Anisotropic fluorescence emission of ionic complex induced by the orientation of azobenzene unit

Article abstract of DOI:10.1021/ma400247h

A series of azobenzene-containing fluorescent complexes with stilbene fluorescent unit were prepared based on the ionic self-assembly approach. The incorporation of the stilbene unit did not change the lamellar structure of the azobenzene-containing compl

Full text of DOI:10.1021/ma400247h

Article
pubs.acs.org/Macromolecules
Anisotropic Fluorescence Emission of Ionic Complex Induced by the
Orientation of Azobenzene Unit
Jun Wu, Xuemin Lu,* Zeyuan Yi, Feng Shan, and Qinghua Lu*
School of Chemistry and Chemical Engineering, the State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong
University, Shanghai 200240, P. R. China
S
* Supporting Information
ABSTRACT: A series of azobenzene-containing fluorescent
complexes with stilbene fluorescent unit were prepared based
on the ionic self-assembly approach. The incorporation of the
stilbene unit did not change the lamellar structure of the
azobenzene-containing complex. Under pulsed laser irradi-
ation, the azobenzene group oriented in the direction
perpendicular to the laser polarization, and as a result, the
fluorescent stilbene unit was cooperatively oriented following
the direction of azobenzene group orientation. This oriented complex films presented anisotropic emission of fluorescence, and
the anisotropic ratio of fluorescence increases with the increase of the content of azobenzene unit in the complex.
electron transfer process.23−25 In fact, the quenching effects
INTRODUCTION
■
only can occur under certain condition, such as the overlap
Materials with anisotropic fluorescence emission played a key
integral between the fluorescence and the azobenzene unit,26,27
role in constructing LED as light source in the fast developing
certain chemical structures of the chromophores,28,29 and
field of flat display technologies.1,2 A direct approach to prepare
special microstructure of the matrix.30−32 Therefore, whether
materials with anisotropic fluorescence emission is to align the
the fluorescence quenching can be avoided through appropriate
fluorescent units in a desired direction. In the case of polymer
molecular design and the polarized fluorescence film with high
materials, orientation of fluorescent unit to obtain high
orientation of fluorescence can be obtained with the aid of
polarization ratio can be carried out by a variety of methods
photoinduced azobenzene orientation are still worthy of further
such as stretching,3 rubbing,4,5 Langmuir−Blodgett (LB)
exploration.
film,6−8 electrospinning,9−11 etc. However, mechanical damag-
In this work, we reported the photoinduced cooperative
orientation of fluorescent unit, stilbene molecule, in an
azobenzene-containing polymer matrix prepared by the ionic
self-assembly (ISA) approach. This fluorescent ionic complex
ing and time-consuming often occurred among these methods.
Recently, orientation of the fluorescent unit driven by a
photosensitive unit has received intensive attention due to its
noncontact alignment nature.12−14 In such an approach, the
shows lamellar structure and meagerly quenching effect on the
fluorescent unit was cooperatively oriented following the
fluorescence emission. Under laser irradiation, the fluorescent
photoactive movement of the photosensitive molecules. For
unit and the azobenzene group present cooperative orientation
example, Stumpe et al. investigated the cooperative orientation
in the same direction, perpendicular to the laser polarization.
of photosensitive cinnamate group and different dye units
Consequently, a dichroic ratio of fluorescent emission up to 4.6
under light irradiation and the photosensitive polymer emitted
polarized light with a high dichroic ratio.15 Lee et al. also
reported a series of supramolecules comprised of fluoreneviny-
lene and cinnamate side group in which the fluorenevinylene
was cooperatively oriented by the cinnamate group.16,17
As a well-known kind of photosensitive molecule, azoben-
zene and its derivatives have been intensively investigated for
their reversible cis−trans isomerization under light irradia-
tion.18−22 A much higher order parameter of azobenzene group
can be easily obtained compared with other photosensitive
group. However, few reports were presented using its
photoinduced alignment to drive the orientation of fluorescent
unit. This was a point of view most of people thought that the
emission of organic fluorescent molecule was quenched while
adding the azobenzene molecules into the fluorescent polymer
film due to the energy transfer process or oxidative/reductive
was obtained from this kind of fluorescent complex.
