2-(3-Cyano-4-{3-[1-(2-hydroxyethyl)-3,3-dimethyl-1,3-dihydroindol-2-ylidene]prop-2-enyl}-5,5-dimethyl-5H-furan-2-ylidene)malononitrile

The title compound, C25H24N4O2, adopts a cisoid configuration and has twofold orientational disorder of the 2-hydroxyethyl group. The molecule is twisted from planarity so that the dihedral angle between the terminating indol-2-ylidene and the furan-2-ylidene moiety mean planes is 12.75 (7)°. Conformational disorder occurs at the indol-2-ylidene N atom, which results in two orientations for the hydroxyethyl group [occupancy ratio = 0.896 (2):0.104 (2)], and the hydroxy O atom of the 2-hydroxyethyl group is located over three sites [occupancy ratio = 0.548 (2):0.348 (2):0.104 (2)]. An intramolecular C—H⋯O hydrogen bond involving the lowest occupancy hydroxy O atom is observed. In the crystal, the molecules pack in parallel dimeric sheets about centres of symmetry, utilizing O—H⋯N(cyano), C—H⋯N(cyano) and O—H⋯O hydrogen bonds, in two sets parallel to (02-1) and (021) planes.

We thank Dr J. Wikaira of the University of Canterbury, New Zealand, for the data collection.

Comment
Organic nonlinear optical (NLO) chromophores containing donor (D) and acceptor (A) units have been widely reported in the literature as the enabling materials for a range of photonic devices Mao et al., 1998;Harper et al., 1999;Ma et al., 2002). We have previously reported a synthetic methodology (Kay et al., 2001a;Kay et al., 2001b, Teshome et al., 2009Kay et al., 2004;Smith et al., 2010) that allows entry to a number of high figure-of-merit NLO chromophores with aromatisable donors (e.g. 1,4-dihydropyridinylidene, 1,4-dihydroquinolinylidene), and containing the powerful acceptor 4,5,5-trimethyl-3-cyano-2(5H)-furanylidenepropane dinitrile (TCF). While this approach allowed for ease of synthesis and for a controlled increase in the extent of conjugation in the molecules, the resultant 'parent′ merocyanines are prone to significant amounts of aggregation (Teshome et al., 2009). As a continuation of this work, we have further developed our synthetic methodology, and extend the series to include chromophores with a nonaromatisable indoline donor group. Here we have synthesized a new NLO chromophore containing an indoline donor, an acceptor based on the well known moiety (2-(3-cyano-4,5,5-trimethyl-5H-furan-2-ylidene)-malononitrile), hereafter CTF, and a conjugated chain of three carbon atoms between the donor and acceptor. The chromophore also contains an hydroxyethyl substituent on the donor nitrogen atom which will allow for covalent attachment of the molecule to a polymer backbone, if needed, in the future.

Experimental
We have synthesized the title compound by following the procedure in Bhuiyan et al. (2011). Single crystals were grown by slow ether diffusion into a dichloromethane solution of the compound.

Refinement
A total of 4 reflections within 2θ 55° were omitted as being partially screened by the backstop. The 5-membered C14-C16,N4 ring was disordered over two sites [0.896 (2):0.104 (2)], giving two major orientations for the 2-hydroxy-ethyl substituents. The oxygen atoms of the hydroxy group on the major conformation (labelled A) were also disordered over two sites; a restraint was applied (SUMP) to ensure total occupancy of the hydroxy OH atoms (3 sites) was unity. To ensure reasonable connectivity, C25A-H and C25A-O bond lengths were restrained to be the same, and all hydroxy O2-H bonds were fixed at 0.84 Å. The hydroxy H atoms were located and refined with U iso = 1.5U eq (O). Terminal atoms on the 2-hydroxy ethyl substituents except O2A1 were refined with isotropic linked thermal parameters. Hydrogen atoms bound to carbon were constrained to their expected geometries (C-H 0.98, 0.99 Å): all methyl and tertiary H atoms were refined with U iso 1.5 or 1.2 times respectively that of the U eq of their parent atom.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.