Electron spin polarization in the photoexcited triplet state of porphyrins

Electron spin polarization in the photoexcited triplet state of tetraphenyl porphyrin was detected at 100°K using EPR technique. The zero field splitting parameters |D| and |E| the free base porphyrin were found to be 0.0369 ± 0.0005 and 0.0082 ± 0.0005 cm^?1, respectively.     

EPR study of electron spin polarization in the photoexcited triplet state of chlorophyll a and b

The photoexcited triplet states of chlorophyll à and b are studied by the EPR method at ≈85 K using modulated light excitation. Both compounds show anomalous EPR line intensities and transient kinetics, indicating electron spin polarization (ESP) in the photoexcited triplet state. EPR studies, using Mg-tetraphenyl porphyrin (MgTPP) dissolved in n-octane show that ESP occurs also in that solvent. It is shown that the zero field splitting (ZFS) parameters of MgTPP depend strongly on the solvent. From the analysis of the data for chlorophyll a and b we evaluate: (1) the population rate constants (k_p); (2) the ratio between the population rate constants (A_p) (p = x, y, z) and, (3) the spin lattice relaxation rate W. In both chlorophylls the in-plane component, x, is predominantly populated and depopulated. The ZFS parameters have been also determined for the above compounds.     

ESR study of the photoexcited triplet state of substituted 2-acetonaphthones

Electron spin resonance of phosphorescent triplet states of a number of 6-substituted-2-acetonaphthones is studied at 77 K in toluene glass. The life-times of the triplet states and zero-field parameter D* have been measured from Δm_s = ± 2 resonance. The life-times and the zero-field splitting parameters indicate that the lowest triplet states have mainly π, π* character.     

Anisotropic spin—lattice relaxation in the triplet state of naphtralene-h8 in n-pentane

The temperature dependence of the individual spin—lattice relaxation rate constants (W_ij) between the lowest triplet sublevels of naphthalene-h₈ in a polycrystalline Shpol'skii matrix of n-pentane have been measured between 1.2 and 2.4 K in zero field. The total sublevel decay constants are evaluated and found to be independent of temperature. The W_ij are found to be highly anisotropic, but the anisotropy pattern differs from that observed previously in a durene matrix.     

Room-temperature kinetics of the photoexcited triplet state of acridine in fluorene crystals as obtained from electron spin echo studies

It is shown that electron spin echo results allow an analysis of the complete kinetics of excited triplet spin states at room temperature, not only with respect to all relevant kinetic parameters of the triplet decay and spin relaxation but also the spin selectivity of the triplet population and decay processes.     

Anisotropy of the triplet spin—lattice relaxation in molecular crystals at low temperatures and high magnetic fields

The anisotropy of the triplet relaxation rates in molecular crystals at low temperatures (T = 1.6 K) and very high magnetic fields (B = 5.2–10 T) is explained by a model, which assumes time dependent matrix elements of the electronic dipole tensor and of the electronic g-tensor. The time dependencies may be due, for instance, to librations of the molecule or to changes in the electronic configuration; they arise from a direct process. This fact is used in order to reduce the number of parameters. The relaxation rates are given as functions of the direction cosines of the magnetic field and of eight parameters which are determined in a least-squares fit for the system quinoxaline in perdeuterated naphthalene. Some uncertainties concerning the numerical values of the parameters would be reduced by measurements with the magnetic field directed along the principal axes of the electronic dipole tensor.     

Anisotropic spin lattice relaxation of triplet quinoxaline in a naphthalene host crystal at high magnetic field

The spin lattice relaxation in the excited triplet state of quinoxaline molecules in a perdeutero-naphthalene host crystal was investigated at high magnetic field (B = 5 T) between T = 1.7 K and T = 4.2 K. Relaxation rates were deduced from the non-monoexponential phosphorescence decay of the individual Zeeman sublevels. Besides the so far investigated Δm = 1 transition the Δm = 2 transition was observed and attributed to a purely direct process. Its anisotropy is different to that of the Δm = 1 direct process at 1.7 K. The temperature effect upon Δm = 1 relaxation anisotropy is discussed.     

