Dual species emission from single polyfluorene molecules: signatures of stress-induced planarization of single polymer chains

Hexylation Stabilises Twisted Backbone Configurations in the Prototypical Low-Bandgap Copolymer PCDTBT

Conjugated donor-acceptor copolymers hold great potential as materials for high-performance organic photovoltaics, organic transistors and organic thermoelectric devices. Their low optical bandgap is achieved by alternation of donor and acceptor moieties along the polymer chain, leading to a pronounced charge-transfer character of electronic excitations. However, the influence of appended side chains and of chemical defects of the backbone on their photophysical and conformational properties remains largely unexplored on the level of individual chains. Here, we employ room temperature single-molecule photoluminescence spectroscopy on four compounds based on the prototypical copolymer PCDTBT with systematically changed chemical structure. Our results show that an increasing density of statistically added hexyl chains to the TBT comonomer distorts the molecular conformation, likely through the increase of average dihedral angles along the backbone. We find that, although the conformation becomes more twisted with high hexyl density, the side chains appear to stabilize the backbone in this twisted conformation. In addition, we demonstrate that homocoupling defects along the backbone barely influence the PL spectra of single chains, and thus intra-chain electronic properties.

Coplanar Conformational Structure of π-Conjugated Polymers for Optoelectronic Applications

Hierarchical structure of conjugated polymers is critical to dominating their optoelectronic properties and applications. Compared to nonplanar conformational segments, coplanar conformational segments of conjugated polymers (CPs) demonstrate favorable properties for applications as a semiconductor. Herein, recent developments in the coplanar conformational structure of CPs for optoelectronic devices are summarized. First, this review comprehensively summarizes the unique properties of planar conformational structures. Second, the characteristics of the coplanar conformation in terms of optoelectrical properties and other polymer physics characteristics are emphasized. Five primary characterization methods for investigating the complanate backbone structures are illustrated, providing a systematical toolbox for studying this specific conformation. Third, internal and external conditions for inducing the coplanar conformational structure are presented, offering guidelines for designing this conformation. Fourth, the optoelectronic applications of this segment, such as light-emitting diodes, solar cells, and field-effect transistors, are briefly summarized. Finally, a conclusion and outlook for the coplanar conformational segment regarding molecular design and applications are provided.

Thermal Degradation of Photoluminescence Poly(9,9-dioctylfluorene) Solvent-Tuned Aggregate Films

The progression of the green emission spectrum during the decomposition of polyfluorenes (PFs) has impeded the development and commercialization of the materials. Herein, we constructed a solvent-tuned aggregated PFO film with the aim of retarding the material’s thermal degradation behavior which causes a significant decline in optical properties as a result of phase transformation. The tuning of the aggregate amount and distribution was executed by applying a poor alcohol-based solvent in chloroform. It emerges that at a lower boiling point methanol evaporates quickly, limiting the aggregate propagation in the film which gives rise to a more transparent film. Furthermore, because of the modulated β-phase conformation, the absorption spectra of PFO films were red-shifted and broadened. The increase in methanol percentage also led to a rise in β-phase percentage. As for the thermal degradation reactions, both pristine and aggregated PFO films exhibited apparent changes in the UV-Vis spectra and PL spectra. In addition, a 97:3 (chloroform:methanol) aggregated PFO film showed a more defined emission spectrum, which demonstrates that the existence of β-phase is able to suppress the unwanted green emission.

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.

Control of Intrachain Morphology in the Formation of Polyfluorene Aggregates on the Single-Molecule Level

