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Langmuir 1999, 15, 6510-6514
Synthesis, Characterization, and NLO Properties of a Phenothiazine-Stilbazole Monolayer Wenxi Huang and Merritt Helvenston* Department of Chemistry, New Mexico Highlands University, Las Vegas, New Mexico 87701
Joanna L. Casson, Rong Wang, Jean-Franc¸ ois Bardeau, Yongwoo Lee, Malkiat S. Johal, Basil I. Swanson, Jeanne M. Robinson, and DeQuan Li* Chemical Science & Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received March 1, 1999. In Final Form: May 17, 1999 4-(trans-4-(4-(10-phenothiazinyl)butyramido)styryl)pyridine (PBSP) has been synthesized and covalently bound to a coupling layer of (p-(chloromethyl)phenyl)trichlorosilane (CPTS) on a quartz substrate to produce a polar-ordered thin film. The addition of the PBSP molecules on the coupling layer was monitored by FTIR spectroscopy, UV-visible spectroscopy, AFM, and second harmonic generation (SHG). The observation of an amine band and strong methylene bands in the infrared spectra indicates the presence of the PBSP layer. The UV-visible spectrum shows a red shift in the charge-transfer band, consistent with the addition of the PBSP and quaternization of the pyridyl group. AFM shows that there is an incomplete coverage of the PBSP layer on the coupling layer. The nonlinear optical properties of the PBSP monolayers were measured by SHG. The second harmonic (SH) signal increased by 1 order of magnitude after the addition of the PBSP chromophore layer. The dependence of the SH intensity on incident angle and input polarization was modeled. The average molecular orientation angle of the choromophore was determined to be 55°, and a value of χzzz ) χzxx ) 2.8 × 10 -8 esu was calculated.
1. Introduction Well-ordered organic films chemically bound to solid substrates have received much attention over the past decade.1,2 Chemically synthesized films are generally more stable than those assembled by weak dispersion forces, such as in the Langmuir-Blodgett technique.3-7 The former provides greater control of NLO-active chromophore density, orientational order, and desired physical and chemical properties.1,4,8-16 Greater control of the surface structure is possible when producing films by * To whom correspondence should be addressed. (1) New Developments in Construction and Function of Organic Thin Films; Mobius D., Miller, R., Eds.; Elsevier: Amsterdam, 1996. (2) Organic Thin Films for Waveguiding Nonlinear Optics; Kajzar, F., Swalen, J. D., Eds.; Gordon and Breach Publishers: Langhorne, PA, 1996. (3) Johal, M. S.; Parikh, A. N.; Lee, Y.; Casson, J. L.; Foster, L.; Swanson, B. L.; McBranch, D. W.; Li, D. Q.; Robinson, J. M. Langmuir 1999, 15, 1275. (4) Roscoe, S. B.; Yitzchaik, S.; Kakkar, A.; Marks, T. J.; Xu, Z.; Zhang, T.; Lin, W.; Wong, G. K. Langmuir 1996, 12, 5338. (5) Freeman, T. L.; Evans, S. D.; Ulman, A. Langmuir 1995, 11, 4411. (6) Ulman, A. Organic Thin Films and Surfaces: Directions for the Nineties; Academic Press: New York, 1995. (7) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (8) Ulman, A.; Tillman, N. Langmiur 1989, 5, 1418. (9) Lee, H.; Kepley, L. J.; Hong, H. G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (10) Allara, D. L.; Atre, S. V.; Elliger, C. A.; Snyder, R. G. J. Am. Chem. Soc. 1991, 113, 1852. (11) Davies, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Nature 1998, 396, 60. (12) Li, D. Q.; Swanson, B. I.; Robinson, J. M.; Hoffbauer, M. A. J. Am. Chem. Soc. 1993, 115, 6975. (13) Kakkar, A. K.; Yitzchaik, S.; Roscoe, S. B.; Marks, T. J.; Lin, W.; Wong, G. K. Thin Solid Films 1994, 242, 142. (14) Kakkar, A. K.; Yitzchaik, S.; Roscoe, S. B.; Kubota, F.; Allan, D. S.; Marks, T. J.; Lin, W.; Wong, G. K. Langmuir 1993, 9, 388. (15) Li, D. Q.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 7389.
