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Here, we present the investigations of photo-isomerization behavior and the nonlinear optical properties of azobenzene derivative LB films. The few-layer LB films of AOB-t4 and BNB-t4 exhibit positive nonlinear refraction and two-photon absorption properties as revealed by picosecond Z-scan. The increased conjugation by introducing an oxadiazole group improves the photo-isomerization rate and the nonlinear optical properties, due to a weaker intermolecular interaction and the formation of J-aggregates within AOB-t4 LB film. The third-order susceptibility of cis-AOB-t4 9-layer LB film reaches 1.866 × 10−9 esu and the two-photon absorption coefficient is on the order of 10−8 m/W. Interestingly, the 15-layer AOB-t4 LB film shows negative nonlinear refraction and saturable absorption. Taken together, we have demonstrated the switchable nonlinear optical absorption and refraction properties of AOB-t4 LB film with changing film thickness, which is of significance for nonlinear optics and photonics applications.
© 2017 Optical Society of America
1. Introduction
Nonlinear optical materials with large third-order optical nonlinearities and sensitive response have been applied in optical signal processing, optical limiting, photonic switching and optical devices [1, 2]. Azobenzene derivatives play an important role in these applications because of the large nonlinear optical coefficient and fast optical response. The photoisomerization of azo-containing molecules provides an easier way to modify their linear and nonlinear polarizability as well as optical nonlinear refraction [3–5]. In the past decades, much effort in the field has been dedicated to investigating and improving the nonlinear properties of optically controllable azobenzene molecules in solution and assembled structures [6–10]. While many previously reported works of azobenzene derivatives focused on solution phase, the macroscopic properties of azobenzene derivative in film are critically dependent on the molecular assembly behaviors and structures. The Langmuir-Blodgett (LB) technique provides a good platform to construct molecular assemblies due to its capability in controlling molecular orientation and packing mode [11–13], which has been proven in fabricating monolayer and multilayer structured films of azobenzene derivatives [14].
In this work, we prepared two kinds of LB film with different numbers of layers composed of azobenzene derivatives, N-(3,4,5-tributoxyphenyl)-N’-4-[(4-hydroxyphenyl) azophenyl]1,3,4-oxadiazole (AOB-t4) and N-(3,4,5-tributoxyphenyl)-N’-4-[(4-hydroxyphenyl)azophenyl] benzohydrazide (BNB-t4), as shown in Fig. 1 [15]. The photoinduced isomerization behavior and the nonlinear optical property of these LB films were then systemically studied. Compared to the BNB-t4 LB film, the AOB-t4 LB film demonstrates a faster response to UV irradiation as well as higher nonlinear optical refraction and absorption parameters with its increased conjugation degree. Meanwhile, the trans-cis photo-isomerization of azobenzene unit and the formation of J-aggregate in film also demonstrate the effect on nonlinear optical property. The obtained third-order susceptibility χ(3) of cis-AOB-t4 9-layer LB film reaches 1.866 × 10−9 esu and the two-photon absorption coefficient is on the order of 10−8 m/W. Particularly, we successfully tuned the nonlinear optical response of AOB-t4 film by varying the film thickness, switching from two-photon absorption to saturable absorption as well as from self-focusing to self-defocusing.
2. Experiment section
In the experiments, all the chemicals and reagents were obtained from commercial suppliers and used as instructed. The UV-vis absorption spectra were recorded on a Perkin-Elmer Lambda 35 spectrometer. The 365 nm UV light for photoisomerization was generated by a 500 W high-pressure mercury lamp, and the UV light intensity was set to be 38.9 mW/cm2 at film surface. Fourier transform infrared (FT-IR) spectra of powder were measured using a Perkin-Elmer spectrometer with KBr pellets. The Z-scan nonlinear optical measurements were performed using a mode-locked Nd:YAG laser as the light source with near Gaussian spatial beam profile at 532 nm, 30 ps pulse width and 10 Hz repetition frequency. The laser beam was focused onto the sample by a lens with 25 cm focal length. The beam waist was measured to be 10.6 μm and the corresponding Rayleigh length was 663 μm in the air. The transmittance/incidence energy of laser pulses was monitored with an EPM 2000 energy meter in both open and close aperture setups.
