Single-molecule electroluminescence and photoluminescence of polyfluorene unveils the photophysics behind the green emission band

Binary Host-induced Exciplex Enabled High Color-Rendering Index of 94 for Carbon Quantum Dot-Based White Light-Emitting Diodes

White light-emitting diodes (WLEDs) with high color-rendering index (CRI, >90) are important for backlight displays and solid-state lighting applications. Although the well-developed colloidal quantum dots (QDs) based on heavy metals such as cadmium and lead are promising candidates for WLEDs, the low CRI still remains a significant limitation. In addition, the severe toxicity of heavy metals greatly limits their widespread use. Herein, the study demonstrates low-cost and environmentally friendly carbon quantum dots (CQDs)-based WLEDs that exhibit a high CRI of 94.33, surpassing that of conventional cadmium/lead-containing QD-based WLEDs. This achievement is attained through the employment of a binary host-induced exciplex strategy. The high hole/electron mobility and suitable energy levels of the donor and acceptor give rise to a broadband orange-yellow emission stemming from the exciplex. As the host, the binary exciplex is capable of contributing blue and orange-yellow emission components while efficiently mitigating the aggregation-induced quenching of CQDs. Meanwhile, CQDs effectively address the deep-red emission gap, enabling the realization of CQDs-based WLEDs with high CRI. These WLEDs also exhibit a remarkably low turn-on voltage of 2.8 V, a maximum luminance exceeding 2000 cd m- 2, a correlated color temperature of 4976 K, and Commission Internationale de l'Eclairage coordinates of (0.34, 0.32).

Polyfluorene-poly(ethylene oxide) diblock copolymers: synthesis and electron transport behavior

Under mild reaction conditions, we synthesized diblock copolymers of poly(9,9-dioctylfluorene)-block-poly(ethylene oxide) (PFO-b-PEO) via end-capping poly(9,9-dioctylfluorene) (PFO) with poly(ethylene oxide) (PEO) on one end. We investigated the thermal, optical, electrochemical and crystalline properties as well as electron transport performance of these polymers. Our results demonstrate that PFO-b-PEO diblock copolymers with short PEO chains (M n = 1000 and 2000 g mol-1) exhibit higher electron mobilities compared to the PFO homopolymer and longer PEO chain (M n = 4000 g mol-1) attached copolymers. This enhanced electron mobility is attributed to the higher crystallinity induced by the shorter PEO chain end-capping.

A Holistic Review of C = C Crosslinkable Conjugated Molecules in Solution-Processed Organic Electronics: Insights into Stability, Processibility, and Mechanical Properties

Solution-processable organic conjugated molecules (OCMs) consist of a series of aromatic units linked by σ-bonds, which present a relatively freedom intramolecular motion and intermolecular re-arrangement under external stimulation. The cross-linked strategy provides an effective platform to obtain OCMs network, which allows for outstanding optoelectronic, excellent physicochemical properties, and substantial improvement in device fabrication. An unsaturated double carbon-carbon bond (C = C) is universal segment to construct crosslinkable OCMs. In this review, the authors will set C = C cross-linkable units as an example to summarize the development of cross-linkable OCMs for solution-processable optoelectronic applications. First, this review provides a comprehensive overview of the distinctive chemical, physical, and optoelectronic properties arising from the cross-linking strategies employed in OCMs. Second, the methods for probing the C = C cross-linking reaction are also emphasized based on the perturbations of chemical structure and physicochemical property. Third, a series of model C = C cross-linkable units, including styrene, trifluoroethylene, and unsaturated acid ester, are further discussed to design and prepare novel OCMs. Furthermore, a concise overview of the optoelectronic applications associated with this approach is presented, including light-emitting diodes (LEDs), solar cells (SCs), and field-effect transistors (FETs). Lastly, the authors offer a concluding perspective and outlook for the improvement of OCMs and their optoelectronic application via the cross-linking strategy.

