Synthesis and characterization of photoactive azobenzene-based chromophores containing a bulky cholesteryl moiety

Article abstract of DOI:10.1016/j.molstruc.2012.02.032

This study describes the synthesis of a series of azobenzene-based chromophores bearing pendent bulky cholesteryl groups, using esterification reactions. The chromophores were composed of liquid crystalline mesophases with six or eleven methylene segments as spacers, and with electron-donating (OCH₃) and electron-withdrawing (NO₂) terminal groups. The target compounds were characterized by nuclear magnetic resonance spectroscopy, differential scanning calorimetry, polarizing optical microscopy, absorption, and photoluminescence spectroscopies. All the azobenzene derivatives with six or eleven methylene segments revealed chiral nematic phases. We investigated the effects of these photochromic compounds' structures on E/Z photoisomerization under UV irradiation. Chromophores containing the electron-withdrawing nitro-group (NO₂) underwent a faster rate of Z to E isomerization in darkness than the electron-donating (OCH₃) groups did; the isomerization process proceeded via a rotation mechanism. Self-assembled aggregates of C6 solution exhibited enhanced fluorescence in THF/water mixtures at 10% water fraction.

Full text of DOI:10.1016/j.molstruc.2012.02.032

Journal of Molecular Structure 1015 (2012) 129–137
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and characterization of photoactive azobenzene-based chromophores
containing a bulky cholesteryl moiety
b
a
Po-Chih Yang a, , Ya-Ling Lu , Chung-Yuan Li
⇑
a Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li 32003, Taiwan
b Department of Applied Chemistry, Chaoyang University of Technology, Taichung 41349, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 August 2011
Received in revised form 3 January 2012
Accepted 13 February 2012
Available online 22 February 2012
This study describes the synthesis of a series of azobenzene-based chromophores bearing pendent bulky
cholesteryl groups, using esterification reactions. The chromophores were composed of liquid crystalline
mesophases with six or eleven methylene segments as spacers, and with electron-donating (AOCH3) and
electron-withdrawing (ANO2) terminal groups. The target compounds were characterized by nuclear
magnetic resonance spectroscopy, differential scanning calorimetry, polarizing optical microscopy,
absorption, and photoluminescence spectroscopies. All the azobenzene derivatives with six or eleven
methylene segments revealed chiral nematic phases. We investigated the effects of these photochromic
compounds’ structures on E/Z photoisomerization under UV irradiation. Chromophores containing the
electron-withdrawing nitro-group (ANO2) underwent a faster rate of Z to E isomerization in darkness
than the electron-donating (AOCH3) groups did; the isomerization process proceeded via a rotation
mechanism. Self-assembled aggregates of C6 solution exhibited enhanced fluorescence in THF/water
mixtures at 10% water fraction.
Keywords:
Azobenzene
Photochemistry
Synthesis
UV–vis spectroscopy
Isomerization
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
light-emitting diodes [18], and photovoltaic cells [19]. Because of
their unique properties that originate from quantum effects, fluo-
Liquid crystalline compounds (LCs) have been receiving increas-
ing attention in the manipulation of optimized high-efficiency
electro-optic devices because of their ability to self-assemble, their
fluidity, and facile defect-free orientation under certain conditions
[1,2]. The development of photosensitive media employing LCs for
data recording, optical storage, and reproduction is one of the most
rapidly developing areas in the physical chemistry area of low
molecular mass and polymer LCs [3–6]. Cholesteric LCs incorporat-
ing photoisomerizable mesogenic fragments such as azobenzene
[7,8] and stilbene [9,10], that can reverse their configurations
when irradiated with light at various wavelengths, can be used
to provide a motor function for contractions in photoresponsive
materials [11]. Several groups have reported the preparation and
applications of photoresponsive materials composed of fast photo-
chemical segments [12–14]. Azobenzene-containing chromoph-
ores undergo isomerization from the E-isomer to the Z-form
under irradiation by light, whereas the Z-isomer may return to
the E form by either photochemical or thermal stimuli [15].
Fluorescent organic nanoparticles have also attracted consider-
able research attention because of their potential applications,
including fluorescent biological labels [16], optical sensors [17],
rescent organic nanoparticles have significant potential, due to
the wide variety of organic synthesis and nanocomposite prepara-
tion methods available. Systematic investigations into fluorescent
organic nanoparticles began after Nakanishi and co-workers [20],
reported that phthalocyanine and perylene nanoparticles exhibit
different and size-dependent fluorescent properties from those of
their bulk samples. Generally, the fluorescence quantum yields of
organic chromophores decrease in the solid but exhibit high fluo-
rescence quantum yields in solution. This occurs because of aggre-
gation in the solid state, and mainly arises from intermolecular
vibronic interactions, which induce non-radiative deactivation
processes such as excitonic coupling, excimer formation, and exci-
tation energy migration to impurity traps [21], resulting in de-
crease in fluorescence efficiency. However,
a few cases of
enhanced emission in the solid states of specific organic fluoro-
phores, such as arylethynyl compounds, pentaphenylsiloles, and
poly(phenyleneethynylene) compounds, have been reported and
interpreted as the intra- and inter-molecular effects exerted by
fluorophore aggregation [22–24]. Thus, an aggregation-induced
fluorescence enhancement was proposed [23]. Intermolecular ef-
fects by aggregation influence the fluorescence changes in p-conju-
gated organic chromophores. Effects on fluorescence changes by
intermolecular interactions are correlated with the aggregation
morphology such as H-type or J-type aggregation [21].
⇑
Corresponding author. Tel.: +886 3 4638800; fax: +886 3 4559373.
