Fine-Tuning the Microstructure and Photophysical Characteristics of Fluorescent Conjugated Copolymers Using Photoalignment and Liquid-Crystal Ordering

Replicating the microstructural basis and the near 100% excitation energy transfer efficiency in naturally occurring light-harvesting complexes (LHCs) remains challenging in synthetic energy-harvesting devices. Biological photosynthesis regulates active ensembles of light-absorbing and funneling chlorophylls in proteins in response to fluctuating sunlight. Here, use of long-range liquid crystal (LC) ordering to tailor chain orientation and packing structure in liquid crystalline conjugated polymer (LCCP) layers for bio-mimicry of certain structural basis and light-harvesting properties of LHCs is reported. It is found that long-range orientational ordering in an LC phase of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) copolymer stabilizes a small fraction of randomly-oriented F8BT nanocrystals dispersed in an amorphous matrix of F8BT chains, resembling a self-doped host-guest system whereby excitation energy funneling and photoluminescence quantum efficiencies are enhanced significantly by triggering 3D donor-to-acceptor Förster resonance energy transfer (FRET) and dominant intrachain emission in the nano-crystal acceptor. Further, photoalignment of nematic F8BT layers is combined with LC orientational ordering to fabricate large-area-extended monodomains exhibiting >60% crystallinity and ≈20 nm-long interchain packing order. Remarkably, these monodomains demonstrate strong linearly polarized emission, whilst also promoting a new band-edge absorption species and an extra emissive interchain excited state as compared to the non-aligned films.

Charge Concentration Limits the Hydrogen Evolution Rate in Organic Nanoparticle Photocatalysts

Time-resolved microwave conductivity is used to compare aqueous-soluble organic nanoparticle photocatalysts and bulk thin films composed of the same mixture of semiconducting polymer and non-fullerene acceptor molecule and the relationship between composition, interfacial surface area, charge-carrier dynamics, and photocatalytic activity is examined. The rate of hydrogen evolution reaction by nanoparticles composed of various donor:acceptor blend ratio compositions is quantitatively measured, and it is found that the most active blend ratio displays a hydrogen quantum yield of 0.83% per photon. Moreover, it is found that nanoparticle photocatalytic activity corresponds directly to charge generation, and that nanoparticles have 3× more long-lived accumulated charges relative to bulk samples of the same material composition. These results suggest that, under the current reaction conditions, with ≈3× solar flux, catalytic activity by the nanoparticles is limited by the concentration of electrons and holes in operando and not a finite number of active surface sites or the catalytic rate at the interface. This provides a clear design goal for the next generation of efficient photocatalytic nanoparticles.

Negative diffusion of excitons in quasi-two-dimensional systems

We show how two different mobile-immobile type models explain the observation of negative diffusion of excitons reported in experimental studies in quasi-two-dimensional semiconductor systems. The main reason for the effect is the initial trapping and a delayed release of free excitons in the area close to the original excitation spot. The density of trapped excitons is not registered experimentally. Hence, the signal from the free excitons alone includes the delayed release of not diffusing trapped particles. This is seen as the narrowing of the exciton density profile or decrease of mean-squared displacement which is then interpreted as a negative diffusion. The effect is enhanced with the increase of recombination intensity as well as the rate of the exciton-exciton binary interactions.

Non-Markovian diffusion of excitons in layered perovskites and transition metal dichalcogenides

The diffusion of excitons in perovskites and transition metal dichalcogenides shows clear anomalous, subdiffusive behaviour in experiments. In this paper we develop a non-Markovian mobile-immobile model which provides an explanation of this behaviour through paired theoretical and simulation approaches. The simulation model is based on a random walk on a 2D lattice with randomly distributed deep traps such that the trapping time distribution involves slowly decaying power-law asymptotics. The theoretical model uses coupled diffusion and rate equations for free and trapped excitons, respectively, with an integral term responsible for trapping. The model provides a good fitting of the experimental data, thus, showing a way for quantifying the exciton diffusion dynamics.