Temperature dependence of the spin-lattice relaxation rates in the triplet state of pyrazine at low temperatures

The electron spin-Lattice relaxation, (SLR) rate constants have been measured for the triplet state at pyrazine-d₄ in a cyclohexane matrix from 1 to 8 K by resolving the phosphorescent decay curve into three exponential components. A matrix kinetics is presented that gives the relationship between the SLR rate constants and the observed decay component rate constants and the decay component intensities of the three zero-field levels. An exact numerical procedure based on the Newton-Raphson technique is described in detail for systems of two and three coupled levels. This procedure was used to find the SLR rate constants starting from either the decay component rate constants or the decay component intensities. Both the matrix kinetics and the numerical procedure should be useful in many other studies involving multilevel systems whose sublevels are connected by some combination of SLR and microwaves. The observed SLR rate constants are best explained as a function of temperature as the sum of a direct process (linear in T) and a Raman process (T² dependence). Their relative magnitudes indicate that the Raman process is probably antharmonic. Vancus mechanisms for SLR in this system are examined, and a short review of SLR theory and experiment is given. The nature of the zero-field sublevels in pyrazine-J₄ is also discussed as is the effect of the choice of matrix solvent upon SLR rate constants and the phosphorescent spectrum.     

Librational model for the direct spin—lattice relaxation process in isolated triplet molecules in a naphthalene-mixed crystal

For the spin-lattice relaxation (SLR) in molecular crystals a model is presented, which attributes the SLR mechanism to phonon-stimulated molecular librations. Its applicability to the direct process is shown by comparison with the experimental data on quinoxaline guest molecules in a perdeuterated napthalene host crystal.     

Proton spin–lattice relaxation study of a partial bilayer smectic A phase

Abstract

Measurements of proton spin-lattice relaxation rates for the partial bilayer smectic A phase of 4-((4′-n-hexadecyloxybenzylidene)-amino) benzonitrile obtained at different Larmor frequencies and temperatures show that the essential relaxation mechanisms in the MHz frequency region are translational self-diffusion and local molecular reorientations similar to those in monolayer smectics. The values of the diffusion constant obtained from the fit of the theory to the experimental data show a range from 2.6 × 10^?11 m² s^?1 at 95°C to 1.7 × 10^?11 m² s^?1 at 75°C. A dynamic process specific to the partial bilayer smectic A phase seems to influence relaxation below 10 MHz. It can be associated either with the dimerization of molecules in the layers or with a higher value of the low cut-off frequency of order director fluctuations than that found in monolayer smectic A phases.     

ODMR in zero field of the photoexcited triplet state of naphthalene and quinoxaline dissolved in argon matrices

ODMR of zero-field transitions in the triplet states of naphthalene and quinoxaline in an argon host are reported. Excitation with a Hg are leads to linewidths considerably smaller than those obtained for the same molecules dissolved in glassy matrices. In naphthalene selective laser excitation reduces the linewidth to a value almost equal to those observed in single crystals.     

Determination of the individual spin-lattice relaxation rates between the spin levels of phosphorescent triplet-state molecules

It is shown how one can determine the individual spir-lattice relaxation rates of a phosphorescent triplet-stale molecule from analysis of the recovery curve in the phosphorescence intensity upon a microwave sweep through one of the zero-field transitions.     

An electron spin echo and laser photolysis study of chemically induced electron spin orientation in the triplet ground state of diphenylmethylene

The electron spin in the triplet ground state of diphenylmethylene is found to be oriented predominantly in the plane perpendicular to the long molecular axis after the photo-dissociation reaction of diphenyldiazomethane. The spin selectivity of the intersystem crossing which occurs when the methylene relaxes to its ground state is suggested as the orientation mechanism. The combined spin echo and laser photolysis are shown to be very suitable for this kind of investigation.     

