Synthesis, structure, thermal and nonlinear optical properties of a series of novel D-π-A chromophores with varying alkoxy substituents

Article abstract of DOI:10.1016/j.molstruc.2011.09.032

Six new derivatives of the previously reported chromophore 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile (OH1) with potential application in electrooptics were synthesized and their crystal structures characterized using a single c

Full text of DOI:10.1016/j.molstruc.2011.09.032

Journal of Molecular Structure 1006 (2011) 356–365
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, structure, thermal and nonlinear optical properties of a series of novel
D–p–A chromophores with varying alkoxy substituents
Ilya V. Kosilkin a, Emily A. Hillenbrand a, Paul Tongwa b, Alexandr Fonari b, Joel Zazueta b, Marina S. Fonari b,c
,
Mikhail Antipin b,d, Larry R. Dalton a, Tatiana Timofeeva b,
⇑
a University of Washington, Seattle, WA 98195, USA
b New Mexico Highlands University, Las Vegas, NM 87701, USA
c Institute of Applied Physics Academy of Sciences of Moldova, Academy Str., 5 MD2028 Chisinau, Republic of Moldova
d A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova Str. 28, 119991 Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Six new derivatives of the previously reported chromophore 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclo-
hex-2-enylidene)malononitrile (OH1) with potential application in electrooptics were synthesized and
their crystal structures characterized using a single crystal X-ray technique. The presented chromophores
Received 28 June 2011
Received in revised form 14 September
2011
Accepted 14 September 2011
Available online 19 September 2011
have a D–p–A structure with a phenol donor, ring locked polyene conjugated bridge and dicyanomethy-
lydene acceptor. In order to potentially alleviate problems with the quality of melt grown films of OH1,
the melting point was tuned through the introduction of alkoxy groups of various lengths and onto var-
ious positions on the chromophore core as well as through the modification of the bridge structure. We
show that the melting point of these materials can be significantly decreased with the increased number
of carbon atoms in alkoxy group. However, the molecular arrangement was found to be highly sensitive
to the addition of substituents, and regardless of the size and position of the alkoxy groups, all presented
chromophores formed centrosymmetric structures. The distinctions in crystal packing are explained by
the mutual arrangement of donor (OH) and H-acceptor (OAlk, C(CN)2) groups in the molecules. Static first
order hyperpolarizabilities along with molecular geometry in isolated state were calculated using DFT.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Ring-locked polyenes
Synthesis
X-ray structure
DFT calculations
NLO chromophores
Electrooptics
1. Introduction
ores having strong dipole moments remains a challenge. One of the
first examples of a molecule that packed in a noncentrosymmetric
The field of organic electrooptic (EO) materials has significantly
advanced over the last several decades. Chromophores with excep-
tional nonlinear optical (NLO) properties have been synthesized and
tested in various device designs employing the EO effect [1–3]. To
achieve EO performance, noncentrosymmetric ordering of dipolar
chromophores is necessary. Different approaches for such align-
ment have been proposed and studied. One of the most widely used
is electric field poling. However, this method lacks efficiency, which
results in lower than expected EO performance [4,5]. Alternative ap-
proaches have also been studied; among them is the use of chro-
mophores with structures that favor molecular arrangement in
noncentrosymmetric fashion upon packing in a crystal lattice [6–
15]. This approach, however, is not widely applied since the relation-
ship between molecular structure and crystal packing is not well
understood. There are a large number of papers that discuss the pre-
diction of molecular arrangement in crystalline form taking into ac-
count various forces (e.g. H-bonding and dipole–dipole interactions)
[16]. However, the noncentrosymmetric organization of chromoph-
manner and showed strong nonlinearity was the organic EO chro-
mophore 4-N,N-dimethylamino-40-N0-methyl stilbazolium tosylate
(DAST) [17,18]. However, its low solubility and poor processability
limited its application. Other examples can also be found in the lit-
erature, but low molecular hyperpolarizabilities (b) of those materi-
als resulted in quite modest overall EO performance [9,10].
A new class of chromophores consisting of a phenol (or aniline)
donor, ring locked
p-conjugated bridge, and dicyanomethylydene
acceptor has several members that exhibit noncentrosymmetric
molecular organization in a crystal lattice [11–15]. Among them
are 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)
malononitrile (OH1) [11], (2-(3-(4-hydroxystyryl)-5-methylcyclo-
hex-2-enylidene)malononitrile) (OH2) [19] and 2-(3-(4-hydroxy-
styryl)cyclohex-2-enylidene)malononitrile (OH3) [14,19]. These
materials have been shown to have large nonlinearities in thin
films grown from crystal melt or using a slow solution evaporation
technique. Unfortunately, the crystal melt approach giving good
molecular arrangement produced films of OH1 with internal
microcracks [20]. This behavior was attributed to the excessive
film shrinking upon cooling caused by the rather high melting
point of OH1. The presence of microcracks significantly increases
⇑
Corresponding author. Tel.: +1 505 454 3362.
