Holographic investigations of azobenzene-containing low-molecular-weight compounds in pure materials and binary blends with polystyrene

Article abstract of DOI:10.1002/chem.201100941

This paper reports on the synthesis and the thermal and optical properties of photochromic low-molecular-weight compounds, especially with respect to the formation of holographic volume gratings in the pure materials and in binary blends with polystyrene. Its aim is to provide a basic understanding of the holographic response with regard to the molecular structure, and thus to show a way to obtain suitable rewritable materials with high sensitivity for holographic data storage. The photoactive low-molecular-weight compounds consist of a central core with three or four azobenzene-based arms attached through esterification. Four different cores were investigated that influence the glass transition temperature and the glass-forming properties. Additional structural variations were introduced by the polar terminal substituent at the azobenzene chromophore to fine-tune the optical properties and the holographic response. Films of the neat compounds were investigated in holographic experiments, especially with regard to the material sensitivity. In binary blends of the low-molecular-weight compounds with polystyrene, the influence of a polymer matrix on the behavior in holographic experiments was studied. The most promising material combination was also investigated at elevated temperatures, at which the holographic recording sensitivity is even higher. Fast holographic response: The formation of volume gratings in photochromic low-molecular-weight compounds is investigated. The compounds consist of a central core with azobenzene-based arms attached through esterification (see image). Films of the neat compounds and binary blends of the low-molecular-weight compounds with polystyrene are investigated in holographic experiments.

Full text of DOI:10.1002/chem.201100941

DOI: 10.1002/chem.201100941
Holographic Investigations of Azobenzene-Containing Low-Molecular-
Weight Compounds in Pure Materials and Binary Blends with Polystyrene
Hubert Audorff,[a] Roland Walker,[b] Lothar Kador,*[a] and Hans-Werner Schmidt[b]
Abstract: This paper reports on the
synthesis and the thermal and optical
properties of photochromic low-molec-
ular-weight compounds, especially with
respect to the formation of holographic
volume gratings in the pure materials
and in binary blends with polystyrene.
Its aim is to provide a basic under-
standing of the holographic response
with regard to the molecular structure,
and thus to show a way to obtain suita-
ble rewritable materials with high sen-
sitivity for holographic data storage.
The photoactive low-molecular-weight
compounds consist of a central core
with three or four azobenzene-based
arms attached through esterification.
Four different cores were investigated
that influence the glass transition tem-
perature and the glass-forming proper-
ties. Additional structural variations
were introduced by the polar terminal
substituent at the azobenzene chromo-
phore to fine-tune the optical proper-
ties and the holographic response.
Films of the neat compounds were in-
vestigated in holographic experiments,
especially with regard to the material
sensitivity. In binary blends of the low-
molecular-weight compounds with
polystyrene, the influence of a polymer
matrix on the behavior in holographic
experiments was studied. The most
promising material combination was
also investigated at elevated tempera-
tures, at which the holographic record-
ing sensitivity is even higher.
Keywords: azo compounds · binary
blends · holography · photochrom-
ism
· low-molecular-weight com-
pounds
Introduction
is parallel to the transition dipole moment, into the plane
perpendicular to the polarization axis of the incident light.
Electronic excitation is then no longer possible (“orienta-
tional hole burning”).[7] The resulting effects such as pro-
nounced birefringence and the formation of a refractive-
index modulation in the bulk of the sample upon holograph-
ic illumination were mainly investigated in polymer systems,
in which the azobenzene chromophores are usually cova-
lently linked as side groups.[8–13] Azobenzene chromophores
were also introduced into low-molecular species to yield
photochromic molecular glasses.[1,3] The focus of holographic
studies on these materials has mainly been the formation of
surface relief gratings (SRGs).[14–19] In films of the investigat-
ed compounds, SRGs can form very efficiently.[3,16,20] By
choosing appropriate polarization configurations of the
holographic writing beams, however, SRG formation can be
suppressed and the inscription of pure phase gratings in the
bulk is possible. An important difference relative to their
polymeric counterparts is that low-molecular materials are
expected to exhibit faster photophysical response because of
the absence of polymer chain entanglements.[20,21] So far,
only few reports on holographic volume gratings in amor-
phous low-molecular azobenzene compounds have been
published,[22,23] although they are promising candidates for
many potential applications such as optical switches, security
devices, and holographic data storage.
