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Ruddlesden–Popper phase quasi-2-D organolead halide perovskites like n-butylammonium methylammonium lead iodide (nBA-MAPI) exhibit improved moisture stability over typical 3-D perovskite materials,...

With more than 10 times the capacity of the graphite used in current commercial batteries, lithium metal is ideal for a high-capacity battery anode; however, lithium metal electrodes suffer from safety and efficiency problems that prevent their wide industrial adoption. Their intrinsic high reactivity towards most liquid organic electrolytes leads to lithium loss and dendrite growth, which result in poor efficiency and short circuiting. However, the multitude of factors that contribute to dendrite formation make determining a nucleation mechanism extremely difficult. Here, we study the intricate science of dendrite nucleation on metallic lithium by using an array of analytical techniques that provide simultaneous ultra-high spatial sensitivity and chemical selectivity. Our results reveal a 3D picture of the chemical make-up of the native Li surface and the resulting solid electrolyte interphase (SEI) with better than 200 nm resolution. We find that, contrary to the general understanding, the initial surface chemistry, not the topography, is the dominant factor leading to dendrite nucleation. Specifically, inhomogeneously distributed organic material in the native surface leads to inhomogeneously dispersed LiF-rich SEI regions where dendrite nucleation is favored. This has significant implications for battery research as it elucidates a mechanism for inhomogeneous SEI formation, something that is accepted, but not well understood, and highlights the importance of Li surface preparation for experimental studies, which is implicit in battery research, but not directly addressed in the literature. By homogenizing the initial lithium surface composition, and thus the SEI composition, we increase the number of dendrite nucleation sites and thereby decrease the dendrite size, which significantly increases the electrode lifetime.

This contribution describes the design and synthesis of β-sheet-mimicking synthetic polymers comprising distinct poly(p-phenylene vinylene) (PPV) and poly(norbornene) (PNB) backbones with multiple turns. The rod–coil–coil–rod tetrablock copolymers, synthesized using ring-opening metathesis polymerization (ROMP) and featuring orthogonal face-to-face π–π stacking and phenyl/perfluorophenyl interactions, show persistent folding both in bulk and at the single molecule level, irrespective of the number of β-turns. Single molecule polarization studies reveal that the copolymers are more anisotropic than the corresponding homopolymers. Examination of the spectral signatures of the single molecules shows a dominant emissive chromophore in the linked materials compared to the homopolymer. The lack of significant spectral changes of the folded materials along with the existence of a dominant emission spectrum supports the proposed structure of well-aligned, minimally-interacting chromophores. Utilization of this reliably folding, phenyl/perfluorophenyl functionality could provide an extremely useful tool in future functional materials design.

A definitive comprehension of morphology and photophysics in conjugated polymers at multiple length scales demands both single molecule spectroscopy and well-controlled molecular architectures., Conjugated polymers (CPs) possess a wide range of desirable properties, including accessible energetic bandgaps, synthetic versatility, and mechanical flexibility, which make them attractive for flexible and wearable optoelectronic devices. An accurate and comprehensive understanding about the morphology–photophysics relations in CPs lays the groundwork for their development in these applications. However, due to the complex roles of chemical structure, side-chains, backbone, and intramolecular interactions, CPs can exhibit heterogeneity in both their morphology and optoelectronic properties even at the single chain level. This molecular level heterogeneity together with complicated intermolecular interactions found in bulk CP materials severely obscures the deterministic information about the morphology and photophysics at different hierarchy levels. To counter this complexity and offer a clearer picture for the properties of CP materials, we highlight the approach of probing material systems with specific structural features via single molecule/aggregate spectroscopy (SMS). This review article covers recent advances achieved through such an approach regarding the important morphological and photophysical properties of CPs. After a brief review of the typical characteristics of CPs, we present detailed discussions of structurally well-defined model systems of CPs, from manipulated backbones and side-chains, up to nano-aggregates, studied with SMS to offer deterministic relations between morphology and photophysics from single chains building up to bulk states.

We demonstrate that bisulfite can be used for reduction of a highly electrophilic anthraquinone derivative, N,N'-dimethyl-2,6-diaza-9,10-anthraquinonediium (DAAQ), and subsequent autoxidation generates an equivalent of hydrogen peroxide. The mechanism for DAAQ reduction by bisulfite, DAAQ electrochemistry, and use of a simple test strip assay for H2O2, are described.

With power conversion efficiencies (PCEs) of

In this manuscript, we report a design strategy to control polychromophore polymer folding at the single molecule level through hydrogen-bonding (H-bonding) interactions. Polymers composed of bis(2-ethylhexyl)-p-phenylene vinylene (BEH-PPV) oligomers (trimers) containing H-bonding capable side-chains (e.g., carboxylic acid or urea units) were prepared by copolymerization with flexible linkers. Control polymers with masked H-bonding side-chains (e.g., t-butyl ester) were also prepared. Different polymer folding schemes are proposed including random or non-folding for the control polymers, and side-on and face-on geometries for the carboxylic acid and urea motifs, respectively. Single molecule excitation polarization spectroscopy demonstrates that the highest folding order is achieved with the urea containing side-chains and isotropic folding is found in polymers that do not contain H-bonding units. We propose that the effects of the chromophore conformational geometries and the type of H-bonding are additive and lead to highly aligned urea-containing polymers. In addition, an examination of the individual polymers’ spectral signatures via single molecule spectroscopy showed an overall red-shift of ∼0.06 eV in the 0–0 origin peak for the urea-containing polymer compared to the other two polymer systems. This spectral shift is attributed to planarization of the backbone and further supports the presence of a highly ordered urea-containing polymer structure. Utilization of this H-bonding inclusion synthetic strategy to control polymer secondary structure could provide important design elements for future functional material design.

