Coumarin-containing polymers for high density non-linear optical data storage

Article abstract of DOI:10.3390/molecules21020147

Optical data storage was performed with various thin polymer films containing coumarin-based derivatives and by using femtosecond laser pulses as well as two-photon absorption processes. Exploring the photodimerization attribute of coumarin derivatives and using appropriate irradiation wavelengths, recording/erasing processes could be carried out in the same area. Second harmonic generation microscopy was used to read the stored information.

Full text of DOI:10.3390/molecules21020147

molecules
Article
Coumarin-Containing Polymers for High Density
Non-Linear Optical Data Storage
Denis Gindre * , Konstantinos Iliopoulos 1,†, Oksana Krupka 2,†, Marie Evrard 1,†
1
, ,†
,
Emilie Champigny 1 and Marc Sallé
,†
1,†
1
Laboratory MOLTECH-Anjou, CNRS UMR 6200, University of Angers, 2 Bd Lavoisier,
9045 Angers CEDEX, France; kostiliopoulos@gmail.com (K.I.); marie.evrard@univ-angers.fr (M.E.);
4
emilie.champigny@gmail.com (E.C.); marc.salle@univ-angers.fr (M.S.)
Department of Chemistry, Taras Shevchenko National University of Kyiv, 60 Volodymyrska, 01033 Kyiv,
Ukraine; oksana_krupka@yahoo.com
2
*
†
Correspondence: denis.gindre@univ-angers.fr; Tel./Fax: +33-241-735-228
These authors contributed equally to this work.
Academic Editor: Pascal Richomme
Received: 8 December 2015 ; Accepted: 18 January 2016 ; Published: 26 January 2016
Abstract: Optical data storage was performed with various thin polymer films containing
coumarin-based derivatives and by using femtosecond laser pulses as well as two-photon absorption
processes. Exploring the photodimerization attribute of coumarin derivatives and using appropriate
irradiation wavelengths, recording/erasing processes could be carried out in the same area. Second
harmonic generation microscopy was used to read the stored information.
Keywords: optical data storage; coumarin; two-photon absorption; second harmonic generation
1
. Introduction
A key challenge of the 21st century concerns the management of information. The exponential
increase of the amount of digital information requires continuous improvement of storage capacities.
The last few years have seen intensive research focusing on novel materials and/or new experimental
techniques suitable for optical data storage (ODS) as a complementary approach to magnetic
hard drives and high capacities flash memories. New materials, based for instance on DNA [
or nanostructured glass [ ], have been used for ODS and there has been concurrent progress
made for advanced techniques [ ]. Despite these ongoing advances, the market demand for light
1]
2
3
and inexpensive high capacity media remains strong and organic materials based on polymers
functionalized with active chromophores constitute a promising approach. In particular, the possibility
to stimulate photochromic materials in order to modulate their linear or nonlinear optical properties
constitutes an important breakthrough regarding applications. For those related to ODS, two-photon
processes are often used as they provide high storage capacity, due to the quadratic dependence on the
incident light intensity. The main advantage of two-photon absorption-induced photochemistry lies in
the precise three dimensional (3D) spatial control of the photoreactions. The use of two-photon
processes properties to improve data storage was introduced by the Rentzepis group [
several works have been reported since, including rewritable ODS based on spironaphthoxazine [
two-photon optical storage by fluorescence modulation [ ], as well as 3D ODS using photochromic
materials [ ]. Another important light-related effect is second-harmonic generation (SHG), which
4
,5
] and
6],
7,8
9
corresponds to a nonlinear optical process of conversion of two photons at fundamental frequency
into one photon at doubled frequency. Importantly, photoinduced chemical modifications carried out
at the molecular level in the bulk of the material can induce a local modification of the SHG response
of the material. This detection parameter, which is inherently nonlinear and harder to measure than
Molecules 2016, 21, 147; doi:10.3390/molecules21020147
www.mdpi.com/journal/molecules

