THE DARK SINGLET STATE AS DOORWAY STATE OF INTERSYSTEM CROSSING IN DNA MONOMERS

Abstract

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 S$_2$($\pi\pi^\ast$) state to the S$_1$ state. Quantum chemical calculations that include bulk and explicit solvent interactions show that the S$_1$ state has significant n$\pi^\ast$ character. Population of the S$_1$ state is followed by intersystem crossing (ISC) to the T$_1$($\pi\pi^\ast$) state on a time scale of hundreds of picoseconds. Vanishingly small fluorescence yields were measured, supporting the dark character of the S$_1$ 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. %Acknowledgements: CR wants to thank the Deutsche Forschungsgemeinschaft (DFG) for their support. C. E. C. -H. thanks the Department of Chemistry, Case Western Reserve University for support and the Ohio Supercomputer Center for generous allotment of computer time. %Time required: 15 min %Session in which paper is recommended for presentation: 2 - Electronic (UV\\VIS\\NIR)