Aggregation-induced emission enhancement of chiral boranils

Article abstract of DOI:10.1039/c8nj03228a

New boranils based on chiral benzylamines have been synthesized and their photophysical properties studied. These BF₂-complexes exhibit a bright blue fluorescence in solution and in the solid state, and exhibit aggregation-induced emission enhancement. The chirality of the ligand, even if it is not directly located at the boron center, has consequences on the circular dichroism of the complexes.

Full text of DOI:10.1039/c8nj03228a

NJC
View Article Online
View Journal | View Issue
PAPER
Aggregation-induced emission enhancement
of chiral boranils†
ab
Cite this: New J. Chem., 2018,
42, 18166
b
a
Patrıcia A. A. M. Vaz,
Joao Rocha,
Artur M. S. Silva
and
´
˜
ab
Samuel Guieu
*
New boranils based on chiral benzylamines have been synthesized and their photophysical properties
studied. These BF -complexes exhibit a bright blue fluorescence in solution and in the solid state, and
Received 29th June 2018,
Accepted 7th October 2018
2
exhibit aggregation-induced emission enhancement. The chirality of the ligand, even if it is not directly
located at the boron center, has consequences on the circular dichroism of the complexes.
DOI: 10.1039/c8nj03228a
rsc.li/njc
1
. Introduction
Four-coordinate organoboron complexes have emerged as
promising materials for optoelectronics, including organic
light-emitting diodes (OLEDs), organic field-effect transistors,
1–3
photo-responsive materials, and sensory and imaging materials.
Fig. 1 Structures of compounds (+)-3a-(R), (À)-3a-(S) and 3b.
Concerning OLED applications, although numerous red and green
emitters showing good electroluminescence performance have
4
been reported, efficient blue emitters are still missing, in
boron complexes exhibiting AIEE have been reported and they are
5
–7
particular deep blue emitters for electroluminescent devices.
36,37
mainly based on polymeric compounds.
8
–11
Complexes of boron-dipyrromethene (BODIPY) derivatives,
Herein, the synthesis and a comparative study of two
enantiomeric boranils and a similar achiral compound are
presented (Fig. 1). The dyes exhibit bright blue fluorescence
in solution and in the solid state, presenting a strong AIEE
effect.
12–15
16–22
boranils
and boron b-diketonates
feature high fluores-
cence quantum yields and an excellent photostability. While many
four-coordinate organoboron complexes exhibit excellent fluores-
cence properties in dilute solutions, few examples are available of
emissive dyes in the solid state. This is mainly due to aggregation-
caused quenching (ACQ), prompted by inherent strong inter-
molecular p–p stacking interactions that consume the excitation
energy. Nevertheless, certain dyes present a behaviour opposite to
2
. Experimental section
23–27
2.1 General
ACQ, i.e., aggregation-induced emission (AIE)
induced emission enhancement (AIEE).
or aggregation-
28–34
In this case, the dye All reagents were purchased from Sigma-Aldrich and used
exhibits quenched or faint emission in solution but strong without any further purification. The compounds were purified
fluorescence upon aggregation. Based on theoretical and experi- by column chromatography on silica gel (Merck, 35–70 mesh),
mental studies, the restriction of intramolecular rotation (RIR) by preparative TLC carried out on 20 cm  20 cm glass plates,
has been proposed as the main cause of this phenomenon. coated with silica gel (0.5 mm thick) or by crystallization.
1
19
13
Because these materials emit both in solution and in the aggre-
H, F and C NMR spectra were recorded on a Bruker 300
35
1
19
13
gate state they are of much interest. Very few examples of chiral and 500 [300.13 MHz ( H), 282.41 MHz ( F), 75.47 MHz ( C) or
1
13
5
00.13 MHz ( H)]. Unequivocal C assignments were made on
the basis of 2D HSQC and HMBC experiments ( H/ C) (the
delays for long-range JC/H couplings were optimized for 7 Hz
experiments). CDCl was used as the solvent and tetramethyl-
silane (TMS) as the internal standard. Chemical shifts d are
reported in parts per million (ppm) relative to TMS (d = 0), and
the values of coupling constants ( J) are given in Hertz (Hz).
a
1
13
QOPNA, Department of Chemistry, University of Aveiro, Aveiro, Portugal.