LB films were prepared on a KSV 5000 system. The quartz plates were cleaned by a mixture of H2SO4/H2O2 (3:1, v/v) [16], and dried with nitrogen gas. The azobenzene derivative was dissolved in chloroform with a concentration of 5 × 10−5 mol/L, and a certain volume of the solution was spread on the ultrapure water subphase. After evaporating the chloroform for 15 min, the film was slowly compressed at a constant barrier speed. The monolayer film was then transferred onto solid substrates by a conventional vertical dipping method at a surface pressure of 35 mN/m for BNB-t4 and 40 mN/m for AOB-t4, respectively. The Y-type multilayer film was fabricated with a transfer rate close to 5 mm/min, and its absorption spectrum was measured on a Perkin-Elmer Lambda 35 spectrometer. FT-IR spectral measurement of BNB-t4 LB film deposited on a CaF2 plate was performed on a Perkin-Elmer spectrometer (Spectrum One B). All of the prepared monolayer or multilayer LB film was of bilayer, deposited on both sides of quartz plate or CaF2 plate.
3. Results and discussion
3.1 π-A isotherms of the monolayers
According to the chemical structure of the two azobenzene derivatives, the hydroxyl group (-OH) and the three butoxy chains are expected to act as the hydrophilic head and the hydrophobic tail, respectively. The π-A isotherms shown in Fig. 2(a) indicate that both azobenzene derivatives can form stable monolayers on the surface of water. For the monolayer of AOB-t4 on pure water, the onset of isotherm is at 0.50 nm2 and the surface pressure increases almost linearly from 0.33 to 0.19 nm2 upon compression, indicating the formation of liquid-condensed phase. Thus, the obtained limiting molecular area is 0.33 nm2 and the collapse pressure is 55.87 mN/m. Similarly, the corresponding limiting molecular area is 0.47 nm2 and the collapse pressure is 54.73 mN/m for the BNB-t4 monolayer. Based on a previous study on the orientation of an amphiphile with one azobenzene-containing alkyl chain at the air/water interface, the minimum molecular area of the azobenzene chromophore in the Langmuir monolayer was determined to be 0.25 nm2 [17]. Considering the molecular structures of BNB-t4 and AOB-t4, the observed differences in π-A isotherms can be attributed to different intermolecular interactions during monolayer formation. Specifically, in addition to the hydrophobic interactions between the three butoxy chains and the π-π interactions between the azobenzene units, the intermolecular H-bonding between the hydrazide subgroups of BNB-t4 and the π-π interaction between the oxadiazole units of AOB-t4 offer the forces to stabilize monolayer formation.
Fig. 2 (a) π–A isotherms of trans-AOB-t4 and trans-BNB-t4 monolayers at the air-water interface. Normalized absorption spectra of (b) 1-layer trans-AOB-t4 LB film and trans-AOB-t4 chloroform solution (5 × 10−5 M), and (c) 1-layer trans-BNB-t4 LB film and trans-BNB-t4 chloroform solution (5 × 10−5 M). (d) FT-IR spectra of trans-BNB-t4 powder and trans-BNB-t4 15-layer LB film.
To determine the structural characteristics of the aggregation effect, we then compared the UV-visible absorption spectra of the LB film with that in solution. As shown in Fig. 2(b), the π–π* absorption maximum is located at 356 nm for AOB-t4 in chloroform solution (5 × 10−5 M) and 362 nm in monolayer LB film at room temperature. The obvious redshift in LB film of AOB-t4 relative to the dilute solution suggests the formation of J-aggregation mainly through π–π interactions [18]. Similar characteristics were also observed by comparing the spectrum of BNB-t4 in chloroform solution with that in the LB film, as shown in Fig. 2(c), and the corresponding maximal π-π* absorption in BNB-t4 LB film is shifted from 355 nm to 360 nm. A relatively smaller molecular area in π-A isotherm and a larger redshift in absorption spectrum observed for AOB-t4 film compared to those for BNB-t4 film can be thus attributed to the increased π-electron conjugation in AOB-t4 containing oxadiazole subgroup and the stronger π–π stacking intermolecular interactions. In other words, the hydrogen bonding, π–π stacking and hydrophobic interactions all contribute to modulate the molecular arrangement and eventual assembly of AOB-t4 and BNB-t4 LB films.
