Topology-dependent emissive properties of zirconium-based porphyrin MOFs

Article abstract of DOI:10.1039/c6cc07343c

Highly ordered chromophoric linkers positioned within the metal-organic frameworks (MOFs) have the potential to mimic natural light-harvesting complexes. Herein we report topological control over the photophysical properties of MOFs via modular interchrom

Full text of DOI:10.1039/c6cc07343c

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DOI: 10.1039/C6CC07343C
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Topology-Dependent Emissive Properties of Zirconium-Based
Porphyrin MOFs
Pravas Deria,*,a Jierui Yu,a Rajesh P. Balaraman,a Jamil Mashni,a and Sandra N. Whitea,†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Highly ordered chromophoric linkers positioned within the metal- around modular pores,9 and thus make these systems
organic frameworks (MOFs) have the potential to mimic natural fundamentally different than stacked aromatic chromophores
light-harvesting complexes. Herein we report topological control possessing various excited state quenching mechanisms.10
A
over the photophysical properties of MOFs via modular high chromophore density, critical for efficient photon
interchromophoric electronic coupling to manifest different absorption in LH model system, can be achieved in a MOF.
steady-state singlet emissive spectra and their corresponding While exciton hopping among the closely spaced chromophores
fluorescence lifetimes.
within these solid materials were found to be quite efficient,8
experiment probing the spectral evolution as a function of
interchromophoric orientation is surprisingly rare. Difficulty in
generating frameworks consisting of identical building blocks,
particularly chromophoric linkers, may pose a major reason
behind it. Recall the antenna complex in purple bacteria, where
one kind of chromophore was engineered to generate coherent
or collective property1a, 2 via a strong electronic coupling (~300
cm⁻¹) among the precisely positioned bacteriochlorophyll-a.
This essentially produces new chromophores with a gradient in
energy gap directing the sequential energy migration among
the various LH complexes.1a
The light-harvesting (LH) antenna complex defines a highly
efficient energy transfer machinery that near quantitatively
delivers absorbed light to the reaction center.1 Highly ordered
chlorophyll and carotenoid pigments ensure sequential
migration of photoinduced energy.1b,
architectures have motivated many scientific explorations over
the past decades to design artificial antenna systems such as
covalently linked polymers,3 dendrimers and various
supermolecular arrays,4 and a wide range of self-assembled
supramolecular constructs.5 Considering the synthetic ease and
control over interchromophoric interactions, the self-
assembled supramolecular approach seems to be promising as
it often leads to hierarchical solid state ordering via noncovalent
interactions.6
2
These nanoscale
Metal-organic frameworks (MOFs), a new class of porous
hybrid material,7 have received discernible attention in recent
time for the construction of solid-state crystalline LH model
system.8 These coordination polymers often consist of
chromophoric linkers connected through inorganic nodes. A
precise control over interchromophoric ordering within its
crystalline frameworks7c can play a key role in delineating new
solid-state platform for anisotropic energy transfer processes.8f
Furthermore, frameworks position chromophores periodically
a.Department of Chemistry and Biochemistry, Southern Illinois University, 1245
Lincoln Drive, Carbondale, Illinois 62901, United States.
† NSF-REU from Department of Chemistry, Alabama Agricultural and Mechanical
University, 4900 Meridian Street, Normal, Al 35762.
Fig 1. Molecular structures of porphyrin-based MOFs (a) NU-902 and (b) MOF-525
(M = H2 or Zn) and their corresponding building blocks.
