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Organometal halide perovskites show promising features for cost-effective application in photovoltaics. The material instability remains a major obstacle to broad application because of the poorly understood degradation pathways. Here, we apply simultaneous luminescence and electron microscopy on perovskites for the first time, allowing us to monitor in situ morphology evolution and optical properties upon perovskite degradation. Interestingly, morphology, photoluminescence (PL), and cathodoluminescence of perovskite samples evolve differently upon degradation driven by electron beam (e-beam) or by light. A transversal electric current generated by a scanning electron beam leads to dramatic changes in PL and tunes the energy band gaps continuously alongside film thinning. In contrast, light-induced degradation results in material decomposition to scattered particles and shows little PL spectral shifts. The differences in degradation can be ascribed to different electric currents that drive ion migration. Moreover, solution-processed perovskite cuboids show heterogeneity in stability which is likely related to crystallinity and morphology. Our results reveal the essential role of ion migration in perovskite degradation and provide potential avenues to rationally enhance the stability of perovskite materials by reducing ion migration while improving morphology and crystallinity. It is worth noting that even moderate e-beam currents (86 pA) and acceleration voltages (10 kV) readily induce significant perovskite degradation and alter their optical properties. Therefore, attention has to be paid while characterizing such materials using scanning electron microscopy or transmission electron microscopy techniques. ispartof: Journal of Physical Chemistry Letters vol:7 issue:3 pages:561-566 ispartof: location:United States status: published

Imperfections in the spotlight: fluorescence microscopy was used to detect defects in metal-organic frameworks formed during synthesis. In contrast to currently available techniques, confocal fluorescence microscopy offers the advantage of three-dimensional imaging at the single-crystal level combined with the sensitivity required to study the start of defect formation.

In previous reports it was shown that the photopolymerization of N,N-didodecyl-methylethylmethacrylate ammonium chloride, a reverse micelle forming surfactant, leads to a transparent polymerized inverse micellar solution1,2. The polymerization leads to high molecular weight polymer as determined by GPC. However recent results indicate that GPC, due to the association of the polymer overestimates the molecular weight. In addition a more extensive characterization of the polymeric aggregates is presented. The polymerization of the inverse micellar solution leads to a more stable and less dynamic inverse micellar system. However, the obtained aggregates are much larger then the monomeric inverse micelles. On the basis of the aggregation behaviour of the polymer and the characteristics of the polymer aggregates a model describing the structure of the polymer aggregates is proposed. Finally the monolayer formation and properties of the monomer and polymer are compared.

HcRed is a dimeric intrinsically fluorescent protein with origins in the sea anemone Heteractis crispa. This protein exhibits deep red absorption and emission properties. Using a combination of ensemble and single molecule methods and by varying environmental parameters such as temperature and pH, we found spectroscopic evidence for the presence of two ground state conformers, trans and cis chromophores that are in thermal equilibrium and that follow different excited-state pathways upon exposure to light. The photocycle of HcRed appears to be a combination of both kindling proteins and bright emitting GFP/GFP-like proteins: the trans chromophore undergoes light driven isomerization followed by radiative relaxation with a fluorescence lifetime of 0.5 ns. The cis chromophore exhibits a photocycle similar to bright GFPs and GFP-like proteins such as enhanced GFP, enhanced YFP or DsRed, with radiative relaxation with a fluorescence lifetime of 1.5 ns, singlet-triplet deactivation on a microsecond time scale and solvent controlled protonation/deprotonation in tens of microseconds. Using single molecule spectroscopy, we identify trans and cis conformers at the level of individual moieties and show that it is possible that the two conformers can coexist in a single protein due to the dimeric nature of HcRed.

The synthesis and photophysics of two new aminopropenyl naphthalene diimide (SANDI) dyes are reported. A general and convenient method for the synthesis of the precursor mono-, di-, and tetrabrominated 1,4,5,8-naphthalene tetracarboxylic dianhydrides is described. The two core-substituted SANDIs exhibit many of the photophysical properties required for fluorescence labeling applications including high photostability and high fluorescence quantum yields (>0.5) in the visible region of the spectrum. The emission wavelength is sensitive to the number of substituents on the NDI core, and the fluorescence decay times are in the range of approximately 8-12 ns for both compounds in the solvents investigated. Preliminary fluorescence emission data from single molecules of the compounds embedded in poly(methyl methacrylate) films are also reported and show that single molecules have very low yields of photobleaching, particularly the di-substituted system. Furthermore, only a small proportion (

Many of the biological processes taking place in cells are mediated by enzymatic reactions occurring in the cell membrane. Understanding interfacial enzymatic catalysis is therefore crucial to the understanding of cellular function. Unfortunately, a full picture of the overall mechanism of interfacial enzymatic catalysis, and particularly the important diffusion processes therein, remains unresolved. Herein we demonstrate that single-molecule wide-field fluorescence microscopy can yield important new information on these processes. We image phospholipase enzymes acting upon bilayers of their natural phospholipid substrate, tracking the diffusion of thousands of individual enzymes while simultaneously visualising local structural changes to the substrate layer. We study several enzyme types with different affinities and catalytic activities towards the substrate. Analysis of the trajectories of each enzyme type allows us successfully to correlate the mobility of phospholipase with its catalytic activity at the substrate. The methods introduced herein represent a promising new approach to the study of interfacial/heterogeneous catalysis systems.

