Asymmetric Difluoroboron Quinazolinone-Pyridine Dyes with Large Stokes Shift: High Emission Efficiencies Both in Solution and in the Solid State

Article abstract of DOI:10.1002/chem.201803428

Boron difluoroquinazolinone-pyridine (BODIQPy) and substituted derivatives have been designed and synthesized. Strong emission with high fluorescence quantum yield both in solution (up to 0.99) and in solid state (up to 0.60) was achieved in these asymmetric BODIQPys. For 6-halogen substituted BODIQPys, the heavy atomic effect of bromine and iodine was suppressed in solution due to the strong electron-withdrawing ability of the BODIQPy core. As a result, 6-iodine substituted BODIQPy shows the unusual fluorescence quantum yields (>0.70) in dichloromethane and tetrahydrofuran. In the solid state, the asymmetric structure induced a unique dimeric structure, and efficient luminescence was also observed. The formation of halogen bonds between carbonyl oxygen and bromine or iodine regulated the stacking mode and enhanced intermolecular interaction, resulting in a decline in the fluorescence quantum yield. In addition, a large Stokes shift was also achieved in these asymmetric BODIQPys.

Full text of DOI:10.1002/chem.201803428

DOI: 10.1002/chem.201803428
Communication
&
Dyes and Pigments
Asymmetric Difluoroboron Quinazolinone-Pyridine Dyes with
Large Stokes Shift: High Emission Efficiencies Both in Solution
and in the Solid State
Jia Zhou,[a] Lu Liu,[a] Ying Pan,[a] Qiaoyu Zhu,[a] Yajie Lu,[a] Jiacheng Wei,[a] Kang Luo,[a]
Yang Fu,*[a] Cheng Zhong,*[b] Yiyuan Peng,[a] and Zhibin Song*[a]
cluding high photochemical stability, large absorption coeffi-
cient, and high fluorescence quantum yield in solution.[5] But
Abstract: Boron difluoroquinazolinone-pyridine (BODIQ-
two deficiencies of BODIPY dyes are their generally small
Stokes shift (5–20 nm, in most cases)[6] and low fluorescence in
the solid state.[7]
Py) and substituted derivatives have been designed and
synthesized. Strong emission with high fluorescence quan-
tum yield both in solution (up to 0.99) and in solid state
(up to 0.60) was achieved in these asymmetric BODIQPys.
For 6-halogen substituted BODIQPys, the heavy atomic
effect of bromine and iodine was suppressed in solution
due to the strong electron-withdrawing ability of the
BODIQPy core. As a result, 6-iodine substituted BODIQPy
shows the unusual fluorescence quantum yields (>0.70)
in dichloromethane and tetrahydrofuran. In the solid
state, the asymmetric structure induced a unique dimeric
structure, and efficient luminescence was also observed.
The formation of halogen bonds between carbonyl
oxygen and bromine or iodine regulated the stacking
mode and enhanced intermolecular interaction, resulting
in a decline in the fluorescence quantum yield. In addi-
tion, a large Stokes shift was also achieved in these asym-
metric BODIQPys.
Recently, new types of difluoroboron (BF2) complexes
through ligand design,[8] such as anilido-pyridine,[9] diketone,[10]
pyridomethene,[11] pyrimidine,[12] and pyrazine,[13] have been
developed to realize large Stokes shifts (>80 nm). However,
most of them exhibit weak or quenched fluorescence in the
solid state due to the aggregation-caused quenching (ACQ). A
successful strategy to increasing emission efficiency of BF2
complexes in the solid state is to introduce a structure with ag-
gregation-induced emission (AIE) property.[14] However, the
fluorescence of those compounds with AIE properties is usually
weak in solution.[15] To design and synthesis of asymmetric di-
fluoroboron complexes is a promising strategy to achieve ef-
fective emission both in solution and in the solid state, which
provides possibilities to reduce the intermolecular p–p stack-
ing interactions and make more energetic difference between
the ground and excited states.[9,16] Until now, to the best of
our knowledge, the difluoroboron complexes with high effec-
tive emission both in solution and solid state were rare.[17]
Herein, we report the facile synthesis and fluorescence proper-
ties of asymmetric difluoroboron quinazolinone-pyridine com-
plexes (BODIQPys), which exhibit ultra-high emission efficien-
cies with particularly high Stokes shifts both in solution and in
the solid state. And the experimental and theoretical simula-
tion results revealed that this asymmetric structure design
could afford controllable luminescence in different states.
