Rational design of a high-efficiency, multivariate metal-organic framework phosphor for white led bulbs

Article abstract of DOI:10.1039/c9sc05721h

Developing rare-earth element (REE) free yellow phosphors that can be excited by 455 nm blue light will help to decrease the environmental impact of manufacturing energy efficient white light-emitting diodes (WLEDs), decrease their cost of production, and accelerate their adoption across the globe. Luminescent metal-organic frameworks (LMOFs) demonstrate strong potential for use as phosphor materials and have been investigated intensively in recent years. However, the majority are not suitable for the current WLED technology due to their lack of blue excitability. Therefore, designing highly efficient blue-excitable, yellow-emitting, REE free LMOFs is much needed. With an internal quantum yield of 76% at 455 nm excitation, LMOF-231 is the most efficient blue-excitable yellow-emitting LMOF phosphor reported to date. Spectroscopic studies suggest that this quantum yield could be further improved by narrowing the material's bandgap. Based on this information and guided by DFT calculations, we apply a ligand substitution strategy to produce a semi-fluorinated analogue of LMOF-231, LMOF-305. With an internal quantum yield of 88% (λ_em = 550 nm) under 455 nm excitation, this LMOF sets a new record for luminescent efficiency in yellow-emitting, blue-excitable, REE free LMOF phosphors. Temperature-dependent and polarized photoluminescence (PL) studies have provided insight on the mechanism of emission and origin of the significant PL enhancement.

Full text of DOI:10.1039/c9sc05721h

View Article Online
View Journal
Chemical
Science
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: W. P. Lustig, Z.
Shen, S. Teat, N. Javed, E. Velasco, D. O'Carroll and J. Li, Chem. Sci., 2020, DOI: 10.1039/C9SC05721H.
Volume
Number
9
1
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
7
January 2018
Pages 1-268
Chemical
Science
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
rsc.li/chemical-science
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2041-6539
EDGE ARTICLE
Xinjing Tang et al.
Caged circular siRNAs for photomodulation of gene
expression in cells and mice
rsc.li/chemical-science

Page 1 of 18
Chemical Science
View Article Online
DOI: 10.1039/C9SC05721H
Rational Design of a High-Efficiency, Multivariate Metal-
Organic Framework Phosphor for White LED Bulbs
William P. Lustig,a Zeqing Shen,a Simon J. Teat,b Nasir Javed,a Ever Velasco,a
Deirdre M. O’Carroll,a and Jing Li*,a
a Chemistry & Chemical Biology, Rutgers University. 123 Bevier Rd, Piscataway, NJ 08854
b Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
* Email: jingli@rutgers.edu
Abstract
Developing rare-earth element (REE) free yellow phosphors that can be excited by 455 nm blue light will
help to decrease the environmental impact of manufacturing energy efficient white light-emitting diodes
(WLEDs), decrease their cost of production, and accelerate their adoption across the globe. Luminescent
metal-organic frameworks (LMOFs) demonstrate strong potential for use as phosphor materials and have
been investigated intensively in recent years. However, the majority are not suitable for the current WLED
technology due to their lack of blue excitability. Therefore, designing highly efficient blue-excitable,
yellow-emitting, REE free LMOFs is much needed. With an internal quantum yield of 76% at 455 nm
excitation, LMOF-231 is the most efficient blue-excitable yellow-emitting LMOF phosphor reported to
date. Spectroscopic studies suggest that this quantum yield could be further improved by narrowing the
material’s bandgap. Based on this information and guided by DFT calculations, we apply a ligand
substitution strategy to produce a semi-fluorinated analogue of LMOF-231, LMOF-305. With an internal
quantum yield of 88% (em = 550 nm) under 455 nm excitation, this LMOF sets a new record for
luminescent efficiency in yellow-emitting, blue-excitable, REE free LMOF phosphors. Temperature-
dependent and polarized photoluminescence (PL) studies have provided insight on the mechanism of
emission and origin of the significant PL enhancement.
1. Introduction
White LED (WLED) bulbs are a cost- and energy-efficient solid state lighting (SSL) technology that make up
an increasing share of the global lighting market. The US Department of Energy estimates that WLEDs will
comprise 84% of the US lighting market by 2030, reducing light energy consumption by 261 terawatt-
hours per year, or 40% of the annual lighting energy consumption, which in turn accounts for
approximately 6% of the total US energy consumption.1-2 Globally, lighting accounts for 15% of all energy
used, and while government regulations will likely phase out inefficient incandescent lighting in some
areas over the next several years, WLED bulbs must continue to compete with cheaper incandescent bulbs
in much of the world.3 Decreasing the cost of WLED bulbs will speed their global adoption, resulting in
significant energy savings. In addition, switching to phosphor materials that are more environmentally
benign is equally important. One possible method to achieve these goals is through the development of
low-cost and rare-earth-element (REE) free phosphors. Currently, the most cost-effective WLED design is
comprised of a blue-emitting LED chip, coated with a blue-excitable REE based yellow phosphor or
1