To further investigate hydrogen bonding in LB film of BNB-t4, we compared FT-IR measurements of BNB-t4 in LB film and in powder. Figure 2(d) shows the FT-IR spectra of BNB-t4 powder in a KBr pellet and the 15-layer LB film onto a CaF2 substrate. For BNB-t4 powder, the spectrum shows the hydrogen-bonded O-H stretching vibration at 3388 cm−1, the N-H stretching vibrations at 3258 cm−1, and amide I at 1643 and 1678 cm−1. These data indicate that the N-H groups are associated with C = O groups by means of N-H∙∙∙O = C hydrogen bonding [19]. For the BNB-t4 15-layer LB film, the peak of O-H stretching vibration shifts to a lower wavenumber, and merges with N-H stretching vibrations into one peak located at 3245 cm−1. This shift to a lower wavenumber can be attributed to stronger intermolecular hydrogen bonding in LB film than in powder.
3.2 Photoresponsivity of LB films upon UV irradiation
Figure 3(a) shows the typical absorption spectra of AOB-t4 in 1-layer LB film with different time periods of 365 nm irradiation. Evidently, the maximum π–π* absorption band at 362 nm of trans-azobenzene moiety decreases with irradiation time, and the π–π* absorption band of cis-azobenzene moiety at around 272 nm increases concomitantly. The conversion efficiency to cis-AOB-t4 can reach ca. 83% at the photo-stationary state within 40 min [20].
Fig. 3 UV-vis absorption spectra of (a) 1-layer trans-AOB-t4 LB film with different UV irradiation time; (b) trans-AOB-t4 LB films with different layers, Inset: absorbance at 532 nm vs number of layers; (c) 9-layer trans-AOB-t4 LB film with different UV irradiation time; (d) multilayer trans-BNB-t4 LB films, Inset: absorbance at λmax vs number of layers; (e) 9-layer trans-BNB-t4 LB film with different UV irradiation time. (f) The correlation of UV-vis absorption changes with UV irradiation time for 9-layer trans-AOB-t4 and trans-BNB-t4 LB films, respectively.
To examine the influence of film thickness on photo-isomerization of AOB-t4 and BNB-t4 in LB films, we prepared multilayer LB films. As shown in Figs. 3(b) and 3(d), the UV-vis absorbance at λmax of the LB films increases linearly with the number of bilayers, and no absorption shift can be observed. These results indicate that the monolayer can be uniformly transferred onto the solid substrate. Meanwhile, it should be noted that a tailing absorption band around 532 nm can be observed with increasing of film thickness. The absorption spectra of 9-layer AOB-t4 LB film with different time period of 365 nm irradiation are shown in Fig. 3(c). We observed that the photo-induced trans-to-cis isomerization still takes place in multilayer films, and the conversion efficiency from trans- to cis-AOB-t4 reaches 71% within 90 min UV light irradiation, which is much higher than that for BNB-t4 with 57% efficiency, as shown in Fig. 3(e). Quantitatively, the experimental data can be used to calculate the corresponding first-order rate constant of π–π* transition (A), according to the Eq. (1) as follow [21]:
where [t]0, [t] and [t]∞ are an initial concentration of a trans-azobenzene, the changing trans-azobenzene concentration under different UV irradiation time, and an equilibrium trans-azobenzene concentration of photostationary state, respectively. The absorption plots of LB films for the two azobenzene derivatives to UV irradiation time can be well fitted as a linear relationship, as shown in Fig. 3(f). As a consequence, the corresponding A can be determined to be 0.021 min−1 for AOB-t4, and 0.014 min−1 for BNB-t4, respectively. The results clearly demonstrate that the photoisomerization from trans-to-cis isomer for AOB-t4 9-layer LB film is faster than that for BNB-t4 9-layer LB film. As mentioned above, the FT-IR spectrum of BNB-t4 LB film shows the hydrogen-bonded O-H stretching vibration and intermolecular H-bonding between the hydrazide subgroups. In contrast, the π–π interactions through the azobenzene and oxadiazole units among the adjacent molecules play an important role in the formation of AOB-t4 LB film. Therefore, the faster rate of trans-cis isomerization of AOB-t4 9-layer LB film can be attributed to a weaker intermolecular interaction than that in BNB-t4 9-layer LB film. Therefore, the differences between AOB-t4 and BNB-t4 LB films in minimum molecular area, absorption maximum shifting, and the response rate to UV irradiation are consistent with their differences in molecular structures, molecular interactions and self-assembly features.3.3 Nonlinear Optical Properties of LB films