E-mail address: pcyang@saturn.yzu.edu.tw (P.-C. Yang).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2012.02.032

130
P.-C. Yang et al. / Journal of Molecular Structure 1015 (2012) 129–137
Unsubstituted azobenzenes show extremely little fluores-
2. Experimental
cence with a small quantum yield of approximately 2.53 ꢀ
10ꢁ5 [25]. A few azobenzene derivatives in rigid matrices at
low temperature, ortho-metalated azobenzenes, and self-assem-
bled aggregates of azobenzene derivatives exhibit fluorescence
[26–28]. Han et al. [29] reported increasing fluorescence quan-
tum yields of UV-exposed azobenzene molecules with increasing
electron-donating abilities of the substituents. Han et al. attrib-
uted this unusual fluorescence enhancement to the light-driven
self-assembly of Z-isomers with significant lifetime and large
dipole moment.
In our previous report [30], we demonstrated that self-assem-
bled azobenzenes Z-isomers, incorporating the diacetylene group
(AC„CAC„CA), exhibit fluorescence enhancement upon UV irra-
diation; this fluorescence originates from the formation of Z-iso-
mer aggregates. Our system, based on configuration changes of
azobenzene-based chromophores, exhibits simple photochemistry
and offers ease of chemical modification. The current study exam-
ines the effect of terminal groups and spacer length of chromo-
phore structure on photoreactivity and mesomorphic properties.
We also investigate the solvent dependence of azobenzene aggre-
gates’ fluorescence enhancement. The effect of the structure of
these photochromic compounds on E/Z photoisomerization under
UV irradiation is discussed in detail.
2.1. Materials
Synthetic routes for the target photochromic compounds are
shown in Scheme 1. The intermediates, 4-hydroxy-40-nitro-azo-
benzene (1) and 4-hydroxy-40-methoxy-azobenzene (2), 1-hydro-
xy-6-(4-nitro-azobenzene-40-oxy) hexane (3), 1-hydroxy-11-(4-
nitro-azobenzene-40-oxy) undecane (4), 1-hydroxy-6-(4-meth-
oxy-azobenzene-40-oxy) hexane (5), 1-hydroxy-11-(4-methoxy-
azobenzene-40-oxy) undecane (6), and the target photochromic
compounds were synthesized following the processes reported in
the literature [31] and our previous reports [32,33]. All organic sol-
vents and reagents were purchased from Acros, Alfa, and Aldrich
Chemical Co. and used without further purification. Dichlorometh-
ane (CH2Cl2) was distilled over calcium hydride under argon
immediately before use.
2.2. Measurements
All new compounds were identified by NMR, FT-IR spectra, and
elemental analyses (EAs). 1H NMR (400 MHz) and 13C NMR
(100.6 MHz) spectra were recorded on a Bruker AMX-400 FT-
NMR, and chemical shifts were reported in ppm using tetramethyl-
Scheme 1. Synthetic routes of azobenzene-based chromophores.

P.-C. Yang et al. / Journal of Molecular Structure 1015 (2012) 129–137
131
silane (TMS) as an internal standard. FTIR spectra were measured
as KBr disk using a Jasco VALOR III FTIR spectrometer; 32 scans
were collected at a resolution of 1 cmꢁ1. Elemental analyses were
carried out on a Heraeus CHNAO rapid elemental analyzer. Optical
rotations were measured at 30 °C in CHCl3 using a Jasco DIP-370
polarimeter at the sodium D-line (k = 589 nm) with a precision of
0.001°. The measurements were performed using 1% solutions
of substances in CHCl3. UV/visible absorption spectra were mea-
sured using a Jasco V-550 spectrophotometer and photolumines-
matic), 7.88–7.90 (d,J = 8.52 Hz, 2H, aromatic), 7.90–7.92
(d,J = 8.64 Hz, 2H, aromatic). 13C NMR (100.6 MHz, CDCl3): 11.9,
18.8, 19.2, 22.4, 22.4 (CH3); 21.2, 24.2, 24.5, 27.7, 32.2, 32.4, 36.7,
38.0, 39.8, 39.8, 40.0 (CH2); 55.4 (OCH3); 42.4 (quaternary); 28.1,
31.7, 36.0, 50.4, 56.6, 57.0, 75.1 (CH); 122.5, 138.7 (vinylic);
113.3, 120.3, 124.8, 126.8 (aromatic); 146.3, 149.2, 155.1, 166.8
(aromatic quaternary); 153.8 (C@O). Anal. Calcd. (%) for
C41H56N2O4: C, 76.77; H, 8.74; N, 4.37. Found: C, 76.82; H, 8.75;
N, 4.39.
cence (PL) spectra were obtained using
a
Jasco FP-750
fluorescence spectrophotometer. Fluorescence quantum yields of
compounds in chloroform using 9,10-diphenylanthracene
(kex = 350 nm) as the standard were estimated at room tempera-
ture by the dilution method (1 ꢀ 10ꢁ7 M, assuming a quantum effi-
ciency of unity) [34]. Thermal analysis was performed using a
differential scanning calorimeter (Perkin Elmer DSC 7) at a scan-
ning rate of 10 K minꢁ1 under nitrogen atmosphere. The meso-
phase transitions were investigated by an Olympus BH-2
polarizing light microscope (POM) equipped with a Mettler FP-82
hot stage and a heating rate of 10 K minꢁ1. UV light (365 nm; Mod-
el UVG-54, UVP) with an intensity of 0.6 mW was used as a pump-
ing light to induce photoisomerization of chiral compounds with
N@N bonds in CHCl3 solutions.
2.3.3. Synthesis of (ꢁ)-cholesteryl 6-(4-nitro-azobenzene-40-oxy)
hexyl carbonate, N6
N6 was prepared by a procedure similar to that for N0, using
1-hydroxy-6-(4-nitro-azobenzene-40-oxy) hexane (3) instead of
4-hydroxy-40-nitro-azobenzene (1).