Polymer Electrolyte Dielectrics Enable Efficient Exciton-Polaron Quenching in Organic Semiconductors for Photostable Organic Transistors

The photoelectric response of organic field-effect transistors (OFETs) will cause severe photoelectric interference, which hinders the applications of OFETs in the light environment. It is highly challenging to relieve this problem because of the high photosensitivity of most organic semiconductors. Here, we propose an efficient "exciton-polaron quenching" strategy to suppress the photoelectric response and thus construct highly photostable OFETs by utilizing a polymer electrolyte dielectric─poly(acrylic acid) (PAA). This dielectric produces high-density polarons in organic semiconductors under a gate electric field that quench the photogenerated excitons with high efficiency (∼70%). As a result, the OFETs with PAA dielectric exhibit unprecedented photostability against strong light irradiation up to 214 mW/cm², which far surpasses the reported values and solar irradiance value (∼138 mW/cm²). The strategy shows high universality in OFETs with different OSCs and electrolytes. As a demonstration, the photostable OFET can stably drive an organic light-emitting diode (OLED) under light irradiation. This work presents an efficient exciton modulation strategy in OSC and proves a high potential in practical applications.

Operando Characterization of Organic Mixed Ionic/Electronic Conducting Materials

Operando characterization plays an important role in revealing the structure-property relationships of organic mixed ionic/electronic conductors (OMIECs), enabling the direct observation of dynamic changes during device operation and thus guiding the development of new materials. This review focuses on the application of different operando characterization techniques in the study of OMIECs, highlighting the time-dependent and bias-dependent structure, composition, and morphology information extracted from these techniques. We first illustrate the needs, requirements, and challenges of operando characterization then provide an overview of relevant experimental techniques, including spectroscopy, scattering, microbalance, microprobe, and electron microscopy. We also compare different in silico methods and discuss the interplay of these computational methods with experimental techniques. Finally, we provide an outlook on the future development of operando for OMIEC-based devices and look toward multimodal operando techniques for more comprehensive and accurate description of OMIECs.

A Single Molecule Polyphenylene-Vinylene Photonic Wire

Nanoscale transport of light through single molecule systems is of fundamental importance for light harvesting, nanophotonic circuits, and for understanding photosynthesis. Studies on organization of molecular entities for directional transfer of excitation energy have focused on energy transfer cascades via multiple small molecule dyes. Here, we investigate a single molecule conjugated polymer as a photonic wire. The phenylene-vinylene-based polymer is functionalized with multiple DNA strands and immobilized on DNA origami by hybridization to a track of single-stranded staples extending from the origami structure. Donor and acceptor fluorophores are placed at specific positions along the polymer which enables energy transfer from donor to polymer, through the polymer, and from polymer to acceptor. The structure is characterized by atomic force microscopy, and the energy transfer is studied by ensemble fluorescence spectroscopy and single molecule TIRF microscopy. It is found that the polymer photonic wire is capable of transferring light over distances of 24 nm. This demonstrates the potential residing in the use of conjugated polymers for nanophotonics.

Multimode Time-Resolved Superresolution Microscopy Revealing Chain Packing and Anisotropic Single Carrier Transport in Conjugated Polymer Nanowires

Here, we developed a novel, multimode superresolution method to perform full-scale structural mapping and measure the energy landscape for single carrier transport along conjugated polymer nanowires. Through quenching of the local emission, the motion of a single photogenerated hole was tracked using blinking-assisted localization microscopy. Then, utilizing binding and unbinding dynamics of quenchers onto the nanowires, local emission spectra were collected sequentially and assembled to create a superresolution map of emission sites throughout the structure. The hole polaron trajectories were overlaid with the superresolution maps to correlate structures with charge transport properties. Using this method, we compared the efficiency of inter- and intrachain hole transport inside the nanowires and for the first time directly measured the depth of carrier traps originated from torsional disorder and chemical defects.

