Quinoline-based azo derivative assembly: Optical limiting property and enhancement mechanism

Article abstract of DOI:10.1016/j.dyepig.2013.07.007

Abstract To further investigate the dependence of optical limiting properties on molecular structures, especially the strong intermolecular influence, such as the influences of hydrogen bond, dipole-dipole interaction and their synergetic effect on optical limiting properties, three polar D-π-A conjugated quinoline-based heterocyclic azo derivatives (3a, 3b and 3c) with different hydrogen-bond recognition abilities were designed and synthesized. Their structures and properties were characterized and evaluated by infrared spectroscopy, thermogravimetric analysis, and ultraviolet-visible spectrometry. The strengthening mechanism of optical limiting property was investigated in detail based on theoretical calculation and experimental results of molecular assembly induced by synergetic effect of hydrogen bond and dipole-dipole interaction. The results indicate that the optical limiting properties of these polar D-π-A conjugated optical materials were significantly affected by molecular structure, strong intermolecular interaction and their synergetic enhancement effect.

Full text of DOI:10.1016/j.dyepig.2013.07.007

Accepted Manuscript
Quinoline-based azo derivative assembly: Optical limiting property and enhancement
mechanism
Zhengquan Yan, Shanyi Guang, Hongyao Xu, Xiangyang Liu Xu
PII:
S0143-7208(13)00258-1
10.1016/j.dyepig.2013.07.007
DYPI 4015
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 20 May 2013
Revised Date: 1 July 2013
Accepted Date: 4 July 2013
Please cite this article as: Yan Z, Guang S, Xu H, Xu XL, Quinoline-based azo derivative assembly:
Optical limiting property and enhancement mechanism, Dyes and Pigments (2013), doi: 10.1016/
j.dyepig.2013.07.007.
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ACCEPTED MANUSCRIPT
Quinoline-based azo derivative assembly: Optical limiting
property and enhancement mechanism
Zhengquan Yan,a,b Shanyi Guang,a Hongyao Xu,a* and Xiangyang Liu Xua
a College of Material Science and Engineering & State Key Laboratory for Modification of
Chemical Fibers and Polymer Materials, College of Chemistry & biology Engineering,
Donghua University, Shanghai 201620, China
b
Anhui Provincial Laboratory of Biomimetic Sensor and Detecting Technology &
Department of Material and Chemical Industry, West Anhui University, Lu’an 237012, China
To further investigate the dependence of optical limiting properties on molecular structures, especially the
strong intermolecular influence, such as the influences of hydrogen bond, dipole-dipole interaction and their
synergetic effect on optical limiting properties, three polar D-π-A conjugated quinoline-based heterocyclic
azo derivatives (3a, 3b and 3c) with different hydrogen-bond recognition abilities were designed and
synthesized. Their structures and properties were characterized and evaluated by infrared spectroscopy,
thermogravimetric analysis, and ultraviolet-visible spectrometry. The strengthening mechanism of optical
limiting property was investigated in detail based on theoretical calculation and experimental results of
molecular assembly induced by synergetic effect of hydrogen bond and dipole-dipole interaction. The results
indicate that the optical limiting properties of these polar D-π-A conjugated optical materials were
significantly affected by molecular structure, strong intermolecular interaction and their synergetic
enhancement effect.
TOC Graph

ACCEPTED MANUSCRIPT
Quinoline-based azo derivative assembly: Optical limiting property
and enhancement mechanism
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Zhengquan Yan,a,b Shanyi Guang,a Hongyao Xu,a* and Xiang-Yang Liua
aCollege of Material Science and Engineering & State Key Laboratory for Modification of Chemical
Fibers and Polymer Materials, College of Chemistry & biology Engineering, Donghua University,
Shanghai 201620, China1
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bAnhui Provincial Laboratory of Biomimetic Sensor and Detecting Technology & Department of
Material and Chemical Industry, West Anhui University, Lu’an 237012, China
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Abstract: To further investigate the dependence of optical limiting properties on molecular
structures, especially the strong intermolecular interactions, such as the influences of hydrogen bond,
dipole-dipole interaction and their synergetic effect on optical limiting properties, three polar D-π-A
conjugated quinoline-based heterocyclic azo derivatives (3a, 3b and 3c) with different
hydrogen-bond recognition abilities were designed and synthesized. Their structures and properties
were characterized and evaluated by infrared spectroscopy, thermogravimetric analysis, and
ultraviolet-visible spectrometry. The strengthening mechanism of optical limiting property was
investigated in detail based on theoretical calculation and experimental results of molecular assembly
induced by synergetic effect of hydrogen bond and dipole-dipole interaction. The results indicate that
the optical limiting properties of these polar D-π-A conjugated optical materials were significantly
affected by molecular structure, strong intermolecular interaction and their synergetic enhancement
effects.
