Non‐covalent Versus Covalent Control of Self‐Assembly and Chirality of Nile Red‐modified Nucleoside and DNA

A DNA‐based covalent versus a non‐covalent approach is demonstrated to control the optical, chirooptical and higher order structures of Nile red ( Nr ) aggregation. Dynamic light scattering and TEM studies revealed that in aqueous media Nr ‐modified 2′‐deoxyuridine aggregates through the co‐operative effect of various non‐covalent interactions including the hydrogen bonding ability of the nucleoside and sugar moieties and the π‐stacking tendency of the highly hydrophobic dye. This results in the formation of optically active nanovesicles. A left‐handed helically twisted H‐type packing of the dye is observed in the bilayer of the vesicle as evidenced from the optical and chirooptical studies. On the other hand, a left‐handed helically twisted J‐type packing in vesicles was obtained from a non‐polar solvent (toluene). Even though the primary stacking interaction of the dye aggregates transformed from H→J while going from aqueous to non‐polar media, the induced supramolecular chirality of the aggregates remained the same (left‐handed). Circular dichroism studies of DNA that contained several synthetically incorporated and covalently attached Nr ‐modified nucleosides revealed the formation of helically stacked H‐aggregates of Nr but—in comparison to the noncovalent aggregates—an inversed chirality (right‐handed). This self‐assembly propensity difference can, in principle, be applied to other hydrophobic dyes and chromophores and thus open a DNA‐based approach to modulate the primary stacking interactions and supramolecular chirality of dye aggregates.     

Light Harvesting and White‐Light Generation in a Composite of Carbon Dots and Dye‐Encapsulated BSA‐Protein‐Capped Gold Nanoclusters

Several strategies have been adopted to design an artificial light‐harvesting system in which light energy is captured by peripheral chromophores and it is subsequently transferred to the core via energy transfer. A composite of carbon dots and dye‐encapsulated BSA‐protein‐capped gold nanoclusters (AuNCs) has been developed for efficient light harvesting and white light generation. Carbon dots (C‐dots) act as donor and AuNCs capped with BSA protein act as acceptor. Analysis reveals that energy transfer increases from 63 % to 83 % in presence of coumarin dye (C153), which enhances the cascade energy transfer from carbon dots to AuNCs. Bright white light emission with a quantum yield of 19 % under the 375 nm excitation wavelength is achieved by changing the ratio of components. Interesting findings reveal that the efficient energy transfer in carbon‐dot–metal‐cluster nanocomposites may open up new possibilities in designing artificial light harvesting systems for future applications.     

Synthesis and Photovoltaic Properties of Boron β‐Ketoiminate Dyes Forming a Linear Donor‐π‐Acceptor Structure

Organoboron complexes are of interest as chromophores for dye sensitizers owing to their light‐harvesting and carrier‐transporting properties. In this study, compounds containing boron β‐ketoiminate (BKI) as a chromophore were synthesized and used as dye sensitizers in dye‐sensitized solar cells. The new dyes were orange or red crystals and showed maximum absorptions in the 410–450 nm wavelength region on titanium dioxide substrates. These electrodes exhibited maximum efficiencies of over 80% in incident photon‐to‐current conversion efficiency spectra, suggesting that the continuous process of light absorption‐excitation‐electron injection was effectively performed. Open‐circuit photovoltages were relatively high owing to the large dipole moments of the BKI dyes with a linear molecular structure. Thus, a maximum power conversion efficiency of 5.3% was successfully observed. Comparison of BKI dyes with boron β‐diketonate dyes revealed certain differences in solution stability, spectral properties, and photovoltaic characteristics.     

