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The temperature and solvent dependence of the photophysical behaviour of 3-phenyl-1-N-dimethyl-aminopropane and 4-phenyl-1-N-dimethylaminobutane are investigated. The formation of a polar excited state complex is shown to be thermodynamically controlled.

The excimer formation of 1-pyrenylpentametyldisilane (P2) in isooctane is studied as a function of concentration and temperature. This contribution shows for the first time how the global compartmental analysis can be used to determine the temperature dependence of the excited state processes of an excimer forming species, i.e. the activation energy of excimer formation and excimer dissociation, the stabilisation enthalpy of the excimer (ΔH0) and the entropy change of the excimer formation. The activation energy of excimer formation is found to be larger than the activation energy for viscous flow.

Highly fluorescent organic nanoparticles with size of about 300 nm were prepared by nanosecond laser ablation of micrometer-sized powder of dendronized perylenediimide dispersed in water. The nanoparticle colloidal solution provided a fluorescence quantum yield of 0.58. The absorption and emission spectral studies demonstrated that the bulky dendron groups at the side bays of perylenediimide chromophore efficiently suppress the interchromophoric interactions in the nanoparticles. Fluorescence measurement on several single nanoparticles underlines that the prepared nanoparticles are bright and photo-stable enough to be a useful probe for single particle fluorescence investigation.

— The synthesis, absorption and emission properties of threo and erythro N-acetyl-bis-(pyrenylalanine)-methylester are discussed. The intensity of excimer emission is very dependent on the tendency of the solvent to interact with the peptide function by hydrogen bonding. Hydrogen accepting solvents shift the equilibrium between the different conformations, adopted by these dipeptides, towards a random coil conformation, which is unfavourable for excimer formation. In strongly hydrogen donating solvents however, a very intense excimer emission is observed due to a reduction of the partial double bond character of the peptide bond by an hydrogen bridge with the solvent molecules.

A series of polyphenylene dendrimers (PDs) incorporating additional pentafluorophenyl units have been synthesized. A key building block thereby is the tetrakis(pentafluorophenyl)cyclopentadienone, which serves as the end-capper for making the outer shells of the dendrimers. The variation of the peripheral pentafluorophenyl content was varied using desymmetrized tetraphenylmethane cores for the divergent construction of the dendrimers. The self-assembled nanostructures, prepared by drop-casting solutions of the second-generation species on highly oriented pyrolytic graphite (HOPG), have been investigated by tapping mode atomic force microscopy (AFM). Besides isotropic globular aggregates and percolated networks of nanometer size anisotropic objects, the herein presented dendrimers self-organize into micrometer long nanofibers. The formation of a certain morphology depends upon the amount of pentafluorophenyl rings at the periphery of the dendrimers. Furthermore, it has been observed that by the progressive...

The time-dependent spectral properties of a rigid, extended system consisting of three pentaphenylene units end-capped with perylene monoimide were investigated in detail by femtosecond transient absorption and single photon timing measurements. In polar solvents, the molecular system shows the occurrence of photoinduced charge transfer. Upon gradually increasing the excitation power, annihilation involving two states with charge-transfer character has been observed. Quantum-chemical calculations performed on the system consisting of two pentaphenylene units end-capped with perylene monoimide strongly support the experimental data. ispartof: Journal of the American Chemical Society vol:129 issue:3 pages:610-619 ispartof: location:United States status: published

Evidence for intramolecular photoinduced electron transfer (ET) in synthetic systems consisting of a triphenylamine-perylenediimide donor-acceptor dendrimer or a triphenylamine-peryleneimide dendrimer at the ensemble and single-molecule (SM) level is presented. Moreover, for the first time a direct observation of the forward as well as the backward ET step is made in a single emitting entity. Fluctuations in the values of the rate constants for forward and backward ET were observed, induced by the local environment as well as by conformational changes of the dendrimer itself. The results obtained in a weakly coupled system can also be extended to a strongly coupled donor-acceptor system based on peryleneimide and penta-phenylene.