Push–pull conjugated polymers have shown high performance in organic photovoltaic devices and yet, there have been fewer studies of the fundamental structure–property relationship in this class of materials compared to prototypical conjugated homopolymers MEH-PPV and P3HT. Here we report a single molecule study of a push–pull polymer poly[N-(1-octylnonyl)-2,7-carbazole]-alt-5,5-[4′,7′-di(thien-2-yl)-2′,1′,3′-benzothiadiazole] (PCDTBT) to understand the origin of the substantial band gap narrowing and the sensitivity of peak position to chain length even for long polymer chains. In bulk solution, the peak of the emission spectrum was observed to shift from a higher energy emission state to a lower energy state above a critical molecular weight. This same trend was observed at the single molecule level where there were two clear emission species. At low MW the single molecules showed emission predominately from the higher energy state. Above the critical molecular weight, the single molecules changed and showed emission from the low energy species. At the transition MW, a mixture was observed with molecules emitting from one or the other of the two species. The transition from the high energy to low energy state at a particular molecular weight is likely the result of substantial intermolecular self-interactions between chain segments that leads to excitation delocalization. This is made possible at even higher molecular weight and lengths where there are more energy transfer pathways for the already long polymer chains.

Plastic photovoltaic devices (PVs) were fabricated by spray-depositing copper indium diselenide (CuInSe2) nanocrystals into micrometer-scale groove features patterned into polyethylene terephthalate (PET) substrates. Each groove has sidewall coatings of Al/CdS and Au and performs as an individual solar cell. These PV groove features can be linked electrically in series to achieve high voltages. For example, cascades of up to 15 grooves have been made with open-circuit voltages of up to 5.8 V. On the basis of the groove geometry, the power conversion efficiencies (PCEs) of the devices reached as high as 2.2%. Using the active area and photovoltaic response of devices determined from light-beam-induced current (LBIC) and photoreflectivity measurements gave PCE values as high as 4.4%.

The synthesis and morphological investigation of a series of polychromophore polymers composed of oligomeric bis(2-ethylhexyl)-p-phenylenevinylene (BEH-PPV) (trimer, pentamer, and heptamer) monomers copolymerized with rigid morphological directing groups (morphons) are described. The polymerization was carried out using a Sonogashira cross-coupling polymerization between monomers composed of an iodo-terminated BEH-PPV oligomer and a bis(phenylacetylene)-containing morphon. The rigid morphons are prepared from adamantane and diamantane frameworks that are composed solely of sp3 carbons, inhibit conjugation between BEH-PPV oligomers, and direct the local polymer morphology in either a bent or linear vector, respectively. The morphological properties of the polychromophore polymers were interrogated via single molecule fluorescence spectroscopy, thin-film absorption and fluorescence spectroscopy, and atomic force microscopy. Significant morphological variation was found upon substituting the morphon, as well...

Lateral bulk heterojunctions (LBHJ) provide a tool to directly probe the active area of photovoltaic devices using microscopy techniques. Here, we use scanning photocurrent microscopy (SPCM) to probe an organic photovoltaic (OPV) device with poly[{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl} (PTB7):[6,6]phenyl-C61-butyric acid methyl ester (PC 61 BM). The effects of the additive 1,8-diiodooctane (DIO) on the recombination dynamics and morphology are probed in real space in a LBHJ structure. By using SPCM, we can see a larger increase in the space charge region for samples with DIO when compared to those without DIO. This indicates that the additive improves film morphology leading to increased charge extraction efficiency and decreased recombination.

Recent experiments have reported that the self-assembly of conjugated polymers mimicking rod-coil-rod triblock copolymers (BCPs) in selective solvents exhibits unique aggregate morphologies. However, the nature of the arrangement of the polymers within the aggregates and the spatial organization of the aggregates remain an unresolved issue. We report the results of coarse-grained Langevin dynamics simulations, which investigated the self-assembly behavior of rod-coil-rod BCPs in a coil-selective solvent. We observe a rapid formation of cylindrically shaped multichain clusters. The initial stages of formation of the aggregates was seen to be independent of the strength of the solvent selectivity. However, for higher solvent selectivities, the clusters were seen to merge into larger units at later stages. A reduction in rod to coil block ratio was observed to decrease the size and number of clusters. In the limit of a highly concentrated solution, we observe the formation of a networked system of distinct clusters, which however retain the cylindrical arrangement observed at lower polymer concentrations.

