Iron(II) coordination compounds with improved octahedral character and photophysical properties for solar energy harvesting

A series of novel Fe(II) coordination complexes were synthesized and investigated for enhanced structural and photophysical properties. Spacer sulfur atoms were introduced in a terpyridine ligand motif, and a near-perfect octahedral coordination environment resulted in the stabilization of the spin-crossover transition between LS and HS in the crystallographic state. Two nonequal Fe(II) centers were contained within the unit cell of the homoleptic complex Fe(TPS)₂, with bond lengths unambiguously attributed to different spin-states on the nonequal Fe(II) centers within the same crystal at 100 K, and a gradual increase to LS character at higher temperatures. Two pincer-type ligands were synthesized in an effort to improve the excited-state lifetime decay pathway in Fe(II) complexes. Both ligand motifs incorporated a coordinating amido bridging moiety, which resulted in panchromatic light absorption, with charge-transfer bands in the far-vis and NIR regions. Furthermore, computational calculations confirmed that both complexes’ HOMOs are composed of contributions from both metal and ligand orbitals, as opposed to metal-centered as in most classical complexes. A pincer ligand with flanking NHCs was successful in raising the ⁵MC energy to a noncompetitive height with respect to the ³MLCT state, resulting in deactivation of the excited state via the ³MC state. When the NHCs were substituted for pyridines in the second ligand framework, the decay pathway proceeded through the ³MLCT to the ³MC and ⁵MC states. In both cases, the excited state ³MLCT decays were more than an order of magnitude larger than those in the prototypical comparison complex, Fe(tpy)₂.Science, Faculty ofChemistry, Department ofGraduat