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

Abstract

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 μ Δ ⎯⇀ ⎯ = 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).T he electron spin in the metal-to-ligand charge-transfer (MLCT) excited states utilized for applications in displays, sensing, solar energy conversion, and organic synthesis differ from that of the ground state. Stark and visible absorption spectroscopies directly probe the "Franck−Condon" (FC) excited state of [M(bpy) 3 ] 2+ complexes, where bpy is 2,2′-bipyridine and M is Fe, Ru, and Os. It is shown herein that Stark spectroscopy is a more sensitive technique than absorption spectroscopy for ΔS ≠ 0 electronic transitions and provides experimental values of the excited state dipole moment. In particular, a previously unresolved low energy transition for solvated [Fe(bpy) 3 ] 2+ * indicates a paramagnetic FC excited state. Group theoretical and Liptay analysis provides a large dipole moment change, μ |Δ | ⎯⇀ ⎯

A Roadmap Towards Visible Light Mediated Electron Transfer Chemistry with Iridium(III) Complexes

“…Depending on the nature of the chelating ligands, these correspond to a mixture of metal‐to‐ligand (MLCT) and ligand‐to‐ligand (LLCT) transitions, which are usually characterized by slightly smaller molar absorption coefficients (5000–10000 M −1 cm −1 ). Due to the large spin orbit coupling of the iridium center, small contributions from direct triplet excitation (<1000 M −1 cm −1 ) may be observed in the visible range [19] . While the absorption bands are typically similar for the different families of complexes, they may vary in terms of energy, shape and molar absorption coefficients as an effect of the number of Ir−C bonds and the nature of the different chelating ligands.…”

Perspectives on Dye Sensitization of Nanocrystalline Mesoporous Thin Films

“…Electrons injected into TiO 2 emanate an electric field that significantly influences the absorption spectra of surface anchored sensitizers. , The electro-absorption features provide a useful means to quantify the impact of electric fields on sensitizer orientation, ion migration (termed screening), and interfacial electron transfer. − The feature has also been utilized to quantify the field strength of rigid-rod sensitizers set at variable distances from the TiO 2 surface . A simplified basis for the electro-absorption is shown in Figure a.…”

“…This uncertainty inspired construction of a traditional Stark apparatus that has shown Δμ⃗ to be sensitive to functional groups and to spin changes that accompany light absorption. 93,94 Molecular sensitizers with well-defined Δμ⃗ values positioned precisely at the TiO 2 electrolyte interface can serve as in situ probes of the electric fields present in regenerative and photoelectrosynthetic cells.…”

A biohybrid strategy for enabling photoredox catalysis with low-energy light

“…56 However, the extremely low molar absorptivity ($10 2 M À1 cm À1 ) of this state limits its light-harvesting ability. [57][58][59] Upconversion of triplet states in a sensitizer/photocatalyst mixture was introduced as an alternative strategy to use near-infrared light, but with low photon conversion efficiency. [60][61][62] Finally, sensitization of a transition-metal photoredox catalyst through energy transfer from light-harvesting ligands was demonstrated, but its impact on reactivity was not investigated.…”