The temperature dependence of the steady-state excitonic absorption and emission spectral features of the J-aggregates of the amphiphilic cyanine dye 3,3′-bis(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) was examined over the temperature range from 77 to 298 K. Two C8S3 J-aggregate structures were investigated: well-separated, double-walled nanotubes and bundles of agglomerated nanotubes that spontaneously assemble over long periods of storage. Absorption and emission spectral line broadening and Stokes shift are presented, and the responses of both aggregates are evaluated as a function of temperature. Both J-aggregates exhibit two fluorescence bands. We found that, across the measured temperature range, the ratio of the nanotube’s emission bands is well described with Boltzmann statistics, while that of the bundles is not. Additionally, the relative quantum yield of the nanotubes increased dramatically upon cooling, while the bundles’ quantum yield exhibited a significan...

Exciton transport lengths in double-walled and bundled cylindrical 3,3'-bis- (2-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimida-carbocyanine (C8S3) J-aggregates were measured using direct imaging of fluorescence from individual aggregates deposited on solid substrates. Regions identified in confocal images were excited with a focused laser spot, and the resulting fluorescence emission was imaged onto an electron multiplying charged coupled device camera. A two-dimensional Gaussian fitting scheme was used to quantitatively compare the excitation beam profile to the broadened aggregate emission profiles. The double-walled tubes exhibit average exciton transport lengths of 140 nm, while exciton transport in the bundled nanotubes was found to be remarkably long, with distances reaching many hundreds of nanometers. A steady-state one-dimensional diffusion model for the broadening of the emission profiles yields diffusion coefficients of 120 nm(2) ps(-1) for the nanotubes and 7000 nm(2) ps(-1) for the aggregate bundles. The level of structural hierarchy dramatically affects the exciton transport capabilities in these artificial light-harvesting systems, and energy migration is not limited to a single dimension in J-aggregate bundles.

Mechanically sensitive biocomposites comprised of fluorescent proteins report stress through distinct pathways. Whereas a composite containing an enhanced yellow fluorescent protein (eYFP) exhibited hypsochromic shifts in its fluorescence emission maxima following compression, a composite containing a modified green fluorescent protein (GFPuv) exhibited fluorescence quenching under the action of mechanical force. These ratio- and intensiometric sensors demonstrate that insights garnered from disparate fields (that is, polymer mechanochemistry and biophysics) can be harnessed to guide the rational design of new classes of biomechanophore-containing materials.

A fully consistent model for the exciton band structure of double-walled 3,3′-bis(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) J-aggregates was developed using reduced linear dichroism (LDr) spectroscopy on flow aligned samples. Chemical oxidation was utilized to “turn off”outer wall optical absorption and produce stable aggregate samples with a simplified absorption profile associated only with the nanotube inner wall. The oxidized aggregates were aligned in a flow cell to collect LDr spectra; these spectra reveal a series of both polarized and isotropic transitions. Four spectral transitions, assigned to be purely parallel or perpendicular to the aggregate long axis, that fit both the experimental LDr and isotropic spectra were used create a model for oxidized J-aggregate excitonic absorption. The LDr spectral study was repeated using pristine J-aggregates, and the spectrum for the full double-walled J-aggregates could be fit using six total transitions: four from the ox...

It has been proposed that interface morphology affects the recombination rate for electrons and holes at donor-acceptor heterojunctions in thin film organic photovoltaic cells. The optimal morphology is one where there is disorder at the heterointerface and order in the bulk of the thin films, maximizing both the short circuit current and open circuit voltage. We show that an amorphous, buried functionalized molecular squaraine donor layer can undergo an "inverted" quasi-epitaxial growth during postdeposition processing, whereby crystallization is seeded by a subsequently deposited self-assembled nanocrystalline acceptor C60 cap layer. We call this apparently unprecedented growth process from a buried interface "inverse quasi-epitaxy" where the crystallites of these "soft" van der Waals bonded materials are only approximately aligned to those of the cap. The resulting crystalline interface hastens charge recombination, thereby reducing the open circuit voltage in an organic photovoltaic cell. The lattice registration also facilitates interdiffusion of the squaraine donor and C60 acceptor, which dramatically improves the short circuit current. By controlling the extent to which this crystallization occurs, the voltage losses can be minimized, resulting in power conversion efficiencies of ηP = 5.4 ± 0.3% for single-junction and ηP = 8.3 ± 0.4% for tandem small-molecule photovoltaics. This is a general phenomenon with implications for all organic donor-acceptor junctions. That is, epitaxial relationships typically result in a reduction in open circuit voltage that must be avoided in both bilayer and bulk heterojunction organic photovoltaic cells.

The power conversion efficiency of photovoltaic devices made with ink-deposited Cu(InxGa1-x)Se2 (CIGS) nanocrystal layers can be enhanced by sintering the nanocrystals with a high temperature selenization process. This process, however, can be challenging to control. Here, we report that ink deposition followed by annealing under inert gas and then selenization can provide better control over CIGS nanocrystal sintering and yield generally improved device efficiency. Annealing under argon at 525 °C removes organic ligands and diffuses sodium from the underlying soda lime glass into the Mo back contact to improve the rate and quality of nanocrystal sintering during selenization at 500 °C. Shorter selenization time alleviates excessive MoSe2 formation at the Mo back contact that leads to film delamination, which in turn enables multiple cycles of nanocrystal deposition and selenization to create thicker, more uniform absorber films. Devices with power conversion efficiency greater than 7% are fabricated using the multiple step nanocrystal deposition and sintering process.