Electronic Supplementary Information (ESI) available: [materials, instrumentation,
and procedures; characterization (NMR, PXRD, and SEM-EDS); emission spectra,
TCSPC, and computation details]. See DOI: 10.1039/x0xx00000x
Here we report topological control over the photophysical
properties by probing the singlet state emissive properties of
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two Zr –based MOFs consisting of
tetrakis(4- collected in gas phase under argon [to VeielwimAritnicaletOenlinae
DOI: 10.1039/C6CC07343C
carboxyphenyl)porphyrin (TCPP)-derived linkers with different solvatochromic effect]. Emission spectra of MOF-525(H2) and
network topologies such as NU-902 (scu) and MOF-525 (ftw). NU-902(H2) show different extent of red shift along with
We present the evolution of steady-state and time-resolved different relative intensity of the Q0,0 and Q0,1 bands compared
emission spectra and fluorescence lifetime as a function of to a free TCPPOMe(H2) linker in dichloromethane solvent. We
interchromophoric orientation and macrocyclic ring planarity attribute this red-shifted Q band emission to different degree of
with varying framework topology. Furthermore, we excitonic coupling among the closely placed chromophores as a
investigated the relative efficiency of exciton migration within a function of their orientation. For example, in MOF-525, a
given MOF with metallated and free-base porphyrin linkers. dominant coupling should stem from one of the Qx or Qy derived
Four MOF samples: NU-902(H2) and MOF-525(H2) consisting of transition dipole of adjacent chromophores aligned at 90° (Fig.
free-base TCPP(H2), and NU-902(Zn) and MOF-525(Zn)
2; vide infra). Likewise, a stronger coupling can be expected for
consisting of zinc(II)-metallated TCPP(Zn) linkers were studied NU-902 among the adjacent chromophores that are positioned
here. These samples were prepared according to literature at 10.5 Å with 60° torsional angle.
procedure as the free-base versions.11 Metalation with zinc(II)
was achieved post-synthetically.11a (see ESI section S3
-S4).
Fig. 2. DFT optimized structures of closely positioned porphyrin dimers in (a) MOF-
525(H2) [90°, dM-M = 13.5Å] and (b) NU-902(H2) [60°, dM-M = 10.5Å]; H atoms
connected to C and O are omitted for clarity. [See ESI section S5]
The underlying topological networks of MOF-525 (cubic;
ftw
) and NU-902 (rhombic; scu) manifest the smallest
porphyrin-porphyrin torsional angle of 90° and 60° respectively.
Linker geometry and orientation within the frameworks were
obtained via constrained DFT (CAM-B3LYP//LANL2DZ; see ESI
section S5) optimization of two adjacent TCPP-M units with the
smallest torsional angle, where the carboxylate (O2C-) moieties
were fixed at positions with their corresponding
crystallographic coordinates. DFT optimization within the
framework connectivity was necessary to affirm the position of
the H-atoms and to obtain the atomic coordinates of a stable
conformation (instead of the thermally allowed probable
coordinates reported in the crystallographic data) of the linkers.
As highlighted in Figs. 2 and SI-6, such cofacial orientation leads
to interchromophoric center-to-center distance (dM-M) = 13.5
and 10.5 Å for MOF-525 and NU-902 respectively. Furthermore,
the node connectivity and symmetry of the linkers (12,4 for
MOF-525 and 8,4 for Nu-902), for these topological nets, have
a subtle stereochemical impact on the porphyrin ring planarity.
For the ftw topology, the carboxylates (O-C-O) are required to
be coplanar with the cubic faces that are lined up with the
porphyrin macrocycle planes. This enforces the linker to adopt
an energetically demanding conformation where the meso-
phenyl groups need to be non-planar to both the carboxylate
and porphyrin planes; these constrains eventually lead to a
nonplanar porphyrin. On the contrary, the NU-902 requires the
carboxylate to be at ~55° angle to the macrocycle allowing the
linker to adopt a stable conformation, where the carboxylate
and meso- phenyl are nearly coplanar.
Fig. 3. Steady state (a) emission and (b) excitation spectra for NU-902(H2) and
MOF-525(H2). c) TD-DFT computed electronic spectra of TCPP(H2) and TCPP(H2)
dimer for NU-902(H2) shown in Fig.2b (see text).