Keywords: Quinoline-based azo derivative; Self-assembly; Optical limiting; Mechanism; Hydrogen
bond; Synergetic effect
1 Corresponding: E-mail: hongyaoxu@163.com; Tel: +86-2167792874
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1. Introduction
Optical materials, in particular, optical limiting materials for protection of human eyes, optical
sensors, and sensitive optical components from laser damage in both civilian and military
applications[1-4], have attracted more and more considerable interest in recent years with the fast
development of modern laser technologies. Different conjugated organic materials that exhibit strong
optical limiting property and fast response time have been designed and synthesized, i.e., azo
compounds[5, 6], porphyrins[7], phthalocyanines[8], polydiacetylenes[9] and polyacetylenes[10] and
so on. The results show that extending the conjugation length[11], introducing strong electron donor
(D) and acceptor (A) groups to the conjugated system[12], strengthening the molecular planarity[13]
and making the system multi-branch[14], multi-dimension or dendrimer[3] can all improve the
optical limiting properties. Recently, Li[15] proposed the concept of ‘‘suitable isolation group’’ based
on the facts that the nonlinear optical (NLO) effect would be enhanced dramatically with further
increases in loading density[16, 17], meaning that the frequently observed asymptotic dependence of
electro-optic activity on the chromophore number density could be overcome through rational design.
The proposed concept would provide some useful information for the rational design of new NLO
materials. The basic principle is to extend more π-delocalization, which results in the lower energy
gap between the highest occupied molecular orbits (HOMO) and the lowest unoccupied molecular
orbits (LUMO) and effective intermolecular charge transfers (ICT). However, the above
investigations are mainly based on the molecular structural design. The influence of intermolecular
interactions, i.e., hydrogen bond, dipole-dipole interaction and their synergetic enhancement effect,
on optical limiting properties was not completely understood.
In this work, three polar D-π-A conjugated heterocyclic azo molecules with different terminal
hydrogen bond recognition abilities: 6-quinaldinyl-4’-[N,N-dimethyl-amino]azo-benzene (3a),
6-quinaldinyl-4’-[N-methyl-N-hydroxyethyl amino] azo-benzene (3b), and 6-quinaldinyl-4’-[N,
N-di-hydroxyethyl amino]-azo-benzene (3c), in which quinolyl groups are used as electron acceptors
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(A) and aniline derivative groups as electron donors (D), are designed and synthesized for further
investigating the influence of molecular structure, hydrogen bond, dipole-dipole interaction and
their synergetic enhancement effect on optical limiting properties.
2. Experimental
2.1. Reagents and apparatus
4-Nitro aniline, crotonaldehyde, sodium nitrite, N,N-dimethyl-aniline, N-methyl-N-hydroxyethyl
aniline, N,N-dihydroxyethyl aniline and all the other reagents were of analytical grade and purchased
from Shanghai Chemical Reagent Company. They were used directly without any further
purification.
FT-IR spectra were recorded as KBr pellets on a Perkin Elmer Model 882 infrared spectrometer
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by infrared spectrometer scanning from 4000-500 cm–1. H NMR (300 MHz) and 13C NMR (100
MHz) spectra were recorded using a Bruker AMX-500 spectrometer operating at 400 MHz, with
tetramethyl-silane(TMS) as the reference and chloroform-d as solvent. Elemental analyses were
conducted with an Elemental Vario EL-III apparatus. Mass spectra were carried out by using
Shimadzu GCMS-QP2010 Spectrometry. The aggregation morphologies were visualized using a
CMM-88E metallographic electron optical microscope (OM). Thermogravimetric analyses (TGA)
were performed on a Perkin Elmer TGA under nitrogen at a heating rate of 10 °C/min.