An elegant method for the electrodeposition of MoS2 thin films using room temperature ionic liquids (RTIL) as an electrolyte was developed. Simple molecular precursors of Mo and S were added in different concentrations to tune the composition and deposition process. The electrodeposition of MoS2 was confirmed with both Raman spectroscopy and XPS. Analysis showed that the electrodeposited MoS2 films form a flower shape morphology with edge active sites that promote the hydrogen evolution reaction (HER). Furthermore, this technique enables selective tuning of the film thickness and demonstrates high photoluminescence activity with a decrease in the number of layers.

Single-molecule excitation polarization anisotropy and molecular dynamics simulations reveal that the folding of polymers composed of conjugated phenylene vinylene (PPV) oligomers joined by flexible linkers can be influenced by the length of the conjugated segments. By varying the number of PPV repeat units from three to five to seven, both the structure and the spectral properties of the polymer can be controlled at the synthetic level. The stronger interactions between longer conjugated units of the polymers lead to more ordered conformations. The mean modulation depth of a septamer containing PPV (M = 0.75) was found to be even higher than that of the traditional homopolymer MEH-PPV (M = 0.66), which suggests that these new polymers provide access to highly aligned nanostructures not typically found in homopolymer systems.

Herein we describe the use of regioregular (rr-) and regiorandom (rra-) P3HT as models to study energy transfer in ordered and disordered single conjugated polymer chains. Single molecule fluorescence spectra and excitation/emission polarization measurements were compared with a Förster resonance energy transfer (FRET) model simulation. An increase in the mean single chain polarization anisotropy from excitation to emission was observed for both rr- and rra-P3HT. The peak emission wavelengths of rr-P3HT were at substantially lower energies than those of rra-P3HT. A simulation based on FRET in single polymer chain conformations successfully reproduced the experimental observations. These studies showed that ordered conformations facilitated efficient energy transfer to a small number of low-energy sites compared to disordered conformations. As a result, the histograms of spectral peak wavelengths for ordered conformations were centered at much lower energies than those obtained for disordered conformations. Collectively, these experimental and simulated results provide the basis for quantitatively describing energy transfer in an important class of conjugated polymers commonly used in a variety of organic electronics applications.

In contrast to the detailed understanding of inorganic materials, researchers lack a comprehensive view of how the properties of bulk organic materials arise from their individual components. For conjugated polymers to eventually serve as low cost semiconductor layers in electronic devices, researchers need to better understand their functionality. For organics, traditional materials science measurements tend to destroy the species of interest, especially at low concentrations. However, fluorescence continues to be a remarkably flexible, relatively noninvasive tool for probing the properties of individual molecules and allows researchers to carry out a broad range of experiments based on a relatively simple concept. In addition, the sensitivity of single-molecule spectroscopy allows researchers to see the properties of an individual component that would be masked in the bulk phase. In this Account, we examine several photophysical properties of different conjugated polymers using single-molecule spectroscopy. In these experiments, we probed the relationship between the conformation of single conjugated polymer chains and the distance scale and efficiency of energy transfer within the polymer. Recent studies used polarization anisotropy measurements on single polymer chains to study chain folding following spin-casting from solution. This Account summarizes the effects of monomer regioregularity and backbone rigidity, by comparing a regiorandom phenylene vinylene (MEH-PPV) with both a regiorandom and regioregular thiophene (P3HT). Synthesis of novel polymers allowed us to explore the role of different conformation-directing inclusions in a PPV backbone. We showed that these inclusions control the conformation of individual chains and that molecular dynamics can predict these structural effects. In situ solvent vapor annealing studies explored the dynamics of polymer chains as well as the effect of solvent evaporation on the structural equilibrium of the polymer. We observed that a slower rate of solvent evaporation results in a narrow population of highly ordered polymer chains. These highly ordered single chains serve as a model system to probe the effect of conformation on energy transfer following excitation in single MEH-PPV polymer chains in two distinct experiments. In the first, we correlated the anisotropy of the fluorescence emission of individual chains with the anisotropy of their fluorescence excitation. Using this data, we derived a model for energy transfer in a conjugated polymer, simulating chromophores along a chain, coupled via Förster energy transfer. In the second experiment, super-resolution measurements demonstrated the ability of single-molecule spectroscopy to directly visualize energy transfer along a polymer chain embedded in a model device environment. A capacitive device allowed for controlled localization of hole polarons onto the polymer chain. These positive charges subsequently quenched local excitations, providing insight into the range of energy transfer in these single polymer molecules. As researchers continue to characterize conjugated polymer films and develop methods for creating multichain systems, single-molecule techniques will provide a greater understanding of how polymer morphology influences interchain interactions and will lead to a richer description of the electronic properties of bulk conjugated polymer films.

