2-Cyano-5-({4-[N-methyl-N-(2-hydroxyethyl)amino] phenyl}diazenyl)pyridine

In the title compound, C15H15N5O, the benzene and pyridine rings make a dihedral angle of 30.86 (7)°. In the crystal, chains of molecules are wrapped around the screw axes into compressed helices, through hydrogen bonding between the hydroxy and cyano groups. The chains are linked by weak C—H⋯N and C—H⋯O interactions. The π conjugated unit of the molecule is almost perpendicular to the helix axis, and the formation of the helix is allowed by a gauche-type torsion angle in the hydroxyethyl tail. In this way, consecutive chromophore units along the chain are placed in a strict antiparallel arrangement, and this is energetically favoured because of the high dipole moment of the molecule.

Compound (I), 2-cyano-5-[4-(N-methyl-N-(2-hydroxyethyl)amino)-1-diazenylphenyl]pyridine, is a typical push-pull azo-dye, containing the dialkylamino as donor group and the cyano as acceptor. Moreover, the cyano group is attached to an electron poor pyridine ring, and this should increase the electron withdrawing character. This compound has been used in the synthesis of cross-linked systems showing both piezoelectric and quadratic NLO behaviour (Centore et al., 2012).
The molecular structure of (I) is shown in Fig. 1. The geometry around the donor N1 atom is substantially planar indicating sp 2 hybridization (the sum of valence angles at N1 is 360°) and the pattern of bond lenghts within the adjacent phenyl ring shows a certain degree of quinoidal character. All these structural features are in accordance with the expected π conjugation and push-pull character of the chromophore group.
The two aromatic rings are not coplanar, the dihedral angle between the mean planes being 30.86 (7)°; this twist, which is mainly due to a torsion around the bond N3-C10, is not expected to negatively affect the quadratic NLO performances of the molecule, as it has been proved both theoretically and experimentally for similar chromophores (Castaldo et al., 2002;Locatelli et al., 2005).
Nonlinear optical properties of (I) have been determined by means of electro-optical absorption spectroscopy measurements in dioxane solution (Centore et al., 2009). Relevant data are given in the Experimental part. We note that the dipole moment of the ground state is rather high and also the change in dipole moment between the ground and the first excited state is high, coherently with the expected charge-transfer character of the HOMO-LUMO transition. The quadratic NLO activity of (I), measured by the µβ 0 product, is also significant, if we consider the simple chemical structure of the compound, and is comparable with the NLO reference compound DANS (Singer et al., 1989).
Molecules in the crystal form rows through hydrogen bonds between hydroxy and cyano groups of consecutive molecules, Fig. 2 and Table 1. The chains, which have graph set symbol C 1 1 (17), are wrapped around crystallographic binary screw axes. Actually, the pitch of the helix (b=7.240 (4) Å) is very short, as compared with the length of the molecule (N5···O1= 14.818 (8) Å), so the helix is very compressed; in fact, the π conjugated unit of the molecule is almost perpendicular to the helix axis, and the formation of the helix is is allowed by a gauche-type torsion angle in the hydroxyethyl tail. In this way, consecutive chromophore units along the chain are placed in a strict antiparallel arrangement, and this is energetically favoured because of the high dipole moment of the molecule. Along (b+c) and a+b directions, the chains are held by weaker interactions involving C-H aromatic donor and pyridine N acceptor groups or C-H aliphatic donor and O acceptor groups ( Fig. 3 and Table 1). Two dimeric ring patterns can be recognized, having graph set symbols R 2 2 (16) and R 2 2 (8); both are formed across crystallographic inversion centers.
A search within CSD (version 5.33) (Allen, 2002) has shown that these patterns are unprecedented in compounds containing N-methyl-N-2-hydroxyethylamino and azopyridyl-benzene mojeties.

Refinement
The H atom of the hydroxy group was located in difmap. All other H atoms were generated stereochemically. All H atoms were refined by the riding model with U iso =1.2×U eq of the carrier atom (1.5 for H atoms of the methyl group).

Figure 1
ORTEP view of the molecular structure of (I). Thermal ellipsoids are drawn at 30% probability level.  Row of H-bonded molecules of (I).

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.