Yield: 76.5%. K 138.7 °C Nꢂ 189.1 °C I (heating). [
a]
D = ꢁ4.4°. FT-IR
(KBr,
m
max/cmꢁ1): 2942, 2851 (CH2), 1742 (C@O), 1603, 1471 (CC in
Ar), 1257 (COC), 1344 (NO2). 1H NMR (CDCl3, d in ppm): 0.67 (s,
3H, CH3), 0.86–1.00 (m, 12H, CH3), 1.05–2.41 (m, 28H, CH2), 4.06
(t,J = 6.35 Hz, 2H, AOCH2A), 4.14 (t,J = 6.50 Hz, 2H, AOCH2A),
4.45–4.49 (m, 1H, OCH in chol.), 5.38–5.39 (m, 1H, CHCH2 in chol.),
7.00–7.03 (d,J = 8.80 Hz, 2H, aromatic), 7.94–7.96 (d,J = 9.40 Hz,
2H, aromatic, AArN@NArA), 7.97–7.99 (d,J = 9.30 Hz, 2H, aromatic,
AArN@NArA), 8.35–8.37 (d,J = 8.80 Hz, 2H, aromatic, ortho to NO2).
13C NMR (100.6 MHz, CDCl3): 11.8, 18.7, 19.2, 22.5, 22.8 (CH3); 21.0,
23.8, 24.3, 25.7, 27.7, 28.6, 29.0, 31.9, 36.2, 36.5, 38.1, 39.5, 39.7
(CH2); 67.6, 68.2 (OCH2); 42.3 (quaternary); 28.0, 31.8, 35.8, 50.0,
56.1, 56. 7, 77.7 (CH); 122.9, 139.4 (vinylic); 114.9, 123.1, 124.7,
125.6 (aromatic); 146.8, 148.2, 154.7, 162.8 (aromatic quaternary);
156.1 (C@O). Anal. Calcd. (%) for C46H65N3O6: C, 73.01; H, 8.60; N,
5.56. Found: C, 72.98; H, 8.56; N, 5.57.
2.3. Synthesis of photochromic compounds (Scheme 1)
2.3.1. Synthesis of (ꢁ)-cholesteryl (4-nitro-azobenzene-40oxy)
carboxylate, N0
4-Hydroxy-40-nitro-azobenzene (1) (2.0 g, 8.22 mmol), pyridine
(1.63 g, 20.61 mmol) and dry dichloromethane (50 ml) were put
inside of a 150 ml two-necked flask equipped with a magnetic stir-
rer. Then cholesteryl chloroformate (7.38 g, 16.44 mmol) was
added dropwise under vigorous stirring to the resulting solution
under nitrogen atmosphere. After the reaction mixture was stirred
under nitrogen atmosphere for 12 h at 30 °C, the solution was
poured into water and extracted with chloroform. The organic
layer was washed several times with water, dried over anhydrous
MgSO4, and evaporated to dryness. The crude product was purified
by column chromatography (silica gel, CH2Cl2/n-hexane = 1/1).
2.3.4. Synthesis of (ꢁ)-cholesteryl 6-(4-nitro-azobenzene-40-oxy)
undecyl carbonate, N11
N11 was prepared by a procedure similar to that for N0, using
1-hydroxy-11-(4-nitro-azobenzene-40-oxy) undecane (4) instead
of 4-hydroxy-40-nitro-azobenzene (1).
Yield: 66.2%. K 123.3 °C Nꢂ 162.9 °C I (heating). [
a]
D = ꢁ5.2°. FT-
IR (KBr,
m
max/cmꢁ1): 2934, 2853 (CH2), 1733 (C@O), 1606, 1478 (CC
Yield: 76.5%. Tm = 219.5 °C. [
a]
D = ꢁ5.7°. FT-IR (KBr,
m
max/cmꢁ1):
in Ar), 1254 (COC), 1344 (NO2). 1H NMR (CDCl3, d in ppm): 0.60 (s,
3H, CH3), 0.78–0.93 (m, 12H, CH3), 1.00–2.33 (m, 28H, CH2), 3.99
(t,J = 6.56 Hz, 2H, AOCH2A), 4.04 (t,J = 6.68 Hz, 2H, AOCH2A),
4.37–4.43 (m, 1H, OCH in chol.), 5.32 (m, 1H, CHCH2 in chol.),
6.95–6.97 (d,J = 8.92 Hz, 2H, aromatic), 7.88–7.90 (d,J = 8.28 Hz,
2H, aromatic, AArN@NArA), 7.90–7.92 (d,J = 8.48 Hz, 2H, aromatic,
AArN@NArA), 8.28–8.30 (d,J = 8.88 Hz, 2H, aromatic, ortho to
NO2). 13C NMR (100.6 MHz, CDCl3): 11.7, 18.5, 19.2, 22.5, 22.6
(CH3); 21.2, 24.2, 24.5, 25.9, 28.3, 28.6, 28.6, 28.6, 28.7, 28.9,
29.0, 29.1, 31.7, 32.2, 36.1, 36.5, 37.9, 39.4, 39.7 (CH2); 67.5, 68.2
(OCH2) ; 42.3 (quaternary); 27.8, 31.8, 35.4, 49.8 , 56.0, 56.3, 77.0
(CH); 122.6, 138.2 (vinylic); 114.2, 123.0, 124.1, 125.3 (aromatic);
146.8, 148.1, 154.1, 162.4 (aromatic quaternary); 155.5 (C@O).
Anal. Calcd. (%) for C51H75N3O6: C, 74.08; H, 9.11; N, 5.08. Found:
C, 74.04; H, 9.15; N, 5.13.
2941, 2868 (CH2), 1763 (C@O), 1608, 1498 (CC in Ar), 1252 (COC),
1346 (NO2). 1H NMR (CDCl3, d in ppm): 0.62 (s, 3H, CH3), 0.79–1.02
(m, 12H, CH3), 1.05–2.45 (m, 28H, CH2), 4.52–4.57 (m, 1H, OCH in
chol.), 5.36 (m, 1H, CHCH2 in chol.), 7.32–7.34 (d,J = 8.78 Hz, 2H,
aromatic), 7.93–7.95 (d,J = 8.80 Hz, 2H, aromatic, AArN@NArA),
7.96–7.98 (d,J = 8.72 Hz, 2H, aromatic, AArN@NArA), 8.31–8.33
(d,J = 8.86 Hz, 2H, aromatic, ortho to NO2). 13C NMR (100.6 MHz,
CDCl3): 12.0, 18.8, 19.3, 22.4, 22.4 (CH3); 21.3, 24.2, 24.5, 27.7,
32.2, 32.5, 36.6, 38.0, 39.7, 39.8, 40.0 (CH2); 42.4 (quaternary);
28.2, 31.7, 36.0, 50.3, 56.6, 56.2, 76.1 (CH); 122.6, 138.6 (vinylic);
120.2, 125.1, 125.8, 126.7 (aromatic); 147.6, 153.3, 155.3, 155.4
(aromatic quaternary); 153.9 (C@O). Anal. Calcd. (%) for
C40H53N3O5: C, 73.19; H, 8.08; N, 6.40. Found: C, 73.06; H, 8.12;
N, 6.45.
