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Orientational Phase Transitions in Merocyanine Monolayers on Acidic Aqueous Subphases Shujie Lin† and Stephen R. Meech* School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. Received July 13, 1999. In Final Form: November 8, 1999 The headgroup orientation of a merocyanine dye, 1-docosyl-4-(4-hydroxystyryl)pyridinium bromide, has been studied as a function of surface pressure on an acidified and a buffered subphase at pH 4 by second harmonic generation (SHG). The protonated form of the dye is SHG active for a fundamental wavelength of 800 nm. Phase transitions observed in the pressure-area isotherms correlate with changes in headgroup orientation, thus yielding microscopic information on the nature of the surface phase transition. The polarization resolved SHG data are analyzed to recover the orientation of the merocyanine headgroup as a function of surface pressure. The complex index of refraction of the surface film has been taken into account. Both the measured pressure-area isotherms and the orientation of the headgroup are found to be dependent on the nature of the subphase. In particular, at low pressure the acidified subphase yields a more vertical mean orientation (24°) of the headgroup than the buffered surface (42°), while at high pressures, above the expanded-to-condensed phase transition, the mean orientation of the headgroup is nearly independent of subphase (>50°). This behavior is discussed in terms of complex formation between the headgroup and the phthalate ions in the buffered subphase and shows how complexation of the adsorbate from the subphase can influence adsorbate orientation.
1. Introduction The merocyanine and hemicyanine dyes are representative of a class of organic molecules that are of potential interest in the fabrication of nonlinear optical devices.1 One reason for this interest lies in the large hyperpolarizability of these molecules, which arises from the intense charge-transfer nature of their electronic transitions.2 In addition amphiphilic derivatives of the mero- and hemicyanine dyes can be deposited in Langmuir-Blodgett films, which could then be produced in such a way as to lift the inversion symmetry of the medium, an important consideration if the large molecular hyperpolarizability is to be used to create a medium with a large second-order nonlinear susceptibility.1 In addition to this technological interest, monolayers of these dyes have been used to probe the acid-base properties of the interface and to investigate complex formation between the adsorbate monolayers and species in the subphase.3,4 For example, Hall et al. presented pressure-area isotherms and electronic spectra of the merocyanine dye 1-docosyl-4-(4-hydroxystyryl)pyridinium bromide (HSP) as a function of subphase pH and complexation.4 This dye (the structure of the protonated form is shown in Scheme 1) is also the subject of the present paper. In this work the orientation of the headgroup will be investigated using surface second harmonic generation (SHG) as a function of both surface pressure and subphase * Author for correspondence. E-mail:
[email protected]. † Current address: University Chemical Laboratories, Lensfield Road, Cambridge CB2 1EW, U.K. (1) Chemla, D. S.; Zyss, J. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic Press: New York, 1987. (b) Williams, D. J. Angew. Chem., Int. Ed. Engl. 1984, 23, 690 (c) Marowsky, G.; Chi, L. F.; Mo¨bius, D.; Steinhoff, R.; Shen, Y. R.; Dorsch, D.; Rieger, B. Chem. Phys. Lett. 1988, 147, 420. (d) Girling, I. R.; Kolinsky, P. V.; Cade, N. A.; Earls, J. D.; Peterson, I. R. Opt. Commun. 1985, 55, 289. (2) Oudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66, 2664. (3) Xiao, X.-D.; Vogel, V.; Shen, Y. R. Chem. Phys. Lett. 1989, 163, 555. (4) Hall, R. A.; Thistlewaite, P. J., Grieser, F.; Kimizuka, N.; Kunitake, T. Langmuir 1994, 10, 3743.
Scheme 1
composition. In the following it will be demonstrated that these SHG experiments provide microscopic information on the structural changes that accompany the surface phase transitions, observed in the surface pressure-area isotherms. Second-order nonlinear optical studies of interfaces provide a wealth of new information on adsorbates that is not readily available using other methods.5 Sum frequency generation (SFG) provides details on the vibrational spectra of adsorbates, yielding molecular structural information. On the other hand, the surface SHG technique, employed here, has been widely applied in studies of the orientation of molecules on surfaces.5 The SHG technique has many merits for surface studies, and these have been reviewed in considerable detail.5 The SHG intensity, I(2ω) is given by (2) 2 2 I(2ω) ) C|χs,eff | I (ω)
(1)
where C is a constant containing the optical frequencies, the angle of incidence, and the dielectric properties of the interface, I(ω) is the intensity of the incident radiation, and χ(2) s,eff is the effective surface second-order nonlinear optical susceptibility. This is a third-rank tensor that in general has 27 elements, but which, on surfaces that have azimuthal symmetry, reduces to only three independent (2) (2) 5 nonzero elements, χ(2) zzz, χzxx, and χxzx. These three elements of the macroscopic, laboratory frame, nonlinear (5) (a) Meech, S. R. In Advances in Multiphoton Processes and Spectroscopy; Lin, S. H., Villaeys, A. A., Fujimura, Y., Eds.; World Scientific: Singapore, 1993; p 281. (b) Heinz, T. F. In Nonlinear Electromagnetic Phenomena; Ponath, H.-E., Stegeman, G. I., Eds.; Elsevier: Amsterdam, 1991; p 351. (c) Eisenthal, K. B. Annu. Rev. Phys. Chem. 1992, 43, 627. (d) Shen, Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327.
10.1021/la990932k CCC: $19.00 © 2000 American Chemical Society Published on Web 01/20/2000
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optical susceptibility are related to the molecular hyperpolarizability βi′j′k′ through the relation ijk χ(2) ijk ) Na〈Ti′j′k′〉βi′j′k′
(2)
where a prime indicates molecular frame coordinates, Na is the adsorbate number density (molecules m-2), and the coordinate transformation matrix, 〈T〉, transforms between laboratory and molecular frames. Careful analysis of measurements of χ(2) ijk permits, when some approximations concerning the molecular hyperpolarizability are made, the determination of the adsorbate orientation.5 Since it is only the merocyanine headgroup of HSP that has any appreciable hyperpolarizability, the SHG data will necessarily reflect the orientation of the headgroup (Scheme 1). In the following it will be shown that careful analysis of the SHG data yields information on the headgroup orientation, and its dependence on surface pressure and subphase composition. Such data is not available from surface pressure measurements alone. Thus, the SHG data provide new microscopic information that complements the surface pressure data. In the next section the experimental procedures are described. After that the data and the analysis methods are presented, and the results are rationalized in terms of a simple model. The paper closes with some conclusions. 2. Experimental Methods The laser source was a regeneratively amplified mode-locked titanium sapphire laser that has been described in detail elsewhere.6 The output was at 800 nm, with a pulse width of
