Chlorosomes are light-harvesting antennae of photosynthetic bacteria containing large numbers of self-aggregated bacteriochlorophyll (BChl) molecules. They have developed unique photophysical properties that enable them to absorb light and transfer the excitation energy with very high efficiency. However, the molecular-level organization, that produces the photophysical properties of BChl molecules in the aggregates, is still not fully understood. One of the reasons is heterogeneity in the chlorosome structure which gives rise to a hierarchy of structural and energy disorder. In this report, we for the first time directly measure absorption linear dichroism (LD) on individual, isolated chlorosomes. Together with fluorescence-detected three-dimensional LD, these experiments reveal a large amount of disorder on the single-chlorosome level in the form of distributions of LD observables in chlorosomes from wild-type bacterium Chlorobaculum tepidum . Fluorescence spectral parameters, such as peak wavelength and bandwidth, are measures of the aggregate excitonic properties. These parameters obtained on individual chlorosomes are uncorrelated with the observed LD distributions and indicate that the observed disorder is due to inner structural disorder along the chlorosome long axis. The excitonic disorder that is also present is not manifested in the LD distributions. Limiting values of the LD parameter distributions, which are relatively free of the effect of structural disorder, define a range of angles at which the excitonic dipole moment is oriented with respect to the surface of the two-dimensional aggregate of BChl molecules. Experiments on chlorosomes of a triple mutant of Chlorobaculum tepidum show that the mutant chlorosomes have significantly less inner structural disorder and higher symmetry, compatible with a model of well-ordered concentric cylinders. Different values of the transition dipole moment orientations are consistent with a different molecular level organization of BChl's in the mutant and wild-type chlorosomes.
We report results on circular dichroism (CD) measured on single immobilized chlorosomes of a triple mutant of green sulfur bacterium Chlorobaculum tepidum . The CD signal is measured by monitoring chlorosomal bacteriochlorphyll c fluorescence excited by alternate left and right circularly polarized laser light with a fixed wavelength of 733 nm. The excitation wavelength is close to a maximum of the negative CD signal of a bulk solution of the same chlorosomes. The average CD dissymmetry parameter obtained from an ensemble of individual chlorosomes was gs = -0.025, with an intrinsic standard deviation (due to variations between individual chlorosomes) of 0.006. The dissymmetry value is about 2.5 times larger than that obtained at the same wavelength in the bulk solution. The difference can be satisfactorily explained by taking into account the orientation factor in the single-chlorosome experiments. The observed distribution of the dissymmetry parameter reflects the well-ordered nature of the mutant chlorosomes.
b S Supporting Information C onjugated polymers continue to attract attention of physicists, chemists, and material scientists both because of their fascinating photophysical properties and because of their potential for applications in organic optoelectronic devices and sensors. The study of the photophysical properties is complicated by the amorphous nature of most conjugated polymers, which produces a wide distribution of chain conformations and resulting microscopic properties and interactions. The optical properties are determined by conjugated segments over which the π-electrons are delocalized. Near proximity of segments located on different chains or on different parts of the same chain can result in photophysical interactions, such as energy transfer, ground-or excited-state aggregate formation, or charge transfer.Single-molecule spectroscopy has been providing exceptional insight into the physical properties of polymers and other soft and complex matter. 1À5 Basic understanding of the photophysics of conjugated polymers has been formed based on the early studies on single chains of the prototypical conjugated polymer, poly[2-methoxy-5-(2 0 -ethylhexyl)oxy-1,4-phenylenevinylene] (MEH-PPV). Fluorescence from single molecules of MEH-PPV showed step-like intermittency (blinking) and photobleaching. 6 These are features that have been previously observed for single small dye molecules but were not expected for a conjugated polymer chain of the molecular weight of 900 000. The blinking has been ascribed to the localization of the exciton on one or a few conjugated segments, which effectively reemit the energy. The localization is caused by efficient energy transfer within the polymer chain due to small intersegment distances in a compact defect-cylinder conformation. 7 Later, it has been shown that the blinking can be partially suppressed in single MEH-PPV chains, which retain extended random coil conformation when cast from a good solvent. 8 This basic picture of "blinking = compact conformation (poor solvent)" and "nonblinking = extended conformation (good solvent)" has been later confirmed by other groups on various conjugated polymer systems. 9À17 At the same time, there are indications that the picture may be more complex. The blinking depends not only on the polarity but also on the molecular weight of the matrix polymer in which single conjugated chains are dispersed. 9 Also, it was found that two different extended conformations can coexist in the same good-solvent matrix, and relatively subtle differences between the conformations can lead to the appearance of the blinking. 18 There is also the fundamental question of what determines the solvent quality for MEH-PPV. Aromatic solvents such as toluene interact preferentially with the aromatic backbone of the main chain, whereas nonaromatic solvents have a preferential interaction with the side groups. 19 This has resulted in conflicting reports on the solvent quality of polystyrene (PS) for MEH-PPV and other conjugated polymers. 15,18,20 An answer on...
Perylenediimide (PDI) molecules are promising building blocks for photophysical studies of electronic interactions within multichromophore arrays. Such PDI arrays are important materials for fabrication of molecular nanodevices such as organic light-emitting diodes, organic semiconductors, and biosensors because of their high photostability, chemical and physical inertness, electron affinity, and high tinctorial strength over the entire visible spectrum. In this work, PDIs have been organized into linear (L3) and trefoil (T3) trimer molecules and investigated by single-molecule fluorescence microscopy to probe the relationship between molecular structures and interchromophoric electronic interactions. We found a broad distribution of coupling strengths in both L3 and T3 and hence strong/weak coupling between PDI units by monitoring spectral peak shifts in single-molecule fluorescence spectra upon sequential photobleaching of each constituent chromophore. In addition, we used a wide-field defocused imaging technique to resolve heterogeneities in molecular structures of L3 and T3 embedded in a PMMA polymer matrix. A systematic comparison between the two sets of experimental results allowed us to infer the correlation between intermolecular interactions and molecular structures. Our results show control of the PDI intermolecular interactions using suitable multichromophoric structures.
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