2.3.2. Synthesis of (ꢁ)-cholesteryl (4-methoxy-azobenzene-40-oxy)
carboxylate, C0
2.3.5. Synthesis of (ꢁ)-cholesteryl 6-(4-methoxy-azobenzene-40-oxy)
hexyl carbonate, C6
C0 was prepared by a procedure similar to that for N0, using 4-
hydroxy-40-methoxy-azobenzene (2) instead of 4-hydroxy-40-ni-
tro-azobenzene (1).
C6 was prepared by a procedure similar to that for N0, using 1-
hydroxy-6-(4-methoxy-azobenzene-40-oxy) hexane (5) instead of
4-hydroxy-40-nitro-azobenzene (1).
Yield: 61.8%. Tm = 171.7 °C. [
a]
D = ꢁ6.1°. FT-IR (KBr,
m
max/cmꢁ1):
Yield: 60.5%. K 160.0 °C Nꢂ 185.4 °C I (heating). [
a]D = –7.2°. FT-IR
2940, 2863 (CH2), 1760 (C@O), 1579, 1481 (CC in Ar), 1248 (COC).
1H NMR (CDCl3, d in ppm): 0.69 (s, 3H, CH3), 0.86–1.05 (m, 12H,
CH3), 1.08–2.52 (m, 28H, CH2), 3.88 (s, 3H, OCH3), 4.57–4.66 (m,
1H, OCH in chol.), 5.43 (m, 1H, CHCH2 in chol.), 6.99–7.02
(d,J = 8.82 Hz, 2H, aromatic), 7.31–7.34 (d,J = 8.60 Hz, 2H, aro-
(KBr, m
max/cmꢁ1): 2940, 2858 (CH2), 1744 (C@O), 1589, 1471 (CC in
Ar), 1274 (COC). 1H NMR (CDCl3, d in ppm): 0.66 (s, 3H, CH3), 0.84–
1.00 (m, 12H, CH3), 1.10–2.48 (m, 28H, CH2), 3.88 (s, 3H, OCH3),
4.03 (t,J = 6.56 Hz, 2H, AOCH2A), 4.14 (t,J = 6.73 Hz, 2H, AOCH2A),
4.44–4.49 (m, 1H, OCH in chol.), 5.40 (m, 1H, CHCH2 in chol.),

132
P.-C. Yang et al. / Journal of Molecular Structure 1015 (2012) 129–137
Yield: 60.4%. K 101.9 °C Nꢂ 168.5 °C I (heating). [
IR (KBr,
a
]
D = ꢁ4.6°. FT-
m
max/cmꢁ1): 2935, 2851 (CH2), 1739 (C@O), 1584, 1476 (CC
in Ar), 1276 (COC). 1H NMR (CDCl3, d in ppm): 0.60 (s, 3H, CH3),
0.78–1.00 (m, 12H, CH3), 1.02–2.33 (m, 28H, CH2), 3.81 (s, 3H,
OCH3), 3.96 (t,J = 6.54 Hz, 2H, AOCH2A), 4.04 (t,J = 6.72 Hz, 2H,
AOCH2A), 4.37–4.39 (m, 1H, OCH in chol.), 5.32 (m, 1H, CHCH2
Fig. 1. CAH HMQC NMR spectrum of N6.
6.95–6.98 (d,J = 8.80 Hz, 2H, aromatic), 6.99–7.02 (d,J = 8.64 Hz, 2H,
aromatic), 7.83–7.85 (d,J = 8.50 Hz, 2H, aromatic), 7.88–7.90
(d,J = 8.64 Hz, 2H, aromatic). 13C NMR (100.6 MHz, CDCl3): 11.9,
18.8, 19.2, 22.4, 22.4 (CH3); 21.2, 24.3, 24.5, 25.6, 27.2, 28.9, 28.9,
29.2, 32.2, 32.4, 36.6, 39.1, 39.8, 39.9, 40.0 (CH2); 55.5 (OCH3);
67.4, 68.1 (OCH2); 42.4 (quaternary); 28.1, 31.8, 36.1, 50.4, 56.5,
56.9, 75.1 (CH); 122.5, 138.7 (vinylic); 110.4, 113.3, 123.4, 124.9
(aromatic); 146.3, 146.3, 166.5, 166.9 (aromatic quaternary);
155.7 (C@O). Anal. Calcd. (%) for C47H68N2O5: C, 76.11; H, 9.18; N,
3.78. Found: C, 76.14; H, 9.23; N, 3.75.
2.3.6. Synthesis of (ꢁ)-cholesteryl 6-(4-methoxy-azobenzene-40-oxy)
undecyl carbonate, C11
C11 was prepared by a procedure similar to that for N0, using 1-
hydroxy-11-(4-methoxy-azobenzene-40-oxy) undecane (6) instead
of 4-hydroxy-40-nitro-azobenzene (1).
Fig. 2. DSC thermograms of (a) N6 and (b) C6. Inset: polarizing microscopic
textures at the corresponding state indicated in the figures (100ꢀ magnification).
Table 1
Thermal properties of azobenzene-based chromophores.
d
No.
Ra
[a
]
D
(°)b
Transition temperature (°C)c
D
D
Th
Tc
d
(corresponding enthalpy
Table 2
changes (J gꢁ1
)
Optical properties of azobenzene-based chromophores.
