D. Stehlik

Optical nuclear polarization as a consequence of the non-crossing rule (level-anti-crossing): I. Analytical treatment of ONP in the level-crossing region

Abstract The general formal theory for Optical Nuclear Polarization (ONP) through an optical excitation cycle is worked out for the special case of ONP as a consequence of the non-crossing rule (Level-Anti-Crossing, LAC). In doped crystals certain guest-host complexes in a triplet state play a crucial role; they have rather low D and E values and consequently theiir LAC regions are at much lower field strengths that expects for normal molecules in a triplet state. The field and orientation dependence of ONP is treated analytically for a two-electron one-proton model by means of perturbation theory. One finds then that the field dependence is given by a superposition of two bell-shaped functions, opposite in sign and differing in centre and width. Under certain circumstances the ONP curves become a superposition of two lorentzian functions is the high-field approximation. The widths and the distance between the centres are determined by the elements of the hyperfine tensor. The superposition pattern mentioned here is found experimentally in the strongest ONP effects measured so far in doped crystals.

Radical pair formation from excited states in doped aromatic crystals. I. Epr studies of the guest−host system acridine−fluorene

Abstract Following light absorption in acridine doped fluorene single crystals formation of a triplet state complex (heteroexcimer) has been established earlier. More detailed EPR investigations presented here permit the clarification of the nature of this complex. Hyperfine structure has been resolved for three nuclear spins which could be assigned by selective deuteration experiments: the nitrogen and meso-proton spin located in the guest-constituent of the complex and one proton spin originating from the CH 2 -group of a fluorene host molecule. The complete hyperfine tensors could be evaluated. The anisotropy is found to be that of a planar aromatic CH-fragment for both observed proton spins. As a consequence of the CH-fragment assignment the central CH 2 -group of the fluorene constituent of the complex has to change from a sp 3 to a planar sp 2 configuration indicating a photoinduced radical pair formation involving hydrogen abstraction from a fluorene and hydrogen addition to an acridine molecule. The model is able to explain: (a) the high spin density located at the observed CH-fragments; (b) the geometry of the molecular partners involved in the complex, and (c) the fine structure tensor on the basis of a simple point charge dipole-dipole coupling of the two unpaired electron spins.

Optical nuclear polarization as a consequence of the non-crossing rule (Level-Anti-Crossing): III. Experimental results and evidence for guest-host complexes in doped fluorene crystals

Abstract Fluorene crystals doped with the guest molecules acridine, phenazine and anthracene have been studied before due to their strong Optical Nuclear Polarization (ONP) in low magnetic fields around 100 gauss. In this paper, these results together with supporting new data are interpreted in terms of ONP as a consequence of Level-Anti-Crossing (LAC) within the excited triplet state of a guest-host complex. The results permit a first detailed check of the theoretical model of ONP by LAC including the influence of electronic relaxation. Furthermore, they render the spectroscopic parameters needed for the characterization of the triplet-state complexes: • The zero-field splitting tensor and the orientation of its principal axes system with respect to the crystalline axes. • The proton hyperfine tensor, which is essentially due to one-proton spin in the CH2-group of the fluorene host molecule contributing to the complex.

Optical nuclear polarization as a consequence of the non-crossing rule (level-anti-crossing): II. The influence of electronic relaxation

Abstract The theory for Optical Nuclear Polarization (ONP) as a consequence of Level-Anti-Crossing (LAC) as outlined in the first part of this series, is extended by considering the influence of relaxation between the electron spin states Tx. Ty and Tz. Unselective relaxation as defined by equal rates of relaxation among the spin states, Tμ, in general reduces the nuclear polarization. In addition, it broadens the shape of the ONP as a function of the magnetic field. Similar destructive effects are found for selective relaxation with the exception of one specific type of selectiveness. It will be referred to as constructive relaxation, because it is even able to enhance ONP by LAC together with a more pronounced broadening of the field dependence. For instance when a polarizing field is taken in the z direction, a large relaxation rate Wxy (relaxation between Tx and Ty) enhances the effect. Large rates Wxz and Wyz would reduce the ONP. The constructive relaxation is treated analytically by perturbation theory. From the experimental ONP results one has to conclude that often of the three relaxation rates is considerably larger than the other two.

Optical nuclear polarization in molecular crystals through an optical excitation cycle

Abstract The theory of optical nuclear polarization (ONP) as outlined in two earlier papers is extended and generalized to a description of the complete optical excitation cycle. The effects of selective population and depopulation of all electronic and nuclear spin substates of the excited triplet state are considered as well as the influence of relaxation and cross relaxation processes. Two models are discussed in detail, one spin system composed to two electrons (total spin S = 1) and one proton (I = 1 2 ), the other of two electrons and two protons. It is argued that for most cases of interest, i.e., sufficiently large magnetic fields (⪢ 10 G), a superposition of all two electron one proton systems gives a rather accurated result for the polarization in many proton molecular entity. The model has been applied to the ONP results observed in pure phenazine single crystals. The numerical results reproduce the experimentally observed field and orientation dependence rather closely. The only open parameter needed is the ratio of the electronic relaxation rate and the overall triplet state decay rate.

