Photophysics of a Water−Soluble Rylene Dye: Comparison with Other Fluorescent Molecules for Biological Applications

Theoretical and experimental investigation on the intersystem crossing kinetics in benzothioxanthene imide luminophores, and their dependence on substituent effects

Substituent induced distortion effects play a crucial role in enhancing the intersystem crossing kinetics in benzothioxanthene imide derivatives.

Facile Synthesis and Evaluation of Electron Transport and Photophysical Properties of Photoluminescent PDI Derivatives

Perylenediimides (PDIs) have emerged as potential materials for optoelectronic applications. In the current work, four PDI derivatives, substituted at imide nitrogen with 2,6-diisopropyl phenyl, 2-nitrophenyl, diphenylmethylene, and pentafluorophenyl groups, have been synthesized from perylene 3,4,9,10-tetracarboxylic dianhydride using a facile imidization synthesis process. PDI derivatives have been spectroscopically characterized for their structure and optical properties. Temperature-variable absorption and emission spectroscopy study confirmed the H-aggregation property. H-aggregation along with strong emission suggests the slipped π-π stacking of PDI molecules. Electrochemical analysis was performed for their redox behavior and calculation of lowest unoccupied molecular orbital and highest occupied molecular orbital energy levels. Scanning electron microscopy showed the formation of ordered structures. The PDI derivatives showed excellent electron conductivity without doping and 5-10× higher electron mobility than that of state-of-the-art fullerene acceptor phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). Finally, the charge generation and charge transfer phenomenon was studied by transient absorption spectroscopy (TAS). TAS showed ultrafast charge transfer from the poly(3-hexyl)thiophene (P3HT) donor polymer to PDI and formation of long-lived charge-separated states similar to fullerene derivative PC₆₁BM/P3HT blends. Such PDI derivatives with excellent solubility and photophysical and electronic properties are potential n-type materials to be used in organic electronic devices.

Perylenetetracarboxy-3,4:9,10-diimide derivatives with large two-photon absorption activity

Perylenetetracarboxy-3,4:9,10-diimides with large TPA cross-sections.

Design and Synthesis of Reactive Perylene Tetracarboxylic Diimide Derivatives for Rapid Cell Imaging

A new water-soluble reactive perylene tetracarboxylic diimide derivative (PDI-pfp) is designed and synthesized that can realize fast imaging of the endoplasmic reticulum in living cells. The PDI-pfp comprises three functional moieties: perylene tetracarboxylic diimide as fluorescent backbone, poly(ethylene glycol) for providing good water disperse ability, and pentafluorophenol active ester as the reactive group under physiological condition. On the basis of covalent reaction between the active ester group of PDI-pfp and amine groups on cytomembrane, PDI-pfp can rapidly interact with cytomembrane, followed by uptake by living MCF-7 cells within 1 min and also exhibit low cell cytotoxicity. Furthermore, it is proved that PDI-pfp acts as a universal imaging agent for other types of cells. This fluorescent probe is of great potential for the application in the rapid imaging of organelles in cells.

Fundamental Properties of One-Dimensional Zinc Oxide Nanomaterials and Implementations in Various Detection Modes of Enhanced Biosensing

Recent bioapplications of one-dimensional (1D) zinc oxide (ZnO) nanomaterials, despite the short development period, have shown promising signs as new sensors and assay platforms offering exquisite biomolecular sensitivity and selectivity. The incorporation of 1D ZnO nanomaterials has proven beneficial to various modes of biodetection owing to their inherent properties. The more widely explored electrochemical and electrical approaches tend to capitalize on the reduced physical dimensionality, yielding a high surface-to-volume ratio, as well as on the electrical properties of ZnO. The newer development of the use of 1D ZnO nanomaterials in fluorescence-based biodetection exploits the innate optical property of their high anisotropy. This review considers stimulating research advances made to identify and understand fundamental properties of 1D ZnO nanomaterials, and examines various biosensing modes utilizing them, while focusing on the unique optical properties of individual and ensembles of 1D ZnO nanomaterials specifically pertaining to their bio-optical applications in simple and complex fluorescence assays.

