Low-temperature Triplet Research Articles

Characterization of the low-temperature triplet state of chlorophyll in photosystem II core complexes: Application of phosphorescence measurements and Fourier transform infrared spectroscopy

Phosphorescence measurements at 77 K and light-induced FTIR difference spectroscopy at 95 K were applied to study of the triplet state of chlorophyll a (3Chl) in photosystem II (PSII) core complexes isolated from spinach. Using both methods, 3Chl was observed in the core preparations with doubly reduced primary quinone acceptor QA. The spectral parameters of Chl phosphorescence resemble those in the isolated PSII reaction centers (RCs). The main spectral maximum and the lifetime of the phosphorescence corresponded to 955±1nm and of 1.65±0.05ms respectively; in the excitation spectrum, the absorption maxima of all core complex pigments (Chl, pheophytin a (Pheo), and β-carotene) were observed. The differential signal at 1667(−)/1628(+)cm−1 reflecting a downshift of the stretching frequency of the 131-keto C=O group of Chl was found to dominate in the triplet-minus-singlet FTIR difference spectrum of core complexes. Based on FTIR results and literature data, it is proposed that 3Chl is mostly localized on the accessory chlorophyll that is in triplet equilibrium with P680. Analysis of the data suggests that the Chl triplet state responsible for the phosphorescence and the FTIR difference spectrum is mainly generated due to charge recombination in the reaction center radical pair P680+PheoD1−, and the energy and temporal parameters of this triplet state as well as the molecular environment and interactions of the triplet-bearing Chl molecule are similar in the PSII core complexes and isolated PSII RCs.

Heisenberg models forCaV4O9: Expansions about high-temperature, plaquette, Ising, and dimer limits

We study antiferromagnetic, S=1/2 Heisenberg models with nearest- and second-neighbor interactions on the one-fifth-depleted square lattice which describes the spin degrees of freedom in the spin-gap system ${\mathrm{CaV}}_{4}$${\mathrm{O}}_{9}$. High-temperature expansions are used to calculate the temperature-dependent susceptibility, and Ising expansions are used to study phase boundaries and properties of the N\'eel-ordered phase, while plaquette and dimer expansions are used to calculate the ground-state properties and excitation spectra of magnetically disordered phases. The temperature dependences of the susceptibility and the spin gap are compared with experimental data on ${\mathrm{CaV}}_{4}$${\mathrm{O}}_{9}$. Within the parameter space we have considered, the data are best accounted for by a Hamiltonian in which the second-neighbor interactions are equal to (or perhaps somewhat less than) half the nearest-neighbor exchange, but the fits are not entirely satisfactory. The low-temperature triplet and singlet excitation spectra have many striking features which should be observable in neutron and Raman scattering experiments and would allow for more confident estimation of model parameters.

Emission and decay time studies of Sn 2+ centers in SrCl 2

Optical, kinetic and polarization measurements of the emission of SrCl 2 - Sn single crystals in the temperature range 4–295 K are reported. The weak polarization degree of the low temperature triplet emission along the C 4 axis of the emitting centers of O h symmetry could be explained by the assumption that the JT trigonal wells on the 3T 1 u APES have a higher energy than the tetragonal wells.

On the active site of the Phillips catalyst for ethylene polymerization

The FTIR spectra of CO adsorbed on the chromium(II)-A surface compound (the active site of the Phillips catalyst) before and after short polymerization with ethylene have been reexamined. No difference in the CO spectra of the room temperature triplet at 2190, 2185, and 2179 cm −1 was found for that part of chromium which reacts with ethylene compared to the CO spectra of the unreacted surface compound. The same result was observed with the low temperature CO IR triplet at 2120, 2100, and 2035 cm −1. After low temperature polymerization with ethylene a new CO IR band at 2176.82 cm −1 was observed. After high temperature reaction with ethylene a symmetrical and rather sharp CO IR band at 2171.52 cm −1 was noted at room temperature instead. It is concluded that in both cases rather well-structured chromium surface compounds are formed. The CO IR bands of the room temperature triplet shifted from 2190.79, 2184.74 and 2178.69 cm −1 to 2192.28, 2185.03, and 2180.76 cm −1 after seven oxidation (800 °C, O 2)/reduction (350 °C, CO) cycles. The CO IR bands of the low temperature triplet (−145 °C) separated into two triplets with higher (2121.12, 2102.06, and 2040.37 cm −1) and lower (2117.12, 2098.47 and 2031.34 cm −1) positions than the original one. It is concluded that the surface structure of the support has a rather large influence on the electronic density of the chromium(II) surface ions.

Low-temperature delayed fluorescence in 1,4-dibromonaphthalene

The occurrence of low-temperature triplet—triplet annihilation in 1,4-dibromonaphthalene is strongly correlated with the presence of molecules of the isomer 1,5-dibromonaphthalene. The main part of the delayed fluorescence originates from this guest molecule. The crystal structure of 1,5-dibromonaphthalene has been determined.

