One-pot synthesis and characterization of novel boronates for the growth of single crystals with nonlinear optical properties

Article abstract of DOI:10.1016/j.dyepig.2010.02.007

Novel boronates were synthesized by the single step reaction of 2,4-pentanedione, aminophenol and phenylboronic acid in good yield. X-ray diffraction analysis showed that the compounds crystallized in noncentrosymmetric space groups, which was used for the growth of organic crystals with luminescent and nonlinear optical properties. For some boronates it was possible to obtain mm scale single crystals which displayed good transparency from 550 to 1200?nm. These crystals exhibited efficient second-order nonlinearity (second-harmonic generation) and third-order nonlinearity (two-photon excited fluorescence). In the case of second-harmonic generation, the observed effect was four times larger than that generated by urea which was used as a reference. Additionally, the crystals were used to prepare aqueous colloidal nanocrystals that exhibited superior fluorescence properties than those of the boronates when dissolved in organic solvents. Finally, the thermal stability of single crystals was determined using TG and?TGA.

Full text of DOI:10.1016/j.dyepig.2010.02.007

Dyes and Pigments 87 (2010) 76e83
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
One-pot synthesis and characterization of novel boronates for the growth
of single crystals with nonlinear optical properties
,
a
a
Mario Rodríguez a, Gabriel Ramos-Ortíz a , Martha I. Alcalá-Salas , José Luis Maldonado ,
*
Karla A. López-Varela a, Yliana López b, Oscar Domínguez b, Marco A. Meneses-Nava a,
Oracio Barbosa-García a, Rosa Santillan b , Norberto Farfán
,
c
*
a Centro de Investigaciones en Óptica, A.P. 1-948, 37000 León, Gto., Mexico
b Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, 07000, Apdo. Postal. 14-740, México D. F., Mexico
c Facultad de Química, Departamento de Química Orgánica, Universidad Nacional Autónoma de México, México D.F. 04510, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel boronates were synthesized by the single step reaction of 2,4-pentanedione, aminophenol and
phenylboronic acid in good yield. X-ray diffraction analysis showed that the compounds crystallized in
noncentrosymmetric space groups, which was used for the growth of organic crystals with luminescent
and nonlinear optical properties. For some boronates it was possible to obtain mm scale single crystals
which displayed good transparency from 550 to 1200 nm. These crystals exhibited efficient second-order
nonlinearity (second-harmonic generation) and third-order nonlinearity (two-photon excited fluores-
cence). In the case of second-harmonic generation, the observed effect was four times larger than that
generated by urea which was used as a reference. Additionally, the crystals were used to prepare aqueous
colloidal nanocrystals that exhibited superior fluorescence properties than those of the boronates when
dissolved in organic solvents. Finally, the thermal stability of single crystals was determined using TG
and TGA.
Received 18 January 2010
Received in revised form
17 February 2010
Accepted 23 February 2010
Available online 4 March 2010
Keywords:
Organic materials
Boronates
Nanomaterials
Nonlinear optics
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
the best of our knowledge, reports are not available on the growth
and characterization of the luminescent and nonlinear optical
In recent years, organic noncentrosymmetric materials have
received considerable attention due to potential applications in
nonlinear optics (NLO) namely, electro-optic light modulation,
frequency conversion, parametric light generation and terahertz
(THz) wave generation [1e3]. The use of organic molecules and
metal-organic coordination networks as main construction blocks
offer synthetic flexibility in terms of the design of novel materials
that display levels of optical nonlinearity which, in many cases, are
larger than those of inorganic materials. In addition, the nonlinear
optical response of such compounds is of electronic origin which is
of interest for the development of high-bit-rate devices.
The development of discrete organic molecules combined with
various strategies to produce noncentrosymmetric packing, via the
use of chiral fragments [4] and others as halogen hydrogen bonding
[5], meta substitution [6], diminution of dipole moment [7] etc., have
generated many organic noncentrosymmetric crystals. However, to
properties of noncentrosymmetric organic crystals based on boron-
containing molecules. This contrasts with the fact that in the field of
nonlinear optics, boron has been extensively used for the growth of
inorganic crystals, such as the series of nonlinear borate crystals:
b-BaB2O4 (BBO), LiB3O5 (LBO), BiB3O6 (BiBO), Nd:YAl3e(BO3)4
(NYAB), etc. [8]. In these inorganic crystals, boron is used because of
its ability to coordinate with either three or four oxygen atoms, this
being advantageous in the formation of structural units comprising
several differental BxOy groups. This structural versatility of boron
has been exploited in the growth of inorganic crystals of large
effective nonlinear coefficient, wide transparency range, high
damage threshold and good mechanical properties [8,9].