Light-Emitting Electrochemical Cells Based on Nanogap Electrodes

In a light-emitting electrochemical cell (LEC), electrochemical doping caused by mobile ions facilitates bipolar charge injection and recombination emissions for a high electroluminescence (EL) intensity at low driving voltages. We present the development of a nanogap LEC (i.e., nano-LEC) comprising a light-emitting polymer (F8BT) and an ionic liquid deposited on a gold nanogap electrode. The device demonstrated a high EL intensity at a wavelength of 540 nm corresponding to the emission peak of F8BT and a threshold voltage of ∼2 V at 300 K. Upon application of a constant voltage, the device demonstrated a gradual increase in current intensity followed by light emission. Notably, the delayed components of the current and EL were strongly suppressed at low temperatures (<285 K). The results clearly indicate that the device functions as an LEC and that the nano-LEC is a promising approach to realizing molecular-scale current-induced light sources.

Simultaneous Force and Fluorescence Spectroscopy on Single Chains of Polyfluorene: Effect of Intra-Chain Aggregate Coupling

Conjugated polymer chains in compact conformations or in films exhibit spectral features that can be attributed to interactions between individual conjugated segments of the chain, including formation of aggregates or excimers. Here, we use atomic force microscopy (AFM) on single chains of the conjugated polymer polyfluorene (PFO) to control the intersegment interactions by mechanically unfolding the chain. Simultaneously with the force spectroscopy we monitor fluorescence from the single PFO chains using a fluorescence microscope. We found that mechanical stretching of the chain causes disappearance of the green emission band. This observation provides evidence that the green emission originates from an intrachain aggregated state on the self-folded chain, which is decoupled by the stretching. In addition, the stretching upon laser irradiation leads to the appearance of additional features in the force spectra, small force peaks in the initial stages of the unfolding. These features are attributed to a combination of excitonic and van der Waals coupling of a ground-state intrachain aggregate.

Excitonically Coupled Simple Coacervates via Liquid/Liquid Phase Separation

Viscoelastic liquid coacervate phases that are highly enriched in nonconjugated polyelectrolytes are currently the subject of highly active research from biological and soft-materials perspectives. However, formation of a liquid, electronically active coacervate has proved highly elusive, since extended π-electron interactions strongly favor the solid state. Herein we show that a conjugated polyelectrolyte can be rationally designed to undergo aqueous liquid/liquid phase separation to form a liquid coacervate phase. This result is significant both because it adds to the fundamental understanding of liquid/liquid phase separation but also because it opens intriguing applications in light harvesting and beyond. We find that the semiconducting coacervate is intrinsically excitonically coupled, allowing for long-range exciton diffusion in a strongly correlated, fluctuating environment. The emergent excitonic states are comprised of both excimers and H-aggregates.

Real-time Observation of Structural Dynamics Triggering Excimer Formation in a Perylene Bisimide Folda-dimer by Ultrafast Time-Domain Raman Spectroscopy

In π-conjugated organic photovoltaic materials, an excimer state has been generally regarded as a trap state which hinders efficient excitation energy transport. But despite wide investigations of the excimer for overcoming the undesirable energy loss, the understanding of the relationship between the structure of the excimer in stacked organic compounds and its properties remains elusive. Here, we present the landscape of structural dynamics from the excimer formation to its relaxation in a co-facially stacked archetypical perylene bisimide folda-dimer using ultrafast time-domain Raman spectroscopy. We directly captured vibrational snapshots illustrating the ultrafast structural evolution triggering the excimer formation along the interchromophore coordinate on the complex excited-state potential surfaces and following evolution into a relaxed excimer state. Not only does this work showcase the ultrafast structural dynamics necessary for the excimer formation and control of excimer characteristics but also provides important criteria for designing the π-conjugated organic molecules.

