G. A. Crosby

Photophysical investigations of rhenium(I)Cl(CO)3(phenanthroline) complexes

Abstract The electronic states of three complexes with the generic formula Re(I)Cl(CO) 3 (s-phen) [s-phen=4,7-dimethyl-1,10-phenanthroline, 5,6-dimethyl-1,10-phenanthroline, and 3,4,7,8-tetramethyl-1,10-phenanthroline] have been characterized by absorption and luminescence spectroscopy and the T-dependence of the decay times in rigid glasses in the range 4–77 K. Methyl substitution leads to composite phosphorescence bands that arise from 3 MLCT and 3 ππ* terms that are separated by Franck–Condon barriers. Evidence is also presented for the existence of FC barriers in singlet manifolds. Criteria are defined for the occurrence of FC barriers and slow rates of radiationless transitions between terms arising from disparate electronic configurations. The energy splittings and the decay constants of the individual sublevels of the 3 MLCT manifolds have been determined.

Nature of the emitting 3MLCT manifold of rhenium(I) (diimine) (CO) 3Cl complexes

Abstract Spectroscopic investigations of four Re(NN) (CO) 3 Cl complexes (NN = 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, 1,10-phenanthroline,or 2,9-dimethyl-1,10-phenanthroline) in the 77—4 K range reveal a single 3 MLCT emitting configuration. Temperature dependences of the phosphorescence lifetimes have been analyzed in terms of a multiple state model that yields rate constants and level splittings. A model based on C 2v symmetry that depends on second-order spin-orbit coupling between dπ* configurations is proposed to rationalize the results. Implications for the well-studied Ru(NN) 2+ 3 and Os(II) (NN) 2+ 3 manifolds are discussed.

Assignment of the triplet manifolds of d8 complexes and linear chains via the effects of temperature and magnetic fields on the phosphorescence lifetimes

Abstract Characteristics of the electronic excited states of transition-metal complexes are reviewed. Attention is focussed on the experimental means of identifying the orbital natures of the emitting manifolds. Two new criteria for distinguishing among excited triplet terms are enunciated: (a) the profile of the phosphorescence lifetime vs. the temperature in the range 77−4 K, and (b) the functional dependence of the phosphorescence decay rate on an external magnetic field at ∼ 4 K temperatures. These criteria are applied to the red emission emanating from solid platinum(II) complexes that form linear chains [Pt(2,2′-bipyridine)(CN)2; Pt(2,2′-bipyridine)Cl2] and to Pt(2-phenylpyridine)2 that crystallizes with discrete dimeric units. The origin of the phosphorescence from all three substances is assigned to a 3 d σ ∗ p σ term from a consideration of the criteria given above. The red emission from solutions and rigid glasses of Pt(4,4′-dimethyl-2,2′-bipyridine)(ecda) and Pt(4,7-diphenyl-1,10-phenantroline)(ecda) [ecda = 1-ethoxycarbonyl-1-cyanoethylene-2,2-dithiolate] is reassigned to a 3 d σ ∗ p σ term of a binuclear defect state since the T-dependence criterion does not support the current assignment to a 3MLCT of a monomeric species.

Thermal redistribution of energy from an excited 3ππ∗ term to a nearby chemically reactive 3dd level in [Rh(III)(NN)3](PF6)3 complexes

Abstract Below ∼ 150 K [Rh(NN) 3 ](PF 6 ) 3 complexes display 3 gpπ ∗ phosphorescences characteristic of the NN diimine ligand [NN = 1,10-phenanthroline (phen) or 2,2′-bipyridine (bpy)]. Above ∼ 150 K and below the melting point of glycerol, a temperature-dependent first-order photoreaction occurs. Temperature dependencies of these photochemical reactions conform to the Arrhenius equation with activation energies in the 2–3 × 10 3 cm −1 range. These results are interpreted in terms of thermal redistribution from an excited 3 ππ ∗ term to a nearby chemcially reactive 3 dd manifold. The relationship of the Arrhenius activation energies to the electronic states of the parent complexes and the paths of intramolecular energy migration are discussed.