Low-molecular-weight materials, especially molecular
glasses, are a new class of materials that are the subject of
intense current studies, mainly with respect to their electron-
ic, optical, and optoelectronic properties. Like polymers,
they are characterized by their glass transition temperature
(Tg) and form a stable amorphous phase.[1–3] In contrast to
functional polymers,[4–6] low-molecular-weight materials pos-
sess a well-defined molecular structure, no molecular-weight
distribution, and no undefined or undesired end groups,
which results in more uniform physical properties. There-
fore, low-molecular glasses are ideal candidates for compa-
rative investigations.
Another subject of current interest are photochromic
compounds that carry azobenzene groups. Upon illumina-
tion with polarized light, the azobenzene moiety undergoes
multiple reversible trans–cis–trans isomerization cycles,
which finally reorient the long axis of the trans form, which
[a] Dr. H. Audorff, Prof. L. Kador
University of Bayreuth
Bayreuth Institute of Macromolecular Research (BIMF)
Universitꢀtsstrasse 30, 95440 Bayreuth (Germany)
Fax : (+49)921-55-3250
E-mail: lothar.kador@uni-bayreuth.de
[b] R. Walker, Prof. H.-W. Schmidt
In the present paper, holographic experiments on photo-
chromic low-molecular-weight materials are reported. Thin
films of the pure materials and of blends with polystyrene
were investigated. Holographic volume gratings in materials
University of Bayreuth
Macromolecular Chemistry I and
Bayreuth Institute of Macromolecular Research (BIMF)
Universitꢀtsstrasse 30, 95440 Bayreuth (Germany)
12722
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12722 – 12728

FULL PAPER
Benzene-based 1,3,5-trisester 1b: 1,3,5-Tri(carboxylic acid chloride)ben-
zene (1) (0.50 g, 1.9 mmol) was coupled with 4-[(4-methoxy)phenylazo]-
phenol (1.42 g, 6.2 mmol) according to GPM 1 to yield the title com-
with different azobenzene chromophores and various cores
were inscribed and studied with a view toward their holo-
graphic sensitivity. The temporal behavior of the formation
and decay of the refractive-index gratings at ambient condi-
tions and elevated temperatures is discussed.
1
pound as an orange powder (0.84 g, 53%) . H NMR (250 MHz, CDCl3):
ꢀ
d=3.91 (s, 9H; CH3), 7.02 (d, J=3 Hz, 6H; Ar H), 7.43 (d, J=3 Hz,
ꢀ
ꢀ
6H; Ar H), 7.94 (d, J=3 Hz, 6H; Ar H), 8.01 ppm (d, J=3 Hz, 6H;
ꢀ
Ar H); elemental analysis calcd (%) for C48H36N6O9: C 68.58, H 4.32, N
9.99; found: C 68.77, H 4.45, N 9.67.
Cyclohexane-based 1,3,5-trisester 2a: 1,3,5-Cyclohexanetricarbonyl chlo-
ride (2) (0.75 g, 2.8 mmol) was coupled with 4-(phenylazo)phenol (1.81 g,
9.1 mmol) according to GPM 1 to yield the title compound as an orange
powder (1.14 g, 55%) . 1H NMR (250 MHz, CDCl3): d=1.84–1.99 (m,
3H; CH), 2.67–2.76 (m, 3H; CH), 2.79–2.89 (m, 3H; CH), 7.21 (d, J=
Experimental Section
Materials and methods: All starting materials were commercially avail-
able and were used without further purification. Dry toluene and dry tet-
rahydrofuran (THF) were heated at reflux over potassium. All synthe-
ꢀ
ꢀ
3 Hz, 6H; Ar H), 7.45–7.48 (m, 9H; Ar H), 7.87 (dd, J=3 Hz, J=1 Hz,
ꢀ
ꢀ
sized compounds were identified by 1H NMR spectroscopy using
a
6H; Ar H), 7.94 ppm (d, J=3 Hz, 6H; Ar H); elemental analysis calcd
(%) for C45H36N6O6: C 71.42, H 4.79, N 11.10; found: C 69.71, H 5.00, N
11.16.
