EPR study of spin polarization of charge transfer triplet excitons in anthracene-tetracyanobenzene single crystals

The population rates of the sublevels of triplet excitons in anthracene-1,2,45-tetracyanobenzene are determined from the angular dependence of the polarization of EPR spectra. Their values are closely related to those of the acceptot molecular Anomalous tnintensities of the polarized lines are attributed to a cross-relaxation process involving a triplet trap.     

Chemically induced dynamic electron polarization investigation of the triplet-radical system in the solution of the triplet quencher

The chemically induced dynamic electron polarization (CIDEP) of the triplet molecule/triplet quencher/2,2,6,6-te-tramethyl-1-piperidinyloxyl (TEMPO) systems were measured using the high time-resolved ESR spectrometer.The competition between the radical-triplet pair mechanism (RTPM) and triplet mechanism (TM) or radical pair mechanism (RPM) polarization in the solution of the triplet quencher was investigated,and the relationship between reaction rate of the radical-triplet pair and quenching rate of triplet was deduced.     

Radical pair substitution in chemically induced dynamic nuclear polarization. Co-operative effects

The problem of radical pair substitution in Chemically Induced Dynamic Nuclear Polarization is reconsidered. The singlet—triplet evolution in the radical pairs is described in a continuous fashion, assuming non-disturbance of the electron spin state during the scavenging reaction. The CIDNP effect of the recombination product of the secondary pair is demonstrated to result from the “co-operative effect” of singlet—triplet evolution in both the primary and the secondary pair. The hypothetical one proton case, in which the primary pair has different g-factors and a zero hyperfine interaction, and the secondary pair equal g-factors and a non-zero hyperfine interaction is treated qualitatively. Some examples are discussed in which older models lead to a faulty interpretation of experimental results.     

Cidep after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism

Benzil ketyl radicals are generated by laser flash irradiation of benzil in 2-propanol at T = -50 °C and are observed by time-resolved ESR spectroscopy. Their electron spin polarization is found to consist of a fast and slowly rising emissive component. The fast component is due to polarized ketyl radicals formed by a two-photon process from an excited triplet state. The slow one is attributed to ketyl radicals which are generated by a slow photoreduction of benzil in its lowest triplet state. Their emissive polarization stems predominantly from the radical-triplet pair mechanism (RTPM). Rate constants of the relevant processes are determined.     

Radical pair formation from excited states in doped aromatic crystals. II. Optical nuclear polarization (ONP) studies

The use of ONP to determine absolute signs of fine and hyperfine tensor elements in excited triplet states is developed theoretically and demonstrated experimentally. Convenient ONP-rules are formulated which relate the desired signs of the tensor elements to quantities which can be extracted from the measured ONP field dependence. Application is made to the ONP originating from the radical pair triplet state formed after photochemical hydrogen abstraction from a fluorene host molecule and hydrogen transfer to an acridine guest molecule in doped single crystals. Negative fine and hyperfine tensor elements are determined and confirm the structural model of a radical pair product inferred from EPR-data.     

Symmetry assignments of excited states of [Pt2H8P8O20]4− by polarization ratio spectroscopy

Linear polarization ratios were measured on the title compound. The symmetry of the lowest excited singlet state was confirmed as ¹A_2u. Emission from the triplet term was negatively polarized when excitation occurred in the strong singlet but positively polarized upon excitation directly to the triplet, confirming the E_U symmetry assignment of the emitting triplet component.     

Chemically induced electron spin polarization from photolysis with plane polarized light

Experiments confirm the prediction that the magnitude of electron polarization in radicals produced by photolysis of a carbonyl compound depends upon the polarization of the incident light and thereby provide conclusive evidence for the triplet electron polarization mechanism.     

Electron spin polarization in photosynthetic bacteria. Anisotropic chemical reactivity

It is shown that electron spin polarization can be used to probe the anisotropy of singlet-triplet interconversion of radical pairs involved in photosynthetic charge separation. Anisotropic polarization may be observed with non-oriented reaction centres, provided an anisotropic interaction (e.g. zero-field splitting or g-tensor anisotropy) produces resolvable structure in the EPR spectrum of the reaction intermediates. Two examples, both for prereduced bacterial reaction centres, are discussed: (i) the triplet state of the primary donor (a bactenochlorophyll dimer) and (ii) the reduced secondary acceptor (a semiquinone). Computer simulations are used to understand the observed behaviour and yield information on the magnetic and electronic interactions involved in electron transport.     

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.     

Microwave-induced optical nuclear polarization (MI ONP) of benzophenone in dibromodiphenylether

In this paper we report the results of microwave-induced optical nuclear polarization experiments on photoexcited mixed crystals of dibromodiphenylether doped with benzophenone. The highest polarization obtained is an enhancement with respect to the Boltzmann polarization of = 820. This corresponds to an absolute nuclear spin polarization of 11%. In addition, we have measured the dependence of the optically detected magnetic resonance spectrum and of the nuclear spin polarization on the microwave power. The results are in quantitative agreement with a theory which is based on the induction of forbidden simultaneous spin flips of the triplet electrons with one or more protons by one microwave quantum.     

Redistribution of emitting state population in conjugated polymers probed by single-molecule fluorescence polarization spectroscopy

Fluctuations in the fluorescence polarization degree and direction are reported for the first time for single conjugated polymer molecules embedded in a polystyrene matrix at room temperature. The polymer molecule, a polythiophene derivative, clearly emits as a multi-chromophore ensemble showing that the energy does not funnel to any specific low-energy trap. The fluorescence instead originates from thermally populated exciton states with different relative orientations of the transition dipole moments. The fluctuations in the fluorescence polarization are explained in terms of changes in the relative contributions of the different exciton states to the signal due to conformational fluctuations of the molecule or selective exciton quenching by triplet states.     