E-mail address: tvtimofeeva@nmhu.edu (T. Timofeeva).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.09.032

I.V. Kosilkin et al. / Journal of Molecular Structure 1006 (2011) 356–365
357
light scattering and thus optical loss. In this study we explored the
introduction of alkoxy groups of variable length to the donor and to
two different ring locked conjugated bridge structures as a means
to tune the melting point of these materials. In this report the syn-
thesis, X-ray structural characterization, and computational analy-
sis of these six new materials are discussed. Although the addition
of these substituents in all cases disrupted the noncentrosymmet-
ric packing seen in the parent chromophore, the results
nevertheless provide insight into the relationship between the po-
sition of the substituent and the resulting crystal structure.
afforded 0.030 g of red crystalline solid. Mp 216–219 °C. Yield: 45%.
1H NMR (300 MHz, CDCl3): d 7.38 (d, 2H, J = 8.5 Hz), 7.21 (d, 1H,
J = 16 Hz), 6.87 (d, 1H, J = 16 Hz), 6.79 (d, 2H, J = 8.6 Hz), 3.68 (s,
3H), 2.60 (s, 2H), 2.43 (s, 2H), 1.02 (s, 6H). MS: m/z = 319.1 (M-). IR,
m
(NaBr) 3411 (OH), 3301, 2939 (OCH3), 2232 (CN), 1597, 1547
(Ar), 1532, 1517, 1443, 1375, 1370(C(CH3)2), 1344, 1269,
1247(OH), 1208, 1172, 1161, 1105(C@C), 970, 828 cmꢀ1
.
2.2.2. Synthesis of 2-(3-(4-hydroxy-3-methoxystyryl)-5,5-
dimethylcyclohex-2-enylidene)malononitrile (2)
2. Experimental
4-Hydroxy-3-methoxybenzaldehyde (0.367 g, 2.41 mmol), 9a
(0.500 g, 2.68 mmol), piperidine (0.265 mL, 2.68 mmol) in 10 mL
of ethanol afforded 0.305 g of red crystalline solid. Mp 198–199 °C.
Yield: 40%. 1H NMR (500 MHz, CDCl3): d 7.10–6.86 (m, 4H), 5.88 (s,
1H), 4.01 (s, 3H), 2.63 (s, 2H), 2.49 (s, 2H), 1.11 (s, 6H). MS:
2.1. General description
The chemical structures of the synthesized chromophores are
shown in Scheme 1. The chromophores have a common ring locked
m/z = 319.2 (Mꢀ). IR,
m (NaBr) 3452(Ar-OH), 2962(OCH3), 2936,
p-conjugated bridge with a dicyanomethylydene electron acceptor
2222 (CN), 1596, 1555(Ar), 1507, 1426, 1338, 1311, 1282, 1264,
1244, 1215, 1196, 1157, 1122(C@C), 1032, 972, 861 cmꢀ1
.
and phenolic electron donor. To study how the molecular structure
affects the melting point and crystal packing, six chromophores
with varied positions and lengths of the alkoxy group, and having
two different types of central ring were synthesized. Chromophore
2.2.3. Synthesis of 2-(3-(3-ethoxy-4-hydroxystyryl)-5,5-
dimethylcyclohex-2-enylidene)malononitrile (3)
1 has the methoxy group attached to the ring locked part of the p-
3-ethoxy-4-hydroxybenzaldehyde (0.400 g, 2.41 mmol), 9a
(0.500 g, 2.68 mmol), piperidine (0.265 mL, 2.68 mmol) in 10 mL
of ethanol afforded 0.470 g of red crystalline solid. Mp 170–172 °C.
Yield: 58%. 1H NMR (500 MHz, CDCl3): d 7.08–6.94 (m, 4H), 5.93 (s,
1H), 4.22 (q, 2H, J = 7.0 Hz), 2.62 (s, 2H), 2.48 (s, 2H), 1.52 (t, 3H,
conjugated bridge. The remaining chromophores have alkoxy
group attached to the donor at the ortho position to the OH group,
with the number of carbon atoms varying from one to three. Chro-
mophores 5 and 6 have hydrogen atoms in place of methyl groups
in the central ring.
J = 7.0 Hz), 1.10 (s, 6H). MS: m/z = 333.2 (M-). IR,
m (NaBr) 3405(Ar-
OH), 2958, 2927(Ar-OCH3), 2872, 2219 (CN), 1558(Ar), 1507, 1480,
Synthesis of chromophores 1–6 and intermediates is presented
in Scheme 2 and consists of Knoevenagel condensation of the
appropriate benzaldehyde with 8, 9a or 9b where the latter were
obtained using the following procedures. Compound 8 was synthe-
sized starting from 4-bromo-3,5,5-trimethylcyclohex-2-enone that
was first converted into 7, followed by a microwave assisted con-
densation with malononitrile. Isophorone (or 3-methyl-2-cyclo-
hexenone) was condensed with malononitrile using Dean–Stark
technique to afford 9a (or 9b [21]). Compounds 2, 5 and 3, 6 were
synthesized using commercially available methyl- and ethyl vanil-
lin respectively [11]. Chromophore 4 was synthesized using 4-hy-
droxy-3-propoxybenzaldehyde 10 that was obtained by the
reaction of 3-bromo-4-hydroxybenzaldehyde with sodium n-prop-
oxide [22]. The melting points of all materials were measured using
differential scanning calorimetry using a DSC Q20 (TA instruments).
