Structures and nonlinear optical properties of molecular crystals DMCC and DBCC

Article abstract of DOI:10.1016/j.molstruc.2007.01.041

Structures and nonlinear optical properties of dimethylcroconate (DMCC) and dibenzoylcroconate (DBCC) are reported. The SHG active DMCC crystal packed in non-centrosymmetric P2₁2₁2₁ space group. The DBCC crystal has a host

Full text of DOI:10.1016/j.molstruc.2007.01.041

Available online at www.sciencedirect.com
Journal of Molecular Structure 871 (2007) 1–5
www.elsevier.com/locate/molstruc
Structures and nonlinear optical properties of molecular
crystals DMCC and DBCC
a
a,b,
c
a
Hong-Yu Chen , Qi Fang
Xiu-Feng Cheng , Zhi-Qiang Liu
*, Hong Lei , Wen-Tao Yu ,
a
a
a
State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, PR China
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Science, 200032, PR China
b
c
School of Information Science and Engineering, Shandong University, Jinan, Shandong 250100, PR China
Received 6 December 2006; received in revised form 20 January 2007; accepted 20 January 2007
Available online 30 January 2007
Abstract
Structures and nonlinear optical properties of dimethylcroconate (DMCC) and dibenzoylcroconate (DBCC) are reported. The SHG
active DMCC crystal packed in non-centrosymmetric P212121 space group. The DBCC crystal has a host–guest structure and crystallized
in centrosymmetric P21/n space group. In DBCC crystal, the host DBCC molecules build channels along the b direction with the guest
benzene molecules trapped in the channels. DSC, XRD, SHG and structural studies reveal that these benzene molecules can be removed
at 90 ꢀC, accompanying the loss of the centro-symmetry and resulting in the SHG active new crystalline phase.
ꢁ 2007 Elsevier B.V. All rights reserved.
Keywords: Dimethylcroconate; Dibenzoylcroconate; Structure; Nonlinear optical property
1. Introduction
cy being about 1000 times that of urea reference and the
metal–organic complexes with large b values being the
order of 1000 · 10ꢀ30 e.s.u. [8,9]. In exploring the struc-
ture–property relationships, the most successful theoretical
approach may be the double-energy model for D-p-A type
of dipolar molecules [10]. This was followed by an interest-
ing idea of the non-polar and non-centrosymmetric
octupoles [11]. An octupolar NLO crystal TTB (1,3,5-tricy-
ano-2,4,6-tris(p-diethylaminostyryl)benzene) was recently
reported to have an extraordinarily large SHG efficiency
which is double of that of the well known dipolar NLO
crystal DAST [12].
In the past three decades, molecular nonlinear optical
(NLO) materials have been intensively explored because
of their large NLO response, structural diversities and
intriguing structure–property relationships [1–5]. Second
harmonic generation (SHG) is a basic 2-order NLO
process, which is characterized by molecular 2-order polar-
izability b (known as superpolarizability) and crystalline
2-order susceptability v(2). For molecular crystals, their
macro SHG intensities are dominated by the b value and
the molecular packing style in the crystal.
Among the most significant experimental results in 2-or-
der organic NLO field are, for example, the neutral organic
crystals [6] and organic salts [7] with a typical SHG efficien-
The croconate compounds belong to a member of so
called oxocarbons CnOn(n = 3 for deltate, 4 for squarate,
5 for croconate, 6 for rhodizonate). All atoms of these
oxocarbons are coplanar and their dianians are well
delocalized [13]. A theoretical study indicates that the
oxocarbons have moderate b values of the order of
10–100 · 10ꢀ30 e.s.u., which increase with increasing the
ring size [14]. For squarate compounds, there is only one
*
Corresponding author. Address: State Key Laboratory of Crystal
Materials, Shandong University, Jinan, Shandong 250100, PR China.
Fax: +86 531 88362782.
E-mail address: fangqi@icm.sdu.edu.cn (Q. Fang).
0022-2860/$ - see front matter ꢁ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.01.041

