Star-shaped molecules containing both azo chromophores and carbazole units as a new type of photoresponsive amorphous material

Article abstract of DOI:10.1039/c3tc30536h

We synthesized a series of star-shaped molecules containing both azo chromophores and carbazole units (nCz-AZ-X), where n is the number of carbazole units (n = 3 and 6) and X represents cyano (CN) and nitro (NT) as the electron-withdrawing groups on the azobenzenes. The azo compounds existed as amorphous solids at room temperature with glass transition temperatures (T _g) of 89, 86, 74 and 73 °C for 3Cz-AZ-CN, 3CZ-AZ-NT, 6Cz-AZ-CN and 6Cz-AZ-NT, respectively. Thin solid films of the azo molecular glasses were obtained by spin-coating. The formation of the photoinduced self-structured surface patterns was investigated by irradiating the solid thin films of the azo molecular glasses with a uniform laser beam (532 nm, 200 mW cm^-2) at normal incidence. The formation of surface-relief-gratings (SRGs) was studied by exposing the thin films to an interference pattern of the laser beams (532 nm, 80 mW cm^-2). The formation of both the self-structured surface patterns and SRGs showed a close correlation with the electron-withdrawing groups of the azo chromophores and the content of the carbazole units in the molecules. The development of these new star-shaped molecules can add a new member to the category of azo molecular glasses and lead to a deeper understanding of the photoinduced effects and their correlation with molecular structures. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3tc30536h

Journal of
Materials Chemistry C
View Article Online
View Journal
PAPER
Star-shaped molecules containing both azo
chromophores and carbazole units as a new type of
photoresponsive amorphous material
Cite this: DOI: 10.1039/c3tc30536h
*
Jianjun Yin, Gang Ye and Xiaogong Wang
We synthesized a series of star-shaped molecules containing both azo chromophores and carbazole units
(nCz-AZ-X), where n is the number of carbazole units (n ¼ 3 and 6) and X represents cyano (CN) and nitro
(NT) as the electron-withdrawing groups on the azobenzenes. The azo compounds existed as amorphous
solids at room temperature with glass transition temperatures (Tg) of 89, 86, 74 and 73 ꢀC for 3Cz-AZ-CN,
3CZ-AZ-NT, 6Cz-AZ-CN and 6Cz-AZ-NT, respectively. Thin solid films of the azo molecular glasses were
obtained by spin-coating. The formation of the photoinduced self-structured surface patterns was
investigated by irradiating the solid thin films of the azo molecular glasses with a uniform laser beam
(532 nm, 200 mW cmꢁ2) at normal incidence. The formation of surface-relief-gratings (SRGs) was
studied by exposing the thin films to an interference pattern of the laser beams (532 nm, 80 mW cmꢁ2).
The formation of both the self-structured surface patterns and SRGs showed a close correlation with the
electron-withdrawing groups of the azo chromophores and the content of the carbazole units in
the molecules. The development of these new star-shaped molecules can add a new member to the
category of azo molecular glasses and lead to a deeper understanding of the photoinduced effects and
their correlation with molecular structures.
Received 21st March 2013
Accepted 17th April 2013
DOI: 10.1039/c3tc30536h
www.rsc.org/MaterialsC
pattern formation is another type of light-induced surface
modulation.17 It is characterized by the spontaneous formation
1 Introduction
In the past decades, azo polymers (polymers containing
aromatic azo chromophores) have attracted considerable
attention for their interesting properties and various applica-
tions.1–8 When irradiated with light at appropriate wavelengths,
the azobenzene moieties in the polymers undergo reversible
trans–cis isomerization, which triggers a variety of photo-
responsive variations.4–8 Photoinduced surface pattern forma-
tion includes some of the most interesting properties of azo
polymers.9,10 Surface-relief-grating (SRG) formation refers to the
reversible surface modulation on azo polymer lms when irra-
diated with interfering laser beams.5–7,9,10 Sinusoidal surface
patterns with modulated depth on the submicrometer scale can
be inscribed on azo polymer lms at a temperature well below
the glass transition temperatures (Tgs) of the polymers and
erased by heating the samples to a temperature above their
Tgs.6,9,10 Photoinduced SRG formation has been intensively
studied for its potential application in areas such as informa-
tion storage, diffractive optical elements, sensors and actua-
tors.5–8 Although it has been actively studied in recent years, the
mechanism of SRG formation still remains a puzzling issue
requiring further elucidation.11–16 Self-structured surface
of submicrometer hexagonal patterns on the azo polymer lms
aer irradiation with a uniform single laser beam at normal
incidence. The patterns comprise regularly spaced pillar-like
structures with a saturated height of about 100 nm.17,18
Compared with the extensive exploration of the SRGs, the
research effort devoted to the photoinduced self-structured
surface pattern formation is rather limited and even less is
known about its formation mechanism.
