Langmuir 1993,9, 1974-1977
1974
Nonlinear Optical Properties of Azo Dye Monolayers: The Effect of Tilt Angle on the Local Field Gerard Cnossen,. Karel E. Drabe, and Douwe A. Wiersma Department of Chemical Physics, Ultrafast Laser and Spectroscopy Laboratory, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Monique A. Schoondorp and Arend Jan Schouten Department of Polymer Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Jos B. E. Hulshof and Ben L. Feringa Groningen Centre for Catalysis and Synthesis, Department of Organic and Molecular Inorganic Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Received August 3,1992. In Final Form: June 16, 1993 We report on the second-order nonlinear optical susceptibility x(2)(2w,w,w) of dye-doped LangmuirBlodgett monolayers. xt2)is found to exhibit a nonlinear dependence on surface density,which is attributed to microscopic local-fields. In order to calculate the microscopic local-field for the disordered systems, a novel Monte-Carlo type of calculation is described, in which the tilt angle of the dye unit is a sensitive parameter. It is shown also that the tilt angle of rod-shaped chromophores in Langmuir-Blodgett films can be measured by angular-dependent absorption measurements.
Introduction Second-harmonic generation (SHG) is a widely applied technique to study properties of monolayers and interfaces. Due to its surface selectivity, information can be obtained about the structure and ordering of molecules at interfaces or in monolayers.' In order to interpret the experimental observations in terms of molecular properties, the interactions between the molecules have to be considered. In the case of nonlinear optical experiments at high concentrations of optically active species, the resonant interaction of an excited molecule with neighboring molecules has to be taken into account. These interactions strongly depend on the ordering of and distances between the optical nonlinear molecules. Perhaps the most extreme effect of these interactions occurs in aggregates, where the fundamental excitations are exciton-likeand the proper interpretation of nonlinear susceptibilities may require consideration of biexcitonic and two-exciton optical nonlinearities. The aim of this work is to study the influence of surface density (Ne)and the orientation of dye molecules on the second-order nonlinear optical susceptibility x ( ~ ) ( ~ w , o , w ) . We report on the investigation of monolayers of two azo dyes, whose structural formulas are shown in Figure 1. While the chromophoric groups in these molecules are identical, the monolayer structure of these dyes is different due to the presence of a second alkyl chain in compound 11. Experimentally, the structure of the monolayer is revealed by angle-dependent linear absorption spectroscopy and by SHG. Recently several papers have reported on measurements of the SHG efficiency as a function of surface densitya2In these studies the second-harmonic efficiency was found to decrease at high surface densities,caused by aggregation of the dye molecules. Interpretation of SHG experiments is difficult here because the structure and size of the (1) Shen, Y. R. Nature (London) 1989,337,519. Shen, Y.R.Annu.
Rev. Phys. Chem. 1989,40, 327.
0743-746319312409-1974$04.00/0
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II Figure 1. Chemical formulas of the dye molecules studied.
aggregates, and the fraction of aggregated dye molecules is unknown. In our work aggregation of dye molecules does not occur as attested by the absorption spectra of the samples. We ) molecular orientation interpret the dependence of x ( ~on and surface density in terms of local-field corrections. Using this approach, we find excellent agreement between our data and numerical calculations,justifying, a posteriori, our approach. This Letter is organized in the following way. We first recapitulate briefly our method of calculating local-field corrections. A full account has been published elsewhere.3 Secondly, a method is presented to obtain the tilt angle of the dye units in the monolayer from a linear absorption experiment. In the final section we discuss the results of (2) Girling, I. R.; Cade, N. A.; Kolinsky, P. V.; Jones, R. J.; Peterson, I. R.; Ahmad, M.M.;Neal, D. B.; Petty, M.C.; Roberta, G. G.; Feast, W. Am. 1987, B4,950. Berkovic, G.; Rasing, Th.; Shen, Y. R. J. J. Opt. SOC. J. Opt. SOC. Am. 1987, B4, 946. Hayden, L. M.Phys. Reu. 1988, B38, 3718. Schildkraut, J. S.;Penner, T. L.; Willand, C. S.; Wan, A. Opt. Lett. 1988,13,134. Marowsky, G.; Stainhoff, R. Opt. Lett. 1988,13,707. Shirota, K.; Kajikawa, K.; Takezoe, H.; Fukada, A. Jpn. J.Appl. Phys. 1990. 29. 750. ( 3 ) Cnossen, G.;Drabe, K. E.;Wiersma, D. A. J. Chem. Phys. 1992,97, 4512.
0 1993 American Chemical Society
Letters
Langmuir, Vol. 9, No. 8,1993 1975
the linear absorption measurements and the determination of xzyy@) as a function of surface density.
Theory (A) Local-FieldCalculations. To calculate the linear and nonlinear optical properties of a dye-doped LB film, detailed knowledge about its structure is essential. We therefore assume the following generally accepted monolayer model to hold.4 In this isotropic monolayer model the tilt angle t) of each dye unit with respect to the surface normal (the z axis) is assumed to be equal for every molecule, whereas the distribution of projection angles 4 in the azimuthal plane (the substrate surface) is random.3 As will be shown below, the tilt angle of the dye unit is an important parameter in the calculation of the microscopic local-field. The local-field is calculated iteratively by summing the externally applied field and the reaction field of the surrounding dip0les.~15The oscillating dipolar reaction at site n due to an induced oscillating dipole field Pdip(rn) pm at site m is calculated according tos
aoo 700 61
0
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2
3
Surface Density ( 1Oi4 cm-')
Figure 2. Calculated values of &w(2)divided by cos J, sin2 J, as a function of surface density. The y-axis is normal to the plane of incidence; the z-axis is normal to substrate surface. The
Here the unit vector I) points from r,,,to r,, and lr,,ml is the distance between sites m and n. Note that the time dependence of the electric Eand the dipole p field are not incorporated in eq 1. Retardation effects have been discarded, as the requirement k-r
