(E)-2,6-Dibromo-4-{2-[1-(1H,1H,2H,2H-perfluorooctyl)pyridinium-4-yl]ethenyl}phenolate methanol disolvate, a fluoroponytailed solvatochromic dye

The title compound crystallizes as a methanol disolvate and exhibits short hydrogen bonds and a disordered perfluoroalkyl chain.


Chemical context
Dyes with a fundamental type of conjugated system as in the title compound have long been known (Hü nig & Rosenthal, 1955). It was intended to combine the structural features of a delocalized -electron system with those of polyfluorinated compounds in order to derive a new material with advantageous properties such as altered solubility (Hoang & Mecozzi, 2004) and affinity (Wagner et al., 2016) profiles, given that the physical and chemical properties of organic compounds are strongly affected by the introduction of fluorinated substituents. Fluorosurfactants have a tendency towards micelle formation in biphasic or ternary solvent mixtures. Thus, the utilization of solvatochromic surfactants as self-indicating micelle reporters (Kedia et al., 2014) is an attractive analytical concept for fluorous-phase-related materials science.

Structural commentary
The title compound comprises a delocalized -electron system, involving either a zwitterionic benzoid or a non-polar ISSN 2056-9890 quinoid resonance structure. Inspection of bond lengths leads to the conclusion that it is not a typical cyclohexadienone system (Chandran et al., 2008;Chiverton et al., 1991) but rather a benzoid system similar to 2,6-dibromophenol predominant (Eriksson & Eriksson, 2001;Lu et al., 2011;Lehmler & Parkin, 2005). The heterocyclic ring also resembles a typical pyridinium system. Furthermore, the shortest C C bond in the bridge linking the two rings is between C6 and C7 with a length of 1.337 (6) Å , whereas the adjacent bonds are considerably longer. The framework thus is not quinoid but benzoid. The conjugated moieties of the dye molecule are almost planar and the mean planes of the benzene and pyridine rings form an angle of 2.97 (2) , whilst the fluorinated chains protrude from the plane.
The carbon atoms C17-C21 and fluorine atoms F3-F13 of the polyfluorinated tail are disordered over sets of sites with an occupancy ratio for the two disorder fragments of 0.538 (7):0.462 (7). The chain adopts a slightly helical conformation (Fournier et al., 2010) with an average C-C-C-C twist angle (deviation from 180 ) of 3 . Typically, -electron donor-acceptor-substituted conjugated systems exhibit solvatochromism. Solutions of the title compound display absorption maxima at 610 nm (blue) in THF and 502 nm (red) in MeOH. Here, increased solvent polarity leads to higher transition energy (negative solvatochromism). A quinoid system based on 2,6-dibromophenol displaying positive solvatochromism has been reported previously (Laus et al., 2003).

Supramolecular features
The three components of the title compound are linked into a finite hydrogen-bonded chain. The two solvent molecules are connected by an O1S-H1SÁ Á ÁO2S bond, and additionally the interaction O2S-H2SÁ Á ÁO1 links the second solvent molecule with the main molecule (Table 1, Fig. 1). In addition, there are significantstacking interactions between the benzene and pyridine rings. These are weakly connecting in the b-axis direction. Centroid-centroid distances Cg1Á Á ÁCg2 i and Cg1Á Á ÁCg2 ii are 3.525 (3) and 3.605 (3) Å , respectively [Cg1 and Cg2 are the centroids of the benzene and pyridine rings, respectively; symmetry codes: (i) 1 À x, À 1 2 + y, 3 2 À z; (ii) 1 À x, 1 2 + y, 3 2 À z]. The packing of the molecules is displayed in Fig. 2.

Figure 2
View of the planar chromophore moieties and the attached perfluoroalkyl chains. Thestacking interactions between the benzene and pyridine rings are shown in red dashed lines. Solvent molecules are omitted for clarity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were identified in difference maps. Methyl H atoms were idealized and included as rigid groups allowed to rotate but not tip and refined with U iso (H) set to 1.5U eq (C) of the parent carbon atom. All other H atoms bonded to carbon atoms were positioned geometrically and refined with U iso (H) set to 1.2U eq (C) of the parent carbon atom. Hydrogen atoms in OH groups were refined with restrained distances [O-H = 0.84 (1) Å ] and their U iso parameters were refined freely.
The terminal C 5 F 11 unit of the polyfluorinated tail was found to be disordered over two orientations. The two disorder components, each consisting of 16 atomic positions, were refined using 401 distance restraints (SADI) for chemically equivalent C-C, C-F and FÁ Á ÁF bonds, and the final occupancy ratio was 0.538 (7):0.462 (7). All disordered atoms were refined anisotropically. The extension of the modelled disorder increased the number of refined parameters substantially. Consequently, the obtained data/parameter ratio is lower than normally expected.   (Farrugia, 2012); software used to prepare material for publication: Mercury (Macrae et al., 2008). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.74 e Å −3 Δρ min = −0.72 e Å −3 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.