Study of the Heterocyclic-Substituted Platinum-1,2-Enedithiolate 3ILCT Excited States by Transient Absorption Spectroscopy

The photoluminescent 1,2-enedithiolate complexes, (dppe)Pt{S₂C₂(2-quinoxaline)(H)}, L₂Pt{S₂C₂(2-pyridinium)(H)}]⁺ where L₂ = dppm and dppe, L₂Pt{S₂C₂(4-pyridinium)(H)}]⁺, L₂Pt{₂C₂(N-Methyl-4-pyridinium)(H)}]⁺ and L₂Pt{S₂C₂(CH₂CH₂-N-2-pyridinium)}]⁺ where L₂ = dppm, dppe, and dppp are room temperature dual emitters where the emissions have thiolate to heterocycle * intraligand charge transfer character (ILCT) singlet and triplet character. The pyridinium complexes have strong triplet-triplet absorption bands at approximately 400, 520 and 630 nm with a weaker band at 800 nm while (dppe)Pt{S₂C₂(2-quinoxaline)(H)} has strong triplet-triplet absorption bands at 385 and 550 nm with weaker bands at 610 and 805 nm. By fitting the decay of the transients to single exponential kinetics, the ³ILCT* lifetimes of the pyridinium complexes where determined to be 0.7 to 15.9 s (DMSO) while the ₃ILCT* lifetime of (dppe)Pt{S₂C₂(2-quinoxaline)(H)} was determined to be 2.8 s (CH₃CN). The transient absorption spectra of the complexes is affected by the appended heterocycle rather than the bulk of ancillary phosphine ligand or whether the heterocycle is protonated or alkylated.