The self-assembly of a series of hexadehydrotribenzo[12]annulene (DBA) derivatives has been investigated by scanning tunneling microscopy (STM) at the liquid/solid interface in the absence and presence of nanographene guests. In the absence of appropriate guest molecules, DBA derivatives with short alkoxy chains form two-dimensional (2D) porous honeycomb type patterns, whereas those with long alkoxy chains form predominantly dense-packed linear type patterns. Added nanographene molecules adsorb in the pores of the existing 2D porous honeycomb type patterns or, more interestingly, they even convert the guest-free dense-packed linear-type patterns into guest-containing 2D porous honeycomb type patterns. For the DBA derivative with the longest alkoxy chains (OC20H41), the pore size, which depends on the length of the alkoxy chains, reaches 5.4 nm. Up to a maximum of six nanographene molecules can be hosted in the same cavity for the DBA derivative with the OC20H41 chains. The host matrix changes its structure in order to accommodate the adsorption of the guest clusters. This flexibility arises from the weak intermolecular interactions between interdigitating alkoxy chains holding the honeycomb structure together. Diverse dynamic processes have been observed at the level of the host matrix and the coadsorbed guest molecules.

The excited state dynamics of two generations perylenediimide chromophores substituted in the bay area with dendritic branches bearing triphenylamine units as well as those of the respective reference compounds are investigated. Using single photon timing and multi-pulse femtosecond transient absorption experiments a direct proof of a reversible charge transfer occurring from the peripheral triphenylamine to the electron acceptor perylenediimide core is revealed. Femtosecond pump-dump-probe experiments provide evidence for the ground state dynamics by populating excited vibronic levels. It is found by the means of both techniques that the rotational isomerization of the dendritic branches occurs on a time scale that ranges up to 1 ns. This time scale of the isomerization depends on the size of the dendritic arms and is similar both in the ground and excited state.

Lipoplex formation for normal and cholesterol-modified oligonucleotides is investigated by fluorescence correlation spectroscopy (FCS). To overcome the problems related to the fitting of autocorrelation curves when fluorescence bursts are present, the baseline fluorescence levels and the fluorescence bursts in the same trace were separately analyzed. This approach was not previously used in FCS studies of lipoplexes and allowed a more detailed characterization of this heterogeneous system. From the baseline levels, the number of free/bound DNA molecules and the presence of tens to hundreds of nanometer-sized lipoplexes were estimated using various mathematical models. Analysis of the fluorescent bursts provided an indication about the sizes of the lipoplexes, the number of DNA molecules in these aggregates, and the relative amount of lipids in each aggregate. An explanation for the higher transfection efficiency previously reported for one of the cholesterol-modified oligonucleotide compounds was found in relation to the formation of large size lipoplexes.

The ever-improving time and space resolution and molecular detection sensitivity of fluorescence microscopy offer unique opportunities to deepen our insights into the function of chemical and biological catalysts. Because single-molecule microscopy allows for counting the turnover events one by one, one can map the distribution of the catalytic activities of different sites in solid heterogeneous catalysts, or one can study time-dependent activity fluctuations of individual sites in enzymes or chemical catalysts. By experimentally monitoring individuals rather than populations, the origin of complex behavior, e.g., in kinetics or in deactivation processes, can be successfully elucidated. Recent progress of temporal and spatial resolution in single-molecule fluorescence microscopy is discussed in light of its impact on catalytic assays. Key concepts are illustrated regarding the use of fluorescent reporters in catalytic reactions. Future challenges comprising the integration of other techniques, such as diffraction, scanning probe, or vibrational methods in single-molecule fluorescence spectroscopy are suggested.

The excited-state properties of two peryleneimide chromophore end-capped pentaphenylene compounds were investigated in detail using femtosecond transient absorption and single-photon timing experiments. Singlet-singlet annihilation was found to promote one chromophore into a higher excited state and results in the formation of an ultra-short-living intermediate charge-transfer (CT) state in the S(n)-S(1) deactivation pathway. In low-polarity solvents, this CT state is found to be energetically higher than the first excited state and thus cannot be populated via one-photon excitation. The observed CT state decays with a time constant of about 1 ps to form the lowest singlet excited state. These results demonstrate the potential use of the singlet-singlet annihilation as a novel tool in studying reactions occurring in states that are energetically above the S(1).

