# Photodriven Electron Transfer Processes with MLCT and Spin-Flip Excited States of d$^6$ and d$^3$ Polypyridine Complexes

Bürgin, Tobias Hendrik. Photodriven Electron Transfer Processes with MLCT and Spin-Flip Excited States of d$^6$ and d$^3$ Polypyridine Complexes. 2022, Doctoral Thesis, University of Basel, Faculty of Science.

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Official URL: https://edoc.unibas.ch/90097/

The tridentate ligand 2,6-bis(8′-quinolinyl)pyridine (dqp) is a central motif encountered in all projects presented in this thesis. Due to its favorable ligand geometry, it ensures a large ligand field splitting which leads to long excited state lifetimes as well as high photorobustness in complexes with Ru(II) and Cr(III) metal centers. Cr(III) polypyridine complexes have been mainly investigated for their photoluminescence properties, but they remained underexplored in photodriven electron transfer reactions. The spin-flip nature of their lowest electronically excited states stands in contrast to the metal-to-ligand charge transfer (MLCT) excited state of their d$^6$ counterparts, such as Ru(II) polypyridines, which are often employed as catalysts in photoinduced transformations.
In the first project (chapter 3), the driving-force dependence for photoinduced electron transfer of 10 different electron donors to [Cr(dqp)$_2$]$^3$$^+ was found to be very similar to that of a Ru(II) polypyridine reference compound. Electron transfer to the spin-flip excited state of [Cr(dqp)_2]^3$$^+$ is therefore facilitated to the same extent as in the case of MLCT-excited Ru(II) complexes. In addition, [Cr(dqp)$_2$]$^3$$^+ features a long excited-state lifetime and high photostability even under aerobic conditions. Consequently, [Cr(dqp)_2]^3$$^+$ was employed as catalyst in photoredox reactions under air but also under anaerobic conditions that have not previously been explored for this compound class.
The focus of the second project (chapter 4) is on intramolecular photodriven electron transfer in donor-photosensitizer-acceptor (DPA) triads. Heteroleptic rod-like [M(dqp-D)(dqp-A)]$^n$$^+-type complexes with either Ru(II) or Cr(III) metal centers were synthesized and investigated for vectorial electron transfer from the electron donor to the acceptor. Whereas photodriven charge separation was observable in pump-probe experiments for the Ru(II)-based molecular compounds, no evidence for a photoinduced electron-hole pair is found with the Cr(III)-based analogues. The Cr(III)-based photosensitizer is a strong photo-oxidant, but this property mainly stems from a positively shifted ligand-centered ground-state reduction potential. Therefore, initial photoinduced reduction of the photosensitizer by the electron donor is readily achieved, but subsequent electron transfer to the electron acceptor is thermodynamically unfavorable and fast unproductive back-electron transfer to the oxidized donor is expected to occur. The Ru(II)-based triads were also investigated in pump-pump-probe experiments in which a second excitation event with time delay after formation of the charge separated state was performed. The charge separated state including two holes and two electrons is not observed, but mainly additional simple electron-hole pairs from unreacted triads were formed. The last project (chapter 5) aims to combine the high triplet energy observed in a recently investigated Ru(II) bis-diimine bis-isocyanoborato complex with the long excited state lifetime of [Ru(dqp)_2]^2$$^+$. The heteroleptic complex mer-(PPN)[Ru(dqp)(BCF)$_3$] (PPN = bis(triphenylphosphin)iminium ion, BCF = [CNB(C$_6$F$_5$)$_3$]$^−$) was successfully synthesized by borylation of the cyanide precursor. A surprisingly large Stokes shift and fast photodegradation was observed for Ru(dqp)(BCF)$_3$]$^−$ in solution. Both observations are tentatively attributed to mer-to-fac isomerization in the relevant $^3$MLCT-excited state with fac-Ru(dqp)(BCF)$_3$]$^−$ having a lower triplet energy and a more distorted octahedral geometry compared to the corresponding mer-isomer.
In summary, this thesis provides an insight into the photophysics and photochemistry of novel Ru(II) and Cr(III) complexes based on the structural motif of dqp ligands and highlights analogies and differences in the photoredox reactivity of their $^3$MLCT and spin-flip excited states.