Synthesis and Nonlinear Optical Behavior of Thermally Stable Chromophores Based on 9,9-Dimethyl-9 H -fluoren-2-amine: Improving Intrinsic Hyperpolarizability through Modulation of "Push-Pull".

Improvement in the first hyperpolarizability (β HRS ) as well as intrinsic hyperpolarizability (β int ) of chromophores based on 9,9-dimethyl-9 H -fluoren-2-amine through modulation of the conjugation pathway is described. A series of six novel chromophores with "linear" conjugation showed significant enhancement of β HRS as well as β int compared to the counterparts lacking a "linear" conjugation but having an identical combination of donor, acceptor, and the intervening π-conjugated linker. The hyperpolarizability (β HRS as well as β int ) values of the new series measured using hyper-Rayleigh scattering exceeded the apparent limit set by the latter set of fluorene-based chromophores. The experimental results are analyzed and interpreted in the context of linear optical properties, single-crystal X-ray analysis, electrochemistry, etc. and corroborated by theoretical studies. We find that modulation of the "push-pull" of the conjugation pathway in these donor-acceptor chromophores compares favourably with the corresponding changes in the optical gaps, transition dipole moments, and dipole moment difference between the ground and excited states.