N0
N6
N11
C0
NO2
ꢁ5.7
ꢁ4.4
ꢁ5.2
ꢁ6.1
ꢁ7.2
ꢁ4.6
K 219.5 (55.1) I
–
–
a
c
d
e
No.
UV–vis kmax (nm) (log
e)
PL kmax (nm)b
UPL
t365
tdark
I 194.0 (ꢁ46.5) K
K 138.7 (67.2) Nꢂ 189.1 (4.1) I
I 187.6 (ꢁ3.9) Nꢂ 108.8 (ꢁ43.0) K
K 123.2 (53.0) Nꢂ 162.9 (2.7) I
I 161.0 (ꢁ2.9) Nꢂ 100.6 (ꢁ46.0) K
K 171.7 (58.2) I
50.4
78.8
39.6
60.4
–
N0
N6
N11
C0
C6
C11
342 (4.36)
380 (4.23)
381 (4.21)
349 (4.52)
359 (4.51)
359 (4.51)
413, 437, 465 s
413, 438, 466 s
413, 437, 461 s
415, 436
414, 434
416, 438
0.022
0.035
0.042
0.016
0.017
0.028
8 m
1 m
1 m
2 m
1 m
50 s
730 m
60 m
60 m
70 h
54 h
26 h
NO2
NO2
OCH3
OCH3
OCH3
I 107.8 (ꢁ41.7) K
–
C6
K 160.0 (68.0) Nꢂ 185.4 (3.9) I
I 183.1 (ꢁ3.9) Nꢂ 135.6 (ꢁ59.9) K
K 101.9 (44.8) Nꢂ 168.5 (3.0) I
I 165.1 (ꢁ3.8) Nꢂ 86.9 (ꢁ34.1) K
25.4
47.5
66.6
78.2
Maximum wavelength of the p–
pꢂ transition of compounds in CHCl3 solution;
a
concentration: 1 ꢀ 10ꢁ5 M. Extinction coefficient (Mꢁ1 cmꢁ1
) in the maxima
C11
absorption of UV–vis spectra.
b
The excitation wavelengths were the maxima absorption of UV–vis spectra
(concentration: 1 ꢀ 10ꢁ5 M). Superscript s means the wavelength of the shoulder.
a
Terminal groups of the synthesized compounds.
Specific rotation of compounds, [
meters (l = 1 dm), and
10 ml chloroform.
c
b
These values of quantum efficiency were measured using 9,10-diphenylan-
a
]
D
=
a
/(l ꢀ
q); l is the path length in deci-
thracene as a standard (concentration: 1 ꢀ 10ꢁ7 M, assuming a quantum efficiency
q is the density of the liquid in g/ml; concentration: 0.1 g in
of unity).
d
c
Time to reach the photostationary state of UV irradiation (365 nm).
Time to reach the photostationary state in the dark after UV irradiation of
Phase transition temperature during second heating and first cooling scans at a
e
rate of 10 K minꢁ1; K, crystal; Nꢂ, chiral nematic (cholesteric); I, isotropic phase.
d
365 nm.
D
Th = Tiso ꢁ TK?Nꢂ in heating cycle;
D
Tc = Tiso ꢁ TK?Nꢂ in cooling cycle.

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133
in chol.), 6.90–6.92 (d,J = 8.80 Hz, 2H, aromatic), 6.92–6.94
3. Results and discussion
(d,J = 8.66 Hz, 2H, aromatic), 7.78–7.80 (d,J = 8.52 Hz, 2H, aro-
matic), 7.80–7.82 (d,J = 8.66 Hz, 2H, aromatic). 13C NMR
(100.6 MHz, CDCl3): 11.9, 18.8, 19.2, 22.4, 22.5 (CH3); 21.2, 24.2,
24.5, 25.9, 28.4, 28.5, 28.5, 28.5, 28.6, 28.9, 28.9, 29.0, 29.1, 32.1,
32.4 36.6, 39.1, 39.8, 39.8, 40.0 (CH2); 55.4 (OCH3); 67.4, 68.1
(OCH2); 42.4 (quaternary); 28.2, 31.8, 36.0, 24.3, 50.3, 57.0, 75.2
(CH); 122.5, 138.7 (vinylic); 110.4, 113.3, 123.4, 124.9 (aromatic);
146.3, 146.4, 166.5, 166.9 (aromatic quaternary); 155.6 (C@O).
Anal. Calcd. (%) for C52H78N2O5: C, 76.92; H, 9.62; N, 3.45. Found:
C, 76.88; H, 9.65; N, 3.47.
3.1. Synthesis and characterization
A series of azobenzene-based chromophores containing termi-
nal electron-donating methoxy (AOCH3) and electron-withdraw-
ing nitro (ANO2) moieties were prepared to investigate the effect
of E/Z photoisomerization on the optical spectra under UV irradia-
tion. Scheme 1 describes the route followed for the synthesis of
these chromophores. The intermediates, bearing hydroxyl groups,
and six or eleven methylene segments as spacers, 3–6, were
Fig. 3. UV–vis spectra before and after UV irradiation with 365 nm for (a) N0, (b) N6, (c) N11, (d) C0, (e) C6, and (f) C11.

134
P.-C. Yang et al. / Journal of Molecular Structure 1015 (2012) 129–137
synthesized using 1, 2 and 6-chloro-1-hexanol, or 11-bromo-1-
undecanol as reactants, in the presence of KOH in EtOH, under
reflux. The six target chromophores were synthesized by esterifica-
tion of 1–6 with cholesteryl chloroformate, using pyridine as a
base, in dry dichloromethane solution at 30 °C under nitrogen
atmosphere. Chemical structures and constitutional composition
of the synthesized compounds were confirmed by elemental anal-
ysis, FTIR, and NMR spectroscopies. The 1H NMR spectra of two
representative compounds (N6 and C6) in CDCl3 are shown in
Fig. SI-1a. In N6, the aromatic protons at Ha appear as a doublet
at the most downfield position (8.35–8.37 ppm). Compared to
N6, the aromatic protons Hc and Hd in C6 (Fig. SI-1b) appear at
higher field (6.95–7.02 ppm) than the other aromatic protons at
Ha and Hb for C6 do; this is because of the adjacent electron-donat-
ing (AOCH3) terminal groups. 1H and 13C assignments of N6 were
confirmed by a combination of DEPT-135 (Fig. SI-2), and CAH
HMQC (Fig. 1) methods, as were the assignments of N0, C0, C6,
N11 and C11. The proton resonances in the CAH HMQC spectrum
show good correlation with the corresponding 13C resonances.
