Triplet and fluorescing states of the CP47 antenna complex of photosystem II studied as a function of temperature.

Fluorescence emission and triplet-minus-singlet (T-S) absorption difference spectra of the CP47 core antenna complex of photosystem II were measured as a function of temperature and compared to those of chlorophyll a in Triton X-100. Two spectral species were found in the chlorophyll T-S spectra of CP47, which may arise from a difference in ligation of the pigments or from an additional hydrogen bond, similar to what has been found for Chl molecules in a variety of solvents. The T-S spectra show that the lowest lying state in CP47 is at approximately 685 nm and gives rise to fluorescence at 690 nm at 4 K. The fluorescence quantum yield is 0.11 +/- 0.03 at 4 K, the chlorophyll triplet yield is 0.16 +/- 0.03. Carotenoid triplets are formed efficiently at 4 K through triplet transfer from chlorophyll with a yield of 0.15 +/- 0.02. The major decay channel of the lowest excited state in CP47 is internal conversion, with a quantum yield of about 0.58. Increase of the temperature results in a broadening and blue shift of the spectra due to the equilibration of the excitation over the antenna pigments. Upon increasing the temperature, a decrease of the fluorescence and triplet yields is observed to, at 270 K, a value of about 55% of the low temperature value. This decrease is significantly larger than of chlorophyll a in Triton X-100. Although the coupling to low-frequency phonon or vibration modes of the pigments is probably intermediate in CP47, the temperature dependence of the triplet and fluorescence quantum yield can be modeled using the energy gap law in the strong coupling limit of Englman and Jortner (1970. J. Mol. Phys. 18:145-164) for non-radiative decays. This yields for CP47 an average frequency of the promoting/accepting modes of 350 cm-1 with an activation energy of 650 cm-1 for internal conversion and activationless intersystem crossing to the triplet state through a promoting mode with a frequency of 180 cm-1. For chlorophyll a in Triton X-100 the average frequency of the promoting modes for non-radiative decay is very similar, but the activation energy (300 cm-1) is significantly smaller.     

Triplet states of bacteriochlorophyll and carotenoids in chromatophores of photosynthetic bacteria

Chromatophores from photosynthetic bacteria were excited with flashes lasting approx. 15 ns. Transient optical absorbance changes not associated with the photochemical electron-transfer reactions were interpreted as reflecting the conversion of bacteriochlorophyll or carotenoids into triplet states. Triplet states of various carotenoids were detected in five strains of bacteria; triplet states of bacteriochlorophyll, in two strains that lack carotenoids. Triplet states of antenna pigments could be distinguished from those of pigments specifically associated with the photochemical reaction centers. Antenna pigments were converted into their triplet states if the photochemical apparatus was oversaturated with light, if the primary photochemical reaction was blocked by prior chemical oxidation of P-870 or reduction of the primary electron acceptor, or if the bacteria were genetically devoid of reaction centers. Only the reduction of the electron acceptor appeared to lead to the formation of triplet states in the reaction centers.In the antenna bacteriochlorophyll, triplet states probably arise from excited singlet states by intersystem crossing. The antenna carotenoid triplets probably are formed by energy transfer from triplet antenna bacteriochlorophyll. The energy transfer process has a half time of approx. 20 ns, and is about 1 × 10³ times more rapid than the reaction of the bacteriochlorophyll triplet states with O₂. This is consistent with a role of carotenoids in preventing the formation of singlet O₂ in vivo. In the absence of carotenoids and O₂, the decay half times of the triplet states are 70 μs for the antenna bacteriochlorophyll and 6–10 μs for the reaction center bacteriochlorophyll. The carotenoid triplets decay with half times of 2–8 μs.With weak flashes, the quantum yields of the antenna triplet states are in the order of 0.02. The quantum yields decline severely after approximately one triplet state is formed per photosynthetic unit, so that even extremely strong flashes convert only a very small fraction of the antenna pigments into triplet states. The yield of fluorescence from the antenna bacteriochlorophyll declines similarly. These observations can be explained by the proposal that singlet-triplet fusion causes rapid quenching of excited singlet states in the antenna bacteriochlorophyll.     

