Josef Kuthan

Novel preparation and photochromic properties of 2,4,4,6-tetraaryl-4H-thiopyrans

1,3,3,5-Tetraarylpentane-1,5-diones 2 and 5 react with tetraphosphodecasulfide in xylene at elevated temperature to give the corresponding 2,4,4,6-tetraaryl-4H-thiopyrans 4 and 7. The mechanism of this transformation involves a fast initial step of dehydration of 2 and 5 to 2,4,4,6-tetraaryl-4H-pyrans 3, followed by a slower transformation to the 4H-thiopyran analogues 4. Using 2D NMR techniques, a complete assignment of the 1H and 13C NMR spectra of the 1-oxide and 1,1-dioxide analogues of 4(1, X = SO and SO2) as well as of compounds 3c, j, I, m, 4a–m, 6(the pyran analogue of 7) and 7 has been carried out. Compounds 4 and 7 undergo a reversible photochemical colour change after UV illumination. The maxima of the new absorption bands are situated at 540–660 nm. The non-exponential time dependence of the photodecolouration of 4a in the solid state is analysed in terms of dispersive first-order reaction kinetics. Dioxygen accelerates the decolouration process; the half-life at 299 K lies between 6600 s in air and several days in a vacuum (10–3 Pa). The trapping of photoproducts 10a, 10e and 11a enables us to postulate the whole photolysis sequence.

Photochromism of sydnones: Structural evidence for the blue colored species from 3-(3-pyridyl)sydnone

Abstract Positions of absorption bands in the electronic spectra of 3-(3-pyridyl)sydnone I, its valence isomer IV and the assumed decarboxylation product V were calculated using semiempirical molecular orbital method CNDO/S-CI. Based on a comparison between the experimentally recorded UV—VIS spectra and the respective theoretical spectra of heterocycle I and the blue product of its photochromic transformation the latter compound could be attributed to the structure IV. The molecular and electron structures of CNDO and MNDO models of valence isomers I and IV in relation to the isomerization I ⇌ IV is discussed.

METHANEDIOL DECOMPOSITION MECHANISMS : A STUDY CONSIDERING VARIOUS AB INITIO APPROACHES

The alternative decomposition reactions CH2(OH)2 → CH2O + H2O and CH2(OH)2 + H2O → CH2O + 2H2O are investigated using the semiempirical PM3 as well as the ab initioHF/3-21G, HF/6-31G, HF/6-31G**, and MP2/6-31G** calculations. Reactants, products, and appropriate transition states are located on corresponding potential energy surfaces and compared with those reported in earlier studies.

Photochromism of 4H-Thiopyrans and 1,4-Dihydropyridines

Abstract New type of photochromic reactions of 3, 5-bridged isomers of thiopyrans and phenyl migration from position 4 to 3 of dihydropyridine ring are presented.

Oxidation of 1-substituted 2,4,6-triphenylpyridinium salts

Abstract During the oxidation of 1,2,4,6-tetrasubstituted pyridinium salts 1 with potassium hexacyanoferrate(III) in alkali medium a contraction of pyridine ring is observed leading to 1,2,3,5-tetrasubstituted pyrroles 2 .

Photophysics of PBD derivatives. II: The character of the lowest excited triplet state of 2-(biphenyl-4'-yl)-5-phenyl-1,3,4-oxadiazole

Abstract The low temperature phosphorescence spectra, polarization and the lifetime of the phosphorescence for five derivatives of 2-(biphenyl-4′-yl)-5-phenyl-1,3,4-oxadiazole (PBD) were measured. Two chemical pathways were found to force molecules of the PBD-type to phosphoresce: introducing a substituent with a localized low-energy nπ* state or substitution by a heavy atom (bromine). In both cases, the emitting, i.e. the lowest excited triplet state, is of the ππ* character. The T 1 characteristics of the non-phosphorescent PBD were estimated by analogy. The vibrational structure of the phosphorescence spectral bands yields the information on the planarity of PBD-type chromophore in the excited T 1 state.

