Mean-®eld theory of acentric order of chromophores with displaced dipoles Yuriy V. Pereverzev, Oleg V. Prezhdo * , Larry R. Dalton Department of Chemistry, University of Washington, Seattle, WA 98195-1700, USA Received 23 January 2001 Abstract A mean-®eld model of acentric order of dipolar chromophores in polymeric electro-optic materials is developed by extension of our earlier results [Y.V. Pereverzev, O.V. Prezhdo, Phys. Rev. E 62 2000) 8324] to allow for position and orientation ¯exibility of chromophore dipoles with respect of the molecular frame. The extended model quantitatively reproduces the experimental data for two sample chromophore systems. The model indicates that the disappearance of the acentric order and the related turnover and decay of the electro-optic coecient are associated with a second-order phase transition in the dipolar system from para- to antiferroelectric state. Ó 2001 Elsevier Science B.V. All rights reserved. Quasilinear chromophore molecules dissolved in polymer matrices belong to the class of liquid crys- talline systems, whose various properties are being actively investigated by both theoretical and experi- mentaltechniques.Underthein¯uenceoftherepulsiveexcludedvolumeforceaswellasinthepresenceofa more complicated anisotropic Gay±Berne interaction, systems of quasilinear molecules undergo phase transitions with changing temperature and molecular concentration. In particular, with temperature de- crease and concentration increase, the isotropic phase transforms into the nematic phase and, later, into the smectic phase. The phases dier in the orientation and spatial order of the chromophore molecules. The order is maximized at low temperatures and high concentrations. Inadditiontothespatialorder,anelectrostaticasymmetryappearsinthepresenceofelectrondonorand acceptor fragments at the ends of chromophore molecules. A number of valuable macroscopic electric and optical properties are thus produced. Donor±acceptor quasilinear chromophores are characterized by large values of molecular dipole moments, often exceeding 10 Debye. Due to the large molecule dipoles, donor± acceptor chromophore systems are asymmetric in external electric ®elds. Donor and acceptor fragments in dipolar chromophores are also responsible for large values of molecular polarizabilites and hyperpolariz- abilities and the resulting non-linear response of molecules to external ®elds. The combination of the macroscopic asymmetry in the chromophore system with the microscopic electro-optical non-linearities is responsible for a number of macroscopic non-linear optical phenomena that emerge under the in¯uence of weak electromagnetic ®elds. 1 June 2001 Chemical Physics Letters 340 2001) 328±335 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +1-206-522-1602. E-mail address: prezhdo@u.washington.edu O.V. Prezhdo). 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0009-261401)00411-0