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Laser irradiation using a Nd3+:YAG laser (532 nm, 8 ns, 10 Hz) on an ethanol suspension of micrometer-sized pentacene crystals induced absorption spectral changes in the solution, as a result of the formation of pentacene nanoparticles. The nanoparticles were platelike crystals. The nanoparticles formed by irradiation with a fluence of 50 mJ/cm2 for 30 min were 4–13 nm in height and 10–70 nm in width. The threshold laser fluence for the formation was determined to be ca. 20 mJ/cm2, and irradiation with fluences above 80 mJ/cm2 induced the decomposition of the pentacene nanoparticles. The nanoparticle size decreased with an increase in the laser fluence, which was reflected in the longest-wavelength peak position of the absorption bands of the pentacene nanoparticles. On the basis of these results, the mechanism of pentacene nanoparticle formation by laser irradiation is discussed in connection with those of vanadyl phthalocyanine and quinacridone systems reported by Masuhara and coworkers [J. Phys. Chem. A 106 (2002) 2135; Jpn. J. Appl. Phys. 42 (2003) 2725; Jpn. J. Appl. Phys. 45 (2006) 384].

Real-time measurement of the catalysis and substrate kinetics of a single-enzyme hydrolysis reaction is demonstrated with confocal fluorescence microscopy (CFM; see picture, green = CFM beam). A single lipase is shown to have a broad range of conformations; each conformation contributes to the overall enzymatic activity, an observation that is often masked by ensemble measurements.

The spectroscopic properties of seven varieties of wire-type dendrimers, which consist of different generation numbers of dendritic wedges and molecular weights, were examined in tetrahydrofuran solution, in the solid state, and in good/poor solvent mixture for the purpose of revealing the relation between molecular conformation and spectral behavior. The second and third generation wire-type dendrimers showed a sharp aggregate band, which is ascribed to the interchain π stacked aggregate, in the solid state and in the good/poor solvent mixture, while the fourth generation wire-type dendrimer did not show the aggregate band. The absence of the aggregate band indicates that the bulky fourth generation dendritic wedges prevent the conjugated backbone from forming the interchain π stacked aggregate even in the solid state and in the good/poor solvent mixture. In the experiment on poor solvent-induced aggregation, we observed the progressive spectral change in the process of forming the aggregate, and simulta...

Laser manipulation and laser trapping techniques were applied to μm sized gel-like wire-type dendrimer (L3PPE130 and L4PPE8) assemblies, which are formed at drying process of their tetrahydrofuran (THF) solution on a glass substrate. Deformation and extension of single gel-like assemblies were demonstrated with irradiation of a focused 1064 nm laser beam by scanning a microscope stage. In repetitive irradiation experiments, contraction and swelling of the gel-like assembly were observed in response to the switch on and off of the laser beam, respectively, and in the case of the L3PPE130 gel-like assembly, fluorescence spectra detected at a focal point changed accompanying the contraction and swelling behavior. This spectral change means molecular aggregates were formed transiently by photon pressure of the focused near-infrared laser beam.

The photoconversion of a crystalline-phase precursor, 6,13-dihydro-6,13-ethanopentacene-15,16-dione (PDK), into pentacene was examined. Upon irradiation with 470 nm light, the photoconversion proceeded even in the crystalline phase. Although the rate of the crystalline phase photoconversion was much slower than that in solution, quantitative photoconversion was achieved upon lengthy photoirradiation. The photoconversion of PDK was also confirmed at the single μm-sized crystal level by observing fluorescence images and spectra of the PDK crystal using confocal microscopy. Destruction of the crystal was observed when some of the PDK molecules in the crystal were photoconverted into pentacene. These results indicate that the crystalline-phase photoconversion of PDK proceeded through the destruction of the crystal. Quantitative photoconversion was achieved as a result.