The synthesis, properties, and conformation of shape-persistent macrocycles with extraannular oligo-alkyl groups and an intraannular bis(hydroxymethyl)biphenylene diester are investigated. The key step in the preparation is the template-supported Glaser coupling of rigid bisacetylenes which allows obtaining the cyclic product in 81% yield. Contrary to the initial expectations, the compounds do not exhibit liquid crystalline phases. This could be explained by a nonplanar conformation of the molecules because the biphenylene bridge is longer than the diameter of the rings. On the other hand, scanning tunneling microscopy investigations indicate that the intraannular biphenylene bridge can be located in the same plane as the aromatic backbone of the macrocycles on graphite. Modeling involving molecular mechanics and molecular dynamics simulations unraveled this apparent inconsistency by showing that the molecules adopt a nonplanar conformation in the absence of the solid support whereas planarization takes p...

Films of regioregular chiral poly[(S)-3-(3,7-dimethyloctyl)thiophene] (see Figure) prepared via spin-coating and characterized by polarized UV-vis spectroscopy show that order along the film normal is present (which is proportional to the spinning rate). Phase-sensitive second-harmonic generation experiments demonstrate this order to be polar. ispartof: Advanced Materials vol:17 issue:6 pages:708- status: published

The combination of transient absorption and fluorescence experiments performed on a femtosecond to nanosecond time scale was used to characterize the electron-transfer process in mono- (N1P1) and multichromophoric (N1P3) triphenylamine core dendrimers carrying one and three peryleneimide electron acceptor chromophores, respectively. Comparison of the monochromophoric N1P1 to the multichromophoric N1P3 allowed us to investigate the influence of the number of chromophores upon the electron transfer kinetics. The solvent effect on the electron transfer process was investigated by comparing the results obtained in the highly polar solvent benzonitrile with those in the less polar solvents as diethyl ether, 2-methyltetrahydrofuran, tetrahydrofuran, and toluene. By means of fluorescence anisotropy and transient absorption anisotropy experiments, the occurrence of energy hopping in the multichromophoric N1P3 has been demonstrated. Furthermore singlet−singlet annihilation, observed earlier in a model compound wit...

The two-dimensional pattern formation of chiral oligo(p-phenylenevinylene) derivatives of different lengths containing a self-complementary hydrogen bonding motif at the liquid/solid interface has been investigated and compared using scanning tunneling microscopy. Hydrogen bonding leads to dimer formation and the chirality of the molecules is expressed at the level of packing and orientation of the dimers with respect to the substrate symmetry. Differences in expression of molecular chirality are observed as a function of molecular length and the number of stereocenters they carry. Mixing of oligomers of different length does not lead to phase separation, but to the formation of heterodimers, stressing the important role of hydrogen bonding in the self-assembly process in both solution and at the liquid/solid interface.

We have demonstrated that it is possible to program pi-conjugated molecules to self-assemble into cylindrical aggregates in solution. By incorporating energy or electron traps in our stacks, energy and electron transfer processes in these one-dimensional assemblies have been studied in solution. The transfer of the single OPV cylinders from solution to a solid support as isolated objects was only possible when specific concentrations and specific solid supports were used. So far, however, we have not been able to measure any current through our fibers. (C) 2004 Elsevier B.V All rights reserved.

The two-dimensional pattern formation of hydrogen bonding isophthalic acid derivatives at the liquid/solid interface has been investigated using scanning tunneling microscopy. By varying the location and nature of alkyl substituents on the aromatic core in combination with the intrinsic hydrogen bonding properties of the isophthalic acid units, the two-dimensional supramolecular ordering has been controlled, leading to several different motifs.

