Charge Transfer Switching in Donor–Acceptor Systems Based on BN-Fused Naphthalimides

Six-membered
azaborine rings have been straightforwardly fused
on naphthalimide-based donor–acceptor systems, and a series
of BN-containing heteroaromatic compounds BN1–BN3 were constructed. Electron-donating triphenylamines were
functionalized in the extended direction of the 3- or/and 4-position
of the naphthalimide unit. For comparison, reference BN0 without triphenylamine was also prepared. The intramolecular charge
transfer (ICT) interactions in the resulting BN-fused naphthalimides
(BN0–BN3) together with their precursors
(N0–N3) and fluoride-coordinated
analogues (FBN0–FBN3) have been systematically
investigated by photophysical, electrochemical, and theoretical approaches.
It is found that the fusion of the azaborine ring has a great effect
on the ICT properties of the D–A systems based on BN-fused
naphthalimides. For the precursors without boron, the extension of
an electron donor from the 3-position of naphthalimide is superior
in enhancing the D–A interactions. On the contrary, upon fusion
of the azaborine ring on naphthalimide, the dominant orientation of
the ICT interactions conversely converts to the extended direction
of the 4-position of naphthalimide in the D–A molecules based
on BN-fused naphthalimides. Most interestingly, upon coordinating
the boron by a fluoride ion, the ICT interactions are dramatically
enlarged and the substitution position of the triphenylamino group
has a negligible effect on the ICT properties of the fluoride-coordinated
analogues.