The nonlinear optical properties of the two azobenzene derivative films were investigated with the picosecond single beam Z-scan measurements. Figures 4 and 5 show the open-aperture (OA) curves and closed aperture (CA) curves divided by OA curve (CA/OA) of 9-layer AOB-t4 and BNB-t4 LB films before and after UV irradiation, respectively. All of OA curves for trans-AOB-t4, cis-AOB-t4, trans-BNB-t4 and cis-BNB-t4 LB films exhibit nonlinear absorption features. Specifically, these absorptions can be assigned to two-photon absorption with a positive coefficient β for the 9-layer LB film because the maximal π-π* absorption of AOB-t4 is around 362 nm in Fig. 3, corresponding to 3.43 eV, which is larger than one photon energy (2.33 eV) but smaller than two-photon energy of the laser beam at 532 nm. The nonlinear absorption coefficient β can be obtained by fitting OA experimental data with the Eq. (2) [22]:
where q0 (z) = βI0Leff /(1 + Z2/Z02); the I0, Z0 represent the intensity of incident and diffraction length of the beam, respectively; Leff = (1-e-αL)/α is the effective length of the sample; α is the linear absorption coefficient. The calculated molecular lengths of trans- and cis-AOB-t4 are 2.52 nm and 2.12 nm, respectively (estimated by DFT molecular modeling). Supposing this molecular length not changed in layer, we can multiply this length by the number of layer to estimate the film thickness. The effective thickness of 9-layer trans- and cis-AOB-t4 LB bilayer films are 45.36 nm and 38.16 nm, respectively. Similarly, the molecular lengths of 2.37 nm and 2.02 nm for trans- and cis-BNB-t4, respectively, were used to estimate the thickness of BNB-t4 LB film.Fig. 4 The normalized Z-scan curves of (a) OA 9-layer trans-AOB-t4 LB film; (b) CA/OA 9-layer trans-AOB-t4 LB film; (c) OA 9-layer cis-AOB-t4 LB film and (d) CA/OA 9-layer cis-AOB-t4 LB film. The single pulse energy was 5 μJ at 532 nm.
Fig. 5 The normalized Z-scan curves of (a) OA 9-layer trans-BNB-t4 LB film; (b) CA/OA 9-layer trans-BNB-t4 LB film; (c) OA 9-layer cis-BNB-t4 LB film and (d) CA/OA 9-layer cis-BNB-t4 LB film. The single pulse energy was 5 μJ at 532 nm.
Interestingly, all of the normalized transmittance for CA Z-scan curves of trans-AOB-t4 and cis-AOB-t4, trans-BNB-t4 and cis-BNB-t4 LB films demonstrate a large pre-focal valley followed by a post-focal feature, suggesting a strong self-focusing effect corresponding to positive nonlinear refraction. To extract the nonlinear refractive index n2 from the Z-scan data, pure nonlinear refraction curves can be obtained from the division of CA data by OA data according to the following Eq. (3) [22]:
where ΔTP-V is the difference between the normalized peak and valley, and s is the aperture linear transmittance. Accordingly, the real part and imaginary part of the nonlinear optical susceptibility χ(3) can be evaluated from the nonlinear refractive index n2 and nonlinear absorption coefficient β, defined by the following Eq. (4) and Eq. (5) [23]: where c is the velocity of light in vacuum, n0 is the linear refractive index of sample, and ω = 2πc/λ is the angular frequency of light field. The absolute value of χ(3) was calculated from.The obtained nonlinear optical parameters β and n2, together with the deduced values of the third-order susceptibility χ(3) are presented in Table 1. Both AOB-t4 and BNB-t4 9-layer LB films show large third-order susceptibility χ(3), on the order of 10−9 esu for cis-AOB-t4 LB film and 10−10 esu for cis-BNB-t4 LB film. The χ(3) value of cis-AOB-t4 LB film after UV irradiation is 1.53 times larger than that of trans-AOB-t4 film, while the enhancement factor is 1.64 for BNB-t4 LB films. Evidently, the enhanced nonlinear optical susceptibility arises from the trans-cis photo-isomerization of these two azobenzene derivatives. During photo-isomerization, the distance between the two benzene rings connected by nitrogen double bond in an azobenzene unit is reduced. As a result, the reduction of molecular dipole moment decreases the linear polarizability, therefore producing a larger positive nonlinearity [24]. Meanwhile, the χ(3) of trans-AOB-t4 LB film is 3.70 times larger than that of trans-BNB-t4 LB film, and 3.45 times larger for the cis-isomer film. The higher χ(3) values of the AOB-t4 LB films compared with that of BNB-t4 LB films can be attributed to the higher π-electron conjugation in AOB-t4 containing oxadiazole subgroup, which is consistent with the aforementioned observations in UV-vis spectra.