Chemical Science
Page 2 of 18
View Article Online
DOI: 10.1039/C9SC05721H
multicomponent phosphors; the combination of blue light from the chip and yellow light from the
phosphor coating produces white light. This is known as a phosphor-converted WLED, or PC-WLED. The
yellow phosphor used in commercial devices is mainly YAG:Ce—a cerium-doped yttrium aluminum garnet.
However, cost and supply risks for yttrium and cerium may continue to rise due to the high and increasing
demand of these REEs in many high-tech devices, and mining the REEs is environmentally very damaging.4-
5 Developing new, REE free phosphor materials can both provide new cost flexibility to WLED bulbs and
reduce the environmentally hazardous mining operations.
Metal-organic frameworks (MOFs) are crystalline materials composed of metal ions linked into a
framework by organic ligands with multiple binding sites. These framework structures are generally
porous and possess highly tunable characteristics, and the properties of a given MOF depend not only on
the properties of the metal and ligand molecules used to construct it, but also on the connectivity,
topology, and interaction between framework elements that comprise the structure. Due to this
tunability, MOFs have been explored for a wide variety of applications, including gas storage and
separation, catalysis, medical imaging, drug delivery, sensing, toxin adsorption, and energy efficient
lighting.6-17
Luminescent metal-organic frameworks (LMOFs) are one of the most promising candidate materials as
new phosphors. Their luminescence can arise from a variety of mechanisms and interactions within the
structure. Emission can be centered on ligand molecules, metal ions, occur via charge or energy transfer
between different ligands or between ligands and metal ions, and involve guest molecules located within
the porous framework.18 These various mechanisms provide multiple ways to influence luminescence in
MOFs,19 and are part of the reason why LMOFs are such attractive phosphors. One effective strategy for
designing an LMOF phosphor is through a chromophoric ligand approach, in which an organic
chromophore with desirable emission properties is converted into a ligand molecule which can be built
into a MOF.20 The inclusion of a ligand into a rigid framework can often improve quantum yield through
immobilization/rigidification of the chromophore, which reduces or eliminates vibrational and rotational
pathways leading to nonradiative exciton recombination.21-22
A good example of the effectiveness of this strategy is LMOF-231, or Zn2(tcbpe) (H4tcbpe = 1,1,2,2-
tetrakis(4-(4-carboxy-phenyl)phenyl)ethene).21 The blue-excitable, yellow-emitting chromophore ligand
H4tcbpe was designed by starting from the known AIE-active tetraphenylethene (tpe) core and using DFT
calculations to as a screening method to identify molecules with the appropriate electronic structure for
yellow emission. LMOF-231 boasts an impressive quantum yield of 76% under 455 nm blue excitation.
However, despite being the highest quantum yield reported for an LMOF phosphor with properties
appropriate for use in a PC-WLED, the commercial phosphor YAG:Ce has a higher quantum yield of 96%
at the same excitation energy. And despite being reported in 2015, no further performance improvements
have been made in LMOF phosphors for PC-WLEDs. More significant spectroscopic investigations of
energy transfer and other optoelectronic processes in MOFs may provide the opportunity to deepen our
understanding of luminescence mechanisms in these materials, and provide opportunities to further
enhance performance.23-25
In this work, photoluminescence (PL) spectroscopy is used to probe the luminescence mechanism in
LMOF-231, and DFT calculations are used to identify a secondary ligand with the appropriate electronic
structure to improve the LMOF’s photoluminescence quantum yield (PLQY). Based on the information
from these spectroscopic and theoretical studies, a ligand substitution strategy is used to incorporate this
2

Page 3 of 18
Chemical Science
View Article Online
DOI: 10.1039/C9SC05721H
secondary ligand into LMOF-231, creating LMOF-305, which sets a new record for LMOF based PC-WLED
yellow phosphors with an internal quantum yield (IQY) of 88% under 455 nm excitation and an ideal
emission peak of 550 nm, and polarized PL spectroscopy is used to validate the emission mechanism.
2. Experimental
2.1. Materials and Synthesis
All materials were used as received from Sigma Aldrich. The chromophoric ligands H4tcbpe and H4tcbpe-
F were synthesized using a previously reported method with some modifications (Scheme 1), as was
LMOF-231.21, 26
Scheme 1. Synthesis of H4tcbpe (R = H) and H4tcbpe-F (R = F).
LMOF-305 was synthesized solvothermally. H4tcbpe (0.0305 g, 0.0375 mmol), H4tcbpe-F (0.0332 g, 0.0375
mmol), and Zn(NO3)2·6H2O (0.149 g, 0.400 mmol) were dissolved in a glass vial with dimethylacetamide
(DMA, 2 mL). The resulting solution was heated in a reaction oven at 120 ° C for 24 hours, after which the
product was collected via filtration as large, slightly yellow translucent crystals. Solvent exchange was
performed by immersion in approximately 15 mL ethyl acetate for 24 hours, with the solvent being
refreshed every two hours for the first eight hours, after which the sample was heated under vacuum at
50 ° C overnight to give the outgassed product with approximately 58 % yield with respect to H4tcbpe.
2.2 Single crystal structure determination.
The selected crystal was mounted on MiTeGen® loops in Paratone oil on a Bruker D8 diffractometer
equipped with a PHOTON100 CMOS detector and Oxford Cryosystems Cryostream 800 plus, on Beamline
11.3.1 of the Advanced Light Source at LBNL. A sphere of data were collected at 100K using Bruker APEX3
software27 in shutterless mode with ω rotations at fixed φ values at λ= 0.7749Å, from a channel cut Silicon
[111] monochromator. The intensity data were integrated and corrections applied with SAINT v8.34a,28
absorption and other corrections were made using TWINABS 2012/1.29 Dispersion corrections appropriate
for this wavelength were calculated using the Brennan method in XDIP with in WinGX.30 The structures
3

