supplementary materials

The design of organic polar crystals for quadratic non-linear optical applications is supported by the observation that the organic molecules containing π-electron systems asymmetrized by electron donor and acceptor groups are highly polarizable entities in which problems of transparency and crystal growth may arise from their molecular crystal packing (Pecaut & Bagieu-Beucher, 1993). It is known that nitrophenols act not only as π-acceptors to form various π-stacking complexes with other aromatic molecules, but also as acidic ligands to form salts through specific electrostatic or H-bonding interactions (In et al., 1997). The bonding of electron-donor acceptor complexes strongly depends on the nature of the partners. The linkage could involve not only electrostatic interactions, but also the formation of molecular complexes (Zadrenko et al., 1997). It has been reported that proton transferred thermochromic complexes were formed between phenols and amines in apolar solvents at low temperature if an appropriate H-bonding network between the phenols and amines was present to stabilize it (Mizutani et al., 1998). Pyridinium picrate has been reported in two crystalline phases and it appears in both phases as an internally linked H-bonded ion pair. These two phases are referred to as molecular crystals rather than salts based on their structural arrangements (Botoshansky et al., 1994). A similar structural arrangement has also been reported for 4-dimethylaminopyridinium picrate (Vembu et al., 2003). The monoclinic polymorph of the title compound (CSD Reference Code: REYDEE) has been reported previously (Takayanagi et al., 1996). We have structurally elucidated the orthorhombic polymorph of the title compound as a forerunner to assessing its optical properties and report its structure here.

The asymmetric unit of (I) contains one N,N-Dimethylanilinium cation, and one 2,4,6-trinitrophenolate anion. (Fig.1). The crystal structure of (I) is stabilized by N—H···O and C—H···O interactions. The range of H···O distances (Table 1) found in (I) agrees with those found for N—H···O (Jeffrey, 1997) and C—H···O hydrogen bonds (Desiraju & Steiner, 1999). The N7—H7···O16 and N7—H7···O19 interactions form a pair of bifurcated donor bonds that link the N,N-dimethylanilinium cation and 2,4,6-trinitrophenolate anion and also form a motif of graph set R²₁(6) (Bernstein et al., 1995; Etter, 1990). Another pair of bifurcated donor bonds consists of the C2—H2···O16 and C2—H2···O24 interactions that also link the cation and the anion and form a R²₁(6) motif. The C8—H8A···O19 and C9—H9C···O19 interactions constitute a pair of bifurcated bonds forming a R¹₂(6) motif that link the cation and the anion. The N7—H7···O19 and C8—H8A···O19 interactions constitute a pair of bifurcated acceptor bonds that form a R¹₂(5) motif. The above two motifs, R¹₂(6) and R¹₂(5), together form a R¹₂(5) motif by the interplay of the trifurcated acceptor bonds formed by N7—H7···O19, C9—H8A···O19 and C9—H9C···O19 interactions. There are five intermolecular C—H···O interactions (Table 1) that contribute to the supramolecular aggregation of the title compound. The intramolecular N—H···O interactions mentioned above also contribute to the formation of cooperative H-bonded network (Fig. 2). There is an offset π···π stacking interaction, Cg1···Cg2 (x, -1+y, z) at 3.697Å with α = 3.19, β = 24.88 and γ = 24.00 and the two perpendicular distances being 3.377 and 3.354Å. There is also an edge to face π···π stacking interaction, Cg1···Cg2 (1.5-x, -0.5+y, 0.5+z) at 3.673Å with α=13.75, β=25.68 and γ=12.78 and the two perpendicular distances being 3.582 and 3.310Å. Cg1 and Cg2 are the centroids of the C1···C6 and C10···.C15 rings.

The interplay of strong N—H···O and weak C—H···O, π···π interactions with different strengths, directional preferences and distance presents a complex mosaic of interactions. The three dimensional arrangement of the 2,4,6-trinitrophenolate and N,N-dimethylanilinium moieties in the unit cell, shows that the title compound is an internally linked hydrogen bonded ion pair and hence can be regarded as a molecular crystal rather than a salt.