6957
J. Phys. Chem. 1995, 99, 6957-6960
Optical Second-Harmonic Generation from Langmuir-Blodgett Films of an Asymmetrically Substituted Phthalocyanine Yunqi Liu,*?+Yu Xu: Daoben Zhu: Tatsuo Wada: Hiroyuki Sasabe,* Xinsheng Zhao,g and Xiaoming Xie* Institute of Chemistry, Academia Sinica, Beijing 100080, China: Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako, Saitama 351-01, Japan; and Department of Chemistry, Beijing University, Beijing 100871, China Received: September 22, 1994; In Final Form: January 26, 1995@
Phthalocyanine, which is a n-conjugated macrocyclic molecule, usually shows third-harmonic generation. After chemical modification by attaching donor-acceptor substituents to its peripheral ring, intramolecular charge transfer through the n-conjugated system is created. This makes phthalocyanine a second-harmonicgeneration (SHG) active molecule. The Langmuir-Blodgett (L-B) film technique offers a way to create noncentrosymmetric structure, which in principle is a requirement for SHG materials. Here we report the L-B film fabrication of an asymmetrically substituted phthalocyanine consisting of three tert-butyl groups as donor substituents and one nitro group as an acceptor substituent, namely, nitrotri-tert-butylphthalocyanine both in pristine form and in alternating form alternating with either tetra-tert-butylphthalocyanine or arachidic acid. SHG from these L-B films was investigated. We find that the tilt angle of such phthalocyanine molecules on the substrate is almost the same as that at the air-water interface, indicating that the configuration of these molecules was maintained during the deposition process. We demonstrate an alternative strategy for improving the SHG behavior in L-B films. A nearly quadratic dependence of second-harmonic intensity on the number of bilayers is observed up to nine bilayers.
Introduction
t-Bu
Nonlinear optical (NLO) organic thin films have collected much interest due to their potential application for optical processing and storage of data or images. In recent years, there has been increasing research activity on phthalocyanine or porphyrin derivatives, which are two-dimensional n-conjugated molecules. They are attractive because of their relatively large x(3)values (10-10-10-12 esu),Iw4strong absorption bands in the visible and near-infrared region that can be used for resonance enhancement of x ( ~fast ) , response time? and excellent chemical and physical stability. However, the molecules studied so far were mainly of symmetrical structure and therefore exhibited only third-order NLO properties. Suslick et aL6 have synthesized a series of “push-pull” porphyrins containing both donor and acceptor substituents and have examined their secondharmonic-generation (SHG) behavior in solution. There has been relatively little work published to date on SHG of phthalocyanines both in theory’ and in e ~ p e r i m e n t . ~ . ~ The Langmuir-Blodgett (L-B) method plays an important role in the fabrication of well-ordered organic thin films. In particular, for second-order macroscopic NLO, it is necessary to have an intramolecular charge transfer molecule and a noncentrosymmetric film structure. These can be realized by chemical modification of phthalocyanine molecules and by X-type or Z-type L-B deposition models. Furthermore, it has been predicted theoretically that the SH intensity for ideally ordered thin films (thickness much less than the coherence length) should increase proportional to the square of the film thickness.I0 However, quadratic relationships are not always observed in practice.”-I3
’ Academia Sinica.
* RIKEN. 8
Beijing University. Abstract published in Advance ACS Abstracts, April 1, 1995.
t-Bu
t-Bu Figure 1. Chemical structure of NtBuPc molecule.
One of our goals is to extend research on NLO properties of such molecules from their third-harmonic generation (THG) to SHG properties with asymmetrically substituted derivatives. In previous work^,^^^'^ we have reported the synthesis of a novel asymmetric phthalocyanine with donor-acceptor substituents and the primary results of its NLO properties. Although a Z-type L-B film was prepared, no quadratic dependence of SH intensity on the number of layers was observed. In this study, we report some new results conceming the molecular configuration in monolayers and SHG from the L-B films.
Experimental Section Nitrotri-tert-butylphthalocyanine(NtBuPc), shown in Figure 1, was synthesized by a mixed condensation of 5-nitro-1,3diiminoisoindoline and 5-tert-butyl-l,3-diiminoisoindoline.’4 L-B films of NtBuPc were fabricated on KSV-5000 (KSV, Finland) alternating t r 0 ~ g h s . lA~ chloroform solution of NtBuPc was spread onto the deionized double-distilled water surface at
0022-365419512099-6957$09.00/0 0 1995 American Chemical Society
6958 J. Phys. Chem., Vol. 99, No. 18, I995
Liu et al. compared with the SH signal from the reference quartz crystal for which x ( ~is) known (dll = 1.2 x esu). The SHG measurements for NtBuPc's and reference were done under the same conditions for all controllable variables, taking n, = 1.51 and nzO = 1.52 for the reference quartz plate. The refractive indices for NtBuPc are unknown, but from Schechtman's studies on unsubstituted metal-free phthalocyanine, whose structure is similar, n is taken to be n, = 2.20 and = 1.50." The second-order molecular hyperpolarizability @) was then derived using the equationI8
s C
.-sC
I v)
.-c 8
-m
d
L I I I - 8 0 -60 -40 -20
I
I
0
20
I
40
60 80
Incident angle 0 (deg.)
