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Cite this: DOI: 10.1039/c5nj01299f
Surface relief grating growth in thin films of
mexylaminotriazine-functionalized glass-forming
azobenzene derivatives†
Othmane R. Bennani,
ab
Tayel A. Al-Hujran,
c
Jean-Michel Nunzi,
a
Ribal Georges Sabat
b
and Olivier Lebel*
c
Azobenzene-containing materials exhibit various photomechanical properties, including the formation of
surface relief gratings (SRG) when irradiated with two interfering laser beams. In a recent study, a novel
glass-forming derivative of Disperse Red 1 (DR1) with a mexylaminotriazine group was synthesized in high
yield with a simple and efficient procedure, and showed the ability to form high-quality amorphous thin
films with a high resistance to crystallization. Irradiation of films of this material yielded SRG with growth
rates comparable to other reported azo materials. Herein, a series of closely related molecular glasses
containing azobenzene chromophores with various absorption maxima ranging from 410 to 570 nm were
synthesized, and their physical and photomechanical properties were studied. All materials studied showed
the ability to form stable glassy phases, and irradiation with lasers emitting at various wavelengths allowed to
perform a comparative study of SRG growth within a series of analogous chromophores.
Introduction
The cis–trans isomerisation of azobenzene and its derivatives upon
irradiation with visible light is a well-documented phenomenon.
1
Typically, azobenzene derivatives can rapidly isomerize back and
forth between both forms, which, when occurring in the solid
state, leads to migration at the molecular level due to the motion
of the azo groups, eventually leading to photomechanical phenom-
ena, including the formation of surface relief gratings (SRG).
2–4
SRG typically occur when a material containing azobenzene moi-
eties is irradiated with two interfering coherent laser beams, upon
which the molecular motion causes the formation of peaks and
troughs in the surface of the material which mimic the inter-
ference pattern.
5
Several applications have been proposed for SRG,
including resonant waveguide filters,
6
plasmonic biosensors,
7
DNA-
tunable dye laser,
8
organic light emitting diodes,
9
light-harvesting
structures for photovoltaic cells,
10,11
as well as many other
applications.
12
As most simple azobenzene derivatives readily crystallize,
studies of their photophysical behavior in the solid state use
materials where the chromophores are either dispersed in or
bonded to polymers,
1,13,14
or incorporated into small mole-
cules that can readily form glassy phases.
15–17
The latter class
of materials, called molecular glasses or amorphous mole-
cular materials, possesses several advantages over polymers
linked to their smaller size and monodisperse nature, which
ensures that samples are homogeneous and constituted of a
single, pure component.
18,19
Not only does this make purifica-
tion and characterization easier, it also increases the reprodu-
cibility of their behavior. On the other hand, small molecules
tend to crystallize more easily, but with careful molecular
design it is possible to generate molecular glasses that do
not crystallize under ambient conditions.
20–23
Generally, small
molecules that possess an irregular and non-planar shape,
can adopt different conformations, and interact weakly and
indiscriminately with neighboring molecules show a higher
propensity to form glassy phases.
20
While most azobenzene
derivatives tend to crystallize, the presence of bulky bis-
(9,9-dimethylfluorenyl)amino,
15
bis(4-biphenyl)amino,
16
or
triphenylmethyl
17
groups on azo chromophores slows down
crystallization kinetics enough to allow the compounds to be
isolated in glassy form.
Mexylaminotriazines are one family of molecular glasses
that, despite defying some of the traditional structural
a
Department of Chemistry, Department of Physics, Queen’s University, Kingston,
K7L 3N6, Canada. E-mail: nunzijm@queensu.ca; Fax: +1-613-533-6669;
Tel: +1-613-533-6749
b
Department of Physics, Royal Military College of Canada, Kingston, ON, K7K 7B4,
Canada. E-mail: sabat@rmc.ca; Fax: +1-613-541-6040;
Tel: +1-613-6541-6000 ext. 6721
c
Department of Chemistry and Chemical Engineering, Royal Military College of
Canada, Kingston, ON, K7K 7B4, Canada. E-mail: Olivier.Lebel@rmc.ca;
Fax: +1-613-542-9489; Tel: +1-613-541-6000 ext. 3694
†Electronic supplementary information (ESI) available: Synthetic procedures and
spectral data for compounds 4b–4e, additional data on SRG growth for compounds 1
and 4a–4e, NMR spectra for compounds 4a–e. See DOI: 10.1039/c5nj01299f
Received (in Montpellier, France)
22nd May 2015,
Accepted 14th September 2015
DOI: 10.1039/c5nj01299f
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features shared by other molecular glasses, shows extreme
resistance to crystallization.