Bruker AC250 spectrometer (250 MHz). To confirm purity, SEC meas-
urements were employed by using a Waters model 515 HPLC pump with
a UV detector at 254 nm (Waters model 486) and a differential RI detec-
tor (Waters model 410). The column setup consisted of a guard column
(PSS: SDV gel; length 5 cm, diameter 0.8 cm, particle size 5 mm, pore
size 100 ꢂ) and two separation columns (PL: PL gel; length 60 cm, diam-
eter 0.8 cm, particle size 5 mm, pore size 100 ꢂ). Stabilized THF at a con-
stant flow rate of 0.5 mLminꢀ1 was used as eluent. Characterization of
the thermal properties was carried out by differential scanning calorime-
try measurements using a Perkin–Elmer DSC7 (standard heating rate:
10 Kminꢀ1) using approximately 10 mg of the compounds in 40 mL pans.
Verification of the data obtained from the DSC was performed using an
optical microscope with crossed polarizers (Nikon Diaphot 300) on a
heating table (Mettler FP82 with Mettler FP80 control unit). UV/Vis ab-
sorption spectra of the compounds were recorded in the range from 250
to 650 nm using a Hitachi U-3000 spectral photometer. Elemental analy-
sis was carried out using a Euro EA 3000 analyzer (Hekatech).
Cyclohexane-based 1,3,5-trisester 2b: 1,3,5-Cyclohexanetricarbonyl chlo-
ride (2) (0.60 g, 2.2 mmol) was coupled with 4-[(4-methoxy)phenylazo]-
phenol (1.66 g, 7.3 mmol) according to GPM 1 to yield the title com-
1
pound as an orange powder (1.10 g, 59%) . H NMR (250 MHz, CDCl3):
d=1.95–2.00 (m, 3H; CH), 2.73–2.78 (m, 3H; CH), 2.85–2.92 (m, 3H;
ꢀ
CH), 3.92 (s, 9H; CH3), 7.04 (d, J=3 Hz, 6H; Ar H), 7.29 (d, J=3 Hz,
ꢀ
ꢀ
6H; Ar H), 7.92–7.98 ppm (m, 12H; Ar H); elemental analysis calcd
(%) for C48H43N6O9: C 67.99, H 5.11, N 9.91; found: C 67.95, H 5.11, N
9.78.
Triphenylamine-based 4,4’,4’’-trisester 3a: The synthesis and characteriza-
tion have already been reported elsewhere.[21]
Triphenylamine-based 4,4’,4’’-trisester 3b: The synthesis and properties
have already been reported elsewhere.[21]
Spirobischroman-based 6,6’,7,7’-tetraester 4a: 6,6’,7,7’-Tetrahydroxy-
4,4,4’,4’-tetramethyl-2,2-spirobischroman (4) (0.90 g, 2.4 mmol) was cou-
pled with 4-(phenylazo)benzoyl chloride (2.60 g, 10.6 mmol) according to
GPM 2 to yield the title compound as an orange powder (0.79 g, 27%) .
1H NMR (250 MHz, CDCl3): d=1.45 (s, 6H; CH3), 1.68 (s, 6H; CH3),
General preparation method (GPM 1) for ester coupling of tris-carboxyl-
ic acids: The corresponding tris-carboxylic acids were placed in a flame-
dried Schlenk flask and thionylchloride (10 mL) was added. The suspen-
sion was stirred for 24 h under reflux at 708C. During this time the car-
boxylic acids slowly dissolved. Thereafter the thionylchloride was dis-
tilled off and the crude product was used without further purification.