Amorphous molecular materials, also known as “molecular
glasses”, are low molecular-weight organic compounds that
exist as stable amorphous materials near and above room
temperature.19,20 By vapor deposition or spin-coating, trans-
parent amorphous solid lms of these materials can be
prepared. In recent years, incorporating azo chromophores into
the amorphous molecular materials has offered a promising
new possibility for developing photoresponsive materials.21–23
Compared with azo polymers, the amorphous molecular
materials containing azo chromophores (azo molecular glasses)
can show several attractive characteristics such as high chro-
mophore density and absence of chain entanglement.19b More-
over, the well-dened structures of azo molecular glasses enable
them to be an ideal candidate for mechanism study. Several
types of azo molecular glasses have been developed through
different molecular design motifs and synthetic methods.19–30
SRGs can be inscribed on an azo molecular glass lm with a fast
Department of Chemical Engineering, Key Laboratory of Advanced Materials (MOE),
Tsinghua University, Beijing 100084, P. R. China. E-mail: wxg-dce@mail.tsinghua.
edu.cn; Fax: +86 10 6278 1003; Tel: +86 10 6279 6171
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C

View Article Online
Journal of Materials Chemistry C
Paper
growth rate and large surface modulation.26–30 However, as a with plenty of water. The nal product was vacuum dried at
new class of potential photo- and electro-active organic mate- 70 ꢀC for 24 h. Yield: 75%. 1H NMR (DMSO-d6) d (ppm): 1.32 (m,
rials, only limited types of the azo molecular glasses have been 2H), 1.52 (m, 2H), 1.77 (m, 2H), 2.14 (t, 2H), 4.38 (t, 2H), 7.19 (t,
developed.
According to the strategy for molecular design, the amor-
2H), 7.44 (t, 2H), 7.59 (d, 2H), 8.14 (d, 2H), 11.99 (br, 1H).
2-(Phenylamino)ethanol. A mixture of aniline (9.30 g,
phous molecular materials should incorporate rigid moieties to 0.1 mol), 2-chloroethanol (8.05 g, 0.1 mol), anhydrous K2CO3
maintain the amorphous state at room temperature.19,20 Typi- (13 g, 0.1 mmol) and KI (1 g) was heated to 80 ꢀC under nitrogen
cally, biphenyl and biphenylene units have been widely used as protection with vigorous stirring for 12 h. The residue was
building blocks to construct molecular glasses. For most diluted with ethyl acetate. The solution was ltered and the
reported azo molecular glasses, the biphenyl or biphenylene solvent was then removed by evaporation in a vacuum. The
units and azobenzene are tightly anchored by rigid tris(phenyl) crude product was further puried by column chromatography
cores.19–25 At the current stage, new building blocks are still (ethyl acetate–hexane, v1/v2 ¼ 1 : 2). Yield: 41%. 1H NMR
needed to further develop azo molecular glasses. Carbazoles (CDCl3-d) d (ppm): 3.32 (t, 2H), 3.80 (t, 2H), 6.65 (d, 2H), 6.73
have been widely investigated as building blocks to prepare (t, 1H), 7.18 (t, 2H).
photoconductive polymers, non-linear optical polymers, and
1,3,5-Tris(2-hydroxy-3-(methyl(phenyl)amino)propyl)-1,3,5-
organic/polymeric materials for plastic electronics and photo- triazinane-2,4,6-trione (Tr-AN). Tr-AN was obtained from the
voltaics.31–33 Introducing carbazole moieties into azo molecular reaction between 1,3,5-tris(oxiran-2-yl-methyl)-1,3,5-triazinane-
glasses can be an attractive strategy to develop new types of 2,4,6-trione and N-methylaniline according to the literature.30
1
photoresponsive materials. These materials can show a sensi- Yield: 85%. H NMR (DMSO-d6) d (ppm): 2.92 (s, 9H), 3.20 (m,
tive response to light irradiation to form SRGs, self-structured 3H), 3.45 (m, 3H), 3.68 (m, 3H), 3.90 (m, 3H), 4.07 (m, 3H), 4.91
surface patterns and exhibit other new functions. However, to (d, 3H), 6.57 (t, 3H), 6.66 (d, 6H), 7.12 (t, 6H).
our knowledge, a molecular glass containing both azo chro-
mophore and carbazole group in the molecular structure has 1,3,5-triazinane-2,4,6-trione (6-Tr-AN). This compound was
not been reported in the literature yet. synthesized via a procedure similar to that described above for
1,3,5-Tris(2-hydroxy-3-((2-hydroxyethyl)(phenyl)amino)propyl)-
In this study, we have developed a new molecular design Tr-AN preparation by using N-(2-hydroxyethyl)aniline instead of
strategy to synthesize star-shaped azo molecules to contain both N-methylaniline in the reaction. Yield: 75%. 1H NMR (DMSO-d6)
azo chromophores and carbazole groups as amorphous mate- d (ppm): 3.22 (m, 3H), 3.40 (m, 3H), 3.48–3.60 (m, 12H), 3.71 (m,
rials. The molecules were constructed by connecting three azo 3H), 3.91 (m, 3H), 4.09 (m, 3H), 4.71 (br, 3H), 5.06 (br, 3H), 6.54
chromophores to a cyanuric acid core and carbazole moieties (t, 3H), 6.66 (d, 6H), 7.10 (t, 6H).
were introduced into the molecules through exible spacers.