Shape persistent perylene diimide (PDI) multichromophores incorporating ethynylene bridges have been synthesized in high yield via palladium-catalyzed Hagihara coupling, which provides compounds with no rotational or constitutional isomerism in contrast to polyphenylene dendrimers. Their excited-state pathways have been studied at the ensemble and at the single-molecule level and compared to several model compounds. In an apolar solvent, energy hopping and/or energy transfer between the chromophoric units are the dominating processes. In a polar medium, energy hopping is still operative, but electron transfer from the phenyl ethynylene bridge to the chromophore occurs if the former is connected to the bay area of PDI. This effect should be considered when further developing this type of multichromophore, as this nonradiative deactivation process might be unwanted for applications such as optical and electronic devices. At the single-molecule level, the fluorescence intensity traces are characterized by rich on-off dynamics, which we attribute to oxygen-enhanced intersystem crossing leading to the formation of a long-lived dark charge-separated state.

The thermal motion of polymer chains in a crowded environment is anisotropic and highly confined. Whereas theoretical and experimental progress has been made, typically only indirect evidence of polymer dynamics is obtained either from scattering or mechanical response. Toward a complete understanding of the complicated polymer dynamics in crowded media such as biological cells, it is of great importance to unravel the role of heterogeneity and molecular individualism. In the present work, we investigate the dynamics of synthetic polymers and the tube-like motion of individual chains using time-resolved fluorescence microscopy. A single fluorescently labeled polymer molecule is observed in a sea of unlabeled polymers, giving access to not only the dynamics of the probe chain itself but also to that of the surrounding network. We demonstrate that it is possible to extract the characteristic time constants and length scales in one experiment, providing a detailed understanding of polymer dynamics at the single chain level. The quantitative agreement with bulk rheology measurements is promising for using local probes to study heterogeneity in complex, crowded systems.

Shape-persistent macrocycles with interiors in the nanometer regime were prepared by the oxidative cyclization of the appropriate bisacetylene precursors under high-dilution conditions. These compounds contain polycyclic aromatic hydrocarbons in the ring backbone and are decorated with extra annular oligoalkyl or silyl side groups. Interestingly, after depositing them on different surfaces and investigating the self-assembled structures by means of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), various nanostructures were observed. STM showed that these macrocycles are organized in two-dimensional (2D) layers, whereas AFM showed, in addition, the formation of 2D crystallites and one-dimensional fibrils. These results reveal the importance of the extra annular substitution of the macrocycles in creating patterned surfaces and nanoscale objects.

The self-assembly of a series of hexadehydrotribenzo[12]annulene (DBA) derivatives has been scrutinized by scanning tunneling microscopy (STM) at the liquid-solid interface. First, the influence of core symmetry on the network structure was investigated by comparing the two-dimensional (2D) ordering of rhombic bisDBA 1a and triangular DBA 2a (Figure 1). BisDBA 1a forms a Kagomé network upon physisorption from 1,2,4-trichlorobenzene (TCB) onto highly oriented pyrolytic graphite (HOPG). Under similar experimental conditions, DBA 2a shows the formation of a honeycomb network. The core symmetry and location of alkyl substituents determine the network structure. The most remarkable feature of the DBA networks is the interdigitation of the nonpolar alkyl chains: they connect the pi-conjugated cores and direct their orientation. As a result, 2D open networks with voids are formed. Second, the effect of alkyl chain length on the structure of DBA patterns was investigated. Upon increasing the length of the alkyl chains (DBAs 3c-e) a transition from honeycomb networks to linear networks was observed in TCB, an observation attributed to stronger molecule-substrate interactions. Third, the effect of solvent on the structure of the nonpolar DBA networks was investigated in four different solvents: TCB as a polar aromatic solvent, 1-phenyloctane as a solvent having both aromatic and aliphatic moieties, n-tetradecane as an aliphatic solvent, and octanoic acid as a polar alkylated solvent. The solvent dramatically changes the structure of the DBA networks. The solvent effects are discussed in terms of factors that influence the mobility of molecules at the liquid-solid interface such as solvation.