Photobleaching of organic fluorophores: quantitative characterization, mechanisms, protection

Photochemical stability is one of the most important parameters that determine the usefulness of organic dyes in different applications. This Review addresses key factors that determine the dye photostability. It is shown that photodegradation can follow different oxygen-dependent and oxygen-independent mechanisms and may involve both ¹S₁-³T₁ and higher-energy ¹S_n-³T_n excited states. Their involvement and contribution depends on dye structure, medium conditions, irradiation power. Fluorescein, rhodamine, BODIPY and cyanine dyes, as well as conjugated polymers are discussed as selected examples illustrating photobleaching mechanisms. The strategies for modulating and improving the photostability are overviewed. They include the improvement of fluorophore design, particularly by attaching protective and anti-fading groups, creating proper medium conditions in liquid, solid and nanoscale environments. The special conditions for biological labeling, sensing and imaging are outlined.

Enhancing Photoluminescence Quenching in Donor-Acceptor PCE11:PPCBMB Films through the Optimization of Film Microstructure

We show that a precise control of deposition speed during the fabrication of polyfullerenes and donor polymer films by convective self-assembly leads to an optimized film microstructure comprised of interconnected crystalline polymer domains comparable to molecular dimensions intercalated with similar polyfullerene domains. Moreover, in blended films, we have found a correlation between deposition speed, the resulting microstructure, and photoluminescence quenching. The latter appeared more intense for lower deposition speeds due to a more favorable structuring at the nanoscale of the two donor and acceptor systems in the resulting blend films.

Superresolution mapping of energy landscape for single charge carriers in plastic semiconductors

The performance of conjugated polymer devices is largely dictated by charge transport processes. However, it is difficult to obtain a clear relationship between conjugated polymer structures and charge transport properties, due to the complexity of the structure and the dispersive nature of charge transport in conjugated polymers. Here, we develop a method to map the energy landscape for charge transport in conjugated polymers based on simultaneous, correlated charge carrier tracking and single-particle fluorescence spectroscopy. In nanoparticles of the conjugated polymer poly[9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-{2,1′-3}-thiadiazole)], two dominant chain conformations were observed, a blue-emitting phase (λ_max = 550 nm) and a red-emitting phase (λ_max = 595 nm). Hole polarons were trapped within the red phase, only occasionally escaping into the blue phase. Polaron hopping between the red-emitting traps was observed, with transition time ranging from tens of milliseconds to several seconds. These results provide unprecedented nanoscale detail about charge transport at the single carrier level.

To understand the complex nanoscale structure-property relationships in conjugated polymers for device applications, new methods for tracking charge transport are required. Here, the authors employ superresolution mapping to study the charge carrier dynamics in conjugated polymer nanoparticles.

Constructing Nanostructured Donor/Acceptor Bulk Heterojunctions via Interfacial Templates for Efficient Organic Photovoltaics

We demonstrate that a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/diindenoperylene (PEDOT:PSS/DIP) interfacial bilayer could serve as a structural template to enable the morphological control of bulk heterojunctions (BHJs) by co-evaporation of tetraphenyldibenzoperiflanthene:fullerene (DBP:C₆₀), which greatly improves the device performances. Especially, we show that isolated crystalline domains of C₆₀ can be well-controlled at the nanoscale during the co-evaporation. Photoluminescence spectra indicate the realization of DIP/DBP cascade energy architecture, which significantly facilitates both the energy transfer and photocurrent generation. In addition, with bias-dependent external quantum efficiency analysis, we reveal that such a cascade energy device architecture greatly suppresses the energy recombination in both carrier and exciton transfer, resulting in a high open-circuit voltage and a high fill factor. By carefully optimizing the interfacial and BHJ layers, we achieved a high-performance organic photovoltaic cell with a power conversion efficiency of 5.0 ± 0.3%.