Chemical Science
Page 4 of 18
View Article Online
DOI: 10.1039/C9SC05721H
were solved with a dual space method with SHELXT 2014/4 and refined using SHELXL 2014/7.31 Once the
refinement had converged, SQUEEZE was used to mask the electron density in the pores.32
2.3 Physical characterizations. A Rigaku Ultima IV diffractometer was used to collect all powder X-ray
diffraction (PXRD) data. Data collection was performed at room temperature using Cu Kα radiation (λ =
1.5406 Å), scanning across a 2θ range from 3° to 35° with a scan speed of 2° 2θ/min and a step size of
0.2°. A TA Instruments Q5000 was used to perform thermogravimetric analyses (TGA) of all samples.
Samples were loaded into a Pt pan and heated under constant N2 flow (20 mL/min) from 30 °C to 600 °C,
with the temperature increasing at a constant rate of 10 °C/min. SEM images were obtained using a Zeiss
Sigma Field Emission SEM with Oxford INCA PentaFETx3 EDS system.
2.4 Photoluminescence characterizations. Steady-state PL emission and excitation spectra were collected
from dry powder samples prepared as described in using a Varian Cary Eclipse spectrophotometer at room
temperature. Diffuse reflectance data were collected at room temperature using a Shimadzu UV-3600
spectrophotometer. Internal quantum yield was measured at room temperature for dry powder samples
with a Hamamatsu C9220-03 spectrophotometer, using a 150 W Xenon monochromatic light source and
integrating sphere. Polarized photoluminescence measurements were carried out in reflection mode of
an inverted fluorescence microscope (Zeiss, AxioVert), using a 470 nm ( 20 nm) excitation light and a 515
nm long-pass filter in the collection path. The excitation and emission passed thought the same
microscope objective and a dichroic mirror was used to separate the excitation light from the emission
light. Single crystals of LMOF-305 were positioned with their c-axis perpendicular to the excitation and
collection directions. Polarizers were added to the excitation and collection paths to analyze the
polarization of the emitted light from the crystal. Polarization bias in the equipment was quantified using
an amorphous conjugated polymer sample to give an instrument correction factor. Subsequently, the
polarized PL spectra from LMOF-305 crystals were corrected for polarization bias in the measurement
using the instrument correction factor.
2.5 Lifetime measurements. Temperature-dependent photoluminescence lifetime measurements were
carried out in the solid state using powder samples that had been pressed into pellets on a cryostat (Cryo
Industries of America, Inc., Model 6NSVT Variable Temperature Optical Top-Loading Nitrogen Cryostat)
and temperature controller (Cryo-con 32B Temperature Controller). A pulsed laser excitation wavelength
of 440 nm was employed with a laser repetition rate of 10kHz or 20kHz. The PL emission was collected
from the sample by a convex lens and directed through a 550 ± 44 nm band-pass filter to an APD detector
for luminescence decay measurements. The PL lifetime decay data were recorded using the time-
correlated single photon counting method via a TimeHarp 260 Nano (PicoQuant) with down to 1 ns
resolution. The PL lifetimes of the samples at different temperatures were extracted by fitting the PL decay
curves with second-order exponential decay functions.
2.6 Density functional theory (DFT) calculations. DFT calculations were performed on Gaussian 09, using
the B3LYP3 hybrid functional and DGDZVP basis set.33-38 The geometries of all ligand molecules were
optimized, with a frequency calculation performed after geometry optimization to confirm that the
calculations resulted in a true minimum.
3. Results and Discussion
4

Page 5 of 18
Chemical Science
View Article Online
DOI: 10.1039/C9SC05721H
3.1 Structure and luminescence mechanism of LMOF-231
LMOF-231 or Zn2(tcbpe) is composed of a zinc-carboxylate chain forming an infinite secondary building
unit running in the c direction, with each tcbpe4- ligand linking four of these chains linked together. The
Zn2+ ions display a distorted tetrahedral geometry, with each ion bound to oxygen from four different
ligand molecules, and the ligand molecules organized into close-packed columns running in the c direction
(Figure 1).
Within each ligand, the dihedral angles of the four phenyl rings connected to the ligand’s central ethenyl
moiety vary. Two phenyl rings bonded to one of the ethenyl carbons having dihedral angles of 42.2°
relative to the central ethene, while the two phenyl rings bonded to the other ethenyl carbon have
dihedral angles of 58.7° (Figure 1). Each ligand is symmetrically related to its neighbors above and below
it in the column through an inversion center; this results in the neighboring phenyl rings on different
ligands being arranged in an edge-face-edge fashion, with close H-H interactions (atom-atom distances of
approximately 2.4 Å) preventing significant rotation of the phenyl rings in the structure (Figure 1).
Additionally, DFT calculations indicated that the lowest energy configuration of a single ligand molecule
has ethene-phenyl dihedral angles of 49.4°. The observed displacement from that value supports the
identification of rigidifying H-H interactions.
The average layer spacing between ligands within these stacks, determined by simplifying the tcbpe ligand
into a 2D rectangular surface with its corners defined by the four carboxylate carbons and measuring the
distance separating these surfaces, is 5.4 Å. And since the ligands are arranged in a tilted fashion, the
closest centroid-centroid distance between phenyl rings in neighboring ligands is 5.93 Å, which is distant
enough to prevent significant pi-pi interactions that could lead to quenching.39.
5

Chemical Science
Page 6 of 18
View Article Online
DOI: 10.1039/C9SC05721H
a
b
c
d
e
f
Figure 1. a) A single tcbpe molecule coordinated to eight Zn2+ ions, with the pink-colored phenyl rings having a
dihedral angle of 42.2° with respect to the central ethene, and the blue colored phenyl rings having dihedral angles
of 58.7° with respect to the central ethene. b) A segment of LMOF-231’s infinite secondary building unit, which runs
parallel to the c axis. c) A section of LMOF-231 viewed along the b axis, showing a single column of tcbpe ligands
linking four zinc-carboxylate chains parallel to the c axis. Pink-colored phenyl rings indicate an ethene-phenyl
dihedral angle of 42.2°, while blue-colored phenyl rings indicate an ethene-phenyl dihedral angle of 58.7°. d) The
complete structure of LMOF-231 viewed along the c axis. e) The structure of LMOF-231 viewed along the b axis using
a space-filling model to illustrate the tight packing of the ligand molecules. f) The central tetraphenylethene cores
from three neighboring tcbpe ligands within a column, illustrating the edge-face-edge arrangement, with green lines
showing close H-H interactions. Pink-colored phenyl rings indicate an ethene-phenyl dihedral angle of 42.2°, while
blue-colored phenyl rings indicate an ethene-phenyl dihedral angle of 58.7°. Hydrogen have been omitted from
figures 1a, 1c, and 1d for clarity (Zn = blue, O = red, C = grey, H = white).
6

