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

The contributions of the naphthalenediimide dianion in its singlet (1NDI2-*) and triplet (3NDI2-*) excited states to intermolecular electron transfer (ET) were investigated for the first time via picosecond and nanosecond transient absorption measurements of NDI2- in the presence of various electron acceptors. Upon photoexcitation at 610 nm, excited-state absorption bands of NDI2- were observed with lifetimes of 8.3 ns and 3.1 μs, corresponding to 1NDI2-* and 3NDI2-*, 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 NDI2- solution, the decay of 1NDI2-* and the rise of NDI•- were accelerated, confirming ET from 1NDI2-*. In contrast, upon addition of 0.2 mM 2-NN, the decay of 3NDI2-* became faster, whereas that of 1NDI2-* remained essentially unchanged, demonstrating that ET proceeded predominantly from 3NDI2-*. A similar ET behavior was observed with other acceptors. When the free energy change (-ΔGinterET) was sufficiently large, ET proceeded at rates approaching the diffusion-controlled limit (kdiff = 3.3 × 109 M-1 s-1) in dimethyl sulfoxide. These findings elucidate the previously unexplored excited-state dynamics and photoinduced intermolecular ET characteristics of NDI2-, thereby providing a basis for subsequent photochemical investigations of excited multicharged ions and their prospective utilization in functional semiconductors.