Single Crystal Matrices Research Articles

Matrix dependence of the electron spin resonance spectra of the radical trapped in different single crystals

Nitrogen and proton coupling tensors have been measured for the [Formula: see text] radical trapped in five different single crystal matrices. As all isotropic coupling constants are the same within experimental error (aN = 21.2 G, aH = 28.4 G), no measurable crystal field effect is found. In these solids the motions of the radical depend on the dimension of the trapping cage and the motions of the surrounding molecules.

Influence of the Host Lattice upon EPR Coupling Parameters and d-d Transitions of Planar Copper (II) Complexes

The EPR and optical spectra of two similar square planar CuO2N2 chelates, copper hydroxyquinolinate (CuOx2) and copper picolinate (CuPic2) have been investigated. By studying both molecules in a series of several paramagnetic and diamagnetic single crystal matrices, powders and solutions, the dependence of the g tensor, metal hyperfine (hfs) tensor, nitrogen hfs tensor and the optical d-d transitions on the host lattice was determined. The strong interactions of polar matrix groups with the substituted metal complex cause the major change in the magnetic parameters and the d-d optical transitions of these centrosymmetric planar d9 complexes. In both series of host metal complexes, the copper ligands changed stepwise from a nearly octahedral arrangement (strong axial coordination) to nearly square planar arrangement (negligible axial coordination). This change is accompanied by a total decrease in the Δg parameters of 30%–35%, a total increase in the ligand spin density of 15%, and a total increase in the d-d transition frequencies of 25%. The observed increase in covalency of the singly occupied orbital with increasing axial distortion of the ligand octahedron is in qualitative agreement with results of semiempirical MO calculations. The orthorhombic distortion of the g and A tensors shows significant coorelation with the Fermi contact parameter κ for the copper nucleus. A 2p/2s hybridization ratio of 5.8 for the nitrogen lone pair component of the singly occupied σ orbital has been found for CuOx2, as well as for CuPic2. The nitrogen hfs principal axis (Bx) coincides with one of the in plane g axes (gx) for CuPic2, while the gx and Bx axes differ by 12°–17° in the case of CuOx2. The nitrogen spin density is significantly smaller for CuOx2 than for CuPic2 at equal g shifts, indicating a stronger interaction of the phenolic oxygen in CuOx2 relative to the carboxylate oxygen in CuPic2 with the metal ion. These experimental results show the shortcomings of the simple LCAO model generally used to describe the electronic structure of metal complexes. The use of free ion (or atom) wavefunctions for example leads to large overestimations of the π delocalization parameters and of the amount of g dependence on covalency. In order to obtain agreement with experiment, a scaling parameter (as proposed by K. Ruedenberg in 1962) for the metal 3d part of the involved MO's should be introduced to account for the virtual expansion of the atomic orbitals in antibonding orbitals. Variations of the effective charge appear to be negligible. This expansion results in a smaller spin-orbit constant and a smaller <r−3>3d parameter. This effect contributes substantially to the generally observed reduced spin-orbit and metal hfs interactions in covalent transition metal complexes.

A study of some singlet triplet electronic states of pyrene

The polarized fluorescence spectra of pyrene pyrene- d 10 as guests in biphenyl and fluorene single crystal matrices at about 10°K are presented. An incomplete vibrational analysis is given in terms of known i.r. and Raman data. The spectrum is complex: As well as the normally allowed Franck-Condon and Herzberg-Teller vibrations, an exceptionally large number of modes of all symmetries appear in the spectra being induced by the crystal forces of the host material. An upper estimate for these interactions is 10 cm −1 probably arises mainly from repulsive contributions. In its first excited electronic state, pyrene is probably not situated at the inversion site of the biphenyl lattice. The mirror-image relationship between fluorescence and absorption, normally evident in the spectra of aromatic molecules, is lost for pyrene, and is probably due to wholesale changes primarily in the off-diagonal elements of the force constant matrix. The first transition is assigned 1B 2 u ← 1 A g and the second 1 B 1 u ← 1 A g , vibrations belonging to the first excited electronic state and having B 3 g total symmetry provide a vibronic interaction with the second electronic state. The more nearly degenerate the interacting states become the more intense is the coupling, A g modes of the first electronic state (built on false origins, the most important being a B 3 g fundamental at about 1564 cm −1 in pyrene- h 10) have about the same activity as their counterparts of the second electronic state in the energy region above the second origin; this activity of the A g , modes is independent of their Franck-Condon overlap with the vibrationless ground electronic state. A measurement of the absolute intensity of absorption indicates that the pyrene molecule causes a local expansion of the fluorene lattice in the ab plane. Triplet-triplet absorption bands at 18,400 23,850 cm −1 were directly measured in the fluorene lattice and were tentatively assigned 3 B 3 g ← 3 B 1 u (short-axis polarized) and 3 A g ← 3 B 1 u (long-axis polarized) respectively. This result agrees with recent photoselection measurements of polarization and calculations.

The electronic spectrum of acenaphthylene

The electronic absorption spectrum of acenaphthylene has been measured in the vapor and pure crystal phases, and in single crystal matrices of acenaphthene, fluorene, naphthalene, and durene. Acenaphthylene preferentially occupies one of the two inequivalent types of sites in the acenaphthene lattice. The first three transitions are polarized, respectively, along the long, short, and long molecular axes. Intensity stealing by b 2 vibrations is observed in the first system. An X-ray investigation of the acenaphthylene crystal shows that the molecular C 2 and crystal b axes coincide.

Influence of the Host Lattice upon the EPR Coupling Parameters of Copper-8-hydroxyquinolinate

Copper-8-hydroxyquinolinate was substituted into single crystals of phthalimide, phthalic acid, and 8-hydroxyquinoline in order to study matrix effects on the magnetic properties of the metal complex. The substitution yielded highly ordered solid solutions. The doped purely organic single-crystal matrices gave rise to well-resolved EPR spectra allowing complete resolution of 63Cu and 65Cu and of the nitrogen hfs structure. The g parameters and the copper and nitrogen hfs constants turned out to be sensitive to the surrounding host lattice. It is shown that the covalent bonding in d9 systems can be influenced by the matrix and that planar paramagnetic complexes act as probes to study the crystal field of a matrix at certain sites of a lattice. Evidence is given that the position of the g axes in the complex plane depends on the individual bonding conditions of a copper complex and is not a general property of all planar copper complexes with nearly orthorhombic symmetry.