Fluorescent Polymer Nanoparticles Based on Dyes: Seeking Brighter Tools for Bioimaging

Speed, resolution and sensitivity of today's fluorescence bioimaging can be drastically improved by fluorescent nanoparticles (NPs) that are many-fold brighter than organic dyes and fluorescent proteins. While the field is currently dominated by inorganic NPs, notably quantum dots (QDs), fluorescent polymer NPs encapsulating large quantities of dyes (dye-loaded NPs) have emerged recently as an attractive alternative. These new nanomaterials, inspired from the fields of polymeric drug delivery vehicles and advanced fluorophores, can combine superior brightness with biodegradability and low toxicity. Here, we describe the strategies for synthesis of dye-loaded polymer NPs by emulsion polymerization and assembly of pre-formed polymers. Superior brightness requires strong dye loading without aggregation-caused quenching (ACQ). Only recently several strategies of dye design were proposed to overcome ACQ in polymer NPs: aggregation induced emission (AIE), dye modification with bulky side groups and use of bulky hydrophobic counterions. The resulting NPs now surpass the brightness of QDs by ≈10-fold for a comparable size, and have started reaching the level of the brightest conjugated polymer NPs. Other properties, notably photostability, color, blinking, as well as particle size and surface chemistry are also systematically analyzed. Finally, major and emerging applications of dye-loaded NPs for in vitro and in vivo imaging are reviewed.

Broadband single-molecule excitation spectroscopy

Over the past 25 years, single-molecule spectroscopy has developed into a widely used tool in multiple disciplines of science. The diversity of routinely recorded emission spectra does underpin the strength of the single-molecule approach in resolving the heterogeneity and dynamics, otherwise hidden in the ensemble. In early cryogenic studies single molecules were identified by their distinct excitation spectra, yet measuring excitation spectra at room temperature remains challenging. Here we present a broadband Fourier approach that allows rapid recording of excitation spectra of individual molecules under ambient conditions and that is robust against blinking and bleaching. Applying the method we show that the excitation spectra of individual molecules exhibit an extreme distribution of solvatochromic shifts and distinct spectral shapes. Importantly, we demonstrate that the sensitivity and speed of the broadband technique is comparable to that of emission spectroscopy putting both techniques side-by-side in single-molecule spectroscopy.

Effects of crystallographic facet-specific peptide adsorption along single ZnO nanorods on the characteristic fluorescence intensification on nanorod ends (FINE) phenomenon

The precise effect of crystallographically discriminating biomolecular adsorption on the fluorescence intensification profiles of individual zinc oxide nanorod (ZnO NR) platforms was elucidated in this study by employing peptide binding epitopes biased towards particular ZnO crystal surfaces and isolating the peptides on given crystalline facets of ZnO NRs. Subsequently, the fluorescence emission profiles of the preferentially bound peptide cases on the basal versus prismic planes of ZnO NRs were carefully evaluated both experimentally and via computer simulations. The phenomenon of fluorescence intensification on NR ends (FINE) was persistently observed on the individual ZnO NR platforms, regardless of the location of the bound peptides. In contrast to the consistent occurrence of FINE, the degree and magnitude of FINE were largely influenced by the discriminatory peptide adsorption to different ZnO NR facets. The temporal stability of the fluorescence signal was also greatly affected by the selectively located peptides on the ZnO NR crystal when spatially resolved on different NR facets. Similarities and differences in the spatial and temporal fluorescence signal of the crystalline NR facet-specific versus -nonspecific biomolecular adsorption events were then compared. To further illuminate the basis of our experimental findings, we also performed finite-difference-time-domain (FDTD) calculations and examined the different degrees of FINE by modelling the biased peptide adsorption cases. Our multifaceted efforts, providing combined insight into the spatial and temporal characteristics of the biomolecular fluorescence signal characteristically governed by the biomolecular location on the specific NR facets, will be valuable for novel applications and accurate signal interpretation of ZnO NR-based biosensors in many rapidly growing, highly miniaturized biodetection configurations.