In this paper, we investigate if the rate constants and spectral parameters that describe models for intramolecular three-state excited-state events can be uniquely determined from ideal time-resolved fluorescence data. For the general model, in which there are reversible transformations between the three excited species, values of nine rate constants and four spectral parameters related to excitation and emission must be defined. Without prior knowledge of rate constants and spectral parameters there is an infinite number of these model parameters that fit the time-resolved fluorescence data and hence the model is not identifiable. Even simplified models leading to single exponential decay kinetics of some excited species concentrations are unidentifiable if such prior knowledge is not available. It is shown for each simplified model what kinetic information can be extracted from perfect time-resolved fluorescence data when the individual excited species concentrations can be monitored. Guidelines to the experimentalist are given for using a priori knowledge to make a distinction among the different models.

In this paper we present a joint experimental and theoretical approach for the study of the assembly of end-substituted oligothiophenes at surfaces with different polarities (i.e., mica vs graphite). Scanning probe microscopy studies of (sub)monolayer deposits show various types of structures (one-dimensional fibrils, two-dimensional regular layers, and monolayers), depending on the nature of the end groups and the substrate. Using molecular modeling with an atomistic approach, we focus on the interplay between the molecule-molecule (and segment-segment) interactions and the molecule-substrate interactions and their influence on the observed morphologies and the stacking geometry. Such information is relevant for controlling the structural order in thin layers of thiophene oligomers for use in field-effect transistor applications, for example, by modifying the nature of dielectric material over which those compounds are deposited.

Three novel pyrene dyes substituted by four oligo-ether side chains of varying size have been synthesized. Surprisingly, two of these fluorescent derivatives are liquid at room temperature. The photophysical study of one of these dyes, which is miscible in common organic solvents such as hexane, chloroform and toluene, but also in water, has been performed. It has been shown that this pyrene derivative exhibits a high quantum yield of fluorescence, Φf ∼ 1, and has a high extinction coefficient, i.e. ɛ = 100,000 mol−1 cm−1 l, in diluted n-hexane solution. In water, aggregation takes place and the fluorescence quantum yield decreases considerably. Moreover, the spectroscopic properties of the aggregates formed in water are very close to those of the neat liquid. Therefore, the basic structure of the aggregates formed in water and in the pure liquid are expected to be similar.

The blinking behavior of single Atto565 molecules on a glass surface is studied under air or nitrogen atmospheres using confocal microscopy. The broad distributions for both on- and off-time durations obey power law kinetics that are rationalized using a charge tunneling model. In this case, a charge is transferred from the Atto565 molecule to localized states found on the glass surface. Subsequent charge recombination by back charge tunneling from trap to Atto565 cation (i.e., dark state) restores the fluorescence. The off-time distribution is independent of excitation intensity (I), whereas the on-time distribution exhibits a power law exponent that varies with I. Two pathways have been identified to lead to the formation of the radical dark state. The first involves direct charge tunneling from the excited singlet S1 state to charge traps in the surrounding matrix, and the second requires charge ejection from the triplet T1 state after intersystem crossing from S1. Monte Carlo simulation studies complement the two-pathway model. Photobleaching curves of both single and ensemble molecules do not exhibit monoexponential decays suggesting complex bleaching dynamics arising from triplet and radical states.

We describe the synthesis, supramolecular ordering on surfaces and in solution, and photophysical characterization of OPV4UT-PERY, an oligo(p-phenylenevinylene) (OPV) with a covalently attached perylene bisimide moiety. In chloroform, the molecule forms dimers through quadruple hydrogen bonding of the ureido-s-triazine array. This is supported by scanning tunneling microscopy (STM) studies, which reveal dimer formation at the liquid (1,2,4-trichlorobenzene)/solid (graphite) interface. Moreover, contrast reversal in bias-dependent STM imaging provides information on the ordering and different electronic properties of the oligo(p-phenylenevinylene) and perylene bisimide moieties. In dodecane, the molecule self-assembles into H-type aggregates that are still soluble as a result of the hydrophobic shell formed by the dodecyloxy wedges. The donor-acceptor molecule is characterized by efficient energy transfer from the photoexcited OPV to the perylene bisimide. Mixed assemblies with analogous OPVs lacking the perylene bisimide unit have been prepared in dodecane solution and energy transfer to the incorporated perylene bisimides has been studied by fluorescence spectroscopy.

The effect of solvent on the two-dimensional (2D) supramolecular ordering of monodendron 1 at the liquid-solid interface has been systematically investigated by means of scanning tunneling microscopy (STM). Solvents range from those with hydrophilic solvating properties, such as alkylated alcohols and acids, to hydrophobic solvents such as alkylated aromatics and alkanes. Dramatic differences in the 2D ordering are observed depending on the nature of the solvent. Of particular interest is the fact that in hydrophobic solvating solvents, such as aliphatic and aromatic hydrocarbons, solvent molecules are coadsorbed in the 2D molecular network while this is not the case for alkylated alcohols or acids. Furthermore, in the case of the coadsorbing solvents, a striking influence of the alkyl chain length has been observed on the 2D pattern formed. The solvent and alkyl chain length dependences are discussed in terms of molecule-molecule interactions (homo and hetero) and molecule-substrate interactions.