1439, 1398, 1383(C(CH3)2), 1346, 1307, 1277, 1237, 1217, 1187,
1156, 1124(C@C), 957 cmꢀ1
.
2.2.4. Synthesis of 2-(3-(4-hydroxy-3-propoxystyryl)-5,5-
dimethylcyclohex-2-enylidene)malononitrile (4)
4-Hydroxy-3-propoxybenzaldehyde (1.00 g, 5.55 mmol), 9a
(1.088 g, 5.84 mmol), piperidine (0.577 mL, 5.84 mmol) in 20 mL
of ethanol afforded 0.905 g of red crystalline solid. Mp 160–162 °C.
Yield: 47%. 1H NMR (300 MHz, CDCl3): d 6.95 (m, 4H), 5.95 (s, 1H),
4.10 (t, 2H, J = 6.6 Hz), 2.61 (s, 2H), 2.47 (s, 2H), 1.92 (sextet, 2H,
J = 7.1 Hz), 1.11 (t, 3H, J = 7.4 Hz), 1.09 (s, 6H). MS: m/z = 347.2
(Mꢀ). IR,
m (NaBr) 3368(Ar-OH), 2966, 2933(Ar-OCH2CH3), 2876,
2231, 2213 (CN), 1597, 1559(Ar), 1506, 1472, 1439, 1400,
1387(C(CH3)2), 1345, 1306, 1282, 1240, 1196, 1123(C@C), 960 cmꢀ1
.
2.2. Synthesis
Typical procedure of Knoevenagel condensation. A round bottom
flask was flame dried, cooled under stream of nitrogen gas, charged
with 8 (9a, 9b) and appropriate benzaldehyde. Ethanol was added,
followed by piperidine. The mixture was heated to 60 °C and stirred
for 40 min. Ethanol was then removed in vacuo affording a black
sticky solid that was purified two times on a silica gel column elut-
ing with dichloromethane ? dichloromethane/methanol (v/v: 95/5).
2.2.5. Synthesis of 2-(3-(4-hydroxy-3-methoxystyryl)cyclohex-2-
enylidene)malononitrile (5)
4-Hydroxy-3-methoxybenzaldehyde (0.289 g, 1.89 mmol), 9b
(0.334 g, 2.11 mmol), piperidine (0.208 mL, 2.11 mmol) in 10 mL
of ethanol afforded 0.163 g of red crystalline solid. Mp 196–200 °C.
Yield: 26%. 1H NMR (300 MHz, CDCl3): d 6.96 (m, 4H), 5.88 (s, 1H),
4.00 (s, 3H), 2.81 (dd, 2H, J = 5.7 Hz, J = 7.2 Hz), 2.65 (t, 2H,
2.2.1. Synthesis of 2-(3-(4-hydroxystyryl)-2-methoxy-5,5-
J = 6.1 Hz), 1.98 (quintet, 2H, J = 6.3 Hz). MS: m/z = 291.1 (M-). IR,
m
dimethylcyclohex-2-enylidene)malononitrile (1)
(NaBr) 3387(Ar-OH), 3025, 2962, 2933(Ar-OBu), 2872, 2221 (CN),
4-Hydroxybenzaldehyde (0.025 g, 0.208 mmol),
0.231 mmol), piperidine (0.023 mL, 0.231 mmol) in 5 mL of ethanol
8 (0.050 g,
1558(Ar), 1530, 1512, 1457, 1429, 1360, 1295, 1283, 1230, 1222,
1180, 1162(C@C), 1038, 968 cmꢀ1
.
where:
1
R1 = H
R2 = OCH3 R3 = CH3
2
3
4
5
6
R1 = OCH3
R1 = OCH2CH3
R1 = OCH2CH2CH3 R2 = H
R1 = OCH3
R1 = OCH2CH3
R2 = H
R2 = H
R3 = CH3
R3 = CH3
R3 = CH3
R3 = H
R2 = H
R2 = H
R3 = H
Scheme 1. Molecular structure of chromophores 1–6.

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Scheme 2. Synthesis of chromophores 1–6 and intermediates.
2.2.6. Synthesis of 2-(3-(3-ethoxy-4-hydroxystyryl)cyclohex-2-
enylidene)malononitrile (6)
eluting with hexanes/ethyl acetate (v/v: 8/2) affording 0.222 g of
white solid. Yield: 25%. 1H NMR (300 MHz, CDCl3): d 3.66 (s, 3H),
2.64 (s, 2H), 2.27 (d, 2H, J = 1.1 Hz), 1.99 (s, 3H), 1.04 (s, 6H).
3-ethoxy-4-hydroxybenzaldehyde (0.397 g, 2.39 mmol), 9b
(0.420 g, 2.65 mmol), piperidine (0.262 mL, 2.65 mmol) in 10 mL
of ethanol afforded 0.140 g of red crystalline solid. Mp 184–187 °C.
Yield: 18%. 1H NMR (300 MHz, CDCl3): d 6.95 (m, 4H), 5.93 (s, 1H),
4.23 (q, 2H, J = 7.0 Hz), 2.81 (dd, 2H, J = 5.8 Hz, J = 7.2 Hz), 2.65 (t,
2H, J = 6.2 Hz), 1.98 (quintet, 2H, J = 6.3 Hz), 1.53 (t, 3H, J = 7.0 Hz).