24
Unlike classic molecular glasses,
mexylaminotriazines show relatively small size, higher symme-
try, relatively rigid structures, and the potential to self-assemble
along pairs of hydrogen bonds. Instead, these compounds resist
crystallization because of hindered conformational equilibria
thatlimitspackinginanorderedfashion,andhydrogen
bonding to help slow their crystallization kinetics by reducing
their mobility in the solid state.
25
Derivatives incorporating
various structural elements can be easily synthesized from
cyanuric chloride, 3,5-dimethylaniline and other amines.
26
By
varying the amino substituents, it is possible to control the
physical properties of the materials (solubility, glass transi-
tion temperature (T
g
), etc.), and it is possible to introduce
reactive functional groups that can be used to anchor other
functional compounds, including chromophores.
27
This modular
approach allows the design and synthesis of novel glass-forming
materials in an efficient fashion, with high yields, and with
minimal screening.
Materials containing several azobenzene chromophores
with various absorption ranges, polarity, and steric bulk
have been studied so far, but in most cases, with different
polymer backbones or small-molecule structures.
1,13,14
Com-
parisons between different chromophores are complicated
by differences in their molecular structures that extend
beyond the chromophore. Furthermore, while azobenzene
chromophores with absorptions between 400 and 500 nm
have been extensively studied,
2–5
azobenzene derivatives can
be synthesized with absorption ranges higher than 700 nm,
28
and few studies exist on the photomechanical properties
of azobenzene chromophores with absorption maxima over
500 nm.
3,29–31
A previous report introduced a mexylaminotriazine derivative
of Disperse Red 1 (1),‡which could be easily prepared in yields
over 90% and that showed to be capable of readily forming
amorphous thin films which could yield surface relief gratings
(SRGs) upon irradiation.
32
In the present work, the same
synthetic strategy was used to generate mexylaminotriazine
glasses functionalized with various azo chromophores with a
wide range of absorption maxima spanning 410 to 570 nm.
Their photomechanical behavior was studied using different
lasers, and it was shown that while most materials studied
showed the capability to inscribe SRGs with a rate loosely
proportional to their absorbance with the corresponding lasers,
azothiazole-based chromophores showed very slow SRG
growth, even at wavelengths near their absorption maxima.
This behavior is likely due to a slow cis–trans isomerisation rate,
though it is unclear at this stage if cis–trans isomerisation is
hindered by an intrinsically high isomerisation barrier, the
formation of aggregates in the solid state, or hydrogen bonding
in the material.
Experimental section
General
4-(N-(2-Hydroxyethyl)-N-ethylamino)azobenzene (3a),
33
4-(N-(2-
hydroxyethyl)-N-ethylamino)-30,50-dichloroazobenzene (3b),
34
4-(N-
(2-hydroxyethyl)-N-ethylamino)-40-phenylazoazobenzene (3c),
34
2-[4-(N-(2-hydroxyethyl)-N-ethylamino)phenylazo]-6-nitrobenzo-
thiazole (3d)
35
and 2-[4-(N-(2-hydroxyethyl)-N-ethylamino)-
phenylazo]-5-nitrothiazole (3e)
36
were prepared according to the
literature. 2-Methylamino-4-mexylamino-6-(2-aminoethylamino)-
1,3,5-triazine (2) and Disperse Red 1 glass 1were purchased from
Solaris Chem, Inc., N-ethyl-N-(2-hydroxyethyl)aniline, aniline,
3,5-dichloroaniline, 2-amino-5-nitrothiazole and 2-amino-6-nitro-
benzothiazole were purchased from Sigma-Aldrich, 4-amino-
azobenzene hydrochloride was purchased from TCI America,
N,N0-carbonyldiimidazole (CDI) was purchased from Oakwood
Chemicals, and all solvents were purchased from Caledon Labs.
