Interesting spin state properties of iron(II) polypyridine complex substituted by fluorine: A theoretical study

Abstract

The special iron(II) polypyridyl complex substituted by fluorine ([Fe(dftpy)2]2+, dftpy = 6,6″-difluoro-2,2’; 6′2″-terpyridine) with uncommon mixed ground states has been detected recently, but the related explanation and characterization are not enough, especially for the influence of substituents. On the basis of previous studies, the key problem of mixed spin states is expected to be described very well by means of theoretical calculations. In this work, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been carried out to study the characteristics of the excited states in detail. Comparing with the parent complex [Fe(tpy)2]2+ (tpy = 2,2′:6′,2″-terpyridine) and corresponding bromine or chlorine substituted derivates, the ground states of [Fe(dftpy)2]2+ stay around the mixture of singlet state and quintet state, but also there is rare high spin excited state lifetime. Among the halogen substituted complexes, [Fe(dftpy)2]2+ has the shortest MLCT state lifetime of 14.0 ps which has been much longer than subpicosecond lifetime of [Fe(tpy)2]2+. The reason is explored by the combination of electronic structures, absorption properties, extended transition state coupled with natural orbitals for chemical valence (ETS-NOCV) and potential energy curves (PECs). We can find that the bond lengths of Fe-Nt play a significant role on the change of metal centered (MC) ground states. With Fe-Nt extended by fluorine atoms, the quintet state becomes lower than the singlet state. Due to the deformation of structures, the interactions between metal and ligands diminish and give rise to weaker d orbital splitting than that of [Fe(tpy)2]2+, but slightly impact the pairwise orbital deformation density characteristics. And the PEC of 5MC intersects with 1,3MC which renders faster non-radiative deactivation through low-lying energy crossing points.