The synthesis and photophysical characterization of a series of low-spin d6 cyclometallated organometallic compounds was carried out in efforts to investigate intermolecular excited-state proton-transfer reactions. [IrIII(ppyOH)2(N^N)]PF6 , where ppyOH is 3-(pyridin-2-yl)phenol and (N^N) is 1,10-phenanthroline (Ir-1), 2,2’-biquinoline (Ir-2), or 4,4’-di-tert-butyl-2,2’-dipyridyl (Ir-3) were successfully synthesized and characterized by spectroscopic methods. The ground-state and excited-state pKa values, pKa and pKa*, respectively, were determined 10.3 and 9.1 for Ir-1, 11.0 and 10.5 for Ir-2, and 10.4 and 10.4 for Ir-3 by performing acid/base titrations with concomitant ultraviolet–visible absorption and photoluminescence spectroscopy measurements. All were determined to have very weak room temperature emission. A comparison between the photoluminescence measurements of Ir-1 and it’s unsubstituted analog, [Ir(ppy)2phen]PF6, supports that the addition of an electron-donating group, causes a narrowing of the gap between the HOMO and LUMO orbitals, and a significant decrease in emission intensity, consistent with Jortner’s Energy Gap Law. For compounds Ir-1 and Ir-2, ΔpKa &lt; –1, which suggests they are photoacids, while for compound Ir-3, ΔpKa ≈ 0, indicating that it is not a photoacid. We also desired to introduce photobasicity into these compounds and therefore replaced bq in Ir-2 with that 2,2’-biquinoline-4,4’-dicarboxylic acid (dcbq), (Ir-4), so that the LUMO was localized on a ligand containing a protic functional group (i.e dcbq). Thus, in the excited state, the electron density would increase on that ligand and promote protonation of the functional group and thereby photobasicity, which was observed, ΔpKa = +0.4. In conclusion, these compounds form the basis of the first series of inorganic coordination compound photoacids/bases.

Why MLCT excited-states make poor photoacids, and how to overcome it

251st National Meeting (2016)

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