EXPERIMENTAL SECTION
■
Materials. Poly(acrylic acid) (PAA) aqueous solution (25 wt %,
Mw 240 000) was purchased from Alfa Aesar. Sodium polyacrylate was
obtained from the neutralization of PAA with sodium hydroxide.
Sodium nitrite, phenol, potassium carbonate, 4-methoxyaniline, 33 wt
% trimethylamine alcoholic solution, piperidine, and organic solvents
involved in this work were purchased from the Sinopharm Chemical
Reagent Co. N-Methyl-N-(2-hydroxyethyl)-4-aminobenzaldehyde, 4-
nitrophenylacetic acid, 1,6-dibromohexane, and 6-bromohexanoyl
chloride were obtained from J&K Reagent Co. All the reagents were
Received: February 2, 2013
Revised: April 12, 2013
© XXXX American Chemical Society
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Scheme 1. Synthetic Route to the Fluorescence Unit (Nstil) and Azobenzene Moiety (MAZO)
used without further purification unless stated otherwise. 4-(N-(2-
Hydroxyethyl)-N-methylamino)-4′-nitrostilbene (1) and trimethyl-6-
(4-methoxyazobenzene-4′-oxy)hexylammonium bromide (MAZO)
were synthesized according to the reported methods.33−35
measured by using UV−vis interferometry (model F20, Filmetrics,
Inc.).
Instruments and Characterization. FTIR spectra were recorded
on a PerkinElmer Spectrum 100 FTIR spectrometer on pressed thin
transparent disks of the samples mixed with KBr. Nuclear magnetic
resonance (1H NMR) studies were carried out with a Varian Mercury
Plus 400 MHz spectrometer using DMSO-d6 or CDCl3 as solvent at
room temperature, and the chemical shifts were referenced relative to
tetramethylsilane. Thermogravimetric analysis (TGA) was performed
using a TA TGA Q5000IR instrument. Samples were heated at the
speed of 20 °C min−1 from room temperature to 700 °C in nitrogen.
Differential scanning calorimetry was performed on a TA DSC Q2000
instrument with a heating rate of 10 °C min−1 in nitrogen. Polarized
optical microscopy (POM) was performed on a Leica DMLO
microscope with a Leitz 350 hot stage. Small-angle X-ray scattering
(SAXS) experiments were carried out on an Anton Paar SAXS
instrument at room temperature. Wide-angle X-ray diffraction
(WXRD) was performed on a Rigaku D/MAX-2200/PC diffrac-
tometer at 25 °C (Cu Kα radiation λ = 0.154 nm, U = 40 kV, I = 20
mA). The samples were obtained by the solution casting method. The
solvent was volatilized slowly under 60 °C for 24 h and further dried in
vacuum at 70 °C overnight. Absorption spectra of the PMmNn films
and solution were performed on PerkinElmer LAMBDA 750 UV−vis
and 20 UV−vis spectrophotometers, respectively. The degree of
molecular orientation was evaluated by the polarized UV−vis spectra
using a PerkinElmer LAMBDA 750 UV−vis spectrophotometer with
polarization angle scanning from 10° to 330° equipped with a
polarizer.
2-(N-Methyl-N-(4-nitrostilbene)-4′-amino)ethyl 6-Bromohexa-
noate (2). To a solution of 1 (1.00 g, 3.36 mmol) in 40 mL of dry
THF, triethylamine (470 mg, 4.70 mmol, 1.4 equiv) was added at
room temperature, and the solution was stirred for 15 min. The
reaction mixture was cooled to 0 °C in an ice bath, and 6-
bromohexanoyl chloride (1.00 g, 4.70 mmol, 1.4 equiv) was added
slowly by syringe. The resulting mixture was heated to room
temperature and stirred for 24 h. After filtration, the filtrate was
evaporated by vacuum at room temperature. The residue was dissolved
in ethyl acetate and washed with deionized water for three times. Then
organic layer was dried using anhydrous MgSO4. The dark brown solid
product was obtained after evaporating the solvent and further purified
1
by column chromatography on a SiO2 column. Yield 1.15 g, 72%. H
NMR (400 MHz, CDCl3, δ (ppm)): 8.17−8.19 (d, 2H), 7.55−7.58 (d,
2H), 7.43−7.45 (d, 2H), 7.18−7.20 (d, 1H), 6.91−6.95 (d, 1H),
6.73−6.76 (d, 2H), 4.27−4.30 (t, 2H), 3.64−3.57 (t, 2H), 3.35−3.38
(t, 2H), 3.04 (s, 3H), 2.25−2.29 (t, 2H), 1.55−1.61 (m, 2H), 1.42−
1.44 (m, 2H), 1.22−1.25 (m, 2H).
