A facile synthesis of novel near-infrared pyrrolopyrrole aza-BODIPY luminogens with aggregation-enhanced emission characteristics

Article abstract of DOI:10.1039/c7cc04568a

Here we report the facile synthesis of two triphenylethylene-modified pyrrolopyrrole aza-BODIPY dyes with an aggregation-enhanced emission feature. NIR-emitting nanoparticles with remarkable photostability properties and applications in bioimaging were ge

Full text of DOI:10.1039/c7cc04568a

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DOI: 10.1039/C7CC04568A
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A facile synthesis of novel near-infrared pyrrolopyrrole aza-
BODIPY luminogens with aggregation-enhanced emission
characteristics
Received 00th January 20xx,
Accepted 00th January 20xx
Lanqing Li, Lingyun Wang *, Hao Tang and Derong Cao
DOI: 10.1039/x0xx00000x
www.rsc.org/
Here we report the facile synthesis of two triphenylethene-
modified pyrrolopyrrole aza-BODIPY dyes with an aggregation-
enhanced emission feature. A NIR-emitting nanoparticles with
remarkable photostability properties and applications in
bioimaging was generated due to its good dispersity in water and
biocompatibility.
The development of fluorogens with near-infrared (NIR)
emission is nowadays one of the hottest topics of investigation
in the field of bio/chemosensors and bioimaging.1-3 NIR
excitation and emission (650-900nm) offers many important
advantages: (i) great sample penetration depths due to weak
absorption and strongly reduced scattering; (ii) sensitive
detection because of negligible NIR auto-fluorescence
background; and (iii) less photodamage compared to visible
excitation. As a new class of NIR dyes and fluorophores
developed by Shimizu group in 2013,4 pyrrolopyrrole aza-
BODIPY dyes (PPABs) have attracted considerable attention in
the fields of molecular electronics, molecular probes and
bioimaging because of its broad ranges of absorption and
emission in the visible and near-infrared regions, high chemical
stability and photostability. However, due to its big conjugated
structure and rigid planar plane, PPAB usually undergoes
compact packing and strong π-π stacking interactions, leading
to the notorious aggregation-caused quenching effect (ACQ).
Its bright emission in solution is partially or completely
quenched upon aggregation.4, 5 This has seriously restricted its
application. Thus, it is of significance to examine the
availability of tuning the emission performance of PPAB by
rational modifications.
Chart 1 Molecular structures of PPAB-2 and PPAB-3.
AIE luminogens (AIEgens) show negligible emission in dilute
solution but enhanced fluorescence in the aggregated state
due to the restriction of intramolecular rotation (RIR).6-8
Triphenylethene (TPEH) and its derivatives with a bulky size
and propeller shape are representatives of AIE active
molecules, which possess prominent AIE properties and enjoy
the easiness to be synthesized. TPEH is also used as an AIE-
generator to modify traditional luminogens such as BODIPY,
dibenzofuran and dibenzothiophene, and successfully
converted them from ACQ to AIE molecules.9, 10 However,
most of the AIEgens developed so far exhibit green or red
emission and NIR emissive AIEgens often needed costly raw
materials for preparation and involved multistep syntheses.
PPAB-based NIR emissive AIEgens have not been reported till
now. Therefore, it is highly desirable to exploit AIEgens with
efficient NIR emission using cost effective and facile synthetic
procedures.
In general, changes in both the heteroaromatic ring units
and substituents on the pyrrolopyrrole moiety can be utilized
to control the fluorescence of PPAB molecules. Herein, two
NIR emissive AIEgens (PPAB-2 and PPAB-3, Chart 1) were
prepared by facile synthesis for the first time. The strategy
used here is to modify the PPAB core by linking one or two
TPEH moieties at its pyrrolopyrrole moiety directly in one step.
The development of aggregation-induced emission (AIE)
materials provides a new strategy to solve the ACQ problem.