Molecules 2016, 21, 147
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other linear optical contrasts, is interesting in terms of high density volumetric optical storage. On
this basis, some of us reported ODS by modulation of the SHG response occurring upon the cis-trans
isomerization of azo-dyes chromophores grafted in a polymeric matrix [10
approach based on quantum dots [13].
–12]. Others proposed an
Coumarins and their analogues are well-known for their photochemical and photophysical
properties [14 16]. In particular, some coumarin derivatives are able to undergo a photoinduced
cyclodimerization [17 18] to a cyclobutane-based dimer. The reverse process (photocleavage)
can be induced upon irradiation with higher energy photons (wavelength = 254 nm) [19 21].
We previously reported the remarkable ability of a coumarin-based copolymer to undergo a
reversible writing-erasing process [22 24] and its potential use in particular for anti-counterfeiting
–
,
–
–
applications [25]. Optical storage based on modulation of the SHG response in coumarin-based
materials involves the photo-induced reaction shown in Scheme 1. A coumarin derivative
cyclodimerizes to a stable cyclobutane-based dimer upon irradiation at
λ > 300 nm (four isomeric
configurations, i.e., syn head-to-head, anti head-to-head, syn head-to-tail, and anti head-to-tail are
possible), whereas the reverse photocleavage reaction occurs at shorter wavelengths. When a donating
1
substituent is located on the 7-position of the coumarin unit (typically R = OR (alcoxy)), the
corresponding chromophore presents a suitable -delocalization profile over the entire molecule with
a suitable dipole moment to get a SHG response. This is no longer the case with the dimeric form, once
the cyclobutane ring is formed, the latter fragment preventing the -delocalization. Consequently it is
π
π
possible, providing a previous spatial organization of the coumarin dipoles by means of an externally
applied electric field (corona poling) [26], to address locally the SHG response in the material, from
a high value (monomer form) to a low value (dimer form). It has to be noted that when applied to
the case of a functionalized polymer bearing pendant coumarin units, the cyclodimerization process,
being intermolecular, gives rise to a cross-linking of the corresponding polymer, an issue which is well
documented [14,18,27].
Scheme 1. Reversible photodimerization process of coumarin (four possible isomers of the dimeric
form can be obtained).
Even if coumarins appear more appealing than azobenzenes with respect to ODS applications
because of their greater transparency in the visible range, the fact remains that their nonlinear responses
are weak and especially the contrast between the response of the monomeric coumarin form and
the cross-linked form remains low. The objective of this study is therefore to determine the best
composition of the copolymer that affords the highest contrast of the SHG response. In this full-paper,
we report an exhaustive study related to the synthesis, characterization and nonlinear optical (NLO)
properties of a family of methacrylic copolymers incorporating coumarin side-groups as NLO-active
fragments. A comparative study addressing the NLO response of the corresponding films as a function
of their molecular formula has been carried out and allows to address the critical parameters for
application in ODS.

Molecules 2016, 21, 147
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2
. Results and Discussion
We first developed the synthesis of a series of twenty-one methacrylic copolymers P1
–P21 bearing
coumarin side groups.
2
.1. Synthesis and Characterization of Copolymers P1–P21
The coumarin-based methacrylic monomers 3a
methacryloyl chloride with the coumarin alcohol derivatives 1a
–c
and 4a
–
c
were synthesized by reaction of
and 2a , respectively, in the
–
c
–c
presence of triethylamine (Scheme 2). The corresponding copolymers were obtained by radical
polymerization of these monomers with methyl methacrylate (MMA) or butyl methacrylate (BMA),
using 2,2-azobis(isobutyronitrile) (AIBN) as radical initiator. The structures of the twenty-one
1
copolymers were confirmed by H-NMR spectroscopy and a reasonable accord was found between
the observed n/m values in the polymers ((n/m)obs in Tables 1 and 2) and the respective amounts of
both monomers which were introduced ((n/m) ). Nevertheless, as expected from their less hindered
0
character, the BMA or MMA motifs are generally found in slight excess in the final composition of the
polymers (m values). Glass transition temperatures (T
higher ratio in coumarin motif results in a higher T when MMA (R = CH ) is used. Also, introduction
of conformationally flexible alkyl chains as linkers of the coumarin chromophore (y = 1 and x = 5)
and/or in the methacrylate co-monomer (R = n-butyl) leads to a systematically lower value of T
g
) are collected in Tables 1 and 2. As expected, a
g
2
3
2
g
.
Finally, the copolymers’ molecular weights are in the 11700–52100 range and polydispersity indices
are 1.4–2 as determined by gel permeation chromatography (Tables 1 and 2).
Scheme 2. Synthesis of coumarin-functionalized methacrylic ester co-polymers (MMA: methyl
methacrylate; BMA: butyl methacrylate).
Table 1. Results of radical polymerization of 10% methacrylic monomers (case of monomers 3a and 4a
˝
(
y = 0)) in DMF at 80 C (argon atmosphere, AIBN 1%).
(n/m)obs b
Mn c
Mw c
Mv c
Ipd c
Tg d
Polymer
R1
R2
(n/m)0 a
P1 e
P2
P3
P4
P5
P6
P7
P8
P9
H
H
H
H
H
-
-
-
-
-
-
120
112
88
83
85
CH3
CH3
CH3
1:1
1:3
1:5
1:1
1:3
-
1:1
1:3
1:5
1:1
1:3
1:1.4
1:2.95
1:5.57
1:1.16
1:3.9
-
1:1.18
1:3
1:5.52
1:1.3
1:3
14,700
14,500
21,900
15,900
27,200
-
21,200
18,900
20,500
18,600
17,000
28,100
24,800
34,900
31,400
52,100
-
34,000
33,000
33,200
36,000
32,100
25,600
22,900
32,500
28,100
47,000
-
31,000
30,400
30,800
32,800
29,300
1.9
1.7
1.6
1.9
1.9
-
1.6
1.7
1.6
1.9
1.9
(CH
(CH
2
2
)
)
3
CH
CH
3
3
H
3
73
e
CH3
CH3
CH3
CH3
CH3
CH3
152
143
110
103
81
CH3
CH3
CH3
P10
P11
P12
(CH
(CH
2
2
)
)
3
CH
CH
3
3
3
76
a
b
n/m stoichiometry in the copolymer as determined from 1H-NMR
Molar ratio of monomers;
integration; Measured by gel permeation chromatography; d Measured by differential scanning calorimetry;
c
e
No co-monomer added; the resulting polymer is not soluble in THF.

