Synthesis of organic phenothiazine-based molecular glasses and effect of racemic/homochiral aliphatic chain on near-infrared photorefractive property

Article abstract of DOI:10.1016/j.jpcs.2012.04.008

Organic near-infrared photorefractive molecular glasses with a phenothiazine moiety are designed and synthesized through the introduction of linear, racemic/homochiral asymmetrically branched aliphatic chains into photorefractive chromophore as an auxiliary group. The compounds are characterized with ¹H-NMR, IR, FAB-MS, UV-vis, TG, DSC, etc. The effect of different aliphatic chains on the absorption and thermal properties is investigated in detail. The molar absorption coefficiency at the absorption maximum wavelength showed that the homochiral asymmetrically branched aliphatic chain has a strong hypochromic effect in the dilute solution when it is introduced into photorefractive chromophore. The DSC measurement indicated that the introduction of asymmetrically branched aliphatic chain is the key issue to design organic molecular glasses whether it is racemic or homochiral. The effect of racemic/homochiral asymmetrically branched aliphatic groups on photorefractive property is investigated carefully with poly(N-vinylcarbazole) (PVK) as a photoconductor and with (2,4,7-trinitro-9-fluorenylidene) malononitrile (TNFM) as a photosensitizer. The results suggested that the racemic group is more beneficial to the improvement of photorefractive performance than the homochiral when the homochiral cannot induce rigid photorefractive chromophore to be much more ordered.

Full text of DOI:10.1016/j.jpcs.2012.04.008

Journal of Physics and Chemistry of Solids 73 (2012) 1136–1145
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Synthesis of organic phenothiazine-based molecular glasses and effect of
racemic/homochiral aliphatic chain on near-infrared
photorefractive property
Yingliang Liu, Ryushi Fujimura, Kazuki Ishida, Nobuhiro Oya, Naoko Yoshie, Tsutomu Shimura n,
Kazuo Kuroda
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Organic near-infrared photorefractive molecular glasses with a phenothiazine moiety are designed and
synthesized through the introduction of linear, racemic/homochiral asymmetrically branched aliphatic
chains into photorefractive chromophore as an auxiliary group. The compounds are characterized with
1H-NMR, IR, FAB-MS, UV–vis, TG, DSC, etc. The effect of different aliphatic chains on the absorption and
thermal properties is investigated in detail. The molar absorption coefficiency at the absorption
maximum wavelength showed that the homochiral asymmetrically branched aliphatic chain has a
strong hypochromic effect in the dilute solution when it is introduced into photorefractive chromo-
phore. The DSC measurement indicated that the introduction of asymmetrically branched aliphatic
chain is the key issue to design organic molecular glasses whether it is racemic or homochiral. The
effect of racemic/homochiral asymmetrically branched aliphatic groups on photorefractive property is
investigated carefully with poly(N-vinylcarbazole) (PVK) as a photoconductor and with (2,4,7-trinitro-
9-fluorenylidene) malononitrile (TNFM) as a photosensitizer. The results suggested that the racemic
group is more beneficial to the improvement of photorefractive performance than the homochiral when
the homochiral cannot induce rigid photorefractive chromophore to be much more ordered.
& 2012 Elsevier Ltd. All rights reserved.
Received 26 November 2011
Received in revised form
16 February 2012
Accepted 19 April 2012
Available online 9 May 2012
Keywords:
A. Amorphous materials
A. Optical materials
A. Organic compounds
A. Thin films
B. Chemical synthesis
1. Introduction
dopant in organic composite materials can block the crystallizing
of crystalline compounds in film, temporarily affording the uni-
Organic molecular glass has recently attracted much attention
due to its property tunability through the modification of chemi-
cal structure and the thermal/optical stability in film, which is a
key issue for practical application of organic optoelectronic
devices, such as organic photorefractive devices (OPRs) [1–5],
organic light-emitting diodes (OLEDs) [6–9], organic photovoltaic
cells (OPVs) [10,11], organic field-effect transistors (OFETs)
[12,13], etc. Since especially, organic glasses for photorefractive
materials were first reported by Lundquist et al. [14], organic
photorefractive glasses have made much progress not only in the
synthesis of materials [15–17] but also in the physic mechanism
of devices [18–23]. Simultaneously, the photorefractive response
wavelength is also extended to the near-infrared region [24]. In
general, organic molecular glasses (also called as monolithic
molecules) are referred to as a class of organic compounds being
able to keep an amorphous state without any dopant, which is
different from the organic composite glass. In principle, the
form film. With time passing by, the crystalline compound still
tends to form a crystal microparticle, which is a crucial factor of
the instability for optoelectronic devices. However, organic mole-
cular glasses will overcome this disadvantage extraordinarily
keeping themselves in amorphous state. In this occasion, the
solid film is optically uniform without light-scattering and trans-
mission-attenuating. Therefore, organic photorefractive mole-
cular glasses have become a prospective candidate for the
practical application of photorefractive devices.
Notably, the optoelectronic properties are rarely investigated
from organic chiral molecular glasses because it is very difficult
for the enantiomerically pure compounds to form an amorphous
phase state. Recently, chiral molecular glasses have been pre-
sented with one thiophene-based enantiopure helicene forming a
rare chiral molecular glass while its chiroptical properties were
enhanced [25]. The chirality effect on nonlinear optical properties
has been studied extensively [26–35]. The chirality effect on
photorefractive performance was also reported in organic photo-
refractive liquid crystal [36–39]. As one of the promising opto-
electronic materials, organic photorefractive molecular glass is
also scarcely investigated up to date without exception through
n Corresponding author. Tel.: þ81 3 5452 6137; fax: þ81 3 5452 6140.
E-mail addresses: shimura@iis.u-tokyo.ac.jp, liuylxn@sohu.com (T. Shimura).
0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jpcs.2012.04.008

Y. Liu et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1136–1145
1137
the introduction of chiral group into the chromophore due to its
more complicated mechanism although high-efficient erasable
polarization gratings have been optically recorded in a molecular
glass composed of azobenzene and chiral isosorbide moieties
[40]. It is well known that charge-transfer complexes between
donor and acceptor usually take a role of the photosensitizer,
which is one of the important component in photorefractive
composite film. When organic chiral molecular glass is applied
as photorefractive chromophore, it is possible for chiral molecular
glass to affect the photosensitive effect of the photosensitizer, i.e.,
charge-transfer complex between chiral molecular glass and
other acceptors, which is generally (2,4,7-trinitro-9-fluorenyli-
dene) malononitrile (TNFM),2,4,7 trinitro-9-fluorenone (TNF) and
so on. To the best of our knowledge, the investigation in point is
not reported even if chiral charge-transfer complexes have been
studied on several occasions [41–43].
recorded using a cuvette of thickness 1 cm on a HITACHI U-3010
spectrophotometer. The transmittance spectra were performed
in-situ with the photorefractive devices as a sample by an U-4000
spectrophotometer. The DPS-3001 laser diode with a laser wave-
length of 780 nm from NIHON KAGAKU ENG Corp. was used
during the photorefractive measurement. The electric field was
applied on the photorefractive devices with HEOPS-5B6 high
voltage amplifier as a high voltage power supply. The current of
photorefractive devices was monitored with a KEITHLEY 6517A
electrometer. The piezomirror was translated with a piezoelectric
stage driven by low voltage PZT driver and TEKITRONIX AFG 3021
function generator. The shutter was controlled by SIGMA KOKI
S-65L shutter controller. The total power of transmitted signals
was monitored by identical NEW FOCUS 2033-M large-area
photoreceivers and recorded by LeCroy LC 334AM 500 MHz
oscilloscope.
In this paper, organic near-infrared photorefractive molecular
glasses with a phenothiazine moiety are designed and synthe-
sized through the introduction of linear, racemic/homochiral
asymmetrically branched aliphatic chains into photorefractive
chromophore as an auxiliary group. The compounds are charac-
terized with 1H-NMR, IR, FAB-MS, UV–vis, TG, DSC, etc. Especially,
the effect of different aliphatic chains on absorption and thermal
properties is investigated in detail, including linear, racemic/
homochiral asymmetrically branched aliphatic chains. Addition-
ally, the effect of racemic/homochiral asymmetrically branched
aliphatic chains on photorefractive property is also measured
carefully with the PVK as a photoconductive polymer and with
TNFM as a photosensitizer.
