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.

Spatial Correlations Drive Long-Range Transport and Trapping of Excitons in Single H-Aggregates: Experiment and Theory

Describing long-range energy transport is a crucial step, both toward deepening our knowledge on natural light-harvesting systems and toward developing novel photoactive materials. Here, we combine experiment and theory to resolve and reproduce energy transport on pico- to nanosecond time scales in single H-type supramolecular nanofibers based on carbonyl-bridged triarylamines (CBT). Each nanofiber shows energy transport dynamics over long distances up to ∼1 μm, despite exciton trapping at specific positions along the nanofibers. Using a minimal Frenkel exciton model including disorder, we demonstrate that spatial correlations in the normally distributed site energies are crucial to reproduce the experimental data. In particular, we can observe the long-range and subdiffusive nature of the exciton dynamics as well as the trapping behavior of excitons in specific locations of the nanofiber. This trapping behavior introduces a net directionality or asymmetry in the exciton dynamics as observed experimentally.

Directed Gradients in the Excited-State Energy Landscape of Poly(3-hexylthiophene) Nanofibers

Funneling excitation energy toward lower energy excited states is a key concept in photosynthesis, which is often realized with at most two chemically different types of pigment molecules. However, current synthetic approaches to establish energy funnels, or gradients, typically rely on Förster-type energy-transfer cascades along many chemically different molecules. Here, we demonstrate an elegant concept for a gradient in the excited-state energy landscape along micrometer-long supramolecular nanofibers based on the conjugated polymer poly(3-hexylthiophene), P3HT, as the single component. Precisely aligned P3HT nanofibers within a supramolecular superstructure are prepared by solution processing involving an efficient supramolecular nucleating agent. Employing hyperspectral imaging, we find that the lowest-energy exciton band edge continuously shifts to lower energies along the nanofibers' growth direction. We attribute this directed excited-state energy gradient to defect fractionation during nanofiber growth. Our concept provides guidelines for the design of supramolecular structures with an intrinsic energy gradient for nanophotonic applications.

Dye-induced photoluminescence quenching of quantum dots: role of excited state lifetime and confinement of charge carriers

We investigate the role of quantum confinement and photoluminescence (PL) lifetime of photoexcited charge carriers in semiconductor core/shell quantum dots (QDs) via PL quenching due to surface modification. Surface modification is controlled by varying the number of dye molecules adsorbed onto the QD shell surface forming QD-dye nanoassemblies. We selected CuInS₂/ZnS (CIS) and InP/ZnS (InP) core/shell QDs exhibiting relatively weak (664 meV) and strong (1194 meV) confinement potentials for the conduction band electron. Moreover, the difference in the emission mechanism gives rise to a long and short excited state lifetime of CIS (ca. 290 ns) and InP (ca. 37 ns) QDs. Dye molecules of different ionic characters (rhodamine 575: zwitterionic and rhodamine 560: cationic) are used as quenchers. A detailed analysis of Stern-Volmer data shows that (i) quenching is generally more pronounced in CIS-dye assemblies as compared to InP-dye assemblies, (ii) dynamic quenching is dominating in all QD-dye assemblies with only a minor contribution from static quenching and (iii) the cationic dye shows a stronger interaction with the QD shell surface than the zwitterionic dye. Observations (i) and (ii) can be explained by the differences in the amplitude of the electronic component of the exciton wavefunction near the dye binding sites in both QDs, which results in the breaking up of the electron-hole pair and favors charge trapping. Observation (iii) can be attributed to the variations in electrostatic interactions between the negatively charged QD shell surface and the cationic and zwitterionic dyes, with the former exhibiting a stronger interaction. Moreover, the long lifetime of CIS QDs facilitates us to easily probe different time scales of the trapping processes and thus differentiate the origins of static and dynamic quenching components that appear in the Stern-Volmer analysis.

