Photoluminescence properties of new BF2 complexes with pyrazolone ligands: Dependence on volume and electronic effect of substituents

Article abstract of DOI:10.1016/j.jfluchem.2010.05.006

Novel fluorine-boron complexes with donor-acceptor architecture in which pyrazoline-1,3-diones were chosen as electron donors have been synthesized and well characterized. Correlation of the luminescence properties of the complex 2c and its crystal structures was discussed. Well-ordered molecular packing in the crystal results in strong charge transfer interactions characterized by long excited-state lifetime. These fluorine-boron complexes show photophysical properties highly dependent on the solvent polarity and aggregation states. The substituents on the pyrazoline were found to have a significant impact on the solid-state luminescent properties. As a result, some significant differences in charge transfer modes were observed in the solid state among these complexes.

Full text of DOI:10.1016/j.jfluchem.2010.05.006

Journal of Fluorine Chemistry 131 (2010) 883–887
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Short communication
Photoluminescence properties of new BF2 complexes with pyrazolone ligands:
Dependence on volume and electronic effect of substituents
Hui-hui Zhang a, Xiong Hu a, Wei Dou a, Wei-sheng Liu a,b,
*
a College of Chemistry and Chemical Engineering and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
b State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Novel fluorine–boron complexes with donor–acceptor architecture in which pyrazoline-1,3-diones were
chosen as electron donors have been synthesized and well characterized. Correlation of the
luminescence properties of the complex 2c and its crystal structures was discussed. Well-ordered
molecular packing in the crystal results in strong charge transfer interactions characterized by long
excited-state lifetime. These fluorine–boron complexes show photophysical properties highly
dependent on the solvent polarity and aggregation states. The substituents on the pyrazoline were
found to have a significant impact on the solid-state luminescent properties. As a result, some significant
differences in charge transfer modes were observed in the solid state among these complexes.
ß 2010 Elsevier B.V. All rights reserved.
Received 4 February 2010
Received in revised form 12 April 2010
Accepted 20 May 2010
Available online 1 June 2010
Keywords:
Photoluminescence properties
Donor–acceptor architecture
Pyzarolone
BF2 complexes
Charge transfer
Crystal structure
1. Introduction
complexes have excellent photophysical properties. For instance,
these materials exhibit interesting fluorescence emission, first-
Recently, some multifunctional fluorophores with donor–
acceptor (D–A) architecture have been reported for their ambipolar
charge transport property and high luminescence efficiency [1–4].
In addition, compounds with D–A architecture always show
particular and strong intermolecular and intramolecular interac-
tion due to their planar structure and large polarity in the solid
state. Many fluorine–boron complexes with D–A structure have
been designed [1,4] because that trivalent boron is a well known
electron-deficient acceptor and can receive part of the negative
and second-order nonlinear optical properties [13–15], ion sensing
ability [16–18], and can serve as emissive and/or electron-
transport layers in OLEDs [19,20]. In order to expand their
application potential, the relationship between structure and
photochemistry properties needs to be further investigated. Some
reports showed that annelation of electron donor to the
dioxaborine ring, formed into D–A structure, resulted in enhanced
absorption and fluorescence of the corresponding dyes and
improving resistance to hydrolysis [21,22]. In the present work,
pyrazoline-1,3-diones were chosen as electron donors which are
new heterocyclic donors with unique photochemical properties
[23–28] and then a new D–A structure (Fig. 1) was constructed.
Herein, we examined the effects of the substituents of the
pyrazoline ligands on the luminescent properties of the light-
emitting materials.
charge from an electron donor via certain delocalized
p-bridges.
There are three main types of fluorine–boron complexes, classified
as N,N double-dentate, N,O double-dentate and O,O double-
dentate compounds. For the former two kinds of fluorine–boron
complexes, BODIPY (boradipyrromethene) [5–8] and 1,3,2-dioxa-
borine [9–12] are their corresponding representatives. Further-
more, boron-dipyrromethenes (BODIPY) have been studied widely
because of their excellent fluorescent properties. However, there
are not many reports available on the generation of difluoroboron
complexes with O,O double-dentate ligands, although dioxaborine
2. Results and discussion
2.1. Crystal analysis
In the solid state, X-ray crystallographic data can generally
provide detailed and conclusive structural information for a given
molecule. The crystal of 2c for X-ray diffraction was obtained
through volatilization of benzene solvent at room temperature.
The structural model refinement details carried out by the least-
squares method are given in Table 1. As shown in Fig. 2, the unit
* Corresponding author at: College of Chemistry and Chemical Engineering and
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Tianshui
South Street NO. 222, Lanzhou 730000, China. Tel.: +86 931 8915151;
fax: +86 931 8912582.
E-mail addresses: liuws@lzu.edu.cn, zhanghh06@163.com (W.-s. Liu).
0022-1139/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2010.05.006