The self-assembled structures by evaporation of tetrahydrofuran (THF) solution of wire-type dendrimers have been studied by some spectroscopic and microscopic techniques. Doughnut-like structures were obtained for some dendrimers with generation number of 2 and about 28 nm length by depositions their THF solutions on a quartz substrate, while yarn-like and particle-like structures were obtained for other dendrimers. The size of the doughnut structure varied from 1 to 8 μm in diameter and between 30 and 150 nm in height. Confocal fluorescence microscopy, atomic force microscopy (AFM), and near-field scanning optical microscopy (NSOM) were applied to elucidate assembled structures and electronic properties in addition to conventional fluorescence spectroscopy. Polarization and local fluorescence measurements at some parts of doughnuts and particles were demonstrated, however, no correlation between molecular orientation/intermolecular interaction and doughnut structures was found. The doughnut formation mec...

Photon-correlation measurements were applied to single chains of poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] of various molecular weights in a fluid solvent in order to investigate the...

Solvents with limited solubility for N,N'-di(octyl)perylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C8) were used for laser ablation in liquid. Laser irradiation of the suspension induced an absorption spectral change of the solution. The absorbance of free PTCDI-C8 molecules in the solution obtained by laser irradiation was unusually larger than that of the suspension before laser irradiation, suggesting that a supersaturated solution was formed by laser irradiation of PTCDI-C8 nanoparticles prepared by laser ablation.

We demonstrated that the emission from a single organic nanocrystal consisting of organic dye molecules shows photon antibunching. That is, an assembling structure consisting of many chromophores can also be made to behave as a single-photon source by controlling the size. We found that most of single nanocrystals within a size range from 35 to 85 nm show photon antibunching even when more than one exciton in a crystal is generated by an intense single excitation pulse. On the other hand, single micrometer-sized crystals showed no antibunching. The present results indicate that molecular assemblies can also be considered as candidates for new single-photon sources.

Renewable resource of carbon-free energy is crucial in the current scenario of global warming and our growing energy needs. Besides, the significance of safer energy resources has been underscored by many nuclear mishaps, including the recently crippled Fukushima Nuclear power plant in Japan. Sunlight is the most promising alternative to nuclear fuels and greenhouse-gas-emitting fossil and alcohol fuels. (Published: 11 June 2013) Citation: Nano Reviews 2013, 4 : 21079 - http://dx.doi.org/10.3402/nano.v4i0.21079

Novel bisimide-fused acenes were synthesized viabismuth triflate mediated double-cyclization reaction of acid chlorides and isocyanates. Their optical and electrical properties revealed significantly smaller HOMO–LUMO gaps compared with those of their parent acenes. Fabricated OFET based on tetracene bisimide showed n-type OFET outputs.

Novel bisimide-fused acenes were synthesized via bismuth triflate mediated double-cyclization reaction of acid chlorides and isocyanates. Their optical and electrical properties revealed significantly smaller HOMO-LUMO gaps compared with those of their parent acenes. Fabricated OFET based on tetracene bisimide showed n-type OFET outputs.

Ordered fibril- and particle-like assemblies of poly(2,7-(9,9-bis(2-ethylhexyl)fluorene)) can be formed by photon force of a focused near-infrared laser beam during the drying process of its tetrahydrofuran solution on a glass substrate. These formations have been achieved controllably by combining laser irradiation with convection in the cast solution; that is, when viscous drag of the solution in the convection is stronger than the photon force, the fibril-like assemblies can be formed. Molecular orientation in the assemblies differs from that in self-assembled fibril-like structures, and maybe it can be controlled by the polarization direction of the focused laser beam. We have demonstrated that the length and width of the assemblies can be controlled by the irradiation time, the laser power, the concentration of the solution, and the convection rate in the solution. On the other hand, when the viscous drag of the solution in the convection is weak compared to the photon force, particle-like assemblies in which molecular orientation is controlled by polarization direction are formed.

