J. Phys. Chem. B 2000, 104, 11859-11863
11859
Characterization, Second-Harmonic Generation, and Gas-Sensitive Properties of Langmuir-Blodgett Films of 1,8-Naphthalimide-tri-tert-butylphthalocyanine Yunqi Liu,* Wenping Hu, Yu Xu, Shenggao Liu, and Daoben Zhu Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed: May 22, 2000; In Final Form: September 13, 2000
Langmuir-Blodgett (LB) films of a new metal-free asymmetrically substituted phthalocyanine (Pc), 1,8naphthalimide-tri-tert-butylphthalocyanine (NaBuPc), were fabricated and characterized by UV-vis spectrometry, atomic force microscopy, and small-angle X-ray diffraction (SAXRD). The results proved that the asymmetrically substituted Pc not only possesses good solubility in common organic solvents but also possesses an ideal LB film forming ability. From its surface pressure-area isotherm and SAXRD pattern, the configuration of the phthalocyanine molecules both at the air-water interface and in the LB films was demonstrated at a tilted angle (72°) to the water surface or solid substrate. The second-harmonic generation (SHG) of a threepfp layer LB film of NaBuPc was measured. Its SH intensity relative to a Y-cut quartz wedge I2ω is (2) approximately 1.76. The values of second-order susceptibility (χ ) and the molecular hyperpolarizability (β) from the multilayers are 8.32 × 10-9 and 5.97 × 10-30 esu, respectively. Gas sensors based on the LB films of NaBuPc demonstrated good gas sensitivity to nitrogen dioxide at room temperature. A linear relationship between the logarithm of the concentration of NO2 and the resistance of the sensor was observed, providing a basis for the application as gas sensors in the future.
Introduction Phthalocyanines (Pcs) and metallophthalocyanines have been investigated as molecular materials for photoelectronic devices, such as nonlinear optics1,2 (NLO) and gas sensors,3-5 for many years because of their thermal and chemical stability.6 However, most research is concentrated on symmetrical Pcs. Asymmetrical Pcs have relatively little attention due to the difficulties of synthesis.7 Our previous work revealed many special properties of asymmetrically substituted Pcs, for example second-order NLO behavior8,9 and gas sensitivity,10,11 as a result of the donor or acceptor substituents in their peripheral ring.12 Recently, we have succeeded for the first time in preparing a new asymmetrical Pc derivative with one naphthalimide group and three tert-butyl groups, namely, 1,8-naphthalimide-tri-tert-butylphthalocyanine (NaBuPc). In this paper, we report the fabrication and characterization of Langmuir-Blodgett (LB) films comprised of this material. The NLO and gas sensitive properties based on these LB films at room temperature are also investigated. Experimental Section The chemical structure of NaBuPc is shown in Figure 1; its synthesis has been reported previously.13 The LB films of NaBuPc were fabricated on KSV-5000 (Finland) computer controlled Langmuir alternating troughs. A chloroform solution of NaBuPc (V ) 150 µL, C ) 7.2 × 10-4 M) was spread onto the deionized double-distilled water surface at 21 °C. After the solvent evaporated completely, the floating layer at the airwater interface was compressed at a speed of 20 mm/min to obtain the surface pressure-area isotherm. The monolayers on the subphase were transferred onto hydrophilic substrates at a * E-mail:
[email protected] Figure 1. Chemical structure of NaBuPc.
surface pressure of 22 mN/m. The upstroke speeds used for Z-type model deposition were in the range of approximately 0.5-2 mm/min. Moreover, the monolayer of NaBuPc also could be deposited onto hydrophobic substrates as Y-type deposition at an up speed of 0.5-2 mm/min and a down speed of 15-18 mm/min. UV-vis spectra of the LB films were recorded on an HP8451A spectrometer. Atomic force microscope (AFM) experiments were performed using Nanoscope III (Digital Instruments, Inc.). The cantilevers were in contact mode with a length of 200 µm and a force constant of 0.12 N/m. Small-angle X-ray diffraction (SAXRD) measurement was performed on a D-maxγB X-ray diffractometer using Cu KR radiation. Second-harmonic generation (SHG) measurements were carried out in transmission geometry using 1.064 µm output
10.1021/jp0018798 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/28/2000
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Figure 2. Surface pressure-area isotherm of NaBuPc at 21 °C.