chemical reactivity than by physisorption. Assemblies containing moieties such as calixarene-based molecular pyramids,16 porphyrin derivatives,12 rodlike alkynyl chromophores,14 and stilbazole chromophores,13 all of which show strong nonlinear optical (NLO) behavior, have been synthesized by this approach. Engineering highly NLO-active assemblies can lead to thin films with potentially useful properties. For instance, materials with electron donor and acceptor groups bridged by a conjugated system have potential application as “molecular wires”, where electrons can tunnel or hop through the conjugated bridge.17 This has recently been demonstrated by Davies et al11 in thin films of pphenylenevinylene oligomers. These systems yield highly polarizable chromophores that have nonlinear susceptibilities that can be easily measured by second harmonic generation (SHG).18 Phenothiazine and its derivatives are of widespread chemical interest. For example, they have useful pharmacological properties19 and can potentially be used in solar energy conversion.20-22 There is also interest in these systems as charge-transfer complexes with several acceptors due to their low ionization potentials. (16) Yang, X.; McBranch, D. W.; Swanson, B. I.; Li, D. Q. Angew. Chem., Int. Ed. Engl. 1996, 35, 5 (5), 538. (17) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100, 13148. (b) Ratner, M. A.; Jortner, J. Molecular Electronics; Blackwell: Oxford, U.K., 1997. (c) Wasielewski, M. R. Chem, Rev. 1992, 92, 435. (18) Shen, Y. R. Principles of Nonlinear Optics; Wiley: New York, 1984. (19) Hawkins, D.; Pauling, L. Orthomolecular Psychiatry; Azzaro, M. A., Cambon, F., Gouezo, R., Guedj, W. H., Eds.; Freeman: San Francisco, CA, 1973. (20) Albery, W. J.; Foulds, A. W.; Hall, K. J.; Hillmam, A. R.; Edgell, R. G.; Orchard, A. F. Nature 1979, 282, 793. (21) Christ, C. S., Jr.; Yu, J.; Zhao, X.; Palmore, G. T. R.; Wrighton, M. S. Inorg. Chem. 1992, 31, 4439. (22) Argazzi, R.; Bignozzi, C. A. J. Am. Chem. Soc. 1995, 117, 11815.
10.1021/la990237c CCC: $18.00 © 1999 American Chemical Society Published on Web 07/30/1999
A Phenothiazine-Stilbazole Monolayer
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Figure 1. Synthetic route to 4-(trans-4-(4-(10-phenothiazinyl)butyramido)styryl)pyridine (PBSP).
