Fluorescence.detected X-band epr spectroscopy of the photosynthetic bacterial triplet state

The X-band FDMR spectrum of the bacterial triplet in reduced Rhodopseudomonas sphaeroides at 5 K has been obtained. Pure S-T₀ mixing is sufficient and k_z <k_x,k_y necessary to explain the polarization pattern and intensity ratios. The kinetic fluorescence response is sigmoidal due to the second-order kinetics of antenna-reaction center energy transfer.     

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.     

Time-resolved electron spin resonance of gallium and germanium porphyrins in the excited triplet state

Gallium and germanium porphyrin complexes in the lowest excited triplet (T1) state have been studied by time-resolved electron spin resonance (TRESR). It is found that for Ge(TPP)(OH)2 (TPP = dianion of tetraphenylporphyrin) intersystem crossing (ISC) from the lowest excited singlet (S1) state to the T1x and T1y sublevels is faster than that to the T1z sublevel (T1x, T1y, and T1z are sublevels of the T1 state), while the ISC of ZnTPP and Ga(TPP)(OH) is selective to the T1z sublevel. This is interpreted by a weak interaction between the dpi orbital of germanium and LUMO (eg) of the porphyrin ligand, resulting in small spin-orbit coupling (SOC). The interpretation is supported by molecular orbital calculations. The ISC of Ge(OEP)(OH)2 (OEP = dianion of octaethylporphyrin) and Ge(Pc)(OH)2 (Pc = dianion of tetra-tert-butylphthalocyanine) is found to be selective to the T1z sublevel in contrast to Ge(TPP)(OH)2. This dependence on the porphyrin ligand is reasonably explained by a difference between the 3(a(1u)eg) (the OEP and Pc complexes) and 3(a(2u)eg) (the TPP complex) configurations. This is the first observation of a difference in selective ISC between the 3(a(1u)eg) and 3(a(2u)eg) configurations. The TRESR spectrum of Ge(TPP)Br2 is different from those of Ge(TPP)Cl2 and Ge(TPP)(OH)2, and is interpreted by SOC between the T1 and T2 states. From ESR parameters the square of the coefficient of the eg orbital on bromine is evaluated as 0.018 in the T1 state.     

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.     

Transient electron spin resonance spectroscopy of the carotenoid triplet state in Rhodopseudomonas Sphaeroides wild type

We have determined the triplet state spin sublevel dynamics of spheroidene contained within the photosynthetic bacterium Rhodopseudonionas sphaeroides wild type. The triplet state decay dynamics were measured using direct-detection, rapid-transient, high-field, electron spin resonance spectroscopy on chromatophores isolated from the bacterium. The transient signals were analyzed as a function of microwave power to obtain the individual triplet state spin sublevel decay rate constants and the spin—lattice relaxation time. The individual triplet state spin sublevel decay constants yielded a value of 4.2±0.3 μs for the overall triplet state lifetime, which agrees with the lifetime measured previously by other workers using optical spectroscopic methods.     

Absorbance-detected electron spin echo spectroscopy of non-radiative triplet states in zero field : An application to the primary donor of the photosynthetic bacterium rhodobacter sphaeroides R-26

Electron spin echoes of triplet states in zero field were optically detected using the singlet ground-state absorbance. The technique has been applied to reaction centers of the photosynthetic bacterium Rhodobacter sphaeroides R-26. The spin-spin relaxation time T₂ of the triplet state of the primary electron donor P (a bacteriochlorophyll dimer) was 1.16±0.05 μs, independent of temperature in the range 1.2–2.1 K.     

Long-distance electron transfer from a triplet excited state

Electron transfer from the triplet excited state of N,N,N',N'-tetramethylphenylene diamine to phthalic anhydride has been monitored by phosphorescence emission decay. The kinetics of the transfer process were observed directly and the rate constant depends exponentially on the reacting distance, k(r) = 1 × 10⁴ exp(−0.58r) s⁻¹. The electron transfer rate has been found to be invariant over the temperature interval 77–143 K.     

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.     

Coherent averaging of electron spins by rotary echoes in excited triplet states

A theoretical treatment of rotary echoes derived from average hamiltonian formalism in photoexcited triplet states is presented. From a general relaxation matrix, it is shown that relaxation fields perpendicular to the driving field H_eff are completely averaged, whereas relaxation fields parallel to H_eff are not. Rotary echoes are found to differ from other coherence techniques only in the geometry of the experiment and not in the averaging properties per se. Off-resonance driving field conditions are shown to impair the efficiency of the averaging and to cause beats in the observed echo decay. Finally, inhomogenous line broadening is shown to produce a damped echo decay with beats even when the field is on-resonance.     

Two-dimensional electron spin echoes: magnetization transfer and molecular dynamics

The utility of inversion recovery and stimulated echoes in studying slow motional dynamics is analyzed. A comparison between theory and experiment is provided for a nitroxide radical in viscous solvent. A new two-dimensional stimulated echo technique, suggested by the theory, is applied to study NO₂ physisorbed on vycor.     

Optically detected zero-field triplet state magnetic resonance in photosynthetic bacteria

Triplet state transitions of the photosynthetic bacteria Rhodospirillum Rubrum, Rhodopseudomonas Spheroides and Chromatium Vinosum in chemically reduced preparations have been observed by zero-field optical detection of magnetic resonance at 2 K. For each bacterial preparation two sharp, structureless, zero-field EPR transitions were observed as microwave-induced decreases in the fluorescence intensity of the frozen cellular preparations. The depopulating rate constants for the spin sublevels of the triplet states observed in R Rubrum and R Spheroides were also measured. The similarities of the triplet state frequencies, spectral features and intersystem crossing rates suggest a common structure for the reaction centers in the photosynthetic bacteria.     

Investigation of the structure of the reaction center in photosynthetic systems by optical detection of triplet state magnetic resonance

Properties expected of the triplet state of a pair of interacting pigment molecules in photosynthetic systems are described in terms of the triplet state properties of the monomer pigment. The triplet state zero-field splittings and intersystem crossing rate constants measured in isolated chlorophylls and the same properties measured in vivo are utilized to elucidate the geometry of the eraction center in photosynthetic systems.     

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.     

The triplet state of photosynthetic pigments. I. Pheophytins

ZFS constants (in cm^?1) and decay rate constants for the lowest triplet state of pheophytins have been determined by ESR: pheophytin a: D = 341 ± 3,E = 33 ± 3, K_T = 1050 s^?1; pheophytin b: D = 358 ± 8, E = 46 ± 5, K_T = 630 s^?1; bacteriopheophytin: D = 256 ± 4, E = 54 ± 5/37 ± 5, K_T ≈ 4000 s^?1. In addition values for the decay rate constants and relative populating rates of the individual spin levels have been obtained; these numbers turn out to be appreciably different from those for the corresponding chlorophylls. For the series pheophytin a, b and bacteriopheophytin we find parallel behaviour with the corresponding chlorophylls. The effects of side group substitution and pyrrole ring reduction on the ZFS constant D can be understood by including configuration interaction between the excited states using the 4-orbital model. The change of the mean triplet decay constant K_T upon side group substitution and pyrrole ring reduction follows an energy gap law. Substitution of the central Mg-ion by two protons, however, causes K_T to increase; this is attributed to the introduction of an extra promoting mode - of the NH-group - and/or to the presence of low lying nπ^* states in pheophytins.     

Photoinduced electron transfer from electron donor to bis-carbocyanine dye in excited triplet state

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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.