[
H.- Zhang et al. / Journal of Fluorine Chemistry 131 (2010) 883–887
2
Fig. 1. Structures of 2a, 2b and 2c.
cell contains two molecules in the crystal. Similar to the BF2
complexes reported [29], the central six member ring containing
O1–C7–C8–C16–O2 is almost coplanar with the adjacent pyr-
azolyl ring with the average deviation from the mean plane being
Fig. 2. Perspective view and labeling scheme for molecule 2c in unit cell, thermal
ellipsoids are at 30% probability level for C, O, N, B and F atoms. Selected bond lengths
˚
(A): 1.300(3) (C7–O1), 1.379(3) (C7–C8), 1.394(3) (C8–C16), 1.300(3) (C16–O2);.
˚
0.048 A. The C7–C8 and C8–C16 bond lengths represent a bonding
order of about 1.5, which indicates that the complexation of BF2
2.2. Electronic absorption and fluorescence spectra
increase the delocalization area of the
p electrons in the b-
diketone moiety. 1-Phenyl is almost coplanar with pyrazolyl ring
with a twisting angle of 9.138 due to the existence of C–H. . .O and
Electronic absorption of the dioxaborine compounds in differ-
ent solvents was measured in the region of 280–450 nm. As shown
in Table 2, the maximum peaks show hypsochromic shifts with
increase of the polarity of solvents, which indicates that these
C–H. . .N hydrogen bonds. These phenomena indicate strong
p-
electron delocalization within the pyrazoline plane and the 1,3-
diketone plane. The B–O bonds in the BF2-chelating moiety are
peaks originate from the
p–p* transition. For 2c, the absorption
˚
˚
equivalent with their lengths for 1.495(4) A and 1.493(3) A,
respectively. The BF2 group in 2c acts as a bifurcated acceptor with
an angle of 113.0 (2)8, and one of F atoms forms C–H. . .F weak
hydrogen bonds with the adjacent carbon of the phenyl ring with
maximum is considerably red-shifted compared to those of 2a and
2b, due to the increased conjugation of dioxaborine and pyrazoline
ring.
In toluene, compound 2a, 2b and 2c exhibit fluorescence
emission maximum at 355 nm when excited at 311 nm, which can
˚
˚
the distance of 3.180(4) A and 3.237(4) A (Fig. 3a). Due to dual C–
H. . .F hydrogen-bonding interactions, the dioxaborine chelate
ring is not planar. The relatively large deviation is observed
between the plane of O1–B–O2 and the plane of O1–C7–C8–C16–
O2 with a dihedral angle of 30.398. Therefore, in sharp contrast to
be assigned to
p–p* transition (Fig. 4). They show weak
fluorescence intensities with 4.4%, 7.6% and 7.0% of absolute
quantum yields for 2a, 2b and 2c, respectively. Their broaden peaks
in the longer wavelength region suggest mixing of the ICT
character from N1–C1–N2–C3 in the pyrazoline. However, in
CHCl3, THF and acetonitrile, the emissions from 2a, 2b and 2c
cannot be observed, which may be due to strong ICT [33,34]. Unlike
in solution, 2a, 2b and 2c show fairly strong fluorescence intensity
in the solid state, as displayed in Fig. 5. However, due to their
strong absorption in the solid state, they exhibit lower absolute
quantum yields than those in the toluene solution, with 3.0% for 2a,
1.0% for 2b, and 5.0% for 2c. The emission maximum of 2c red-shifts
to 503 nm, and the emission bands of 2b and 2a are at 470 nm and
527 nm, respectively.
There are three features in their fluorescence spectra: a large
Stokes shift, broad and fairly structureless emission spectra, and no
mirror image relationship between the excitation and emission
spectra. Their excitation bands get much broader and more
intensive than in solution and red-shifted remarkably (Fig. 6).
These features imply existence of charge transfer (CT) [35,36]. To
some reported BF2 complexes [9,10,30,31], regular
p–p stacking
interactions between the layers cannot be observed in the solid
state for 2c. In the crystal, two molecules form a dimmer through a
non-classical C–H. . .
p hydrogen-bonding interaction which
probably was induced by C–H. . .N hydrogen bond [32] (Fig. 3b).
C20–H20. . .
C16) distance is 2.884 A and the C20–H20. . .
p
aromatic centroid (C10, C11, C12, C13, C14, and
˚
p centroid angle is
128.538. The C–H. . .p interaction may result in the effective
intermolecular charge transfer between two pyrazoline mole-
cules. Adjacent molecules are linked by C–H. . .F and C–H. . .N
hydrogen bonds which work as a major influencing factor on their
p–p stacking, and thus two dimensional (2D) antiparallel
columns are generated as shown in Fig. 3c. The well-ordered
molecular packing is a favorable and critical factor in determining
their electron-transporting properties for electron-optical mate-
rials based on small molecules.
Table 1
Crystal data and structure refinement for complex 2c.
Empirical formula
Formula weight
Crystal system
Space group
C
22H15BF2N2O2
Dc (g cmꢀ3
(Mo K
) (mmꢀ1
F(0 0 0)
)
1.377
388.17
m
a
)
0.100
Triclinic
400
P-1
Data collected/unique
Limiting indices
4699/3358
ꢀ10 ꢁ h ꢁ 11
ꢀ6 ꢁ k ꢁ 11
ꢀ13 ꢁ l ꢁ 13
R1 = 0.0433, wR2 = 0.0967
R1 = 0.0715, wR2 = 0.1146
262
˚
a (A)
9.458 (7)
9.910 (7)
11.165 (8)
87.085 (11)
79.256 (10)
65.652 (10)
936.3 (11)
0.30 ꢂ 0.26 ꢂ 0.24
2
˚
b (A)
˚
c (A)
a
b
g
(8)
(8)
(8)
R1, wR2 (I>2s(I))
R1, wR2 (all data)
Parameter
V (A3)
Goodness of fit on F2
1.035
˚
Crystal size (mm)
D
(e Aꢀ3) (max, min)
˚
0.15/ꢀ0.21
–
Z
–

