Charge-Transfer and Ligand-Localized Photophysics in Luminescent Cyclometalated Pyrazolate-Bridged Dinuclear Platinum(II) Complexes

Abstract

We present the synthesis, photophysical characterization, and electrochemistry of three series of cyclometalated binuclear platinum(II) complexes, each bridged by two 3,5-disubstituted pyrazolate ligands (μ-R2pz). These neutral compounds have the general formula [C∧NPt(μ-R2pz)]2, where C∧N is a cyclometalating ligand corresponding to 2-(2′-thienyl)pyridine (thpy), 1-phenylisoquinoline (piq), or 7,8-benzoquinoline (bzq) with R = H, Me, iPr, Ph, corresponding to series I–III dimers, respectively. Systematic variation of the cyclometalating ligands in addition to the bridging pyrazolates renders colorful structures exhibiting a range of electrochemical and spectroscopic behavior with absorption and photoluminescence properties tuned over a wide portion of the visible spectrum. Steric bulk introduced into the 3,5-positions on the pz bridges readily modulates intramolecular d8–d8 metal–metal σ interactions strongly affecting the frontier orbitals’ electronic structure, manifested by changes in absorption and emission energy, excited-state lifetime, and photoluminescence quantum yield. Cyclic voltammetry revealed the presence of two very closely spaced reversible C∧N ligand-based reductions ranging between −1.97 and −2.56 V vs Fc+/Fc, and the first metal–metal-centered oxidation wave was found to be reversible in dichloromethane and irreversible in coordinating THF in most instances. All the complexes of series I displayed triplet ligand-localized excited states at all temperatures, while an increase of steric bulk in the pz bridge in the two other molecular series resulted in a variation of photophysical behavior ranging from charge transfer to ligand localized, including admixture behavior.

Excited State Equilibrium Induced Lifetime Extension in a Dinuclear Platinum(II) Complex

“…The product was isolated with the C-and N-coordinated units from each ppy ligand disposed trans to one another. 11 Absorption spectra were acquired using an Agilent 8453 diode array spectrophotometer. Room temperature static emission spectra were collected with a FLS980 fluorometer (Edinburgh Instruments) equipped with a 450 W Xe arc lamp and a Peltier cooled PMT (R928P Hamamatsu).…”

“…The transient absorption difference spectrum was measured 5 μs after excitation (50 ns gate width), and the transient absorption decay kinetics were probed at 720 nm. 11 The reaction mixture was concentrated, and the product was precipitated by the addition of 1:1 ethanol−water. The solid obtained was filtered and dried in vacuum.…”

“…Previous studies on pyrazolate-bridged cyclometalated Pt(II) dimers have shown that increasing the steric bulk of the substituents on the pyrazolate bridging ligands systematically decreases the Pt−Pt distance, which leads to a red-shift in both the absorption and emission energies of these dimeric complexes. [5][6][7]11,12 Exploitation of this phenomenon permits access to cyclometalated Pt(II) complexes possessing visible absorptions and photoluminescence properties that are easily and predictably tuned with minor structural modification.…”

Binuclear platinated 2-phenylbenzothiazole complexes with bridging 2-mercapto derivatives of thiazoline, 1-methylimidazole, and pyrimidine: Structures and optical and electrochemical properties

“…The planar structure of the cyclometallated Pt(II) complexes allows their association due to the and π-π interactions of the orbitals of the metal and ligand, which can lead to the formation of the Pt-Pt chemical bond and a change in the nature and energy of the highest occupied (HOMO) and lowest unoccu pied (LUMO) molecular orbitals of the complexes, which is accompanied by the shift of the phosphores cence spectrum and oxidation and reduction poten tials of Pt(II) [7][8][9][10][11]. The modification of the optical and electrochemical properties of the binuclear cyclo metallated Pt(II) complexes with different distances between the platinum centers determined by the nature of the metallated and bridging ligands was shown [8,10,12].…”

The effect of the nature of peripheral platinated and bridging mercapto ligands on the optical and electrochemical properties of binuclear Pt(II) complexes with a metal-metal chemical bond

“…The planar structure of cycloplatinated complexes of heterocyclic ligands determines the possibility of inter action of d Z2 orbitals of two metal centers of {Pt(C^N)} fragments with formation of a metal-metal bond in [Pt(C^N)(µ L^L)] 2 binuclear complexes [1][2][3][4][5][6][7]. The phosphorescence of these complexes at room tempera ture provides wide possibilities of their application as components of optoelectronic devices [8][9][10].…”