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

[Image: see text] Improvement in the first hyperpolarizability (β(HRS)) as well as intrinsic hyperpolarizability (β(int)) of chromophores based on 9,9-dimethyl-9H-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.