Spectrophotometric titration and a binding isotherm were used to accurately assess the loading capacity of generation four polyamido(amine) (PAMAM) dendrimer templates with terminal alcohol groups (G4-OH). Preparation of bimetallic G4-OH dendrimer-encapsulated metal nanoclusters (DENs) necessitates knowledge of the precise metal-ion binding capacity. The binding of metal ions such as Pt(2+) and Pd(2+) has proven difficult to assess via UV-vis spectroscopy because the absorbance shifts associated with metal-ion binding within the dendrimer template are masked by the absorbance of the PAMAM dendrimer itself. In contrast, the binding of Cu(2+) to G4-OH PAMAM dendrimer results in a strong, distinct absorption band at 300 nm, making UV-vis spectrophotometric titration with copper straightforward. Here we use copper binding as a means to assess the number of binding sites remaining within the PAMAM G4-OH dendrimer after the complexation of a specified molar excess of Pd(2+) or Pt(2+). In addition, we use a binding isotherm to mathematically estimate the loading capacity of the dendrimer in each case. The loading capacities for M(2+) in the G4-OH dendrimer were found to be ∼16 for copper alone, ∼21 for copper combined with palladium, and ∼25 for copper combined with platinum.

Solution processing of oligothiophene molecules is shown to produce a range of particles with distinct morphologies. Once isolated on a substrate, the optical and electronic properties of individual particles were studied. From polarized scanning confocal microscopy experiments, distinct particles that are identifiable by shape were shown to have similar emission spectra except in regard to the 0-0 vibronic band intensity. This suppression of the 0-0 vibronic band correlates to the amount of energetic disorder present in a weakly coupled H-aggregate. The studied particles ranged from moderate to almost complete suppression of the 0-0 vibronic band when compared to the emission spectrum of the isolated molecule in solution. All particles were found to have a high degree of geometric order (molecular alignment) as observed from the fluorescence dichroism (FD) values of around 0.7-0.8 for all the studied morphologies. The structural and electronic properties of the particles were investigated with Kelvin probe force microscopy (KPFM) to measure the local contact potential (LCP) difference, a quantity that is closely related to the differences in intermolecular charge distribution between the oligothiophene particles. The LCP was found to vary by as much as 70 mV between different oligothiophene particles and a trend was observed that correlated the LCP changes with the amount of energetic disorder present, as signified by the suppression of the 0-0 vibronic peak in the emission spectra. Combined polarized scanning confocal microscopy studies, along with KPFM measurements, help to provide fundamental insights into the role of morphology, molecular packing, and intermolecular charge distributions in oligiothiophene particles.

Graphene offers great potential for electrodes in flexible organic optoelectronic devices. Moreover, it may function as a permeation barrier to protect a device from chemical degradation under ambient conditions. Here, we report on the chemical and structural stability of graphene in situ on a conjugated polymer film. Fluorescence and scanning force microscopies were used to probe the degradation kinetics of the fluorescent polymer protected from ambient by graphene. We demonstrate that defect-free single-layer graphene efficiently protects the polymer from oxygen and water in the ambient, reaching the technological requirements on ultrabarriers, but we also observe a growing number of individual permeable defects in the single-layer graphene resulting from photoinduced structural degradation of the graphene. In contrast, double-layer graphene remains free of permeable defects, which we attribute to the structural independence of the two single layers. This suggests that graphenes can function as both a t...

The effect of molecular weight on the formation of β phase poly(9,9'-dioctylfluorene) (PF8) was studied in dilute solutions. Temperature-dependent fluorescence experiments of unique synthetic batches as well as size-excluded single batches of polyflourene were studied. Each batch had unique molecular weight, tetrahedral defect concentration, and polydispersity index (PDI). Polyflourene was found to exhibit a temperature-dependent transition between two phases with distinct electronic transition signatures: the α (primary) phase and the β (secondary) phase. In dilute solutions, the temperature at which the polymer exhibited a conversion between these phases showed a clear dependence on molecular weight. We model this transition temperature for β phase formation using the mean field theory for the coil-globule transition developed by Isaac Sanchez. Results show that temperature affects the average end-to-end distance corresponding to increases in secondary electronic absorption and that the dependence on temperature related to the coil-globule transition.

The redox chemistry of supramolecular nanotubes self-assembled from amphiphilic cyanine dye 3,3′-bis(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) in aqueous solutio...

The regioregularity of a conjugated polymer can greatly affect bulk film morphologies and properties. However, it remains unclear how regioregularity affects the conformation of isolated individual chains where interchain interactions are absent. Here, the effect of the regioregularity on the conformations adopted by regioregular (rr-) and regiorandom (rra-) poly(3-hexylthiophene) (P3HT) chains was studied using single molecule fluorescence excitation polarization spectroscopy. While every rr-P3HT chain within an ensemble was found to fold into a highly ordered conformation, single rra-P3HT chains adopted a wide variety of conformations, ranging from highly ordered to isotropic. This distribution is likely due to variations in the positions of nonhead-to-tail linkages of the side-chains along the backbone of the different polymer chains. Molecular dynamics simulation on atomistic models of rr- and rra-P3HT chains supports the effect of regioregularity on the collapsed conformations. These results demonstr...

