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

Abstract

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.