According to NewMoney.gov:
“The blue 3-D Security Ribbon on the front of the new $100 note contains images of bells and 100s that move and change from one to the other as you tilt the note. The Bell in the Inkwell on the front of the note is another new security feature. The bell changes color from copper to green when the note is tilted, an effect that makes it seem to appear and disappear within the copper inkwell.
The new design for the $100 note retains three effective security features from the previous design: the portrait watermark of Benjamin Franklin, the security thread, and the color-shifting numeral 100.
The back of the note has a new vignette of Independence Hall featuring the rear, rather than the front, of the building. Both the vignette on the back of the note and the portrait on the front have been enlarged, and the oval that previously appeared around both images has been removed.”