3Cz-AN. 3Cz-AN was synthesized by esterication reaction
Transparent solid lms could be obtained by spin-coating with between Tr-AN and 6-(H-carbazol-9-yl)hexanoic acid. Dicyclo-
solutions of these azo compounds. The structure and property hexylcarbodiimide (DCC, 1.03 g, 5 mmol) was added to a
relationship of the azo molecular glasses, in particular for magnetically stirred dichloromethane solution (30 mL) of 4-
surface modulation in response to light irradiation, was inves- dimethylaminopyridine (DMAP, catalytic amount), 6-(H-carba-
tigated in detail.
zol-9-yl)hexanoic acid (1.40 g, 5 mmol) and Tr-AN (0.93 g,
1.5 mmol) at 0 ꢀC. Aer the solution was stirred at room
temperature for 12 h, it was ltered and the solvent was
removed by evaporation in a vacuum. The crude product was
further puried by column chromatography (CH2Cl2, the rst
2 Experimental
2.1 Materials and synthesis
1
ꢀ
Tetrahydrofuran (THF) was rst reuxed with cuprous chloride component). Yield: 85%. H NMR (DMSO-d6), 80 C, d (ppm):
for 1 h and distilled; then reuxed with sodium for 6 h and 1.10 (6H), 1.29 (6H), 1.59 (6H), 1.90–1.94 (6H), 2.72 (9H), 3.41
distilled before use. N,N-Dimethylformamide (DMF) was azeo- (3H), 3.82 (3H), 4.05 (3H), 4.19 (6H), 5.25 (3H), 6.51 (3H), 6.62
tropically distilled with benzene six times for dehydration and (6H), 7.02 (6H), 7.14 (6H), 7.38 (6H), 7.42 (6H), 8.08 (6H).
then puried by vacuum distillation. Dichloromethane (DCM)
6Cz-AN. This compound was synthesized via a procedure
was washed with 98% H2SO4 and distilled. All other chemicals similar to that described above for 3Cz-AN preparation by using
1
and solvents were commercially purchased and used as received 6-Tr-AN instead of Tr-AN in the reaction. Yield: 83%. H NMR
without further purication.
(DMSO-d6), 80 ꢀC, d (ppm): 1.06–1.67 (36H), 1.92–2.07 (12H),
6-(H-Carbazol-9-yl)hexanoic acid. Carbazole (3.34 g, 3.34–3.44 (12H), 3.81 (3H), 4.00 (6H), 4.10 (3H), 4.13 (6H), 4.26
20 mmol) was added to a mixture of potassium hydroxide (6H), 5.23 (3H), 6.48 (3H), 6.66 (6H), 7.00 (6H), 7.12 (12H), 7.37–
powder (4 g) and anhydrous DMF (50 mL), which was magnet- 7.45 (24H), 8.06 (12H).
ically stirred at room temperature for 1 h. 6-Bromohexanoic
3Cz-AZ-CN. 4-Aminobenzontrile (0.30 g, 2.5 mmol) was
acid (4.88 g, 25 mmol) was slowly added into the reaction mixed with sulfuric acid (0.5 mL) and glacial acetic acid (7 mL).
mixture. The mixture was stirred at room temperature for 3 h The diazonium salt was prepared by slowly adding an aqueous
and then poured into plenty of water. The insoluble solid was solution of sodium nitrite (0.2 g, 2.9 mmol in 1 mL of water) into
removed from the aqueous solution by ltration. The clear the 4-aminobenzontrile solution. The mixture was stirred at
solution was then adjusted to neutral pH. The precipitate 5 ꢀC for 5 min and then added dropwise into a solution of 3Cz-
formed in the solution was collected by ltration and washed AN (0.70 g, 0.5 mmol) in DMF (50 mL). The solution was stirred
J. Mater. Chem. C
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
at 0 ꢀC for 12 h and then poured into plenty of water. The The surface proles of the surface-relief-gratings and self-
precipitate collected by ltration was then dissolved in THF and structured surface patterns were monitored using an atomic
precipitated again with petroleum ether. The nal product was force microscope (AFM, Nanoscope IIIa, tapping mode).
vacuum dried at 70 ꢀC for 24 h. Yield: 87%. DSC: Tg 89 ꢀC. Mw/
Mn (GPC): 1.06. IR (KBr, cmꢁ1): 3051 (carbazole), 2935, 2846
2.3 Film preparation
(–CH2), 2225 (C^N), 1736, 1697 (C]O), 1597, 1516, 1462 (Benz
Solid lms of the azo molecular glasses with smooth surfaces
ring, carbazole). 1H NMR (DMSO-d6), 80 ꢀC, d (ppm): 1.05 (6H),
were prepared by spin-coating. The azo compounds were dis-
1.25 (6H), 1.52 (6H), 1.93 (6H), 2.90 (9H), 3.63–3.68 (6H), 3.92
solved in anhydrous N,N-dimethylformamide (DMF) to obtain
(3H), 4.11 (9H), 5.33 (3H), 6.78 (6H), 7.11 (6H), 7.32 (12H), 7.70
homogeneous solutions with concentrations about 15 wt%. The
(12H), 7.80 (6H), 8.05 (6H).
solutions were ltered through 0.45 mm membranes and spin-
3Cz-AZ-NT. This compound was synthesized by the azo-
coated onto glass slides. The lm thickness was controlled to be
coupling reaction between 3Cz-AN and the diazonium salt of
in a range from 200 nm to 400 nm by adjusting the spinning
4-nitroaniline via a similar method used for preparing 3Cz-AZ-
speed. The lms were dried at 60–70 ꢀC under vacuum for 48 h.
