Miloš Nepraš

Electronic excited states of azo compounds: Strong ππ* fluorescence of bis-4,4′-diethylaminoazobenzene

Abstract The low-temperature absorption and luminescence spectra of bis-4,4′-diethylaminoazobenzene (BDAAB) in 2-methyltetrahydrofurane (MTHF) have been measured. On the basis of the spectral characteristics, this luminescence is interpreted as the fluorescence from the first excited singlet state (ππ*). A comparison of the spectral and photophysical properties of BDAAB with those of other para-substituted trans-azobenzenes enables us to draw some conclusions concerning the structure and fluorescence of these systems.

Excited states of azo compounds. II: Vibrational structure of the electronic absorption spectra of 4,4'-di-substituted azobenzene derivatives

Abstract The electronic absorption spectra of azobenzene and its fourteen symmetrical and non-symmetrical derivatives substituted in both para positions by electron-donating substituents were studied in 2-methyl-tetrahydrofuran solvent at room (293 K) and low (103 K) temperatures. The main effect destroying the vibronic structure of the absorption spectra of the azobenzene derivatives was found-the ground state non-planarity caused by torsions around the NPh bond. The central azo bond stretching was interpreted as the dominant vibrational mode in absorption spectrum. It has been shown that increasing electron-donating ability of azobenzene substituents in para positions increases the planarity of the molecules in the ground state and decreases the difference between equilibrium azo bond lengths in the ground and first excited singlet ππ * states.

Structure and properties of fluorescent reactive dyes : Electronic structure and spectra of some benzanthrone derivatives

Absorption and fluorescence spectra of N-(disubstituted-1,3,5-triazinyl)-3-aminobenzanthrones, 3-acetylaminobenzanthrone and 3-methoxybenzanthrone have been investigated. Excitation energies and the character of the first absorption band have been calculated by the PPP-MO method. Some data on substituent and solvent effects on fluorescence spectral position and quantum yields are reported. The connection between electron density distribution in the electronic ground and excited state, and spectroscopic properties of those compounds, is explained.

N-Triazinyl Derivatives of 1- and 9-aminoanthracene: Synthesis and Photo-Physical Properties

New N-triazinyl derivatives were synthesized by reaction of cyanuric chloride with 1- and 9-aminoanthracenes and subsequent nucleophilic substitution of chlorine atoms on triazinyl ring with methoxy and/or phenylamino groups. The compounds were characterized by 1H and 13C NMR and mass spectra. The influence of the chemical structure and solvent polarity on the UV/Vis absorption and fluorescence spectra and fluorescence quantum yields were investigated. Semi-empirical computations revealed highly polar CT states in singlet excited state manifold connected with charge-transfer from the hydrocarbon moiety to the triazinyl ring. The relationships between the CT-to-emitting state energy gap, solvent polarity and fluorescence quantum yield were discussed.

Synthesis, visible absorption spectra and application properties of disperse dyes derived from 1-indanylidenemalononitrile

The series of dyes derived from 1-indanylidenemalononitrile were prepared and characterised. The purity of the dyes was checked by TLC and elemental analysis; the structure of substances was confirmed by 1 H and 13 C NMR spectra. The relationships among the structure of dyes, their absorption characteristics and the polarity of a solvent were investigated. The full optimised geometry was computed by AM1 method, the theoretical characteristics of electronic transitions were studied by PPP-MO, CNDO/S and INDO/S procedures. Due to possible application of the studied dyes in textile practice, the colouristic characteristics were determined as well. # 2002 Published by Elsevier Science Ltd.

Photophysics of PBD derivatives. I. The fluorescence of para-biphenylyl-substituted 2-(biphenyl-4′-yl)-5-phenyl-1,3,4-oxadiazoles

Abstract The room and low temperature absorption and fluorescence spectra and the fluorescence quantum yields and lifetimes of ten derivatives of 2-(biphenyl-4′-yl)-5-phenyl-1,3,4-oxadiazole with a substituent in the para-biphenylyl position are reported. The theoretical spectral characteristics of the electronic transitions of the derivatives with strong electro-releasing (-NH2) and strong electron-withdrawing (-NO2) substituents have been calculated by the PPP method. Differences between the spectral characteristics of the phenyl- and biphenyl-substituted PBDs are explained on the basis of the charge transfer character of the first electronic transition. No fluorescence has been observed for the carbonyl, thiocarbonylamide and nitro derivatives. An efficient non-radiative deactivation mechanism involving low energy nπ* states localized on the substituent is assumed for these cases. The planarity of the compounds under study both in their ground and the lowest excited singlet ππ* state is discussed on the basis of the vibrational structure of the absorption and fluorescence spectra and on the basis of quantum chemical calculations.

Fluorinated diphenylpolyenes: Ground- and excited-state geometry studied by absorption and fluorescence spectroscopy

Abstract Low-temperature absorption and fluorescence spectra of all-trans-1,4-diphenyl-1,2,3,4-tetrafluoro-1,3-butadiene (FDPB) and 1,6-diphenyl-1,2,3,4,5,6-hexafluoro-1,3,5-hexatriene (FDPH) were measured in hydrocarbon and 2-methyltetrahydrofuran solvents. The hypsochromic shift of the absorption bands, as compared with their hydrocarbon analogues, indicates a strong non-planarity in the S 0 ground state. Large Stokes shifts (about 6900 cm −1 for FDPB and about 11000 cm −1 for FDPH) depending very slightly on the solvent polarity can be explained by changes in the nuclear geometry when passing from the S 0 to the S 1 state.

Fluorescence Anisotropy of Branched Molecules Containing 1-Aminopyrene Chromophores

Fluorescence anisotropy measurements were performed on a set of multichromophoric compounds, which contain a different number of aminopyrenyl moieties linked to a triazine ring, in order to reveal the nature of both the electronic excited states and relaxation pathways of the compounds. Our experimental results complement quantum chemical calculations. We propose that the lowest excited state from which fluorescence proceeds is localized on a single individual aminopyrene moiety. In contrast, excitation to a higher excited state is likely followed by a migration of energy to another nearby aminopyrene chromophore before the internal conversion to the emitting state takes place. We suggest that this migration is responsible for the experimentally measured decrease of fluorescence anisotropy of the studied compounds.