Supramolecular assembly of borate with quaternary ammonium: Crystal structure and tunable luminescent properties

Article abstract of DOI:10.1016/j.jssc.2013.01.032

A new borate [C₆H₁₆N][B₅O ₆(OH)₄] (1) is synthesized hydrothermally by the reaction of isopropyltrimethylammonium hydroxide with boric acid. It crystallizes in the triclinic space group P-1 with the parameters a=9.1578(10) A, b=9.372(9) A, c=9.9812(10) A, α=66.508(2)°, β=74.751(2)°, γ=81.893(2)°. The [B₅O₆(OH)₄] ^- anions are interlinked via hydrogen bonding forming a 3D supramolecular network containing large cavities, where reside the (CH ₃)₃(i-C₃H₇) N⁺ cations. This borate shows tunable luminescent properties with temperature, heating-treatment, exciting-light, and solvents. The fluorescent intensity of 1 enhances 6-fold with decreasing the temperature from 25 K to 78 K. By treatment under different temperatures, the luminescence of 1 shifted from blue to white and the sample treated at 230 °C emits bright white light to naked eyes. The hybrid borate can disperse in different solvents, and shows a red-shifted and intense emission in polar solvents.

Full text of DOI:10.1016/j.jssc.2013.01.032

Journal of Solid State Chemistry 200 (2013) 99–104
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Supramolecular assembly of borate with quaternary ammonium: Crystal
structure and tunable luminescent properties
n
n
Jie Liang, Yong-gang Wang, Ying-xia Wang , Fu-hui Liao, Jian-hua Lin
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new borate [C H N][B O (OH) ] (1) is synthesized hydrothermally by the reaction of isopropyltri-
6
16
5
6
4
Received 26 October 2012
Received in revised form
methylammonium hydroxide with boric acid. It crystallizes in the triclinic space group P-1 with the
˚
˚
˚
parameters a¼9.1578(10) A, b¼9.372(9) A, c¼9.9812(10) A,
a¼66.508(2)1, b¼74.751(2)1, g¼81.893(2)1.
1
9 January 2013
ꢀ
The [B
5
O
6
(OH)
4
]
anions are interlinked via hydrogen bonding forming a 3D supramolecular network
Accepted 20 January 2013
Available online 29 January 2013
þ
containing large cavities, where reside the (CH
3
)
3
(i-C
3
H
7
) N cations. This borate shows tunable luminescent
properties with temperature, heating-treatment, exciting-light, and solvents. The fluorescent intensity of 1
enhances 6-fold with decreasing the temperature from 25 K to 78 K. By treatment under different
temperatures, the luminescence of 1 shifted from blue to white and the sample treated at 230 1C emits
bright white light to naked eyes. The hybrid borate can disperse in different solvents, and shows a red-shifted
and intense emission in polar solvents.
Keywords:
Pentaborate
Supramolecular interaction
Luminescence
Hydrogen bonding
Solvent effect
&
2013 Elsevier Inc. All rights reserved.
1
. Introduction
good luminescent properties. The first reported organic-inorganic
hybrid borate was (H en) (Hen)
which can emit blue light under the excitation of near-UV light at
room temperature, and showed a temperature-variable emitting
property from blue to white [23]. Later, other two borates,
2
2
2
B
16
O
27 (en¼ethylenediamine),
Light-emitting diodes (LEDs) are highly promising for display and
lighting technology, due to their low power consumption and high
light efficiency [1,2]. The exploration for phosphors which have
strong absorbency at the wavelengths of LEDs and then emit suitable
lights has attracted considerable attentions. There are three ways to
produce white light: multi-phosphors excited by a UV LED, combina-
tion of a blue LED with yellow phosphors, and mixed multi-
components LEDs [3,4]. However, problems such as color dependence
on the driving voltage and complicated color balance do exist in such
methods. To solve these problems fundamentally, single-emitting-
component (SEC) phosphors which can intrinsically emit white light
are advocated. The research on the SEC phosphors has carried out on
various materials, including organic molecules, inorganic materials
doped with rare-earth ions, nanomaterials and organic-inorganic
hybrid materials [5–20]. The organic-inorganic composites are parti-
cularly attractive, because they can preserve the advantages of both,
such as hardness, chemical resistance and optical function of inor-
ganics, and flexibility, lightness and processability of organics.
