A polymorph of 2,4-dinitro-phenyl-hydrazine.

The crystal structure of a previously unreported polymorph (form II) of 2,4-dinitro-phenyl-hydrazine (DNPH), C6H6N4O4, was determined at 90 K. The first polymorph (form I) is described in the monoclinic space group P21/c [Okabe et al. (1993 ▶). Acta Cryst. C49, 1678-1680; Wardell et al. (2006 ▶). Acta Cryst. C62, o318-320], whereas form II is in the monoclinic space group Cc. The mol-ecular structures in forms I and II are closely similar, with the nitro groups at the 2- and 4-positions being almost coplanar with the benzene ring [dihedral angles of 3.54 (1) and 3.38 (1)°, respectively in II]. However, their packing arrangements are completely different. Form I exhibits a herringbone packing motif, whereas form II displays a coplanar chain structure. Each chain in form II is connected to adjacent chains by the inter-molecular inter-action between hydrazine NH2 and 2-nitro groups, forming a sheet normal to (101). The sheet is stabilized by N-H⋯π inter-actions.

Excited-state intramolecular proton transfer and charge transfer in 2-(2'-hydroxyphenyl)benzimidazole crystals studied by polymorphs-selected electronic spectroscopy.

Crystal structures of polymorphs of 2-(2'-hydroxyphenyl)benzimidazole (HPBI), Forms α and ß, are analyzed by X-ray crystallography. The fluorescence excitation (FE) and fluorescence spectra of the polymorphs are separately observed at temperatures 77-298 K. It has been found that the electronic spectra of the two crystal forms are significantly different from each other. Photo-excitation of the enol forms in Forms α and ß induces the excited-state intramolecular proton transfer (ESIPT) to produce the S(1) state of the keto forms. In the FE spectra of Forms α and ß, the S(1) ← S(0) (ππ*) transition of the keto form is observed in the 360-420 nm region in addition to that of the enol form in the 250-420 nm region. In the FE spectrum of Form ß a new band peaking at 305 nm is observed, which is assigned to the S(1) ← S(0) transition of a non-planar enol form based on the observation of dual fluorescence in the UV and visible regions and quantum chemical calculation on the transition energy against the twisted angle between the benzimidazole and hydroxyphenyl rings. The fluorescence quantum yield (φ(T)) for the keto form is remarkably dependent on polymorphs at room temperature; φ(T) = 0.53 for Form α is much larger than φ(T) ≤ 0.23 for Form ß. At 77 K the φ(T) values for Forms α and ß increase to 0.67 and ≤0.57, respectively. The changes in the φ(T) values are associated with the intramolecular charge transfer (ICT) state. The potential barrier height between the S(1)-keto and S(1)-ICT states is significantly lower for Form ß than for Form α. At 77 K the S(1)-keto → S(1)-ICT process followed by S(1)-ICT → S(0)-keto internal conversion is significantly suppressed in Form ß. We compare difference in the dynamics between Forms α and ß in the electronic ground and excited states.

(S)-Alanine-(S)-2-phen-oxy-propionic acid (1/1).

In the title co-crystal, C(3)H(7)NO(2)·C(9)H(10)O(3), the (S)-alanine mol-ecule exists in the zwitterionic form stabilized by two pairs of N(+)-H⋯O(-) hydrogen bonds and an electrostatic inter-action between the ammonium center and the carboxyl-ate anion, forming a sheet along the ab plane. The carboxyl group of the (S)-2-phen-oxy-propionic acid mol-ecule is connected to the top and bottom of the sheet via N(+)-H⋯O=C and O-H⋯O(-) [R(2) (2)(7) graph set] hydrogen bonds, giving an (S,S)-homochiral layer, in which both methyl groups of (S)-alanine and the phenyl rings of (S)-2-phen-oxy-propionic acid are oriented in the same direction along the b axis.

1,4-Bis(1,1-dimethyl-prop-yl)-2,5-dimeth-oxy-benzene.

The title compound, C(18)H(30)O(2), was prepared by Friedel-Crafts alkyl-ation of 1,4-dimeth-oxy-benzene with 2-methyl-2-butanol. The complete mol-ecule is generated by the application of a crystallographic centre of inversion. The two meth-oxy groups are oriented in the same plane of the aromatic ring [C-C-O-C torsion angle = 9.14 (16)°]. While one methyl group of the tert-pentyl substituent is coplanar with the benzene ring [C-C-C-C = 0.45 (15)°] and lies towards the less-hindered H atom, the other methyl and ethyl groups are directed to either side of the benzene ring [C-C-C-C torsion angles = 118.78 (12) and 59.11 (14)°, respectively]. In the crystal, the hydro-phobic mol-ecules pack to form a brick-wall-like architecture.

Photochromism of polymorphic 4,4'-methylenebis(N-salicylidene-2,6-diisopropylaniline) crystals.

4,4[prime or minute]-Methylenebis(N-salicylidene-2,6-dialkylaniline) derivatives were prepared and their structures were determined by (1)H NMR, IR, DSC and X-ray crystallographic analyses. The 2,6-diisopropylaniline derivative yielded definite polymorphic crystals: the space groups of the crystals were C2/c and P2(1)/n, respectively. It was found that both polymorphs were similarly photochromic but the thermal stability of the photochrome was different due to the variance of the shape of molecule and cavity in each crystal.

Crystalline photochromism of N-salicylidene-2,6-dialkylanilines: advantage of 2,6-dialkyl substituents of aniline for preparation of photochromic Schiff base crystals.

N-(3,5-Dihalosalicylidene)-2,6-dialkylaniline derivatives were prepared and their structure and photochromic properties were investigated. From the X-ray crystallography, it was revealed that steric repulsion between the azomethine hydrogen atom and the alkyl groups at the 2,6-positions of the aniline ring lead to non-planar molecular structure, which was effective for the crystals to exhibit photochromism. The relationship between photochromicity and crystal packing of N-(3,5-dichlorosalicylidene)-2,6-dialkylanilines series was also discussed. The 2,6-dialkylaniline method was suggested to be applicable to the preparation of photochromic Schiff bases with various substituents in the salicylidene rings.

Deuterium isotope effect on the solid-state thermal isomerization of photo-coloured cis-keto species of N-salicylideneaniline.

Deuterium isotope kinetic effect in the solid state was observed by using thermal fading reaction of photo-coloured species derived from N-salicylideneaniline deuterohydroxyl derivative; thus, the existence of a cis-keto form in the photo-coloured Schiff base crystals was suggested experimentally along with 6-methyl-substitution effect on the stability of the photoproduct.

The Proton Cryptate of Hexaethylenetetramine.

The next higher homologue of hexamethylenetetramine was synthesized as the proton cryptate H+ @1⋅Br- (shown schematically), and its X-ray structure determined. The proton trapped by the lone pairs accumulated at the center of the T-symmetric tetraaza cage could not be exchanged or removed, even after heating for three days in 3 M NaOD.