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

Abstract

The three organic two-photon-absorbing cycloalkanone chromophores 2,4-bis-[4-(diethylamino)benzylidene]cyclobutanone, C 26 H 32 N 2 O (I), 2,5-bis[4-(diethylamino)benzylidene]cyclopentanone, C 27 H 34 N 2 O (II), and 2,6-bis[4-(diethylamino)benzylidene]cyclohexanone, C 28 H 36 N 2 O (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 (D 1 and D 2 ) and represent examples of the D 1 --A--D 2 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 Et 2 N-C 6 H 4 -C C-C( O)-C C-C 6 H 4 -NEt 2 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 [AÁ Á ÁB] dimers via intermolecular C-HÁ Á ÁO hydrogen bonds. Furthermore, the [AÁ Á ÁB] dimers in I are bound by intermolecular C-HÁ Á ÁO hydrogen bonds into two-tier puckered layers, whereas in the crystals of II and III, the [AÁ Á ÁB] 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.