Delayed fluorescence S 2 → S 0 from 4,6,8-trimethylazulene due to homo-triplet—triplet annihilation

A delayed fluorescence S 2 → S 0 from the second excited singlet state (S 2) of 4,6,8-trimethylazulene (TMA), resulting from homo-triplet—triplet annihilation T 1 + T 1 ∼→ S 2 + S 0, has been observed. No delayed fluorescence from an upper excimer state of TMA was found. The extrapolated low-temperature triplet lifetime of TMA is ≈ 140 μs.

Magnetic field effect on the low temperature triplet state population of an organic molecule

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2184. Excitation of high-frequency oscillations in a plasma with accelerating Hall layer: V I Arefev and K P Kirdyashev,Zh Tekh Fiz,45 (3), 19s75, 527–535 ( in Russian)

The FTIR spectra of CO adsorbed on the chromium(II)-A surface compound (the active site of the Phillips catalyst) before and after short polymerization with ethylene have been reexamined. No difference in the CO spectra of the room temperature triplet at 2190, 2185, and 2179 cm−1 was found for that part of chromium which reacts with ethylene compared to the CO spectra of the unreacted surface compound. The same result was observed with the low temperature CO IR triplet at 2120, 2100, and 2035 cm−1. After low temperature polymerization with ethylene a new CO IR band at 2176.82 cm−1 was observed. After high temperature reaction with ethylene a symmetrical and rather sharp CO IR band at 2171.52 cm−1 was noted at room temperature instead. It is concluded that in both cases rather well-structured chromium surface compounds are formed. The CO IR bands of the room temperature triplet shifted from 2190.79, 2184.74 and 2178.69 cm−1 to 2192.28, 2185.03, and 2180.76 cm−1 after seven oxidation (800 °C, O2)/reduction (350 °C, CO) cycles. The CO IR bands of the low temperature triplet (−145 °C) separated into two triplets with higher (2121.12, 2102.06, and 2040.37 cm−1) and lower (2117.12, 2098.47 and 2031.34 cm−1) positions than the original one. It is concluded that the surface structure of the support has a rather large influence on the electronic density of the chromium(II) surface ions.

The structure of low temperature triplet energy traps in organic solids from pmdr spectroscopy 11. 1,3,5-Trichloro and hexachlorobenzenes

The new technique of polarized microwave PMDR spectroscopy is used to determine the structure of the low temperature (1.6 – 4.2°K) triplet energy traps. The structure of 1.3,5-trichlorobenzene (Tri-CB) and hexachlorobenzene (HCB) molecules in hexamethylbenzene (HMB) host as well as in their own neat crystals (present as x-traps) is determined from the linear polarization characteristics of their optically detected microwave zerofield (zf) transitions as well as from the analysis of their phosphorescence emission. The former technique gives the direction of the principal magnetic axes of these trap molecules in the crystal as well as the relative order of their zf levels. ln HMB host, deviation from trigonal symmetry is found to be only slight for HCB and absent for Tri-CB. ln the neat crystals, large deviations from trigonal symmetry are observed for the traps of both HCB and Tri-CB. In HCB x- traps, the HCB molecule is found to be slightly contracted along the CCl axis near parallel to the c′ crystal axes. Deviation From planarity is also strongly suggested by the large value of the zf parameter D. ln addition, the principal magnetic axis of the 2| E| moment for HCB x-traps is found to lie only 5° off the molecular N axis. The | D| + | E| and | D| — | E| moments, however, are 15° away from the A and B molecular axis, respectively. In Tri-CB neat crystal. two traps are observed optically with their phosphorescence origins 10 cm −1 apart. The zf parameter E is found to have a non-zero value and is opposite in sign for the two traps. If the distortion is to be blamed on pseudo-Jahn-Teller forces, the results lead to the conclusion that the x-(shallow) trap is contracted while the y- (deep) trap is expanded along an in-plane axis going through the CCl bonds near parallel to the a axis of the Tri-CB crystal. The plane containing the | D| + | E| and | D| — | E| moments of the x-trap suffers a rotation around the N-molecular axis, which is almost parallel to the 2| E| moment. Th | D| + | E| moment is 10° off the A axis and the | D| — | E| moment is 10° off the B axis. The 2| E| transition moment of the y-trap lies off the molecular N axis and the plane containing the | D| + | E| and | D| — | E| moments moves upward from the The results of these and other studies suggest that low temperature trapping in neat crystals of this type results from crystal induced geometrical and orientational changes in the molecules at point defects. The observed traps are those molecules for which the crystal field induced deformation leads to a lowering in their singlet-triplet transition energy as compared with that for the host lattice.

The determination of the structure of low temperature triplet energy traps using PMDR methods. I. The magnetic axes of hexachlorobenzene excimers

Using linearly polarized microwave radiation, the optically detected microwave signal of the zero-field transitions of the hexachlorobenzene broad excimer-type emission is studied as a function of the angle between the microwave H field and the crystal axes. The dependence of the PMDR signal on the angle between the H field of the linearly polarized microwave and crystal axes is examined. From these studies, the exact orientation of the three orthogonal magnetic axes of a phosphorescent triplet state in a single crystal host can be elucidated if three microwave polarization ratios are determined. The method is applied to determine the principal magnetic axes of the triplet state giving rise to the excimer type emission of the hexachlorobenzene crystal at 1.6°K.