Equatorial restriction of the photoinduced Jahn-Teller switch in Mn(iii)-cyclam complexes.

Ultrafast transient absorption spectra were recorded for solutions of [Mn(cyclam)(HO)(OTf)][OTf] (cyclam = 1,4,8,11-tetraazacyclotetradecane and OTf = trifluoromethanesulfonate) in water to explore the possibility to restrict the equatorial expansion following photoexcitation of the d ← d electronic transition, often resulting in a switch from axial to equatorial Jahn-Teller distortion in Mn complexes. Strong oscillations were observed in the excited state absorption signal and were attributed to an excited state wavepacket. The structural rigidity of the cyclam ligand causes a complex reaction coordinate with frequencies of 333, 368, 454 and 517 cm, and a significantly shorter compressed-state lifetime compared to other Mn complexes with less restricted equatorial ligands. Complementary density functional theory quantum chemistry calculations indicate a switch from an axially elongated to a compressed structure in the first excited quintet state Q, which is accompanied by a modulation of the axial tilt angle. Computed harmonic frequencies for the axial stretching mode (∼379 cm) and the equatorial expansions (∼410 and 503 cm) of the Q state agree well with the observed coherences and indicate that the axial bond length contraction is significantly larger than the equatorial expansion, which implies a successful restriction of the wavepacket motion. The weak oscillation observed around 517 cm is assigned to a see-saw motion of the axial tilt (predicted ∼610 cm). The results provide insights into the structural perturbations to the molecular evolution along excited state potential energy surfaces of Mn octahedral complexes and can be used to guide the synthesis of optically controlled Mn-based single-molecule magnets.