Identification of different emitting species in the red fluorescent protein DsRed by means of ensemble and single-molecule spectroscopy

The photophysics and photochemistry taking place in the DsRed protein, a recently cloned red fluorescent protein from a coral of the Discosoma genus, are investigated here by means of ensemble and single-molecule time-resolved detection and spectroscopic measurements. Ensemble time-resolved data reveal that 25% of the immature green chromophores are present in tetramers containing only this immature form. They are responsible for the weak fluorescence emitted at 500 nm. The remaining 75% of the immature green chromophores are involved in a fluorescence resonance energy transfer process to the red species. The combination of time-resolved detection with spectroscopy at the single-molecule level reveals, on 543-nm excitation of individual DsRed tetramers, the existence of a photoconversion of the red chromophore emitting at 583 nm and decaying with a 3.2-ns time constant into a super red one emitting at 595 nm and for which the decay time constant ranges between 2.7 and 1.5 ns. The phenomenon is further corroborated at the ensemble level by the observation of the creation of a super red form and a blue absorbing species on irradiation with 532-nm pulsed light at high excitation power. Furthermore, single-molecule experiments suggest that a similar photoconversion process might occur in the immature green species on 488-nm excitation.

Polyphenylene dendrimers with perylene diimide as a luminescent core

A novel synthesis is presented of a fourfold ethynyl-substituted perylene diimide dye 4, which acts as a core molecule for the buildup of polyphenylene dendrimers. Around the luminescent core 4, a first-generation (5), a second-generation (6), and a third-generation (7) polyphenylene dendritic environment consisting of pentaphenylbenzene building blocks are constructed. The dendrimers 5 and 6 are synthesized by an exclusively divergent route, whereas for 7, a combination of a divergent and convergent approaches is applied. Absorption and emission spectra of 5-7 in different solvents and in a film have been measured and compared to a nondendronized model compound 13. In solution, the internal chromophore is scarcely influenced by the dendritic scaffold; however, in the solid state, aggregation of the perylene diimide is prevented very effectively by the four rigid dendrons. Additionally, fluorescence quantum yields in solution have been determined for 5-7 and 13; they decrease as the number of generation increases.

Single-molecule spectroscopy of a dendrimer-based host-guest system

We report on a single-molecule study of a host-guest system that consists of a second-generation polyphenylene dendrimer and the cyanine dye Pinacyanol. The use of single-molecule spectroscopy enables us to obtain more detailed information on the properties of the host-guest system and can be used to confirm solution data. At low dye to dendrimer ratios the system is present as a one-to-one complex, while for higher ratios an ion-pair system is formed. Changes in the spectral properties of the single molecules are explained by differences in local polarisability. The difference of the triplet lifetimes of single free dye molecules and of associated ones is interpreted as deriving from a larger free volume for the dye molecules in the dendritic host relative to the rigid polymer matrix.

A new analysis method of single molecule fluorescence using series of photon arrival times: theory and experiment

Up to now, single molecule fluorescence experiments were performed by dividing the time into a set of intervals and to observe the number of fluorescence photons arriving in each interval. It is obvious that the detected photons carry less information than the arrival times of the photons themselves. From the arrival times, one can still calculate the number of photons in any user-defined interval; whereas, when only the number of photons in an interval are recorded, information about their positions in time is lost. Therefore, we present a new analysis method of single molecule fluorescence data based on the positions in time of the detected fluorescence photons. We derive mathematically different statistical characteristics describing the single molecule fluorescence experiment assuming an immobilized molecule. The theory of point processes using the generating functionals formalism is ideally suited for a consistent description, linking the statistical characteristics of the excitation and detected photons to the statistical characteristics of the single motionless molecule. We then use computer-generated data sets mimicking the single molecule fluorescence experiment to explore the parametric estimation of mono- and bi-exponential single molecule impulse response functions (SMIRFs) via the following statistical characteristics: the probability density distributions (pdd) of the single and first photocount time positions in a user-defined detection interval, the probability distribution of the number of photocounts per user-defined detection interval, the time correlation function and the pdd of the time interval between two consecutive photocounts. It is shown that all of the above characteristics ensure a satisfactory recovery of the decay time of mono-exponential SMIRFs for a broad range of excitation intensities and widths of user-defined detection intervals. For bi-exponential SMIRFs, the selection of the experimental conditions is more critical and dependent on the detection procedure. At lower excitation intensities it is advantageous to use the pdds of the single and first photocount time occurrences in the user-defined detection interval. To show the practical usefulness of the new analysis method, series of photon arrival times from immobilized single molecules of DiI and rhodamine 6G were analyzed to estimate triplet lifetimes and intersystem crossing yields.

