).
Tetrabutylammonium
hexafluorophosphate (TBAPF6; 1 M in argon purged
dichloromethane (DCM) solvent) was used as
electrolyte.
targeted
2-((4’-ethoxycarbonyl)phenyl)-5,10,15,20-
Excitation-emission mapping spectra were collected
on Edinburgh FS5 spectrofluorimeter equipped with a
front-face sample module and corrected using the
instrumental correction functions for both excitation and
emission light signal. Absolute quantum yields (QY) were
determined using a 150 mm integrating sphere. The
crystalline powder samples immersed in desired solvents
tetraphenylporphyrin (TPPH2-COOMe) was collected
and purified via column chromatography (100% DCM ).
Yield 86%. H NMR (400 MHz, CDCl3 as 7.26 ppm): δ-
2.57 (s, 2 H); 1.42 (t, 3H) 4.41 (q, 2H); 7.15-7.27 (m, 3
H); 7.35 (d, 2 H); 7.65-7.68 (m, 9H);7.79 (d, 2H); 7.85 (d,
2 H); 8.17-8.21 (m, 6 H); 8.71-8.84 (m, 7 H).
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exerted by the adjacent meso-phenyl limits the maximum
’ -carboxy)phenyl)-5,10,15,20-
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2-((4
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loading of TPPZn-COO (i.e. 0.5/node) despite all our
aggressive SALI conditions. Thus, the steric requirement
of the 훽-connected TPPZn-COO self-limits its
installation, which, at least technically, ensures a
homogeneous distribution of the TPPZn and does not
lead a system where multiple TPPZn moieties are
installed at one node and none elsewhere. Indeed, the
SEM-EDS based line and areal scans (Figure S7) on
various TPPZn@NU-1000 crystallites display that the
Zn/Zr ratio is maintained throughout the crystals.
Nitrogen isotherm of TPPZn@NU-1000 (Figure S10)
reflects a significant reduction of BET surface area
compared with the pristine NU-1000, from 1800 to 1540
m2g-1. The major change, however, was observed in the
mesopore uptake, which is reflected a drop in the pore
volume from 1.31 cm3/g in the pristine MOF to 0.98
cm3/g in the composite, whereas the micropore region is
less affected. These data suggest the mesopore
channels of the NU-1000 are partially occupied by
TPPZn moiety. Given that the proximity and the
orientation play a significant role in both the EnT and CT
process, we sought to determine the position of TPPZn
moiety relative to the adjacent TBAPy through DFT
optimization. As shown in Figure 2, the TPPZn moiety
leans towards one of the TBAPy units instead of staying
at the middle of two adjacent TBAPy units. A short
Tetraphenylporphyrin (TPPH2-COOH): TPPH2-COOH
was prepared through the hydrolysis of TPPH2-COOEt.
For this, TPPH2-COOEt (76 mg; 0.1 mmol) was
hydrolyzed with potassium hydroxide (280 mg; 5 mmol)
in
a mixed solvent comprised of tetrahydrofuran,
methanol, and water (30 mL at 1:1:1 ratio). The titled
product was precipitated by a carefully administered
slow acidification of the reaction mixture to pH = 4.
Yielded TPPH2-COOH was harvested by filtration;
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washed with water and air-dried. Yield 81%. H NMR
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(400 MHz, CDCl3 as 7.26 ppm): δ -2.76 (s, 2 H); 7.24-
7.34 (m, 3 H); 7.41 (d, 2 H); 7.71 (d, 2H); 7.80-7.85 (m,
9H); 7.90 (d, 2 H); 8.21-8.26 (m, 6 H); 8.63-8.86 (m, 7
H). MS (ES+) m/z: 735.2751 [(Μ), calcd 735.2760;
Figure S4]
TPPZn@NU-1000: TPPH2-COOH (15 mg; 0.02
mmol) was reacted with activated phase-pure NU-1000
(60 mg; 0.027 mmol SBU) in dimethylformamide (2 mL)
at 80 °C for 72 hours. The purple TPPH2@NU-1000
powder was sedimented and washed by centrifuging the
respective dimethylformamide and acetone suspensions.