2
H.-Y. Chen et al. / Journal of Molecular Structure 871 (2007) 1–5
report about the measured b values but the crystals are vir-
tually SHG inactive because of the unfavorable molecular
packing [15]. As for croconate derivatives, to our knowl-
edge, there is no experimental NLO result reported.
In this work, we synthesized and grew two single crystals
of the croconate derivatives, dimethlycroconate (DMCC)
and dibenzoylcroconate (DBCC). The DBCC forms cen-
trosymmetric crystal in which benzene molecules are incor-
porated in a ratio of 2 (DBCC):1 (benzene) and its crystal
structure is first reported here. The DMCC forms non-cen-
trosymmetric crystal and its structure has been briefly
reported by our group [16]. The SHG intensities of DMCC
and DBCC were measured and the structure–property
investigation has been carried out.
P212121 space group with a = 7.181(5), b = 8.069(5),
3
˚
˚
c = 13.015(5) A, v = 754.1(8) A , Z = 4, R1 = 0.040 and
wR2 = 0.117 (I > 2r(I)). DBCC crystal has a host–guest
structure formulated as C19H10O7Æ0.5C6H6 and crystallized
in P21/n space group with a = 11.203(2), b = 5.7765(9),
3
˚
˚
c = 28.073(5) A, b = 97.525(9)ꢀ, v = 1801.1(5) A , Z = 4,
R1 = 0.060 and wR2 = (I > 2r(I)).
The molecular ground-state dipole moment l was cal-
culated by using Gaussian 03 program within the frame-
work of HF/6-31G [17]. The geometry conformations for
the calculations are the X-ray molecular structures of
DMCC and DBCC in this work. The excited-state
dipole moment le, the oscillator strength f and energy
differences between various states were calculated by
the same program within the framework of RCIS/6-
31G [17]. Based on these calculated results, the hyperpo-
larizabilitiy tensor components bijk were then calculated
by the sum-over-states method [18]. A total of 20 states
were taken.
2. Experimental and calculation
The synthesis for the two title compounds is outlined in
Scheme 1. A solution of dipotassium croconate (5 g) in
water (300 ml) was mixed with another solution of silver
nitrate (9 g) in water (50 ml) in the dark. Orange disilver
croconate immediately precipitated and the mixture was
kept stirring for 20 min. The orange solid was then filtered
out and dried at 50–60 ꢀC for several hours, resulting 7.5 g
of disilver croconate.
To synthesis DMCC, 7.4 g anhydrous disilver croconate
was placed in a Soxhlet extractor, which was fitted on a
flask containing methyl iodide (7 ml) and 200 ml anhy-
drous benzene (freshly distillated with Na). The reaction-
and-extraction process under reflux lasted for 24 h. The
solvent was removed and 2.6 g of yellow DMCC solid
was obtained (yield 73%). DBCC was synthesized in the
similar way by reacting 7.5 g disilver croconate with
12 ml benzoyl chloride and 4.4 g of pale-yellow tiny needle
crystals were obtained (yield 68%). Yellow parallelepiped
DMCC crystals were obtained by spontaneously evaporat-
ing its benzene solution at room temperature in a water-
free atmosphere. Yellow rod DBCC crystals were obtained
by slowly cooling its benzene solution from 70 ꢀC to room
temperature.
The b value listed in Table 2 is the vector component bl
along the direction of the molecular dipole moment l,
defined as [19]
X
where bi ¼
bl ¼
libi=jlj; i ¼ x; y; z
ð1Þ
ð2Þ
i
X
1
3
ðbijj þ bjij þ bjjiÞ; i; j ¼ x; y; z
j
For DMCC and DBCC, the molecular dipole moment is
along the y direction shown in Fig. 1.
3. Results and discussion
3.1. Crystal structure
DMCC molecules are packed in non-centrosymmetric
space group P212121, while DBCC molecules are packed
in centrosymmetric space group P21/n. As shown in
Fig. 1, DMCC molecule has very good planarity, but
DBCC molecule is non-planar. The flexibility of the K
shaped DBCC molecule has been overcome to some extent
by some intramolecular connections.
X-ray refraction data of the DMCC and DBCC single
crystals were collected by x scan method by a Bruker P4
diffractometer with Mo Ka radiation at room temperature.
DMCC crystal has a formula of C7O5H6 and packed in
The croconate moieties in the two compounds show C2v
symmetry. As listed in Table 1, for DMCC the five CAC
O
O
O
CH3
CH3
O
O
O
O
CH3I
O
DMCC
DBCC
OK
OK
OAg
OAg
Ag+
O
O
O
O
C
O
O
O
C
O
Cl
O
C
O
O
Scheme 1.

