Covalent organic frameworks: A platform for the experimental establishment of the influence of intermolecular distance on phosphorescence

Article abstract of DOI:10.1039/c8tc01559g

We report herein a two-dimensional (2D) COF (BZL-COF) based on an representative crystallization-induced phosphorescence (CIP) organic phosphor, benzil, for phosphorescence study. In BZL-COF, the CIP building blocks are arranged in an eclipsed fashion sim

Full text of DOI:10.1039/c8tc01559g

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10.1039/C8TC01559G.
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ISSN 2050-7526
PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Covalent organic framework: a platform for experimental
establishment of the influence of intermolecular distance on
phosphorescence
Received 00th January 20xx,
Accepted 00th January 20xx
Shan Wang,‡a Li Ma,‡a Qianyou Wang,a Pengpeng Shao,a Dou Ma,a Shuai Yuan,a Peng Lei,a Pengfei
Li,a,b Xiao Feng,*a and Bo Wanga
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report herein a two-dimensional (2D) COF (BZL-COF) based on skeleton and specific polymeric packings. Although there is a
an representative crystallization-induced phosphorescence (CIP) rapid expansion of their applications in many intriguing fields,
organic phosphor, benzil, for phosphoresence study. In BZL-COF, fundamental studies resort to the unique structural features of
the CIP building blocks are arranged in an eclipsed fashion similar COFs are still in its infancy.
to the face-to-face mode in H-aggregates. The phosphorescence
Organic phosphors have attracted significant attention in
behavior is significantly changed from on to off at cryogenic electronic,18 optical,19 and biological20,21 applications, since
temperature along with the altering of interlayer distance from they cause less environment footprint and their emission
3.7 Å to 3.4 Å by removing the solvent molecules trapped between property are more easily to be fine-tuned compared with
the 2D polymeric layers. In comparison, single crystal of model traditional inorganic and organometallic complexes.22,23
compound that adopts an H-type aggregation with
a
Pioneering molecular design strategies, such as introducing
intermolecular distance of 4.1 Å can emit strong phosphoresence heavy halogen atom,24 intermolecular electronic coupling,25
at room temperature. Our study shows that in addition to the host-guest system26 and rational molecular design27 have been
restriction of intramolecular rotation and H-aggregated packing, proposed to pursue long lifetime and high quantum efficiency
suitable intermolecular distance also plays vital role for the organic phosphors. Nonetheless, the inherent behaviors of the
enhancing of phosphorescence.
phosphors in the solution may largely differ from them in solid
state, because the intermolecular interactions and molecular
packing has significant impact on their optical properties.28-30
The formation of crystallization-induced phosphorescence
(CIP)31,32 and H-aggregate33 in solid states is considered to be
the most plausible reason to achieve high performance for
“untraditional” phosphors, for instance, benzophenone,31
isophthaic,34 4,6-diphenyl-2-carbazolyl-1,3,5-triazine33 and
Covalent organic frameworks (COFs) are crystalline
porous materials that constructed by pure organic building
molecules based on the reticular chemistry.1-5 Their structures
are entirely composed of light elements and joint together via
covalent bonds, imparting COFs with high architectural and
chemical robustness. The characteristic features of COFs, such
as large specific surface area, tunable structure, low density
and ease of functionalization, allow for their use in widespread
applications of gas storage and separation,6,7 catalysis,8,9
sensing,10 energy storage11 and optoelectronics,12,13 etc. COF
chemistry extends the predictably linear linking of the
monomers to two and three dimensions and enables
manipulating organic matter packings in the framework level
beyond atom and molecule.14-17 Therefore, COFs are able to
serve as ideal research platform for the investigation of
chemical and physical phenomenon within infinitely extended
(9H-carbazol-9-yl)(4-chlorophenyl)-methanone.35
In
CIP
concept, the efficient emission is stemmed from the restriction
of molecular free rotation and the blocking of non-radiation
channels.36-38 The formation of H-aggregate is suggested to
allow the stabilization of triplet excitons, and thus, realizing
ultralong lifetime. Since it remains a difficult task to predict
and regulate the packing of small molecules, and more tough
for macromolecules, comprehensive investigations of the
influence of the interlayer distance and molecular
configuration on the phosphorescence property are still
challenging.
