Haim Levanon

Isolated ultracold porphyrins in supersonic expansions. I. Free‐base tetraphenylporphyrin and Zn‐tetraphenylporphyrin

In this paper we report the results of an experimental study of the fluorescence excitation spectra and of the time‐resolved emission of Zn‐tetraphenylporphyrin (ZnTPP) and of free‐base tetraphenylporphyrin (H2TPP) seeded in pulsed supersonic expansions of He. We have studied the S0→S1 transition (the Q band) and the S0→S2 transition (the Soret, B band) of ZnTPP, as well as the S0→S1x transition (the Qx band), the S0→S1y transition (the Qy band), and the S0→S2x transition (the Bx band) of H2TPP. Information was obtained on the electronic energy levels, the vibrational level structure, the details of low‐frequency nuclear motion, and some characteristics of electronic relaxation.

ESR and NMR of Mn(II) Complexes in Methanol

ESR spectra of Mn2+ in methanol solutions of Mn(ClO4)2 containing various amounts of LiCl, NaNCS, LiBr, LiNO3, and LiClO4 were studied. In all solutions, variations in the integrated intensity of the ESR signal upon addition of the electrolyte were observed. This effect was interpreted in terms of the formation of asymmetrically substituted octahedral Mn2+ complexes, in many of which the zero‐field splitting interaction causes fast electronic relaxation and therefore an ESR signal too broad to observe. Some of the mixed solvent‐anion complexes do however yield resolved ESR signals; in particular, a well‐defined spectrum due to [Mn(MeOH)4(NCS)2] could be characterized. From the decrease of the ESR signal the equilibrium constants for the formation of monosubstituted octahedral complexes, [Mn(MeOH)5X]+, were estimated. The equilibrium constants for Cl−, NCS−, Br−, NO3−, and ClO4− are respectively >100, >100, 10, 5, and 1M−1. At high LiCl concentrations tetrahedral [MnCl4]2−, exhibiting resolved ESR lines, i...

ESR Study of Complex Formation and Electronic Relaxation of Fe3+ in Aqueous Solutions

ESR measurements of aqueous solutions of Fe3+ containing various electrolytes were made at X‐ and Q‐band frequencies. It was found that the line shape and integrated intensity depend on the pH and on the type and concentrations of counteranions in the solution. Addition of LiCl and NH4NCS result in reduction in the integrated intensity and subsequent disappearance of the ESR signal. This is interpreted in terms of the formation of monosubstituted complexes which give signals too broad to observe. From the results, formation constants have been calculated. Successive addition of NH4F results in the formation of complexes of the type [Fe(H2O)6−nFn]3−n having different ESR linewidth ranging from 10 to 1000 G. There is a strong reduction in the linewidth on going from X‐ band to Q‐band frequencies. The results are explained in terms of electronic relaxation via the modulation of the zero field splitting (zfs) parameter. From the analysis of the data it was possible to derive values for the zfs interactions an...

ESR and Electronic Relaxation of Cr3+ and Fe3+ in Water and Water–Glycerol Mixtures

ESR intensities and linewidths at X band and linewidths at Q band of [Cr(H2O)6]3+ and [Fe(H2O)6]3+ in water‐glycerol mixtures are reported. At X band the linewidth increases with glycerol concentration and there is a concommitant decrease in the integrated intensity of the ESR line. The linewidth at Q band is independent of the glycerol content of the solvent. The results are interpreted in terms of the general theory of electronic relaxation. It is shown that the dominant electronic relaxation in solutions of Cr3+ and Fe3+ hydrates is due to modulation of the zero‐field splitting interaction. This interaction and its time dependence result from collisions of the hydrated complex with solvent molecules. Using equations derived for relaxation by rotational tumbling, a correlation time of about 5 × 10−12 sec in water is found for both complexes. The decrease in the integrated intensity with increasing viscosity at X band is due to the smearing out of all but the 12 → − 12 transitions. At Q band the Cr3+ res...

Erratum: Analysis of the transient EPR signals in the photoexcited triplet state. Application to porphyrin molecules

Anomalous EPR line intensities observed in the photoexcited triplet state of porphyrins at 100°K are discussed. A kinetic model consisting of the singlet ground state and the first excited triplet state is developed. The EPR signal intensities are expressed in terms of the kinetic rate constants and are given as linear combinations of two exponents. With these expressions the transient effects on the line intensities can be explained. Experimental results for two porphyrin molecules H2TPP (tetraphenyl porphyrin free base) and H2(PF)TPP (pentafluorotetraphenyl porphyrin free base) are presented and compared with the theory. From the analysis we evaluated the spin lattice relaxation rate W, the depopulation rate constants kp and the ratio between the population rate constants Ap: For H2TPP, kx =(6±2)×102 sec−1, ky =(3±1)×102 sec−1, kz =(1.5±0.5)×102 sec−1, Ax:Ay:Az≃1:0.6:0.2, and W =3.3×103 sec−1. For H2(PF)TPP, kx =(5±2)×102 sec−1, ky =(1.4±0.4)×102 sec−1, kz =(1.0±0.3)×102 sec−1, Ax:Ay:Az≃1:0.2:0.3, and W...