Single-molecule techniques offer a unique tool for studying the dynamical behavior of individual molecules and provide the possibility to construct distributions from individual events rather than from a signal stemming from an ensemble of molecules. In biological systems, known for their complexity, these techniques make it possible to gain insights into the detailed spectrum of molecular conformational changes and activities. Here, we report on the direct observation of a single lipase-catalyzed reaction for extended periods of time (hours), by using confocal fluorescence microscopy. When adding a profluorescent substrate, the monitored enzymatic activity appears as a trajectory of “on-state” and “off-state” events. The waiting time probability density function of the off state and the state-correlation function fit stretched exponentials, independent of the substrate concentration in a certain range. The data analysis unravels oscillations in the logarithmic derivative of the off-state waiting time probability density function and correlations between off-state events. These findings imply that the fluctuating enzyme model, which involves a spectrum of enzymatic conformations that interconvert on the time scale of the catalytic activity, best describes the observed enzymatic activity. Based on this model, values for the coupling and reaction rates are extracted.

We use single-molecule fluorescence lifetimes to probe dynamics of photoinduced reversible electron transfer occurring between triphenylamine (donor) and perylenediimide (acceptor) in single molecules of a polyphenylenic rigid dendrimer embedded in polystyrene. Here, reversible electron transfer in individual donor-acceptor molecules results in delayed fluorescence that is emitted with a high photon count rate. By monitoring fluorescence decay times on a photon-by-photon basis, we find fluctuations in both forward and reverse electron transfer spanning a broad time range, from milliseconds to seconds. Fluctuations are induced by conformational changes in the dendrimer structure as well by polystyrene chain reorientation. The conformational changes are related to changes in the dihedral angle of adjacent phenyl rings located in the dendritic branch near the donor transferring the charge, a torsional motion that results in millisecond fluctuations in the “through-bond” donor-acceptor electronic coupling. Polymer chain reorientation leads to changes in the local polarity experienced by the donors and to changes in the solvation of the charge-separated state. As a result, switching between different donor moieties within the same single molecule becomes possible and induces fluctuations in decay time on a time scale of seconds.

Using single molecule fluorescence spectroscopy we have investigated fluorescence resonance energy transfer (FRET) occurring between two peryleneimide (PI) chromophores in a series of synthetic systems: PI end-capped fluorene trimers, hexamers and polymers for which the interchromophoric distance vary from 3.4 to 5.9 and 42 nm, respectively. By monitoring in parallel the fluorescence intensity and the number of independent emitting chromophores from each molecule, we could discriminate between competitive Foerster-type energy transfer processes such as energy hopping, singlet-singlet annihilation and singlet-triplet annihilation for the PI end-capped fluorine compounds. Due to different energy transfer efficiencies, variations in the interchromophoric distance enable switching between these processes. The single molecule fluorescence data reported here suggest that similar energy transfer pathways have to be considered in the analysis of single molecule trajectories of donor/acceptor pairs, as well as in the case of more complex systems like natural multichromophoric systems, such as light harvesting antennas or oligomeric fluorescent proteins.

Single-molecule fluorescence spectroscopy was performed on a number of multi-peryleneimide substituted polyphenylene dendrimers for the purpose of investigating the parameters which influence the efficiency of the dendrimers as single-photon sources at room temperature. Analysis of fluorescence intensity, lifetime, and interphoton arrival time distribution revealed that all measured first-generation dendrimers behave as single-photon emitters when more than one chromophore is excited by a single excitation pulse regardless of the number of constituent chromophores. This is a result of efficient singlet-singlet annihilation, which becomes less efficient in higher-generation dendrimers when the interchromophoric distance increases. The efficiency of the investigated dendrimers as single-photon sources depends on several other parameters, such as the nature of the surrounding polymer matrix, the number of chromophores, and the extent of interchromophoric interactions. These parameters mainly affect the frequency of singlet-triplet annihilation, which in turn dominates the quality of these multichromophoric dendrimers as single-photon sources. The results reported here are important not only for the design of single-photon sources based on single organic molecules but also for a fundamental understanding of natural and artificial multichromophoric systems.