Stereoisomer-Independent Stable Blue Emission in Axial Chiral Difluorenol

Bulky conjugated molecules with high stability are the prerequisite for the overall improvement of performance in wide-bandgap semiconductors. Herein, a chiral difluorenol, 2,2′-(9,9′-spirobi[fluorene]-2,2′-diyl)bis(9-(4-(octyloxy)phenyl)-9H-fluoren-9-ol) (DOHSBF), is set as a desirable model to reveal the stereoisomeric effects of wide-bandgap molecules toward controlling photophysical behavior and improving thermal and optical stability. Three diastereomers are obtained and elucidated by NMR spectra. Interestingly, the effect of modifying the stereo-centers is not observed on optical properties in solutions, pristine films, or post-treated film states. All three diastereomers as well as the mixture exhibit excellent spectral stability without undesirable green emission. Therefore, this stereoisomer-independent blue-emitting difluorenol will be a promising candidate for next-generation wide-bandgap semiconductors that would have extensive application in organic photonics.

Photoinduced Energy Transfer in Linear Guest-Host Chromophores: A Computational Study

Polymer-based guest-host systems represent a promising class of materials for efficient light-emitting diodes. The energy transfer from the polymer host to the guest is the key process in light generation. Therefore, microscopic descriptions of the different mechanisms involved in the energy transfer can contribute to enlighten the basis of the highly efficient light harvesting observed in this kind of materials. Herein, the nature of intramolecular energy transfer in a dye-end-capped conjugated polymer is explored by using atomistic nonadiabatic excited-state molecular dynamics. Linear perylene end-capped (PEC) polyindenofluorenes (PIF), consisting of n (n = 2, 4, and 6) repeat units, i.e., PEC-PIF_n oligomers, are considered as model systems. After photoexcitation at the oligomer absorption maximum, an initial exciton becomes self-trapped on one of the monomer units (donors). Thereafter, an efficient ultrafast through-space energy transfer from this unit to the perylene acceptor takes place. We observe that this energy transfer occurs equally well from any monomer unit on the chain. Effective specific vibronic couplings between each monomer and the acceptor are identified. These oligomer → end-cap energy transfer steps do not match with the rates predicted by Förster-type energy transfer. The through-space and through-bond mechanisms are two distinct channels of energy transfer. The former dominates the overall process, whereas the through-bond energy transfer between indenofluorene monomer units along the oligomer backbone only makes a minor contribution.

Real-Time Monitoring of Formation and Dynamics of Intra- and Interchain Phases in Single Molecules of Polyfluorene

Poly(9,9-dioctylfluorene) (PFO) is one of the most important conjugated polymer materials, exhibiting outstanding photophysical and electrical properties. PFO is also known for a diversity of morphological phases determined by conformational states of the main chain. Our goal in this work is to address some of the key questions on formation and dynamics of one such conformation, the β-phase, by following in real time the evolution of fluorescence spectra of single PFO chains. The PFO is dispersed in a thin polystyrene film, and the spectra are monitored during the process of solvent vapor annealing with toluene. We confirm unambiguously that the PFO β-phase segments are formed on a true single-chain level at room temperature in the solvent-softened polystyrene. We further find that the formation of the β-phase is a dynamic and reversible process occurring on the order of seconds, leading to repeated spontaneous transitions between the glassy and β-phase segments during the annealing. Comparison of PFO with two largely different molecular weights (M_w) shows that chains with lower M_w form the β-phase segments much faster. For the high M_w PFO chains, a detailed Franck-Condon analysis of the β-phase spectra shows a large distribution of the Huang-Rhys factor, S, and even dynamic changes of this factor occurring on a single chain. Such dynamics are likely a manifestation of changing coherence length of the exciton. Further, for the high M_w PFO chains we observe an additional conformational state, a crystalline γ-phase. The γ-phase formation is also a spontaneous reversible process in the solvent-softened matrix. The phase can form from both the β-phase and the glassy phase, and the formation requires high M_w to enable intersegment interactions in a self-folded chain.