The photophysical properties of seven new 8-(p-substituted)phenyl analogues of 4,4-difluoro-3,5-dimethyl-8-(aryl)-4-bora-3a,4a-diaza-s-indacene (derivatives of the well-known fluorophore BODIPY) in several solvents have been studied by means of absorption and steady-state and time-resolved fluorimetry. For each compound, the fluorescence quantum yield and lifetime are lower in solvents with higher polarity owing to an increase in the rate of nonradiative deactivation. Increasing the electron withdrawing strength of the p-substituent on the phenyl group in position 8 also leads to lower fluorescence quantum yields and lifetimes. When the p-substituent on the phenyl group in position 8 is a tertiary amine [8-(4-piperidinophenyl), 8-(4-N,N-dimethylaminophenyl), and 8-(4-morpholinophenyl)], the low quantum yields of these compounds in more polar solvents can be rationalized by the inversion of the energy levels of an apolar, highly fluorescent and a polar, nonfluorescent excited state, where charge transfer from the tertiary amine to the BODIPY unit occurs. These amine analogues can be protonated at low pH in aqueous solution. Fluorescence titrations yielded pK(a) values of their conjugate ammonium salts which are in agreement with the electron donating tendency of the amine group: piperidino (4.15)dimethylamino (2.37)morpholino (1.47), with the pK(a) values in parentheses. The rate constant of radiative deactivation (k(f)) is the same for all compounds in all solvents studied (k(f) = 1.4 x 10(8) s(-1)).

Cholesterol modified mono-, di-, and tetrameric oligonucleotides were synthesized and hybridized with antisense oligonucleotides to study their incorporation in cationic liposomes together with the influence of this dendrimeric delivery system on biological activity. Electrostatic interactions seem to play the most important role during complexation with cationic lipids. This oligonucleotide formulation gives a small but significant increase in the inhibition of P-glycoprotein expression in a cellular system.

We report on the ensemble and single-molecule (SM) dynamics of Förster resonance energy transfer (FRET) in a multichromophoric rigid polyphenylenic dendrimer (triad) with spectrally different rylene chromophores featuring distinct absorption and emission spectra which cover the whole visible spectral range: a terrylenediimide (TDI) core, four perylenemonoimides (PMIs) attached at the scaffold, and eight naphthalenemonoimides (NMIs) at the rim. For FRET from PMI to TDI taking place with an efficiency of 99.5%, single triad molecules optically excited at 490 nm show fluorescence exclusively from the TDI side in the beginning of their emission. On 360-nm excitation, NMI chromophores transfer their excitation energy either directly or in a stepwise fashion to the core TDI, the latter case involving scaffold-substituted PMIs as intermediate acceptors. Indeed, SM experiments on 360-nm excitation evidence highly efficient FRET from NMI chromophores to the TDI core since individual triad molecules show fluorescence exclusively either from TDI or from an intermediate (oxidized) species but never from PMI. Because PMI and TDI are chromophores with high fluorescence quantum yields and high resistance to photobleaching compared to NMI, 360-nm excitation of a single triad molecule leads to bleaching of NMI chromophores with no chance for PMI to be observed. The spatial positioning and the spectral properties of the chosen rylene chromophores make this multichromophoric system an efficient light collector, able to capture light over the whole visible spectral range and to transfer it finally to the core TDI, the latter releasing it as red fluorescence.

Rigid polyphenylene dendrimers substituted with dyes are suitable substrates for the investigation of interchromophore photophysical processes as they contain discrete numbers of chromophores in well-defined spatial relationships to each other. In this contribution, we provide an overview of the synthesis of such materials and show how a combination of ensemble and single-molecule spectroscopic investigations can be used to elucidate the interchromophore excitation energy transfer processes in them.

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 photophysical properties of dendrimers with 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole, also known as DPP, as a core were studied. DPP itself has a high quantum yield of fluorescence, as well as a high molar decadic absorption coefficient, e (e = 33 000 L.mol -1 .cm -1 ). The dendrimers, which differ by their substitution pattern, either at the nitrogen (DPPN) or at the carbon (DPPC), are studied in various solvents with different viscosity and solvent-probe interaction and this for several generations and thus varying molecular volume. The stationary measurements show no difference in the absorption and emission spectra for the different generations nor a solvent-effect. Single photon timing is used to determine the fluorescence lifetime, τ. For both dendrimers, the decay curves can be fitted to a monoexponential model. By means of fluorescence depolarization and fluorescence correlation spectroscopy, the hydrodynamic volumes (V h ) are determined. The results for both dendrimers as well as for both techniques are compared. The hydrodynamic radii of DPPN are similar when obtained by FCS compared with anisotropy. When the properties of the different dendrimers are compared, it can be shown that the hydrodynamic volumes of the first generation DPPC dendrimer are much larger compared to V h of the DPPN dendrimers. The volumes for the third generation are comparable for both substitution patterns.