On the other hand, in organic synthesis, boron complexes have
resulted in compounds with interesting nonlinear optical response
[10] and electronic properties [11]. For instance, three-coordinated
boron species have received considerable attention due to the fact
that their vacant p-orbitals are strong
p-electron acceptors which
can lead to significant electronic delocalization within adjacent
organic conjugated systems, which, in turn, results in high
nonlinearity [10c]. Four-coordinated boron has also been incorpo-
rated into NLO organic systems and it was demonstrated that the
* Corresponding authors.
E-mail addresses: garamoso@cio.mx (G. Ramos-Ortíz), rsantill@cinvestav.mx
(R. Santillan).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.02.007

M. Rodríguez et al. / Dyes and Pigments 87 (2010) 76e83
77
second-order NLO character of these boron complexes was
enhanced with respect to their corresponding boron-free counter-
parts [12]. The present research group has studied both the second-
and third- order nonlinearity of four-coordinated boron systems
prepared from tridentate ligands [13,14]; these studies showed that
the appropriate combination of groups (donor-acceptor) and the
formation of the N / B coordinative bond optimize quadratic
nonlinear effects such as the second-harmonic generation.
This work concerns the one-step synthesis of a novel series of
boronates (1ae1d) as well as mm scale crystal growth from these
boron complexes. This new series of boronates are interesting since
the crystal packing of some of them resulted in non-
centrosymmetric solids. Photoluminescence and nonlinear optical
properties of these crystals are reported.
1387,1307,1285,1250,1175,1116,1022, 957 cmꢁ1. 1H NMR (300 MHz,
CDCl3) : 2.25 (3H, s, H-1), 2.45 (3H, s, H-5), 5.57 (1H, s H-3), 6.84
d
(1H, t, J ¼ 8.0 Hz, H-9), 7.04 (1H, dd, J ¼ 8.1, 1.0 Hz, H-7), 7.20 (1H, t,
J ¼ 8.1 Hz, H-8), 7.21e7.24 (3H, m, H-m, p), 7.34 (1H, d, J ¼ 7.8 Hz,
H-10), 7.36e7.38 (2H, m, H-o) ppm. 13C NMR (75 MHz, CDCl3)
d: 21.3
(C-1), 22.9 (C-5) 103.6 (C-3),114.1 (C-7),117.9 (C-10),118.7 (C-9),127.4
(C-m), 127.7 (C-p), 128.9 (C-8), 130.9 (C-o), 132.5 (C-6), 156.8 (C-11),
161.2 (C-2),173.3 (C-4) ppm.11B-NMR (96 MHz, CDCl3)
d: 7.9 ppm. EM
(20 eV) m/z (%): 277 (Mþ, 0.3), 201 (9), 200 (Mþ ꢁ C6H5,100),199 (26),
185 (10), 77 (8), 43 (5). Anal. Calc. for C17H16BNO2; C, 74.25; H, 6.23; N,
4.81. Found: C, 74.32; H, 6.49; N, 4.83.
2.3.2. 2-Chloro-8,10-dimethyl-6-phenyl-5,7-dioxa-11-aza-
6-bora-benzocyclononene (1b)
Compound 1b was prepared from the reaction of 2-amino-4-
chlorophenol 1.40 g (9.9 mmol), 2,4-pentanedione 1.00 g (9.9 mmol)
and phenylboronic acid 1.20 g (9.9 mmol), to give 2.80 g (9.0 mmol,
2. Experimental
2.1. Instrument
90% yield) of 1b as a yellow solid. M.P.: 137e138 ꢀC. IRn
(KBr):
max
3009, 1615, 1518, 1473, 1430, 1317, 1258, 1180, 1125, 943 cmꢁ1
.
