Varying Diphosphine Bridge Length in Cyclometalated Platinum(II) Dimers Tunes Photophysics by Modulating Orbital...

Compared to previously explored bidentate cyclometalating motifs in Pt(II) dimers, tridentate cyclometalating ligands such as 4-tolyl-6-phenyl-2,2'-bipyridine are believed to improve d_z²-d_z² angular orbital overlap between the Pt centers and promote metal-metal-to-ligand charge transfer (MMLCT) at greater metal distances. To evaluate this, a series of Pt(II) dimers bridged by diphosphines with varying alkyl spacers, Ph₂P(CH₂)_nPPh₂ [n = 1 (1), 2 (2), and 3 (3)], were prepared to investigate the lowest-energy photophysics as a function of Pt-Pt distance. Additionally, a mononuclear complex with an ancillary triphenylphosphine (4) was studied as a baseline for mixed ligand-centered/metal-to-ligand charge transfer (LC/MLCT) character. The photophysical behaviors of 3 and 4 were nearly identical, with both exhibiting "mononuclear-like" LC/MLCT photophysics. Conversely, 1, featuring the shortest metal separation and greatest orbital overlap, showed red-shifted MMLCT spectral features. The assigned ³MMLCT excited state was populated with a 155 ps time constant, likely through intersystem crossing (ISC) from the nested ¹MMLCT state. Finally, 2 exhibited both ³LC/³MLCT- and ³MMLCT-based photophysics, with the two triplet states in thermal equilibrium separated by a 744 cm^-1 energy gap. Relaxation from this superposition of states occurs concurrently on the microsecond time scale, likely following internal conversion from a higher-energy ³LC excited state.