All reagents were used without further purification. Reactions
were performed under ambient atmosphere unless otherwise
specified. Glass transition temperatures were determined with
a TA Instruments 2010 Differential Scanning Calorimeter calibrated
with indium at a heating rate of 5 1Cmin
1
from 30 to either 150
(for compounds 4d–e) or 200 1C. Values were reported after an
initial heating and cooling cycle as the half-height of the heat
capacity change averaged over two heating runs. FTIR spectra were
acquired with thin films cast from CH
2
Cl
2
on KBr windows using a
Perkin-Elmer Spectrum 65 spectrometer. UV-Visible absorption
spectra were acquired using a Hewlett-Packard 8453 spectrometer.
1
H NMR spectra were acquired on a 400 MHz Bruker AV400
spectrometer at 363 K, while
13
C NMR spectra were recorded on
a 300 MHz Varian Oxford spectrometer at 298 K.
Synthesis
Synthesis of azo glass 4a. To a stirred suspension of N,N0-
carbonyldiimidazole (0.729 g, 4.50 mmol) in dry THF (5 mL) in
a dry round-bottomed flask equipped with a magnetic stirrer
was slowly added a solution of 4-(N-(2-hydroxyethyl)-N-ethyl-
amino)azobenzene (0.876 g, 3.25 mmol) in dry THF (10 mL) at
ambient temperature, then the mixture was stirred for 18 h
under nitrogen atmosphere. CH
2
Cl
2
and H
2
O were added, then
the layers were separated, and the organic layer was washed two
more times with copious amounts of H
2
O. The organic extract
was recovered, dried over Na
2
SO
4
, filtered, and the solvent was
evaporated. The crude residue was redissolved in THF (20 mL),
‡Marvin was used for drawing, displaying and characterizing chemical struc-
tures, substructures and reactions, Marvin 15.1.12, 2015, ChemAxon (http://www.
chemaxon.com).
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then 2-methylamino-4-mexylamino-6-(2-aminoethylamino)-1,3,5-
triazine(1.00g,3.48mmol)wasaddedandthemixturewas
refluxed for 18 h. The solvent was evaporated, then 1 M aqueous
HCl was added, then the precipitated product was collected by
filtration and washed with 1 M aq. HCl and H
2
Ountiltheeffluent
was colorless. The residue was dissolved in acetone, then CH
2
Cl
2
and aq. NaHCO
3
were added. The layers were separated, the
organic layer was dried over Na
2
SO
4
, filtered, and the volatiles
were thoroughly evaporated under reduced pressure to yield
1.68 g of pure glass 4a (2.89 mmol, 89%). T
g
63 1C; FTIR
(CH
2
Cl
2
/KBr) 3401, 2973, 2928, 2854, 1701, 1599, 1565, 1515,
1439, 1397, 1359, 1324, 1299, 1255, 1189, 1153, 1137, 1090,
1065, 1039, 840, 809, 767, 736, 689 cm
1
;
1
H NMR (400 MHz,
DMSO-d
6
, 363 K) d8.28 (br s, 1H), 7.78 (m, 4H), 7.52 (t,
3
J=
7.3 Hz, 2H), 7.41 (t,
3
J= 7.3 Hz, 1H), 7.41 (s, 2H), 6.88 (d,
3
J=
9.1 Hz, 2H), 6.83 (br s, 1H), 6.57 (s, 1H), 6.39 (br s, 1H), 6.31
(br s, 1H), 4.20 (t,
3
J= 5.8 Hz, 2H), 3.64 (t,
3
J= 5.8 Hz, 2H), 3.50
(q,
3
J= 7.1 Hz, 2H), 3.43 (q,
3
J= 5.8 Hz, 2H), 3.26 (q,
3
J= 5.8 Hz,
2H), 2.84 (d,
3
J= 4.5 Hz, 3H), 2.24 (s, 6H), 1.18 (t,
3
J= 7.1 Hz, 3H)
ppm;
13
C NMR (75 MHz, DMSO-d
6
)d166.5, 166.1, 164.4, 156.7,
152.9, 150.7, 142.9, 141.0, 137.4, 129.9, 129.6, 125.4, 123.1,
122.2, 117.6, 111.7, 61.5, 49.2, 45.4, 40.4, 40.4, 27.7, 21.7,
12.4 ppm; UV-Vis (CH
2
Cl
2
): l
max
(e) 413 nm (8700); HRMS (ESI)
m/z:[M+H]
+
calcd for C
31
H
39
N
10
O
2
: 605.3071, found: 605.3081.