Page 7 of 18
Chemical Science
View Article Online
DOI: 10.1039/C9SC05721H
LMOF-231 emits yellow light at 550 nm under both blue and UV excitation, with an IQY of 96% and 76%
under 420 and 455 nm excitation, respectively.40 Given the scale of Stokes shift, it is apparent that the
absorbance of both 420 and 455 nm photons excites an electron from the ground state to a higher energy
state than the state involved in emission, after which the electron rapidly relaxes to the emissive state. As
the emission energy (Figure S2) is independent of the excitation wavelength, it is clear that electrons
excited by both 420 nm and 455 nm relax to the same state before emitting (Figure 2).
The only clear difference between excitation at 420 nm and excitation at 455 nm is the energy level of the
orbital that the excited electron resides immediately after absorbance of the photon, which is higher for
the 420 nm photon. However, the fact that absorbance of the 420 nm photon resulted in emission with a
higher quantum yield suggests that the “higher energy pathway” through which it decays is more efficient
than the “lower energy pathway” available to the 455 nm photon (Figure 2).
There are slight differences between the photoluminescent lifetimes of LMOF-231 under 380 nm (UV) and
440 nm (blue) excitation, supporting the existence of these two pathways (Table 1). In both cases, the
average amplitude weighted lifetime for LMOF-231 is approximately 4.14 ns under UV excitation and 3.81
ns under blue excitation—with the total decay being the sum of two processes (τ1 and τ2). However,
under UV excitation, the faster τ1 process is more significant, while the slower τ2 dominates under blue
excitation. Additionally, both τ1 and τ2 are approximately 1 ns slower under UV excitation than under
blue excitation.
Table 1. Room temperature excited state lifetime data for LMOF-231 under 440 nm excitation.
Average
Excitation
amplitude
weighted τ
4.14 ns
τ1 (ns)
τ2 (ns)
UV
Blue
3.02 (56.3%)
1.98 (35.6%)
5.59 (43.7%)
4.80 (64.4%)
3.81 ns
This suggests that the quantum yield of LMOF-231 under 455 nm excitation could potentially be improved
by altering the material’s electronic structure such that absorbance of a 455 nm photon activates the
“higher energy pathway”, mimicking absorbance of a 420 nm photon. One possible way to accomplish
this is through an adapted bandgap modulation approach,41 in which a functionalized tcbpe ligand with
an offset HOMO-LUMO energy gap relative to the non-functionalized ligand is introduced into the LMOF
as a secondary ligand to create a new dual-ligand MOF. This could achieve the desired change in one of
two ways. If the ligand with higher-lying HOMO-LUMO energy levels is excited by a 455 nm photon, that
excited electron could be transferred to the ligand with the lower-lying HOMO-LUMO energy levels at a
higher energy state than was possible in the original absorbing ligand (Figure 2c). Alternatively,
absorbance of a photon may directly excite an electron between the primary and secondary ligands, with
the offset HOMO-LUMO gaps of the two permitting the 455 nm photon to directly excite the electron into
the “higher energy pathway” (Figure 2d). As the hole will likely remain located in the higher-lying HOMO
of the originally excited ligand, emission would need to occur between the two ligands. Experimentally,
these two mechanisms can be distinguished by their optical bandgaps; for the first mechanism, the optical
bandgap will be identical to native LMOF-231, while in the second mechanism, a decreased optical
bandgap would be observed for the dual-ligand MOF. In either case, this interligand emission (Figure 2c
7

Chemical Science
Page 8 of 18
View Article Online
DOI: 10.1039/C9SC05721H
& 2d) would likely occur simultaneously with intraligand emission from both the primary and secondary
ligands (Figure 2b).
a
b
c
d
Figure 2. (a) Schematic illustrating a possible fluorescence mechanism for the more efficient “higher-energy
pathway” in LMOF-231 following absorbance of a 420 nm photon. (b) Schematic illustrating a possible fluorescence
mechanism for the less efficient “lower-energy pathway” in LMOF-231 following absorbance of a 455 nm photon.
(c) Schematic demonstrating a possible fluorescence mechanism in a dual-ligand MOF composed of both tcbpe and
a functionalized tcbpe with an offset HOMO-LUMO, in which absorbance on a 455 nm photon on one ligand is
followed by electron transfer to a neighboring functionalized tcbpe ligand with lower-lying HOMO/LUMO energy
levels. (d) Schematic demonstrating a possible fluorescence mechanism in a dual-ligand MOF composed of both
tcbpe and a functionalized tcbpe with an offset HOMO-LUMO, in which absorbance on a 455 nm photon results in
direct excitation from the non-functionalized ligand (tcbpe) to the functionalized ligand, injecting an excited electron
directly in to the “higher-energy pathway”.
However, for bandgap modulation with a functionalized tcbpe as a secondary ligand to be effective, the
luminescence process in the modulated LMOF must involve electron or energy transfer between the
primary and secondary ligands. Given the close spacing between neighboring tcbpe ligands within LMOF-
231 and the electron-rich aromatic core of the ligand, it was hypothesized that the luminescence process
may involve interactions between neighboring ligands. This suggests that bandgap modulation would be
an effective strategy, as long as the functionalized tcbpe ligand replace some of the non-functionalized
tcbpe ligands without disturbing the overall framework structure.
3.2 Development of secondary ligand
Density Functional Theory (DFT) calculations were used as a screening method to identify the
functionalized tcbpe-analogue with the appropriate electronic structure, and the version of tcbpe
fluorinated at the ortho position relative to the carboxylate was found to meet the design requirements.
Calculations indicated that this ligand, 1,1,2,2-tetrakis(4-(3-fluoro-4-carboxy-phenyl)phenyl)ethane or
H4tcbpe-F, possessed HOMO and LUMO energy levels that are approximately 0.23 and 0.22 eV lower than
those of H4tcbpe, respectively. This is ideal, as the offset energy levels in combination with a multi-ligand
excitation process should allow the ligand to function as a bandgap modulator for excitation, while the
nearly identical HOMO-LUMO energy gap should limit changes to the emission wavelength.
8

Page 9 of 18
Chemical Science
View Article Online
DOI: 10.1039/C9SC05721H
Following synthesis, spectroscopic study indicated that the electronic properties of the fluorine-
functionalized ligand were consistent with the relevant calculations. Diffuse reflectance data indicated
that the two ligands had nearly identical optical bandgaps, and their excitation and emission spectra were
nearly identical as well (Figure 3).
a
b
Figure 3. a) Absorbance-analogue Kubelka-Munk function for H4tcbpe and H4tcbpe-F. b) Excitation (dotted line)
spectra for H4tcbpe and H4tcbpe-F at 550 nm emission, and emission spectra (solid line) for H4tcbpe and H4tcbpe-F
under 455 nm excitation.
3.3 Structure of LMOF-305
The multivariate LMOF-305 or Zn2(tcbpe)0.8(tcbpe-F)0.2 was synthesized solvothermally with a 1:1 molar
ratio for H4tcbpe and H4tcbpe-F. The resulting single crystals grew up to 2 mm long, in bloom-like bunches
of rectangular crystals. Both PXRD and single crystal analysis confirmed that inclusion of the tcbpe-F ligand
did not alter the crystal structure or phase of the LMOF. As shown in Table S1, LMOF-305 crystallizes in
the same space group as that of LMOF-231 with the same metal and ligand arrangement, except the
distribution of ligand, where 20% of the tcbpe in LMOF-231 (Figure 1) is replaced by tcbpe-F in LMOF-305.
To determine stability under storage conditions, PXRD and QY data was measured on a sample of LMOF-
305 that had been stored under air in ambient conditions for two months; the results are summarized in
Supporting Information section S3.
Fragments of multiple crystals were analyzed using SEM-EDS to ensure that the distribution of tcbpe-F
within the LMOF was homogenous (Figure 4). The elemental analysis data confirmed that, although the
synthetic ratio between H4tcbpe and H4tcbpe-F being 50:50, the resulting LMOF-305 crystal was
composed of approximately 80% H4tcbpe and 20% H4tcbpe-F. This was consistent with the single crystal
data, which yielded a 20% occupancy of F atoms at the relevant crystallographic site.
Despite the relatively low loading of the fluorinated ligand into the LMOF, the replacement of 20% of the
tcbpe ligands with tcbpe-F leads to a structure in which approximately 50% of the remaining non-
functionalized tcbpe ligands have one neighboring tcbpe-F ligand either above or below within the c-
oriented ligand stacks. This assumes that two tcbpe-F ligands rarely neighbor each other, which is
9

