Distance Dependence for Intramolecular Electron Transfer from Excited Naphthalenediimide Dianion

The properties of excited naphthalenediimide dianion (NDI^2-*) and its intramolecular electron transfer (ET) dynamics were comprehensively explored for the first time using subnanosecond and subpicosecond transient absorption spectroscopies on NDI^2- and NDI^2--acceptor (A) dyads. Upon photoexcitation, NDI^2- produced singlet (S₁) and triplet (T₁) states with lifetimes of 13 and >200 ns, respectively. The fluorescence lifetime of NDI^2- was determined to be 13.3 ns, with a quantum yield of 0.71, notably exceeding the values reported for other dianionic species. Efficient intramolecular ET from NDI^2-* within several picoseconds to tens of picoseconds was confirmed in all of the dyads. The investigation into the distance dependence of ET from excited dianions revealed that both forward ET and backward electron transfer (BET) slowed in the dyads with increasing donor-acceptor separation imposed by the varied spacer. In particular, the BET observed herein displayed significant distance dependence and a markedly extended lifetime, probably because of a BET pathway with an increased energy barrier. These results provide a clear description of the previously uncharted excited-state and photoinduced ET features of NDI^2-, laying the foundation for further photochemical studies of excited multi-ions and their potential applications in advanced semiconducting materials.