Multicolour single molecule emission and excitation spectroscopy reveals extensive spectral shifts

We explore the distribution and shape of single molecule spectra at room temperature, when embedded in a polymer host. Multicolour excitation and emission spectroscopy is implemented to capture the full inhomogeneous distribution. We observe dramatic spectral changes in a distribution of single quaterrylene diimide (QDI) molecules isolated in a PMMA matrix. The molecules are strongly blue shifted with respect to the ensemble absorption maximum and spread over a staggering 200 nm range. Despite these strong shifts, the shape of the emission spectra does not differ much between individual molecules. We demonstrate that a considerable number of molecules may be invisible in single molecule experiments, as they typically rely on only a single excitation wavelength, which predetermines which subensemble is probed in the experiment. Lastly, we make a first step towards single molecule excitation spectroscopy under ambient conditions, which allows us to determine the spectral range at which individual molecules absorb light most efficiently. We show how single molecule emission and excitation spectroscopies can complement each other and a combination of both techniques can help in understanding the origin of underlaying spectral properties of individual molecules.

Insight into factors affecting the presence, degree, and temporal stability of fluorescence intensification on ZnO nanorod ends

We have carried out a combined experimental and simulation study identifying the key physical and optical parameters affecting the presence and degree of fluorescence intensification measured on zinc oxide nanorod (ZnO NR) ends. Previously, we reported on the highly localized, intensified, and prolonged fluorescence signal measured on the NR ends, termed fluorescence intensification on NR ends (FINE). As a step towards understanding the mechanism of FINE, the present study aims to provide insight into the unique optical phenomenon of FINE through experimental and simulation approaches and to elucidate the key factors affecting the occurrence, degree, and temporal stability of FINE. Specifically, we examined the effect of the length, width, and growth orientation of single ZnO NRs on the NR-enhanced biomolecular emission profile after decorating the NR surfaces with different amounts and types of fluorophore-coupled protein molecules. We quantitatively and qualitatively profiled the biomolecular fluorescence signal from individual ZnO NRs as a function of both position along the NR long axis and time. Regardless of the physical dimensions and growth orientations of the NRs, we confirmed the presence of FINE in all ZnO NRs tested by using a range of protein concentrations. We also showed that the manifestation of FINE is not dependent on the spectroscopic signatures of the fluorophores employed. We further observed that the degree of FINE is dependent on the length of the NR with longer NRs showing increased levels of FINE. We also demonstrated that vertically oriented NRs exhibit much stronger fluorescence intensity at the NR ends and a higher level of FINE than the laterally oriented NRs. Additionally, we employed finite-difference time-domain (FDTD) methods to understand the experimental outcomes and to promote our understanding of the mechanism of FINE. Particularly, we utilized the electrodynamic simulations to examine both near-field and far-field emission characteristics when considering various scenarios of fluorophore locations, polarizations, spectroscopic characteristics, and NR dimensions. Our efforts may provide deeper insight into the unique optical phenomenon of FINE and further be beneficial to highly miniaturized biodetection favoring the use of single ZnO NRs in low-volume and high-throughput protein assays.

Tuning the color and photostability of perylene diimides inside polymer nanoparticles: towards biodegradable substitutes of quantum dots

Fluorescent organic nanoparticles (NPs) are attractive alternatives to quantum dots due to their potential biodegradability. However, preparation of fluorescent organic NPs is challenging due to the problem of self-quenching of the encapsulated dyes. Moreover, the photostability of organic dyes is much lower than that of quantum dots. To address both problems, we studied encapsulation into biodegradable polymer PLGA NPs of perylene diimide (PDI) derivatives, which are among the most photostable dyes reported to date. Two PDIs were tested, one bearing bulky hydrophobic groups at the imides, while the other was substituted in both imide and bay regions (Lumogen Red). Encapsulation of the former resulted in aggregation, which was accompanied by the emission color change from green to red, some decrease in the fluorescence quantum yield and a significant drop in the photostability, unexpected for PDI dyes. In contrast, Lumogen Red showed nearly no aggregation inside polymer NPs and maintained high quantum yield and photostability. According to wide-field fluorescence microscopy with a 532 nm excitation laser, our 40 nm PLGA NPs loaded with 1 wt% Lumogen Red were >10-fold brighter than quantum dots (QD-585). These NPs were stable in biological media, including serum, and entered spontaneously into HeLa cells by endocytosis showing no sign of cytotoxicity. Due to excellent photostability, these nanoparticles could be considered as biodegradable substitutes of quantum dots in bioimaging.

