Thermally activated delayed fluorescence of a Zr-based metal-organic framework

Article abstract of DOI:10.1039/c7cc08595h

The first metal-organic framework exhibiting thermally activated delayed fluorescence (TADF) was developed. The zirconium-based framework (UiO-68-dpa) uses a newly designed linker composed of a terphenyl backbone, an electron-accepting carboxyl group, and an electron-donating diphenylamine and exhibits green TADF emission with a photoluminescence quantum yield of 30% and high thermal stability.

Full text of DOI:10.1039/c7cc08595h

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Thermally activated delayed fluorescence of a Zr-based metal-
organic framework
H. Mieno,a R. Kabe*a,b, M. D. Allendorf c and C. Adachi*a,b,d
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first metal-organic framework exhibiting thermally activated introducing luminescent organic molecules into MOFs as
delayed fluorescence (TADF) was developed. The zirconium-based linkers or guests.9 Such solid-state luminescent MOFs are
framework (UiO-68-dpa) uses a newly designed linker composed useful for bioapplications, including bioimaging10 and drug
of a terphenyl backbone, an electron-accepting carboxyl group, delivery11, photocatalysts12 and chemical sensors13. Although a
and an electron-donating diphenylamine and exhibits green TADF wide variety of luminescent MOFs have been demonstrated to
emission with a photoluminescence quantum yield of 30% and date and we previously showed that guest molecules within
high thermal stability.
MOF pores can exhibit TADF,7 a MOF exhibiting intrinsic TADF
has never been reported. MOFs exhibiting TADF could be an
avenue to achieving higher thermal and chemical stability than
in conventional TADF molecules and may offer new ways to
tune TADF properties, such as delayed lifetime and emission
spectrum, by varying the selection of metals and size of pores.
Here, we design a new linker with TADF properties for use
in MOFs. The linker, H2tpdc-dpa, has both electron-donating
and electron-accepting moieties with a large dihedral angle
between them. Based on this TADF linker, we developed a
zirconium- based MOF that is the first to exhibit intrinsic TADF
at room temperature.
Organic molecules exhibiting thermally activated delayed
fluorescence (TADF) are being actively developed for highly
efficient organic light-emitting diodes (OLEDs).1 Capable of
harvesting 100% of excitons, TADF molecules produce delayed
fluorescence when singlet excitons produced by the reverse
intersystem crossing of triplet excitons radiatively decay.
Delayed fluorescence is also useful for bioimaging applications
because the delayed emission can be temporally separated
from the excitation light.2,3 However, most organic emitters
needed to be dispersed into solvents or host molecules to
avoid concentration quenching of the emission. Furthermore,
the charge-transfer emission of TADF molecules is strongly
affected by the polarity of solvents or host molecules and
often quenched in polar solvents like water, complicating film
optimization and hindering biological applications.
Most TADF molecules consist of electron donor and
acceptor units separated by
a large dihedral angle to
effectively separate the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
of the molecule. The resulting small spatial overlap between
the HOMO and the LUMO leads to a small energy gap between
the lowest singlet excited state and the lowest triplet excited
state (ΔEST).
The high surface area, porosity, thermal stability, chemical
stability, and structural tunability of metal-organic frameworks
(MOFs) possibly offer a way to modify some of these effects.4
MOFs are hybrid porous materials composed of metal-clusters
and organic linkers. Because their rigid frameworks inhibit
nonradiative deactivation5,6 and prevent aggregation of
emitters,7,8 solid-state emission can be observed by
Organic linkers used in MOFs often contain a carboxyl unit
as the functional group that coordinates with the metal center.
Since a carboxyl group acts as an electron acceptor, TADF
emission might be obtained by adding an electron-donating
unit to the linker to induce the spatial separation of the HOMO
and LUMO. For example, the organic linker H2tpdc-NH2, which
was used in the MOF UiO-68-NH2, contains an electron-
accepting carboxylic acid and an electron-donating amino
group.14 The ΔEST of this linker was calculated to be as high as
0.66 eV using density functional theory (B3LYP/6-31G level).
