Eugene Hadjoudis

Photochromism and thermochromism of Schiff bases in the solid state: structural aspects

This tutorial review describes in a brief historical perspective the most important organic compounds that exhibit photochromism in the crystalline state since its discovery in 1867 up to now and considers in detail Schiff bases of salicylaldehyde with amines (anils). The latter comprise a chemical system undergoing hydrogen-atom tautomerism between enol and keto forms and show the phenomena of solid state photochromism and thermochromism. The system has been investigated extensively. Thus it has been shown that the photochromic property is a characteristic of the molecules but their chromobehaviour is influenced by the crystal structure of the compounds. Anils, apart from their fundamental interest, have potential for various applications.

Photochromism and thermochromism of solid trans-N,N′-bis-(salicylidene)-1,2-cyclohexanediamines and trans-N,N′-bis-(2-hydroxy-naphylidene)-1,2-cyclohexanediamine

Abstract The photochromic and thermochromic properties of trans - N , N ′-bis(salicylidene)-1,2-cyclohexanediamine ( 1 ), trans - N , N ′-bis(3,5-dichloro-salicylidene)-1,2-cyclohexanediamine ( 2 ), trans - N , N ′-bis(3,5-di- t -butyl-salicylidene)-1,2-cyclohexanediamine ( 3 ) and trans - N , N ′-bis(2-hydroxy-naphylidene)-1,2-cyclohexanediamine ( 4 ) were investigated by UV and fluorescence spectroscopies in the crystalline state at various temperatures and the molecular structures of 2 and 4 were determined by single-crystal X-ray diffraction. The existence of the two Schiff base groups on a single molecule does not seem to differentiate the chromobehavior of the present compounds (except possibly for 3 ) with respect to the usual Schiff bases of salicylaldehyde. It is suggested that for this class of compounds also, what determines the thermochromic behavior is the enhanced basicity of the nitrogen atom, due the absence of π,π- and n ,π-conjugation with an aryl ring bound to it. The role of the crystal structure in this case is important only in so far as it affects the electron density on the nitrogen atom. Photochromism, however, is structure dependent and requires space for the generation of the photoproduct that involves cis to trans isomerization in the excited state.

The crystal structure of the inclusion complex of cyclomaltoheptaose (β-cyclodextrin) with 4-tert-butylbenzoic acid

The crystal of the 1:1 complex of 4-tert-butyltoluene with cyclomaltoheptaose (beta-cyclodextrin, beta CD) is triclinic P1 with a = 15.562(2), b = 15.564(4), c = 15.835(3) A, alpha = 102.11(2), beta = 102.15(1), gamma = 103.64(2) degrees, V = 3505(1) A3, and Z = 2. The two independent beta CD molecules in the asymmetric unit form a dimer by hydrogen bonding involving HO-3, which accommodates two molecules of the guest. The hydrophobic guests are enclosed completely in the beta CD cavities with the tert-butyl groups in the hydrophobic region beneath the primary hydroxyl groups. The aromatic rings have two orientations and their toluene methyl moieties could not be located but were calculated to be at the interface of the two monomers. The dimers form channels along the c axis. The inter-dimer space is filled with 17 molecules of water distributed over 25 sites. A dense network of hydrogen bonds is formed, involving the beta CD hydroxyl groups and water molecules.

Inclusion complexes of cyclomaltoheptaose (β-cyclodextrin) and its methylated derivatives with the main components of the pheromone of the olive fruit fly

The inclusion complexes of cyclomaltoheptaose (beta CD) and heptakis(2,3,6-tri-O-methyl)cyclomaltoheptaose (TM-beta CD) with the four major components of the pheromone of the olive fruit fly (Dacus oleae), namely 1,7-dioxaspiro[5.5]undecane, (-)-alpha-pinene, nonanal, and ethyl dodecanoate, and the complex of heptakis(2,6-di-O-methyl)cyclomaltoheptaose (DM-beta CD) with 1,7-dioxaspiro[5.5]undecane were studied. The complexes were characterised in the solid state by differential scanning calorimetry and X-ray powder diffraction. In aqueous solution, the structure of the complexes was investigated by 1H NMR spectroscopy. In solution, 1,7-dioxaspiro[5.5]undecane, (-)-alpha-pinene, and nonanal enter the cavity of the cyclo-oligosaccharides. Association constants for some of these complexes were also measured. The complexes of ethyl dodecanoate did not provide evidence of their structure in solution. This was attributed to the existence of negligible amounts of these complexes in water due to the combined effects of low solubility and low association constant.

