Red fluorescent zwitterionic naphthalenediimides with di/mono-benzimidazolium and a negatively-charged oxygen substituent

Article abstract of DOI:10.1039/d1cc03586j

The C-H/C-X cross-coupling of a benzimidazolium salt with 2Br-NDI afforded two unprecedented zwitterionic NDIs with di/mono-benzimidazolium and an extra negatively-charged oxygen substituent. They exhibited intensified red fluorescence in polar solvents and negative solvatochromism due to an intramolecular charge transfer process, and could specifically label lysosomes and the endoplasmic reticulum in living A549 cells, respectively. They represent a rare case of NDI-derived ionic fluorophores.

Full text of DOI:10.1039/d1cc03586j

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Red fluorescent zwitterionic naphthalenediimides
with di/mono-benzimidazolium and a negatively-
charged oxygen substituent†
Cite this: DOI: 10.1039/d1cc03586j
Received 5th July 2021,
Accepted 16th August 2021
Chunqin Wang,a Songyan Han,a Tianbao Wang,a Xuesong Zheng,a Linsen Zhou,b
a
a
Yanhong Liu,a Cheng Zhang
and Ge Gao
*
DOI: 10.1039/d1cc03586j
rsc.li/chemcomm
The C–H/C–X cross-coupling of a benzimidazolium salt with 2Br-NDI and anion affinity.9 More recently, NDIs with cationic functional-
afforded two unprecedented zwitterionic NDIs with di/mono- ities have drawn much attention. Mukhopadhyay et al. reported the
benzimidazolium and an extra negatively-charged oxygen substituent. in situ synthesis of an extraordinarily stable radical cation of
They exhibited intensified red fluorescence in polar solvents and nega- diphosphonium-substituted NDI.10 This kind of NDI could be
tive solvatochromism due to an intramolecular charge transfer process, further reduced into air-stable, highly electron-rich doubly zwitter-
and could specifically label lysosomes and the endoplasmic reticulum in ionic derivatives.11 By replacement of one phosphonium moiety
living A549 cells, respectively. They represent a rare case of NDI-derived with an alkyl/aryl amino group, stable, high-SOMO zwitterionic
ionic fluorophores.
radicals were obtained.12 The same group later showcased that
diviologen-modified NDIs could accept up to six electrons within a
1,4,5,8-Naphthalenediimide and its derivatives (NDIs) are pla- narrow window of about 1 V.13 Tam and Wu et al. synthesized
nar, electron deficient and p-conjugated molecules,1 which various dipyridinium-functionalized NDIs with different counter
have been extensively studied during the past decades due to anions, and found interesting anion shuttling between the NDI
their widespread application in the fields of optoelectronic core and the pyridinium moieties.14 It needs to be pointed out that
devices,2 chemical sensors,3 organic dyes,4 and bio-imaging.5 these cationic NDIs are not emissive.
Unmodified NDI generally has an absorption below 400 nm
Due to our continuous interests in ionic functional
and weakly emits in the range of 400–450 nm.6 While substitu- materials,15 we previously used the imidazolium unit as an
tion on the diimide nitrogen atom shows limited influence on electron-withdrawing functionality to build a typical D–p–A
the optical and electrochemical properties of NDIs, profound fluorophore, which featured blue-colored aggregation-induced
impacts are found following core modification. For example, emission.16 In 2018, we developed a Cu-catalyzed C–H/C–X
electron donating alkoxy and alkylamino groups substituted at cross-coupling reaction of imidazolium salts with aryl halides,
the 2-, 3-, 6- and 7-positions significantly boost the fluorescence by which a phenylene-bridged dibenzimidazolium compound
and bathochromically shift the emissions via intramolecular was constructed to show electrochromic characteristics.17 We
charge transfer (ICT).7 Expansion of the p-conjugation by therefore wanted to explore the unknown properties of electron
fusion with (hetero)aromatic rings can narrow the energy band deficient NDI-bridged dibenzimidazolium compounds synthe-
gap and enhance p–p interactions, which may eventually sized using this method. To the best of our knowledge, no
improve the intermolecular charge transport for high perfor- benzimidazolium-substituted NDI has been reported yet.18
mance organic field-effect transistors (OFETs).8
Herein, we would like to present the synthesis of two unex-
On the other hand, the introduction of electron withdrawing pected zwitterionic di/mono-benzimidazolium C-substituted
groups such as trifluoromethyl, cyano, sulfoxide and sulfone NDIs with an extra negatively-charged oxygen substituent,
decreases the LUMO energy level, resulting in increased p-acidity which emit red fluorescence in polar solvents and specifically
label different organelles in living cells.
