Ok i admit it, i can't really understand what you want to do. When i read you message it sounds like you want the capacitor to be discharged as quickly as possible. If this is what you want, you could make the pin low output, wait a short time and then switch it back to an input. This will release the charge in a controlled manner. If, on the other hand, you want the capacitor to keep the charge longer you should make an OPampbuffer as suggested.

/Ingvar