Key Laboratory of Functional Molecular Engineering of Guangdong Province,
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou, 510641, China. E-mail: lingyun@scut.edu.cn; Fax: +86 20
87110245; Tel: +86 20 87110245
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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spectroscopic methods are given in Experimental Section in ESI
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DOI: 10.1039/C7CC04568A
(Figs. S1 to S15).
PPABs exhibit an intense band at around 650 nm with broad
and weak bands at around 330 and 430 nm. In THF, the main
absorption band shifts to lower-energy visible region upon
changing substituent groups of pyrrolopyrrole ring unit from
tert-butyls (PPAB-1: 659 nm) to tert-butyl/TPEH (PPAB-2: 668
nm) and TPEHs (PPAB-3: 677 nm), manifesting that TPEH has
extended the conjugation of the resultant molecules (Fig. 1a).
The fluorescence emission is also red-shifted in this order (672
nm (PPAB-1), 683 nm (PPAB-2) and 691 nm (PPAB-3), Fig. 1b).
The Stokes shifts are as follows: 13 nm (PPAB-1), 15 nm (PPAB-
2
), and 14 nm (PPAB-3). The fluorescence quantum yields (φF)
in THF determined by the absolute method are 58.5% (PPAB-1),
0.8% (PPAB-2), and 1.1% (PPAB-3). These results indicate that
the peripheral rotors, phenyl rings of TPEH rotate against the
central PPAB stator to annihilate the excitons in a non-
radiative fashion in solution state makes the PPAB-2 and
PPAB-3 weakly emissive.
The sensitivity of these PPAB-based luminogens to polar
solvents was investigated by measuring their photophysical
properties in various organic solvents with different polarities.
The UV-vis spectra of PPAB-1 nearly unaffected from the
change of solvent polarity. Similarly, only slight changes in
emission maximum were observed. The bathochromic shift
was identified to be 11 nm in solvents from hexane to DMSO
for PPAB-1. Moreover, similar weakly solvatochromism was
also observed with emission red-shift of 11 nm (PPAB-2) and
15 nm (PPAB-3), respectively (Fig. S16 to S18, Table S2, ESI†).
The AIE character of PPAB-2 and PPAB-3 was checked in
CH2Cl2/hexane mixtures, where CH2Cl2 and hexane were used
as good solvent and poor solvent, respectively. As shown in Fig.
2a, the emission of PPAB-2 is weak in CH2Cl2 solution and
increases slowly until volume percentage of hexane (fh)
reaches 70%. Afterwards, the emission intensifies swiftly.
PPAB-3 behaves similarly (Fig. 2b). The highest emission
enhancement was recorded for PPAB-2 and PPAB-3 with a fh
of 90%, which is 93-fold and 70-fold higher than those in
CH2Cl2 solution, respectively (Fig. S19, ESI†). Both PPAB-2 and
PPAB-3 were weakly emissive in diluted solutions with φF of
0.7% and 0.4%, while become quite emissive in the aggregate
state at 90% fh with φF of 21.9% and 18.8%, respectively. 31
and 47 times higher than that recorded in CH2Cl2 solution for
PPAB-2 and PPAB-3 were present, respectively. Thus, PPAB-2
and PPAB-3 feature the unique AIE characteristics because of
the restricted intramolecular rotation (RIR) of phenyls in TPEH
moiety.
Fig. 1 (a) Normalized UV-vis absorption and (b) fluorescence spectra of PPAB-1
(black), PPAB-2 (red), PPAB-3 (blue) in THF.
A
traditional PPAB compound
synthesized for the purpose of comparison.
The detailed synthetic routes of PPAB-1
in Scheme S1 in the ESI†. 3,6-Bis(4-octyloxyphenyl)-2,5-
(PPAB-1, ESI) was also
~
PPAB-3 are shown
dihydropyrrolo[3,4-c]pyrrole-1,4-dione
(1) was synthesized
according to the literature procedure,11 and used as starting
material because of its moderate solubility in common organic
solvents.
