Performance of optical materials with the derivative of planar π-conjugated groups: Recent advances and future prospects

Abstract

Planar π-conjugated groups that possess not only large hyperpolarizability but also optical anisotropy are proved to be good functional motifs for optical materials with outstanding nonlinear optics and/or birefringence. To date, several comprehensive reviews have reported planar π-conjugated groups including [BO₃], [B₃O₆], [CO₃], [NO₃], and [H_xC₃N₃O₃] (x varying from 0 to 3), which can be regarded as desirable fundamental building units (FBUs) for the enhancement of the second harmonic generation (SHG) response and birefringence. Herein, we systematically review the derivative of planar π-conjugated groups, for example, the derivative of BO₃-based groups [BO_x(OH)_3−x] (x = 0, 1, 2), analogous [C(NH₂)₃] units, and C–O groups like [CO₂(OH)], [C₂O₄], [C₆O₆], [CH₃COO], [HCOO], [HC₄O₄], etc. In this review, compounds involving the derivative of planar π-conjugated groups are classified and summarized on the basis of their crystal structures and the accessible experimental and calculated data are discussed in detail. Considering the structure–performance relationship, the prospects of the above-mentioned materials with various categories are evaluated.