Aza-boron-diquinomethene complexes bearing N-aryl chromophores: Synthesis, crystal structures, tunable photophysics, the protonation effect and their application as pH sensors

Article abstract of DOI:10.1039/c4tc02955k

A series of aza-boron-diquinomethene complexes (1a-1e) bearing different N-aryl chromophores were synthesized and characterized by multinuclear NMR spectroscopy, X-ray crystallography, optical absorption and emission spectroscopy, and elemental analysis. These robust thermal complexes possess tunable intense luminescence from blue to red with relatively high emission quantum yields. The introduction of different N-aryl chromophores into the aza-BODIQU core significantly tuned the emission colors. The relationship between their structures and properties was investigated systematically via spectroscopic methods and simulated by density functional theory (DFT) calculations. Additionally, the application of 1c as a pH sensor with a remarkable colour-changing property has been investigated. All these results indicate that these complexes exhibit robust thermal stability, tunable photophysical properties, relatively high photoluminescence quantum yields and protonation effect, making these complexes potential candidates for pH sensors, bioimaging probes and organic light-emitting materials.

Full text of DOI:10.1039/c4tc02955k

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ARTICLE TYPE
Aza-boron-diquinomethene complexes bearing N-aryl chromophores: synthesis,
crystal structures, tunable photophysics, protonation effect and their application
for pH sensor
Xiaolin Zhu,a Hai Huang,a Rui Liu,*a Xiaodong Jin,a Yuhao Li,b Danfeng Wang,a Qiang Wang,a Hongjun
Zhu*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A series of aza-boron-diquinomethene complexes (1a-1e) bearing different N-aryl chromophores were synthesized and characterized by
multinuclear NMR spectroscopy, X-ray crystallography, optical absorption and emission spectroscopy, and elemental analysis. These
10 robust thermal complexes possess tunable intense luminescence from blue to red with relatively high emission quantum yields. The
introduction of different N-aryl chromophores to the aza-BODIQU core significantly tuned the emission colors. The relationship between
structure and property was investigated systematically via spectroscopic methods and simulated by density functional theory (DFT)
calculations. Additionally, the application of 1c for pH sensor with remarkable colour-changing property has been investigated. All these
results indicate that these complexes exhibit the robust thermal stability, tunable photophysical properties, relatively high
15 photoluminescence quantum yields and protonation effect, making these complexes potential candidates in pH sensor, bioimaging probe
and organic light-emitting materials.
substituents that can be used as efficient organic light-emitting
materials.12 All these complexes exhibit robust thermal stability,
Introduction
intense blue to green emission and relatively high
Research on boron dipyrromethene (BODIPY) complexes have
45 photoluminescence quantum yields. Moreover, their low-lying
20 attracted a great deal of attention in the past decade, due to their
LUMO (lowest unoccupied molecular orbital) energy levels (~ -
broad applications in molecular probes,1 photodynamic therapy,2
2.63 to -2.92 eV), are desirable for electron transfer. In addition,
laser dyes,3 nonlinear optics,4 and dye-sensitized solar cells,5 etc. 6
we also reported an aza-BODIQU complex bearing N-substituted
The versatile applications of BODIPYs are intrinsically based on
phenothiazine chromophores as multi-stimuli-responsive
their narrow absorption and emission bands, large molar
50 luminescent material, which can be utilized in data encryption
25 absorption coefficients, high fluorescence quantum yields and
and decryption based on the protonation-deprotonation effect.13 It
outstanding chemical stability. Recently, structural modification
has been clearly demonstrated that diquinolinamine could be an
of BODIPYs for fine-tuning of their optical and physical
excellent building block in the construction of aza-BODIQU
properties has been extensively explored in energy conversion
based functional luminescent materials.
devices (organic light-emitting devices, OLEDs, and organic
55 Although the previous work is intriguing, the investigation on the
30 photovoltaics, OPV),7 solar energy conversion,8 and NIR-
aza-BODIQUs based luminescent materials is still limited.
absorbing systems.9 To meet the different requirements for
Furthermore, most of the reported aza-BODIQU complexes only
diverse applications, considerable work has been carried out on
exhibit luminescence in the region of blue-green. Clearly, aza-
tailoring of BODIPY cores, such as converting the traditional
BODIQU based luminescent materials with a tunable emission
boron dipyrromethene to boron dipyrroamine, boron
60 region are desirable, challengeable and potentially valuable. We
35 diindolamine, boron dipyridoamine, and boron diquinolinamine,
also seek to improve their luminescent properties by structural
modification. To this end, we choose an alternative strategy for
ect.1-9 Among them, diquinolinamine derivatives have been the
subject of significant study by virtue of their structure stability,
good electron-transport properties and high glass-transition
temperatures.10, 11
40 During our previous work, we reported a series of aza-boron-
diquinomethene (aza-BODIQU) complexes with different aryl-
developing
a series of new aza-BODIQU complexes by
introducing electron-donating chromophors on the aza-BODIQU
65 core directly. We envision that by incorporating N-aryl
chromophors (N-arylaminos) with different electron-donating
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Scheme 1. Synthetic routes of complexes 1a-1e.