In this work, the synthesis of a new NLO-active molecule, 4-(trans-4-(4-(10-phenothiazinyl)butyramido)styryl)pyridine (PBSP) (Figure 1), is described. The NLOactive films are synthesized by covalent linkage of PBSP on both sides of the fused quartz substrates that have been pretreated with a coupling reagent, (p-(chloromethyl)phenyl)trichlorosilane (CPTS). The nucleophilic substitution reaction of the PBSP molecule creates a strong electron-accepting quaternary ammonium group. The addition of the chromophoric layer is monitored by atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, UV-visible spectroscopy, and SHG. The π-conjugated system from the amide oxygen to the pyridinium nitrogen atom constitutes the polarizable NLO-active portion of the structure. The NLO characteristics of the ground state are described in the present work. After resonant excitation of the NLO chromophore, the phenothiazine moiety is anticipated to reduce the NLO chromophore via an electron transfer. The challenging timeresolved NLO studies associated with the electrontransfer reaction will be the subject of future work. 2. Experimental Section All synthetic procedures described below were carried out under a nitrogen atmosphere. All solvents and chemicals were AR-grade obtained from Aldrich. The solvents, benzene, chloroform, and N,N-dimethylacetamide, were dried by stirring with P2O5, followed by vacuum distillation, and stored under argon. 1H and 13C NMR spectra were recorded on a Bruker Model AC-300 MHz spectrometer. 2.1. Chromophore Synthesis. 4-(trans-4-Aminostyryl)pyridine (1). A solution containing 4-bromoaniline (1.72 g, 10 mmol), 4-vinylpyridine (1.31 g, 12.5 mmol), palladium acetate (0.02 g, 0.10 mmol), tri-o-tolylphosphine (0.12 g, 0.40 mmol), and triethylamine (60 mL) was heated under nitrogen at 80 °C for 24 h. The cooled reaction mixture was diluted with water and methylene chloride. The methylene chloride layer was separated, washed with water, and dried over anhydrous magnesium sulfate. Evaporation of the solvent and recrystallization from a mixture of 2-propanol and DMF gave 0.72 g of the product (37%). The key features in the NMR spectra were the following: 1H NMR (DMSOd6) δ 8.42 (d, 2H), 7.35 (m, 5H), 6.84 (d, 1H), 6.68 (d, 2H), 5.44 (s, 2H). 13C NMR (DMSO-d6) δ 150.26, 148.27, 146.69, 136.29, 128.97, 123.18, 120.58, 118.76, 113.66). 10-Phenothiazinebutyrate Ethyl Ester (2) and its Acid (3). Phenothiazine (3.08 g, 15.5 mmol) and BrCH2CH2CH2CO2Et (4.00 g, 20.5 mmol) were heated with anhydrous K2CO3 (0.7 g, 5 mmol) and Cu powder (0.3 g, 5 mmol) as a catalyst at 160
Figure 2. Molecular self-assembly of 4-(trans-4-(4-(10-phenothiazinyl)butyramido)styryl)pyridine (PBSP) on a quartz substrate. °C for 4 h. GC-MS indicated the mixture contained 71% of the desired product (2). The crude product was used for hydrolysis without further purification. A suspension of 5.0 g of 10-phenothiazinebutyrate ethyl ester (2), 100 mL of ethanol, and 20 g of sodium hydroxide in 200 mL of water was stirred under reflux for 2 h. After hot filtration to remove residual ester (2), the filtrate was acidified to pH 3 to precipitate the acid (3). The product was recrystallized again by dissolution in boiling water with excess base and was reprecipitated by acidification to pH 3. Crystals were washed with deionized water and then 80% alcohol with a trace of bisulfite. This afforded 1.9 g of off-white crystals of 10-phenothiazinebutyric acid with a melting point range of 159-160 °C . The total yield was 43%. The key features in the NMR spectra were the following: 1H NMR (DMSO-d6) δ 7.18- 6.92 (m, 8H), 4.33 (t, 2H), 2.64 (t, 2H), 1.93 (t, 2H); 13C NMR (DMSO-d6) δ 174.41, 144.77, 127.73, 127.25, 123.76, 122.67, 155.93, 45.68, 30.57, 21.71. 4-(trans-4-(4-(10-Phenothiazinyl)butyramido)styryl)pyridine (5). PCl5 (1.03 g, 5 mmol) was added to a suspension of 10-phenothiazinylbutyric acid (1.40 g, 4.9 mmol) in anhydrous benzene (20 mL). After the solution was stirred for 2 h at room temperature, the solvent was removed under vacuum. 