CN. Yield: 80%. DSC: Tg 86 ꢀC. Mw/Mn (GPC): 1.04. IR (KBr,
cmꢁ1): 3050 (carbazole), 2933, 2862 (–CH2), 1736, 1697 (C]O),
1601, 1518, 1454 (Benz ring, carbazole), 1340 (–NO2). H NMR
(DMSO-d6), 80 ꢀC, d (ppm): 1.07–1.15 (6H), 1.28–1.32 (6H), 1.54–
1
2.4 Laser irradiation
A linearly or circularly polarized beam from a diode-pumped
1.59 (6H), 1.90–1.98 (6H), 2.84–2.89 (9H), 3.48–3.71 (6H), 3.92
(3H), 4.12 (9H), 5.31 (3H), 6.79 (6H), 7.10 (6H), 7.32 (12H), 7.72–
7.80 (12H), 8.03 (6H), 8.19–8.25 (6H).
frequency doubled solid state laser (532 nm) was used as the
light source. The laser beam with an intensity of 200 mW cmꢁ2
was obtained aer being spatially ltered, expanded and colli-
mated. For inducing the self-structured surface patterns, the
laser beam was incident perpendicularly on the lm surfaces.30
SRG inscription was carried out by the experimental setup and
conditions similar to those reported before.9,10 A p-polarized
beam from the diode-pumped frequency doubled solid state
laser at 532 nm with an intensity of about 80 mW cmꢁ2 was used
as the writing beam. The writing beam was split by a mirror
such that a half of the beam reected onto the lm surface was
coincident with the other half of the beam to form an inter-
ference pattern. The formation of SRGs was probed with an
unpolarized low power He–Ne laser beam at 633 nm by
measuring the diffraction efficiency of the rst-order diffracted
beam in a real time mode.
6Cz-AZ-CN. This compound was synthesized via a procedure
similar to that described above for 3Cz-AZ-CN preparation by
using 6Cz-AN instead of 3Cz-AN in the reaction. Yield: 79%.
DSC: Tg 74 ꢀC. Mw/Mn (GPC): 1.05. IR (KBr, cmꢁ1): 3049
(carbazole), 2933, 2862 (–CH2), 2224 (C^N), 1734, 1697 (C]O),
1597, 1512, 1452 (Benz ring, carbazole). 1H NMR (DMSO-d6),
80 ꢀC, d (ppm): 1.01–1.65 (36H), 1.96–2.08 (12H), 3.53–3.66
(12H), 3.91 (3H), 4.04–4.07 (12H), 4.17 (3H), 4.22 (6H), 5.31 (3H),
6.84 (6H), 7.12 (12H), 7.28 (12H), 7.35–7.42 (12H), 7.66 (12H),
7.75 (6H), 8.06 (12H).
6Cz-AZ-NT. This compound was synthesized via a procedure
similar to that described above for 3Cz-AZ-NT preparation by
using 6Cz-AN instead of 3Cz-AN in the reaction. Yield: 86%.
DSC: Tg 73 ꢀC. Mw/Mn (GPC): 1.05. IR (KBr, cmꢁ1): 3049
(carbazole), 2933, 2860 (–CH2), 1736, 1697 (C]O), 1599, 1514,
3 Result and discussion
3.1 Synthesis and characterization
1
1452 (Benz ring, carbazole), 1338 (–NO2). H NMR (DMSO-d6),
80 ꢀC, d (ppm): 1.04–1.67 (36H), 1.99–2.10 (12H), 3.57–3.69
(12H), 3.95 (3H), 3.97–4.10 (12H), 4.23 (9H), 5.38 (3H), 6.87 (6H), The synthetic route of the star-shaped molecules containing
7.11 (12H), 7.29 (12H), 7.36–7.44 (12H), 7.69 (12H), 8.02 (6H), both azo chromophores and carbazole moieties is given in
8.16 (12H).
Scheme 1. The syntheses started from the preparation of two
precursors Tr-AN and 6-Tr-AN to obtain the core structures. The
precursor Tr-AN was obtained by the ring open reaction of 1,3,5-
triglycidyl isocyanurate (TGIC) with N-methylaniline (AN). The
2.2 Characterization
1H NMR spectra were obtained on a JOEL JNM-ECA300 NMR experimental details of this step have been reported in our
spectrometer. IR spectra were determined using a Nicolet 560- recent publication.30 6-Tr-AN was synthesized by a similar
IR FT-IR spectrophotometer by incorporating the powder procedure but using N-(2-hydroxyethyl)aniline instead of
samples in KBr disks. The UV-vis spectra of the samples were N-methylaniline. The carbazole moieties were introduced by the
recorded using a Perkin-Elmer Lambda Bio-40 spectrophotom- esterication reaction between the 6-(H-carbazol-9-yl)hexanoic
eter. The molecular weights (MWs) and distributions were acid and the precursors. The reactions were catalyzed by
determined using a gel permeation chromatographic (GPC) 4-dimethylaminopyridine (DMAP) through a routine esterica-
apparatus, which was operated at 25 ꢀC using THF as the eluent tion procedure activated by dicyclohexyl-carbodiimide (DCC).
