Crystal structures, syntheses, and spectroscopic and electrochemical measurements of two push–pull chromophores: 2-[4-(dimethylamino)benzylidene]-1H-indene-1,3(2H)-dione and (E)-2-{3-[4-(dimethylamino)phenyl]allylidene}-1H-indene-1,3(2H)-dione

The title pull–push chromophores, 2-[4-(dimethylamino)benzylidene]-1H-indene-1,3(2H)-dione (ID[1]) and (E)-2-{3-[4-(dimethylamino)phenyl]allylidene}-1H-indene-1,3(2H)-dione (ID[2]), with donor–π-bridge–acceptor structures, are almost planar for the molecule with a short π-bridge (ID[1]) but less planar for the molecule with a longer bridge (ID[2]).


Chemical context
Organic molecules containing donor and acceptor groups connected by a conjugated -bridge (push-pull chromophores) are important in many areas of materials chemistry, especially organic electronics and optoelectronics. Applications of pull-push molecules can be related to their properties such as intramolecular charge transfer and specific molecular arrangements in the solid state. Intramolecular charge transfer from donor to acceptor via a -bridge defines their colour, light absorption and emission, hyperpolarizability and other optoelectronic effects. Applications of pull-push chromophores include non-linear optics (NLO; Ortiz et al., 1994), as luminescent sensors (Duarte et al., 2011;Qin et al., 2015), solid-state lasers (Samuel & Turnbull, 2007), organic light-emitting diodes (Muller et al., 2003), organic field-effect transistors (Suponitsky et al., 2006;Oliveira et al., 2018) and many more. The spectroscopic properties of pull-push molecules are related to the donor and acceptor strength in these molecules and to the length of the -bridge. Many such compounds have been studied, but not all of their crystal structures have been reported. Such compounds are important for their NLO properties (Andreu et al., 2003;Raimundo et al., 2002). Herein, we report on the crystal structures, syntheses and spectroscopic and electrochemical properties of the title donor--bridge-acceptor structures, ID[1] and ID [2]. The structures of three polymorphs of ID[1] have been reported previously; the -polymorph (Magomedova & Zvonkova, 1978), the -polymorph  and the -polymorph (Magomedova, Neigauz et al., 1980). We have repeated the structural study of ID[1] in order to establish exactly which polymorph we obtained. It was then characterized by spectroscopic and electrochemical measurements.

Structural commentary
The molecular structures of ID[1] and ID[2] are illustrated in Fig. 1. The structural analysis of ID[1] synthesized by us established that it is the -polymorph , and it was then characterized with spectroscopic (x4) and electrochemical (x5) measurements.
Both molecules have acceptor--bridge-donor structures. It was found, as in our previous studies , that with an increase of the length of the -conjugated bridge the molecule becomes less planar, and the angles between the different planar fragments (acceptor-bridge, bridge-donor) become larger (Table 1).
Compound ID[1] has an almost planar structure, with the benzene ring (C10-C15) being inclined to the mean plane of the indene ring system (C1-C9) by 3.19 (4) . In ID[2] the deviation from planarity is somewhat larger with the benzene ring (C10-C15) being inclined to the mean plane of the indene ring system (C1-C9) by 13.06 (8) ; see further details in Table 1.  Table 1 Dihedral angles between molecular fragments ( ) and mean deviations (Å ) of atoms from these fragments.
In the crystal of ID[2], molecules form stacks with parallel molecular positions, and shifted positions of stacks extended along the b-axis direction within the acentric space group P2 1 . The ID[2] molecules are packed in a herringbone fashion (Fig. 3). Here, the angle between two molecules from different stacks is ca 60.8 . The molecules are linked by C-HÁ Á ÁO hydrogen bonds (Table 3), forming a 2 1 helix that propagates along the b-axis direction. The molecules in the helix are linked by offsetinteractions with, for example, a centroid-centroid distance Cg1Á Á ÁCg1 i of 3.9664 (13) Å [symmetry code: (i) x, y À 1, z] separating the indene ring systems (C1-C9), with an offset of 1.869 Å .

Spectroscopic studies
Absorbance spectra were obtained for both ID[1] and ID[2] in chloroform and acetonitrile. For donor-acceptor polyenes, the dominating feature of the absorbance spectrum is the -* transition that results from charge transfer from donor to acceptor. According to recent studies , it proves that dimethylaminophenyl polyenals have reversed solvatochromism, which is proved by the maxima of the absorption values (

Electrochemical measurements
Donor-acceptor polyenes can be characterized by electrochemical measurements to show their ability to transfer electrons. The voltammagrams (Fig. 5) demonstrate a completely reversible oxidation process and a partially reversible reduction process. When only swept between 0 V and 1.7 V the oxidation process is reversible (Fig. 5a),     (Table 2) are shown as dashed lines. For clarity, only the H atoms involved in the intermolecular interactions have been included. Table 3 Hydrogen-bond geometry (Å , ) for ID [2].

Database survey
Also, out of all 27 hits there are three hits, (MBYINO: Magomedova et al., 1978;MBYINO01: Magomedova, Neigauz et al., 1980;MBYINO02: Magomedova & Zvonkova, 1980), which are the , and ID[1] polymorphs, respectively, published over 40 years ago. It should be mentioned that the crystal packing in the and polymorphs is centric (space group P2 1 /c), while in the polymorph it is acentric (space group Pna2 1 ). The crystal structure of ID[1] we obtained corresponds to the polymorph, i.e. the centrosymmetric modification MBYINO02. The dihedral angles between the benzene and indene rings for two independent molecules in MBYINO are ca 4.35 and 7.79 , compared to ca 7.36 in MBYINO01, and 3.54 in MBYINO02 (cf. 3.19 (4)

Synthesis and crystallization
For the synthesis of the title compounds, two aldehydes were used: 4-(dimethylamino)benzaldehyde (A1; purchased from Aldrich) and 4-(dimethylamino)cinnamaldehyde (A2), which was synthesized as described previously . 2,3-Indanedione was purchased from Aldrich and used without further purification.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. For both structures, the C-bound hydrogen atoms were positioned geometrically and refined using a riding model: C-H = 0.95-0.98 Å with U iso (H) = 1.5U eq (C-methyl) and 1.2U iso (C) for other H atoms.

(E)-2-{3-[4-(Dimethylamino)phenyl]allylidene}-1H-indene-1,3(2H)-dione (ID2)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.