Ultraviolet-visible (UV-vis) spectra were recorded on a Lambda 35 UV/Vis spectrometer (Perkin
Elmer Precisely) using a 1-cm square quartz cell. The optical limiting properties were measured in
DMSO solutions. Testing was performed using a frequency-doubled, Q-switched, mode-locked
Continuum ns/ps Nd:YAG laser, which provided linearly polarized 4 ns optical pulses at 532 nm
wavelength with a repetition of 1 Hz. The transverse mode of the laser pulses was nearly Gaussian.
The input laser pulses were split into two beams by an attenuator (Newport). One was employed as
reference to monitor the incident laser energy, and the other was focused onto the sample cell by
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using a lens with 40 cm focal length. The sample was housed in quartz cells with a thickness of 4
mm. The incident and transmitted laser pulses were monitored by utilizing two energy detectors, D1
and D2 (Rjp-735 energy probes, Laser Precision). The optical limiting properties of the samples were
carried out using the same laser system at the input energy 184 mJ. The experimental set up was
similar to that given in the literature[18].
2.2. Synthesis and self-assembly
The target heterocyclic azo-derivatives with different terminal hydrogen bond recognition abilities
were prepared according to Scheme 1[5, 19, 20].
Insert Scheme 1
2.2.1 Preparation of 6-nitro-quinaldine (1)
A 100 mL three-necked bottle was charged with hydrochloride (44.8 mL, 6 mol/L), 4-nitro aniline
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(1232 mg, 8.9 mmol) and acetic acid (0.5 mL, 8.9 mmol), which was refluxed at 100 C for 0.5 h.
After iodine/potassium iodide (50 mg/132 mg, 0.4 mmol/0.8 mmol) and 10 mL toluene added, a
mixture of toluene (2 mL) and crotonaldehyde (1.5 mL, 17.8 mmol) was added drop wise over a
period of 1 h and refluxed for another 6 h. The reaction mixture was cooled to room temperature, and
adjusted to alkaline using aqueous ammonia for precipitating the product completely. The
precipitation was filtered and purified by column chromatography on silica gel (eluent: petroleum
ether/ethyl acetate = 1:5.5, v/v) to give light yellow crystal 6-nitro-quinaldine 1.
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Yield, 93.1%. IR (KBr), υ (cm–1): 3096 (Ar-H), 2959 (CH3), 1534, 1382 (-NO2). H NMR (400
MHz, CDCl3) δ: 8.12 (s, J=8.4 Hz, 1 H, H5), 7.97 (d, J=2.4 Hz, 1 H, H4), 7.95 (d, J=2 Hz, 1 H, H3),
7.53 (d, J=8.2 Hz, 1 H, H8), 7.38 (d, J=8.4 Hz, 1 H, H7), 2.78 (s, 3 H, CH3). Anal. Calcd. for
C10H8N2O2: C, 63.82; H, 4.28; N, 14.89. Found: C, 63.93; H, 4.27; N, 14.74.
2.2.2 Preparation of 6-amino-quinaldine (2)
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To a 50 mL round bottom bottle, glacial acetic acid (10 mL) and 6-nitro quinaldine (1) (400 mg,
2.13 mmol) were added. The mixture was then stirred and added iron powder (480 mg, 8.60 mmol)
slowly. After turning to light yellow completely, the reaction mixture was filtered and washed
thoroughly with ethanol. The filtrate was distilled to dryness under reduced pressure conditions and
the residue was purified by column chromatography on silica gel (eluent: ethyl acetate) to give white
crystal 6-amino quinaldine 2.
Yield, 71.9%. IR (KBr), υ (cm–1): 3462, 3428 (NH2), 2920 (CH3). 1H NMR (400 MHz, CDCl3) δ:
7.93 (d, J=8.4 Hz, 1 H, H3), 7.76 (d, J=8.5 Hz, 1 H, H2), 7.73 (d, J=4.2 Hz 1 H, H7), 7.32 (d, J=4.8Hz,
1H, H8), 7.26 (s, 1 H, H5), 5.02 (s, 2 H, NH2), 2.739 (s, 1H, CH3). Anal. Calcd. for C10H10N2: C,
75.92; H, 6.37; N, 17.71. Found: C, 75.03; H, 6.41; N, 17.80.