2.2.9. Synthesis of 2-(3,5,5-trimethylcyclohex-2-
enylidene)malononitrile (9a)
9a was synthesized according to the procedure previously de-
scribed in the literature. The spectral analysis data matched pub-
lished values [23].
MS: m/z = 305.1 (Mꢀ). IR,
m(NaBr) 3353(Ar-OH), 2938(Ar-OCH2CH3),
2230, 2217 (CN), 1548(Ar), 1524, 1507, 1479, 1454, 1395, 1361,
1340, 1322, 1294, 1264, 1235, 1201, 1184, 1123(C@C), 963 cmꢀ1
.
2.2.7. Synthesis of 2-methoxy-3,5,5-trimethylcyclohex-2-enone (7)
A round bottom flask was flame dried and cooled under a stream
of nitrogen gas. Na was added (1.65 g 74.3 mmol) followed by MeOH
(300 mL). After Na dissolved, a 0.367 M methanol solution of 4-bro-
mo-3,5,5-trimethylcyclohex-2-enone (7.98 g, 36.7 mmol) was
added dropwise. The reaction was stirred for 72 h at RT. The solution
was then diluted with ethyl acetate, and washed with DI water
(7 ꢁ 100 mL). The organic fraction was dried over MgSO4. Solvent
was removed in vacuo. The crude material was purified on silica
gel column eluting with hexanes/ethyl acetate (v/v: 8/2) affording
2.80 g of colorless oil. Yield: 45%. 1H NMR (300 MHz, CDCl3): d 3.66
(s, 3H), 2.34 (s, 2H), 2.30 (s, 2H), 1.92 (s, 3H), 1.07 (s, 6H).
1
2
3
4
5
6
2.2.8. Synthesis of 2-(2-methoxy-3,5,5-trimethylcyclohex-2-
enylidene)malononitrile (8)
A round bottom flask was flame dried and cooled under stream
of nitrogen gas. 7 (0.681 g, 4.05 mmol), malononitrile (0.803 g,
12.15 mmol) followed by 5 mL of ethanol was added. After the sol-
ids dissolved, 12.2 mL of freshly prepared 0.5 M sodium ethoxide
solution was added. The reaction ran at 70 °C for 1.5 h using micro-
wave radiation (30 W). The crude reaction mixture was diluted
with ethyl acetate and washed with DI water (7 ꢁ 50 mL). The or-
ganic fraction was dried over MgSO4. Solvent was removed in va-
cuo. The crude material was purified on a silica gel column
300
350
400
450
500
550
600
650
Wavelength, nm
Fig. 1. Normalized UV–Vis absorption of chromophores 1–6 in chloroform. The
presented curves were obtained at
a chromophore concentration of around
2 ꢁ 10ꢀ5 M.

I.V. Kosilkin et al. / Journal of Molecular Structure 1006 (2011) 356–365
359
Table 1
2.2.10. Synthesis of 2-(3-methylcyclohex-2-enylidene)malononitrile
(9b)
9b was synthesized according to the procedure previously
kmax and corresponding molar extinction coefficients for compounds 1–6.
Compound
kmax (nm)
e
(L molꢀ1 cmꢀ1
)
described in the literature. The spectral analysis data matched
published values [23].
1
2
3
4
5
6
OH1
OH3
430
429
431
432
428
428
417
416
27,800
28,000
26,500
34,900
26,000
24,700
37,600
41,600
2.2.11. Synthesis of 4-hydroxy-3-propoxybenzaldehyde (10)
A round bottom flask was flame dried and cooled under stream
of nitrogen gas. Na (2.50 g, 106.95 mmol) was added followed by n-
propanol (50 mL). The mixture was brought to reflux allowing
about 10 mL of n-propanol to distill off. A suspension of CuBr
(1.40 g, 9.90 mmol), 3-bromo-4-hydroxybenzaldehyde (5.00 g,
24.87 mmol) in 30 mL of DMF was then added at once to the first
solution. The resulting mixture was refluxed for 10 h. It was then
cooled to room temperature, diluted with DI water, acidified to
pH 3 with 1 N HCl, and extracted with ethyl acetate (3 ꢁ 30 mL).
1
2
3
4A
5
6A
Fig. 2. ORTEP drawings of 1–6 on the best plane projection with the atom numbering scheme. Thermal ellipsoids are drawn with the 50% probability level. H atoms are
shown as small spheres of arbitrary radii. For 4 and 6 only independent molecule labeled A is shown.

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I.V. Kosilkin et al. / Journal of Molecular Structure 1006 (2011) 356–365
Organic extracts were combined and dried over MgSO4. Solvent
was removed in vacuo. The crude material was purified on a silica
gel column eluting with hexanes/ethyl acetate (v/v: 8/2) affording
1.25 g of off-white solid. Yield: 28%. 1H NMR (500 MHz, CDCl3): d
9.84 (s, 1H), 7.44–7.41 (m, 2H), 7.07 (d, 1H, J = 8.3 Hz), 6.26 (s,
1H), 4.11 (t, 2H, J = 6.6 Hz), 1.90 (sextet, 2H, J = 7.1 Hz), 1.08 (t,
3H, J = 7.4 Hz).
2.4. Computational methodology
The Gaussian09 program suite [27] was used for all calculations.