Discrete π‐Stacks from Self‐Assembled Perylenediimide Analogues

The formation of well‐defined finite‐sized aggregates represents an attractive goal in supramolecular chemistry. In particular, construction of discrete π‐stacked dye assemblies remains a challenge. Reported here is the design and synthesis of a novel type of discrete π‐stacked aggregate from two comparable perylenediimide (PDI) dyads ( PEP and PBP ). The criss‐cross PEP ‐ PBP dimers in solution and ( PBP ‐ PEP )‐( PEP ‐ PBP ) tetramers in the solid state are well elucidated using single‐crystal X‐ray diffraction, dynamic light scattering, and diffusion‐ordered NMR spectroscopy. Extensive π–π stacking between the PDI units of PEP and PBP as well as repulsive interactions of swallow‐tailed alkyl substituents are responsible for the selective formation of discrete dimer and tetramer stacks. Our results reveal a new approach to preparing discrete π stacks that are appealing for making assemblies with well‐defined optoelectronic properties.     

Efficient Excitation‐Energy Transfer in Ion‐Based Organic Nanoparticles with Versatile Tunability of the Fluorescence Colours

Organic nanoparticles consisting of 3,3′‐diethylthiacyanine (TC) and ethidium (ETD) dyes are synthesized by ion‐association between the cationic dye mixture (10 % ETD doping) and the tetrakis(4‐fluorophenyl)borate (TFPB) anion, in the presence of a neutral stabilizing polymer, in aqueous solution. Doping with ETD makes the particle size smaller than without doping. Size tuning can also be conducted by varying the molar ratio (ρ) of the loaded anion to the cationic dyes. The fluorescence spectrum of TC shows good overlap with the absorption of ETD in the 450–600 nm wavelength region, so efficient excitation‐energy transfer from TC (donor) to ETD (acceptor) is observed, yielding organic nanoparticles whose fluorescence colours are tunable. Upon ETD doping, the emission colour changes significantly from greenish‐blue to reddish or whitish. This change is mainly dependent on ρ. For the doped nanoparticle sample with ρ=1, the intensity of fluorescence ascribed to ETD is ～150‐fold higher than that from pure ETD nanoparticles (efficient antenna effect). Non‐radiative Förster resonance‐energy transfer (FRET) is the dominant mechanism for the ETD fluorescence enhancement. The organic nanoparticles of a binary dye system fabricated by the ion‐association method act as efficient light‐harvesting antennae, which are capable of transferring light energy to the dopant acceptors in very close proximity to the donors, and can have multi‐wavelength emission colours with high fluorescence quantum yields.     

Excitation Energy Transfer from Semi‐Conducting Polymer Nanoparticles to Surface‐Bound Fluorescent Dyes

Summary: The first examples of the dye‐coated semi‐conducting polymer nanoparticles as well as experiments to demonstrate the excitation energy transfer from the excited chromophor of the nanoparticle to the fluorescent dye are described. We have demonstrated that the blue fluorescence of the dye‐coated polyfluorene nanoparticles is only slightly quenched after dye deposition. However, a new emission band of the surface‐bound dye (Rhodamine 6G or Rhodamine TM) appears in the wavelength region of 530–600 nm. These results clearly indicate an effective excitation energy transfer from the excited PF chromophores to the fluorescent dye.

Emission spectra of PF2/6 nanoparticle dispersion and of Rhodamine 6G‐coated nanoparticle dispersion.     

Polyelectrolyte‐Assisted Noncovalent Functionalization of Carbon Nanotubes with Ordered Self‐Assemblies of a Water‐Soluble Porphyrin

The self‐assembly and induced supramolecular chirality of meso‐tetrakis(4‐sulfonatophenyl)porphyrin (TSPP) on both single‐wall (SWCNT) and multiwall carbon nanotubes (MWCNT) are investigated. Under mild pH conditions (pH 3), TSPP forms aggregates when CNTs are dispersed in an aqueous solution containing positively charged polyelectrolytes such as poly‐L ‐lysine (PLL) or poly(allylamine hydrochloride) (PAH). Evidence for the geometry of the porphyrin aggregates is obtained from absorption spectra, whereby the fingerprints of J‐ and H‐aggregates are clearly seen only in the presence of smaller‐diameter nanotubes. J‐aggregates are better stabilized with PLL, whereas in the presence of PAH mainly H‐aggregates prevail. Excited‐state interactions within these nanohybrids are studied by steady‐state and time‐resolved fluorescence. The porphyrin emission intensity in the nanohybrid solution is significantly quenched compared to that of TSPP alone, and this implies strong electronic interaction between CNTs and porphyrin molecules. Fluorescence lifetime imaging microscopy (FLIM) further supports that porphyrin arrays are associated with the MWCNT sidewalls wrapped in PLL. In the case of the SWCNT hybrid, spherical structures associated with longer fluorescence lifetime appeared after one week, indicative of H‐aggregates of TSPP. The latter are the result of π–π stacking of porphyrin units on neighboring nanotubes facilitated by the strong tendency of these nanotubes to interact with each other. These results highlight the importance of optimum dimensions and surface‐area architectures of CNTs in the control/stability of the porphyrin aggregates with promising properties for light harvesting.     

