Crystal structure and fluorescence study of (μ-N-[(3,5-dimethyl-1H-pyrrol-2-yl)methylidene]-N-{4-[(3,5-dimethyl-1H-pyrrol-2-yl)methylideneazaniumyl]phenyl}azanium)bis[difluoridoboron(IV)]

In the title compound, each boron atom is four-coordinated by two fluorine atoms, a pyrrole N atom and an imine N atom. Both imine CH=N groups adopt a trans conformation. In the crystal, the molecules self-assemble into a pillar structure through C—H⋯F hydrogen bonds and π–π interactions.


Chemical context
Fluorescent materials are gradually becoming a necessity in modern chemistry and biology because of their unique advantages in the characterization of life activities in living organisms (Zhang et al., 2019). Boron-dipyrromethene (BODIPY) is a frequently reported fluorescent structure (Boens et al., 2015). Its planar structure endows BODIPY compounds with strong fluorescence emission under the action of excitation light. Such compounds also have high molar absorption coefficient, good light stability and excitation wavelengths in the visible to near infrared region. In addition, their structures can easily be modified and they are not easily affected by the environment (Loudet & Burgess, 2007). The success of BODIPY dyes has led to research on similar structures such as aza-BODIPY structures (Bodio & Goze, 2019), boron complexes of iminopyrrolide ligands (BOIMPY; Suresh et al., 2012Suresh et al., , 2015Lee et al., 2016), bis(difluoroboron)-1,2-bis{(pyrrol-2-yl)methylene}hydrazine (BOPHY; Boodts et al., 2018) structures and other novel organoboron fluorescence materials (Frath et al., 2014).
BOIMPY has a similar structure to BODIPY, in which the pyrrole ring is located in the same plane as the aromatic ring, the boron atom and the methylene bridge. More importantly, BOIMPY has the advantage of lower molecular symmetry, which can overcome the shortcoming of the short Stokes shifts of BODIPY (Lee et al., 2016). In contrast to BODIPY, studies on BOIMPY are still rare. Herein, we report the synthesis, crystal structure and spectroscopic properties of a new BOIMPY compound, bis(difluoroboron)bis(pyrrol-2-yl)methylenediaminophenylene.

Supramolecular features
In the crystal, the molecules are linked by C6-H6BÁ Á ÁF2 hydrogen bonds between methyl group and the fluorine atom (Table 1), andinteractions between benzene rings [Cg1Á Á ÁCg1(Àx + 1, y + 1 2 , Àz + 1) = 3.7521 (2) Å ; Cg1 is the centroid of the C8-C13 ring] into one-dimensional pillars along the b-axis direction. Within the pillar, neighbouring molecules are oriented in opposite directions (Fig. 2). The pillars are held together by van der Waals interactions, forming a herringbone structure. A perspective view of the crystal packing within the unit cell is depicted in Fig. 3.

Figure 3
Part of the packing diagram for the title compound. TUJFOV (Suresh et al., 2015)]. In their crystals, the respective dihedral angles between the 2-formiminopyrrolyl unit and the phenyl ring are À47.2 (3) and 46.1 (11) .

UV-vis spectrum and fluorescence spectra
The UV-vis spectrum and fluorescence spectra of the title compound are shown in Figs. 4 and 5, respectively. The UV-vis spectrum is solvent independent. A THF solution of the title compound displays intense broad absorption at 474 nm, which can be assigned to the n-* transition of the iminopyrrolyl group. The title compound has two emission peaks at 528 nm and 574 nm. It can be seen that the fluorescence intensity of title compound is greatly affected by the solvents. In the polar solvent DMSO, the fluorescence intensity is much weaker than that in the apolar solvent CHCl 3 , which is similar to a previous report (Li et al., 2018). The title compound shows substantial bathochromic shifts in both absorption and emission when compared to the diphenylboron analogues reported by Gomes and coworkers (Suresh et al., 2012), which can be ascribed to the planar structure of the title compound.

Synthesis and crystallization
To a solution of bis(pyrrol-2-yl)methylenediaminophenylene (1 mmol, 0.32 g) and triethylamine (4.2 mmol, 6 mL) in dry dichloromethane (15 mL) was slowly added boron trifluoride ethyl ether (7.2 mmol, 2 mL). The resulting solution was stirred overnight, and then saturated potassium carbonate solution was added and stirred for 30 minutes. The resulting solution was extracted and evaporated under vacuum to dryness. The residue was purified by column chromatography eluting with CH 2 Cl 2 and petroleum ether (v:v 1:2) to give an orange product, m.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2

Figure 4
UV-vis spectrum of the title compound in THF solution (1 Â 10 À5 M).

Figure 5
Fluorescence spectra of the title compound in different solutions (1 Â 10 À5 M).
Idealized methyl groups were refined as rotating groups.

Funding information
Funding for this research was provided by: National Natural Science Foundation of China (award No. 21172174).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (