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.
Electronic communications between S- and/or Se-heteroarenes containing cyclopenta[b]thiopyran and cyclopenta[b]selenopyran moieties have been investigated.
In efforts to enhance the activity of liposomal drugs against solid tumors, three novel lipids that carry imidazole-based headgroups of incremental basicity were prepared and incorporated into the membrane of PEGylated liposomes containing doxorubicin (DOX) to render pH-sensitive convertible liposomes (ICL). The imidazole lipids were designed to protonate and cluster with negatively charged phosphatidylethanolamine-polyethylene glycol when pH drops from 7.4 to 6.0, thereby triggering ICL in acidic tumor interstitium. Upon the drop of pH, ICL gained more positive surface charges, displayed lipid phase separation in TEM and DSC, and aggregated with cell membrane-mimetic model liposomes. The drop of pH also enhanced DOX release from ICL consisting of one of the imidazole lipids, sn-2-((2,3-dihexadecyloxypropyl)thio)-5-methyl-1H-imidazole. ICL demonstrated superior activities against monolayer cells and several 3D MCS than the analogous PEGylated, pH-insensitive liposomes containing DOX, which serves as a control and clinical benchmark. The presence of cholesterol in ICL enhanced their colloidal stability but diminished their pH-sensitivity. ICL with the most basic imidazole lipid showed the highest activity in monolayer Hela cells; ICL with the imidazole lipid of medium basicity showed the highest anticancer activity in 3D MCS. ICL that balances the needs of tissue penetration, cell-binding, and drug release would yield optimal activity against solid tumors.
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