Controlling photophysical properties of ultrasmall conjugated polymer nanoparticles through polymer chain packing

Applications of conjugated polymer nanoparticles (Pdots) for imaging and sensing depend on their size, fluorescence brightness and intraparticle energy transfer. The molecular design of conjugated polymers (CPs) has been the main focus of the development of Pdots. Here we demonstrate that proper control of the physical interactions between the chains is as critical as the molecular design. The unique design of twisted CPs and fine-tuning of the reprecipitation conditions allow us to fabricate ultrasmall (3.0–4.5 nm) Pdots with excellent photostability. Extensive photophysical and structural characterization reveals the essential role played by the packing of the polymer chains in the particles in the intraparticle spatial alignment of the emitting sites, which regulate the fluorescence brightness and the intraparticle energy migration efficiency. Our findings enhance understanding of the relationship between chain interactions and the photophysical properties of CP nanomaterials, providing a framework for designing and fabricating functional Pdots for imaging applications.

Synthesis of small conjugated polymer nanoparticles (Pdots) with bright and stable fluorescence is an active challenge. Here, the authors introduce a strategy to fabricate ultrasmall Pdots with high fluorescence intensity by using twisted, rather than planar, conjugated polymers, lending new insight into the molecular design of Pdots.

Super-Resolution Imaging and Plasmonics

This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.

Enhancing the photostability of poly(phenylene ethynylene) for single particle studies

Enhanced photostability of conjugated polyelectrolytes achieved by using anti-fading agents opens the way for advanced single molecule fluorescence imaging studies.

Field-Controlled Charge Separation in a Conductive Matrix at the Single-Molecule Level: Toward Controlling Single-Molecule Fluorescence Intermittency

The fluorescence intermittency or "blinking" of single molecules of ATTO647N (ATTO) in the conductive matrix polyvinylcarbazole (PVK) is described in the presence of an external applied electric field. It is shown that due to the energy distribution of the highest occupied molecular orbital (HOMO) level of PVK, which is energetically close to the HOMO of ATTO, sporadic electron transfer occurs. As a result, the on/off dynamics of blinking can be influenced by the electric field. This field will, depending on the respective position and orientation of the dye/polymer system with respect to those of the electrodes, either enhance or suppress electron transfer from PVK to ATTO as well as the back electron transfer from reduced ATTO to PVK. After the charge-transfer step, the applied field will pull the hole in PVK away from the dye, increasing the overall time the dye resides in a dark state.

Anisotropic Conjugated Polymer Chain Conformation Tailors the Energy Migration in Nanofibers

Conjugated polymers are complex multichromophore systems, with emission properties strongly dependent on the electronic energy transfer through active subunits. Although the packing of the conjugated chains in the solid state is known to be a key factor to tailor the electronic energy transfer and the resulting optical properties, most of the current solution-based processing methods do not allow for effectively controlling the molecular order, thus making the full unveiling of energy transfer mechanisms very complex. Here we report on conjugated polymer fibers with tailored internal molecular order, leading to a significant enhancement of the emission quantum yield. Steady state and femtosecond time-resolved polarized spectroscopies evidence that excitation is directed toward those chromophores oriented along the fiber axis, on a typical time scale of picoseconds. These aligned and more extended chromophores, resulting from the high stretching rate and electric field applied during the fiber spinning process, lead to improved emission properties. Conjugated polymer fibers are relevant to develop optoelectronic plastic devices with enhanced and anisotropic properties.

Molecular Water Lilies: Orienting Single Molecules in a Polymer Film by Solvent Vapor Annealing

The microscopic orientation and position of photoactive molecules is crucial to the operation of optoelectronic devices such as OLEDs and solar cells. Here, we introduce a shape-persistent macrocyclic molecule as an excellent fluorescent probe to simply measure (i) its orientation by rotating the excitation polarization and recording the strength of modulation in photoluminescence (PL) and (ii) its position in a film by analyzing the overall PL brightness at the molecular level. The unique shape, the absorption and the fluorescence properties of this probe yield information on molecular orientation and position. We control orientation and positioning of the probe in a polymer film by solvent vapor annealing (SVA). During the SVA process the molecules accumulate at the polymer/air interface, where they adopt a flat orientation, much like water lilies on the surface of a pond. The results are potentially significant for OLED fabrication and single-molecule spectroscopy (SMS) in general.