(1 mL minꢁ1). The instrument was equipped with a refractive The target molecules were nally obtained from the two kinds
index (RI) detector (Wyatt Optilab rEX) and tted with a PLgel of carbazole-containing precursors by the azo-coupling reac-
5 mm mixed-D column calibrated using linear polystyrene tions using diazonium salts of the two aniline derivatives. This
standards. The thermal properties of the azo molecular glasses reaction route could introduce azo chromophores with different
were investigated with a TA instrument (DSC 2910 and Hi-Res electron-withdrawing groups at the nal stage of the prepara-
TGA 2950) at a heating rate of 10 ꢀC minꢁ1 under N2 protection. tion under mild conditions. The diazonium salts readily attack
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C

View Article Online
Journal of Materials Chemistry C
Paper
Scheme 1 (a) Synthetic route of the star-shaped molecules 3Cz-AZ-CN and 3Cz-AZ-NT. (b) Synthetic route of the star-shaped molecules 6Cz-AZ-CN and 6Cz-AZ-NT.
the benzene rings of the anilino groups at positions with high did not show optical birefringence caused by preferential
electron densities. The bulkiness of the attacking groups and molecular orientation. Thermogravimetric analysis (TGA) was
the resulting steric hindrance allow the electrophilic substitu- used to investigate the thermal stability of the azo compounds.
tion to take place exclusively at the para position. This synthetic TGA curves of the azo compounds are shown in Fig. 2 and the
route can be used to prepare a series of star-shaped molecules thermal decomposition temperatures (Tds) obtained from TGA
bearing different electron-withdrawing groups. The chemical are summarized in Table 1. The Tds of 3Cz-AZ-CN, 3CZ-AZ-NT,
structure of the star-shaped molecules was conrmed by spec-
troscopic analyses as shown by the data given in the Experi-
mental section.
3.2 Thermal and spectral properties
The thermal transition and phase behavior of the star-shaped
molecules were investigated by differential scanning calorim-
etry (DSC) and polarized optical microscopy (POM). The DSC
curves of the four azo compounds given in Fig. 1 show the
typical glass transition behavior. The glass transition tempera-
tures (Tgs) obtained from DSC are summarized in Table 1. The
Tgs of the 3Cz-AZ-CN, 3CZ-AZ-NT, 6Cz-AZ-CN and 6Cz-AZ-NT are
89, 86, 74 and 73 ꢀC, respectively. The 6Cz-AZ-X (X ¼ CN and NT)
molecules have relatively lower Tgs, which could be attributed to
more carbazole moieties in the molecules. POM observations
indicated that before and aer the glass transition, all samples
of 3Cz-AZ-X and 6Cz-AZ-X (X ¼ CN and NT) were isotropic and Fig. 1 DSC curves of the azo molecular glasses.
J. Mater. Chem. C
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
Table 1 Tgs, Tds and lmaxs of 3Cz-AZ-CN, 3CZ-AZ-NT, 6Cz-AZ-CN and 6Cz-AZ-
strongly affected by the electron-withdrawing groups on the
azobenzene moieties, which can be attributed to their inductive
and conjugative effects on both the ground and excited states.
On the other hand, the band positions are also related to the
molecular structure, such as those observed for 3CZ-AZ-NT and
6Cz-AZ-NT. The lmaxs of 3Cz-AZ-CN, 3CZ-AZ-NT, 6Cz-AZ-CN and
6Cz-AZ-NT are 445, 458, 443 and 469 nm (Table 1).
NT
Sample
Tg (ꢀC)
Td (ꢀC)
lmax (nm)a
3Cz-AZ-CN
3Cz-AZ-NT
6Cz-AZ-CN
6Cz-AZ-NT
89
86
74
73
279
312
322
325
445
458
443
469
a
In DMF solution.
3.3 Photoinduced self-structured pattern formation
To compare the photoresponsive properties of the azo
compounds, the self-structured surface pattern formation on
3Cz-AZ-CN, 3CZ-AZ-NT, 6Cz-AZ-CN and 6Cz-AZ-NT lms was
studied. The solid lms of the azo molecular glasses were
irradiated with a uniform laser beam (532 nm, 200 mW cmꢁ2) at
the normal incidence. Fig. 4 shows the typical AFM images of
the photoinduced surface patterns on the 3Cz-AZ-X (X ¼ CN and
NT) lms aer irradiating for 30 min. The surface patterns
comprise regularly spaced structures. Some localized arrange-
ment of the surface structures can be recognized from the
images. With the same irradiation time and light intensity,
obvious differences in their morphology can be seen for the
surface patterns induced by irradiation with linearly or circu-
larly polarized laser lights.
For 3Cz-AZ-CN, when irradiated with linearly polarized light,
the surfaces show partially merged pillar-like structures with
some localized hexagonal arrangement (Fig. 4(a)). The align-
ment of the hexagonal patterns shows some correlation with the
polarization direction of the laser beam. The period of the
regularly spaced pillars is 540 ꢂ 10 nm, which is estimated
along ꢂ60 degrees with respect to the light polarization direc-
tion. The amplitude of the surface modulation is about 60 nm.
When irradiated with circularly polarized light, no such local-
ized hexagonal patterns can be seen (Fig. 4(b)). The surface
Fig. 2 TGA curves of the star-shaped molecules.
6Cz-AZ-CN and 6Cz-AZ-NT are 279, 312, 322 and 325 ꢀC,
respectively.