Highly Efficient Aggregation-Induced Electrochemiluminescence of Polyfluorene Derivative Nanoparticles Containing Tetraphenylethylene

The aggregation-induced electrochemiluminescence (AIECL) of polyfluorene derivative nanoparticles containing tetraphenylethylene (TPE) in aqueous media is reported in this work. The TPE unit limits the intramolecular free rotation of phenyl rings, as well as the π-π stacking interactions of molecules, which significantly enhances ECL signal of the polyfluorene nanoparticles. With co-reactants of tri-n-propylamine (TPrA) and S2O82-, the copolymer nanoparticles show visualized ECL emissions at both positive and negative potentials. The ECL efficiency of copolymer nanoparticles in solid state is 163% compared with that of standard ECL species, Ru(bpy)32+. And at negative potential, the ECL intensity of copolymer nanoparticles is even stronger with 6.5 times compared with that at positive potential. The ECL generation mechanisms are analyzed detailed by annihilation and co-reactant route transient ECL test (millisecond scale). This work provides a reference for the organic structure design for AIECL and shows promising potential in luminescent device and biological applications.

Single-particle electroluminescence of CsPbBr3 perovskite nanocrystals reveals particle-selective recombination and blinking as key efficiency factors

Halide perovskites nanocrystals (NCs) are being explored as promising materials for optoelectronic applications, such as light-emitting devices or lasers. However, electroluminescence devices prepared from such NCs have long suffered from low efficiency and there has been no systematic study on the nanoscale origin of the poor efficiencies. Here, we use single-particle spectroscopy to compare electroluminescence and photoluminescence on the level of individual NCs of the perovskite CsPbBr₃. The NCs form aggregates in a conducting matrix used as an emission layer in an electroluminescence device. In electroluminescence, only a small fraction of the NCs within the aggregate is emitting as a result of efficient charge migration, accumulation and selective recombination on larger NCs, leading to pronounced blinking and decreased efficiency. Under the condition of comparable excitation rates in both electroluminescence and photoluminescence, the intrinsic quantum yield in electroluminescence is on average 0.36 of that in photoluminescence.

Halide perovskite nanocrystals show high photoluminescence (PL) but their electroluminescence (EL) devices suffered from low external quantum efficiency. Here Sharma et al. study both individual and statistical PL and EL behaviors of these nanocrystals and reveal the origin of the lower EL efficiency.

Hierarchical Uniform Supramolecular Conjugated Spherulites with Suppression of Defect Emission

Easily processed, well-defined, and hierarchical uniform artificial architectures with intrinsic strong crystalline emission properties are necessary for a range of light-emitting optoelectronic devices. Herein, we designed and prepared ordered supramolecular spherulites, comprising planar conformational molecules as primary structures and multiple hydrogen bonds as physical cross-links. Compared with serious aggregation-induced fluorescence quenching (up to 70%), these highly ordered architectures exhibited unique and robust crystalline emission with a high PLQY of 55%, which was much higher than those of other terfluorenes. The primary reasons for the high PLQY are the uniform exciton energetic landscape created in the planar conformation and the highly ordered molecular packing in spherulite. Meanwhile, minimal residual defect (green-band) emissions are effectively suppressed in our oriented crystalline framework, whereas the strong and stable blue light radiations are promoted. These findings may confirm that supramolecular ordered artificial architectures may offer higher control and tunability for optoelectronic applications.

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Coplanar molecular conformation is stabilized in supramolecular crystalline frameworks

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Spiro-terfluorene can self-assemble into hierarchical well-defined spherulites

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Ordered and uniform condensed structures suppress defect emission

Optically switchable organic light-emitting transistors

Organic light-emitting transistors are pivotal components for emerging opto- and nanoelectronics applications, such as logic circuitries and smart displays. Within this technology sector, the integration of multiple functionalities in a single electronic device remains the key challenge. Here we show optically switchable organic light-emitting transistors fabricated through a judicious combination of light-emitting semiconductors and photochromic molecules. Irradiation of the solution-processed films at selected wavelengths enables the efficient and reversible tuning of charge transport and electroluminescence simultaneously, with a high degree of modulation (on/off ratios up to 500) in the three primary colours. Different emitting patterns can be written and erased through a non-invasive and mask-free process, on a length scale of a few micrometres in a single device, thereby rendering this technology potentially promising for optically gated highly integrated full-colour displays and active optical memory.

