Yasen Velkov

Cubic Nonlinear Optical Properties of Platinum-Terminated Polyynediyl Chains

The wavelength dependence of the cubic nonlinearity of ligated platinum-terminated polyynes trans, trans-{(p-MeC6H4)3P}2(p-MeC6H4)Pt(C[triple bond]C)n Pt(p-C6H4Me){P(p-C6H4Me)3}2 (n = 3-6, 8, 10, 12) has been examined by femtosecond Z-scan studies in the wavelength range 520-1500 nm and the results rationalized by density functional theory calculations on the model complexes trans, trans-(H3P)2(C6H5)Pt(C[triple bond]C)n Pt(C6H5)(PH3)2 (n = 2-8, 10, 12). Although the final states for one- and two-photon transitions are not the same in these centrosymmetric molecules, the Z-scan studies reveal coincidences in one-photon absorption with features in the frequency dependencies of both real and imaginary parts of the cubic hyperpolarizability, as well as inflections in the frequency dependencies of the real part of gamma that correspond to resonances in the imaginary part of gamma. The theoretical studies suggest that the linear absorption spectra are dominated by X(1)A g --> n(1)B(3u) transitions, with the first state of B(3u) symmetry playing a steadily diminishing role upon oligoyne chain lengthening. The theoretical studies also predict a red-shift of two-photon absorption (TPA) profile with increasing conjugation length, and a significant enhancement on proceeding from the shortest to the longest chromophore, trends that are observed experimentally. The experimental low-energy TPA maxima for these complexes can be approximated by a simple Gaussian profile. The sp-carbon chain-length dependence of linear and nonlinear absorption maxima enable an estimate (neglecting saturation) of 660 and 1000 nm for the infinite carbon chain, carbyne.

Melatonin: Quantum-chemical and biochemical investigation of antioxidant activity.

Experimental and theoretical investigation of the antioxidant activity of melatonin is carried out. The theoretical approach comprises the evaluation of several appropriate descriptors of scavenging activity with the help of quantum-chemistry methods. The values obtained are compared with available data for substances with established antioxidant properties. One of the most widely used markers for in vivo free radical oxidation processes is malondialdehyde (MDA) as an end product of membrane lipid peroxidation. Experimental support of the computed scavenging parameters is provided by estimation of the effect of supplementary melatonin therapy on the plasma levels of MDA in CRF patients on maintenance HD therapy. Different reaction paths have been considered and related to the obtained data, allowing speculations about the reaction mechanism and the antioxidant potential of melatonin for practical purposes.

X-ray absorption of N2 accompanied by infrared-induced transitions between the ungerade and gerade core levels

We study a two-colour pump–probe scheme of x-ray absorption accompanied by core-hole hopping in the field of a strong IR laser. The process is exemplified for fixed-in-space and randomly oriented homonuclear diatomic molecules N2 near the 1σu → 1πg x-ray absorption transition. The laser field mixes the core holes of opposite parities and causes Rabi splitting of the core-excited states. The IR field results in spectral broadening and shifts of the x-ray resonances as well as decrease of x-ray photoabsorption. The Stark broadening of the x-ray absorption spectrum depends on the orientation of the molecule and the angle between the polarization vectors of the x-ray and IR fields. The spectral changes caused by the IR field are weaker for randomly oriented molecules in comparison with fixed-in-space molecules.