Trimethyl-5-(2-(N-Methyl-N-(4-nitrostilbene)-4′-amino)-
ethoxycarbonyl)amylammonium Bromide (Nstil). 1.1 g (2.32 mmol)
of 2 was dissolved in 50 mL of tetrahydrofuran. To the solution, 4.15 g
(23.2 mmol) of 33 wt % trimethylamine alcoholic solution was added,
and the solution was refluxed for 48 h under dry nitrogen. The crude
product was washed thoroughly with ethyl acetate to remove the
Photoinduced Orientation of the Complexes under Laser
Irradiation. The photoinduced orientation experiment was done with
an s-polarized Nd:YAG laser (355 nm), with a pulse duration of 5 ns
and repetition rate of 10 Hz . The spin-coated films were fixed on an
X−Y moving platform with the speeds being 0.1 and 5 mm s−1 in X
and Y directions, respectively. The incident angle θ was fixed at 15°
with respect to the normal of the PMmNn films.
1
unreacted material. Yield: 1.05 g, 85%. H NMR (400 MHz, DMSO-
d6, δ (ppm)): 8.16−8.18 (d, 2H), 7.73−7.76 (d, 2H), 7.48−7.50 (d,
2H), 7.39−7.43 (d, 1H), 7.08−7.13 (d, 1H), 6.74−6.76 (d, 2H),
4.18−4.21 (t, 2H), 3.62−3.65 (t, 2H), 3.17−3.21 (m, 2H), 3.98 (s,
9H), 2.95 (s, 3H), 2.19−2.26 (t, 2H), 1.58−1.62 (m, 2H), 1.46−1.50
(m, 2H), 1.16−1.22 (m, 2H).
Synthesis of Ionic Complexes (PMmNn) and Films. The letters
“m” and “n” in the abbreviation PMmNn denote that the molar ratio of
the azobenzene moiety MAZO to the luminescence moiety Nstil in
the complex is m:n. Take PM0.8N0.2 as an example; 0.107 g (0.2 mmol)
of Nstil was dissolved in 40 mL of 70 °C water, and then the hot
solution was added slowly to a 10 mL (10 mg mL−1, 1 mmol of
acrylate groups calculated by the polymer repeating unit) sodium
polyacrylate (PANa) aqueous solution with vigorous stirring in a 300
mL beaker. The mixture was kept at 70 °C. In another beaker, 0.360 g
(0.8 mmol) of MAZO was dissolved in 150 mL of hot water. Then the
previous solution was added dropwise to the MAZO hot aqueous
solution. The resulting precipitate was collected by filtration and
washed thoroughly with deionized water to remove residual salts and
noncomplexed precursors and then dried in vacuum at 60 °C for 24 h.
The molar ratio of PANa, MAZO, and Nstil was 10:8:2. The PMmNn
films were prepared by spin-coating a NMP solution (concentration:
40 mg mL−1) onto a quartz plate, glass slide, and silica substrates and
further dried in vacuum at 70 °C for 8 h to remove the residual
solvent. The thickness of the resultant film was about 800−1000 nm
The photoinduced orientation in the PMmNn films was evaluated
by the in-plane order parameter (S) by
A − A⊥
S =
Alarge + 2Asmall
where A∥ and A⊥ are the UV−vis absorbance of parallel and
perpendicular to laser polarization direction (E), respectively. Alarge and
Asmall are associated with the larger and smaller absorbance between A∥
and A⊥.