X-ray photoelectron spectroscopy (XPS) has been used to characterize poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDT/PSS), one of the most common electrically conducting organic polymers. A correlation has been established between the composition, morphology, and polymerization mechanism, on the one hand, and the electric conductivity of PEDT/PSS, on the other hand. XPS has been used to identify interfacial reactions occurring at the polymer-on-ITO and polymer-on-glass interfaces, as well as chemical changes within the polymer blend induced by electrical stress and exposure to ultraviolet light. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2561-2583, 2003

Fluorescence lifetime measurements have been performed on single tetraphenoxy−perylenetetracarboxy diimide molecules embedded in two different polymer matrices. Single molecule decay times are found to fluctuate in time as a result of the dye molecule−polymer chains interaction, causing conformational rearrangements of the dye molecule. The rearrangements are shown to take place on a 2 ms time scale at some locations of the dye molecule within the polymer matrix. This behavior illustrates the spatially heterogeneous dynamics present in polymers at room temperature and provides a quantified estimation of the time scales involved in the process.

The nanophase separation behavior of two semi-fluorinated isophthalic acid derivatives, F8H10-ISA and F12H11-ISA, at the liquid−solid interface is studied by in situ scanning tunneling microscopy. Two polymorphs of F8H10-ISA were observed at the interface. One polymorph, which is not stable in time, showed nanosegregation of the perfluorinated segments of the alkyl chains, while the second polymorph showed a completely interdigitating lamellar structure with solvent co-deposition. This co-deposition of solvent molecules is believed to induce extra stability in the monolayer which in turn aids the molecules in overcoming the repulsive interactions induced by the perfluorinated segment of the molecule. F12H11-ISA did not exhibit nanosegregation at the liquid−solid interface, despite the longer perfluorinated segment (increase by 4 carbon atoms) with respect to the non-fluorinated segment (increase by 1 carbon atom). Instead, clustering of the molecules into units consisting of eight molecules was observed. ...

Ensemble and single molecule spectroscopic measurements on a bichromophoric dendrimer system were performed. Although Forster type energy processes such as energy hopping and singlet−singlet annihilation can be observed at the ensemble and single molecule level, only single molecule measurements can visualize the presence of singlet−triplet annihilation. The flux dependent population of the triplet state suggests facilitated formation of the triplet through a higher singlet excited state. The presence of synthesis inherent structural isomers, not to be distinguished at the ensemble level, could be demonstrated by analyzing modulated fluorescence intensity trajectories. These data show the complementarity of both the ensemble and single molecule approaches in the study of detailed photophysics of a multichromophoric system.

Hybrid clay-organic monolayers prepared using the Langmuir−Blodgett (LB) method are examined by polarized infrared attenuated total reflection spectroscopy (ATR), polarized grazing angle infrared reflection absorption spectroscopy (IRRAS), and atomic force microscopy (AFM). These spectra represent the first reported IR spectra of clay monolayers functionalized with cationic surfactants. The intensities of the ν(Si−O) bands of SapCa-1 saponite and SWy-1 montmorillonite on both Ge and ZnSe substrates were analyzed quantitatively using the thin film approximation. The measured intensity on both substrates corresponds to the calculated intensity of one clay layer (0.96 nm), indicating that a monolayer of clay was formed on ZnSe as well as on Ge. In the case of saponite, a trioctahedral smectite, the ν(OH) band at 3680 cm-1 and the out-of-plane ν(Si−O) band at 1063 cm-1 show dichroic ratios of 0.33 and 0.23, respectively, consistent with the nearly perpendicular orientation of these bonds in the film. For SWy-...

The light-induced and tip-induced polymerization of two diacetylene derivatives has been investigated at the air/solid interface by scanning tunneling microscopy (STM). One molecule is an isophthalic acid derivative with one diacetylene group, while the other one, a terephthalic acid derivative, contains two diacetylene functions. The design of the molecules (symmetry, presence of alkyl chains and hydrogen bonding moieties) leads to the formation of highly ordered two-dimensional patterns where the diacetylene groups stack in an optimal fashion. Both diacetylene derivatives can be polymerized by UV light irradiation, and locally the polymerization can be initiated and controlled by the STM tip. In addition to the formation of one-dimensional organic structures, it was possible to create two-dimensional organic nanostructures.