E-mail: sguieu@ua.pt
b
CICECO, Department of Chemistry, University of Aveiro, Aveiro, Portugal
Electronic supplementary information (ESI) available: NMR spectra of new
3
†
compounds, absorption and emission spectra. CCDC 1852252–1852256. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8nj03228a
18166 | New J. Chem., 2018, 42, 18166--18171
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
Paper
High-resolution mass spectra analysis (HRMS-ESI ) was performed
on an LTQ OrbitrapTM XL hybrid mass spectrometer. Ultraviolet- CHQN), 7.22–7.36 (m, 7H, Harom), 6.96 (d, JH–H 8.2 Hz, 1H,
visible (UV-vis) spectra in solutions were obtained on a Shimadzu Harom), 6.86 (td, JH–H 7.5, JH–H 1.0 Hz, 1H, Harom), 4.54
NJC
+
1
H NMR (500 MHz, CDCl
3
) d 13.56 (s, 1H, OH), 8.40 (s, 1H,
3
3
4
3
3
UV-2501 PC spectrophotometer (1 cm path length quartz cell) and (q, JH–H 6.7 Hz, 1H, CH), 1.63 (d, JH–H 6.7 Hz, 3H, CH3).
the UV-vis absorption spectra of the solids were measured at room
13
C NMR (75 MHz, CDCl ) d 163.6 (CHQN), 161.2 (C–OH),
3
temperature on a JASCO V-560 instrument. The excitation and 144.0 (Carom) 132.4 (CHarom), 131.5 (CHarom), 128.8 (2C,
emission spectra, also in solutions, were recorded with a Jobin CHarom), 127.4 (CHarom), 126.5 (2C, CHarom), 119.0 (Carom),
Yvon FluoroMax-3 spectrofluorometer and a JASCO spectro- 118.7 (CHarom), 117.1 (CHarom), 68.7 (CH), 25.1 (CH
3
). HRMS-
+
+
fluorometer. Fluorescence quantum yields, j
f
, were determined ESI m/z for [C15
(E)-2-[(Benzylimino)methyl]phenol 2b . Benzylamine (1 equiv.,
H15NO + H] calcd 226.1232, found 226.1224.
38
using fluorescein in 0.1 M NaOH water solution as a fluores-
cence standard (j = 0.90). The CD spectra in solutions were 0.546 mL, 5.00 mmol) and salicylaldehyde (1 equiv., 0.522 mL,
f
obtained on a J-1500 CD spectrometer. The absolute emission 5.00 mmol) were added to methanol (10 mL), and the reaction
quantum yields in the solid state were measured using a system mixture was heated to reflux for one hour. After being cooled down
(Quantaurus-QY Plus C13534, Hamamatsu) with a 150 W xenon to room temperature, the solvent was removed under reduced
lamp coupled to a monochromator for wavelength discrimination, pressure to afford the product 2b as a yellow oil. The compound
an integrating sphere as the sample chamber, and a multichannel was purified by flash column chromatography over silica gel, using
analyzer for signal detection. Melting points were determined dichloromethane as the eluent (1.07 g, 5.0 mmol, quantitative).
on a BUCHI Melting point apparatus and are uncorrected. For Yellow oil.
crystal structure determination, a suitable single-crystal was
(R)-2,2-Difluoro-3-(1-phenylethyl)-2H-benzo[e][1,3,2]oxazaborinin-
mounted on a glass fibre with the help of silicon grease. Data 3-ium-2-uide 3a-(R). Compound 2a-(R) (1 equiv., 0.7 mmol,
were collected at 180(2) K on a Bruker X8 Kappa APEX II charge 155 mg) was dissolved in 1,2-dichloroethane (5 mL), into
coupled device (CCD) area-detector diffractometer (MoKa which triethylamine (4 equiv., 2.8 mmol, 0.4 mL) and boron
graphite-monochromated radiation, l = 0.71073 Å). Single trifluoride etherate complex (4 equiv., 2.8 mmol, 0.4 mL) were
crystals suitable for X-ray diffraction were grown from CH
2
Cl
2
added. The mixture was refluxed under an atmosphere of
solutions of 2a-(R), 2a-(S), 3a-(R), 3a-(S) and 3b. Full details are nitrogen for 24 h. After that, the mixture was poured into water
given in the ESI.† Crystallographic data for the structures (100 mL), and was extracted with dichloromethane, followed by
reported in this paper have been deposited with the Cambridge drying over anhydrous Na SO . After the removal of the solvent,
2
4
Crystallographic Data Centre as supplementary publication no. the crude product was purified by column chromatography
CCDC 1852252–1852256.† (silica gel) using dichloromethane as the eluent to afford 3a-(R)
(
151 mg, 0.6 mmol, 79%). White solid. m.p.: 165–166 1C.