Table 1. Nonlinear optical parameters of 9-layer LB films for the two azobenzene derivatives.
To investigate the assembly effect on nonlinear optical properties, we performed the identical Z-scan analysis on the two molecules in solution, where the trans-AOB-t4 and trans-BNB-t4 in chloroform solution (2 × 10−3 M) were put in a cuvette of 1 mm optical length. Both OA curves of trans-AOB-t4 and trans-BNB-t4 exhibit the normalized valleys, indicating the presence of two-photo absorption with a positive coefficient β. The normalized transmission curves for both trans-AOB-t4 and trans-BNB-t4 show a pre-focal valley followed by a post-focal peak. The determined nonlinear optical parameters β, n2, and the third-order susceptibility χ(3) are presented in Table 2. Notably, both trans-AOB-t4 and trans-BNB-t4 in chloroform solution show a much smaller third-order susceptibility χ(3) on the order of 10−13 esu, compared with trans-AOB-t4 (10−9 esu) and trans-BNB-t4 (10−10 esu) 9-layer LB films. Meanwhile, the two-photon absorption coefficient β demonstrates 4 orders enhancement for trans-AOB-t4 LB film relative to that in chloroform solution. To determine the number of molecules contributing to the nonlinear susceptibility, we calculated the two-photon absorption cross section (δ) for trans-AOB-t4 both in solution and in LB films with the Eq [25]: hνβ = δN, where hν is the incident photon energy, N is the molecular density. The obtained δ value of trans-AOB-t4 9-layer LB film is 11184.2 GM (1 GM = 1 × 10−50 cm4 s photon−1 molecule−1), much larger than 2948.3 GM in solution. Importantly, this result suggests that the enhanced nonlinear optical properties of AOB-t4 and BNB-t4 LB film arise from the J-aggregate assembly structures.
Table 2. Nonlinear optical parameters of trans-AOB-t4 and trans-BNB-t4 in chloroform solution.
The observed assembly effect on nonlinear optical property inspires us to examine the dependence of nonlinearity on the number of AOB-t4 LB layers, and the results are shown in Fig. 6. Interestingly, the OA curves for 7-layer and 9-layer AOB-t4 LB films exhibit the two-photon absorption valley, indicating that the two-photon absorption has a positive coefficient β. However, the OA curves exhibit a small normalized peak when the layer number was increased to over 11 and the magnitude increases with the number of layers, indicating saturable absorption behavior with a negative coefficient β. In other words, the curves presented in Fig. 6 demonstrate a clear evolution from valley (positive) to peak (negative) with the increasing number of LB layers. As shown above in Fig. 3, the absorption at 532 nm is very weak when the number of layers is less than 7, and a broad band absorption in the region from 450 nm to 550 nm clearly appears when the layer number is over 9. Indeed, there exists a two-photon absorption of π-π* transition around 362 nm and a one-photon absorption at 532 nm for AOB-t4 LB film, and the sign of nonlinear optical absorption is determined by the competition between the two-photon absorption and saturable absorption. For the thinner LB films, the two-photon absorption dominates, while the saturable absorption plays the major role for the thicker LB films. Therefore, the nonlinear optical response of AOB-t4 LB film can be tuned by simply changing the number of layers. When the number of layer increases from 7 to 15, the normalized transmission changing from valley to peak indicates the dependence of nonlinear optical response on layer thickness. Similar switchable nonlinear absorption was previously observed in semiconductor multilayer WS2 and MoS2 films, using the Z-scan technique with femtosecond pulse from the visible to near-infrared range [26]. Apparently, the origin of such an effect could vary depending on the nature of systems. Sahraoui and associates studied the third order nonlinearities of two azobenzene-iminopyridine molecular systems, employing Z-scan technique at 532 nm with 30 ps pulse width [7]. Their results showed a high dependence of optical nonlinearity on both conjugation length of molecule and the number of the electron accepting units. In addition, Couris et al. investigated the nonlinear optical response of three π-conjugated azobenzene derivatives under picosecond laser excitation by means of Z-scan technique [3], and proposed that the cis-isomer should have a significantly larger optical nonlinearity than the trans-isomer. Therefore, the results presented in our work are in agreement with that reported in the literatures.