The difference in relative intensity of the Q0,0 and Q0,1
derived vibronic emission bands are stemming from the varying
degree of ring planarity adopted by the TCPP linker within the
two topological networks. The degree of ring planarity impacts
the symmetry and hence on the allowance of the lowest energy
electronic transitions. Considering excitation spectra as
complementary to an electronic absorption spectra, a clear
difference in the intensity and transition energies for various
vibronic transitions [contributing to their lowest emission bands
~730 nm] can be observed. To understand how the
interchromophoric interactions impact their spectral evolution,
we have computed the electronic transitions via TD-DFT
method (CAM-B3LYP//LANL2DZ; see ESI section-S5) on their
corresponding DFT-optimized structures shown in Figs. 2 and SI-
6
. Computed electronic transitions for NU-902(H2) and
TCPP(H2) are summarized in Fig. 3c and represented with
oscillator strengths and extinction spectra. These data clearly
suggest sizable red shifted transitions for the dimer in both the
Q and Soret band regions, and highlight appearance of new
Next, we examined how these structural features critically
impact the evolution of their spectroscopic signatures. Fig. 3
highlights the steady state emission and excitation spectra
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transitions due to exciton splitting. Frontier molecular orbitals insight, we examined wavelength dependent dynamics of the
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of these dimers indeed show a sizable electron density shared emissive states with time-resolved emiDssOioI:n10s.1p0e39c/tCra6.CCF0o7r34th3Ce
by the two porphyrin units (see Fig. SI-7). The zinc(II) metallated free-base MOF samples, the emissive state decays to both the
MOFs, however, show subtle difference in that the degree of v₀ and v₁ vibrational levels of the ground state are followed with
redshift relative to the free TCPPOMe(Zn) is more pronounced identical time constants (Figs 4b, SI-10) indicating that
and the spectral evolutions for MOF-525(Zn) and NU-902(Zn) interchromophoric interactions within the constrained
are more similar (Fig. SI-8) than that observed for the free-base conformations provided by the frameworks do not alter the
samples (Fig. 3a). Such spectral evolution is plausible due to a transition probability to both the vibrational levels of the
better degree of ring planarity and higher symmetry of ground state. Thus the evolution of their time-resolved
macrocycles resulting in better excitonic coupling that the free- emission spectra mirrors their steady-state spectra (Figs. 4b,
base variant.
3a). However, for NU-902(Zn) and MOF-525(Zn), the dynamics
To gain insight into the effect of these spectral features on of the emissive state radiative decay to the v₀ and v₁ vibrational
their corresponding excited state properties, we studied levels of the ground state are different (Fig. SI-10). With a faster
transient fluorescence decay along with the evolution of time- emissive decay to the lower energy v₀ vibrational level of the
resolved fluorescence spectra for both the free base and zinc- ground state, the evolution of time-resolved emission spectra
metallated MOF samples. Summarized in Fig. 4 (and Figs. SI-10, are non-identical to their corresponding steady state spectra
11) are the gas phase transient fluorescence decay profiles as
(Figs. 4d, SI-8, 11). This observation can be rationalized based
well as time-resolved emission spectra collected under argon. on the fact that an enhanced chromophore symmetry within
In general, the fluorescence lifetime of the MOF samples are these constrained framework geometries may have facilitated
significantly shorter compared to the corresponding free linkers a better Franck-Condon overlap between two v₀ vibrational
in solution and follow a trend with the relative redshifts found levels of emissive excited and ground electronic states. This
in their steady-state emission spectra (Figs 3
compared to a free TCPPOMe(H2) linker (
fluorescence lifetimes for MOF-525(H2) and NU-902(H2) were the free TCPP(Zn) (Figs
found to be ~5.6 and 4.6 ns (considering only the major To verify if these spectroscopic features are indeed
,
SI-8). For example, assertion would be associated with manifestation of an

= 9 ns), the increased Q₀ ₀ intensity for the MOF sample relative to that of
,
.
SI-8a, SI-13c)
component). We attribute these shorter lifetimes to their manifested by the chromophore cofacial dimers lined at the
diminished electronic transition energy-gaps (S₁-S₀)12 stemming closest proximity, we spectroscopically studied PCN-222(M)
from the interchromophoric interaction: NU-902(H2) has [M=H2, Zn(II); csq net with identical molecular formula as that
stronger excitonic coupling than MOF-525(H2) to render a low of NU-902(H2)].11b As shown in Fig. SI-12, PCN-222 consists of
Q0,1 transition, which, in turn, shorten the excited state lifetime. identical TCPP(H2) linker and Zr6 node forming triangular and
hexagonal channels. Any two of the three TCPP linkers within
the triangular channel possess spatial orientation identical to
that present in NU-902 (Figs. 2, SI-12). Both steady state and
transient emissive spectroscopic features for PCN-222 were
found to be essentially identical to the NU-902 samples (Fig. SI-
13).