Quenching of fluorescence by triplet excited states in chloroplasts

The fluorescence quantum yield in spinach chloroplasts at room temperature has been studied utilizing a 0.5-4.0 mus duration dye laser flash of varying intensities as an excitation source. The yield (phi) and carotenoid triplet concentration were monitored both during and following the laser flash. The triplet concentration was monitored by transient absorption spectoscopy at 515 nm, while the yield phi following the laser was probed with a low intensity xenon flash. The fluorescence is quenched by factors of up to 10-12, depending on the intensity of the flash and the time interval following the onset of the flash. This quenching is attributed to a quencher Q whose concentration is denoted by Q. The relative instantaneous concentration of Q was calculated from phi utilizing the Stern-Volmer equation, and its buildup and decay kinetics were compared to those of carotenoid triplets. At high flash intensities (greater than 10(16) photon . cm-2) the decay kinetics of Q are slower than those of the carotenoid triplets, while at lower flash intensities they are similar. Q is sensitive to oxygen and it is proposed that Q, at the higher intensities, is a trapped chlorophyll triplet. This hypothesis accounts well for the continuing rise of the carotenoid triplet concentration for 1-2 mus after the cessation of the laser pulse by a slow detrapping mechanism, and the subsequent capture of the triplet energy by carotenoid molecules. At the maximum laser intensities, the carotenoid triplet concentration is about one per 100 chlorophyll molecules. The maximum chlorophyll ion concentration generated by the laser pulses was estimated to be below 0.8 ions/100 chlorophyll molecules. None of the observations described here were altered when a picosecond pulse laser train was substituted for the microsecond pulse. A simple kinetic model describing the generation of singlets and triplets (by intersystem crossing), and their subsequent interaction leading to fluorescence quenching, accounts well for the observations. The two coupled differential equations describing the time dependent evolution of singlet and triplet excited states are solved numerically. Using a single-triplet bimolecular rate constant of gammast = 10(-8) cm3 . s-1, the following observations can be accounted for: (1) the rapid initial drop in phi and its subsequent levelling off with increasing time during the laser pulse, (2) the buildup of the triplets during the pulse, and (3) the integrated yield of triplets per pulse as a function of the energy of the flash.     

Temperature-dependent triplet and fluorescence quantum yields of the photosystem II reaction center described in a thermodynamic model.

A key step in the photosynthetic reactions in photosystem II of green plants is the transfer of an electron from the singlet-excited chlorophyll molecule called P680 to a nearby pheophytin molecule. The free energy difference of this primary charge separation reaction is determined in isolated photosystem II reaction center complexes as a function of temperature by measuring the absolute quantum yield of P680 triplet formation and the time-integrated fluorescence emission yield. The total triplet yield is found to be 0.83 +/- 0.05 at 4 K, and it decreases upon raising the temperature to 0.30 at 200 K. It is suggested that the observed triplet states predominantly arise from P680 but to a minor extent also from antenna chlorophyll present in the photosystem II reaction center. No carotenoid triplet states could be detected, demonstrating that the contamination of the preparation with CP47 complexes is less than 1/100 reaction centers. The fluorescence yield is 0.07 +/- 0.02 at 10 K, and it decreases upon raising the temperature to reach a value of 0.05-0.06 at 60-70 K, increases upon raising the temperature to 0.07 at approximately 165 K and decreases again upon further raising the temperature. The complex dependence of fluorescence quantum yield on temperature is explained by assuming the presence of one or more pigments in the photosystem II reaction center that are energetically degenerate with the primary electron donor P680 and below 60-70 K trap part of the excitation energy, and by temperature-dependent excited state decay above 165 K. A four-compartment model is presented that describes the observed triplet and fluorescence quantum yields at all temperatures and includes pigments that are degenerate with P680, temperature-dependent excited state decay and activated upward energy transfer rates. The eigenvalues of the model are in accordance with the lifetimes observed in fluorescence and absorption difference measurements by several workers. The model suggests that the free energy difference between singlet-excited P680 and the radical pair state P680+l- is temperature independent, and that a distribution of free energy differences represented by at least three values of about 20, 40, and 80 meV, is needed to get an appropriate fit of the data.     