Molecules 2016, 21, 147
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Table 2. Results of radical polymerization of 10% methacrylic ester monomers (case of monomers 3b
and 4b,c (y = 1)) in DMF at 80 C (argon atmosphere, AIBN 1%).
,c
˝
(n/m)obs b
Mn c
Mw c
Mv c
Ipd c
Tg d
Polymer R1
x
R2
(n/m)0 a
P13
P14
P15
P16
P17
P18
P19
P20
P21
CH3
CH3
CH3
CH3
CH3
H
H
H
H
1
5
5
1
5
1
1
5
5
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
1:3
1:1
1:3
1:1
1:5
1:3
1:1
1:3
1:1
1:4
-
16,900
25,000
36,600
11,700
29,700
27,000
17,200
30,400
23,500
16,900
23,000
24,000
10,100
24,700
14,700
18,900
-
1.4
1.8
2
90
40
47
85
80
93
81
73
50
1:1.32
1:3.21
1:1.11
1:6
1:2.56
1:1.2
1:3
13,600
17,900
6000
16,500
13,000
9700
2
1.8
2.0
1.7
1.9
1.95
16,000
12,000
1:1.75
21,000
a
c
Molar ratio of monomers; b n/m stoichiometry in the copolymer as determined from H-NMR integration;
1
Measured by gel permeation chromatography; d Measured by differential scanning calorimetry.
2
.2. UV-Vis Spectroscopy
Photoinduced dimerization and photocleavage studies were conducted on thin films of the
copolymers. Their optical absorption spectra show one major absorption region, assigned to the
absorption of the conjugated chromophore. During irradiation at > 300 nm, a decrease of the
absorbance assigned to * transition of the coumarin unit is observed, as illustrated for instance
λ
π–π
with the case of polymer P17 which evolves to the corresponding cross-linked form (Figure 1a).
The decreasing of the optical density is observed for all polymers studied, as expected from the
light-induced dimerization reaction to the corresponding less conjugated cyclobutane dimeric form.
Conversely, irradiation of the latter film at
λ = 254 nm (Figure 1b) shows the progressive restoration
of the * signature of the coumarin backbone which corresponds to the reverse reaction, i.e.,
π–π
photocleavage of the dimeric form. The trends observed are the same for all polymer films studied,
which all exhibit both reverse photoinduced processes, depending on the irradiation wavelength
(Figures S1–S6).
Figure 1.
at > 300 nm; (
54 nm.
(
a
) Evolution of the absorption spectrum of a P17 thin film (1.2
µm) with time upon irradiation
λ
b
) Changes of the absorption spectrum of cross-linked P17 thin film upon irradiation at
2
2
.3. Second Order Nonlinear Response
The thin films exhibit a centrosymmetric attribute as the molecules are randomly oriented. In
order to break this centrosymmetry, we used corona poling, which allows an alignment of the molecules
by means of an externally applied electric field (see Experimental Section). Note that we excluded
˝
g
values (typically <80 C) since it is
from these poling experiments all polymers presenting low T
well-established that in this case, a dipole relaxation occurs subsequently to the corona poling, a