1H
All starting materials were purchased from Aldrich. Solvents
were used without further purification. Melting points were
recorded on an Electrothermal 9200 apparatus and are uncor-
rected. Infrared spectra were measured on FT-IR PerkineElmer GX
spectrophotometer using KBr pellets. 1H, 11B and 13C NMR spectra
were recorded on Bruker avance DPX 300 and Jeol Eclipse þ400
spectrometers. Chemical shifts (ppm) are relative to (CH3)4Si for 1H
and 13C and to BF3(OEt2) for 11B. Ultraviolet spectra were obtained
with a Perkin Elmer Lambda 12 spectrophotometer. Mass spectra
NMR (400 MHz, CDCl3) : 2.24 (3H, s, H-1), 2.44 (3H, s, H-5), 5.60
d
(1H, s H-3), 6.94 (1H, d, J ¼ 8.8 Hz,H-10), 7.15 (1H, dd, J ¼ 8.5, 2.2 Hz,
H-9), 7.30 (1H, s, H-7), 7.20e7.31 (3H, m, H-m, p), 7.29-7-31 (2H, m,
H-o) ppm. 13C NMR (100 MHz, CDCl3)
d: 21.3 (C-1), 23.1 (C-5) 103.9
(C-3), 114.6 (C-10), 118.0 (C-7), 123.1 (C-8), 127.5 (C-m), 127.9 (C-p),
128.5 (C-9), 130.8 (C-o), 133.1 (C-6), 155.3 (C-11), 162.0 (C-2), 174.5
(C-4) ppm. 11B NMR (128 MHz, CDCl3)
d: 8.1 ppm. EM (20 eV) m/z
(%): 313 (Mþ, 1), 311 (3), 236 (Mþ ꢁ C6H5, 50), 234 (Mþ ꢁ C6H5, 100),
219 (2), 199 (21), 77 (1), 43 (3). Anal. Calc. for C17H15BNO2Cl; C,
65.53; H, 4.85; N, 4.50. Found: C, 65.28; H, 4.95; N, 4.49.
were recorded on
a Hewlett Packard 5989A spectrometer.
Elemental analyses were carried out on a Thermo Finnigan Flash EA
1112 elemental microanalyzer.
2.3.3. 2-Methyl-8,10-dimethyl-6-phenyl-5,7-dioxa-11-aza-
6-bora-benzocyclononene (1c)
2.2. X-ray data collection and structure determination
Compound 1c was prepared from the reaction of 2-amino-4-
methylphenol 1.20
g (9.9 mmol), 2,4-pentanedione 1.00 g
In all cases the single crystals suitable for X-ray structural
studies were obtained by slow evaporation from mixtures of CHCl3
or CH2Cl2. The crystal data were recorded on an Enraf Nonius
(9.9 mmol) and phenylboronic acid 1.20 g (9.9 mmol), to give 2.80 g
(9.6 mmol, 97% yield) of 1c as a yellow solid. M.P.: 120e121 ꢀC. IRn
max
(KBr): 3006, 2916, 1618, 1587, 1521, 1365, 1310, 1430, 1178, 1128,
Kappa-CCD (
l
MoKa
¼ 0.71073 Å, graphite monochromator,
946 cmꢁ1 1H NMR (300 MHz, CDCl3)
d: 2.22 (3H, s, H-1), 2.33 (3H, s,
T ¼ 293 K-CCD). The crystals were mounted on a Lindeman tube. All
reflection data set were corrected for Lorentz and polarization
effects. The first structure solution was obtained using the SHELXS-
97 and SIR2004 programs and then the SHELXL-97 programs was
applied for refinement and output data. All software manipulations
were done under the WinGX environment program set [15].
Molecular perspectives were drawn under DIAMOND drawing
application. All heavier atoms were found by Fourier map differ-
ence and refined anisotropically. The hydrogen atoms were
geometrically modelled and are not refined.
Me), 2.43 (3H, s, H-5), 5.53 (1H, s, H-3), 6.93 (1H, d, J ¼ 8.1 Hz, H-10),
6.99 (1H, dd, J ¼ 8.3 Hz, H-9), 7.15 (1H, s, H-7), 7.21-7-22 (2H, m, H-o)
7.33e7.35 (3H, m, H-m, p) ppm. 13C NMR (75 MHz, CDCl3)
d: 21.3
(C-1), 21.3 (C-12), 22.9 (C-5), 103.5 (C-3), 113.6 (C-10), 118.5 (C-7),
127.4 (C-9), 127.66 (C-m), 128.0 (C-p), 129.4 (C-8), 130.9 (C-o), 132.3
(C-6), 154.6 (C-11), 160.7 (C-2), 173.0 (C-4) ppm. 11B-NMR (96 MHz,
CDCl3) d
: 7.8 ppm. EM (20 eV) m/z (%): 291 (Mþ, 4), 215 (13), 214
(Mþ ꢁ C6H5, 100), 199 (3). Anal. Calc. for C18H18BNO2; C, 74.25; H,
6.23; N, 4.81. Found: C, 74.32; H, 6.69; N, 4.83.