Triplet state dynamics in peridinin-chlorophyll-a-protein: a new pathway of photoprotection in LHCs?

This work investigates the interaction of carotenoid and chlorophyll triplet states in the peridinin-chlorophyll-a-protein (PCP) of Amphidinium carterae using step-scan Fourier transform infrared spectroscopy. We identify two carotenoid triplet state lifetimes of approximately 13 and approximately 42 mus in the spectral region between 1800 and 1100 cm(-1) after excitation of the 'blue' and 'red' peridinin (Per) conformers and the Q(y) of chlorophyll-a (Chl-a). The fast and slow decaying triplets exhibit different spectral signatures in the carbonyl region. The fast component generated at all excitation wavelengths is from a major conformer with a lactone stretching mode bleach at 1745 cm(-1). One (1720 cm(-1)) and two (1720 cm(-1) and 1741 cm(-1)) different Per conformers are observed for the slow component upon 670- and 530-480-nm excitation, respectively. The above result implies that (3)Per triplets are formed via two different pathways, corroborating and complementing visible triplet-singlet (T-S) spectra (Kleima et al., Biochemistry (2000), 39, 5184). Surprisingly, all difference spectra show that Per and Chl-a modes are simultaneously present during the (3)Per decay, implying significant involvement of (3)Chl-a in the (3)Per state. We suggest that this Per-Chl-a interaction via a delocalized triplet state lowers the (3)Per energy and thus provides a general, photoprotection mechanism for light-harvesting antenna complexes.     

Biradicals by triplet recombination of radical ion pairs.

Radical ion pairs generated by photoinduced electron transfer may undergo return electron transfer (RET) in pairs of singlet or triplet multiplicity. RET efficiencies are determined by the free energy of RET and the topologies of the potential surfaces of parent molecule, radical ion and triplet state. If radical ion geometries are different from the corresponding triplet states, RET occurs either with cleavage ("dissociative" RET; 1,2-diphenylcyclopropane radical cations) or formation of C-C bonds ("associative" RET; norbornadiene radical cation). Radical ions of some strained ring compounds spontaneously undergo ring-opening; RET to such species form ring-opened triplets without major geometry changes. CIDNP spectroscopy offers unique insights into triplet RET.     

Quenching of fluorescence by triplet excited states in chloroplasts

The fluorescence quantum yield in spinach chloroplasts at room temperature has been studied utilizing a 0.5–4.0 μs duration dye laser flash of varying intensities as an excitation source. The yield (Ф) and carotenoid triplet concentration were monitored both during and following the laser flash. The triplet concentration was monitored by transient absorption spectroscopy at 515 nm, while the yield Ф following the laser was probed with a low intensity xenon flash. The fluorescence is quenched by factors of up to 10–12, depending on the intensity of the flash and the time interval following the onset of the flash. This quenching is attributed to a quencher Q whose concentration is denoted by Q. The relative instantaneous concentration of Q was calculated from Ф utilizing the Stern-Volmer equation, and its buildup and decay kinetics were compared to those of carotenoid triplets. At high flash intensities (10¹⁶ photon · cm⁻²) the decay kinetics of Q are slower than those of the carotenoid triplets, while at lower flash intensities they are similar. Q is sensitive to oxygen and it is proposed that Q, at the higher intensities, is a trapped chlorophyll triplet. This hypothesis accounts well for the continuing rise of the carotenoid triplet concentration for 1–2 μs after the cessation of the laser pulse by a slow detrapping mechanism, and the subsequent capture of the triplet energy by carotenoid molecules.

At the maximum laser intensities, the carotenoid triplet concentration is about one per 100 chlorophyll molecules. The maximum chlorophyll ion concentration generated by the laser pulses was estimated to be below 0.8 ions/100 chlorophyll molecules. None of the observations described here were altered when a picosecond pulse laser train was substituted for the microsecond pulse.