ability and shortening the π-conjugation length between aza-
BODIQU core and peripheral chromophores, the HOMO (highest
occupied molecular orbital)/LUMO energies of these complexes
will be significantly tuned, hence, their emission region could be
Scheme 1 illustrates the synthesis of aza-BODIQU complexes
(1a-1e). The Buchwald-Hartwig coupling reactions yielded the
desired final products in 21-73% yields. All the products are air-
stable and soluble in CH2Cl2, ethylacetate (EA), tetrahydrofuran
5
extended accordingly. We also anticipate that the ground-state 30 (THF), dimethylformamide (DMF) and dimethyl sulphoxide
1
absorption and fluorescence quantum yield of the new
10 complexes could be enhanced.
(DMSO) in varying degrees. H NMR, 13C NMR, 19F NMR and
elemental analyses confirmed the proposed structures for all
complexes except 1a. 13C NMR data of 1a was not obtained due
to its poor solubility, although different solvents were attempted.
In this work, a series of aza-BODIQU complexes (1a-1e) with
different arylamino-substituents, structurally, N-benzoimidazolyl,
N-carbazolyl, 3,6-di-tert-butyl-N-carbazolyl, N-naphthalen-2-
yl(phenyl)amino and N-phenothiazinyl groups were synthesized
15 (Scheme 1). Compared to the previous aza-BODIQU complexes
which mainly show blue-green emission, the emission colors of
these complexes (1a-1e) are ranging from blue to red. The
photophysical, crystal structures and electrochemical properties
of complexes 1a-1e were investigated systematically.
20 Additionally, fluorescence of 1c changes based on protonation
effect have been investigated, which show that different
acidic/basic atmospheres can induce a switch in the emission
from green to red.
35 X-ray crystallography
Single crystals of compound 1b, 1c and 1e suitable for X-ray
analysis were successfully obtained and their crystal structures
were determined. Disappointingly, efforts to grow good-quality
crystals of compounds 1a and 1d were unsuccessful. The
40 molecular structures of compound 1b, 1c and 1e are shown in Fig.
1. 1b, 1c and 1e crystallize in the triclinic P-1, monoclinic P21/c,
and triclinic P-1 space groups, respectively. Their bond lengths
and angles are within the typical range. The details of the
crystallographic data for these crystals are summarized in Table
45 S4 (ESI ).
†
Results and discussion
As anticipated, all these complexes adopt twisted
conformations. In complex 1b, the two N-carbazole rings are
twisted out of the planar core structure, with dihedral angles of
25 Synthesis and characterization
2
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63.4o and 67.1o, respectively. For complex 1c, the 3,6-di-tert-
Table 1. Thermal properties of aza-BODIQUs 1a-1e a.
butyl-N-carbazole rings form dihedral angles of 55.9o and 40.6o
with the aza-BODIQU core. Whereas for 1e, in which the two N-
aryl chromophores are N-phenothiazine rings, the bowl-shaped
phenothiazine rings are almost vertically twisted from the planar
aza-BODIQU core (Fig. 1). This quite obvious intramolecular
torsion indicates that such a noncoplanar structure will prevent
the molecules from packing in a close π-π stacking mode, which
are proven by their packing motifs in packing diagrams (Fig.
Complex
T
m (°C)
1a
1b
1c
1d
1e
266
383
250
283
315
5
10 S15-17, ESI ). Strong intermolecular π-π interactions and other
†
strong intermolecular interactions are almost absent. At the same
time, the intramolecular bond rotational and vibrational freedom
is still restricted by the intermolecular C-H…π hydrogen bonds
formed between the aromatic hydrogen and the aromatic rings.
15 And one 1b molecule forms two C-H…F hydrogen bonds with
a DSC scans of 1a-1e recorded under nitrogen during the second heating
cycle at a scan rate of 10 °C min−1
.
Thermal properties
two of its neighbor molecules (Fig. S15, ESI ). The distances of
†
the intramolecular C-H…F hydrogen bonds range from 2.38 Å to
2.52 Å. The weak interactions between two neighbor molecules
are from 3.73 to 3.79 Å. Similarly, complex 1c molecule forms
20 two C-H…F hydrogen bonds with its two neighbor molecules.
The distances of the intramolecular C-H…F hydrogen bonds
range from 2.30 Å to 2.54 Å. The weak interactions between two
35 The thermal properties of compounds 1a-1e were investigated by
differential scanning calorimetry (DSC) analyses, the results are
listed in Table 1. These complexes are thermally stable and lack
any detectable phase transitions from 50 °C to 230 °C.
Furthermore, all these complexes exhibit endothermic peaks,
40 good thermal stability with high melt transitions temperatures (>
250 °C), which indicate excellent thermal stability and are
suitable for the application of optical devices, such as the OLEDs.
neighbor molecules are from 3.71 to 3.73 Å (Fig. S16, ESI ).