2
.2 Synthesis
1
H NMR (500 MHz, CDCl
3
) d 8.01 (s, 1H, CHQN), 7.56 (ddd,
3
3
4
(R,E)-2-{[1-(Phenylethyl)imino]methyl}phenol 2a-(R). (R)-(+)-a-
JH–H 8.6, JH–H 7.7, JH–H 1.6 Hz, 1H, Harom), 7.37–7.48 (m, 5H,
3
4
Methylbenzylamine (1.5 equiv., 1.5 mL, 8.76 mmol) and salicyl- Harom), 7.24 (dd,
aldehyde (1 equiv., 0.60 mL, 5.84 mmol) were added to methanol (d, JH–H 8.6 Hz, 1H, Harom), 6.91 (td, JH–H 7.7, JH–H 0.8 Hz,
20 mL), and the reaction mixture was heated to reflux for 4 h. 1H, Harom), 5.58 (q, JH–H 6.9 Hz, 1H, CH), 1.87 (d, JH–H
After being cooled down to room temperature, the solvent was 6.9 Hz, 3H, CH3). C NMR (75 MHz, CDCl ) d 163.4 (CHQN),
J
H–H 7.7,
JH–H 1.6 Hz, 1H, Harom), 7.09
3
3
4
3
3
(
1
3
3
removed under reduced pressure to afford the product 2a-(R) as a 159.0 (C–O), 138.7 (Carom) 138.4 (CHarom), 131.6 (CHarom),
yellow solid. The compound was purified by recrystallization in 129.4 (2C, CHarom), 129.0 (CHarom), 128.2 (2C, CHarom),
ethanol (1.29 g, 5.72 mmol, 98%). Yellow solid. m.p. 75–76 1C; 120.0 (CHarom), 119.6 (CHarom), 115.5 (Carom), 59.0 (CH),
1
19
2
H NMR (300 MHz, CDCl
3
) d 13.55 (s, 1H, OH), 8.40 (s, 1H, 20.8 (CH
3
). F NMR (282 MHz, CDCl
3
) d À160.0 (d,
J
F–F
3
2
+
CHQN), 7.22–7.36 (m, 7H, Harom), 6.96 (d,
Harom), 6.87 (td,
J
H–H 8.2, 1H, 15.4 Hz, 1F), À160.2 (d, JF–F 15.4 Hz, 1F). HRMS-ESI m/z for
3
4
+
JH–H 7.5,
J
H–H 1.0 Hz, 1H, Harom), 4.54 [C15
H14BF
2
NO + H] calcd 274.1218, found 274.1200.
3
3
(
q, JH–H 6.7 Hz, 1H, CH), 1.63 (d, JH–H 6.7 Hz, 3H, CH3).
(S)-2,2-Difluoro-3-(1-phenylethyl)-2H-benzo[e][1,3,2]oxazaborinin-
C NMR (75 MHz, CDCl ) d 163.6 (CHQN), 161.2 (C–OH), 3-ium-2-uide 3a-(S). Compound 2a-(S) (1 equiv., 0.7 mmol,
1
3
3
144.0 (Carom) 132.4 (CHarom), 131.5 (CHarom), 128.8 (2C, 150 mg) was dissolved in 1,2-dichloroethane (5 mL), into
CHarom), 127.4 (CHarom), 126.5 (2C, CHarom), 119.0 (Carom), which triethylamine (2 equiv., 1.4 mmol, 0.2 mL) and boron
18.8 (CHarom), 117.1 (CHarom), 68.7 (CH), 25.1 (CH ). HRMS- trifluoride etherate complex (4 equiv., 2.8 mmol, 0.4 mL) were
15NO + H] calcd 226.1232, found 226.1218. added. The mixture was refluxed under an atmosphere of
1
3
+
+
ESI m/z for [C15
H
(
S,E)-2-{[(1-Phenylethyl)imino]methyl}phenol 2a-(S). (S)-(À)-a- nitrogen for 24 h. After that, the mixture was poured into water
Methylbenzylamine (1 equiv., 1.0 mL, 5.84 mmol) and salicyl- (100 mL), and was extracted with dichloromethane, followed by
aldehyde (1 equiv., 0.60 mL, 5.84 mmol) were added to methanol drying over anhydrous Na SO . After removal of the solvent, the
2
4
(20 mL), and the reaction mixture was heated to reflux overnight. crude product was purified by column chromatography (silica
After being cooled down to room temperature, the solvent was gel) using dichloromethane as the eluent to afford 3a-(S)
removed under reduced pressure to afford the product 2a-(S) as a (147 mg, 0.5 mmol, 77%). White solid. M.P.: 165–166 1C.