Fluorescence Properties of the Complexes. Fluorescence
spectra of the films and solution were recorded with a PTI steady-
state and time-resolved fluorescence spectrofluorometer (QM/TM/
IM), and polarized excited photoluminescence measurements of the
films were performed on the same instrument equipped with a
polarizer. The complex films, spin-coated from the NMP solution with
the same concentration of PMmNn (wt %), were used to test the
fluorescent intensity (excitation slit: 12 nm; emission slit: 16 nm), the
excitation wavelength was 470 nm. In order to comparison, the
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Scheme 2. Synthetic Route to the Ionic Complexes PMmNn
fluorescence of PMmNn complex was also measured in the NMP
solution with the same concentration of Nstil unit (0.5 mmol L−1),
which was calculated by the average molecule weight of repeating unit.
Excitation and emission slits were set to 6 and 12 nm, respectively.
The excitation wavelength was 550 nm. The fluorescence dichroic
ratio Rex was calculated by the equation
band of symmetric and asymmetric vibrations of Ar−O from
MAZO unit was found at 1253 and 1027 cm−1, respectively.
These results confirmed the success attachment of Nstil and
MAZO units to the PANa backbone.
Figure 2 presents the H NMR spectra of the MAZO, Nstil,
and PMmNn complexes. Take PM0.6N0.4 as an example; all the
1
Rex = I /I⊥
where I∥ and I⊥ are the fluorescence intensity measured in the
direction parallel and perpendicular to the direction of laser
polarization orientation, respectively. Rex is calculated at maximum
emission of the irradiated film by polarized fluorescence spectra.
Cyclic Voltammetry (CV). Cyclic voltammetry experiments were
performed on CHI 630E electrochemical analyzer (Shanghai Chenhua
Co.) for compounds 2 and 4 in THF solution at room temperature.
0.1 M tetrabutylammonium hexafluorophosphate was added to the
THF solution as supporting electrolytes. The working electrode was a
platinum electrode. A platinum spiral was used as the counter
electrode and Hg/Hg2Cl2 electrode was used as the reference
electrode. The scan rate was 50 mV s−1.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Nstil, MAZO, and
PMmNn. FTIR and 1H NMR were used to confirm the
successful formation of PMmNn complexes. As shown in Figure
1, the stretching vibration band of the carbonyl group in the
1
Figure 2. H NMR spectra of PM0.6N0.4, MAZO, and Nstil.
proton signals were marked clearly. The proton signals of the
phenyl groups in the MAZO and Nstil moieties corresponded
to the chemical shifts of low-field region (>5 ppm) in the
complex, while the chemical shifts of high-field region (<5
ppm) were from the proton signal of the polyelectrolyte main
chain, the soft spacer, and methyl of the MAZO and Nstil
moieties in the side chain. Compared to signal from the MAZO
and Nstil, the proton signals form the PM0.6N0.4 complex had a
slight shift, especially for the quaternary ammonium proton
signals in the MAZO and Nstil moieties, indicating that the
chemical environment of the protons changes slightly due to
the formation of complex PMmNn. Moreover, the proton
signals from the side-chain moieties became broader than the
signal from the resource molecules MAZO and Nstil. These
results above presented powerful evidence that the small
molecules had attached to PANa main chain successfully.
Furthermore, due to the partial overlap of the proton signals of
MAZO and Nstil units in PM6N4, the overall peaks area of “e”,
“f ”, and “f′” had been integrated, which was as 4 times as the
Figure 1. FT-IR spectra of PM0.9N0.1, PM0.8N0.2, PM0.7N0.3, PM0.6N0.4,
MAZO, and Nstil.
stilbene unit was found in 1730 cm−1 whose intensity increased
with the content of Nstil unit increasing. In addition, the
stretching vibration bands of CC double bonds in phenyl
group were also found at ca. 1602 and 1500 cm−1. The
absorption band at 1340 cm−1 was associated with the
stretching vibration of nitro group. Moreover, the absorption
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peak area of “h′”. So the exact ratio of MAZO unit to Nstil unit
in the complex PM0.6N0.4 was calculated to be 6:4.