The effect of vibronic relaxation on the spin coherence in photo-excited triplet states

We present a theoretical model for describing spin coherence in photo-excited triplet states in zero-field under the influence of vibronic relaxation. From a comparison with experimental results in parabenzoquinone we obtain information about the properties of the excited vibronic level.     

Pi topology and spin alignment in unique photoexcited triplet and quintet states arising from four unpaired electrons of an organic spin system

Syntheses, electronic structures in the ground state, unique photoexcited states, and spin alignment are reported for novel biradical 1, which was designed as an ideal model compound to investigate photoinduced spin alignment in the excited state. Electron spin resonance (ESR), time-resolved ESR (TRESR), and laser-excitation pulsed ESR experiments were carried out. The magnetic properties were examined with a SQUID magnetometer. In the electronic ground state, two radical moieties interact very weakly (almost no interaction) with each other through the closed-shell diphenylanthracene spin coupler. On photoirradiation, a novel lowest photoexcited state with the intermediate spin (S = 1) arising from four unpaired electrons with low-lying quintet (S = 2) photoexcited state was detected. The unique triplet state has an interesting electronic structure, the D value of which is reduced by antiferromagnetic spin alignment between two radical spins through the excited triplet spin coupler. The general theoretical predictions of the spin alignment and the reduction of the fine-structure splitting of the triplet bis(radical) systems are presented. The fine-structure splitting of the unique photoexcited triplet state of 1, as well as the existence of the low-lying quintet state, is interpreted well on the basis of theoretical predictions. Details of the spin alignment in the photoexcited states are discussed.     

Spin relaxation in the phosphorescent triplet state of several pyridyl carbonyl compounds

Spin relaxation is observed in benzophenone, isomers of benzoylpyridines and dipyridyl ketones. Models are used to describe optically detected echo shapes observed by transient techniques in the phosphorescent triplet state. Framed after the density matrix approach, the echo shapes were found to adequately fit the analytical expressions. Hahn spin echo, Carr-Purcell multiple echo, and rotary echo pulse sequences were used to study the nuclear contribution to the electron spin dephasing and the proximity effect of the nitrogen nuclei to the localized triplet excitation in the carbonyl portion of the molecule.     

Proton spin lattice relaxation rates of substituted ferrocene derivatives; assignment of resonances and determination of motional correlation times

Proton spin lattice relaxation rates have been used to assign the proton resonances of the substituted cyclopentadienyl ring of two ferrocenyl-sugar conjugates. Comparison between the R₁-values of the protons of the substituted and unsubstituted rings shows that the latter ring is spinning more rapidly about the Cp-Fe-Cp axis.     

Effects of spin transitions degeneracy in pulsed EPR of the fullerene C70 triplet state produced by continuous light illumination

X-band echo-detected electron paramagnetic resonance (ED EPR) spectra of triplet state of fullerene C(70) generated by continuous light illumination were found to correspond below 30K to a non-equilibrium electron spin polarization. Above 30K spectra are characteristic of Boltzmann equilibrium. Spectra were simulated fairly well with zero-field splitting parameters D=153 MHz and E and distributed within the range of 6-42 MHz. The origin of E distribution is attributed to the Jahn-Teller effect, which in glassy matrix is expected to depend on the local surrounding of a fullerene molecule (a so-called E-strain). In the center of ED EPR spectra a narrow hole was observed. With increase of the microwave pulse turning angle this hole transforms into a single narrow absorptive line. Numerical simulations by density matrix formalism confirm that central hole originates from a simultaneous excitation of both allowed electron spin transitions of the triplet (T(0)?T(+) and T(0)?T(-)), because of their degeneracy at this spectral position. Also explanations are given why this hole has not been observed in the previously reported experiments on continuous wave EPR and on ED EPR under laser pulse excitation.