Zinc metalation of the appended TPPH2 units in NU-
1000 was performed by reacting the TPPH2@NU-1000
(30 mg; 0.0135 mmol TPPH2-COO‾) with zinc chloride
(5mg; 0.037 mmol) in chloroform and methanol solvent
mixture (3.5 mL; 6:1) at RT for 48 hours. The titled
center-to-center distance of 8.5
electronic interaction between the two complementary
donor-acceptor pair.
Å ensures good
TPPZn@NU-1000 was obtained after
methanol wash of the resulting purple powder.
a thorough
RESULTS and DISCUSSIONS
The complementary carboxy-terminated TPPZn-
COOH counterpart, in its free-base form, was
successfully synthesized via a Suzuki coupling of the
mono-bromo
TPPH2
compound
with
(4-
(ethoxycarbonyl)phenyl)boronic acid (Scheme 1). The
ester was subsequently hydrolyzed to generate free-
base variant, TPPH2-COOH ligand.
Scheme. 1 Synthesis of TPPH2-COOH
Figure 2. DFT-optimized structure of chromophoric units
within TPPZn@NU-1000 from two viewing directions.
Ground-state electronic energy levels were
determined from electrochemical measurements of the
individual unit: cyclic voltammograms display a TBAPy
centered NU-1000 oxidation and reduction events at
+1.35 V and -0.74 V (vs Ag/AgCl), respectively (Figure
3b). With an optical band gap of 3.10 eV that is required
to vertically excite the NU-1000 (i.e. λmax at 390 nm of the
UV-vis adsorption spectrum; Figure S5), the excited-
state oxidation potential of NU-1000 is determined to be
-1.75 V. In contrast, the TPPZn features a comparable
electrochemical bandgap with reduction and oxidation
potential of -1.28 V and +0.89 V (Figure 3a). Based on
the lowest energy Q band transition of TPPZn at 599 nm
(2.07 eV), the excited-state oxidation potential of the
porphyrin component is -1.18 V. It is interesting to note
that the CV of the TPPZn@NU-1000 composition
(Figure S12) did not show any significant shift in
potential for its first redox events (i.e. a NU-1000 –
centered reduction at ca -0.75 V and TPPZn –centered
To construct a LHC like system, we targeted a low
TPPZn- /TBAPy loading ratio. Thus, we reacted 1:1
TPPH2-COOH to Zr-SBU –the resulting TPPH2@NU-
1000 was analyzed to have ca half TPPH2-COOH per
Zr-SBU (0.55). The free-base variant was then metalated
with zinc(II) salt at room temperature. UV-vis spectral
analysis of the acid decomposed TPPZn@NU-1000
revealed that ca one TPPZn-COO unit is loaded per two
Zr-SBUs (Figure S5; TPPZn: TBAPy ≈ 1:4). The EDS
analysis (Figure S6) corroborate the TPPZn-COO
loading with a Zn to Zr atomic ratio of 0.6 per one SBU.
The steric hindrance at the carboxy-phenyl binding site
oxidation at +0.9 V); however,
a charged MOF
composite displays progressive permselective behavior
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for the counter ion only to raise the overpotential for the 1000, can thus have sufficient driving force for electron
2nd/3rd redox events. While such effect is well
documented in literature,49 this data suggest that no
sizable ground-state electronic interaction is occurring
between the TPPZn and NU-1000 –a similar observation
was found in ferrocene decorated NU-1000, where the
redox potential of the ferrocene moiety was unchanged
even when installed within the MOF pore.47, 49-50
transfer to NU-1000 (Figure 4b). Note that a more
positive ground-state oxidation potential of the NU-1000
supports this CT from the TPPZn* to NU-1000. This
energy level alignment between two complementary
chromophores defines a solid composition that can
operate EnT or CT process depending on the excitation
wavelength. A direct excitation at the Soret (~420 nm) or
Q-band (520-550 nm) (Figure S13, S14) will result in
TPPZn*-COO to proceed a CT to NU-1000. Thus, 400-
600 nm excitation, defined by high oscillator strength of
the respective pigment units will end up delivering the
excitons at the TBAPy-TPPZn charge-separation pair
(Figure S14).