2.2.3 Preparation of 6-quinaldinyl-4’-[N,N-dimethyl-amino] azobenzene (3a)
6-Aminoquinaldine (2) (316 mg, 2 mmol) was dissolved in an ice-water solution of sodium nitrite
(170 mg, 2.5 mmol) in 3 mL of water. After cooling to 0 oC, concentrated hydrochloric acid (1.2 mL)
was added to the solution under continuous stirring for 30 min. The excess nitrous acid was
destroyed with about 5 mg urea. The mixture was then added drop wise to 10 mL buffered aqueous
solution (acetic acid/sodium acetate, pH ≈ 6) containing N,N-dimethyl aniline (267 mg, 2.2 mmol)
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and stirred for another 2 h at 0-5 C. The resultant precipitation was filtered and rinsed with enough
water. The crude product was purified by column chromatography on silica gel (eluent: petroleum
ether/ethyl acetate = 1:1, v/v) to provide red crystal 3a.
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Yield, 87.1%. IR (KBr), υ (cm–1): 3056 (ArH), 2923 (CH3), 1599 (N=N). H NMR (400 MHz,
CDCl3) δ: 8.76 (s, 1H, H5), 8.43 (d, J=7.9 Hz, 1 H, H4), 8.24 (d, J=7.8 Hz, 1 H, H3), 8.14 (d, J=8.8
Hz, 2 H, H8), 7.94 (d, J=8.8 Hz, 2 H, H7), 7.35 (d, J=8.4 Hz, 2 H, H9), 6.79 (d, J=8.8 Hz, 2 H, H10),
3.12 (s, 6 H, N-CH3), 2.82 (s, 3H, Ar-CH3). 13C NMR(100MHz, CDCl3) δ: 159.8, 154.3, 151.1,
149.2, 145.8, 136.6, 131.7, 126.9, 126.2, 126.1, 125.8, 123.0, 122.9, 112.7, 112.7 (Ar-C), 42.3,
42.3(N-CH3), 24.3(ArCH3). MS: m/z 290.4 [M]+. Anal. Calcd. for C18H18N4: C, 74.46; H, 6.25; N,
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19.30. Found: C, 74.53; H, 6.29; N, 19.39.
2.2.4 Preparation of 6-quinaldinyl-4’-[N-methyl-N-hydroxyethyl amino] azo benzene (3b)
3b was prepared as above from N-methyl-N-hydroethyl aniline and 6-Aminoquinaldine. The
product was red crystal. Yield, 84.7%. IR (KBr), υ (cm–1): 3363 (OH), 2922 (CH3), 1599 (N=N). 1H
NMR (400 MHz, CDCl3) δ: 8.27 (d, J=7.8 Hz, 1 H, H4), 8.15 (d, J=7.9 Hz, 1 H, H3), 7.93 (s, 1 H,
H5), 7.91 (d, J=8.5 Hz, 2 H, H8), 7.37 (d, J=8.4 Hz, 2 H, H7), 6.85 (m, J=3.2 Hz, 4 H, H9,10), 3.90 (t,
J=8.4Hz, 2 H, CH2OH), 3.64 (t, J=8.6Hz, 2 H, N-CH2), 3.33 (s, 1H, OH), 3.15 (s, 3H, N-CH3), 2.82
(s, 3H, ArCH3). 13C NMR(100MHz, CDCl3) δ: 159.9, 152.3, 151.0, 149.3, 144.8, 136.6, 131.7, 126.8,
125.8, 125.1, 125.1, 124.8, 122.9, 111.8, 111.8 (Ar-C), 64.0(N-CH2-), 58.8(O-CH2-), 41.9(N-CH3),
24.3(ArCH3). MS: m/z 320.2 [M]+. Anal. Calcd. for C19H20N4O: C, 71.23; H, 6.29; N, 17.49. Found:
C, 71.16; H, 6.33; N, 17.55.
