Chain length-dependent photoinduced formation of azobenzene aggregates

Article abstract of DOI:10.1039/b511462d

We describe photoinduced formation of blue fluorescent aggregates of azobenzene derivatives, whose isolated molecules in solution are not fluorescent at ambient temperature, with different alkyl chain lengths. In contrast to nonfluorescent CO (4-hydroxyaz

Full text of DOI:10.1039/b511462d

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Chain length-dependent photoinduced formation of azobenzene
aggregates
Mina R. Han* and Masahiko Hara
Received (in Montpellier, France) 11th August 2005, Accepted 9th December 2005
First published as an Advance Article on the web 22nd December 2005
DOI: 10.1039/b511462d
We describe photoinduced formation of blue fluorescent aggregates of azobenzene derivatives,
whose isolated molecules in solution are not fluorescent at ambient temperature, with different
alkyl chain lengths. In contrast to nonfluorescent C0 (4-hydroxyazobenzene), C10 and C24 with
long alkyl chains started being blue fluorescent upon UV light irradiation for 60 min and the
fluorescence intensities were more enhanced with increasing UV light irradiation time. Such a
striking fluorescence enhancement by UV light irradiation is closely associated with the unique
formation of self-assembled nanoaggregates (50–70 nm and 30–60 nm in diameter for C10 and
C24, respectively) of cis-azobenzene derivatives with long alkyl chains as a result of
intermolecular interaction of azobenzene moieties with long alkyl chains. Moreover, photoinduced
blue fluorescence kept stable even after sufficient thermal cis-to-trans isomerization.
chain length, upon UV light irradiation in organic solution
Introduction
C10 and C24, with long alkyl chains, spontaneously assembled
into aggregates, which showed blue fluorescence.
Functionalization of self-assembling systems has been a chal-
lenging research topic in the field of chemical biology and
materials science because it leads to significant supramolecular
structures exhibiting selective binding affinity, synthetic bi-
layer membranes, photoluminescence, and optical data
storage.1–6 Incorporation of photochromic azobenzene
compounds into such supramolecular structures can give rise
to artificial photoresponsive systems. The photoisomerization
of azobenzene and its derivatives influences versatile physical
properties such as wettability, viscosity, and aggregation
behaviors because of changes in molecular structure and
dipole moment (from 0 of trans form to ca. 3 debye of cis
form) between the rod-shaped trans form and the bent-shaped
cis form.7
Experimental
Materials and instrumentation
Dichloromethane was freshly distilled prior to use under a
nitrogen atmosphere in order to remove extra oxygen and
water. After a 30-sec nitrogen purge, a screw-cap quartz
cuvette containing a dichloromethane solution of an azo-
benzene derivative was sealed with Parafilm.
Photoisomerization measurements were taken using a
Mineralights lamp (365 nm, Model UVGL-25, UVP, Upland,
CA 91786, USA) as a light source. 1H NMR spectra were
recorded on JEOL JNM-EX270 (270 MHz) in CDCl3 (Merck,
99.8%) and CD2Cl2 (Aldrich, 100 atom% D). Absorption and
fluorescence spectra were obtained using a Shimadzu UV-
3100PC UV-VIS-NIR scanning spectrophotometer and a
JASCO FP-6500 spectrofluorometer, respectively. The SEM
(scanning electron microscopy) was recorded on a Hitachi
S-5200, after putting one drop of an azobenzene solution on a
clean glass substrate and coating it with platinum about 2 nm
thick using the Hitachi E-1030 ion-sputter.
While it is well known that in general the azobenzene
molecule in solution is not fluorescent with reasonable quan-
tum yield, only a few exceptional azobenzenes incorporated
into Nafion (Nafion-H1) membrane8 and into an alumino-
phosphate framework (AlPO4-5)9 were reported to be fluorescent
at room temperature. However, the fluorescence was not due
to azobenzene itself but due to the protonated azobenzenes.
