L. Angeloni

Role of internal conversion on the excited state properties of trans-styrylpyridines

Abstract A theoretical and photophysical study of the three isomeric trans- n -styrylpyridines ( n = 2,3,4) has been carried out to evaluate the role of internal conversion (IC) on their deactivation pathways from the singlet manifold. Calculations by the QCFF/PI and CNDO/S methods and application of current theories of radiationless transitions allowed the rate constants for IC to be evaluated. The fluorescence lifetimes measured in the picosecond region and the fluorescence and trans → cis photoisomerization quantum yields from previous works allowed the rate constants for the non-radiative and non-reactive deactivation processes to be calculated. The experimental results were in a reasonable agreement with the theoretical predictions on the role of IC (markedly more important for the 2 and 4 derivatives) and confirmed that intersystem crossing is a low efficiency process for these azastilbenes. On the basis of the temperature and the solvent viscosity effects on the electronic spectra, an interpretation of the excitation energy effect on the quantum yields of the 3 isomer was attempted.

IR and Raman spectra of A 2,2′-bipyridine single crystal: internal modes☆

The polarized light IR and Raman spectra of 2,2′-bipyridine single crystals were obtained. The oriented gas model was applied and the proportionality factors for the intensity of the IR and Raman bands calculated and compared with experiment. The difficulties encountered while applying the oriented gas model to molecules with C2h symmetry and to monoclinic structures were discussed. An assignment of the internal modes based on the polarized light spectra has been given. The far infrared spectrum of the molecule was also measured and the ring—ring torsional model of Au species was identified at 73 cm−1.

Normal mode analysis of 2,2′-bipyridine—I. Internal modes

Abstract The vibrational assignment of the internal modes of 2,2′-bipyridine is discussed using i.r. and Raman spectra in polarized light for a single crystal and for two different orientations of polycrystalline films. Predictions based on zero-order frequency calculations provide a useful guideline for the identification of the in plane fundamentals, while no satisfactory force field was available for the out-of-plane vibrations. A refined set of valence force constants was then obtained which, under the assumption of a planar stable conformation both in solution and in solid, can be used for further calculations of the whole crystal spectrum.

Interference effects in the infrared spectra in polarized light of monoclinic crystals

Abstract Interferences due to anharmonic interactions between a broad band and discrete energy states have been observed in the infrared spectra of some carboxylic acid salts. In particular, the spectra in polarized light of the ac face of the monoclinic crystals show continuous changes in frequency and band shape in function of the orientation of the electric vector of the incident light. The theory of these spectral interferences developed by Fano for the electronic transition has been extended to the vibrational case in which polarized light is used and it has been shown that the different line profiles of a same band for different orientations of the electric vector can be reproduced.

The dynamics of succinonitrile in the plastic phase by subpicosecond time- resolved optical Kerr effect

The relaxation kinetics of succinonitrile in the plastic crystalline phase has been investigated between 250 and 320 K by transient optical Kerr effect with femtosecond pulses. Three different noninstantaneous contributions to the signal time profile have been identified. The fastest one is a subpicosecond decay, attributed to the relaxation of the damped librational and torsional vibrations of the molecules. The intermediate decay time, ranging from 4 ps at 323 K to 30 ps at 250 K, is interpreted as due to rotations of the trans molecules about the shortest inertia axis which bring the molecule from one cube diagonal to another. The slowest decay time ranges from 28 ps at 323 K to 190 ps at 250 K and agrees very well with previous measurements with different experimental techniques. This decay is interpreted as due mostly to rotational motions of the gauche molecules jumping from one equilibrium position to another in the unit cell. The activation energy for the two processes is 3.7 and 4.3 kcal/mol, res...

Relaxation dynamics of fermi doublets in CS2 crystals

Abstract The vibrational relaxation of CS 2 crystals in the Fermi resonance region is probed by picosecond CARS spectroscopy. The vibrational decay times of the Fermi doublet components have been measured as a function of temperature between 40 and 160 K. Decay mechanisms are proposed to explain the experimental data.