2.2.5 Preparation of 6-quinaldinyl-4’-[N,N-dihydroxyethyl amino]azobenzene (3c)
3c was prepared as above from N,N-dihydroxyethyl aniline and 6-Aminoquinaldine. The product
was red crystal. Yield, 85.8%. IR (KBr), υ (cm–1): 3346 (OH), 2921 (CH3), 1597 (N=N). H NMR
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(400 MHz, CDCl3) δ: 8.19 (d, J=7.8 Hz, 1 H, H4), 8.01 (d, J=7.9 Hz, 1 H, H3), 7.91 (s, 1 H, H5), 7.89
(d, J=8.5 Hz, 2 H, H8), 7.33 (d, J=8.4 Hz, 2 H, H7), 6.79 (m, J=2.2 Hz, 4 H, H9,10), 3.97 (t, J=7.6Hz, 4
H, CH2OH), 3.74 (t, J=7.5Hz, 4 H, NCH2), 3.59 (s, 2H, OH), 2.77 (s, 3H, CH3). 13C NMR(100MHz,
CDCl3) δ: 159.8, 151.8, 151.0, 149.2, 144.8, 136.6, 131.6, 126.8, 125.1, 125.1, 124.8, 123.0, 122.8,
111.6, 111.6 (Ar-C), 61.8, 61.8(N-CH2-), 58.9, 58.9(O-CH2-), 24.5(ArCH3). MS: m/z 350.5 [M]+.
Anal. Calcd. for C20H22N4O2: C, 68.55; H, 6.32; N, 15.99. Found: C, 68.51; H, 6.37; N, 16.04.
2.2.6 Self-assembly procedure
According to the reported method in the literature[4, 21], in a typical experiment, injection of 5
mL +3a, 3b or 3c solutions with the concentration of 10−4 mol·L−1 in tetrahydrofuran solution (THF,
a good solvent for 3a, 3b and 3c) into water (a bad solvent for 3a, 3b and 3c) got a mixture. The
mixture was sonicated to disperse evenly in water-bath urtrasonic apparatus at room temperature for
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0.5 h and then all the mixtures were on standing for 48 h for self-assembly effectively. After that, the
assemblies were filtered and stored at room temperature (20 oC).
2.2.7 Preparation of the samples
To measure the UV-vis absorption spectra and the optical limiting properties of 3a, 3b and 3c in
solutions, the above self-assemblies of 3a, 3b and 3c were dissolved in DMSO to get 1.0×10−4
mol·L−1 solutions, respectively. According to our recent research result [4], we conclude that there
must be the existence of supramolecular self-assemblies for 3b and 3c to some extent, especially for
3c in its solution with the increasing terminal hydrogen bond recognition ability.
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3. Results and discussion
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3.1. Synthesis of 3a, 3b and 3c
Three polar quinoline-based heterocyclic azo derivatives (3a~3c) with different terminal hydrogen
bond recognition groups were designed and synthesized through a multi-step reaction route shown in
Scheme 1[5, 19, 20]. 6-Nitroquinaldine was synthesized using crotonaldehyde and p-nitroaniline as
raw materials in the presence of acetic acid and toluene. It was found that the optimal reaction
conditions for diazotization of 6-aminoquinaldine with N,N-bis-substituent aniline derivatives were
in the buffer solution of acetic acid and sodium acetate (pH~6), which could keep diazo salt in high
concentration and gave quinoline-based azo derivatives 3a~3c in high yields.
1
All the intermediates and target compounds were characterized by FT-IR, H NMR and element
analysis and satisfactory analysis data corresponding to their molecular structures were obtained as
shown in section 2.
3.2. Characterization of the hydrogen bond between polar D-π-A azo molecules (3a, 3b and 3c)
Theoretically, when the hydrogen-bond proton donor is the same, the higher alkaline and the less
space steric effect of the proton acceptor, the more easily and strongly hydrogen bonds can be
formed. As the alkaline of N-atom in quinoline ring is stronger and its space steric effect is less than
those of O-atom in the terminal hydroxyl group, the intermolecular hydrogen bond interaction
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mainly happens between the terminal hydroxyl groups as proton donors and N-atoms in quinoline
rings as proton acceptors in polar 3a, 3b or 3c molecules based on the thermodynamic equilibrium
and kinetic effect, does not bring forth between two of the terminal hydroxyl groups (shown in
Figure 1).