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Steady-state excitation-emission emissive mapping
spectra (EEMS) was employed to verify the EnT and CT
process as deduced from the experimentally measured
energy levels. The EEMS plots for pristine NU-1000 and
TPPZn-COOEt (Figure 5a, b) were collected in 2-methyl
tetrahydrofuran (MeTHF) solvent which confirms their
maximum emission at 465 and 660 nm upon the
excitation at 400 and 420 nm, respectively. In
TPPZn@NU-1000, the characteristic emission of NU-
1000 at 465 nm (λex = 400 nm) is significantly diminished
(i.e. 40% for the pristine NU-1000 to mere 0.1% in the
TPPZn@NU-1000); instead, an intense TPPZn derived
emission at 664 nm (Figure 5c) is observed. This
spectral shift indicates a Förster resonance energy
transfer (FRET) process between NU-1000* → TPPZn.
Note that this TPPZn derived intense emission at 660
nm in TPPZn@NU-1000 sample upon 400 nm excitation
is not stemming from the Soret band excitation which
appears at 420 nm excitation.
Figure 3. The CV of a) TPPZn-COOEt and b) NU-1000
showing the respective redox potentials. Experimental
conditions: 1 M TBAPF6 in DCM; scan rate = 100 mV/s; GC
or FTO electrode for TPPZn or NU-1000; [TPPZn-COOEt] =
1 mM.
Figure 4. Energy diagram illustrating the electrochemically
determined chromophore valence and conduction band
energies for the NU-1000 (black) and TPPZn (red) along
with their corresponding excited-state oxidation potentials
(cyan). Photo-induced EnT and CT processes are shown
with blue and black arrows, respectively.
Figure 5. Excitation-emission mapping spectra of (a) NU-
1000; (b) TPPZn-COOEt; (c, d) TPPZn@NU-1000 recorded
in MeTHF or CF3Tol solvent. Note that the QY-normalized
intensities are scaled (color-coded) differently for better
visualization.
Figure 4 summarizes these energy levels: the type-I
energy level alignment of the excited NU-1000* and
ground-state TPPZn moiety is perfect for the NU-1000*
→ TPPZn EnT process. Thus, optical excitation of the
NU-1000 at 400 nm should cause a significant red-
shifted emission at 660 nm stemming from the porphyrin
counterpart (Figure 5a). For the excited TPPZn* unit,
whose excited-state oxidation potential lies 440 mV
higher than the ground-state reduction potential of NU-
Since the FRET does not involve a charge-
separated polar intermediate or transition state, solvent
polarity (or acceptor number, AN) does not affect the
EEMS features recorded in CF3Tol (Figure 5d). The
lower NU-1000 centered emission observed in CF3Tol
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(Figure 5d) relative to that observed in MeTHF solvent was observed. This data indicates directly excited
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(Figure 5c) has its origin in the solvent dependent
emission of the pristine NU-1000 (i.e. QY(CF3Tol) = 35%
and QY(MeTHF) = 42%).31 Nevertheless, these solvent-
polarity dependent TPPZn@NU-1000 EEMS data clearly
display a significantly low TPZZn-derived emission in
CF3Tol solvent possibly due to an efficient CT process.
TPPZn undergo TPPZn* → NU-1000 CT process.