6 9 2 2 2 6 4 20 5 6 4
B O (en) @(H en)Cl and [C N H ]0.5[B O (OH) ] with tunable
luminescent properties were also revealed [24,25]. The organic
templated borates unfolded new applications in fabricating white
LEDs as single component phosphors. Since there were no metal
luminescent centers in these compounds, it was suggested that
structure defects caused by heat treatment may play a main role
for the self-activating photoluminescence (PL) property. However,
the luminescent mechanism remains unclear and the exploration
of more hybrid borates are expected.
6 5 6 4
Here, we report a new hybrid borate [C H16N][B O (OH) ] (1)
synthesized hydrothermally at 135 1C by employing isopropyltri-
methylammonium as a template. It exhibits a tunable lumines-
cent emission with not only the heating treatment, but also with
the temperature, exciting source, and solvents.
Borates, served as luminescent hosts, have been receiving
intense interests. Most of the widely used borate phosphors are
doped by rare earth ions as luminescent centers [21,22]. Recently,
in the study of borates with novel structures induced by organic
molecules, three organic templated borates were revealed with
2
. Experimental section
2.1. Synthesis
The borate was synthesized under hydrothermal condition and
n
the typical composition of the gels was H
:0.5:2.5ꢀ5. The SDAOH for 1 was M iPNOH. In a typical
procedure, 0.3710 g H BO was added into the solution of SDAOH
3
BO
3
:SDAOH:H
2
O¼
Corresponding authors. Fax: þ86 10 62751708.
1
3
E-mail addresses: wangyx@pku.edu.cn (Y.-x. Wang), jhlin@pku.edu.cn
(
J.-h. Lin).
3
3
0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jssc.2013.01.032

1
00
J. Liang et al. / Journal of Solid State Chemistry 200 (2013) 99–104
under stirring at room temperature; then the extra water was
evaporated in an oven to reach the gel composition as given
above; finally the mixture was transferred into a Teflon-lined
stainless steel autoclave and heated in an oven at 135 1C with a
of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (Fax: þ441223336-033; Email: deposit@ccdc.cam.ac.uk).
The powder X-ray diffraction profile was collected on a Rigaku
D/Max-2000 diffractometer using a rotating anode (CuKa, 40 kV
1
5 rpm rotating speed for about 10 days. The solid products were
and 100 mA), a graphite monochromator and a scintillation
detector. The powder X-ray pattern coincides well with the
patterns simulated by the structure data from single crystal
X-ray diffraction, indicating that the product is pure, as shown
in Fig. S1.
filtered and washed repeatedly with distilled water and ethanol,
and dried at 60 1C overnight.
Isopropyltrimethylammonium hydroxide was synthesized as
follows. First, isopropyldimethylamine was obtained by Eschweiler–
Clarke reaction from isopropylamine, formic acid and formaldehyde.
Then the tertiary amine reacted with methyl iodide to obtain
isopropyltrimethylammonium iodide. The solid product was filtered,
washed with acetone, and characterized by chemical analysis and
solid-state NMR spectroscopy. The hydroxide form of the SDA was
obtained by ion exchange of an aqueous solution of the iodide salt
2.3. Characterizations and physical measurements
Scanning electron microscopy (SEM) images were taken by a
Quanta 200F scanning electron microscope. The elemental analy-
sis of carbon, nitrogen and hydrogen was carried out on an
Elementar Vario EL III microanalyzer. Elemental analysis data
2
with Ag O.