The molecular structures of the synthesized compounds were also
supported by elemental analysis.
contain suitable polarity and spacer length at the center and termi-
nals. It appears that compounds lacking the long methylene chain
as a flexible linkage stack via mesogenic side chains, and thus,
there is an absence of mesophase in compounds N0 and C0. How-
ever, N6, N11, C6, and C11, which all incorporate six or eleven
methylene segments as spacers, exhibit mesomorphic phases.
The four chromophores show enantiotropic mesophases, and re-
veal chiral nematic phases (Nꢂ; oily streaky texture), which we
attributed to the introduction of a flexible spacer between the mes-
ogenic core and the terminal groups to decouple side chain mo-
tions from the terminal groups in the mesophase, during heating
and cooling cycles. The LC phases were confirmed using DSC anal-
yses and were compared with polarizing optical microscopy (POM)
textures, as reported in the literature [35].
Fig. 2 shows DSC thermograms and POM textures for N6 and C6
at a heating rate of 10 K minꢁ1. During the heating scan, N6 melted,
and a chiral nematic mesophase was observed between 138.7 and
189.1 °C before isotropization occurred. During the cooling scan,
we observed an isotropic-to-chiral nematic phase transition at
187.6 °C and crystallization at 108.8 °C. C6, with an electron releas-
ing methoxy (AOCH3) segment, exhibited enantiotropic meso-
phase in the temperature ranges 160.0–185.4 °C, and 135.6–
183.1 °C during heating and cooling cycles, respectively (Fig. 4b).
In comparison, N6 showed broader phase transition temperature
ranges than C6 did, suggesting that the electron-withdrawing nitro
(ANO2) moiety enhances the head-to-tail molecular interaction,
3.2. Thermal and liquid crystalline properties
Phase transition temperatures and the phases of the synthe-
sized compounds are summarized in Table 1. The data listed in Ta-
ble 1 show that the effect of molecular structure on melting
temperature, mesomorphic properties, and even molecular
arrangement of the chromophores was considerable. Chromoph-
ores N0 and C0, without the presence of flexible spacer lengths
do not show any LC mesophase. In general, LC mesophase must
because of the existence of strong nitro p–p interactions. However,
introduction of the longer eleven methylene section as a spacer in
N11 might decrease head-to-tail molecular interactions, resulting
in a reduction in phase transition temperature ranges. By contrast,
chromophores C6 and C11 that incorporate terminal electron-
Fig. 4. Stability of UV–vis spectra of (a) N0, (b) N6, (c) C6, and (d) C11 in the dark.

P.-C. Yang et al. / Journal of Molecular Structure 1015 (2012) 129–137
135
donating methoxy moieties, exhibit different behavior to that of N6
and N11. Compound C11 showed broader phase transition temper-
ature ranges compared with those of C6. The results suggest that
the rigidity of the mesogenic core, the flexible spacer length, and
types of terminal unit have a large influence on intermolecular
and dipole-induced dipole interactions, leading to the observed
variation in phase transition temperatures. It also indicates that
the terminal substituent is important in size and in the nature of
its polarity. Consequently, the intermolecular interaction, dipole-
induced dipole interactions might play an important role in deter-
mining the type of mesophase textures and influence the physical
properties of the LC compounds [36]. Furthermore, as shown in Ta-
ble 1, the enthalpy changes for the chiral nematic to isotropic
phase transition in the range 2.7–4.1 J gꢁ1 were smaller than those
of the chiral nematic to crystallization phase transition (around
34.1–68.0 J gꢁ1), consistent with the ordering of the molecular
arrangement.
for Z–E isomerization within 60 min, as shown in Fig. 4b and
Table 2. Fig. 4c and d shows a steady state for Z–E isomerization
after 54 h and 26 h for C6 and C11, respectively, implying that
N6 and N11, which incorporate the nitro terminal group, use a
rotation mechanism with low potential energy profile to achieve
a faster rate of Z–E isomerization in darkness than C6 and C11 do
[31].
As shown in Table 2 and Fig. 5, the fluorescence spectra of N
and C series azobenzene-based compounds in CHCl3 at ambient
temperatures, exhibit emission maxima at approximately
413–438 nm. The N series fluorescence peaks include two well
defined, and one shoulder at approximately 413, 437, and
465 nm; these might arise from the different vibrational–rota-
tional levels of the excited states and ground electronic states.
The PL quantum yields of photochromic compounds, estimated
using 9,10-diphenylanthracene as reference, were at the range
of 0.016–0.042.
Right and left rotations of plane-polarized light passing through
the compounds occurred, depending on the arrangement of ligands
around the chiral center. In principle, the characteristic net vector
of the rotation of plane-polarized light varies with polarity of the
chiral molecules [37]. The specific rotation of cholesteryl chlorofor-
mate is ꢁ28°. As set out in Table 1, the specific rotations of chiral
chromophores derived from cholesteryl chloroformate have nega-
tive values. Specifically, it was found that chirality could be a fac-
tor, which influences the physical properties of the compounds.
Cleavage of the CACl bond and the binding of an azobenzene group
significantly affect the chirality of these compounds. Our findings
suggest that the specific rotation is influenced by the coherence
of polarity changes due to chemical bonding during the esterifica-
tion reaction.