Molecules 2016, 21, 147
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behaviour which is not compatible with SHG. We therefore discarded several polymers from the
following NLO studies, in particular those bearing long alkyl linkers (e.g., P14 P15 P17 P20 P21
Table 2).
Two different sequences to perform ODS were performed. The first one was based on: (i) irradiation
,
,
,
,
)
(
of the spin-coated film to induce photo-dimerization, (ii) corona-poling, (iii) SHG scanning to read
the data. The second sequence involves inversion of steps (i) and (ii). We found out that the latter
sequence results in a very significant improvement of the ODS quality, manifested by a higher SHG
contrast between irradiated and non-irradiated areas. This difference is assigned to a more efficient
orientation of the molecules upon corona-poling at the coumarin monomeric state compared to the
cyclobutane dimeric one, which is a result of the higher dipole moment of the π-conjugated monomer.
On this basis, we decided to perform the ODS studies with samples which are corona-poled before the
recording process.
Two different experimental approaches were used for the data recording/reading. In the first
case, the sample was displaced relative to the laser beam by means of a three-dimensional stage, while
in the second approach the sample was kept immobile and rapidly scanned by the laser beam using an
X–Y scanner. We found that the latter set-up offered a higher speed (a few microseconds for one pixel
compared to a few ms with the first configuration) for both the data writing and reading [24]. For ODS
applications, efficient SHG is required as well as a significant SHG contrast between the “0” and the
“
1” bits, which correspond to the two different molecular states. We previously determined [23] the
absolute values of the second order NLO susceptibilities, before and after the dimerization process,
for three coumarin-based polymers (P8 P9 P11). These values were obtained by the Maker fringes
,
,
technique using picosecond laser pulses. However the low repetition rate of a picosecond laser system
cannot be adapted for data storage purposes. We therefore used a Ti:Sapphire laser source (Tsunami,
Spectra-Physics, Santa Clara, CA, USA) with 120 fs pulse duration and a repetition rate of 80 MHz for
our developed ODS setup (see Experimental Section). An additional clear advantage of femtosecond
laser pulses is that an intense light peak power is obtained at the focal point of an objective lens,
offering the possibility to induce two-photon absorption phenomena, which in turn induces a higher
localized interaction area. In this way a 3D optical data storage in the bulk of the thin films could
be achieved.
2
.4. Comparative Investigation of the Optical Data Recording Efficiency
A detailed study of the influence of the molecular structure on the recording efficiency was
carried out, in order to find the most promising systems for optical storage devices. Many films of
coumarin-polymers were thus prepared and corona-poled. The goal of this study was to determine
the highest SHG contrast between the non-irradiated areas (corresponding to bit “0”, i.e., coumarin
monomers) and the laser-irradiated areas (corresponding to bit “1”, i.e., dimerized coumarin). Special
attention was paid to keep the experimental conditions unchanged during the different experiments
in order to enable direct comparison between the samples. The fixed parameters include the laser
excitation wavelength (fixed at 700 nm), the microscope objective lens (SLMPLN20X, Olympus,
Tokyo, Japan, numerical aperture = 0.25) and the laser power (50 mW). The bits “0” and “1” were
determined by the SHG level of the non-irradiated (high SHG signal) and the irradiated zones
(low SHG signal) respectively.
The SHG contrasts for six polymers films (Scheme 3) are shown in Table 3, where the values of
measured SHG signals in arbitrary units and the corresponding normalized values are reported as
well as the contrast, defined by the ratio (SHGMax SHGMin)/(SHGMax + SHGMin). The ideal material
´
for ODS applications should be have a contrast of 100%.
In order to address the influence of the linker length, comparative studies were first performed
between P13 and P9 on one hand, and between P18 and P3 on the other hand. These co-polymers all
exhibit the same n/m (= 1/3) ratio and whereas the former pair involves a methylcoumarin derivative,
the latter one is related to unsubstituted coumarin. In addition, in each pair one member corresponds

Molecules 2016, 21, 147
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to a coumarin unit directly connected through an ester group to the polymeric backbone, whereas the
second member integrates an ethandiyl linker. The SHG studies led on the first pair reveals two main
features: the bit “0” level of P13 is much lower than for P9 and the contrast between bit “1” and bit
“0” is larger. In the case of the second pair, a very significant contrast (ie gap between bit “1” and bit
“0”) and a high SHG value are found in the case of the P18 film, while those values are very low for
P3. From these comparisons, it appears obvious that the addition of the ethandiyl linker results in a
much better contrast between the SHG levels of the two bits, thus offering higher optical recording
efficiency. This observation is ascribed to a better conformational mobility in the case of P18 and P13,
as promoted by the alkyl linker, which presumably leads to a higher efficiency of the intermolecular
cyclodimerization reaction upon irradiation.
Table 3. SHG values for bit 1 and bit 0 and contrast for different coumarin-based polymers.
SHG bit 1
SHG bit 0
(arb. units)
Normalized
SHG bit 1
Normalized
SHG bit 0
Polymer
Contrast (%)
(arb. units)
P9
P13
P3
P18
P2
3.38
3.23
3.24
4.2
3.23
2.33
3.44
3.34
3.33
8.75
3.35
2.41
0.983
0.967
0.973
0.480
0.964
0.967
1
1
1
1
1
1
0.88
1.67
1.37
35.14
1.82
1.69
P8
Scheme 3. Coumarin-based copolymers investigated for optical storage applications.
A comparative study was also carried out between P8 and P2, in order to address the effect of
the NLO-active coumarin structure on the SHG contrast. Both copolymers are similar in terms of
monomer ratio (n/m = 1/1) and do not present any linker between the polymeric backbone and the
NLO-phore. They only differ by the structure of the latter, which is coumarin (P2) and methylcoumarin
(P8) respectively. From these studies, it appears that coumarin derivative (P2) exhibits higher SHG
values. Nevertheless, both materials P2 and P8 present similar and low contrasts, which indicates
a very moderate influence of this structural parameter. An ultimate parameter concerns the impact
of the (n/m) value. As mentioned above, the latter has a clear direct effect on the Tg value of the
corresponding copolymers, which regularly decreases upon dilution of the coumarin units with
increasing the ratio of MMA units (Tables 1 and 2). As for the NLO contrast, no clear tendency could
be drawn by comparing couples which only differ by the (n/m) ratio (i.e., P8 and P9 on one hand or
P2 and P3 on the other hand). Altogether, those comparative studies demonstrate the key importance
of the conformational flexibility around the NLO-phore when designing the material. In other words,