†
Whereas for 1e, the complexes adopt a stacking mode of H-type
25 aggregation along a axis via intermolecular C-H…F, π-π and C-
H…π interaction, and the vertical distance between them was
Electronic absorption
measured to be 3.51
Å
(Fig. S17, ESI ). These weak
†
The UV-Vis absorption of aza-BODIQUs (1a-1e) obeys
45 Lambert-Beer’s law in the concentration range studied (1×10-6
-
intermolecular interactions plus the twisted intramolecular
geometries are the factors that account for the strong fluorescence
30 of 1b, 1c and 1e in the solid state.
Fig. 1. Molecular structures of 1b (CCDC 885792), 1c (CCDC 1021584) and 1e (CCDC 996071) obtained by X-ray diffraction.
(a)
(c)
(b)
1a
1b
1c
1d
1e
1.0
0.8
0.6
0.4
0.2
0.0
1a
1b
1c
1d
1e
150000
100000
50000
0
ε
400
450
500
550
300
400
500
600
400
500
600
Wavelength (nm)
Wavelength (nm)
Wavelenth (nm)
50 Fig. 2. (a) UV-Vis absorption spectra of 1a-1e in CH2Cl2 (1×10-5 M); (b) UV-Vis absorption spectra of 1a-1e in solid state; (c) UV/Vis spectra of 1c in CH2Cl2 with
addition of -TsOH /CH3CN solution.
p
1×10-4 mol L-1) in CH2Cl2, suggesting that no dimerization or
oligomerization occurs within this concentration range in CH2Cl2
. The UV-Vis absorption spectra of 1a-1e in CH2Cl2 and solid
shows sharp and strong absorption (ε = 1.65 × 105 L mol-1 cm-1)
in the range of 391-443 nm, which is ascribed to the 1π-π*
transitions localized on the conjugated aromatic rings,
55 state are presented in Fig. 2. The photophysical parameters are 60 considering its intense and structured absorption bands and minor
listed in Table 2. As shown in Fig. 2a and Table 2, complex 1a
solvatochromic effect (Fig. S18, ESI ). In the other hand,
†
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complexes 1b-1e show relatively broad bands (ε = 1.4 × 104 - 1.1
× 105 L mol-1 cm-1) in the range of 438-505 nm, which can be
ascribed to the 1π-π* mixed intramolecular charge transfer (1ICT)
ability. The dramatic bathochromic shifts of 1b-1d in emission
spectra are observed (Fig. S19, ESI ), owning to the intra-
molecular electron push-pull effect. Whereas for 1e, because of
an aza-BODIQU core structure and two N-substituted
†
1
transitions. The solvent effect also supports the nature of π-π*
5
mixed 1ICT transitions for their absorption bands (Fig. S18, 40 phenothiazine chromophores with
a twisted bowl-shaped
ESI
†
). The possible mixture of the 1ICT character could be
configuration, thus, complex 1e displays AIE effect and multi-
ascribe to the strong electron-donating N-Aryl chromophors. It is
further supported by the acid titration study with p-TsOH, which
shows red-shifted ICT for 1b-1d in acidic solution (Fig. S23,
stimuli responsive fluorescent properties.13 All the complexes
exhibit ΦPL in the range of 0.24-0.78. Their ΦPL in CH2Cl2
solution follow the trend of 1b > 1c > 1d > 1e > 1a, in which
1
10 ESI
†
). As shown in Fig. 2c, for 1c, upon addition of p-TsOH, the 45 complex 1b exhibits the highest photoluminescence quantum
absorption band at about 466 nm decreases with peak red-shift,
yield (ΦPL = 0.78) (Table 2). The minor solvatochromic effect and
accompanied by the increase of a new absorption band at about
337 nm. A similar phenomenon was observed for 1b and 1d. In
addition, compared with the absorption spectra in solution,
structured emission bands observed for 1a indicate the 1π-π*
dominated features of their emitting states (Fig. S19, ESI
However, the emission bands of complexes 1c and 1d
†
).
15 broader absorption bands with significant bathochromic shifts are 50 bathochromically shift in more polar solvents (i.e., CH2Cl2 and
observed in the solid state (Fig. 2b and Table 2). This could be
attributed to the molecular stacking in the solid state, which
suggests an increased π-conjugation length in their solid state due
to the π-stacking of the planar conformations of aza-BODIQUs.
CH3CN) compared to those in solvents with lower polarity (i.e.,
hexane and toluene). The drastic positive solvatochromic effect
observed in complexes 1c and 1d should be attributed to the
electron-donating groups (N-naphthalen-2-yl(phenyl)amino, and
55 3,6-di-tert-butyl-N-carbazolyl substituents), which enhances
electron transfer from the symmetrical peripheral chromophores
20 Photoluminescence in solution
1
to the aza-BODIQU motif. To verify the involvement of ICT
The photoluminescence of complexes 1a-1e in different solvents
were investigated at room temperature. Their normalized
emission spectra in CH2Cl2 at a concentration of 1 × 10-5 mol/L
are illustrated in Fig. 3a. The emission band maxima and
25 quantum yields are listed in Table 2. As we anticipated,
connecting different electron-donating chromophors on the aza-
BODIQU core directly can prominently help to achieve the aim
that aza-BODIQU complexes 1a-1e emit tunable emission
varying from blue to red. Complex 1a emits blue light in CH2Cl2
30 solution, and its stokes shift is quite small. Taking the feature into
character into the emitting states, the emission of the acid titration
study of 1a-1d has been investigated. Fig. 3c shows the emission
60 spectra of 1c with addition of p-TsOH. Upon addition of p-TsOH,
the emission of 1c at about 507 nm decreases, accompanied by
the increase of a new relatively weak emission at about 627 nm,
which exhibits a ca. 120 nm red-shift. This can be rationalized by
the fact that protonation of the nitrogen atoms on the aza-
65 BODIQU core increases the electron-withdrawing ability, which
results in enhanced 1ICT character and emission red-shifted.