3.4. Fluorescence enhancement effect on PL properties
In this study, we conducted fluorescence spectroscopy on azo-
benzene solutions to further investigate solvent effects on lumi-
nescence properties. The percentages of THF/water mentioned
in the text refer to volume ratios of water. The solutions were left
to equilibrate at room temperature for 30 min after preparation
before carrying out further studies. Fig. 6 shows the UV–vis and
PL spectra of C6 in THF/water (good/non-solvent) mixtures with
various volume ratios of water. In dilute THF solution, the azo-
benzene molecules were well-dissolved and isolated. C6
3.3. Optical properties
We studied the E/Z isomerization of the azobenzene-based
chromophores substituted with various terminal groups, to ex-
plore the photoreactivity of these compounds. Optical properties
were investigated using UV–vis and PL spectroscopies. E–Z photo-
isomerization of chromophores was studied by UV absorption
spectroscopy at 365 nm in CHCl3. Table 2 summarizes the UV–vis
absorption data for the photochromic compounds. These com-
pounds exhibit strong UV–vis absorption bands at 342–381 nm,
which we attribute to a
p–
pꢂ transition of the chromophore in
the E-isomer. Weak absorption bands at approximately 440–
450 nm resulted from an n–pꢂ transition in the E-isomer. The
absorption maxima for N6 and N11, which incorporate the elec-
tron-withdrawing nitro group, are located at approximately 380–
381 nm, and are red-shifted (ca. 21–22 nm) relative to C6 and
C11, which incorporate the electron-donating methoxy group.
Our results suggest that the electronic transition in N6 and N11 in-
volves a migration of electron density from a donor group (AORA)
toward an acceptor group (ANO2), leading to the bathochromic
shifts seen for N6 and N11. The UV–vis spectra in Fig. 3 shows
the variation in shift for various UV irradiation times for all of
our synthesized compounds. Fig. 4 illustrates the stability of four
representative compounds in darkness. UV irradiation induces a
decrease in the absorption intensity of the absorption maxima,
and an increase in absorption at approximately 450 nm during
irradiation. As shown in Fig. 3, an E–Z photostationary state was
obtained for N6 and N11 within 1 min of irradiation. The observed
variation in absorptions for the various compounds might result
from the geometric change from the E-isomer to the Z-isomer of
the azo compounds. The E–Z isomerization rate and time of photo-
stationary state under UV irradiation were influenced by various
potential energy profiles between E/Z isomers and dipolar transi-
tion state [31]. In darkness, N6 and N11 achieved thermal stability
Fig. 5. Normalized UV–vis and PL spectra of (a) N and (b) C series compounds in
chloroform.

136
P.-C. Yang et al. / Journal of Molecular Structure 1015 (2012) 129–137
fluorophore aggregation. The enhanced fluorescence emission in
azo nanoparticles might also result from the combined effects
of aggregation-induced planarization and J-aggregation forma-
tion, which restrict parallel face-to-face intermolecular interac-
tions and the formation of excimers in the aggregated state
[38,39]. Enhanced fluorescence did not occur in N series solu-
tions. The results are thus consistent with the substituent effects
of azobenzene derivatives seen for fluorescence intensity, sug-
gesting that the electron-donating group at the terminal position
is important for intense emission [40]. A detailed investigation of
PL solvent-dependence and scanning electron microscopy mor-
phologies of azobenzene nanoparticles is now in progress.
4. Conclusions
We successfully synthesized and characterized six azoben-
zene-substituted chromophores. The optical properties of the
photochromic compounds were evaluated using UV–vis and PL
spectroscopies. All synthesized compounds bearing six or eleven
methylene segments revealed chiral nematic phases, and exhib-
ited an oily streaky texture. Introduction of the nitro-group in
a terminal position resulted in increased Z to E isomerization
rates in darkness. We have demonstrated that C6 azobenzene
nanoparticles aggregate in THF/water solution at a 10% water
fraction.
Acknowledgment
The authors are thankful for the National Science Council of the
Republic of China (Taiwan) for its financial aid through project NSC
99-2113-M-115-001.
Appendix A. Supplementary material
Fig. 6. Dependence of THF/water mixture on (a) UV–vis and (b) PL spectra of C6.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2012.02.032.
exhibited a typical azobenzene absorption spectrum with a weak
References
n–pꢂ transition at approximately 440–450 nm and an intense
p–
pꢂ transition moment along the long-axis at 358 nm. Increase of
water content in the solution (i.e. THF/water = 9/1) resulted in a
red shift (ca. 14 nm) of the main absorption band centered at
approximately 372 nm (Fig. 6a). This bathochromic effect might
indicate that effective conjugation length of C6 is extended from
the isolated twisted molecule to the planar one in nanoparticles.
Aggregates were spontaneously formed upon adding non-solvent
to the THF solution of C6. The bulky cholesteryl and polar car-
bonate groups in C6 might play important roles of restricting
the parallel face-to-face intermolecular interactions in the aggre-
gated state. Preventing parallel orientation of conjugated chro-
mophores tends to favor J-aggregation with enhanced
fluorescence emission instead of H-aggregation in the solid state.
Extended conjugation and J-aggregation lead to the dramatic in-
crease of the fluorescence intensity in 10% volume fractions of
water addition of C6 nanoparticles as shown in Fig. 6b. However,
with subsequent increase in water content (i.e. THF/water = 3/7
and 1/9), the blue shift of the absorption band was accompanied
by a decrease in intensity. The solutions containing aggregates
exhibited a blue shift due to the scattering of incident light.
The original quantum yield of C6 in THF solution is low at
0.017. When the water fraction in the THF/water mixture reaches
10%, the quantum yield rises to 0.203, approximately 12 times
greater than that in THF alone. The quantum yield increases sig-
nificant upon adding up to 10% water, but decreases again at
greater dilutions. This also indicates that fluorescence enhance-
ment effect on C6 in 10% THF/water mixture might be due to
[1] M. O’Neill, S.M. Kelly, Adv. Mater. 15 (2003) 1135–1146.
[2] A.J.J.M. van Breemen, P.T. Herwig, C.H.T. Chlon, J. Sweelssen, H.F. Schoo, M.
