Excited-state structural characterisation of a series of nanosecond-lived [Fe(terpy)2]2+ derivatives using X-ray Solution Scattering

[Fe(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 (EW) or Electron Donating (ED) 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 (MD) calculations. The quintet state lifetime is determined to vary by more than a factor of ten (from 1.5 to 16 nanoseconds) 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.