2.3.4. 2-Nitro-8,10-dimethyl-6-phenyl-5,7-dioxa-11-aza-
6-bora-benzocyclononene (1d)
2.3. Synthesis
Compound 1d was prepared from the reaction of 2-amino-4-
nitrophenol 1.50 g (9.9 mmol), 2,4-pentanedione 1.00 g (9.9 mmol)
and phenylboronic acid 1.20 g (9.9 mmol), to give 2.60 g (8.0 mmol,
The following procedure was employed in the preparation of all
boron compounds described herein. Equimolar amounts of ami-
nophenol, 2,4-pentanedione and phenylboronic acid were placed
in a flask to which was added 20 mL of benzene and the ensuing
solution was refluxed for 2h. Excess solvent was eliminated with
a DeaneStark trap and the solid precipitate was collected by
filtration under vacuum and washed with hexane (see Scheme. 1).
80% yield) of 1d as a green solid. M.P.: 178e179 ꢀC. IRn
(KBr):
max
3009, 1610, 1511, 1433, 1387, 1368, 1336, 1311, 1280, 1186, 927, 888,
753, 705 cmꢁ1, 1H NMR (300 MHz, CDCl3)
d: 2.30 (3H, s, H-1), 2.57
(3H, s, H-5), 5.72 (1H, s, H-3), 7.06 (1H, d, J ¼ 8.8 Hz, H-10),
7.21e7.32 (5H, m, H-o, m, p), 8.20 (1H, d, J ¼ 2.4 Hz, H-7), 8.24 (1H,
dd, J ¼ 8.8, 2.4 Hz, H-9) ppm. 13C NMR (75 MHz, CDCl3)
d: 21.8 (C-1),
2.3.1. 8,10-Dimethyl-6-phenyl-5,7-dioxa-11-aza-6-
23.5 (C-5), 104.7 (C-3), 113.4 (C-10), 114.0 (C-9), 126.1 (C-7), 127.9
bora-benzocyclononene (1a)
(C-m), 128.6 (C-p), 131.0 (C-o), 132.7 (C-6), 140.0 (C-8), 162.5 (C-11),
Compound 1a was prepared from the reaction of 2-aminophenol
1.00 g (9.9 mmol), 2,4-pentanedione 1.00 g (9.9 mmol) and phenyl-
boronic acid 1.20 g (9.9 mmol), to give 2.20 g (8.2 mmol, 80% yield) of
1a as a yellow solid. M.P.: 149e150 ꢀC. IRnmax (KBr): 3003, 1596, 1513,
164.1 (C-2), 176.4 (C-4) ppm. 11B NMR (96 MHz, CDCl3)
d: 8.8 ppm.
EM (20 eV) m/z (%): 322 (Mþ,2), 318 (56), 317 (27), 215 (14), 214
(100), 213 (33), 128 (10). Anal. Calc. for C17H15BN2O4: C; 63.39. H;
4.69, N; 8.70, Found: C; 63.39. H; 5.08, N; 8.83.

78
M. Rodríguez et al. / Dyes and Pigments 87 (2010) 76e83
Scheme 1. Synthesis of boronates 1ae1d in one step from the reaction of 2,4-pentanedione, aminophenol and phenylboronic acid.
CCDC 758819 (1a), 758820 (1b) and 758821 (1c) contain the
spectra were assigned employing 1He13C HETCOR and HMBC NMR
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
technique. Finally, the 11B NMR analysis of compounds 1ae1d
confirmed the presence of tetra-coordinate boron atom for the
observed chemical shift between 7.8 and 8.8 ppm.