(
2
wt%) for 1 are: calcd.: N 4.37, C 22.45, H 6.62; found: N 4.32, C
1.86, H 6.10. The thermalgravimetric analysis (TGA) of the
sample was done on the instrument TGA Q50 V20.6 in Ar atmo-
sphere with a heating rate of 10 1C/min from 25 to 800 1C. The
excitation and emission spectra at different temperatures were
conducted on a NanoLog infrared fluorescence spectrometer
equipped with a 450-W Xe lamp and a R928P PMT detector.
The temperature was controlled with a precision of 0.1 K with
Oxford itc503 temperature controllers. The quantum yield mea-
surements were recorded on an absolute PL quantum yield
spectrometer C11347 (Hamamatsu). The fluorescent lifetime
was conducted on a LifeSpec Red Spectrometer with excitation
wavelength at 440 nm. The CIE-1931 chromaticity coordinates
were calculated using a ColorCoordinate.exe program. Fourier
Transform Ion Cyclotron Resonance Mass Spectrometer (FT-MS)
were performed on Bruker APEX IV mass spectrometer, using
electrospray ionization technique.
2
.2. Crystallographic studies
3
Crystal of 1 (colorless, dimensions 0.28 ꢁ 0.27 ꢁ 0.17 mm ) was
carefully selected under an optical microscope and glued to thin glass
fiber with epoxy resin. Single X-ray crystal data were collected on
a Bruker SMART APEX CCD diffractometer with graphite-mono-
˚
chromated Mo-K
a radiation (l¼0.71073 A) under 296 K. Intensity
data sets were collected using the w scan technique and processed
with the INTEGRATE program of the APEX2 software [26] for
reduction and cell refinement. Multi-scan absorption corrections
were applied by using the SCALE program for area detector. The
2
structure was solved with direct method and refined on F with full-
matrix least-squares methods using SHELXS-97 and SHELXL-97
programs, respectively [27,28]. Non-hydrogen atoms were refined
with anisotropic displacement parameters, and hydrogen atoms on
˚
carbons were placed in idealized positions (C–H¼0.93 or 0.96 A) and
included as riding with Uiso(H)¼1.2 or 1.5 Ueq (non-H). The crystal-
lographic parameters for 1 are summarized in Table 1, and the
selected bond lengths and bond angles are listed in Table S1 in the
Supplementary materials. Crystallographic data (excluding structure
factors) for the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication, No. CCDC 886664. Copies of the data can be obtained free
3
. Results and discussion
3.1. Crystal structure
Compound 1 crystallinzes in the space group P-1 with unit
˚
˚
˚
parametes of a¼9.1578(10) A, b¼9.372(9) A, c¼9.9812(10) A,
a
¼66.508(2)1, b¼74.751(2)1, g¼81.893(2)1. Single X-ray struc-
ture determination reveals that it is a supramolecular network of
Table 1
ꢀ
Crystallographic parameters of 1.
H-bonded isolated [B
5
O
6
(OH)
4
]
anions, with quaternary ammo-
nium occupying cavities within the network. An assymetric unit
Formula
C
6 5
NB O
10
H
20
þ
ꢀ
contains one [C
shown in Fig. 1a, the polyborate anion is composed of four
BO (OH) triangles and one BO tetrahedron, which form two
[B ] cycles linked by the common BO tetrahedron. The B–O
distances for the trigonal boron atoms vary from 1.349(3) to
6
H
16N] cation and one [B
5
O
6
(OH)
4
]
anion. As
Formula weight
Temperature (K)
Wavelength ( A˚ )
320.28
296(2)
2
4
0
.71073
Triclinic
P–1 (no. 2)
3
O
3
4
Crystal system
Space group
a ( A˚ )
˚
1
1
.384(3) A, and those of the tetrahedral boron atoms vary from
9
9
9
.1578(10)
˚
.440(3) to 1.485(3) A. The O–B–O angles at the trigonal boron
atoms range from 116.0(2)1 to 123.1(2) and angles at the
b ( A˚ )
c ( A˚ )
.3720(9)
o
.9812(10)
tetrahedral boron atoms range from 107.73(18)1 to 111.74(11)1.