2.2. Reagents and materials
Phenothiazine, 1,10-dibromooctane, p-nitrophenylacetonitrile,
(n-Bu)4NOH(10%), 1-bromo-2-ethylhexane, 1-chloro-2-methylbu-
tane, poly(N-vinylcarbazole) (PVK), (2,4,7-trinitro-9-fluorenyli-
dene) malononitrile (TNFM) were bought from Tokyo Chemical
Industry (TCI) and (S)-(þ)-1-bromo-2-methylbutane from Sigma-
Aldrich. Other chemicals and solvents were purchased from Wako
chemical company. All the chemicals were used without any
purification. The filter with a pore size of 0.2 mm, which was built
of polytetraflouroethylene (PTFE) membrane with a polypropy-
lene housing, was purchased from Whatman incorporation.
2.3. Synthesis
2. Experimental section
All the intermediates and target compounds are synthesized
according to the synthetic route in Scheme 1. Basically, all the
target compounds follow the same or similar synthetic procedure.
First, different aliphatic chains are linked onto phenothiazine by
alkylation reaction between phenothiazine and alkylhalide. Then,
an aldehyde group is added to phenothiazine by Vilsmeier
reaction of N-alkyl-phenothiazine. Finally, the acceptor is con-
nected with phenothiazine by the below Knoevenagel reaction.
The specific synthetic procedures are given.
2.1. Characterization and instruments
1H-NMR spectroscopy was recorded on JEOL JNM-AL400 using
chloroform-d solution at room temperature. Infrared (IR) spectro-
scopy was measured using a JEOL WINSPEC100 spectrometer. The
fast atom bombardment mass spectrometry (FAB-MS) was per-
formed on JEOL JMS-600H mass spectrometer using 3-nitroben-
zylalcohol as
a matrix. The molecular weight of PVK was
monitored by gel permeation chromatography (GPC) on Toshoh
HLC 822 using the chloroform as an eluent (1 mL/min) at 40 1C
and with the polystyrenes as calibration standards. Thermo-
gravimetry (TG) was investigated on Thermo Plus TG 8120 at
heating and cooling rates of 10 1C/min under N2 atmosphere.
Differentionial scanning calorimetry (DSC) was carried out with
Perkin-Elmer Pyris I at heating and cooling rates of 10 1C/min
under N2 atmosphere. Ultraviolet–visible (UV–vis) spectra were
2.3.1. General procedure for synthesis of compound 1
A typical procedure is given with compound 1a as an example.
Phenothiazine (3.0 g, 15 mmol) was completely dissolved in DMF
(30 ml) and then NaH (1.2 g) was added. After the reaction was
carried out in an ice bath under a vigorous stirring for half an
hour, 1,10-dibromooctane (13.5 g, 45 mmol) was immediately
added to the mixture. The reaction was sequentially going on at
Scheme 1. Synthetic route of phenothiazine-based compounds with different aliphatic chains.

1138
Y. Liu et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1136–1145
room temperature for another 4 h. Finally, the mixture was
poured into distilled water (600 ml), and extracted with n-hexane
for three times (400 ml each). The extracting solution was dried
with anhydrous magnesium sulfate overnight. After the magne-
sium sulfate was filtrated out, n-hexane was evaporated from the
filtrate under reduced pressure. The crude product was purified
by silica-gel column chromatography. When compounds 1b, 1c,
1d were synthesized, alkane bromide was added with a excess of
20% instead of 200%.
1.76 (p, J¼7.2 Hz, 2H, CH2), 1.82 (p, J¼7.2 Hz, 2H, CH2), 3.54 (t,
J¼6.8 Hz, 2H, CH2Cl), 3.90 (t, J¼7.2 Hz, 2H, CH2N), 6.90
(d, J¼6.8 Hz, 1H, PTzH), 6.92 (d, J¼8.0 Hz, 1H, PTzH), 6.98
(t, J¼7.6 Hz, 1H, PTzH), 7.12 (d, J¼7.6 Hz, 1H, PTzH), 7.18
(t, J¼8.0 Hz, 1H, PTzH), 7.59 (s, 1H, PTzH), 7.64 (d, J¼8.0 Hz,
1H, PTzH), 9.80 (s, 1H, CHO). IR (KBr,
n
/cmꢁ1): 2926 (
nas, CH),
2853 (
n
s, CH), 2723 (CH on CHO), 1688 (CQO), 1597, 1572,
1556 (benzene ring), 1464 (CH2), 1369, 1337, 1286 (C–N),
1250 (m-disubstitution on the benzene ring), 748 (R(CH2)4-C).
Compound 2b: Rf value: 0.42 (chloroform). Compound 2b was
obtained as a yellow liquid in 89.4% yield. FAB-MS: m/z¼339.1
Compound 1a: Chloroform was used as eluent (Rf value:
0.53). Compound 1a was not identified and directly used in
the next reaction.
[M]þ
Chloroform-d,
.
UV–vis (THF): lmax¼377 nm. 1H-NMR (400 MHz,
d): 0.87 (m, 6H, CH3 ꢀ 2), 1.26 (m, 4H,
Compound 1b: n-hexane was used as eluent (Rf value: 0.15).
Compound 1b was obtained as a buff viscous liquid in
CH2 ꢀ 2), 1.34–1.46 (m, 4H, CH2 ꢀ 2), 1.92 (m, 1H, CH), 3.78
(d, J¼6.8 2H, CH2N), 6.92 (d, J¼8.0 Hz, 1H, PTzH), 6.94 (d, J¼
8.4 Hz, 1H, PTzH), 6.98 (t, J¼7.6 Hz, 1H, PTzH), 7.14 (d,
J¼8.0 Hz, 1H, PTzH), 7.17 (t, J¼8.0 Hz, 1H, PTzH), 7.62 (s, 1H,
PTzH), 7.66 (d, J¼8.4 Hz, 1H, PTzH), 9.80 (s, 1H, CHO). IR (KBr,
59.8% yield. FAB-MS: m/z¼311.2 [M]þ. UV–vis (THF): lmax
¼
309 nm. 1H-NMR (400 MHz, Chloroform-d,
d): 0.86 (m, 6H,
CH3 ꢀ 2), 1.26 (m, 4H, CH2 ꢀ 2), 1.36–1.47 (m, 4H, CH2 ꢀ 2),
1.93 (m, 1H, CH), 3.72 (b, 2H, CH2N), 6.88 (d, J¼8.8 Hz, 2H,
PTzH ꢀ 2), 6.90 (b, 2H, PTzH ꢀ 2), 7.14 (t, J¼7.6 Hz, 4H,
n
/cmꢁ1): 3059 (CH on the phenothiazine ring), 2957, 2926 (nas
,
CH), 2856 (
ns, CH), 2723 (CH on CHO), 1688 (C¼O), 1597,
PTzH ꢀ 4). IR (KBr,
n
/cmꢁ1): 3063 (CH on the phenothiazine
ring), 2957, 2926 (nas, CH), 2856 (ns, CH), 1593, 1568, 1485
1572, 1493 (benzene ring), 1460 (CH2)’’, 1378 (CH3), 1348
(CHR3), 1333, 1310, 1286 (C–N), 1250, 1225 (C-CH3), 746
(R(CH2)4-C).
(benzene ring), 1456 (CH2), 1380 (CH3), 1329, 1285 (C–N),
1250, 1221 (C–CH3), 748 (R(CH2)4–C).
Compound 1c: Rf value: 0.15 (n-hexane). Compound 1c was
obtained as a viscous liquid in 16.0% yield. FAB-MS: m/
Compound 2c: Rf value: 0.29 (chloroform). Compound 2c was
obtained as a yellow liquid in 91.3% yield. FAB-MS: m/z¼297.1
[M]þ
.
UV–vis (THF): lmax¼376 nm. 1H-NMR (400 MHz,
z¼269.2 [M]þ
.
