1 – Optical Spectra and Electronic Structure of Porphyrins and Related Rings

Phosphorescence is a phenomenon of delayed luminescence that corresponds to the radiative decay of the molecular triplet state. As a general property of molecules, phosphorescence represents a cornerstone problem of chemical physics due to the spin prohibition of the underlying triplet-singlet emission and because its analysis embraces a deep knowledge of electronic molecular structure. Phosphorescence is the simplest physical process which provides an example of spin-forbidden transformation with a characteristic spin selectivity and magnetic field dependence, being the model also for more complicated chemical reactions and for spin catalysis applications. The bridging of the spin prohibition in phosphorescence is commonly analyzed by perturbation theory, which considers the intensity borrowing from spin-allowed electronic transitions. In this review, we highlight the basic theoretical principles and computational aspects for the estimation of various phosphorescence parameters, like intensity, radiative...

Theory and Calculation of the Phosphorescence Phenomenon

A subazaporphyrin (SubAP), tert-butylated and crowned subphthalocyanines (tBSubPc and SubCRPc), a μ-oxo dimer of tert-butylated SubPc {(tBSubPc)2O}, a subnaphthalocyanine (SubNc), and monosubstituted type unsymmetrical phthalocyanine (Pc) and naphthalocyanine (Nc) analogues have been synthesized. In particular, unsymmetrical Pc's and Nc's have been prepared in moderate yields by the ring expansion reaction of structurally distorted SubPc's and SubNc's with isoindolediimine derivatives in dimethyl sulfoxide−chloronaphthalene (or chlorobenzenes or aromatic hydrocarbons such as toluene and xylene) mixtures. The compounds have been characterized by electronic absorption, magnetic circular dichroism (MCD), fluorescence emission, and nuclear magnetic resonance spectroscopy. Both the Soret bands and Q-bands shift to longer wavelength and gain intensity in the order SubAP, SubPc, and SubNc. Fluorescence quantum yields and lifetimes generally decrease with decreasing molecular symmetry. Circular dichroism and NMR ...

Synthesis, Spectroscopy, and Molecular Orbital Calculations of Subazaporphyrins, Subphthalocyanines, Subnaphthalocyanines, and Compounds Derived Therefrom by Ring Expansion1

It is known that the porphyrin free base (H2P) can be incorporated in an n-octane Shpol'skii matrix. In slowly grown crystals the solute molecules occur in two orientations, which are co-planar and have their N-H … H-N axes at right angles; in the S 1↔S 0 ‘quasi-line’ absorption and fluorescence spectra they can be distinguished by a relative shift of 65 cm-1 of their spectral features. By irradiation with light the two species are transformed into one another via a shift of the two inner protons. Here a study is presented of the photoisomerism following selective excitation with a dye laser into the 0-0 band of the S 1←S 0 transition of either of the two forms, for H2P and D2P (two inner hydrogens replaced by deuterium). At 4·2 K the reaction proceeds in both directions; for H2P the reaction in one direction appears to be 10 per cent faster than in the other, for D2P the absolute rates are nearly ten times lower than for H2P and not measurably different in the two directions. The reactions are not signif...

Laser-induced photochemical isomerization of free base porphyrin in an n-octane crystal at 4·2 K

The homogeneous linewidth of the 0–0 band of the S1←S0 transition of free‐base porphyrin (H2P) in an n‐octane matrix and its temperature dependence (T=1.5–4.2 K) have been measured by means of photochemical hole‐burning. The linewidth extrapolated to T=0 is ∼9 MHz for the two types of sites which have been investigated, and it appears to be determined entirely by the decay time of the emitting state S1 (17 ns). The temperature dependence of the linewidth is strongest for the thermodynamically less stable sites. The increase in linewidth with temperature may arise from a relaxation process involving a low frequency resonance mode (∼5 cm−1) localized at the H2P molecules. At 77 K the homogeneous linewidths are comparable to the inhomogeneous widths of 3–4 cm−1; hence, hole–burning is no longer observed.

Homogeneous linewidth of the S1←S0 transition of free‐base porphyrin in an n‐octane crystal as studied by photochemical hole‐burning

E.S.R. experiments have been performed on the lowest triplet state of free base porphin (H2P) in a n-octane single crystal at 1·3 K. The results demonstrate that the large majority of guest molecules occur in two orientations. While the molecules in these two orientations are coplanar, they have their N-H H-N axes at right angles. The fine structure results show that the two molecular orientations have zero-field splittings that differ by a few per cent in magnitude. Further, the 65 cm-1 doublet separation which appears in the fluorescence spectrum of H2P is related to the occurrence of these two orientations. Resolved hyperfine structure is obtained for the two in-plane canonical orientations of the magnetic field and also when the field bisects the angle between these two directions. From an analysis of the fine structure and hyperfine structure results it is established that the zero-field splitting pattern is described by the parameters (average over the two orientations) the x axis is taken along the...