A series of azobenzene-cored polyphenylene dendrimers have been synthesized. The photoresponsive properties of these dendrimers have been investigated by optical spectroscopy and gel permeation chromatography (GPC). These polyphenylene dendrimers exhibit a photoresponsive behavior upon UV and visible irradiation, resulting in a reversible and appreciable change of the dendrimer structure. Furthermore, the influence of dendrimer generation and degree of branching on the kinetic characteristics of the photoisomerization is discussed. The crystal structures of two precursors and two solvate structures of the first generation are presented.

The light stability of two 3,4-ethylenedioxythiophene-based derivatives, i.e. the oligomeric compound 5,5″-diphenyl-2,2′-5′,2″-ter(3,4-ethylenedioxythiophene) (1, 3-EDT) and the corresponding polymer of 2-tetradecyl-2,3-dihydro-thieno[3,4-b][1,4]dioxine (2, PEDT-C14), was investigated. Their stability upon irradiation with a xenon lamp was studied by means of UV-Vis spectroscopy. These measurements lead to the proposal of a photooxidation mechanism that seems to be operative in chlorinated solvents, followed by H-abstraction. Furthermore, the experiments point out that it is mainly UV light that causes the instability of the oxidatively doped compounds.

The time-resolved fluorescence kinetics of a first and a second generation dendrimer with peryleneimide chromophores at the rim and a terrylenediimide chromophore in the core were investigated using femtosecond fluorescence upconversion and single photon timing techniques. By global analysis of an extentensive data set with λem ranging from 540 to 780 nm, combining both techniques and comparing the kinetic results obtained previously for peryleneimide and terrylenediimide reference compounds, two independent energy transfer processes could be identified. The time constants were 4 and 25 ps for the first generation dendrimer and 22 and 68 ps for the second generation, respectively. They scale as expected for Forster energy transfer processes between chromophores at the distances present in the dendrimers. Each of these processes can possibly be related to different isomers present in the samples. Analysis of the in-the-bay area substituted terrylenediimide model compound reveals subsets of molecules showin...

Self-assembled nanostructures of various polyphenylene dendrimers, prepared by drop casting dendrimer solutions on the surface of highly oriented pyrolytic graphite (HOPG), have been investigated by noncontact atomic force microscopy (NCAFM). Besides forming globular clusters, all the polyphenylene dendrimers studied self-assemble into micrometer long nanofibers irrespective of their cores (e.g., tetraphenylmethane, biphenyl, or azobenzene). The effect of the dendrimer generation and the degree of branching on the formation of dendrimer nanofibers is studied.

Polyphenylene dendrimer molecules, G2Td(COOH)16, were ionized (G2 ions) and then adsorbed on a carboxylic acid (COOH) group terminated self-assembled monolayer (SAM) on a Au substrate through the linkage with Cu2+ ions. The individual G2 ions were observed by noncontact atomic force microscopy (NCAFM). The strong interaction between the G2 and Cu2+ ions, the latter of which are preadsorbed on the COOH SAM, led to a compression or deformation of the SAM and resulted in a decreased height of the G2 ions measured by NCAFM as compared to a theoretical model of the dendrimer molecules. The present method offers an approach to bind dendrimer molecules to a solid substrate and to study the nanoscale behavior of two kinds of thiol molecules in replaced and mixed SAMs.

The detection of single molecules is first exemplified for a model compound and then applied to a macromolecular system of a dendrimer with multiple chromophores at the rim.