from an Nd:YAG mode locked laser. The experimental set up was similar to that described in our previous paper.9,14 Linearly polarized light parallel (p) to the plane of incidence was directed at a variable incident angle θ or a fixed angle of 45° onto the vertically mounted samples. An infrared blocking filter and a 532 nm interference filter were used to ensure that only SH radiation was detected. The SHG signal was measured with a photomultiplier and a boxcar integrator. The gas sensors were fabricated on glass substrates covered with gold interdigital electrodes and LB films as shown in our previous paper.15 A standard gas diluting system was used in this study. Measurements were performed during adsorption and desorption operations as a function of time in NO2/N2 environment. To clean the surface of LB films, the sample chamber was pumped to vacuum and N2 was used as inert gas. The electrical conductivity of sensor was measured by a Solartron Schlumberger 7081 precision voltmeter (UK). Results and Discussion Characterization of LB Films. As a result having three tertbutyl groups attached peripherally, the title compound is soluble in common organic solvents, such as chloroform (∼10-2 M), benzene, and toluene. Such good solubility is suitable for fabricating thin films by the LB technique. Figure 2 shows the surface pressure-area isotherm of NaBuPc. The steeply inclining part corresponds to the formation of the solid monolayer and the high surface pressure of the collapse point of the monolayer up to 33 mN/m indicate the good film-forming behavior of the phthalocyanine compound. In general, the isotherm for Pcs is reproducible for every first compression; however, a slight difference between the first and subsequent compressions has been reported.7,16 This phenomenon was also observed for NaBuPc. During the first compression the molecules are free to move and align themselves to each other or even to form aggregates when compressed. On expansion of the area, some of the aggregates that were formed remain as small domains. As a result of these domains, the isotherm is shifted to lower mean-molecular area in the second and subsequent compressions. From the surface pressure-area isotherm, the limiting area per molecule is estimated to be 0.7 nm2 (70 Å2). This value is useful for estimating the configuration of phthalocyanine molecules at the air-water interface.17,18 If the molecules of NaBuPc are oriented flat on the water surface, the average area per molecule would be approximately 219.4 Å2 (15.9 Å × 13.8 Å) according to the Corey-Pauling-Koltun (CPK) model,7,19 assuming that NaBuPc molecules are a rectangle with one side approximately13.8 Å and another about 15.9 Å. However, if the molecules of NaBuPc are densely stacked in a face-to-face orientation and edge-on to the water surface, the average area per molecule would be approximately
Figure 3. UV-vis absorption spectrum of NaBuPc LB films. (a) Absorption spectrum of a 12-layer LB film. (b) Relationship between the absorbance and the number of LB layers: b, at 338 nm; 9, at 628 nm.
46.9 Å2 (3.4 Å × 13.8 Å), assuming that the thickness of NaBuPc molecule is about 3.4 Å17 and that the naphthalimide group is the hydrophilic moiety (because of the hydrophobic properties of other three tert-butyl substituents on the NaBuPc peripheral ring). Since the limiting area per molecule (70 Å2) from the isotherm is between the above two calculated values, it is reasonable to consider that the phthalocyanine molecules are in a tilted arrangement,19 and the tilt angle of NaBuPc molecules is calculated to be about 72° to the water surface. The LB films of NaBuPc were capable of being deposited onto quartz, mica, and glass substrates. The transfer ratio was around 0.8-1.0. UV-vis optical absorbance measurement is usually used to assess the transfer behavior of the floating Langmuir films at the air-water interface to the solid substrates. Figure 3a shows the UV-vis absorption spectrum of a 12-layer LB film of NaBuPc. The spectrum consists of two intense absorption bands in both the visible and near UV regions, the low energy band and the high energy band are attributed to πfπ* Q-band and the πfπ* Soret band with maximum absorbance at 628 and 338 nm, respectively. If the absorbance values at 338 and 628 nm are selected as references to investigate the relationship between the absorbance and the number of transferred layers, linear relationships are presented in Figure 3b, which indicates that each monolayer contributes, on the average, an equal amount to the absorbance, meaning the monolayers were reproducibly and uniformly transferred onto the substrates. The atomic force microscopy topography of NaBuPc LB monolayer on muscovite substrate is shown in Figure 4. Molecular structure of individual molecules was observed for this LB film, which indicates a smooth and steady film can be obtained through LB technique. From this picture the diameter of NaBuPc molecules was measured to be about 0.84 nm. If
LB Films of NaBuPc
J. Phys. Chem. B, Vol. 104, No. 50, 2000 11861
Figure 4. AFM image of the NaBuPc LB film, scan size: 16 × 16 nm2, AFM: contact mode, set point: 0 V, scan rate: 20.35 Hz.
