l-Tryptophan 4-nitrophenol trisolvate

The title compound, C11H12N2O2·3C6H5NO3, comprises a zwitterionic amino acid formed by two nearly planar groups: (i) the indole ring and Cβ, and (ii) the carboxyl group, Cα, as well as the amine N atom, with r.m.s. deviations of 0.0084 and 0.0038 Å, respectively. The angle between these idealized planes is 39.47 (9)°. The amine group of the amino acid is in a syn (−sc) arrangement relative to the ring system. The overall crystal structure results from the packing of sheets parallel to the (001) planes. These sheets are formed by a pair of screw axis related parallel networks bound by hydrogen-bond and π–π stacking interactions. The intermolecular cohesion of all organic residues in each of the latter two-dimensional networks is achieved via strong hydrogen bonding, nitro–π and π–π stacking interactions.

supplementary materials . E68, o920 [doi:10.1107 Non-linear optical phenomena (NLO) form the basis for a wide range of devices including frequency doublers, electrooptic modulators, optical limiters, high speed optical gates and parametric amplifiers. Organic molecules containing π electron systems asymetrized by donor and acceptor groups are highly polarizable entities which may give rise to organic polar crystals for NLO applications. The properties of individual molecules and their organization in the bulk crystalline structure are the key factors that determine the properties of the resulting molecular materials. An essential condition to obtain even-order NLO processes in materials is a noncentrosymmetric crystal structure. However, optimal molecular orientations are required if appreciable effects are to be achieved in molecular materials (Chemla & Zyss, 1987;Zyss & Ledoux, 1994;Zyss & Nicoud, 1996). In this context the crystal structure of the title compound resulting from cocrystallization of a chiral molecule, L-tryptophan, and p-nitrophenol, which has a high ground state dipole moment is reported. No other crystal structures with three neutral independent p-nitrophenol molecules were found in a search to the CSD database, version 1.13 (Allen, 2002).
The aminoacid is a zwitterion with overall neutral charge, which means that no Brønsted-Lowry acid-base reaction has occurred between L-tryptophan and p-nitrophenol molecules; this is further confirmed by inspection and comparison of all three C-O distances of the p-nitrophenol molecules which range from 1.341 (3) Å to 1.351 (3) Å.
The L-tryptophan has a conformation similar to the one found in the L-form of DL-formate (Bye et al., 1973), with the amine group in a syn (-sc) arrangement relative to the indole and, as in most cases (Bakke & Mostad, 1980), with C1 trans to C4. The zwitterion can be divided in two planar groups forming an angle of 39.47 (9)°. The first group consists of the indole ring and C3; the second group includes the carboxyl group and the amine N atom. Root mean square deviations of the atoms in these latter planes are 0.0084 and 0.0038 Å, respectively.
Hydrogen bonded two-dimensional networks, parallel to the (001) planes, involving all four independent molecules, can be found in the crystal structure of the title compound. Figure 2 shows such a two-dimensional network of H-bonded, nitro-π and π-π stacked molecules. In the latter networks, chains formed by the aminoacid and one p-nitrophenol (X41-X46, where X stands for C, N or O) running along the a axis can be found. These chains are linked through two H-bonds: N1-H1A···O41 i or N1-H1A···O42 i and O43-H43A···O2 iv . Other chains, formed by the same aminoacid and p-nitrophenol molecules, with a more mirregular shape, run along the b axis and are also connected through two H-bonds: the same bifurcated N1-H1A···O41 i or N1-H1A···O42 i and N2-H2A···O42. The other two p-nitrophenol molecules (X21-X26 and X31-X36, where X stands for C, N or O) can be seen as dimmers, H-bonded via O23-H23A···O32 iv , that connect to the latter mdescribed base of perpendicularly running chains through H-bonds N1-H1B···O21, N2-H2A···O22 iii and O33-H33···O2 iii . All but one of the H-bonds found are used to build the two-dimensional network, which includes all the molecules present in the assymetric unit. Nitro-π and π-π interactions further stabilize the stacking of the latter dimmers and the indole ring along the a axis, via N31-O31···Cg C21-C26 v , N21-O21···Cg pyrrole , N21-O21···Cg ind. benz. and correspondent π-π intermolecular contacts. Geometric details of the π-π and nitro-π interactions can be found in tables A supplementary materials sup-2 Acta Cryst. (2012). E68, o920 and B. An H-bonded and also π-π stacked pair of the latter two-dimensional networks (screw axis related to each other), constitutes a sheet (parallel to the (001) planes); the relevant intermolecular contacts in the binding of this pair of neighbouring two-dimensional networks are the H-bond N1-H1C···O1 ii and π-π stacking interactions Cg ind. benz. ···Cg C21-C26 vi and Cg C41-C46 ···Cg C41-C46 vii (geometric details can be found in table A). The overall three-dimensional structure can be viewed as a repetition along the c axis of the latter sheets, as shown in Fig. 3.
Optical second harmonic generation (SHG) was measured on polycrystalline samples using the standard Kurtz and Perry technique (Kurtz and Perry, 1968). The material was particle sized using a set of standard microsieves (Retsch) having mesh width between 50 µm and 160µm. The sample cell consisted of a microscope slide with a depression of 3 mm diameter and 0.5 mm thickness covered with a flat microscope slide. The generated second harmonic signal was compared with that generated by polycrystalline urea with the same grain size and similar sample preparation. The Ltryptophan tris(4-nitrophenol) SHG response was twice that of urea.

Experimental
Single crystals were produced by dissolving 5.0 mmol of L-tryptophan and 10 mmol of p-nitrophenol in 30 ml of hot methanol. Yellow needles were formed by slow cooling at room temperature.

Refinement
The structure was solved by direct methods using SHELXS97 (Sheldrick, 2008). All H(C/N) atoms were placed at The refined model structure is non-centrosymmetric with only atoms which are poor anomalous scatterers for the wavelength used, therefore Friedel pairs were merged before the final refinement.   Representation of the two-dimensional networks, paralell to the (001) planes, of H-bonded molecules. Different sorts of chains can be individualized. Nitro-π and π-π stacking are also evident.   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.14 e Å −3 Δρ min = −0.15 e Å −3

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O1 0.53801 (10