To further confirm the ratio of the MAZO and Nstil units in
the complexes, UV−vis absorption spectra of MAZO, Nstil, and
PMmNn have been measured in NMP solutions (Figure 3).
Figure 4. (left) DSC thermograms of the second cooling curves of
PMmNn with the cooling rate being 10 °C min−1. (right) POM images
of PMmNn power obtained during cooling from their isotropic phase
at the temperature 43, 70, 49, and 47 °C for PM0.9N0.1 (1), PM0.8N0.2
(2), PM0.7N0.3 (3), and PM0.6N0.4 (4), respectively. The bar is 100 μm.
Figure 3. Normal UV−vis spectra of MAZO, PM0.9N0.1, PM0.8N0.2,
PM0.7N0.3, and PM0.6N0.4 and Nstil in NMP solution.
complexes. The clearing points of the liquid crystalline
complexes were in the range of 72 °C (PM0.9N0.1) to 105 °C
(PM0.6N0.4) depending on the content of Nstil moieties (Table
1), and the liquid crystal temperature range became wide with
MAZO and Nstil showed their absorption peaks at 361 and 445
nm, respectively. Therefore, the absorption band at 361 nm of
PMmNn was attributed to the absorption of MAZO. Mean-
while, the absorption band at 445 nm was mainly associated
with the absorption of Nstil unit and partly with the n−π*
Table 1. Layer Spacing d Calculated from SAXS Profiles,
Phase Transition Temperature, and Corresponding Enthalpy
(ΔH) in the Cooling Process of the Ionic Complexes
PMmNn
transition of MAZO unit. The absorption at 445 nm (A445nm
)
increased with the content of Nstil in PMmNn while
normalizing the absorption at 361 nm (A361nm). In addition,
the UV−vis spectra of the mixture of MAZO and Nstil have
also been measured in NMP solution (Figure S2 in Supporting
Information). The mixture (abbreviated with MmNn, the “m”
and “n” here indicate that the molar ratio of MAZO to Nstil in
the mixture was m:n) showed two absorption peaks at 361 and
445 nm like PMmNn. According to Lambert−Beer’s law, the
concentration is proportional to the absorbance. Therefore, the
ratio value A*445nm/A3*61nm represents the concentration ratio of
Nstil to MAZO unit in the complex. The detailed calculations
for A*445nm/A*361nm of PMmNn and MmNn are shown in the
Supporting Information, and their values are also collected in
Table S1. A calibration curve was obtained from the mixture
MmNn for the A*445nm/A3*61nm value vs the content of Nstil in
the complexes (Figure S2). The A4*45nm/A3*61nm value of
PM0.8N0.2 and M0.8N0.2 was 0.135 and 0.131, respectively,
which indicated the ratio of MAZO to Nstil moiety in
PM0.8N0.2 was 8:2. Similar results were observed when other
complexes were submitted to the same treatment. These results
a
d
phase transition temp
corresponding ΔH
b
complexes (nm)
(°C)
(J g−1
)
PM0.9N0.1
PM0.8N0.2
PM0.7N0.3
PM0.6N0.4
2.94
3.01
3.16
3.27
I 72 Sm 29 Cr
I 80 Sm 29 Cr
I 95 Sm 26 Cr
I 105 Sm 22 Cr
7.66, 1.98
4.40, 1.15
3.89, 1.10
7.59, 0.76
a
Cr = crystalline phase; Sm = smectic phase; I = isotropic phase.
Transition temperatures are the peak temperature of second cooling
b
curves of DSC profiles. Enthalpies are also calculated from DSC
profiles.
the content of Nstil unit in PMmNn increasing. In addition, the
low enthalpy change of PM0.6N0.4 from liquid crystalline phase
to crystalline phase indicated the low degree of crystallization
(Table 1).
The complexes PMmNn were further submitted to SAXS
measurement to investigate their microstructure. Take
PM0.9N0.1 as an example; two clear scattering peaks at q1 =
2.13 nm−1 and q2 = 4.26 nm−1 can be detected in the SAXS
pattern (Figure 5a), indicating the presence of only one long-
range lamellar microstructure with a spacing d = 2.94 nm (d =
2π/q1). The d value was larger than the calculated molecular
length for the fully extended side-chain unit of MAZO (2.39
nm) or Nstil (2.75 nm), revealing a partial interdigitated
structure (Figure 5b). This result was similar to the previous
report where a lamellar microstructure with partly overlap of
side chain was postulated in the ionic complexes.36,37
Furthermore, due to the incorporation of Nstil unit, the
1
are consistent with that from H NMR, indicating that the
MAZO and Nstil attached to PANa backbone according to the
initial calculated proportion.
