388
Langmuir 1993,9,388-390
Chromophoric Self-Assembled Nonlinear Optical Multilayer Materials. Synthesis, Properties, and Structural Interconversions of Assemblies with Rodlike Alkynyl Chromophores Ashok K. Kakkar, Shlomo Yitzchaik, Stephen B. Roscoe, Fuyuhiko Kubota, David S. Allan, and Tobin J. Marks* Department of Chemistry and the Materials Research Center, Northwestern University, Evanston, Illinois 60208-3113
Weiping Lin and George K. Wong' Department of Physics and the Materials Research Center, Northwestern University, Evanston, Illinois 60208-3113 Received November 30, 1992. In Final Form: December 30,1992
The communication describes an approach to the synthesis of thin-film second-order nonlinear optical (NLO) materiala in which self-assembled chromophore-containing multilayer structures are built up on clean glass or quartz surfaces in the repeatingreactionsequencepCl~Si(CH2)2C~H&H2Cl (couplinglayer), 4-[NJV-bis(3-hydroxypropyl)aminolphenylethynyl-4'-pyridine(chromophore layer), Cl&3OSiClzOSiCls (capping layer). The multilayer structures have been characterized by optical and X-ray photoelectron spectroscopy, advancing contact angle measurements, and polarized second harmonic generation (SHG) properties. It is found that the initially formed coupling-chromophoremonolayer exhibits a rather weak SHG response which increases substantially upon capping with the Si reagent. The proposed structural reorganization of the chromophore dipoles (&, direction) to an average orientation more coincident with the surface normal is supported by optical spectroscopy. The square root of the SH intensity scales linearly with the number of chromophore layers up to nine layers. For a single coupling-chromophore esu at hw = 1.17 eV. layer, x::: = 6 X Challenges in the construction of efficient moleculebased thin film second-order nonlinear optical (NLO) materials' include maximizing the number density of constituent high-@chromophoresas well as achieving and preserving maximum microstructural acentricity. We recently reported an approach to such materials2s3which employs established silicon self-assembly technolo& as well as a bifunctional precursor which is transformed into
Scheme I HO OH
\r'
Qk
CHpCl
Q -
(Ccl,),
1
N
Sic13
+
(1) (a)MaterialsforNonlinear Optics: ChemicolPerspectiws; Marder, S. R., Sohn, J. E., Stucky, G. D., Eds.; ACS Symposium Series 455; AmericanChemicalSociety:Waahington,DC,1991. (b) Nonlinear Optical Properties of Organic Materiale IV; Singer, K. D., Ed.; SPIE Proc. 1991, 1560. (c) Nonlinear Optical Properties of Organic Materials III; Khanarian, G., Ed.; SPIEProc. 1990,1337. (d) Nonlinear Optical Properties of Organic Materiale 11; Khanarian, G., Ed.; SPIE Roc. 1990,1147. (e) Nonlinear Optical Effects in Organic Polymers; Messier, J., Kajar, F., Prasad, P., Ulrich, D., Eds.; Kluwer Academic Publishers: Dordrecht, 1989. (0Organic Materiale for Nonlinear Optics; Hann, R. A., Bloor, D., Eds.; Royal Society of Chemistry: London, 1988. (g) Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Prase: New York, 1987; Vols. 1 and 2. (2) (a) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990,112,7389. (b) Li, D.; Marks, T. J.; Zhang, C.; Yang,J.; Wong, C. K. In ref 1, p 341. (c) Allan, D. S.; Kubota, F.; Marks, T. J.; Zhang, T. J.; Lin, W. P.; Wong, G. K. In ref lb, p 362. (d) Allan, D. S.; Kubota, F.; Orihaaki, Y.; Li, D.; Marks, T. J.; Zhang, T. G.; Lin, W. P.; Wong, G. K. Polym. Prepr. 1991, 32, 86. (e) Allan, D. S.; Kubota, F.; Kakknr, A. K.; Marks, T. J.; Zhang, T. J.; Lin, W. P.; Shih, M.;Wong, G.K.; Dutta, P. Moter. Res. SOC.Symp. Proc. 1992,247,779. (3) For an alternativeapproach,see Katz, H.E.;Scheller, G.; Putvinski, T. J.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991,254,
i. Toluene, 25OC (15 min.); 115"Ccure(15 min.). ii. Reflux in n-ROH (48 h).
iii. Cl3SiOSiClzOSiCl3in THF (15 min.); 115°Ccure (15 min.).
