Excited-state structural characterization of a series of nanosecond-lived [Fe(terpy)2]²⁺ derivatives using x-ray solution scattering

[ F e (terpy)₂ ]²⁺ (terpy = 2,2′:6′,2″-terpyridine) is a transition metal complex where the spin state is photoswitchable and where the properties of the metal-centered quintet excited state (⁵MC) can be tuned by substituting different electron withdrawing or electron donating groups on the 4′ position of the terpyridine. To better understand the physics determining the photoswitching performance, a deeper insight into the positions of the relevant potential energy surfaces and the molecular structure of the ⁵MC state is needed. We present a structural investigation based on Time Resolved x-ray Solution Scattering (TR-XSS) by which we determine the average d_Fe-N bond-length elongation following population of the ⁵MC state as well as the lifetime of this state in a series of seven modified [Fe(terpy)₂]²⁺ systems in aqueous solution following photo-excitation. The analysis of the TR-XSS data is supported by Density Functional Theory (DFT) and Molecular Dynamics calculations. The quintet state lifetime is determined to vary by more than a factor of 10 (from 1.5 to 16 ns) based on the electron withdrawing/donating properties of the substituting group. Both the DFT calculations and the structural analysis of the experimental data show that the main photo-induced change in metal-ligand bond lengths Δd_Fe-N is ∼0.2 Å for all systems.