Thermal Analysis and Phase Behavior. TGA and DSC
were employed to investigate the stability and phase behavior of
the complexes. TGA results showed that all the complexes were
stable up to 165 °C (Figure S3). In order to avoid the
interference of the thermal history, DSC thermograms of the
second heating and cooling curves of PMmNn were employed.
All of the complexes displayed two clear phase transition
processes during the heating (Figure S4b) and cooling process
(Figure 4), indicating their liquid crystalline behavior existing.
The different textures could be observed reversibly by POM
during the heating and cooling process, as shown in the Figure
4. The POM images were captured at the temperatures
between the two peaks of the DSC second cooling curves,
which further confirmed their liquid crystalline properties of the
space d increased linearly in the following order: PM0.9N0.1
<
PM0.8N0.2 < PM0.7N0.3 < PM0.6N0.4. WXRD analysis (Figure S6)
showed a broad peak at wide angle 20° and several sharp peaks,
indicating that the crystalline exists in the complexes and the
degree of crystallization decreased with the content of Nstil unit
increasing in PMmNn.
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Figure 5. (a) SAXS patterns of PMmNn at room temperature after annealing at 70 °C for 10 h. (b) Possible molecular packing mode for the
PMmNn complexes.
Figure 6. (a) Changes of polarized absorption spectra in the film of PM0.9N0.1 irradiated with 35 mW cm−2. A∥ and A⊥ are the absorbance parallel
and perpendicular to the laser polarization direction, respectively. (b) Angular dependent UV−vis spectra of PMmNn films at 320 nm; PM0.9N0.1,
PM0.7N0.3, and PM0.6N0.4 films were irradiated with 35 mW cm−2 while PM0.8N0.2 film was irradiated with 46 mW cm−2.
Figure 7. Schematic illustration of the photoinduced orientation process of the PMmNn films.
Anisotropic Orientation of PMmNn Films under Laser
Irradiation. In this section, an s-polarized pulsed laser (355
nm) was used as the light source. As shown in Figure 6a, after
irradiated with pulsed laser at 35 mW cm−2, the absorbance of
PM0.9N0.1 film parallel to laser polarization (A∥) was smaller
than that in the direction perpendicular to laser polarization
(A⊥). Figure 6b gives the angular dependent UV−vis
absorption spectra of the irradiated PMmNn films at 320 nm.
The absorption varied as a function of the polarization angle
(polarization angle was defined as the angle between the
direction of laser polarization and the direction of the
polarizer). A periodical change in the absorbance with a period
of 180° was seen. The maximum absorbance was present as the
polarization angle was 90° and 270° while the minimum
absorbance was at 180°. These results proved that the
azobenzene group oriented in the direction perpendicular to
the laser polarization direction. The calculated absolute value of
in-plane orientation order parameter S were 0.200, 0.225, 0.238,
and 0.285 for PM0.6N0.4, PM0.7N0.3, PM0.8N0.2, and PM0.9N0.1
films, respectively, decreasing with the content of Nstil unit
increasing. It can be explained from the following two facts:
first, the incorporation of Nstil group into the PMmNn film
leads to the decrease of the azobenzene group, which caused
the decrease of amount of oriented group. However, Nstil is no
response to light due to almost no adsorption at 355 nm.
Second, the order parameter S of azobenzene and stilbene has
no significant change after annealing at 70 °C for 10 min, which
indicated that the PMmNn complex does not possess the
nature of the thermally induced ordered amplification effect of
liquid crystalline polymer.
The orientation of Nstil unit in the complex was also
detected by polarized absorption at the wavelength of 460 nm.