Controlling the morphology of π-conjugated polymers for organic optoelectronic devices has long been a goal in the field of materials science. Since the morphology of a polymer chain is closely intertwined with its photophysical properties, it is desirable to be able to change the arrangement of the polymers at will. We investigate the π-conjugated polymer poly(9,9-dioctylfluorene) (PFO), which can exist in three distinctly different structural phases: the α-, β-, and γ-phase. Every phase has a different chain structure and a unique photoluminescence (PL) spectrum. Due to its unique properties and the pronounced spectral structure-property relations, PFO can be used as a model system to study the morphology of π-conjugated polymers. To avoid ensemble averaging, we examine the PL spectrum of single PFO chains embedded in a non-fluorescent matrix. With single-molecule spectroscopy the structural phase of every single chain can be determined, and changes can be monitored very easily. To manipulate the morphology, solvent vapor annealing (SVA) was applied, which leads to a diffusion of the polymer chains in the matrix. The β- and γ-phases appear during the self-assembly of single α-phase PFO chains into mesoscopic aggregates. The extent of β- and γ-phase formation is directed by the solvent-swelling protocol used for aggregation. Aggregation unequivocally promotes formation of the more planar β- and γ-phases. Once these lower-energy more ordered structural phases are formed, SVA cannot return the polymer chain to the less ordered phase by aggregate swelling.

Gel Trapping Enables Optical Spectroscopy of Single Solvated Conjugated Polymers in Equilibrium

Single-molecule studies have provided a wealth of insight into the photophysics of conjugated polymers in the solid and desolvated state. Desolvating conjugated chains, e.g., by their embedding in inert solid matrices, invariably leads to chain collapse and the formation of intermolecular aggregates, which have a pronounced effect on their properties. By contrast, the luminescent properties of individual semiconducting polymers in their solvated and thermodynamic state remain largely unexplored. In this paper, we demonstrate a versatile gel trapping technique that enables the chemistry-free immobilization and interrogation of individual conjugated macromolecules, which retain a fully equilibrated conformation by contrast to conventional solid-state immobilization methods. We show how the technique can be used to record full luminescence spectra of single chains, to evaluate their time-resolved fluorescence, and to probe their photodynamics. Finally, we explore how the photophysics of different conjugated polymers is strongly affected by desolvation and chain collapse.

Interplay Between J- and H-Type Coupling in Aggregates of π-Conjugated Polymers: A Single-Molecule Perspective

Strong dipole–dipole coupling within and between π‐conjugated segments shifts electronic transitions, and modifies vibronic coupling and excited‐state lifetimes. Since J‐type coupling between monomers along the conjugated‐polymer (CP) chain and H‐type coupling of chromophores between chains of a CP compete, a superposition of the spectral modifications arising from each type of coupling emerges, making the two couplings hard to discern in the ensemble. We introduce a single‐molecule H‐type aggregate of fixed spacing and variable length of up to 10 nm. HJ‐type aggregate formation is visualized intuitively in the scatter of single‐molecule spectra.

Temporally Programmed Disassembly and Reassembly of C3Ms

Responsive materials, which can adapt and operate autonomously under dynamic conditions, are a stepping stone towards functional, life-like systems inspired by fueled self-assembly processes in nature. Complex coacervate core micelles (C3Ms) comprising oppositely charged macromolecules constitute a novel class of polymeric micelles ideally suited for use as responsive nanoscopic delivery vehicles of hydrophilic and hydrophobic cargo. To fully exploit their potential, it is important that the C3Ms form and fall apart in an autonomous fashion as orchestrated by dynamic cues in their environment. Herein a means to temporally program the self-regulated C3M coassembly pathway, using a modulated base-catalyzed feedback system, is presented. Incorporated in the C3Ms is a pH responsive polyfluorene-based conjugated polyelectrolyte (CPF) as a building block and trace amounts of a molecular sensor (doxorubicin HCl) as cargo, both of which report on micellar coassembly and disassembly via binding-induced fluorescence quenching. CPF additionally reports on the pH of its microenvironment as its pH-dependent conformational states are mirrored in the transitions of its vibronic bands. This experimental design enables one to monitor solution pH, C3M disassembly and reassembly, as well as cargo release and recapture noninvasively in a closed system with real time florescence experiments.