Polyglycerol-Dendronized Perylenediimides as Stable, Water-Soluble Fluorophores

The synthesis and photophysical properties of water-soluble, fluorescent polyglycerol-dendronized perylenediimides 1-4 are reported. The polyglycerol dendrons, which are known to be highly biocompatible, are found to confer high water-solubility on the perylenediimide in aqueous media while retaining its excellent fluorescent properties. Furthermore, intramolecular cross-linking of the polyglycerol dendrons using the ring-closing metathesis reaction not only enhances the photostability but also reduces the size of perylenediimide-cored dendrimers. The permeability of the various dendritic shells is probed using heavy metal ion quenchers and compared to non-dendritic but water-soluble perylenediimide 5.

Spectroscopic properties, excitation, and electron transfer in an anionic water-soluble poly(fluorene-alt-phenylene)-perylenediimide copolymer

An anionic fluorene-phenylene poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl}-based copolymer containing on-chain perylenediimine (PDI) chromophoric units, PBS-PFP-PDI, was synthesized and its photophysical properties studied as aggregates and isolated chains in water and dioxane/water (1:1) solution. UV-vis and emission spectroscopy measurements, time-correlated single photon counting, and wide field imaging have been employed to investigate the excited-state behavior of the PBS-PFP-PDI copolymer, including the effect of environment on the energy and electron transfer to the on-chain PDI chromophore. Although the Förster overlap integral is favorable, no evidence is found for intramolecular singlet excitation energy transfer in isolated copolymer chains in solution. Fluorescence is suggested to involve an interchain process, thus revealing that isolated copolymer chains in solution do not undergo efficient intramolecular energy transfer. However, quenching of the PBS-PFP excited state by PDI is observed in aqueous media and ultrafast pump-probe studies in water or dioxane-water solutions show that electron transfer occurs from the phenylene-fluorene units to the PDI. The extent of electron transfer increases with aggregation, suggesting it is largely an interchain process. The interaction of the negatively charged PBS-PFP-PDI copolymer with the positively charged surfactant hexadecyltrimethylammonium bromide (CTAB) in solution has also been studied. The copolymer PBS-PFP-PDI aggregates with the surfactant already at concentrations below the critical micelle concentration (cmc) and the nonpolar environment allows intermolecular energy transfer, observed by the weak emission band located at 630 nm that is associated with the emission of the PDI chromophore. However, the fact that the PDI photoluminescence (PL) lifetime (~1.4 ns) obtained in the presence of CTAB is considerably shorter than that of the nonaggregated chromophore (~5.4 ns) suggests that even in this case there is considerable PL quenching, possibly through some charge transfer route. The increase of the PBS-PFP-PDI photoluminescence intensity at surfactant concentrations above the cmc indicates deaggregation of polyelectrolyte within the initially formed polyelectrolyte-surfactant aggregates.

Monovalent, clickable, uncharged, water-soluble perylenediimide-cored dendrimers for target-specific fluorescent biolabeling

Herein we report the synthesis of water-soluble polyglycerol-dendronized perylenediimides with a single reactive group that undergoes high-yielding click reactions. Single-molecule studies and target-specific biolabeling are reported, including the highly specific labeling of proteins on the surface of living bacterial and mammalian cells.