Spark Discharge Doping-Achieving Unprecedented Control over Aggregate Fraction and Backbone Ordering in Poly(3-hexylthiophene) Solutions

The properties of semiconducting polymers are strongly influenced by their aggregation behavior, that is, their aggregate fraction and backbone planarity. However, tuning these properties, particularly the backbone planarity, is challenging. This work introduces a novel solution treatment to precisely control the aggregation of semiconducting polymers, namely current-induced doping (CID). It utilizes spark discharges between two electrodes immersed in a polymer solution to create strong electrical currents resulting in temporary doping of the polymer. Rapid doping-induced aggregation occurs upon every treatment step for the semiconducting model-polymer poly(3-hexylthiophene). Therefore, the aggregate fraction in solution can be precisely tuned up to a maximum value determined by the solubility of the doped state. A qualitative model for the dependences of the achievable aggregate fraction on the CID treatment strength and various solution parameters is presented. Moreover, the CID treatment can yield an extraordinarily high quality of backbone order and planarization, expressed in UV-vis absorption spectroscopy and differential scanning calorimetry measurements. Depending on the selected parameters, an arbitrarily lower backbone order can be chosen using the CID treatment, allowing for maximum control of aggregation. This method may become an elegant pathway to finely tune aggregation and solid-state morphology for thin-films of semiconducting polymers.

Disorder in P3HT Nanoparticles Probed by Optical Spectroscopy on P3HT-b-PEG Micelles

We employ photoluminescence (PL) spectroscopy on individual nanoscale aggregates of the conjugated polymer poly(3-hexylthiophene), P3HT, at room temperature (RT) and at low temperature (LT) (1.5 K), to unravel different levels of structural and electronic disorder within P3HT nanoparticles. The aggregates are prepared by self-assembly of the block copolymer P3HT-block-poly(ethylene glycol) (P3HT-b-PEG) into micelles, with the P3HT aggregates constituting the micelles' core. Irrespective of temperature, we find from the intensity ratio between the 0-1 and 0-0 peaks in the PL spectra that the P3HT aggregates are of H-type nature, as expected from π-stacked conjugated thiophene backbones. Moreover, the distributions of the PL peak ratios demonstrate a large variation of disorder between micelles (inter-aggregate disorder) and within individual aggregates (intra-aggregate disorder). Upon cooling from RT to LT, the PL spectra red-shift by 550 cm^-1, and the energy of the (effective) carbon-bond stretch mode is reduced by 100 cm^-1. These spectral changes indicate that the P3HT backbone in the P3HT-b-PEG copolymer does not fully planarize before aggregation at RT and that upon cooling, partial planarization occurs. This intra-chain torsional disorder is ultimately responsible for the intra- and inter-aggregate disorder. These findings are supported by temperature-dependent absorption spectra on thin P3HT films. The interplay between intra-chain, intra-aggregate, and inter-aggregate disorder is key for the bulk photophysical properties of nanoparticles based on conjugated polymers, for example, in hierarchical (super-) structures. Ultimately, these properties determine the usefulness of such structures in hybrid organic-inorganic materials, for example, in (bio-)sensing and optoelectronics applications.

All-optical manipulation of singlet exciton transport in individual supramolecular nanostructures by triplet gating

Directed transport of singlet excitation energy is a key process in natural light-harvesting systems and a desired feature in assemblies of functional organic molecules for organic electronics and nanotechnology applications. However, progress in this direction is hampered by the lack of concepts and model systems. Here we demonstrate an all-optical approach to manipulate singlet exciton transport pathways within supramolecular nanostructures via singlet-triplet annihilation, i.e., to enforce an effective motion of singlet excitons along a predefined direction. For this proof-of-concept, we locally photo-generate a long-lived triplet exciton population and subsequently a singlet exciton population on single bundles of H-type supramolecular nanofibres using two temporally and spatially separated laser pulses. The local triplet exciton population operates as a gate for the singlet exciton transport since singlet-triplet annihilation hinders singlet exciton motion across the triplet population. We visualize this manipulation of singlet exciton transport via the fluorescence signal from the singlet excitons, using a detection-beam scanning approach combined with time-correlated single-photon counting. Our reversible, all-optical manipulation of singlet exciton transport can pave the way to realising new design principles for functional photonic nanodevices.