Enantiomerically pure oligo(p-phenylene vinylene) diaminotriazine derivatives and a short structurally related achiral diaminotriazine derivative, all having a rigid backbone in common, are studied to self-assemble at the solution-graphite interface by scanning tunneling microscopy. As a function of the length of the backbone, different two-dimensional motifs are formed (dimers and rosettes) that are rationalized in terms of the balance between different intermolecular interactions, in this case, intermolecular hydrogen bonding and the packing requirements of the alkyl chains on a graphite surface. In addition, the effect of molecular chirality on monolayer chirality is investigated, revealing molecular size-dependent expressions of the monolayer chirality.

By means of scanning tunneling microscopy (STM), the self-assembly of two organic donor-acceptor-donor triads (donor=oligo(p-phenylene vinylene) (OPV); acceptor=perylene diimide (PDI)) and their mixtures has been investigated at the liquid/solid interface. Both triads differ in the nature of the substituents and, therefore, in the redox properties of the central perylene diimide unit (H or Cl). Thanks to the submolecular resolution, the distinct electronic properties of the units, within a triad and between the two triads, are reflected by the relative STM contrast in the bias-dependent imaging experiments. Moreover, scanning tunneling spectroscopy reveals an inverse rectifying behavior of the OPV and H-substituted PDI units, which is discussed in the framework of quasi-resonant tunneling. A striking difference is observed for the Cl-substituted triad.

A novel perylene imide and oligo-pentaphenyl bisfluorene containing molecule is shown to undergo electron transfer to form an emissive charge transfer state in di-benzyl ether and THF. At the single molecule level in a PMMA film, fluorescence spectra characteristic of both emissive states (locally excited and charge transfer) are observed with 44% of the molecules studied showing switching between the two states. These results demonstrate that charge transfer fluorescence from single molecules can be used to report on the properties and dynamics of a molecule’s immediate surroundings or nano-environment.

By means of scanning tunneling microscopy, we have explored the two-dimensional self-assembly of functional bicomponent hydrogen-bonding dye systems, leading to well-defined patterns, different from those of the individual components, and providing design rules to immobilize multicomponent systems at the liquid-solid interface.

When dioctadecyl dimethylammonium bromide (DODAB) is compressed on a subphase containing 3,3'-disulfopropyl-5,5'-dichlorothiacyanine (THIAMS), adsorption of the dye to the DODAB monolayer results in the formation of J-aggregates which spontaneously organize into polygonal domains of micron size. The features of the domains depend on the surface pressure. The fluorescence of the individual domains is polarized. The shapes of the domains determined by fluorescence microscopy and atomic force microscopy (AFM) are identical. The domains differ however significantly from those observed after injection of a 3,3'-disulfopropyl-5,5'-dichloro-9-ethylthiacarbocyanine (THIATS) or THIAMS solution below a precompressed DODAB film, as well as from the domains observed upon compression of a DODAB monolayer on a subphase containing 10(-6) M THIATS.

We report on single-molecule fluorescence measurements performed on the phycobiliprotein allophycocyanin (APC). Our data support the presence of a unidirectional Forster-type energy transfer process involving spectrally different chromophores, alpha84 (donor) and beta84 (acceptor), as well as of energy hopping amongst beta84 chromophores. Single-molecule fluorescence spectra recorded from individual immobilized APC proteins indicate the presence of a red-emitting chromophore with emission peaking at 660 nm, which we connect with beta84, and a species with the emission peak blue shifted at 630 nm, which we attribute to alpha84. Polarization data from single APC trimers point to the presence of three consecutive red emitters, suggesting energy hopping amongst beta84 chromophores. Based on the single-molecule fluorescence spectra and assuming that emission at the ensemble level in solution comes mainly from the acceptor chromophore, we were able to resolve the individual absorption and emission spectra of the alpha84 and beta84 chromophores in APC.

