As incorporating ortho-carboranes into cyclometalated Ir(III) complexes, Ir-p-Cb and Ir-m-Cb, their excited-states were significantly altered by ortho-carboranes and the emissions were completely quenched in dichloromethane solution at room temperature for both complexes. On the other hand, their benzene analogs, Ir-p-Ph and Ir-m-Ph, maintained intensive emissions with emission quantum yields (Φem) of 0.88 and 0.83, respectively. This interesting phenomenon was explained that the excited states of Ir-p-Cb and Ir-m-Cb possess the second charge transfer state (Ir(IV)-ppy-Cb●–) followed by first MLCT (metal-to-ligand charge transfer) state (Ir(IV)ppy●–-Cb). The presence of second charge transfer state was confirmed by transient absorption spectroscopy. Nano-second transient absorption spectrum of Ir-p-Cb showed new absorption band centered at 455 nm which is assigned to o-Cb●– radical anion as evidenced by SEC spectra. Moreover, Ir-p-Cb and Ir-m-Cb showed significantly different temperature dependent emission behaviors. Ir-p-Cb showed very weak phosphorescence at 465 and 505 nm with new charge transfer emission peak at 711 nm while strong phosphorescent was observed at 476 and 512 nm for Ir-m-Cb at 77 K in mTHF solution. This result indicates that their activation energies from MLCT to the charge separated state for Ir(IV)−Cb●– are different. Namely, second charge separated state of Ir(IV)−Cb●– in Ir-p-Cb is activationless process while Ir-m-Cb has a significant energy barrier to the Ir(IV)−Cb●– state from the first MLCT state, Ir(IV)ppy●–-Cb. Successful intermolecular electron transfers from the Ir-p-Cb to the MV2+ was observed by transient absorption spectra. Electrocatalytic hydrogen generation was successfully demonstrated with Ir-p-Cb as an electron mediator in the presence of Pt dispersed in polyvinyl-alcohol (PVA-Pt) as electrocatalyst.

Photosensitization behavior of supramolecular Ir(III)-carborane complex in application to photocatalytic hydrogen production: Efficient visible light-driven charge separation and mediation

255th National Meeting (2018)

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