Nuclear Zeeman and dipolar relaxation due to slow motion in aromatic single crystals

Abstract Nuclear relaxation in the rotating frame was measured to investigate slow but large amplitude mobility in naphthalene- and biphenyl-type crystals. Reorientation of the whole molecule around molecular axes appears to be the most probable motion. While increasing fluorine substitution slows down the mobility in biphenyl the opposite is true for naphthalene crystals. According to the effective external fields acting on the sample the following relaxation rates were measured: 1 T 1 : relaxation of Zeeman order in the high magnetic field H0; 1 T 1ϱ : relaxation of Zeeman order in the rotating frame effective field Hϱ under rf-irradiation; 1 T 1D : relaxation of dipolar order in the local dipolar field H L , 1 T 2 : spin-spin relaxation rate. For the dipolar relaxation rate in some crystals deviations by orders of magnitude were found with respect to the following theoretical predictions: (1) All relaxation rates are expected to be essentially equal in the “extreme narrowing” regime: ω0τ r ⪡ 1. (2) The ratio of the relaxation maxima ( 1 T 1 ) m :( 1 T 1ϱ ) m :( 1 T 1D ) m , should be given approximately by that of the inverse Larmor frequencies in the effective field 1 ω 0 : 1 ω ϱ : 1 ω L ∗ .

Excitation of triplet excitons in aromatic single crystals by guest-host energy transfer

Abstract Selective excitation of singlet states of one guest molecule (phenazine or acridine as donor) in a host single crystal (fluorene or diphenyl) populates the lowest triplet state of a second guest molecule (naphthalene- d 8 or pyrene- d 10 as acceptor). Phosphorescence as well as ESR were used to detect the population of the triplet states of the acceptor. The results can be interpreted only as energy transfer from the donor singlet to the acceptor triplet state either via intermolecular intersystem crossing from a donor singlet state to the lowest triplet state of the host or via intramolecular intersystem crossing and triplet-triplet energy transfer to the host from a higher triplet state or from a highly excited vibrational level of the triplet ground state of the donor, followed both by triplet exciton diffusion and exciton-trapping in the acceptor triplet state. High quantum yields up to 5% can be estimated for the observed energy transfer process.

The staphylococcal PEP dependent phosphotransferase system, proton magnetic resonance (PMR) studies on the phosphoryl carrier protein HPr: Evidence for a phosphohistidine residue in the intact phospho-HPr molecule

The phosphorylated forms of the P-carrier proteins HPr and F III can be demonstrated using 32P PEP as phosphoryl donor. After alkaline hydrolysis in 3 M NaOH lOO”C, 3 hr 1-[32P]histidine was isolated from 32 P-HPr whereas 3-[32 P] histidine was isolated from 32P-F III, leading to the conclusion that the phosphoryl group in HPr was linked to the N-l of the imidazol ring of the histidine residue in the protein

On the analysis of competitive binding of various ligands to cooperative and independent binding sites of macromolecules.

Abstract The paper deals with the practical evaluation of multiple binding equilibria of macromolecules and different ligands competing for the same binding sites. The necessary formalism is reviewed and set up for the equilibria involving a macromolecule with various classes of independent binding sites and/or a class of cooperative sites and up to three different ligands in competition for them. In particular, it was necessary to extend the Hill approximation to treat simultaneous competition for cooperative as well as independent binding sites, while earlier attempts are shown to be inadequate. Criteria are developed for a qualitative analysis of complex binding patterns using the Scatchard-plot of the experimental data in order to obtain a model of the binding structure and an adequate set of input parameters for the numerical analysis. Numerical examples refer to the binding of calcium and magnesium to the sarcoplasmic reticulum as studied by competitive replacement of manganese ions [3].

Optical nuclear polarization of protons in fluorene-d8,h2 single crystals

The optical nuclear polarization has been studied in single crystals of octadeutero-9,9′-proto-fluorene (fluorene-d8,h2). In this case, only two protons with spin I = 12 per molecule interact with the excited triplet states of spin S = 1. From the proton NMR data it is concluded that the crystalline proton positions agree with those predicted for the known crystal structure of fluorene-h10. Proton ONP was observed at room temperature as a function of the magnetic field Hp as well as of its orientation in all crystalline planes. The maximum ONP is pL ≈ 10−3, corresponding to a spin temperature of 8 mK in the optimal polarization field Hp = 80 G. The quantum efficiency, i.e., the number of polarized proton spins per absorbed light quantum, is about 10−2. The basic experimental results can be understood with a general ONP mechanism which is reviewed briefly. Some striking properties of the field and orientation dependence are tentatively related to particular cross-relaxation processes, which occur if the transition frequencies between different pairs of the electron and nuclear spin sublevels are so close that they overlap within their respective linewidth.