The effect of the fused-ring substituent on anthracene chalcones: crystal structural and DFT studies of 1-(anthracen-9-yl)-3-(naphthalen-2-yl)prop-2-en-1-one and 1-(anthracen-9-yl)-3-(pyren-1-yl)prop-2-en-1-one

Two new anthracene chalcones, namely 1-(anthracen-9-yl)-3-(naphthalen-2-yl)prop-2-en-1-one and 1-(anthracen-9-yl)-3-(pyren-1-yl)prop-2-en-1-one, have been successfully synthesized and the effect of the different fused ring substituent system attached to the anthracene chalcone derivative investigated. These compounds show a very narrow band gap due to the large p-conjugated systems, making them promising candidates as optoelectronic materials. Hirshfeld surface analysis has been carried out to show the contribution of intermolecular contacts and weak interactions to supramolecular stabilization.


Chemical context
Naphthalene, anthracene and pyrene are three types of polycyclic aromatic hydrocarbons that consist of two, three and four fused benzene rings sharing a common side. Polyaromatic hydrocarbons or -conjugated materials are an important class of organic compounds because of their significant conductivity properties that have led to tremendous advancements in the field of organic electronics (Li et al., 2016). Most conjugated materials used in such applications rely on linear electron-rich fragments (Lin et al., 2017). Furthermore, -conjugated systems have been studied extensively for their optoelectronic properties because they give the possibility of low-cost, large-area, and flexible electronic devices. Over the past decade, significant research into new -conjugated systems has been ongoing due to the rapidly growing number of applications in electronic devices such as semiconducting materials, organic light-emitting diodes (OLEDs; Kulkarni et al., 2004) and organic field-effect transistors (OFETs; Torrent & Rovira, 2008;Wu et al., 2010). Recently, we found that the presence of fused-ring systems at both terminal rings of chalcone derivatives to be useful in obtaining good quality single crystals with an easy-to-synthesize method. In this work, we report the synthesis and combined experimental and theoretical studies of anthracene chalcones containing a naphthalene (I) or pyrene (II) fused- ISSN 2056-9890 ring system. Additionally, the UV-Vis absorption and Hirshfeld surface analyses are discussed.

Structural commentary
The molecular and optimized structure of compounds (I) and (II) is shown in Fig. 1. The optimization of the molecular geometries leading to energy minima was achieved using DFT [with Becke's non-local three parameter exchange and the Lee-Yang-Parr correlation functional (B3LYP)] with the 6-311++G (d,p) basis set as implemented in Gaussian09 program package (Frisch et al., 2009). From the results it can be concluded that this basis set is well suited in its approach to the experimental data. The slight deviations from the experimental values are due to the fact that the optimization is performed in an isolated condition, whereas the crystal environment affects the X-ray structural results (Zainuri et al., 2017).
In both compounds, the C2-C3, C4-C5, C9-C10 and C11-C12 bond distances [mean value 1.3614 (18) Å for (I) and 1.351 (3) Å for (II)] are significantly shorter than the C-C bond distances in the central rings of the anthracene units [1.412 (8) and 1.403 (7) Å for (I) and (II) respectively]. This observation is consistent with an electronic structure for the  anthracene units where a central ring displaying aromatic delocalization is flanked by two isolated diene units (Glidewell & Lloyd, 1984).

Absorption Spectrum and Frontier Molecular Orbital
The theoretical maximum absorption wavelengths ( calc ) was obtained by time-dependent DFT (TD-DFT) calculations using B3LYP and the calculated values were compared with the experimental values. The calculations of the molecular orbital geometry show that the absorption maxima of the molecules correspond to the electron transition between frontier orbitals such as the transition from HOMO to LUMO. As can be seen from the UV-Vis spectra (Fig. 4) Table 2 Hydrogen-bond geometry (Å , ) for (II).
Cg4 is the centroid of the C18-C20/C25-C27 ring Through an extrapolation of the linear trend observed in the optical spectra (Fig. 4), the experimental energy band gaps are 3.18 and 2.76 eV for (I) and (II) respectively. The predicted energy gaps of 3.15 and 2.95 eV are comparable to the experimental energy gaps. The energy gap for (II) is smaller because the fused ring system of the pyrene substituent has a larger -conjugated system compared to the naphthalene fused ring system in (I). In addition, a previous study from Nietfeld et al. (2011) comparing the structural, electrochemical and optical properties between fused and non-fused ring compounds shows that the former have a lower band gap than other structures. The value of the optical band gaps observed for compound (I) and (II) indicate the suitability of these compounds for optoelectronic applications.

Hirshfeld Surface Analysis
The d norm and shape-index (Wolff et al., 2012) surfaces for compounds (I) and (II) are presented in Fig. 5a and 5b, respectively. C-HÁ Á ÁO and C-HÁ Á Á contacts are shown on the d norm mapped surfaces as deep-red depression areas in Fig. 5a. The C-HÁ Á ÁO contacts are only present in compound (I). The C-HÁ Á Á interactions are indicated through a combination of pale orange and bright-red spots, which are present on the Hirshfeld Surface mapped over the shape index surface and identified by black arrows (Fig. 5b).
In the fingerprint plot ( Fig. 5c), the HÁ Á ÁH, HÁ Á ÁO, CÁ Á ÁH and CÁ Á ÁC interactions are indicated together with their relative percentage contribution. The HÁ Á ÁH contacts have the largest overall contribution to the Hirshfeld surface and dominate in the crystal structure. The contribution of HÁ Á ÁO/ OÁ Á ÁH contacts to the Hirshfeld surface, showing two narrow spikes, provides evidence for the presence of intermolecular C-HÁ Á ÁO interactions in compound (I). Furthermore, the significant C-HÁ Á Á interactions in both (I) and (II) are indicated by the wings at d e + d i 2.7 Å .

Synthesis and crystallization
A mixture of 9-acetylanthracene (0.5 mmol) and 2-napthaldehyde or 1-pyrenecarboxaldehyde (0.5 mmol) for compounds (I) and (II), respectively, was dissolved in methanol (20 ml). A catalytic amount of NaOH (5 ml, 20%) was added to the solution dropwise with vigorous stirring. The reaction mixture was stirred for about 6 h at room temperature. After stirring, the contents of the flask were poured into ice-cold water (50 ml). The resultant crude products were filtered, washed successively with distilled water and recrystallized from acetone to get the corresponding chalcones. Single crystals of (I) and (II) suitable for X-ray diffraction analysis were obtained by slow evaporation of an acetone solution. UV-Vis absorption spectra and electron distribution of the HOMO and LUMO energy levels of (a) compound (I) and (b) compound (II).

1-(Anthracen-9-yl)-3-(pyren-1-yl)prop-2-en-1-one (II)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.15 e Å −3 Δρ min = −0.14 e Å −3 Special details Experimental. The following wavelength and cell were deduced by SADABS from the direction cosines etc. They are given here for emergency use only: CELL 0.71104 11.238 12.388 17.234 90.027 107.859 90.073 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.  (2)