K. P. Balashev

Photophysics, photochemistry and electrochemistry of mixed-ligand platinum(II) complexes with 2-phenylpyridine and 2-(2′-thienyl)pyridine as cyclometalating ligands

The spectroscopic and electrochemical properties of two series of some [Pt(ĈN)XY]z complexes are studied where (ĈN) is cyclometalated 2-phenyl-pyridinate, Ppy−, and 2-(2-thienyl)pyridinate, Tpy−; X and Y represent various monodentate and bidentate ligands: Cl−, CN−, CO, (Mor)3P=trismorpholinophosphine, Bpy=2,2-bipyridine, Phen=1,10-phenanthroline, En=ethylenediamine, Edp=1,2-bis(diphenylphosphino)ethane, Etdp=1,2-bis(diphenylphosphino)ethene, Edt=1,2-bis(phenylthio)ethane, Mnt2−=1,2dicyano-1,2-ethylenedithiolate. The lowest excited state in the [Pt(Ppy)XY]+ complexes with an unsaturated ligand, (NN) being Bipy and Phen, is assigned to a 3MLCT {d(Pt)−π∗(NN)} transition, while for the Tpy− complexes it corresponds to a 3{d(Pt)−π*(ĈN)} transition. The lowest excited state of the [Pt(ĈN)Mnt]− complexes is assigned to a 3{p(S)/d(Pt)−π∗Mnt)} transition. In the related complexes, in which a saturated chelating ligand replaces the unsaturated one, (NN)En, (SS)Edt, (PP) Edp, and in Pt(ĈN)(P(Mor)3)Cl, Pt(ĈN)COCl, [Pt(ĈN)(CN)2]−, [Pt(ĈN)Cl2]− the lowest excited state is assigned to a 3{d(Pt)−π∗(ĈN)} transition. When replacing the cyclometalating ligand Ppy− with Tpy− the 3MLCT {d(Pt)−π∗(ĈN)} absorption and emission bands are red shifted by ca. 3000 cm−1. The redox potentials, however, are very similar and depend only on the additional ligands X and Y showing the linear relationship relative to Levers electrochemical parameters. The values of the calculated radiative rate constants are sensitive both to the identity of the X,Y ligands (Edp<Edt<(CN−,CN−)<En<((Mor)3P,Cl−)<(CO,Cl−)<(Cl−,Cl−)) and the (ĈN) ligands (Tpy−<Ppy−). This effect is suggested to be due to a reduction in the amount of charge transfer to the (ĈN) ligand induced by an increase in π-back-bonding to the X,Y-ligands. The photochemical activity of this class of complexes in electron transfer reactions as well as in ground state quenching has been demonstrated.

Optical and electrochemical properties of cyclometalated Pd(II) and Pt(II) complexes with a metal-metal chemical bond

Cyclometalated [M(C^N)(μ-(N-S))]2 complexes ((M = Pd(II), Pt(II)), (C^N)− are the deprotonated forms of 2-tolylpyridine and benzo[h]quinoline, and (N-S)− are pyridine-2-thiolate and benzothiazole-2-thiolate ions) are studied by 1H NMR, IR, electronic-absorption, and emission spectroscopy, as well as by voltammetry. It is shown that the formation of the metal-metal chemical bond and the σdz2* orbital as a HOMO of complexes leads to the long-wavelength spin-allowed (410–512 nm) and spin-forbidden (595–673 nm) optical transitions σdz2*−π(C^N)* in the absorption and phosphorescence spectra, as well as to the two-electron and successive one-electron oxidation with the formation of binuclear Pt(III) and Pd(III) complexes. The substitution of Pt(II) by Pd(II) is characterized by hypso- and bathochromic shifts of the spin-allowed and forbidden σdz2*−π(C^N)* optical transitions in the absorption and phosphorescence spectra of complexes, by phosphorescence quenching of Pd(II) complexes in liquid solutions, and by an anodic shift of the oxidation potential of Pd(II) complexes compared with Pt(II) complexes.

Mixed-ligand complexes of Pt(II) and Pd(II) with 2-phenylbenzothiazole

The mixed-ligand cyclometalated [M(Bt)(μ-Cl)]2 and [(M(N∧N))(Bt)]+ complexes (M = Pd(II), Pt(II); Bt− is the deprotonated form of 2-phenylbenzothiazole; and ( N∧N) is ethylenediamine (En) and orthophenanthroline (Phen)) are studied and described by 1H NMR spectroscopy, electronic absorption and emission spectroscopy, and voltammetry. The one-electron reduction of complexes is attributed to the electron transfer to the π * orbitals of both diimine and cyclometalated ligands. The long-wavelength absorption bands and vibrationally structured luminescence bands are assigned to optical transitions that are localized mainly on the M(Bt) metal-complex fragment.

Effect of heterocyclic diimine ligands with donor and acceptor substituents on the spectroscopic and electrochemical properties of Au(III) complexes

The composition, structure, and properties of a series of Au(III) complexes with heterocyclic diimine ligands [Au(N^N)Cl2]+, where (N^N) = 2,2′-bipyridine (Bipy), 4,4′-dimethyl-2,2′-bipyridine (DmBipy), 2,2′-biquinoline (Bqx), 1,10-phenanthroline (Phen), 2,9-dimethyl-1,10-phenanthroline (DmPhen), and 4,7-diphenyl-1,10-phenanthroline (DphPhen), were characterized by 1H NMR, electronic absorption, and emission spectroscopy and also by cyclic voltammetry. The influence of donor and acceptor substituents on the spectroscopic and electrochemical properties of the Au(III) complexes was revealed.