On the other hand, even though the uncomplexed azobenzene
compound was nonfluorescent in solution, cyclopalladated
azobenzene derivatives were weakly fluorescent (F
B
10ꢀ4).10 By cyclometallation, the photoisomerization of a
trans-blocked azobenzene ligand was suppressed and the
efficiency of emission could increase significantly.11
Syntheses
4-(12-Bromododecyloxy)azobenzene (C12-Br). C12-Br was
synthesized by reacting 4-(hydroxyl)azobenzene (C0, 2.50 g,
0.0126 mol) with 1,12-dibromododecane (12.4 g, 0.0379 mol)
in 100 mL of acetone in the presence of excess anhydrous
K2CO3 (3.48 g, 0.0250 mol) and a catalytic amount of
potassium iodide. The reaction mixture was stirred at 60 1C
for 11 h. After cooling the mixture to room temperature,
acetone was removed under reduced pressure. The residue
was washed three times with water, dried in vacuum and
Here we describe significant photoinduced fluorescence en-
hancement of simple azobenzene derivatives with different
alkyl chain lengths. Unlike nonfluorescent C0 without an alkyl
Local Spatio-Temporal Functions Laboratory, Frontier Research
System, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
E-mail: minahjp@riken.jp; Fax: (þ81) 48 462 4695;
Tel: (þ81) 48 462 1111
1
recrystallized from 2-propanol (3.1 g, yield: 55%). H NMR
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(270 MHz, CDCl3) d 1.2–1.6 (m, 16H, CH2), 1.6–1.9 (m, 4H,
BrCH2CH2, ArOCH2CH2), 3.40 (t, 2H, BrCH2), 4.04 (t, 2H,
ArOCH2), 7.00 (d, 2H, Ar-H), 7.39–7.52 (m, 3H, Ar-H),
7.85–7.92 (dd, 4H, Ar-H).
4-[12-(Dodecyldithio)dodecyloxy]azobenzene (C24). Sodium
thiosulfate pentahydrate (0.836 g, 0.0337 mol) in 6 mL of
distilled water was added to C12-Br (1 g, 0.00225 mol) in 50
mL of DMF solution under a nitrogen atmosphere and the
reaction mixture was stirred at 60 1C for 6 h. After cooling to
room temperature, 2 mL of water was added to the mixture
and the precipitate was filtered out to obtain Bunte salt (0.088
g). A solution of Bunte salt in 30 mL of DMF was added to a
solution of dodecanethiol (0.499 g, 0.00247 mol), methanol (2
mL) and NaOH (0.109 g) in 1 mL of distilled water, and the
reaction mixture was stirred for 3 h under a nitrogen atmo-
sphere. Chloroform (30 mL) and water (30 mL) were poured
into the mixture, and the organic layer was collected and
evaporated. The residue was purified by silica gel column
chromatography using hexane–chloroform (10 : 1) as eluent,
to afford an orange crystal (0.25 g, yield: 18%). 1H NMR (270
MHz, CDCl3) d 0.88 (t, 3H, CH3), 1.2–1.6 (m, 34H, CH2), 1.61–
1.69 (m, 4H, SSCH2CH2), 1.76–1.87 (m, 2H, ArOCH2CH2),
2.68 (t, 4H, SSCH2), 4.04 (t, 2H, ArOCH2), 7.00 (d, 2H, Ar-
H), 7.39–7.52 (m, 3H, Ar-H), 7.86–7.92 (dd, 4H, Ar-H). IR
(ATR): 2918, 2849 (C–H stretching), 1604, 1587, 1500 (ben-
zene ring), 1253 (Ph–O stretching) cmꢀ1. FAB-MS (m/z): [M þ
H]1 found, 599.40 (¼M þ 1), calcd for C36H58N2OS2, 598.40.
Anal. Calcd: C, 72.19; H, 9.76; N, 4.68; S, 10.71. Found: C,
71.84; H, 9.80; N, 4.69; S, 10.58%.
Scheme 1 Synthesis of C10 and C24.
was generated by UV light irradiation and underwent very fast
thermal cis-to-trans isomerization (within B5 min) at ambient
temperature (inset of Fig. 1).
When initial dilute solutions (6 ꢂ 10ꢀ5 M for C0 and C10,
and 5 ꢂ 10ꢀ5 M for C24) were excited at 365 nm, no
fluorescence was observed at ambient temperature, regardless
of alkyl chain length. After 3-min UV irradiation for generat-
ing a cis-rich photostationary state, the solutions (C0, C10,
and C24) did not fluoresce as well.
4-(Decyloxy)azobenzene (C10). C10 was synthesized by a
procedure analogous to that for C12-Br. 1H NMR (270 MHz,
CD2Cl2) d 0.89 (t, 3H, CH3), 1.2–1.6 (m, 14H, CH2), 1.76–1.87
(m, 2H, ArOCH2CH2), 4.05 (t, 2H, ArOCH2), 7.00 (d, 2H, Ar-
H), 7.41–7.54 (m, 3H, Ar-H), 7.85–7.93 (dd, 4H, Ar-H). Anal.
Calcd: C, 78.06; H, 8.93; N, 8.28. Found: C, 77.58; H, 8.94; N,
8.30%.