1
yellow solid. The compound was purified by recrystallization in
3
H NMR (500 MHz, CDCl ) d 8.02 (s, 1H, CHQN), 7.56
3
3
4
ethanol (1.08 g, 4.80 mmol, 82%). Yellow solid. m.p.: 75–76 1C. (ddd, JH–H 8.6, JH–H 7.5, JH–H 1.60 Hz, 1H, Harom), 7.37–7.48
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018 New J. Chem., 2018, 42, 18166--18171 | 18167

View Article Online
NJC
Paper
3
4
(
m, 5H, Harom), 7.24 (dd, JH–H 7.5, JH–H 1.6 Hz, 1H, Harom), obtained in high yields. The structure of the compounds was
3
3
4
7
1
3
.09 (d, JH–H 8.6 Hz, 1H, Harom), 6.91 (td, JH–H 7.5, JH–H 0.8 Hz, confirmed by standard spectroscopic techniques including
3
3
1
13
19
H, Harom), 5.47 (q, J
6.9 Hz, 1H, CH), 1.87 (d, J
6.9 Hz,
H NMR, C NMR, F NMR, and HRMS (see ESI†). The
H–H
H–H
13
1
H, CH3). C NMR (75 MHz, CDCl ) d 163.4 (CHQN), 159.0 formation of the complexes was confirmed by H NMR, which
3
(
C–O), 138.7 (Carom), 138.4 (CHarom), 131.6 (CHarom), 129.4 (2C, showed the disappearance of the OH singlet, and a shift in the
1
9
CHarom), 129.0 (CHarom), 128.2 (2C, CHarom), 120.0 (CHarom), peak of the imine proton. The presence of two F NMR
1
19
3
19.6 (CHarom), 115.5 (Carom), 59.0 (CH), 20.8 (CH ). F NMR doublets confirmed the complexation because in the chiral
2
2
(282 MHz, CDCl
3
) d À160.0 (d, JF–F 15.6 Hz, 1F), À160.2 (d, JF–F complexes the two fluorides are diastereotopic. The crystal
+
+
1
5.6 Hz, 1F). HRMS-ESI m/z for [C15
H
14BF
2
NO + HF + H] calcd structures of the asymmetric ligands and the three boranils
294.1280, found 294.9195.
unequivocally confirmed their structures, and the configuration
3
-Benzyl-2,2-difluoro-2H-benzo[e][1,3,2]oxazaborinin-3-ium- of the asymmetric carbon in the case of (+)-2a-(R), (À)-2a-(S),
2
-uide 3b. Compound 2b (1 equiv., 3.0 mmol, 633 mg) was (+)-3a-(R) and (À)-3a-(S).
dissolved in dichloromethane (10 mL), and triethylamine
2 equiv., 6.0 mmol, 1.16 mL) and boron trifluoride etherate
3.2 Photophysical properties
(
complex (2 equiv., 6.0 mmol, 0.835 mL) were added. The The absorption and emission spectra of (+)-3a-(R), (À)-3a-(S)
solution was stirred at room temperature for 1 h, and then and 3b are shown in Fig. 2. The three boranils possess similar
the solvent was removed under reduced pressure. The product molecular structures, with the only difference being in the alkyl
was purified by flask column chromatography over silica gel, bridge between the imine and phenyl moieties. As expected,
eluent dichloromethane, to afford the product as off-white they present almost identical absorption and emission spectra.