Photodynamic therapy and two-photon bio-imaging applications of hydrophobic chromophores through amphiphilic polymer delivery

The synthesis and photophysical properties of two lipophilic quadrupolar chromophores featuring anthracenyl (1) or dibromobenzene (2) were described. These two chromophores combined significant two-photon absorption cross-sections with high fluorescence quantum yield for 1 and improved singlet oxygen generation efficiency for 2, in organic solvents. The use of Pluronic nanoparticles allowed a simple and straightforward introduction of these lipophilic chromophores into biological cell media. Their internal distribution in various cell lines was studied using fluorescence microscopy and flow-cytometry following a successful staining that was achieved upon 2 h of incubation. Finally, multiphoton excitation microscopy and photodynamic therapy capability of the chromophores were demonstrated by cell exposure to a 820 nm fs laser and cell death upon one photon resonant irradiation at 436 ± 10 nm, respectively.

Novel pyridinium dyes that enable investigations of peptoids at the single-molecule level

Single-molecule microscopy is a powerful tool for investigating various uptake mechanisms of cell-penetrating biomolecules. A particularly interesting class of potential transporter molecules are peptoids. Fluorescence labels for such experiments need to comply with several physical, chemical, and biological requirements. Herein, we report the synthesis and photophysical investigation of new fluorescent pyridinium derived dyes. These fluorescent labels have advantageous structural variations and spacer units in order to avoid undesirable interactions with the labeled molecule and are able to easily functionalize biomolecules. In our case, cell-penetrating peptoids are successfully labeled on solid supports, and in ensemble measurements the photophysical properties of the dyes and the fluorescently labeled peptoids are investigated. Both fluorophores and peptoids are imaged at the single-molecule level in thin polymer gels. With respect to bleaching times and fluorescence lifetimes the dye molecules and the peptoids show only slightly perturbed optical behaviors. These investigations indicate that the new fluorophores fulfill well single-molecule microscopy and solid-phase synthesis requirements.

Photophysics of new photostable rylene derivatives: applications in single-molecule studies and membrane labelling

Three new photostable rylene dyes for applications in single molecule studies and membrane labelling have been synthesized and their photophysical properties were characterized. These dyes differ in the number of polyethylene glycol (PEG) chains attached to the core structure which is either a perylene derivate or a terrylene derivate. One perylene and one terrylene dye is modified with two PEG chains, and another terrylene derivate has four PEG chains. The results show that the terrylene dye with four PEG chains (4-PEG-TDI) forms soluble nonfluorescing H-aggregates in water, so that the absorption bands are blue-shifted with respect to those of the fluorescing monomeric form. The presence of a surfactant such as Pluronic P123 leads to the disruption of the aggregates due to the formation of monomers in micelles and a strong increase in fluorescence. Application for labelling cell membranes can be considered for this dye since it adsorbs in a similar way as monomer to a lipid bilayer. Furthermore a single-molecule study of all three rylene dyes in polymeric films of PMMA showed excellent photostability with respect to photobleaching, far above the photostability of other common water-soluble dyes, such as Oxazine-1, Atto647N, Cy5, Alexa647 and Rhodamin6G. Especially 4-PEG-TDI seems to be a promising dye for membrane labelling with its high photostability.

Perylene-labeled silica nanoparticles: synthesis and characterization of three novel silica nanoparticle species for live-cell imaging

The increasing exposure of humans to nanoscaled particles requires well-defined systems that enable the investigation of the toxicity of nanoparticles on the cellular level. To facilitate this, surface-labeled silica nanoparticles, nanoparticles with a labeled core and a silica shell, and a labeled nanoparticle network-all designed for live-cell imaging-are synthesized. The nanoparticles are functionalized with perylene derivatives. For this purpose, two different perylene species containing one or two reactive silica functionalities are prepared. The nanoparticles are studied by transmission electron microscopy, widefield and confocal fluorescence microscopy, as well as by fluorescence spectroscopy in combination with fluorescence anisotropy, in order to characterize the size and morphology of the nanoparticles and to prove the success and homogeneity of the labeling. Using spinning-disc confocal measurements, silica nanoparticles are demonstrated to be taken up by HeLa cells, and they are clearly detectable inside the cytoplasm of the cells.