FRET acceptor suppressed single-particle photobleaching in semiconductor polymer dots

Brightness and photostability are key parameters for fluorescent probes in optical imaging. This Letter describes Förster resonance energy transfer (FRET) as a useful strategy to enhance the photostability of fluorescent nanoparticles. Small molecules as FRET acceptors were doped into semiconductor polymer dots (Pdots), yielding apparent suppression of their rapid photobleaching in single-particle imaging. For 20 nm-diameter particles, the photobleaching percentage decreased from 71.8% to 47.2% after dye doping, while the single-particle brightness remained unchanged. The photostability of large Pdots was also enhanced by FRET at the expense of a moderate decrease in per-particle brightness as compared to the pure Pdots. This study indicates that FRET is a facile, yet effective, approach to mediate the brightness and photostability of fluorescent nanoparticles. Considering the combined factors of brightness and photostability, the dye-doped Pdots of ∼20  nm diameter are the most suitable for long-term imaging and tracking applications.

Alignment and Patterning of Ordered Small-Molecule Organic Semiconductor Micro-/Nanocrystals for Device Applications

Large-area alignment and patterning of small-molecule organic semiconductor micro-/nanocrystals (SMOSNs) at desired locations is a prerequisite for their practical device applications. Recent strategies for alignment and patterning of ordered SMOSNs and their corresponding device applications are highlighted.

Synergetic Influences of Mixed-Host Emitting Layer Structures and Hole Injection Layers on Efficiency and Lifetime of Simplified Phosphorescent Organic Light-Emitting Diodes

We used various nondestructive analyses to investigate various host material systems in the emitting layer (EML) of simple-structured, green phosphorescent organic light-emitting diodes (OLEDs) to clarify how the host systems affect its luminous efficiency (LE) and operational stability. An OLED that has a unipolar single-host EML with conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (

PEDOT

PSS) showed high operating voltage, low LE (∼26.6 cd/A, 13.7 lm/W), and short lifetime (∼4.4 h @ 1000 cd/m(2)). However, the combined use of a gradient mixed-host EML and a molecularly controlled HIL that has increased surface work function (WF) remarkably decreased operating voltage and improved LE (∼68.7 cd/A, 77.0 lm/W) and lifetime (∼70.7 h @ 1000 cd/m(2)). Accumulated charges at the injecting interfaces and formation of a narrow recombination zone close to the interfaces are the major factors that accelerate degradation of charge injection/transport and electroluminescent properties of OLEDs, so achievement of simple-structured OLEDs with high efficiency and long lifetime requires facilitating charge injection and balanced transport into the EML and distributing charge carriers and excitons in EML.

Influence of the Conjugation Length on the Optical Spectra of Single Ladder-Type (p-Phenylene) Dimers and Polymers

We employ low-temperature single-molecule photoluminescence spectroscopy on a π-conjugated ladder-type (p-phenylene) dimer and the corresponding polymer methyl-substituted ladder-type poly(p-phenylene), MeLPPP, to study the impact of the conjugation length (π-electron delocalization) on their optical properties on a molecular scale. Our data show that the linear electron-phonon coupling to intramolecular vibrational modes is very sensitive to the conjugation length, a well-known behavior of organic (macro-) molecules. In particular, the photoluminescence spectra of single dimers feature a rather strong low-energy (150 cm(-1)) skeletal mode of the backbone, which does not appear in the spectra of individual chromophores on single MeLPPP chains. We attribute this finding to a strongly reduced electron-phonon coupling strength and/or vibrational energy of this mode for MeLPPP with its more delocalized π-electron system as compared to the dimer. In contrast, the line widths of the purely electronic zero-phonon lines (ZPL) in single-molecule spectra do not show differences between the dimer and MeLPPP; for both systems the ZPLs are apparently broadened by fast unresolved spectral diffusion. Finally, we demonstrate that the low-temperature ensemble photoluminescence spectrum of the dimer cannot be reproduced by the distribution of spectral positions of the ZPLs. The dimer's bulk spectrum is rather apparently broadened by electron-phonon coupling to the low-energy skeletal mode, whereas for MeLPPP the inhomogeneous bulk line shape resembles the distribution of spectral positions of the ZPLs of single chromophores.