The EnT and CT rates for this rigid node-bound
system are estimated from the following equation:
훷0
= 1 + τ0 k
(1)
훷푠
where 훷0 and τ0 are the intrinsic fluorescence quantum
yield and lifetime without complementary species,
respectively; 훷s is the fluorescence QY of the NU-1000
or TPPZn-derived emission envelope in TPPZn@NU-
1000 composition; k is the EnT or CT rate (see SI Table
S1). The maximum kEnT of ca 4.7×1011 s-1 (휏EnT = 2 ps)
reflects an efficient NU-1000* → TPPZn energy transfer
process. The efficient EnT process can be attributed to
the proximity between the counterparts (dTBAPy-TPPZn = 8.5
Å) and a significant overlap integral (Figure S13). In
contrast, appropriate alignment of the ground and
excited-state energy levels of the TBAPy-TPPZn were
Analysis of the transient emission profiles
unambiguously reveals the distinct EnT and CT features
and dynamics. Selective excitation of the NU-1000 at
403 nm (Figure 6a) display a significantly faster
emission decay when probed at 465 nm. The lifetime
profile was found not to be solvent polarity dependent
when examined by filling the MOF pores with MeTHF or
CF3Tol solvent separately. This data underscores the
major decay pathways of the NU-1000* proceed through
a non-polar FRET pathway observed with EEMS data. In
contrast, the emission decay profile probed for the
TPPZn moiety at 660 nm upon NU-1000 excitation (λex =
403 nm) manifested a solvent dependent transient profile
(Figure 6b) as the TPPZn S1-state decay rate to the CT
state varies by the extent of its solvent stabilization.
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∆퐺
expected to drive the CT process. The
CT ≈ -0.7 eV for
the TPPZn*-COO → NU-1000 electron transfer was
determined by the standard Weller equation:56
푒2
푒2
푠푅퐷퐴 8휋 ∈ 0 푟퐷
1
1
1
1
∆퐺
= e(
퐸퐷표푥-퐸퐴푟푒푑) - ∆퐸
0,0 - 4휋 ∈
-
(
-
푟퐴 ) (
-
)
푠
(2)
∈
∈
∈
0
푟푒푓
퐸퐷
퐸퐴
where 표푥 and 푟푒푑 are the electrochemically determined
oxidation and reduction potentials of donor and acceptor,
∆퐸
respectively;
is first excitation energy of donor; rA,
0,0
rD, and RDA are radius of the acceptor (the TBAPy; 7.4
Å), the donor (TPPZn; = 6.2 Å), and the center-to-center
distance between donor and acceptor (= 8.5 Å),
respectively (these parameters were taken from the
휖0
corresponding DFT optimized structures);
is the
the static
permittivity of free space (8.85 × 10 -12);
휖 푖푠
푠
휖
dielectric constant of solvent;
(8.93) is the dielectric
푟푒푓
constant of the solvent (DCM) in which the redox
potential was measured. The proximity and sizable
driving force manifested a kCT = 6.2×108 s-1 and 1.2×1010
s-1 (i.e. τCT ~ 1.6 ns and 80 ps) in MeTHF and CF3Tol
solvents, respectively (see SI Table 1). Note that the
highest rate recorded in CF3Tol solvent is about one
order magnitude slower than that was observed for our
previous Fc@NU-1000 system (kCT ≈ 1×1011 s-1; τCT ~ 10
ps) due to negligible internal reorganization energy for
the Fc-centered redox process. The charge separation
time constants are within the predicted range reported in
푟퐷퐴
literature for
≃10 Å.57-58 Furthermore, the titled
composition TPPZn@NU-1000 displays about 20 fold
boost in kCT going from MeTHF to CF3Tol solvent,
whereas the related Fc@NU-1000 did not show any
appreciable solvent response. This is due to the fact that
the polar hydroxyl and aqua ligands at the Zr-oxo SBUs
impede the CT-rate (requiring more reorganization
energy) for the Fc@NU-1000, where the Fc-moiety
resides very close to the Zr-oxo SBU (dFe-Zr = 5.6 Å in
Fc@NU-1000; compared to the dZn-Zr = 12.75 Å in
TPPZn@NU-1000).
Figure 6. Transient emission decay profiles of NU-1000,
TPPZn, and TPPZn@NU-1000 in two solvents under two
different excitation and probing conditions (a) λex =403 nm,
λprobe = 465 nm; (b) λex =403 nm, λprobe = 660 nm
To validate the CT process from the TPPZn*-COO
→ NU-1000, we selectively excite the porphyrin species
at 507 nm and probed at 660 nm (Figure S15a).