Photophysical and Fluorescence Anisotropic Behavior of Polyfluorene β-Conformation Films

We demonstrate a systematic visualization of the unique photophysical and fluorescence anisotropic properties of polyfluorene coplanar conformation (β-conformation) using time-resolved scanning confocal fluorescence imaging (FLIM) and fluorescence anisotropy imaging microscopy (FAIM) measurements. We observe inhomogeneous morphologies and fluorescence decay profiles at various micrometer-sized regions within all types of polyfluorene β-conformational spin-coated films. Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) and poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF) β-domains both have shorter lifetime than those of the glassy conformation for the longer effective conjugated length and rigid chain structures. Besides, β-conformational regions have larger fluorescence anisotropy for the low molecular rotational motion and high chain orientation, while the low anisotropy in glassy conformational regions shows more rotational freedom of the chain and efficient energy migration from amorphous regions to β-conformation as a whole. Finally, ultrastable ASE threshold in the PODPF β-conformational films also confirms its potential application in organic lasers. In this regard, FLIM and FAIM measurements provide an effective platform to explore the fundamental photophysical process of conformational transitions in conjugated polymer.

Switching between H- and J-type electronic coupling in single conjugated polymer aggregates

The aggregation of conjugated polymers and electronic coupling of chromophores play a central role in the fundamental understanding of light and charge generation processes. Here we report that the predominant coupling in isolated aggregates of conjugated polymers can be switched reversibly between H-type and J-type coupling by partially swelling and drying the aggregates. Aggregation is identified by shifts in photoluminescence energy, changes in vibronic peak ratio, and photoluminescence lifetime. This experiment unravels the internal electronic structure of the aggregate and highlights the importance of the drying process in the final spectroscopic properties. The electronic coupling after drying is tuned between H-type and J-type by changing the side chains of the conjugated polymer, but can also be entirely suppressed. The types of electronic coupling correlate with chain morphology, which is quantified by excitation polarization spectroscopy and the efficiency of interchromophoric energy transfer that is revealed by the degree of single-photon emission.

Understanding the Structural Evolution of Single Conjugated Polymer Chain Conformers

Single molecule photoluminescence (PL) spectroscopy of conjugated polymers has shed new light on the complex structure–function relationships of these materials. Although extensive work has been carried out using polarization and excitation intensity modulated experiments to elucidate conformation-dependent photophysics, surprisingly little attention has been given to information contained in the PL spectral line shapes. We investigate single molecule PL spectra of the prototypical conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) which exists in at least two emissive conformers and can only be observed at dilute levels. Using a model based on the well-known “Missing Mode Effect” (MIME), we show that vibronic progression intervals for MEH-PPV conformers can be explained by relative contributions from particular skeletal vibrational modes. Here, observed progression intervals do not match any ground state Raman active vibrational frequency and instead represent a coalescence of multiple modes in the frequency domain. For example, the higher energy emitting “blue” MEH-PPV form exhibits PL maxima at ~18,200 cm⁻¹ with characteristic MIME progression intervals of ~1200–1350 cm⁻¹, whereas the lower energy emitting “red” form peaks at ~17,100 cm⁻¹ with intervals in the range of ~1350–1450 cm⁻¹. The main differences in blue and red MEH-PPV chromophores lie in the intra-chain order, or, planarity of monomers within a chromophore segment. We demonstrate that the Raman-active out-of-plane C–H wag of the MEH-PPV vinylene group (~966 cm⁻¹) has the greatest influence in determining the observed vibronic progression MIME interval. Namely, larger displacements (intensities)—indicating lower intra-chain order—lower the effective MIME interval. This simple model provides useful insights into the conformational characteristics of the heterogeneous chromophore landscape without requiring costly and time-consuming low temperature or single molecule Raman capabilities.

Excitonic linewidth of organic quantum wires generated in reduced dimensionality matrices

Nanostructured crystalline film achieving a 2D bath for single conjugated polymer chain linewidth spectroscopy.

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.

Chromophore Bending Controls Fluorescence Lifetime in Single Conjugated Polymer Chains

Single-molecule spectroscopy of conjugated polymers offers unique insight into the interplay between the spatial arrangement of monomer units-twisting and bending-and the characteristics of the primary excitonic photoexcitation, provided that a single conjugated segment can be isolated. β-phase polyfluorene constitutes an ideal model to study variations in intermonomeric coupling, determined by nanoscale molecular shape, on the fundamental optical transition. If structural relaxation in the excited state is weak, exciton self-trapping occurs stochastically along the conjugated segment. Bending of the π-system is then revealed by a reduction in single-photon polarization anisotropy, correlating directly with increased fluorescence lifetime. Strong relaxation raises emission anisotropy because of deterministic exciton localization, decelerating fluorescence decay due to a decrease in exciton coherence length.

