Conjugates of tetraphenylethene and diketopyrrolopyrrole: Tuning the emission properties with phenyl bridges

Article abstract of DOI:10.1039/c4cc03024a

Diketopyrrolopyrrole (ACQ-gen) and tetraphenylethenes (AIE-gen) are linked together with phenyl bridges. The derivatives show substantially enhanced and red-shifted emission in the solid state.

Full text of DOI:10.1039/c4cc03024a

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Conjugates of tetraphenylethene and
diketopyrrolopyrrole: tuning the emission
Cite this:DOI: 10.1039/c4cc03024a
properties with phenyl bridges†
Received 23rd April 2014,
Accepted 10th June 2014
Xiao Yuan Shen,‡a Yi Jia Wang,‡a Haoke Zhang,a Anjun Qin,ab Jing Zhi Sun*a and
Ben Zhong Tang*abc
DOI: 10.1039/c4cc03024a
www.rsc.org/chemcomm
Diketopyrrolopyrrole (ACQ-gen) and tetraphenylethenes (AIE-gen) derivatives emit strong fluorescence in dilute solution and show
are linked together with phenyl bridges. The derivatives show excellent thermal and photo-stability. They play an active role in
substantially enhanced and red-shifted emission in the solid state.
photovoltaic cells, field-effect transistors and fluorescent probes.7
As a consequence of the well-conjugated DPP core, however, they
Great efforts have been devoted to the development of high- display a typical ACQ effect. Thus, it is of significance to examine
performance organic luminogens that can be used in organic the availability of tuning the emission performance of DPP by
light-emitting diodes (OLEDs), bio- and chemo-sensors.1 For a rational modifications.
long time, it has been noted that lots of organic luminogenic
The strategy used here is to modify the DPP core by linking two
molecules suffer from the ‘‘aggregation-caused quenching’’ TPE moieties at its a-positions directly (DTPE–DPP, 1) and indirectly
(ACQ) effect. That is, their aggregates or solids show drastic (DTPE-Dph-DPP, 2) via phenyl groups. Considering that DPP is a
emission quenching due to the adverse intermolecular interac- typical electron acceptor, we also designed and synthesized other
tions.2 This has seriously restricted their application in the solid DPP derivatives, which were further modified with electron
state. The discovery of ‘‘aggregation-induced emission’’ (AIE) donors (D), i.e. N,N-dimethylamine groups on different phenyls of
opens an avenue to improving the fluorescence quantum yield the TPE moieties (molecule 3 and its isomer 4).
of organic molecules in aggregated/solid states.3 AIE active
The molecular structures of 1 to 4 are shown in Chart 1 and
molecules thus have attracted considerable attention in the their synthetic routes are shown in Schemes S1 to S5 in the ESI.†
fields of OLEDs, mechanochromic materials and bioimaging.4
Tetraphenylethene (TPE) and its derivatives are representa-
tives of AIE active molecules, which possess prominent AIE
properties and enjoy the easiness to be synthesized.5 TPE is also
used as an AIE-generator (AIE-gen) to modify traditional lumino-
gens such as triphenylamine, pyrene, and perylenebisimide and
successfully converted them from ACQ to AIE molecules.6 Diketo-
pyrrolopyrrole (DPP) is another classical luminogen. DPP and its
a MOE Key Laboratory of Macromolecule Synthesis and Functionalization,
Department of Polymer Science and Engineering, Zhejiang University,
Hangzhou 310027, China. E-mail: sunjz@zju.edu.cn
b Guangdong Innovative Research Team, State Key Laboratory of Luminescent
Materials and Devices, South China University of Technology,
Guangzhou 510640, China
c Department of Chemistry, Institute for Advanced Study, Institute of Molecular
Functional Materials, and State Key Laboratory of Molecular Neuroscience,
The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,
Hong Kong, China. E-mail: tangbenz@ust.hk
† Electronic supplementary information (ESI) available: Synthetic details;
1H NMR and 13C NMR spectra, mass spectra, CV curves, UV-visible and PL
spectra, and thermal gravimetric curves of the molecules; SEM and confocal FL
images of the molecular aggregates. See DOI: 10.1039/c4cc03024a
Chart 1 Molecular structures of the TPE–DPP conjugates.
