Enhanced MMLCT character in thiol-bridged Pt(II) dimers featuring tridentate cyclometalating ligands

Thiol-pyridine and thiol-quinoline bridging ligands have previously been shown to reduce the metal-metal separation in Pt(II) dimers. Additionally, coordination complexes of the tridentate cyclometalating ligand 4-tolyl-6-phenyl-2,2'-bipyridine (4tpbpy) exhibit enhanced Pt d_z² orbital overlap through improved π-stacking of the cyclometalating ligands (CMLs), lengthening the Pt-Pt distance at which metal-metal-to-ligand charge transfer (MMLCT) transitions can be observed. To collectively study these phenomena, a series of molecules using the 4tpbpy CML with thiol-containing bridges: 2-benzothiazolethiol (2), 2-quinolinethiol (3), 2-pyridinethiol (4), and 6-methyl-2-pyridinethiol (5), was synthesized. Their photophysical properties were compared with a model complex known to exhibit MMLCT photophysics within the same tridentate motif, [Pt₂(4tbpy)₂(µ-dppm)][PF₆]₂ (1) (dppm = bis(diphenylphosphino)methane). The MMLCT energies in 2-5 were significantly lower than in 1 and in similar bidentate systems, as evidenced by red-shifted absorption bands extending past 650 nm and photoluminescence (PL) maxima between 750-790 nm. Femtosecond transient absorption spectroscopy showed that the population and subsequent relaxation of the ³MMLCT excited state in 2-5 are similar to those in 1, with noticeably reduced time constants. The growth of the ³MMLCT excited state in 2-5 was observed with a shortened time constant of ∼55 ps, which subsequently relaxes on the timescale of 2-7 ns, atypical for these triplet charge transfer excited states.