H.-Y. Chen et al. / Journal of Molecular Structure 871 (2007) 1–5
3
Fig. 1. Molecular structures for (a) DMCC and (b) DBCC.
Table 1
Bond lengths related to croconate moiety in the title compounds
DMCC
DBCC
O1AC3 1.319(4) C3AC4 1.373(5)
O2AC4 1.315(4) C3AC7 1.452(5)
O5AC7 1.222(5) C4AC5 1.454(5)
O3AC5 1.219(4) C5AC6 1.502(5)
O4AC6 1.202(4) C6AC7 1.509(5)
C1AO1 1.196(5) C1AC3 1.521(6)
C2AO2 1.208(5) C1AC2 1.527(6)
C3AO3 1.203(5) C2AC4 1.481(6)
C4AO4 1.364(5) C3AC5 1.472(6)
C5AO5 1.348(5) C4AC5 1.344(6)
bonds lengths of the five-membered ring are within the
˚
range 1.373(5)–1.509(5) A and the C@O bonds lengths
˚
are within 1.202(4)–1.319(4) A. For DBCC, the five CAC
bonds and the five C@O bonds of the croconate moiety
are characterized by 1.344(6)–1.527(6) and 1.196(5)–
1.364(5) A, respectively. The difference for the same type
˚
Fig. 2. Packing diagram of DBCC crystal.
bond of the croconate moiety in DBCC is much larger than
that in DMCC. This means the electronic delocalization of
the croconate moiety in DMCC is relatively larger.
The calculated ground-state dipole moment of the
DBCC is almost double that of DMCC (see Table 2). This
may be the reason why DBCC crystal has a centrosymmet-
ric structure.
are all along the b axis direction to accommodate the guest
benzene molecules and have a transverse area about 40 A .
2
˚
3.2. Linear and nonlinear optical properties
It is noteworthy that DBCC crystal has an host–guest
structure with a stoichiometric ratio of 2 (DBCC):1 (ben-
zene) (see Fig. 2). Two DBCC molecules coupled to form
a near-square and all these squares form channels, which
The absorption spectra of DMCC and DBCC in aceto-
nitrile are shown in Fig. 3. The cut-off wavelength kcut-off
for the two compounds is about 420 and 320 nm, respec-
tively. And their absorption maximum wavelength kmax is
Table 2
Experimental absorption wavelength and the calculated electronic struc-
tural parameters
1.2
kcut-off lg (D) le (D)a fge
bl
SHGd
b
kmax
(nm) (nm)
(10ꢀ30
e.s.u.)c
0.8
DMCC
DMCC 301
404
313
4.770
8.970
6.910 0.7885 15.31
10.398 0.6328 25.09
0.9 Urea
0.6 Urea
0.4
DBCC
234
DBCC
a
According to the results of the RCIS/6-31G scheme, the charge-
transfer excited-state j eæ of DMCC mainly consists of the LUMO orbital;
while that of DBCC consists of many orbitals.
0.0
250
300
350
400
450
500
b
Oscillator strength.
The calculated bl is at the k = 1064 nm of fundamental light.
The SHG intensities (1080 nm fi 540 nm) of the powder crystal
wavelength (nm)
c
d
Fig. 3. Absorption spectra for DMCC and DBCC in acetonitrile at
concentration of 10ꢀ5 mol Lꢀ1
.
samples are the relative values with urea powder as the standard.