Mesoscopic quantum emitters from deterministic aggregates of conjugated polymers

An appealing definition of the term "molecule" arises from consideration of the nature of fluorescence, with discrete molecular entities emitting a stream of single photons. We address the question of how large a molecular object may become by growing deterministic aggregates from single conjugated polymer chains. Even particles containing dozens of individual chains still behave as single quantum emitters due to efficient excitation energy transfer, whereas the brightness is raised due to the increased absorption cross-section of the suprastructure. Excitation energy can delocalize between individual polymer chromophores in these aggregates by both coherent and incoherent coupling, which are differentiated by their distinct spectroscopic fingerprints. Coherent coupling is identified by a 10-fold increase in excited-state lifetime and a corresponding spectral red shift. Exciton quenching due to incoherent FRET becomes more significant as aggregate size increases, resulting in single-aggregate emission characterized by strong blinking. This mesoscale approach allows us to identify intermolecular interactions which do not exist in isolated chains and are inaccessible in bulk films where they are present but masked by disorder.

Long-range energy transport in single supramolecular nanofibres at room temperature

Efficient transport of excitation energy over long distances is a key process in light-harvesting systems, as well as in molecular electronics. However, in synthetic disordered organic materials, the exciton diffusion length is typically only around 10 nanometres (refs 4, 5), or about 50 nanometres in exceptional cases, a distance that is largely determined by the probability laws of incoherent exciton hopping. Only for highly ordered organic systems has the transport of excitation energy over macroscopic distances been reported--for example, for triplet excitons in anthracene single crystals at room temperature, as well as along single polydiacetylene chains embedded in their monomer crystalline matrix at cryogenic temperatures (at 10 kelvin, or -263 degrees Celsius). For supramolecular nanostructures, uniaxial long-range transport has not been demonstrated at room temperature. Here we show that individual self-assembled nanofibres with molecular-scale diameter efficiently transport singlet excitons at ambient conditions over more than four micrometres, a distance that is limited only by the fibre length. Our data suggest that this remarkable long-range transport is predominantly coherent. Such coherent long-range transport is achieved by one-dimensional self-assembly of supramolecular building blocks, based on carbonyl-bridged triarylamines, into well defined H-type aggregates (in which individual monomers are aligned cofacially) with substantial electronic interactions. These findings may facilitate the development of organic nanophotonic devices and quantum information technology.

Giant photoluminescence blinking of perovskite nanocrystals reveals single-trap control of luminescence

Fluorescence super-resolution microscopy showed correlated fluctuations of photoluminescence intensity and spatial localization of individual perovskite (CH3NH3PbI3) nanocrystals of size ∼200 × 30 × 30 nm(3). The photoluminescence blinking amplitude caused by a single quencher was a hundred thousand times larger than that of a typical dye molecule at the same excitation power density. The quencher is proposed to be a chemical or structural defect that traps free charges leading to nonradiative recombination. These trapping sites can be activated and deactivated by light.

Enhanced and tunable photoluminescence of polyphenylenevinylenes confined in nanocomposite films

Conformation of macromolecules and interchain interactions determine spectral properties of conjugated polymers (CP). An achievement of spatial confinement of isolated chains is one of the routes to use this feature of CP for their purposeful usage. In the present work, CP/O300 nanocomposites based on CP - poly(p-phenylenevinylene) and poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) - and silica nanoparticles (O300) are prepared. In comparison with many previously known hybrid nanomaterials synthesized with the similar purpose, CP/O300 nanocomposites are characterized by the essentially enhanced and tunable photoluminescence. The greatest change of color coordinates is observed for poly(p-phenylenevinylene)-based nanocomposites due to specific preparation method and interaction with the inorganic component. The main emission from CP in the CP/O300 nanocomposites is owing to 0-0 transitions, while 0-1 transitions, associated with aggregate states of the CP chains, are suppressed.