Intramolecular kinetic processes in a series of second-generation polyphenyl dendrimers with multiple peryleneimide chromophores attached to the para position of the outer phenyl ring were investigated by steady-state and femtosecond to nanosecond time-resolved fluorescence spectroscopy. The results obtained were compared to the ones of the corresponding first-generation dendrimer series. The energy-hopping rate constant, khopp, observed from anisotropy decay times was found to be 5 times smaller than that of the first-generation series and scales well with the difference in average distance between the chromophores. In addition to the processes observed in first-generation dendrimers in the ultrafast time domain by fluorescence up-conversion, a second annihilation process is found in the second-generation multichromophoric dendrimer. The observation of two singlet-singlet annihilation processes in this compound can be explained by the presence of a mixture of constitutional isomers leading to a broader distribution of distances between neighboring chromophores compared to first-generation multichromophoric dendrimers.

The aggregation properties of derivatives of linear trans-quinacridone, an archetypal pigment, have been explored in two and three dimensions. The sparse solubility of the parent quinacridone pigment in common organic solvents can be traced back to the formation of a network of hydrogen bonds. Introducing aliphatic substituents on specific positions of the quinacridone core leads to an increased solubility without hampering the formation of intermolecular hydrogen bonds. With UV−Vis absorption, steady-state fluorescence, and infrared spectroscopy, the aggregation behavior of these substituted quinacridone derivatives is studied. On the basis of these data, models are proposed for the aggregate structure in solution. In addition, the two-dimensional ordering on graphite of these compounds and N,N‘-dialkylated analogues has been investigated with scanning tunneling microscopy. The comparison between the two-dimensional organization and the solution data allows one to identify the extent of intermolecular in...

The kinetics of a newly synthesized series of a first generation of polyphenylene dendrimers in which one phenyl in a dendritic arm was para-substituted by a peryleneimide chromophore are reported. One such peryleneimide chromophore is attached to 1, 3 or 4 arms. The results are compared to a series of polyphenylene dendritic compounds, which are identical except for the substitution at a meta-position of a phenyl ring. The para-substitution yields a better spatial definition of the peryleneimide units relative to one another and the influence of this aspect on the kinetics is studied. Four different kinetic components were resolved for both groups of dendrimers. An ultra-short component varying from 500 fs to 2 ps and attributed to intramolecular vibrational redistribution (IVR) is identical for both series. The decay time of a second component, which is comprized of the vibrational relaxation and a singlet–singlet annihilation process observed in both substituted dendrimer series, is shorter in the para-substituted dendrimers compared to the meta-substituted ones. It is also shown that the annihilation process, which is only present in the multi-chromophoric compounds of both the series and resolved with an excitation energy dependent study, has definitely a larger contribution in the partial amplitudes for the para-substituted compounds. This is related to the relative orientation of the transition dipoles of the chromophores in both the series.

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

The photochemical dimerization of two cinnamate derivatives, C18CinnC18 and C18CinnC10, has been investigated at the liquid/graphite interface by scanning tunneling microscopy (STM). The unit cell parameters of the two derivatives as deduced from the STM images indicate that the photodimerization reaction should not occur according to the topochemical postulates. Nevertheless, C18CinnC18 does photodimerize upon irradiation of its monolayer, while C18CinnC10 does not.

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

In this contribution we investigated the two-dimensional (2D) supramolecular organization and electronic properties of two bis(urea)-substituted oligothiophene derivatives, containing two or three thiophene units (T2 and T3, respectively), at the solution/graphite interface with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Because of the π-stacking of the oligomers the observed zero conductance band gap in the I(V) curves of a ribbon is found to be considerably smaller than for an isolated oligothiophene molecule, indicating that there exists an effective conjugation in the π-stacked ribbons on the surface.

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

Physisorbed monolayer films of a chiral terephthalic acid derivative have been imaged on highly oriented pyrolytic graphite (HOPG) at the solution−substrate interface using scanning tunneling microscopy. The molecule comprises a nonchiral aromatic moiety and a chiral handle. It is found to form several two-dimensional polymorphs. The packing arrangement of the molecule is primarily directed by the aromatic part, whereas the chiral part may assume different conformations on the graphite surface. The high-resolution data confirm that for all polymorphs molecular chirality is transferred to the two-dimensional adlayer structure in an enantiospecific way. This enantiospecificity is attributed to adsorbate−substrate interactions.

