Research
Nanophotonics and molecular electronics are rapidly evolving fields with many promising applications ahead. The challenge is to identify suitable materials and nanostructures to enable or to optimise the desired functionality. The research in our group focuses on organic functional materials and supramolecular (nano)systems that are relevant for these fields. Our aim is to understand the optical and electronic properties of such materials and to gain detailed insights into the basic underlying principles of energy transport and light propagation within tailored model systems. We also perform in-situ investigations to study material properties under operando conditions in real devices, e.g., organic electrochemical transistors. Ultimately, our work should help to design new, more efficient architectures, e.g. for nanophotonic and -electronic applications as well as for light-energy conversion.
On the material side we focus on conjugated (co-)polymers, non-fullerene acceptors, small organic molecules and supramolecular structures thereof. As methods we employ a broad range of complementary optical spectroscopy and microscopy techniques both on the ensemble and single molecule (single object) level. We combine such experiments with numerical modelling of optical spectra.
Structure-Property Relationships in Conjugated (Co-)Polymers
Conjugated polymers represent an important class of functional materials with applications in e.g. organic solar cells, sensors, thermoelectric generators and transistors. In particular, so-called copolymers represent the current state-of-the-art materials in these fields. A monomer of a copolymer comprises electron-donating and -accepting moieties and excited states possess a strong charge-transfer character due to a strong redistribution of electron density upon (photo-)excitation. Our work on this material class focusses on establishing structure-property relationships at a truly molecular level: We want to understand how the photophysics is determined by the chemical structure of the backbone and the side chains, by the conformation of the conjugated backbone as well as by assembly into (well-defined) small aggregates.
As a recent example, we studied the prototypical copolymer PCDTBT using single-molecule spectroscopy (Staeter et al., ChemPhysChem 2024). We found that despite being a long polymer with many monomers, all chains exhibited the characteristic features of a single molecule with on-off blinking and one-step photobleaching (Figure 1). This behaviour indicates that each chain possesses a defined site, comprising few (3-4) monomers, from which emission originates. Systematically changing the side groups by hexylation, a statistical analysis of emission spectra of single PCDTBT chains revealed that there is a blue-shift of the emission with increasing hexyl side group density. This blue-shift results from an increasing torsion of the backbone, and thus a decreasing delocalisation of the emitting site, induced by side groups. At the same time, the distribution of transition energies was significantly narrower for the PCDTBT with highest hexyl density, which we related to a reduction of the variation of torsion angles along the backbone, i.e., the backbone is ‘stabilised’ by side groups.
Figure 1. a) Chemical structure of the copolymer PCDTBT. b) Wide-field PL image of a single-molecule sample of PCDTBT (top) and selected time-traces of the PL of different single PCDTBT chains (bottom), featuring characteristic on-off blinking and one-step photobleaching. c) Transition energy distributions derived from single-molecule PL spectra of PCDTBT with 0%, 50% and 100% hexyl density at positions marked with a red R in panel a. Adapted from Staeter et al. ChemPhysChem 2024.
We also study the aggregation behaviour of (co-)polymers with varied side groups and chemical structure of the backbone (Beer et al., JPC A 2021; Woering et al., Chem. Commun. 2026). For instance, time- and spectrally resolved PL measurements on solutions of dihydroanthracene-based copolymers allowed us to deconvolve emission from single strands, disordered aggregates and ordered pi-stacked aggregates that all co-exist in solution. Depending on the chemical structure, however, the relative contributions of these emissive species varied. These results showed how rational design of conjugated polymers can be used to optimise their structure for optoelectronic applications.
Currently, we investigate copolymers, such as PM6 and derivatives, as well as so-called non-fullerene acceptors, such as Y6, that are used in highly efficient organic solar cells. Our single-molecule spectroscopy work on these materials will help to obtain a clearer picture of the various processes occurring in real device applications, and may help to identify strategies for optimisation of device performance.
Selected References:
Woering, E. F.; Taddeucci, A.; Pucci, A.; Hildner, R.; Carlotti, M. Tuneable Emission from Disordered to Ordered Aggregates in Substituted 9,10-Dihydroanthracene Polymers, Chem. Commun. 2026, 62, 3265-3268.
Staeter, S.; Woering E. F.; Lombeck, F.; Sommer, M.; Hildner, R. Hexylation Stabilises Twisted Backbone Configurations in the Prototypical Low-Bandgap Copolymer PCDTBT. ChemPhysChem 2024, 25, e202300971.
Beer, P.; Reichstein, P. M.; Schötz, K.; Raithel, D.; Thelakkat, M.; Köhler, J.; Panzer, F.; Hildner, R. Disorder in P3HT Nanoparticles Probed by Optical Spectroscopy on P3HT-B-PEG Micelles, J. Phys. Chem. A 2021, 125, 10165.