Simulations of singlet exciton diffusion in organic semiconductors: a review

Recent advances in exciton diffusion simulations in conjugated materials are presented in this review.

Single Lévy states-disorder induced energy funnels in molecular aggregates

Using fluorescence super-resolution microscopy we studied simultaneous spectral, spatial localization, and blinking behavior of individual 1D J-aggregates. Excitons migrating 100 nm are funneled to a trap appearing as an additional red-shifted blinking fluorescence band. We propose that the trap is a Frenkel exciton state formed much below the main exciton band edge due to an environmentally induced heavy-tailed Lévy disorder. This points to disorder engineering as a new avenue in controlling light-harvesting in molecular ensembles.

High intrachain order promotes triplet formation from recombination of long-lived polarons in poly(3-hexylthiophene) J-aggregate nanofibers

Photoluminescence (PL) of single poly(3-hexylthiophene) (P3HT) J-aggregate nanofibers (NFs) exhibits strong quenching under intensity-modulated pulsed excitation. Initial PL intensities (I(0)) decay to steady-state levels (ISS) typically within ∼ 1-10 μs, and large quenching depths (I(0)/I(SS) >2) are observed for ∼ 70% of these NFs. Similar studies of polymorphic, H-aggregate type P3HT NFs show much smaller PL quenching depths (I(0)/I(SS) ≤ 1.2). P3HT chains in J-type NF π-stacks possess high intrachain order, which has been shown previously to promote the formation of long-lived, delocalized polarons. We propose that these species recombine nongeminately to triplets on time scales of >1 ns. The identity of triplets as the dominant PL quenchers was confirmed by subjecting NFs to oxygen, resulting in an instantaneous loss of triplet PL quenching (I(0)/I(SS) ∼ 1). The lower intrachain order in H-type NFs, similar to P3HT thin-film aggregates, localizes excitons and polarons, leading to efficient geminate recombination that suppresses triplet formation at longer time scales. Our results demonstrate the promise of self-assembly strategies to control intrachain ordering within multichromophoric polymeric aggregate assemblies to tune exciton coupling and interconversion processes between different spin states.

Femtosecond Pump-Push-Probe and Pump-Dump-Probe Spectroscopy of Conjugated Polymers: New Insight and Opportunities

Conjugated polymers are an important class of soft materials that exhibit a wide range of applications. The excited states of conjugated polymers, often referred to as excitons, can either deactivate to yield the ground state or dissociate in the presence of an electron acceptor to form charge carriers. These interesting properties give rise to their luminescence and the photovoltaic effect. Femtosecond spectroscopy is a crucial tool for studying conjugated polymers. Recently, more elaborate experimental configurations utilizing three optical pulses, namely, pump-push-probe and pump-dump-probe, have been employed to investigate the properties of excitons and charge-transfer states of conjugated polymers. These studies have revealed new insight into femtosecond torsional relaxation and detrapping of bound charge pairs of conjugated polymers. This Perspective highlights (1) the recent achievements by several research groups in using pump-push-probe and pump-dump-probe spectroscopy to study conjugated polymers and (2) future opportunities and potential challenges of these techniques.

Super-resolution molecular and functional imaging of nanoscale architectures in life and materials science

Super-resolution (SR) fluorescence microscopy has been revolutionizing the way in which we investigate the structures, dynamics, and functions of a wide range of nanoscale systems. In this review, I describe the current state of various SR fluorescence microscopy techniques along with the latest developments of fluorophores and labeling for the SR microscopy. I discuss the applications of SR microscopy in the fields of life science and materials science with a special emphasis on quantitative molecular imaging and nanoscale functional imaging. These studies open new opportunities for unraveling the physical, chemical, and optical properties of a wide range of nanoscale architectures together with their nanostructures and will enable the development of new (bio-)nanotechnology.