A theoretical study of the methanetriol decomposition mechanism

Abstract The linear synchronous-transit method (LST) based on the PRDDO wavefunctions was used to investigate the possibility of various methanetriol dehydration mechanisms. The decomposition paths for the systems HC(OH)3, (HO)2CHOH2+, HC(OH)3-H2O, HC(OH)3-H3O+ and HC(OH)3-OH− exhibit transition states except the two alternative reaction channels found for the positively charged reactants. The existence of methanetriol and its ionized forms is discussed.

A theoretical study of the methanediol decomposition mechanism

Abstract The linear-synchronous-transit method (LST) based on the PRDDO nonempirical wavefunctions was used to investigate the background of the methanediol dehydration. While the decomposition paths of the systems CH2(OH )2-H2O, CH2 (OH)2-H3O+, and CH2 (OH)2-OH− exhibit no transition states, certain activation energies are predicted for the dehydrations of the single particles CH2 (OH)2 and HOCH2OH2+.

Substituted 2,4,4,6-tetraphenyl-4H-selenopyrans: preparation, photocolouration and 4H-selenopyran ring geometry; an X-ray and DFT calculation study

1,5-Diaryl-3,3-diphenylpentane-1,5-diones 3a–d react with Al2Se3–HCl–AcOH reagent to give the 2,6-diaryl-4,4-diphenyl-4H-selenopyrans 4a–d in satisfactory yields while the same procedure starting from 1,5-diketone 5 affords small amounts of spiroheterocycle 6. 2,4,4,6-Tetraphenyl-4H-selenopyran 4a can be brominated or nitrated at positions 3 and 5 to give substituted derivatives 7–9. Dibromo derivatives 4c and 7 were converted with copper(I) cyanide to dicyano derivatives 4e and 10,11, respectively. Similarly to 4H-pyrans 1 and 4H-thiopyrans 2, the 1-selena analogues 4a–d and 8 exhibit solid state photocolouration which was investigated in terms of dispersive first-order reaction kinetics. The influence of selenium heteroatom on the geometry of the heterocyclic ring of molecules 4a,c and 6 is discussed on the basis of X-ray structure determinations, quantum DFT and semiempirical PM3 calculations.

Photocolouration of Hypervalent Heterocycles

The notion of hypervalent molecule is usually used to describe a chemical entity which can be characterized by two or more mesoionic valence structures and/or by a structure in which valence states of some atoms do not follow the classical rules concerning the chemical bonding [1]. Typical examples represent some isomers of heterocyclic molecules, e.g. dihydropyridines (cf. Scheine 1). While L2-dihydroisomer (1) and 1,4dihydroisomer (2) do not show any hypervalence, it is possible in the case of 1,3dihydroisomer (cf. structures (3a) - (3d)) The chemical formulas (3a) - (3d) represent formal structures; usually, information concerning a real molecular structure, i.e., the conclusion whether an N-hypervalent structure (3a) or a hybrid of mesoionic rezonant structures (3b) - (3d) is more probable, must be obtained by quantum chemical methods. It could be mentioned that “hypervalent” molecules often show properties different from non-hypervalent ones [2,3] and they are very often unstable and coloured [4]. These materials, in agreement with theory, have relatively high internal energies and it is probable that also molecules of some photoproducts can be considered as hypervalent ones. In connexion with this fact, the phototropic behaviour of 2,4,4,6-tetraphenyl 4H-pyrans (4. X = 0), its thio- (X = S), and aza- (X = NR, with R specified in Table l) analogues seems to be of interest. Somc of these compounds, which become reversibly coloured under UV or solar irradiation, can be utilized in some optical devices. In this paper, the photocolouration of compounds of the type (44) is described and the mechanism of the photochromism briefly discussed. A more extensive discussion of the photochromic behaviour, including also results of model quantum chemical calculations. is given in the accompanying paper [5].