Effect of the orientational disorder on the hyperpolarizability measurement of amphiphilic push-pull chromophores in Langmuir-Blodgett monolayers

First-order hyperpolarizabilties of two new amphiphilic push-pull chromophore derivatives of hemicyanine dyes prepared in Langmuir-Blodgett films and designed for membrane potential imaging of living cells are measured by surface second-harmonic generation (SHG). Assuming uniaxial symmetry, an effective hyperpolarizability coefficient beta(e) is defined, and a procedure to measure this coefficient is described. Relationships between beta(e), and the true hyperpolarizability coefficient beta are derived. on the basis of a stochastic model taking account of the tilt angle disorder of the chromophores. Maximum discrepancies between the true and the effective hyperpolarizabilities are calculated as a function of the orientation parameter D, and it is shown that the relative difference may not exceed 40% for usual distribution functions of the tilt angle. As a result, the dominant hyperpolarizability coefficient beta((2))(z,z,z) of the chromophores in LB films is found to be in the range 0.3-0.7 x 10(-27) esu at 1064 nm fundamental wavelenjih. (c) 2004 Elsevier B.V. All rights reserved.