Transparent miniature dielectric resonator for electron paramagnetic resonance experiments

A novel miniature (∼2×2×1 mm) dielectric resonator for X-band electron paramagnetic resonance experiments is presented. The resonator is based on a single crystal of KTaO3, which is excited to its TE01δ resonance mode by means of a simple iris-screw coupling. Several configurations of resonators are considered and discussed with respect to their filling factor, power conversion ratio, and optical excitation efficiency. Our findings are presented in terms of both experimental and theoretical studies. For small samples, the high filling factor of this resonator results in a signal increase by a factor of 140–800 (assuming nonsaturating conditions) as compared to a rectangular X-band cavity. The high power conversion factor (∼40 G/W), should enable one to perform pulse experiments employing power amplifiers, with ∼100-fold less peak power used for rectangular cavities. With an antireflective layer, the crystal’s transparency enables efficient laser illumination of the sample in light-induced experiments.

Photo‐induced electron transfer in covalently linked donor–acceptor assemblies in liquid crystals. Time‐resolved electron paramagnetic resonance

Intramolecular electron transfer (ET) in cis and trans isomers of a covalently linked porphyrin–cyclohexylene–quinone, oriented in liquid crystals (LCs), is monitored by time‐domain electron paramagnetic resonance (EPR) spectroscopy over an extended range of temperature, i.e., 210<T<320 K. The spectra of both isomers exhibit different line shape behavior and temperature dependence, as compared to those found in isotropic solutions (ethanol). Whereas in ethanol the range of detection is quite narrow, i.e., 130<T<160 K, the one in the LC environment includes also the nematic fluid phase, thus allowing one to monitor the ET products in this phase. The difference in spectra between the two environments is analyzed in terms of the magnetization projection on the LCs director, L. The different spectra of the two isomers are interpreted in terms of their different molecular geometry. In the case of the trans isomer, both triplet‐ and probably singlet‐initiated ET routes can concurrently be detected, and the free...

Exploring the photoexcited triplet states of aluminum and tin corroles by time-resolved Q-band EPR

The photoexcited triplet states of three 5,10, 15-tris(pentafluorophenyl)corroles (tpfc), hosting Sn(IV) and Al(III) in their core, namely, Sn(Cl)(tpfc), Al(pyr)2(tpfc) and Al(pyr)2(tpfc-Br8), were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal E7. Only two of these metallocorroles, namely, Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8), exhibit TREPR spectra following pulsed laser excitation. This result is rationalized in terms of a very low quantum yield of triplet formation in Al(pyr)2(tpfc). Analysis of the spin polarized Q-band (34 GHz) EPR spectra of Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8) provides detailed information on the magnetic and kinetic parameters of the triplet states as well as on the molecular ordering of the complexes in the liquid crystal. With the assignment of the zero-field splitting parameterD<0 for the Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8), one can evaluate the dominant intersystem crossing path for these metallocorroles. Analysis reveals that in Sn(Cl)(tpfc) the in-plane triplet sublevels are preferentially populated, i.e.,AX, AY≫AZ. This can be rationalized in terms of weak electronic interactions between the Sn(IV) ion and the corrole π-system, consistent with the domed structure of Sn(Cl)(tpfc). In Al(pyr)2(tpfc-Br8), however, the out-of-plane triplet sublevel is predominantly populated, i.e.,AZ>AX, AY, which is attributed to a large increase in the spin-orbit coupling strength arising from the peripheral bromine atoms on the corrole skeleton.

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

Abstract The photoexcited triplet states of chlorophyll a 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.

The photoexcited triplet state as a probe of dynamics and phase memory in a multiphase liquid crystal: Time‐resolved electron paramagnetic resonance spectroscopy

Differences between mesophases and hysteresis effects can be examined by electron paramagnetic resonance (EPR) detection of light‐induced transient triplets oriented in liquid crystals (LC). Time‐resolved EPR spectroscopy (direct detection) has been applied to study the photoexcited triplet state dynamics of tetraphenylporphyrin (free‐base) oriented in a multiphase LC, ZLI‐1167, having a negative anisotropic susceptibility, Δχ<0. The feasibility of triplet detection over a temperature range of 100–370 K, enables one to monitor conspicuous changes of the triplet line shape associated with the different mesophases, i.e., cryst.↔(287 K) smec.↔(305 K) nem.↔(356 K) isot. Line shape analysis shows that all phases consist of domains having their directors spanned in a plane normal to the external magnetic field, as expected for Δχ<0. The triplet spectra in the fluid smectic phase depend on sample rotation, indicating the known rigidity of this phase. On the other hand, the spectra in the nematic phase are indepe...