c13
b
-80-60-40
-20
0
20 4 0 60 80
Incident angle 0 (deg.) Figure 2. SH intensities as a function of incident angle of p-polarized fundamental light for monolayer of NtBuPc: (a) monolayer on both sides of substrate; (b) monolayer on one side of substrate. 0 , I p; 0 , I.*;;
18 f 2 "C. Monolayer was transferred onto a hydrophilic quartz substrate at a surface pressure of 20 "/m. In the case of alternating L-B films, either tetra-tert-butylphthalocyanine (BuPc) or arachidic acid (AA) monolayers were formed in another trough. The SHG measurement was carried out in a transmission geometry using 1.064 pm output from an Nd:YAG modellocked laser, and the experimental setup was similar to that described in our previous paper.I5 Linearly polarized light which was parallel (p) to the plane of incidence was directed at a variable incident angle 8 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.
Results and Discussion Phthalocyanine exhibits electron transitions in the visible (Q band at 550-750 nm) and in the near-W (B or Soret band at 300-400 nm) region. In the spectral range 400-550 nm is the optical window of the ground-state absorption.I5 In the SHG experiment, the macroscopic second-order susceptibility ~ ( of~ the1 L-B film of NtBuPc was determined according to the method developed by Ashwell et al.I6
($1IJ2 I20
Oc
2 nUJ n20
where I is the film thickness, Z0 is the incident intensity, and n, and nzO are the refractive indices at 1.064 and 532 nm, respectively. The SHG signal from NtBuPc monolayer was
+
Here f w,2w = [(n0,2w)2 2]/3 is a local field factor, and u is the surface density of the monolayer. Thus, by comparison with the signal from the quartz reference and using the values above, we obtain x(2)= (2-3) x esu and /3 = (2-3) x esu. Figure 2 shows fringing patterns for L-B films of NtBuPc. The p-polarized SH intensities were measured with s-polarized (0)and p-polarized fundamental light (0).The rotation axis was vertical and perpendicular to the laser beam. In Figure 2a, both sides of the substrate were deposited a monolayer, but in Figure 2b, the monolayer on one side was erased with chloroform. A periodic fringe pattern was observed for monolayer deposited on both sides of the substrate (Figure 2a). This phenomenon has been reported in the l i t e r a t ~ r e , ' ~and , ' ~ the interference fringes originate from dephasing of the SH generated at the front and back sides of the substrate rather than from a Maker fringe, since the SHG coherence length is much larger than the optical path in the L-B films. On the other hand, such a periodic fring pattem disappears when a monolayer is on only one side as shown in Figure 2b. Marowsky et a1.20 have derived an equation for estimating the tilt angle for molecules on a substrate. For an incident angle of 8 = 45", the equation is expressed as
(3) where I :2;" and Z i.7;"are the SH intensities, and 9 is tilt angle with respect to the substrate surface normal. From Figure 2b, I and I iT are determined to be 3.8 and 0.8, respectively; thus, the tilt angle can be calculated to be 32" (Figure 3b). The orientation of molecules at the air-water interface can also be estimated from its surface pressure-area isotherm. Surprisingly, the tilt angle of NtBuPc molecules on the substrate is almost the same as that at the air-water interface. The limiting molecular area is 56 A2 determined from the surface pressurearea i~otherm,'~ and the molecular size is 46.9 A2 calculated by the Corey-Pauling-Koltun (CPK) molecular m ~ d e l . ' ~ % * ~ Therefore, the tilt angle of NtBuPc molecules at the air-water interface is 33" (cos q5 = 46.9/56, Figure 3a). This result indicates that the orientation of the phthalocyanine molecules is maintained during the transfer process. It should be point out that eq 3 is derived under some assumptions, one of which is that the molecule has a rodlike structure. Although phthalocyanine is a two-dimensional molecule, in our case the molecule is mononitro substituted, and the net dipole moment that is expected to be along the obvious mixed-substitution axis is one-dimensional. The tilt angles can also be investigated by other techniques, including X-ray and electron diffraction, ESR spectroscopy, electronic and infrared polarized light spectroscopies, and electron microscopy. Such a tilt orientation for the L-B films
J. Phys. Chem., Vol. 99, No. 18, 1995 6959
L-B Films of a Substituted Phthalocyanine Normal
tt a 1 bilayer
3 bilayers
1 bilayer
3 bilayers
8
Water
Normal
Figure 4. Schematic structures of alternating L-B films: (a) altemating with BuPc. (b) Alternating with AA: 0-, AA molecules; 0-, NtBuPc molecules; -, BuPc molecules.
b
I
Substrate
I
Figure 3. Schematic representation of NtBuPc monolayer at air-water interface (a) or on substrate (b).