Triplet states as non-radiative traps in multichromophoric entities: single molecule spectroscopy of an artificial and natural antenna system

Energy transfer in antenna systems, ordered arrays of chromophores, is one of the key steps in the photosynthetic process. The photophysical processes taking place in such multichromophoric systems, even at the single molecule level, are complicated and not yet fully understood. Instead of directly studying individual antenna systems, we have chosen to focus first on systems for which the amount of chromophores and the interactions among the chromophores can be varied in a systematic way. Dendrimers with a controlled number of chromophores at the rim fulfill those requirements perfectly. A detailed photophysical study of a second-generation dendrimer, containing eight peryleneimide chromophores at the rim, was performed 'J. Am. Chem. Soc., 122 (2000) 9278'. One of the most intriguing findings was the presence of collective on/off jumps in the fluorescence intensity traces of the dendrimers. This phenomenon can be explained by assuming a simultaneous presence of both a radiative trap (energetically lowest chromophoric site) and a non-radiative trap (triplet state of one chromophore) within one individual dendrimer. It was shown that an analogue scheme could explain the collective on/off jumps in the fluorescence intensity traces of the photosynthetic pigment B-phycoerythrin (B-PE) (Porphyridium cruentum). The different values of the triplet lifetime that could be recovered for a fluorescence intensity trace of B-PE were correlated with different intensity levels in the trace, suggesting different chromophores acting as a trap as function of time.

Polyphenylene dendrimers with different fluorescent chromophores asymmetrically distributed at the periphery

A new synthetic approach leading to asymmetrically substituted polyphenylene dendrimers is presented. Following this method, polyphenylene dendrimers decorated with an increasing number of chromophores at the periphery have been obtained up to the second generation. Especially the synthesis of a polyphenylene dendrimer bearing three donor chromophores and one acceptor chromophore has been realized. Intramolecular energy transfer within this molecule is demonstrated by applying absorption and fluorescence measurements.

Intramolecular energy hopping and energy trapping in polyphenylene dendrimers with multiple peryleneimide donor chromophores and a terryleneimide acceptor trap chromophore

Intramolecular Förster-type excitation energy transfer (FRET) processes in a series of first-generation polyphenylene dendrimers substituted with spatially well-separated peryleneimide chromophores and a terryleneimide energy-trapping chromophore at the rim were investigated by steady-state and time-resolved fluorescence spectroscopy. Energy-hopping processes among the peryleneimide chromophores are revealed by anisotropy decay times of 50--80 ps consistent with a FRET rate constant of k(hopp) = 4.6 ns(-1). If a terryleneimide chromophore is present at the rim of the dendrimer together with three peryleneimide chromophores, more than 95% of the energy harvested by the peryleneimide chromophores is transferred and trapped in the terryleneimide. The two decay times (tau(1) = 52 ps and tau(2) = 175 ps) found for the peryleneimide emission band are recovered as rise times at the terryleneimide emission band proving that the energy trapping of peryleneimide excitation energy by the terryleneimide acceptor occurs via two different, efficient pathways. Molecular- modeling-based structures tentatively indicate that the rotation of the terryleneimide acceptor group can lead to a much smaller distance to a single donor chromophore, which could explain the occurrence of two energy-trapping rate constants. All energy-transfer processes are quantitatively describable with Förster energy transfer theory, and the influence of the dipole orientation factor in the Förster equation is discussed.

An Experimental Comparison of the Maximum Likelihood Estimation and Nonlinear Least-Squares Fluorescence Lifetime Analysis of Single Molecules

Two procedures based on the weighted least-squares (LS) and the maximum likelihood estimation (MLE) method to confidently analyze single-molecule (SM) fluorescence decays with a total number (N) of 2,500-60,000 counts have been elucidated and experimentally compared by analyzing measured bulk and SM decays. The key observation of this comparison is that the LS systematically underestimates the fluorescence lifetimes by approximately 5%, for the range of 1,000-20,000 events, whereas the MLE method gives stable results over the whole intensity range, even at counts N less than 1,000, where the LS analysis delivers unreasonable values. This difference can be attributed to the different statistics approaches and results from improper weighting of the LS method. As expected from theory, the results of both methods become equivalent above a certain threshold of N detected photons per decay, which is here experimentally determined to be approximately 20,000. In contrast to the bulk lifetime distributions, the SM fluorescence lifetime distributions exhibit standard deviations that are sizably larger than the statistically expected values. This comparison proves the strong influence of the inhomogenuous microenvironment on the photophysical behavior of single molecules embedded in a 10-30-nm thin polymer layer.

Microscopy and optical manipulation of dendrimer-built vesicles

A fifth-generation poly(propylene imine) dendrimer decorated with palmitoyl- and azobenzene-containing alkyl groups forms giant vesicles in aqueous solutions with diameters from 50 nm up to 20 mm and a multilaminar onion-like structure. Dense and ordered arrangement of the azobenzene chromophores in the bilayer structure leads to fluorescence with lmax= 600 nm. The fluorescence intensity can be increased by irradiation with blue light, and at low pH a distinctive blue shift of the spectrum is observed. With the aid of a single-beam optical tweezers it is possible to trap vesicles and direct them in a billiard-like fashion against each other using forces in the range of several pN. In collision experiments, the vesicles behave like hard spheres, and merging is not observed.

Scanning tunneling microscopy: a unique tool in the study of chirality, dynamics, and reactivity in physisorbed organic monolayers

Scanning tunneling microscopy (STM) is applied to study organic monolayers, physisorbed at the liquid-graphite interface. Due to the very local nature of the probing, the structure of these adlayers has been imaged with very high detail. The high resolution allowed us to investigate the effect of molecular chirality on the monolayer formation and provided a unique way to study chemical reactions at the liquid-graphite interface. Making use of a fast scanning mode, dynamic processes in these adlayers have been visualized.