Similar phenomenon was observed for 1b and 1d, (Fig. S24,
1
account, we can assign the observed emission to the π-π* state.
ESI ) whereas for 1a, the emission at about 473 nm decreases,
†
Meanwhile, the emission of 1b-1d are significantly red-shifted to
green and yellow, due to the ICT process caused by the N-
carbazolyl, 3,6-di-tert-butyl-N-carbazolyl and N-naphthalen-2-
35 yl(phenyl)amino substituents with strong electron-donating
accompanied by the increase of a new relatively weak emission at
1
70 about 428 nm, which exhibits a ca. 45 nm blue-shift. These ICT
features are also supported by the DFT calculation results.
(a)
(b)
(c)
1.0
0.8
0.6
0.4
0.2
0.0
1a
1b
1c
1d
1e
3500
1.0
0.8
0.6
0.4
0.2
0.0
3500
1a
1b
506 nm
3000
2500
3000
2000
1c
1d
1500
1000
2500
500
1e
0
0
2
4
6
8
10
2000
1500
1000
500
0
Cp-TsOH/C1f
400
500
600
700
500
600
700
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Fig. 3. Emission spectra of 1a-1e in CH2Cl2 (1×10-5 M) (a); in solid state (b); (c) emission spectra of 1c in CH2Cl2 with addition of
p-TsOH /CH3CN solution.
75
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Fig 4. The optimized geometries and the molecular orbital surfaces of the HOMOs and LUMOs for aza-BODIQU 1a-1e obtained at the B3LYP/6-31G* level.
Theoretical calculations
indicates that such a noncoplanar propeller-shaped structure will
prevent the molecules from packing in a close π-π stacking mode.
To understand the nature of the ground state and the low-lying
excited states, quantum chemical calculations were performed for
complexes 1a-1e. The ground-state electron density distribution
Thus, an AIE future is expected to observe from complex 1e, and
5
40 our previous work confirmed this.13 In addition, as is evident
from Table 3, the trend of the predicted Egcal values is consistent
of the HOMO and LUMO are illustrated in Fig. 4. The HOMO-
with the estimated Egopt from the ground-state absorption in Table
cal
LUMO energy differences (energy band gaps, calculated Eg
)
2.
are presented in Table 3. The LUMOs of complexes 1a-1e are
10 almost delocalized on the aza-BODIQU cores. Due to the
difference of dihedral angle (36o to 98o) between the N-
substituents rings and the core structure, the π-electrons in the
HOMO of 1a-1d are delocalized over the entire molecule
backbone, offering effective orbital interactions among the
15 stacked π-systems. Meanwhile, the HOMOs of 1e are localized
on the N-aryl rings. Therefore, the HOMO→LUMO transition in
1a-1e should be ascribed to a mixture of 1π-π* mixed with
pH sensor
45 Because of 1c showing the most significant protonation effect
among 1a-1d, the pH sensor experiments of 1c switching
fluorescence from green to red were carried out. To certify the
reversible transformation by protonation and deprotonation, the
1H NMR spectra of protonation and deprotonation have been
50 studied (Fig. 5). Addition of a drop of trifluoroacetic acid (TFA)
into a chloroform solution of 1c protonates its central N atom and
generates 1c-H+ (Scheme 2). These protons shift downfield in the
1H NMR spectra after protonation because of the transformation
1
different ICT characters, which is consistent with their UV-vis
absorption assignments. The results clearly demonstrate that
20 strong electron-donating substituent, such as N-naphthalen-2-
yl(phenyl)amino for 1d, increases the energy level of HOMO
more than that of LUMO. Thus the HOMO-LUMO gap
Table 2. Optical properties of aza-BODIQUs 1a-1e in CH2Cl2 solution and in
55 solid-state.
1
decreases, causing a red-shift of the π-π*/1ICT absorption band.
This not only follows the trend observed from the UV-vis
25 absorption measurement but also explains the bathochromic shifts
observed in the absorption, especially for 1d bearing strong
λmaxabs (nm) a
λmaxem (nm) b
CH2 Solid-
Egopt (eV) c
d
Complex
ΦPL
εmax
(104 M-1
cm-1)
Solid-
CH2Cl2 state
Solid-
state
electron-
donating
N-naphthalen-2-yl(phenyl)amino
substituents19. In addition, as is evident from Table 3, the trend of
CH2Cl2
Cl2
state
cal
opt
the predicted Eg values is in consistent with the estimated Eg
30 from the ground-state absorption.