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Insert Fig. 1
It is well known that FT-IR spectroscopy is a suitable technique for investigating particular
intermolecular interaction owing to the fact that any alteration in intermolecular and/or
intramolecular interaction can induce the corresponding FT-IR absorption change in its strength,
peak position or peak area of the characteristic functional group[4]. To certify the intermolecular
hydrogen bonding distribution in 3a, 3b and 3c, their FT-IR spectra were recorded at room
temperature first and shown in Figure 2, respectively.
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Insert Fig. 2
From Figure 2, it is easy to find that there is no absorbance for 3a in the region of 3100~3800 cm–1,
i.e., the interaction force between 3a molecules is weak dipole-dipole interaction and van der waals
force only. For 3b and 3c, the absorbance at ca. 3600 cm–1, which belongs to the O-H stretching
vibration of free hydroxyl groups, is very weak. It means that there are few free hydroxyl groups in
solid 3b and 3c. Moreover, we can find two obvious absorption peaks at ca. 3415 and 3269 cm–1,
respectively. The peak at 3415 cm–1 is assigned to the “oligomers” resulting from hydrogen bond
interaction between two molecules of 3b or 3c, whereas the one at 3269 cm–1 is attributed to the
formation of ‘‘polymeric’’ species[4]. The peak intensity at 3269 cm–1 becomes stronger in 3c than
that in 3b, hinting that the stronger H-bond interaction happens in 3c than that in 3b. Simultaneously,
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it is also found that the absorbance peak attributing to C=N stretching vibration changes from 1590
cm–1 for 3a to 1597 cm–1 for 3b, and further to 1599cm–1 for 3c (Fig. 2), due to the strengthening
electron- attractive ability of the N-atom in the quinoline ring once the formation of H-bonds. It also
declares the N-atom in the quinoline ring has participated in forming H-bonds in 3b and 3c. The
FT-IR spectra of 3b and 3c at 60 oC were also recorded shown in Figure 3.
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Insert Fig. 3
At 60 oC, for 3b, the peak intensities at 3415 cm–1 and 3269 cm–1 both decrease compared with
those at room temperature but their relative intensities change inversely (Figure 3a), exhibiting that
“polymeric species” possesses higher stability than “oligomers” pattern. That is, the hydrogen bond
intensity in the “polymeric species” pattern is stronger and more stable than that in “oligomers”
pattern. The similar conclusion can be drawn from FTIR spectra of 3c (Figure 3b).
Another proof for the formation of hydrogen bond interactions with different intensities in 3a, 3b
and 3c is their thermal properties, which are directly related to the strength of intermolecular
interaction and the regularity of molecular arrangement[22, 23]. Thermal decomposition
temperatures (Td, 5 wt % loss) of 3a, 3b and 3c are 166.5, 200.2, and 245.5 oC (shown in Figure 4),
respectively.
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Insert Fig. 4
Td improves with increasing terminal hydrogen bond recognition ability. Thermal properties of 3a,
3b and 3c further confirm that the strength of intermolecular force and the regularity of molecular
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arrangements of 3a~3c are in the order of 3c䟟3b䟟3a. It is consistent with analyses of FT-IR
spectra. The intermolecular hydrogen bond interactions of 3a, 3b and 3c with different intensities are
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also confirmed directly by their induced aggregates morphologies. To do this, the morphologies of
the resultant self-assemblies were examined by optical microscopy (OM) shown in Figure 5.
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Insert Fig. 5
From Figure 5 we can find that the morphology of 3a agglomeration is random nano-particle
shape owing to the weak van der waals interaction and dipole-dipole interaction between 3a
molecules. With the introduction of hydrogen bond group and increase of hydrogen bond recognition
ability of group, the morphology of molecular agglomeration becomes from zero-dimensional nano
particles for 3a, to one-dimensional fibrous structure for 3b, and further to two-dimensional line-like
crystals for 3c.