Synthesis of compounds 4b–4e. Compounds 4b–4e were
synthesized by procedures similar to those used for compound
4a. Synthetic details can be found in ESI.†
Thin film deposition
Solutions of compounds 1and 4a–e were prepared at a concen-
tration of 3 wt% in CH
2
Cl
2
and mechanically shaken for 1 hour.
They were then filtered through 50-micron syringe filters. Thin films
were then prepared by spin-coating using a Headway Research spin-
coater: 3 mL of solution was deposited on a 3 3cm
2
BK7 glass
slide, followed by spinning at a rate of 1500 rpm for 40 seconds. After
deposition, all films of azo glasses 1and 4a–e were dried in a Yamato
ADP-21 oven at 95 1C for 10 minutes. This procedure yielded
uniform films having an average thickness around 400 nm as
measured using a Sloan Dektak II D profilometer (model 139961).
Surface relief grating writing
Thin films of azo glasses 1and 4a–e were irradiated with two
interfering beams from three different laser systems using a
Lloyd mirror set-up.
5
The first laser was a 2.5 W Argon-Ion Lexel
model 85, which emitted laser lines at 488 nm, 496.5 nm and
514 nm. The irradiance of the other lines of this Argon-Ion laser
was too low to inscribe gratings. The second laser used was a
532 nm wavelength Coherent Verdi diode-pumped laser (model
0174-525-52, 5 Watts), and the third laser was a 150 mW Jodon
He–Ne laser emitting at 632 nm. The light beam of each laser
was collimated and circularly polarized. A variable iris was used
to set the laser beam diameter at around 1 cm. The laser
irradiance was measured using a Coherent Powermax Wand.
The interference pattern by the Lloyd mirror from each laser
line resulted in sinusoidal light intensity variations along the
vertical axis. Upon exposure to the azo glass films, surface relief
gratings in the form of half discs with an area of approximately
10.5 cm
2
were recorded. The gratings’ pitch could be varied
by rotating the Lloyd mirror with respect to the writing beam,
but for this series of experiments, the pitch was kept constant
at 500 nm for all SRGs. The depth of the gratings was depen-
dent on the laser exposure time and irradiance.
The time-dependent diffraction efficiency was monitored
in situ during laser irradiation. A probe low-power He–Ne laser
was incident on the portion of the thin film where the SRG was
forming. As the grating appeared, the first order diffracted
beam from the He–Ne laser was mechanically chopped and
then incident on a silicon photodetector. The signal of the
photodetector was then measured by a lock-in amplifier and
recorded by a computer. The diffraction efficiency was obtained
by dividing the signal of the first by the zeroth diffraction order.
Atomic force microscopy
AFM scans were performed using an Ambios Q-Scope atomic force
microscope, in tapping mode with a scanning rate of 1 Hz using
40 N m
1
force constant Quesant Premounted cantilever probes.
Results and discussion
Synthesis
Azo chromophore-substituted glasses 4a–e were synthesized by
a procedure similar to the one used for Disperse Red 1 derivative
1
32
from the respective N-ethyl-N-(2-hydroxyethyl)amino azo
derivatives 3a–e
33–36
and amino-functionalized mexylaminotri-
azine derivative 2in the presence of N,N0-carbonyldiimidazole
(CDI) (Scheme 1). A solution of chromophore in THF was first
added to a suspension of CDI in THF at ambient temperature to
generate an imidazolylcarbamate intermediate, which was then
heated with glass 1in THF to give adducts 4a–e. For thiazole-
containing dyes 3d–e, the starting materials and intermediates
were much less soluble in THF than azobenzene derivatives
3a–c, therefore the reaction with CDI was performed in DMF
instead of THF, and the imidazolylcarbamate intermediates were
isolated by precipitation. Additionally, while azobenzene-based
glasses 4a–c gave very high yields (near 90%), thiazole-containing
azo glasses 4d–e were obtained in 73–74% yields, as intractable
degradation products were also obtained. In the case of
5-nitrothiazolyl derivative 4e,thoseimpuritiescouldbeconveni-
ently removed by dissolving the crude product in dichloromethane.
However, for 6-benzothiazolyl analogue 4d, those impurities were
also soluble in dichloromethane, and the product had to be purified
by filtration on a short silica pad using CHCl
3
/acetone 9 : 1. The
identity of the products could be easily confirmed by the appearance
of a carbamate CQO band near 1705 cm
1
by FTIR spectroscopy, or
by the shift of the –CH
2
–peakfrom2.8ppmto3.2ppm,andthe
disappearance of the –NH
2
and –OH peaks from both reactants, by
1
H NMR spectroscopy.