Chemical Science
Page 10 of 18
View Article Online
DOI: 10.1039/C9SC05721H
reasonable given the low functionalized ligand loading level and the fact that attempts to further increase
the percentage of H4tcbpe-F within the structure were not successful, which is likely due to steric crowding
around the zinc-carboxylate chain in the presence of multiple fluorinated ligands. This is consistent with
the thermal stability of LMOF-305 as measured by TGA, which was slightly reduced relative to LMOF-231
and would be expected in the event that the inclusion of the fluorine increased steric strain near the zinc-
carboxylate building unit (Figure 4c). TG analysis also indicated that less solvent was present within the
LMOF pore, which is consistent with the presence of fluorine slightly decreasing the available space
(Figure 4c).
a
b
c
10

Page 11 of 18
Chemical Science
View Article Online
DOI: 10.1039/C9SC05721H
Figure 4. a) PXRD patterns of LMOF-231 (red) and LMOF-305 (blue) overlaid with the simulated PXRD pattern of
LMOF-231 (black). b) SEM-EDS images of fragments from three different LMOF-305 crystals, mounted on graphite
tape, showing the distribution of C, Zn, N, O, and F atoms within the samples. c) Thermogravimetric (TG) curves for
LMOF-231 (red) and LMOF-305 (blue).
3.4 Optical properties of LMOF-305
Following the successful inclusion of H4tcbpe-F into the Zn2(tcbpe) framework at 20% occupancy to give
LMOF-305, the material’s optical bandgap decreased by approximately 0.3 eV based on diffuse reflectance
measurement, and the absorption intensity at 455 nm increased significantly (Figure 5a). The excitation
spectrum similarly redshifted to increase coverage at 455 nm, while changes to the emission spectrum
were minor, as desired (Figure 5b). Most importantly, the quantum yield of LMOF 305 under 455 nm
excitation increased to 88%, a 12% increase over that of LMOF-231 (76%) under the same excitation
energy.
Figure 1. (a) UV-Vis absorbance analogue Kubelka-Munk function derived from diffuse reflectance for LMOF-231
(red) and LMOF-305 (blue). (b) Excitation (dotted) and emission (solid) spectra for LMOF-231 (red), LMOF-305 (blue),
and the commercial phosphor YAG:Ce (black). Excitation spectra were monitored at 550 nm emission, and emission
spectra were collected under 455 nm excitation.
Given that the emission is almost the same for the two LMOFs yet the Stokes shift is much smaller for the
dual-ligand LMOF-305 (Figure 5b), the density of electronic states at energies near the lowest tcbpe LUMO
state must be enhanced either due to reduction in motion of the ligands, which is consistent with the
expected steric influence of tcbpe-F on the framework PBUs as discussed previously, or due to additional
tcbpe-F states that are very close in energy to the tcbpe LUMO.
In order to demonstrate the effect of tcbpe-F on the PL of LMOF-305, three additional samples were
prepared using tcbpe:tcbpe-F molar ratios of 85:15, 90:10, and 95:5, and their quantum yields were
measured. Additionally, a physical mixture consisting of 80% H4tcbpe ligand and 20% H4tcbpe-F ligand—
the ratio present in LMOF-305—was prepared and optically qualified. As the tcbpe-F content of the LMOF
samples increased, the quantum yield under 455 nm excitation also increased, and the physical ligand
11

Chemical Science
Page 12 of 18
View Article Online
DOI: 10.1039/C9SC05721H
mixture showed no improvement. These results, summarized in Table 2 below and in Figures S3 and S4,
support the argument that an interaction between tcbpe and tcbpe-F is responsible for LMOF-305’s
elevated quantum yield, and that the specific arrangement imposed by the LMOF framework is required
to enable that interaction.
Table 2. Quantum yield data for a physical mixture of 80:20 tcbpe:tcbpe-F, LMOF-305, and three LMOF samples
prepared with varying tcbpe-F amounts.
Quantum Yield
(455 nm ex.)
Sample
80:20 H4tcbpe:H4tcbpe-F
LMOF-305’ (5% H4tcbpe-F)
LMOF-305’’ (10% H4tcbpe-F)
LMOF-305’’’ (15% H4tcbpe-F)
LMOF-305 (20% H4tcbpe-F)
52%
57%
69%
83%
88%
To further probe the emission mechanism, temperature dependent photoluminescence lifetime data
were collected for the bulk ligands H4tcbpe and H4tcbpe-F, as well as LMOF-231 and LMOF-305, from 77
K to 295 K under 440 nm excitation. Room temperature lifetime data for all four samples is presented
below in Table 3, and the lifetime data at temperatures ranging from 77 K to 273 k, as well as the
luminescence decay profiles, are given in Supporting Information section S5.
Both LMOF samples possessed similar average amplitude weighted lifetimes of approximately 4 ns (3.81
ns for LMOF-231 and 3.98 ns for LMOF-305) at 273 K, with the total decay being the sum of two processes
(τ1 and τ2). LMOF-305 showed longer lifetimes for τ1 and τ2, with τ1 (3.05 ns) being 54 % longer than that
of LMOF-231 (1.98 ns) and τ2 (6.00 ns) being 25 % longer than that of LMOF-231 (4.80 ns). Despite this,
the average amplitude-weighted lifetimes were very similar because the longer τ2 lifetime was dominant
in LMOF-231, while the shorter τ1 lifetime was dominant in LMOF-305.
Importantly, the lifetime behavior displayed by LMOF-305 under 440 nm excitation is very similar to the
previously-discussed lifetime behavior of LMOF-231 under 380 nm excitation. This is strong evidence that
inclusion of the fluorinated ligand helped push emission towards the more efficient “higher-energy
pathway”. It should also be noted that the lifetime of LMOF-305 is independent of excitation wavelength
(Figure S8, Table S6), which further indicates that the luminescence mechanism for the material is
identical for both blue and UV photons, unlike LMOF-231.
A similar trend can also be observed when comparing the lifetime of H4tcbpe and H4tcbpe-F; both the τ1
and τ2 processes are slower at room temperature in H4tcbpe-F, but the relatively faster τ1 process plays
a larger role in the fluorinated ligand. This suggests that the H4tcbpe-F ligand plays a relatively large role
in the emission from LMOF-305, despite the fact that it is present at a much lower concentration than
H4tcbpe, and is convincing evidence that excitation energy moves to the H4tcbpe-F ligand before emission.
Furthermore, the fact that LMOF-305 has both a higher quantum yield than LMOF-231 and slower excited
state decay processes indicates that the inclusion of H4tcbpe-F within LMOF-231 weakens nonradiative
excited state recombination.
12