6-(tert-butyl)benzo[d]thiazol-2-amine
(2a)
substituted with a tert-butyl group was used to improve the
solubility of the condensation products. To obtain TPEH-based
luminogens with flexible structures, the intermediate of 6-(1, 2,
2-triphenylvinyl)benzo[d]thiazol-2-amine (2b) was prepared in
a facile approach in two steps. PPAB with a symmetric
substitution pattern is synthesized using a one-step reaction
under mild conditions. In the presence of titanium
tetrachloride and triethylamine,
1 was reacted with 2a in
refluxing toluene, and the mixture was further reacted with
BF3.OEt2 to provide PPAB-1 in 19% yield. 2b was successfully
The photostability of PPAB-1
continuous 365 nm irradiation was investigated by using
Indocyanine Green (ICG) in water as reference for
, PPAB-2 and PPAB-3 under
reacted with
in 10% yield. Asymmetric PPAB-2 was synthesized by
assembling the chromophores in a stepwise manner: the
monosubstituted 1:1 condensation product ( ) derived from
1 under similar reaction conditions to give PPAB-
a
3
comparison, because ICG is a NIR fluorescent dye approved for
clinical use by the Food and Drug administration (FDA).12 With
increasing irradiation time, the maximal absorbance of the ICG
aqueous solution decreased quickly. In contrast, only negligible
3
1
with 2a was isolated and used as the starting material to react
with a second equivalent of 2b followed by complexing with
BF3.OEt2 to obtain asymmetric PPAB-2 in 15% yield. The
structural characterization and purity data of the products by
decrease of absorbance could be observed in the PPAB-1
,
PPAB-2 and PPAB-3 toluene solutions under the same
irradiation conditions (Fig. S20, ESI†). After 120 min of
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summarized in Table S3. These results and variation of the
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energy gaps values are in good agreemeDnOt Iw: 1i0th.10t3h9e/Cr7eCdCs0h4if5t68oAf
the absorption of these PPAB molecules and broadly
reproduced by the theoretical calculations. The HOMO–LUMO
energy gap decreases from PPAB-1 (1.537 eV) to PPAB-2
(1.524 eV) and PPAB-3 (1.516 eV), which corresponds well to
the red-shift of the lowest-energy absorption in this order
(Table S3, ESI†).
To further demonstrate the potential of AIE-active PPAB
derivatives for bioimaging applications, PPAB-2 was tested as a
fluorescent reagent for fabrication of AIE nanoparticles (NPs)
to improve its water solubility and biocompatibility. The NPs
formed by PPAB-2 molecules were prepared through
nanoprecipitation in the presence of Pluronics F127 as the
stabilizer.14 The absorption spectrum appeared slightly
broader after PPAB-2 molecules were encapsulated with
Pluronics F127 and dispersed in water. The PPAB-2 NPs show
absorption maximum at 677 nm and emit NIR fluorescence
peaked at 696 nm (Fig. S23, ESI†), which is red-shifted
compared with its dilute THF solution. More importantly, a
remarkable 1.2-fold fluorescence enhancement was observed.
Laser light scattering results showed that PPAB-2 NPs has the
average hydrodynamic diameter around 41 nm with
a
polydispersity index of 0.313 (Fig. S24, ESI†). PPAB-3 NPs
behaves similarly, which shows main absorption band at 690
nm and a strong NIR emission peak at 710 nm (Fig. S25, ESI†).
The average diameter of PPAB-3 NPs was measured as 22 nm
(Fig. S26, ESI†). Since the nanoparticles contains both
hydrophilic and hydrophobic domains, water insoluble PPAB-2
and PPAB-3 will mainly distribute in the apolar hydrophobic
domains with single molecules or in the hydrophilic domains
with aggregates, both of which can show strong fluorescence.