of phthalocyanine derivatives, for example BuPc, has been reported in the literature.22 For the practical application of noncentrosymmetrical L-B films for second-order NLO, the L-B films are required to have (1) optimum second-order susceptibilities x ( ~ (2) ) , high physical and optical stability, and (3) a superposition of SH radiation generated in each asymmetric L-B layer. The moderately high SH intensity &(2) = (2-3) x esu) of NtBuPc monolayer coupled with the high physical and chemical stability common to all phthalocyanine derivatives, as well as if the SH intensity from the multilayers possesses a quadratic dependence on the number of layers, would make NtBuPc a promising candidate for SHG material. Quadratic SHG enhancement has been reported for long-chain substituted chromophore molecule^.^^-^^ Unfortunately, we failed to see the expected increase in SHG with the number of layers in L-B films with Z-type deposition.I5 In this study, an altemating Z-type deposition was used. The structure of the films is shown in Figure 4. All active layers of NtBuPc were up-stroke-deposited and in Z-type models (head-to-tail). Because molecular dipoles prefer an antiparallel alignment, the L-B films were fabricated with NtBuPc as an active layer, alternating with either BuPc or AA as a passive layer. Figure 5 shows a log-log plot of SH intensity versus the number of bilayers. In contrast to our previous work, these films exhibit nearly quadratic dependence of SH intensity on the film thickness up to nine bilayers. The dashed line (slope = 2) illustrates an ideal quadratic relationship. This result demonstrates that an alternating deposition model is a useful method to improve the superposition behavior of molecular hyperpolarizability in the L-B films. Such an improvement is probably due to two aspects. (1) Upon comparison of the two deposition models, the transfer ratio (area lost from the waterbome film during L-B deposition divided by the substrate geometric area onto which the film is to be deposited) of NtBuPc
3 5 7 9 Number of bilayers Figure 5. Dependence of SH intensity from NtBuPc L-B films alternating with either BuPc (a) or AA (b) on the number of bilayers. (c) Ideal quadratic relationship with slope = 2. 1
in the altemating Z-type model (transfer ratio %0.8) is higher than that in the Z-type model (transfer ratio %0.6), although it does not yet reach unity. This means that molecular density is increased in the subsequent layers. (2) Although phthalocyanines are not a typical L-B molecule, such as AA or stearic acid, they might form a stable monolayer at the air-water interface because of both the interaction between the hydrophilic moiety and water molecules and their x-electronic interaction under surface pressure (Figure 3a). As we know, the n-electronic interaction for dimerization of phthalocyanine molecules is comparable in strength to hydrogen bond formation.26 For the NtBuPc molecule, the nitro group is a moderately strong hydrophilic group (the best one is carboxyl group). A disordered arrangement of the molecules may be happen in the monolayer. Once such a disordered arrangement exists in the first layer, the second layer will become even worse and then the third, etc., layers. Especially this is true in our case, since the NtBuPc molecules are in a tilted arrangement on the substrate. Obviously, alternating layers with AA or BuPc can avoid continuous degradation of alignment in the subsequent layer. Furthermore, the passive layers can also reduce the tendency of the molecules to invert during deposition and hence avoid the cancellation of the nonlinearities between the successive layers.
6960 J. Phys. Chem., Vol. 99, No. 18, I995 In conclusion, after chemical modification of n-conjugated phthalocyanine by connecting with donor-acceptor substituents, the nitrotri-tert-butylphthal~y~e becomes a second-harmonicgeneration active molecule. An altemative strategy for achieving macroscopic noncentrosymmetric order in L-B multilayers is demonstrated. The moderately high SH intensity, superposition behavior of SHG in L-B films, and high stability suggest that L-B films of NtBuPc are a promising material for SHG.
Acknowledgment. This work was supported by the National High-Technique Program, a Fundamental Research Project of Academia Sinica, and Natural Science Foundation of China. References and Notes (1) Hosoda, M.; Wada, T.; Yamada, A.; Garito, A. F.; Sasabe, H. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. B: Nonlinear Opt. 1992, 3, 183191. (2) Hosoda, M.; Wada, T.; Yamada, A,; Garito, A. F.; Sasabe, H. Jpn. J . Appl. Phys. 1991, 30, L1486-1488. (3) Nonvood, R. A,; Sounik, J. R. Appl. Phys. Lett. 1992, 60, 295297. (4) Shirk, J. S.; Lindle, J. R.; Bartoli, F. J.; Hoffman, C. A,; Kafafi, Z. H.; Snow, A. W. Appl. Phys. Lett. 1989, 55, 1287-1288. ( 5 ) Ho, Z. Z.; Peyghambarian, N. Chem. Phys. Lett. 1988, 148, 107111. (6) Suslick, K. S.; Chen, C. T.; Meredith, G. R.; Cheng, L. T. J . Am. Chem. SOC.1992, 114, 6928-6930. (7) Li, D.; Ratner, M. A.; Marks, T. J. J. Am. Chem. SOC.1988, 110, 1707- 1715. (8) Hoshi, H.; Nakamura, N.; Maruyama, Y. J . Appl. Phys. 1991, 70, 7244-7248. (9) Neuman, R. D.; Shah, P.; Akki, U. Opt. Lett. 1992, 17, 798-800.
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