1a
1b
1c
443
457
466
406
415
481
16.5 473
11.4 483
491
529
522
2.76
2.59
2.52
2.36
2.24
2.15
0.24
0.78
0.73
It is worthy to notice the optimized geometric structure of 1e. The
core structure of aza-BODIQU is planar, and the two N-
substituted phenothiazine rings are nearly coplanar (the dihedral
angle between two phenyl rings is 35o). However, the dihedral
35 angles between the N-substituted phenothiazine rings and the core
structure are 98o. This quite severe intramolecular torsion
6.2
493
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40 hydrochloride (HCl) vapour, the fluorescence was tuned to red
emission. Then fuming this test paper with triethylamine (TEA)
for 5 min, the bright green fluorescence could be observed again.
The switch between red and green can be repeated for more than
three times. To further understand the protonation effect, DFT
45 calculations of 1c and 1c-H+ were performed. As shown in Fig. 8,
the dihedral angles between the 3,6-di-tert-butyl-N-carbazolyl
and the core structure in the optimized structure of 1c-H+ are both
43.0o, which are smaller than those in 1c (52.3o and 53.2o). Thus,
after protonation, its conformation becomes more planar and the
50 C-N single bonds connecting the N-substituents and the core are
1d
1e
505
440
465
418
7.5
1.4
540
423
570
540
2.32
2.70
1.97
2.48
0.51
0.28
a Recorded in 1 × 10−5 M CH2Cl2 solution at r.t.b Recorded in 10−7 M CH2Cl2
solution of at r.t. c Optical band gaps determined from the absorption edge of
d
the normalized absorption spectra. Determined in CH2Cl2 using 9,10-
diphenylanthracene (
ΦPL = 0.90 in cyclohexane) as standard at r.t.
5
Table 3. DFT calculation results of HOMO/LUMO energy levels.
also shorter (Table S5, ESI
electronic communications in 1c-H+ with a high conjugation and
hence a smaller energy band gap (Fig. S24, ESI ). Thus, the
emission switches to red region. All these results demonstrate that
55 complex 1c is a fluorescence pH sensor with remarkable colour-
changing.
†
). These results indicate better
Complex
LUMO (eV)
-2.57
HOMO (eV)
-5.90
Egcal (eV)a
3.33
†
1a
1b
1c
1d
1e
-2.60
-5.89
3.29
-2.51
-5.66
3.15
-2.25
-5.35
3.10
-2.43
-5.06
2.63
Scheme 2. Reversible transformation between 1c and 1c-H+ by repeated
protonation and deprotonation.
a Carried out at the B3LYP/6-31G* level of theory.
of 1c change to an electron-deficient form 1c-H+. After adding
excess triethylamine (TEA), the 1H NMR spectra is fully
recovered, suggesting that the reversible transformation between
10 1c and 1c-H+. Based on the significant protonation effect, we
envision that the fluorescence of 1c could be switched by the
solutions with different pH. Thus, we explored its application as a
fluorescent pH sensor, using THF-buffer mixtures (1:9, v/v) with
different pH as model compounds. As shown in Fig. 6a, 1c in
15 THF-buffer mixture (1:9, v/v) at pH 1 exhibits weak red
fluorescence due to its transformation from 1c to 1c-H+ under
such acidic condition. At pH 2-4, the fluorescence of 1c blue-
shifted with the emission intensity increased, because the ratio of
1c-H+ decreases in the solutions. When pH increased from 5 to 10,
20 the fluorescence intensity of 1c at 507 nm was obviously
enhanced. At pH = 10, the emission intensity become maximum.
At pH 10-12, the emission intensity got a little weaker than the
emission intensity at pH 10.
60
Fig. 5. 1H NMR spectra of 1c in CDCl3 containing (A) 0 and (B) 10 μL TFA (C)
adding 50 μL triethylamine (TEA) into (B).
The pH sensor was also performed on the solid supports.
25 Prepared filter paper of complex 1c and buffer solutions with
different pH were employed for experiments. As shown in Fig. 6b,
the test paper shows faint red fluorescence under 365 nm UV
light by addition of buffer solution at pH = 1. From pH 2 to 4, the
emission blue-shift from yellow to green with intensity enhanced
30 gradually. At pH > 5, the green emission was enhanced gradually
until when pH=10 the emission got maximum, which is similar as
the detections in the solutions. Evidently, the detections both in
the solution and solid-state can also work well as a sensitive pH
sensor. However, the detection in solid-state is much more rapid
35 and convenient than that in the solution since the concentration in
solid-state is much higher than in solution.