Controlling n-Type Molecular Doping via Regiochemistry and Polarity of Pendant Groups on Low Band Gap Donor-Acceptor Copolymers

We demonstrate the impact of the type and position of pendant groups on the n-doping of low-band gap donor–acceptor (D–A) copolymers. Polar glycol ether groups simultaneously increase the electron affinities of D–A copolymers and improve the host/dopant miscibility compared to nonpolar alkyl groups, improving the doping efficiency by a factor of over 40. The bulk mobility of the doped films increases with the fraction of polar groups, leading to a best conductivity of 0.08 S cm^–1 and power factor (PF) of 0.24 μW m^–1 K^–2 in the doped copolymer with the polar pendant groups on both the D and A moieties. We used spatially resolved absorption spectroscopy to relate commensurate morphological changes to the dispersion of dopants and to the relative local doping efficiency, demonstrating a direct relationship between the morphology of the polymer phase, the solvation of the molecular dopant, and the electrical properties of doped films. Our work offers fundamental new insights into the influence of the physical properties of pendant chains on the molecular doping process, which should be generalizable to any molecularly doped polymer films.

Energy transport and light propagation mechanisms in organic single crystals

Unambiguous information about spatiotemporal exciton dynamics in three-dimensional nanometer- to micrometer-sized organic structures is difficult to obtain experimentally. Exciton dynamics can be modified by annihilation processes, and different light propagation mechanisms can take place, such as active waveguiding and photon recycling. Since these various processes and mechanisms can lead to similar spectroscopic and microscopic signatures on comparable time scales, their discrimination is highly demanding. Here, we study individual organic single crystals grown from thiophene-based oligomers. We use time-resolved detection-beam scanning microscopy to excite a local singlet exciton population and monitor the subsequent broadening of the photoluminescence (PL) signal in space and on pico- to nanosecond time scales. Combined with Monte Carlo simulations, we were able to exclude photon recycling for our system, whereas leakage radiation upon active waveguiding leads to an apparent PL broadening of about 20% compared to the initial excitation profile. Exciton-exciton annihilation becomes important at high excitation fluence and apparently accelerates the exciton dynamics leading to apparently increased diffusion lengths. At low excitation fluences, the spatiotemporal PL broadening results from singlet exciton diffusion with diffusion lengths of up to 210 nm. Surprisingly, even in structurally highly ordered single crystals, the transport dynamics is subdiffusive and shows variations between different crystals, which we relate to varying degrees of static and dynamic electronic disorders.

Enhancing Long-Range Energy Transport in Supramolecular Architectures by Tailoring Coherence Properties

Efficient long-range energy transport along supramolecular architectures of functional organic molecules is a key step in nature for converting sunlight into a useful form of energy. Understanding and manipulating these transport processes on a molecular and supramolecular scale is a long-standing goal. However, the realization of a well-defined system that allows for tuning morphology and electronic properties as well as for resolution of transport in space and time is challenging. Here we show how the excited-state energy landscape and thus the coherence characteristics of electronic excitations can be modified by the hierarchical level of H-type supramolecular architectures. We visualize, at room temperature, long-range incoherent transport of delocalized singlet excitons on pico- to nanosecond time scales in single supramolecular nanofibers and bundles of nanofibers. Increasing the degree of coherence, i.e., exciton delocalization, via supramolecular architectures enhances exciton diffusivities up to 1 order of magnitude. In particular, we find that single supramolecular nanofibers exhibit the highest diffusivities reported for H-aggregates so far.

Self-Interference of Exciton Emission in Organic Single Crystals Visualized by Energy-Momentum Spectroscopy

We employ energy-momentum spectroscopy on isolated organic single crystals with micrometer-sized dimensions. The single crystals are grown from a thiophene-based oligomer and are excellent low-loss active waveguides that support multiple guided modes. Excitation of the crystals with a diffraction-limited laser spot results in emission into guided modes as well as into quasi-discrete radiation modes. These radiation modes are mapped in energy-momentum space and give rise to dispersive interference patterns. On the basis of the known geometry of the crystals, especially the height, the characteristics of the interference maxima allow one to determine the energy dependence of two components of the anisotropic complex refractive index. Moreover, the method is suited to identify the orientation of molecules within (and around) a crystalline structure.

Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes

The backbone conformation of conjugated polymers affects, to a large extent, their optical and electronic properties. The usually flexible substituents provide solubility and influence the packing behavior of conjugated polymers in films or in bad solvents. However, the role of the side chains in determining and potentially controlling the backbone conformation, and thus the optical and electronic properties on the single polymer level, is currently under debate. Here, we investigate directly the impact of the side chains by studying the bulky-substituted poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) and the common poly(3-hexylthiophene) (P3HT), both with a defined molecular weight and high regioregularity, using low-temperature single-chain photoluminescence (PL) spectroscopy and quantum-classical simulations. Surprisingly, the optical transition energy of PDOPT is significantly (∼2,000 cm-1 or 0.25 eV) red-shifted relative to P3HT despite a higher static and dynamic disorder in the former. We ascribe this red shift to a side-chain induced backbone planarization in PDOPT, supported by temperature-dependent ensemble PL spectroscopy. Our atomistic simulations reveal that the bulkier 2,5-dioctylphenyl side chains of PDOPT adopt a clear secondary helical structural motif and thus protect conjugation, i.e., enforce backbone planarity, whereas, for P3HT, this is not the case. These different degrees of planarity in both thiophenes do not result in different conjugation lengths, which we found to be similar. It is rather the stronger electronic coupling between the repeating units in the more planar PDOPT which gives rise to the observed spectral red shift as well as to a reduced calculated electron-hole polarization.

Excited state dynamics and conformations of a Cu(ii)-phthalocyanine-perylenebisimide dyad

We investigate the excited state dynamics and the conformations of a new molecular donor-bridge-acceptor system, a Cu(ii)-phthalocyanine (CuPc) covalently linked via a flexible aliphatic spacer to a perylenebisimide (PBI). We performed time-resolved polarization anisotropy and pump-probe measurements in combination with molecular dynamics simulations. Our data suggest the existence of three conformations of the dyad: two more extended, metastable conformations with centre-of-mass distances >1 nm between the PBI and CuPc units of the dyad, and a highly stable folded structure, in which the PBI and CuPc units are stacked on top of each other with a centre-of-mass distance of 0.4 nm. In the extended conformations the dyad shows emission predominantly from the PBI unit with a very weak contribution from the CuPc unit. In contrast, for the folded conformation the PBI emission of the dyad is strongly quenched due to fast energy transfer from the PBI to the CuPc unit (3 ps) and subsequent intersystem-crossing (300 fs) from the first excited singlet state of CuPc unit into its triplet state. Finally, the CuPc triplet state is deactivated non-radiatively with a time constant of 25 ns.

Real-Time Observation of Iodide Ion Migration in Methylammonium Lead Halide Perovskites

Organic-inorganic metal halide perovskites (e.g., CH₃ NH₃ PbI_3-x Cl_x ) emerge as a promising optoelectronic material. However, the Shockley-Queisser limit for the power conversion efficiency (PCE) of perovskite-based photovoltaic devices is still not reached. Nonradiative recombination pathways may play a significant role and appear as photoluminescence (PL) inactive (or dark) areas on perovskite films. Although these observations are related to the presence of ions/defects, the underlying fundamental physics and detailed microscopic processes, concerning trap/defect status, ion migration, etc., still remain poorly understood. Here correlated wide-field PL microscopy and impedance spectroscopy are utilized on perovskite films to in situ investigate both the spatial and the temporal evolution of these PL inactive areas under external electric fields. The formation of PL inactive domains is attributed to the migration and accumulation of iodide ions under external fields. Hence, we are able to characterize the kinetic processes and determine the drift velocities of these ions. In addition, it is shown that I₂ vapor directly affects the PL quenching of a perovskite film, which provides evidence that the migration/segregation of iodide ions plays an important role in the PL quenching and consequently limits the PCE of organometal halide-based perovskite photovoltaic devices.

Low loss optical waveguiding in large single crystals of a thiophene-based oligomer

Active optical waveguides based on functional small organic molecules in micro/nano regime have attracted great interest for their potential applications in high speed miniaturized photonic integrations.

Tracing Single Electrons in a Disordered Polymer Film at Room Temperature

The transport of charges lies at the heart of essentially all modern (opto-) electronic devices. Although inorganic semiconductors built the basis for current technologies, organic materials have become increasingly important in recent years. However, organic matter is often highly disordered, which directly impacts the charge carrier dynamics. To understand and optimize device performance, detailed knowledge of the transport mechanisms of charge carriers in disordered matter is therefore of crucial importance. Here we report on the observation of the motion of single electrons within a disordered polymer film at room temperature, using single organic chromophores as probe molecules. The migration of a single electron gives rise to a varying electric field in its vicinity, which is registered via a shift of the emission spectra (Stark shift) of a chromophore. The spectral shifts allow us to determine the electron mobility and reveal for each nanoenvironment a distinct number of different possible electron-transfer pathways within the rugged energy landscape of the disordered polymer matrix.