Page 13 of 18
Chemical Science
View Article Online
DOI: 10.1039/C9SC05721H
Table 3. Room temperature excited state lifetime data for LMOF-231, LMOF-305, and the two ligands under 440 nm
excitation
Average
Sample
amplitude
weighted τ
3.44 ns
3.57 ns
3.80 ns
τ1 (ns)
τ2 (ns)
H4tcbpe
2.11 (42.2%)
2.34 (50.5%)
1.98 (35.6%)
3.05 (66.2%)
4.49 (55.8%)
4.83 (49.5%)
4.80 (64.4%)
6.00 (33.8%)
H4tcbpe-F
LMOF-231
LMOF-305
4.05 ns
As discussed in section 3.1, emission from LMOF-305 is expected to result from a combination of
intermolecular processes (electron transfer between tcbpe and tcbpe-F), and intramolecular processes
located in a single ligand molecule. Polarized photoluminescence measurements were carried out on
LMOF-305 single crystals to investigate the relationship between these processes.
When excitation light was polarized parallel to the c-axis of the crystal, the PL emission intensity was found
to be generally lower in intensity compared to the PL emission when the excitation light was polarized
perpendicularly to the c-axis (Figure 6a). This suggests that absorption is slightly stronger in the direction
perpendicular to the c-axis (i.e., almost parallel to the a-axis). This may be because the conjugated ligands
absorb more strongly when light is polarized parallel to the ligand chain which is oriented almost
perpendicular to the c-axis. In most cases, when the excitation light polarization was parallel to the c-axis,
the emission light was more strongly polarized in the perpendicular direction (Figure 6b). Therefore, the
dominant excitation and emission polarizations were orthogonal to one another, which indicates
electronic interaction between ligands (intermolecular). However, when excitation light was polarized
perpendicularly to the c-axis, the results were less straight-forward: in some cases the emission polarized
along the c-axis was strongest (i.e., the emission polarization was orthogonal to the excitation
polarization); in other cases the emission polarized perpendicular to the c-axis was stronger or was
completely depolarized (Figure S9). These results suggest that the emission originates from combinations
of both intermolecular (tcbpe to tcbpe-F) and intramolecular ligand excited states.
13

Chemical Science
Page 14 of 18
View Article Online
DOI: 10.1039/C9SC05721H
a
b
c
Figure 6. a) Non-polarized emission spectra from LMOF-305 under excitation polarized parallel with (black) and
perpendicular to (red) the crystallographic c axis. b) Polarized emission spectra from LMOF-305 oriented parallel
with (green) and perpendicular to (blue) the crystallographic c axis, produced under polarized excitation that was
oriented parallel to the crystallographic c axis, with a photograph of a LMOF-305 single crystal used in the polarized
photoluminescence experiment inset in the upper right. c) Diagram illustrating measurement geometry and crystal
orientation with the excitation source polarized parallel with (top) and perpendicular to (bottom) the
crystallographic c index, used to collect polarized emission spectra oriented parallel with (top) or perpendicular to
(bottom) the crystallographic c axis. As the excitation and emission polarization filters are independent, emission
spectra were also collected with the excitation and emission filters orthogonal to each other; in the interest of saving
space, these measurement geometries are not shown.
4. Conclusions
In order to develop LMOF-based phosphor materials capable of competing with the commercial yellow
phosphor YAG:Ce for applications in PC-WLEDs, it is necessary to be able to obtain a yellow-emitting LMOF
with quantum yield comparable to YAG:Ce under blue (455 nm) excitation. We have previously
synthesized LMOF-231, based on the chromophoric ligand H4tcbpe. With a quantum yield of 76% (ex =
455 nm), it became the highest performing yellow phosphor among all LMOFs reported to date. However,
further improvement was necessary to increase the material’s competitiveness. To address this,
spectroscopic measurements were used to assess the potential excitation mechanisms, and DFT
calculations were performed to help design a ligand that could be doped into the LMOF-231 structure and
14

Page 15 of 18
Chemical Science
View Article Online
DOI: 10.1039/C9SC05721H
function as a bandgap modulator. The ligand H4tcbpe-F was synthesized, and spectroscopic studies
indicated that it possessed the qualities predicted by DFT calculations; namely, reduced HOMO and LUMO
energies and a largely unchanged HOMO-LUMO energy gap. H4tcbpe-F was successfully doped into LMOF-
231 at 20% occupancy, and the resulting LMOF-305 showed increased blue light absorption and a
significantly higher quantum yield of 88.2% at 450 nm excitation, the highest value among all yellow-
emitting LMOF phosphors reported to date. In addition, we conducted an in-depth spectroscopic study
employing temperature dependent and polarized photoluminescence methods in order to investigate and
to gain a fundamental understanding of the PL mechanism and the origin for strongly enhanced quantum
yield of this LMOF. This work serves as an example of how understanding luminescence mechanisms can
guide the rational design of materials with greatly improved performance.
Acknowledgement
WPL, EV and JL acknowledge the financial support from the National Science Foundation (Grant No. DMR-
1507210). This research used resources of the Advanced Light Source, which is a DOE Office of Science
User Facility under contract no. DE-AC02-05CH11231.
15

