Presently described are multilayer optical films, oriented polyester films, negatively birefringent copolyester polymers, fluorene monomers, and polyester polymers prepared from such fluorene monomers. In one embodiment, the multilayer optical film comprises at least one first birefringent optical layer; and at least one second optical layer having a lower birefringence than the first optical layer; wherein at least one of the optical layers comprises a negatively birefringent polyester polymer comprising a backbone and repeat units comprising at least one pendent aromatic group that is conformationally locked relative to the backbone.