a Key Laboratory of Green Chemistry and Technology of Ministry of Education,
College of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu 610064,
P. R. China. E-mail: gg2b@scu.edu.cn
Starting from a commercially available 1,4,5,8-naphthalene-
tetracarboxylic dianhydride (NDA), 2,6-dibromo-1,4,5,8-naphthalene-
tetracarboxylic dianhydride (2Br-NDA) was obtained by bromination
using 1,3-dibromo-5,5-dimethylhydantoin (DBH) according to the
literature procedure.19 Subsequent condensation of 2Br-NDA with
n-butylamine afforded the key intermediate 2Br-NDI in moderate
yield (see the ESI†).20 The C–H/C–X cross-coupling reaction of
b Institute of Materials, China Academy of Engineering Physics, Mianyang 621907,
P. R. China
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization data. CCDC 2091134. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/d1cc03586j
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in Fig. 1. It is obvious to see the additional oxygen atom at the
C3-position. No isolated counter anion was found in the crystal
lattice, which unambiguously confirmed its zwitterionic struc-
ture. The bond length between C2 and C9 is 1.479 Å, which is
within the range of a typical C–C single bond. The core NDI is
not planar but slightly twisted. The dihedral angle between the
benzimidazolium ring and the NDI plane is 85.11, indicating
very limited electron communication between these two units.
The cationic benzimidazolium ring could probably only endow
NDI with a pure electron-withdrawing inductive effect.
The optical properties of 1 and 2 were first studied in DMF
(Fig. 2). Their DMF solutions were purple blue-colored and the
UV-vis spectra similarly showed two absorption bands.
The band in the UV area between 300–400 nm corresponds to
the characteristic absorption of the NDI core.1,6 The main band
is bathochromically shifted into the visible region at around
580 nm, suggesting a strong ICT process. A shoulder peak
emerges at lower wavelengths, presumably related to a vibro-
nic fine structure in resonance with the naphthale-
nolate skeleton.21 The molar extinction coefficient (e) of 2 is
Scheme 1 Synthesis of compounds 1–3. Conditions: (a) 2Br-NDI
(0.1 mmol), 1,3-diethylbenzimidazolium bromide (0.3 mmol), Cu2O
(40 mol%), a base (0.3 mmol), and DMF (1 mL), N2, 120 1C, 24 h; (b) 2Br-NDI
(0.1 mmol), benzimidazole (0.4 mmol), Cu2O (40 mol%), KOH (0.48 mmol), and
DMSO (1 mL), N2, 120 1C, 48 h; (c) bromoethane (0.5 mL) and acetonitrile
(1 mL), 80 1C, 16 h. Insets: Photos of 1–3 in the solid state.
2Br-NDI with 1,3-diethylbenzimidazolium bromide under the 36 400 MÀ1 cmÀ1, which is about three times larger than that of
catalysis of Cu2O in the presence of a base provided two purple- 1 (12 000 MÀ1 cmÀ1). 1 and 2 both emitted red fluorescence
colored spots on the TLC plates, which differed from the known with maximum emissions at 655 and 641 nm, respectively. The
cationic NDI derivatives. These two compounds were isolated, quantum yield of 2 was measured to be 3.0%, while 1 showed a
purified using column chromatography and characterized using lower value of 0.35%, which is probably due to more nonradia-
NMR and high-resolution mass spectroscopy (HRMS). To our tive decay arising from one more benzimidazolium substituent.
surprise, however, besides the designed mono- and dibenzimida- The similar spectra of 1 with a bromide anion and 2 without a
zolium substitutions on the NDI core, a C–H oxygen substitution counter anion suggested that there is no anion–p interaction
occurred next to the benzimidazolium moiety concomitantly, between the NDI core and the bromide anion.
resulting in two zwitterionic NDI derivatives 1 and 2 (Scheme 1).