However, obviously different from nonfluorescent C0 even
after prolonged UV light (365 nm light, 3 mW cmꢀ2) irradia-
tion, C10 and C24 with long alkyl chains started being weakly
Results and discussion
The structures of azobenzene derivatives used in this investi-
gation are shown in Scheme 1. All the azobenzene compounds
exhibited good solubility in dichloromethane to give solutions
characterized by the typical azobenzene monomer-like absorp-
tion spectra with an intense pꢀp* absorption band at 342 nm
(for C0) and 348 nm (for C10 and C24) and a weak nꢀp*
absorption band at about 400–500 nm (Fig. 1).12 Exposure of
azobenzene solutions to UV light at 365 nm for 3 min for
inducing sufficient trans-to-cis photoisomerization led to a
marked reduction in the trans-azobenzene pꢀp* absorption
band and a concomitant increase in the nꢀp* absorption band
at 440 nm. Dark incubation of the UV-exposed C10 and C24
solutions resulted in slow thermal cis-to-trans back isomeriza-
tion, keeping two sharp isosbestic points at 304 and 408 nm
(Fig. 1), indicating that conversion between only two species,
trans and cis forms of C10 and C24, occurs in solution in the
early stage of UV light irradiation. In contrast to sufficient
trans-to-cis photoisomerization of C10 and C24 with long
alkyl chains, as small as about 20% of the cis form of C0
Fig. 1 Changes in UV-Vis absorption spectra of C24 in dichloro-
methane as a function of time for thermal cis-to-trans isomerization in
the dark after 365 nm light irradiation for 3 min. Inset: Changes in
normalized absorbance at lmax of (a) C0, (b) C10 and (c) C24 as a
function of time for thermal cis-to-trans back isomerization after
365 nm light irradiation for 3 min.
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solution at a certain concentration was almost constant. The
photographs in Fig. 4 display blue fluorescence from UV-
exposed C10 and C24 solutions, compared with no emission
from the C0 solution. The fluorescence quantum yields, esti-
mated from 9,10-diphenyl anthracene in cyclohexane as the
reference,13 of UV-exposed C10 and C24 solutions were 1.1 ꢂ
10ꢀ3 and 2 ꢂ 10ꢀ3, respectively. These quantum yields are very
high when compared with those of earlier reports (B10ꢀ7
–
10ꢀ5).7 This unique fluorescence enhancement is not likely due
to photodecomposition or further undesirable photoreaction
of azobenzene units under UV light irradiation, but more
likely due to the formation of self-assembled aggregates.
The formation of spherical aggregates of UV-exposed azo-
benzene derivatives with long alkyl chains in organic solution
was confirmed by using scanning electron microscopy (SEM).
The micrographs in Fig. 5 show spherical aggregates of
approximately 50–70 nm and 30–60 nm in diameter for C10
and C24, respectively. However, no spherical aggregates were
observed in the samples non-irradiated with UV light. This
observation strongly suggests that the remarkable increases in
fluorescence intensities are closely related to the photoinduced
formation of aggregates, that is, aggregation-induced emis-
sion.14,15
Fig. 2 Changes in fluorescence intensities of C24 (¼5 ꢂ 10ꢀ5 M) as a
function of 365 nm light irradiation time at different irradiation
intensities of UV light.
fluorescent at about 440–470 nm upon UV light irradiation for
60 min. With increasing UV light irradiation time, the fluor-
escence intensities of C10 and C24 solutions were more
enhanced and saturated within 180 min and 300 min, respec-
tively. Their fluorescence maxima were red-shifted to 465 nm
with increasing UV light irradiation time (Fig. 3e and 3f).
Futhermore, with respect to a rate dependence on UV light
intensity, it was observed that an induction period became
shorter and an increasing rate of photoinduced fluorescence
intensity was faster with increasing light intensity (Fig. 2). The
finally saturated fluorescence intensity of the UV-exposed
To understand the reason for the formation of aggregates by
UV light irradiation, additional UV-Vis absorption spectra
were taken. In contrast to no apparent spectral change for
nonfluorescent C0, upon prolonged UV light irradiation of
both C10 and C24 solutions, an obvious deviation from two
isosbestic points was observed (Fig. 3b and 3c), implying that
other reactions such as photoinduced aggregation could fol-
low the first stage of typical trans-to-cis isomerization. Further
Fig. 3 Upper panel: changes in UV-Vis absorption spectra of (a) C0, (b) C10, and (c) C24 in dichloromethane. (7 days: dark incubation for 7 days
after UV light irradiation, 3 mW cmꢀ2). Lower panel: fluorescence spectra of (d) C0, (e) C10, and (f) C24 at ambient temperature (365 nm
excitation). Insets show uncorrected fluorescence spectra of the initial azobenzene solution and solvent (dichloromethane) itself.