Starting from the crystallographic data, molecular geometries were
optimizedwithout any symmetry restrictionsat the DFT level of the-
ory using the B3LYP functional [28–30] and 6-311G⁄⁄ basis set. Fre-
quency calculations follow the geometry optimization in order to
verify the nature of the stationary point. Optimized molecular struc-
tures are denoted with index C. As a result of the optimization, sym-
metry independent molecules 4 (A and B), and 6 (A and B) reached
the same true stationary points, being referred to as 4c and 6c. In or-
der to estimate the energy difference between 6c and the isomer
with ethoxy group at the other ortho-position to the hydroxyl group
(as in the rest of the discussed molecules), the new isomers were
generated by the rotation around C(4)AC(7) (labeled 6d) and
C(8)AC(9) (labeled 6e) bonds, respectively. Also, the potential en-
ergy of the molecule vs. torsion angles C(5)AC(4)AC(7)AC(8) and
C(7)AC(8)AC(9)AC(10) was calculated for structure 6. Ground di-
pole moment and static first order hyperpolarizability were calcu-
lated using B3LYP/6-311++G⁄⁄, with an electric field step of
0.001 au, as described elsewhere [31,32].
2.3. Single crystal X-ray diffraction analysis
Single crystal X-ray diffraction experiments for 1–6 were carried
out with a Bruker SMART APEX II diffractometer with CCD area detec-
tor (graphite monochromated Mo Ka radiation, k = 0.71073 Å), x-
scans with a 0.5° step in at 100 K. The semi-empirical method SAD-
x
ABS [24] was applied for absorption correction for all six compounds. A
crystal of 5 was found to be pseudomerohedrally twinned. By running
the TwinRotMat program within the PLATON package [25] the twinned
matrices [ꢀ1.000 0.000 0.000 0.000 ꢀ1.000 0.000 ꢀ0.448 ꢀ0.592
1.000] and [ꢀ0.999 0.001 ꢀ0.005 ꢀ0.015 ꢀ1.015 0.059 ꢀ0.503
ꢀ0.503 1.014] were obtained for two minor components (29.6% and
13.3%, respectively). From a total of 6085 reflections, 1353 reflections
(partial overlaps) were removed from the hkl file and the SHELX HKLF
5 format reflection data file was generated for the refinement. The
structure solution and refinement for all the materials proceeded sim-
ilarly using the SHELX–97 program package [26]. The structures were
solved by direct methods and refined by the full-matrix least-squares
technique against F2 with the anisotropic temperature parameters
for all non-hydrogen atoms. The hydrogen atoms on the hydroxyl
groups for all but structure 5 were located from the Fourier electron
density map and refined isotropically. For 5, the hydrogen position
was calculated from the geometrical point of view. All C-bound hydro-
gen atoms were refined within the riding model. Data reduction and
further calculations were performed using Bruker SAINT+ and SHELXTL
NT program packages [26]. Compounds 1–3 and 5 crystallize with one
molecule in the asymmetric unit, while compounds 4 and 6 crystallize
with two molecules (designated as A and B) in the asymmetric unit.
Crystallographic data for the structures in this paper have been depos-
ited with the Cambridge Crystallographic Data Centre as supplemen-
tary publication numbers CCDC 826400–826405. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 (0)1223 336033 or e-mail: depos-
it@ccdc.cam.ac.uk). The selected geometry parameters for 1–6 are gi-
ven in Table S1.
3. Results and discussion
3.1. UV–Vis absorption
The UV–Vis absorption spectra in chloroform were measured for
all six chromophores discussed in this work as well as for OH1 and
OH3 (for comparison). The absorption curves are presented in
Fig. 1. The presented materials show a strong p–
p⁄ transition with
kmax centered at around 430 nm (Table 1). In all materials, the
absorption maximum is red-shifted compared to the unsubstituted
analogs in agreement with the Dewar rules. A slight bathochromic
shift is also found as the number of carbon atoms in the alkoxy group
is increased, which can be explained by the increased electron
donating power of longer substituents. For the same reason, the
absorption maximum of the OH3 analogs is blue-shifted compared
to the OH1 analogs. The shift is small and is in the range of 2–4 nm.
3.2. Melting points analysis
As expected, a correlation between the length of the alkoxy
group and the melting point of the final chromophores was found.
Table 2
Selected crystal data, details of data collection and structure refinement of 1–6.
Compound
1
2
3
4
5
6
Empirical formula
Fw
C
20H20N2O2
C20H20N2O2
320.38
C21H22N2O2
334.41
C22H24N2O2
348.43
C18H16N2O2
299.33
C19H18N2O2
306.35
320.38
Mp (°C)
PI
219–216
67.9
199–198
70.2
172–170
68.0
162–160
67.7
200–196
71.0
187–184
70.5
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21/c
8.8140(7)
21.5829(18)
9.8468(8)
90
114.007(2)
90
1711.1(2)
4
Monoclinic
C2/c
22.440(13)
15.810(9)
9.555(6)
90
101.849(10)
90
3318(3)
8
Monoclinic
P21/c
9.8071(16)
13.479(2)
14.152(2)
90
103.337(2)
90
1820.4(5)
4
Triclinic
P ꢀ 1
Triclinic
P ꢀ 1
Triclinic
P ꢀ 1
7.679(4)
13.453(7)
19.355(10)
95.007(9)
94.912(9)
104.140(9)
1919.7(18)
4
4.996(6)
10.938(13)
13.890(16)
76.10(2)
84.18(2)
84.85(2)
731.3(15)
2
7.5115(18)
13.204(3)
16.823(4)
88.163(5)
78.489(6)
74.852(5)
1577.9(6)
4
a
(deg.)
b (deg.)
c
(deg.)