a
b
g
(1)
(1)
(1)
66.508(2)
74.751(2)
81.983(2)
757.41(13)
ꢀ
As mentioned above, the [B
5
O
6
(OH)
4
]
anion is of remarkably
regular geometry and it can donate four hydrogen atoms whereas
accept hydrogen atoms by any of the ten oxygen atoms to form
hydrogen bonds. Accroding to Schubert’ nomenclature of the
available oxygen sites in pentaborate anions (see Fig. 1a), differ-
ent H-bond interactions between neighbouring pentaborate
A˚ 3
Volume (
)
Z
2
3
r
m
calcd (g/cm )
(MoK ), (mm
1.404
0.121
336
ꢀ
1
a
)
F(0 0 0)
No. total reflns.
No. uniq. reflns. (Rint
anions can be classified [29]. As shown in Fig. 1b, one
4518
3345
2292
ꢀ
)
[B
5
O
6
(OH)
4
]
connects with three adjacent clusters via pairwise
No. obs. [IZ2
R1, wR2 [IZ2
R1, wR2 [all data]
s
s
(I)]
(I)]
(b
-a
) hydrogen bonds to yield 12-ring channels along the b axis.
The ribbons of pentaborates are linked by two pairwise (
interactions from the same boroxyl (B ) ring of a pentaborate in
the [1–10] direction, whereas the other boroxyl ring of the
0.0833, 0.1904
0.0591, 0.1717
1.076
b-a)
3 3
O
GOF

J. Liang et al. / Journal of Solid State Chemistry 200 (2013) 99–104
101
ꢀ
Fig. 1. (a) View of the building unit [B
5
O
6
(OH)
4
]
. Hydrogen bonding network of 1 along the (b) b (c) c and (d) a direction.
Table 2
Hydrogen bond interactions for 1, where, D and A are hydrogen bond donor and
a
acceptor atoms, respectively .
o
D–HyA
˚
˚
˚
+(DHA)/
d(D–H)/A
d(HyA)/A
d(D–A)/A
O
O
O
O
7
9
–H
–H
7
yO
yO
2
[#1]
[#2]
0.82
0.82
0.82
0.82
1.91
1.89
1.92
2.06
2.732(2)
2.696(2)
2.729(2)
2.849(2)
174.5
169.8
169.4
161.7
9
3
10–H10yO
–H yO [#4]
6
[#3]
8
8
6
[
#1] ꢀx, ꢀyþ2, ꢀzþ2; [#2] ꢀxþ1, ꢀyþ2, ꢀzþ1; [#3] ꢀx, ꢀyþ2, ꢀzþ1; [#4]
x, yꢀ1, z.
a
Symmetry transformations used to generate equivalent atoms.
pentaborate crosslink the ribbons via a third pairwise (b-a)
interaction along the c axis. The O–HyO distances in the
b-a
˚
interactions vary from 2.696(2) to 2.732(2) A, whereas the
Fig. 2. Excitation (left,
l
em¼435 nm) and emission (right,
lex¼365 nm ) spectra of
O–HyO angles range from 169.41 to 174.51. In Fig. 1c, the further
1
at different temperatures.
pairwise (
b-g) hydrogen bonds by the fourth adjacent cluster in
the b axis results in the formation of a 3D supramolecular
network. The
b
-
g
H-bond interaction is relatively longer with a
˚
between 360 and 500 1C are due to the thermal decomposition
of the organic amine and the removal of another two water
molecules. The obtained sample from TG analysis is black amor-
phous phase as shown in Fig. S3.
O–HyO distance of 2.849(2) A and a O–HyO angle of 161.71. The
details of hydrogen bonds for 1 are given in Table 2. The organic
molecules are located at the channels to compensate the negative
charges and interact with the inorganic framework.