Compounds 1 and 2 were soluble in common polar solvents
1 was a claret-colored solid with a metallic luster, while 2 was a such as chloroform, ethyl acetate, acetonitrile, acetone,
1
rust-colored solid. In the H NMR spectra, 1 showed a singlet at dimethyl sulfoxide, and methanol, etc. Furthermore, 1 could
8.39 ppm, corresponding to only one proton being left on the NDI even be dissolved in water (Fig. S1, ESI†). A negative solva-
core. For 2, two doublets at 8.66 and 8.22 ppm are related to two tochromism22 was observed (Fig. S2, S3 and Table S3, ESI†).
adjacent protons on the NDI core, implying that debromination The maximum absorption of 1 was hypochromically shifted
occurred. The HRMS analysis showed ion peaks at 739.3608 and 63 nm from 604 nm in dichloromethane to 541 nm in water,
567.2600, respectively, which were in agreement with the struc- while a 47 nm shift was recorded for 2 from 583 nm in
tures of 1 (calcd [M À BrÀ]+: 739.3602) and 2 (calcd [M + H]+: dichloromethane to 539 nm in methanol. These zwitterionic
567.2602). By varying the base used (Table S1, ESI†), the produc- molecules were expected to be stabilized by polar solvents,
tion of 1 and 2 could be tuned: as high as 54% yield of 2 was resulting in a lowered HOMO energy as well as an enlarged
reached when sodium acetate was used, and 1 was only detected HOMO–LUMO gap, which eventually led to hypochromic shifts
in a trace amount; when potassium hydroxide was employed, 1 of the absorption. Although the emissions were less shifted by
was isolated in 45% yield accompanied by 2 in 11% yield. We did variation of the solvent, the intensities were enhanced with the
not find any compound without the extra oxygen substituent
as we initially expected. For comparison, dibenzimidazolium
N-substituted NDI 3 with no oxygen substituent was obtained
via a sequential N-arylation and quarternization procedure
(Scheme 1, for details, see the ESI†). 3 was a wheat-colored solid.
It is noteworthy that heating 2Br-NDI or 3 alone in the absence of
the benzimidazolium salt under otherwise identical conditions
resulted in no reaction and recovery of the starting material,
indicating a unique activating effect of the C-substituted benzi-
midazolium on the NDI core for oxygen substitution.
Single crystals of 2 were luckily obtained in a mixed solution
of methanol, acetonitrile and ethyl acetate, and the structure
was resolved using single crystal X-ray diffractometry as shown hydrogen atoms are omitted for clarity.
Fig. 1 ORTEP drawing with ellipsoids drawn at the 50% level for 2. The
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revealed that the dihedral angles between the C-substituted benzi-
midazolium moiety and the NDI plane were 831 and 851, respec-
tively (Fig. S8, ESI†), which is consistent with the single crystal
structure of 2. Their HOMOs and LUMOs are therefore mainly
located on the NDI skeleton and the oxygen substituent with a
small separation, suggesting an ICT process from the oxygen
substituent to the NDI core. The similar MO distributions of 1
and 2 explain their similar optical spectra. The DFT-optimized
geometry of 3 disclosed a 611 dihedral angle between the
N-substituted benzimidazolium moiety and the NDI plane, leading
to the distribution of the HOMO mainly on the benzimidazolium
moiety and the LUMO on the NDI skeleton (Fig. S7 and S8, ESI†).
Therefore, the unique red fluorescence of 1 and 2 originates from
the ICT process between the strongly electron-donating negatively-
charged oxygen atom and the NDI core without any contribution
from the C-substituted benzimidazolium moieties. In compound 3,
Fig. 2 UV-vis (solid lines) and emission spectra (dotted lines) of 1 and 2
(1 Â 10À5 M) in DMF at r.t. The absorption of compound 3 was measured in
methanol. Insets: Photos of solutions of 1–3 under (a) ambient light and (b)
365 nm irradiation.
increase of solvent polarity. The quantum yield of 1 in water the two N-substituted benzimidazolium moieties served as typical
was enhanced about 10 times compared to that in DMF to electron-withdrawing groups through the resonated and positively-
3.42%, and that of 2 in MeOH was enhanced about 2 times to charged nitrogen atoms, quenching the inherently very weak
6.43%. This phenomenon is in contrast to that observed for fluorescence of the NDI core.
alkylamino-substituted NDI fluorophores, which exhibit con-
Time-dependent density functional theory (TD-DFT) calcula-
siderably diminished fluorescence in protic solvents.7a The tions were performed at the optimally tuned PBE1PBE/6-
optical property of the reference compound 3 was measured 31g(d,p) level of theory on the above optimized geometries of
in methanol because it decomposed slowly in DMF. The 1–3. The maximum absorption of 1 at 587 nm was characterized
methanol solution was almost colorless and its absorption by HOMO - LUMO (S2, 558 nm, oscillator strength f = 0.2384).
