Quasi-equilibrium Processes of Electronic Excitation Relaxation in the 1.5-Diazidopyrimidine Molecule

Abstract

This work reports combined experimental and theoretical investigations of quasi-equilibrium relaxation of electronic excitation in 1.5-diazidopyrimidine (1.5-DAP) and discusses its practical relevance. Analysis of the energy structure of the molecule shows that the first two singlet excited states (S₁ and S₂) arise from quantum transitions to dissociative pσ* states: S1 is associated with excitation localized on the azido group at position 5, whereas S₂ is localized on the azido group at position 1; the corresponding wave functions are spatially separated. The next two singlet states correspond to quantum transitions to pp*-states that form the absorption spectrum of the molecule. The relaxation of electronic excitation of the molecule is described in terms of a quasi-equilibrium pathway that enables localization of excitation at higher energy levels. Because there is an energy barrier in S₁ prevents N–N bond cleavage along this channel, the process of excitation relaxation proceeds to T₃(pp*) and subsequently to T₂(pσ*), which corresponds to excitation localized on the azido group at position 5. Consequently, the molecule dissociation occurs only via the single dissociative state T₂(pσ*). Relaxation of excitation from T₂ to the lowest triplet excited state T₁(pp*) is found to be forbidden, which provides a high quantum yield of azido-group photodissociation. It is shown that all other channels of electronic excitation relaxation led to the relaxation of the molecule to the T₂ state, i.e. the process is quasi-equilibrium. The consequence of such localization is the dissociation of the molecule with the release of a nitrogen molecule and nitrene. Nitrene, being a highly reactive particle, is widely used in molecular and nanoelectronics to create photoresists, as well as intermediates in nanotechnology.