V (Å3)
Z
dcalc (g cmꢀ3
)
1.244
0.081
1.003
1.283
0.084
0.708
1.220
0.079
1.048
1.206
0.078
0.904
1.328
0.088
1.064
1.290
0.085
0.918
l
(mmꢀ1
)
Goodness-of-fit on F2
Data/restraints/parameters
3019/0/224
0.0455, 0.0946
0.0785, 0.1132
3263/0/224
0.0495; 0.0893
0.2190; 0.1276
3575/0/233
0.0440; 0.1141
0.0562; 0.1206
7527/0/483
0.0689; 0.1693
0.1352; 0.2205
4898/0/203
0.1500; 0.3681
0.2322; 0.4179
5495/0/425
0.0487; 0.0996
0.0985; 0.1227
R1; wR2 (I > 2r (I))
R1; wR2 (all data)

I.V. Kosilkin et al. / Journal of Molecular Structure 1006 (2011) 356–365
361
Table 3
X-ray and DFT calculated interplanar angles.
Compounds Dihedral angle,
Dihedral angle,
Deviation of C(13) atom
A/B (°)
A/C (°)
from PC (Å)
1
1c
2
2c
3
3c
4A
4B
4c
5
5c
6A
6B
6c
6d
6e
0.7(4)
3.34
7.5(6)
0.34
8.2(1)
0.52
12.5(2)
14.7(4)
0.67
1.8(9)
0.52
2.3(2)
6.9(1)
0.51
17.5(3)
2.23
4.3(6)
3.11
5.3(1)
3.29
7.6(2)
9.5(4)
3.30
10.7(6)
4.07
5.4(2)
12.8(1)
4.22
0.718(3)
0.712
0.665(5)
0.644
0.684(2)
0.644
0.731(4)
0.686(4)
0.644
0.706(9)
0.637
0.587(3)
0.717(3)
0.639
Fig. 3. General structure of the reported chromophores, showing planes A, B, and C
that were used for calculation of the interplanar angles (Table 3); bonds presented
in bold were used for BLA calculation (Tables 4 and S1).
melting point of the parent material OH3 is 277–279 °C, dropping
to 196–200 °C and to 184–187 °C for methoxy and ethoxy deriva-
tives respectively. Surprisingly, chromophore 1 with the methoxy
group attached to the central ring has Mp about 15 °C higher than
that of the parent OH1. The results obtained in this study suggest
that the melting point of this class of chromophores can be suc-
cessfully tuned through the introduction of substituents with vary-
ing length of alkyl chains.
0.56
2.68
4.08
15.22
0.636
0.632
The unsubstituted chromophore OH1 has a melting point of 203–
204 °C [11]. Addition of the methoxy group to the donor ring low-
ers Mp by only several degrees (2, 198–199 °C). However, further
increase of the number of carbon atoms in the substituent has a
more pronounced effect. The ethoxy derivative 3 melted at 170–
172 °C and n-propoxy derivative 4 at 160–162 °C. The same trend
was found for the chromophores analogous to OH3. The published
3.3. X-ray structure description
Compounds 1–6 (Fig. 2) crystallize in the centrosymmetric
space groups, with one molecule per asymmetric unit in 1–3, 5
and two crystallographically independent molecules in 4 and 6
Table 4
BLA and calculated
l
and b values of 1–6.
Compound
BLA (Å)
Crystala
b (ꢁ10ꢀ30 esu)
Crystala
l (Debye)
Crystala
Calculated
Calculated
Calculated
1
2
3
4
5
6
0.110
0.071
0.079
0.078
0.073
0.073
0.073
85.6
105.1
102.1
101.1
117.5
120.5
120.9
116.7
126.2 (119.7, 110.4)
8.54
8.24
7.40
9.33
8.14
8.21
8.22
8.21
12.54 (8.28,7.89)
0.078 (0.097)b
0.096
119.4 (114.5)b
108.7
8.08 (7.36)b
7.63
0.073
0.080 (0.085)b
0.074 (0.072c, 0.075)
117.0 (116.8)b
12.19 (12.29)b
a
Denotes geometries obtained from X-ray experiment.
Value was computed for symmetry independent molecule B.
Computed for 6d and 6e isomers, respectively.
b
c
Table 5
Intermolecular short contacts in 1–6.