4-(trans4-Aminostyryl)pyridine (0.96 g, 4.9 mmol) dissolved in DMF (10 mL) and (dimethylamino)pyridine (0.61 g, 5 mmol) was added to neutralize the residual acid. The solution was stirred at room temperature for 4 h and then poured into water (200 mL). The precipitates were filtered out and dried. The final product of 4-(trans-4-(4-(10-phenothiazinyl)butyramido)styryl)pyridine was further purified by recrystalization from DMF. The total yield was 64%. The compound decomposed around 200 °C, before its melting point. The key features in the NMR spectra were the following: 1H NMR (DMSO-d6) δ 10.23(br, 1H), 8.77-6.91(m, 18H), 4.23 (t, 2H), 2.74 (t, 2H), 1.91 (t, 2H); 13C NMR (DMSO-d6) (168.98, 149.57, 144.31, 143.73, 138.91, 132.13, 130.91, 127.35, 127.09, 126.03, 124.15, 122.24, 121.01, 118.88, 114.82, 42.16, 34.05, 21.04. 2.2. NLO-Active Self-Assembly. Figure 2 shows the general strategy for the formation of the self-assembled monolayers. The typical thickness of the fused quartz substrate (General Electric) was 1 mm. The fused quartz was ultrasonically cleaned in 10% detergent solution for 10 min and then refluxed in piranha solution (98% sulfuric acid and 30% hydrogen peroxide, 7:3 v/v) for 20 min. Finally, the substrate was thoroughly rinsed with copious amounts of deionized water followed by acetone and then dried with nitrogen. All substrates were used immediately after cleaning. Coupling Layer Formation of (p-(Chloromethyl)phenyl)trichlorosilane (CPTS) on Surfaces. A 0.5 mL volume of CPTS was syringed into a Schlenk tube containing 5 mL of dry
6512 Langmuir, Vol. 15, No. 19, 1999
Figure 3. AFM image of (a) coupling layer and (b) coupling layer and chromophore layer. Compared to the flat surface of quartz glass, the first layer exhibits a uniform grain and a thickness of 0.4 nm. The PBSP molecules partially cover the first layer and form larger domains, giving rise to an additional height of about 0.9 nm relative to the first layer. chloroform under an inert atmosphere. The quartz substrates were coated by immersing them in the chloroform solution for 12 h at room temperature. The substrates were transferred to another chloroform solution and cleaned by repeated sonication. The coated substrates were rinsed with acetone and dried with nitrogen. Chromophore Layer Addition. The substrates were immersed in a 1.5 mM solution of PBSP in DMF and heated at 100 °C for 24 h. The chromophore-coated substrates were transferred into a neat DMF solvent and then cleaned by repeated sonication. The substrates were thoroughly rinsed with acetone and dried with nitrogen. 2.3. Second Harmonic Generation. Details of the SHG experiments have been described elsewhere.3,18,23 Briefly, a Q-switched mode-locked Nd:YAG laser (Continuum) provided ∼25 ps pulses at 10 Hz for fundamental excitation at 1064 nm. The sample was excited with p or s polarized light. The SHG measurements were carried out in transmission geometry under nonresonant conditions. The energy density at the sample was ∼2.5 mJ/mm2. The sample was rotated in 1.0° steps under computer control and the transmitted p-polarized SHG signal at 532 nm was measured as a function of incident angle from normal (0°) to 70°. The SHG signal was separated from the fundamental beam, passed through a polarization analyzer, and finally detected using a PMT (Thorn EMI). The SH data points were averaged over 30 laser pulses using a gated integrator (SRS 250). The SHG signal from the sample was referenced to that from a Y-cut quartz crystal.
3. Results and Discussion The surface was functionalized in two steps (Figure 2). In the first step, surface hydroxyl groups displace the active chlorines on silicon of the bifunctional coupling reagent. Subsequent quaternization of the pyridine in the chromophore (5) by the surface bound benzyl chloride completes the heterobilayer. The CPTS layer on the quartz substrate provides a uniform surface for attaching PBSP by nucleophilic substitution. Both the coupling layer and the final film were studied by AFM, UV-visible spectroscopy, FTIR spectroscopy, and SHG. These techniques indicate that the second layer is formed, albeit incompletely, via nucleophilic substitution. 3.1. Atomic Force Microscopy. The AFM images were acquired on a NanoScope III system from Digital Instruments with a 125 mm × 125 mm scanner. A commercial silicon tip was used, and scanning was carried out in tapping mode at a resonant frequency of 340 kHz in air. Figure 3 shows the tapping mode AFM images of the film topology on quartz before and after the addition of the (23) Li, D. Q.; Swanson, B. I.; Robinson, J. M.; Hoffbauer, M. A. SPIE (Nonlinear Opt. III) 1992, 1626, 424.