Ultrathin organic multilayers, consisting of polyelectrolytes and J-aggregates of a cyanine dye, were obtained using the layer by layer alternate adsorption technique. Buildup of the multilayer is followed by UV−vis absorption spectroscopy. The fluorescence properties of the J-aggregates are studied by steady-state fluorescence spectroscopy and picosecond fluorescence decay measurements. Fluorescence of the cyanine dye, when adsorbed on the substrate, results in a narrow emission band with a small Stokes shift which is characteristic for J-aggregate emission. Time-resolved fluorescence measurements yield a very short average decay time, which can be attributed to the J-aggregates. These results describing the fluorescence properties of multilayer assemblies containing J-aggregates allow one to compare them to those of J-aggregates of the cyanine dye in solution.

Spectra of single molecules of different rhodamine dyes deposited on a glass surface and on a methyl-terminated silanised glass surface were recorded by means of confocal fluorescence microscopy. The observation of an emission, blue shifted in comparison to the solution spectrum of these rhodamine dyes, is reported. It is suggested that this shorter wavelength emission is related to the contact ion pair formation of the single rhodamine dye molecule.

The photophysical and hydrodynamic properties of dendrimers (GnPZn and GnTPPH2) with zinc porphyrin (PZn) and tetraphenylporphyrin (TPP) cores are studied in tetrahydrofuran (THF) and dimethylformamide (DMF). UV-vis absorption spectra of GnPZn exhibit a small red shift of the Soret band upon increasing the generation as a result of interactions between the dendrons and the core. All fluorescence decays obtained from global analysis show a monoexponential profile. The intrinsic viscosity obtained for GnPZn from the hydrodynamic volume (Vh) passes through a maximum as a function of generation (G) in agreement with earlier experimental findings and calculations suggesting that the internal density profile of dendrimers decrease monotonically outward from the center of the molecule. Within the investigated range (G ) 1-3), GnTPPH2 exhibits an approximately constant intrinsic viscosity due to the linear dependence between the hydrodynamic volume and the molecular weight. The differences observed between GnPZn and GnTPPH2 are correlated to structural differences in their cores. The additional phenyl group of the TPP in GnTPPH2 increases the distance between the branches and the porphyrin moiety compared to GnPZn, resulting in a more flexible structure. The enhanced flexibility allows the terminal groups to sample more conformational space and therefore decreases the volume of the dendrimer as compared to the theoretical fully extended structure where Vh ∝ G 3 . A comparison of the results obtained from analysis of fluorescence anisotropy decays with previously reported viscometry measurements shows a dependence of the structural collapse on the core size.

In this contribution we report on a structural investigation of the two-dimensional (2D) supramolecular organization of three bis-urea substituted thiophene derivatives, containing one, two, or three thiophene units, at the solution/graphite interface with scanning tunneling microscopy (STM). The compounds under investigation form highly ordered physisorbed monolayers. It is found that hydrogen bonding between the urea groups of adjacent molecules controls the spatial arrangement on the graphite surface. Molecular modeling and theoretical calculations demonstrate that the thiophene rings are tilted with respect to the surface and have partially overlapping π-systems. This control of the 2D self-assembly is promising for future studies on the electronic properties of these molecules.