A simple kinetic model describing the generation of singlets and triplets (by intersystem crossing), and their subsequent interaction leading to fluorescence quenching, accounts well for the observations. The two coupled differential equations describing the time dependent evolution of singlet and triplet excited states are solved numerically. Using a singlet-triplet bimolecular rate constant of γ_st = 10⁻⁸ cm³ · s⁻¹, the following observations can be accounted for: (1) the rapid initial drop in Ф and its subsequent levelling off with increasing time during the laser pulse, (2) the buildup of the triplets during the pulse, and (3) the integrated yield of triplets per pulse as a function of the energy of the flash.     

Experimental study of the excited-state properties and photostability of the mycosporine-like amino acid palythine in aqueous solution.

Characterization of the excited states of the mycosporine-like amino acid palythine (lambda(max) = 320 nm) in aqueous solutions was achieved experimentally. The low value for the photodegradation quantum yield, (1.2 +/- 0.2) x 10(-5), confirms that palythine is highly photostable in air saturated-aqueous solutions. Laser flash photolysis of acetone in the presence of palythine allowed for the observation of a transient spectrum which is consistent with the triplet-triplet absorption of palythine. Kinetic treatment of the transient signals yields a lifetime of the triplet state of ca. 9 micros and a triplet energy around 330 kJ mol(-1). The photoacoustic calorimetry results are consistent with non-radiative decay as the major fate of excited palythine. A comparison of the photodegradation quantum yields and photophysical properties of palythine with those previously determined for the other mycosporine-like amino acids, shinorine and porphyra-334, suggests that geometrical isomerization around the C=N bond may contribute to the rapid deactivation of this group of molecules.     

The photochemistry and photophysics of all-trans-1,4-diindanylidenyl-2-butene, a rigid analogue of all-trans-1,6-diphenyl-1,3,5-hexatriene.

all-trans-1,4-Diindanylidenyl-2-butene (ttt-stiff-5-DPH), a torsionally constrained analogue of all-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH), was synthesized and studied in order to evaluate the role of phenyl-vinyl torsional motions in the photophysical and photochemical responses of the DPH chromophore. Spectroscopic and photoisomerization measurements reveal that the behavior of the rigid DPH analogue is very similar to that of the parent DPH. This similarity is obtained despite the fact that the alkyl substitution from the five-membered rings selectively lowers the energy of the 1 (1)B(u)* state, leading to inversion of the order of the 1 (1)B(u)* and 2 (1)A(g)* energy levels in hydrocarbon solvents. In stiff-5-DPH, as in DPH, an increase in solvent polarity enhances terminal over central bond photoisomerization. Analyses of fluorescence and photoisomerization quantum yields show that, as in DPH, the torsional relaxation channel on the singlet excited state manifold is inefficient, falling far short of accounting for all radiationless decay. Significant ( approximately 50 and 80% of all singlet decay in Bz and AN, respectively), photochemically unproductive, radiationless decay channels exist in both molecules. Competing one bond photoisomerizations give the two major photoproducts: tct-stiff-5-DPH and ctt-stiff-5-DPH. They were isolated in pure form and were spectroscopically characterized. Biacetyl-sensitization was used to study the behavior of the stiff-5-DPH triplet state. As in the parent DPH, stiff-5-DPH triplets undergo relatively efficient concentration dependent geometric photoisomerization.     

Picosecond and microsecond pulse laser studies of exciton quenching and exciton distribution in spinach chloroplasts at low temperatures

Studies of the fluorescence quantum yield and decay times, determined at the emission maxima of 685 and 735 nm, using picosecond laser pulses for excitation, indicate that the pigments which are responsible for the 735 nm emission derive their energy by transfer of singlet excitons from the light-harvesting pigments and not by direct absorption of photons. Microsecond pulse laser studies of the fluorescence quantum yields at these two fluorescence wavelengths indicate that long lived quenchers (most probably triplet states), which quench singlet excitons, accumulate preferentially within the long wavelength pigment system which gives rise to the 735 nm emission band.     

Further evidence for dissipative energy migration via triplet states in photosynthesis. The protective mechanism of carotenoids in Rhodopseudomonas spheroides chromatophores.