Raithel, D.; Simine, L.; Pickel, S.; Schötz, K.; Panzer, F.; Baderschneider, S.; Schiefer, D.; Lohwasser, R.; Köhler, J.; Thelakkat, M.; Sommer, M.; Köhler, A.; Rossky, P. J.; Hildner, R. Direct Observation of Backbone Planarization via Side-Chain Alignment in Single Bulky-Substituted Polythiophenes. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 2699–2704.
Cooperation Partners:
Michael Sommer (Chemnitz)
Remco Havenith (Groningen)
Ryan Chiechi (North Carolina State University)
Andrea Pucci, Marco Carlotti (Pisa)
Hyperspectral Imaging of Nanostructures and Films of Functional Materials
Our experiments on single (co)polymers and (co)polymers in solution are complemented by hyperspectral imaging on films of conjugated polymers and nanostructures thereof. Figure 2 shows an example of this approach to detect local variations in chemical doping levels of copolymer films based on naphthaldimid-bithiophenes (NDI-T2) for organic thermoelectric generators (Ye et al., Macromolecules 2021). We recorded absorption spectra of a ~20x20 μm2 area with a spatial resolution of ~250 nm, which showed variations across the sample. In particular, the ratio of the absorption at 600 and 850 nm reports on the relative doping levels of the NDI-T2 chains. Displaying this ratio as a function of position in 2D maps allows to visualise the spatial distribution of highly doped and less doped regions within the films. Moreover, this local doping level and its spatial distribution depends on the side-group structure of the NDI-T2 copolymer.
Figure 2: a) Absorption spectra of films of NDI-T2 derivatives chemically doped with 42 mol% n-DMBI measured with diffraction-limited resolution (~250 nm) on different positions of films. Top: Both NDI and T2 had alkyl side groups. Bottom: NDI had alkyl groups and T2 ethylene glycol groups. The blue shaded area represents the spatial variation of the spectra. b) 2D-representation of the absorption ratio at 600 and 850 nm (black dots in a) as a function of the position. This ratio indicates local differences in the doping level. Insets: AFM height profiles of the films showing a correlation of doping level and surface morphology. Adapted from Ye, Staeter et al. Macromolecules 2021.
For the homopolymer P3HT we investigated the excited-state landscape upon formation of aligned, several μm long nanofibres (Staeter et al., JACS 2023; Wenzel et al., Macromolecules 2022 and 2024). These nanofibres were grown by exploiting a supramolecular nucleating agent as a seed. Using hyperspectral (absorption and PL) imaging, we found that a directed gradient is imprinted into the excited-state energy landscape, i.e., the excited state energy decreased along the nanofibres’ long axis. Such gradients can be exploited for directional energy transport in artificial light-harvesting structures.
In current work we perform in-situ hyperspectral imaging on organic electrochemical transistors under operando conditions. We want to understand changes of optical properties upon electrochemical doping of active organic layers as a function of position and of applied gate voltages, with the goal to derive structure-property relationships and to derive strategies to optimise materials for applications.
Selected References:
F. A. Wenzel, S. Stäter, P. O’Reilly, K. Kreger, J. Köhler, R. Hildner, and H.-W. Schmidt, Isolated Hierarchical Superstructures with Highly Oriented P3HT Nanofibers, Macromolecules 2024, 57, 10389.
S. Stäter, F. A. Wenzel, H. Welz, K. Kreger, J. Köhler, H.-W. Schmidt, and R. Hildner, Directed Gradients in the Excited-State Energy Landscape of Poly(3-Hexylthiophene) Nanofibers, J. Am. Chem. Soc. 2023, 145, 13780.
F. A. Wenzel, H. Welz, K. P. van der Zwan, S. Stäter, K. Kreger, R. Hildner, J. Senker, and H.-W. Schmidt, Highly Efficient Supramolecular Nucleating Agents for Poly(3-Hexylthiophene), Macromolecules 2022, 55, 2861.
G. Ye, J. Liu, X. Qiu, S. Stäter, L. Qiu, Y. Liu, X. Yang, R. Hildner, L. J. A. Koster, and R. C. Chiechi, Controlling N-Type Molecular Doping via Regiochemistry and Polarity of Pendant Groups on Low Band Gap Donor–Acceptor Copolymers, Macromolecules 2021, 54, 3886.
Cooperation Partners:
Hans-Werner Schmidt (Bayreuth)
Eva Herzig (Bayreuth)
Jan-Anton Koster (Groningen)
Ryan Chiechi (North Carolina State University)
Sebastien Sanaur (IMT Gardanne)
Natalie Banerji/Julien Rehault (Bern)
Sophia C. Hayes (Nikosia)
Supramolecular Nanostructures Based on Small Organic Molecules
A further major research topic in our group is the investigation of the photophysical properties of well-defined supramolecular nanostructures as well as of the transport of electronic excitations along nanostructures. Energy transport in supramolecular systems is a fundamental process to understand the initial light-driven steps in artificial and natural structures for light-energy conversion. We aim at establishing design principles to create supramolecular structures that are optimised for highly efficient, long-range energy transport.