The obtained carboxylic acid chlorides (1 equiv) were dissolved in tolu-
ene (150 mL) under an argon atmosphere. A threefold molar excess
amount of anhydrous triethylamine per acid chloride group and the cor-
responding azobenzene compound (1.1 equiv) for each acid chloride
group were added. The dark-red solution was stirred for 24 h under
reflux at 1108C. The solution was filtered, washed with additional tolu-
ene, and thereafter the solvent was distilled off. The crude product was
dissolved in THF and precipitated in EtOH. The precipitate was filtered
using a frit and dried in high vacuum. Remaining impurities were re-
moved by a second precipitation in EtOH.
ꢀ
ꢀ
2.15 (q, J=6 Hz, 4H; CH2), 6.87 (s, 2H; Ar H), 7.37 (s, 2H; Ar H),
ꢀ
ꢀ
7.48–7.50 (m, 12H; Ar H), 7.85–7.92 (m, 16H; Ar H), 8.18–8.24 ppm
ꢀ
(m, 8H; Ar H); elemental analysis calcd (%) for C73H56N8O10: C 72.75,
H 4.68, N 9.30; found: C 72.99, H 4.69, N 9.29.
Spirobischroman-based 6,6’,7,7’-tetraester 4b: 6,6’,7,7’-Tetrahydroxy-
4,4,4’,4’-tetramethyl-2,2-spirobischroman (4) (0.23 g, 0.6 mmol) was cou-
pled with 4-[(4-methoxy)-phenylazo]benzoyl chloride (0.75 g, 2.7 mmol)
according to GPM 2 to yield the title compound as an orange powder
(0.40 g, 49%) . 1H NMR (250 MHz, CDCl3): d=1.46 (s, 6H; CH3), 1.69
(s, 6H; CH3), 2.17 (q, J=6 Hz, 4H; CH2), 3.93 (s, 12H; CH3), 6.88 (s,
ꢀ
ꢀ
ꢀ
2H; Ar H), 7.01 (d, J=3 Hz, 6H; Ar H), 7.38 (s, 2H; Ar H), 7.86 (d,
ꢀ
ꢀ
J=3 Hz, 6H; Ar H), 7.92 (d, J=3 Hz, 6H; Ar H), 8.20 ppm (d, J=
ꢀ
3 Hz, 6H; Ar H); elemental analysis calcd (%) for C77H64N8O14: C
69.78, H 4.87, N 8.45; found: C 69.48, H 5.04, N 7.97.
General preparation method (GPM 2) for ester coupling of tetraols: In a
flame-dried Schlenk flask the corresponding tetraols (1 equiv) were dis-
solved in THF (150 mL) under an argon atmosphere. A threefold molar
excess amount of anhydrous pyridine per hydroxyl group and the corre-
sponding azobenzene acid chloride compound (1.1 equiv) per hydroxyl
group were added. The dark-red solution was stirred for 24 h under
reflux at 708C. The solution was filtered, washed with additional THF,
and thereafter the solvent was distilled off. The crude product was dis-
solved in THF and precipitated in EtOH. The precipitate was filtered
using a frit and dried in high vacuum. Remaining impurities were re-
moved by a second precipitation in EtOH.
Sample preparation: Thin films of the neat materials were prepared by
spin-coating a filtered solution of 6 wt% of the compound in THF at
1000 rpm for 60 s on commercially available glass substrates. To produce
samples of the materials blended with polystyrene, a filtered solution of
3 wt% low-molecular-weight compound and 3 wt% PS (BASF 165H) in
THF was spin-coated at 1000 rpm for 60 s. The film thickness was mea-
sured using a Dektak profilometer (Veeco 3300ST). All samples showed
good optical quality and absence of light scattering.