Cβ conformer formation in poly(9,9-dioctylfluorene) single chains facilitated by endcapping with an electron deficient moiety

Using single molecule spectroscopy, we reveal that endcapping poly(9,9-dioctylfluorene) (PFO) with an electron deficient moiety (EDM) can promote the population of C_β conformers at the intra-chain level.

Light-harvesting capabilities of low band gap donor–acceptor polymers

The ability of three new and six existing donor–acceptor polymers to absorb strongly at low photon energies is studied experimentally and theoretically, and is related to their chemical structures.

Methods for controlling structure and photophysical properties in polyfluorene solutions and gels

Knowledge of the phase behavior of polyfluorene solutions and gels has expanded tremendously in recent years. The relationship between the structure formation and photophysics is known at the quantitative level. The factors which we understand control these relationships include virtually all important materials parameters such as solvent quality, side chain branching, side chain length, molecular weight, thermal history and myriad functionalizations. This review describes advances in controlling structure and photophysical properties in polyfluorene solutions and gels. It discusses the demarcation lines between solutions, gels, and macrophase separation in conjugated polymers and reviews essential solid state properties needed for understanding of solutions. It gives an insight into polyfluorene and polyfluorene beta phase in solutions and gels and describes all the structural levels in solvent matrices, ranging from intramolecular structures to the diverse aggregate classes and network structures and agglomerates of these units. It goes on to describe the kinetics and thermodynamics of these structures. It details the manifold molecular parameters used in their control and continues with the molecular confinement and touches on permanently cross-linked networks. Particular focus is placed on the experimental results of archetypical polyfluorenes and solvent matrices and connection between structure and photonics. A connection is also made to the mean field type theories of hairy-rod like polymers. This altogether allows generalizations and provides a guideline for materials scientists, synthetic chemists and device engineers as well, for this important class of semiconductor, luminescent polymers.

Solvent Vapor Annealing of Single Conjugated Polymer Chains: Building Organic Optoelectronic Materials from the Bottom Up

Optoelectronic devices based on organic materials show a strong relationship between the morphological structure of the material and the function of the device. One of the grand challenges in improving the efficiencies of these devices is hence achieving morphological control throughout the entire course of processing. One of the most important postprocessing methods is solvent vapor annealing, which has repeatedly demonstrated its utility in improving the efficiency of organic-material-based devices by changing bulk-film morphology. This Perspective discusses the recent impact of single-molecule spectroscopy techniques in unraveling morphological changes and molecular dynamics and presents solvent vapor annealing as a tool to build organic optoelectronic materials from the bottom up. In particular, we discuss examples of how solvent vapor annealing at the single-chain level can be split into two different regimes, (i) the solvation regime, in which intrachain interactions and molecular dynamics during solvent vapor annealing can be probed, and (ii) the aggregation regime, in which the influence of interchain interactions can be probed. Finally, it will be shown that solvent vapor annealing in the aggregation regime can be used to build highly ordered mesoscopic objects with distinct properties such as long-range energy transfer.

Network structure of polyfluorene sheets as a function of alkyl side chain length