3.2. X-ray analysis
3. Results and discussion
The crystals for X-ray diffraction analysis were obtained by slow
evaporation of concentrated solutions of each boronate (1ae1d) in
dichloromethane or ethyl acetate at room temperature. For
compounds 1a, 1b and 1d it was possible to obtain crystals suitable
for X-ray diffraction. A summary of selected angles, bond distances
and torsion angles are contained in Table 2. The X-ray diffraction
analysis confirmed the structure of the boronates containing
a tetra-coordinated boron atom (Fig. 1), with three covalent bonds
(two BeO bonds and one BeC bond) and additionally a coordinative
N / B bond with a length of 1.575(3), 1.577(2) and 1.5768(19) Å for
1a, 1b and 1d, respectively. The values for the coordinative bond
lengths are shorter than the values shown by boronates assembled
from salicylidenimino alcohol ligands (from 1.586(2) to 1.861(5) Å)
[14,16], but they are similar to those reported for boronates
prepared from the reaction of a ketoenamine ligand with two units
3.1. Synthesis and spectroscopic characterization
of the boron complexes 1ae1d
The series of boronates 1ae1d were prepared from the reaction
in one step of 2,4-pentanedione, aminophenol and phenylboronic
acid, at reflux of benzene for 2 h (see Scheme. 1). These new [4.3.0]
heterobicyclic boronates were obtained in good yields, as colored
solids which are soluble in common organic solvents. The mole-
cules were re-crystallized several times in dichloromethane to
perform the physical and chemical analysis. As a first insight into
the formation of compounds 1ae1d the IR spectral analysis showed
stretching bands from 1511 to 1521 cmꢁ1 which are assigned to the
C]N bond and from 1587 to 1615 cmꢁ1 assigned to the C]C double
bond. In general, the mass spectra of boronates show the peak for
the molecular ion and the base peak corresponding to loss of the
arylboronic fragment [14].
The analysis of the spectroscopic data established the formation
of 1ae1d (Table 1). The 1H NMR spectra for these new boronates
showed signals for two different methyl groups H-1 and H-5 in
2.22e2.30 and 2.43e2.57 ppm, respectively. Furthermore, the
signal for H-3 presents a chemical shift from 5.53 to 5.72 ppm that
evidences the presence of enol-imine tautomer in the boronate
structure. The 1H signals corresponding to the aromatic part of the
molecule were assigned based on the coupling constant values and
with the 1H-1H COSY NMR technique. The 13C NMR spectra of
boronates 1ae1d showed two methyl signals with a chemical shift
of 21.3e22.0 and 22.9e23.7 ppm corresponding to C-1 and C-5,
respectively. On the other hand, the signal corresponding to the
iminic carbon appears from 160.7 to 164.1 ppm and the C-4 base
enol moiety from 173.0 to 176.4 ppm. All signals of the 13C NMR
Table 2
Selected bond lengths (Å), angles (ꢀ) and torsion angles (ꢀ) for boronates 1a, 1b
and 1d.
Compounds
1a
1.575 (3)
1b
1.577 (2)
1d
N(1)eB(1)
N(1)eC(6)
N(1)eC(2)
C(2)eC(3)
C(3)eC(4)
C(4)eC(5)
C(4)eO(1)
B(1)eO(1)
B(1)eO(2)
B(1)eCPh
1.5768 (17)
1.4104 (17)
1.3162 (17)
1.421 (2)
1.367 (2)
1.493 (2)
1.3259 (19)
1.4735 (19)
1.4992 (19)
1.607 (2)
1.413 (3)
1.313 (3)
1.421 (3)
1.360 (4)
1.484 (4)
1.326 (3)
1.476 (3)
1.481 (3)
1.602 (3)
1.4121 (19)
1.314 (3)
1.418 (3)
1.360 (3)
1.321 (3)
1.321 (3)
1.479 (2)
1.488 (2)
1.610 (2)
O(1)eB(1)eN(1)
O(1)eB(1)eO(2)
O(1)eB(1)eCPh
O(2)eB(1)eN(1)
O(2)eB(1)eCPh
N(1)eB(1)eCPh
C(2)eN(1)eB(1)
C(2)eC(3)eC(4)
107.32 (17)
111.73 (18)
111.46 (19)
100.79 (17)
112.33 (17)
112.67 (18)
120.10 (18)
120.8 (2)
107.68 (12)
111.63 (13)
111.12 (13)
100.88 (12)
112.30 (12)
112.75 (12)
121.00 (13)
120.8 (2)
107.94 (10)
111.31 (12)
111.67 (12)
100.18 (11)
111.74 (11)
113.44 (11)
121.21 (11)
120.75 (14)
Table 1
NMR Chemical shifts for compounds 1ae1d obtained in a Bruker avance DPX 300
spectrometer in CDCl3 as a solvent.