Molecules 2016, 21, 147
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having a high SHG state for a given material is not enough for OSD purposes and a good contrast
between both bits is required. This can be obtained through incorporation of a linker between the
NLO-active unit and the polymer skeleton, which allows a more efficient dimerization reaction upon
irradiation. Conversely, as mentioned above, one must keep in mind that introduction of a long alkyl
˝
chain results in low T values as in the case of P14, P15, P17, P20, P21 (T = 40–80 C) which integrate
g
g
a hexandiyl linker, which are unsuitable for a stable SHG response. Finally, it appears obvious from
these studies that the molecular system P18 gets the closest behaviour to the ideal system and is the
most promising candidate for binary ODS storage.
2
.5. Image Storage
In order to demonstrate the possibility to perform ODS from a thin P18 film, we recorded a 2D
binary image (barcode, shown in Figure 2). During the recording process, the laser beam was switched
on or off in each pixel accordingly to the value (black or white) of the pixel of the reference image
(
wavelength 700 nm, laser power 80 mW, writing scan rate 1
binary information was done by SHG imaging (wavelength 800 nm, laser power 40 mW, scan rate
µm/s). The well-contrasted resulting SHG image corresponds to a film area of 100 50 . The
dark areas correspond to dimerized-coumarin areas for which a lower SHG response is observed
bit “1”), while the brighter areas corresponds to non-dimerized ones (bit “0”) where the laser was
µm/s). The reading process of the stored
5
ˆ
µm²
(
switched off. Other ODS applications can be seen in previously published papers [22,24].
Figure 2. Normalized SHG image of a barcode (scale bar: 10 µm).
3
. Experimental Section
3
.1. Characterization Techniques
1
Compounds 1a and 2a are commercially available. H-NMR (500 MHz) spectra were measured
on a Bruker Avance DRX-500 spectrometer (Bruker Corp., Karlsruhe, Germany) using CDCl as
3
solvent. Chemical shifts are given in ppm from the internal standard tetramethylsilane (TMS). The
molecular weights of all polymers were obtained by Gel Permeation Chromatography with a SYSTEM
AS1000 autosampler (Thermo Scientific Corp., Waltham, MA, USA), equipped with a guard column
(
PL gel 5 µm guard, 5
ˆ
0.75 cm, Polymer Laboratories, Santa Clara, CA, USA) followed by 2 columns
m MIXED-D columns, 0.2 30 0.75 mm), with a SpectraSYSTEM
(
Polymer Laboratories, 2 PL gel 5
µ
ˆ
ˆ
´
1
RI-150 and a SpectraSYSTEM UV2000 detectors. The eluent used is THF at a flow rate of 1 mL
¨
min
mol ) were used for calibration. The mass spectra
were obtained on a Trace DSQ GC-MS mass spectrometer (Thermo Scientific Corp.). The glass
transition temperatures (T ) of all polymers are determined by Differential Scanning Calorimetry
with Q20 Differential Scanning Calorimetry model (TA Instruments, New Castle, PA, USA) with
˝
´1
at 35 C. Polystyrene standards (580–4.83 g
¨
g