The cell viability was assayed to estimate the toxicity of PPAB-
Fig. 2 PL spectra of PPAB-2 (a) and PPAB-3 (b) (10 μM) in various hexane fractions
of CH2Cl2-hexane mixtures. Inset: their associated fluorescent photos in absence
and presence of 90% hexane taken under 365 nm UV irradiation.
continuous irradiation, the decrease ratio of the absorbance at
665, 673 and 682 nm for PPAB-1, PPAB-2 and PPAB-3 solution
was about 1.47%, 1.23% and 0.14%, respectively, much smaller
than that of ICG solution at 779 nm (21.01%). These results
indicated PPAB-1, PPAB-2 and PPAB-3 feature excellent
photostability characteristics. Hence, they are preferable for
long-term tracking due to the increase of circulation half-life
and excellent photostability compared to ICG.13
To gain insight into the molecular structures of the PPAB-
based luminogens, theoretical calculations were performed.
Density functional theory (DFT) with a B3LYP/6-31G (d) basis
set was used and the results of the molecular energy levels are
shown in Fig. S21 and Table S3 (ESI†). The electron densities of
the HOMO and LUMOs of PPAB-1 are mainly located on the
PPAB core. However, the electron densities of HOMO are
stretched to TPEH units for PPAB-2 and PPAB-3, which
indicated that there is partial electronic conjugation between
the TPEH moieties and PPAB core. The energy gap (Eg) of
2
NPs quantitatively by CCK-8 assay, in which the formation of
formazan dye depended on the mitochondria activity. The data
of OD 450 was directly proportional to the number of living
cells. Fig. S27 (ESI*) shows the viability of HeLa cells after
treatment with PPAB-2 NPs (1 μM, 10 μM) for a period of 1, 2
and 4 days. Results indicate that PPAB-2 NPs has a good
biocompatibility on HeLa cells. The number of cells increases
significantly from 1 to 4 days, suggesting that the cells
proliferate in all groups. Moreover, we introduced PPAB-2 NPs
into HeLa cells and realized the imaging by confocal
fluorescence microscopy. After the cells were incubated with
PPAB-2 NPs (50 μM) for 12 h, strong fluorescence was
exhibited in the cytoplasm (Fig. S28, ESI*). When incubation
time was extended to 48 h, fluorescence was further enhanced
PPAB-1, PPAB-2 and PPAB-3 are 2.049, 1.987 and 1.940 eV,
respectively (Table S3, ESI†). Moreover, a cyclic voltammogram
(CV) technique was used to measure their energy levels
experimentally. As shown in Fig. S22, all of the PPAB
compounds exhibited one or two reversible oxidations and
reductions. The experimental HOMO and LUMO energy levels
determined by the first oxidation and reduction potentials are
Fig. 3 Real-time fluorescence images showing PPAB-2 NPs (50 µM) stained
HeLa cells at room temperature. “B”: cells nuclei stained by 4’,6-diamidino-
2-phenylindole (DAPI), “R”: red fluorescence image of PPAB-2 NPs, “L”:
bright field, respectively.
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13 (a) W.-H. Jian, T.-W. Yu, C.-J. Chen, W.-C. Huang, H.-C. Chiu
(Fig. 3). As a control, PPAB-1 NPs in cell-imaging was also be
studied. Only weak red fluorescence was exhibited in the
cytoplasm when the cells were incubated with PPAB-1 NPs (50
μM) for 48 h (Fig. S29, ESI*). The results indicated that AIE-
active PPAB-2 NPs are preferable for long-term tracking of cell
bioimaging.
In summary, two AIE-active and NIR emissive PPABs have
been efficiently synthesized from diketopyrrolopyrrole and
heteroaromatic amines. While the one-pot synthesis described
leads to symmetric PPAB-3, the stepwise synthesis of the
chromophores now permits to obtain asymmetric PPAB-2. The
efficient NIR emission makes PPAB-2 NPs as a valuable AIEgen
for bioimaging due to its low cytotoxicity and excellent
photostability. Inspired by this, it is promising to generate
more AIE-active and NIR-emissive PPABs for organelle-specific
imaging in living cells through rational structure design.
Further studies on the development of such materials are
ongoing in our laboratory.
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Acknowledgements
The supports by the Natural Science Foundation of Guangdong
Province (2015A030313209, 2016A030311034), the Fundamental
Research Funds for the Central Universities (2017ZD075) are
gratefully acknowledged.
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