Setup for holographic experiments: Holographic experiments were per-
formed with the setup described in the literature.[24] The gratings were in-
scribed with the 488 nm line of an Ar+ ion laser. A beam splitter gener-
ated two coherent s-polarized beams, which were superimposed in the
plane of the sample. The beam diameter was about 1.4 mm and the inten-
sity of each beam was adjusted to 1 Wcmꢀ2. The angles of incidence with
respect to the surface normal were ꢁ148, thereby resulting in a grating
period of 1 mm. An s-polarized diode laser beam at 685 nm was used for
monitoring the diffraction efficiency (DE) in situ without affecting the
writing process. The signals of the transmitted and the diffracted beam
Synthesis of low-molecular-weight materials
Benzene-based 1,3,5-trisester 1a: 1,3,5-Tri(carboxylic acid chloride)ben-
zene (1) (0.50 g, 1.9 mmol) was coupled with 4-(phenylazo)phenol
(1.23 g, 6.2 mmol) according to GPM 1 to yield the title compound as an
orange powder (0.57 g, 40%). 1H NMR (250 MHz, CDCl3): d=7.45–7.48
ꢀ
ꢀ
(m, 15H; Ar H), 7.94 (dd, J=3 Hz, J=1 Hz, 6H; Ar H), 8.06 (d, J=
ꢀ
ꢀ
3 Hz, 6H; Ar H), 9.31 ppm (s, 3H; Ar H); elemental analysis calcd (%)
for C45H30N6O6: C 71.99, H 4.03, N 11.19; found: C 71.15, H 4.37, N
11.31.
Chem. Eur. J. 2011, 17, 12722 – 12728
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www.chemeurj.org
12723

L. Kador et al.
were measured with two photodiodes. From the diffraction efficiency, the
refractive-index modulation was calculated according to Kogelnikꢃs
theory.[25] The temperature-dependent measurements were performed in
a similar way with the sample placed in a temperature-controlled box of
anodized aluminum. Its temperature could be adjusted between 20 and
1208C with an accuracy of 18C.
low-molecular-weight compounds described in the literature
possess a widely conjugated p-electron system that also
comprises the azobenzene chromophore. In contrast, the
conjugation in the molecular glass formers presented here is
largely interrupted by the ester linkage. Two absorption
maxima can be distinguished in the absorption spectra. The
maximum at shorter wavelengths is assigned to an intense
p–p* transition and the second one to a weak n–p* transi-
tion (see Table 1). The exact wavelengths of both transitions
could be influenced by the core compound. For the materi-
als 3a and 3b, the maximum of the p–p* transition is
strongly redshifted, since the absorption of the triphenyla-
mine core overlaps that of the azobenzene chromophore.
However, the transition maxima could shift because of the
location of the carbonyl unit in relation to the chromophore
or its substituent. For all compounds with chromophore b,
the methoxy substituent leads to a redshift of the p–p* ab-
sorption maxima and a blueshift of the n–p* maxima.
Results and Discussion
Thermal and optical properties of the azobenzene-contain-
ing low-molecular-weight compounds: The materials de-
scribed in this work are based on a central core unit func-
tionalized with azobenzene chromophores. Four different
core compounds were chosen to cover a broad spectrum of
glass transition temperatures. The azobenzene side arms
were linked to the cores through esterification, which pro-
vides easy access to a large variety of compounds. Further-
more, azobenzene moieties with different substituents can
be readily introduced to study their effects on the optical
and photophysical properties. The influence of substitution
at the azobenzene moiety on the photoresponsive properties
of the whole molecule has only been studied in few reports
so far.[3,26,27]
The thermal behavior of all neat compounds was investi-
gated with common methods such as DSC and TGA. In ad-
dition, the solid-state properties of all compounds were con-
firmed by polarizing microscopy. A summary of the thermal
and optical properties is given in Table 1.
The materials discussed in this work have glass transition
temperatures in the range of 54–1138C. The changes of the
glass transition temperature due to the methoxy substituents
at the azobenzene moiety are minor; the Tg values of all
compounds with chromophore b differ by no more than 48C
from those of the compounds with chromophore a. Com-
pound 3a shows a polymorphic melting behavior that is
often found in low-molecular-weight compounds.[4,28] With
the exception of 1a and 2a, the compounds vitrify upon
cooling without crystallization; therefore, six of the eight
materials can be considered molecular glasses.