(a)
8000
6000
4000
2000
0
450
500
550
600
650
700
Wavelength (nm)
Moreover, the prepared test paper of 1c can also used for acid and
base vapours detection. The 1c test paper shows strong green
fluorescence under 365 nm UV light (Fig. 7). After exposing in
6
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Journal of Materials Chemistry C
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DOI: 10.1039/C4TC02955K
(b)
Fig. 7. (a) Emission spectra of initial, HCl fumed and TEA fumed test paper
30 of 1c; (b) 1c paper samples (left to right: blank, HCl vapor fumed sample,
TEA vapor fumed sample after fumed by HCl vapor ) taken under UV light.
450
500
550
600
650
700
Wavelength (nm)
Fig. 6. Effect of pH on (a) PL spectra of 1c in THF-buffer mixtures (1/9, v/v)
and (b) deposited on filter paper. Insets in (a and b): fluorescent photos at
pH 1 (left) to 12 (right) taken under 365 nm UV irradiation.
5
Experimental section
Fig. 8. The dihedral angles of the rings between nitrogen heterocyclic rings
and cores according to the ground-state optimized structure of 1c and 1c-H+
35 in gas state.
Synthesis
All solvents and reagents employed were purchased from Aldrich
or Sinopharm Chemical Regent Co. Ltd. and used as received
unless otherwise stated. Tetrahydrofuran (THF) and toluene were
using an HP-8453 UV/Vis/near-IR spectrophotometer (Agilent)
with a 1 cm quartz cell. Photoluminescence spectra were carried
out on
a
F-4600 FL Spectrophotometer (Hitachi).
10 distilled under N2 over sodium benzophenone ketyl. Tetra-n-
butylammonium perchlorate (TBAP) and ferrocene were purified
by recrystallization twice from ethanol. Silica gel (300-400 mesh)
used for chromatography was purchased from Sinopharm
Chemical Reagent Co. Ltd. Buffer solutions with pH 1 to 12 were
15 purchased from Sigma-Aldrich.
Photoluminescence quantum yields were carried out on a F-4600
40 FL Spectrophotometer (Hitachi). Powder X-ray diffraction (XRD)
patterns of the samples were collected on a Bruker D8 Advance
powder diffractometer. Electrochemical experiments were carried
out using a CHI 660C electrochemistry workstation (CHI-USA).
A standard one-compartment three-electrode cell was used with a
45 Pt electrode as the working electrode, a Pt wire as the counter
electrode and an Ag/AgNO3 electrode (Ag in 0.1 M AgNO3
solution) as the reference electrode. TBAP (0.1 M) was used as
the supporting electrolyte. The scan rate was 100 mV S-1.
Differential scanning calorimetry (DSC) was conducted on a
50 DSC instruments (NETZSCH DSC 204).
Instruments
1H NMR and 13C NMR spectra were recorded on a Bruker 300
MHz or 400 MHz spectrometer using DMSO-d6 or CDCl3 as the
solvent. Chemical shifts were referred to the internal standard
20 tetramethylsilane (TMS). 19F NMR spectra were recorded at
room temperature with CFCl3 as internal standard at 376 MHz.
Melting points (m.p.) were taken on an X-4 microscope
electrothermal apparatus (Taike China). The elemental analyses
were performed with a Vario El III elemental analyzer. Mass
25 spectra were obtained on a VG12-250 mass spectrometer and an
Agilent 1100 mass spectrometer. UV-Vis spectra were recorded
X-ray diffraction crystallography
Crystals 1b, 1c and 1e were grown by the slow evaporation of
their solutions in the mixture solvent of ethyl acetate and
petroleumether at room temperature. All diffraction data were
55 collected on a Rigaku Saturn diffractometer with a CCD area
detector. All calculations were performed by using SHELXL97
and the crystallographic software packages. CCDC 885792 (1b),
CCDC 1021584 (1c) and CCDC 996071 (1e) contain the
supplementary crystallographic data for this paper.
1c
(a)
1c-HCl
1c-HCl-Et N
1.0
0.8
0.6
0.4
0.2
0.0
3
60 DFT calculations
All the calculations were carried out using Gaussian 09 program
package. The geometries of complexes 1a-1e were fully
optimized at B3LYP/6-31G* level. Harmonic vibrational
frequencies were calculated at the same level to check whether
65 the obtained structure is a minimum. The frontier molecular
orbitals were calculated for each of the compounds. In this work,
all the calculations were performed in vacuum without
considering the solvent.
500
550
600
Wavelength (nm)
650
700
750
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7

Journal of Materials Chemistry C
Page 8 of 10
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DOI: 10.1039/C4TC02955K
Synthesis of difluoro-boron complex of bis(6-bromoquinolin- 55 P21/c, a = 22.4457(17) Å, b = 11.3035(7) Å, c = 20.0181(14) Å, α
2-yl)amine (BrQNBF)
= 90°, β = 92.623(5)°, γ = 90°, V = 5073.6(6) Å3, Z = 4, ρcalcd
=
1.144 g/cm3, T = 296 K, Crystal size 0.30 × 0.15 × 0.10 mm3, R1
Compound
quinoline),
2
(6-bromoquinoline N-oxide),
(6-bromoquinolin-2-amine),
3
(2,6-dibromo-
(bis(6-bromo-
= 0.1316, wR2 = 0.2674, [I > 2sigma(I)].
