Site-Specific Photodecomposition in Conjugated Energetic Materials

Nonadiabatic
excited-state molecular dynamics (NEXMD) has been
used to study photodecomposition in a class of recently synthesized
bicyclic conjugated energetic materials (CEMs) composed of fused tetrazole
and tetrazine derivatives with increasing oxygen substitutions. Modification
by oxygen functionalization has already been demonstrated to increase
the two-photon absorption intensity in the target CEMs while
simultaneously improving oxygen balance. Photodecomposition mechanisms
in materials that undergo nonlinear absorption could be used to achieve
controlled, direct optical initiation. Here, we use NEXMD simulations
to model the nonradiative relaxation and photodecomposition in CEMs
following photoexcitation by a simulated Nd:YAG laser pulse. Excess
electronic energy is quickly converted into vibrational energy on
a sub-100 fs time scale resulting in bond dissociation. We find that,
for the studied tetrazine derivatives, the bicyclic framework
is an important structural feature that enhances the photochemical
quantum yield and the high atomic oxygen content increases the relaxation
lifetime and opens additional photodissociation pathways targeting
the oxygen-substituted sites. The presented analysis scheme based
on bond orders in the swarm of NEXMD trajectories is a useful tool
for determining photochemical reactions.