crystals after crystallization in DCM (350 mg, 1.40 mmol, The absorption and emission maxima of the two enantiomers
1
4
7%). White solid. m.p.: 128–129 1C. H NMR (500 MHz, CDCl ) are 351 nm and 430 nm, respectively. The absorption and
3
3
3
4
d 7.94 (br s, 1H, CHQN), 7.57 (ddd, JH–H 8.6, JH–H 7.3, JH–H emission maxima of 3b are slightly red shifted (352 and
3
1
7
1
.7 Hz, 1H, Harom), 7.50–7.42 (m, 3H, Harom), 7.39 (dd, JH–H 433 nm, respectively). All compounds exhibit a bright blue
.7, JH–H 1.7 Hz, 2H, Harom), 7.25 (dd, JH–H 7.1, JH–H 1.7 Hz, fluorescence under irradiation at 254 nm or 365 nm. The
H, Harom), 7.10 (dd, JH–H 8.6, JH–H 1.0 Hz, 1H, Harom), 6.92 fluorescence quantum yields of the boranils in CH Cl solution
ddd, JH–H 7.3, JH–H 7.1, JH–H 1.0 Hz, 1H, Harom), 4.95 (s, 2H, are ca. 34%, and are similar for the chiral and achiral dyes
4
3
4
3
4
2
2
3
3
4
(
1
3
CH2). C NMR (75 MHz, CDCl ) d 163.7 (CHQN), 159.1 (C–O), (Table 1).
3
138.5 (CHarom), 133.1 (Carom), 131.6 (CHarom), 130.3
The photophysical properties of the dyes in solution are
(
1
CHarom), 129.7 (CHarom), 129.4 (CHarom), 120.1 (CHarom), almost independent of the solvent (Table 1), as the absorption
1
9
2
19.6 (CHarom), 115.6 (Carom), 55.2 (CH ). F NMR (282 MHz, and emission maxima, molar extinction coefficients and
2
2
CDCl
3
) d À161.44 (d, JF–F 14.7 Hz, 1F), À161.55 (d, JF–F 14.7 quantum yields are very similar in toluene, CH Cl , methanol
2
2
+
+
2
Hz, 1F). HRMS-ESI m/z for [C14H12BF NO + H] calcd 260.1053, and acetonitrile. This was expected when compared to other
found 260.1045.
1
5
boranils reported in the literature.
Solid-state emission spectra are shown in Fig. 2. The emission
maxima are similar to the maxima measured in solution,
indicating that the dyes do not form excimers in the solid state.
The quantum yields are lower, 8–13% and 11% for the enantiomers
and the achiral dye, respectively (Table 1). The measurement of the
3
. Results and discussion
3.1 Synthesis
Boranils were obtained following a two-step strategy (Scheme 1).
First, the imines 2a-(R), 2a-(S) and 2b were prepared by a
condensation between salicylaldehyde and the appropriate
amines in refluxing methanol. The chiral amines were optically
pure, and a single enantiomer was obtained after the conden-
sation step, as confirmed by single crystal X-ray diffraction
(
see ESI†). The boron complexation was performed using
BF O under basic conditions (triethylamine) in refluxing
ÁEt
,2-dichloroethane. After purification, the three boranils were
3
2
1
Fig. 2 Absorption (full lines) and fluorescence spectra (short dotted lines)
excited at the corresponding labs max of compounds (+)-3a-(R), (À)-3a-(S)
and 3b in CH
(at 1 Â 10 mol L ).
2
Cl
2
À1
and in the solid state at room temperature
À5
Scheme 1 Synthesis of boranils (+)-3a-(R), (À)-3a-(S) and 3b.
18168 | New J. Chem., 2018, 42, 18166--18171
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
Paper
NJC
Table 1 Photophysical properties of (+)-3a-(R), (À)-3a-(S) and 3b
a
abs
b
À1
À1
c
em
d
f
Dyes
l
e (M cm
)
l
j
(%)
Solvent
3a-(R)
3a-(S)
3b
354
52 000
32 500
24 300
24 500
n.a.
46 600
31 800
14 900
24 900
n.a.
22 000
25 900
31 800
20 800
n.a.
433
430
431
430
420
433
430
432
430
420
436
433
434
432
434
31
32
33
35
13
32
34
36
37
8
35
36
37
40
11
Toluene
351
350
347
2 2
CH Cl
MeCN
MeOH
Solid
n.a.
354
Toluene
3
3
3
51
50
47
2 2
CH Cl
MeCN
MeOH
Solid
n.a.
352
Toluene
3
3
3
52
50
46
2 2
CH Cl
MeCN
MeOH
Solid
n.a.