Physical properties
As with DR1-substituted analogue 1,glasses4a–e proved
capable of readily forming glassy phases with no signs of
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crystallizationuponheatingatarateasslowas0.51Cmin
1
.
Compounds 4a–c were stable up to 200 1C, while thiazole-
substituted derivatives 4d–e started showing decomposition
around 180 and 150 1C, respectively. DSC scans of compounds
4a–e are shown in Fig. 1, while T
g
values are listed in Table 1.
DSC scans shown were recorded after an initial heating scan
to erase the thermal history of the samples and during which
no melting transitions were observed. T
g
values are close to
the one previously reported for DR1 glass 1(71 1C) and showed
a slight increase with increasing polarity from 63 1Cfor
phenyl-substituted dye 4a to 73 1C for 6-nitrothiazolyl
analogue 4e. Only bis(azo) derivative 4c showed a slightly
lower T
g
value than expected (64 1C), possibly owing to the
higher flexibility of the chromophore. The polarity of the azo
chromophore thus impacts the T
g
of the material, albeit very
slightly compared to substituents on the triazine ring.
All five azo glasses reported herein show appreciable solu-
bility in various organic solvents and can be easily processed
from solution by spin-coating. The films thusly obtained are
high-quality, uniform, and devoid of crystalline imperfections
(this was confirmed by the absence of peaks by PXRD). The
monodisperse nature of the materials resulted in processing
behavior that was consistent and reproducible from one sample
to another.
UV-Visible spectra of compounds 4a–e were recorded both in
CH
2
Cl
2
solution and as thin films, and the values are reported
in Table 2. The absorption maxima for all five compounds are
very close to reported values for the precursor dyes (Fig. 2a), as
was the case for Disperse Red 1-based glass 1.
32
Absorption
maxima for the dyes in thin films were very close (within 9 nm
in each case) of their values in solution (Fig. 2b). Those shifts
are interpreted as the solvatochromic shift expected with the
polarity change from the solution (CH
2
Cl
2
) to the thin film
(triazine-glass) environment.
Photomechanical properties
Surface relief gratings were inscribed on the various thin films
as detailed previously. The diffraction efficiency of the gratings
is strongly dependent on their depth, hence, indirectly related
Scheme 1 Synthesis of azo-functionalized glasses 4a–e.
Fig. 1 DSC scans of compounds 4a–e, recorded at a heating rate of
51Cmin
1
after an initial heating scan. T
g
are indicated.
Table 1 Glasses transition temperatures (T
g
) for azo glasses 4a–e
Compound T
g
(1C)
4a 63
4b 68
4c 64
1
a
71
4d 72
4e 73
a
Ref. 32.
Table 2 Absorption bands for azo-functionalized glasses 1and 4a–e in
CH
2
Cl
2
solution and as solid thin films
Compound
l
max
(CH
2
Cl
2
)
(nm)
e(CH
2
Cl
2
)
(cm
1
M
1
)
l
max
(film)
(nm)
4a 413 8700 416
4b 435 20 000 432
4c 475 28 000 470
1
a
485 28 000 476
4d 539 24 000 538
4e 571 18 000 576
a
Ref. 32.
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to the laser exposure time and irradiance. Nonetheless, too
much irradiance too quickly can saturate the sample and destroy
the gratings, thus reducing the overall diffraction efficiency. In
order to provide an accurate comparison between the various
compounds, all samples were prepared in an identical manner
and the diffraction efficiencies were obtained at the same laser
powers. Furthermore, the measurements were conducted on
various samples under the same conditions and the results were
averaged and compiled in the plots that follow.
The diffraction efficiencies with various Argon-Ion laser
lines (488, 496.5 and 514 nm) are shown in Fig. 3. Diffraction
efficiencies are also compiled for every SRG in Table S1 (see
ESI†). The 488, 496.5 and 514 nm laser lines had irradiances of
80, 31 and 95 mW cm
2
respectively. SRGs formed on only azo
glasses 1and 4a–c at these wavelengths with azo glass 1being
much stronger than the other azo glasses 4a–c in the 488 nm
laser line. The diffraction efficiencies of glasses 4a–c were pro-
portional to their relative absorptions at the given wavelengths.