Higher generations of poly(propylene imine) dendrimers functionalized with aliphatic chains form large micrometer-sized spherical objects in aqueous solution below pH 8. These spheres are giant vesicles with a multilaminar onion-like structure. The size distribution and the structure of the vesicles depend on the pH of the solution and the endgroups at the periphery of the dendrimer. The vesicles containing azobenzene units (2 and 3) fluoresce with a maximum at ¿max = 600 nm. This emission can be attributed to the dense and ordered arrangement of the azobenzene chromophores in the bilayer structure. Laser irradiation of a small area of giant vesicles of 2 or 3 with 1064 and/or 420 nm light leads to changes in the morphology of the vesicles. Infrared light induces a rearrangement, whereas the azobenzene units isomerize under the influence of 420 nm light. Both irradiations lead to a change in refractive index in the illuminated area. Irradiation using 420 nm light is accompanied by an increase in the emission intensity. In aqueous solutions at pH 1, the increase in fluorescence intensity is concurrent with a blue shift of the emission maximum to 540 nm. This blue shift is not observed when the experiment is performed in Milli Q water (pH 5.5). The enhanced fluorescence can be attributed to reorganization of the chromophores within the giant vesicle. The increase in emission proves that the giant vesicle is a kinetically formed system that reaches a thermodynamically more relaxed state after light-induced isomerization.

A semifluorinated isophthalic acid derivative and its mixtures with a number of nonfluorinated analogues have been investigated with scanning tunneling microscopy (STM) at the solution−graphite interface. The specific contrast arising from fluorine atoms in STM images allows us to use this functionality as a probe to aid in the analysis of the data obtained for the mixtures under investigation. Unlike the findings reported for other systems, we did not observe any segregation of fluorinated and nonfluorinated alkyl segments into microdomains that can be attributed to the incompatibility between the two components.

Electron Transfer Reactions in Micelles: Reactivity of Pyrene and Tris(2,2'-bipyridyl)ruthenium(II) Excited States Investigated by Time-Resolved Fluorescence Quenching

alpha.-Terthiophene in Micellar Solutions: Influence of Micellar Size and Charge on the Terthiophene Photophysics and Photochemical Reactivity

Fiber-shaped aggregates were obtained from aqueous solutions of 5-octadecyloxyisophthalic acid neutralized with 1.5 equiv of KOH. The structure of the aggregates in the resulting suspensions was investigated by X-ray and electron diffraction (ED) studies; the morphological observations reported here stem from atomic force microscopy (AFM) studies and from polarized optical microscopy (OM). The aggregate formation was examined as a function of the concentration and temperature of the isophthalate solutions or suspensions. The structure elucidation shows that the aggregates consist of crystals as the smallest subunits. Their structure was determined, and the cell parameters of the unit cell could be established. Furthermore, it is demonstrated that the material obtained from the aqueous suspensions, by pulling fiber-shaped species mechanically, is a continuous sequence of crystals.

Physisorbed monolayer films of both enantiomers of a chiral terephthalic acid derivative, 2,5-bis[[10-(2-methylbutoxy)decyl]oxy]terephthalic acid, have been characterized on graphite (HOPG) at the ...

Fast electronic processes taking place immediately after excitation of sensitized silver halide emulsions were studied with picosecond time-correlated single-photon counting (SPC) and femtosecond transient absorption (TA). The fluorescence decays of the J aggregate obtained by SPC measurements were also analyzed as a sum of three exponentials. For the J aggregates of a benchmark dye, a dimethyl-μ-ethyl-thiacarbocyanine, a component with a 20 ps decay time, which was absent in samples containing no AgBr crystals, was attributed to J aggregates of the sensitizer adsorbed on the AgBr microcrystals. The fluorescence decay results were compared with photographic quantum yields in an attempt to show the relevance of the spectroscopic measurements. The second component (150 ps) is a combination of fluorescence decays of the unaggregated sensitizer molecules not adsorbed on the AgBr microcrystals and monomers of the dye on AgBr surface. The fluorescence decay was found to be independent of the crystal coverage. P...

Nonamphiphilic N,N‘-bis(2,6-dimethylphenyl)-3,4,9,10-perylenetetracarboxylic diimide (DMPI) has been incorporated in Langmuir−Blodgett (LB) films mixed with stearic acid (SA). Steady-state spectroscopic studies of the LB-deposited films on quartz substrates indicate an extensive broadening and shift of the absorption and emission spectra, suggesting molecular aggregation. Perhaps the most interesting spectral feature observed in this study is the observation of a broad emission band with its maximum at 650 nm. Concentration dependent studies demonstrate that, with increasing mole fraction of DMPI in the mixed film, the broad band emission at 650 nm increases in intensity while the higher energy band at 570 nm decreases. Excitation spectra corresponding to the two emission bands at 570 and 650 nm are observed to be different, indicating that the species corresponding to the emission at 570 and 650 nm are different. One plausible explanation is that the band at 570 nm corresponds to the monomeric species wh...