The formation of self-organized structures in poly(9,9-di-n-alkylfluorene)s ∼1 vol % methylcyclohexane (MCH) and deuterated MCH (MCH-d(14)) solutions was studied at room temperature using neutron and x-ray scattering (with the overall q range of 0.00058-4.29 Å(-1)) and optical spectroscopy. The number of side chain carbons (N) ranged from 6 to 10. The phase behavior was rationalized in terms of polymer overlap, cross-link density, and blending rules. For N=6-9, the system contains isotropic areas and lyotropic areas where sheetlike assemblies (lateral size of >400 Å) and free polymer chains form ribbonlike agglomerates (characteristic dimension of >1500 Å) leading to a gel-like appearance of the solutions. The ribbons are largely packed together with surface fractal characteristics for N=6-7 but become open networklike structures with mass fractal characteristics for N=8-9, until the system goes through a transition to an isotropic phase of overlapping rodlike polymers for N=10. The polymer order within sheets varies allowing classification for loose membranes and ordered sheets, including the so-called β phase. The polymers within the ordered sheets have restricted motion for N=6-7 but more freedom to vibrate for N=8-9. The nodes in the ribbon network are suggested to contain ordered sheets cross-linking the ribbons together, while the nodes in the isotropic phase appear as weak density fluctuations cross-linking individual chains together. The tendencies for macrophase separation and the formation of non beta sheets decrease while the proportion of free chains increases with increasing N. The fraction of β phase varies nonlinearly, reaching its maximum at N = 8.

Exciton localization in disordered poly(3-hexylthiophene)

Singlet exciton localization in conformationally disordered poly(3-hexylthiophene) (P3HT) is investigated via configuration interaction (singles) calculations of the Pariser-Parr-Pople model. The P3HT structures are generated by molecular dynamics simulations. The lowest-lying excitons are spatially localized, space filling, and nonoverlapping. These define spectroscopic segments or chromophores. The strong conformational disorder in P3HT causes breaks in the pi-conjugation. Depending on the relative values of the disorder-induced localization length and the distances between the pi-conjugation breaks, these breaks sometimes serve to pin the low-lying localized excitons. The exciton confinement also causes a local spectrum of low-lying exciton states. Coulomb-induced intra- or interchain interactions between spectroscopic segments in close spatial proximity can delocalize an exciton across these segments, in principle causing phase coherent transition dipole moments.

Observation of the missing mode effect in a poly-phenylenevinylene derivative: effect of solvent, chain packing, and composition

Optical emission spectra of poly[2-methoxy-5-[3('),7(')-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in dilute solutions exhibit a vibronic progression interval (∼1225 cm(-1)) that does not correspond to any ground state vibrational mode frequency. This phenomenon is assigned as the missing mode effect (MIME) in which five key displaced polymer backbone vibrational modes in the range of 800-1600 cm(-1) contribute to the MIME interval. Emission spectra are calculated by analytically solving the time-dependent Schrödinger equation using estimates of mode-specific vibrational displacements determined independently from preresonance Raman intensities. Emission spectra of MDMO-PPV thin films and nanoparticles are measured and lineshapes show an increase of the MIME frequency to ∼1340 cm(-1) in addition to changes in vibronic intensity distributions and energies. Composite blend thin films consisting of MDMO-PPV and a fullerene derivative (1:1 w/w) exhibit a substantially larger MIME interval (∼1450 cm(-1)) that arises from an increase in polymer chain planarity. This structural change is most apparent from large decreases of the excited state displacement of an out-of-plane C-H bending mode (961 cm(-1)) that becomes forbidden in the planar structure.

Effect of exciton self-trapping and molecular conformation on photophysical properties of oligofluorenes

Electronic absorption and fluorescence transitions in fluorene oligomers of differing lengths are studied experimentally and using density functional theory (DFT) and time-dependent DFT. Experimental values are determined in two ways: from the measured molar absorption coefficient and from the radiative rate deduced from a combination of fluorescence quantum yield and lifetime measurements. Good agreement between the calculated and measured transition dipoles is achieved. In both theory and experiment a gradual increase in transition dipoles with increasing oligomer length is found. In absorption the transition dipole follows an approximately n(0.5) dependence on the number of fluorene units n for the range of 2 < or = n < or = 12, whereas a clear saturation of the transition dipole with oligomer length is found in fluorescence. This behavior is attributed to structural relaxation of the molecules in the excited state leading to localization of the excitation (exciton self-trapping) in the middle of the oligomer for both twisted and planar backbone conformations. Twisted oligofluorene chains were found to adopt straight or bent geometries depending on alternation of the dihedral angle between adjacent fluorene units. These different molecular conformations show the same values for the transition energies and the magnitude of the transition dipole.