Considering that the introduction of halogen may bring di-
verse effects on photophysical properties of BODIQPys. The 6-
halogen substituted BODIQPys were also designed. The prepa-
ration of five BODIQPys can be achieved through two-step re-
actions (Scheme 1). The quinazolinone-pyridine based ligands
The development of new fluorescent dyes with good emission
efficiency both in solution and in the solid state have attracted
increasing attention due to their potential applications in
many research fields, such as molecular and biomolecular sen-
sors,[1] solid-state solar concentrators,[2] organic light-emitting
diodes[3] and photodynamic therapy.[4] As a class of the most
important fluorescent chromophores, boron dipyrromethene
(BODIPY) dyes is known to have excellent characteristics in-
[a] J. Zhou, L. Liu, Y. Pan, Q. Zhu, Y. Lu, J. Wei, K. Luo, Y. Fu, Prof. Dr. Y. Peng,
Prof. Dr. Z. Song
Key Laboratory of Functional Small Organic Molecules
Ministry of Education, College of Chemistry and Chemical Engineering
Jiangxi Normal University
Nanchang 330022 (P.R. China)
E-mail: yangfu@jxnu.edu.cn
zbsong@jxnu.edu.cn
[b] Dr. C. Zhong
College of Chemistry and Molecular Sciences
Wuhan University, Wuhan, 430072 (P. R. China)
E-mail: zhongcheng@whu.edu.cn
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201803428.
Scheme 1. Synthesis of difluoroboron quinazolinone-pyridine complex.
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were obtained from o-anthranilamides and picolinaldehyde
through Brønsted acid-driven aerobic oxidation in high
yields.[18] The corresponding difluoroboron complexes were
prepared with BF3·OEt2 in CH2Cl2 with Et3N at room tempera-
ture.[11,14,19] All the structures of five BODIQPys can be recog-
nized, because the NÀH proton signal of the quinazolinones
1
near 12 ppm in H NMR spectra disappeared since the coordi-
nation with BF2, and further confirmed by high resolution ESI-
MS and single-crystal structure analysis (see Supplementary In-
formation).
The UV/Vis spectrums of all five difluoroboron complexes
were examined in different solvents. In acetonitrile, five BOD-
IQPys showed better molar absorptivity. Compared to those
uncoordinated precursors (Figures S1 and S2, Supporting Infor-
mation), the highest absorption peak of all difluoroboron com-
plexes shows a slight redshift (20–30 nm), which could be in-
duced by the coordination of the BF2, and the planarity and p-
conjugation of the molecule has been enhanced after the at-
tachment of BF2 (Figure 1). It is noteworthy that the introduc-
Figure 2. Absorption (solid line) and emission (dashed line) spectra of 3a
(black line), 3b (red line), 3c (blue line), 3d (pink line), and 3e (green line) in
acetonitrile. Inset: pictures of 3a–3d solutions excited at 365 nm using a UV
lamp.
emission efficiencies in solution under the concentration of
10À5 m. (Table 1). It is noteworthy that the introduction of a
bromine atom at the 6-position of BODIQPy, which usually
produces a heavy atomic effect, did not reduce the fluores-
cence quantum yield. And 3e, which is 6-iodine substituted
BODIQPy still had a high fluorescence quantum yield (>0.7) in
CH2Cl2 and THF, compared to existing iodine-substituted
BODIPY dyes or their analogues.[6] The theoretical calculations
(computation details are listed in the Supporting Information)
shows that the HOMOs are delocalized over the quinazolinone
moieties, whereas the LUMOs are delocalized over the pyridine
moieties. The first singlet (S1) state is mainly composed by cal-
culation as a result of HOMO to LUMO transition thus a charge
transfer state (Table S12 and Figure S38), which is responsible
for the large Stokes shift. The frontier orbital distribution indi-
Figure 1. Optical properties of a selection of BF2 complexes and BODIQPys.
Table 1. Spectroscopic properties of 3a–3d in solution and in the solid
state.
tion of halogen atoms leads to a significant enhancement of
the molar extinction coefficient, and the luminescence proper-
ties of the difluoroboron complexes can also be improved (Fig-
ure S2). The luminescent properties of these BODIQPys in dif-
ferent polar solvents were investigated. The luminescence effi-
ciencies of 3a–3d had not changed much in selected solvents
and the mixed-solvent (MeCN/H2O) (Figures S5–S16, Support-
ing Information). Only the fluorescence emission of 3e de-
creased with the increase of water ratio in MeCN/H2O mixtures
(Figures S17–S19). These results indicated that all five com-
pounds exhibit bright emission from 437 to 459 nm (Figure 2
and Table 1), which were all located in the blue-light region.
The observed Stokes shifts of 3a–3e are all around 100 nm in
solution. More importantly, there is almost no overlap between
absorption and emission spectra, which prevents self-absorp-
tion and improves the reliability of fluorescence detection in
solution (Figure 1, and Figures S6, S9, S12, and S15). Thus, the
BODIQPys derivatives are promising for the future applications
as biosensors and luminescent materials.
