Determining the Quintet Lifetimes in Side-ring Substituted [Fe(terpy)2]2+ Complexes

We previously studied the effect of 4' substitution in iron(II)-bis-terpyridine complexes, showing that the photoexcited high-spin quintet-state is stabilized by electron-donating substituents. In this paper we explore the effects of electron-donating ( X = NH2 , Cl ) and withdrawing ( X = NO2 ) substituents in the 5,5" positions on the stability and lifetime of the quintet-state. We used a simple densitiy-functional theory (DFT) based method that had been proven fairly accurate in the case of 4' substitution to estimate the energy barrier of the quintet-singlet transition and thereby predict the quintet state lifetime. We synthetized the complexes and used ultrafast transient optical absorption spectroscopy to experimentally determine the quintet lifetimes, in order to test the applicability of these quantum-chemistry based predictive methods for these side-ring substitution cases. UV-Visible spectra of the complexes have shown that the metal-to-li­gand charge transfer (MLCT) and ligand-localized transitions of these complexes change according to the previous observations. We have shown that in the 5,5" positions, electron withdrawing groups stabilize the quintet state, while donating groups destabilize it. This is in stark contrast to the effects previously observed for the 4' case, and indicates that unlike the latter case, the simple concept of inductive and mesomeric effects may not be adequate to describe the changes due to 5,5" substitution, warranting further study of the area.