Figure 5. Small-angle X-ray diffraction pattern of a 28-layer LB film of NaBuPc on glass.
we further assume that the molecules of NaBuPc are cubic structure, the area per molecule should be about 70 Å2. This is completely in accordance with the result estimated from the surface pressure-area isotherm of NaBuPc, indicating that a highly oriented LB film was obtained on this substrate. Figure 5 depicts an SAXRD pattern of a 28-layer Y-type LB film of NaBuPc on glass. The SAXRD pattern shows several reflections at 2θ ) 2.14, 4.54, and 9.98° corresponding to (001), (002), and (005) Bragg diffraction, respectively, demonstrating a well-defined layered structure of the LB film. From the diffraction pattern, the bilayer d spacing of the LB film is calculated to be about 40 Å, thus, the thickness of a singlelayer film is 20 Å. As mentioned above the NaBuPc molecules are densely stacked in a face-to-face orientation and edge-on to the water surface with a tilt angle of about 72° using a naphthalimide group as the hydrophilic moiety. The longer diagonal distance across the NaBuPc molecule is approximately
21.1 Å. Therefore, the thickness of the NaBuPc monolayer on the water surface should be about 20 Å. Obviously, the thickness values obtained both from surface pressure-area isotherm and from SAXRD agree well. The SAXRD result, on one hand provides additional evidence for the configuration of NaBuPc molecules at the air-water interface. On the other hand, it indicates that the orientation of the phthalocyanine molecules was maintained during the LB transfer process. Second Harmonic Generation from the LB Films. For SHG NLO materials, there are three requirements: (1) a highly polarizable π-electron conjugation system; (2) an electrondonating substituent and an electron-withdrawing substituent on the aromatic ring or other conjugated system; (3) a noncentrosymmetric structure. Phthalocyanine is an 18 π-electron conjugated molecule, and the other two requirements can be realized by chemical modification of phthalocyanine molecules and by LB technique with X- or Z-type deposition models. As we previously reported,7-9,15 the synthesized novel asymmetrical phthalocyanines with donor-acceptor substituents could result in ideal NLO properties. We fabricated the LB films of NaBuPc by Z-type deposition to investigate their SHG activity. The macroscopic second-order susceptibility (χ(2)) and the molecular hyperpolarizability (β) of the LB films on a quartz substrate were determined using the following equations:20-22
I2ω ∝
β)
2 (χ(2) θ lIω)
n2ωn2ω
χ(2)l f 2ω(f ω)2 F
(1)
(2)
where l is the film thickness, Iω is the incident intensity, nω and n2ω are the refractive indices at 1.064 µm and 532 nm,
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TABLE 1: Parameters of Quartz and a Three-Layer LB Film of NaBuPca materials
thickness (nm)
nω
n2ω
pfp I2ω
F (mol./cm2)
χ(2) (esu)
β (esu)
quartz NaBuPc
2.05 × 104 6.0
1.51 2.02
1.52 1.50
7.55 × 105 1.76
1014/0.70
1.2 × 10-9 8.32 × 10-9
5.97 × 10-30
a n : refractive index at 1.064 µm. n : refractive index at 532 nm. Ipfp: SH intensity, F: surface density. χ(2): macroscopic second-order ω 2ω 2ω susceptibility. β: molecular hyperpolarizability.
Figure 6. Influences of NO2 on the resistance of the NO2 gas sensor consisting of a five-layer LB film of NaBuPc in 10 ppm NO2/N2 at room temperature.
f ω,2ω ) [(nω,2ω)2 + 2]/3 is a local field factor and F is the surface density of the monolayer. By comparison with the signal from pfp the quartz (Y-cut) reference, the SH intensity I2ω and other (2) parameters are listed in Table 1. The χ and β values were obtained according to eqs 1 and 2. From Table 1, a 3-layer LB film of NaBuPc possesses positive macroscopic SHG activity: its χ(2) and β values are 8.32 × 10-9 and 5.97 × ∼10-30 esu, respectively. The SHG results indicate that the asymmetrically substituted NaBuPc is a SHG active molecule. Gas Sensitivity of a NaBuPc Gas Sensor at Room Temperature. At room temperature, the response and recovery curves of the gas sensor of a 5-layer LB film of NaBuPc in 10 ppm NO2/N2 atmosphere are shown in Figure 6. When NO2 was introduced into the test cell, the resistance of the sensor decreased. The equilibrium was reached in less than 1 min, demonstrating relatively large adsorption and desorption velocities for NO2 molecules on NaBuPc films. This is probably due to the fact that the LB sensor films are ultrathin. They can be rapidly saturated by the gas molecules, reaching a maximum response value. After several cycles, the response of the sensor remained almost at a same level, a tremendous improvement compared with our previous work10,11,15 and that of others reported in the literature.23 In those cases, the sensitivities were only partially recovered and decreased gradually. This may be because of the adsorption heat, active energy, and desorption energy of the system. The dramatic resistance decrease of the gas sensors could be explained in terms of a charge-transfer interaction between the phthalocyanine (electron donor) and NO2 (electron acceptor). Although we have been unable to directly establish the nature of the adsorbed species, NO2 is known to form a solid complex with electron donors,24 acting as a planar π-electron acceptor. Battisti et al.5 used surface-enhanced Raman scattering to study the reversibility of NO2 adsorption on a single LB film of phthalocyanine, demonstrating the formation of a complex where charge had been transferred from the macrocycle (Pc) to the electron acceptor molecule (NO2). Tert-butyl groups of NaBuPc are electron donors, which may donate electrons to the phthalocyanine π-system, enhance its donor strength, strengthen the chemisorption and finally reduce the resistance
Figure 7. 85% response and recovery time of the NaBuPc gas sensor in 10 ppm NO2/N2 versus the layer number of LB films.
