Photophysical properties of 4,4′-di-tert-butyl-2,2′-bipyridine supported 6-membered 2,2′-diphenyl-X platinacycles (X = CH2, NMe, O)

Abstract

The six-membered platinacycle complex, Pt(^tBu₂bpy)(C₆H₄OC₆H₄) (6) (^tBu₂bpy = 4,4′-di-tert-butyl-2,2′-bipyridine) has been prepared from Pt(^tBu₂bpy)Cl₂ and 2,2′-dilithio-diphenyl ether. Platinacycle 6 and its analogs with X = CH₂ (4) and NMe (5) exhibit intense solid-state photoluminescence and nearly identical crystal structures. The photophysical properties of 4–6 in the visible range are dominated by mixed metal–ligand-to-ligand charge transfer (MLL′CT) transitions involving high-lying filled mixed metal–ligand orbitals (ML), composed primarily of platinacyclic ring-based d- and π-orbitals, and a low lying vacant π* orbital (L′) of the ^tBu₂bpy ligand. Lone pair donation from the bridging oxygen atom and especially the NMe group increases the energy of the mixed metal–ligand orbital (ML) without altering the energy of the ^tBu₂bpy π* orbital. As a result, the MLL′CT state energy decreases and the absorption and emission wavelengths are red-shifted. DFT and TD-DFT calculations support the experimental results. Additional calculations on the unknown platinacycles with X = CO (7) and SO₂ (8) predict a blue-shift for the MLL′CT absorption and emission. Two nearly equal energy triplet minima were located on the DFT triplet surface for 4–6. One of these (4T–6T) has a geometry very similar to the ground-state singlet (as represented by 4–6) and is associated with the emissive ³MLL′CT excited state. The other triplet-state (4T′–6T′) has a distorted structure where the platinacycle ring is twisted out of the Pt(^tBu₂bpy) plane. Thermal access to this distorted triplet may be responsible for the loss of photoluminescence in room temperature solutions of 4–6.