NMR shifts (ppm)
IR (cmꢁ1
)
H-1
H-3
H-5
C-2
C-3
C-4
11B
C]C
C]N
N(1)eC(2)eC(3)eC(4)
C(2)eC(3)eC(4)eO(1)
O(1)eB(1)eN(1)eC(2)
C(2)eN(1)eC(6)eC(7)
N(1)eC(6)eC(7)eO(2)
ꢁ19.7 (4)
11.0 (4)
37.3 (3)
ꢁ19.93 (3)
9.9 (3)
34.37 (19)
25.3 (3)
ꢁ20.5 (2)
155.49 (15)
32.19 (17)
29.3 (2)
1a
1b
1c
1d
2.25
2.24
2.22
2.30
5.57
5.60
5.53
5.72
2.45
2.44
2.43
2.57
161.2
162.0
160.7
164.1
103.6
103.9
103.5
104.7
173.3
174.5
173.0
176.4
7.9
8.1
7.8
8.8
1596
1615
1587
1610
1513
1518
1521
1511
ꢁ160.6 (2)
1.9 (2)
ꢁ0.59 (17)
ꢁ0.04 (15)

M. Rodríguez et al. / Dyes and Pigments 87 (2010) 76e83
79
Fig. 1. Perspective views of the molecular structures of compounds 1a, 1b and 1d. Their ellipsoids are shown at the 20% probability level.
of phenylboronic acid [1.574(4) Å] [17]. The analysis of the covalent
BeO bond for boronates 1a,1b and 1d showed shorter bond lengths
for the BeO(1) distance than the values reported previously in
other boronates (1.496(6) Å) [17], indicating that the boronates
reported herein exist as the enolimine tautomers.
3.3. Growth of single crystals
The crystals of compounds 1b and 1d were grown further to
larger size by using conventional crystal growth techniques.
A saturated solution of 1b and 1d (0.5 g) in chloroform (3 mL) was
The three boron compounds 1a, 1b and 1d analyzed in this work
crystallized in the space group P 212121 that belongs to the ortho-
rhombic crystal system. The crystal features and collection data are
summarized in Table 3. All crystals contain one crystallographic
independent molecule in the asymmetric unit. It is worth to notice
that this series of boronates crystallize in a noncentrosymmetric
space group, this can be attributed to various facts including the
Table 3
Crystal data for boronates 1a, 1b and 1d.
Compound
1a
1b
1d
Chemical formula
Mol wt.
Space group
a (Å)
b (Å)
c (Å)
C17H16BNO2
277.12
P 212121
8.4610 (1)
11.554 (2)
14.889 (3)
90
C17H15BClNO2
311.58
P 212121
8.4550 (1)
12.5096 (2)
14.4744 (3)
90
90
90
1531.48
4
1.351
C17H15BN2O4
322.12
P 212121
8.5364 (2)
12.5227 (6)
14.4121 (3)
90
90
90
1541.70
4
1.388
small size of the electronic p-system, the low dipole moment value
and the strong intermolecular hydrogen bond. The introduction of
the B-phenyl moiety contained in compound 1a did not affect the
packing, in fact the X-ray diffraction analysis showed that its
precursor ligand also crystallizes in a noncentrosymmetric space
group (P 212121) [18a]; in the case of the ligand 1b the crystalliza-
tion occurs in the space group P 21/a, but the addition of
the B-phenyl group favored the crystallization in the non-
centrosymmetric space group P 212121 [18b].
The supramolecular analysis for 1a, 1b and 1d shows that the
stabilization of the crystal structure is due to non classical hydrogen
bonds. In the case of compound 1b that contains a chlorine atom,
this promotes strong C(13)eH(13)/Cli(1) interactions (i ¼ ꢁ1/
2 þ x, 3/2 ꢁ y, ꢁz) with a C(13)/Cl(1) distance of 3.6347(19) Å and
an angle of 137.25(12)ꢀ. The expansion of the previous interaction
leads to the formation of molecular chains containing molecules
related by screw axis (Fig. 2).
a
b
g
(o)
(o)
(o)
90
90
V (Åꢁ3
)
1455.5 (4)
4
1.265
Z
rcalcd. (g/cm3)
Q
Range (ꢀ)
3e27
4e27
3e27
No. of reflections
Measured
Unique
Observed
R [I > 2
6558
1879
1512
0.0376
0.0977
204
ꢁ0.197
0.216
1.056
3462
3462
3092
0.0364
0.0882
201
ꢁ0.213
0.131
1.044
8587
3273
2984
0.0374
0.0991
251
ꢁ0.127
0.116
1.026
s
(I)]
Rw (all data)
Parameters
rmin (e Åꢁ3
)
)
rmax (e Åꢁ3
GOOF

80
M. Rodríguez et al. / Dyes and Pigments 87 (2010) 76e83
Fig. 2. Molecular interactions in the crystal of compound 1b.
prepared. The solution was stirred at slow rate and refluxed at 77 ꢀC
for 4 h. The refluxed solution was covered and kept undisturbed for
slow cooling and slow evaporation of the solvent at room
temperature. Yellow-color crystals were obtained in the form of
plates, needles or bulk structures after a few days. Fig. 3 shows
some crystals obtained with this method.
of the crystal is detected from 220 to 260 ꢀC. The DTA analysis for
compound 1b showed an endothermic peak in 252 ꢀC.