Because the arrangement of molecular components in
the 2D COF can be atomically and metrically well defined in
either eclipsed or staggered fashion, we intentionally select 2D
COF to experimentally establish the influence of molecular
packing on phosphorescence behavior. Benzil derivatives are
typical CIP phosphor that can emit strong phosphorescence in
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solid state but are non-luminescent in dilute solution.39 Herein, diphenylbenzil (PBZL), which can emit intense green emission
we chose benzil derivative as the building unit to construct a at room temperature in the single crystal form that adopts H-
novel [3+2] boronate-linked 2D COF (denoted as BZL-COF). The aggregation with an interlayer distance of 4.1 Å (Scheme 1c).
BZL-COF consists of stackings of periodically extended Inconsistent with the concept of CIP and H-aggregation-
polymeric sheets, in which the benzil unit at edges are enhanced
phosphorescence,
the
ability
to
emit
covalently linked with triphenylene at vertices to form phosphorescence is gradually vanished along with the more
hexagonal polygons. The 2D polymeric sheets pack in an powerful restriction of intramolecular rotation and the forming
eclipsed fashion similar to the face-to-face mode in H- of more closely packed H-aggregates. In addition, we prepared
aggregates and produce self-sorted and unidirectional benzil amorphous microporous polymer based on benzil derivative
columnar arrays. Interestingly, we found that the layer-to- (BZL-POP) to avoid the formation of the closed packing
layer distance for the as-synthesized sample is 3.7 Å, and the (Scheme 1b). The BZL-POP showed yellow phosphorescence at
value of which reduced to 3.4 Å after vacuum drying (Scheme 77 K and non-emissive at 298 K, attributing to the relatively
1a). We attribute it to the removing of solvent molecules low rotation barrier of BZL building blocks in the polymer
trapped between the layers during the activation process. matrix at room temperature. Therefore, we conclude that in
Notably, the as-prepared BZL-COF can emit bright yellow addition to the restriction of intramolecular rotation and the
phosphorescence at cryogenic temperature, whereas the as- formation of H-aggregated packing, a proper intermolecular
activated 2D COF is nonemissive to naked eye. As control distance is also required to ensure the achieving of the high
experiment, we obtained a small model organic molecule, 4,4’- quantum yield of phosphorescence.
Scheme 1 (a-b) Synthesize procedures of the Dio⊂BZL-COF, BZL-COF and BZL-POP. (c) Chemical structures, top views and side views of
Dio⊂BZL-COF, BZL-COF, PBZL and BZL-POP.
The BZL-COF was synthesized by co-condensation of 4,4’- molecular connectivity and bonding modes were assessed by
2,3,6,7,10,11- Fourier transform IR (FT-IR) and 13C cross-polarization with
mixture of magic-angle spinning (CP-MAS) NMR spectroscopies (Fig. 1).
benzildiboronic
acid
(BZLBA)
and
hexahydroxytriphenylene (HHTP) in
a
dioxane/mesitylene (1/1 by vol.) at 100 oC for 72 h. The The FT-IR spectra clearly indicate the total consumption of the
resultant deep-green powders were collected by starting material, as the peak over 3000 cm-1 (around 3400 cm-
centrifugation and washed by dioxane and acetone to remove 1, the hydroxyl group for HHTP and BZLBA) is absent. The
unreacted monomers and oligomers. Then the BZL-COF was formation of the expected boron-based five-membered ring
activated by drying under vacuum at 120 oC for 12 h. The groups can be observed from the peak at 1352 cm-1,
2 | J. Name., 2012, 00, 1-3
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corresponding to the stretching of the borate ester bond (B-O). model using N2 at 77 K on carbon kernel shows a diameter of 4
The 13C CP-MAS NMR data further confirms the molecular nm (Fig. 2b), which is consistent with the pore size in the
structure. It displayed signals at 189.6, 145.7, 132.9, 125.9, constructed crystal mode based on the bnn topology.
123.2 and 100.3 ppm, which are assignable to the carbonyl
We found a significant shift from 24.9o to 26.4o of the
carbons and aromatic carbons. Elemental analysis (EA) PXRD peak assigned to the (001) facet after activation (Fig. 2c).
demonstrates that the C and H contents are agreed well with It indicates that the solvent molecules are trapped between
the calculated values for an infinite 2D sheet (Table S1). Field- the layers before activation, leading to a larger interlayer
emission scanning electron microscopy (FE/SEM) images reveal distance of 3.7 Å. The weakened intensities of the peaks
that the COF particles adopt a micrometer-scale sphere assigned to (100), (110) and (200) facets are resulted from the
morphology (Fig. S1a-b). The fully activation of BZL-COF was encapsulation of disordered solvent molecules in the
confirmed by thermogravimetric analysis (TGA), as evidenced channels.40 Since acetone are easily volatilized, the interlayer
by no weight loss until decomposition at 480 oC under N2 distance will be shortened upon long-time exposure to air.
atmosphere (Fig. S2a).