Structural Aspects of Permethylated Cyclodextrins and Comparison with their Parent Oligosaccharides, as Derived from Unequivocally Assigned 1H and 13C NMR Spectra in Aqueous Solutions

The complete assignments of the 1H and 13C NMR spectra of α‐, β‐ and γ‐cyclodextrins (CDs) and their permethylated derivatives were derived from dedicated experiments. Existing literature data were complemented with detailed analyses, including unequivocal assignment of the methoxy groups. The structures of the macrocycles in solution, as derived from the NMR data, resemble those in the solid state. The increased distortions observed for the larger permethylated CDs are expected to affect seriously their complexing ability.

Solid State Anil Photochromism

Abstract We report in this paper on the work in photochromisn and thermochromism of anils in our laboratory and put this work in context with the past. Thus, results with anils containing a hetero-atom in the aniline ring, anils with the thenylamine moiety instead of aniline and anils with the CH2 grouping between the nitrogen in the bridge and the aniline or the thenylamine group are compared with those of N-salicylideneanilines.

The crystal structure of the inclusion complex of cyclo-maltoheptaose (β-cyclodextrin) with 3,3-dimethylbutylamine

The crystal of the 1:1 complex of 3,3-dimethylbutylamine with cyclomaltoheptaose (beta CD, beta-cyclodextrin) is monoclinic C2 with a = 19.187(9), b = 24.56(1), c = 15.893(7) A, beta = 108.77(4) degrees, V = 7091 A3, and Z = 4. Two beta CD molecules, held together by intermolecular hydrogen bonds involving HO-3, form dimers, in the cavities of which two 3,3-dimethylbutylamine and two water molecules are accommodated. The guest molecule is completely enclosed in the cavity. The amino group is located at the secondary-hydroxyl-group side, and is hydrogen-bonded to the entrapped water molecules. The dimers form channels along the c axis. The inter-dimer space is filled with 10.7 water molecules that are distributed over 14 sites, and there is a dense network of hydrogen bonds involving the water molecules and the beta CD hydroxyl groups.

Photoactivity with in Cyclodextrin Cavities: Inclusion Complexes of Anils 1

Abstract Photochromic percinnamoylated cyclomaltoheptaose (β-cyclodextrin) solid inclusion complexes with guests N-salicylideneaniline and N-5-chlorosalicylideneaniline were prepared and characterized. The guests of these inclusion complexes can be easily “locked” with UV irradiation in the solid state by formation of cyclobutane bridges among the percinnamate residues rendering them more stable than before and stronger in fluorescence. The reverse reaction, rupture of the cyclobutane bridges, has not been achieved in the solid state and more work toward this goal is in progress. 1. Presented at the XVIIIth International Carbohydrate Symposium, Milan, Italy, July 21-26, 1996.

Solid State Photochromism of Anils

Abstract The anils of ortho-hydroxybenzaldehyde comprise a chemical system which exhibits the properties of solid state photochromism and thermochromism. These properties are exclusive in this system and therefore there is a problem which may be described as the relation between photochromism and thermochromism on the one hand and crystal structure on the other. Thus in this work the topochemical factors responsible for these properties are explored and “structural engineering” approach are applied in order to prepare solid anils which are photochromic and/or thermochromic at will. Literature data on crystal structure and photo-behaviour of anils are presented and discussed under the same light in order to have a more complete understanding of the problem.

Photoactivity within cyclodextrin cavities : inclusion complexes of anils with cyclodextrins

Abstract Complexation of the thermochromic compounds N-5-chloro-salicylideneaniline and N-salicylidene-2-aminopyridine with cyclodextrins results in disappearance of thermochromic properties and appearance of photochromism. From uv, visible and fluorescence spectra in the crystalline state and supporting evidence from NMR spectra in solution the structure of the complexes is proposed.