3
3
.3. Variable luminescent properties
3.2. Thermal properties
.3.1. Tunable PL by variation of temperature
The TG curve of 1 shows that the compound is stable up to
The photoluminescent property of 1 at room temperature is
about 235 1C (see Fig. S4). A weight loss of about 45.48% is present
in the range of 235 1C to 700 1C, which corresponds to the
decomposition of the organic template and the removal of four
water molecules by the dehydration of hydroxyls (theoretical:
poor, but becomes more and more stronger with decreasing
temperature, as shown in Fig. 2. The excitation spectra of 1 reach
to a maximum around 365 nm, and show an almost steady
absorption thereafter. The excitation spectra of 1 cover the range
of UV to visible region, and the emission spectra present as broad
bands covering the visible region with the maximum at 427 nm
under the excitation of 365 nm light. The fluorescent intensity of
1 has a 6-fold enhancement (see Fig. 2) with decreasing the
4
5.93%). In the DSC curve, there are four endothermic peaks
between 230 and 500 1C. The first peak at 280 1C is attributed to
the dehydration of the isolated pentaborates with loss of two
water molecules, while the later three endothermic peaks

1
02
J. Liang et al. / Journal of Solid State Chemistry 200 (2013) 99–104
temperature from 298 K to 78 K. Generally it is believed that the
decrease of temperature can relieve thermal motions and non-
radiative relaxations, and then increase the intensities [30,31].
the thermal stable temperature (235 1C), defect concentration
increases and luminescence intensity becomes higher. However,
when the treating temperature is beyond the thermal stable
temperature, there are so many defects that quench the lumines-
cent intensities. The PL lifetime is less than 10 ns for all of the
materials, suggestive of their fluorescence character. The 1-230
(representing the sample heated at 230 1C) emits bright white
light to naked eyes. It has an external PL quantum yield of about
2.1% when illuminated with 365 nm light. The single crystal
structure of 1-230 shows no significant differences to that of 1,
The emission band of 1 is similar to those of (H
2
en)
2 2
(Hen)
B
16
O
27, B
6
O
9
(en) (H en)Cl and [C (OH)
2
2
2
6
N
4
H
20
]
0.5[B
5
O
6
4
], which
imply that the luminescence properties of these hybrids are
common [23–25]. The difference in maximum emission positions
among these hybrid borates may be due to the different
hydrogen-bonding interactions between borate and amines.
and only becomes slightly compact with hydrogen bond distances
shorter by average of 0.02 A˚ between adjacent pentaborate
3.3.2. Tunable PL by variation of heat treatment
The as-synthesized samples of 1 were heated at 190 1C, 210 1C,
anions.
2
30 1C and 250 1C for 3 h, and the treated products were white,
faint-yellow and gray-yellow as shown in Fig. 3a. Powder X-ray
diffraction patterns of these samples are similar to that of 1,
despite that the peaks of the sample heated at 250 1C are
broadened (Fig. S4), which indicate that the calcined samples
keep the structure except the sample heated at 250 1C is involved
some structural defects. Under a UV lamp (ꢂ365 nm), these
samples display blue to near-white emission as shown in
Fig. 3a. The excitation and emission spectra of these samples
are shown in Fig. 3b. With the increase of heat-treated tempera-
ture, the intensities of the emission band are enhanced and reach
to the maximum for the sample heated at 230 1C, and then
decrease for the sample treated at 250 1C. The emission bands
of these heat-treated samples also show a red shift of 9 nm,
3
.3.3. Tunable PL by variation of exciting light
Since 1-230 has no significant structural differences with 1,
and shows an excellent luminescent performance, it was taken for
further study. It is interesting that its emission spectra vary with
the excitation wavelength. As shown in Fig. 4, with the increase of
the excitation wavelength from 300 to 400 nm, the emission
spectra show a pronounced red shift of 75 nm, and the intensities
of the emission enhance by almost 10-fold. The CIE chromaticity
coordinates of the emissions excited by 300 and 400 nm light are
approximately (0.19, 0.31) and (0.27, 0.32), respectively.
