Relevance of pi sigma(*) states in the photoinduced processes of adenine, adenine dimer, and adenine-water complexes

Abstract

Ab initio calculations and time-resolved photoionization spectroscopy were carried out to characterize the role of the lowest two pi sigma(*) excited states for the photoinduced processes in the adenine monomer, adenine dimer, and adenine-water clusters. The calculations show-with respect to the monomer-a stabilization of 0.11-0.14 eV for the pi sigma(*) states in different isomers of adenine dimer and an even bigger stabilization of 0.14-0.36 eV for isomers of adenine-(H2O)(1) and adenine-(H2O)(3). Hence, the stabilized pi sigma(*) states should play an important role in the excited-state relaxation of partially or fully solvated adenine. This conclusion is supported by experimental results: In the adenine monomer, strong n pi(*) state signals are observed. Those signals are reduced in adenine dimer and vanish in water clusters due to the competing relaxation via the pi sigma(*) states.