KLOC-6 is a so-called fullerene ketolactam. These fullerene ketolactams are strong electron acceptors, because they have a LUMO level approximately 0.2 V below that of PCBM. This makes the ketolactams a stronger acceptor than C60 itself, which is quite unusual for a C60-fullerene derivative. These strong acceptors are highly desirable when electron-donating polymers with low-lying LUMO levels are used.
Because of the low-lying LUMO level, KLOC -6 is also a possible replacement for PCBM in certain organic electronics devices.