Compound
DAHꢂ ꢂ ꢂA
Hꢂ ꢂ ꢂA (Å)
Dꢂ ꢂ ꢂA (Å)
Angle (DAHꢂ ꢂ ꢂA) (°)
Symmetry transformation for acceptor
1
O(1)AH(1O)ꢂ ꢂ ꢂN(1)
1.88(4)
2.47
2.12(4)
2.22(5)
2.55
2.16(2)
2.24(3)
2.19(5)
2.16(4)
1.93(6)
2.03(5)
2.58
2.20
2.12
2.57
2.24(3)
2.23(3)
2.02(3)
2.13(3)
2.49
2.728(3)
3.428(3)
2.632(4)
2.876(5)
3.467(6)
2.6367(16)
3.037(2)
2.638(3)
2.642(3)
2.888(4)
2.897(4)
3.501(4)
2.644(8)
2.832(9)
3.477(12)
2.682(2)
2.658(2)
2.822(3)
2.971(3)
3.345(3)
3.542(3)
3.487(3)
3.443(3)
3.294(3)
175(3)
165
116(4)
131(4)
167
114(2)
154(2)
105(4)
111(3)
158(4)
154(4)
157
114
145
166
112(3)
110(3)
154(3)
158(3)
144
ꢀx, ꢀy ꢀ 1, 2 ꢀ z
ꢀx, 0.5 + y, 1.5 ꢀ z
x, y, z
C(20)AH(20A)ꢂ ꢂ ꢂO(1)
O(1)AH(1A)ꢂ ꢂ ꢂO(2)
2
O(1)AH(1A)ꢂ ꢂ ꢂN(2)
x, y ꢀ 1, z
C(8)AH(8)ꢂ ꢂ ꢂN(1)
ꢀx, 1 ꢀ y, 1 ꢀ z
3
4
O(1)AH(1A)ꢂ ꢂ ꢂO(2)
x, y, z
O(1)AH(1A)ꢂ ꢂ ꢂN(1)
x + 1, 1/2 ꢀ y, 1/2 + z
x, y, z
x, y, z
O(1A)AH(1AA)ꢂ ꢂ ꢂO(2A)
O(1B)AH(1BA)ꢂ ꢂ ꢂO(2B)
O(1A)AH(1AA)ꢂ ꢂ ꢂN(1B)
O(1B)AH(1BA)ꢂ ꢂ ꢂN(1A)
C(16B)AH(16C)ꢂ ꢂ ꢂO(1A)
O(1)AH(1A)ꢂ ꢂ ꢂO(2)
1 ꢀ x, 1 ꢀ y, 1 ꢀ z
2 ꢀ x, 1 ꢀ y, ꢀz
x, y ꢀ 1, z
5
6
x, y, z
O(1)AH(1A)ꢂ ꢂ ꢂN(2)
x ꢀ 2, y ꢀ 1, z
ꢀx, 1 ꢀ y, 1 ꢀ z
x, y, z
x, y, z
x, y + 2, z + 1
x, y, z
1 ꢀ x, 2 ꢀ y, 1 ꢀ z
ꢀx, ꢀy, ꢀz
1 ꢀ x, ꢀy, ꢀz
1 ꢀ x, ꢀ1 ꢀ y, ꢀz
x, 1 + y, z
C(8)AH(8)ꢂ ꢂ ꢂN(1)
O(1A)AH(1A)ꢂ ꢂ ꢂO(2A)
O(1B)AH(1B)ꢂ ꢂ ꢂO(2B)
O(1A)AH(1A)ꢂ ꢂ ꢂN(2B)
O(1B)AH(1B)ꢂ ꢂ ꢂN(2A)
C(13A)AH(13A)ꢂ ꢂ ꢂO(1A)
C(12B)AH(12C)ꢂ ꢂ ꢂO(1B)
C(13B)AH(13D)ꢂ ꢂ ꢂO(1B)
C(10B)AH(10B)ꢂ ꢂ ꢂN(1B)
C(15B)AH(15D)ꢂ ꢂ ꢂN(1B)
2.57
2.57
2.56
2.58
168
155
155
129

362
I.V. Kosilkin et al. / Journal of Molecular Structure 1006 (2011) 356–365
Fig. 4. Self-assembling organization via hydrogen bonding in 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), 6 (f).
(Table 2). The single crystal X-ray structural analysis gives an esti-
mate on how the position and length of the alkoxy substituent im-
pacts molecular planarity in 1–6 (Fig. 2).
Compound 2 has the highest coplanarity between the conju-
gated bridge, cyclohexene ring and acceptor moiety (Fig. 3) as indi-
cated by the dihedral angles PA/PC (Table 3), whereas compound 1

I.V. Kosilkin et al. / Journal of Molecular Structure 1006 (2011) 356–365
363
Fig. 5. The overlapping diagrams of 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), 6 (f) demonstrating the pronounced p–p stacking interactions.
has the lowest coplanarity. The degree of planarity in 1–6, defined
by the dihedral angles between the three planar fragments (Fig. 3),
correlates well with the bond-length alternation (BLA) in the con-
jugated bridge. BLA is defined as an average change of the length
between adjacent single and double bonds [33]. BLA ranges from
0.085 Å in 2 to 0.118 Å in 1 (Tables 4 and S1). These values are sim-
ilar to those of the analogous compounds reported earlier [34–42].