The UV-vis spectra of the azo compounds in DMF solutions
are given in Fig. 3. The spectra show typical characteristics of a
pseudo-stilbene-type azo compound.2 The absorption bands
corresponding to the p–p* transition appear in the visible
spectral region and the azo compounds exhibit a bright colour
owing to the strong absorption. The weak n–p* transition band
overlaps with the p–p* transition band and cannot be seen
separately. The positions of the p–p* transition bands are
Fig. 4 Typical AFM images (10 mm ꢃ 10 mm) of the photoinduced self-structured
surface patterns on 3Cz-AZ-X films induced by light with the different polariza-
tions: (a) 3Cz-AZ-CN, linearly polarized; (b) 3Cz-AZ-CN, circularly polarized; (c)
3Cz-AZ-NT, linearly polarized and (d) 3Cz-AZ-NT, circularly polarized. The inten-
sity of the laser beam was 200 mW cmꢁ2 and the irradiation time was 30 min.
Fig. 3 UV-vis spectra of the star-shaped molecules in DMF solutions.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C

View Article Online
Journal of Materials Chemistry C
Paper
patterns appear like ripple stripes with some periodicity
between them. The amplitude of the surface modulation
induced by the circularly polarized light is about 80 nm.
For 3Cz-AZ-NT, when irradiated with light under the same
conditions, the amplitude of the surface modulation is much
smaller compared to 3Cz-AZ-CN. Aer being irradiated with
linearly polarized light, the heights of pillars on 3Cz-AZ-NT
lms are only about 7 nm, which shows the period of 510 ꢂ
10 nm. For the sample irradiated with a circularly polarized
laser beam, the amplitude is about 15 nm. Similar to 3Cz-AZ-
CN, the morphology also shows a close reliance on the light
polarization condition (Fig. 4(c) and (d)).
Fig. 5 shows the typical AFM images of the surface
morphology on the 6Cz-AZ-X (X ¼ CN and NT) lms aer irra-
diating for 30 min. Among these two compounds, only 6Cz-AZ-
CN shows weak ability to form the self-structured pattern with
the pillar height about 7 nm, when irradiated with circularly
polarized light. For 6Cz-AZ-CN irradiated with linearly polarized
light and 6Cz-AZ-NT irradiated with both linearly and circularly
polarized light, only irregular surface roughness can be seen
aer the light irradiation.
The above observation clearly indicates the correlation of
pattern formation with the azo chromophore. The photoin-
duced pattern formation behavior of the star-shaped molecules
is obviously different for these two kinds of azo chromophores,
which show difference only in the electron-withdrawing groups.
The two molecules bearing the cyano-substituted azo chromo-
phore show the higher efficiency to form the self-structured
surface pattern. On the other hand, the chromophore density
also plays an important role in the formation of these regular
surface patterns. A high enough chromophore density is
necessary for the self-structured pattern formation. This point
is consistent with our previous reports about photoinduced self-
structured pattern formation for linear azo polymers.34
3.4 Photoinduced surface-relief-gratings
Photoinduced SRG formation on nCz-AZ-X (n ¼ 3 or 6, X ¼ CN or
NT) lms was investigated by using interfering p-polarized laser
beams (532 nm, 80 mW cmꢁ2) as the inscribing light and a He–
Ne laser beam at 633 nm as the probe beam. The SRG formation
was monitored by measuring the rst-order diffraction effi-
ciencies of the probe beam in a real-time manner. The proles
and trough depths of the gratings were detected by AFM. Upon
exposure to the interference pattern of laser beams at the
modest intensity, SRGs were formed on lms of all four azo
molecular glasses. Fig. 6 shows a typical AFM image of the
sinusoidal surface relief structures with regular spaces formed
on the 6Cz-AZ-NT lms. The spatial period depends on the
wavelength and the incident angle of the writing beams. Like
those reported for azo polymer lms, the SRGs are stable below
the Tgs of the amorphous materials and can be removed by
heating the samples to a temperature above their Tgs.
For a comparative study, lms of the four azo molecular
glasses were exposed to laser light when the experimental
conditions, such as the laser intensity, incident angle and lm
thickness, were controlled to be identical. Fig. 7 shows the rst-
order diffraction efficiency (DE) as a function of irradiation time
for nCz-AZ-X (n ¼ 3 or 6, X ¼ CN or NT). The result shows that
the SRG-forming rate is closely related to structures of the
molecular glasses and the electron-withdrawing groups of the
azo chromophores.
Molecules containing the cyano-substituted azo chromo-
phores, 3Cz-AZ-CN, shows a faster growth rate during the SRG
formation than 6Cz-AZ-CN owing to the higher azo chromo-
phore density of the former. However, an opposite tendency is
observed for the molecules containing the nitro-substituted azo
chromophores. 3Cz-AZ-NT shows a slower SRG-growth rate than
that of 6Cz-AZ-NT. An important point can be seen by
comparing the difference between the molecules with the same
azo density but bearing the cyano- or nitro-substituted chro-
mophores. For 3Cz-AZ-X (X ¼ CN or NT), 3Cz-AZ-CN forms SRG
much faster than 3Cz-AZ-NT does, which is coincident with
most of the previous reports for azo molecular glasses and azo
polymers.28,29,35,36 However, the SRG growth rate of 6Cz-AZ-NT is
extraordinarily high, which is almost the same as that of 3Cz-
AZ-CN and much higher than that of 6Cz-AZ-CN. This
Fig. 5 Typical AFM images (10 mm ꢃ 10 mm) of the photoinduced self-structured
surface patterns on 6Cz-AZ-X films induced by light with the different polariza-
tions: (a) 6Cz-AZ-CN, linearly polarized; (b) 6Cz-AZ-CN, circularly polarized; (c)
6Cz-AZ-NT, linearly polarized and (d) 6Cz-AZ-NT, circularly polarized. The inten-
sity of the laser beam was 200 mW cmꢁ2 and the irradiation time was 30 min.