Stark Effect and Environment-Induced Modulation of Emission in Single Halide Perovskite Nanocrystals

Organic-inorganic halide perovskites have emerged as promising materials for next-generation solar cells. In nanostructured form also, these materials are excellent candidates for optoelectronic applications such as lasers and light-emitting diodes for displays and lighting. While great progress has been achieved so far in optimizing the intrinsic photophysical properties of perovskite nanocrystals (NCs), in working optoelectronic devices, external factors, such as the effects of conducting environment and the applied electric field on exciton generation and photon emission, have been largely unexplored. Here, we use NCs of the all-inorganic perovskite CsPbBr₃ dispersed polyvinyl carbazole, a hole-conductor, and in poly(methyl methacrylate), an insulator, to examine the effects of applied electric field and conductivity of the matrix on the perovskite photophysics at the single-particle level. We found that the conducting environment causes a significant decrease of photoluminescence (PL) brightness of individual NCs due the appearance of intermediate-intensity emitting states with significantly shortened lifetime. Applied electric field has a similar effect and, in addition, causes a nonlinear spectral shift of the PL maxima, a combination of linear and quadratic Stark effects caused by environment-induced polarity and field-related polarizability. The environment and electric-field effects are explained by ionization of the NCs through hole transfer and emission of the resulting negatively charged excitons.

Effect of side chains on color purities of mono-triphenylamine-functionalized polyspirobifluorenes for pure blue polymer light-emitting diodes

Large and rigid substituents may facilitate the improvement of the color purity of polyfluorenes.

Ultrastable Supramolecular Self-Encapsulated Wide-Bandgap Conjugated Polymers for Large-Area and Flexible Electroluminescent Devices

Controlling chain behavior through smart molecular design provides the potential to develop ultrastable and efficient deep-blue light-emitting conjugated polymers (LCPs). Herein, a novel supramolecular self-encapsulation strategy is proposed to construct a robust ultrastable conjugated polydiarylfluorene (PHDPF-Cz) via precisely preventing excitons from interchain cross-transfer/coupling and contamination from external trace H₂ O/O₂ . PHDPF-Cz consists of a mainchain backbone where the diphenyl groups localize at the 9-position as steric bulk moieties, and carbazole (Cz) units localize at the 4-position as supramolecular π-stacked synthon with the dual functionalities of self-assembly capability and hole-transport facility. The synergistic effect of the steric bulk groups and π-stacked carbazoles affords PHDPF-Cz as an ultrastable property, including spectral, morphological stability, and storage stability. In addition, PHDPF-Cz spin-coated gelation films also show thickness-insensitive deep-blue emission with respect to the reference polymers, which are suitable to construct solution-processed large-scale optoelectronic devices with higher reproducibility. High-quality and uniform deep-blue emission is observed in large-area solution-processed films. The electroluminescence shows high-quality deep-blue intrachain emission with a CIE (0.16, 0.12) and a very narrow full width at half-maximum of 32 nm. Finally, large-area and flexible polymer light-emitting devices with a single-molecular excitonic behavior are also fabricated. The supramolecular self-encapsulation design provides an effective strategy to construct ultrastable LCPs for optoelectronic applications.

Macrocyclic poly(p-phenylenevinylene)s by ring expansion metathesis polymerisation and their characterisation by single-molecule spectroscopy

Ring expansion metathesis polymerisation (REMP) has proven to be a viable approach to prepare high purity macrocyclic phenylenevinylene polymers.

Ring expansion metathesis polymerisation (REMP) has proven to be a viable approach to prepare high purity cyclic polymers. Macrocyclic polymers with a fully conjugated defect free backbone are of particular interest as these polymers have no end groups that can act as charge traps. In this work soluble macrocyclic poly(p-phenylenevinylene)s (cPPVs) have been prepared directly via the REMP of substituted paracyclophanedienes. Single-molecule spectroscopy of the two topological forms of PPV i.e., linear (lPPV) and cyclic (cPPV) revealed that lPPV exists in an extended conformation whereas the cPPV adopts a restricted ring-like conformation. Despite such large differences in the chain conformation, the spectral properties of the two compounds are unexpectedly very similar, and are dominated by torsional deformations in relatively short conjugated segments.

