A leaning amine-ketone dyad with a nonconjugated linker: solvatofluorochromism and dual fluorescence associated with intramolecular charge transfer.

Dyad 4, comprising a triphenylamine (TPA) electron donor and 1,4-pentadien-3-one (pentadienone) electron acceptor tethered by a nonconjugated linker, displays solvatofluorochromism (SFC) and dual fluorescence associated with intramolecular charge transfer (ICT) in the excited state. While the fluorescence arises from a locally excited state of 4 (LE-4*) in saturated hydrocarbon solvents, the fluorescence from the ICT state of 4 (ICT-4*) occurs in aprotic solvents. ICT-4* has a much greater dipole moment than its corresponding ground state. The results of theoretical calculations suggest that the conversion of LE-4* to ICT-4* involves a unique structural change like a leaning of the pentadienone moiety. Two factors are responsible for the significant SFC displayed by 4, the first being the high electron-donating and -accepting abilities of the respective locally excited TPA and pentadienone moieties in LE-4* and the other being a rigid ethano bridge that links the two moieties in ICT-4*. The former property facilitates photoinduced electron-transfer (PET) and the latter prevents full single electron transfer (SET) by prohibiting direct π-conjugation and the spatial approach of the two dyad components. Consequently, these electronic and geometrical features lead to SFC arising from a large dipole moment change caused by ICT and partial intramolecular SET.