Photophysical properties of the azobenzene-containing low-
molecular-weight materials: A comprehensive investigation
of the photophysical response by holographic methods was
conducted on thin films that consisted of the pure photoac-
tive compounds (see Scheme 1) and of blends with polystyr-
ene. The applied ss polarization of the writing beams is
known to produce surface relief gratings (SRGs) very inef-
fectively.[20,29] Furthermore, the irradiation times were far
too short to form SRGs with a reasonable amplitude. There-
fore, it was possible to investigate pure volume gratings. The
growth of the refractive-index modulation can be sufficiently
fitted with a stretched-exponential function, which is com-
monly used to describe the kinetics in disordered systems
[Eq. (1)]:[30,31]
ꢄ
ꢂ ꢀ
ꢁ
ꢃꢅ
0:9
t
t1
ð1Þ
nlðtÞ ¼ nl max 1 ꢀ exp ꢀ
Here n1max is the maximum achievable refractive-index mod-
ulation and t1 is the time constant of the buildup. The neces-
sity of introducing a stretching exponent b<1 indicates a
size distribution of the voids of free volume around the
chromophores. Since the expo-
The optical properties of all compounds were investigated
by UV/Vis spectroscopy in solution. Most photoresponsive
Table 1. Data of the thermal and optical properties of the synthesized azobenzene-containing low-molecular-
nent turned out to be the same
for all fits, we can assume that
the size distribution is roughly
independent of the substituent
and the core. The buildup of
the refractive-index modulation
results from the formation of a
transient cis–trans population
lattice and an orientation lattice
of the thermally stable trans
form of azobenzene.
weight compounds.[a]
[d]
First heating[b]
Second heating[b]
l
(p–p*max
)
l
(n–p*max
)
OD per mm
Tm [8C]
Tc [8C]
Tg [8C]
Tc [8C]
Tm [8C]
[nm][c]
[nm][c]
1a
1b
2a
2b
3a
3b
4a
4b
247
253
185
190
160, 175
127
211
206
125
n.d.
n.d.
n.d.
n.d.
n.d.
74[e]
70
102
104
97
247
251
163
185
n.d.
n.d.
257
n.d.
324
352
324
351
359
366
326
358
438
432
438
433
438
434
449
442
0.10
0.37
0.08
0.09
0.16
0.18
0.08
0.19
54[e]
58
98
92
89
109
113
n.d.
n.d.
212
n.d.
257
n.d.
[a] Tm, melting temperature; Tc, crystallization temperature; Tg, glass transition temperature; l
ACHTUGNRTNE(NUNG p–p*max),
The decay of the refractive-
index modulation after turning
the writing laser off shows a
wavelength of maximum of the p–p* transition; l(n–p*max), wavelength of maximum of the n–p* transition;
AHCTUNGTRENNUNG
OD per mm, optical density per micrometer thickness; n.d., not distinguished. [b] Determined by DSC with
heating and cooling rates of 10 Kminꢀ1 under N2. [c] Measured in 10ꢀ5 molar solution in CHCl3. [d] Deter-
mined from thin films (for preparation see the Experimental Section). [e] Obtained after quenching the more complex behavior than
sample.
12724
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Chem. Eur. J. 2011, 17, 12722 – 12728

Azobenzene-Containing Compounds
FULL PAPER
pffiffi
@
h
@t
ð3Þ
S ¼
I0d
in which h is the diffraction ef-
ficiency, I0 the laser intensity,
and d the sample thickness.[33]
The sensitivity depends mainly
on the writing time and the cor-
responding
refractive-index
modulation. It is usually mea-
sured at the beginning of the
writing process, at which point
it has its maximum value Smax
.
For a systematic comparison
of all eight materials, they were
blended with 50 wt% of poly-
styrene to obtain films of suffi-
cient optical quality, since 1a
and 2a did not form scatter-free
films in neat form. The growth
and decay curves of the refrac-
tive-index modulation could be
fitted with Equations (1) and
(2), respectively. As a typical
Scheme 1. Schematic illustration of the photo-addressable azobenzene-containing low-molecular-weight com-
pounds.
the growth. It can be fitted with two stretched-exponential
functions [Eq. (2)]:
example, these curves are shown in Figure 1 for the blend of
2b.