3.3.4. Tunable PL by variation of solvent
1
1 nm, 12 nm, 5 nm compared to that of 1, which is consist with
The organic-inorganic hybrid borate can dissolve in different
solvents, and shows a solvent-dependent luminescent property.
Fig. 5a (left) shows the solutions of 1-230 in water (faint-yellow),
alcohol (faint-yellow), acetone (colorless) and protonic diethyl
the previous reports [23–25]. The heating-temperature depen-
dent PL may be related with the defect concentrations caused by
heat treatment. That is, when the activated temperature rises to
Fig. 3. (a) Photos of 1 heat-treated at different temperatures (top: samples, bottom: samples under UV light). (b) Excitation (left,
ex¼365 nm) spectra.
lem¼440 nm) and emission (right,
l

J. Liang et al. / Journal of Solid State Chemistry 200 (2013) 99–104
103
ether (colorless). Fourier Transform Ion Cyclotron Resonance
Mass Spectra (FTMS) of the four solutions reveal that large mass
units corresponding to one or one-half pentaborate are still
reserved in the solutions, thus rule out the disintegration of the
compound into basic groups (see Fig. S5). These solutions show a
variable luminescent intensities under the excitation of 365 nm
light (Fig. 5a, right): bright and blue in protonic solvents (water
and alcohol), weak and blue in moderate polar solvent (acetone)
and very weak in nonpolar solvent (diethyl ether).
of the four solutions become relatively narrower covering only
the UV region with a maximum at ꢂ370 nm. The absence of
absorption bands in the visible region intimate additional inter-
actions happened between solvent and solute which prevent the
absorption. The emission spectra of 1-230 change in both band
locations and intensities, and vary in different solvents. In
ethanol, it shows an intense emission with maximum at
447 nm, whereas a red-shifted and more stronger emission at
454 nm is observed in a water solution. When hydrochloric acid is
added into the water solution, a blue-shifted emission at 446 nm
is obtained, whereas the addition of potassium hydroxide has no
obvious effect on the emission spectra. In acetone, the emission
has a slightly blue shift with the peak at 436 nm, and in diethyl
ether, a large blue-shifted emission at 415 nm with very weak
intensity is observed. The strong emission in polar solvents may
be due to the competitions between polyborate-amino and
polyborate-solvent hydrogen bonds. In the nonpolar solvent like
diethyl ether, both intermolecular and intramolecular hydrogen-
bond interactions are almost prohibited, thus present a very weak
emission. The solute-solvent interactions and their effects on
organic spectra has been demonstrated several decades ago, but
this is the first time that they are extended to inorganic–organic
borates [32,33]. It is interesting that the solids recovered from
solutions can still emit white light under UV irradiation, which
further suggests that the hybrid borate can be exploited for the
generation of white light emission.
The luminescent spectra of 1-230 in these solutions are shown
in Fig. 5b. Compared with that in solid state, the excitation spectra
It is really difficult to explain the luminescent mechanism of the
organic templated borates. In previous reports, it was attributed the
PL of these borates to the structural defects caused by heat treatment.
We performed a theoretical calculation on the pentaborate group and
the result shows that the band gap for both 1 and 1-230 is above
Fig. 4. The emission spectra of 1-230 excitated at different wavelengths.
Fig. 5. (a) Photos of 1-230 in different solvents. Left: solutions, right: solutions under UV light (
lex¼365 nm). (b) The excitation (lem¼454, 447, 436, 420 nm for water,
alcohol, acetone and diethyl ether solutions) and emission spectra of 1-230 in different solvents.

1
04
J. Liang et al. / Journal of Solid State Chemistry 200 (2013) 99–104
5
.0 eV, which rule out that these borates have emission in the visible
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4
. Conclusions
1
In conclusion, we have synthesized a new organic–inorganic
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