Indolic squaraines as two-photon absorbing dyes in the visible region: X-ray structure, electrochemical, and nonlinear optical characterization

In this article, we have shown that it is possible to design large optical gap squaraines displaying a sizable fs regime TPA activity in the technologically relevant 700-800 nm region. Also, we unequivocally assigned, through single crystal X-ray analysis, the regiochemistry of the squaric acid condensation reaction at the 3-position of unsubstituted indole derivatives. Finally, we demonstrated that derivative 1 TPA activity in the optical limiting relevant ns regime exceeds that of one of the most promising quadrupolar derivative so far employed.