Electron spin resonance in the photo-excited triplet state of free base porphin in a single crystal of n-octane

E.S.R. experiments performed at 1·3 K by optical detection are reported for the photo-excited triplet state of palladiumporphin in a single crystal of n-octane, and the observation of a level anticrossing signal is described. In the crystal the orbital degeneracy of the 3 E u state of the free molecule is lifted by the crystal field and in n-octane the energy difference between the two orbital components |x> and |y> is found to be 58 ± 2 cm-1. The spinorbit coupling (SOC) and the orbital Zeeman interaction couple the triplet manifolds of |x> and |y>, and for a proper understanding of the magnetic properties of these states it is necessary to work in the basis of the six spin-orbit functions deriving from the 3 E u state of the free molecule. It is shown that either of the two triplet states can be described by an effective spin hamiltonian of the common form and expressions for the zero-field parameters D and E and the principal values of the g tensor are given. The experimental values of the parameters i...

Electron spin resonance of the lowest triplet state of palladiumporphin in a n-octane crystal at 1·3 K

E.S.R. experiments at 1·3 K are reported for the photo-excited triplet state of zinc porphin (ZnP) and magnesium porphin (MgP) in single crystals of n-octane. The effect of partial and complete deuteriation, and the influence on the E.S.R. spectra of the addition of ethanol or benzene as a second solvent are given. It is found that most of the ZnP molecules occupy one orientation, whereas MgP + ethanol occurs in two orientations for which the molecules are coplanar but differ by a rotation of 90° about the out-of-plane axis. For ZnP, MgP, and free base porphin (H2P) the zero-field principal axes are parallel to within 5° and run through opposite nitrogen atoms. By E.S.R. combined with X-ray diffraction an insight into the orientation of the porphin molecules in the n-octane host is obtained. The fine structure results show a small (ZnP) to negligible (MgP) influence of the spin-orbit coupling of the metal atom. The observed hyperfine structure in ZnP closely resembles that in MgP and originates from four ...

The metastable triplet state of zinc porphin and magnesium porphin: A study by E.S.R. in an n-octane crystal at 1·4 K

The phosphorescence spectrum of p-xylene-h 10 in a p-xylene-d 10 host is first presented. Secondly we report E.S.R. experiments performed on p-xylene-α,α′-d 6 in p-xylene-d 10; from an analysis of the orientations of the principal axes in combination with the crystal structure and of the hyperfine splittings it is concluded that the in-plane principal axes of the zfs tensor make an angle of 68° with the molecular axes. Thirdly we discuss Microwave-Induced Delayed Phosphorescence (MIDP) experiments on p-xylene-h 10 in p-xylene-d 10 in which the relative radiative rate constants for decay from the zero-field spin components of the triplet state to single vibronic bands of the ground state have been obtained. The results of the MIDP and E.S.R. measurements are shown to be mutually consistent in terms of a model in which the effect of the crystal field and the methyl substituents on the pseudo-Jahn-Teller unstable electronic structure of the triplet state of benzene is described.

The phosphorescent triplet state of p-xylene in an isotopically mixed crystal at 1·2K: an investigation by E.S.R. and MIDP

The spin density and zero-field splitting (ZFS) of the lowest triplet state of a light metal porphin are determined by means of a Pariser-Parr-Pople ground state SCF π-electron MO-calculation with CI. Only the pz -orbitals of the carbon and nitrogen atoms are considered. The semi-empirical values of the integrals involving nitrogen orbitals are varied between the values commonly used for pyrrole and pyridine-type nitrogen. The two lower triplet states (corresponding to the excitations 3(eg ←a 2u ) and 3(eg ←a 1 u ) in Gouterman's four orbital model) are considered, both for a b 1g and a b 2g the spin densities determined by McLachlan's extended Hartree-Fock method lead to a perfect explanation of the hyperfine structure observed for zinc porphin and magnesium porphin in n-octane. The ZFS parameters are calculated by a semi-empirical method; satisfactory agreement with experiment is obtained only if the CI expansion is truncated at the point where the weight of the individual configurations becomes less th...

J.A. Kooter

Papers

Electron spin resonance in the photo-excited triplet state of free base porphin in a single crystal of n-octane

Electron spin resonance of the lowest triplet state of palladiumporphin in a n-octane crystal at 1·3 K

The metastable triplet state of zinc porphin and magnesium porphin: A study by E.S.R. in an n-octane crystal at 1·4 K

The phosphorescent triplet state of p-xylene in an isotopically mixed crystal at 1·2K: an investigation by E.S.R. and MIDP

Calculation of the spin density and spin-spin dipolar interaction in the triplet state of metal porphins