Repeat 1
- 111
1485.
(4) (e) Netzer, L.; Iscovici, R.; Sagiv,J. Thin SolidFilms 1983,99,235. (b) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100 (2), 465. (c) Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985, 132, 153. (d) Waaserman, 5. R.; Tao, Y.-T.; Whitesides, G.M. Langmuir 1989,5,1074. (e) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G.M.J.Am. Chem. SOC.1989,111,321. (0 Ulman, A. Adu. Mater. 1990,2, 573, and references therein. (g) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991, Part 3.
a high-@chromophore only upon quaternizatiordanchoring to the surface (Scheme I).5+6 Importantly, this new
0743-7463/93/2409-0388$04.00/0Q 1993 American Chemical Society
Letters
Langmuir, Vol. 9, No.2, 1993 389
approach does not require electric field poling1 to create an acentric microstructure. The first embodiment employed stilbazole-based chromophore precursor 1.2f6With the goal of exploring the generality of this approach and building block-macroscopic NLO relationships, we now communicate reaulta with chromophore precursor 2', which offera upon quatemization, a rodlike acetylenic chromophore architecture and (vs 1) comparable ,!3zzz with a 30-nm blue-shifted ,A, (for greater optical transparenCY).^^^ NLO evidence for an unusual monolayer structural reorganization is also presented. HO
OH
HO
OH
6.0
1
-5
'
'
'
'
'
,
'
'
A.m.0- Before Capping
'
6.0
11 '
3 5.0
5.0
$
4.0
.e
4.0
v
3 .O 0
3:
.->m
2.0
2.0
1.0
1.o
c)
3
2
0.0 . _
0.0
0
Q 1
2
Multilayer construction begins with deposition of 2-trichlorosilyl-l-(4-chloromethyl)phenyl)ethane on both sides of clean glass slides (i, Scheme I; toluene, 25 OC (15 min), air cure, 115 OC (15min)) followed by introduction of 2 (ii, Scheme I; 1-PrOH,60 "C (48 h)). Finally, capping with octachlorotriailoxane (iii, Scheme I; THF, 25 O C (15 min)); air cure, 115OC (15 min)) completes the first cycle, which can then be repeated. Evolution of the multilayer architecture is monitored by measurement of advancing aqueouscontact angles, which are in accord with expected surface wettabilities214and which repeat regularly in each i-iii cycle (reaction product, 8, (HzO)): clean SiO2, 15'; benzylic chloride, 74O; hydroxypropyl, 46'; Si-OH, 18'. The aseembly process is also evidenced by progressive growth of the film 2-chromophore optical maximum at h = 480 nm, by a 201 198eV shift in the ClapXPS binding energy together with the appearance of an NI,feature at 400 eV after step ii (Scheme I), and by polarized SHG measurements (vide infra). The multilayer films adhere strongly to glass and to other substrates and cannot be
-
(5) For the NJV-dimethyl-N-methyl analogue of I-CH3+,&z2ca'd = 178 lo-."cm5 esu-1 ( h a = 0.65 eV) and X m a x c a l=~ 479 nm by the ZINDO/ SOS formalism.sb-d We thank Dr. D. A. Kanie for these calculations. (b) Kanis, D. R.; Ratner, M. A.; Marks, T. J.; Zerner, M. C. Chem. Mater. 1991,3,19. (c) Kanis, D. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. SOC. 1990,112,8203. (d) Kanis, D. R.; Ratner, M. A,; Marks, T. J. Int. J. Quantum Chem. 1992,43, 61. (6) For powder SHG data on related stilbazolium chromophores, see (a) Marder, S.R.; Tiemann, B. G.; Perry, J. W.; Cheng, L.-T.; Tam, W.; Schaefer, W. P.; Marsh, R. E. In ref la, p 187. (b) Marder, S. R.; Perry, J. W.; Schaefer, W. P. Science 1989,245,626. (7) Prepared by Horner-Emmons coupling of 4-(N&diallylaminohnzaldehyde with diphenyl-l-chloro-l-(4-pyridyl)methanephosphonate, followed by 9-BBNhydroboration/reduction and H2Oz/NaOH.1H NMR (400 MHz, DMSO-&): 6 8.56 (d, 2 H, J 5.2 Hz), 7.42 (d, 2 H, J = 5.1 Hz), 7.38 (d, 2 H,J = 8.3 Hz), 5.95-5.75 (m, 2 HI, 5.22-4.98 (m, 2 H),3.98 (d, 4 H, J = 4.4 Hz). I3CNMR (100 MHz, DMs0-d~):6 149.69, X
15
30 45 60 Angle of Incidence (degrees)
75
Figure 1. SHG intensity as a function of fundamental beam incident angle from aglass slide having a self-assembled2-derived monolayer on either side: (A) before step iii, Scheme I; (B) after step iii, Scheme I. The interference patternarises from the phase difference between the SHG wave8 generated at either side of the substrate during propagation of the fundamental wave.