This large value is the result of a small dihedral angle between
the donor and acceptor moieties (Fig. S1) and is too high to
enable the efficient conversion of triplet excitons to singlet
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excitons at room temperature. To obtain a smaller ΔEST, we
designed the novel linker H2tpdc-dpa, which consists of
carboxyl and diphenyl amino moieties. A large steric hindrance
between the donor and acceptor moieties induces
a
separation of HOMO and LUMO, leading to a moderately small
calculated ΔEST of 0.2 eV (Fig. 1).
Both H2tpdc-NH2 and H2tpdc-dpa linkers and their Zr-based
MOFs (UiO-68-NH2 and UiO-68-dpa, respectively) were
synthesized and characterized (Scheme S1). The powder X-ray
diffraction pattern of UiO-68-dpa reveals the crystallinity of
the MOF, and the diffraction peaks are similar to those of UiO-
68-NH2, which has two different-shaped cavities in the
structure (Figs. 1 and S2).14 The nitrogen isotherms of UiO-68-
dpa, with a step at P/P0 of 0.2, are also consistent with the
presence of two types of cavities (Fig. S3). The BET surface
area of UiO-68-dpa was calculated to be 1455 m2/g, which is
smaller than that of UiO-68-2CH3 (2470 m2/g)15 because the
diphenylamine of H2tpdc-dpa partially occupies the cavities.
Scanning electron microscopy images reveal an octahedral
morphology for UiO-68-dpa with a particle size of less than 10
μm (Fig. S4). Thermogravimetric analysis of UiO-68-dpa shows
no substantial weight loss up to 700 K even though H2tpdc-dpa
decomposes at around 650 K, indicating that UiO-68-dpa has
good thermal stability (Fig. S5). The small weight loss at 400 K
is due to residual dimethylformamide (DMF).
when doped in a solid-state film of polymethyl methacrylate
(PMMA), the emission peak maximum (Fig. S7) was red-shifted
from 461 nm to 500 nm by increasing the concentration from 2
wt% to 50 wt%, indicating that H2tpdc-dpa aggregates at
higher concentrations. We investigated the emission decay of
H2tpdc-dpa doped at different doping concentrations (2 wt%, 5
wt%, and 10 wt%) into PMMA (Fig. S8). All films exhibit double
exponential decay corresponding to prompt fluorescence and
the delayed fluorescence. These results indicate that H2tpdc-
dpa exhibits TADF characteristics in both the isolated and
aggregated states. Although the prompt emission decay is
almost constant at all concentrations, the delayed emission
lifetime decreases with increasing doping concentration
because of concentration quenching.
UV-vis absorption and emission spectra of H2tpdc-dpa in
tetrahydrofuran (THF) solution and UiO-68-dpa dispersed in
THF are shown in Fig. 2, and the photophysical properties are
summarized in Table 1. The absorption spectrum of H2tpdc-
dpa shows two absorption peaks at 291 and 370 nm, which are
attributable to π-π* and charge-transfer transitions,
respectively. The H2tpdc-dpa linker exhibits broad charge-
transfer emission with a peak at 481 nm in THF solution. The
phosphorescence spectrum of H2tpdc-dpa in THF was
observed at 77 K (Fig. S6), and the ΔEST was estimated to be
0.24 eV from the offset between the fluorescence and
phosphorescence spectra, which is good agreement with the
calculation results.