4
5
5
quinolin-2-yl)amine) and BrQNBF (difluoro-boron complex of
Difluoro-boron
complex
of
bis(6-(N-naphthalen-2-yl-
bis(6-bromoquinolin-2-yl)amine) were synthesized according to 60 phenylamino) quinolin-2-yl) amine (1d)
the reference.12
145 mg of yellow solid was obtained, yield 64%, m.p. = 283-284
General procedure for synthesis of 1a-1e
oC. 1H NMR (400 MHz, CDCl3 ) δ 8.47 (d, J = 9.4 Hz, 2H), 7.76
(dd, J = 17.7, 8.6 Hz, 6H), 7.62 (d, J = 7.7 Hz, 2H), 7.58-7.48 (m,
4H), 7.46-7.36 (m, 4H), 7.31 (dd, J = 8.0, 5.4 Hz, 8H), 7.19 (d, J
65 = 7.7 Hz, 4H), 7.15-7.08 (m, 4H). 13C NMR (100 MHz, CDCl3) δ
153.14, 147.22, 144.84, 139.67, 134.42, 134.20, 130.42, 129.60,
129.31, 127.73, 127.10, 126.50, 125.98, 124.84, 124.49, 123.75,
123.04, 122.80, 121.05, 120.47. 19F NMR (376 MHz, CDCl3) δ -
127.18 (q, JB,F = 73.6 Hz). Elemental analysis calcd. (%) for
70 C50H34BF2N5: C, 79.68; H, 4.55; N, 9.29, found C 79.59; H, 4.51;
N, 9.25.
To a round bottom flask was charged BrQNBF (237 mg, 0.5
10 mmol), arylamines(1.1 mmol), t-BuONa (120 mg, 1.2 mmol),
Pd(OAc)2 (6 mg, 0.025 mmol, 5% mol), t-Bu3P (0.05 mmol) and
3 mL toluene14. The reaction mixture was refluxed under N2 for
12 h. After cooling to r.t., it was diluted with brine (20 mL) and
extract with CH2Cl2 (3 × 30 mL). The organic layer was then
15 dried with Na2SO4 and the solvent was removed under reduced
pressure. The product was purified by silica gel column
chromatography (CH2Cl2 / hexane).
Difluoro-boron complex of bis(6-(N-phenothiazinyl) quinolin-
Difluoro-boron
complex
of
bis(6-(N-benzoimidazolyl)
2-yl) amine (1e)
quinolin-2-yl) amine (1a)
o
121 mg of yellow solid was obtained, yield 65%, m.p. > 300 C.
20 128 mg of yellow solid was obtained, yield 54%, m.p. = 264-266
oC. 1H NMR (400 MHz, CDCl3) δ 8.48 (d, J = 9.6 Hz, 4H), 7.93
(d, J = 9.2 Hz, 4H), 7.84 (d, J = 2 Hz, 4H), 7.80 (d, J = 2.4 Hz,
2H), 7.78 (d, J = 2.4 Hz, 2H), 7.21 (d, J = 8.8Hz, 4H). 19F NMR
(376 MHz, CDCl3) δ -126.25 (q, JB,F = 78.0 Hz).
75 1H NMR (400 MHz, CDCl3) δ 8.82 (d, J = 9.1 Hz, 2H), 8.04 (d, J
= 9.1 Hz, 2H), 7.81-7.68 (m, 4H), 7.29 (s, 2H), 7.12 (dd, J = 7.3,
1.6 Hz, 4H), 6.99-6.82 (m, 8H), 6.42 (d, J = 7.7 Hz, 4H). 13C
NMR (100 MHz, CDCl3) δ 143.75, 140.33, 138.53, 132.59,
127.12, 126.69, 124.20, 123.30, 122.23, 117.53. 19F NMR (376
80 MHz, CDCl3) δ -126.26 (q, JB,F = 73.3 Hz). Elemental analysis
calcd. (%) for C58H34BF2N5: C, 81.98; H, 4.03; N, 8.24, found C
82.09; H, 4.11; N, 8.15. Crystal data for 1e (C42H26BF2N5S2): Mr
= 713.61, triclinic, space group P-1, a = 8.2448(13) Å, b =
13.187(2) Å, c = 19.061(3) Å, α = 100.053(14)°, β = 95.703(13)°,
85 γ = 95.546(12)°, V = 2016.7(6) Å3, Z = 2, ρcalcd = 1.175 g/cm3, T
= 293(2) K, R1 = 0.0952, wR2 = 0.2014, [I > 2sigma(I)].
25 Difluoro-boron complex of bis(6-(N-carbazolyl) quinolin-2-
yl)amine (1b)
o
221 mg of yellow solid was obtained, yield 69%, m.p. > 300 C.
1H NMR (400 MHz, CDCl3) δ 8.91-8.88 (m, 2H), 8.18 (d, J = 7.7
Hz, 4H), 8.09 (d, J = 9.1 Hz, 2H), 7.97 (dd, J = 9.2, 2.4 Hz, 2H),
30 7.92 (d, J = 2.3 Hz, 2H), 7.52-7.40 (m, 8H), 7.40-7.29 (m, 6H).