This is to be expected since azo glass 1has its absorbance
maxima closest to this wavelength. In Fig. 3b, azo glass 4c
shows the strongest diffraction signal despite having its
Fig. 2 UV-Visible spectra for azo glasses 1and 4a–e, (a) in 0.01 mM
CH
2
Cl
2
solution, and (b) as solid thin films.
Fig. 3 Diffraction efficiency as a function of exposure time for films of azo
glasses 1and 4a–c at the wavelengths (a) 488 nm, (b) 496.5 nm, and
(c) 514 nm.
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absorbance maxima around 470 nm. As the inscribing laser
wavelength increases, the diffraction signal from azo glasses
4a–b decreases despite the stronger irradiance at the 514 nm
laser line. This is because their absorbance decreases dramati-
cally around 500 nm, as seen in Fig. 2. Unexpectedly, there were
no SRGs formed in azo glasses 4d–e at any of the Argon-Ion
wavelengths. This behavior cannot be rationalized from absor-
bance alone, because the molar absorptivity coefficients (e)of
compounds 4d range between 12 000 and 20 000 M
1
cm
1
at
the three wavelengths studied, while those of compound 4e
range from 7000 to 10 000 M
1
cm
1
. Those values are similar
to higher than those for compound 4b (2300–9500), and orders
of magnitude higher than that of compound 4a (200–1000),
which both show SRG growth. A representative atomic force
microscopy (AFM) image of a SRG inscribed on compound 4c
with the 488 nm laser line and an exposure time of 240 seconds
is shown in Fig. 4. It shows a pitch of 750 nm and a grating
depth close to 200 nm.
The diffraction efficiencies of the SRGs with the 532 nm
coherent diode-pumped green laser at irradiances of 157, 344
and 521 mW cm
2
are shown in Fig. 5 for compounds 1and
4a–c, and in Fig. 6 for compounds 4d–e. As diffraction effi-
ciency is correlated linearly with grating depth, SRG heights
were measured by AFM for gratings inscribed on compounds 1
and 4a–c at 532 nm at an irradiance of 344 mW cm
2
and an
exposure time of 300 s. Grating height is plotted as a function
of diffraction efficiency in Fig. 7. It should be noted that the
maximum diffraction efficiencies obtained in this work are
different for compound 1from what has been published earlier
by our group.
32
This is because the diffraction efficiency is
strongly dependent on the probe laser polarization. Therefore,
a different polarization will yield significantly different effi-
ciency values, irrelevant of the SRG depths. Nonetheless, the
experimental set-up throughout this work was kept unchanged,
so it should provide an accurate comparison between the
different compounds presented here. Despite the extremely
small absorbance of the azo glasses 4a–b at 532 nm, there are
still SRGs forming, although at low efficiency values. As the
laser irradiance is increased, deeper gratings form within a
shorter time. However, at an irradiance of 521 mW cm
2
, the
diffraction signal from azo glasses 4a–b becomes more erratic,
Fig. 4 Atomic force microscopy (AFM) scan of a surface relief grating
written on a thin film of compound 4c with an Argon-Ion 488 nm laser line
and an exposure time of 240 s. The pitch of the SRG is 750 nm with a
200 nm depth.
Fig. 5 Diffraction efficiency as a function of exposure time for films of azo
glasses 1and 4a–c at the wavelength 532 nm and at the irradiances
(a) 157 mW cm
2
, (b) 344 mW cm
2
, and (c) 521 mW cm
2
.
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and the efficiencies decrease overall. Therefore, it can be
concluded that the optimal irradiance for these samples is
around 300 mW cm
2
, since the SRGs form quickly and exhibit
high efficiency. On the other hand, compounds 4d–e did not
form any gratings at the aforementioned irradiances, even
though the laser wavelength corresponds to the absorption
maximum of compound 4d.However,byincreasingthe
irradiance of the laser to approximately 800 mW cm
2
,
shallow SRGs with very low diffraction efficiencies appeared
as seen in Fig. 6. These gratings were barely visible to the
naked eye. The observed diffraction efficiency for compound
4e was almost ten times smaller than that of compound 4d,
a consequence of its lower absorption at the incident wave-
length.Longerirradiationtimesorhigherirradiancesresulted
in the bleaching of the films, as well as a decrease in the
diffraction efficiency.