UV–vis (THF): lmax¼308 nm. 1H-NMR
Chloroform-d, d): 0.91 (t, J¼ 7.2 Hz, 3H, CH3), 0.96
(400 MHz, Chloroform-d,
d
): 0.90 (t, J¼7.2 Hz, 3H, CH3), 0.96
(d, J¼6.8 Hz, 3H, CH3), 1.24 (m, 1H, CH), 1.56 (m, 1H, CH),
2.00 (m, 1H, CH), 3.66 (p, J¼8.0 Hz, 1H, CH), 3.83 (p, J¼6.4 Hz,
1H, CH), 6.89 (d, J¼8.0 Hz, 1H, PTzH), 6.94 (d, J¼8.0 Hz, 1H,
PTzH), 6.98 (t, J¼7.2 Hz, 1H, PTzH), 7.14 (d, J¼8.0 Hz, 1H,
PTzH), 7.18 (t, J¼8.0 Hz, 1H, PTzH), 7.62 (s, 1H, PTzH), 7.64 (d,
(d, J¼7.2 Hz, 3H, CH3), 1.22 (m, 1H, CH), 1.56 (m, 1H, CH), 2.02
(m, 1H, CH), 3.59 (b, 1H, CH), 3.77 (b, 1H, CH), 6.85 (d, J¼
7.6 Hz, 2H, PTzH ꢀ 2), 6.90 (b, 2H, PTzH ꢀ 2), 7.14 (t, J¼7.2 Hz,
2H, PTzH ꢀ 2), 7.16 (t, J¼6.8 Hz, 2H, PTzH ꢀ 2). IR (KBr,
n
n
/cmꢁ1): 3063 (CH on the phenothiazine ring), 2959, 2926
J¼7.2 Hz, 1H, PTzH), 9.80 (s, 1H, CHO). IR (KBr,
2928 ( as, CH), 2873 (
n
/cmꢁ1): 2960,
ns, CH), 2725 (CH on CHO), 1684 (C¼O),
(
as, CH), 2872 (
n
s, CH), 1593, 1571, 1484, 1457 (benzene ring),
n
1249 (C–N), 1250, 1223 (C–CH3), 748 (CH).
1596, 1572, 1493, 1462 (benzene ring), 1375 (CH3), 1345
(CHR3), 1310, 1288 (C–N), 1251, 1226 (C–CH3), 747 (CH).
Compound 2d: Rf value: 0.50 (chloroform). Compound 2d was
obtained as a yellow liquid in 91.0% yield. FAB-MS: m/z¼297.3
Compound 1d: Rf value: 0.21 (n-hexane). Compound 1d was
obtained as a buff viscous product in 57.7% yield. FAB-MS:
m/z¼269.2 [M]þ
.
UV–vis (THF): lmax¼308 nm. 1H-NMR
(400 MHz, Chloroform-d,
d
): 0.91 (t, J¼7.2 Hz, 3H, CH3), 0.97
[M]þ
.
UV–vis (THF): lmax¼377 nm. 1H-NMR (400 MHz,
(d, J¼6.8 Hz, 3H, CH3), 1.23 (m, 1H, CH), 1.56 (m, 1H, CH), 2.02
(m, 1H, CH), 3.60 (b, 1H, CH), 3.78 (b, 1H, CH), 6.87 (d, J¼
8.0 Hz, 2H, PTzH ꢀ 2), 6.92 (b, 2H, PTzH ꢀ 2), 7.15 (t, J¼7.6 Hz,
Chloroform-d, d): 0.91 (t, J¼ 7.6 Hz, 3H, CH3), 0.97 (d,
J¼6.8 Hz, 3H, CH3), 1.23 (m, 1H, CH), 1.56 (m, 1H, CH), 2.00
(m, 1H, CH), 3.65 (p, J¼8.0 Hz, 1H, CH), 3.82 (p, J¼6.8 Hz, 1H,
CH), 6.91 (d, J¼9.6 Hz, 1H, PTzH), 6.93 (d, J¼8.4 Hz, 1H, PTzH),
6.98 (t, J¼6.8 Hz, 1H, PTzH), 7.14 (d, J¼7.6 Hz, 1H, PTzH), 7.17
(t, J¼8.0 Hz, 1H, PTzH), 7.61 (s, 1H, PTzH), 7.64 (d, J¼8.0 Hz,
4H, PTzH ꢀ 4). IR (KBr,
zine ring), 2959, 2926 (
n
/cmꢁ1): 3063 (CH on the phenothia-
n
as, CH), 2853 (ns, CH), 1593, 1570,
1485, 1456 (benzene ring), 1248 (C–N), 1248, 1223 (C–CH3),
748 (CH).
1H, PTzH), 9.80 (s, 1H, CHO). IR (KBr,
phenothiazine ring), 2961, 2926 ( as, CH), 2855 (
n
/cmꢁ1): 3059 (CH on the
s, CH), 2723
n
n
(CH on CHO), 1686 (C¼O), 1597, 1572, 1491, 1462 (benzene
ring), 1375 (CH3), 1344 (CHR3), 1310, 1288 (C–N), 1252, 1198
(C–CH3), 748 (CH).
2.3.2. General procedure for synthesis of compound 2
A typical procedure was given with compound 2a as an
example. Compound 1a in the above step was all dissolved in
DMF (30 ml) and cooled to 0 1C with an ice bath. Then, phosphoryl
chloride (23.1 g, 150 mmol) was added dropwise to the mixture
under a vigorous stirring. After that, the reactant was heated to
70 1C under the protection of N2 for 4 h. The mixture was finally
poured into distilled water (500 ml) and extracted with chloro-
form. The extracting solution was dried overnight with anhydrous
magnesium sulfate. After magnesium sulfate was filtrated out,
chloroform was removed under reduced pressure. The residue
was dissolved in a minimal amount of chloroform and purified by
silica-gel column chromatography using chloroform as an eluent.
2.3.3. General procedure for synthesis of compound 3
A typical procedure was given with compound 3a as an
example. (n-Bu)4NOH(10%, 1 ml) was dropwise added to a stirred
solution of compound 2 (2.21 g, 5.5 mmol) and p-nitrophenyla-
cetonitrile (0.89 g, 5.5 mmol) in THF (10 ml). Then, the mixture
was heated to 60 1C under the protection of N2 atmosphere. After
the reaction was carried out for 5 h, the mixture was cooled to
room temperature and precipitated with distilled water (600 ml).
The viscous precipitate with a purple black was dissolved in
chloroform and dried overnight with anhydrous magnesium
sulfate. After filtration, most of the solvent was evaporated under
a reduced pressure. A small quantity of chloroform was reserved
in order to dissolve the product. Finally, the crude product was
Compound 2a: Rf value: 0.12 (chloroform). Compound 2a was
obtained as a yellow solid in 43.3% yield. FAB-MS: m/z¼401.2
[M]þ
Chloroform-d,
.
UV–vis (THF): lmax¼379 nm. 1H-NMR (400 MHz,
d): 1.28 (m, 8H, (CH2)4), 1.43 (m, 4H, CH2 ꢀ 2),

Y. Liu et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1136–1145
1139
purified by silica-gel column chromatography using chloroform
as an eluent.
(d, J¼7.2 Hz, 3H, CH3), 1.25 (m, 1H, CH), 1.56 (m, 1H, CH), 2.02
(m, 1H, CH), 3.66 (p, J¼8.0 Hz, 1H, CH), 3.85 (p, J¼7.2 Hz, 1H,
CH), 6.91 (d, J¼8.0 Hz, 1H, PTzH), 6.94 (d, J¼8.8 Hz, 1H, PTzH),
6.99 (t, J¼7.2 Hz, 1H, PTzH), 7.17 (d, J¼7.6, 1H, PTzH), 7.19 (t,
J¼8.0 Hz, 1H, PTzH), 7.50 (s, 1H, PTzH), 7.66 (s, 1H, QCH),
7.80 (d, J¼8.4 Hz, 2H, ArH ꢀ 2), 7.88 (d, J¼8.8 Hz, 1H, PTzH),
Compound 3a: Rf value: 0.55 (chloroform). Compound 3a was
obtained as a red purple powder in 53.3% yield. FAB-MS: m/
z¼545.3 [M]þ
.