‡ X. Y. Shen and Y. J. Wang contributed equally to this work.
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Also shown in Chart 1 is the molecular structure of the
The fluorescent behaviors observed above for molecule 1 are
intermediate DBr-Dph-DPP, which is a crucial intermediate quite different from our previous reports on TPE-modified
and used for property comparison with its derivatives. The pyrene, perylene-bisimide and triphenylamine derivatives, where
experimental details and the data of the structure characteriza- the linking of TPE moieties onto these cores has converted them
tion are provided in ESI.† The purity of the resultants and from ACQ to AIE active molecules (Chart S1, ESI†).6,8 It seems
intermediates has been confirmed by elemental analysis, that the TPE-modification has no effect on the emission beha-
1H NMR, 13C NMR and high resolution mass spectroscopic viors of DPP. To get an understanding of this abnormal pheno-
techniques (Fig. S1 to S8, ESI†). Due to the N-term modified menon, we carefully examined the electronic structure of
octyl chain, all of them have good solubility in common organic molecule 1 and found that the two phenyl groups in TPEs, which
solvents. This allows their electronic structures to be evaluated are directly linked to the DPP moiety, have conjugated with DPP.
with optical absorption, fluorescent emission and cyclic voltam- In other words, they are merged into a new fluorogen. 1 should
metry (CV) measurements in solution.
be considered as a diphenyl-DPP core (Dph-DPP) modified by two
The results of CV measurements demonstrate that all of these 1,2,2-triphenylethene groups but not a DPP core modified by two
DPP derivatives have a pair of redox waves under negative bias TPE moieties (cf. Chart S2 and Fig. S12, ESI†).
(Fig. S9, ESI†), suggesting that the electron acceptor character of the
This is readily confirmed by examining the fluorescent
DPP core remains unchanged after the chemical modifications. behavior of DBr-Dph-DPP. As revealed by the data in Tables
Thermo-gravimetric analysis results indicate that the decomposi- S1, S2 and Fig. S13 (ESI†), in dilute THF solution, DBr-Dph-DPP
tion point for molecules DBr-DPh-DPP, 1, 2, and 3 is 318, 389, 419 emits yellow-greenish fluorescence with a peak at 543 nm (lem
)
and 407 1C, respectively (Fig. S10, ESI†). These data indicate that and the FF is 95.2%. But FF falls to 0.95% in the THF–water
the TPE-modified DPP derivatives have improved thermal stability. mixture at fw of 95% and Fq is calculated to be 99.0%. These
The property that we are most interested in is their fluorescent data indicate that DBr-Dph-DPP is a typical ACQ molecule. The
behaviors. As demonstrated by the spectra in Fig. 1 and Fig. S11 red-shift of the emission of 1 relative to DBr-Dph-DPP origi-
and the data in Table S1 (ESI†), in tetrahydrofuran (THF) solution, nates from the modification of Dph-DPP with two segments of
molecule 1 emits yellow fluorescence (lem = 572 nm) and the a,b,b-triphenylethene. Meanwhile, the modification results in
quantum efficiency (FF) is 13.6%. When relative low fraction of the decrease of FF from 95.2% for DBr-Dph-DPP to 13.6% for 1
water ( fw, by volume), a non-solvent to 1, is introduced into the (Table S2, ESI†). This is ascribed to the intramolecular rotation
THF solution, the change in emission intensity is quite small. of the phenyl groups, which partially exhaust the energy of the
When fw reaches 50%, the emission intensity decreases evidently. excited state. On the whole, it is obvious that molecule 1 is
When fw is over 70%, the emission becomes much weaker and the similar to DBr-Dph-DPP for their fluorescent behaviors.