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Figure 4. UV-visible data of (a) the coupling layer and (b) coupling layer with chromophore layer. The red-shifted chargetransfer band at 405 nm in (b) is attributed to the pyridinium group generated after the addition of PBSP. The spectra were recorded at room temperature.
chromophoric layer. Compared to the flat surface of quartz glass, the first layer exhibits uniform grain structures with an average grain size of 45 nm and a thickness of about 0.4 nm. There is a random distribution of the second (PBSP) layer on top of the coupling layer. The PBSP molecules partially cover the first layer and form larger domains, giving rise to an additional height of about 0.9 nm relative to the first layer. 3.2. UV-Vis Spectroscopy. UV-visible spectra were collected with a Varian-1C instrument. The coupling layer is bound to the surface oxide (SiO2) via a siloxane linkage. After attachment of the chromophore to the coupling layer, a characteristic change in its electronic spectrum was observed (Figure 4). The coupling layer showed absorption peaks at 196 and 223 nm. After the addition of PBSP, a characteristic charge-transfer band at 405 nm was observed. This band is red-shifted in comparison with the absorption of the PBSP in solution (332 nm). The shift can be attributed to the conversion of the pyridyl group into the pyridinium group, which confirms the quaternization reaction.24 3.3. FTIR Spectroscopy. The infrared spectra of the thin films on quartz were collected using a Bruker IFS55 Fourier transform infrared spectrometer equipped with a nitrogen purge sample chamber and a DTGS detector. The samples were placed perpendicular to the unpolarized IR beam. The transmission spectra were recorded with a resolution of 2 cm-1. Figure 5 shows the IR spectra of the film between 2800 and 3500 cm-1, before and after the addition of the chromophore layer. For the coupling layer, the IR spectrum (Figure 5a) shows two prominent features at 2849 and 2920 cm-1, which can be assigned to the symmetric and asymmetric stretching vibrations of the methylene unit in the CPTS molecule, respectively.25 After the addition of the second layer, three additional bands are observed: a weak shoulder at 2956 cm-1 and features around 3186 and 3394 cm-1. The latter two bands are attributed to the symmetric and asymmetric N-H stretching vibrations, respectively (Figure 5b). The methylene stretching modes at 2849 and 2920 cm-1 are much stronger after the addition of the second layer. The presence of the new bands and the intensity enhancement of the methylene stretching bands are consistent with the addition (24) Silverstein, R. M.; Clayton Bassler, G.; Morrill, T. C. Spectrometric Identification of Organic Compounds; John Wiley & Sons: New York, 1981. (25) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85.
A Phenothiazine-Stilbazole Monolayer
Figure 5. Infrared spectra of (a) the first coupling layer (CPTS) and (b) after addition of the PBSP on a quartz substrate. Spectra were recorded at room temperature at 2 cm-1 resolution.