Photocontrol Over Self‐Assembled Nanostructures of π–π Stacked Dyes Supported by the Parallel Conformer of Diarylethene

Diarylethenes (DAEs) have rarely been used in the design of photoresponsive supramolecular assemblies with a well‐defined morphology transition owing to rather small structural changes upon photoisomerization. A supramolecular design based on the parallel conformation of DAEs enables the construction of photoresponsive dye assemblies that undergo remarkable nanomorphology transitions. The cooperative stacking of perylene bisimide (PBI) dyes was used to stabilize the parallel conformer of DAE through complementary hydrogen bonds. Atomic force microscopy, UV/Vis spectroscopy, and molecular modeling revealed that our DAE and PBI building blocks coassembled in nonpolar solvent to form well‐defined helical nanofibers featuring J‐type dimers of PBI dyes. Upon irradiating the coassembly solution with UV and visible light in turn, a reversible morphology change between nanofibers and nanoparticles was observed. This system involves the generation of a new self‐assembly pathway by means of photocontrol.     

A Versatile Long‐Wavelength‐Absorbing Scaffold for Eu‐Based Responsive Probes

Coumarin‐sensitized, long‐wavelength‐absorbing luminescent Eu^III‐complexes have been synthesized and characterized. The lanthanide binding site consists of a cyclen‐based chelating framework that is attached through a short linker to a 7‐hydroxycoumarin, a 7‐B(OH)₂‐coumarin, a 7‐O‐(4‐pinacolatoboronbenzyl)‐coumarin or a 7‐O‐(4‐methoxybenzyl)‐coumarin. The syntheses are straightforward, use readily available building blocks, and proceed through a small number of high‐yielding steps. The sensitivity of coumarin photophysics to the 7‐substituent enables modulation of the antenna‐absorption properties, and thus the lanthanide excitation spectrum. Reactions of the boronate‐based functionalities (cages) with H₂O₂ yielded the corresponding 7‐hydroxycoumarin species. The same species was produced with peroxynitrite in a ×10⁶–10⁷‐fold faster reaction. Both reactions resulted in the emergence of a strong ≈407 nm excitation band, with concomitant decrease of the 366 nm band of the caged probe. In aqueous solution the methoxybenzyl caged Eu‐complex was quenched by ONOO^?. We have shown that preliminary screening of simple coumarin‐based antennae through UV/Vis absorption spectroscopy is possible as the changes in absorption profile translate with good fidelity to changes in Eu^III‐excitation profile in the fully elaborated complex. Taken together, our results show that the 7‐hydroxycoumarin antenna is a viable scaffold for the construction of turn‐on and ratiometric luminescent probes.     

Synthesis and Spectral Properties of 4‐(2,5‐Dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO)

An interesting flourophore, 4‐(2,5‐dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) was synthesized by mixing an equivalent molar quantity of hippuric acid and 2,5‐dimethoxybenzaldehyde in acetic anhydride in the presence of anhydrous sodium acetate. The absorption and fluorescence characteristics of 4‐(2,5‐dimethoxy‐phenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) were investigated in different solvents. DMPO dye exhibits red shift in both absorption and emission spectra as solvent polarity increases, indicating change in the dipole moment of molecules upon excitation due to an intramolecular charge transfer interaction. The fluorescence quantum yield depends strongly on the properties of the solvents, which was attributed to positive and negative solvatokinetic effects. A crystalline solid of DMPO gave strong excimer like emission at 630 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B‐type class of Steven's Classification. DMPO displayed fluorescence quenching by triethylamine via nonemissive exciplex formation.     