3.3. UV-vis absorption spectra
To illustrate the intermolecular interaction intensity in 3a, 3b and 3c, the UV-vis absorption
spectra of 3a, 3b and 3c in DMSO at room temperature were measured (Figure 6). As shown in
Figure 6, the UV-vis absorption peaks are at 446, 304 and 269 nm for 3a, 450, 308 and 270 nm for
3b, and 453, 313 and 273 nm for 3c, respectively, i.e., the corresponding absorption peaks are
red-shift to longer wavelength with increase in terminal hydrogen bond recognition ability. Optical
band gaps for 3a, 3b and 3c were calculated to be 2.32, 2.29 and 2.27 eV, respectively according to
the onset absorption edge at higher wavelength in the intensity of 5% λmax as illustrated in the
literature reported[20].
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Insert Fig. 6
Red-shift of UV-absorption peaks and decrease in optical band gaps may be explained by the
formation of much stronger intermolecular hydrogen bond interaction described above. It is well
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known that once the formation of hydrogen bond interaction, the electron-attractive ability of N-atom
in quinoline ring will get stronger and the arrangement of molecules will be more regular, which
both facilitate electron dispersal in molecules and further result in larger π-electron delocalization.
The band gap between the highest occupied molecular orbits (HOMO) and the lowest unoccupied
molecular orbits (LUMO) is lowered, which facilitates intermolecular charge transfers (ICT) in the
excited state.
3.4. Optical Limiting Properties
To further explore the influence of the intermolecular interaction on optical properties, the optical
limiting properties of 3a, 3b and 3c with the same nonlinear optical chromphore content in DMSO
were evaluated by Z-scan technique under an open aperture configuration. As shown in Figure 6, 3a,
3b and 3c in DMSO show nearly no linear absorption at 532 nm, which promises low intensity loss
and little temperature change by photon absorption during the NLO measurements. In theory, the
normalized transmittance for the open aperture configuration can be written as[24].
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(1)
where q0(z) = βI0(t)Leff /(1 + z2/z0 ), β is the nonlinear absorption coefficient, I0(t) is the intensity
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of laser beam at focus (z = 0), Leff = [1– exp(–α0 L)]/ α0 is the effective thickness with α0 the linear
absorption coefficient, L is the sample thickness, z0 is the diffraction length of the beam, and z is the
sample position. The results from Z-scan experiments are demonstrated in Figure 7.
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Insert Fig. 7
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The solid lines in Figure 7 are theoretical curves from equation (1). The data collected under the
open-aperture configuration indicate that they all exhibit obvious nonlinear absorption properties
with effective β values of 0.989 × 10–11 m W–1 (3a), 1.41 × 10–11 m W–1 (3b), and 1.55 × 10–11 m W–1
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(3c), respectively. The result is consistent with our report before [25] that nonlinear properties appear
the trend of 3c > 3b > 3a with increasing the terminal hydrogen bond recognition ability, but are
stronger than those of corresponding azobenzene-based dyes. The reason may be that the
introduction of polar quinolyl group can make the electron cloud distribution of target molecules
more uniformitive, which facilitates intermolecular charge transfers (ICT) in the excited state and
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improves their optical limiting characteristics.
Simultaneously, it is found that optical limiting characteristics of these materials are also
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influenced by the regularity of the supramolecular self-assemblies. Optical limiting behaviors of 3a,
3b and 3c in DMSO are shown in Figure 8, evaluated at 532 nm using 4 ns laser pulses with the
same linear transmittance (T=45%).
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Insert Fig. 8
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The limiting threshold (incident fluence at which the output fluence starts to deviate from
linearity) and amplitude (maximum output fluence) of 3a, 3b and 3c are 0.85 J/cm2 and 0.37 J/cm2,
0.52 J/cm2 and 0.30 J/cm2, 0.48 J/cm2 and 0.27 J/cm2, respectively. Just like UV-vis and nonlinear
absorption properties, the lowest limiting threshold and limiting amplitude of the three materials both
improve with increasing the terminal hydrogen bond recognition ability. With increase of
intermolecular interaction, the larger π-electron delocalization may be shown in molecules along
with lower energy gap between HOMO and LUMO, further facilitating intermolecular charge
transfers (ICT) in the excited state[26], and enhancing optical limiting property.
