Complete Monitoring of Coherent and Incoherent Spin Flip Domains in the Recombination of Charge-Separated States of Donor-Iridium Complex-Acceptor Triads

The spin chemistry of photoinduced
charge-separated (CS) states
of three triads comprising one or two triarylamine donors, a cyclometalated
iridium complex sensitizer and a naphthalene diimide (NDI) acceptor,
was investigated by transient absorption spectroscopy in the ns−μs
time regime. Strong magnetic-field effects (MFE) were observed for
two triads with a phenylene bridge between iridium complex sensitizer
and NDI acceptor. For these triads, the lifetimes of the CS states
increased from 0.6 μs at zero field to 40 μs at about
2 T. Substituting the phenylene by a biphenyl bridge causes the lifetime
of the CS state at zero field to increase by more than 2 orders of
magnitude (τ = 79 μs) and the MFE to disappear almost
completely. The kinetic MFE was analyzed in the framework of a generalized
Hayashi–Nagakura scheme describing coherent (S, T0 ↔ T±) as well as incoherent (S, T0 ⇌ T±) processes by a single rate constant k±. The magnetic-field dependence of k± of the triads with phenylene bridge spans
2 orders of magnitude and exhibits a biphasic behavior characterized
by a superposition of two Lorentzians. This biphasic MFE is observed
for the first time and is clearly attributable to the coherent (B < 10 mT) and incoherent (10 mT < B < 2 T) domains of spin motion induced by isotropic and anisotropic
hyperfine coupling. The parameters of both domains are well understood
in terms of the structural properties of the two triads, including
the effect of electron hopping in the triad with two donor moieties.
The kinetic model also accounts for the reduction of the MFE on reducing
the rate constant of charge recombination in the triad with the biphenyl
bridge.