Setayesh, W.M. Hardeman, C.A. Martin, D.M. de Leeuw, J.J.P. Valeton, C.W.M.
Bastiaansen, D.J. Broer, A.R. Popa-Merticaru, S.C.J. Meskers, J. Am. Chem. Soc.
128 (2006) 2336–2345.
[3] G. Schwarz, H.R. Kricheldorf, J. Polym. Sci. Part A: Polym. Chem. 34 (1996) 603–
611.
[4] S.H. Chen, H. Shi, B.M. Conger, J.C. Mastrangelo, T. Tsutsui, Adv. Mater. 8 (1996)
998–1001.
[5] C.T. Chen, Y.T. Chou, J. Am. Chem. Soc. 122 (2000) 7662–7672.
[6] Y. Wu, A. Kanazawa, T. Shiono, T. Ikeda, Q. Zhang, Polymer 40 (1999) 4787–
4793.
[7] A. Bobrovsky, V. Shibaev, Polymer 47 (2006) 4310–4317.
[8] N. Tamaoki, S. Song, M. Moriyama, H. Matsuda, Adv. Mater. 12 (2000) 94–97.
[9] R. Giménez, M. Millaruelo, M. Piñol, J.L. Serrano, A. Viñuales, R. Rosenhauer, T.
Fischer, J. Stumpe, Polymer 46 (2005) 9230–9242.
[10] R. Giménez, M. Piñol, J.L. Serrano, A.I. Viñuales, R. Rosenhauer, J. Stumpe,
Polymer 47 (2006) 5707–5714.
[11] Y. Wu, J. Mamiya, T. Shiono, T. Ikeda, Q. Zhang, Macromolecules 32 (1999)
8829–8835.
[12] P.M. Hogan, A.R. Tajbakhsh, E.M. Terentjev, Phys. Rev. E 65 (2002) 041720.
[13] Y. Yu, M. Nakato, T. Ikeda, Nature 425 (2003) 145.
[14] M.H. Li, P. Keller, B. Li, X.G. Wang, M. Brunet, Adv. Mater. 15 (2003) 569–572.
[15] N. Tamai, H. Miyasaka, Chem. Rev. 100 (2000) 1875–1890.
[16] M. Bruchez, M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science (1998) 2013–
2016.
[17] A.N. Shipway, E. Katz, I. Willner, ChemPhysChem 1 (2000) 18–52.
[18] A. Hagfeldt, M. Grätzel, Chem. Rev. 95 (1995) 49–68.
[19] M.C. Schlamp, X. Peng, A.P. Alivisatos, J. Appl. Phys. 82 (1997) 5837–5842.
[20] H. Kasai, H. Kamatani, Y. Yoshikawa, S. Okada, H. Oikawa, A. Watanabe, O. Ito,
H. Nakanishi, Chem. Lett. (1997) 1181–1182.
[21] J.B. Birks, Photophysics of Aromatic Molecules, Wiley-Interscience, London,
1970.
[22] M. Levitus, K. Schmieder, H. Ricks, K.D. Shimizu, U.H.F. Bunz, M.A. Garcia-
Garibay, J. Am. Chem. Soc. 123 (2001) 4259–4265.

P.-C. Yang et al. / Journal of Molecular Structure 1015 (2012) 129–137
137
[23] J. Luo, Z. Xie, J.W.Y. Lam, L. Cheng, H. Chen, C. Qiu, H.S. Kwok, X. Zhan, Y. Liu, D.
[32] J.H. Liu, P.C. Yang, J. Appl. Polym. Sci. 91 (2004) 3693–3704.
[33] P.C. Yang, M.Z. Wu, J.H. Liu, Polymer 49 (2008) 2845–2856.
[34] H.J. Son, W.S. Han, K.R. Wee, S.H. Lee, J. Mater. Chem. 19 (2009) 8964–8973.
[35] I. Dierking, Textures of Liquid Crystals, Wiley-VCH, Weinheim, 2003.
[36] K. Rameshbabu, P. Kannan, R. Velu, P. Ramamurthy, Liq. Cryst. 32 (2005) 823–
832.
Zhu, B.Z. Tang, Chem. Commun. (2001) 1740–1741.
[24] R. Deans, J. Kim, M.R. Machacek, T.M. Swager, J. Am. Chem. Soc. 122 (2000)
8565–8566.
[25] H. Rau, Angew. Chem. Int. Ed. Engl. 12 (1973) 224–235.
ˇ
[26] M. Nepraš, S. Lunák Jr., R. Hrdina, J. Fabian, Chem. Phys. Lett. 159 (1989) 366–
370.
[37] D.P. Shomaker, Experimental Physical Chemistry, McGraw-Hill, 1989. pp. 728–
729.
[38] D. Oelkrug, A. Tompert, J. Gierschner, H. Egelhaaf, M. Hanack, J. Phys. Chem. B
102 (1998) 1902–1907.
[39] G. Narayan, N.S.S. Kumara, S. Paul, O. Srinivas, N. Jayaraman, S. Dasa, J.
Photochem. Photobiol. A: Chem. 189 (2007) 405–413.
[40] J. Yoshino, A. Furuta, T. Kambe, H. Itoi, N. Kano, T. Kawashima, Y. Ito, M.
Asashima, Chem. Eur. J. 16 (2010) 5026–5035.
[27] Y. Wakatsuki, H. Yamazaki, P.A. Grutsch, M. Santhanam, C. Kutal, J. Am. Chem.
Soc. 107 (1985) 8153–8159.
[28] M. Han, M. Hara, J. Am. Chem. Soc. 127 (2005) 10951–10955.
[29] M.R. Han, Y. Hirayama, M. Hara, Chem. Mater. 18 (2006) 2784–2786.
[30] P.C. Yang, Y.H. Wang, H. Wu, J. Appl. Polym. Sci. 124 (2012) 4193–4205.
[31] C.H. Ho, K.N. Yang, S.N. Lee, J. Polym. Sci. Part A: Polym. Chem. 39 (2001) 2296–
2307.