UV–vis (THF): lmax¼458 nm. 1H-NMR
(400 MHz, Chloroform-d,
d
): 1.28 (m, 8H, (CH2)4), 1.42 (m,
8.29 (d, J¼8.8 Hz, 2H, ArH ꢀ 2). IR (KBr,
as, CH), 2855 ( s, CH), 2210 (CN), 1585, 1566, 1518, 1499
n
/cmꢁ1): 2959, 2926
4H, CH2 ꢀ 2), 1.75 (p, J¼7.2 Hz, 2H, CH2), 1.82 (p, J¼7.2 Hz, 2H,
CH2), 3.52 (t, J¼6.8 Hz, 2H, CH2Cl), 3.88 (t, J¼7.2 Hz, 2H,
CH2N), 6.87 (d, J¼8.4 Hz, 1H, PTzH), 6.90 (d, J¼8.8 Hz, 1H,
PTzH), 6.96 (t, J¼7.2 Hz, 1H, PTzH), 7.10 (d, J¼7.2 Hz, 1H,
PTzH), 7.17 (t, J¼8.0 Hz, 1H, PTzH), 7.49 (s, 1H, PTzH), 7.61
(s, 1 H, ¼CH), 7.78 (d, J¼8.8 Hz, 2H, ArH), 7.85 (d, J¼8.8 Hz,
(n
n
(benzene ring), 1462 (CH2), 1340 (CHR3), 1207 (C–N), 853
(p-disubstitution on the benzene ring), 750 (CH).
2.4. Device fabrication
1H, PTzH), 8.27 (d, J¼8.8 Hz, 2H, ArH). IR (KBr,
as, CH), 2851 ( s, CH), 2212 (CN), 1604, 1564, 1516 (benzene
n
/cmꢁ1): 2924
The photorefractive composite film was fabricated by solution-
casting approach. Firstly, organic photorefractive molecular
glasses and photoconductive polymer (PVK) were respectively
dissolved in THF at a concentration of 5 mg/ml. The photosensi-
tizer (TNFM) was also prepared as a THF solution with a
concentration of 1 mg/ml. Afterwards, the solutions of organic
photorefractive molecular glasses, PVK and TNFM were mixed
accordingly to a certain volume proportion and filtered by a PTEF
(
n
n
ring), 1470 (CH2), 1399, 1338 (C–N), 1364 (NO2), 850 (p-
disubstitution on the benzene ring).
Compound 3b: Rf value: 0.57 (chloroform). Compound 3b was
obtained as a wine-colored powder in 50.3% yield. FAB-MS: m/
z¼483.2 [M]þ
.
UV–vis (THF): lmax¼455 nm. 1H-NMR
(400 MHz, Chloroform-d,
d): 0.88 (m, 6H, CH3 ꢀ 2), 1.28 (m,
4H, CH2 ꢀ 2), 1.38–1.49 (m, 4H, CH2 ꢀ 2), 1.94 (m, 1H, CH), 3.78
(d, J¼6.8 2H, CH2N), 6.92 (d, J¼8.4 Hz, 1H, PTzH), 6.94 (d, J¼
8.4 Hz, 1H, PTzH), 6.98 (t, J¼7.3 Hz, 1H, PTzH), 7.14
(d, J¼8.0 Hz, 1H, PTzH), 7.18 (t, J¼7.2 Hz, 1H, PTzH), 7.50
(s, 1H, ¼CH), 7.66 (s, 1H, PTzH), 7.80 (d, J¼7.2 Hz, 2H,
ArH ꢀ 2), 7.93 (d, J¼8.0 Hz, 1H, PTzH), 8.27 (d, J¼7.2 Hz, 2H,
filter with a pore size of 0.2 mm. After a majority of solvent was
evaporated, the solution was added dropwise to the ITO substrate,
which was heated to 40 1C with a digital hot plate in order to
speed up the solvent evaporation under good ventilation. Subse-
quently, the solid sample was dried overnight in vacuum in order
to remove the residual solvent. After that, another piece of ITO
glass was covered on the solid sample at 70 1C, where the
potential crystal would be eliminated. The photorefractive com-
posite film was immediately pressed while the film thickness was
ArH ꢀ 2). IR (KBr,
n nas, CH), 2856 (ns, CH),
/cmꢁ1): 2957, 2926 (
2210 (CN), 1585, 1566, 1518, 1497 (benzene ring), 1460 (CH2),
1338 (CHR3), 1205 (C–N), 852 (p-disubstitution on the ben-
zene ring).
controlled through a spacer of 100 mm. Finally, the device was
Compound 3c: Rf value: 0.53 (chloroform). Compound 3c was
obtained as a wine-colored powder in 58.9% yield. FAB-MS: m/
suddenly cooled down in order to keep the photorefractive
composite film in a good amorphous state. The fabricated devices
were kept in the refrigerator for photorefractive characterization.
z¼441.1 [M]þ
.
UV–vis (THF): lmax¼454 nm. 1H-NMR
(400 MHz, Chloroform-d,
d): 0.91 (t, J¼7.2 Hz, 3H, CH3), 0.97
(d, J¼7.2 Hz, 3H, CH3), 1.24 (m, 1H, CH), 1.55 (m, 1H, CH), 2.01
(m, 1H, CH), 3.67 (p, J¼7.6 Hz, 1H, CH), 3.82 (p, J¼6.4 Hz, 1H,
CH), 6.91 (d, J¼8.0 Hz, 1H, PTzH), 6.93 (d, J¼8.8 Hz, 1H, PTzH),
6.99 (t, J¼7.2 Hz, 1H, PTzH), 7.13 (d, J¼7.2, 1H, PTzH), 7.17 (t,
J¼8.0 Hz, 1H, PTzH), 7.49 (s, 1H, PTzH), 7.65 (s, 1H, ¼CH), 7.79
(d, J¼8.8 Hz, 2H, ArH ꢀ 2), 7.88 (d, J¼8.8 Hz, 1H, PTzH), 8.26
2.5. Photorefractive characterization
The photorefractive device was characterized with an experi-
mental setup as in Fig. 1. A tilted grating is written inside the
photorefractive devices at an oblique incidence angle. Two equal
p-polarized beams (beams 1 and 2) with a wavelength of
(d, J¼8.8 Hz, 2H, ArH ꢀ 2). IR (KBr,
n ,
/cmꢁ1): 2961, 2928 (nas
l
¼780 nm and the intensity of 9.0 mW, which are obtained by
CH), 2873 (ns CH), 2212 (CN), 1585, 1568, 1517, 1497
,
a nonpolarized beam splitter, are crossed inside the device at
external incidence angles of 301 and 601, respectively. The
photocurrent and the photorefractive property are measured
under the external electric field. First of all, the dark current (Id)
is monitored by measuring the current on a series-connected
(benzene ring), 1462 (CH2), 1339 (CHR3), 1208 (C–N), 853 (p-
disubstitution on the benzene ring), 751 (CH).
Compound 3d: Rf value: 0.72 (chloroform). Compound 3d was
obtained as a wine-colored powder in 57.2% yield. FAB-MS:
m/z¼441.1 [M]þ
.
UV–vis (THF): lmax¼454 nm. 1H-NMR
resistance of 4.7 ꢀ 106
O without any illumination. Then, the
current under illumination (Ii) is measured by the same approach
(400 MHz, Chloroform-d,
d): 0.92 (t, J¼7.2 Hz, 3H, CH3), 0.99
Fig. 1. Optical setup for photorefractive characterization.

1140
Y. Liu et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1136–1145
only when beam 1 is applied. Finally, the photocurrent (Ip) is
calculated by the formula Ip ¼ Ii – Id. The photorefractive property
was investigated by the measurement of steady two beam
coupling (TBC). When the electric field is switched on and the
energy transfer between both beams reaches a stable state, one
beam is moved at a constant speed by a piezomirror under the
control of piezodriver and function generator. The total power of
modulated transmitted signals is monitored by identical photo-
detectors and recorded by a digital oscilloscope.
different absolute configurations would be obtained. The R- or
S-enantiomer oppositely interacts with the light in different
absorption coefficiencies, which might result in different photo-
refractive performances. In order to investigate the effect of chiral
difference on the molecular state and the photorefractive prop-
erty, the racemic and homochiral asymmetrically branched
aliphatic chains with the same chemical structure are also linked
with the phenothiazine-based photorefractive chromophore. The
specific synthetic route of all the phenothiazine-based com-
pounds was shown in Scheme 1. The chemical structures of all
the compounds, which were characterized by 1H-NMR, IR and
FAB-MS, are in accord with the chemical structure in Scheme 1.
