Relaxation of Electronic Excitation in Triazide Molecules

As a result of studies of the relaxation mechanisms of electronic excitation in triazide molecules: triazidopyrimidine (TAP) and triazido-S-triazine (TAT), it was shown that triplet and singlet excited states are states of the ππ* and πσ* type. Both molecules have a flat shape and the same structure of π-molecular orbitals (π-MO). However, they differ significantly in terms of symmetry, which determines the properties of excited states and the relaxation of electrons from excited states. The lowest excited state of both molecules is the binding T₁(ππ*) state. It is followed in TAP by the dissociative T₂(πσ*)-state. In TAT, the T₅(πσ*) state is dissociative. Both molecules are excited to the singlet S₄(ππ*) state, from which the molecule relaxes non-equilibrium through intermediate states to the triplet dissociative state. From this state, triazidopyrimidine dissociates with a quantum yield of h = 1.0, while the quantum yield of triazido-S-triazine dissociation will be significantly reduced due to relaxation to the lower triplet states of the same symmetry. Among the singlet states in both molecules, there is a dissociative state, but on the way to dissociation there is an energy barrier capable of slowing down the corresponding process. When the N-N bond in both molecules is extended to 1.75 Å, the dissociative state crosses the main S₀ state of the molecule, as a result of which the dissociation product (nitrene) appears in the main triplet state. It is shown that the excitation of a solution of TAP in ethanol increases the quantum yield of photodissociation up to 3.0 due to secondary processes. Such a process is absent in the case of TAT solution.