Design of Near-Infrared-Absorbing Unsymmetrical Polymethine Dyes with Large Quadratic Hyperpolarizabilities

Fifteen
heptamethine dyes in which a tricyanofuran acceptor is
linked to donors of different strengths were synthesized, and their
absorption, thermal, electrochemical, and second-order nonlinear optical
properties were investigated. While the variation of the peripheral
bulky substituents allowed a decrease of the intermolecular interactions
in the solid state, subtle engineering of the heterocyclic donor provided
dyes with electronic structures that varied between dipolar and cyanine-like
(i.e., with little bond length alternation and with ground and excited
states having similar dipole moment) and remarkably high quadratic
hyperpolarizabilities (μβ1.9) of up to 115 000
× 10–48 esu.