New excited state intramolecular proton transfer dyes based on naphthalenediimides (NDI) and its population of triplet excited state

The dyes of Naphthalenediimides (NDI) conjugated and non-conjugated with 2-(2-Hydroxyphenyl)Benzothiazole (N-1~N-5) were prepared. The compounds showed dramatically red-shifted UV–vis absorption (483–611 nm) and emission (538–635 nm) compared with traditional ESIPT compound (HBT). Among them, N-1 shows obvious dual emission at 538 nm (enol form), 682 nm (keto form) in PhCH3 and large Stokes shift (up to 144 nm), which is one of the evidence for the characteristics of ESIPT. Time-resolved transient absorption demonstrated triplet excited states were populated upon photoexcitation for all the compounds. Especially for N-3 and N-4, the life time of triplet excited states (τT) are 48.0 μs and 148.5 μs, respectively. The lifetime of triplet excited state were also observed in N-1 (τT = 72.0 μs) and N-2 (τT = 121.1 μs). The population of triplet excited states of all the compounds were testified by photooxidation for 1, 3-diphenylisobenzofuran. N-3 and N-4 exhibit much efficient photooxidative ability than N-1, N-2 and N-5.