change in FF vs. fw is shown in Fig. 1B. The FF is only 1.27% at fw
The comparative study on fluorescent behaviors of DBr-Dph-
of 95%. The above data indicate a typical ACQ behavior and DPP and 1 suggests that the direct linking of TPE to DPP cannot
the quenching factor (Fq) is calculated to be 90.7% (ESI†). The convert the DPP derivative from an ACQ to an AIE active molecule
decrease of FF against the increase of fw can be ascribed to the and obtain a DPP-based fluorescent material with high FF in the
formation of aggregates in the THF–water mixtures, which solid state. The assimilation of DPP to TPE plays a key role. It is
quenches the fluorescence via the intermolecular p–p interaction. reasonable that the insertion of a spacer between DPP and TPE
Besides the decrease in FF, the emission peak red-shifted from 572 moieties may break the assimilation effect. Considering that a
to 585 nm with the increase of fw from 0 to 95%. The FF of the series of excellent studies have shown experimentally and theore-
solid film of 1 is 11.8%, because in this case molecule 1 locates in a tically that the phenyl ring twisting plays a key role in the organic
low polar environment without being surrounded by high polar photo/electronics.9,10 Herein the electronic communication
water molecules (Table S1, ESI†). These changes will be discussed between DPP and TPE is crucial for tuning to the photo/electronic
below together with molecules 2 and 3.
properties of DPP with TPE, phenyl groups, a p-conjugate building
block is used as the spacer. The target molecule was derived by
substituting the two bromide atoms on DBr-Dph-DPP with two
TPE moieties (2, Chart 1).
The phenyl bridge has evidently tuned the fluorescent
behaviors as compared to molecule 2 to 1 and DBr-Dph-DPP.
Molecule 2 emits yellow fluorescence (lem = 568 nm) with
moderate efficiency (FF = 43.8%) in THF solution (Fig. 2 and
Fig. S13, S15, Tables S1 and S3, ESI†). The red-shift of emission
relative to DBr-Dph-DPP is associated with the segmental
electronic conjugation between DPP and TPE moieties, as
predicted by theoretical calculation (Fig. 3). In the free state
(e.g., in dilute solution), the dihedral angle between the bridge
and the adjacent phenyl in the TPE moiety is about 371,
suggesting the partial conjugation between the TPE and DPP
moieties. In comparison with 1, only a 4 nm difference in the
Fig. 1 Change in fluorescence (FL) intensity (A) and quantum efficiency
(FF) (B) of molecule 1 in THF–water mixtures with different water fractions
(fw). Concentration: 10 mM; lex: 365 nm.
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shifts to 592 nm from 568 nm in THF. These feature changes
are attributed to two effects; (1) the formation of molecular
aggregates and (2) the change in environmental polarity upon
increasing fw.
In aggregated states, on one hand, the molecules take a
closer packing mode, which may reduce the dihedral angle
between the bridge and the adjacent phenyl in the TPE moiety.
We carried out a calculation by setting the dihedral angle at
151, and the result predicts a small energy gap (Fig. 3), corre-
sponding to a red-shifted emission. On the other hand, the
polar environment is facile to the photoinduced charge transfer
(PICT) in the excited state, which helps the red-shift of the
fluorescence spectrum for polar fluorogens. Increasing fw is
equivalent to enhancing the environmental polarity. As a result,
the emission red-shifts accordingly. The slight emission
enhancement at high fw is due to the less polar local environ-
ment inside the molecular aggregates. Attachment of electron
donor (D) groups onto the DPP core will enlarge the polarity
and thereby enhance the PICT process, thus leading to further
red-shifted emission. This will be validated and discussed
below (Tables S1, S4 and S5, ESI†).
Fig. 2 Change in FL intensity (A) and FF (B) of molecule 2 in THF–water
mixtures with different fw values; the inset images show the corresponding
FL photographs. Concentration: 10 mM; lex: 365 nm.
