Photoinduced\nElectron Transfer Dynamics in Triarylamine–Naphthalene\nDiimide Cascades

Abstract

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