3.5. Optical characterization
The UVevisible absorption spectra of the boronates 1a, 1b, 1c
and 1d exhibited similar spectral features (see Fig. 5) with main
peaks at 400, 407, 409 and 390 nm, respectively. For these
compounds the characteristic absorption band can be assigned to
3.4. Thermal analysis
The thermogravimetric (TG) analysis of crystals 1b and 1d was
carried out in the temperature range from 25 to 450 ꢀC under
nitrogen atmosphere at a heating rate of 10 ꢀC minꢁ1 using
a Thermobalance Mettler Toledo TGASDTA851e thermal analyzer.
As an example, the thermogram for compound 1d and its differ-
ential thermogravimetric curve are shown in Fig. 4. In this TG
analysis there is no significant weight loss until 240 ꢀC but
continuous weight loss can be detected from 240 to 290 ꢀC. At the
latter temperature the weight loss is 55% and the resulting residue
undergoes degradation up to 500 ꢀC. The DTA analysis showed an
endothermic peak in 245.7 ꢀC corresponding to the melting
process. Similarly, in the case of compound 1b, the main weight loss
a ligand-centered electronic S0 / S1 (pep*) transition. The optical
region of transparency for single crystals obtained from these
boronates is a key feature to be examined. For instance, the trans-
parency properties for single crystals grown from the boronates 1b
and 1d are shown in the inset of Fig. 5. Both crystals are semi-
transparent in a wide range of the visible spectrum with cutoff
wavelengths at 500 and 460 nm, respectively. The small disconti-
nuities at 860 nm observed in the spectra presented in this figure
are due to a change in the spectrophotometer detector.
It is worth to mention that the boronates 1ae1d exhibited very
weak photoluminescence in solution. Nevertheless, the photo-
luminescence properties were remarkably enhanced in single

M. Rodríguez et al. / Dyes and Pigments 87 (2010) 76e83
81
Fig. 3. Photograph of single crystals of compounds 1b (in the left) and 1d (in the right).
crystals. For instance, the crystals grown from the boronates 1b and
1d emit intense green fluorescence upon excitation at 370 nm
(excitation from a UV lamp). The emission peaks for these crystals
are 544 nm and 509 nm, respectively (see Fig. 6). As expected, the
emission peak of crystal 1b is red-shifted with respect to the peak
corresponding to crystal 1d since its cutoff frequency in the
absorption spectrum is located at larger wavelength.
On the other hand, the single crystals also emitted fluorescence
under multi-photon excitation. By pumping these crystals with
femtosecond laser pulses (100 fs, 80 MHz repetition rate) at
800 nm, a relative intense fluorescence was seen by the naked eye
in a room with normal illumination conditions. Since these crystals
are transparent at 800 nm, thus it follows that the fluorescence can
only occur by means of multi-photon process, i.e., two-photon
absorption (TPA). This is because the two-photon energy at 800 nm
falls into the one-photon absorption band of the boronates
(see Fig. 5). The photoluminescence spectra of the crystals 1b and
1d excited by two photons (800 nm) are compared with that
observed with those excited by one photon (see Fig. 6). It is
observed that the spectral shape is practically the same, indicating
that the light emission induced by one-photon and two-photons
originates from the same excited state in the crystals.