Enhancing the photoluminescence emission of conjugated MEH-PPV by light processing

We show here that treatment of thin films of conjugated polymers by illumination with light leads to an increase of the intensity of their photoluminescence by up to 42%. The corresponding enhancement of absorbance was much less pronounced. We explain this significant enhancement of photoluminescence by a planarization of the conjugated polymer chains induced by photoexcitations even below the glass transition temperature, possibly due to an increased conjugation length. Interestingly, the photoluminescence remains at the enhanced level for more than 71 h after treatment of the films by illumination with light, likely due to the fact that below the glass transition temperature no restoring force could return the conjugated chains into their initial conformational state.

Singlet-triplet annihilation limits exciton yield in poly(3-hexylthiophene)

Control of chain length and morphology in combination with single-molecule spectroscopy techniques provides a comprehensive photophysical picture of excited-state losses in the prototypical conjugated polymer poly(3-hexylthiophene) (P3HT). Our examination reveals a universal self-quenching mechanism, based on singlet-triplet exciton annihilation, which accounts for the dramatic loss in fluorescence quantum yield of a single P3HT chain between its solution (unfolded) and bulklike (folded) state. Triplet excitons fundamentally limit the fluorescence of organic photovoltaic materials, which impacts the conversion of singlet excitons to separated charge carriers, decreasing the efficiency of energy harvested at high excitation densities. Interexcitonic interactions are so effective that a single P3HT chain of order 100  kDa weight behaves like a 2-level system, exhibiting perfect photon antibunching.

Localization in space and time in disordered-lattice open quantum dynamics

We study a two-dimensional tight-binding lattice for excitons with on-site disorder, coupled to a thermal environment at infinite temperature. The disorder acts to localize an exciton spatially, while the environment generates dynamics which enable exploration of the lattice. Although the steady state of the system is trivially uniform, we observe a rich dynamics and uncover a dynamical phase transition in the space of temporal trajectories. This transition is identified as a localization in the dynamics generated by the bath. We explore spatial features in the dynamics and employ a generalization of the inverse participation ratio to deduce an ergodic timescale for the lattice.

Influence of Molecular Conformations and Microstructure on the Optoelectronic Properties of Conjugated Polymers

It is increasingly obvious that the molecular conformations and the long-range arrangement that conjugated polymers can adopt under various experimental conditions in bulk, solutions or thin films, significantly impact their resulting optoelectronic properties. As a consequence, the functionalities and efficiencies of resulting organic devices, such as field-effect transistors, light-emitting diodes, or photovoltaic cells, also dramatically change due to the close structure/property relationship. A range of structure/optoelectronic properties relationships have been investigated over the last few years using various experimental and theoretical methods, and, further, interesting correlations are continuously revealed by the scientific community. In this review, we discuss the latest findings related to the structure/optoelectronic properties interrelationships that exist in organic devices fabricated with conjugated polymers in terms of charge mobility, absorption, photoluminescence, as well as photovoltaic properties.

Unraveling the Electronic Heterogeneity of Charge Traps in Conjugated Polymers by Single-Molecule Spectroscopy

Charge trapping is taken for granted in modeling the characteristics of organic semiconductor devices, but very few techniques actually exist to spectroscopically pinpoint trap states. For example, trap levels are often assumed to be discrete in energy. Using the well-known keto defect in polyfluorene as a model, we demonstrate how single-molecule spectroscopy can directly track the formation of charge and exciton traps in conjugated polymers in real time, providing crucial information on the energetic distribution of trap sites relative to the polymer optical gap. Charge traps with universal spectral fingerprints scatter by almost 1 eV in depth, implying that substantial heterogeneity must be taken into account when modeling devices.

Efficient small-molecule photovoltaic cells using a crystalline diindenoperylene film as a nanostructured template

A cascade-type small-molecule organic photovoltaic cell using a crystalline diindenoperylene film as a nanostructured template is demonstrated. This cell architecture simultaneously realizes organic nanostructure and cascade energy concepts, which significantly improves the photocurrent generation and fill factor, leading to a power conversion efficiency of 5.2±0.3%.