Determination of excitation energies and transition moments in a second order polarization propagator approach. Application to the Be atom and the CH+ molecule

Excitation energies and transition moments have been determined from a second order polarization propagator and the effect of including energy shifted denominators has been investigated. Results for Be and CH⁺ show a change of 2–5% for the transition moments and of 0–20% for the singlet excitation energies in a second order approach compared to a first order (time-dependent Hartree-Fock) approximation. Both Be and CH⁺ are triplet unstable in the time-dependent Hartree-Fock approximation. The triplet instability is removed for Be but not for CH⁺ in a second order polarization propagator approach.     

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.     

Thermally enhanced photoinduced molecular reorientation in a photo-crosslinkable polymer liquid crystal based on polarization triplet energy transfer

Polarization-selective photosensitization and molecular reorientation in photo-crosslinkable polymer liquid crystal (PMCB6M) films, based on polarization-selective triplet energy transfer using LP-365?nm and LP-405?nm light with different photosensitizers, are described. A dramatic improvement in the photoreactivity of the film was observed with a small amount of photosensitizer; and a high degree of molecular reorientation and slantwise reorientation were generated when annealing the resulting films at elevated temperatures. The efficiency of reorientation depended on the photoinduced dichroism of the film.     

Dynamic nuclear polarization of proton spins via photoexcited triplet states: the system phenanthrene in fluorene

The proton spins in a single crystal of fluorene have been polarized by dynamic nuclear polarization via the photoexcited triplet state of phenanthrene guest molecules. It is possible to reach an enhancement of the proton polarization of 90 at 1.2 K where the thermal equilibrium polarization is 0.033%.     

Theory of stimulated nuclear polarization in high magnetic fields

A theory of stimulated nuclear polarization (SNP) is proposed for the first time. The calculations are performed by the method of summation of radical pair (RP) re-encounter contributions to the recombination of neutral radicals in high magnetic fields in the framework of a continuous diffusion model. Some rules for the sign of the SNP effect are formulated. Based on the calculations for model systems, we have studied the effect of different parameters of RPs on the SNP effects: the rate constants of RP recombination from singlet and triplet states, the rates of RP decay, e.g. due to the reaction of radicals with acceptors, and the effective in-cage RP lifetime. It is shown that in SNP the phenomenon of spin-looking manifests itself: when the amplitude of an applied microwave field exceeds the hf splitting in the ESR spectrum, the sign of polarization changes. The mutual effect of nuclei on the magnitude and sign of SNP is studied. For non-viscous liquids (η ≈ 1 cP), the analytical expressions for the SNP effects are obtained by perturbation theory. It is demonstrated that over a wide range of RP parameters, the results for SNP obtained on the basis of these analytical expressions are in good agreement with numerical results. The comparison of theoretical and experimental results exhibits their full conformity.     

Thermally enhanced photoinduced molecular reorientation in a photo-crosslinkable polymer liquid crystal based on polarization triplet energy transfer

Polarization-selective photosensitization and molecular reorientation in photo-crosslinkable polymer liquid crystal (PMCB6M) films, based on polarization-selective triplet energy transfer using LP-365 nm and LP-405 nm light with different photosensitizers, are described. A dramatic improvement in the photoreactivity of the film was observed with a small amount of photosensitizer; and a high degree of molecular reorientation and slantwise reorientation were generated when annealing the resulting films at elevated temperatures. The efficiency of reorientation depended on the photoinduced dichroism of the film.     

A study on the energy transfer and migration in the molecules of polymeric triplet sensitizers: (2) A study on delay fluorescence and polarization spectra

The triplet energy migration of polymers and copolymers of vinyl benzophenone (VBP) and vinyl naphthalene (VN) has been studied by measuring delayed fluorescence and polarization spectra in glassy dilute solution at 77 K. Strong delayed fluorescence of PVN proves the existence of triplet energy migration and T-T annihilation in the polymer chain. Efficient intersystem crossing of “BP” and efficient energy migration and transfer between chromophores along the polymer chain result in the absence of delayed fluorescence for copolymer P (VN-VBP) studied in this work. The order of benzophenone phosphorescence intensity: BP>Co (VBP-St)>PVBP indicates the T-T annihilation decreasing the phosphorescence of PVBP. Fluorescence and phosphorescence polarization data of polymers are smaller than that of their model compounds. It is evident that energy migration exists in the polymer chain.     

Electron spin polarization generated by transport of singlet and quintet multiexcitons to spin-correlated triplet pairs during singlet fissions

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells. Transport of spin-entanglement in the triplet–triplet pair state via one singlet exciton is a promising phenomenon for several energy conversion applications including quantum information science. However, direct observation of electron spin polarization by transport of entangled spin-states has not been presented. In this study, time-resolved electron paramagnetic resonance has been utilized to observe the transportation of singlet and quintet characters generating correlated triplet–triplet (T + T) exciton-pair states by probing the electron spin polarization (ESP) generated in thin films of 6,13-bis(triisopropylsilylethynyl)pentacene. We have clearly demonstrated that the ESP detected at the resonance field positions of individual triplet excitons is dependent on the morphology and on the detection delay time after laser flash to cause SF. ESP was clearly explained by quantum superposition of singlet–triplet–quintet wavefunctions via picosecond triplet-exciton dissociation as the electron spin polarization transfer from strongly exchange-coupled singlet and quintet TT states to weakly-coupled spin-correlated triplet pair states. Although the coherent superposition of spin eigenstates was not directly detected, the present interpretation of the spin correlation of the separated T + T exciton pair may pave new avenues not only for elucidating the vibronic role in the de-coupling between two excitons but also for scalable quantum information processing using quick T + T dissociation via one-photon excitation.

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells.