Synthesis, crystal structure and aggregation-induced emission of a new pyrene-based compound, 3,3-diphenyl-2-[4-(pyren-1-yl)phenyl]acrylonitrile

There are alternating relatively strong and weak intermolecular π–π interactions between adjacent pyrene ring systems, forming a one-dimensional supramolecular structure. The compound is weakly fluorescent in THF solution, but it is highly emissive in the condensed phase, revealing distinct aggregation-induced emission (AIE) characteristics.


Chemical context
Over the last several decades, research on organic fluorescent materials has gained important momentum because of their wide range of applications in organic light-emitting diodes (OLED), organic field-effect transistors (OFET), organic lasers, fluorescent sensors and solar cells and so on (Indumathi et al., 2017;Mishra et al., 2011;Nie et al., 2017;Sasabe et al., 2011;Zhao et al., 2010). As a well known fluorophore, pyrene and its derivatives have attracted much attention owing to its pure blue fluorescence with high quantum yield, exceptionally long fluorescence lifetime, excellent thermal stability and high charge-carrier mobility (Figueira-Duarte et al., 2011;Luo et al., 2001;Zhang et al., 2016dZhang et al., , 2017. However, pyrene-based compounds show notorious aggregation-caused quenching (ACQ), which severely limits their application range. Encouragingly, the discovery of aggregation-induced emission (AIE) by Tang and co-workers has opened up a new approach for excellent emission materials in the solid state (Yuan et al., 2013). Indeed, propeller-like conformations such as tetraphenylethene (TPE) and triphenylacrylonitrile (TPAN) have been widely used for the design of AIE-active compounds because of their easy preparation and outstanding AIE effects (Han et al., 2016;Jadhav et al., 2015;Lu et al., 2015;Tasso et al., 2015;Zhang et al., 2016a). Compared to the propeller-shaped AIE-active moiety TPE, TPAN also exhibits typical crystallization-induced emission (CIE) behaviours, so the combination of TPAN with other fluorophores can readily generate mechanochromic materials, displaying reversible solid-state emission upon mechanical stimuli and solvent evaporation (Hirata et al., 2006;Zhang et al., 2016b). As a result of their promising potential applications in optical recording and as fluorescent switches and security inks, these mechanochromic materials have attracted considerable attention (Srinivasan et al., 2009;Zhang et al., 2018). Herein, we report the synthesis and crystal structure of a new pyrene-based triphenylacrylonitrile, 2-[4-(1-pyrenyl)phenyl]-3,3-diphenylacrylonitrile, using a Suzuki cross-coupling reaction between 2-(4-bromophenyl)-3,3-diphenylacrylonitrile and 1-pyrenylboronic acid, which may exhibit both AIE and mechanochromic characteristics.

Structural commentary
The single X-ray diffraction analysis agrees well with the expected structure of the title compound, as shown in Fig. 1. The 2,3,3-triphenylacrylonitrile unit, which exhibits the typical propeller-shaped structure, is linked by a planar pyrenyl unit at one phenyl segment. The length of the central C2-C3 bond is 1.3623 (14) Å , which is typical for a double C C bond. The C-N bond length is 1.1479 (14) Å , which is comparable with those of other cyanide-containing organic or inorganic compounds, showing the existence of a cyanide group. The pyrenyl ring system is almost strictly planar, with the largest derivation from the mean plane being 0.027 (3) Å for atom C31.

Aggregation-induced emission
The corresponding emission spectra of the title compound in aqueous THF with different water/THF ratios at a concentration of 5 Â 10 À5 M are shown in Fig. 3. It can be seen that the title compound shows weak fluorescence when the water fraction is below 70%, which is ascribed to the active intramolecular rotations of the genuinely dissolved luminogens in these mixtures. The yellow fluorescence starts to increase gradually at a water content of 80%, at which the luminogens begin to aggregate, and reaches a maximum, which is nearly 50 times stronger than that in the pure THF solution, when the water content is 90%. The title compound therefore exhibits typical aggregation-induced emission (AIE) activity. The supramolecular structure of the title compound built up throughand C-HÁ Á ÁN interactions.

Database Suvey
The structure of the title compound can be compared with our previously reported seriors of pyrenyl-based triarylamines in which two compounds crystallize in the same P1 space group (Zhang et al., 2016c). In these compounds, the substituent groups are all at the 1-position of the pyrene ring system. Importantly, because of the existence of the relatively larger planar pyrene ring system, there are intermolecularinteractions between adjacent pyrene ring systems, providing evidence that the presence of a pyrene ring system is favorable for the formation of strong intermolecular interactions.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms were placed in calculated positions C-H = 0.93 Å ) and refined using a riding model with U iso (H) = 1.2U eq (C).

3,3-Diphenyl-2-[4-(pyren-1-yl)phenyl]acrylonitrile
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.