The protection action of carotenoids against irreversible photodestruction was discovered in photosynthetic bacteria by Stanieda and coworkers. In green plant material it was found by Wolff and Witt (1969) Z. Naturforsch, 24b, 1031-1037 and (1972) Proc. 2nd. Int. Congr. Photosynthesis Res. Stresa (Forti, G., Avron, M. and Melandri, A., eds.), Vol. 2, pp. 931-936, Dr. W. Junk, N. V. Publ. The Hague) that the formation of special carotenoid triplet states (via very rapid energy transfer from excited chlorophylls) and their fast radiationless decay in tau1/2 approximately 3 microns is at least one mechanism for the protective action of carotenoids to irreversible photooxidation of the chlorophylls. Hence, it is anticipated that the same mechanism might be realized also in bacteria. The present study gives evidence for such a "triplet valve" to be established also in bacteria. This conclusion was derived from the following observations: 1. The light-induced difference spectrum shows a bleaching of a carotenoid at three characteristic wavelength between 400 and 500 nm. A positive peak around 533 nm indicates the formation of a carotenoid triplet state. 2. The absorption changes can be induced by red light which excites only bacteriochlorophyll. This indicates an energy transfer from bacteriochlorophyll to carotenoids. 3. The light-induced carotenoid triplets decay radiationless in 3 microns in air-saturated aqueous suspensions of the chromatophores. 4. The carotenoid triplet formation occurs only at actinic flash intensities where the photosynthesis becomes saturated. 5. Addition of dithionite, which blocks photosynthesis, markedly increases the extent of carotenoid triplet formation. The different types of exciton migration within the photosynthetic unit are discussed, especially the routes leading to the dissipation of excess excitation energy.     

Novel emission properties of melem caused by the heavy metal effect of lanthanides(III) in a LB film.

A novel emissive molecular system is constructed by the intercalation of the fluorophore melem (triamino-tri-s-triazine) within a Langmuir-Blodgett (LB) film of stearic acid with the periodic arrangement of lanthanides (Ln(III)), mainly Pr(III) with supporting of Eu(III). From emission spectra, decay curves, quantum yields and XPS measurements, it is clarified that the external heavy metal effect of Pr(III) on melem is much stronger in the film than in the bulk solid state, resulting in producing an unusual triplet state of melem. The triplet state of melem in the LB film donates the excitation energy to Pr(III) in the LB film, which is completely different from the energy transfer pathway of Pr-melem complex in the solid state through the singlet state of melem.     

The triplet exciton of chlorophyll a and carotenoid in solution and in photosynthetic antenna proteins

We have observed the development and decay of triplet excitons formed in the ‘antenna’ chlorophyll $\text{a}\text{b}$ protein complex by high-intensity laser excitation. The carotenoid triplet (³Car) appeared 5 ns after excitation in the protein isolation, commonly termed CP-II; the risetime in a larger antenna particle, called LHC (light-harvesting complex) was 12 ns. The quantum yield of ³Car in CP-II decreased 11-fold as intensity was increased from 10¹⁶ to 2 · 10¹⁷ photons/cm² per pulse. The effect is attributed to exciton annihilation during the initial period of triplet formation. Above 5 · 10¹⁶ photons/cm² per s, the ³Car lifetime decreases substantially from its low intensity value of 8.7 μs. A comparison of the transient absorption spectrum of CP-II with those of chlorophyll and carotenoid in vitro indicates that ‘trapped’ chlorophyll triplets formed at high intensities. We present a simple model of destructive interaction between ³Car and chlorophyll triplets which is compatible with the observed increased rate of ³Car decay. Indirect evidence suggests similar effects occur in LHC.     