On the Complementary Photoluminescent and Electroluminescent Images of Poly(phenylenevinylene)‐Based Light‐Emitting Diodes with a Thin Active Layer

A clear complementary relationship between photoluminescent (PL) and electroluminescent (EL) images was observed for organic light‐emitting diodes (OLEDs) based on poly(phenylenevinylene) (PPV) and dye‐doped PPV. So‐called ‘black spots' (dark circular regions observed on the active area of running OLEDs) become bright ones, when the photoluminescence of the same area is excited. A very small thickness of the active layer (ca. 10 nm) was the crucial point to observe this anticorrelation between EL and PL. A substantial increase of the PL yield (‘anti‐burning' effect) was observed after strong light exposure (ca. 10 mJ/cm²) of the polymer covered by an aluminium layer. The same light exposure without aluminium protection resulted in complete photobleaching of the polymer. The presence of a thin insulating layer between the polymer and aluminium was proposed to be responsible for these effects. This layer prevents electron injection and PL quenching due to exciton dissociation at the metal‐polymer interface. The former effect leads to black spots in the EL image, the latter one gives rise to bright spots on the PL image situated on the same places. The intermediate layer can be also induced by light exposure. A very efficient energy transfer from the polymer to the dye and to the J‐aggregates of the dye was demonstrated in PPV/dye composite films.     

Dry Sintering Meets Wet Silver‐Ion “Soldering”: Charge‐Transfer Plasmon Engineering of Solution‐Assembled Gold Nanodimers From Visible to Near‐Infrared I and II Regions

Achieving highly tunable and localized surface plasmon resonance up to near infrared (NIR) regions is a key target in nanoplasmonics. In particular, a self‐assembly process capable of producing highly uniform and solution‐processable nanomaterials with tailor‐made plasmonic properties is lacking. We herein address this problem through a conjunctive use of wet Ag⁺ soldering and dry thermal sintering to produce nanodimer‐derived structures with precisely engineered charge‐transfer plasmon (CTP). The sintered dimers are water soluble, featuring gradually shifted CTP spanning an 800 nm wavelength range (up to NIR II). Upon silica removal, the products are grafted by DNA to offer surface functionality. This process is also adaptable to DNA‐linked AuNP dimers toward plasmonic meta‐materials via DNA‐guided soldering and sintering.     

Formation of regioregular head‐to‐tail poly[3‐(4‐butylphenyl)thiophene] by an oxidative coupling polymerization with vanadium acetylacetonate

The mechanism for the formation of head‐to‐tail (H–T) poly[3‐(4‐butylphenyl)thiophene] by oxidative coupling polymerization with a catalytic amount of vanadium acetylacetonate was investigated. Polymerization was carried out in the presence of vanadium acetylacetonate, trifluoromethane sulfonic acid, and trifluoroacetic anhydride under an oxygen atmosphere in 1,2‐dichloroethane at room temperature. Polymers and oligomers obtained after several polymerization times were characterized by gel permeation chromatography, IR, and NMR spectroscopies. With these findings and the reactivity of monomer and dimers based on ab initio density functional theory, the polymerization was found to proceed mainly through the formation of H–T linkages due to the high spin density at the 2‐position of 3‐(4‐butylphenyl)thiophene and the calculated total energy of dimers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2287–2295, 2001     

Crew‐cut aggregates from self‐assembly of blends of polystyrene‐b‐poly(acrylic acid) block copolymers and homopolystyrene in solution

