Intermolecular Electron Transfer Processes from the Excited Singlet and Triplet States of the Naphthalenediimide Dianion

The contributions of the naphthalenediimide dianion in its singlet ( 1 NDI 2– *) and triplet ( 3 NDI 2– *) excited states to intermolecular electron transfer (ET) were investigated for the first time via picosecond and nanosecond transient absorption measurements of NDI 2– in the presence of various electron acceptors. Upon photoexcitation at 610 nm, excited-state absorption bands of NDI 2– were observed with lifetimes of 8.3 ns and 3.1 μs, corresponding to 1 NDI 2– * and 3 NDI 2– *, respectively. In addition, absorption attributable to NDI •– was observed, indicating partial ET from the excited state to the solvent. Upon addition of 20 mM 2-naphthonitrile (2-NN) as an electron acceptor to the NDI 2– solution, the decay of 1 NDI 2– * and the rise of NDI •– were accelerated, confirming ET from 1 NDI 2– *. In contrast, upon addition of 0.2 mM 2-NN, the decay of 3 NDI 2– * became faster, whereas that of 1 NDI 2– * remained essentially unchanged, demonstrating that ET proceeded predominantly from 3 NDI 2– *. A similar ET behavior was observed with other acceptors. When the free energy change (−Δ G interET ) was sufficiently large, ET proceeded at rates approaching the diffusion-controlled limit ( k diff = 3.3 × 10 9 M –1 s –1 ) in dimethyl sulfoxide. These findings elucidate the previously unexplored excited-state dynamics and photoinduced intermolecular ET characteristics of NDI 2–, thereby providing a basis for subsequent photochemical investigations of excited multicharged ions and their prospective utilization in functional semiconductors.