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Here, we take PM0.6N0.4 as an example. As shown in Figure 3,
the wavelength of pulsed laser (355 nm) is far from the
maximum absorption of stilbene, which avoids the absorption
of stilbene. For stilbene compound without photosensitive
group, orientation of stilbene group also may be obtained under
light irradiation. However, it was very weak or negli-
gible.14,38−40 Here, the calculated absolute value of order
parameter S at 460 nm was 0.151 corresponding to the stilbene
group (Figure S7), much higher than that of pure stilbene film
under light irradiation, indicating that the Nstil unit was driven
to align via MAZO units due to the cooperative orientation. On
the basis of these results, a schematic illustration of the
photoresponsive processes of PMmNn complex films under
irradiation with 355 nm linear pulse laser is given in Figure 7.
Fluorescence Properties of the PMmNn Films. Figure 8
and Figure S8 show the fluorescence and UV−vis spectra of the
670 nm, corresponding to a red emission. Unexpectedly, the
photoluminescence intensity increased with reducing the
percentage content of Nstil in the complex films. The reason
for this phenomenon was that, under the irradiation of
excitation light at 470 nm, the photodimerization of stilbene
molecule was much easier than it fluoresces at high
concentration of Nstil unit in ionic complexes.41
The PMmNn films with low content of Nstil still show strong
fluorescence intensity though slight quenching also occurred
compared with the ionic complex PM0N1 film without MAZO
unit. For example, the fluorescence was quenched about 17%
for PM0.9N0.1 with respect to PM0N1 film (Figure S9).
Meanwhile, the fluorescent properties of PMmNn in NMP
solution were also investigated, as shown in Figure S10. For the
same concentration of Nstil unit, the fluorescence of PMmNn
in solution was also observed with slight quenching compared
to PM0N1 solution, i.e., 18% for PM0.9N0.1. These results are
completely different with previous report on the azobenzene-
containing fluorescent polymer.42,43 That can be explained in
the following aspects: the maximum absorption of azobenzene
group was at 361 nm and the fluorescence emission of stilbene
was 670 nm, and no overlap occurred between the fluorescence
and the azobenzene unit. This feature avoids the energy transfer
between the two chromophores. Moreover, the CV curves for
azobenzene compound 4 shows a reduction peak at −1.62 V
and stilbene compound 2 shows an oxidation peak at +0.95 V
(Figure S11). The free energy ΔG for the fluorescent
quenching process via the oxidative electron transfer is
approximately +0.45 eV (eq S1 in Supporting Information),
so that the fluorescence quenching by electron transfer may not
occur.24
Figure 8. Fluorescence spectra of the complex films with different
Nstil content. The excitation wavelength is 470 nm.
Figure 9 shows the anisotropic fluorescence emission of the
PMmNn films with different Nstil content. The maximum
fluorescence intensity was observed when the polarization of
the excitation light was in the direction parallel to the alignment
spin-coating films, respectively. The PMmNn films exhibited
strong light emission under the excitation wavelength at 470
nm. The fluorescence bands were located at the wavelength of
Figure 9. Polarized fluorescence spectra of the PMmNn films. I∥ and I⊥ are the fluorescence obtained by the polarization parallel and perpendicular
to alignment direction of stilbene unit, respectively. All spectra were measured at room temperature, and the excitation wavelength was 470 nm.
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direction of azobenzene chromophore (I∥) while the minimum
fluorescence intensity in the direction perpendicular to the
alignment direction of azobenzene chromophore (I⊥). With the
increase of the Nstil content in the PMmNn films, the ratio Rex
decreased as 4.60, 4.16, 3.96, and 3.80 for PM0.9N0.1, PM0.8N0.2,
PM0.7N0.3, and PM0.6N0.4, respectively. This trend is consistent
with the order parameter of azobenzene group with the change
of Nstil content, further proving that the orientation of
fluorescent unit Nstil was driven by the orientation of
azobenzene group under linear UV light irradiation. Fur-
thermore, the Rex value did not vary with the measurements and
could keep for at least 3 weeks without any change. This
dichroic ratio is higher than that when a cinnamate group was
employed to cooperatively orient the same fluorescent units
(Rex is about 2.8).14,15 Moreover, this is significantly different
from the Eu-doped azobenzene polymer film (organic−
inorganic hybrid fluorescent material).44,45 The high Rex can
be attributed to the high sensitivity of azobenzene group and
the liquid crystalline nature of the complex. Laser treatment can
cause strong thermal effect accelerating the photoinduced
orientation of azobenzene groups. This result proved that laser
treatment of azobenzene containing liquid crystalline materials
is more effective in obtaining fluorescent film with high ratio of
polarization emission.
