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.