The photophysical properties of a new water-soluble fluorescent dye based on the perylene diimide chromophore are investigated at the ensemble and single molecule level. Water solubility was obtained by attaching hydrophilic substiments in the bay region. The fluorophore has an absorption maximum at 567 nm and a high quantum yield of fluorescence (0.6). In addition, the two-photon absorption cross section has higher or comparable values with the ones reported in the literature for other water-soluble dyes used in biological studies. The most important finding is its improved photostability in single molecule experiments when compared to other water-soluble dyes. This is demonstrated by the increased observation time of immobilized single molecules, which is almost 10 times higher than for commonly studied fluorophores. A possible reason is the lower intersystem crossing rate constant of perylene diimide in comparison, for example, to rhodamine derivatives. A similar conclusion can be drawn for this chromophore in an aqueous solution, because single molecule measurements using fluorescence correlation spectroscopy demonstrate a very low contribution of the triplet state in the autocorrelation function. The possibility of imaging the new molecule in living cells is demonstrated by one-photon confocal microscopy and by fluorescence lifetime microscopy with two-photon excitation.

Fluorescein is a complex fluorophore which at the physiological pH can exist as mono- and dianion forms. In a previous article (J. Phys. Chem. A 2001, 105, 6320−6332), we showed that fluorescein displays an excited-state proton transfer reaction which interconverts the mono- and dianion forms in the presence of a suitable proton donor−acceptor. Because fluorescein is a frequently used fluorescent label in biological systems, it is of interest to know if amino acids with acidic side chains are able to induce fluorescein excited-state proton transfer reactions. We have selected (±)-N-acetyl aspartic acid, N-AcAsp, as a model donor−acceptor which mimics the interaction of the aspartic acid residues with the fluorescent label in native proteins. We present absorption and emission properties which show that N-AcAsp at 1 M concentration induces the excited-state proton transfer reaction between the mono- and dianion of fluorescein. The kinetics of this excited-state process was characterized from time-resolved ...

Novel perylene-3,4,9,10-tetracarboxdiimides (PDI) dyes functionalized with polyphenylene dendrimers attached at the bay region are reported. Derivatives of PDI bearing polyphenylene dendrimers up to the second generation, substituted with an increasing number of triphenylamine (TPA) moieties at the periphery, as well as a related nondendronized model compound were prepared. Intramolecular energy transfer was demonstrated by the observation of PDI emission on excitation of the triphenylamines, and electron transfer was detected by comparing photoluminescence quenching in solvents of different polarity.

Oligo(p-phenylenevinylene)s with pendant diaminotriazine moieties can self-assemble through hydrogen-bonding interactions to give hexameric rosettes. These rosettes further organize into large supramol. tubes with perfect space filling.

Anionic meso-tetrakis(4-sulfonatophenyl)porphine (TSPP) interacts with poly(amido) amine (PAMAM) dendrimers of generations 2.0 and 4.0 in aqueous solution to form several dendrimer-associated species, which depend on the relative concentrations of dendrimer to porphyrin (D/P ≡ [PAMAM]/[TSPP]). At D/P ratios above the isoelectric point of the charge balance between the dendrimer and porphyrin, only two spectroscopic species were detected. An equilibrium model assuming a dendrimer-induced TSPP H-dimer coexisting with dendrimer-associated TSPP monomer afforded a good description of the experimental data, giving equilibrium constants (Kd) for the dimer dissociation of 0.78 and 2.67 for generations 2.0 and 4.0 dendrimers, respectively. The decomposition of the Soret band afforded TSPP H-aggregates' spectra with bandwidth values in the range 910−1210 cm-1, clearly larger than the monomer bandwidth around 711−800 cm-1. The fluorescence decays are nearly exponential with a long decay time component, which varies ...

We demonstrate measurements of the efficiency of competing Förster-type energy-transfer pathways in single bichromophoric systems by monitoring simultaneously the fluorescence intensity, fluorescence lifetime, and the number of independent emitters with time. Peryleneimide end-capped fluorene trimers, hexamers, and polymers with interchromophore distances of 3.4, 5.9, and on average 42 nm, respectively, served as bichromophoric systems. Because of different energy-transfer efficiencies, variations in the interchromophore distance enable the switching between homo-energy transfer (energy hopping), singlet-singlet annihilation, and singlet-triplet annihilation. The data 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 natural and synthetic multichromophoric systems such as light-harvesting antennas, oligomeric fluorescent proteins, and dendrimers. Here we report selectively visualization of different energy-transfer pathways taking place between identical fluorophores in individual bichromophoric molecules.