Chemical Science
Page 16 of 18
View Article Online
DOI: 10.1039/C9SC05721H
6. References
1.
Energy Savings Forecast of Solid-State Lighting in General Illumination Applications. US
Department of Energy: 2014.
2.
3.
Annual Energy Outlook 2019. US Energy Information Administration: 2019.
Horowitz, N., LED Lighting Could Save Developing Countries $40 Billion/yr. Natural Resources
Defense Council, Inc. 2017.
4.
Charalampides, G.; Vatalis, K.; Karayannis, V.; Baklavaridis, A., Environmental Defects And
Economic Impact On Global Market Of Rare Earth Metals. IOP Conf. Ser.: Mater. Sci. Eng. 2016,
161, 012069.
5.
6.
7.
deꢀBoer, M. A.; Lammertsma, K., Scarcity of Rare Earth Elements. ChemSusChem 2013, 6 (11),
2045-2055.
Wang, H.; Li, J., Microporous Metal–Organic Frameworks for Adsorptive Separation of C5–C6
Alkane Isomers. Accounts of Chemical Research 2019, 52 (7), 1968-1978.
Bao, Z.; Chang, G.; Xing, H.; Krishna, R.; Ren, Q.; Chen, B., Potential of microporous metal–
organic frameworks for separation of hydrocarbon mixtures. Energy & Environmental Science
2016, 9 (12), 3612-3641.
8.
9.
Adil, K.; Belmabkhout, Y.; Pillai, R. S.; Cadiau, A.; Bhatt, P. M.; Assen, A. H.; Maurin, G.; Eddaoudi,
M., Gas/vapour separation using ultra-microporous metal–organic frameworks: insights into the
structure/separation relationship. Chemical Society Reviews 2017, 46 (11), 3402-3430.
Trickett, C. A.; Helal, A.; Al-Maythalony, B. A.; Yamani, Z. H.; Cordova, K. E.; Yaghi, O. M., The
chemistry of metal–organic frameworks for CO2 capture, regeneration and conversion. Nature
Reviews Materials 2017, 2 (8), 17045.
10.
11.
12.
Wang, C.; An, B.; Lin, W., Metal–Organic Frameworks in Solid–Gas Phase Catalysis. ACS Catalysis
2019, 9 (1), 130-146.
Cui, Y.; Li, B.; He, H.; Zhou, W.; Chen, B.; Qian, G., Metal–Organic Frameworks as Platforms for
Functional Materials. Accounts of Chemical Research 2016, 49 (3), 483-493.
He, C.; Liu, D.; Lin, W., Nanomedicine Applications of Hybrid Nanomaterials Built from Metal–
Ligand Coordination Bonds: Nanoscale Metal–Organic Frameworks and Nanoscale Coordination
Polymers. Chemical Reviews 2015, 115 (19), 11079-11108.
13.
14.
Zhang, Y.; Yuan, S.; Day, G.; Wang, X.; Yang, X.; Zhou, H.-C., Luminescent sensors based on
metal-organic frameworks. Coordination Chemistry Reviews 2018, 354, 28-45.
Hurlock, M. J.; Kan, Y.; Lécrivain, T.; Lapka, J.; Nash, K. L.; Zhang, Q., Molecular Association-
Induced Emission Shifts for E/Z Isomers and Selective Sensing of Nitroaromatic Explosives.
Crystal Growth & Design 2018, 18 (10), 6197-6203.
15.
16.
17.
18.
Li, J.; Wang, X.; Zhao, G.; Chen, C.; Chai, Z.; Alsaedi, A.; Hayat, T.; Wang, X., Metal–organic fra
mework-based materials: superior adsorbents for the capture of toxic and radioactive
metal ions. Chemical Society Reviews 2018, 47 (7), 2322-2356.
Bobbitt, N. S.; Mendonca, M. L.; Howarth, A. J.; Islamoglu, T.; Hupp, J. T.; Farha, O. K.; Snurr, R.
Q., Metal–organic frameworks for the removal of toxic industrial chemicals and chemical
warfare agents. Chemical Society Reviews 2017, 46 (11), 3357-3385.
Lustig, W. P.; Li, J., Luminescent metal–organic frameworks and coordination polymers as
alternative phosphors for energy efficient lighting devices. Coordination Chemistry Reviews
2018, 373, 116-147.
Lustig, W. P.; Mukherjee, S.; Rudd, N. D.; Desai, A. V.; Li, J.; Ghosh, S. K., Metal–organic
frameworks: functional luminescent and photonic materials for sensing applications. Chemical
Society Reviews 2017, 46 (11), 3242-3285.
16

