The UV/Vis peak at 270 nm in phenol occurs at 287 in phenolate. What causes this red-shift? Well, the HOMO-LUMO gap must have been reduced. You know that one mechanism to reduce the HOMO-LUMO gap consists in the extension of the pi-system (e.g. the alkene series, benzene to phenol). Is this a likely mechanism to explain the difference between phenol and phenolate? The pi-systems contain the same number of electrons on the same number of atoms. The reason why the HOMO-LUMO gap is reduced in going from phenol to phenolate lies with the increase in the HOMO energy. The formation of the anion causes electron-electron repulsion and increases the HOMO energy.