Most 1,4,5,8-naphthalenediimide (NDI) derivatives, especially those with mild -acidity cannot achieve photoinduced electron-transfer (PET). Here we report our investigations on the binding interactions of a NDI diammonium dichloride salt (NDICl2) with cucurbit[n]uril (CB[n], n = 8, 10) and the formation process of a NDI radical anion upon photoexcitation of the NDI derivative in the presence of CB[n] (n = 8, 10). As a comparison, the influence of CB[7] on the PET process was also evaluated. The results show that the NDI core can be partially or fully encapsulated in CB[8] or CB[10] to form inclusion complexes at molar ratios of 1:1 (CB[8]NDI2+) or 1:2 (CB[10]2NDI(2+)). In the presence of the host, quick formation of the NDI radical anion was observed with respect to no radical anion formation without the host. According to the spectral results, interesting CB[8]-promoted charge-transfer interactions between the NDI radical anion and NDI2+ are assumed during