Time-resolved photoluminescence (PL) and transient absorption (TA) spectroscopy are conducted in order to get knowledge on the excited state of CdSe nanorods (NR), and to assess the impact of Au nanoparticles (NP) on the carrier dynamics of hybrid Au-CdSe NRs. The decay dynamics measured in solution show an increase of non-radiative decay channels in the presence of Au NPs, whose characteristic lifetimes vary from a few ps to tens of ps. The ultrafast electron transfer from CdSe NRs to Au NPs efficiently competes with intraband relaxation dynamics, allowing observation of the hot-electron transfer process. Furthermore, the time-averaged PL decay of CdSe NRs shows a strongly multiexponential feature that was analyzed by single-particle spectroscopy. The PL decay of individual NRs fluctuates in time and is correlated with the PL intensity. We show that the time-averaged decay of bare CdSe NRs is composed of (i) a long lifetime component corresponding to bright CdSe NRs (ON state) and (ii) a short lifetime component corresponding to charged NRs that open additional fast non-radiative channels (OFF state). When Au NPs are attached to CdSe NRs, the ON state PL decays still show a long lifetime component, suggesting that the length of the NRs may hinder electron transfer if the exciton is formed far from the Au NPs. Finally, quantitative analysis of the OFF state decays shows that electron transfer occurs even in the presence of fast non-radiative pathways in charged systems.

Single-photon emission behavior in the enhanced fluorescence from single colloidal quantum dots (QDs) near silver nanoparticles (AgNPs) was investigated using a single molecule fluorescence spectroscopy technique. It was found that the degree of fluorescence enhancement from single QDs with AgNPs increased with decrease in the lifetime and the probability of single-photon emission, that is, highly enhanced fluorescence with a shortened lifetime exhibited a low probability of single-photon emission. The present results yield new insights into fundamentals of QD-metal nanostructure interactions, and are also important to understand the mechanism of the fluorescence enhancement by localized surface plasmon of metal nanostructures.

The influence of molecular weight (MW) and chain conformation on single-photon emission from isolated chains of poly[2-methoxy,5-(2′-ethylhexyloxy)-p-phenylene-vinylene] have been investigated. The probability of single-photon emission clearly increased with decreasing MW, and when the polymer chains adopted the “collapsed” conformation, this probability was larger than for the “extended” conformation. Even single polymer chains of high MW behaved as single-photon sources by adopting the collapsed conformation. These results indicate that multi-quantum systems consisting of a large number of chromophores can be made to behave as single-photon sources by appropriately controlling their size.

Dopant-induced laser ablation was applied to thin polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer films doped selectively with tetrakis(4-carboxyphenyl)porphine (TCPP) into the nanoscale spherical domain of P4VP-island parts. Under dry-film conditions, the ablation of the island parts was not induced. On the other hand, in methanol solvent, only the P4VP-island parts doped with TCPP were ablated selectively. The dopant-induced laser ablation is attributed to the fact that the P4VP-island parts are in the swollen state, where P4VP chains cross-linked by TCPP-dopant molecules exist with a high density in methanol. This is the first report concerning the site-selective laser ablation of diblock copolymer films.

A novel dendrimer containing eight perylene diimide chromophores has been synthesized and studied by ensemble and single-molecule spectroscopic techniques. Photon anti-bunching (coincidence) measurements on single molecules embedded in zeonex polymer films show that the dendrimer behaves as a deterministic (triggered) single photon source with only one fluorescence photon being emitted following pulsed laser excitation, even when more than one chromophore is excited. This behaviour is due to efficient singlet–singlet annihilation being operative in this dendrimer. Preliminary results indicate that the triplet lifetime and yield for this molecule are similar to the values for a molecule containing a single perylene diimide chromophore.