13C NMR (100 MHz, CDCl3) δ 154.35, 140.75, 140.29, 137.21,
134.75, 130.12, 126.22, 125.91, 125.68, 123.91, 123.71, 123.60,
120.50, 120.43, 109.55. 19F NMR (376 MHz, CDCl3) δ -126.10
Conclusions
(q, JB,F
=
73.4 Hz). Elemental analysis calcd. (%) for
A series of new aza-boron-diquinomethene complexes (1a-1e)
bearing different N-arylamines has been designed and
35 C42H26BF2N5: C, 77.67; H, 4.03; N, 10.78 , found C 77.49; H,
4.11; N, 10.85. Crystal data for 1b (C48H38BF2N5O2): Mr = 90 synthesized. Their thermal stabilities and photophysical
765.64, triclinic, space group P1(2), a = 9.0720(18) Å, b =
15.181(3) Å, c = 16.186(3) Å, α = 65.63(3)°, β = 86.22(3)°, γ =
77.79(3)°, V = 1984.05(70) Å3, Z = 2, ρcalcd = 1.281 g/cm3, T =
40 293(2) K, Crystal size 0.11 × 0.26 × 0.28 mm3, R1 = 0.0767, wR2
= 0.1565, [I > 2sigma(I)].
properties were investigated systematically. All these complexes
show robust thermal stability with high melt transitions
temperatures (> 250 °C). All complexes exhibit strong 1π-π*
transfer mixed with varying degrees of 1ICT characters and
95 intense blue to red 1π-π*/1ICT emission with different
photoluminescence quantum yields (0.24-0.78). Their
photoluminescence quantum yields follow the trend of 1b > 1c >
1d > 1e > 1a. These results indicate that the direct connection of
N-arylamino groups onto the core structure is a succeed strategy
100 which would be useful for rational design of new aza-BODIQU
molecules. In addition, the application of 1c for pH sensor based
on its remarkable colour-changing property has been investigated,
which can be used in both the solution and solid states as well as
acid and base vapours. All these results indicate that these
105 complexes exhibit the robust thermal stability, tunable
photophysical properties, relatively high photoluminescence
quantum yields and protonation effect, make these complexes
potential candidates for applications in pH sensor, bioimaging
Difluoro-boron
complex
of
bis(6-(3,6-di-tert-butyl-N-
carbazolyl) quinolin-2-yl) amine (1c)
196 mg of yellow solid was obtained, yield 73%, m.p. = 250-251
45 oC. 1H NMR (400 MHz, CDCl3) δ 8.87 (d, J = 9.2 Hz, 2H), 8.18
(d, J = 1.6 Hz, 4H), 8.07 (d, J = 9.1 Hz, 2H), 7.97 (dd, J = 9.3,
2.4 Hz, 2H), 7.90 (d, J = 2.4 Hz, 2H), 7.50 (dd, J = 8.7, 1.9 Hz,
4H), 7.43 (d, J = 8.6 Hz, 4H), 7.33 (d, J = 9.1 Hz, 2H). 13C NMR
(100 MHz, CDCl3) δ 154.27, 143.46, 140.29, 139.10, 136.93,
50 135.29, 129.85, 125.96, 125.11, 123.77, 123.53, 116.48, 109.01,
34.83, 32.05. 19F NMR (376 MHz, CDCl3) δ -126.27 (q, JB,F
=
73.4 Hz). Elemental analysis calcd. (%) for C58H58BF2N5: C,
79.71; H, 6.69; N, 8.01, found C 79.59; H, 6.76; N, 8.15. Crystal
data for 1c (C58H58BF2N5): Mr = 873.90, monoclinic, space group
8
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Journal of Materials Chemistry C
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DOI: 10.1039/C4TC02955K
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Acknowledgment
13 X. L. Zhu, R. Liu, Y. H. Li, H. Huang, Q. Wang, D. F. Wang, X. Zhu,
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The authors greatly acknowledge the financial support in part by
Postgraduate Innovation Fund of Jiangsu Province (2013,
CXZZ13_0459; 2014, KYLX_0774), Natural Science Foundation
of the Jiangsu Higher Education Institutions of China
(13KJB150018), Natural Science Foundation of Jiangsu Province
(BK20130926).
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10 Notes and references
a Department of Applied Chemistry, College of Sciences, Nanjing Tech
University, Nanjing 211816, P. R. China. E-mail: zhuhj@njtech.edu.cn;
rui.liu@njtech.edu.cn.
b Department of Chemistry, Fudan University, Shanghai 200433, P. R.
15 China.
†
Electronic Supplementary Information (ESI) available: structural
characterization data, crystal stacking spectra, calculation and
photophysical studies. CCDC 885792, 1021584 and CCDC 996071. For
ESI and crystallographic data in CIF or other electronic format see DOI:
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Page 10 of 10
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DOI: 10.1039/C4TC02955K
A series of new aza-boron-diquinomethene complexes (1a-1e) with tunable
photophysical properties has been designed and synthesized.