Excitation energy dependence of ultrafast Jahn-Teller switching in Mn(III) acetylacetonate.

The photoinduced Jahn-Teller (JT) switch in Mn(acac)3 provides a model for how ultrafast structural dynamics can be harnessed to control magnetic anisotropy in single-molecule magnets (SMMs). However, the role of excitation energy in governing the efficiency and pathway of this transformation remains unclear. Here, ultrafast transient absorption spectroscopy (380-940 nm) probes the excitation-energy dependence of the JT switch. Selective excitation of Q1, Q2, and Q3,4 states shows that all pathways converge to a long-lived compressed-state photoproduct, but with distinct relaxation timescales, vibrational dynamics, and yields. Vibrational coherence analysis reveals modes at 170, 208, and 254 cm-1, assigned to the elongated, Q1 excited, and compressed state configurations, respectively. The 208 cm-1 mode is observed only under direct Q1 excitation, indicating excitation-dependent access to the JT-reactive coordinate. The compressed-state yield exhibits a strong dependence on excitation energy. For excitation into the lowest Q1 state (940-700 nm), the yield increases and follows a one-dimensional Landau-Zener model, where higher wavepacket velocity along the JT coordinate enhances nonadiabatic transition probability. The yield plateaus between 700-640 nm as the Q2 state is accessed and excess energy is likely transferred into non-reactive modes. Upon excitation into the Q3,4 manifold, the yield increases sharply, reaching a maximum near 470 nm before decreasing at higher energies, revealing the onset of multidimensional dynamics and competing relaxation pathways. These results identify excitation energy as a control parameter for the nonadiabatic structural dynamics, providing design rules for optically steering magnetic anisotropy in SMMs.