Synthesis and structural study of organic two‐photon‐absorbing cycloalkanone chromophores

The three organic two‐photon‐absorbing cycloalkanone chromophores 2,4‐bis4‐(diethylamino)benzylidene]cyclobutanone, C₂₆H₃₂N₂O ( I ), 2,5‐bis4‐(diethylamino)benzylidene]cyclopentanone, C₂₇H₃₄N₂O ( II ), and 2,6‐bis4‐(diethylamino)benzylidene]cyclohexanone, C₂₈H₃₆N₂O ( III ), were obtained by a reaction between 4‐(diethylamino)benzaldehyde and the corresponding cycloalkanone and were characterized by single‐crystal X‐ray diffraction studies, as well as density functional theory (DFT) quantum‐chemical calculations. Molecules of this series have three main fragments, i.e. central acceptor (A) and two terminal donors (D₁ and D₂) and represent examples of the D₁–π–A–π–D₂ molecular design. All three compounds crystallize with two crystallographically independent molecules in the asymmetric unit ( A and B ) and are distinguished by the conformations of both the molecular Et₂N—C₆H₄—C=C—C(=O)—C=C—C₆H₄—NEt₂ backbone (arcuate or linear) and the terminal diethylamino substituents (syn‐ or antiperiplanar to the plane of the molecule). The central four‐ and five‐membered rings in I and II are almost planar, and the six‐membered ring in III adopts a sofa conformation. In the crystals of I – III , the two independent molecules A and B form hydrogen‐bonded A … B ] dimers via intermolecular C—H…O hydrogen bonds. Furthermore, the A … B ] dimers in I are bound by intermolecular C—H…O hydrogen bonds into two‐tier puckered layers, whereas in the crystals of II and III , the A … B ] dimers are stacked along the c and a axes, respectively. Taking into account the decreasing steric strain upon expanding the central ring, compound I might be more efficient as a two‐photon absorption chromophore than compounds II and III , which corresponds to the results of spectroscopic studies.