Langmuir 1994, 10, 1337-1339
1337
Ion Exchange Processes and Environmental Effects in Chromophoric Self-Assembled Superlattices. Manipulation of Microstructure and Large Enhancements in Nonlinear Optical Response Stephen B. Roscoe, Shlomo Yitzchaik, Ashok K. Kakkar, 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 December 16, 1993. I n Final Form: February 18, 1994@ This contribution reports on the anion exchange characteristics of self-assembled monolayers composed of high-@stilbazolium chloride moieties as assessed by X-ray photoelectron spectroscopy, optical spectroscopy,and polarized second harmonic generation (SHG) spectroscopy. At moderate chromophore surface coverages (-1 X 1014molecules/cm2)in acetonitrile or methanol, replacement of -60% of the C1counterions by I-, p-aminobenzenesulfonate, or 4-[4-[(diethylamino)phenyllazo]benzenesulfonate (ethyl orange) is observed. However, negligible exchange is detected at higher chromophore surface coverages ( 1 2 X 1014 molecules/cm2)or when the monolayer is functionalized with a siloxane overlayer. Polarized SHG measurements at 1064 nm reveal ion exchange-induced increases in xl;: of 34%, 25%, and 44% for I-, p-aminobenzenesulfonate, and ethyl orange, respectively. However, changes in the average chromophore orientation angle with respect to the surface normal are 1 2 O , implicating changes in anioncation pairing as the source of the large enhancements in x:::. monolayer level and large, anion-induced enhancements in second-order NLO response. Chromophoric monolayers were assembled using the siloxane4methodology described previously (steps i, ii, iii, and, optionally, iv; Scheme 11.l For most experiments, coupling agent coverages (step i) were deliberately maintained near 1 X 1014 molecules/cm2 by controlling the reaction conditions (the maximum coverage possible is near 3 X lOl4).lC Ion exchange experiments were carried out anaerobically by exposing functionalized glass substrates to dry acetonitrile solutions of (n-Bu)4N+I-(60 "C, 48 h) or methanol solutions of the sodium salts of p-aminobenzenesulfonate (2-9(25 "C, 72 h) or ethyl orange (3-) (60 "C, 72 h). The course of the ion exchange/anion incorporation was monitored by XPS, transmission optical spectroscopy, and polarized second harmonic generation (SHG)spectroscopy. In the case of uncapped (deletion Abstract published in Advance ACS Abstracts, April 1, 1994. of step iv) la, lb-derived monolayers, diminution of the (1)(a) 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. (b) C1-zp XPS signals a t 203 eV is accompanied either by Yitzchaik, S.;Roscoe, S. B.; Kakkar, A. K.; Allan, D. S.; Marks, T. J.; Xu, growth of I-(3d62,3ds 2) features at 621 and 633 eV or of an Z.; Zhang, T.; Lin, W.; Wong, G. K. J.Phys. Chem. 1993,97,6958. ( c ) SZpfeature at 154 e 6 (Figure 1). Analysis using corrected Yitzchaik,S.;Kakkar,A.K.;Roscoe,S.B.;Orihashi,Y.;Marks,T.J.;Lin, W.; Wong, G.K. Mol. Cryst. Liq. Cryst. 1994,240,9. (d) Allan, D. S.; C1, I, and S signal intensities reveals that only -60% of Kubota, F.; Kakkar, A. K.; Marks, T. J.; Zhang, T.; Lin, W.; Shih, M.; C1ions present are exchanged (displaced) under the Wong, G.K.; Dutta, P. Mater. Res. SOC. Symp. Proc. 1992,247,779.