Stark Spectroscopic Evidence that a Spin Change Accompanies Light Absorption in Transition Metal Polypyridyl Complexes.

The "Franck-Condon" (FC) excited state is the first state created when a molecule absorbs a visible photon. Here we report Stark and visible absorption spectroscopies that interrogate the FC state of rigorously diamagnetic [M(bpy)3]2+ complexes, where bpy is 2,2'-bipyridine and M = Fe, Ru, and Os. Direct singlet-to-triplet metal-to-ligand charge transfer (MLCT) transitions are evident in the 550-750 nm region of the absorbance spectrum of [Os(bpy)3]2+, yet are poorly resolved or absent for [Ru(bpy)3]2+ and [Fe(bpy)3]2+. In the presence of a strong 0.4-0.8 MV/cm electric field, well-resolved transitions are observed for all the complexes in this same spectral region. In particular, an electroabsorption feature at 633 nm (15 800 cm-1) provides compelling evidence for the direct population of a high spin [Fe(bpy)3]2+* MLCT excited state. Group theoretical considerations and Liptay analysis of the Stark spectra revealed dramatic light-induced dipole moment changes in the range [Formula: see text] = 3-9 D with the triplet transitions consistently showing shorter charge transfer distances. The finding that the spin of the initially populated FC excited state differs from that of the ground state, even with a relatively light first row transition metal, is relevant to emerging applications in energy up-conversion, dye sensitization, spintronics, photoredox catalysis, and organic light emitting diodes (OLEDs).