Barriers to energy migration in mixed-ligand zinc(II) complexes

Spectroscopic investigations of Zn(4-Cl-PhS)2(dmphen) (4-Cl-PhS = anion of 4-chlorobenzenethiol; dmphen = 2,9-dimethyl-1,10-phenanthroline) in the p21/N and p21/C crystal phases and in polymethylmethacrylate (PMMA) are reported. The results demonstrate the existence of an intramolecular barrier to energy migration between 3ππ* and ligand-to-ligand charge-transfer levels.

Spectroscopic and magnetic studies of complexes of d10 closed shell ions

Abstract Fluorescence, phosphorescence, and the triplet spin sublevel properties (zero-field splitting, total decay rate constants, and relative radiative decay rate constants for individual sublevels) are investigated for a series of complexes of the type MX 2 (phen), where M is Zn(II) or Cd(II), and X is Cl, Br, or I. The fluorescence and the phosphorescence are almost identical to those of free phen indicating that the emission is from the phen localized ππ * state. The rate constants for spin forbidden transitions increase as the atomic number of halogen increases. All the kinetic data as well as the zero-field splitting parameters are satisfactorily interpreted by a configurational mixing between the phen localized ππ * state and the halogen p to phen π * ligand-ligand charge-transfer state. Zero-field splitting and radiative properties of the triplet spin sublevels of the “ligand localized” state of the complexes of closed shell metal ions such as Zn(II) and Cd(II) are all interpreted satisfactorily in terms of the configurational mixing between phen localized ππ * state and the halogen p to phen π * ligand-ligand charge-transfer state. Metal d orbitals do not participate in determining excited state properties. Apparent difference experimentally observed is traced back to the difference of the electronegativity of Zn(II) and Cd(II).

Zero-field splittings of phenanthroline-localized 3ππ* emitting levels in mixed-ligand zinc(II) complexes

Abstract Zero-field splittings of the phenanthroline-localized 3ππ* state in series of mixed-ligand zinc(II) complexes, Zn(X-PhS)2(phen) (X = F5, 4-Cl, 4-CH3) were observed. The results are interpreted in terms of mixing with the nearby lowest ligand—ligand charge-transfer excited triplet level. By employing reasonable assumptions the mixing coefficients were determined.

Assignment of the luminescing states of [Au(I)Ir(I)(CO)Cl(μ-dppm)2][PF6]

Abstract Excitation spectra, photoemission spectra, and phosphorescence decay times were measured on the title compound. Results are consistent with an Ir(I) → Au(I) orbital promotion leading to a lowest 1 A 1 state and a corresponding low-lying 3 A 1 term that splits into (B 1 , B 2 ) and A 2 states by spin-orbit coupling in approximate C 2v symmetry. Analysis of T -dependent decay times and emission polarization results indicates that the B 1 , B 2 states are nearly degenerate and lie ≈90 cm −1 above the A 2 state that is formally forbidden in C 2v . There is also quadratic dependence of the decay rate of the A 2 state on applied magnetic-field strength.

Spectroscopic and magnetic studies of mixed-ligand complexes of zinc (II)

Zero-field splittings and the kinetic parameters associated with the triplet spin sublevels of both the triplet ππ* and the triplet ligand-ligand charge-transfer states were measured by the optically-detected magnetic-resonance method for a series of complexes with a d10 closed-shell metal ion of the type, Zn(X-PhS)2(phen), where X = F5, 4-C1, H, 4-CH3, or 4-CH3O. Both the zero-field splittings and the kinetic decay parameters are satisfactorily interpreted in terms of mixing between the lowest phen-localized3ππ* and ligand-ligand charge-transfer states.

Spectroscopic and ODMR investigations of the lowest ππ* and ligand-ligand charge-transfer excited states of zinc(II) mixed-ligand complexes

Zero-field splittings of the emitting levels of the title compounds were measured by ODMR. For complexes of the type Zn(X-PhS)2(phen) resonances of the3ππ* phosphorescing levels were observed for X=F5, 4-Cl, 4-CH3. For X=4-Cl, resonances were detected for the3LLCT emitting manifold also. For X=4-CH3O, resonances were detected only for the3LLCT emitting states. A model invoking mixing between the two types of states is proposed that satisfactorily accounts for the dependences of the properties of the emitting manifolds on the donating ability of the X substituent.