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.     

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.     

The triplet state of 4-nitronaphthylamine

Nanosecond spectroscopic and kinetic studies of 4-nitronaphthylamine (4-NO₂NA) in aerated and deaerated nonpolar solvents at room temperature show a transient species with absorption maxima at 470 and 665 nm. The rate constant for the decay of this species in deaerated benzene is 6.7 × 10⁵ sec^?1, while in aerated benzene solutions the species is quenched by oxygen with arate constant k = 2.0 × 10⁹M^?1·sec^?1. The transient absorption at 470and 665 nm is assigned to the lowest triplet excited state of 4-NO₂NA. In polar solvents, however, electronic excitation of 4-NO₂NA does not lead to any detectable transient absorption between 400 and 800 nm for the temperature range of 25 to ?150°C. This is attributed to lack of intersystem crossing of 4-NO₂NA in polar solvents.     

Influence of nature of central metal atom in tetrapyrrole pigments on triplet state deactivation processes

A pulse photolysis method has been used to study the absorption spectra and kinetics of deactivation of triplet molecules of tetra-4-tert-butylphthalocyanine, meso-tetraphenylporphine, and their metal derivatives in degassed liquid toluene solutions. The position of the triplet-triplet absorption maximum is only very slightly dependent on the nature of the central atom. The lifetime of the triplet molecules of the phthalocyanines, in contrast to the porphyrins, decreases with increasing atomic number of the central atom. The lifetime of triplet molecules of the phthalocyanines, in contrast to the porphines, becomes shorter as the atomic number of the central atom increases. The inner heavy atom influences the magnitude of for the diphthalocyanines less than for the monophthalocyanines; this is related to the smaller degree of overlapping of the electron shells of the metal atom and the ligand in the case of the diphthalocyanines.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 21, No. 1, pp. 61–66, January–February, 1985.     

Intersystem crossing in photosynthetic pigments

Intersystem crossing (ISC) of chlorophyll a in solution exhibits a strong solvent dependence and a weak deuterium isotope effect. This is due to the importance of the molecular geometry in promoting ISC. These results are generalized to other closely related photosynthetic pigments.     

Xanthone. I. optical detection of an extremely large sublevel splitting the lowest triplet state

The phosphorescence spectrum of xanthone in n-hexane is resolved into three components, one of which is due to thermally activated emission from a n_π* origin. The other two emissions originate from extremely widely split sublevels of the lowest triplet state, which is of ³ππ* origin. higher energy of T_Lx, and T_1y, and thus below 4K there is thermal depletion of T_1z which has the largest radiative strength. Th consequence of spin-orbit coupling between the T_x and T_y sublevels of the-two very close-lying, lowest triplet states. There are signif mixing of orbital character, a mixing which is governed by spin-orbit coupling rather than vibronic coupling.     

Fullerenes linked to photosynthetic pigments

The synthesis and photochemical characterization of two porphyrin-fullerene dyads, two zinc porphyrin-fullerene dyads, and a carotenobuckminsterfullerene are reviewed. In these molecules, the fullerene first excited singlet state may be formed by direct excitation or by singlet-singlet energy transfer from the attached pigment. In polar solvents, the dominant singlet-state decay pathway is photoinduced electron transfer to yield the pigment radical cation and fullerene radical anion. This charge-separated state has a long lifetime relative to the time constant for charge separation. In toluene, in cases where photoinduced electron transfer is slow for thermodynamic reasons, the fullerene singlet state decays by intersystem crossing, and the resulting triplet energy is partitioned between the components of the dyad according to their triplet energies. The results suggest that fullerenes can be valuable components of photochemically active multicomponent molecular systems.     

The triplet state of 4-nitro-N,N-dimethynaphthylamine

Nanosecond laser photolytic studies of 4-nitro-N,N-dimethylnaphthylamine (4-NDMNA) in nonpolar and polar solvents at room temperature show a transient species with an absorption maximum in the 500-510-nm range. This species is assigned to the lowest triplet excited state of 4-NDMNA. The absorption maximum of this state is independent of solvent polarity, and its lifetime is a function of the hydrogen donor efficiency of the solvent. In n-hexane the lifetime 1/k of the triplet state is 9.1 × 10^?6 sec, while in acetonitrile 1/k is 2.0 × 10^?7 sec. The hydrogen abstraction rate constant k_H of the triplet state with tributyl tin hydride (Bu₃SnH) in n-hexane is 1.7 × 10⁷M^?1·sec^?1, while in the case of isopropyl alcohol as hydrogen donor, k_H is 4.0 × 10⁷M^?1·sec^?1. The activation energy for the hydrogen abstraction by the triplet state from Bu₃SnH in deaerated n-hexane is 0.6 kcal/mol. The lack of spectral shift with increasing solvent polarity, and the appreciable hydrogen abstraction reactivity of the triplet state, also independent of solvent polarity, seem to indicate that this excited state is an n-π* state which retains its n-π* character even in polar media.     

Carotenoid triplet state lifetimes

Carotene and xanthophyll triplet lifetimes are found to depend on the concentration of the parent molecule. These results account for some of the variations in carotenoid triplet lifetimes reported previously. The rate constants obtained for ground state quenching correlate with the number of conjugated double bonds, the longer chain systems having higher quenching rate constants.     

The lowest triplet state of tetramethyl-1,3-cyclobutanedithione. III. Single-crystal Zeeman spectroscopy

Pulsed, high-field Zeeman spectra of the lowest singlet-to-triplet transition in single-crystal tetramethyl-1,3-cyclobutanedithione (TMCBDT) have been measured. The analysis of the spectra was performed using an extension of the theory of Castro and Hochstrasser which allows for an arbitrary alignment of the magnetic field with respect to the principal crystal axes. From this study we provide evidence that in TMCBDT (1) the lowest triplet is ³A_u (D_2h molecular symmetry). (2) only one spin-orbit coupling route is active. (3) a substantial zero-field splitting (2.5 ± 1.0 cm⁻¹) is present. (4) the magnetic and molecular axes are rotated by 90°. and (5) the spin-orbit contribution to the triplet-state g values is small compared to its effect on the zero-field splitting parameters.     

Fluorescence detection of electron spin echoes in the triplet state of nonphosphorescent molecules and a photosynthetic bacterium

We have observed electron spin echo signals in zero magnetic field in the triplet state of the porphin free base and the photo-induced triplet state of the photosynthetic bacterium Rhodopseudomonas spheroides (wild type) via changes in the intensity of the fluorescence. With the help of the echo signals we have been able to determine the spin memory times of the triplet spins.     

The lowest triplet state of tetramethyl-1,3-cyclobutanedithione. II. Calculation of spin-orbit coupling

A calculation of the spin-orbit coupling in the lowest excited triplet state of tetramethyl-1,3-cyclobutanedithione (TMCBDT) has been performed. The results show the following. (1) In the TMCBDT crystal the ground singlet-to-lowest triplet transition moment is predicted to be exclusively ? c polarized, as observed. (2) The assignment of the lowest triplet state should be ³A_u as found earlier for the oxygen analog. TMCBD. (3) The two largest contributions (～ 60%) to the isolated-molecule T₁ → S₀ transition moment come from the two triplet-triplet transitions.

and

, both of which are polarized along the CS bonds. (4) The total contribution to the transition moment parallel to the CS bonds is 76% from the T_l ← T₁ transitions and 24% from the S_γ ← S₀ transitions. And, finally, (5) the calculated oscillator strength of 2 × 10^?4 for the largest T₁ ← S₀ component (along y) falls within the range of typical spin-allowed, singlet-singlet n-⁼ transitions.