(e) present reaction conditions, with residual C1- XPS sigAllan, D. S.; Kubota, F.; Orihashi, Y.; Li, D.; Marks, T. J.; Zhang, T. G.; Lin, W. P.; Wong, G. K. Polym. Prepr. 1991,32(2), 86. (0 Li, D.; Ratner, natures persisting in all cases.5 Clearly not all C1- sites M. A.; Marks, T. J.; Zhang, C.; Yang, J.; Wong, G. K. J.Am. Chem. SOC. are accessible to ion exchange. Transmission optical 1990,112,7389. spectra of the starting stilbazolium chloride monolayers (2) (a) Molecular Nonlinear Optics: Materials, Physics, and Devices; Zyss, J., Ed.; Academic Press: Boston, MA, 1993. (b) Nonlinear Optical exhibit an absorption maximum a t 490 nm. Exchange Properties of Organic Materials V; Williams, D. J., Ed. SPIE Proc. 1992, with I- produces only a slight broadening of the optical 1775. (c) Prasad, N. P.; Williams,D. J.Introduction to Nonlinear Optical spectrum, consistent with results for a monolayer conEffects inMolecules andPolymers; Wiley New York, 1991. (d)Materials for Nonlinear Optics: Chemical Perspectives; Marder, S. R., Sohn, J. E., taining only I- (prepared using l-iodomethyl-4-(2-triStucky, G. D., Eds.; ACS Symposium Series 455; American Chemical iodosilylethy1)benzene in step illf where, , X = 510 nm Society: Washington, DC, 1991. (e) Organic Materials for Nonlinear Robust, intrinsically acentric self-assembled superlattices containing high-@ stilbazolium chromophores (Scheme 1)' represent an alternative approach to poled polymers and Langmuir-Blodgett films for the construction of highly efficient second-order nonlinear optical (NLO) materiak2 The high-@,,,3 anchored chromophore constituents in these multilayer materials have a saltlike architecture, and the potential therefore exists, using anions of varying size, shape, and hyperpolarizability, to probe film microstructure/reactivityrelationships as well as to manipulate macroscopic NLO response. We communicate here the first ion exchange studies on these materials, revealing pronounced sensitivity of anion incorporation patterns to microstructural changes a t the @
Optics I& Hann, R. A., Bloor, D., Eds.; Royal Society of Chemistry: London, 1991. (0 Nonlinear Optical Properties of Organic Materials IV; Singer, K. D., Ed. SPIE Proc. 1991,1560. (3)(a) In units of 1 V cm6 esu-l, ZINDO-derived3b.C8, = 178 ( h a = 0.65 eV); 946 ( h a = 1.17eV). (b) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Int. J. Quantum Chem. 1992,43,61. (c) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. Rev. 1994,94,195.
~~~~
~
(4)(a) Ulman, A. AnIntroduction t o Ultrathin 0rganicFilms;Acadmic Press: New York, 1991,Part 3. (b) Ulman, A. Adu. Mater. 1990,2,573. ( c ) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M. J. Am. Chem. SOC.1989,111, 321. (d) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989,5,1074.(e)Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985,132,153.
0743-7463/94/2410-1337$04.50/00 1994 American Chemical Society
Letters
1338 Langmuir, Vol. 10, No. 5, 1994 Scheme 1
iii
88 i. Toluene, 25"C, 6-36 h; 30 min 115°C cure.
+",
,-
*-
$2
+
ii. Toluene, 80"C, 48-72 h.
7 r
iii. B U ~ N + Iin- acetonitrile, 60°C 48 h; Na'2- in MeOH, 25°C 72 h; N a + J in acetonitrile, 60"C, 72 h. iv. C1$3OSiC12OSiC13 in THF, 25"C, 30 min. 30 min 115°C cure.
