The substituent effect on thermochromism of the isostructural N-salicylidene-2-aminopyridines series has been investigated in the solid state. It was found that the substituents affect the thermochromic phenomenon mainly in low temperatures and in a stronger way when the substitution takes place in the pyridine ring than in the salicylaldehyde ring.

1) The intramolecular hydrogen-transfer 'step occurs via a six-membered ring transition state. 2) The hydrogen-transfer produces quinoid structures. 3) The thermally stable forms are the ortho-Oil substituted aromatic structures. 4) One or more processes occur to cause some stabilization of the photocolored species formed by the endothermic photolysis step. 5) The thermochromic compounds display planar structures while the photochromic ones, non-planar structures. 6) Both photochromic and thermochromic compounds show (in their transfer reaction) a high degree of reversibility.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

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.

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.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Thermochromic N-(5-chlorosalicylidene)aniline was entrapped in a mesoporous silica designated as MCM-41 by grinding the twocomponents in the solid state. In the composite material the title compound lostits thermochromic properties and transformed to a photochromic tautomer with dramaticdifferences in the forward and backward ground state reactions with temperature.The observed change in colour with light is attributed to the cis -keto form ofthe title anil stabilized in the environment of the mesoporous MCM-41 material.

The dual phenomenon of photo/thermochromism has been observed for the first time in inclusion complexes of anils with β-cyclodextrin. Thus, the solid inclusion complex of the strongly thermochromic di-anil of 2-hydroxy-5-methyl-isophthalaldehyde with β-cyclodextrin has been found to be both photochromic and thermochromic contrary to other anils which, either photochromic or thermochromic, when complexed with β-cyclodextrin turn out to be only photochromic.

Photochromic and thermochromicanus have been encapsulated in an organic-inorganichybrid network, produced by theSol-Gel method. It was found that the photochromic anils have been transformed to thermochromic, something observed for the first time in rigid glasses. Spectroscopic investigations indicate a common infrastructure responsible for the coloured xerogel.

Di-anil of 2-hydroxy-5-methyl-isophthaldehyde was investigated for thermochromic and photochromic properties in the solid state. The compound has been found to exhibit remarkable thermochromic behavior, due to its structural characteristics, with strong contribution of the quinoid form in the ground state at room temperature.

Solvatochromism is found stronger in N-(2-hydroxybenzylidene)benzylamine (II) and N-(2-hydroxybenzylidene)-2-phenylethylamine (III) than in N-(2-hydroxybenzylidene)aniline (I) because of the increased basicity of the imine nitrogen in the ground state. Electronic absorption and FT-IR spectroscopic studies have provided evidence for an enol-keto tautomerism in all three compounds investigated in methanol, while both keto and protonated tautomers have been observed in their solutions in 2,2,2-trifluoroethanol and in 1,1,1,3,3,3-hexafluoro-2-propanol. The increase of the C=N stretching frequency upon protonation is found to be 21-18 cm(-1) in the compounds investigated in acidic solvents. The higher frequency of the quadrant doublet in the FT-IR spectrum of I is associated with the aromatic stretching vibrations of the aniline ring.

The binding of (1R)-(+)- and (1S)-(-)-alpha-pinene to alpha-, beta- and gamma-cyclodextrin and to the corresponding permethylated derivatives TM alpha-, TM beta- and TM gamma-cyclodextrin, has been studied in aqueous solution by NMR spectroscopy. The stoichiometries and the association constants have been measured, The signals of both (+)- and (-)-alpha-pinene were found to follow the Slow exchange, whereas those of the cyclodextrins the fast exchange regime, indicating that the type of exchange is independent of the magnitude of binding constants, which vary along the series, and also a result of other factors, in addition to the associated rate constants, The structures of the respective complexes have been derived from 2D ROESY experiments, The cyclodextrins preferentially bind with the (1S)-(-)-alpha-pinene, but only alpha-CD exhibits remarkable enantioselectivity, These observations show clearly that formation of a hydrogen bond or the presence of an aromatic ring, previously invoked as some of the requirements for enantioselectivity with CDs, are not necessary, In addition to enantioselectivity, site selectivity was also observed for alpha- and TM alpha-CD with the preferred (1S)-(-)-enantiomer, the stoichiometry being 2:1 in both cases.