Effects of molecular architecture on morphology and photophysics in conjugated polymers: from single molecules to bulk

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.

Photophysics and electroluminescence of single nanocrystals of halide perovskites and related nanomaterials

We report simultaneous photoluminescence and electroluminescence single-particle study of nanocrystals of inorganic halide perovskite CsPbBr3, as well as of ternary I-III-IV semiconductor quantum dots.

Geminal cross-coupling synthesis, ion-induced emission and lysosome imaging of cationic tetraarylethene oligoelectrolytes

Novel ratiometric AIE-active tetraarylethene oligoelectrolytes synthesized by geminal cross-coupling show ion-induced emission and are applied as bioprobes for lysosome tracing.

Suppression of Polyfluorene Photo-Oxidative Degradation via Encapsulation of Single-Walled Carbon Nanotubes

Polyfluorenes have achieved noteworthy performance in organic electronic devices but exhibit undesired green band emission under photo-oxidative conditions that have limited their broad utility in optoelectronic applications. In addition, polyfluorenes are well-known dispersants of single-walled carbon nanotubes (SWCNTs), although the influence of SWCNTs on polyfluorene photo-oxidative stability has not yet been defined. Here we quantitatively explore the photophysical properties of poly[(9,9-bis(3^'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) under photo-oxidative conditions when it is in van der Waals contact with SWCNTs. Photoluminescence spectroscopy tracks the spectral evolution of the polymer emission following ambient ultraviolet (UV) exposure, confirming that PFN exhibits green band emission. In marked contrast, PFN-wrapped SWCNTs possess high spectral stability without green band emission under the same ambient UV exposure conditions. By investigating a series of PFN thin films as a function of SWCNT content, it is shown that SWCNT loadings as low as ∼23 wt % suppress photo-oxidative degradation. These findings suggest that PFN-SWCNT composites provide an effective pathway toward utilizing polyfluorenes in organic optoelectronics.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.

Single-particle spectroscopy of I-III-VI semiconductor nanocrystals: spectral diffusion and suppression of blinking by two-color excitation

Ternary I-III-VI semiconductor nanocrystals have been explored as non-toxic alternatives to II-VI semiconductors for optoelectronic and sensing applications, but large photoluminescence spectral width and moderate brightness restrict their practical use. Here, using single-particle photoluminescence spectroscopy on nanocrystals of (AgIn)xZn2(1-x)S2 we show that the photoluminescence band is inhomogeneously broadened and that size distribution is the dominant factor in the broadening. The residual homogeneous linewidth of individual nanocrystals reaches up to 75% of the ensemble spectral width. Single nanocrystals undergo spectral diffusion which also contributes to the inhomogeneous band. Excitation with two lasers with energies above and below the bandgap reveals coexistence of two emitting donor states within one particle. Spectral diffusion in such particles is due to temporal activation and deactivation of one such state. Filling of a trap state with a lower-energy laser enables optical modulation of photoluminescence intermittency (blinking) and leads to an almost two-fold increase in brightness.

Operating organic light-emitting diodes imaged by super-resolution spectroscopy

Super-resolution stimulated emission depletion (STED) microscopy is adapted here for materials characterization that would not otherwise be possible. With the example of organic light-emitting diodes (OLEDs), spectral imaging with pixel-by-pixel wavelength discrimination allows us to resolve local-chain environment encoded in the spectral response of the semiconducting polymer, and correlate chain packing with local electroluminescence by using externally applied current as the excitation source. We observe nanoscopic defects that would be unresolvable by traditional microscopy. They are revealed in electroluminescence maps in operating OLEDs with 50 nm spatial resolution. We find that brightest emission comes from regions with more densely packed chains. Conventional microscopy of an operating OLED would lack the resolution needed to discriminate these features, while traditional methods to resolve nanoscale features generally cannot be performed when the device is operating. This points the way towards real-time analysis of materials design principles in devices as they actually operate.