3. Results and discussion
3.1. Molecular design
3.2. Absorption property
The photorefractive chromophore is first designed on the basis
of the general molecular design principle of nonlinear chromo-
phore, that the nonlinear optical molecules are based upon
The absorption property of photorefractive compounds 3a, 3b,
3c, 3d was characterized by the measurement of UV–vis
spectra in the THF dilute solution with a concentration of
1.14 ꢀ 10ꢁ5 mol/l, as shown in Fig. 3(a). The result indicated that
all the compounds have two absorption peaks within the visible
region. One is located at 321 nm and the other, i.e., the absorption
maximum wavelength, at 458 nm for compound 3a, 456 nm for
compound 3b, 454 nm for compound 3c, 454 nm for compound
3d respectively. Simultaneously, the relationship between the
absorbance and the solution concentration was measured at the
absorption maximum wavelength of compounds 3a, 3b, 3c, 3d
using the THF solutions with different concentrations as plotted
in Fig. 3(b). Obviously, the absorbance of all the compounds takes
on a linearly monotonous increment with the increase of solution
concentration. Sequentially, the molar absorption coefficiency at
the absorption maximum wavelength was calculated with the
aromatic
p-electron systems asymmetrically end-capped with
electron donating and accepting groups [44–47]. Herein, the
excellent electron-donating ability of phenothiazine in the photo-
refractive chromophore provides the near-infrared response
probability of the photorefractive devices. Both nitryl and cyano
act as acceptors. The specific molecular structure was illustrated
in Fig. 2 (left). Besides, the aliphatic chains with a linear, racemic/
homochiral asymmetrically branched chemical structure as
shown in Fig. 2 (right) are introduced into the photorefractive
chromophore as an auxiliary group, which will induce different
molecular states, such as crystalline or glassy state. It was well
known that there is a stereogenic center in the asymmetrically
branched aliphatic chain, i.e., a stereogenic carbon, which has four
different substituents. If these four different substituents are
arranged around the stereogenic carbon in different orders such
as clockwise or counterclockwise, R- or S-enantiomer with
formula
e ¼A/bc. Here, e is the molar absorption coefficiency, A is
the absorbance at the absorption maximum, b is the cuvette
Fig. 2. Molecular design of organic near-infrared phenothiazine-based photorefractive compounds.
4
3
2
1
0
1.0
3a
3b
0.8
3c
3d
0.6
0.4
0.2
0.0
3a
3b
3c
3d
300
400
500
600
0
10
20
30
40
Concentration (mol/l) / 10-5
Wavelength (nm)
Fig. 3. UV–vis spectra of phenothiazine-based compounds (a) (c ¼1.14 ꢀ 10ꢁ5 mol/l) and the relationship of absorbance versus concentration at the absorption maximum
wavelength (b).

Y. Liu et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1136–1145
1141
thickness, and c is the molar concentration. The calculation
results suggested that the molar absorption coefficiencies of
compounds 3a, 3b, 3c, 3d are 5.96 ꢀ 104 l/mol cm, 7.28 ꢀ 104 l/
mol cm, 7.68 ꢀ 104 l/mol cm, 4.82 ꢀ 104 l/mol cm respectively .
Evidently, the molar absorption coefficiency of compound 3d
with a homochiral asymmetrically branched aliphatic chain, (S)-
(þ)-2-methylbutyl group, is less than compound 3a with a linear
aliphatic chain, 10-chlorooctyl group and compounds 3b/3c with
racemic asymmetrically branched aliphatic chain, 2-ethylhexyl/2-
methylbutyl group. The absorbance-versus-concentration plot in
Fig. 3(b) also showed that the absorbance of compound 3d, at
every concentration is clearly less than that of other compounds.
Whereas, the absorbance-versus-concentration curves of other
compounds are very close, especially compounds 3b and 3c,
because they both have a racemic asymmetrically branched
compound 3d. All those results showed that compound 3a with a
linear aliphatic chain degrades more easily and faster than
compounds 3b, 3c, 3d with asymmetrically branched aliphatic
chains. Even, this fast decomposition causes a temporal tempera-
ture withdrawal on the TG curve of compound 3a. Therefore, we
came to a conclusion that the introduction of asymmetrically
branched aliphatic chain enhances the thermal performance,
independent of racemic or homochiral group. Finally, it was noted
that the loss-weight ratio of compounds 3a, 3b, 3c, 3d at the
degradation peak, which is 22.0%, 12.8%, 15.3% and 15.8%, respec-
tively is not more than the weight content of aliphatic chain in the
compounds, which is 32.1%, 23,4%, 16.1% and 16.1% respectively.
This indicats that the degradation at the degradation peak comes
mainly from the decomposition of aliphatic chains.
aliphatic chain. Therefore, we draw
a
conclusion that the
3.4. DSC measurement
homochiral asymmetrically branched aliphatic chain, (S)-(þ)-2-
methylbutyl group, has a hypochromic effect. In other words, the
linear aliphatic chain, 10-chlorooctyl group, and the racemic
asymmetrically branched aliphatic chain, 2-ethylhexyl/2-methyl-
butyl group, has a hyperchromic effect. This probably resulted
from the enantiomers’ absorption of left- and right-circularly
polarized light to different degrees, which are included in the
light source of the spectrophotometer.
All the photorefractive compounds 3a, 3b, 3c, 3d were also
characterized by the DSC measurement under the nitrogen atmo-
sphere at a scanning rate of 10 1C/min. In order to study the effect
of chemical structure of aliphatic chains on the phase state of
photorefractive compounds in detail, the DSC curves were mea-
sured from ꢁ40 1C to the degradation temperature through the
first heating of powder samples, the cooling, and the second
heating with the same programmed temperature controlling, as
shown in Fig. 5. The results indicated that compound 3a with a
linear aliphatic chain is a crystalline compound. Its melting points
at the first and second heating processes are 107.4 1C and 107.0 1C
respectively. The freezing point in the cooling process is 92.4 1C,
which is a little less than the melting points. At the same time, the
enthalpy change at the first heating, cooling, second heating
processes are 35.5 J/g, 37.0 J/g and 35.3 J/g respectively. However,
when the linear aliphatic chain is substituted by the asymme-
trically branched aliphatic chains including racemic and homo-
chiral asymmetrically branched aliphatic chains, crystalline
compound 3a is turned into glassy compounds 3b, 3c and 3d,
i.e., organic molecular glasses, which is obviously proved by the
DSC curves in Fig. 5, especially the second-heating curves with the
same heat history. Together with the TG result that the thermal
performance is enhanced by the introduction of asymmetrically
branched aliphatic chains including racemic and homochiral
groups, it was suggested that glassy compounds 3b, 3c and 3d
are more thermally stable than crystalline compound 3a.
The racemic asymmetrical branched aliphatic chain, 2-ethyl-
hexyl group, which is often used in the course of the synthesis of
organic optoelectronic materials in order to increase the solubility
and film-processing ability, is first introduced into the photore-
fractive chromophore in place of the linear aliphatic chain, 10-
chlorooctyl group in compound 3a. In consequence, organic
molecular glass, compound 3b, is obtained whose glass transition
temperature is 21.1 1C at the first heating, 9.0 1C at the cooling
and 16.6 1C at the second heating, as shown in Fig. 5. Notably,
there is a very small melting point at 65.1 1C with an enthalpy
change of 0.25 J/g in the first-heating DSC curve of compound 3b,
indicative of a weak crystallization tendency. However, this tiny
melting point will disappear by heating the sample over the
melting point, as shown in the second-heating DSC curve of
compound 3b in Fig. 4. This means that the weak crystallization
tendency does not affect the achievement of organic glassy state
as long as the sample is heated over the melting point. In order to
study the volume effect of branched aliphatic chain on the
probability to achieve organic molecular glasses, the smaller
racemic asymmetrical branched aliphatic chain, 2-methylbutyl
group, is introduced into the photorefractive chromophore to take
the place of 2-ethylhexyl group in compound 3b. Although there
is only one melting point at 57.1 1C to appear in the first-heating
3.3. Thermal property
The thermal property of photorefractive compounds 3a, 3b, 3c,
3d was investigated by the TG measurement under the nitrogen
atmosphere at a heating rate of 10 1C/min. The TG and DTG curves
were illustrated in Fig. 4. The TG and DTG curves of compound 3a
with a linear aliphatic chain, 10-chlorooctyl group, have an
evident difference from compounds 3b, 3c, 3d with asymmetri-
cally branched aliphatic chains, regardless of the racemic or the
homochiral group. The degradation temperature of 283 1C for
compound 3a, which is defined as the temperature at the loss-
weight of 5% on the TG curve, is less than 324 1C for compound
3b, 307 1C for compound 3c, 310 1C for compound 3d. The
degradation peak temperature of 300 1C on the DTG curve of
compound 3a is also less than the compounds 3b, 3c, 3d, which
have the same degradation peak temperature of 354 1C. The
degradation peak height of 16.9
compound 3a is obviously higher than 4.8
3b, 5.7 V/mg for compound 3c, 4.8 V/mg for compound 3d. The
m
V/mg on the DTG curve of
mV/mg for compound
m
m
full width at half a maximum of 7 1C for the degradation peak of
compound 3a on the DTG curve is also a lot evidently narrower
than 22 1C for compound 3b, 24 1C for compound 3c, 24 1C for
0
15
10
5
TG DTG
-20
-40
3a
3b
3c
3d
0
-5
100
200
300
400
Temperature (°C)
Fig. 4. TG and DTG curves of phenothiazine-based compounds.