Electric-field-induced fluorescence quenching in polyfluorene, ladder-type polymers, and MEH-PPV: evidence for field effects on internal conversion rates in the low concentration limit

Electric field-induced fluorescence quenching has been measured for a series of conjugated polymers with applications in organic light-emitting diodes. Electrofluorescence measurements on isolated chains in a glassy matrix at 77 K show that the quenching efficiency for poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) is an order of magnitude larger than that for either a ladder-type polymer (MeLPPP) or polyfluorene (PFH). This effect is explained in terms of the relatively high probability of field-enhanced internal conversion deactivation in MEH-PPV relative to either MeLPPP or PFH. These data, obtained under dilute sample conditions such that chain-chain interactions are minimal, are contrasted with the much higher quenching efficiencies observed in the corresponding polymer films, and several explanations for the differences are considered. In addition, the values of the change in dipole moment and change in polarizability on excitation (|Δμ| and tr(Δα), respectively) are reported, and trends in these values as a function of molecular structure and chain length are discussed.

Electrothermal manipulation of individual chromophores in single conjugated polymer chains: controlling intrachain FRET, blinking, and spectral diffusion

Single molecule spectroscopy of individual chains of a conjugated polymer opens up deep insight into electronic localization phenomena, which govern the underlying optical properties of these complex and disordered materials. We explore the nature of a single chromophore arising in a delocalized pi-electron system by applying periodic electrothermal perturbations at low temperatures. Brief heating of the chromophore leads to a dramatic increase in the transition line width and is generally accompanied by a random jump of the emission energy. This observation demonstrates that chromophores on a polymer chain are not only defined by structural disorder but also by the subtleties of the local dielectric environment. The effect of thermal perturbation becomes more complex when long polymer chains are considered, which can potentially support the formation of multiple chromophores. Here, a momentary increase in temperature can promote intrachain energy transfer to quenching sites, leading to a strong modulation of emission intensity with temperature. Unexpectedly, such energy transfer can serve to either raise or lower the transition line width and quantum yield of the ensemble with increasing temperature, depending on the specific energetics of the chromophores in the system, which in turn vary with time. The controlled perturbation of both the emission spectrum and the intensity by brief heating of the polymer chain offers insight into possible microscopic origins of fluorescence blinking and spectral diffusion, which ultimately impact on the efficiency and spectral purity of devices.

Single-molecule spectroscopy for plastic electronics: materials analysis from the bottom-up

pi-conjugated polymers find a range of applications in electronic devices. These materials are generally highly disordered in terms of chain length and chain conformation, besides being influenced by a variety of chemical and physical defects. Although this characteristic can be of benefit in certain device applications, disorder severely complicates materials analysis. Accurate analytical techniques are, however, crucial to optimising synthetic procedures and assessing overall material purity. Fortunately, single-molecule spectroscopic techniques have emerged as an unlikely but uniquely powerful approach to unraveling intrinsic material properties from the bottom up. Building on the success of such techniques in the life sciences, single-molecule spectroscopy is finding increasing applicability in materials science, effectively enabling the dissection of the bulk down to the level of the individual molecular constituent. This article reviews recent progress in single molecule spectroscopy of conjugated polymers as used in organic electronics.

Single-molecule spectroscopy on a ladder-type conjugated polymer: electron-phonon coupling and spectral diffusion

We employ low-temperature single-molecule spectroscopy combined with pattern recognition techniques for data analysis on a methyl-substituted ladder-type poly(para-phenylene) (MeLPPP) to investigate the electron-phonon coupling to low-energy vibrational modes as well as the origin of the strong spectral diffusion processes observed for this conjugated polymer. The results indicate weak electron-phonon coupling to low-frequency vibrations of the surrounding matrix of the chromophores, and that low-energy intrachain vibrations of the conjugated backbone do not couple to the electronic transitions of MeLPPP at low temperatures. Furthermore, these findings suggest that the main line-broadening mechanism of the zero-phonon lines of MeLPPP is fast, unresolved spectral diffusion, which arises from conformational fluctuations of the side groups attached to the MeLPPP backbone as well as of the surrounding host material.