Controlling the energetics of spin crossover (SCO) in Fe(II) polypyridine complexes is critical for designing new multifunctional materials or tuning the excited-state lifetimes of iron-based photosensitizers. It is well established that the Fe−N “breathing” mode is important for intersystem crossing from the singlet to the quintet state, but this does not preclude other, less obvious, structural distortions from affecting SCO. Recent work has shown that halogenation at the 6 and 6" positions of tpy (tpy = 2,2';6',2"-terpyridine) in [Fe(tpy)2]2+ dramatically increased the lifetime of the excited MLCT state and also had a large impact on the ground state spin-state energetics. To gain insight into the origins of these effects, we used density functional theory calculations to explore how halogenation impacts spin-state energetics and molecular structure in this system. Based on previous work we focused on the ligand “rocking” motion associated with SCO in [Fe(tpy)2]2+ by constructing one-dimensional potential energy surfaces (PESs) along the tpy rocking angle for various spin states. It was found that halogenation has a clear and predictable impact on ligand rocking and spin-state energetics. The rocking is correlated to numerous other geometrical distortions, and these should have significant impacts on the reorganization energies for spin-state changes. We have therefore quantified these trends in reorganization energy and also driving force for various spin-state changes and used these results to interpret the experimentally measured excited state lifetimes.

Halogenation affects driving forces, reorganization energies and “rocking” motions in strained [Fe(tpy)2]2+ complexes

256th National Meeting (2018)

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