Equatorial restriction of the photo-induced Jahn-Teller switch in Mn(III)-cyclam complexes Ryan Phelps, Alvaro Etcheverry-Berrios, Euan K. Brechin and Olof Johansson University of Edinburgh, UK Mn-based Single molecule magnets (SMMs) are promising candidates for the next generation of information storage devices. The photoexcitation of Mn III SMMs and octahedral monomers causes a switch from axially elongated to compressed Jahn-Teller (JT) distortion, 1-2 which is expected to modulate the magnetic anisotropy on femtosecond timescale. Here, ultrafast transient absorption spectroscopy is used to explore the effect of restricting the equatorial expansion following the photoexcitation of [Mn III (cyclam)(H 2 O) 2 ] 3+ (cyclam = 1,4,8,11-tetraazacyclotetradecane). We observe oscillations with frequencies of 333, 368, 454 and 517 cm -1 in the transient absorption spectra which we attribute to the generation of a vibrational wavepacket along the excited state potential energy surface. The rigid cyclam ligand causes a complex reaction coordinate, and the excited state lifetime issignificantly shorter compared to Mn III -complexes that have more flexible equatorial ligands. 1 Density functional theory quantum chemistry calculations indicate a switch in the JT distortion from an axially elongated to compressed structure, and is accompanied by a adjustment of the axial tilt angle. The computed frequencies for the axial stretching mode (379 cm -1 ) and the equatorial expansions (410 and 503 cm -1 ) of the Q 1 state agree well with the observed coherences. The weak 517 cm ‑1 oscillation is assigned to a see-saw motion of the axial tilt which is predicted ~610 cm -1 . The results provide insights into the structural relationship to the molecular evolution along potential energy surfaces of photo-excited Mn III -octahedral complexes, and can be used to guide the synthesis of optically controlled Mn III -based SMMs.
Figure 1 – Fast fourier transform (FFT) of the residuals extracted from biexponential kinetic fits across the probe spectral range. (B) Computed structural displacements and vibrational frequencies of the Q 1 state of [Mn(cyclam)(H 2 O) 2 ] 3+ at the PBE0/Def- 2-SVP level of theory. (A) Ground state geometry of [Mn(cyclam)(H 2 O) 2 ] 3+ . Green arrows show the displacement to form the relaxed Q 1 state geometry. The displacement has been scaled by a factor of 2 for clarity. (C) Kinetic trace and fit of the excited state absorption taken as an average between 370 – 380 nm. Dots are experimental data points, and the blue curve is a sum of the kinetic fit to a biexponential decay (τ 1 = 0.9 ± 0.1 ps and τ 2 = 3.7 ± 0.1 ps) and exponentially dampened cosine functions with frequencies (ω) and dephasing times (τ d ) (numerical results summarised in the inset). References 1. Liedy, F.; Eng, J.; McNab, R.; Inglis, R.; Penfold, T. J.; Brechin, E. K.; Johansson, J. O. Vibrational coherences in manganese single-molecule magnets after ultrafast photoexcitation. Nature Chemistry 2020, 12 , 452-458. 2. Barlow, K.; Eng, J.; Ivalo, I.; Coletta, M.; Brechin, E. K.; Penfold, T. J.; Johansson, J. O. Photoinduced Jahn–Teller switch in Mn(iii) terpyridine complexes. Dalton Transactions 2022, 51 , 10751-10757.
FES07
© The Author(s), 2023
Made with FlippingBook Learn more on our blog