N
2-
(Figure 2A). In contrast, the incorporation of 2- (Amm = 280 nm) gives rise to a high-energy shoulder in the monolayer spectrum (Figure ZB),while exchange with 3(Amm = 470 nm) results in enhanced absorption (Figure 2C). Interestingly, two architectural modifications in Scheme 1 have marked effects on the ion exchange process as assessed by the optical spectra,XPS, and NLO response (vide infra). By use of I- and 2- as probes, negligible ion exchange/anionincorporation is detected (evenunder more forcing conditions) in densely-packed films (coupling agent/chromophore 1 2 X 1014molecules/cm2)or in those in which the siloxane "capping" layer (step iv) has been introduced. Second harmonic generation (SHG)measurements were carried out in the transmission mode using a Q-switched Nd:YAG laser operating at 1064 nm in the p-polarized geometry using instrumentation described previously.lI6 Figure 2 shows the SHG response as a function of fundamental beam incident angle (normal to the surface) (5)XPS spectra were recorded on a VG Scientific, Ltd., Escalab Mk Ilspectrometer using Mg Ka irradiation (AlKa for I-exchanged samples). Survey spectra with a step size of 0.5 eV (five scans) and high-resolution spectra (0.1eV, 5-30 scans) for the peaks of interest wererecorded. Relative abundance8 were calculated from high-resolution peak areas using the following sensitivity factors (relative to C 1s): S 2p, 1.7; C13p, 2.4; 13d612, 19.3 supplied by Kratos Analytical Instruments. It is possible to distinguish covalentlybound benzylic chlorine atoms from ionic chloride64b using standard deconvolution techniques. These data indicate that 60 % of the benzylic chlorines become chloride in step ii of Scheme 1. (a) Chastain, J. Handbook of X-Ray Photoelectron Spectroscopy; PerkinElmer Physical Electronics Div.: Eden Prairie, MN, 1992; pp 62-63, and references therein. (b) Yamamoto, Y.; Toyota, E.; Konno, H. Bull. Chem. SOC.J p n . 1991,64,1398, and references therein. (6) (a) Park, J.;Marks, T. J.;Yang, J.;Wong, G. K. Chem. Mater. 1990, 2,229. (b) Dai, D.-R.; Marks, T. J.; Yang, J.; Lundquist, P. M.; Wong, G. K. Macromolecules 1990,23,1894.
-
3-
6i5
621
621
633
639
645
Binding energy (eV) I
Si 2s
?
2 P5
n
d I
after
I09
lj4
159
184
2b9
2j4
Binding energy (eV)
Figure 1. (A) Ion exchange of C1- for I- in a la-derived monolayer as assessed by X-ray photoelectronspectroscopy. I-regions before and after ion exchange are shown normalized to Si 2p. (B) Ionexchange of C1- for p-aminobenzenesulfonate (2-) in a lb-derived monolayer as assessed by X-ray photoelectron spectroscopy. Si, C1, and S regions before and after ion exchange are shown.
from glass slides coated on both sides with la, lb-derived self-assembled monolayers before and after ion exchange.
Langmuir, Vol. 10,No. 5, 1994 1339
Letters
Table 1. Response of Stilbazolium Chromophore Tilt Angle and Second-Order NLO Response to Anion Exchange structure self-assembled la la lb lb
SHG/microstructurebefore ion exchange
x:
anion (A-) I323-
$, deg W O )
10-' esu 2.22 2.57 1.46 1.41
X
37 39 41 40
SHG/microstructureafter ion exchange
x:
x 10-7 esu 2.98 2.99 1.83 2.03
xi2 5%
$, deg (k2O)
increase (*4%)
38 41 42 42
34 16 25 44
terized by a single dominant PzZzcomponent) and minimal dispersion, leads to eq 1where $ is the average orientation
0
10
20
30
40
50
60
70
Angle of Incidence (degrees) Figure 2. Angle-dependent second harmonic generation response and transmissionoptical spectra (insets) of self-assembled monolayers subjected to ion exchange (open circles = before exchange; filled circles = after exchange): A, la-derived C1monolayer exchanged with I-; B, lb-derived C1- monolayer exchanged withp-aminobenzenesulfonate(2-1; C, lb-derived C1monolayer exchanged with ethyl orange (3-),
No in-plane anisotropy of the SHG signal is observed when the slides are rotated about the film normal which indicates that the distribution of chromophore molecular orientation does not have an azimuthal dependence. The interference pattern arises from the phase difference between the two SHG waves generated at either side during the propagation of the fundamental wave. Note that complete destructive interference occurs both before and after ion-exchange suggesting that films on the two sides of the glass are of high and nearly identical microstructural uniformity. The data are reproducible over a range of locations on the same sample before and after ion exchange, and the samples belonging to the same batch exhibit