Interestingly, nanocrystals of 1b and 1d also exhibited strong
fluorescence. For example, the inset in Fig. 6 shows the emission
spectrum from a colloidal solution of nanocrystals (<200 nm in
size) of 1b; it is observed that the peak of fluorescence from these
nanocrystals is blue-shifted approximately 22 nm with respect to
the peak observed for the corresponding single crystal. Fig. 7 shows
a photograph of the luminescence observed from the colloidal
solution; for comparison, this figure also shows the absence of
fluorescence from a chloroform solution of 1b with the same molar
concentration that the colloidal solution of nanocrystals. To
produce these nanocrystals the laser ablation procedure [19] was
employed where 0.3 mg of micrometer-sized crystals (obtained by
hand-grinding single crystals 1b or 1d) were suspended in a poor
solvent (aqueous solution of cetyl trimethyl ammonium bromide,
CTAB, 0.08 mM) and then exposed to the third-harmonic beam
(355 nm) of a nanosecond Nd:YAG laser (7 ns, 10 Hz repetition rate
and 43 mJ/cm2 at sample position) to produce microcrystal frag-
mentation. In this procedure the fragmented material is then
caught by water and stabilized as nanocolloids. In order to get
a homogenous ablation process, the solution was stirred vigorously
using a magnetic stirrer. After the laser ablation process was
completed the colloidal solution was filtered through 200 nm PTFE
membrane filters.
3.6. Studies of the second-harmonic generation (SHG)
SHG measurements performed on single crystals 1b and 1d
were performed to confirm their noncentrosymmetric structures
and to evaluate their potential use as a second-order NLO materials.
Fig. 5. UVevisible absorption spectra for boron compounds 1ae1d. Inset: Trans-
mission spectra of single crystals obtained with 1b and 1d.
Fig. 4. TG and DTA graphs of crystal 1d.

82
M. Rodríguez et al. / Dyes and Pigments 87 (2010) 76e83
As excitation source, a Nd-YAG laser-pumped optical parametric
oscillator (OPO) that delivered pulses of 7 ns at a repetition rate of
10 Hz was employed. When the idler beam of this OPO system was
focused into the crystals (3 mJ/pulse at sample position) and the
wavelength was tuned in the IR wavelength range between 1100
and 1600 nm, the SHG effect was confirmed by detecting mono-
chromatic radiation from green (550 nm) to near IR (800 nm),
respectively. It is worth to point out that at this range of wave-
lengths no fluorescence induced by two-photon absorption was
detected, indicating that the latter effect is only efficient under
excitation around 800 nm. This was expected since the absorption
peaks for these materials are near 400 nm, being 800 nm the two-
photon resonance wavelength as discussed before. Measurement of
the SGH efficiency of 1b and 1d was performed by the Kurtz-Perry
[20] powder technique. For the use of this technique, fine powder
from the crystals was sandwiched between two glass slides. After
the samples, a low pass filter was used to block IR wavelength. The
SHG signal was then focused into the input slit of an imaging
spectrograph and recorded at the exit with a CCD camera. Under
the same experimental conditions, the SHG signal from urea was
obtained. The SHG signal from the crystal of boronates 1b and 1d
resulted in average 4 times larger than that of urea crystals at
1200 nm. By comparing the SHG capacity of urea and that of the
technologically useful potassium dihydrogenphosphate (KDP) [21],
revealed that single crystals grown from compounds 1b and 1d
exhibit SHG efficiency about 26 times higher than KDP.
Fig. 6. Photoluminescence spectra for single crystals 1b and 1d obtained under one-
photon excitation at 370 nm (continuous lines) and two-photon excitation at 800 nm
(open circles). Inset: Photoluminescence spectrum from colloidal solution of nano-
crystals of 1b excited at 370 nm.
4. Conclusions
A new series of boronates obtained by one-pot synthesis were
utilized for the growth of single crystal with the size of few milli-
meters. These compounds tend to crystallize in the non-
centrosymmetric space group P 212121 promoted by non classical
hydrogen bonds and for the small size of the main backbone.
Nonlinear optical studies confirmed that the SHG efficiency of these
crystals is in average 4 times larger than in urea and 26 times larger
than in KDP. Further, these crystals exhibit notorious fluorescence
induced by one-photon and two-photon absorption at the resonant
wavelengths about 400 and 800 nm, respectively. Interestingly, the
photoluminescence properties are conserved in colloidal solution
of nanocrystals. This is the first report about the preparation of
organic crystals in the scale of millimeters with boron derivatives
and offers the opportunity to investigate other organoboron
compounds in nonlinear organic crystals.
Acknowledgements
One of the authors (M. Rodríguez) thanks to CONACyT for
a scholarship. This work was supported by CONACyT (Projects
J49512F and 55250) and UNAM (PAPIIT IN-214010). The authors
thank Martín Olmos for technical assistance, Maria Luisa Rodríguez
for NMR experiments, Marco Leyva for X-ray analysis and Geiser
Cuellar for MS.
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