For the all presented structures the cyclohexene ring was
found to adopt an envelope conformation. The deviation of the
C(13) atom from the PC plane ranges from 0.587(3) Å in 6A to
0.731(4) Å in 4A (Table 3). The O(1)Hꢂ ꢂ ꢂO(2) intramolecular
hydrogen bond in 2–6 is characterized by the Oꢂ ꢂ ꢂO distance
and OAHꢂ ꢂ ꢂO bond angle falling in the ranges 2.632(4)–
2.682(2) Å and 110(3)–116(4)°, respectively (Table 5). Surpris-
ingly, 6 adopted a conformation different from chromophores
2–5 (Fig. 2). The ethoxy substituent in 6 was found at the other
ortho position to the hydroxyl group when compared to 2–5. It
is unclear whether the molecules of 6 adopt this geometry upon

364
I.V. Kosilkin et al. / Journal of Molecular Structure 1006 (2011) 356–365
C5
C4
C7
C8
C5
C4
C8
C9
C7
C7
C8
C9
C10
C10
6c
6d
6e
Fig. 6. Optimized geometries of the isomers of 6 with the partial labeling.
packing in crystal form or they exist in this conformation in
solution as well.
All presented compounds have similar antiparallel crystal
packing induced by the strong dipole–dipole interactions. Differ-
ent degrees of linearity in antiparallel chains can be attributed
to the short intermolecular OAHꢂ ꢂ ꢂN and CAHꢂ ꢂ ꢂN contacts and
different overlapping patterns of the
p-conjugated systems. The
crystal packing in 1–6 is stabilized by the short intermolecular
OAHꢂ ꢂ ꢂN and CAHꢂ ꢂ ꢂN contacts (Fig. 4, Table 5), and intermolec-
ular
p–p stacking. The hydrogen-bonding interactions in 1 cause
molecules to form centrosymmetric dimers, while in 2–6 mole-
cules form infinite H-bonded chains or ribbon-like aggregates
(Fig. 4).
Analysis of the crystal packing reveals similarities between 2
and 5, and between 3 and 4. The antiparallel hydrogen-bonded
chains of the methoxy substituted 2 and 5 are further sewed in rib-
bons via auxiliary weak C(8)Hꢂ ꢂ ꢂN(1) interactions acting across
inversion centers and generating the sequence of centrosymmetric
R44(28) and R22(16) graph sets [43] (Fig. 4b and e). Compounds 3 and
4 have similar non-planar zigzag chains, with the dihedral angle
between the best planes of the adjacent molecules being 25.6° in
3, and 12.6° in 4, with alternation between 4A and 4B (Fig. 4c
and d). In the linear chain of 6, the intermolecular dihedral angle
6A/6B was found to be 17.8° (Fig. 4f). All of the discussed hydro-
gen-bond motifs pack in stacking modes with different overlapping
Fig. 7. Relative potential energy curves as
C(5)AC(4)AC(7)AC(8) (solid line) and C(7)AC(8)AC(9)AC(14) (dashed line) calcu-
lated with 18° step for 6c (also see Fig. 6).
a function of torsion angles
patterns of the p-conjugated systems (Fig. 5). The interplanar sep-
(the hydrogen atom is shared in the intramolecular OAHꢂ ꢂ ꢂO bond)
aration is within 3.383–3.960 Å. The shortest interplanar distance
was found in 5 and the longest in 1 and 4, whose molecules are
characterized by the poorest planarity (Table 3). These values cor-
relate well with the efficacy of the crystal packing as estimated by
the Kitaigorodskii packing index (PI) [44], where compounds with
the most effective stacking demonstrate the highest PIs and vice
versa (Table 2). The estimated efficiency of crystal packing corre-
lates with the melting point of the homologs 2–4 and 5–6: the in-
crease of the hydrocarbon length of the alkoxy-substituents results
in a less effective crystal packing and as a result, a lower melting
point. The higher melting point of 1 can be explained by the nature
of its H-bond network. In the centrosymmetric dimers of 1 the
molecules are held together via the strongest OAHꢂ ꢂ ꢂN hydrogen
bond among the reported compounds, with an O(1)ꢂ ꢂ ꢂN(1) dis-
tance of 2.728(3) Å. For comparison, in 2–6 the OAHꢂ ꢂ ꢂN intermo-
lecular interactions range from 2.822(9) to 3.037(2) Å, which can
be explained by the fact that the H-bonds there are bifurcated
(Table 5).
3.4. Quantum-chemical calculations
The optimized geometries of 1–6 were obtained using DFT cal-
culations and are in a good agreement with the XRD data; however,
some geometry relaxation is observed (Table 3). The observed flat-
tening of computed molecular structures can be explained by the
absence of the intermolecular interactions usually found in crys-
tals. As a result of the geometry optimization of the isomers of 6
(Fig. 6), the largest deviation from planarity was observed in case
of 6d with the torsional angle C(7)AC(8)AC(9)AC(14) being 166°.
This most probably can be attributed to the Hꢂ ꢂ ꢂH repulsion forces.
The calculated lowest energy was found for the isomer 6d where
the ethoxy group has the same orientation as in other molecules
discussed here. The isomer 6c is higher in energy by 0.9 kcal/mol,
and 6e – by 2.6 kcal/mol (Fig. 6). The calculated energy barriers

I.V. Kosilkin et al. / Journal of Molecular Structure 1006 (2011) 356–365
365
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noncentrosymmetric packing necessary for EO application was
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Acknowledgment
The authors are grateful to the NSF for financial support under
Grants DMR 0120967 & 0934212 and CHE 0832622.
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