2-[(3,3-Dimethylindolin-2-ylidene)methyl]-4-[(3,3-dimethyl-3H-indol-1-ium-2-yl)methylidene]-3-oxocyclobut-1-en-1-olate chloroform disolvate

In the title squaraine dye solvate, C26H24N2O2·2CHCl3, the dye molecule is essentially planar, except for the methyl groups, having a maximum deviation over the 26-membered delocalized bond system of 0.060 (2) Å. It possesses crystallographic twofold rotational symmetry with the indole ring systems adopting a syn conformation. The molecular structure features intramolecular N—H⋯O hydrogen bonds enclosing conjoint S7 ring motifs about one of the dioxocyclobutene O atoms, while the two chloroform solvent molecules are linked to the second O atom through C—H⋯O hydrogen bonds.

In the title squaraine dye solvate, C 26 H 24 N 2 O 2 Á2CHCl 3 , the dye molecule is essentially planar, except for the methyl groups, having a maximum deviation over the 26-membered delocalized bond system of 0.060 (2) Å . It possesses crystallographic twofold rotational symmetry with the indole ring systems adopting a syn conformation. The molecular structure features intramolecular N-HÁ Á ÁO hydrogen bonds enclosing conjoint S7 ring motifs about one of the dioxocyclobutene O atoms, while the two chloroform solvent molecules are linked to the second O atom through C-HÁ Á ÁO hydrogen bonds.
Evaporation of a solution of the dye in chloroform gave the title compound solvate as large green-black crystal prisms and its crystal structure is reported on herein. The dye molecule adopts the uncommon syn-conformation with respect to the indolenine rings, having crystallographic twofold rotational symmetry (Fig. 1). The structures of all other members of this series of N-alkyl-substituted squaraine dyes have the inversion-related anti-conformation.
The planarity of the delocalized 26-membered linked ring system in the overall molecule is indicated by maximum deviations of 0.059 (2) (C6 and C6 i ) and 0.060 (2) (C4 and C4 i ) from the least-squares plane [symmetry code (i): -x, y, -z + 3/2]. This planarity is further stabilized by the intramolecular N-H···O hydrogen bonds to O2 of the dioxocyclobutene ring (Table 1), closing conjoint S7 ring motifs.
Inter-species C-H···O hydrogen-bonding interactions link the two chloroform molecules to the second O-atom (O1).

Experimental
Squaric acid (200 mg, 1.75 mmol) was added to 2.0 molar equivalents of 2,3,3-trimethylindolenine (0.56 g, 3.5 mmol) and quinoline (0.45 g, 3.5 mmol) in a 1:1 v/v mix of 1-butanol:toluene (30 ml) and the mixture was then refluxed for 16 h using a Dean and Stark apparatus. Upon cooling, metallic green crystals were collected in vacuo, washed with petroleum ether (60/40), and were used without further purification [Yield: 0.31 g (45%)]. Spectroscopic data are available in the archived CIF. For the X-ray diffraction analysis large green-black lustrous crystal prisms of the title compound were obtained from the room temperature evaporation of a solution of the dye in chloroform. A cleaved crystal specimen was used for the actual analysis.

Refinement
The H atom of the N-H group was located in a difference Fourier but was subsequently refined as a riding atom: N-H = 0.88 Å with U iso (H) = 1.2U eq (N). C-bound H atoms were included in calculated positions and refined as riding atoms: C-H = 0.93 Å (aromatic or ethylenic), 0.96 Å (methyl) or 0.98 Å (methine) with U iso (H) = 1.5U eq (C-methyl) and = 1.2U eq (C) for other H atoms.

Figure 1
The molecular conformation and atom-numbering scheme for the title compound (symmetry code: (i) -x, y, -z + 3/2). The displacement ellipsoids are drawn at the 40% probability level. The intra-and inter-species N-H···O and C-H···O hydrogen bonds are shown as dashed lines.  Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.