1142
Y. Liu et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1136–1145
Compound 3b
First heating
Cooling
Second heating
Compound 3a
First heating
Cooling
Second heating
0
50
100
150
200
0
50
100
150
Temperature (°C)
Temperature (°C)
Compound 3d
Compound 3c
First heating
First heating
Cooling
Second heating
Cooling
Second heating
0
50
100
150
200
0
50
100
150
200
NC
Temperature (°C)
Temperature (°C)
Fig. 5. DSC curves of phenothiazine-based compounds.
DSC curve of compound 3c, its enthalpy change of 6.9 J/g is much
less than 35.5 J/g of compound 3a at the first heating. This
indicated that the introduction of 2-methylbutyl group decreases
the molecular degree of order of compound 3c to a large extent
compared with compound 3a. Like the small melting point in the
first-heating DSC curve of compound 3b, the melting point of
compound 3c at the first heating will also disappear at the cooling
and second-heating processes. The appearance of glass transition
at 46.6 1C in the second-heating DSC curve of compound 3c
suggested that 2-methylbutyl group is potentially the smallest
racemic asymmetrically branched aliphatic chain to synthesize
organic molecular glass for the present photorefractive chromo-
phore. Finally, the homochiral asymmetrically branched aliphatic
chain, (S)-(þ)-2-methylbutyl group, which has the same chemical
conformation as 2-methylbutyl group in compound 3c, is also
connected with the photorefractive chromophore in order to
understand if the introduction of homochiral asymmetrically
branched aliphatic chain may result in organic molecular glass.
The result is similar to the introduction of 2-ethylhexyl group into
compound 3b. The glass transition temperatures of 28.2 1C at the
first heating, 45.4 1C at the cooling and 49.1 1C at the second
heating in the DSC curves of compound 3d in Fig. 4 indicated that
the introduction of homochiral asymmetrically branched aliphatic
chain can still lead to the achievement of organic molecular glass.
Similarly to compound 3b, the small melting point with an
enthalpy change of 0.36 J/g disappears at the cooling and sec-
ond-heating processes. Additionally, the glass transition tempera-
tures of compounds 3b, 3c and 3d at the second-heating (16.6 1C,
46.6 1C and 49.1 1C), which come from the samples with the same
heat history, suggested that the glass transition temperature of
compound 3b with a large-volume asymmetrically branched
aliphatic chain is less than compounds 3c and 3d with a small-
volume asymmetrically branched aliphatic chain, independent of
racemic or homochiral group. Simultaneously, the glass transition
temperature of compound 3d with homochiral asymmetrically
CN
CH2 CH
N
n
NO2
NO2 NO2
PVK
TNFM
Charge Generator
Photoconductor
Fig. 6. Chemical structures of photoconducting polymer (PVK) and charge gen-
erator (TNFM).
branched aliphatic chain is a little more than compound 3c with
racemic asymmetrically branched aliphatic chain. This is on
account of the same chemical conformation but different mole-
cular configuration.
3.5. Photorefractive property
Compounds 3c and 3d with the same chemical conformation
are selected as organic photorefractive molecular glasses for the
photorefractive measurement in order to investigate the effect of
racemic and homochiral asymmetrically aliphatic chains on the
photorefractive property. Poly(N-vinylcarbazole) (PVK) is used as
a photoconductive polymer and (2,4,7-trinitro-9-fluorenylidene)
malononitrile (TNFM) as a photosensitizer. Their chemical struc-
tures were illustrated in Fig. 6. The GPC measurement indicated
that the number average molecular weight, weight average
molecular weight and polydispersity of PVK are 2.49 ꢀ 105,
1.70 ꢀ 106 and 6.86 respectively. In this work, four photorefrac-
tive devices, whose thicknesses were controlled by the spacer of
100 mm, were prepared by the fabrication approach depicted in

Y. Liu et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1136–1145
1143
Table 1
Data of all the photorefractive devices at 780 nma.
1.0
0.8
0.6
0.4
0.2
0.0
Devices
Tg /1C
Transmittance /%
a
/cmꢁ1
G
/cmꢁ1
n1s
1
2
3
4
8.1
7.6
5.5
3.2
75.7
75.9
63.4
63.3
27.8
27.6
45.6
45.7
11.8
17.6
7.4
1.42 ꢀ 10ꢁ4
2.11 ꢀ 10ꢁ4
0.88 ꢀ 10ꢁ4
1.49 ꢀ 10ꢁ4
1(3c-PVK15%)
2(3c-PVK20%)
3(3d-PVK15%)
4(3d-PVK20%)
12.4
a
Tg is the glass transition temperature of the composite,
a
is the absorption
coefficiency in solid film calculated with the formula I/I0¼eꢁ
, G is the
aL
modulated amplitude of the coupling constant at 780 nm and 1.5 kV, and n1 is
the modulated amplitude of the refractive index at 780 nm and 1.5 kV.
600
700
800
Wavelength (nm)
900
1000
15
1(3c-PVK15%)
Fig. 7. Transmittance spectra of all the photorefractive devices.
2(3c-PVK20%)
3(3d-PVK15%)
4(3d-PVK20%)
12
9
Section 2. The device configuration and composite component
were shown as follows:
1. ITO/compound 3c (84%)/TNFM (1.0%)/PVK (15%)/ITO
2. ITO/compound 3c (79%)/TNFM (1.0%)/PVK (20%)/ITO
3. ITO/compound 3d (84%)/TNFM (1.0%)/PVK (15%)/ITO
4. ITO/compound 3d (79%)/TNFM (1.0%)/PVK (20%)/ITO
6
3
The transmittance spectra of the photorefractive devices were
measured as shown in Fig. 7. The results indicated that all the
photorefractive devices have a good transmittance property at
more than 700 nm, which basically belongs to the near-infrared
region. In addition, at 780 nm where the photorefractive property
will be investigated, compound 3c has a better transmittance
property than compound 3d in solid film, which resulted from the
high transmittances of 75.7% for device 1, 75.9% for device 2 and
the low transmittances of 63.4% for device 3, 63.3% for device 4. In
other words, compound 3d has a better absorption property than
compound 3c in solid film. This is proved by the low absorption
coefficiencies of 27.8 cmꢁ1 in device 1, 27.6 cmꢁ1 for device
2 and the high absorption coefficiency of 45.6 cmꢁ1 for device
3, 45.7 cmꢁ1 for device 4, which were calculated by the formula:
0.4
0.6
0.8
1.0
Voltage / kV
1.2
1.4
1.6
Fig. 8. Photocurrent-versus-voltage curves of all the photorefractive devices.
larger than device 3 from compound 3d when the PVK content is
15% and the photocurrent of device 2 from compound 3c is also
larger than device 4 from compound 3d when the PVK content is
20%. This result indicated that compound 3c with racemic
asymmetrically branched chain has a higher optical charge gen-
eration quantum efficiency in solid film than the compound 3d
with homochiral asymmetrically branched chain. Evidently, the
high absorption coefficiency of compound 3d in solid film does
not cause the high photocurrent. Contrarily, the homochiral
asymmetrically branched aliphatic chain in compound 3d leads
to the low optical charge generation quantum efficiency and then
the low photocurrent in the photorefractive device. The racemic
asymmetrically branched aliphatic chain in compound 3c causes
the high optical charge generation quantum efficiency and then
the high photocurrent in the photorefractive device. We assumed
that this result should be derived from the formation of different
exciplexes such as racemic and homochiral charge-transfer com-
plexes, which play a photosensitizer role in photorefractive effect.
