The insertion of organometallic molecules into the hollow core of single-walled carbon nanotubes can drastically change their properties. Using biocompatible standardized suspensions of pristine, opened, and filled nanotubes, a very selective enhancement of the photoluminescence and optical absorption is observed. Via ferrocene encapsulation, the PL signal increases almost by a factor of three for tubes with chiralities such as (8,6) and (9,5). This behavior is attributed to a local electron charge transfer from the ferrocene molecules that balances out the p-type doping of the nanotubes resulting from the modified charge distribution of the surfactant molecules and the opening process. The near infrared photoluminescence of the nanotubes in solution is strongly enhanced when ferrocene is encapsulated. The diameter-dependent charge transfer is additionally confirmed by first principles calculations. These findings highlight an essential ingredient to optimize the application of solvated nanotubes, for instance, as in–vivo near infrared sensors in biomedical research.