Molecules 2016, 21, 147
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a continuous N2 purge. The sample was initially stabilized and after the first scan was made at a
˝
˝
˝
heating rate of 10 C/min up to 200 C then cooled to 20 C, a second scan was performed with the
same parameters to obtain the values of Tg. UV-Vis spectroscopy measurements were performed
at room temperature with thin films deposited on glass substrates with a UV/Vis/NIR Lambda 19
spectrophotometer (PerkinElmer Corp., Waltham, MA, USA). For irradiation experiments, the thin
˝
films were irradiated at 20 C with a UV immersion lamp TQ 150 W (Heraeus, Hanau, Germany)
equipped with a filter (Duran 50, Heraeus) that cuts off wavelengths below 300 nm. The photocleavage
of coumarin dimers was achieved using a low pressure mercury lamp TNN 15/32 (Heraeus), 15 W
˝
with emission wavelength spectrum at 254 nm (20 C, 30 min). In both cases, a 10 cm distance was
kept between the lamp and the surface of film during the irradiation.
3
.2. Thin Films Preparation
The spin-coating deposition technique was used to produce thin films of coumarin-based
copolymers P1–P21 with a controlled thickness, on glass slides BK7. Polymer films were spin-coated
at 800 rpm from 10 wt % 1,1,2-trichloroethane solution previously filtered through a 400 nm pore
˝
size nylon syringe filter. Immediately after deposition, the thin films were dried in vacuum at 35 C
for 5 h to eliminate any remaining solvent. The thickness of the thin solid films was measured by a
profilometer (Dektak 6M Stylus Surface Profilometer, Veeco, Plainview, NY, USA) and was found to be
about 0.8–1.2 µm for all samples.
3
.3. Monomer Synthesis
3
.3.1. 7-(2-Hydroxyethoxy)coumarin (1b)
A solution of 2-bromoethanol (127 mmol), 7-hydroxycoumarin (1a, 10.54 g, 65 mmol) and
˝
potassium carbonate (50.44 g, 365 mmol) in acetone (250 mL) was heated at 80 C for 65 h. The
cooled reaction solution was filtered and then evaporated under vacuum. The crude product was
dissolved in dichloromethane and treated with a saturated ammonium chloride solution. The aqueous
phase was extracted with dichloromethane and the combined organic phases where washed with
water, dried over MgSO and the solvent was evaporated under vacuum. The crude material was
4
then purified by column chromatography on silica gel (ethyl acetate) to afford a white powder after
˝
1
solvent evaporation; m.p. 89–90 C, yield 51%. H-NMR (500 MHz, CDCl )
δ
(ppm): 1.98 (t, 1H, OH),
3
4
7
,02 (m, 2H, -CH OH), 4.15 (t, 2H, -CH -OAr), 6.27 (d, 1H, J = 9.5 Hz, enone), 6.84–6.88 (m, 2H, ArH),
.39 (d, 1H, Ar-H), 7.64 (d, 1H, J = 9.5 Hz, enone). [22]
2
2
3
.3.2. 7-[(6-Hydroxyhexyl)oxy]coumarin (1c)
6-Bromohexan-1-ol (1.57 ml; 12 mmol) was added to a mixture of 7-hydroxycoumarin
(1a, 12 mmol), anhydrous potassium carbonate (5.5 g; 40 mmol) and a catalytic amount of potassium
iodide in a round-bottomed flask with freshly distilled acetone (75 mL). The mixture was heated at
reflux for 30 h. The solid was filtered off and the solvent was evaporated to dryness under vacuum.
The crude product was purified by column chromatography on silica gel (hexane–ethyl acetate, 4:6)
1
to afford 1c as an oil after solvent evaporation; yield 89%. H-NMR (500 MHz, CDCl ):
δ 1.2–1.95
3
(
m, 8H, -CH CH CH CH -), 3.7 (t, 2H, -CH OH), 4.00 (t, 2H, -CH -OAr), 6.2 (d, 1H, =CH-C(O)-),
2 2 2 2 2 2
6
.81 (m, 2H, Ar-H), 7.4 (d, 1H, Ar-H), 7.6 (d, 1H, Ar-H).
3
.3.3. 7-(2-Hydroxyethoxy)4-methylcoumarin (2b)
A solution of 2-bromoethanol (1.36 mL, 19 mmol), 7-hydroxy-4-methylcoumarin (2a, 2.33 g,
˝
1
3 mmol), potassium carbonate (8.32 g, 60 mmol), and acetone (50 mL) was heated at 80 C for 30 h.
The cooled reaction solution was filtered and then evaporated under vacuum. The crude product was
1
purified using column chromatography on silica gel with ethyl acetate as eluent; yield 91%. H-NMR