4
H.-Y. Chen et al. / Journal of Molecular Structure 871 (2007) 1–5
301 nm for DMCC and 234 nm for DBCC. The kcut-off of
the two compounds are short enough for the SHG of
Nd:YAP or Nd:YAG lasers and other green or blue lasers.
The bathochromic effect of DMCC relative to DBCC can
be interpreted as the stronger donor strength of the methyl
group relative to the benzoyl group. Besides, the croconate
moiety of DMCC shows better p-conjugation as revealed
by the X-ray structural results above. One challenge in
molecular NLO field is the so-called trade-off problem
between transparency and efficiency. Interestingly, howev-
er, DBCC has both larger b value (see Table 2) and better
transparency.
a
b
10
15
20
25
30
35
40
45
50
θ
2
/°
SHG intensities of the power DBCC and DMCC crys-
tals were determined by the Kurtz method [20] and urea
sample was used as the reference. The input fundamental
light has a wavelength of 1080 nm from a Nd:YAP laser.
The powder SHG efficiency of DMCC is 0.9 times that
of the powder urea standard. However, the ‘‘centrosym-
metric’’ DBCC power crystals also exhibited a SHG inten-
sity 0.6 times that of the urea. This result is contrary to the
well-established tensor theory about the centrosymmetric
forbidden for the 2-order NLO response.
The crystal structure of DBCC shows that there are two
guest benzene molecules in the unit cell with weak molecu-
lar interaction with the host DBCC molecules. Prompted
by the channels along the b axis (see Fig. 3), these benzene
molecules must be easy to escape. Once the guest molecules
get out of the crystal, the crystal perhaps changes to a new
phase of structure. The measured SHG efficiency may be
the result of the new non-centrosymmetric phase, for the
benzene molecules are just located at the inversion centers
of the DBCC crystal and the loss of these inversion centers
is likely to result in the collapse of the centrosymmetric
packing of host DBCC molecules. Because it is difficult
to obtain the structure of this new phase, we try to find evi-
dences of the supposed phase transition by thermal analysis
and XRD method.
Fig. 5. XRD spectra for DBCC: (a) slightly grinding sample; (b) long time
grinding sample.
90 ꢀC, and its desorption heat is 2.6 kJ/mol. Such low
desorption temperature and small desorption heat make
the benzene molecules easy to get out of the crystal when
it is grinded to powder sample for SHG test.
The DSC curves show partly overlapped double peaks
in the melting process, which can be interpreted as the con-
secutive melting of the no-benzene phase and the original
phase. Two curves in different heating rate show different
relative height of the double peaks. Slower heating
(10 ꢀC/min) activates more benzene molecules to escape,
so the first peak should belongs to the melting of the no-
benzene phase and the second belongs to the melting of
the original phase. Because the melting temperature of
the no-benzene phase is close to the benzene desorption
point, it is difficult to get the pure no-benzene phase.
To find further evidence of the phase transition, X-ray
powder pattern of the crystals were recorded at a scan rate
of 10 ꢀC/min on a Rigaku D/Max 2200PC with Cu Ka
radiation and shown in Fig. 5. The XRD pattern for the
slightly grinded sample of DBCC is similar to the simulated
pattern from the data of the single crystal structure. When
the sample underwent long time grinding, however, its
XRD pattern is obviously different from the previous
one. Especially, the three strong peak positions are differ-
ent and the pattern is more complicated.
The DSC curves for DBCC crystal were recorded in
nitrogen atmosphere at the different heating rate (10 ꢀC/
min and 20 ꢀC/min, respectively). From Fig. 4, we recog-
nized that the desorption of the benzene took place at
Although DBCC has larger b value (see Table 2), its
crystalline SHG intensity is relatively weak. It is because
of the difficulty to isolate the pure non-centrosymmetric
phase. One may think the NLO effect of the DBCC mole-
cule may come from the benzene moiety, but DMCC also
exhibits considerable NLO effect both at the molecular
level and the crystalline level. Thus we conclude that the
croconate moiety can independently serve as NLO genera-
tor or chromophore.
DSC curve at heating rate 20 °c/min
DSC curve at heating rate 10 °c/min
100 150 200 250
4. Conclusion
50
Organic croconate compounds DMCC and DBCC were
synthesized and their crystal structures were determined by
X-ray diffraction. They are quite transparent and exhibit
Temperature °c
Fig. 4. DSC curves for DBCC crystals.

H.-Y. Chen et al. / Journal of Molecular Structure 871 (2007) 1–5
5
moderate micro hyperpolarizability and macro SHG effi-
ciency, indicating that the croconate can independently
serve as a NLO center. To solve the conflict between the
SHG effect and the centrosymmetric structure of DBCC,
we found that the SHG turned out to be the result of a
new phase of DBCC. The turn-and-off of the SHG signal
is accompanied by the out-and-in of the guest benzene mol-
ecules. This interesting phenomena of DBCC crystal may
find some application as a kind of chemosensor to probe
poisonous benzene in air. Noticeably, this kind of applica-
tion does not demand tremendous SHG intensity, which is
once the goal of the organic NLO materials field. SHG here
had rather be a tool than be a goal.
ed with this article can be found, in the online version, at
doi:10.1016/j.molstruc.2007.01.041.
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We gratefully acknowledge the financial support of the
National Natural Science Foundation of China (Nos.
20472044, 50673054), the Ph.D. Foundation of Ministry
of Education of China, and the Foundation of Key Labo-
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´
Appendix A. Supplementary data
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Crystallographic data for the structure reported in this
paper in the form of CIF file have been deposited with
the Cambridge Crystallographic Data Center as supple-
mentary publication number CCDC-614943 for DBCC
and number CCDC-282597 for DMCC. These data can
be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336033). Supplementary data associat-
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