To investigate energy transport we employ spatially and temporally resolved microscopy on single nanostructures (Carta et al., JPC Lett 2024). Initially a local exciton population is created via excitation with a diffraction-limited spot (Figure 3). The transport of excitons along nanofibres is detected via time-resolved detection of the PL away from the initial excitation spot. From such spatio-temporal data exciton diffusivities and transport distances can be derived. On individual nanofibres based on perfectly pi-stacked small molecules we were able to demonstrate partially coherent energy transport over > 4 µm at room temperature. Moreover, we found that on some nanofibres excitons move preferentially to specific positions, where they become trapped. Despite this trapping, excitons can still move up to μm distances along a nanofibre. Such structures represent an ideal model to study and understand energy transport mechanisms in real, disordered molecular systems in great detail.
Figure 3: a) Schematic of nanofibre with pi-stacked small molecules. A single fibre is locally excited by a diffraction-limited spot in the centre of the fibre (blue), and energy transport is monitored by detecting the PL signal at different spots (orange) of the nanofibre as a function of time after excitation. b) Top: Normalised spatio-temporal map of the PL signal measured from a single nanofibre upon local excitation in the centre. Energy transport occurs along the fibre predominantly ‘to the left’. Bottom: Numerical simulations based on a Holstein Hamiltonian. The space axis is from -1 to 1 μm, and the time axis from 0 to 5 ns from top to bottom. Adapted from Carta et al. JPC Lett. 2024.
We also work on nanostructures based on free-base and metal-coordinated porphyrin derivatives with amide groups in the side chains. Using time- and spectrally resolved emission spectroscopy, we found that metal-coordinated porphyrins feature a complex self-assembly behaviour (Touloupas et al., JPC C 2023). Independent of the metal (Zn, Pt), the porphyrins tend to simultaneously form two thermodynamically stable structures that exhibit distinct slip-stacking. Here we aim at investigating isolated structures for their ability to transport energy over long distances and how this transport can be controlled by the assembly of the molecules.
Selected References:
Kuevda, A. V.; Cangahuala, M. K. E.; Hildner, R.; Jansen, T. L. C.; Pshenichnikov, M. S. Linear Dichroism Microscopy Resolves Competing Structural Models of a Synthetic Light-Harvesting Complex, J. Am. Chem. Soc. 2025, 147, 6171.
Carta, A.; Wittmann, B.; Kreger, K.; Schmidt, H.-W.; Jansen, T. L. C.; Hildner, R. Spatial Correlations Drive Long-Range Transport and Trapping of Excitons in Single H Aggregates: Experiment and Theory, J. Phys. Chem. Lett. 2024, 15, 2697.
Touloupas, I.; Weyandt, E.; Meijer, E. W.; Hildner, R. Unusual Photophysical Properties of Porphyrin-Based Supramolecular Polymers Unveiled: The Role of Metal Ligands and Side Group Amide Connectivity, J. Phys. Chem. C 2023, 127, 23323.
Haedler, A. T.; Kreger, K.; Issac, A.; Wittmann, B.; Kivala, M.; Hammer, N.; Köhler, J.; Schmidt, H.-W.; Hildner, R. Long-Range Energy Transport in Single Supramolecular Nanofibres at Room Temperature. Nature 2015, 523, 196–199.
Cooperation Partners:
Hans-Werner Schmidt (Bayreuth)
Eva Herzig (Bayreuth)
Thomas Jansen (Groningen)
Maxim Pshenichnikov (Groningen)
E. W. Meijer (Eindhoven)
Milan Kivala (Heidelberg)
Light Propagation in Single Crystals Based on Conjugated Oligomers
Conjugated polymers and oligomers are also useful building blocks for the self-assembly into (m- to mm-sized) single crystals (Figure 4). Employing Fourier-space microscopy and spectroscopy, we investigate the light propagation (active waveguiding) within such highly anisotropic systems. For instance, the self-interference of light waveguided over different distances can be exploited to visualise the dispersive behaviour of the anisotropic refractive index. Moreover, we can show how the refraction of (waveguided) light at nanoscale edges deviates from classical ray optics.
Figure 4: Left: Stacking of the thiophene derivative 3TBT into single crystals. Right: 3-dimensional representation of an energy-momentum spectrum of a 3TBT crystal with characteristic interference patterns. Adapted from Schoerner et al. ACS Omega 2018.
References:
Wittmann, B.; Wiesneth, S.; Motamen, S.; Simon, L.; Serein-Spirau, F.; Reiter, G.; Hildner, R. Energy Transport and Light Propagation Mechanisms in Organic Single Crystals, J. Chem. Phys. 2020, 153, 144202.
Schörner, C.; Motamen, S.; Simon, L.; Reiter, G.; Hildner, R. Self-Interference of Exciton Emission in Organic Single Crystals Visualized by Energy-Momentum Spectroscopy. ACS Omega 2018, 3, 6728–6736.
Cooperation Partners:
Khalid Naim, Inge Zuhorn (UMCG, Groningen)
Hans-Werner Schmidt (Bayreuth)
Günter Reiter (Freiburg)
Laurent Simon (Mulhouse)