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The title mol­ecule, C15H9NO4, is non-planar, with the unusually large dihedral angle between mean planes of phenyl ring and chromone moiety being 50.73 (5)°., The geometry of the title mol­ecule [systematic name: 2-(2-nitro­phen­yl)-4H-chromen-4-one], C15H9NO4, is determined by two dihedral angles formed by the mean plane of phenyl ring with the mean planes of chromone moiety and nitro group, being 50.73 (5) and 30.89 (7)°, respectively. The crystal packing is determined by π–π inter­actions and C—H⋯O contacts. The results of DFT calculations at the B3LYP/6–31G* level of theory provided an explanation of the unusually large dihedral angle between the chromone moiety and the phenyl group. The electrostatic potential map on the mol­ecular surface was calculated in order to determine the potential binding sites to receptors.

In the title mol­ecule, which is based on a 1,4-distyryl-2,5-di­meth­oxy­benzene core with p-nitro-substituted terminal benzene rings, the dihedral angle between mean planes of the central fragment and the terminal phenyl ring is 16.46 (6)°. The crystal packing is stabilized by π–π inter­actions., The title compound DBNB, C24H20N2O6, has been crystallized and studied by X-ray diffraction, spectroscopic and computational methods. In the title mol­ecule, which is based on a 1,4-distyryl-2,5-di­meth­oxy­benzene core with p-nitro-substituted terminal benzene rings, the dihedral angle between mean planes of the central fragment and the terminal phenyl ring is 16.46 (6)°. The crystal packing is stabilized by π–π inter­actions. DFT calculations at the B3LYP/6–311 G(d,p) level of theory were used to compare the optimized structures with the experimental data. Energy parameters, including HOMO and LUMO energies, their difference, and vertical excitation and emission energies were obtained.

The crystallization from different solvents of homologues of 1,3-diethyl-2-thiobarbituric acid and p-dimethylaminobenzene with different lengths of the polymethine bridge between donor and acceptor groups (TB[n]) produced three pairs of polymorphs with the number of C═C bonds in the chain n = 1, 3, and 5. The homologues with the shortest (one methine unit) π-conjugated bridge were presented by two concomitant (obtained simultaneously) polymorphs. Bond length alternation values calculated using X-ray data and evaluation using NMR data showed that some homologues possesses a very high hyperpolarizability. It was shown that the increase of polarizability is related to the increase in the length of the π-conjugated bridge that was supported by experimental hyper-Rayleigh scattering data. Density functional theory calculations of HOMO/LUMO molecular energy levels indicated that compounds with a longer π-conjugated bridge have a higher thermal stability.

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 (D 1 and D 2) and represent examples of the D 1–π–A–π–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 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.

The title pull–push chromophores, 2-[4-(dimethylamino)benzylidene]-1H-indene-1,3(2H)-dione, C18H15NO2 (ID[1]) and (E)-2-{3-[4-(dimethylamino)phenyl]allylidene}-1H-indene-1,3(2H)-dione, C20H17NO2 (ID[2]), have donor–π-bridge–acceptor structures. The molecule with the short π-bridge, ID[1], is almost planar while for the molecule with a longer bridge, ID[2], is less planar. The benzene ring is inclined to the mean plane of the 2,3-dihydro-1H-indene unit by 3.19 (4)° in ID[1] and 13.06 (8)° in ID[2]. The structures of three polymorphs of compound ID[1] have been reported: the α-polymorph [space group P21/c; Magomedova & Zvonkova (1978). Kristallografiya, 23, 281–288], the β-polymorph [space group P21/c; Magomedova & Zvonkova (1980). Kristallografiya, 25 1183–1187] and the γ-polymorph [space group Pna21; Magomedova, Neigauz, Zvonkova & Novakovskaya (1980). Kristallografiya, 25, 400–402]. The molecular packing in ID[1] studied here is centrosymmetric (space group P21/c) and corresponds to the β-polymorph structure. The molecular packing in ID[2] is non-centrosymmetric (space group P21), which suggests potential NLO properties for this crystalline material. In both compounds, there is short intramolecular C—H...O contact present, enclosing an S(7) ring motif. In the crystal of ID[1], molecules are linked by C—H...O hydrogen bonds and C—H...π interactions, forming layers parallel to the bc plane. In the crystal of ID[2], molecules are liked by C—H...O hydrogen bonds to form 21 helices propagating along the b-axis direction. The molecules in the helix are linked by offset π–π interactions with, for example, a centroid–centroid distance of 3.9664 (13) Å (= b axis) separating the indene rings, and an offset of 1.869 Å. Spectroscopic and electrochemical measurements show the ability of these compounds to easily transfer electrons through the π-conjugated chain.

