Spin-chemical effects on intramolecular photoinduced charge transfer reactions in bisphenanthroline copper(i)-viologen dyad assemblies

Megan S. Lazorski*, Igor Schapiro, Ross S. Gaddie, Ammon P. Lehnig, Mihail Atanasov, Frank Neese, Ulrich E. Steiner, C. Michael Elliott

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

Two covalently linked donor-acceptor copper phenanthroline complexes (C-A dyads) of interest for solar energy conversion/storage schemes, [Cu(i)(Rphen(OMV)24+)2]9+=RC+A48+withRC+= [Cu(i)Rphen2]+involving 2,9-methyl (R = Me) or 2,9-phenyl (R = Ph)-phenanthroline ligands that are 5,6-disubstituted by 4-(n-butoxy) linked methylviologen electron acceptor groups (A2+= OMV2+), have been synthesized and investigatedviaquantum chemical calculations and nanosecond laser flash spectroscopy in 1,2-difluorobenzene/methanol (dfb/MeOH) mixtures. Upon photoexcitation, charge transfer (CT) statesRC2+A+A36+are formed in less than one ns and decay by charge recombination on a time scale of 6-45 ns. The CT lifetime ofRC2+A+A36+has a strong dependence on MeOH solvent fraction when R = Me, but is unaffected if R = Ph. This solvent effect is due to coordination of MeOH solvent inMeC+A48+(i.e.exciplex formation) allowed by conformational flattening of the ligand sphere, which cannot occur inPhC+A48+having bulkierPhphen ligand framework. Interestingly, the decay time of the CT state increases for both species at low magnetic fields with a maximum increase ofca.30% atca.150 mT, then decreases as the field is increased up to 1500 mT, the highest field investigated. This magnetic field effect (MFE) is due to magnetic modulation of the spin dynamics interconverting3CT and1CT states. A quantitative modeling according to the radical pair mechanism involvingab initiomultireference calculations of the complexes revealed that the spin process is dominated by the effect of Cu hyperfine coupling. The external magnetic field suppresses the hyperfine coupling induced spin state mixing thereby lengthening the CT decay time. This effect is counteracted by the field dependent processes of T0-S mixing through the Δg-mechanism and by a local mode spin-orbit mechanism. Further, the maximum MFE is limited by a finite rate of direct recombination of3CT states and the spin-rotational mechanism of spin relaxation. This study provides a first comprehensive characterization of Cu(ii)-complex spin chemistry and highlights how spin chemistry can be used to manipulate solar energy harvesting and storage materials.

Original languageAmerican English
Pages (from-to)5511-5525
Number of pages15
JournalChemical Science
Volume11
Issue number21
DOIs
StatePublished - 7 Jun 2020
Externally publishedYes

Bibliographical note

Publisher Copyright:
© The Royal Society of Chemistry 2020.

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