In other words, racemic charge-transfer complex might induce
high optical charge generation quantum efficiency and homo-
chiral charge-transfer complex might induce low optical charge
generation quantum efficiency. This point needs to be further
investigated in detail although chiral charge-transfer complexes
have been investigated [41–42]. Besides, it was also noted from
the photocurrent curves in Fig. 8 that the high content of
photoconductive polymer PVK results in the high photocurrent.
The photorefractive property was investigated by the mea-
surement of steady two beam coupling, which was described in
Section 2. The total powers of modulated transmitted signals are
measured with one beam moved at a constant speed by a
piezoelectric stage driven by a piezo driver and a function
generator after the energy transfer between both beams reaches
a steady state. The transmitted powers for beams 1 and 2 of
a
I/I0¼eꢁ L. This result is completely contrary to the fact that the
molar absorption coefficiency of compound 3d in the dilute
solution is less than that of the compound 3c. This is induced
by different molecular states, i.e., the single molecular state in the
dilute solution and the condensed state in solid film. The small
transmittance or the large absorption coefficiencies of devices
3 and 4 of compound 3d is caused by the low steric hindrance of
homochiral asymmetrically branched aliphatic group (here, S-
enantiomer). It was well known that the low steric hindrance
results in the better intermolecular interaction of donor–acceptor
dipole compounds. Consequently, the exciplex will be formed
more easily, which is an excited donor–acceptor complex and
dissociated in the ground state. Therefore, the small transmit-
tance or the large absorption coefficiency can be obtained in solid
film of homochiral compound 3d. In contrast, the different
responses of the enantiomers to left- and right-circularly polar-
ized lights does not appear in solid film. At the same time, we
noted that the content difference of PVK induces only a little
change of the glass transition temperature as shown in Table 1.
All the data of the transmittance, the absorption coefficiency in
solid film and the glass transition temperature of the composites
were listed in Table 1.
The photocurrent of all the photorefractive devices was tested
by the measurement approach described in Section 2. The photo-
current-versus-voltage curves were shown in Fig. 8. It was
obvious that the photocurrent of device 1 from compound 3c is

1144
Y. Liu et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1136–1145
Beam 1
Beam 2
Piezo
0.5
0.4
0.3
0.2
0.1
0.0
30
20
10
0
0.70
0.65
0.60
0.55
0.50
-10
0.000
0.001
0.002
time / s
0.003
0.004
0.000
0.001
0.002
time / s
0.003
0.004
Fig. 9. Transmitted power for beams 1 and 2 of device 2 together with piezoelectric function signal as the piezomirror is translated with a piezoelectric stage driven
by a piezodriver and a function generator (a) and the modulated curve of the coupling constant (b) at 1.5 kV.
device 2 at 1.5 kv together with piezoelectric function signal were
illustrated in Fig. 9a. Both beams are evidently diffracted by the
18
1(3c-PVK15%)
grating in existence after the mirror is translated. Generally, the
2(3c-PVK20%)
3(3d-PVK15%)
4(3d-PVK20%)
15
12
9
intensities I1(z) and I2(z) are given as formula (1) and (2) [48]:
1þmꢁ1
az
I1ðzÞ ¼ I1ð0Þ
z eꢁ
ð1Þ
g
1þmꢁ1e
1þm
az
I2ðzÞ ¼ I2ð0Þ
z eꢁ
ð2Þ
6
g
1þmeꢁ
Here,
g
is the coupling constant, z is any position inside the
3
photorefractive device,
the input intensity ratio:
a
is the absorption coefficiency, and m is
0
I1ð0Þ
m ¼
0.4
0.6
0.8
1.0
Voltage / kV
1.2
1.4
1.6
ð3Þ
mm) and I1(0) is equal to
I2ð0Þ
In our case, the thickness is L (100
I2(0), namely, m¼1. Therefore,
Fig. 10. Modulated amplitude of the coupling constant in all the photorefractive
devices at different operating voltages.
I1ðLÞ
g
L
gL
¼ meꢁ ¼ eꢁ
ð4Þ
I2ðLÞ
Simultaneously,
PVK content is 15% The G value of device 2 from compound 3c is
also larger than device 4 from compound 3d when the PVK
content is 20%. This result suggested that the coupling constant
of compound 3c with racemic asymmetrically branched aliphatic
chain has higher modulated amplitude than compound 3d with
2
p
n1
g
¼
sin
f
ð5Þ
l
cos
Here, n1 is the modulated amplitude of refractive index,
the laser wavelength; 2 is the angle between the beams inside
the medium, and is the phase shift between index grating and
y
l
is
y
homochiral asymmetrically branched aliphatic chain. The
G value
f
has a similar change trend to the photocurrent in Fig. 8 with the
increase of applied voltages and the PVK content. The low optical
charge generation quantum efficiency from homochiral asym-
metrically branched aliphatic chain in compound 3d not only
light interference pattern. Finally, we can know that the coupling
constant can be shown as follows:
1
¼ ꢁ In
L
I1ðLÞ
I2ðLÞ
2pn1
g
¼
sin
f
ð6Þ
l
cos y
results in the low photocurrent but also the low
G value. Vice
versa, the high optical charge generation quantum efficiency from
racemic asymmetrically branched aliphatic chain in compound 3c
Here, in order to evaluate the photorefractive property, we
define the modulated amplitude of the coupling constant
below:
G
as
causes the high photocurrent and finally the high
G value. This is
in accordance with the common principle in the photorefractive
materials that the photorefractive performance is proportional to
the photocurrent. Therefore, we deemed that the flexible homo-
chiral asymmetrically branched aliphatic chain cannot make rigid
photorefractive chromophore much more ordered, which will be
one necessary condition to improve the photorefractive perfor-
mance. The racemic group is more beneficial to the improvement
of photorefractive performance than the homochiral.
2
p
n1
G
¼
ð7Þ
l
cos
y
The modulated curve of the coupling constant of device 2 at
1.5 kV was given as an example in Fig. 9b, which is calculated
with a formula
g ¼ ꢁð1=LÞInðI1ðLÞ=I2ðLÞÞ. At this time, the G value is
equal to 17.6 cmꢁ1. The modulated amplitude of the refractive
index is also calculated as 2.11 ꢀ 10ꢁ4 with the above definition
formula (7). All the data of
G and the modulated amplitude of the
refractive index at 780 nm and 1.5 kV were summarized and
listed in Table 1.
4. Conclusions
The modulated amplitude of the coupling constant
different operating voltages of all the photorefractive devices
was plotted in Fig. 10. Evidently, the value of device 1 from
compound 3c is larger than device 3 from compound 3d when the
G at
Organic near-infrared photorefractive molecular glasses with a
phenothiazine moiety were designed and synthesized through
the introduction of linear, racemic/homochiral asymmetrically
G

Y. Liu et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1136–1145
1145
branched aliphatic chains into photorefractive chromophore as an
auxiliary group. All the compounds were characterized with
1H-NMR, IR, FAB-MS, UV–vis, TG, DSC, etc. The results indicated
that all the compounds are in accordance with the chemical
structures in Scheme 1. The effect of different aliphatic chains on
absorption and thermal properties was investigated in detail. The
UV–vis measurement suggested that the homochiral asymmetri-
cally branched aliphatic chain has a strong hypochromic effect in
the dilute solution. The DSC measurement told us that the
introduction of asymmetrically branched aliphatic chains is the
key issue to synthesize organic molecular glasses whether it is
racemic or homochiral. The effect of racemic/homochiral asym-
metrically branched aliphatic group on photorefractive perfor-
mance was measured carefully with PVK as a photoconductive
polymer and with TNFM as a photosensitizer. The results indi-
cated that the homochiral asymmetrically branched aliphatic
chain results in the low optical charge generation quantum
efficiency and then the low photocurrent in the photorefractive
devices, which finally causes the low modulated amplitude of the
coupling constant. Contrarily, the high optical charge generation
quantum efficiency from the racemic asymmetrically branched
aliphatic chain leads to the high photocurrent and finally the high
modulated amplitude of the coupling constant. Therefore, the
racemic asymmetrically branched aliphatic group is more of benefit
to photorefractive performance than homochiral asymmetrically
branched aliphatic group when the homochiral group cannot make
rigid photorefractive chromophore much more ordered.
[11] H. Kgeyama, H. Ohishi, M. Tanaka, Y. Ohmori, Y. Shirota, Appl. Phys. Lett. 94
(2009) 063304.
[12] R. Schroeder, L.A. Majewski, M. Grell, Adv. Mater. 16 (2004) 633–636þ576.