removed with common organic solventsor most acids. They can, however, be removed/etched with dilute HF solutions or by diamond polishing. Transmission SHG measurements were carried out in the p-polarized geometry using the 1064-nm output of a Q-switched NdYAG laser. Rotation of the slides about the film normal reveals no in-plane anisotropy of the SHG signal, indicating that the films possess uniaxialaymmetry and that the distribution of chromophore molecular orientation does not have an azimuthal dependence. Figure 1 shows the SHG intensity as a function of laser incident angle for the initial 2-based monolayer (product of steps i and ii) before and after addition of the octachlorotrisiloxanecapping layer (step iii). The structures exhibit characteristic SHG interference patterns: the near-zero intensity minima indicating that the quality of the monolayers on either side of the glass substrate is nearly identical and uniform. However, in contrast to the adsorption/quaternization of 1, the adsorption/quaternization of 2 under the same reaction conditions produces a monolayer exhibiting a substantially weaker SHG response (-10% of that of 1). NMR-monitoredsolution reactivity studies at 60 OC of 1and 2 + benzyl chloride in CD~CDZOD indicate that the course of the quaternization is as depicted in Scheme I and that 2 is 1.3 times more reactive than 1. The latter result argues against intrinsic differences in 112 reactivity, hence in likely chromophore coverage, as an explanation for the SHG differences. Furthermore, simple addition of the octachlorotrisiloxane reagent to the 2-based monolayer (step iii) results in an 8-fold enhancement of SHG intensity, yielding an SHG response now comparable to 1 (Figure 1B). The SHG data envelope in Figure 1B can be readily fit to eq 1,"
-
148.80,133.53,132.82,131.04,124.69,115.89,111.70,106.85,95.65,84.80,
51.81. Amax (CHCls) = 355 nm. Anal. Calcd for C19H22N202: C, 73.52; H, 7.15; N, 9.03. Found C, 73.20; H, 7.27; N, 8.81. (8)For the NJV-dimethyl-N-methyl analogue of 2, plllcslcd= 171 X 10-'"cm5esu-l (hw = 0.65 eV) and Xmaxea'cd = 466 nm by the ZINDO/SOS formalism."-d (9) For the NJV-dimethyl-N-methyl iodide analogue of 2-CH.3+,Amax (CHCl') = 474 nm. 'H NMR (400MHz, DMSO-d6): 6 8.86 (d, 2 H, J = 6.4 Hz), 8.07 (d, 2 H, J = 6.4 Hz), 7.52 (d, 2 H, J = 8.4 Hz), 6.79 (d, 2 H, J = 8.8 Hz), 4.26 (e, 3 H), 3.03 (8, 6 H). Anal. Calcd for C16H1&I: C, 52.76;H,4.7l;N,7.69m/e,237.33(-1). Found: C,51G75;H,4.66;N,7.53; m/e, 237.
assuming a one-dimensional chromophore (Le., one having a single, dominant ,!3 component)and minimal dispersion. Here $ is the average of the orientation angles, \Ir, of the chromophore dipoles with respect to the surface normal. The analysis yields G = 42.7 f 2.0°, in reasonable agreement with earlier results for 1.28 Although the corresponding signal in Figure 1A is too weak for accurate
Letters
390 Langmuir, Vol. 9,No.2, 1993
, 0.12
0.12 I
0.10
8
A. Uncapped Film B. Capped Film
-- 0.10
0.08
-- 0.08
0.06
-- 0.06
8.0
F 1 t
D
c 2s:
4.0
1
F L
-- 0.04
0.04
8
0.02 350
440
530
620
71 0
0.02 800
Wavelength (nm)
Figure 2. Optical spectra of 2-derived monolayer on quartz: (A) before step iii, Scheme I; (B)after step iii, Scheme I.
fitting to eq 1, it can be concluded that 3 is significantly greater than 42.7O. These results suggest that step iii effects a structural reorganization of the surface-bound chromophores in which the long axes (the &, direction) are reoriented in a direction more along the surface normal (portrayed schematically in two dimensions in eq 2).