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.

Synthesis and photophysical properties of multichromophoric carbonyl-bridged triarylamines

The synthesis and photophysical properties of two novel multichromophoric compounds is presented. Their molecular design comprises a carbonyl-bridged triarylamine core and either naphthalimides or 4-(5-hexyl-2,2'-bithiophene)naphthalimides as second chromophore in the periphery. The lateral chromophores are attached to the core via an amide linkage and a short alkyl spacer. The synthetic approach demonstrates a straightforward functionalization strategy for carbonyl-bridged triarylamines. Steady-state and time-resolved spectroscopic investigations of these compounds, in combination with three reference compounds, provide clear evidence for energy transfer in both multichromophoric compounds. The direction of the energy transfer depends on the lateral chromophore used. Furthermore, the compound bearing the lateral 4-(bithiophene)naphthaimides is capable of forming fluorescent gels at very low concentrations in the sub-millimolar regime whilst retaining its energy transfer properties.

Stepwise decrease of fluorescence versus sequential photobleaching in a single multichromophoric system

For individual molecules from the newly synthesized calix[4]arene tethered perylene bisimide (PBI) trimer, we studied the emitted fluorescence intensity as a function of time. Owing to the zigzag arrangement of PBI dyes in these trimers, the polarization state of the emission provides directly information about the emitting subunit within the trimer. Interestingly, we observed emission from all neutral subunits within a trimer rather than exclusively from the subunit with the lowest site energy. This can be understood in terms of thermally activated uphill energy transfer that repopulates the higher energetic chromophores. Together with the simultaneously recorded polarization-resolved emission spectra, this reveals that the emission from a multichromophoric system is governed by a complex interplay between the temporal variations of the photophysical parameters of the subunits, bidirectional hopping processes within the trimer, and unavoidable photobleaching. Moreover, it is demonstrated that the typically observed stepwise decrease of the signal from a multichromophoric system does not necessarily reflect sequential bleaching of the individual chromophores within the macromolecule.

Ultrafast dynamics of single molecules

Room-temperature studies of single molecules at femtosecond timescales provide detailed observation and control of ultrafast electronic and vibrational dynamics of organic dyes and photosynthetic complexes, probing quantum dynamics at ambient conditions and elucidating its role in chemistry and biology.

Quantum coherent energy transfer over varying pathways in single light-harvesting complexes

The initial steps of photosynthesis comprise the absorption of sunlight by pigment-protein antenna complexes followed by rapid and highly efficient funneling of excitation energy to a reaction center. In these transport processes, signatures of unexpectedly long-lived coherences have emerged in two-dimensional ensemble spectra of various light-harvesting complexes. Here, we demonstrate ultrafast quantum coherent energy transfer within individual antenna complexes of a purple bacterium under physiological conditions. We find that quantum coherences between electronically coupled energy eigenstates persist at least 400 femtoseconds and that distinct energy-transfer pathways that change with time can be identified in each complex. Our data suggest that long-lived quantum coherence renders energy transfer in photosynthetic systems robust in the presence of disorder, which is a prerequisite for efficient light harvesting.

Beating spatio-temporal coupling: implications for pulse shaping and coherent control experiments

Diffraction of finite sized laser beams imposes a limit on the control that can be exerted over ultrafast pulses. This limit manifests as spatio-temporal coupling induced in standard implementations of pulse shaping schemes. We demonstrate the influence this has on coherent control experiments that depend on finite excitation, sample, and detection volumes. Based on solutions used in pulse stretching experiments, we introduce a double-pass scheme that reduces the errors produced through spatio-temporal coupling by at least one order of magnitude. Finally, employing single molecules as nanoscale probes, we prove that such a double pass scheme is capable of artifact-free pulse shaping at dimensions two orders of magnitude smaller than the diffraction limit.

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.

Comparison of the Photophysical Parameters for Three Perylene Bisimide Derivatives by Single-Molecule Spectroscopy

Characterization of the photophysical parameters for three perylene bisimide derivatives is presented. We exploited time-resolved and steady-state spectroscopy on both ensembles and single molecules under ambient as well as cryogenic (1.4 K) conditions. The finding is that these chromophores show extraordinary high fluorescence-emission rates, low intersystem crossing yields to the triplet state, and relatively short triplet lifetimes.