[13] M. Halik, H. Klauk, U. Zschieschang, G. Schmid, C. Dehm, M. Schutz, S. Malsch,
F. Effenberger, M. Brunnbauer, F. Stellacci, Nature 431 (2004) 963–966.
[14] P.M. Lundquist, R. Wortmann, C. Geletneky, R.J. Twieg, M. Jurich, V.Y. Lee,
C.R. Moylan, D.M. Burland, Science 274 (1996) 1182–1185.
[15] S.J. Zilker, U. Hofmann, Appl. Opt. 39 (2000) 2287–2290.
[16] S. Schloter, A. Schreiber, M. Grasruck, A. Leopold, M. Kol’chenko, J. Pan,
C. Hohle, P. Strohriegl, S.J. Zilker, D. Haarer, Appl. Phys. B: Lasers Opt. 68
(1999) 899–906.
[17] Q. Wang, N. Quevada, A. Gharavi, L.P. Yu, ACS Symp. Ser. 726 (1999) 226–236.
[18] F. Fan, J.S. Liu, Y. Zhang, Y. Chen, J. Mod. Opt. 56 (2009) 1392–1395.
[19] M. Asaro, M. Sheldon, Z.G. Chen, O. Ostroverkhova, W.E. Moerner, Opt. Lett.
30 (2005) 519–521.
[20] Z. Chen, M. Asaro, O. Ostroverkhova, W.E. Moerner, M. He, R.J. Twieg, Opt.
Lett. 28 (2003) 2509–2511.
[21] U. Hofmann, M. Grasruck, A. Leopold, A. Schreiber, S. Schloter, C. Hohle,
P. Strohriegl, D. Haarer, S.J. Zilker, J. Phys. Chem. B 104 (2000) 3887–3891.
[22] S.J. Zilker, M. Grasruck, J. Wolff, S. Schloter, A. Leopold, M.A. Kol’chenko,
U. Hofmann, A. Schreiber, P. Strohriegl, C. Hohle, D. Haarer, Chem. Phys. Lett.
306 (1999) 285–290.
[23] M. Grasruck, A. Schreiber, U. Hofmann, S.J. Zilker, A. Leopold, S. Schloter,
C. Hohle, P. Strohriegl, D. Haarer, Phys. Rev. B (Condens. Matter) 60 (1999)
16543–16548.
[24] O. Ostroverkhova, W.E. Moerner, M. He, R.J. Twieg, Appl. Phys. Lett. 82 (2003)
3602–3604.
[25] M. Miyasaka, A. Rajca, M. Pink, S. Rajca, Chem.- Eur. J. 10 (2004) 6531–6539.
[26] M.H. Levi, S.J. Garth, Annu. Rev. Phys. Chem. 60 (2009) 345–365.
[27] K. Sreekumar, D. Davis, S.K. Pati, Synth. Met. 155 (2005) 384–388.
[28] S. Sioncke, T. Verbiest, A. Persoons, Mater. Sci. Eng. R: Rep. R42 (2003)
115–155.
[29] B.J. Coe, E.C. Harper, K. Clays, E. Franz, Dyes Pigm. 87 (2010) 22–29.
[30] M. Kauranen, T. Verbiest, A. Persoons, J. Nonlinear Opt. Phys. Mater. 8 (1999)
171–189.
[31] A. Persoons, M. Kauranen, S. Van Elshocht, T. Verbiest, L. Ma, L. Pu,
B.M.W. Langeveld-Voss, E.W. Meijer, Mol. Cryst. Liq. Cryst. Sci. Technol. Sect.
A: Mol./Cryst/Liq. Cryst. 314 (1998) 395/93–400/98.
Acknowledgment
[32] M. Kauranen, T. Verbiest, S. Van Elshocht, A. Persoons, Opt. Mater. 9 (1998)
286–294.
The authors greatly acknowledge the support from the Japan
Society for the Promotion of Science (JSPS). Simultaneously, we
also thank Prof. Koji Araki and Mr. Isao Yoshikawa in Institute of
Industrial Science, the University of Tokyo for their help in the
measurements of IR spectra, FAB-MS spectra, TG and DSC.
[33] T. Verbiest, S.V. Van Elshocht, M. Kauranen, L. Hellemans, J. Snauwaert,
C. Nuckolls, T.J. Katz, A. Persoons, Science 282 (1998) 913–915.
[34] M. Kauranen, T. Verbiest, E.W. Meijer, E.E. Havinga, M.N. Teerenstra,
A.J. Schouten, R.J.M. Nolte, A. Persoons, Adv. Mater. 7 (1995) 641–644.
[35] C. Dhenaut, I. Ledoux, I.D.W. Samuel, J. Zyss, M. Bourgault, H. Le Bozec, Nature
374 (1995) 339–342.
[36] M. Talarico, R. Termine, P. Prus, G. Barberio, D. Pucci, M. Ghedini,
A. Golemme, Mol. Cryst. Liq. Cryst. 429 (2005) 65–76.
References
[37] R. Termine, A. Golemme, J. Phys. Chem. B 106 (2002) 4105–4111.
[38] R. Termine, A. Golemme, Opt. Lett. 26 (2001) 1001–1003.
[39] R. Termine, B.C. De Simone, A. Golemme, Appl. Phys. Lett. 78 (2001) 688–690.
[40] M.C. Guo, Z.D. Xu, X.Q. Wang, Opt. Mater. 31 (2008) 412–417.
[41] Y. Imai, K. Kamon, S. Kido, T. Harada, N. Tajima, T. Sato, R. Kuroda,
Y. Matsubara, Cryst. Eng. Comun. 11 (2009) 620–624.
[1] C.S. Choi, I.K. Moon, N. Kim, Appl. Phys. Lett. 94 (2009) 053302.
[2] J. Hwang, J. Seo, J. Sohn, S.Y. Park, Opt. Mater. 21 (2003) 359–364.
[3] H. Chun, N.J. Kim, W.J. Joo, J.W. Han, H.O. Chang, N. Kim, Synth. Met. 129
(2002) 281–283.
[4] U. Gubler, M. He, D. Wright, Y. Roh, R. Twieg, W.E. Moerner, Adv. Mater. 14
(2002) 313–317.
[5] J. Sohn, J. Hwang, S.Y. Park, G.J. Lee, Jpn. J. Appl. Phys., 40 (2001) 3301–3304.
[6] S. Tang, M.R. Liu, P. Lu, H. Xia, M. Li, Z.Q. Xie, F.Z. Shen, C. Gu, H.P. Wang,
B. Yang, Y.G. Ma, Adv. Funct. Mater. 17 (2007) 2869–2877.
[7] L.H. Chan, R.H. Lee, C.F. Hsieh, H.C. Yeh, C.T. Chen, J. Am. Chem. Soc. 124
(2002) 6469–6479.
[8] D.F. O’Brien, P.E. Burrows, S.R. Forrest, B.E. Koene, D.E. Loy, M.E. Thompson,
Adv. Mater. 10 (1998) 1108–1112.
[9] M.R. Robinson, S.J. Wang, A.J. Heeger, G.C. Bazan, Adv. Funct. Mater. 11 (2001)
413–419.
[10] H. Kageyama, H. Ohishi, M. Janaka, Y. Ohmori, Y. Shirota, Adv. Funct. Mater.
19 (2009). 3948-2955.
[42] Y. Imai, T. Kinuta, T. Sato, N. Tajima, R. Kuroda, Y. Mtsubara, Z. Yoshida,
Tetrahedron Lett. 47 (2006) 3603–3606.
[43] M. Frantisek, B. Geraldine, G.A. Emanuel, J. Chromatogr.
205–221.
A 122 (1976)
[44] G. Archetti, A. Abbotto, R. Wortmann, Chem.—A Eur. J. 12 (2006) 7151–7160.
[45] S.J. Zilker, Angew. Chem. Int. Ed. Engl. 39 (2000) 73–87.
[46] S.R. Marder, B. Kippelen, A.K.Y. Jen, N. Peyghambarian, Nature 388 (1997)
845–851.
[47] B. Kippelen, F. Meyers, N. Peyghambarian, S.R. Marder, J. Am. Chem. Soc. 119
(1997) 4559.
[48] P. Yeh, Introduction of Photorefractive Nonlinear Optics, A Wiley Interscience
Publication John Wiley & Sons, Inc., 1993.