The results of the fits are listed in Table 2. The compari-
son of the blends with chromophore a shows that 2a exhib-
its the highest sensitivity and the shortest time constant t1.
It is known that a polar end group at the azobenzene
moiety leads to increased absorption of the molecule,[34,35] so
the transition dipole moment can be influenced by slight
modifications of the molecular structure. Different substitu-
ents at the azobenzene chromophore also influence the
growth of phase gratings in the bulk.[36,37] All materials with
chromophore b show a shorter buildup time t1 as well as a
higher refractive-index modulation and sensitivity than the
corresponding materials with chromophore a.
The comparison of the decay times t2 of the unsubstituted
materials reveals a correlation with Tg. Materials with me-
thoxy substituent have longer decay time constants than
their unsubstituted counterparts, which indicates stronger
cooperative effects between the chromophores due to the
larger dipole moment. Shirota reported an increased cis
population in materials without a substituent at the para po-
sition relative to those with a CH3 substituent.[1] The fit pa-
rameter A, which indicates the relative contribution of the
cis–trans population lattice to the total refractive-index
modulation, is also a measure of the overall cis concentra-
tion. Unsubstituted samples indeed feature higher A values
than those with methoxy substituent.
ꢄ
ꢂ
ꢃ
ꢂ ꢀ
ꢁ
ꢃꢅ
ꢆ
ꢇ
0:4
0:25
t
1s
t
t2
nlðtÞ ¼ nl max A exp ꢀ
þ ð1 ꢀ AÞ exp ꢀ
ð2Þ
The fast relaxation component with amplitude A has a time
constant of 1 s in all measurements; the slower one has an
amplitude 1 - A and the variable time constant t2, which is
longer than one second by several orders of magnitude. The
fast process of decay has a contribution of less than 28%.
We interpret it to be the thermal relaxation of the metasta-
ble cis isomers. In solution, in which the back relaxation of
the cis to the trans form is usually measured, the cis state
has much longer lifetimes on the order of minutes. Accord-
ing to Shirota, a solid environment in which the cis isomers
are trapped in constrained geometries fosters a faster relaxa-
tion.[3] Also, interactions between adjacent molecules, which
increase the thermal cis–trans relaxation rate, are possi-
ble.[32]
The slow relaxation process is assigned to a thermally in-
duced reorientation of the trans-azobenzene moieties. We
recorded the decay for at least one day and found that the
refractive-index modulation is not stable at longer times but
eventually decays to zero. This decay to zero can be ex-
plained by the relatively small degree of the initial anisotro-
py induced by the holographic light grating.
The maximum achievable sensitivity Smax was also mea-
sured for the neat compounds with methoxy substituent at
the azobenzene chromophore, that is, 1b, 2b, 3b, and 4b.
The materials with chromophore b were chosen, since the
blends with polystyrene had already shown that they exhibit
higher sensitivities than their counterparts with the unsubsti-
An important parameter for the characterization of holo-
graphic materials is the recording sensitivity S. It describes
the slope of the square root of the holographic growth curve
and can be calculated with the formula [Eq. (3)]:
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L. Kador et al.
Figure 1. Buildup (left) and decay (right) of the refractive-index modulation of a volume phase grating inscribed in a film of 2b blended with 50 wt%
polystyrene. Symbols represent the experimental data, whereas the smooth curves represent the fits with Equations (1) and (2), respectively. The holo-
gram was inscribed at negative times and the writing laser was turned off at t=0. Note the logarithmic time scale of the decay data.
and the undiluted samples. Blending with polystyrene does
not seem to affect the reorientation of the azobenzene chro-
mophores. Hence, one can conclude that the photophysical
properties in the blends are almost the same as in the neat
materials and the comparison between the blends is also
valid for the latter.