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Fig. 4 Fluorescence from the UV-exposed azobenzene solutions
under 365 nm irradiation. (a) C0, (b) C10, and (c) C24.
evidence for unexpected photoinduced reaction in a UV-
exposed solution can be provided by an NMR study. The
NMR spectra indicate that the cis form of C0 does not exist
even after UV light irradiation for 300 min, as seen in Fig. 6A.
This NMR result is in good agreement with the absorption
spectral data of C0, as shown in Fig. 3a. On the other hand,
upon UV light irradiation of C10 and C24 with long alkyl
1
chains, H NMR signals of the aromatic ring protons of the
azobenzene molecules were significantly shifted upfield, not
due to any decomposition of the azobenzene moieties or
disulfide group but rather due to trans-to-cis isomerization
(Fig. 6B and 6C). About 85% and 76% of cis-azobenzenes16 of
C10 and C24, respectively, were present in UV-exposed solu-
tions (Fig. 6b) and the cis-azobenzenes were fully recovered to
the trans-azobenzenes within 7 days in the dark (Fig. 6c).
Dark incubation of the UV-exposed C24 for sufficient
thermal cis-to-trans isomerization resulted in the weak p–p*
absorption band at about 347–351 nm. On the other hand,
after sufficient thermal cis-to-trans isomerization, the p–p*
absorption band (lmax) of C10 was weak and more or less
red-shifted to 354 nm (Fig. 3b). This red-shift of the absorp-
tion band can be explained in terms of the change in the
intermolecular interactions of the azobenzene chromophores
with long alkyl chains from the initial isolated species to self-
assembled aggregates. Such a spectral change arising from
intermolecular stacking between aromatic units to form ag-
gregates has been seen from azobenzene chromophores, silole
derivatives, and biphenylethylene derivatives.14,15,17,18 In ad-
dition, the bent-shaped cis form is more polar and hydrophilic
relative to the rod-shaped trans form, while long alkyl chains
are hydrophobic.19 When such
a polar cis-azobenzene
Fig. 6 1H NMR spectral changes of (A) C0, (B) C10 and (C) C24
in CD2Cl2. (a) Initial, (b) after UV light irradiation for 300 min,
(c) after dark incubation for 7 days after UV light irradiation for
300 min.
Fig. 5 SEM images of (a) C10 and (b) C24 after UV light irradiation
for 240 min and 300 min, respectively.
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molecule with a hydrophobic alkyl chain exists in solutions for
a long time, the molecule seems to spontaneously self-assemble
into aggregates. That is, the first stage of a cis-rich photosta-
tionary state induced by UV light can be slowly followed by
the spontaneous formation of blue fluorescent aggregates of
cis-azobenzenes with long alkyl chains. Furthermore, photo-
induced blue fluorescence remained stable even after sufficient
cis-to-trans thermal isomerization at ambient temperature.
Nevertheless, it is still unusual that cis-azobenzenes with long
alkyl chains self-assemble into aggregates exhibiting blue
fluorescence in organic solution.
2 (a) K. Ichimura, Chem. Rev., 2000, 100, 1847; (b) K. Ichimura,
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Chem. Soc., 2000, 122, 8648.
4 D. K. Hore, A. L. Natansohn and P. L. Rochon, J. Phys. Chem. B,
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5 M. Han and M. Hara, J. Am. Chem. Soc., 2005, 127, 10951.
6 P. H. Rasmussen, P. S. Ramanujam, S. Hvilsted and R. H. Berg, J.
Am. Chem. Soc., 1999, 121, 4738.
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Academic Press, Elsevier Science, USA, 2002.
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In conclusion, on the basis of the results of three azobenzene
derivatives with different alkyl chain lengths, we note that long
alkyl chains play an important role in causing the sufficient
lifetime of cis-azobenzene for assembling into blue fluorescent
aggregates as well as in facilitating intermolecular stacking
between azobenzene moieties and hydrophobic interactions
between long alkyl chains.
12 M. Han and K. Ichimura, Macromolecules, 2001, 34, 82.
13 S. Hamai and F. Hirayama, J. Phys. Chem., 1983, 87, 83.
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Acknowledgements
16 Because NMR measurement of a sample took 10–20 min, the
presence of about 15% and 24% of trans-azobenzenes of C10 and
C24, respectively, is closely related to the extent of thermal cis-to-
trans isomerization for 10–20 min.
This work was supported by the postdoctoral fellowship of
Japan Society for the Promotion of Science (JSPS).
17 M. Shimomura and T. Kunitake, J. Am. Chem. Soc., 1987, 109,
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18 R. Davis, N. P. Rath and S. Das, Chem. Commun., 2004, 74.
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