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H.- Zhang et al. / Journal of Fluorine Chemistry 131 (2010) 883–887
885
Fig. 3. (a) Views of hydrogen-bonding via C–H. . .F–B contacts in crystal 2c; (b) views of C–H. . .
diagrams along the a-axis, showing the contacts and network in crystal 2c.
p
interaction between two pyrazoline molecules in crystal 2c; (c) packing
p–p
[F(ig._4)TD$FIG]
determine their emission origin in the solid state, absorption
spectra in the solid state were measured (Fig. 7). A new fairly
strong absorption bands for 2a, 2b and 2c, due to aggregation of
molecules, appear and are red-shifted compared to their corre-
sponding absorption maxima in solution. The excitation spectra for
2a, 2b and 2c, monitored at
lem = 527 nm, 470 nm and 503 nm,
respectively, are generally similar to their absorption spectra in the
solid state. These observations indicate that the intermolecular
charge transfer takes place in the ground state. Therefore, the
excimer emission can be ruled out, which can be further confirmed
by the evidence that there is no face-to-face
p–p stacking
structure in the solid state for 2c. Excited-state lifetimes were
measured for investigation of the emission transitions (Table 3).
The luminescence decays of 2a can be analyzed as sum of
biexponentials that yields decay time 16.060
116.390 s (39.44%). The luminescence decays of 2c can be
analyzed as a sum of three exponentials that yield decay time
1.062 (12.66%), 10.466 (30.08%), 153.123 (57.26%).
ms (60.66%),
m
m
s
m
s
ms
Fig. 4. Fluorescence spectra of 2a, 2b and 2c in toluene solvent, lexi = 311 nm.
However, 2b has the longest excited lifetime data in the
millisecond range compared with 2a and 2c. The long lifetimes
suggest that the emissions originate from intermolecular charge
transfer state which is a stable and long-lived charge-separated
state due to strong CT interactions inhibiting efficiently the back
electron transfer [37]. From the lifetime data, there probably exist
multiple emission decay modes corresponding to multiple CT
interaction types [36]. Analyzing on solid structure of 2c, there are
two kinds of molecular aggregation: pyrazoline–pyrazoline (Py–
[F(ig._5)TD$FIG]
Py) due to C–H. . .
p and C–H. . .N hydrogen-bonding interaction
and D–A. . .D–A pairs due to C–H. . .F hydrogen-bonding interac-
tion, which causes emissions to red-shift and be broaden to some
Table 2
Maximum absorption peaks of the dyes 2a, 2b and 2c in four representative
solvents (c = 2 ꢂ 10ꢀ6 mol dmꢀ3).
Toluene
THF
CHCl3
Acetonitrile
2a
2b
2c
288
292
327
284
283
323
283
284
322
281
278
315
Fig. 5. Fluorescence spectra of 2a, 2b and 2c in the solid state, lexi (2a) = 470 nm,
lexi (2b) = 373 nm, and lexi (2c) = 420 nm.