Coupling of photophysical and photochemical techniques to microscopy eventually assisted by manipulating techniques, such as laser trapping, has facilitated obtaining information on heterogeneous organic and bio-organic systems by mapping their optical and excited state properties. Scanning confocal microscopy, eventually of laser trapped ensembles, coupled to fluorescence decay analysis and imaging, scanning plate confocal and scanning near field optical microscopy provide combined spectral and spatial resolution down to a few tenths of nanometers. An even better resolution can be achieved using scanning tunneling microscopy. In this contribution a number of organic and macromolecular systems are discussed first in solution and in a next step assembled either in a trap or at a surface. The techniques are illustrated and their limits assessed using latex particles labeled with fluorophores. Time resolved spectroscopy in solution allows the evaluation of migration of the excited state and the collapse of the arms in a dendritic structure. These and other macromolecular structures can be trapped and the obtained assembly visualized and analyzed. Deposition by self-assembly provides the possibility using scanning near field optical microscopy to investigate the excited state properties of ordered arrays. By dilution in a polymer film of dendritic structures single particle, single chromophore and single molecule spectroscopy becomes accessible. Scanning tunneling microscopy is successfully applied to illustrate the visualization and manipulation of structures with subnanometer resolution and the study of their properties including stimulus by light induced transformations.

A new type of porphyrin array has been synthesized by the coupling of six porphyrin moieties to a central benzene core via an ether linkage. The resulting porphryin supermolecule has a diameter up to 80 A and a mass of 8500 daltons. In solution, the six porphyrins around the central benzene ring arrange themselves into three sets of offset overlapping dimers, which are rapidly interconverting at room temperature. Solution UV−vis and fluorescence studies, however, indicate that there are no electronic interactions between the individual porphyrin molecules. Upon spreading a chloroform solution of these porphyrin molecules on a surface, they self-assemble to form ring-shaped architectures on a micrometer scale. Near-field scanning optical microscopy studies reveal that the porphyrin moieties within the rings have an ordered arrangement with respect to their position in the ring after the sample has been annealed at 80 °C for 2 days.

The excited-state relaxation processes of an amino-substituted triphenylbenzene derivative (p-EFTP) and its biphenyl-model compound (p-EFBP) were investigated by femtosecond polarized transient absorption spectroscopy. The S 1 –S n absorption was detected upon pumping at 360 nm and probing at 470 nm. Both compounds presented a polarization-dependent component in the transient decay. For p-EFBP a decay time of 97±20 ps was obtained which agrees with the rotational diffusion time determined by time-resolved fluorescence anisotropy. For the symmetric p-EFTP a decay time of 8±2 ps was detected. This value correlates with the intramolecular dipole reorientation time deduced from time-resolved microwave conductivity measurements.

The photosensitized injection of holes from an adsorbed rhodamine dye into a polycarbonate film, molecularly doped with 5′-[4-[bis(4-ethylphenyl)amino]phenyl]-N,N,N′,N′-tetrakis(4-ethylphenyl)-[1,1′:3′,1″-terphenyl]-4,4″-diamine, has been investigated using photoconductivity. The small quantum yields for sensitized hole injection as well as their dependence upon the applied field are attributed to the competition between the escape of the injected holes and recombination with the reduced molecules at the surface. While in a first approach the experimental results can be explained by extending the Willig–Gerischer model, developed for organic single crystals, to doped polymers, a more accurate description of this system also requires the consideration of the diagonal and non-diagonal disorder of the hole transport material.