Bayreuth 1998 – scientific programme

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CP: Chemische Physik

CP 13: Dynamik molekularer Systeme

CP 13.5: Talk

Wednesday, March 11, 1998, 12:10–12:30, H19

Intersystem Crossing in Flexible Biradicals — •Alexandra Yurkovskaya and Hans-Martin Vieth — Free University of Berlin, Institute of experimental physics, Arnimallee 14, D-14195, Berlin

Time-resolved nuclear magnetic resonance techniques (chemically induced dynamic nuclear polarization (CIDNP) at high magnetic field of 7T, stimulated nuclear polarization (SNP) for protons and C-13 at low magnetic field of 100 mT) and laser flash photolysis (LFP) at variable magnetic field have been used for the comparative study of intersystem crossing in short-lived flexible biradicals of different structures in wide temperature range. At room temperature, the short biradical lifetime, the small MFE on biradical lifetime, andthe low intensity of spin polaization observed at high field for acyl-ketyl and acyl-alkyl biradicals indicate that the main channel of intersystem crossing in intermediate these biradical is spin-orbit coupling (SOC). Slowing down of SNP decay at low temperature reflects decrease of molecular chain mobility. For bis-ketyl biradicals SOC is of minor importance, and magnetic field effect (MFE) is much larger. Model calculations of biradical evolution, based on the numerical solution of the stochastic Liouville equation, were carried out by taking into account the molecular dynamics of the polymethylene chain linking the radical centers, the distance dependent exchange interaction J(r), hyperfine interaction with protons and C-13 and SOC.