The influence of surface pressure and adsorption time on the size and shape of J aggregates of 3,3‘-disulfopropyl-5,5‘-dichlorothiacyanine sodium salt (THIAMS) adsorbed onto dioctadecylammonium bromide (DODAB) monolayer were investigated. After deposition of the monolayers on a hydrophobic glass substrate by the Langmuir−Schaefer (LS) technique, confocal fluorescence microscopy and atomic force microscopy (AFM) were used to study their fluorescence and topography. The difference in size and shape of the J aggregates formed at different surface pressure and adsorption times may be caused by a different rigidity and charge density of the DODAB film. The total thickness of the films amounts to 4.5 ± 0.2 nm which confirms that THIAMS forms 2D aggregates with edge-on orientation when adsorbed to a DODAB monolayer. In the bulk of the J aggregate the fluorescence was polarized in all films investigated. THIAMS molecules adsorbed to the dark areas between the fluorescence domains remained mainly as monomers and d...

Protein-polymer hybrids can act as giant monolayer-forming amphiphiles at the air-water interface.Using biotinylated polystyrene (PSb) as the hydrophobic part and streptavidin (SAv) as a hydrophilic end of the giant amphiphile, monolayer formation and subsequent deposition leads to a well defined solid-supported monolayer, in which the remaining free sites of the polymer-bound SAv are accessible to biotin binding.The deposition conditions are evaluated by investigating the supported films by atomic force microscopy (AFM).Imaging of the PSb without protein reveals uniform monolayer formation.In situ binding of SAv indicates that the biotin is accessible.Deposition of the SAv-polymer hybrid film results primarily in a very similar morphology, indicative of the dominant role of the polymer in the monolayer formation.High resolution AFM revealed a homogeneously dense packing.This is confirmed by combined AFM and scanning confocal fluorescence microscopy (SCFM) on hybrid monolayers containing rhodamine-red-X labeled SAv.The biotin binding capability of the monolayer was assessed by binding of fluorescein labeled biotin and performing dual color excitation and detection.Biotin binding was uniform and homogeneous.Local spectra revealed that the fluorescein was largely quenched by the rhodamine, indicating an efficient energy transfer.

Investigation of the photophysical and photochemical processes that occur in single molecules by means of fluorescence microscopy has rapidly developed over the last decade.[1,2] We have focused on the photophysics of synthetic multichromophore systems and have shown evidence for intramolecular (excitation) energy migration, that is, energy hopping[3,4] and directional F%rster energy transfer.[5] Photoinduced electron transfer is another fundamental physical process, but it is more difficult to monitor by single-molecule spectroscopy. Electron transfer is usually an efficient mechanism for fluorescence quenching which leaves no signal to be detected at the single-molecule level. In some special cases, chargetransfer fluorescence may be observed after photoinduced electron transfer,[6] but its quantum yield is usually quite low and therefore less suited for single-molecule detection. Despite this difficulty some reports on forward electron transfer in single molecules exist in the literature. Interfacial electron transfer from a single molecule of a cresyl violet dye to the indium–tin oxide surface on which it was physisorbed was shown by Lu and Xie.[7] Weaker fluorescence and a decreased fluorescence lifetime with respect to the same molecule on a glass surface demonstrated the occurrence of electron transfer. In a similar way electron transfer between a guanine base and a dye linked to a DNA molecule could also be monitored, and conformational changes of DNA molecules were observed.[8–10] Intramolecular electron transfer in a synthetic system was recently demonstrated by Zang et al.[11] in a compound in which a perylenediimide dye that serves as the electron acceptor upon excitation is connected to an aniline donor. On a sufficiently polar substrate, no fluorescence from single molecules could be observed, but after protonation of the amine by treatment of the sample with hydrogen chloride, the molecules could be visualized. This experiment nicely dem

The influence of the lanthanide ion and the chain length on the thermal properties of bis[2,3,9,10,16,17,23,24-octakis(alkoxy)phthalocyaninato]lanthanide(III) complexes, denoted in the following as [(CnH2n+1O)8Pc]2Ln, have been investigated. Two series of complexes were considered: a first series in which the alkoxy chain was kept constant with different lanthanide ions (Ln = Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), namely, [(C12H25O)8Pc]2Ln, and a second series of bis(phthalocyaninato)erbium(III) double-deckers, [(CnH2n+1O)8Pc]2Er, with different alkoxy chain lengths (n = 4−18). All these metallomesogens exhibit a hexagonal columnar mesophase (Colh) over wide temperature ranges. It was found that the transition temperatures were not affected to a great extent by the lanthanide ion and that they could be tuned by a proper choice of the alkoxy chain length. The orientational behavior of [(C12H25O)8Pc]2Er in contact with a substrate has also been investigated by scanning tunneling microscopy.

Scanning tunneling microscopy has been employed to investigate the ordering of a tetrathiafulvalene derivative at a liquid-solid interface at room temperature. Surprisingly, in addition to the observation of polymorphous two-dimensional pseudocrystals, lower-dimensional assemblies are observed. Scanning tunneling spectroscopy at the individual molecule level reveals rectifying behavior, independent of the molecular ordering.