This study investigates superradiant organic dye J-aggregates as a potential new class of aqueous luminophores for electrogenerated chemiluminescence (ECL). Simultaneous cyclic voltammograms (CVs) and ECL transients are obtained from the self-assembled double-walled tubular J-aggregates formed from the amphiphilic cyanine dye 3,3′-bis(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) immobilized on glassy carbon electrodes in the presence of the oxidative−reductive coreactant 2-(dibutylamino)ethanol (DBAE). ECL is produced by both the direct oxidation of DBAE at the electrode and the catalytic oxidation of DBAE by the C8S3 J-aggregates. Optimization studies of the DBAE concentration and pH of the electrolyte show the most intense ECL signal was obtained with ∼17 mM DBAE as coreactant (saturated solution in 1 M KNO3) at pH 12.85, an effect of DBAE solubility and pKa. The overlaid ECL spectrum and the fluorescence spectrum were in good agreement, confirming that the ECL emission ...

The effects of chain conformation on the photo-oxidation and green emission of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) has been investigated at both single-molecule and ensemble levels. Single-molecule studies reveal the conformation of PFO chains to be more globular when cast from THF than from toluene solutions. Intensity transients of the single molecules show that the elongated molecules cast from toluene have more fluctuations due to limited energy transfer along the polymer chains. The more globular chains show monotonic decays with isotropic polarization. Emission spectra of the single molecules show that photochemical oxidation leads to a reduction in the emission of the molecule with no change in the emission spectra. No green emission is ever detected for single molecules, indicating that formation of emissive ketone defects is a rare event. Ensemble studies of chains show that molecules cast from THF develop some green emission upon photodegradation, while those cast from toluene do not. The increase in green emission in the globular molecules suggests that interchain contacts improve energy transfer for emissive ketone defects in the PFO cast from THF.

The redox and light absorption properties of immobilized aggregates of the cyanine dye 3,3'-bis(2-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine (C8S3) on transparent, conductive indium tin oxide (ITO) electrodes have been studied directly using cyclic voltammetry (CV) in conjunction with UV-vis spectroscopy to elucidate unique mechanistic features of J-aggregate oxidation. C8S3 self-assembles in aqueous solution to form double-walled, tubular J-aggregates with ~13 nm diameter and lengths up to several hundred nanometers. Spectroelectrochemical measurements indicate that irreversible J-aggregate oxidation occurs primarily along the outer wall of the tubular structure as evidenced by the potential-induced irreversible bleaching of J-band absorption. Additionally, J-aggregate oxidation involves both electrochemical and chemical steps in which dimerization and subsequent dehydrogenation of the J-aggregate leads to the formation of a new dehydrogenated dimer oxidation product.

Rotational motions of probe molecules in poly(cyclohexyl acrylate) (PCA) were investigated by both ensemble fluorescence recovery after photobleaching (FRAP) and single-molecule spectroscopy at temperatures near the glass transition temperature (Tg) of the polymer host. FRAP measurements of the ensemble anisotropy decay show a nonexponential decay with beta values of 0.5-0.6 when fit by a stretched exponential function. The relationship between the average relaxation time and temperature follows the Williams-Landel-Ferry equation, whereas beta shows no temperature dependence over this range. The same system was also studied by single-molecule spectroscopy at 2 degrees C above the Tg of PCA. The rotational dynamics of the probe molecule can be measured by the autocorrelation function of the linear dichroism signals. Each single-molecule correlation function was fit to the stretched exponential function. The results from all single-molecule data yield broad distributions of both the correlation times (tau) and beta values. The average of the single-molecule correlation times agrees with the ensemble relaxation time, and the sum of all single correlation functions has a nonexponential decay that is almost identical to the ensemble anisotropy decay. The ensemble beta values are smaller than the average beta values in the single-molecule experiments, demonstrating that the system exhibits heterogeneous dynamics. However, the dynamics are not described by an ensemble of molecules that all have single-exponential correlation functions with different time constants.

The amphiphilic cyanine dye 3,3‘-bis(2-sulfopropyl)-5,5‘,6,6‘-tetrachloro-1,1‘-dioctylbenzimidacarbocyanine (C8S3) self-assembles in aqueous solution to form double-walled, tubular J-aggregates with ∼13 nm diameters and lengths up to several hundred nanometers. The redox and light absorption properties of immobilized J-aggregates on transparent, conductive indium tin oxide (ITO) electrodes have been studied directly using cyclic voltammetry (CV) in conjunction with UV−vis spectroscopy to elucidate unique mechanistic features of J-aggregate oxidation. Morphological properties were examined using in situ atomic force microscopy (AFM). Irreversible J-aggregate oxidation appears to occur primarily along the outer wall of the tubular structure as evidenced by the potential-induced irreversible bleaching of J-band absorption. Voltammetric studies as a function of scan rate and pH indicate that J-aggregate oxidation involves both electrochemical and chemical steps in which dimerization and subsequent dehydrogena...

