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

The three organic two-photon-absorbing cycloalkanone chromophores 2,4-bis[4-(diethylamino)benzylidene]cyclobutanone, C₂₆H₃₂N₂O (I), 2,5-bis[4-(diethylamino)benzylidene]cyclopentanone, C₂₇H₃₄N₂O (II), and 2,6-bis[4-(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.