Fig. 4 Aggregation-induced emission enhancement of (a) 3a-(R), (b) 3a-(S)
and (c) 3b in THF–water mixtures. (d) Aggregation-induced emission
enhancement of all compounds in THF–water mixtures.
a
b
Absorption maximum labs (nm). Molar absorption coefficients
À1
À1
c
d
(
M
cm ). Fluorescence maximum lem (nm). In solution: deter-
f
mined by comparison with fluorescein (j = 0.90 in water with NaOH
À1 39
0
.1 mol L
)
at room temperature; in the solid state: measured using
an integrating sphere, the excitation wavelength was 375 nm.
3.3 Aggregation-induced emission enhancement
To assess the AIEE characteristics of the dyes, the spectra of
solutions/suspensions in THF–water mixed solvents with
quantum yields in the solid state was carried out using a ‘‘pellet’’
obtained by compressing the crystals of the compounds, in order to
get a sample which can be handle in the fluorimeter: this compres-
sing process seems to change the crystalline arrangement of the
molecules, and to induce a quenching of the emission (as seen by
the naked eye, the single crystals of the compounds shine more
brightly than the ‘‘pellet’’ when put under a UV lamp). As the
compression is not controlled, it is somehow different for the three
boranils and this is reflected in the quantum yield measurement.
Fig. 3a shows the circular dichroism (CD) spectra of (+)-3a-
increased water fractions ( f
all compounds, when the water fraction was increased from
to 80 vol%, the emission intensity was almost unchanged.
w
) were recorded (Fig. 4). For
0
However, for a fraction of water above 80%, the compounds
precipitated and self-aggregated, and the fluorescence showed
a considerable enhancement.
The observation of an AIEE effect is in contradiction with the
values of the quantum yields in the solid state, which are lower than
those measured in solution (Table 1). But as explained in the
previous section, the measurement of the quantum yields is carried
out using a pellet, and not using single crystals. It seems that the
boranils presented here are sensitive to the crystalline phase, and
grinding and compressing the crystals to produce a pellet may
induce a change from a crystalline phase to an amorphous one.
The CD spectra of the suspensions were also recorded
(
R), (À)-3a-(S) and 3b in CH
2
Cl
2
. (+)-3a-(R) and (À)-3a-(S) show
mirror image CD spectra with their maxima matching the
maxima of the absorption spectra. As expected, compound 3b
does not present any CD. The chirality of the two enantiomers
was therefore confirmed by CD showing the optical purity of
the compounds.
(Fig. 3b). As expected, achiral compound 3b does not present
any CD, but the enantiomers (+)-3a-(R) and (À)-3a-(S) present
mirror image CD spectra, proving that the chirality of the dyes
is preserved in the aggregate forms.
3.4 Crystal structures
Single crystals suitable for X-ray diffraction were grown by slow
evaporation of a saturated solution in CH Cl . The crystal
2
2
structures of 2a-(R) and 2a-(S) are not discussed here, as they
only confirmed the structure of the ligands and their chirality.
The two enantiomers (+)-3a-(R) and (À)-3a-(S) crystallized in the
1
monoclinic non-centrosymmetric space group P2 , with Z = 2,
while 3b crystallized in the orthorhombic centrosymmetric
space group Pbca with Z = 8 (Fig. 5). The crystal structures of
(
+)-3a-(R) and (À)-3a-(S) confirmed the configuration of the
asymmetric centres. All bond distances and angles are in
4
0
a normal range. B–N and B–O bond lengths in (+)-3a-(R)
(B–O 1.420 Å, B–N 1.550 Å), (À)-3a-(S) (B–O 1.446 Å, B–N
1.571 Å) and 3b (B–O 1.450 Å, B–N 1.572 Å) are in accordance
Fig. 3 CD spectra of compounds 3a-(S), 3a-(R) and 3b in solution
À5
(
CH
2
Cl
2
, c = 10 M) (a) and in suspension (THF–water, 80% and 90% of
water) (b).
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018 New J. Chem., 2018, 42, 18166--18171 | 18169

View Article Online
NJC
Paper
(POCI-01-0145-FEDER-007679;
FCT–UID/CTM/50011/2013),
through national funds and where applicable co-financed by
the FEDER, within the PT2020 Partnership Agreement, and also
to the Portuguese NMR Network. This work was also supported
by the Integrated Programme of SR&TD ‘‘pAGE – Protein
aggregation Across the Lifespan’’ (reference CENTRO-01-0145-
FEDER-000003), co-funded by the Centro 2020 program, Portu-
gal 2020, European Union, through the European Regional
Development Fund. PV thanks the FCT for a doctoral grant
(SFRH/BD/99809/2014).