Hence, we did a solvent exchange of acetone to dioxane to
ensure the interlayer distance remains unchanged during
characterizations, and the sample with dioxane within the
layers and pores is denoted as Dio
⊂BZL-COF. We also tested
the PXRD pattern of BZL-COF and Dio
⊂BZL-COF with different
reaction time (15 min, 6 h and 24 h), the (001) facet all shifted
obviously (Fig. S5). BZL-COF exhibited good stability in
anhydrous organic solvents like acetone, dioxane, and
remained intact when it was stored in air for 72 h (Fig. S6).
Fig.1 (a) FT-IR spectra and (b) 13C CP-MAS solid-state NMR spectra
of BZL- COF, BZL-POP and PBZL.
The successful formation of a highly crystalline framework
was confirmed by powder X-ray diffraction (PXRD) analysis. In
order to elucidate the building block packing modes in COFs,
we employed Materials Studio software to construct two kinds
of stacking models. One is the eclipsed AA stacking sequence
with bnn net in P6 space group, and the other one is the
staggered AB stacking sequence of layers with gra net in
P63/m space group. It is found that the simulated PXRD pattern
of the eclipsed AA stacking is in good agreement with the
experimental one in both peak positions and relative
intensities (Fig. S3). The diffraction signals at 2.4, 4.2, 4.8, 6.5,
8.5 and 26.4 are assignable to the (100), (110), (200), (210),
(220) and (001) facets, respectively. The Pawley refinement
cell parameters are a = b = 41.86 Å, c = 3.39 Å, α = β = 90o, and
γ = 120o, with Rwp = 6.57% and Rp = 5.16%. These results
indicate that hexagonal polygons are joint to form extended
and planar polymeric sheets and further stack along z axis with
an interlayer distance (d001) of 3.4 Å, where benzyl units are on
the pore walls and triphenylene are at the vertices. The N2
sorption analysis of BZL-COF at 77 K exhibits a type IV
isotherm, indicative of a mesoporous material (Fig. 2a).
Brunauer–Emmett–Teller (BET) calculation gives a specific
surface area of 2115 m2 g-1(Fig. 2a and S4). Estimation of the
pore size from nonlocal density functional theory (NLDFT)
Fig.2 (a) Nitrogen adsorption and desorption isotherm of BZL-COF at
77 K. STP = standard temperature and pressure. (b) Pore-size
distribution of BZL-COF, as determined by NLDFT modeling on the
nitrogen-adsorption isotherm. c) Experimentally observed PXRD
pattern of BZL-COF (green curve), Dio⊂BZL-COF (blue curve) and
simulated PXRD patterns (AA and AB stacking mode).
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Fig.3 (a–b) Phosphorescence emission spectra of BZL-COF and Dio⊂BZL-COF measured at different temperatures. (c–f) Time-resolved
emission decay curves for the c) BZL-COF, d) Dio⊂BZL-COF, e) PBZL crystal and f) BZL-POP at 77 K and 298 K.
Interestingly, the Dio
⊂BZL-COF exhibits intense yellow in the phosphorescence behavior is mainly result from the
phosphorescence that can be observed by naked eye when interlayer distance rather than molecular configuration or
irradiated it at 365 nm at cryogenic temperature. In sharp charge transfer between dioxane and BZL-COF.