Figure 6. Transmitted p-polarized SH light intensity at 532 nm for p-polarized excitation as a function of the angle incidence for the PBSP chromophore layer. The solid line between the data points is added for clarity. The solid line describing the overall envelope is generated from theoretical modeling.
of the PBSP chromophore due to the addition of an amine and three methylene groups in the PBSP molecule. In the FTIR spectrum of the second layer, the weak feature near 2956 cm-1 (Figure 5b) can be assigned to an asymmetric methyl stretching mode.26 There is no conclusive evidence of its origin, but the most likely explanation is the presence of an impurity in the film possibly related to DMF solvent. 3.4. Second Harmonic Generation. SHG data were obtained for films comprising the coupling layer alone and the coupling layer with the PBSP chromophore layer. Figure 6 shows the transmitted p-polarized SH intensity from the chromophore monolayers assembled on both sides of the fused quartz substrate for p-polarized incident excitation. The blank fused quartz substrates do not generate any SH signal. Therefore, the observed SH interference fringes arise from the monolayer coated on both sides of the substrate as it is rotated.27 Complete destructive interference was not observed, and the data in Figure 6 have been adjusted for complete interference so that the SHG intensity verses incident angle could be modeled correctly. The incomplete destructive interference (26) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta 1978, 34A, 395. (27) Maker, P. D.; Terhune, R. W.; Nisenoff, M.; Savage, C. M. Phys. Rev. Lett. 1962, 8, 21.
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Figure 7. p-polarized SH intensity from the coupling layer (open circles) and the coupling layer with the PBSP layer (solid circles), as a function of the fundamental polarization angle. The s-to-p configuration corresponds to 0 and p-to-p configuration corresponds to π/2 in the graph.
indicates the film is not identical on both sides of the substrate. Two major features are observed in the data. First, there is an increase in the SH signal at high incident angles.18 Second, there is an order of magnitude increase in the SH intensity after the addition of the chromophore layer. Figure 7 shows the input polarization dependence of the p-polarized SHG intensity from the film before and after the PBSP layer is added. By the modeling of both the interference fringes15,18,28-31 and the polarization dependence,32,33 a value for the average molecular orientation of the chromophore and the material susceptibility (χzzz) can be obtained. Polar-ordered monolayers typically have uniaxial symmetry along the direction of the surface normal, so that there are only two independent tensors χzzz and χzxx ) χzyy ) χxzx ) χxxz ) χyzy ) χyyz. For p-polarized input and output beams, the induced second-order polarization (Pp,p2ω) has contributions from χzzz and χzxx ) χxzx ) χxxz (eq 1),18 where
Pp,p2ω ) (Ap,pχzzz + Bp,pχzxx)(Eω)2
(1)
Ap,p and Bp,p are functions of incident angles and dielectric constants at both the fundamental frequency and the SH frequency. The ratio χzzz/χzxx determines the shape of the theoretical envelope in Figure 6 when the dielectric constants are fixed. The best fit to the data yields χzzz/χzxx ) 1. Since χzzz ) NCβζζζ〈cos3 ψ〉 and χzxx ) NCβζζζ〈sin2 ψ cos ψ〉, the average molecular orientation of the chromophore can be obtained from χzzz/χzxx ) 2〈cot2 ψ〉 ) 2cot2 〈ψ〉, where ψ is the orientation angle between the surface normal and the principal molecular tensor component, βζζζ. An average molecular orientation angle of the chromophore ψ ) 55° was found. The calculation of ψ assumes both a narrow distribution of molecular orientation angle and a single dominant element, βζζζ, of the molecular hyperpolarizability tensors. (28) Heinz, T. F.; Tom, H. W. K.; Shen, Y. R. Phys Rev. A. 1983, 28, 1883. (29) Cnossen, G.; Drabe, K. E.; Wiersma, D. A.; Schoondorp, M. A.; Schouten, A. J., Hulshof, J. B. E.; Feringa, B. L. Langmuir 1993, 9, 1974. (30) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 250, 1485. (31) Bloembergen, N.; Pershan, P. S. Phys. Rev. 1962, 128, 606. (32) Zhang, T. G.; Zhang, C. H.; Wong, G. K. J. Opt. Soc. Am. B 1990, 7, 902. (33) Heflin, J. R.; Liu, Y.; Figura, C.; Marciu.; Claus, R. O. Proc. SPIE-Int. Soc. Opt. Eng. 1997, 3147, 10.