Molecules 2016, 21, 147
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(
500 MHz, CDCl3): δ 1.27 (t, 1H, OH), 2.41 (s, 3H, -CH ), 4.03 (m, 2H, -CH OH), 4.16 (t, 2H, -CH -OAr),
3 2 2
6
.15 (d, 1H, =CH-C(O)-), 6.83–6.90 (m, 2H, ArH), 7.50 (d, 1H, Ar-H).
3
.3.4. 7-[(6-Hydroxyhexyl)oxy]-4-methylcoumarin (2c)
The same procedure as for 1c was used, but employing 7-hydroxy-4-methylcoumarin (2a). The
crude product was purified by column chromatography on silica gel (hexane–ethyl acetate, 4:6), to
˝
1
afford 2c as white crystals; m.p. 81–82 C, yield 82%. H-NMR (500 MHz, CDCl3):
δ
1.40–1.85
(
6
m, 8H, -CH CH CH CH -), 2.38 (s, 3H, -CH ), 3.66 (t, 2H, -CH OH), 4.00 (t, 2H, -CH -OAr),
.17 (d, 1H, =CH-C(O)-), 6.81 (t, 2H, Ar-H), 7.48 (d, 1H, Ar-H). MS, m/z: 276 [M] .
2
2
2
2
3
2
2
+
3
.3.5. 7-Methacryloyloxycoumarin (3a)
7
-Hydroxycoumarin (1a, 17 mmol) was dissolved in N,N-dimethylformamide (DMF, 20 mL) and
˝
triethylamine (20 mmol) was added. The reaction mixture was cooled to 0–5 C and methacryloyl
chloride (1.98 mL; 20 mmol) in DMF (5 mL) was injected gradually to the above solution. After 4 h at
reflux, the solution was poured on ice. The resulting solid product was filtered off, washed with water
and dried. Recrystallization from ethanol gave colorless plates; m.p. 146–147 C, yield 80%. H-NMR
500 MHz, CDCl3): 2.0 (s, 3H, -CH ), 5.9 (s, 1H, CH =), 6.5 (d, 1H, =CH-C(O)-), 6.35 (s, 1H, CH =),
˝
1
(
δ
3
2
2
7
.2 (d, 1H, Ar-H), 7.3 (s, 1H, Ar-H), 8.1 (d, 1H, Ar-H), 7.8 (d, 1H, Ar-H).
3
.3.6. 7-(2-Methacryloyloxyethoxy)coumarin (3b)
This compound was prepared according to [22].
3
.3.7. 7-[6-Methacryloyloxyhexyloxy]coumarin (3c)
7
-[(6-Hydroxyhexyl)oxy]coumarin (1c, 1.33 g; 4.8 mmol) was dissolved in dry methylene chloride
˝
(
30 mL). The reaction mixture was kept at 0–5 C and triethylamine (0.68 mL; 5 mmol) and methacryloyl
chloride (0.48 mL; 5 mmol) were added gradually. The solution was refluxed for 4 h, washed (1 M
hydrochloric acid, 10% sodium hydroxide) and then dried over anhydrous magnesium sulfate.
The crude oil was purified by column chromatography on silica gel using methylene chloride as
eluent; yield 50%. 1H-NMR (500 MHz, CDCl3):
δ 1.2–1.9 (m, 8H, -CH CH CH CH -), 2.0 (s, 3H,
2 2 2 2
H C=C(CH )-), 4.00 (t, 2H, -CH -OPh), 4.2 (t, 2H, -CH -O-C(O)-), 5.6 (s, 1H, CH =), 6.1 (s, 1H, CH =),
2
3
2
2
2
2
6
.2 (d, 1H, =CH-C(O)-), 6.80–6.83 (m, 2H, Ar-H), 7.4 (d, 1H, Ar-H), 7.6 (d, 1H, Ar-H).
3
.3.8. 7-Methacryloyloxy-4-methylcoumarin (4a)
Same procedure as for 3a was employed, but using 7-hydroxy-4-methylcoumarin (2a). Recrystallization
˝
1
from ethanol gave colorless plates m.p. 177 C, 85%. H-NMR (500 MHz, CDCl ):
δ
2.05 (s, 3H, -CH3),
3
2
7
.41 (s, 3H, -CH ), 5.80 (s, 1H, CH =), 6.23 (d, 1H, =CH-C(O)-), 6.35 (s, 1H, CH =), 7.09 (t, 2H, Ar-H),
.60 (d, 1H, Ar-H). MS, m/z: 244 [M] .
3
2
2
+
3
.3.9. 7-(2-Methacryloyloxyethoxy)4-methylcoumarin (4b)
Methacryloyl chloride (0.48 mL) was added dropwise to a mixture of triethylamine (0.69 mL)
˝
and 7-(2-hydroxyethoxy)4-methylcoumarin (2b) in THF (40 mL). After stirring for 12 h at 0 C, the
reaction mixture was diluted with dichloromethane and the organic layer was washed with an aqueous
solution of NaCl and dried over magnesium sulfate, to produce a solid after solvent evaporation. The
crude product was recrystallized from ethanol to afford a colorless microcrystalline solid; 80% yield.
1
H-NMR (500 MHz, CDCl3):
δ 1.96 (s, 3H, CH ), 2.4 (s, 3H, CH ), 4.28 (t, 2H, CH OR), 4.53 (t, 2H,
3 3 2
CH OAr), 5.60 (s, 1H, ethylene), 6.15 (s, 1H, ethylene; d, 1H, J), 6.8–6.9 (m, 2H, Ar-H), 7.5 (d, 1H, Ar-H).
2
3
.3.10. 7-[[[6-(Methacryloyl)oxy]hexyl]oxy]-4-methylcoumarin (4c)
This compound was prepared according to [23].

