Molecular and crystal structure, optical properties and DFT studies of 1,4-dimethoxy-2,5-bis[2-(4-nitrophenyl)ethenyl]benzene

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.


Chemical context
One method for the design of the organic two-photon absorbing (TPA) molecules is Donor--Bridge-Acceptor--Bridge-Donor or Acceptor--bridge-Donor--bridge-Acceptor (He et al., 2008). Specific spectroscopic properties of such molecules make them useful for applications in different areas. For instance, about half a century ago it was found that the title compound and other substituted distyrylbenzenes would be highly efficient wavelength shifters in organic liquid scintillators (Nakaya et al., 1966). It is important to mention that some molecules with such general structure possess not only plasminogen activator (tPA) activity but also demonstrate light emission, which make them useful for organic light-emitting diodes (OLEDs) (Cá rdenas et al., 2019) and/or chemical sensors (Xu et al., 2013). For instance, for a molecule similar to the title molecule, 1,4-dimethoxy-2,5-bis(4 0 -dichlorostyryl)benzene, blue fluorescence emission was found, which makes it a prospective candidate for cell imaging. Another phenyleneethenylene derivative, 2,5-dimethoxy-1,4bis[2-(4-carboxylatestyryl)]benzene, for which two polymorphs and one DMF solvate have been studied, demonstrated three different types of emission, depending on the molecular packing in the crystal (Cá rdenas et al., 2019). On this basis, we considered that an investigation of the molecular structure and crystal packing of the title compound would be useful for correlating its structural characteristics to its spectroscopic properties.

Structural commentary
The molecular structure of DBDB is presented in Fig. 1. The molecule lies on an inversion center and shows a slight deviation from planarity. The dihedral angle formed by mean planes of the central fragment and the terminal benzene ring is 16.46 (6) . The methoxy group is rotated by 3.77 (11) and the nitro group by 15.99 (8) with respect to the central ring and the terminal benzene ring, respectively. In a similar compound with para-chlorine substitution, the angles between the central and terminal aromatic rings are 43.82 and 67.38 (Xu et al., 2013), whereas in closely related structures these angles vary from 11.97 to 35.75 (Cá rdenas et al., 2019), demonstrating the flexibility of this type of molecule, even in the solid state.

Supramolecular features
In the crystal, the DBDB molecules are packed into ladderlike stacks (Fig. 2) along the a-axis direction, which in turn build a parquet-like structure (Fig. 3). An intermolecular distance of 3.451 (1) Å is found between the mean planes of the central rings in the molecular stacks, with a separation between the centroids of the central ring and the terminal benzene ring of 3.899 (1) Å , which suggests the presence ofinteractions between the molecules.

Database survey
A search of the Cambridge Crystallographic Database (CSD version 5.40, update of September 2019; Groom et al., 2016) for the title molecule returned no results. Two entries for compounds with the same core and unsubstituted terminal rings were found. Over 30 entries were found for variously substituted molecules with the same core, of which 10 entries correspond to para-substituted terminal aromatic groups.

Optical studies in solution
A solution of the title compound in dioxane (at 10 mM concentration) in a quartz sample cuvette (10 mm optical path length) was used for optical absorption and emission studies. All measurements were carried out at ambient temperature. The corresponding spectra are shown in Fig Ladder-like stack of DBDB molecules; the distance between the mean planes of the central phenyl rings within the stack is 3.451 (1) Å .

Figure 3
The packing in the crystal of the title compound.

Figure 1
A view of the molecular structure of the title compound with the atomlabeling scheme. Displacement ellipsoids are drawn at the 50% probability level.

Figure 4
Normalized absorption (black) and emission (red) spectra of the title compound measured in dioxane solution.
as well as band shapes are in good agreement with those previously reported (Nakaya et al., 1966). Fluorescence was measured at the excitation wavelength of 434 nm, chosen from the absorption spectrum, and had a maximum at 525 nm. The E 0-0 transition energy was estimated to be at 483 nm (2.57 eV).

DFT calculations
In an effort to further elucidate the nature of the electronic radiative transitions in the title compound, DFT and timedependent (TD) DFT calculations were carried out with GAUSSIAN 16 software (Frisch et al., 2016). The standard B3LYP functional with the 6-311G(d,p) basis set was used to optimize both the ground and first excited states of the title molecule and to obtain vertical excitation and emission energies, HOMO (E HOMO ) and LUMO (E LUMO ) energies and their difference (Fig. 5). All of the calculated parameters are for the gas phase of the title compound. Both optimized geometries were confirmed to be the true minima via vibrational frequency analysis. The summary of calculated energy parameters is presented in Table 1. The calculated geometry parameters (bond lengths and angles) are in good agreement with the experimental data (Table 2).

Synthesis and crystallization
The synthesis of title compound was carried out as described in the literature (Nakaya et al., 1966;Caruso et al., 2005). The obtained material was recrystallized by slow evaporation of ethanol solution giving dark-red block-shaped crystals.

Funding information
SHELXL2017/1 (Sheldrick, 2015b), OLEX2 (Dolomanov et al., 2009); molecular graphics: Mercury (Macrae et al., 2020); software used to prepare material for publication: Mercury (Macrae et al., 2020). 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.