Perpetual Extension of Conjugation of Fluorene-Based Donor–Acceptor Dyads Yield Diminished Nonlinear Optical Response

Abstract

A fundamental limit of π-conjugation seems to dictate the maximum nonlinear optical (NLO) response of fluorene-based “push–pull” (D−π–A) chromophores SPH1–SPH5 and the precursor aldehyde 4. Comparison of the first hyperpolarizability (βHRS) and intrinsic first hyperpolarizability (βint) values of the fluorene-based chromophores having different lengths and/or pathways of π-conjugation between donor and acceptor units, measured using the hyper-Rayleigh scattering (HRS) technique, revealed that the extension of π-conjugation beyond the limit (the horizontal limit!) does not necessarily improve the NLO response, which is determined by the combined influence of red shift of the intramolecular charge transfer (ICT) bands, optical band gap, and dipole moment of these chromophores. Additionally, the spatial distance between the donor and the acceptor units also constitutes an important attribute influencing the βHRS.