Fig. 6 AFM images of the surface-relief-gratings on the 6Cz-AZ-NT film, (a) 2D
view and (b) 3D view. The intensity of the laser beam was 80 mW cmꢁ2 and the
irradiation time was 600 s.
J. Mater. Chem. C
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
4 Conclusions
A new class of star-shaped molecules, containing both azo
chromophores and carbazole groups (nCz-AZ-X, n ¼ 3 or 6, X ¼
CN or NT), was synthesized in this work. The materials show the
behavior of an amorphous solid at room temperature requiring
no special treatments such as quenching. The type of azo
chromophore, the number of the carbazole groups and the
molecular architecture all play important roles to inuence
both the photoinduced self-structured surface pattern forma-
tion and SRG inscription rate. The molecular design using the
exible spacer to incorporate the carbazole groups can signi-
cantly enhance the SRG formation efficiency of the materials
containing the nitro-substituted azo chromophores. This
understanding can be further explored for developing materials
with better performance. The materials could be potentially
used for other applications by further exploring the functions of
carbazole groups.
Fig. 7 Diffraction efficiency as a function of the irradiation time for 3Cz-AZ-CN,
3CZ-AZ-NT, 6Cz-AZ-CN and 6Cz-AZ-NT.
observation indicates that the slower SRG growth rate observed
for the materials containing nitro-substituted azo chromo-
phores could be attributed to the strong dipole–dipole interac-
tion between the azo chromophores. For 6Cz-AZ-NT, the low
chromophore density and the branched structure help to
separate the azo chromophores and can avoid the dipole–dipole
interaction and result in a high SRG growth rate.
Acknowledgements
The nancial support from NSFC under Projects 51233002 and
91027024 is gratefully acknowledged.
Notes and references
1 G. S. Kumar and D. C. Neckers, Chem. Rev., 1989, 89, 1915–
1925.
3.5 Discussion
The above investigation on the photoinduced formation of self-
structured surface patterns and SRGs can shed some new light
on how to further improve the molecular design. It is generally
agreed that the mass-transfer behavior is caused by or directly
related to the trans–cis isomerization of azo chromophores.11–18
Therefore, this effect should be enhanced by increasing the
chromophore density. On the other hand, the strong dipole–
dipole interaction between the chromophores could restrain
the chain motion in some cases. As indicated by previous
studies, SRG can be inscribed on lms of polymers with a
relatively low chromophore density.34,35 Therefore, some buffer
groups can be introduced into the molecular structures to
separate the azo chromophores to avoid strong dipole–dipole
interaction as demonstrated above using 6Cz-AZ-NT. On the
other hand, as the self-structured surface pattern formation
requires a high azo chromophore density,34 azo chromophores
with low dipole moment such as the cyano-substituted one are
more suitable for this application.
2 H. Rau, in Photochemistry and Photophysics, ed. J. F. Rabek,
CRC Press, Boca Raton FL, 1990, vol. 2, ch. 4.
3 A. Natansohn, P. Rochon, J. Grosselin and S. Xie,
Macromolecules, 1992, 25, 2268–2273.
4 S. Xie, A. Natansohn and P. Rochon, Chem. Mater., 1993, 5,
403–411.
5 J. A. Delaire and K. Nakatani, Chem. Rev., 2000, 100, 1817–
1845.
6 A. Natansohn and P. Rochon, Chem. Rev., 2002, 102, 4139–
4175.
7 N. K. Viswanathan, D. Y. Kim, S. P. Bian, J. Williams, W. Liu,
L. Li, L. Samuelson, J. Kumar and S. K. Tripathy, J. Mater.
Chem., 1999, 9, 1941–1955.
8 (a) T. Ikeda, J. Mamiya and Y. Yu, Angew. Chem., Int. Ed.,
2007, 46, 506–528; (b) A. Shishido, Polym. J., 2010, 42, 525–
533; (c) H. F. Yu and T. Ikeda, Adv. Mater., 2011, 23, 2149–
2180; (d) T. Yoshino, M. Kondo, J. I. Mamiya, M. Kinoshita,
Y. L. Yu and T. Ikeda, Adv. Mater., 2010, 22, 1361–1363.
9 P. Rochon, E. Batalla and A. Natansohn, Appl. Phys. Lett.,
1995, 66, 136–138.
Carbazole is well known as a functional group for many
applications. Poly(N-vinylcarbazole) (PVK) has been used as a
photoconductive polymer in photocopiers for years.32 The 10 D. Y. Kim, S. K. Tripathy, L. Li and J. Kumar, Appl. Phys. Lett.,
molecules prepared in this study could be investigated for other
1995, 66, 1166–1168.
functionalities. As the molecules contain both carbazole units 11 (a) C. J. Barrett, A. L. Natansohn and P. L. Rochon, J. Phys.
and azo chromophores, they can possibly be used as photo-
refractive materials by exploiting their space-charge-eld
formation and electro-optic nonlinearity.31,37 Moreover, if the 12 (a) J. Kumar, L. Li, X. L. Jiang, D. Y. Kim, T. S. Lee and
Chem., 1996, 100, 8836–8842; (b) C. J. Barrett, P. Rochon
and A. Natansohn, J. Chem. Phys., 1998, 109, 1505–1516.
photoactive properties of the carbazole units have some inu-
ence on the mass-transfer properties reported above also needs
further investigation.
