Photophysics and Excited-State Properties of Cyclometalated Iridium(III)–Platinum(II) and Iridium(III)–Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine

We investigated the electrochemical and excited-state properties
of 2,3-bis­(2-pyridyl)­pyrazine (dpp)-bridged bimetallic complexes,
(L)2Ir-dpp-PtCl [1, L = 2-(4′,6′-difluorophenyl)­pyridinato-N,C2 (dfppy); 2, L = 2-phenylpyridinato-N,C2 (ppy)] and [(L)2Ir]2(dpp) [3, L = dfppy; 4, L = ppy] compared to monometallic complexes,
(L)2Ir-dpp (5, L = dfppy; 6,
L = ppy) and dpp-PtCl (dpp-PtIICl2; 7). The single-crystal X-ray crystallographic structures of 1, 3, 5, and 6 showed
that 1 and 3 have approximately coplanar
structures of the dpp unit, while the noncoordinated pyridine ring
of dpp in 5 and 6 is largely twisted with
respect to the pyrazine ring. We found that the properties of the
bimetallic complex significantly depended on the electronic and geometrical
modulations of each fragment: (1) electronic structure of the main
L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2)
planarity of the bridging ligand (dpp). Their electrochemical and
photophysical properties revealed that efficient electron-transfer
processes predominated in the bimetallic systems regardless of the
second metal participation. The low efficiencies of photoluminescence
of dpp-bridged Ir–Pt and Ir–Ir bimetallic complexes
(1–4) could be explained by assuming
the involvement of crossing to platinum- and iridium-based d–d
states from the emissive state. Such stereochemical and electronic
situations around dpp allowed thermally activated crossing to platinum-
and iridium-based d–d states from the emissive triplet metal-to-ligand
charge-transfer (3MLCT) state, followed by cleavage of
the dpp-Pt and (L)2Ir-dpp bonds. The transient absorption
study further confirmed that the planarity of the dpp bridging ligand,
which was defined as the magnitude of tilt between the pyridine ring
and pyrazine, had a direct correlation with the degree of nonradiative
decay from the emissive iridium-based 3MLCT to the Ir d–d
or Pt d–d state, leading to photoinduced dissociation of bimetallic
complexes. From the dissociation pattern of metal complexes analyzed
after photoirradiation, we found that their dissociation pathways
were directly related to the quenching direction (either Ir d–d
or Pt d–d) with a significant dependency on the relative 3MLCT levels of the (L)2Ir-dpp component.