Photochromic percinnamoylated cyclomaltoheptaose (beta-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.

The effects of complexation on thermochromic Schiff bases in β- and γ-cyclodextrin and their permethylated derivatives have been investigated in the solid state. The inclusion complexes formed have been found to be photochromic and fluorescent in the temperature range 298 K-77 K contrary to the photochromic Schiff bases which are fluorescent at low temperatures only.

Semiempirical calculations on cyclomaltoheptaose (βCD), 1,7-dioxaspiro[5.5]undecane (1), nonanal (2), and the inclusion complexes of βCD with 1 and 2 were carried out using the AM1 method. The structure of βCD after complete geometry optimization was in very good agreement with crystallographic structures. Moreover, the calculated dipole moment of βCD was found to depend strongly upon the orientation of the primary hydroxyl groups. Different possible positions of the guest molecules in the βCD cavity were examined, a few of them resulting in a gain of energy. These corresponded to partial inclusion of 1 from the secondary side but total inclusion of 2. Conclusions regarding the geometries of the complexes were in satisfactory agreement with the dominant structures in aqueous solutions as derived from NMR experiments. Thermodynamic data (ΔH0, ΔS0) in aqueous solutions were obtained from van't Hoff plots using 1H NMR, and were compared with the computed heats of formation. The forces responsible for host-guest association are discussed in the light of the above results.

The complete assignments of the H-1 and C-13 NMR spectra of alpha-, beta- and gamma-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.

0-salicylidenebenzylamine (II), N-salicylidene-2-phenylethylamine (III), their mono- and di-substituted derivatives, N-salicylidene-2-aminomethylpyridine (XIV) and N-salicylidene-2-aminoethyl-pyridine (XV) were investigated for their thermochromic and photochromic properties in the solid state and in solution, in the temperature range 298 K-77 K. II, III, XV, some of the monosubstituted derivatives of II and all the monosubstituted derivatives of III studied presented both properties, i.e they were both thermochromic and photochromic. The di-substituted derivatives were, practically, photochemically inactive. All the compounds investigated and found to be thermochromic, presented negative thermochromism in non-polar solvents and some in polar solvents also. Negative thermochromism in non-polar solvents was due to aggregation as it was mainly shown by the concentration dependence on the electronic spectra. The same also seemed true in polar proton donating solvents but the picture was diversified by solubility.

The complexation of alpha-, beta-, gamma-, tri-O-methyl-alpha-, tri-O-methyl-beta-, and tri-O-methyl-gamma-cyclodextrin (CD) with several constituents of the sex pheromone of the olive fruit fly, 1,7-dioxaspiro[5.5]undecane (1), nonanal (2), and ethyl dodecanoate (3), was studied in aqueous solution by NMR spectroscopy. Inclusion was clearly evident from the one-dimensional H-1 and C-13 spectra, whereas the proposed mutual disposition of the host and guest molecules was derived from two-dimensional POESY spectra and the respective stoichiometries. The structures of the complexes in conjunction with their measured association constants allowed for comparison of the behavior of the cyclodextrins. The complexing ability was thus significantly enhanced by methylation in the case of alpha-CD, moderately reduced for beta-CD, and largely diminished for gamma-CD. The retention of the nearly symmetrical CD shape combined with the enlargement of the hydrophobic cavity by methylation is considered to be responsible for the properties of tri-O-methyl-alpha-CD. The increased cavity deformation upon enlargement of the cavity diameter, however, further supported by the skeletal distortions detected for the larger methylated macrocycles, overrules the beneficial effects of the cavity increase, resulting in poorer performance for tri-O-methyl-beta-CD and an even poorer performance for tri-O-methyl-gamma-CD. Moreover, the methylated cyclodextrins share common features in their inclusion behavior: ah undergo conformational changes and all bind exclusively from their secondary side as opposed to normal cyclodextrins, which may use their primary side, too, depending on the guest.

The title compound exhibits in its crystalline state both photochromic and thermochromic properties. The crystal structure shows that the salicylaldimino part is planar to permit enol - keto isomerization and thus thermochromism. Moreover, the crystal packing permits the cis - trans isomerization of the keto species to form the photoproduct.

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.