Structure and Ultrafast Dynamics of the Charge-Transfer Excited State and Redox Activity of the Ground State of Mono- and Binuclear Platinum(II) Diimine Catecholate and Bis-catecholate Complexes: A Transient Absorption, TRIR, DFT, and Electrochemical Study

A series of mononuclear complexes of the type [Pt(Bu(2)cat)(4,4'-R-2-bipy)] [where Bu2cat is the dianion of 3,5-Bu-t(2)-catechol and R = H, Bu-t, or C(O)NEt2] and analogous dinuclear complexes based on the back-to-back bis-catechol ligand 3,3',4,4'-tetrahydroxybiphenyl have been studied in detail in both their ground and excited states by a range of physical methods including electrochemistry, UV/vis/near-IR, IR, and electron paramagnetic resonance spectro-electrochemistry, and time-resolved IR (TRIR) and transient absorption (TA) spectroscopy. Density functional theory calculations have been performed to support these studies, which provide a detailed picture of the ground- and excited-state electronic structures, and excited-state dynamics, of these complexes. Notable observations include the following: (i) for the first time, the lowest-energy catecholate -> bipyridine (bpy) ligand-to-ligand charge-transfer (LL'CT) excited states of these chromophores have been studied by TRIR spectroscopy, showing a range of transient bands associated with the bpy radical anion and semiquinone species, and back-electron-transfer occurring in hundreds of picoseconds; (ii) strong electronic coupling between the two catecholate units in the bridging ligand of the dinuclear complexes results in a delocalized, planar (class 3) mixed-valence catecholate2(-)/semiquinone(is approximately equal to) state formed by one-electron oxidation of the bridging ligand; (iii) in the LL'CT excited state of the dinuclear complexes, the bridging ligand is symmetrical and delocalized, whereas the bpy radical anion is localized at one terminus of the complex. This study is the first example of an investigation of excited-state behavior in platinum(II) catecholate complexes, performed with the use of picosecond TRIR and femtosecond TA spectroscopy.