Excited-state dynamics of [Mn(im)(CO)₃(phen)]⁺: PhotoCORM, catalyst, luminescent probe?

Mn(I) α-diimine carbonyl complexes have shown promise in the development of luminescent CO release materials (photoCORMs) for diagnostic and medical applications due to their ability to balance the energy of the low-lying metal-to-ligand charge transfer (MLCT) and metal-centered (MC) states. In this work, the excited state dynamics of [Mn(im)(CO)₃(phen)]⁺ (im = imidazole; phen = 1,10-phenanthroline) is investigated by means of wavepacket propagation on the potential energy surfaces associated with the 11 low-lying S_n singlet excited states within a vibronic coupling model in a (quasi)-diabatic representation including 16 nuclear degrees of freedom. The results show that the early time photophysics (<400 fs) is controlled by the interaction between two MC dissociative states, namely, S₅ and S₁₁, with the lowest S₁-S₃ MLCT bound states. In particular, the presence of S₁/S₅ and S₂/S₁₁ crossings within the diabatic picture along the Mn-CO_axial dissociative coordinate (q_Mn-COaxial) favors a two-stepwise population of the dissociative states, at about 60-70 fs (S₁₁) and 160-180 fs (S₅), which reaches about 10% within 200 fs. The one-dimensional reduced densities associated with the dissociative states along q_Mn-COaxial as a function of time clearly point to concurrent primary processes, namely, CO release vs entrapping into the S₁ and S₂ potential wells of the lowest luminescent MLCT states within 400 fs, characteristics of luminescent photoCORM.