The formation and morphological characteristics of crew‐cut aggregates from blends of polystyrene‐b‐poly(acrylic acid) diblock copolymer and polystyrene homopolymer in solution were studied by static light scattering, transmission electron microscopy and size exclusion chromatography. The crew‐cut aggregates, consisting of a polystyrene core and a poly(acrylic acid) corona, were prepared by direct dissolution of the polymer blends in a selective solvent mixture consisting of 93 wt % dimethylformamide and 7 wt % water. It is found that the aggregation behavior depends strongly on the relative volume fractions of the block copolymer and homopolymer in the blends. This is a result of the difference in solubility between the copolymer and the homopolymer in solution which, in turn, influences their miscibility and mutual solubility and consequently the morphology of the formed crew‐cut aggregates. Specifically, when the homopolymer fraction is low, it is mainly dissolved in the cores of the crew‐cut aggregates formed by the block copolymer. When the homopolymer fraction exceeds its solubility limit in the copolymer micelles, aggregates of another type are formed which contain a major fraction of the homopolymer. These aggregates are usually much larger than the primary micelles and have an internal structure due to the formation of reverse micelles from the dissolved block copolymer chains. The importance of thermodynamic vs. kinetic aspects during the formation of the crew‐cut aggregates is also discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1469–1484, 1999     

Photoconductive Core–Shell Liquid‐Crystals of a Perylene Bisimide J‐Aggregate Donor–Acceptor Dyad

A novel core–shell structured columnar liquid crystal composed of a donor‐acceptor dyad of tetraphenoxy perylene bisimide (PBI), decorated with four bithiophene units on the periphery, was synthesized. This molecule self‐assembles in solution into helical J‐aggregates guided by π–π interactions and hydrogen bonds which organize into a liquid‐crystalline (LC) columnar hexagonal domain in the solid state. Donor and acceptor moieties exhibit contrasting exciton coupling behavior with the PBIs’ (J‐type) transition dipole moment parallel and the bithiophene side arms’ (H‐type) perpendicular to the columnar axis. The dyad shows efficient energy and electron transfer in solution as well as in the solid state. The synergy of photoinduced electron transfer (PET) and charge transport along the narcissistically self‐assembled core–shell structure enables the implementation of the dye in two‐contact photoconductivity devices giving rise to a 20‐fold increased photoresponse compared to a reference dye without bithiophene donor moieties.     

Intriguing H-aggregate and H-dimer formation of coumarin-481 dye in aqueous solution as evidenced from photophysical studies

Photophysical properties of coumarin-481 (C481) dye in aqueous solution show intriguing presence of multiple emitting species. Concentration and wavelength dependent fluorescence decays and time-resolved emission spectra and area-normalized emission spectra suggest the coexistence of dye monomers, dimers, and higher aggregates (mostly trimers) in the solution. Because of the efficient intramolecular charge transfer (ICT) state to twisted intramolecular charge transfer (TICT) state conversion, the dye monomers show very short fluorescence lifetime of ~0.2 ns. Fluorescence lifetimes of dimers (~4.1 ns) and higher aggregates (~1.4 ns) are relatively longer due to steric constrain toward ICT to TICT conversion. Observed results indicate that the emission spectra of the aggregates are substantially blue-shifted compared to monomers, suggesting H-aggregation of the dye in the solution. Temperature-dependent fluorescence decays in water and time-resolved fluorescence results in water-acetonitrile solvent mixtures are also in support of the dye aggregation in the solution. Though dynamic light scattering studies could not recognize the dye aggregates in the solution due to their small size and low concentration, fluorescence up-conversion measurements show a relatively higher decay tail in water than in water-acetonitrile solvent mixture, in agreement with higher dye aggregation in aqueous solution. Time-resolved fluorescence results with structurally related non-TICT dyes, especially those of coumarin-153 dye, are also in accordance with the aggregation behavior of these dyes in aqueous solution. To the best of our knowledge, this is the first report on the aggregation of coumarin dyes in aqueous solution. Present results are important because coumarin dyes are widely used as fluorescent probes in various microheterogeneous systems where water is always a solvent component, and the dye aggregation in these systems, if overlooked, can easily lead to a misinterpretation of the observed results.     