Single-molecule spectroscopy and imaging of biomolecules in living cells

The number of reports per year on single-molecule imaging experiments has grown roughly exponentially since the first successful efforts to optically detect a single molecule were completed over two decades ago. Single-molecule spectroscopy has developed into a field that includes a wealth of experiments at room temperature and inside living cells. The fast growth of single-molecule biophysics has resulted from its benefits in probing heterogeneous populations, one molecule at a time, as well as from advances in microscopes and detectors. This Perspective summarizes the field of live-cell imaging of single biomolecules.

Utilization of micelles formed from poly(ethylene glycol)-block-poly(epsilon-caprolactone) block copolymers as nanocarriers to enable hydrophobic red two-photon absorbing emitters for cells imaging

A hydrophobic two-photon absorbing (2PA) red emitter (R) was successfully incorporated into micelles formed from two block copolymers, poly(epsilon-caprolactone)-block-poly(ethylene glycol)s, for imaging and toxicity studies. In micelles, the chromophore R exhibits a 2PA cross-section of 400 GM (1 GM = 1 x 10(-50) cm(4) s photon(-1) molecule(-1)) at 820 nm, which is among the highest values reported for red 2PA emitters. The micelles with a cationic amino moiety-containing poly(ethylene glycol) corona showed an enhancement of cell internalization and delivered the dye into the cytoplasmic regions of the mouse macrophage RAW 264.7 cells. In comparison, the dye in micelles with neutral poly(ethylene glycol) as corona could not be delivered into the cells. Cytotoxicity of the micelle-R constructs was studied using a 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. More than 90% of the cells were viable after they were stained with the dye-containing micelles at different concentrations (dye concentrations of 2-6 muM and polymer concentrations of 0.05-0.15 mg/mL) for 16 h. This is the first reported application of a hydrophobic 2,1,3-benzothiadiazole-containing 2PA red emitter delivered into the cytoplasm of cells for bioimaging and toxicity assessment.

Influence of lipid heterogeneity and phase behavior on phospholipase A2 action at the single molecule level

We monitored the action of phospholipase A(2) (PLA(2)) on L- and D-dipalmitoyl-phosphatidylcholine (DPPC) Langmuir monolayers by mounting a Langmuir-trough on a wide-field fluorescence microscope with single molecule sensitivity. This made it possible to directly visualize the activity and diffusion behavior of single PLA(2) molecules in a heterogeneous lipid environment during active hydrolysis. The experiments showed that enzyme molecules adsorbed and interacted almost exclusively with the fluid region of the DPPC monolayers. Domains of gel state L-DPPC were degraded exclusively from the gel-fluid interface where the buildup of negatively charged hydrolysis products, fatty acid salts, led to changes in the mobility of PLA(2). The mobility of individual enzymes on the monolayers was characterized by single particle tracking. Diffusion coefficients of enzymes adsorbed to the fluid interface were between 3.2 microm(2)/s on the L-DPPC and 4.9 microm(2)/s on the D-DPPC monolayers. In regions enriched with hydrolysis products, the diffusion dropped to approximately 0.2 microm(2)/s. In addition, slower normal and anomalous diffusion modes were seen at the L-DPPC gel domain boundaries where hydrolysis took place. The average residence times of the enzyme in the fluid regions of the monolayer and on the product domain were between approximately 30 and 220 ms. At the gel domains it was below the experimental time resolution, i.e., enzymes were simply reflected from the gel domains back into solution.

Single-molecule redox blinking of perylene diimide derivatives in water

Dynamic developments in ultrasensitive and superresolution fluorescence microscopy call for improved fluorescence markers with increased photostability and new functionalities. We used single-molecule spectroscopy to study water-soluble perylene dicarboximide fluorophores (PDI), which were immobilized in aqueous buffer by attaching the fluorophore to DNA. Under these conditions bright fluorescence, comparable to that of single-molecule compatible organic fluorophores, is observed with homogeneous spectral and fluorescence decay time distributions. We additionally show how the fluorescence of the PDI can be controlled through photoinduced electron-transfer reactions by using different concentrations of reductants and oxidants, yielding either blinking or stable emission. We explain these properties by the redox potentials of PDI and the recently introduced ROXS (reducing and oxidizing system) concept. Finally, we evaluate how this fluorescence control of PDIs can be used for superresolution "Blink-Microscopy" in aqueous or organic media and more generally for single-molecule spectroscopy.

