Aleksandra Lewanowicz

Chiral β-amino sulfoxides. Synthesis, configurational assignment and conformational analysis based on X-ray, CD, 1H NMR and theoretical calculations

Abstract Enantiomerically pure u and l β-amino sulfoxides have been easily obtained from the respective homochiral α-amino alcohols. The absolute configuration at the created stereogenic centre was assigned by CD spectra and by X-ray analysis. Conformational analysis of the title compounds was carried out using quantum chemical energy-geometry optimization. Thus established conformational behavior explained the strongly configuration dependent NMR spectral patterns observed for the u and l diastereomers.

Photochromic activity of dihydropyridine derivatives: energetics and kinetics of photochemically driven reactions in polycrystalline 1-methyl-2,4,4,6-tetraphenyl-1,4-dihydropydridine

Kinetics of thermally driven reactions in a crystalline photochromic solid 1-methyl-2,4,4,6-tetraphenyl-1,4-dihydropyridine (DHP) has been studied. The processes have been monitored by measuring the isothermal bleaching of coloured species produced upon UV irradiation of DHP, and by measuring the heat flow in non-isothermal differential scanning calorimetry experiments. Signatures of two processes were detected; a combination of quantum-chemical calculations and spectroscopic measurements allowed us to attribute them to reactions occurring in the photochromic system. The model put forward in the paper assumes that the primary photochemically driven reaction (intramolecular phenyl shift) is irreversible; the photochromic cycle consists of two parallel processes: 2,4-ring closing and intramolecular hydrogen shift.

Low-temperature electronic spectra, luminescence properties and electronic structure of 1,8-diazatriphenylene. A comparison with 1,4-diazatriphenylene

Abstract Highly resolved absorption, fluorescence and phosphorescence spectra of 1,8-diazatriphenylene (1,8-DAT) in n-alkane matrices, at 77K, were obtained, and vibrational analyses of the spectra performed. The assignment of fundamentals found in the emission spectra are given and the out-of-plane (op) modes appearing in the phosphorescence spectrum discussed. Contrary to 1,4-diazatriphenylene (1,4-DAT), no op modes appear in the 1,8-DAT fluorescence spectrum, and the vibronic structure of the 1,8-DAT S1 ← S0 spectrum is unperturbed in a wide frequency range. Fluorescence and phosphorescence quantum yields as well as the phosphorescence lifetime of 1,8-DAT in frozen hydroxylic and hydrocarbon solvents were measured, the rate constants of radiative and non-radiative transitions estimated and the data compared with those of 1,4-DAT. Using a modified INDO CI method, the electronic structure of the 1,8-DAT molecule was calculated. Distinct differences between the spectral and luminescence characteristics of 1,8- and 1,4-DAT are explained by larger energy gaps between the emitting S1(π,π∗) and T1(π,π∗) states and the lowest n, π∗ states in the former molecule.

High-resolution electronic spectra and luminescence properties of some benzonaphthyridines at 77 K. absorption and fluorescence spectra of 1,5-, 1,6- a

Abstract Absorption and fluorescence spectra of 1,5-, 1,6- and 4,6-benzo[h]naphthyridines (BN) were examined in Shpolskii matrices and in n-butanol at 77 K. Vibrational analysis of quasilinear spectra of 1,6- and 4,6-BN in n-hexane matrices was performed. Calcula- tions of the electronic structure of the isomers examined were done using a modified INDO CI method. The results of the experiments and calculations prove the π, π * state to be the lowest excited singlet state of 1,6- and 4,6-BN molecules; in 1,5-BN molecule the S 1 (n, π * ) state is strongly perturbed by the nearby s 2 (π π * ) state.

Kinetics of a photochromic reaction in a dibenzoazo crown ether in solution and in polymer matrices

The kinetics of thermal cis–trans isomerisation of photochromic dibenzoazo 16-crown-4 ether was studied in several matrices (isooctane, polystyrene and pMMA samples fabricated in various ways) using UV-vis spectroscopy. The rate constants were found to be almost independent of the nature of the matrix and the method of its fabrication. The average activation energy of the isomerisation amounts to 93±5 kJ mol−1, the pre-exponential factor being of the order of 1010 s−1. The independence of activation energy on the matrix can be explained by taking into account the relatively small difference in the molecular volumes of the two isomers. In polymerised pMMA samples the decays were non-exponential, probably owing to a distribution of rate constants associated with a distribution of microenvironments of photoactive molecules. Copyright

Position-dependent effects of internal heavy atoms on highly resolved electronic spectra and luminescence properties of some quinoxalines substituted at the homocyclic ring

Abstract Highly resolved phosphorescence and S1(n, π∗) ←S0 phosphorescence excitation spectra and some photophysical properties of 5- and 6-monohaloquinoxalines (XQs, X = Cl or Br) are compared. Experimental data are supplemented by theoretical study of the electronic structures of XQs, performed with the use of a modified INDO CI method. Insensitivity of XQs phosphorescence lifetimes to the nature of the solvent is discussed. The substituent position-dependent T1 state energy is recognized as the main factor differentiating the vibronic structure of the phosphorescence spectra and the luminescence properties of 5- and 6-XQs.