The synthesis and crystal structures of 1,3-diethyl-5-{(2E,4E)-6-[(E)-1,3,3-tri­methyl­indolin-2-yl­idene]hexa-2,4-dien-1-yl­idene}pyrimidine-2,4,6(1H,3H,5H)-trione or TMI, C25H29N3O3, and 1,3-diethyl-2-sulfanyl­idene-5-[2-(1,3,3-tri­methyl­indolin-2-yl­idene)ethyl­idene]di­hydro­pyrimidine-4,6(1H,5H)-dione or DTB, C21H25N3O2S, are described. These compounds contain the same indole derivative donor group and differ in their acceptor groups (in TMI it contains oxygen in the para position, and in DTB sulfur) and the length of the π-bridge., Two compounds, 1,3-diethyl-5-{(2E,4E)-6-[(E)-1,3,3-tri­methyl­indolin-2-yl­idene]hexa-2,4-dien-1-yl­idene}pyrimidine-2,4,6(1H,3H,5H)-trione or TMI, C25H29N3O3, and 1,3-diethyl-2-sulfanyl­idene-5-[2-(1,3,3-tri­methyl­indolin-2-yl­idene)ethyl­idene]di­hydro­pyrimidine-4,6(1H,5H)-dione or DTB, C21H25N3O2S, have been crystallized and studied. These compounds contain the same indole derivative donor group and differ in their acceptor groups (in TMI it contains oxygen in the para position, and in DTB sulfur) and the length of the π-bridge. In both materials, mol­ecules are packed in a herringbone manner with differences in the twist and fold angles. In both structures, the mol­ecules are connected by weak C—H⋯O and/or C—H⋯S bonds.

Structural and spectroscopic characteristics of merocyanine dyes are important because of their potential applications in optoelectronics and sensors. Herein we report synthesis, X-ray diffraction characterization of crystal structure, spectroscopic and NMR studies of four merocyanine dyes which, according to their strong donor-acceptor structure, exhibit a second order polarizability (β). Crystallographic studies demonstrated an almost planar structure of all molecules and indicated the occurrence of conformational changes in π-conjugated bridge between donor and acceptor. For three homologues similar packing modes, defined mostly by week C H⋯N interactions, were found in crystals. Bond length alternation (BLA) values have been evaluated using crystallographic and NMR data and have shown a correlation to their nonlinear optical activity. Second order polarizabilities for all compounds were measured using hyper Rayleigh scattering (HRS) in solution. It was found that the merocyanines studied exhibit very large second order polarizabilities (up to 4.1 10−27 esu), making them potentially useful for materials for second harmonic generation (SHG). Unfortunately, they do not demonstrate acentric packing of chromophores, which is required for SHG in the crystalline state, but may show promise for other applications such as poled polymer blends or for SHG sensing in biological environments.

The synthesis, crystal structure studies and solvatochromic behavior of 2-{(2E,4E)-5-[4-(dimethylamino)phenyl]penta-2,4-dien-1-ylidene}malononitrile, C16H15N3 (DCV[3]), and 2-{(2E,4E,6E)-7-[4-(dimethylamino)phenyl]hepta-2,4,6-trien-1-ylidene}malononitrile, C18H17N3 (DCV[4]), are reported and discussed in comparison with their homologs having a shorter length of the π-conjugated bridge. The compounds of this series have potential use as nonlinear materials with second-order effects due to their donor–acceptor structures. However, DCV[3] and DCV[4] crystallized in the centrosymmetric space group P21/c which excludes their application as nonlinear optical materials in the crystalline state. They both crystallize with two independent molecules having the same molecular conformation in the asymmetric unit. The series DCV[1]–DCV[4] demonstrated reversed solvatochromic behavior in toluene, chloroform, and acetonitrile.