Compared to a single component decay profile for
pristine TPPZn*-COOEt in solution phase, the MOF-
bound TPPZn-COO in the solid TPPZn@NU-1000
composite features a slightly faster decay in MeTHF
solvent (i.e. the pores filled with the solvent). In a
relatively polar CF3Tol solvent, a significantly faster
decay of the TPPZn* component in the solid composite
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transient dynamics (Figure S17): the time constants
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3
suggest a 283 ps charge recombination time constant
beside an 80 ps charge separation and 1.47 ns S1 → S0
emission lifetime.
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To further validate the photo-induced CT process in
TPPZn@NU-1000, we have examined photocurrent
generation in a photoelectrochemical setup. In this
experiment, upon selective TPPZn excitation (i.e. 530
nm illumination), the photogenerated electron is
transferred to the NU-1000. The accumulated hole, upon
application of a forward bias display a photocathodic
current (see Figure S18); the magnitude of the
photocurrent is comparable to that reported for a
porphyrin-based MOF.60 In order to test the impact of CT
driving force, we examined the free-base variant, i.e.
TPPH2@NU-1000. We know that the excited-state
oxidation potential of the free-base porphyrin is less
negative (-0.85 V for the TPPH2-COOEt relative to –
1.18V for TPPZn-COOEt) and, thus, it is closer to the
ground-state reduction potential of the NU-1000 (-0.74
V). We examined the TPPH2@NU-1000 system through
similar spectroscopic methods (Figure 8). With much
lower ΔGCT (-0.36 eV), TPPH2*-COO → NU-1000 CT is
relatively inefficient to display any dramatic solvent
dependent TPPH2 –derived emission decay profile.
Nevertheless, TPPH2@NU-1000 composition evinces
efficient NU-1000* → TPPH2 energy transfer.
Figure 7. Representative femtosecond transient absorption
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spectra for TPPZn@NU-1000 suspension in CF3Tol solvent.
Experimental condition: λex = 450 nm, fluence= ~280
µJ/cm2.
Femtosecond
transient
absorption
(fs-TA)
spectroscopic (Figure 7) data for TPPZn@NU-1000,
collected in CF3Tol solvent, unambiguously display
characteristics of photo-induced CT complex formation.
The early time spectrum (tdelay = 0.38 ps) evinces
induced absorption (IA) peak maxima at 570 and broad
1100 nm as well as negative signals at 530, 594, and
660 nm. Based on the steady-state spectra, the 530,
570, and 594 signals mirror the ground-state absorption
(Figure S13), where the negative spectral signature at
660 nm can be attributed to S1 → S0 stimulated emission
(SE). We know that the TBAPy-centered [NU-1000]‾
anion peak appears at 600 nm,47 and a TPPZn-centered
cation radical possess weak transitions red to its Q-
bands and NIR peak ~970 nm.59 Thus, over time (as
shown by the transient spectra collected at 29 and 116
ps), the fs-TA spectral evolution display a red-shifted
spectral envelop that includes the TPPZn derived SE
(660 nm → 700 nm) peaks. To better understand the
complexity of the fs-TA spectral evolution for the
TPPZn@NU-1000, we mapped it with spectral signature
of the relevant components: these include the fs-TA
spectra of NU-1000 and TPPZn components (at tdelay
~
0.38 and 29 ps), absorption spectral signature of the
resulting TPPZn-Ph•+ and NU-1000•⁻ species, as well as
an inverted emission of TPPZn@NU-1000; see Figure
S16 for details. The 450 nm excitation beam did produce
NU-1000*, besides preferably exciting of the TPPZn
component (Figure S14). The fs-TA spectrum of the NU-
1000* (at tdelay ~0.38 and 29 ps) evinced a broad peak at
1100 nm and a stimulated emission ~480 nm.31 The 530
nm negative signal for TPPZn@NU-1000 composition
(at tdelay ~ 0.38 ps) can be explained as a resultant of
TPPZn ground-state bleaching (GSB) at 540 nm and tail
Figure 8. Photophysics of TPPH2@NU-1000: (a) energy
diagram highlighting the ground and excited-state redox
potentials; (b) EEMS; transient emission decay profiles at
(c) 732 and (d) 465 nm probed for TPPH2-COO and NU-
1000 components in and TPPH2@NU-1000 in two solvents
(λex =403 nm)
Conclusion
of 480 nm SE of NU-1000*. Thus, over time, the decay
31, 47
of the NU-1000*
causes the redshift of the 530 nm
In this study we have constructed the first coordination-
chemistry derived crystalline porous composition, where
energy and electron transfer can be performed
selectively by choosing the appropriate excitation
wavelength. In doing so, the composition displays
various LHC-like functionalities. The system was
designed via appropriate ground and excited-state
negative peak (i.e., 530 → 540 nm), reduction of the
1100 nm IA peak accompanied by the appearance of
976 nm broad NIR peak for TPPZn•+ cation radical
species. Due to the complexity of the TA-spectral time
evolution, a global fitting of the transient dynamical data
probed at 541, 693, and 976 nm was used to analyze
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pderia@siu.edu.
