Synthesis, Electrochemistry, Photophysics, and Solvatochromism in New Cyclometalated 6-Phenyl-4-(p-R-phenyl)-2,2′-bipyridyl (R = Me, COOMe, P(O)(OEt)2) (C∧N∧N) Platinum(II) Thiophenolate Chromophores

Abstract

Three new cyclometalated 6-phenyl-4-(p-R-phenyl)-2,2'-bipyridyl (C--N--N) Pt(II) thiophenolate complexes (R = Me (2a), COOMe (2b), and P(O)(OEt)(2) (2c)) have been synthesized and studied. The new C--N--N ligands L2 (R = COOMe) and L3 (R = P(O)(OEt)(2)) undergo cyclometalation with a Pt(II) source to give the Pt(II) chloro complexes 1b and 1c, respectively, which are luminescent in fluid solution with lambda(max) approximately 575 nm, assigned to a metal-to-ligand charge-transfer ((3)MLCT) emissive state. Reaction of the chloro complexes 1a (R = Me), 1b, and 1c with sodium thiophenolate gives 2a-2c, respectively, in good yields. The novel thiophenolate complexes have two interesting absorption bands in their electronic spectra tentatively assigned to a charge-transfer to C--N--N ((1)CT) (lambda(abs) approximately 415 nm) transition and a mixed metal/ligand-to-ligand' charge-transfer (MMLL'CT, lambda(abs) approximately 555 nm) transition, respectively. The MMLL'CT band is solvatochromic with absorption maxima in the range of 496 nm in MeOH to 590 nm in toluene (epsilon approximately 4000 dm(3) mol(-1) cm(-1)), which correlate well with an empirical charge-transfer-based solvent scale. Excitation of 2a-2c into the MMLL'CT band gives emission maxima around 680 nm in frozen CH(2)Cl(2) solution, and no emission in fluid solution. Ligand L2 and complexes 1a.MeCN, 1b, and 2b.CH(2)Cl(2) have been characterized by single crystal X-ray crystallography. The electrochemical properties of ligands L1 (R = Me) and L2 and complexes 1a-1c and 2a-2c have been examined by cyclic voltammetry and are shown to exhibit reversible and quasi-reversible reductions and irreversible oxidations.