J.H. van der Waals

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

It is known that the porphyrin free base (H2P) can be incorporated in an n-octane Shpolskii 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...

The dynamics of populating and depopulating the phosphorescent triplet state as studied by microwave induced delayed phosphorescence

It is known that at low temperatures relaxation between the spin components of a phosphorescent triplet state of an organic molecule may become very slow as compared to their individual rates of decay. Spectacular changes in the intensity emitted can then be induced by suddenly sweeping a microwave field through one of the zero-field resonances during phosphorescence decay. These microwave induced signals arise because molecules are transferred from a populated non-radiative level to an empty radiative one. In this paper a system of experiments based on this phenomenon is presented which enables one to solve the dynamics of populating and depopulating the individual levels of the phosphorescent state. As a demonstration results of experiments on the aza-naphthalenes quinoline and quinoxaline are given.

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

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.

Microwave induced delayed phosphorescence

Abstract When the molecules populating a phosphorescent triplet state decay at very low temperature, thermal equilibrium between the three spin levels is no longer maintained and they decay indepedently. A technique has been developed to utilize this situation for measuring decay rates of individual spin levels by observing the effect on phosphorescence decay of the sudden saturation of a microwave transition between a part of spin levels.

The lowest triplet state of free base porphin

Transient signals in the fluorescence intensity induced by microwaves can be used to acquire quantitative information on the molecular rates of populating and depopulating of the lowest triplet state of organic molecules. This method seems especially promising when the molecule emits no detectable phosphorescence. Part of this paper concerns the introduction of an appropriate mathematical model for describing the fluorescence transients. The model is formulated in conjunction with a specific series of experiments from which one may evaluate the molecular quantities in a systematic way. The kinetics of the lowest triplet state of free base porphin (H2P) are determined explicitly by this method. In the discussion the results for H2P are compared with those for Zn porphin. A satisfactory qualitative explanation based on the theory of radiationless transitions can be given for the great difference in behaviour of these two porphyrin molecules. Short attention is given to effects due to the photochemical shift...

Frequency shift and dephasing of the S1 ← S0 transition of free-base porphin in an n-octane crystal as a function of temperature

Abstract We report a study of the temperature dependence of the homogeneous width and frequency of the S1 ← S0 0-0 transition of free-base porphin as a guest in an n-octane matrix. In the liquid helium temperature region, the measurements have been made by means of photochemical hole-burning, at higher temperatures up to 90 K via an analysis of the fluorescence spectrum. The results are interpreted in terms of the “exchange model” for optical dephasing.

The lowest triplet state of Zn porphin

Phosphorescence microwave double resonance experiments are reported on Zn porphin at 1·2 K. In glassy solution very broad resonance transitions are observed. However, for Zn porphin in a crystalline n-octane matrix—a system known for its sharp optical spectra (Shpolskii effect)—three pairs of microwave transitions with widths of a few MHz are found, all of them corresponding to a decrease in phosphorescence intensity. By studying the behaviour of the signals for various methods of preparation of the sample and as a function of the optical bandwidth of excitation and detection, one pair of transitions could be assigned to monomeric solute molecules. The corresponding zero-field splittings are |X - Z| = 1355, |Y - Z| = 806 MHz. It was further established that by ‘pumping’ either of these transitions a third one can be detected at the difference frequency, so that the order of the levels must be X> Y>Z (or reverse). The results indicate that the molecule no longer possesses a four-fold axis in the excited st...

Spin-forbidden radiationless processes in isoelectronic molecules: Anthracene, acridine and phenazine: A study by microwave induced delayed phosphorescence

Microwave induced delayed phosphorescence (MIDP) experiments have been performed to study the populating and decay of the phosphorescent triplet state T 0 of seven aromatic three-ring molecules : anthracene (-h 10, -h 2d8, and -d10), acridine (-h9 and -d9) and phenazine (-h8 and -d8), all diluted in biphenyl crystals. We chose these molecules because the deactivation of T 0 goes dominantly via radiationless processes and the total decay rates of the individual spin components of T 0 determined in the experiment thus equal the radiationless decay rates. The phosphorescence emission, used as a monitor for the triplet state population, here gives an insignificant contribution to the decay. MIDP experiments only yield the true decay rates if thermal isolation between the levels is maintained throughout, and the present investigations, in which the intensity of excitation has been varied over a wide range, uncover a possible source of systematic error that has been overlooked thus far. The principal results of...

Investigation of the lowest triplet state of free base porphin by microwave induced changes in its fluorescence

Abstract The zero-field splitting of the non-phosphorescent lowest triplet state of free base porphin is measured by observing microwave induced changes in the fluorescence of this molecule as a guest in a Shpolskii-matrix of n -octane at 4.2°K. The doublet character of the fluorescence spectrum of the system also manifests itself in the microwave spectrum. It is proved that this doublet structure is caused by two physically distinct free base porphin molecules in the host lattice.

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

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...