Kinetics of Photochromic Processes in Dihydropyridine Derivatives

Substitued dihydropyridines belong to a group of photoactive compounds whose properties have been relatively little known (cf. the accompanying paper [1] reviewing the structure and properties of dihydropyridines, pyrans, thiopyrans and selenopyrans). An interesting feature of the photochemical activity of many representatives of this group is that reactions following an electronic excitation are reversible (i.e., the compounds exhibit the photochromic effect), and that the effect can be observed both in liquid solutions, in solid matrices (e.g. in polymer foils), and in crystallites. Moreover, the effective rate constants deduced from the kinetics of colouring and bleaching have been shown to differ by several orders of magnitude [2-8], thus opening (at least in principle) an interesting possibility of tailoring the response kinetics of the systems to particular needs. Several molecules belonging to this group have been synthesized to date (cf. [1, 9] and references therein), complex studies, however. have been performed only on few systems. Thus it comes as no surprise that the molecular mechanism of the activity of these molecules has not been unequivocally established. It has been postulated that the initial step in the photochromic cycle involves an intramolecular phenyl shift [9-11] and/or a 3,5-bridge formation [12-16]. Some further steps in the sequence of elementary processes have also been proposed, e.g., a 2,4-bridge formation [2,3,9, 14] or a hydrogen shift [6, 17]. It has also been believed that initial stages of the photochromic process are identical, irrespective of the nature of the heteroatom in the central ring.

Kinetics of Photochromic Processes in Substituted Dihydropyridines in the Solid State and in Solutions

Abstract Results are presented of studies of the kinetics of thermally driven reactions in substituted dihydropyridines: 1-methyl-2,4,4,6-tetraphenyl-1,4-dihydropyridine (DHP), and 4,4-(diphenyl-2,2′-diyl)-2,6-diphenyl-1-methyl-1,4-dihydro-pyridine (BDH). The processes have been monitored by measuring the isothermal bleaching of coloured species produced upon UV irradiation of the samples, and by measuring the heat flow in non-isothermal differential scanning calorimetry experiments. A comparison of quantum-chemical calculations with results of spectroscopic and kinetic measurements seems to indicate that the initial step following an electronic excitation of DHP may be a radical, the absorption at 520 nm being probably associated with S0 and T1 states of one of intermediate products. The primary photochemically driven reaction (intramolecular phenyl shift) is irreversible; the photochromic cycle consists of two parallel processes: 2,4-ring closing and intramolecular hydrogen shift. The results described above are compared with those obtained on BDH–an analogue of DHP in which the potentially mobile phenyl groups are linked with a chemical bond.

Environmentally Induced H-Bond Transformation as a Source of Anil–Type Molecule Specific Solvatochromy

ABSTRACT The absorption, luminescence and excitation spectra of N-triphenylmethylsalicylideneimine at room temperature has been studied in pure isooctan as well as in acetonitrile and methanol solutions in the presence of varying amounts of acid or base by steady-state experiments. The molecular solvatochromy can be explained by considering the environmentally induced H-bond transformation. Experimental data was supported by theoretical consideration. Ab initio methods (LCAO MO SCF) are employed to investigate the “solvent assisted” proton transfer reaction in the ground electronic state. N-salicylidene methylamine was chosen as a model molecule. The relative stability of model molecule binding of 1, 2 and 4 methanol molecules and the mechanism of the proton transfer reaction are discussed. Analysis of the spectra together with the results of quantum chemical calculations related to different species active in the ground electronic state allows to state of the solution equilibrium of the molecule tested.

Temperature enhancement of photoactivity of N-triphenylmethylsalicylidene imine in solid state

Spectral behavior of chromic N-triphenylmethylsalicylidene imine in solid state has been investigated as function of temperature in 300-77 K temperature range. The analysis of the absorption, fluorescence and fluorescence excitation spectra together with the results of microscopic observation has been taken into account in monitoring of changing features. Direct evidence of the keto tautomer (open form) and intermediate formation has been presented, and dynamics of the intramolecular processes have been shortly discussed. The experimental activation energies equal to 5.9 kcal/mol and 9.5 kcal/mol have been extracted from the kinetics of thermally driven changes of luminescence. Those values have been attributed to 540-nm and 510-nm as well as 490-nm emitting species. Besides radiative transition, two main nonradiative deactivation channels, nonradiative transition and photochemical reaction, have been considered in explanation of the thermally enhanced photoactivity of the crystal under study. Contrary to a model anil compound, the photoactivity of studied molecular system with bulky substituent could be realized as dual-type behavior, thermochromic as well as photochromic simultaneously. The changes of the crystal color have been discussed in the frame of the proposed scheme. The scheme represents junction of thermochromic and photochromic behaviors following the proton transfer photochemical reaction acting as the first step of molecular rearrangement.