Orientation of Electro-optic Chromophores under Poling Conditions:  A Spheroidal Model

Organic-based chromophores are used as a basic material for electo-optic (EO) devices. The chromophores, to act in an EO material, must be acentrically aligned, which is generally achieved by applying an electric poling field. Calculating, from first principles, the extent of the alignment has proven to be challenging. Therefore, to gain an understanding of the interplay of the fundamental forces that determine alignment, we have chosen to study a coarse-grained model of chromophores: Each chromophore is replaced by a single spheroid, containing a dipole at its center, which interacts with neighboring spheroids only through electrostatic dipolar interactions and a Lennard-Jones potential at the surface of the spheroid. We have found, using NPT and NVT MC simulations that, when the spheroids are allowed to move off-lattice with any orientation, the acentric order from spheroids with varied aspect ratios gives an averaged order which is always lower than that obtained by spheres. The predictions of the order parameters for spheres using off-lattice models are nearly the same as those previously reported for on-lattice models. However, the predictions of the off-lattice models for oblate spheroids (with a 2 to 1 aspect ratio) is in marked contrast to previously described predictions of on-lattice models: The oblate spheroids have approximately half the order of the spheres when off-lattice models are used, as contrasted with having approximately twice the order when using on-lattice models. We find that the acentric order parameters are rather insensitive to the strength of the van der Waals energy over a wide range of energies. The results compare very favorably with measured electro-optic properties of experimental systems.