Magnetic-field-dependent delayed fluorescence from thermally activated reverse charge separation of spin-correlated charge separated states.

Thermally activated delayed fluorescence (TADF), involving reversible electron transfer between a fluorescing S1 state and a magnetically responsive charge-separated (CS) state, constitutes a sensitive probe of the spin-chemical dynamics in the CS state as well as of the local magnetic molecular environment. With (Cl)TAA-mB-PDI and SQA-pB-PDI-featuring a Cl-substituted triarylamine ((Cl)TAA) or a squaraine unit as electron donors, meta-benzene (mB) or para-benzene (pB) bridges, and a perylene diimide (PDI) unit as electron acceptor-we introduce two novel triads with sufficiently small S1-CS energy gaps to enable TADF. Their excited state properties were investigated by femtosecond and nanosecond time-resolved transient absorption spectroscopy as well as nanosecond time-resolved fluorescence spectroscopy at variable temperatures and in variable magnetic fields. The variable temperature data were used for a full kinetic and thermodynamic characterization of the reversible electron transfer in the excited state. The magnetic-field-dependent kinetic data were thoroughly analyzed using three models of increasing complexity that explicitly include the role of the S1 state: a purely classical model, a mixed quantum - classical model, and a fully quantum-dynamical model. It is shown that the magnetic-field-dependent decay of the total CS state population is well approximated by a mono-exponential function with a magnetic-field-dependent rate constant k CSS(B). Furthermore, the delayed fluorescence proves to be a reliable indicator of the spin-dependent dynamics of the CS state. Together with two structurally related triads lacking TADF, the magnetic field effects of the new systems are placed into a broader framework that categorizes such effects using three characteristic parameters: the resonance field B res = 2J, the resonance line width (fwhm), and the relative magnetically induced kinetic range R k of k CSS(B). We demonstrate how these characteristic parameters depend on the kinetic properties of the systems.