Preparation , Spectroscopy , Physicochemical Properties and X-ray Structure Analysis of 3 , 4 , 5-Trimethoxy N-( 2-hydroxybenzylidene ) aniline

3,4,5-Trimethoxy N-(salicylidene) aniline was synthesized and characterized by IR spectroscopy, H NMR and x-ray single crystal analysis. UV-visible spectroscopy was used for physicochemical tests. X-ray data reveals that the crystalline network cohesion of this compound is essentially assured by CH/pi type hydrogen bounds. It crystallized in the orthorhombic Pbcn space group. Electronic spectroscopy shows that the title compound doesn't present photochromic property but thermochromic one. Intermolecular interactions analysis confirms for this compound a thermochromic structural predisposition.


Introduction
There have been a number of structural studies on crystalline N-salicylideneaniline and its substituted derivatives, in connection with the tautomerism by hydrogen transfer observed in the molecular materials.Indeed, they provide useful models for the investigation of molecular geometry and electronic to determine the mechanistic aspects of thermo induced (thermochromism) or photo induced (photochromism) colour change phenomenon, respectively with temperature or with ultraviolet irradiation.According the literature, in addition to the proton transfer, 180° rotation of the salicylidene moiety would be necessary for the demonstration of photochromic property [1][2][3][4] .Moreover, N-salicylideneanilines exhibit polymorphism and the existence of a wide range of different crystal packing modes among this class of compound induce us to focus not only on the role of the chromophore but to pay attention to topochemical aspects and crystal packing, in order to develop a global crystallochromy analysis 5 .Indeed, this approach would permit to better understand the mechanisms that orient a compound toward one or the other of these two properties in order to be able to predict them.In recent years the basic research on solid photochromic materials is growing since, in addition to scientific interest, there is considerable commercial interest in areas such as communications, computers, microfilm and display systems 6 .During the course of our systematic structural investigation of Schiff bases, it was suggested that the introduction of methoxy substituents with a large degree of floppiness into salicylideneanilines would be effective in producing photochromic crystals.Consequently, in this paper, we have synthesized the title compound introducing three methoxy substituents in the positions 3, 4 and 5 on the aniline phenyl ring for the determination of the effects of these substituents in these positions on the photochromism of Schiff bases.Experimental 2-Hydroxybenzaldehyde and 3,4,5-trimethoxyaniline were procured from Aldrich and used without further purification.All the solvents were dried before use.Melting points were determined in capillary tube using an MPD Mitamura Riken Kogyo (Japan) electrothermal melting point apparatus and are uncorrected.IR spectra in the range 4000-400 cm -1 were obtained on a Mattson 1000 FTIR spectrophotometer, with samples investigated as KBr disks.The 1 H spectra were recorded on a Bruker AMX-400 spectrometer, operating at 400 MHz.UV-visible spectra were obtained on an UV4 model of ATI UNICAM spectrometer.For the photochromic tests, the samples were radiated by an UV selected radiation (λ = 365 nm).

Synthesis
The title compound has been synthesized by reflux heating in dry ethanol (40 mL) of a mixture of 2-hydroxybenzaldehyde (4 mL) and 3,4,5-trimethoxyaniline (7 g) for about 1 h.The reaction mixture has been cooled in an ice bath until the title compound precipitated as a yellow solid.The solid was then recrystallized from ethanol, (yield: 90%, mp 80-81 o C).A good quality of single crystals was obtained by slow evaporation of a methanol solution.

X-ray analysis
The predictions on the basis of spectroscopic data are entirely borne out in the structure of the title compound, as determined by x-ray diffraction.Figure 1 shows the title molecule with the atom-numbering scheme.The crystal data and the selected bond lengths and angles are given in Tables 1 and 2, respectively.The dihedral angles between the different mean plans are given in Table 3.The weak value of α dihedral angle is in the usual ranges [7][8] and encourage the closeness of oxygen atom O 1 and nitrogen atom.Compared to the standards values proposed by Allen et al. 9 , the title compound with  The analysis of the crystalline stacking (Figure 2) watch that the two aromatic nucleus of the salicylidene moieties of the molecules (1 and 2), are in face to face configuration with 3.54Å mean distance.This short plan to plan distance observed inside this crystal, excluded all important rotation possibility of the salicylidene moieties for the demonstration of photochromic property.The relative short mean distance observed between the hydrogen atom H 4 attached to the C 4 carbon atom of the molecule 3 and the mean plan of the aromatic nucleus of the fragment 3,4,5-trimethoxybenzene of the molecule 1 (d = 2.78 Å), describe C-H ... π(arene) type interaction.According to several authors [10][11][12] , CH/pi interactions can play an important role in the orientation of the crystalline stacking and the molecular structures topographies.The crystalline stacking of the title compound shows around each 2 1 cristallographic symmetry axis « herring-bone » structures 13 .Along the C2 symmetry axis, the molecules are associated in infinite chain through C 4 -H 4 ... π hydrogen bonds.These CH/Pi interactions participate efficiently in the stabilization of the crystalline structure in this direction.In the centrosymetric dimer constituted by the molecules 1 and 4, one of the three hydrogen atoms linked to C 16 carbon atom of one molecule, is in interaction with the six carbon atoms of the aromatic moiety of the 3,4,5-trimethoxybenzene fragment of the other molecule.These other C-H ... π(arene) type interactions, with a shorter mean distance d = 2.65 Å, contribute efficiently to assure the cohesion between the different group of molecule in cluster around the different C2 symmetry axis.Consequently, every trimethoxylated phenyl ring is hired as hydrogen bonds acceptor, in a double hydrogen bonds interactions according to the diagram C-H … π … H-C.The three methoxy groups introduced on the aniline ring, particularly the central substituent in the out-of-plane conformation, don't seem to act as space opener in the crystal lattice to permit the demonstration of the photochromic property.

Physicochemical properties
UV-visible spectroscopy seems to be the best approach to analyze these properties.In order to allow the registration of the spectrum in solid state, sufficiently thin and transparent films of the compound were prepared on quartz plates.In the region 400-550 nm, UV-visible spectrum reveal at room temperature, a large absorption band named « thermochromic band » 2 .It characterizes the π system of the quinonic form (Q) presents in the solid state in balance with the phenolic tautomer (P).In order to verify the thermochromic property of the title compound, the test has been achieved at liquid nitrogen temperature (-196 °C).The spectrum illustrated by the Figure 3 shows clearly the disappearance of the "thermochromic band".This important result permits to affirm that the title compound presents thermochromic property.For the photochromic property test, the compound has been irradiated with λ = 365 nm radiation situated inside the π-π* electronic transition of its P form.At room temperature where the compound was irradiated, no modification of the spectrum has been observed.In these conditions we conclude that 3,4,5-trimethoxy N-(salicylidene) aniline is not a photochromic compound.

Supplementary material
Crystallographic data for the structural analysis have been deposited at the Cambridge Crystallographic Data Centre, CCDC No. 641476.Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CBZ1E Z, UK (Fax: +44-1223-336033; email: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).

Conclusion 3 , 4 , 5 -
Trimethoxy N-(2-hydroxybenzylidene) aniline was easily synthesized and characterised by spectroscopy methods.The stacking analysis reveals that the crystalline network cohesion is essentially controlled by CH/pi type hydrogen bounds.3.54 Å plan to plan mean distance observed inside this crystal excluded all important rotation possibility of the salicylidene moieties for the photochromic property demonstration.Uv-visible spectroscopy confirms that this compound presents exclusively thermochromic property.So, the introduction of methoxy substituents in the positions 3, 4 and 5 on the aniline nucleus doesn't have any effect on the photochromic porterty.