Spectroscopic and electrochemical properties of mixed-ligand cycloplatinated complexes based on 2-(2-thienyl)pyridine and 2-phenylpyridine with 1,10-phenantroline and its 1,4-diazine derivatives

A comparative study of dipyrido-and dibenzo-substituted 1,4-diazines {dipyrido[f,h]quinoxaline (dpq), dipyrido[a,c]phenazine (dppz), 6,7-dicyanodipyrido[f,h]quinoxaline (dicnq), dibenzo[f,h]quinoxaline, dibenzo[a,c]phenazine, 6,7-dicyanodibenzo[f,h]-quinoxaline}, o-phenantroline (phen), and also of the complexes [Pt(N∧C)(N∧N)]+[(N∧C)− are deproronated forms of 2-phenylpyridine and 2-(2-thienyl)pyridine; (N∧N) is ethylenediamine, phen, dpq, dppz, dicnq] was carried out by the methods of 1H NMR, electronic absorption, and emission spectroscopy and by cyclic voltammetry. It was found that in frozen solutions of [Pt(N∧C)·(N∧N)]+ complexes the photoexcitation energy decay from two lowest in energy electronic excited states has isolated character and is localized on {Pt(N∧C)} and {Pt(N∧N)} metal-complex fragments: (d-πN∧C*) and (d-πphen*) [(N∧N) = phen, dpq, dicnq)] or (d-πN∧C*) and (π-πdiaz*) [(N∧N) = dppz]. Thermal quenching of the luminescence from the (d-πphen*) and (π-πdiaz*) states gives rise to luminescence of the complexes in liquid solutions at 293 K only from the (d-πN∧C*) state.

Effect of cyclopalladation on the spectroscopic properties of a Coumarin dye

The cyclopalladated [Pd(C6)Cl]2 and [Pd(C6)En]PF6 complexes based on Coumarin 6 are studied by 1H NMR spectroscopy and electronic absorption and emission spectroscopy. It is shown that cyclopalladation leads both to a long-wavelength shift of the spin-allowed intraligand absorption bands of coumarin and to the formation of a characteristic metal-ligand absorption band. It is found that, in comparison with coumarin, the complexes are characterized by a bathochromic shift and by a decrease in the fluorescence quantum yield due to more efficient intersystem crossing to the lower excited triplet state responsible for the low-temperature (77 K) phosphorescence of the complexes.

New luminescing complexes of Pd(II) with 2-(phenyl)benzothiazolate

The luminescent properties of the complexes [PdBt(μ-Cl)]2, [PdEnBt]ClO4, and [PdBryBt]PF6 (where Bt is 2-(phenyl)benzothiazolate-N,C2′-ion, En is 1,2-(diamine)ethane, and Bpy is 2,2′-bipyridyl) are studied and a qualitative diagram of their electronically excited states is presented.

Spectral properties of 2-phenylpyridinate complexes of Au(III) with diimine ligands

The synthesis, identification, and optical spectra of 2-phenylpyridinate complexes of Au(III) with diimine ligands (2,2′-bipyridyl and 6,7-dimethyl-2,3-di(2-pyridyl) quinoxaline) are described. A difference in the nature of the energetically lowest electronically excited states responsible for the luminescence is established.

Optical and spectroscopic properties of substituted oxa- and thiazole luminophores metalated with platinum metals

The cyclometalated [M(pbo)En]PF6(M = Pd(II), Pt(II)), [M(pbo)2En]PF6 (M = Rh(III), Ir(III)), and [Rh(C∧N)2En]PF6 ((C∧N)− are the deprotonated forms of 2-phenylbenzoxazole (pbo), 2,5-diphenyloxazole (dpo), 2-phenylbenzothiazole (pbt), 2-biphenyl-4-yl-5-phenyloxazole (bpo), and 2-biphenyl-4-yl-6-phenylbenzoxazole (bpbo) and En is ethylenediamine) complexes are studied by 1H NMR spectroscopy, IR spectroscopy, and electronic absorption and emission spectroscopy. Metalation of luminophores leads to the formation of five-membered {M(C∧N)} cycles in the composition of plane-square and octahedral complexes of the cis-C,C structure. In addition to the intraligand (IL) π-π* optical transitions in the UV region, the complexes are characterized by long-wavelength metal-to-ligand charge-transfer (MLCT) absorption bands in the region of 366–416 nm. The phosphorescence of the complexes in the visible region (482–531 nm) is attributed to radiative transition from the mixed IL/MLCT electronically excited state. The temperature quenching of the phosphorescence of complexes is attributed to the thermally activated population of metal-centered electronically excited states with subsequent nonradiative deactivation.

Optical and electrochemical properties of cyclometallated Rh(III) and Pd(II) complexes based on phenyl-substituted pyrazole, pyridine, and pyrimidine with ethylenediamine, 2,2′-bipyridine, and 1,10-phenanthroline

Electrochemical and optical properties of cyclometallated Rh(III) and Pd(II) complexes based on phenyl-substituted pyrazole, pyridine, and pyrimidine with ethylenediamine, 2,2′-bipyridine, and 1,10-phenanthroline chelating ligands were investigated. One-electron reduction waves of the complexes were assigned to ligand-centered processes of electron transfer on π* orbitals of heterocyclic chelating and cyclometallated ligands, whereas irreversible oxidation waves, to the processes involving a mixed phenyl-metal π/d orbital. The long-wave absorption band results from by the spin-allowed optical transition between π/d-HOMO and a π* orbital of a cyclometallated ligand. The character of the optical transition responsible for low-temperature (77 K) phosphorescence of the complexes is defined by the ratio of the energy gap between π* orbitals of chelating and cyclometallated ligands and by the energy of the singlet-triplet splitting of electron-excited states.