energy level alignment of tetraphenyl pyrene (TBAPy)
based MOF and tetraphenylporphyrin (TPPZn). A well-
established SALI chemistry was used to precisely and
periodically install the carboxy-terminated TPPZn-COOH
at the Zr-oxo node. The SALI-based post-synthetic
installation was self-limited by the steric requirement of
the 훽-connected TPPZn-COOH leading to a system
where TPPZn:TBAPy ratio of 1:4 was achieved (i.e., only
0.5 TPPZn/Zr-node, instead of a maximum possible
4/node). Such self-limited installation is expected to
ensure a homogeneously distributed TPPZn within the
NU-1000 MOF crystallites, as probed by electron
microscopy-based elemental analysis.
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ACKNOWLEDGMENT
We thank SIUC for the startup support and Advanced Coal
& Energy Research Center for partial support through
Energy Boost Seed Grant. JY acknowledges the B. & M.
Gower fellowship awarded through the Department of
Chemistry and Biochemistry, SIUC. SEM-EDS data were
collected at SIUC IMAGE center (supported by NSF grant
#CHE0959568). Use of the Center for Nanoscale Materials,
an Office of Science user facility, was supported by the U.S.
Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-
06CH11357. We thank Dr. Amar S. Kumbhar, CHANL, UNC
Chapel Hill for acquiring the EDS line scan and areal
mapping.
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Here, the TBAPy-based robust mesoporous MOF,
NU-1000 serves as the antenna component, where the
precisely organized pyrenes in sub-molar concentration
manifest delocalized excitons31 that can be efficiently
displaced.47 Electrochemical, and series of steady-state
and transient spectroscopic data indicate that the
ground- and excited-state potential energy levels of the
TPPZn relative to that of the NU-1000 enable (i) an
efficient delivery of initial photo-generated exciton at the
TBAPy assemblies in NU-1000 to TPPZn (NU-1000* →
TPPZn; kEnT = 4.7 × 1011 s-1), and (ii) the charge
separation from excited TPPZn* to a neighboring
TBAPy/framework (TPPZn* → NU-1000; kCT = 1.2 ×
1010s-1 in CF3Tol solvent). Interestingly, the EnT and CT
timescales (휏EnT = 2 ps; 휏CT = 80 ps) observed within the
chromophore assembly in TPPZn@NU-1000 are
opposite to what is seen for natural LHC in PSII. The
EnT from LH1 to the special-pair reaction center (RC)
commences over ~30 ps followed by a fast charge
separation (CS) within 2 ps –eventually forming RC+
[quinone]ˉ ‘trapped’ charge-separated state over 200 ps in
PSII.61-62 While generating such trapped-state in artificial
system would be another development, the photo-
generate CS-state in TPPZn@NU-1000 (that persists
over 280 ps) can migrate via redox hopping48-50 through
both TBAPy and the TPPZn units to be harvested in
photoelectrochemical setup. It should be noted that the
TPPZn moiety absorbs light complementary to the
TBAPy assembly and can be excited directly through its
low energy Q band -derive transition (530 nm) and thus,
the TPPZn@NU-1000 composition covers a large (350-
650 nm) effective window of the solar spectrum. This
study underscores that the desired EnT and CT rates
can be achieved through rigorous engineering of the
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