To further demonstrate the pivot role of the phenyl bridge, we
attached two N,N-dimethylamine groups onto the two TPE
moieties of 2. Consequently, molecule 3 and its isomer 4 were
derived. 3 and 4 exhibit similar properties despite that the
N,N-dimethylamine groups have been attached to the phenyls
on the opposite sides of the ethenyl of TPE moieties (Chart 1,
Table S5, Fig. S15 to S18, ESI†). Herein, we use 3 as the
representative. The absorption and emission peaks of 3 in THF
solution appear at 500 and 564 nm, which are nearly identical to
Fig. 3 Energy level alignment and electron distribution of molecule 2 with
different conformations. The dihedral angles between the bridge and the
adjacent phenyls in the TPE moiety are set to 151 (left) and 371 (right).
emission peak was recorded, indicating that the addition of the the data of 2 (496 and 568 nm, respectively, Tables S3 to S5, ESI†),
phenyl groups between DPP and two TPE moieties has not though two strong electron donors have been introduced into the
enlarged the effective conjugation length. Due to the steric molecule. In addition, upon changing solvent polarity from hexane
repulsion, the phenyl bridge is not co-planar with DPP and TPE to N,N-dimethylformamide, the wavelengths of the absorption and
moieties (Fig. 3), thus the effects of TPE modification are emission peaks undergo a small red-shift (Fig. S15–S19, ESI†).
abated and the properties of 2 are dominated by the DPP Such a weak solvato-chromic effect is abnormal to the luminogens
moiety. This is also supported by the results of CV measure- furnished with D and A groups.11 A rational explanation is that the
ment (Fig. S9, ESI†).
D–A effects of the two N,N-dimethyl-amines and DPP units are
The FF values for 2 are different from 1 and DBr-Dph-DPP in partially blocked by the phenyl bridges.
both THF solutions and solid films. The FF change is ascribed
If the above deduction is trustworthy, the conformation
to the intramolecular rotations, which can exhaust the energy of between TPE and DPP units should be quite twisted thereby
the excited state. For DBr-Dph-DPP, there are no extra rotational the twisted intramolecular charge transfer (ICT) should be
phenyls around the Dph-DPP core, it has the highest FF (95.2%). observed. In apolar hexane solvent, 3 shows efficient emission
For 1, the rotational phenyls are closely linked to the Dph-DPP core, (FF = 59.6%, Table S4, ESI†); while in polar THF solvent, FF
its FF steeply decreases to 13.6%. For 2, the linkage between the drops to 0.2%. This is direct evidence of ICT, because the polar
Dph-DPP core and the rotational phenyls is separated by the phenyl solvent is profitable to the ICT process,11,12 which results in the
bridge, and rotation-caused energy dissipation is inefficient. Thus, non-irradiative decay. This FF (0.2%) is much lower than that of
its FF is 43.8%, more than 3 times higher than that of molecule 1. DBr-Dph-DPP (95.2%) and 2 (43.8%) in THF solution. It can be
More importantly, the insertion of phenyl bridges between DPP attributed to the synergetic effect of the ICT process and the
and TPE moieties disturbs the intermolecular p–p interaction in the intramolecular rotations of phenyls on TPE moieties. Accor-
solid state, the FF of 2 in cast films increases to 30.2%, the highest dingly, the molecules could have higher fluorescent efficiency
one among all molecules (Fig. S14, Tables S3 and S5, ESI†).
when they are in solid films. Indeed, the FF of cast films is
In THF–water mixtures, FF of 2 decreases to 36.5% when fw measured to be 8.6%, about 43 times higher than that recorded
is 50%; whereas fw Z 60%, the emission is heavily quenched in THF solution, because the restricted intramolecular rotation
and the FF drops to 2.2%. In the solvent mixture with 95% of (RIR) of phenyls has an effect and the lower polar environment
water, the FF rises to 4.1%. Meanwhile, the emission peak in aggregates is disadvantageous to the ICT process.
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enhanced emission in the solid state. Together with the observa-
tion of the key role of the activation barrier in phenyl ring twisting
on organic opto/electronics,9,10 the present attempt, on one hand,
disclosed instructive information about tuning the performance of
organic luminogens by making subtle balance between AIE and
ACQ units; and on the other hand, derived red-emitting molecules
with good quantum efficiency and high stability.
This work was financially supported by the National Science
Foundation of China (51273175) and the National Basic Research
Program of China (973 Program, 2013CB834704).
Fig. 4 Change in FL intensity (A) and quantum efficiency (B) of 3 in
THF–water mixtures with different fw values; the inset images show the
corresponding FL photographs. Concentration: 10 mM; lex: 365 nm.
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Chem. Commun.
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