The ability to dissipate electronic excitation and thus prevent photo-induced damage and provide natural protection and photo-stability is, perhaps, the most significant characteristic of DNA bases as the building blocks of life on Earth. Therefore, it is understandable that studies of the excited states of DNA bases have attracted remarkable research interest. The challenge of experimental studies of DNA photo-physics stems from the intrinsic complexity of the dynamical behaviours of the excited states. In this regard, quantum mechanical methods for studying excited electronic states provide helpful insight. In this study we utilize time dependent density functional theory (TDDFT) to analyse the excited states of the DNA base-pair adenine–thymine in both the canonical Watson–Crick and stacked configurations. The excited state wavefunctions are analysed and visualized in terms of the one-electron transition density matrix (1TDM) and natural transition orbitals. The environmental effect on the excited states is considered by using a non-empirical effective fragment potential method. The extent of de-localization and the charge transfer character of the excited states in the near- and far-ultraviolet region of the electromagnetic spectrum are identified. It is also shown that the environmental effect on the de-localization is significant and varies with the configuration. Additionally, localized Frenkel type states, as well as de-localized states involving multiple molecular fragments, are also identified.