Design, Synthesis, and Structural and Spectroscopic Studies of Push–Pull Two-Photon Absorbing Chromophores with Acceptor Groups of Varying Strength

A new series of unsymmetrical diphenylaminofluorene-based
chromophores
with various strong π-electron acceptors were synthesized and
fully characterized. The systematic alteration of the structural design
facilitated the investigation of effects such as molecular symmetry
and strength of electron-donating and/or -withdrawing termini have
on optical nonlinearity. In order to determine the electronic and
geometrical properties of the novel compounds, a thorough investigation
was carried out by a combination of linear and nonlinear spectroscopic
techniques, single-crystal X-ray diffraction, and quantum chemical
calculations. Finally, on the basis of two-photon absorption (2PA)
cross sections, the general trend for π-electron accepting ability,
i.e., ability to accept charge transfer from diphenylamine was: 2-pyran-4-ylidene
malononitrile (pyranone) > dicyanovinyl > bis­(dicyanomethylidene)­indane
>1-(thiophen-2-yl)­propenone > dicyanoethylenyl >3-(thiophen-2-yl)­propenone.
An analogue with the 2-pyran-4-ylidene malononitrile acceptor group
exhibited a nearly 3-fold enhancement of the 2PA cross section (1650
GM at 840 nm), relative to other members of the series.