Light energy absorbed by a Ch1 molecule creates an exciton which can produce photochemistry, heat or fluorescence. Fluorescence lifetime measurements have demonstrated that the observed fluorescence is a sum of several lifetime components. We present evidence that:

Most current models for energy transfer and trapping in the antenna of purple nonsulfur bacteria involve excitations visiting many reaction centers before trapping occurs. This so-called lake model has been supported by a variety of observations including singlet-singlet annihilation studies (1–3), low intensity picosecond absorption spectroscopy (4), and fluorescence depolarization studies (5).

In cyanobacteria and red algae the fluorescence decay kinetics of whole cells are very complex because of a variety of chromophores in different protein environments. Many time-resolved fluorescence studies of isolated phycobilisomes have been published (1), but only a few deal with whole cells of cyanobacteria (2). Their fluorescence characteristics are complicated by the fact that the phycobilisomes are attached to the thylakoid membrane and are energetically coupled to the chlorophyll a core antenna of PS II. To assign phases of fluorescence decay to particular parts of the cyanobacterial antenna system, we measured the fluorescence characteristics of cyanobacterial mutants that do not have PS II or whose PS II complex has been altered, and compared it with the fluorescence patterns of wild type. The cyanobacterium Synechocystis sp. PCC 6803 is particularly suitable for such studies, since we have generated a number of mutants of this organism that are specifically impaired in certain PS II genes (3,4). From a comparison of the decay kinetics of the mutants with wild type it should be possible to assign fluorescence lifetimes to certain antenna components and to draw conclusions about the coupling between the phycobilisomes and PS II or PS I.

The antenna chlorophylls in higher plants can produce fluorescence that is blue-shifted (anti-Stokes-shifted) relative to the exciting light. Global analysis of the time-resolved fluorescence spectra resolves an ultrafast fluorescence emission component with a lifetime of about 15 ps which is attributed to exciton equilibration in the antenna. These results support the hypothesis that the overall exciton decay kinetics in PS I and PS II is trap-limited as suggested by us earlier.

Abstract— Absorption and fluorescence emission spectra, fluorescence lifetimes, fluorescence quantum yields, photoisomerization quantum yields and triplet quantum yields were measured for Merocyanine 540 (MC540) in ethanol and in large unilamellar dimyristoyl phosphatidylcholine vesicles. The major differences in the photophysics between the two media are the increase of the fluorescence quantum yield from 0.15 in ethanol to 0.6 in vesicles at 25° C, and the appearance of a second fluorescence decay with a lifetime of 1.87 ns in the latter medium. Upper and lower limits for the photoisomerization quantum yields were determined by combining the data from laser flash photolysis and optoacoustic spectroscopy. The decrease in photoisomerization quantum yield upon incorporation of the dye into the lipid bilayer by a factor 2 suggests that this process competes directly with fluorescence. The temperature dependence of the fluorescence and photoisomerization quantum yields in solution supports this model. In both media MC540 has a very low triplet quantum yield with values 0.002 > (> øT > 0.02 in ethanol and 0.01 > øT‐ > 0.09 in liposomes Our data are consistent with the model whereby the dye is incorporated into the lipid bilayer as a monomer with two different orientations and this model is adopted on the basis of the biexponential behaviour of the fluorescence and photoisomer decay.

Phenylglyoxylic acid (1) in aqueous solution is recommended as a convenient chemical actinometer (φ ≈ 0.6 – 0.8) for the region 250 – 400 nm. UV spectroscopy, nuclear magnetic resonance spectroscopy, gas chromatography, high performance liquid chromatography and titrimetry are suitable analytical methods for monitoring the photodecarboxylation of 1 and the formation of benzaldehyde (2). Quantum yield φ determinations and 1H photochemically induced dynamic nuclear polarization (photo-CIDNP) experiments in different media reveal that the photochemistry of 1 is strongly dependent on the nature, water content and pH value of the solvent. Photodecarboxylation of 1 requires the presence of water, and φ is drastically enhanced by increasing the water concentration in acidic solution. On the basis of photo-CIDNP results it is suggested that the photoreduction of 1 in 2-propanol, yielding a 1:1 mixture of meso- and D, L-2,3-diphenyl-tartaric acid, proceeds via a radical-pair mechanism. Two pathways are discussed for the formation of 2 by photodecarboxylation of 1. The main reaction is proposed to occur without polarization under photo-CIDNP conditions while side reactions give rise to polarized benzoin and benzaldehyde proton nuclear magnetic resonance spectra.

Dissociation of the complexes [Rh(η6-arene)L2]ClO4 (arene = mesitylene (Ia), toluene (Ib) and benzene (Ic); L = P(OPh)3) with loss of the arene ligand in CD2Cl2/ acetone-d6 (95/5 mol/mol) has been studied by 31P NMR spectroscopy; for initial 0.025 M concentrations of degrees of dissociation were 0.23, 0.33 and 0.85, respectively. The dissociations were first-order, with half-lifes of 31 (Ia), 1.2 (Ib) and 0.7 (Ic) h.