Fig. 5 Structure of 3a-(R), 3a-(S) and 3b revealed by single-crystal X-ray
diffraction. Ellipsoids are drawn at the 50% level, hydrogen atoms are
shown as spheres of an arbitrary radius of 0.30 Å; C, grey; H, white; N, blue;
O, red; B pink (top). Crystal packing of 3a-(R) and 3b viewed down the
b axis (down).
References
1
2
3
D. Li, H. Zhang and Y. Wang, Chem. Soc. Rev., 2013, 42, 8416.
Y. L. Rao and S. Wang, Inorg. Chem., 2011, 50, 12263.
D. Frath, J. Massue, G. Ulrich and R. Ziessel, Angew. Chem.,
Int. Ed., 2014, 53, 2290.
1
4,15,41
with those reported for similar compounds.
In all
crystals, the boranil cycle and the phenol ring are coplanar,
while the benzylic ring is not. The fluorine atoms are involved
in weak intermolecular hydrogen bonds C–HÁ Á ÁF. There are no
close contacts between aromatic rings.
The crystal structures provide an insight into the AIEE
properties of the dyes. In solution, the torsional vibrations
of the benzyl group may provide a non-radiative relaxation
channel, which lowers the emission efficiency. In the crystals,
these vibrations seem to be restricted, therefore forcing the
dyes to emit. In the meantime, the absence of close contacts
between the aromatic rings excludes the formation of J- or
H-aggregates and excimers as a potential reason for the emission
enhancement observed between solution and the solid state.
4
C. Li, J. Wei, J. Han, Z. Li, X. Song, Z. Zhang, J. Zhang and
Y. Wang, J. Mater. Chem. C, 2016, 4, 10120.
5
6
M. Zhu and C. Yang, Chem. Soc. Rev., 2013, 42, 4963.
T. M. H. Vuong, J. Weimmerskirch-Aubatin, J.-F. Lohier,
N. Bar, S. Boudin, C. Labbe, F. Gourbilleau, H. Nguyen,
T. T. Dang and D. Villemin, New J. Chem., 2016, 40, 6070.
Z. Zhang, H. Zhang, C. Jiao, K. Ye, H. Zhang, J. Zhang and
Y. Wang, Inorg. Chem., 2015, 54, 2652.
7
8
9
A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891.
G. Ulrich, R. Ziessel and A. Harriman, Angew. Chem., Int. Ed.,
2
008, 47, 1184.
0 T. Kowada, H. Maeda and K. Kikuchi, Chem. Soc. Rev., 2015,
4, 4953.
1 N. Boens, V. Leen and W. Dehaen, Chem. Soc. Rev., 2012,
41, 1130.
2 S. Chibani, A. Charaf-Eddin, B. Le Guennic and D. Jacquemin,
J. Chem. Theory Comput., 2013, 9, 3127.
3 D. Frath, S. Azizi, G. Ulrich, P. Retailleau and R. Ziessel, Org.
Lett., 2011, 13, 3414.
1
1
1
1
1
1
1
1
1
1
4
4
. Conclusions
Three new boranils have been prepared, two of which are based
on enantiopure chiral amines. The chiral center is located on
the carbon adjacent to the nitrogen, close to the chromophoric
part of the dyes, and the sigma-pi hyperconjugation may be
responsible for the distinct CD feature observed. These dyes
present a moderate quantum yield in both solution and the
solid state. Consideration of the crystal structures allowed
the AIEE effect observed for the dyes to be rationalized, and
the restriction of intramolecular rotations is proposed as the
main cause of the emission enhancement observed. These
boranils emit brightly in the solid state, and may find application
as blue emitting materials.
4 F. Cardona, J. Rocha, A. M. S. Silva and S. Guieu, Dyes Pigm.,
2014, 111, 16.
5 S. Guieu, F. Cardona, J. Rocha and A. M. S. Silva, New
J. Chem., 2014, 38, 5411.
6 G. Zhang, R. E. Evans, K. A. Campbell and C. L. Fraser,
Macromolecules, 2009, 42, 8627.
7 L. D. Costa, S. Guieu, J. Rocha, A. M. S. Silva and A. C. Tom ´e ,
New J. Chem., 2017, 41, 2186.