With these spectroscopic data in hand, we proceeded to
study the efficiency of singlet state energy transfer processes
within some of these MOF samples via the amplified emissive
quenching experiments. We installed ferrocene (Fc) -based Fc-
COO
established solvent assisted ligand incorporation (SALI; see ESI
section S7) method at the Zr
-oxo node via a robust ZrIV‒
⁻ quencher, at various Fc/por doping ratio, using a well-
₆
carboxylate linkage.13 Analysis of the emission quenching data
(ESI section S7) as a function of Fc/por ratio indicates that the
number of chromophores the exciton visits within its lifetime
are 5 for NU-902(H2) and 4 for NU-902(Zn) with corresponding
absolute displacement estimated to be 2.5 and 2 linkers
respectively. The number of hops made by the exciton is ~6 for
NU-902(H2) and 4 for NU-902(Zn) with estimated hopping times
of 4630/6 = 770 ps and 410/4 = 100 ps, respectively.
Furthermore, considering the saturation emission intensity at
Fig. 4. Transient emission decay profile for (a) free-base and (c) zinc(II)-metallated
NU-902 and MOF-525. Time-resolved emission spectra for (b) MOF-525(H2) and
(d) NU-902(Zn).
The fluorescence lifetime for the corresponding zinc(II)
metallated MOF samples follow an identical trend as
TCPPOMe(Zn)>MOF-525(Zn)>NU-902(Zn); the difference of
emissive life-time between a zinc-metallated MOF sample and
the free TCPP(Zn) linker is discernibly larger relative to that of a
corresponding free-base system. We attribute this to a better
chromophoric interaction in the metallated samples as seen for
their steady-state emission spectra (Fig. SI-8). To gain deeper
0.37 and 0.45 of Fc-COO
estimated electron transfer rate from Fc to porphyrin to be at
least ~0.4 10⁹ s⁻¹ for NU-902(H2) and ~4 10⁹ s⁻¹ for NU-
⁻ doping level (ESI section S7), we
⨯
⨯
902(Zn). These metrics are in good agreement with the related
5,15-dipyridyl(porphinato)zinc(II) based MOF.8f The transient
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4. J. Yang, M.-C. Yoon, H. Yoo, P. Kim and D. Kim, Chem. Soc. Rev.,
emission decay profiles of the Fc-COO⁻ doped NU-902(H2) and
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2012, 41, 4808-4826.
DOI: 10.1039/C6CC07343C
NU-902(Zn) samples (Fig. 5) also correlate that a better exciton
migration efficiency in NU-902(H2), possibly due to a stronger
oscillator strength of the Q band and a better overlap between
the absorption and emission spectra compared to the
metallated MOFs (Fig. SI-14).
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Fig. 5. Transient emission decay profile for (a) free-base and (b) zinc(II) metallated
MOFs NU-902 at various Fc/por doping ratio.
In summary, with the help of computational and
experimental data, we found that the singlet state emissive
spectral features of MOFs are inherently different and a
function of their underlying framework topology. In these solid-
state samples, framework topology dictates the inter-
chromophoric orientation and interactions. Stronger
interchromophoric interaction leads to more red shifted spectra
(smaller S₁-S₀ energy gap) with faster emissive state radiative
decay. Ability to modulate the corresponding emissive state
lifetimes in LH model systems should enable to control the
number of exciton hopping and effective exciton displacement.
Even with identical building blocks, chromophore ring planarity
may vary due to the network symmetry imposed by the
underlying frameworks to manifest modified emission spectra.
Furthermore, for a given topological network, framework
consisting of less symmetric linker manifests more exciton
hopping. The findings reported here thus underscore that
topological control over the photophysical properties of MOFs
can play a critical role in processes relevant for light harvesting
utility.
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We gratefully acknowledge the support from SIU Carbondale
start-up funds and the State of Illinois. PD acknowledges the
Ralph E Powe Jr. Faculty Enhancement Award, Oak Ridge
Associate University, and SNW acknowledges NSF-REU
fellowship. We acknowledge Prof. Qingfeng Ge, SIUC for
providing access to the computational facility.
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Topological control over the photophysical properties of MOFs via
modular interchromophoric electronic interactions
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