Fig. 6 The normalized Z-scan OA curves of trans-AOB-t4 LB films with different thickness. The single pulse energy was 8 μJ at 532 nm.
Furthermore, both the normalized transmittance for CA/OA Z-scan curves of trans-AOB-t4 and cis-AOB-t4 15-layer LB films in Fig. 7 exhibit a large pre-focal peak followed by a post-focal valley, suggesting a strong self-defocusing effect corresponding to negative nonlinear refraction. The obtained nonlinear optical parameters β and n2, together with the deduced values of third-order susceptibility χ(3) are presented in Table 3. The cis-AOB-t4 15-layer LB film shows a large third-order susceptibility χ(3) on the order of 10−9 esu. The value of χ(3) of cis-AOB-t4 15-layer LB film is 1.23 times larger than that of trans-AOB-t4 film, consistent with the aforementioned results for 9-layer LB films. The nonlinear optical absorption coefficient β reverses to negative 10−9 (m/W) due to the saturable absorption for cis-AOB-t4 LB film. These observed switchable nonlinear optical absorption and refractive index with tuning the number of LB layers indicate the AOB-t4 film to be a good candidate for nonlinear optical devices.
Fig. 7 The normalized Z-scan curves of (a) OA 15-layer trans-AOB-t4 LB film, (b) CA/OA 15-layer trans-AOB-t4 LB film, (c) OA 15-layer cis-AOB-t4 LB film and (d) CA/OA 15-layer cis-AOB-t4 LB film. The single pulse energy was 5 μJ at 532 nm.
Table 3. Nonlinear optical parameters of 15-layer trans-AOB-t4 and cis-AOB-t4 LB films.
4. Conclusions
The photoinduced trans- to cis-isomerization behavior and switchable nonlinear optical properties of AOB-t4 and BNB-t4 LB films have been systematically studied here. The molecular structure containing the hydrophilic hydroxyl group (-OH) and the hydrophobic butoxy chain both in AOB-t4 and BNB-t4 enable these two azobenzene derivatives to form stable LB monolayer at the air-water interface. The conjugated group oxadiazole in AOB-t4 film improves the photoresponsive rate and nonlinear optical properties compared with the hydrazide group in BNB-t4 film. As a consequence, the trans-cis photo-isomerization rate of AOB-t4 LB film is greater than that of BNB-t4 LB film, likely due to the weaker intermolecular interaction via π–π stacking in AOB-t4 LB film than that in BNB-t4 LB film. Consequently, the nonlinear optical refraction and two-photon absorption for thinner film are correspondingly enhanced by several times because of the more delocalized conjugated electron in AOB-t4. With increasing thickness of the LB films, the formed J-aggregate effect enlarges the single photon absorption at the laser wavelength and leads to an effective competition between two-photon absorption and saturable absorption. Specifically, the third-order optical nonlinearities of AOB-t4 and BNB-t4 thinner LB films demonstrate positive nonlinear refraction and two-photon absorption, but their thicker films present negative nonlinear refraction and saturable absorption. The reversing nonlinear optical absorption and refraction can be readily switched by tuning thickness of AOB-t4 LB film, which is of significance for nonlinear optics.
Funding
National Natural Science Foundation of China (NSFC) (U1504510, U1604129, U1404619, 21173068).
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