S. Tripathy, Appl. Phys. Lett., 1998, 72, 2096–2098; (b)
S. Bian, J. Williams, D. Kim, L. Li, S. Balasubramanian,
J. Kumar and S. Tripathy, J. Appl. Phys., 1999, 86, 4498–4508.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C

View Article Online
Journal of Materials Chemistry C
Paper
13 T. G. Pedersen, P. M. Johansen, N. C. R. Holme and
P. S. Ramanujam, Phys. Rev. Lett., 1998, 80, 89–92.
14 P. Len, C. Fiorini and J. Nunzi, Pure Appl. Opt., 1998, 7, 71–82.
15 O. M. Tanchak and C. J. Barrett, Macromolecules, 2005, 38,
10566–10570.
T. Takahashi, T. Kadota and Y. Shirota, Adv. Mater., 2002,
14, 1157–1160; (c) H. Nakano, T. Tanino, T. Takahashi,
H. Ando and Y. Shirota, J. Mater. Chem., 2008, 18, 242–246;
(d) T. Tanino, S. Yoshikawa, T. Ujike, D. Nagahama,
K. Moriwaki, T. Takahashi, Y. Kotani, H. Nakano and
Y. Shirota, J. Mater. Chem., 2007, 17, 4953–4963.
16 (a) K. G. Yager, O. M. Tanchak, C. Godbout, H. Fritzsche and
C. J. Barrett, Macromolecules, 2006, 39, 9311–9319; (b) 26 E. Ishow, B. Lebon, Y. N. He, X. G. Wang, L. Bouteiller,
K. G. Yager and C. J. Barrett, Macromolecules, 2006, 39,
9320–9326.
L. Galmiche and K. Nakatani, Chem. Mater., 2006, 18,
1261–1267.
17 C. Hubert, C. Fiorini-Debuisschert, I. Maurin, J. M. Nunzi 27 A. S. Matharu, S. Jeeva, P. R. Huddleston and
and P. Raimond, Adv. Mater., 2002, 14, 729–732. P. S. Ramanujam, J. Mater. Chem., 2007, 17, 4477–4482.
18 C. Hubert, C. Fiorini-Debuisschert, L. Rocha, P. Raimond 28 Y. N. He, X. Y. Gu, M. C. Guo and X. G. Wang, Opt. Mater.,
and J. Nunzi, J. Opt. Soc. Am. B, 2007, 24, 1839–1846. 2008, 31, 18–27.
19 (a) Y. Shirota, J. Mater. Chem., 2000, 10, 1–25; (b) Y. Shirota, J. 29 G. Ye, D. R. Wang, Y. N. He and X. G. Wang, J. Mater. Chem.,
Mater. Chem., 2005, 15, 75–93. 2010, 20, 10680–10687.
20 E. Ishow, C. Bellaıche, L. Bouteiller, K. Nakatani and 30 J. J. Yin, G. Ye and X. G. Wang, Langmuir, 2010, 26, 6755–
J. A. Delaire, J. Am. Chem. Soc., 2003, 125, 15744–15745. 6761.
21 Y. Shirota, K. Moriwaki, S. Yoshikawa, T. Ujike and 31 M. J. Cho, D. H. Choi, P. A. Sullivan, A. J. P. Akelaitis and
H. Nakano, J. Mater. Chem., 1998, 8, 2579–2581. L. R. Dalton, Prog. Polym. Sci., 2008, 33, 1013–1058.
22 (a) A. Perschke and T. Fuhrmann, Adv. Mater., 2002, 14, 841– 32 P. L. T. Boudreault, S. Beaupre and M. Leclerc, Polym. Chem.,
¨
´
843; (b) T. Fuhrmann and T. Tsutsui, Chem. Mater., 1999, 11,
2226–2232.
2010, 1, 127–136.
33 J. Li and A. C. Grimsdale, Chem. Soc. Rev., 2010, 39, 2399–
23 (a) C. Chun, M. Kim, D. Vak and D. Kim, J. Mater. Chem.,
2410.
2003, 13, 2904–2909; (b) M. Kim, E. Seo, D. Vak and 34 X. L. Wang, J. J. Yin and X. G. Wang, Macromolecules, 2011,
D. Kim, Chem. Mater., 2003, 15, 4021–4027. 44, 6856–6867.
24 (a) H. Utsumi, D. Nagahama, H. Nakano and Y. Shirota, 35 Y. N. He, X. G. Wang and Q. X. Zhou, Polymer, 2002, 43,
J. Mater. Chem., 2000, 10, 2436–2438; (b) H. Utsumi, 7325–7333.
D. Nagahama, H. Nakano and Y. Shirota, J. Mater. Chem., 36 Y. N. He, X. G. Wang and Q. X. Zhou, Synth. Met., 2003, 132,
2002, 12, 2612–2619. 245–248.
25 (a) H. Ando, T. Tanino, H. Nakano and Y. Shirota, Mater. 37 O. Ostroverkhova and W. E. Moerner, Chem. Rev., 2004, 104,
Chem. Phys., 2009, 113, 376–381; (b) H. Nakano,
3267–3314.
J. Mater. Chem. C
This journal is ª The Royal Society of Chemistry 2013