Resonance Raman Excitation Profiles of Fe(II)-Terpyridine Complexes: Electronic Effects of Ligand Modifications.

Abstract

Metal 2,2':6',2″-terpyridine (tpy) complexes are readily used as building blocks in metallo-supramolecular polymers that stand out for their photophysical properties in solar energy assemblies. Furthermore, Resonance Raman (RR) excitation profiles are sensitive indicators of the electronic properties of chromophores. Previously, using RR spectroscopy, we studied the [Fe(tpy)2]2+ complex and metallo-supramolecular polymers formed by tpy derivatives and Fe(II) ions. Here, we compare RR spectra of iron (Fe(II)) complexes with 4'-substituted tpy ligands─[Fe(4'-R-tpy)2]2+, with R = H (1a), Cl (2a), 4-chlorophenyl (3a), and 2-thienyl (4a) to describe changes in their electronic structure after functionalization. By combining theoretical calculations, RR, and UV/vis spectra, we elucidated differences in the RR excitation profiles of 1a, 2a, and 4a complexes. In all Raman modes, complexes 1a and 2a showed maximal enhancement only at 532 nm excitation, whereas complex 4a exhibited maximal enhancement selectively at either 532 or 633 nm excitations. Based on our calculations, the mixed metal/ligand character of the highest occupied molecular orbital (HOMO) of 4a complex manifests itself through selective enhancement of vibration modes, mainly localized on the 2-thienyl unit at 633 nm excitation, which may explain the unique behavior of this complex. Therefore, complex 4a is a prospective candidate for further detailed photophysical explorations toward developing sensitizers for solar cells.