The formation of a ketone defect at the 9-site along the backbone of dialkyl polyfluorenes has been shown to be directly involved in the degradation of the polymer's emission from blue to an undesirable green. Films of poly(9,9‘-dihexylfluorene) (PFH) with and without ketone defects were annealed above their liquid crystalline phase transition in an inert argon atmosphere, and their emission spectra were collected in order to study the effect of morphology on the energy transfer to ketone defects. The annealing was performed in situ in the fluorometer, allowing for a direct comparison of the absolute changes in the emission spectra. Annealing of the films resulted in regions of highly aligned polymer chains as confirmed by atomic force microscopy. After annealing, the fluorescence spectra of pristine films (without ketone defects) exhibited no green emission, indicating the lack of thermal oxidation in the inert atmosphere. However, these films did show an increase in fluorescence quantum yield, revealing...

The thermodynamics of the alpha (α) phase to beta (β) phase transition was investigated in solution of poly(9,9′-dioctylfluorene) in a variety of solvents with UV–vis absorption spectroscopy, differential scanning calorimetry, fluorescence spectroscopy, atomic force microscopy, and near-field scanning optical microscopy. The results show that the solvent quality has a strong affect on the α- to β-conformational change. The trend in enthalpies and transition temperatures indicates that the transition results from an increase in intramolecular interactions upon chain collapse at lower temperatures. This transition leads to subsequent gelation and/or aggregation that stabilizes the β-phase at higher temperatures and leads to a large hysteresis in the transition temperature. The enthalpy for the transition from an aggregated β-phase to a fully solvated α-phase is found to be 21.04 kJ mol −1 of monomer for toluene solutions. Differences between the measured heat and those previously reported are discussed.

Conjugated polymers in the solid state usually exhibit low fluorescence quantum yields, which limit their applications in many areas such as light-emitting diodes. Despite considerable research efforts, the underlying mechanism still remains controversial and elusive. Here, the nature and properties of excited states in the archetypal polythiophene are investigated via aggregates suspended in solvents with different dielectric constants (ɛ). In relatively polar solvents (ɛ>∼ 3), the aggregates exhibit a low fluorescence quantum yield (QY) of 2–5%, similar to bulk films, however, in relatively nonpolar solvents (ɛ, Conjugated polymers in thin films exhibit low fluorescence quantum yields, but the mechanism is still unclear. Here, Hu et al. show the trade-off between charge transfer and emissive exciton states, whilst the former can be suppressed via dielectric-induced stabilization for large fluorescence quantum yields.

Even for apparently simple condensed-phase processes, bulk measurements of relaxation often yield nonexponential decays; the rate appears to be dispersed over a range of values. Taking averages over individual molecules is an intuitive way to determine whether heterogeneity is responsible for such rate dispersion. However, this method is in fundamental conflict with ergodic behavior and often yields ambiguous results. This paper proposes a new definition of rate heterogeneity for ergodic systems based on multidimensional time correlation functions. Averages are taken over both time and molecules. Because the data set is not subdivided, the signal-to-noise ratio is improved. Moment-based quantities are introduced to quantify the concept of rate dispersion. As a result, quantitative statements about the fraction of the dispersion due to heterogeneity are possible, and the experimental noise is further averaged. The practicality of this approach is demonstrated on single-molecule, linear-dichroism trajectories for R6G in poly(cyclohexyl acrylate) near its glass transition. Single-molecule averaging of these data does not provide useful conclusions [C. Y. Lu and D. A. Vanden Bout, J. Chem. Phys. 125, 124701 (2006)]. However, full-ensemble, two- and three-dimensional averages of the same data give clear and quantitative results: the rate dispersion is 95% ± 5% due to heterogeneity, and the rate exchange is at least 11 times longer than the mean rotation time and possibly much longer. Based on these results, we suggest that the study of heterogeneous materials should not focus on “ensemble” versus “single-molecule” experiments, but on one-dimensional versus multidimensional measurements.

The optical properties of thin films of poly[p-(2,5-didodecylphenylene)ethynylene] (DPPE) have been investigated. In chloroform solution the DPPE exhibit structured blue emission with a lifetime of 0.4 ns. In contrast to the solution, pristine DPPE films show a broad featureless green fluorescence with a nonexponential decay with time constants of 1.15 ns (8%) and 5.9 ns (92%). Upon annealing, the emission spectrum returns to a structured blue emission similar to the solution, and the fluorescence decay is nonexponential with a component at 0.45 ns (38%) and 90 ps (62%). Both spin-cast and annealed films possess similar absorption spectra, suggesting that the lifetime difference is due to an excimer-like state in the pristine film and efficient fluorescence from isolated chains within the annealed films. The decrease of the fluorescence lifetime from 0.4 ns (DPPE in solution) to 90 ps (annealed DPPE films) is further attributed to the lack of conformational disorder found within the ordered solid state.

In this paper, we examine the feasibility of two-photon initiated polymerization (TPIP) using the enhanced field generated at the apex of a gold tip as the excitation source. We constructed three-dimensional polymeric objects to demonstrate TPIP using a previously described two-photon absorbing chromophore with a large two-photon absorption cross-section, δ, and an acrylate resin with our fabrication instrument. The minimum power capable of TPIP 'writing' was determined with our instrumentation. As a model system for near-field excitation, NFE, we fabricated an array of gold tips on a glass substrate using nanosphere lithography. Using our minimum writing power as a guide, we exposed the two-photon excitable resin spun onto an array of gold tips. Polymeric structures were fabricated on the array of gold tips and the AFM images of the resulting structures suggest increased polymerization in the vicinity of the gold tips.