The photophysical properties of UiO-68-dpa were
measured after activation of UiO-68-dpa at 400 K under
vacuum. The emission spectrum of UiO-68-dpa obtained from
a THF dispersion is broader and red-shifted compared to that
of H2tpdc-dpa in THF solution (Fig. 2). The emission decay
curves of UiO-68-dpa were obtained under vacuum condition
at temperatures ranging from 30 K to 300 K (Fig. 3a). UiO-68-
dpa clearly exhibited green emission with both the prompt and
delayed components (Fig. 3b). The lifetime of the delayed
component at room temperature is 180 μs, much
Although H2tpdc-dpa also exhibits charge-transfer emission
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Table 1. Photophysical properties of H2tpdc-dpa and UiO-68-dpa
Compound
H2tpdc-dpa
UiO-68-dpa
Condition
2wt% doped PMMA film
THF (10-6 M)
λmax (nm)a
461
CIEb
Фair (%)
32
Фinert (%)
39c
42c
τprompt (ns)
τdelayed (ms)
(0.16, 0.18)
(0.18, 0.34)
(0.24, 0.44)
18
-
199
-
481
23
Solid state
501
18
30d
17
0.18
aThe maximum peak of fluorescence. bCommission International de I’Eclairage coordinates (x, y) cMeasured under N2. dMeasured under the vacuum of a turbo
molecular pump.
lower that of a 2wt% H2tpdc-dpa doped PMMA film (199 ms) Further development of thin-film MOFs deposited by layer-by-
(Table 1). The spectrum shift and decreased lifetime of the layer methods provides a path toward the realization of
delayed component may be due to the coordination electroluminescent devices using TADF-emitting MOFs as the
interaction between the carboxylic acid and the Zr ion16 or the emitters.18
interaction between two adjacent organic linkers in the
This work was supported by the Japan Science and
MOF.5,17 The increase of delayed emission intensity from UiO- Technology Agency (JST), ERATO, Adachi Molecular Exciton
68-dpa with temperature is consistent with TADF. Moreover, Engineering Project under JST ERATO Grant Number
the delayed emission intensity is proportional to the excitation JPMJER1305, Japan, the International Institute for Carbon
intensity, so the delayed fluorescence of UiO-68-dpa can be Neutral Energy Research (WPI-I2CNER) sponsored by the
attributed to TADF and not triplet-triplet annihilation (Fig. S9).
Ministry of Education, Culture, Sports, Science and Technology
The delayed emission spectrum of UiO-68-dpa is slightly (MEXT), and MEXT/JSPS KAKENHI Grant Number JP 15K21220,
red-shifted from that of the prompt emission (Fig. 3b). This and Kyushu University Platform of Inter/Transdisciplinary
emission spectrum change may originate from the structural Energy Research, young researcher/doctor student support
relaxation of ligands in the MOF, because the benzene ring at a program. MDA was supported by the Sandia Laboratory
center position of the terphenyl group of H2tpdc-dpa can Directed Research and Development Program. Sandia National
rotate even at low temperatures according to the single-crystal Laboratories is
a multi-mission laboratory managed by
data of UiO-68-NH2.14 In contrast, UiO-68-NH2 shows no National Technology and Engineering Solutions of Sandia, LLC,
delayed fluorescence at room temperature (Fig. S10), which is a wholly owned subsidiary of Honeywell International Inc., for
consistent with the large calculated ΔEST of 0.60 eV for H2tpdc- the U.S. Department of Energy's National Nuclear Security
NH2.
Administration under contract DE-NA0003525. We thank Dr.
Because MOFs can efficiently adsorb molecules such as W. J. Potscavage Jr. for his assistance with the preparation of
gases and solvents owing to their large surface areas, the TADF this manuscript.
properties of UiO-68-dpa may be sensitized to the
environment. Therefore, the emission spectra and decay
Conflicts of interest
profiles of UiO-68-dpa were investigated when introducing
oxygen and organic solvents into the pores of UiO-68-dpa. The
delayed emission was completely quenched in air because
oxygen acts as a triplet quencher (Fig. S11). As a result, the
There are no conflicts to declare.
Notes and references
photoluminescence quantum yield (Ф) decreased from 30% to
18% (Table 1). The emission spectrum of UiO-68-dpa is also
slightly shifted depending on the solvent polarity because of
the charge-transfer transition (Fig. S12). Although these shifts
are small, they suggest a method of tuning the color of light
emitted by an OLED.
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