Switched fluorescence and photosensitization based on reversible ion-pairing.

The close ion-pair of Ir(bdpSO 3 )(cod)(IMes)] is an efficient photosensitizer, while the solvent-separated anion is highly fluorescent. Controlling ion association governs the path of the excitation energy which is channeled either to the S 1 state (fluorescence) or via intersystem crossing into the T 1 state ( 1 O 2 generation). Modulation of NHC donation affects the photosensitizing properties and only electron-rich NHC provide highly efficient photosensitizers.