Rational Design Using Dewar's Rules for Enhancing the First Hyperpolarizability of Nonlinear Optical Chromophores

A rational material design based on Dewar's predictions is introduced in this paper. A number of conjugation-bridge-modified phenylpolyene chromophores were proposed as candidates for nonlinear optical chromophores. Hyperpolarizabilities of these candidates were calculated using density functional theory with a two-state model and finite-field methods. Significant enhancement with up to 72% increase in the first hyperpolarizability was observed. Another design mechanism using the bond length alternation analysis was proposed and supported by the study. In addition to the strength of the acceptor and donor, and the positions modifying the electron delocalization pathway, the density of lower lying excited states is shown to play an important role in the molecular hyperpolarizability. Increasing the density of lower lying excited states can be an effective approach in the design of highly nonlinear chromophores.