3.5. Theoretical calculations
To theoretically confirm the conclusions made above, calculation under the Gaussian 03 software
suite with B3LYP/6-31G level of theory[27], has been carried out to examine the dipole moments,
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LUMO & HOMO energy levels of compound 3 with and without the interaction of hydrogen
bonding. For simplifying the calculation, the basic structural units of the materials were selected and
the bonds at the end of the structural units were saturated with hydrogen atoms. The results are
shown in Figure 9.
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Insert Fig. 9
From Figure 9, it is easy to find that, electron cloud is mainly grouped on the electron-donor
amino-terminal segments for D-π-A-typed azo compounds 3 without the hydrogen bond interaction,
i.e., electron cloud is unevenly distributed, molecular dipole moment is large (6.1032 D), and the
energy gap between the HOMO (–5.0325 eV) and the LUMO (–1.8533 eV) is high (3.1792 eV).
Once the formation of hydrogen bond interaction between N-atom in quinoline ring and H-atom of
hydroxyl group, the electron cloud distribution is maintained at equilibrium owing to the increasing
electron-withdrawing ability of quinolyl group. The total dipole moment of resultant 3 decreases to
4.5211 D, and the energy gap between the HOMO (–5.1240 eV) and the LUMO (–1.9779 eV) is also
reduced to 3.1641 eV, which facilitates intermolecular charge transfers (ICT) in the excited state and
improves its optical limiting characteristics.
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4. Conclusions
In conclusion, three new polar D-π-A conjugated quinoline-based heterocyclic azo derivatives
with different terminal hydrogen-bond recognition abilities were designed and prepared. Owing to
introducing basic quinolyl (which is a kind of strong basic proton-acceptor) and different terminal
hydrogen bond recognition groups, the intermolecular interactions in the target organic molecules are
greatly different, which results in different D-π-A conjugated degrees and molecular polarities. The
results facilitate very well to enlarge the larger π-electron delocalization of molecules, to reduce
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energy gap between HOMO and LUMO and so to facilitate intermolecular charge transfers. All these
result in red-shift of linear absorption spectra and better optical limiting properties. This work will
provide a new method to construct the building-in of optical limiting materials with nice property in
the future.
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Acknowledgments
The authors gratefully acknowledge School of Chemistry and Chemical Engineering in
Shandong University for offering the Gaussian 03 software suite and also acknowledge financial
support from the National Natural Science Fund of China (Grant Nos. 20974018, 20971021,
51073031, 21271040, 21171034 and 21277103), National Oversea Scholar Cooperation Research
Fund of China (No. 50928301), PhD. Program Foundation of Ministry of Education of China
(No.20070255012).
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Reference
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Figure Legends
Scheme 1. Synthetic routes of the target quinoline-based squaraine dyes (3a, 3b and 3c).
Fig. 1. The main hydrogen bonding formed in 3b (left) and 3c (right) in theory.
Fig. 2. The FT-IR spectra of 3a, 3b and 3c at room temperature (25 oC).
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Fig. 3. Part of the FT-IR spectra of 3b (a) and 3c (b) at 60 oC and 25 oC, respectively.
Fig. 4. TGA thermograms of 3a, 3b and 3c at a ramp rate of 10 °C/min in nitrogen flow, respectively.
Fig. 5. Optical microscopic images of self-assemblies 3a, 3b and 3c amplified by 2000×.
Fig. 6. UV-vis spectra of DMSO solutions of 3a, 3b and 3c.
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Fig. 7. Normalized open-aperture Z-scan transmittance of 3a, 3b, and 3c in THF using 4 ns pulses at
532 nm. The curves have been vertically shifted for clarity. Solid lines represent theoretical fits.
Fig. 8. Optical limiting responses to 4 ns, 1Hz pulses of 532 nm laser light in DMSO solutions of
3a~3c with a linear transmittance of 45%.
Fig. 9. The dipole moments, LUMO & HOMO energy levels of 3 with and without the formation of
H-bond implemented in the Gaussian 03 suite of program.
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Highlights
> Three different polar D-π-A conjugated quinoline-based heterocyclic azo derivatives. > Optical
limiting properties are affected by the molecular structures. > The affection mechanism is confirmed
by means of experimental and theoretical proofs.> A well synergetic enhancement effect is
confirmed between polar and H-bond. >
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