of gas sensors. At the same time, the conductivity enhancement of the LB films of NaBuPc by NO2 can be explained in terms of the different nature of molecular orbits involved in donoracceptor interactions. The electron affinity of NO2 is about 2.2 eV,25 acting as a π-electron acceptor. The interaction between a delocalized positive charge over the phthalocyanine ring and a negative charge localized on NO2 is weak. Thus, the induced positive holes on the film surface by NO2 are able to escape the Coulombic field of negatively charged adsorbed species, producing a large conductivity enhancement. With the increase in the number of LB film layers, the response and recovery time of gas sensors changed simultaneously. Figure 7 shows the 85% response and recovery time of gas sensor versus the number of LB films of NaBuPc. The response and recovery times increase approximately linearly with the increasing number of layers. According to diffusion theory, a concentration (C) gradient of NO2 exists between the interdigital electrodes and the surface of LB films. The diffusion of NO2 molecules from the surface of LB films to the electrodes takes time, a factor that is prolonged with increasing film thickness. With increasing gas concentration, the conductivity of NaBuPc films is also increased. The relationship between the conductivity (ln σ) of a five-layer LB film gas sensor and varying NO2 gas concentration (ln C) is shown in Figure 8. The ln σ varies linearly with ln C. If we assume the increasing conductivity is proportional to the number of adsorbed gas molecules on the phthalocyanine surface,24 the results shown in Figure 7 may be described using the Freundlich adsorption isotherm
V ) V∞KCx
(3)
where V is the equilibrium adsorbed amount at a given gas concentration, V∞ is the adsorbed amount at the saturation point, C is the gas concentration, x is the distance of the selected point from the surface of the film, and K is a constant.
log σ ∝ logV ) log(V∞K) + xlogC
(4)
These equations are used to interpret the linear relationship
LB Films of NaBuPc
J. Phys. Chem. B, Vol. 104, No. 50, 2000 11863 Acknowledgment. The authors are grateful to Dr. Fang Tian and Prof. Xinsheng Zhao for AFM and SHG measurements. This project was supported by National Natural Science Foundation of China, the Climbing Program, and Chinese Academy of Sciences. References and Notes
Figure 8. The conductivity (ln σ) of the gas sensor (five-layer NaBuPc LB film) versus the gas concentration (ln C) of NO2.
shown in Figure 8 and suggest that NO2 is chemisorbed by phthalocyanines and the heat of adsorption decreases logarithmically with increasing NO2 surface coverage. Conclusions On the basis of the results of LB films, NLO properties, and gas sensitivities of NaBuPc, three conclusions can be drawn: (1) NaBuPc not only possesses good solubility in common organic solvents but also has an ideal LB film forming ability. The configuration of phthalocyanine molecules both at the airwater interface and in the LB films was demonstrated in a tilted arrangement. (2) The LB films of NaBuPc exhibited secondharmonic generation (SHG) activity. The SH intensity of pfp NaBuPc relative to a Y-cut quartz wedge I2ω is about 1.76 (2) and the second-order susceptibility (χ ) and the molecular hyperpolarizability (β) values of a 3-layer LB film are 8.32 × 10-9 and 5.97 × 10-30 esu, respectively. (3) Molecular gas sensors using LB films of NaBuPc were sensitive to nitrogen dioxide, demonstrating fast response and recovery properties. A linear relationship between the logarithm of NO2 concentration and the conductivity of the sensor can be explained in terms of Freundlich adsorption isotherm. Thus, from upon three points, it is reasonable to infer that it is an important way to synthesize asymmetrically substituted phthalocyanines in order to obtain novel Pc compounds with ideal solubility, LB film forming ability, NLO properties, and gas sensitivity.
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