There is a need to characterize devices during operation in real-time and at nanoscopic length scales. Here, King et al. perform electroluminescence-STED imaging with a polymer based light-emitting diode, revealing nanoscopic defects that would be unresolvable with traditional optical microscopy.

High Color-Purity Green, Orange, and Red Light-Emitting Diodes Based on Chemically Functionalized Graphene Quantum Dots

Chemically derived graphene quantum dots (GQDs) to date have showed very broad emission linewidth due to many kinds of chemical bondings with different energy levels, which significantly degrades the color purity and color tunability. Here, we show that use of aniline derivatives to chemically functionalize GQDs generates new extrinsic energy levels that lead to photoluminescence of very narrow linewidths. We use transient absorption and time-resolved photoluminescence spectroscopies to study the electronic structures and related electronic transitions of our GQDs, which reveals that their underlying carrier dynamics is strongly related to the chemical properties of aniline derivatives. Using these functionalized GQDs as lumophores, we fabricate light-emitting didoes (LEDs) that exhibit green, orange, and red electroluminescence that has high color purity. The maximum current efficiency of 3.47 cd A⁻¹ and external quantum efficiency of 1.28% are recorded with our LEDs; these are the highest values ever reported for LEDs based on carbon-nanoparticle phosphors. This functionalization of GQDs with aniline derivatives represents a new method to fabricate LEDs that produce natural color.

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.

Conjugated Polymer Nanoparticle-Triplet Emitter Hybrids in Aqueous Dispersion: Fabrication and Fluorescence Quenching Behavior

Conjugated polymer nanoparticles based on poly[9,9-bis(2-ethylhexyl)fluorene] and poly[N-(2,4,6-trimethylphenyl)-N,N-diphenylamine)-4,4'-diyl] are fabricated using anionic surfactant sodium dodecylsulphate in water by miniemulsion technique. Average diameters of polyfluorene and polytriarylamine nanoparticles range from 70 to 100 and 100 to 140 nm, respectively. The surface of the nanoparticles is decorated with triplet emitting dye, tris(2,2'-bipyridyl)ruthenium(II) chloride. Intriguing photophysics of aqueous dispersions of these hybrid nanoparticles is investigated. Nearly 50% quenching of fluorescence is observed in the case of dye-coated polyfluorene nanoparticles; excitation energy transfer is found to be the dominant quenching mechanism. On the other hand, nearly complete quenching of emission is noticed in polytriarylamine nanoparticle-dye hybrids. It is proposed that the excited state electron transfer from the electron-rich polytriarylamine donor polymer to Ru complex leads to the complete quenching of emission of polytriarylamine nanoparticles. The current study offers promising avenues for developing aqueous solution processed-electroluminescent devices involving a conjugated polymer nanoparticle host and Ru or Ir-based triplet emitting dye as the guest.

Control of Photoluminescence of Carbon Nanodots via Surface Functionalization using Para-substituted Anilines

Carbon nanodots (C-dots) are a kind of fluorescent carbon nanomaterials, composed of polyaromatic carbon domains surrounded by amorphous carbon frames, and have attracted a great deal of attention because of their interesting properties. There are still, however, challenges ahead such as blue-biased photoluminescence, spectral broadness, undefined energy gaps and etc. In this report, we chemically modify the surface of C-dots with a series of para-substituted anilines to control their photoluminescence. Our surface functionalization endows our C-dots with new energy levels, exhibiting long-wavelength (up to 650 nm) photoluminescence of very narrow spectral widths. The roles of para-substituted anilines and their substituents in developing such energy levels are thoroughly studied by using transient absorption spectroscopy. We finally demonstrate light-emitting devices exploiting our C-dots as a phosphor, converting UV light to a variety of colors with internal quantum yields of ca. 20%.