dose exceeds a certain value, the thermal induced relaxation
plays the major role and leads to the decrease of the
orientation. The contrary factors of the pulse laser resulted in
the parabola-like variation of anisotropic orientation with the
laser fluence.
CONCLUSION
■
In this contribution, we successfully prepared fluorescence
material with high polarized fluorescence emission. The
material bearing azobenzene and stilbene was prepared by the
ionic self-assembly approach. The fluorescence of the ionic
complexes was generated without being quenched by
azobenzene. The cooperative orientation of stilbene occurred
following the alignment direction of azobenzene groups in the
complex films irradiated by linear polarized pulsed laser. The
oriented films gave the anisotropic absorption and fluorescence
emission. High PL dichroic ratio Rex up to 4.6 can be obtained
in the complex films with the low content of Nstil unit.
ASSOCIATED CONTENT
* Supporting Information
■
S
1H NMR spectra of PMmNn; UV−vis spectra of the mixture
MmNn; calculation for the absorption ratio A*445nm/A3*61nm; The
calibration curve of the A*445nm/A3*61nm value vs the content of
Nstil in the complexes; absorption ratio A4*45nm/A3*61nmof
PMmNn and MmNn; TGA curves of MAZO, Nstil and
PMmNn; POM images of PMmNn; DSC thermograms of the
first and second heating curves of PMmNn; wide-angle X-ray
diffraction profile of PMmNn; polarized UV−vis spectra of
PM0.6N0.4; UV−vis spectra of PMmNn films; fluorescence
spectra of the PM0N1 and PM0.9N0.1 films; fluorescence spectra
of the PMmNn in NMP solution; cyclic voltammogram of
compounds 4 and 2 in THF solution; influence of laser fluence
on the in-plane orientation order parameter S of PMmNn. This
material is available free of charge via the Internet at http://
pubs.acs.org.
The influence of laser fluence on the ratio Rex was also
investigated. The polarized fluorescence was measured on the
complex films after irradiating with different laser fluence
(Figure 10). The obtained curves were extremely similar to that
AUTHOR INFORMATION
Corresponding Author
*E-mail xueminlu@sjtu.edu.cn (X.L.), qhlu@sjtu.edu.cn
(Q.L.); Fax +86 (0)21 5474 7535.
■
Figure 10. Influence of laser fluence on the PL dichroic ratio Rex of
PMmNn.
Notes
The authors declare no competing financial interest.
of in-plane orientation order parameter S as a function of the
laser fluence (Figure S12). The dichroic ratio Rex was first
increased with the increase of the irradiation fluence and then
began to decrease with the increase of laser fluence. Because the
stilbene group was cooperatively oriented by azobenzene group
under the irradiation of pulsed laser, the ratio Rex shows trend
with the order parameter S of MAZO unit. And the change in
orientation of azobenzene with laser fluence like parabola is
related to the thermal effect of the nanosecond pulsed laser.
Laser irradiation leads to the anisotropic orientation of
azobenzene group with a preferred direction perpendicular to
the laser polarization. Meanwhile, the film surface was heated to
locally melt due to the strong thermal effect of pulsed laser.46,47
This makes the motion of azobenzene molecules and the
subsequent orientation of azobenzene easy.48 On the contrary,
the thermal effect also leads to the relaxation of the oriented
molecules. At low laser fluence, the thermal relaxation is rather
slow and the order parameter of the orientation (S) increases
with the laser fluence increasing. However, when irradiation
ACKNOWLEDGMENTS
■
This research project was supported by the National Science
Fund for Distinguished Young Scholars (50925310), the
National Science Foundation of China (51173103), 973 project
(2012CB933803), and excellent academic leaders of Shanghai
(11XD1403000).
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