Synthesis, photophysical and photochemical properties of novel soluble tetra[4-(thiophen-3yl)-phenoxy]phthalocyaninato zinc(II) and Ti(IV)O complexes

The synthesis, photophysical and photochemical properties of zinc and oxo-titanium phthalocyanine derivatives 4-(tetra[4-(thiophen-3yl)-phenoxy]phthalocyaninato)zinc(II), (2); and 4-(tetra[4-(thiophen-3yl)-phenoxy]phthalocyaninato)oxo-titanium(IV), (3), are described for the first time. These peripherally substituted complexes (2 and 3) have been synthesized and characterized by elemental analysis, IR, ¹H NMR and electronic spectroscopy. The compounds (2 and 3) have good solubility in organic solvents such as CHCl₃, DCM, DMSO, DMF, THF and toluene and are not aggregated within a wide concentration range. General trends are described for singlet oxygen, photodegradation, fluorescence quantum yields, triplet quantum yields and triplet life times of these complexes in DMSO, DMF and THF. Compound 2 has higher fluorescence quantum yields, triplet quantum yields and triplet life times than 3, however, the former has lower singlet oxygen quantum yields and photodegradation quantum yields than the latter.     

Absorption studies of primary reactions in Photosystem I. Yield and rate of formation of the P-700 triplet state

The quantum yield of P-700 triplet state formation has been found from flash-induced absorption studies in the microsecond time range to be 0.45 and 0.35 at 294 K and 6–20 K, respectively, in CP1-SDS particles which lack the secondary acceptors. From these quantum yield measurements, yields of formation of the P-700 triplet state from the primary biradical (P-700⁺–A⁻₀) were calculated to be around 0.6 at both temperatures, whereas double-laser flash experiments allowed us to derive upper limits for this yield (0.84 at 294 K and 0.79 at 20 K). These values agree with the high values that have been previously calculated from an earlier absorption study (Sétif, P., Hervo, G. and Mathis, P. (1981) Biochim. Biophys. Acta 638, 257–267) but appear significantly higher than the yield calculated from EPR experiments (5–10%) (Gast, P., Swarthoff, T., Ebskamp, F.C.R. and Hoff, A.J. (1983) Biochim. Biophys. Acta 722, 163–175). Possible explanations for this discrepancy are discussed. From absorption studies in the submicrosecond time range as well as from double-laser flash experiments, the lifetime of the biradical (P-700⁺–A⁻₀), from which the P-700 triplet state is formed by recombination, has been measured to increase from 30–50 ns at room temperature up to 120–130 ns between 10 and 110K.     

Photoprotection in the antenna complexes of photosystem II: role of individual xanthophylls in chlorophyll triplet quenching

In this work the photoprotective role of all xanthophylls in LHCII, Lhcb4, and Lhcb5 is investigated by laser-induced Triplet-minus-Singlet (TmS) spectroscopy. The comparison of native LHCII trimeric complexes with different carotenoid composition shows that the xanthophylls in sites V1 and N1 do not directly contribute to the chlorophyll triplet quenching. The largest part of the triplets is quenched by the lutein bound in site L1, which is located in close proximity to the chlorophylls responsible for the low energy state of the complex. The lutein in the L2 site is also active in triplet quenching, and it shows a longer triplet lifetime than the lutein in the L1 site. This lifetime difference depends on the occupancy of the N1 binding site, where neoxanthin acts as an oxygen barrier, limiting the access of O(2) to the inner domain of the Lhc complex, thereby strongly contributing to the photostability. The carotenoid triplet decay of monomeric Lhcb1, Lhcb4, and Lhcb5 is mono-exponential, with shorter lifetimes than observed for trimeric LHCII, suggesting that their inner domains are more accessible for O(2). As for trimeric LHCII, only the xanthophylls in sites L1 and L2 are active in triplet quenching. Although the chlorophyll to carotenoid triplet transfer is efficient (95%) in all complexes, it is not perfect, leaving 5% of the chlorophyll triplets unquenched. This effect appears to be intrinsically related to the molecular organization of the Lhcb proteins.     

2,5-Dimethylphenacyl carbamate: a photoremovable protecting group for amines and amino acids.