8 H. Maeda, Y. Kusunose, Y. Mihashi and T. Mizoguchi,
J. Org. Chem., 2007, 72, 2612.
9 E. Cogn ´e -Laage, J. F. Allemand, O. Ruel, J. B. Baudin,
V. Croquette, M. Blanchard-Desce and L. Jullien, Chem. –
Eur. J., 2004, 10, 1445.
Conflicts of interest
There are no conflicts to declare.
20 S. Guieu, J. Pinto, V. L. M. Silva, J. Rocha and A. M. S. Silva,
Eur. J. Org. Chem., 2015, 3423.
Acknowledgements
21 L. Wang, Z. Zhang, X. Cheng, K. Ye, F. Li, Y. Wang and
H. Zhang, J. Mater. Chem. C, 2015, 3, 499.
Thanks are due to University of Aveiro, FCT/MEC for the
financial support to the QOPNA research Unit (FCT UID/ 22 A. Nagai, K. Kokado, Y. Nagata and Y. Chujo, Macromole-
QUI/00062/2013) and CICECO–Aveiro Institute of Materials cules, 2008, 41, 8295.
18170 | New J. Chem., 2018, 42, 18166--18171
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
Paper
NJC
2
2
2
2
2
2
3 B.-K. An, J. Gierschner and S. Y. Park, Acc. Chem. Res., 2012, 33 J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam and
5, 544. B. Z. Tang, Chem. Rev., 2015, 115, 11718.
4 S. Guieu, J. Rocha and A. M. S. Silva, Tetrahedron Lett., 2013, 34 J. Zhang, H. Kang, N. Li, S. Zhou, H. Sun, S. Yin, N. Zhao and
4, 2870. B. Z. Tang, Chem. Sci., 2017, 8, 577.
5 Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu and 35 Y. Kubota, K. Kasatani, H. Takai, K. Funabiki and
J. Xu, Chem. Soc. Rev., 2012, 41, 3878. M. Matsui, Dalton Trans., 2015, 44, 3326.
6 F. Ciardelli, G. Ruggeri and A. Pucci, Chem. Soc. Rev., 2013, 36 Y. Zhou, J. W. Kim, M. J. Kim, W.-J. Son, S. J. Han,
4
5
42, 857.
H. N. Kim, S. Han, Y. Kim, C. Lee, S.-J. Kim, D. H. Kim,
J.-J. Kim and J. Yoon, Org. Lett., 2010, 12, 1272.
37 H. Maeda, T. Shirai, Y. Bando, K. Takaishi, M. Uchiyama,
A. Muranaka, T. Kawai and M. Naito, Org. Lett., 2013,
15, 6006.
7 X. Zhang, Z. Chi, Y. Zhang, S. Liu and J. Xu, J. Mater. Chem.
C, 2013, 1, 3376.
8 Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2011,
40, 5361.
2
9 S. Guieu, J. Rocha and A. M. S. Silva, Tetrahedron, 2013, 69, 9329. 38 D. Orteg o´ n-Reyna, C. Garc ´ı as-Morales, I. Padilla-Mart ´ı nez,
30 P. A. A. M. Vaz, J. Rocha, A. M. S. Silva and S. Guieu, New
J. Chem., 2016, 40, 8198.
E. Garc ´ı a-B ´a ez, A. Ar ´ı za-Castolo, A. Peraza-Campos and
F. Mart ´ı nez-Mart ´ı nez, Molecules, 2014, 19, 459.
31 W. Z. Yuan, X. Y. Shen, H. Zhao, J. W. Y. Lam, L. Tang, 39 A. M. Brouwer, Pure Appl. Chem., 2011, 83, 2213.
P. L. C. Wang, Y. Liu, Z. Wang, Q. Zheng, J. Z. Sun, Y. Ma 40 A. G. Orpen, L. Brammer, F. H. Allen, O. Kennard,
and B. Z. Tang, J. Phys. Chem. C, 2010, 114, 6090.
D. G. Watson and R. Taylor, J. Chem. Soc., Dalton Trans.,
32 J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
1987, S1–S83.
H. Sing Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem. 41 K. Dhanunjayarao, V. Mukundam, M. Ramesh and
Commun., 2001, 1740. K. Venkatasubbaiah, Eur. J. Inorg. Chem., 2014, 539.
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem., 2018, 42, 18166--18171 | 18171