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In the polarization study, a 1064 nm λ/2 waveplate placed before the film was used to rotate the polarization of the input beam from s to p and then from p to s. In these experiments, the s-in, p-out configuration and the p-in, p-out configuration correspond to 0 and 90°, respectively. At 0°, only the tensor element χzyy contributes to the SH signal (eq 2).
Ps,p2ω ) (Cs,pχzyy)(Eω)2
(2)
At an incident angle, R, the electric field vector of the incident light is projected along the z and x axes, yielding both p- and s-excitations. In other words, the p-polarized SH response is a combination of eqs 1 and 2. In general, the p-polarized SH response at an arbitrary incident polarization angle can be described by eq 3.
IR-p2ω ) (Ps,p2ω cos2 R + Pp,p2ω sin2 R)2
(3)
Again, the symmetry of the thin film was used to derive eq 3. The solid line in Figure 7 is fitted to the experimental data (solid circles) using eq 3. The result yields a ratio of Pp,p2ω/Ps,p2ω ) 1.7, which also can be obtained from eqs 1 and 2. Solving these equations, we obtained a value of χzzz/χzxx ) 1, which is in excellent agreement with the modeling of the envelope function of the interference fringes. By calibration of the observed SH signal to that measured for Y-cut quartz crystal at the first Maker fringe,27 a value of χzzz ) χzxx ) 2.8 × 10 -8 esu was estimated for the STP monolayers. The monolayer thickness was estimated to be 13 Å from AFM. The observation of a relatively large SH response is consistent with the polar alignment of the chromophores with highly delocalized π-electrons. The material susceptibility of the PBSP films is lower than that found for films made from 4-[N,N-bis(3-hydroxypropyl)amino]styryl-4-pyridine(BHASP) which were prepared in exactly the same manner.4 In that study it was found χzzz ) χzxx ) 3 × 10 -7 esu. BHASP contains the same donor-acceptor system as PBSP, but (34) Johal, M. S.; Cao, Y. W.; Chai, X. D.; Smilowitz, L. B.; Robinson, J. M.; Li, T. J.; McBranch, D. W.; Li, D. Q. Chem. Mat.
the headgroup contains much smaller and less bulky hydroxypropyl groups. There are two main reasons for the smaller SH response of PBSP compared to BHASP. First, in PBSP the inductive effect of the carbonyl group adjacent to the amino group reduces its effectiveness as an electron donor. This reduction yields a smaller molecular hyperpolarizability and hence reduces the material susceptibilities χzzz and χzxx. Second, steric hindrance due to the bulkier 10-phenothiazine units inhibits close packing in the PBSP monolayer, leading to a reduction in both the number density and anisotropy. The low surface coverage of the chromophore layer is also confirmed by the AFM measurement of the surface topology. However, both PBSP and BHASP show much stronger NLO-active behavior in comparison to similar systems that lack donor-acceptor properties.30,34 4. Conclusion The surface synthesis of a polar ordered NLO-active film containing electron-donor and -acceptor groups bridged by a conjugated system has been demonstrated. The nucleophilic substitution reaction between a chromophoric molecule (PBSP) and reactant coupling layer was confirmed by AFM, FTIR, SHG, and UV-visible spectroscopy. The AFM images clearly showed a partial coverage of the chromophoric layer. The large red shift in the absorption band of the monolayer compared to pure PBSP in solution indicated that the quaternization reaction occurred. The presence of strong methylene and N-H bands in the infrared spectrum also indicated the presence of the chromophoric layer on the surface. The SHG data shows that the film is polar ordered and strongly polarizable. Acknowledgment. This work was supported by the AFOSR (F49620-98-1-0073) and by Los Alamos National Laboratory Directed Research and Development, under the auspices of the U.S. Department of Energy. J. L. C. also acknowledges partial support from the Underrepresented Females and Minorities program at LANL. We also thank Gary Duerksen and Graham Allan (New Mexico Highlands University) for their assistance in this work. LA990237C