Finally, the diffraction efficiencies of the SRGs with the
632 nm He–Ne laser at an irradiance of 76 mW cm
2
are shown
in Fig. 8. At this wavelength, only azo glasses 1and 4c form
SRGs. However, the exposure time was much longer than with
the other laser wavelengths. Arguably, the absorbance of
compounds 4a–b at this wavelength was too low to undergo
isomerisation on the scale required for SRG formation, while in
the case of compounds 4d–e, a significantly higher irradiance
would have been necessary for SRG growth.
These SRG inscription studies have shown that even though
the absorbance of the chromophore at the wavelengths used
impacts the rate of SRG formation, azo chromophores that
absorb at lower wavelengths can form SRG more efficiently. This
is likely due to the fact that rapid and continuous cis–trans
isomerisation is required for the photomechanical effect respon-
sible for SRG formation to occur,
37
and this isomerisation
reaction is influenced by various parameters, including the
activation energy and the cis–trans equilibrium constant. Photo-
isomerisation requires the absorption of photons to reach the
Fig. 6 Diffraction efficiency as a function of exposure time at the wave-
length 532 nm and at irradiances of 809 or 1509 mW cm
2
for films of azo
glasses (a) 4d, and (b) 4e.
Fig. 7 SRG height as a function of diffraction efficiency for compounds 1
and 4a–c. Heights were measured by AFM on SRG inscribed at the
wavelength 532 nm with an irradiance of 344 mW cm
2
and an exposure
time of 300 s.
Fig. 8 Diffraction efficiency as a function of exposure time for films of
azo glasses 1and 4c at the wavelength 632 nm and at an irradiance of
76 mW cm
2
.
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excited state, but chromophores that undergo isomerisation with
a higher quantum yield, faster kinetics, and with a non-negligible
cis isomer population will form SRG with less overall absorption
of light. On the other hand, the thiazole-containing chromo-
phores of compounds 4d–e show very weak SRG formation, even
at wavelengths near the absorption maximum of compound 4d.
The fact that azo chromophores bearing strongly electron-
donating substituents on one ring and strongly electron-
withdrawing substituents on the other (commonly referred to as
the push–pull effect) show higher dipoles and higher stability of the
trans isomer, has been well-documented.
31
Absorption maxima
increase as a result of the stronger conjugation involving comple-
mentary substituents, which is accompanied by a strong dipole that
stabilizes the trans isomer. The chromophore of compound 4d has
already been documented to exhibit low birefringence upon irradia-
tion in the solid state, which is indicative of cis–trans isomerisa-
tion,
38
and to relax quickly to the trans isomer. It is unclear,
however, if this behavior is due to the intrinsic high dipole of the
chromophores, stronger aggregation of the azo moieties in the solid
state, or even hydrogen bonding between thiazole nitrogen atoms
and the triazine groups. Further studies are currently underway to
investigate this behavior.
Conclusions
The strategy used to synthesize Disperse Red 1-functionalized
glass 1from Disperse Red 1 itself and a mexylaminotriazine
precursor, which is simple and efficient, was used to generate a
series of materials readily capable of forming stable glasses
containing various similar azo chromophores with absorption
maxima ranging from 410 to 570 nm. The compounds share very
closely similar molecular structures, thereby allowing for a com-
parative study of the photomechanical properties of the various
chromophores, in particular the inscription of surface relief
gratings (SRG) and their relative rates of growth under irradiation
with different wavelengths. Interestingly, for azobenzene chromo-
phores, SRG formed even at wavelengths at which the compounds
absorbed very weakly, whereas azothiazole chromophores showed
very slow SRG growth even at wavelengths near their absorption
maxima. The materials studied herein thus constitute an exciting
family of azo materials that can readily form high-quality amor-
phous thin films with varying photophysical properties. While the
azobenzene derivatives can form SRG, the Disperse Red 1 and
azothiazole derivatives are used in nonlinear optics as a result of
their strong dipoles.
Acknowledgements
The authors would like to thank the Academic Research Pro-
gramme (ARP) of RMC and the Discovery Grants Program from
the Natural Sciences and Engineering Research Council
(NSERC #327116) for funding. The authors would also like to
thank Dr Rene
´Gagnon (Universite
´de Sherbrooke) for mass
spectrometry analyses, Dr Hirohito Umezawa (Fukushima
National College of Technology) for acquisition of some AFM
scans, as well as Dr Paul Rochon (Royal Military College of
Canada) for useful discussions.
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