Structural and luminescent properties of co-crystals of tetraiodoethylene with two azaphenanthrenes

Two new co-crystals of tetraiodoethylene with two azaphenanthrenes were successfully synthesized. In the crystals, C—I⋯π and C—I⋯N halogen bonds link the molecules. A study of their luminescence properties indicates that co-crystals 1 and 2 exhibit distinctly different luminescence in the solid state at room temperature.

Two new co-crystals, tetraiodoethylene-phenanthridine (1/2), 0.5C 2 I 4 ÁC 13 H 9 N (1) and tetraiodoethylene-benzo[f]quinoline (1/2), 0.5C 2 I 4 ÁC 13 H 9 N (2), were obtained from tetraiodoethylene and azaphenanthrenes, and characterized by IR and fluorescence spectroscopy, elemental analysis and X-ray crystallography. In the crystal structures, C-IÁ Á Á and C-IÁ Á ÁN halogen bonds link the independent molecules into one-dimensional chains and two-dimensional networks with subloops. In addition, the planar azaphenanthrenes lend themselves tostacking and C-HÁ Á Á interactions, leading to a diversity of supramolecular three-dimensional structural motifs being formed by these interactions. Luminescence studies show that co-crystals 1 and 2 exhibit distinctly different luminescence properties in the solid state at room temperature.

Chemical context
A halogen bond is an attractive non-covalent interaction between an electrophilic region in a covalently bonded halogen atom and a Lewis base. Halogen bonding (XB) is a powerful tool to assemble supramolecular materials and to promote chemical or biological molecular recognition (Desiraju et al., 2013;Cavallo et al., 2016;Gilday et al., 2015;Wang et al., 2016). Over the past few years, XB has been used successfully to assemble luminescent co-crystals (Liu et al., 2017a;d'Agostino et al., 2015;Ventura et al., 2014;Bolton et al., 2011). XB can play multiple roles in co-crystals, for example, as cement to assemble XB donors and acceptors together (Metrangolo et al., 2005), and, importantly, as a heavy-atom source to enhance phosphorescence or delayed fluorescence by efficient spin-orbital coupling (Gao et al., 2012). Phosphorescence or delayed fluorescence materials are very popular for preparing light devices because of the higher internal quantum efficiency of triplet excitons (Brown et al., 1993;Baldo et al., 1999). ISSN 2056-9890 Nitrogen heteroaromatic rings are a common type of luminescence or luminescent precursor materials. However, in general, it is difficult to use them to generate phosphorescence or delayed fluorescence. Haloperfluorobenzenes, as XB donors, have been used in attempts to assemble luminescence co-crystals with azaphenanthrenes (Gao et al., 2017;Wang et al., 2014Wang et al., , 2016Wang & Jin, 2017;Liu et al., 2017b). We report herein the use of tetraiodoethylene (TIE) as a new XB donor in the assembly of co-crystals with two different azaphenanthrenes, namely phenanthridine (PHN) and benzo[f]quinoline (BfQ), which is expected to tune their luminescence behaviour via a change of the co-crystal structures. Single crystal X-ray diffraction (XRD) data reveal that the two cocrystals of TIE with PHN and BfQ reported here have interesting structural properties and exhibit different luminescence behaviour from previous reports. TIE as a quadridentate XB donor allows the formation of three-dimensional halogenbonded networks with XB acceptors, PHN and BfQ. Using the conventional solution-based method, yellow co-crystals suitable for XRD measurement were obtained. The crystal structures of the co-crystals are mainly constructed by C-IÁ Á Á and C-IÁ Á ÁN halogen bonds. Other multiple intermolecular interactions, such asstacking, C-HÁ Á Á, C-HÁ Á ÁI as well as C-HÁ Á ÁH-C interactions, are also observed in the co-crystals.

Structural commentary
The asymmetric units of co-crystals 1 and 2 each comprise one half TIE molecule lying about an inversion centre and one PHN or BfQ molecule in a general position, hence the cocrystals have a 1:2 stoichiometry (Fig. 1). Co-crystal 1 crystallizes in the monoclinic space group C2/c while 2 crystallizes in the triclinic space group P1.

Powder X-ray diffraction pattern
The powder X-ray diffraction (PXRD) experiments were carried out for the title co-crystals using a Bruker D8-ADVANCE X-ray diffractometer (Cu K, = 1.5418 Å ) in the 2 range of 5 to 50 . As shown in Fig. 6, the experimental patterns for 1 and 2 match well with the spectra simulated from the XRD data, which confirms the purity of 1 and 2.

Luminescence behavior of co-crystals 1 and 2
As shown in Fig. 7, the two co-crystals fluoresce with some vibrational fine structure (see also spectroscopic data in Table 2). The two co-crystals also show delayed fluorescence (Fig. 8). For both co-crystals, the emission bands in the region of 450-480 nm should be relative to thestacking patterns. Luminescence from the excimer is possible because of the closestacking distances as shown in Figs. 2-5, besides luminescence from a monomer. Furthermore, TIE-PHN and TIE-BfQ produce weak phosphorescence. The strong XB interaction between the iodine atoms of TIE and the nonbonding orbitals of the azaphenanthrene N atoms should cause the energy of the lowest 1 (n, *) state to drop below that of the 3 (, *) state. It is supposed that for the singlet states the 0-0 transition of emitters in co-crystals is localized at 375 nm and 450 nm, respectively, and for triplet states the 0-0 transition is at about 600 nm. The energy gap between S 1 and T 1 is largely greater than 20 kJ mol À1 , so the delayed fluorescence most likely originates from the triplet-triplet annihilation process, named P-type delayed fluorescence (P-DF). Both delayed fluorescence and phosphorescence are relative to triplet states, so they should be significant for 440 Cui et al. 0.5C 2 I 4 ÁC 13 H 9 N and 0.5C 2 I 4 ÁC 13 H 9 N Acta Cryst. (2020). E76, 438-442 research communications Table 1 Hydrogen-bond geometry (Å , ) for (2).  (4)  127 Symmetry code: (i) x þ 1; y; z.

Figure 5
Crystal packing of 2. The two-dimensional network extends along two directions, by C-I2Á Á ÁH7 interactions andstacking in one direction, and by C-I2Á Á ÁH1 interactions andstacking in the other. improving the exciton emission efficiency of luminescence materials (Adachi et al., 2001). For the luminescence decay, all singlet state decay lifetimes (11.49 ns for 1 and 9.29 ns for 2) are about 10 ns, while the delayed fluorescence lifetime (4.36 ms for 1 and 6.45 ms for 2) is less than the 10 ms level because of the strong heavy-atom effect leading to a faster decay of the triplet state. Additionally, the phosphorescence is too weak to measure its decay lifetime. However, the phosphorescence lifetime can be estimated to be about 20 ms based on the relationship between P-DF and the accompanying phosphorescence (Parker et al., 1962(Parker et al., , 1965.

Figure 7
Total luminescence spectra of co-crystals (a) 1 and (b) 2 (excitation at 300 nm) measured under fluorescence mode.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms attached to C atoms were positioned geometrically and refined as riding on their parent atoms, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C).

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Tetraiodoethylene-phenanthridine (1/2) (1)
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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq I1 0.70990 (2) 0.95088 (10) 0.57060 (2) 0.04011 (16)  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.