Infrared and Raman spectra of potassium hydrogen malonate single crystals

Abstract We have recorded the i.r. absorption, reflection, and Raman spectra of single crystals of KH malonate at room and liquid N 2 temperatures. This and the spectra of polycrystalline KD-malonate- d 2 allow for a complete assignment of the internal modes. From the present data combined with previous results obtained with acid salts of carboxylic acids of different crystal symmetry conclusions are drawn which concern spectral features connected with hydrogen bonding, in particular the strong absorption near 1000 cm −1 and the bands in the carbonyl region.

Protein relaxation in the photodissociation of myoglobin-CO complexes †

Laser-induced optoacoustic spectroscopy has been applied to the study of the photodissociation of myoglobin-CO complexes. Time-resolved optoacoustic signals have been measured from aqueous solutions of horse myoglobin-CO complex (hMbCO) at pH 3.5 and 8, and of sperm whale myoglobin-CO complex (swMbCO) at pH 8, in the temperature range 273-300 K. The signal of hMbCO at pH 8 exhibits three components. The first, which is faster than 20 ns and is associated with a reaction enthalpy of 61 kJ mol(-1), corresponds to Fe-CO bond breakage. The second component has a decay time of 80 ns at 293 K and is associated with an exothermic protein relaxation (-13 kJ mol(-1)) and a volume change of -3 ml mol(-1). The relaxation, which involves a state where the photo-dissociated CO is still in a protein docking site, is thermally activated, with an activation enthalpy of 51 kJ mol(-1). The third component has a decay time of 800 ns at 293 K and an activation enthalpy of 39 kJ mol(-1), and is associated with an endothermic process (26 kJ mol(-1)) and an expansion of 19 ml mol(-1). This process is ascribed to the migration of the photodissociated CO to the bulk solvent. At acidic pH, the latter process becomes faster (230 ns) and the volume change decreases. These features are correlated with the presence of an open form of the protein. swMbCO exhibits two components only, due to the overlap of the two fastest processes. The first involves a reaction enthalpy of 49 kJ mol(-1) and a volume contraction of -4.9 ml mol(-1). The second component (900 ns at 293 K, activation enthalpy 45 kJ mol(-1)) is associated with a reaction enthalpy of 38 kJ mol(-1) and a volume expansion of 15.3 ml mol(-1). These experimental findings have been interpreted by means of a new model, which also takes into account both laser flash photolysis results and structural information. The model is based on a two-dimensional scheme which describes both protein relaxation and the CO pathway following photodissociation.

The disaccharide anthracycline MEN 10755 binds human serum albumin to a non-classical drug binding site

The interaction of the novel disaccharide anthracycline MEN 10755 with human serum albumin (HSA) was investigated by visible absorption and fluorescence spectroscopies and by ultrafiltration. Notably, MEN 10755 binds serum albumin far stronger than doxorubicin. Albumin binding results into a drastic quenching of the intrinsic fluorescence of MEN 10755; a binding constant of 1.1 x 10(5) was determined from fluorescence data. To localize the HSA binding site of MEN 10755 competition experiments were carried out with ligands that are selective for the different drug binding sites of the protein. No relevant competition effects were seen in the case of warfarin, diazepam and hemin, known ligands of sites I, II and III, respectively. Modest effects were observed following addition of palmitic acid that targets the several fatty acid binding sites of the protein. In contrast, extensive displacement of the bound anthracycline was achieved upon addition of ethacrinic acid. On the basis of these results, it is proposed that MEN 10755 binds serum albumin tightly to a non-canonical surface binding site for which it competes specifically with ethacrinic acid.

Temperature-dependent (10-240 K) relaxation of vibrational excitons in KClO4 crystal

Abstract The relaxation times of vibrational excitons in KClO 4 have been measured between 10 and 240 K by picosecond time-resolved CARS spectroscopy. The low temperature data are consistent with energy decay mechanisms based on three-phonon scattering processes. At higher temperature the linewidth of the symmetric stretching ν 1 has a well defined linear dependence on T , showing that three-phonon scattering is still the main relaxation mechanism. In contrast, the ν 2 bending mode has a marked T 2 dependence, showing that four-phonon and higher-order depopulation and dephasing mechanisms control the overall relaxation rate. Finally, the strong temperature dependence of the linewidth of the 2ν 2 overtone is interpreted in terms of the relaxation of an undispersed, quasi-free two-phonon state.