Temporal Fluctuations in Excimer-Like Interactions between π-Conjugated Chromophores

Inter- or intramolecular coupling processes between chromophores such as excimer formation or H- and J-aggregation are crucial to describing the photophysics of closely packed films of conjugated polymers. Such coupling is highly distance dependent and should be sensitive to both fluctuations in the spacing between chromophores as well as the actual position on the chromophore where the exciton localizes. Single-molecule spectroscopy reveals these intrinsic fluctuations in well-defined bichromophoric model systems of cofacial oligomers. Signatures of interchromophoric interactions in the excited state--spectral red shifting and broadening and a slowing of photoluminescence decay--correlate with each other but scatter strongly between single molecules, implying an extraordinary distribution in coupling strengths. Furthermore, these excimer-like spectral fingerprints vary with time, revealing intrinsic dynamics in the coupling strength within one single dimer molecule, which constitutes the starting point for describing a molecular solid. Such spectral sensitivity to sub-Ångström molecular dynamics could prove complementary to conventional FRET-based molecular rulers.

Inhomogeneity in the excited-state torsional disorder of a conjugated macrocycle

The photophysics of conjugated polymers has generally been explained based on the interactions between the component conjugated chromophores in a tangled chain. However, conjugated chromophores are entities with static and dynamic structural disorder, which directly affects the conjugated polymer photophysics. Here we demonstrate the impact of chain structure torsional disorder on the spectral characteristics for a macrocyclic oligothiophene 1, which is obscured in conventional linear conjugated chromophores by diverse structural disorders such as those in chromophore size and shape. We used simultaneous multiple fluorescence parameter measurement for a single molecule and quantum-mechanical calculations to show that within the fixed conjugation length across the entire ring an inhomogeneity from torsional disorder in the structure of 1 plays a crucial role in causing its energetic disorder, which affords the spectral broadening of ∼220 meV. The torsional disorder in 1 fluctuated on the time scale of hundreds of milliseconds, typically accompanied by spectral drifts on the order of ∼10 meV. The fluctuations could generate torsional defects and change the electronic structure of 1 associated with the ring symmetry. These findings disclose the fundamental nature of conjugated chromophore that is the most elementary spectroscopic unit in conjugated polymers and suggest the importance of engineering structural disorder to optimize polymer-based device photophysics. Additionally, we combined defocused wide-field fluorescence microscopy and linear dichroism obtained from the simultaneous measurements to show that 1 emits polarized light with a changing polarization direction based on the torsional disorder fluctuations.

H- and J-aggregation of fluorene-based chromophores

Understanding of H- and J-aggregation behaviors in fluorene-based polymers is significant both for determining the origin of various red-shifted emissions occurring in blue-emitting polyfluorenes and for developing polyfluorene-based device performance. In this contribution, we demonstrate a new theory of the H- and J-aggregation of polyfluorenes and oligofluorenes, and understand the influence of chromosphere aggregation on their photoluminescent properties. H- and J-aggregates are induced by a continuous increasing concentration of the oligofluorene or polyfluorene solution. A relaxed molecular configuration is simulated to illustrate the spatial arrangement of the bonding of fluorenes. It is indicated that the relaxed state adopts a 21 helical backbone conformation with a torsion angle of 18° between two connected repeat units. This configuration makes the formation of H- and J-aggregates through the strong π-π interaction between the backbone rings. A critical aggregation concentration is observed to form H- and J-aggregates for both polyfluorenes and oligofluorenes. These aggregates show large spectral shifts and distinct shape changes in photoluminescent excitation (PLE) and emission (PL) spectroscopy. Compared with "isolated" chromophores, H-aggregates induce absorption spectral blue-shift and fluorescence spectral red-shift but largely reduce fluorescence efficiency. "Isolated" chromophores not only refer to "isolated molecules" but also include those associated molecules if their conjugated backbones are not compact enough to exhibit perturbed absorption and emission. J-aggregates induce absorption spectral red-shift and fluorescence spectral red-shift but largely enhance fluorescence efficiency. The PLE and PL spectra also show that J-aggregates dominate in concentrated solutions. Different from the excimers, the H- and J-aggregate formation changes the ground-state absorption of fluorene-based chromophores. H- and J-aggregates show changeable absorption and emission derived from various interchain interactions, unlike the β phase, which has relatively fixed absorption and emission derived from an intrachain interaction.