Table 2. Summary of the experimental data for the different photo-ad-
dressable low-molecular-weight materials in blends with 50 wt% polystyr-
ene.[a]
Tg [8C] n1max
A
Buildup t1 [s] Decay t2 [s] Smax [cmJꢀ1
]
1a 74
1b 70
2a 54
2b 58
3a 92
3b 89
4a 109
4b 113
0.0046 0.17 0.29
0.0028 n.d. 0.14
0.0032 0.28 0.10
0.0038 0.08 0.09
0.0027 0.19 0.91
0.0056 0.10 0.32
0.0028 0.14 0.28
0.0025 0.13 0.11
0.7ꢄ106
n.d.
366
536
647
846
66
381
225
518
0.2ꢄ106
0.2ꢄ106
5ꢄ106
The values of n1max are smaller than in polymers that bear
azobenzene side groups at a comparable concentration. A
quantitative comparison is difficult, since polymers with
such high azobenzene concentrations and flexible spacers
tend to have liquid-crystalline properties. The decoupling of
the chromophore by a flexible spacer—both in polymers
and in low-molecular-weight compounds—usually favors the
formation of a liquid-crystalline phase and therefore an im-
proved in-plane orientation, a higher refractive-index modu-
lation, and a much better stability of the inscribed gra-
tings.[38] On the other hand, the lack of a spacer in the low-
molecular-weight compounds, along with the absence of
chain entanglements typical for polymers, results in substan-
tially faster writing times. Azobenzene-containing block co-
polymers are a promising material class for rewritable holo-
graphic data storage, but their sensitivities are low.[39,40] The
sensitivity of the neat molecular glass 2b is larger than
1800 cmJꢀ1, which is higher by a factor of five than the
values of block copolymers with a comparable optical densi-
ty per mm thickness. The time constant of hologram inscrip-
tion has been improved by a factor of 10.[39] Therefore, the
low-molecular-weight compounds presented here are an in-
teresting material class and may be used for blending with
photo-addressable polymers to improve the photophysical
response, especially the sensitivity, of the latter.
19ꢄ106
6ꢄ106
1.2ꢄ106
[a] Tg: glass transition temperature measured for the neat material. The
other symbols pertain to Equations (1), (2), and (3).
tuted chromophore a. Films of the neat compounds were
studied in addition to the blends, since they are expected to
possess higher sensitivities. Stable amorphous films were ob-
tained by spin-coating from solution, and they remained
amorphous unless they were heated above their glass transi-
tion temperature. No crystallization was detected even after
months when stored at room temperature. The growth and
decay curves of the refractive-index modulation could be
satisfactorily fitted with Equations (1) and (2), respectively.
The results of the fits are listed in Table 3. The refractive-
index modulation and the sensitivity are twice as high as in
the blends, which is expected, since the concentration of
azobenzene has also increased by a factor of two. The time
constants of the buildup were comparable for the diluted
Table 3. Summary of the experimental data for the different photo-ad-
dressable low-molecular-weight compounds with substituent b as mea-
sured in the neat materials.[a]
Holographic experiments at elevated temperatures: Holo-
graphic measurements on compound 2b diluted in polystyr-
ene were also performed at elevated temperatures. The
growth curves of the refractive-index modulation could
again be fitted with Equation (1). The time constant t1
shortened with increasing temperature, as Figure 2 shows.
Moderate heating to 508C resulted in a buildup with a time
constant that was three times faster. This result can be as-
Tg [8C]
n1max
A
Buildup t1 [s] Decay t2 [s] Smax [cmJꢀ1
]
1b
2b
3b
70
58
89
0.0112 0.19
0.0078 0.10
0.0103 0.10
0.0047 0.12
0.24
0.10
0.27
0.12
3ꢄ104
1ꢄ106
2ꢄ106
4ꢄ104
1102
1843
901
4b 113
925
[a] Tg: glass transition temperature. The other symbols pertain to Equa-
tions (1), (2), and (3).
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Chem. Eur. J. 2011, 17, 12722 – 12728

Azobenzene-Containing Compounds
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This work was financially supported by the German Science Foundation
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Received: March 27, 2011
Published online: September 29, 2011
12728
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Chem. Eur. J. 2011, 17, 12722 – 12728