15min:
THE DARK SINGLET STATE AS DOORWAY STATE OF INTERSYSTEM CROSSING IN DNA MONOMERS..

CHRISTIAN REICHARDT, CARLOS E. CRESPO-HERNÁNDEZ, Case Western Reserve University, Department of Chemistry Center for Chemical Dynamics, 2080 Adelbert Road, Cleveland, Ohio 44106.

The excited state dynamics of 9-methylpurine (9MP) were studied with broadband transient absorption spectroscopy in the time regime from femtoseconds to 3 nanoseconds. Excitation of 9MP in aqueous solutions at 266 nm results in ultrafast internal conversion from the initially excited S2( pi pi^ ast) state to the S1 state. Quantum chemical calculations that include bulk and explicit solvent interactions show that the S1 state has significant n pi^ ast character. Population of the S1 state is followed by intersystem crossing (ISC) to the T1( pi pi^ ast) state on a time scale of hundreds of picoseconds. Vanishingly small fluorescence yields were measured, supporting the dark character of the S1 state as well as the high triplet yield in 9MP. Analogous experiments in acetonitrile show a decrease in the ISC lifetime by almost 50 % but an equally high triplet yield. The results presented in this work demonstrate the important role that the dark singlet state has in modulating the excited-state dynamics of DNA monomers in solution.

%Session in which paper is recommended for presentation: 2 - Electronic (UV
VIS
NIR)