Amphiphilic gradient copolymers of 2‐methyl‐ and 2‐phenyl‐2‐oxazoline: self‐organization in aqueous media and drug encapsulation

Gradient (or pseudo‐diblock) copolymers were synthesized from 2‐methyl‐2‐oxazoline and 2‐phenyl‐2‐oxazoline monomer mixtures via cationic polymerization. The self‐assembling properties of these biocompatible gradient copolymers in aqueous solutions were investigated, in an effort to use the produced nanostructures as nanocarriers for hydrophobic pharmaceutical molecules. Dynamic and static light scattering as well as AFM measurements showed that the copolymers assemble in different supramolecular nanostructures (spherical micelles, vesicles and aggregates) depending on copolymer composition. Fluorescence spectroscopy studies revealed a microenvironment of unusually high polarity inside the nanostructures. This observation is related partly to the gradient structure of the copolymers. The polymeric nanostructures were stable with time. Their structural properties in different aqueous media—PBS buffer, RPMI solution—simulating conditions used in pharmacological/medicinal studies, have been also investigated and a composition dependent behavior was observed. Finally, the hydrophobic drug indomethacin was successfully encapsulated within the gradient copolymer nanostructures and the properties of the mixed aggregates were studied in respect to the initial copolymer assemblies. The produced aggregates encapsulating indomethacin showed a significant increase of their mass and size compared to original purely polymeric ones. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

π‐Conjugated oligomers containing tropone in the main chain: Synthesis,characterization, and optical properties

The synthesis and characterization of tropone‐containing π‐conjugated oligomers were investigated. Two kinds of oligomers [1,4‐phenylene type ( 4a – 4e ) and 2,5‐thienylene type ( 5 )] were successfully obtained by the Wittig polycondensation technique, in which the tropone content could be controlled by the monomer feed ratio for the 1,4‐phenylene‐type oligomers. The absorption maximum blueshifted and the emission intensity decreased with an increase in the tropone content in the oligomers. The emissive color could be tuned by the selection of the aromatic ring; that is, 4a – 4e emitted orange‐yellow light and 5 emitted either orange‐yellow or red light according to the excitation wavelength. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3927–3937, 2002     

Modulating Dye Aggregation by Incorporation into 1D‐MgAPO Nanochannels

The fluorescing dye Pyronine Y has been incorporated by crystallization inclusion into three different one‐dimensional microporous aluminophosphate host materials. A computer‐aided rational choice of the framework of the host material made it possible to modulate the aggregation state of the guest dye molecules. Undesirable H‐type dimers of Pyronine Y are included within the large channels of the AFI structure, which allow the inclusion of any of the aggregated species of the dye. Density functional theory (DFT) calculations show that H‐type aggregate formation is suppressed within the ATS framework. Experimental results indicate that red‐emissive J‐type aggregates are formed instead, offering a one‐directional, organized, multicolour emission system that is interesting for energy transport. Complete suppression of aggregation is achieved by the inclusion of Pyronine Y within the AEL‐type structure, due to its particular topology and channel dimensions This results in a highly fluorescent hybrid system with extraordinarily preferential alignment of the chromophores. Here, we report experimental evidence and modelling insights for how the “cage effect” of the nanochannels can tune the optical properties of the hybrid composite material by influencing the aggregation state of the dye.     

Optical,Electrochemical, and Magnetic Properties of Pyrrole‐ and Thiophene‐Bridged 5,15‐Diazaporphyrin Dimers

The first examples of pyrrole‐ and thiophene‐bridged 5,15‐diazaporphyrin (DAP) dimers are prepared through Stille coupling reactions of nickel(II) and copper(II) complexes of 3‐bromo‐10,20‐dimesityl‐5,15‐diazaporphyrin (mesityl=2,4,6‐trimethylphenyl) with the respective 2,5‐bis(tributylstannyl)heteroles. The effects of the heterole spacers and meso nitrogen atoms on the optical, electrochemical, and magnetic properties of the DAP dimers are investigated by UV/Vis absorption spectroscopy, density functional theory calculations, magnetic circular dichroism spectroscopy, cyclic voltammetry, and EPR spectroscopy. The heterole spacers are found to have a significant impact on the electronic transitions over the entire π‐system. In particular, the pyrrole‐bridged DAP dimers exhibit high light‐harvesting potential in the low‐energy visible/near‐infrared region owing to the intrinsic charge‐transfer character of the lowest excitation.