2,5-Dimethylphenacyl (DMP) carbamates (1a-c) released the corresponding free amines or amino acids in high chemical yields, albeit with quantum yields Phi of only 0.04-0.09, upon irradiation in either aprotic or protic solvents. The photoreaction proceeded principally from the triplet excited state via the E-photoenol. The lifetimes of the triplet enol and the E- and Z-enols in the ground state were determined by laser flash photolysis. The primary photoinitiated transformation liberated a carbamic acid derivative, which subsequently decarboxylated to the amino group-containing compound. Exhaustive irradiation of a DMP-protected aniline (1a) in acetonitrile did not provide aniline in quantitative chemical yields, because it was involved in reductive cleavage of the starting material as an electron donor, thereby decreasing the overall deprotection yield (86%). Phenylalanine methyl ester, liberated from 1c, was, however, obtained in excellent chemical yield (97%). It was also found that the carbamates, while thermally stable, released amines with higher quantum yields in acidic methanol solutions. The DMP chromophore is proposed as an excellent photoremovable protecting group for amino acids and, under specific conditions, for amines in organic synthesis and biochemistry.     

Reactivity of halides with triplets of 6-chloro and 6-bromopicolinic acids: contrasting effects of bromide and chloride.

The reactivity of Br(-) and Cl(-) with triplet of anionic 6-chloropicolinic acid (pH = 5.4) and with triplets of 6-chloro and 6-bromopicolinic acids in zwitterionic forms (pH = 0.9) was studied by laser flash photolysis and steady-state irradiation. Br(-) was found to trap the three triplets. Triplet lifetime measurements gave quenching rate constants of 8 x 10(8) mol(-1) dm(3) s(-1) for the zwitterion of 6-chloropicolinic acid and of 3.4 x 10(5) mol(-1) dm(3) s(-1) for the anionic counterpart. No secondary transient species were observed indicating that the charge transfer intermediates are subject to dissipative processes. Cl(-) trapped triplet of zwitterions only, and reactions were found to be associated with a high quantum yield of radicals. The photolysis of 6-bromopicolinic acid photolysis was drastically enhanced by Cl(-), 6-chloropicolinic acid being produced with a chemical yield of about 90%. The 6-bromo-2-carboxypyridinyl radical could be characterized (lambda(max)/nm = 318 with shoulder at 370 nm and epsilon/mol(-1) dm(3) cm(-1) = 8100).     

Magnetic field dependence of radical-pair decay kinetics and molecular triplet quantum yield in quinone-depleted reaction centers

Radical-pair decay kinetics and molecular triplet quantum yields at various magnetic fields are reported for quinone-depleted reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides R26. The radical-pair decay is observed by picosecond absorption spectroscopy to be a single exponential to within the experimental uncertainty at all fields. The decay time increases from 13 ns at zero field to 17 ns at 1 kG, and decreases to 9 ns at 50 kG. The orientation averaged quantum yield of formation of the molecular triplet of the primary electron donor, ³P, drops to 47% of its zero-field value at 1 kG and rises to 126% at 50 kG. Combined analysis of these data gives a singlet radical-pair decay rate constant of 5 · 10⁷s^?1, a lower limit for the triplet radical-pair decay rate constant of 1 · 10⁸s^?1 and a lower limit for the quantum yield of radical-pair decay by the triplet channel of 38% at zero field. The upper limit of the quantum yield of ³P formation at zero field is measured to be 32%. In order to explain this apparent discrepancy, decay of the radical pair by the triplet channel must lead to some rapid ground state formation as well as some ³P formation. It is proposed that the triplet radical pair decays to a triplet charge-transfer state which is strongly coupled to the ground state by spin-orbit interactions. Several possibilities for this charge-transfer state are discussed.     

Spectral, photophysical and photochemical properties of tetra- and octaglycosylated zinc phthalocyanines

Photophysical and photochemical properties of a series of tetra- and octaglycosylated zinc phthalocyanines (ZnPcs) substituted with glucose and galactose moieties have been reported. Spectral properties of these phthalocyanines are compared in DMSO. Absorption spectra of the non-peripherally tetra-substituted ZnPcs 2 showed a significant red shift in their Q-band maxima as compared to the peripherally substituted analog 1. All the complexes gave high triplet quantum yields ranging from 0.68 to 0.88, whereas triplet lifetimes were in the range of 100-430 μs in argon-saturated solutions. The octagalactosylated ZnPc 3b showed the highest triplet quantum yield and singlet oxygen quantum yield of 0.88 and 0.69, respectively. The fluorescence quantum yields and lifetimes of all the compounds under investigation were within the range of zinc phthalocyanine complexes.