Photoinduced Electron Transfer Dynamics in Triarylamine–Naphthalene Diimide Cascades

A series of dyads and triads of the A-D and A-D1-D2 type, respectively, containing triarylamine (TAA) donors and naphthalene diimide (NDI) acceptors, which are linked via triazole (Tz) heterocycles, were synthesized by Cu­(I)-catalyzed azide alkyne cycloaddition (CuAAC). Upon photoexcitation, these systems undergo charge separation leading to long-lived charge-separated (CS) states. The population of these CS states was monitored using femtosecond and nanosecond transient absorption spectroscopy. The transient signals of the CS states of all triads and dyads feature biexponential decays in the nanosecond time regime with a short and a long component. These biexponential decays are the result of an ISC from the primarily populated ¹CS state into the ³CS, from which charge recombination to the S₀ state is forbidden by spin conservation rules. The existence of ³CS states in the triads was confirmed by strong magnetic field dependent transient absorption kinetics, while for the dyads no effect could be observed due to a much larger singlet–triplet splitting. Thus, although charge recombination from the ¹CS state in the triads is slowed down compared to the dyads, the lifetime of the ³CS states is clearly longer in the dyads. This is the result of the larger singlet–triplet splitting in the dyads which leads to lifetimes of several microseconds