Molecules 2016, 21, 147
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3
.4. Polymerization
The copolymers P1
–
P21 were synthesized by free radical polymerization using coumarin
methacrylic monomers and methyl methacrylate (MMA) or butyl methacrylate (BMA) in 10% DMF
˝
solution with AIBN as radical initiator at 80 C (argon atmosphere). The mixture was degassed with
repeated freeze and thaw cycles and then heated for 24 h. The resulting viscous solution was added
into methanol to precipitate a polymeric material. The precipitation was repeated from DMF into
methanol to give the purified polymer. The copolymerization ratio was calculated on the basis of the
1
integration ratio of H-NMR measurements (Tables 1 and 2).
3
.5. Molecular Orientation in Thin Films by Corona Poling
The dipole moments of the chromophores are initially randomly oriented within the film and
as a result, the macroscopic second order nonlinear optical response of the material is negligible. To
overcome this problem the molecules are oriented by the corona-poling technique [26 28] as illustrated
in Figure 3. The film is heated up to a temperature slightly lower than the glass transition temperature
of the polymer and a strong electric field (4.5 kV) is applied thanks to two 30 m tungsten wires
,
µ
which are placed over the sample. The sample is kept under these conditions for ten minutes. While
keeping the electric field constant, the temperature is smoothly decreased down to room temperature,
resulting to the orientation of the dipole moments and inducing a non-centrosymmetry on the sample,
required for a second order nonlinear optical response. The uniformly poled film can be considered as
a “blank”, exhibiting homogeneous maximum SHG, so all the bits over the surface are set to “0”. Due
to the relatively high glass transition temperatures of the films, the orientation of the molecules (and
SHG values) are expected to be maintained even several months after the corona poling (6 months, as
checked in the case of P18).
Figure 3. Corona-poling setup. The substrate is a BK7 glass slide. The distance between the wire and
the film is approximately 1 cm.
3
.6. Non-Linear Microscopy Setup
A schematic representation of the setup is shown in Figure 4. The writing process (cyclodimerization
induced by two-photon absorption) and read-out process (SHG imaging) are performed using a
tunable (670–1100 nm) Ti:sapphire laser source (Spectra-Physics Tsunami, 120 fs pulse duration,
8
0 MHz repetition rate, linearly polarized) pumped by a 10 W solid-state laser (Millenia Xs, Spectra
Physics). The laser power was adjusted by means of an electro-optic modulator (Pockels cell,
Conoptics, Danbury, CT, USA). A Faraday rotator was used as an optical isolator. The angular
deviation of the infrared beam was controlled with an X-Y scanner (Cambridge Technology, Bedford,
MA, USA) and then focused on the sample by a long working distance microscope objective (Olympus
SLMPLN20X, numerical aperture = 0.25). The lateral resolution is about 1 µm for the illuminated
area. The transmitted NLO signal (SHG) is filtered with IR blocking filters (NT48-071, Edmund
Optics Ltd., York, UK, and BG39, Schott, Mainz, Germany) and detected by a fast photomultiplier
(H744P-40, Hamamatsu) coupled with a photon counter (C9744, Hamamatsu, Hirakuchi, Japan).
The samples (thin films deposited on glass microscope slides) are placed on X, Y, Z, motorized
stages (Newport, Irvine, CA, USA) and driven by a high performance 8-axes motion controller

Molecules 2016, 21, 147
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(XPS, Newport) and computer controlled in order to keep always the focal point in the plane of the
sample [24]. We have developed a Labview™ program (National Instrument, Austin, TX, USA) to
control the micro-positioning of the sample, the light polarization, the electric signals for the scanner
and the photon counter in order to store the signal for each position of the scanning laser beam on the
sample and to produce an SHG image, pixel after pixel, at the maximum frequency of 1 MHz.
Figure 4. Non-linear scanning microscopy setup used to induce localized photo-dimerization and to
detect the SHG response.
4
. Conclusions
A comparative study of several coumarin-based copolymers was performed in terms of their
efficiency for optical data storage applications. The ODS was carried out by locally modifying the SHG
response of these materials by means of two-photon absorption, while the efficiency of each film was
related with the SHG contrast between bits “0” and “1”. A relation between the molecular structure
and the ODS efficiency has been established and demonstrates the critical importance of the molecular
design. In particular, introduction of a short linker between the polymer backbone and the pendant
coumarin unit allows a better conformational freedom of the latter, which results in better contrasts
between SHG of the monomeric and the dimeric forms. This improvement is assigned to a higher
efficiency of the cyclodimerization reaction.
Supplementary Materials: Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/21/
2
/147/s1.
Acknowledgments: This work was supported by the Région des Pays de la Loire (Programs MoltechAnjou 2009
and Movamol 2011) and by OSEO and “Fonds Pays de la Loire Territoires d’Innovation” (Program Stovomol 2013).
Author Contributions: D.G and M.S. conceived and designed the study. O.K. and M.E. performed the synthesis
of all compounds and their characterization. D.G., K.I. and E.C. performed the NLO experiments. D.G. wrote
the paper. M.S., K.I, M.E. and O.K. reviewed and edited the manuscript. All authors read and approved
the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.

Molecules 2016, 21, 147
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Sample Availability: Samples of the compounds P1–P21 are available from the authors.
©
2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons by Attribution
CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
(