Descriptive Chemistry of Self-Assembled Multilayer Second-Order Nlo Materials. Chemical, Microstructural and Performance
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SHLOMO YITZCHAIK, ASHOK K. KAKKAR, STEPHEN B. ROSCOE, TOBIN J. MARKS,* PAUL M. LUNDQUISTt, WEIPING LINt, AND GEORGE K. WONGt* Department of Chemistry, Physicst and the Materials Research Center, Northwestern University, Evanston, Illinois 60208-3113 (USA) ABSTRACT The construction of covalently self-assembled second-order nonlinear optical (NLO) materials containing stilbazole or alkynyl-type high-P3 chromophores acentrically organized on inorganic oxide substrates is discussed. These structurally different chromophores exhibit differing packing densities on the surface when introduced under the same reaction conditions. In the stilbazolium chromophores, ion-exchange of the chloride counter-anion with very large anionic organic dyes results in appreciably enhanced second harmonic generation (SHG) efficiency. The frequency-dependent SHG response for self-assembled monolayers derived from the alkynyl-type chromophore was also investigated. The linear absorption spectra exhibit two maxima in the visible region which are assigned to electronically isolated chromophores (480 nm) and aggregated species (540 nm). The dispersion of the second-order NLO coefficient reveals that both microstructures are NLO-active. The origin of the SHG response from both assemblies is discussed. INTRODUCTION A new approach to the construction of second-order nonlinear optical (NLO) materials was recently reported from our laboratory (Scheme I).1 This method involves the covalent selfassembly of intrinsically acentric multilayers of high-13 chromophores on inorganic oxide substrates. Such robust thin film superlattices exhibit high second harmonic generation (SHG) efficiencies with X ZZZ (2) values for 25 A thick monolayers of 5-7 x 10-7 esu at hwo = 1.17 eV. This level of response is higher than conventional inorganic oxides such as LiNbO 3 and is comparable to the most efficient poled polymers and acentric Langmuir-Blodgett films. 2 ,3 In addition, the self-assembled thin films are thermodynamically equilibrated systems which do not require an external aligning electric field to establish an acentric environment. This represents a major advance over poled NLO polymers where inducing and completely stabilizing net chromophore alignment remains a daunting challenge. 2 ,3 In this contribution, we survey key aspects of thin film construction, focusing on structural variation of chromophore 27 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
building blocks and the accessibility of these systems to ion exchange processes. We demonstrate how these crucial variables affect microstructural features such as surface coverage, average chromophore tilt angle, and NLO properties of the resulting films.
Scheme I R
RRR \,R N N
R N
CH2CI
I CIe-+I CI-
(ýH2)2
" (OH2 )2 (CH,
OH2
o.Si0 0
S-Sk
S1013 +
OH2
(9H 2)2 (9H 2)2
oi0////•/ 0
77Si7o-/S,?
HO OH OH OH -. •Si-Si-Si•ý I
I
0 000 N
I
I
N
Capping layer i- iii
Repeat
L
ER
Chromophore layer
iii 1-2, b-c
CI- +NCI-
(CH2)2(CH2)2
0I 0
Coupling layer
0i-- " 0
28
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