Modeling the Optical Behavior of Complex Organic Media: From Molecules to Materials

Abstract

For the past three decades, a full understanding of the electro-optic (EO) effect in amorphous organic media has remained elusive. Calculating a bulk material property from fundamental molecular properties, intermolecular electrostatic forces, and field-induced net acentric dipolar order has proven to be very challenging. Moreover, there has been a gap between ab initio quantum-mechanical (QM) predictions of molecular properties and their experimental verification at the level of bulk materials and devices. This report unifies QM-based estimates of molecular properties with the statistical mechanical interpretation of the order in solid phases of electric-field-poled, amorphous, organic dipolar chromophore-containing materials. By combining interdependent statistical and quantum mechanical methods, bulk material EO properties are predicted. Dipolar order in bulk, amorphous phases of EO materials can be understood in terms of simple coarse-grained force field models when the dielectric properties of the media are taken into account. Parameters used in the statistical mechanical modeling are not adjusted from the QM-based values, yet the agreement with the experimentally determined electro-optic coefficient is excellent.

Theory-Guided Design of Organic Electro-Optic Materials and Devices

“…If segments connecting chromophores to the core polymer or dendrimer structure are very flexible, the poling-induced organization is essentially the same as found for the chromophore in a composite material, i.e., one can neglect chromophore-host interactions). However, if covalent bond potentials prevent chromophores from pairing centrosymmetrically, then the behavior approaches that expected for independent particles, i.e., a linear dependence of r 33 on N. Such behavior is observed for the three-chromophore-containing dendrimer of Figure 3 and for related dendrimers [65,72]. An advantage of covalent incorporation of chromophores is that high chromophore number densities can be obtained without chromophore phase separation.…”

“…For example, the OEO material (PSLD_33) shown in Figure 3, has a number density of ∼6.5 × 10 20 molecules/cc, i.e., about three times the maximum loading achievable for the core (YLD156-type) chromophore in a chromophore-polymer composite. Decreasing the number density by creating higher generation versions of the PSLD_33 dendrimer results in an observed (and theoretically-predicted) linear variation of r 33 with N as does decreasing number density by dissolving dendrimers in polymer hosts such as APC [65,72]. For such materials, the observed electro-optic activity is typically above 150 pm/V but less than 250 pm/V.…”

“…Course graining of such methods, as illustrated in Figure 3, has permitted extension of these methods to complex polymer and dendrimer OEO materials [62][63][64][65]. Course-grained or pseudo-atomistic MC/MD methods are useful for OEO materials as extended π-electron conjugation inhibits rotation about π-bonds.…”

“…Both fully atomistic MC/MD and pseudo-atomistic PAMC/MD methods have shown that chromophore-host interactions can be neglected to good approximation and poling-induced order is dominated by the competition of chromophore dipole-electric poling field interactions with chromophore-chromophore dipolar interactions. Both analytical [9,10,62] and numerical [11,[63][64][65][66] methods have demonstrated that there are two components to the chromophore-chromophore dipolar interaction potential. One component favors centrosymmetric (centric) chromophore organization while the other favors noncentrosymmetric (acentric) organization with the weighting of the contributions from these two components determined by chromophore shape (steric or nuclear repulsive potentials).…”

Massive Faculty Donations and Institutional Conflicts of Interest

“…He and others wrote a paper about this newfound understanding, unbeknownst to us. 104 1. Not until the very last line do we discover that mistakes were made: the "order in the material is much lower than anticipated from previous estimates".…”