contrast, the activated BZL-COF are completely non-
luminescence (Fig. S7d and S7e). The phosphorescence rotation and the formation of H-aggregates are favored for the
spectrum of Dio BZL-COF shows an intense peak centered at boosting of phosphorescence. However, compared with
545 nm with a life time 1.27 ms at 77 K (Fig. 3b and 3d). The Dio BZL-COF, the activated BZL-COF bears a more closed π-π
ability to exhibit phosphorescence for Dio BZL-COF is stacking and more limited freedom for bond vibration and
As mentioned above, the restriction of intramolecular
⊂
⊂
⊂
originated from the lone electron pairs of the carbonyl groups rotation but possesses a much-weakened phosphorescent
in benzil units that can promote the intersystem crossing (ISC) ability. To further investigate the reason for these abnormal
transition through strong spin-orbit coupling.41 For as- phenomenon, we prepared single crystal of
a
model
activated COFs, the resolved emission bands at 540 nm with a compound (PBZL) and a three-dimensional porous polymer
lifetime of 1.02 ms at 77 K (Fig. 3a and 3c). (BZL-POP) (Scheme 1). The composition and structure of target
To exclude the influence of energy or charge transfer materials were thoroughly characterized by FT-IR, NMR
between dioxane and BZL-COF on the phosphorescence spectroscopy, PXRD and single-crystal X-ray diffraction (Fig 1,
behavior, we re-dispersed the activated BZL-COF into dioxane Fig. S10). In the single crystal of PBZL, the molecules adopt
solution (denoted as Dio+BZL-COF) and examined its physical twisted conformations and stack in face-to-face H-aggregated
property. Dio+BZL-COF mixture still retains an interlayer fashion with an intermolecular distance of 4.1 Å. In addition, C-
distance of 3.4 Å (Fig. S8), indicating that the dioxane H···O (2.729, 3.207 Å) hydrogen bonding interactions are
molecules cannot re-insert into the interlayer due to the observed between adjacent molecules.42 BZL-POP processes
strong π-π interaction between adjacent polymeric layers in an amorphous and porous structure with a BET surface area of
activated COF. In contrast to the strong emission of Dio
COF at 77 K, Dio+BZL-COF shows no obvious phosphorescence (Fig. S11 and S12).
at both room temperature and 77 K (Fig. S7). However, its UV- PBZL exhibits typical CIP characteristic, which is non-
vis absorption spectra are similar with that of Dio BZL-COF emissive in the solution state but emits bright green light in
(Fig. S9). These results demonstrate that the significant change the single-crystal state at room temperature (Fig. S13-S15).
⊂
BZL- 670 m2g-1 and wide pore size distribution from 0.7 to 1.5 nm
⊂
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The PL spectrum of PBZL crystals obtained under nitrogen the nonraditive rates and the changes of phosphorescent
atmosphere shows a peak centered at 537 nm upon excitation lifetimes. According to the theory of restriction of
at 345 nm and 298 K with a lifetime of 0.13 ms. The absolute intramolecular rotation, the quantum yields should be
PL quantum yield (ϕPL) is measured to be 3.42%. To the eye, improved along with the decrease of their interlayer distance.
BZL-POP appears non-emissive when irradiated at 365 nm However, we found that their ability to emit phosphorescence
under ambient conditions, a peak is observed at 565 nm in the is in proportional to their interlayer distance. It maybe
emission spectrum with a low quantum yield (ϕPL= 0.32%). The originated from the concentration quenching effect or static
lifetime measurements showed a relative short lifetime (τ = quenching effect similar to the fluorescence theory.43 This
0.02 ms) (Fig. S16 and Table S2).
experimental study may provide some new insights for the
To investigate the thermal motion effects on the formation of H aggregates and CIP presumes for achieving long
phosphorescence behavior, we tested the photoluminescence lifetime and high quantum yield organic phosphors and the
spectra under cryogenic temperatures. For PBZL crystals and quantum theoretical studies are undergoing. The COFs provide
BZL-POP, obvious shoulder peaks at 582 and 605 nm were an ideal research platform for the investigation of chemical
observed, respectively, with a significantly prolong of lifetime and physical behaviors related to ordered intermolecular and
(PBZL: 298 K, 0.16 ms; 77 K, 2.42 ms; BZL-POP: 298 K, 0.02 ms
77 K, 0.96 ms) and enhancement of intensities upon
decreasing of temperature (Fig. S16). These results indicate
the non-radiative decay of triplet excited states are largely
suppressed in a rigid frozen environment. In another word,
there still remains plenty of positional freedom in both PBZL
crystals and BZL-POP, which allows remarkably thermal motion
at ambient temperatures. These results are agreed with the
large interlayer distance in PZBL crystal and wide pore size
distribution in BZL-POP with amorphous nature. On the
contrary, the phosphorescence behavior of activated BZL-COF
did not change much at cryogenic temperatures (Fig. 3a and
3c), demonstrating that the thermal motion is mostly
restricted by intralayer covalent linking and closely π-π
；
polymeric packings.
Acknowledgements
This work was financially supported by the 973 Program
2013CB834702; the National Natural Science Foundation of
China (Grant No. 21490570, 21625102, 21471018, 21404010,
21674012); 1000 Plan (Youth).
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⊂BZL-COF is lower than that of BZL-
COF and higher than that of PBZL and are in consistent with
the change of interlayer distance. The dynamic photophysical
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Dio
⊂
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