[a]
[c]
labs
lFem
FF[d]
Stokes shift
[cmÀ1
[nm]
[nm]
]
3a
3b
3c
3d
3e
MeCN
CH2Cl2
Solid[b]
MeCN
CH2Cl2
Solid[b]
MeCN
CH2Cl2
Solid[b]
MeCN
CH2Cl2
Solid[b]
MeCN
CH2Cl2
Solid[b]
334
353
386
337
350
372
345
345
374
343
358
334
345
362
317
439
441
506
437
438
513
439
444
498
440
448
480
449
449
478
0.78
>0.99
0.60
0.86
>0.99
0.29
0.89
>0.99
0.28
0.94
>0.99
0.09
7161 (105 nm)
5652 (88 nm)
6143 (120 nm)
6790 (100 nm)
5740 (88 nm)
7388 (141 nm)
6206 (94 nm)
6462 (99 nm)
6657 (124 nm)
6427 (97 nm)
5611 (90 nm)
9106 (146 nm)
6713 (104 nm)
5352 (87 nm)
10625 (161 nm)
0.58
0.73
0.02
[a] Maximum absorption wavelength with the highest extinction coeffi-
cient, measured at a concentration of 1.0ꢁ10À5 moldmÀ3 in CH3CN.
[b] Maximum absorption wavelength with the highest absorption in solid
state. [c] The highest emission wavelengths. [d] Absolute fluorescence
quantum yield.
The fluorescence quantum yields of all five BODIQPys were
then carefully analyzed. All five BODIQPys display very high
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cates that the quinazolinone moiety is the donor, whereas the
pyridine moiety is the acceptor. Compared to the donor group
in BODIPY dyes (pyrrole), quinazolinone is a much weaker
donor. Therefore, the energy of S1 state is much higher than
that of BODIPY dyes, which will result in small internal conver-
sion (IC) rate and thus contribute to the high quantum yield of
BODIQPy. The first triplet (T1) states have similar orbital com-
positions to S1. As a result, the intersystem crossing (ISC) from
S1 to T1 is forbidden according to the El-Sayed rule.[22] As
shown in Table S10, the spin orbital coupling constants be-
tween S1 and T1 are small for 3a–3e, which may be due to
the reduction of the effect of heavy atoms in solution. The cal-
culated oscillator strength of the S1 state increased from 3a to
3e, which may be the result of increasing the electron-donat-
ing ability from F to I (as shown by the NPA charge in
Table S22). The quantum yield of BODIQPy in MeCN followed
the same trend of oscillator strength, except for 3e, which has
the lowest quantum yield due to the non-negligible heavy
atom effects.
(506 nm for 3a, 513 nm for 3b, 498 nm for 3c, 480 nm for 3d,
478 nm for 3e) were redshifted and broadened compared with
the respect to the corresponding emission in solution. It can
be attributed to the increment of the intermolecular interac-
tions from 3a to 3e in the solid state, which were further con-
firmed by structure analysis and theoretical simulations. The
Stokes shift was thus increased in solid state (from 120 to
161 nm) compared with the solution. The exact solid-state
structures of all five difluoroboron complexes have been char-
acterized by the X-ray single-crystal structure analysis (Fig-
ures S20–S37). The single molecule of 3a to 3e in crystal lat-
tices is highly planar with a very small dihedral angle between
the quinazolinone ring and pyridine ring (Table S6), which
means the enhancement of the intramolecular conjugation,
and is consistent with fluorescence results. The bond lengths
of BÀN (quinazolinone nitrogen) are shorter than the bond
length of BÀN (pyridine nitrogen) (Table S7), because quinazoli-
none nitrogen formed covalent bonds, whereas pyridine nitro-
gen formed coordinate bonds with difluoroboron. Although
the single molecular structures of the difluoroboron complexes
are all nearly planar, except for the two coordinated fluorine
atoms, the packing results indicate that the molecular stacking
models of 3a–3e are different (Figure 4 and Figures S21–S37),
which are closely related to the halogen atom on the quinazo-
linone ring. For compound 3a and 3b, the intermolecular
packing exhibit a similar H-aggregation structure,[20] and for
compound 3c–3e, a typical herringbone stacking mode can
be found according to the structure refinement (Table S6). Fur-
thermore, the herringbone packing of the bromine- and
iodine-containing compounds 3d and 3e is slightly different
from 3c. The interaction between halogen and oxygen was
found in the crystal structures from 3c to 3e (Figures S26–S37,
Table S8), due to the relatively high negativity of carbonyl
oxygen (Table S22), which may be the reason for different
single-crystal structures for 3c, 3d, and 3e. The C=O···X in 3c
to 3e shift from 3.222 to 3.091 ꢂ (Supporting Information,
Table S8). These halogen bonds[21] may be the reason for the
different single-crystal structures in 3c to 3e,and also lead to
the different stacking results. Although the packing styles in
3a–3e are different, strong p–p interactions have been found
in all the crystals of 3a–3e. The corresponding p–p stacking
distances ranged from 3.307 to 3.402 ꢂ from 3a to 3e
(Table S6).