%&LLP-
Capping
Support for this model is also provided by transmission optical spectroscopy, which reveals dimunition of absorbance of the 2-based monolayer at 480nm (Figure 2)upon capping, explicable in terms of rotation of the chromophore charge-transfer excitation componentsout of the plane of the substrate. This same behavior is commonly observed in analogous chromophore-containing poled polymer systems where the optical absorbance inversely parallels the degree of preferential chromophore alignment normal to the substrate surface.1° By calibrating the present SHG data against quartz, we esu for a single capped %-based estimatex;:), = 6 X monolayer prepared via Scheme I and of 25 A estimated thickness. This efficiency is comparable to high-quality chromophoric LB films and poled chromophore-functionalized polymers.lJl The procedure of Scheme I was
-
(10) (a)Kajzar,F.;Chollet,P.A.;Ledoux,I.;Moigne,J.Le.;Loxin,A.; Gadret, G. In Organic Molecules for Nonlinear Optics and Photonics; Messier, J., Kajzar, F.,Prasad, P., Eds.; Kluwer Academic Publishers: Dordrecht, 1991; p 403, and references therein. (b) Mortazavi, M. A.; Knoesen, A.; Kowel, S. F.;Higgins, B. G.;Dienes, A. J . Opt. SOC.Am. B 1989,6,733-741. (11) Similar measurements for 1-based films at 1064 and 1900 nm revealed slight resonant enhancement (-40%) of x(*). Since Amax is blueshifted for 2-basedfilms,we anticipate the enhancmentto be even smaller.
/
Y
&
1
IJ
2*o 0.0Y , 0 1 2 3 4 5 6 7 8 9 10 Number of Layers Figure 3. Plot of the square root of SHG intensity versus the number of chromophore layers in 2-derived multilayer superlattices. The straight line is the linear least-squares fit to the experimental data.
next employed iteratively to generate a series of 2-based superlattices with one, three, five, seven, and nine layers. Figure 3 shows a plot of the square root of the SHG intensity versus the number of layers. Since the superlattice thicknesses are small in comparison to the X = 1064 nm light and thus to the anticipated coherence length, I h should scale quadratically with the number of layers-a common diagnostic to assay the structural regularity of repeating NLO structures (e.g., chromophoric LangmuirB1odgettfilms).l2 The adherenceof the present multilayer structures to quadratic behavior is good (Figure 3) indicating that it is possible to maintain the same degree of noncentrosymmetric chromophore ordering in the additions of successive layers. These results demonstrate that the elaboration of chromophoricself-assembledsuperlattices to aikynylconstituents is possible and that regular acentric structures with large second-order optical nonlinearities can be fabricated. In the process, it is also seen that average chromophore orientation with reepect to the surface normal is a microstructural variable which can be modified by the assembly process.
Acknowledgment. This research was supported by the NSF-MRL program through the Materials Research Center of NorthwesternUniversity (GrantDMR8821571) and by the Air Force Office of ScientificReaearch (Contract 90-0071). A.K.K.thanks NSERC of Canada for a postdoctoral fellowship. (12) (a) Wijekoon, W. M. K. P.; Aeghorian, B.; Prasad, P. N.; Geieler, T.;Rosenkilde, 5. Thin Solid Films 1992,208,137. (b)Howarth, V. A.; Asai, N.; Kishii, N.; Fujiwara, I. Appl. Phys. Lett. 1992, 61, 1616. (c) Boilehard,Ch.;Kilpfer,M.;G~ter,P.;Pasquier,C.;Zahir,S.;Seifert,M. Appl. Phys. Lett. 1990, 56, 1204-1206, and references therein. (d) Popovitz-Biro, R.; Hill, K.; Landau, E. M.; Lahav, M.; Leiserowitz, L.; Sagio, J.; Heiung, H.; Meredith, G. R.; Vanherzeele,H. J. Am. Chem. SOC. 1988,110,2672-2674.
