UV spectra and excitation delocalization in DNA : Influence of the spectral width

that has occurred during the past decades opened the possibility to determine the exciton-matrix elements with much higher precision compared to earlier studies. New methodologies, introducing quantum chemistry methods in the calculation of the diagonal and off-diagonal terms, were applied in the investigation of various systems such as molecular aggregates and photosynthetic antennas (see for example refs. [13– 18]). Thus, subtle differences appeared in the properties of various systems and the effect of structural disorder could be evaluated. Following this progress, it became possible to revisit the DNA excited states, examine the various factors that affect their properties and determine their footprint on the absorption spectra. Within this context, we are interested here in the exciton states of the model duplex (dA)10.(dT)10, composed of one strand of adenine bases and one strand of thymine bases. This theoretical study is a continuation of two previous investigations on duplexes consisting of adenine–thymine base pairs [19–20] which were performed in parallel with experimental spectroscopic studies. [21–22] Our first communication [19] focused on the electronic transitions of the monomers that have to be taken into account in the construction of the exciton matrix, and the precision necessary in the calculation of the dipolar coupling. It was shown that it is important to consider the two lowest transitions for adenine, S0!S1 and S0!S2, which can be coupled with the