Transfection of living HeLa cells with fluorescent poly-cytosine encapsulated Ag nanoclusters

The fluorescence of silver clusters encapsulated by single stranded oligo-DNA (24 cytosine base pairs, C(24):Ag(n)) was used to monitor the transfection of this new silver/DNA fluorophore inside living HeLa cells. For this, the C(24):Ag(n) molecules were complexed with a commercially available transfection reagent Lipofectamine and the internalization of C(24):Ag(n) was followed with confocal fluorescence microscopy. Bright near-infrared fluorescence was observed from inside the transfected HeLa cells, when exciting with 633 nm excitation, opening up the possibility for the use of these C(24):Ag(n) clusters for biological labelling and imaging of living cells and for monitoring the transfection process with limited harm to the living cells.

Synthesis of arylated perylene bisimides through C-H bond cleavage under ruthenium catalysis

Treatment of perylene bisimide (PBI) with various arylboronates in the presence of a ruthenium catalyst provides tetraarylated PBIs at 2,5,8,11-positions in good yields with perfect regioselectivity. The electronic nature of the introduced aryl substituents has a significant impact on their optical and electronic properties. This protocol has been applied to the synthesis of a water-soluble emissive PBI derivative.

Highly fluorescent water-soluble polyglycerol-dendronized perylene bisimide dyes

Water-soluble perylene tetracarboxylic acid bisimides with terminally linked polyglycerol dendrons [G1–G4] reveal a strong dendritic effect, enabling outstanding fluorescence quantum yields in water for [G3] and [G4] dendrons.

Hydrophobic dimerization and thermal dissociation of perylenediimide-linked DNA hairpins

The structure and properties of hairpin-forming bis(oligonucleotide) conjugates possessing perylenediimide (PDI) chromophores as hairpin linkers have been investigated using a combination of spectroscopic and computational methods. These conjugates exist predominantly as monomer hairpins at room temperature in the absence of added salt and as head-to-head hairpin dimers in the presence of >50 mM NaCl. The hairpin dimer structure is consistent with the results of small-angle X-ray scattering in aqueous solution and molecular dynamics simulation. The structure of the nonconjugated PDI dimer in water is investigated using potential of mean force calculations. The salt dependence is attributed to increased cation condensation in the hairpin dimer vs monomer. Upon heating at low salt concentrations, the hairpin dimer undergoes sequential dissociation to form the monomer hairpin followed by conversion to a random coil structure; whereas at high salt concentrations both dissociation processes occur over the same temperature range. The monomer and dimer hairpins have distinct spectroscopic properties both in the ground state and excited singlet state. The UV and CD spectra provide evidence for electronic interaction between PDI and the adjacent base pair. Low fluorescence quantum yields are observed for both the monomer and dimer. The transient absorption spectrum of the dimer undergoes time-dependent spectral changes attributed to a change in the PDI-PDI torsional angle from ca. 20 degrees in the Franck-Condon singlet state to ca. 0 degrees in the relaxed singlet state, a process which occurs within ca. 40 ps.

Bright, Red Single-Molecule Emitters: Synthesis and Properties of Environmentally Sensitive Dicyanomethylenedihydrofuran (DCDHF) Fluorophores with Bisaromatic Conjugation

A group of new fluorescent dye materials for single-molecule imaging applications comprised of an amine donor, a π-system comprised of phenyl and thiophene rings and a 2-dicyanomethylene-3-cyano-2,5-dihydrofuran acceptor group have been synthesized. Relative to comparable single-ring compounds these doubly aromatic conjugated fluorophores have red-shifted absorption and emission usually accompanied by significantly increased quantum yields. Solvatochromism studies indicate that the photophysical properties of these dyes are sensitive to the solvent polarity and environmental rigidity. Photophysical studies demonstrate that these DCDHF dye materials are strong single-molecule emitters and the total number of detected photons per molecule is among the highest thus far for this family of fluorophores.