In the solid state, the absorption of 3a to 3e has also been
increased due to the introduction of the halogen (Figure S3),
and 3a–3e also showed pronounced solid-state luminescence
at the same time. The normalized fluorescence spectra are pre-
sented in Figure 3. The emission maxima of the BODIQPys
Figure 3. Emission spectra of 3a (black line), 3b (red line), 3c (blue line), 3d
(pink line), and 3e (green line) in powder. Inset: pictures of 3a–3d powders
excited at 365 nm using a UV lamp.
Figure 4. Molecular structure of 3a–3e (top) and packing (bottom) based on single-crystal X-ray analysis.
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As a result, compared to the high fluorescence efficiency of
all five BODIQPy in solution, the quantum yield of BODIQPy
decreased, from 0.60, to 0.29, 0.28, 0.09 and 0.02, for 3a–3e in
the solid state, respectively. In DFT calculations, the dimeric
structures of p-p stacking were extracted from crystal to simu-
late electronic structures in solid state. The particular planar
“head (quinazolinone) to head” or “head(quinazolinone) to tail
(pyridine)” packing model of asymmetric BODIQPys may re-
sponse for the fluorescence enhancement in the solid-state.
The HOMO and LUMO orbitals of dimers are in the Figure 5,
the HOMO orbitals are delocalized over the central overlapped
Acknowledgements
This work was supported by the National Natural Science
Foundations of China (No. 21606116, 21762020, 51873160), the
Natural Science Foundations of Jiangxi Province (No.
20161BAB213070, 20132BAB213008), and the Department of
Education of Jiangxi Province (GJJ170176, GJJ150323). The the-
oretical calculations in this paper have been done on the su-
percomputing system in the Supercomputing Centre of
Wuhan University.
Conflict of interest
The authors declare no conflict of interest.
Keywords: chromophores
·
difluoroboron
dyes
·
fluorophores · nitrogen heterocycles · N ligands · Stokes shift
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Figure 5. Molecular orbital energy diagrams and isodensity surface plots of
the HOMOÀ1, HOMO, LUMO, and LUMO+1 of 3a–3e in the solid phase.
quinazolinone moieties, whereas the LUMO orbitals are located
on two pyridine moieties on both sides. As a result, this
“donor (quinazolinone) to donor” packing pushed up the
HOMO level, which lead to the small gap and bathochromic
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In summary, five novel difluoroboron quinzolinone-pyridine
complexes were synthesized with high yields. The asymmetri-
cal structure of BODIQPy (3a) showed outstanding emission
efficiency both in solution and in the solid state, with high
quantum yields and large Stokes shifts. The photophysical
properties of halogen atoms at the 6-position of quinazolinone
indicated that the influence of substituents on BODIQPy was
different from BODIPY due to the strong electron-withdrawing
ability of BODIQPy. Therefore, the quinazolinone-based difluor-
oboron complex would be a new and good candidate for or-
ganic optical materials such as transistors or bio-probes. Fur-
ther investigation of such organic boron complexes based on
this work is underway in our laboratory.
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Manuscript received: July 5, 2018
Revised manuscript received: September 10, 2018
Version of record online: && &&, 0000
Chem. Eur. J. 2018, 24, 1 – 6
www.chemeurj.org
5
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Dyes and Pigments
J. Zhou, L. Liu, Y. Pan, Q. Zhu, Y. Lu,
J. Wei, K. Luo, Y. Fu,* C. Zhong,* Y. Peng,
Z. Song*
&& – &&
Asymmetric Difluoroboron
Quinazolinone-Pyridine Dyes with
Large Stokes Shift: High Emission
Efficiencies Both in Solution and in
the Solid State
Fluorescent dye: Asymmetric difluoro-
boron quinazolinone-pyridine dyes
(BODIQPys) were easily obtained
through a two-step reaction. These
asymmetric BODIQPys exhibited high
Stokes shift and strong emission with
high fluorescence quantum yield both
in solution (>0.99) and in the solid
state (up to 0.60).
&
&
Chem. Eur. J. 2018, 24, 1 – 6
www.chemeurj.org
6
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!