This Resource Letter provides a guide to the literature exploring widely used laser-based modern optical microscopic techniques. Journal articles and books are cited for the following topics: confocal, near-field, nonlinear, total internal reflection, photon tunneling, and coherent microscopies. Review articles, books, and websites are included to support references to primary literature sources.

A quick, cost effective, semiquantitative means for gauging the quality of near-field scanning optical microscopy (NSOM) probe apertures has been demonstrated by employing a nanoperforated thin metal film standard sample. Small 182 nm holes were created by evaporating gold over dispersed latex spheres with subsequent removal of the spheres. The size of the NSOM aperture can be determined from a deconvolution of the image size and the known sample size and geometry. Results from the standard correlate well with aperture size measurements made from scanning electron micrographs.

Irreversible photobleaching imparts the unavoidable limitation on single-molecule spectroscopy: when the fluorescent probe molecule becomes nonfluorescent, the experiment is over. A detailed study of the photobleaching rate for rhodamine 6G under vacuum reveals that low excitation rates yield long photochemical survival times and unprecedented numbers of emitted photons. A four-level system is used to model the photobleaching rate, reproducing the experimental bleaching rate and photon yield from single molecule and ensemble experiments. A higher triplet excited state Tn is found to be the predominant reactive state for photobleaching. This model produces a calculated quantum yield for photobleaching on the order of 7×10−7 bleaching events per excitation to Tn.

Single molecule spectroscopy was used to characterize the rotation of fluorescent probe molecules in supercooled o-terphenyl (OTP) just above the glass transition. Rotational motions of spatially i...

Single molecule spectroscopy and polarization holeburning (PHB) are used to measure the dynamics of rotation for rhodamine 6G in poly(methylacrylate). It is found that the ensemble average correlation time obtained from the single molecule experiments is the same as that obtained from bulk fluorescence anisotropy decay. Both reflect the same underlying dynamics as the mechanically measured α relaxation. Fitting α relaxation and PHB data to the Vogel–Tammann–Fulcher (VTF) equation, it is found that the two measurements follow the same temperature dependence, but with absolute times that differ by nearly four orders of magnitude. This difference is discussed in two contexts. In the first the relative ratio of the correlation and α-relaxation times depend on probe size, in which case the large difference is not unexpected given the size of the probe. In the second, the difference is attributed to specific probe/polymer interactions. In this case, a modified VTF equation that includes an activation energy ter...

This paper presents a systematic study of the influence of electron-transport materials on the operation stability of the inverted perovskite solar cells under both laboratory indoor and the natural outdoor conditions in the Negev desert. It is shown that all devices incorporating a Phenyl C61 Butyric Acid Methyl ester ([60]PCBM) layer undergo rapid degradation under illumination without exposure to oxygen and moisture. Time-of-flight secondary ion mass spectrometry depth profiling reveals that volatile products from the decomposition of methylammonium lead iodide (MAPbI3) films diffuse through the [60]PCBM layer, go all the way toward the top metal electrode, and induce its severe corrosion with the formation of an interfacial AgI layer. On the contrary, alternative electron-transport material based on the perylendiimide derivative provides good isolation for the MAPbI3 films preventing their decomposition and resulting in significantly improved device operation stability. The obtained results strongly suggest that the current approach to design inverted perovskite solar cells should evolve with respect to the replacement of the commonly used fullerene-based electron-transport layers with other types of materials (e.g., functionalized perylene diimides). It is believed that these findings pave a way toward substantial improvements in the stability of the perovskite solar cells, which are essential for successful commercialization of this photovoltaic technology.

Conjugated polymers are appealing materials for cost-effective production of flexible electronic devices, but the efficiency of these devices may be compromised by exciton quenching by hole polarons. Exciton migration was monitored directly in single poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) chains embedded in a hole-injection device by applying super-resolution point-spread function fitting techniques to the fluorescence image of each chain as holes were reversibly injected into the polymer. Correlated excitation polarization spectroscopy techniques reported the orientation of the longitudinal axis of the rod-like polymers. The wide-field microscopy images are diffraction-limited in one dimension but slightly elongated in the direction of the longitudinal axis of the rod-like polymer in most cases and at every depth of fluorescence quenching explored.

Near-field scanning optical microscopy (NSOM) is used to characterize nanoscale topographic and fluorescence features in thermally annealed films of the conjugated polymer polyfluorene. Thin films of polyfluorenes with either two hexyl (1), octyl (2), or dodecyl (3) alkyl groups at the 9 position were studied. Annealed films were made by holding the films above their respective liquid crystalline phase transition temperature and then rapidly cooling the films. Upon annealing, the films show large spectroscopic and morphological changes. The emission spectra films of 1 show a large increase in emission at wavelengths greater than 500, while films of 2 and 3 show small or no change in the long wavelength emission. Polarized NSOM images of all three films show that the films organize into highly ordered nanoscale domains. The order in the films is found to be largest in the polymer with the shortest alkyl chains growing progressively less ordered with increasing chain length. Films of 3 have domains on the o...