[
H.- Zhang et al. / Journal of Fluorine Chemistry 131 (2010) 883–887
7
Fig. 7. Absorption spectra of 2a, 2b and 2c in the solid state.
Fig. 6. Excitation spectra of 2a, 2b and 2c in the solid state,
(2b) = 470 nm, and lemi (2c) = 503 nm.
lemi (2a) = 527 nm, lemi
Table 3
The fluorescence decay times of different samples in the solid state at room temperature.
Sample
Excitation wavelength/nm
Monitored wavelength/nm
t/m
sa
2a
2b
2c
470
373
420
527
470
503
16.060 (60.66%)
2085.075
116.390 (39.44%)
10.466 (30.08%)
1.062 (12.66%)
153.123 (57.26%)
a
The data in the bracket show the percentage of excited-state lifetime.
extent (Fig. 3b). However, the volume and the electronic effect of
substituent groups should be considered factors on the molecular
aggregation effects when molecules are approaching among them.
So, 2a, 2b and 2c exhibit different emission spectra remarkably
with the changes of the substituent groups at 3,4-positions. This
suggests that the spatial configuration of these molecules in the
solid state plays an important role in their photoluminescence
properties.
ettes of 1 cm path length at room temperature. Quantum yields
were determined by an absolute method using an integrating
sphere on FLS 920 of Edinburgh Instrument. Fluorescence
lifetimes were recorded on a single-photon-counting spectro-
photometer from Edinburgh Instruments (FLS 920) with
nanosecond pulse lamp as the excitation source. The emission
of a sample was collected by two lenses into a monochromator
(WDG30), detected by a photomultiplier and processed by a
Boxcar Average (EGG model 162) in line with a microcomputer.
Single-crystal X-ray diffraction measurements were carried out
3. Conclusions
on a Bruker SMART 1000 CCD diffractometer operating at 50 kV
˚
and 30 mA using Mo Ka radiation (l = 0.71073 A). Each selected
In conclusion, the fluorine–boron complexes with donor–
acceptor architecture exhibit significant differences in lumines-
cent behaviors with the changes of the electronic effect of
substituents at the 3,4-position on the pyrazoline. Well-ordered
molecular packing in the crystals results in strong
charge transfer interactions characterized by long excited-state
lifetime.
crystal was mounted inside a Lindemann glass capillary for data
collection using the SMART and SAINT software. An empirical
absorption correction was applied using the SADABS program.
All the structures were solved by direct methods and refined by
full-matrix least-squares on F2 using the SHELXTL-97 program
package [38].
4. Experimental
4.2. Synthesis of boron complexes
4.1. Materials and methods
The 4-acyl-5-hydroxyl-3-methyl-phenylpyrazolone, 4-acyl-5-
hydroxyl-1,3-diph enylpyrazolone and 4-benzoyl-5-hydroxyl-1,3-
diphenylpyrazolones (1a, 1b, and 1c) were synthesized by the
procedure reported by Jensen [39]. The dioxaborine complexes (2a,
2b and 2c) were prepared by pyrazolone and 10 equivalents of BF3/
ether adduct in little amount of CH2Cl2 solvent.
All chemicals were obtained from commercial suppliers and
used without further purification. CHCl3 and CH3CN were dried
over CaH2 overnight under nitrogen before distillation. Elec-
tronic absorption spectra in chloroform solution were obtained
by using Varian UV-Cary 100 spectrophotometer. Electronic
absorption spectra in the solid state were measured in Varian
UV-Cary 5000 spectrophotometer with diffuse reflectance
method by using 110 mm integrating sphere accessory. 1H
NMR and 13C NMR spectra were measured on a Varian Mercury
plus 400 MHz spectrometer in CDCl3 solution with TMS as
internal standard. Fluorescence measurements were made on a
Hitachi F-4500 spectrophotometer equipped with quartz cur-
4.2.1. (O1-B)-1-[40-[(Difluoroboryl)oxy]-(5-hydroxy-3-methyl-1-
phenyl-1H-pyrazol-4-yl)] ethan-1-one (2a)
Boron trifluoride–diethyl etherate (3.8 mL, 29.4 mmol) was
added to a solution of 4-acyl-5-hydroxyl-3-methyl-phenylpyr-
azolones (0.65 g, 3 mmol) in dry dichloro- methane (10 mL) under
nitrogen. The reaction mixture was stirred for 3 h. After removal of
the solvent, the residue was filtered and washed with hexane. The

H.- Zhang et al. / Journal of Fluorine Chemistry 131 (2010) 883–887
887
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The work was supported by the NSFC (Grants 20771048 and
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