Page 17 of 18
Chemical Science
View Article Online
DOI: 10.1039/C9SC05721H
19.
20.
21.
Cornelio, J.; Zhou, T.-Y.; Alkaş, A.; Telfer, S. G., Systematic Tuning of the Luminescence Output of
Multicomponent Metal–Organic Frameworks. Journal of the American Chemical Society 2018,
140 (45), 15470-15476.
Lustig, W. P.; Wang, F.; Teat, S. J.; Hu, Z.; Gong, Q.; Li, J., Chromophore-Based Luminescent
Metal–Organic Frameworks as Lighting Phosphors. Inorganic Chemistry 2016, 55 (15), 7250-
7256.
Hu, Z.; Huang, G.; Lustig, W. P.; Wang, F.; Wang, H.; Teat, S. J.; Banerjee, D.; Zhang, D.; Li, J.,
Achieving exceptionally high luminescence quantum efficiency by immobilizing an AIE molecular
chromophore into a metal–organic framework. Chemical Communications 2015, 51 (15), 3045-
3048.
22.
Wei, Z.; Gu, Z.-Y.; Arvapally, R. K.; Chen, Y.-P.; McDougald, R. N.; Ivy, J. F.; Yakovenko, A. A.; Feng,
D.; Omary, M. A.; Zhou, H.-C., Rigidifying Fluorescent Linkers by Metal–Organic Framework
Formation for Fluorescence Blue Shift and Quantum Yield Enhancement. Journal of the
American Chemical Society 2014, 136 (23), 8269-8276.
23.
24.
Hanna, L.; Lockard, J. V., From IR to x-rays: gaining molecular level insights on metal-organic
frameworks through spectroscopy. Journal of Physics: Condensed Matter 2019, 31 (48), 483001.
Dolgopolova, E. A.; Rice, A. M.; Smith, M. D.; Shustova, N. B., Photophysics, Dynamics, and
Energy Transfer in Rigid Mimics of GFP-based Systems. Inorganic Chemistry 2016, 55 (15), 7257-
7264.
25.
26.
Dolgopolova, E. A.; Rice, A. M.; Martin, C. R.; Shustova, N. B., Photochemistry and photophysics
of MOFs: steps towards MOF-based sensing enhancements. Chemical Society Reviews 2018, 47
(13), 4710-4728.
Wang, F.; Liu, W.; Teat, S. J.; Xu, F.; Wang, H.; Wang, X.; An, L.; Li, J., Chromophore-immobilized
luminescent metal–organic frameworks as potential lighting phosphors and chemical sensors.
Chemical Communications 2016, 52 (67), 10249-10252.
27.
28.
Bruker Apex3, Bruker Analytical X-ray Systems Inc.: Madison, WI, 2003.
Bruker SAINT: SAX Area-Detector Integration Program v7.60a; Bruker Analytical X-ray Systems,
Inc.: Madison, WI, 2010.
29.
30.
Blessing, R., An empirical correction for absorption anisotropy. Acta Crystallographica Section A
1995, 51 (1), 33-38.
Farrugia, L., WinGX suite for small-molecule single-crystal crystallography. Journal of Applied
Crystallography 1999, 32 (4), 837-838.
31.
32.
Sheldrick, G., A short history of SHELX. Acta Crystallographica Section A 2008, 64 (1), 112-122.
Spek, A. L., PLATON SQUEEZE: a tool for the calculation of the disordered solvent contribution to
the calculated structure factors. Acta Crystallogr C Struct Chem 2015, 71 (Pt 1), 9-18.
Becke, A. D., Density-functional exchange-energy approximation with correct asymptotic beh
avior. Physical Review A 1988, 38 (6), 3098-3100.
Lee, C.; Yang, W.; Parr, R. G., Development of the Colle-Salvetti correlation-energy formula into
a functional of the electron density. Physical Review B 1988, 37 (2), 785-789.
Frisch, M. J.; Trucks, G. W.; H. B. Schlegel; G. E. Scuseria; M. A. Robb; J. R. Cheeseman; G.
Scalmani; V. Barone; B. Mennucci; G. A. Petersson; H. Nakatsuji; M. Caricato; X. Li; H. P.
Hratchian; A. F. Izmaylov; J. Bloino; G. Zheng; J. L. Sonnenberg; M. Hada; M. Ehara; K. Toyota; R.
Fukuda; J. Hasegawa; M. Ishida; T. Nakajima; Y. Honda; O. Kitao; H. Nakai; T. Vreven; J. A.
Montgomery, J.; J. E. Peralta; F. Ogliaro; M. Bearpark; J. J. Heyd; E. Brothers; K. N. Kudin; V. N.
Staroverov; T. Keith; R. Kobayashi; J. Normand; K. Raghavachari; A. Rendell; J. C. Burant; S. S.
Iyengar; J. Tomasi; M. Cossi; N. Rega; J. M. Millam; M. Klene; J. E. Knox; J. B. Cross; V. Bakken; C.
Adamo; J. Jaramillo; R. Gomperts; R. E. Stratmann; O. Yazyev; A. J. Austin; R. Cammi; C. Pomelli;
J. W. Ochterski; R. L. Martin; K. Morokuma; V. G. Zakrzewski; G. A. Voth; P. Salvador; J. J.
33.
34.
35.
17

Chemical Science
Page 18 of 18
View Article Online
DOI: 10.1039/C9SC05721H
Dannenberg; S. Dapprich; A. D. Daniels; O. Farkas; J. B. Foresman; J. V. Ortiz; J. Cioslowski; Fox,
D. J. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford CT, 2013.
36.
Sosa, C.; Andzelm, J.; Elkin, B. C.; Wimmer, E.; Dobbs, K. D.; Dixon, D. A., A local density
functional study of the structure and vibrational frequencies of molecular transition-metal
compounds. The Journal of Physical Chemistry 1992, 96 (16), 6630-6636.
37.
38.
Becke, A. D., A new mixing of Hartree–Fock and local density-functional theories. The Journal of
Chemical Physics 1993, 98 (2), 1372-1377.
Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E., Optimization of Gaussian-type basis sets
for local spin density functional calculations. Part I. Boron through neon, optimization technique
and validation. Canadian Journal of Chemistry 1992, 70 (2), 560-571.
39.
40.
Huang, Y.; Xing, J.; Gong, Q.; Chen, L.-C.; Liu, G.; Yao, C.; Wang, Z.; Zhang, H.-L.; Chen, Z.; Zhang,
Q., Reducing aggregation caused quenching effect through co-assembly of PAH chromophores
and molecular barriers. Nature Communications 2019, 10 (1), 169.
Hu, Z.; Huang, G.; Lustig, W. P.; Wang, F.; Wang, H.; Teat, S. J.; Banerjee, D.; Zhang, D.; Li, J.,
Achieving exceptionally high luminescence quantum efficiency by immobilizing an AIE molecular
chromophore into a metal-organic framework. Chem. Commun. (Cambridge, U. K.) 2015, 51
(15), 3045-3048.
41.
Gong, Q.; Hu, Z.; Deibert, B. J.; Emge, T. J.; Teat, S. J.; Banerjee, D.; Mussman, B.; Rudd, N. D.; Li,
J., Solution Processable MOF Yellow Phosphor with Exceptionally High Quantum Efficiency. J.
Am. Chem. Soc. 2014, 136 (48), 16724-16727.
18