spectrum exhibited only one absorption band in the UV region, The absorption of 2 at 583 nm was attributed to HOMO -
which is attributed to the NDI core. 3 emitted no fluorescence LUMO (S2, 548 nm, f = 0.2277). However, the HOMO - LUMO
at all.
transition is prohibited in 3 (f o 0.01). A large contribution
Cyclic voltammetry (CV) was then conducted to measure the from the HOMOÀ4 - LUMO transition and a small contribu-
energy levels of 1–3 (Fig. S4, S5 and Tables S4, S5, ESI†). 1 and 2 tion from the HOMOÀ8 - LUMO transition (S16, 368 nm,
showed two reversible reduction peaks during the cathodic f = 0.2799) were responsible for its absorption at 375 nm.
sweep, and no oxidation peak was observed during the anodic
The red fluorescence in highly polar solvents of ionic 1 and 2
sweep. The first reduction potential of 1 was more positive than implied potential applications as biomarkers in living cell label-
that of 2. The LUMO energy levels of 1 and 2 were determined to ling, taking advantage of the low cell damage by visible-light
be À3.89 and À3.60 eV from the E1/2 values of the first excitation, the low interference from background fluorescence
reduction potentials using the equation: ELUMO = À4.80 eV À and the good membrane penetrating ability.15e,23 Hence, the
Ered1. The HOMO energy levels were calculated to be À5.86 and fluorescence imaging capabilities of 1 and 2 were evaluated in
À5.57 eV according to the equation: EHOMO = ELUMO À Eg, A549 cells. The cells were cultivated with 1 in PBS and with 2 in
wherein Eg was estimated from the edge of the absorption PBS containing 1% DMSO for 15 minutes at 37 1C, respectively.
maximum. In comparison with the LUMO (À3.83 eV) and The cell labelling status was viewed under a confocal microscope
HOMO (À7.01 eV) of the parent NDI, the HOMO energy levels to show that both 1 and 2 penetrated and labeled the A549 cells
are significantly elevated while the LUMO energy levels are less with red fluorescence (Fig. S9 and S10, ESI†). The morphological
affected. However, the LUMO of 2 is raised by 0.23 eV. These distribution of the fluorescence suggested that 1 might gather in
results suggested that the negatively charged oxygen atom the lysosomes, while 2 might be located in the endoplasmic
exerts a more profound electron-donating influence on the reticulum (ER). Co-staining experiments using commercially avail-
NDI core than the electron-withdrawing effect of the benzimi- able Lyso-Tracker Green and ER-Trackert Green were subse-
dazolium moieties, in line with the bathochromically-shifted quently conducted, respectively. As seen in Fig. 3, the images
absorption and red-light emission properties of 1 and 2. In the from the red fluorescence channel overlapped well with images
absence of an electron-donating moiety, the LUMO of 3 is from the green fluorescence channel, and the Pearson’s coeffi-
significantly lowered as predicted (see the ESI†).
cient values were calculated to be 0.93 and 0.91 accordingly (Table
To further understand the optical properties of 1–3, DFT S9, ESI†). Moreover, the cytotoxicities of 1 and 2 were also
calculations at the B3LYP/6-31g(d,p) level were performed and the examined using MTS (CellTiter 96 AQueous One Solution Cell
molecular orbital (MO) distributions are shown in Fig. S6 (ESI†). Proliferation Assay) (Fig. S11, ESI†). 1 and 2 showed low cytotoxi-
The HOMO/LUMO energies of 1 and 2 were calculated to be À5.90/ cities as the cell viability was about 80% at a concentration of
À3.35 and À5.80/À2.96 eV (Fig. S7, ESI†), respectively, which are in 4 mM. These results demonstrated the specific fluorescent label-
accordance with the measurements. The DFT-optimized structures ling ability of 1 and 2 towards different organelles in living cells.
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zwitterionic NDIs with electron-withdrawing di-/mono-benzimida-
zolium moieties and an extra electron-donating negatively-
charged oxygen substituent. They have similar LUMO levels and
significantly elevated HOMO levels in comparison with the parent
NDI, suggesting a larger impact of the later. They not only exhibit
unusual enhanced red fluorescence in polar solvents and negative
solvatochromism, but also specifically label lysosomes and the
endoplasmic reticulum, respectively. These dual functional NDIs
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Conflicts of interest
There are no conflicts to declare.
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