Spotting Molecular Excitons: Probing Organic Crystal Textures With Imaging Mueller Matrix Ellipsometry - NANOscientific Community

Johannes Kepler University Linz, Austria; Optical Nanometrology,
National Metrology Institute, Braunschweig, Germany.

Introduction: Turning Light into a Crystal Map
At the 2024 NanoScientific Forum Europe, Dr. Manuela Schiek introduced the audience to the power of Imaging Mueller Matrix Ellipsometry (IMME) for revealing the hidden optical and excitonic properties of complex molecular crystals. Her work focuses on organic semiconductor systems—materials whose optical response is driven by molecular packing and excitonic interactions—demonstrating how IMME can extract complete dielectric tensors and reveal subtle excitonic transitions beyond the reach of standard optical microscopy.

In her talk, Dr. Schiek focused on a noncommercial organic semiconductor known for its intense light–matter interactions. By carefully controlling its crystalline polymorphs through spin-coating and thermal annealing, and by combining polarized microscopy with Mueller matrix ellipsometry, she and her collaborators have mapped Davydov-split excitonic transitions, quantified dielectric tensors for orthorhombic crystals and identified dark states. Her results show how advanced ellipsometric imaging can link molecular orientation, crystal symmetry, and optical anisotropy at a quantitative level.

The Material System: A Platform for Excitonic Coupling
The organic semiconductor at the center of this work—nicknamed “SQIB”—features a donor–acceptor–donor molecular backbone with side chains that dictate the molecular packing. Although the side chains do not directly participate in light absorption, their steric effects steer the crystal structure, aligning transition dipole moments along the molecular backbone.

When crystallized into rotational domains (platelets) through spin-coating and thermal annealing, the material forms a distinct polymorph. The orthorhombic form—common at elevated annealing temperatures—consists of four molecules per primitive unit cell, preferentially oriented with their (110) plane parallel to the substrate. The molecular arrangement creates strong excitonic coupling between adjacent molecules, resulting in multiple Davydov-split excitonic transitions: two upper and two lower components, one being a dark state, with the polarization directions of the remaining three aligned to crystal axes.

This excitonic structure is more than an optical curiosity—it encodes information about molecular packing, crystal symmetry, and intermolecular interactions. Determining the polarization and energies of these transitions, and correlating them to the underlying crystal lattice, is the core challenge that Mueller matrix ellipsometry addresses.

From Polarized Microscopy to Imaging Mueller Matrix Ellipsometry
Initial characterization begins with polarized optical microscopy, where rotating a linear polarizer across a crystalline platelet reveals characteristic color and intensity changes. These variations directly reflect the polarization of optical transitions. By collecting spectra as a function of polarizer angle, the two main Davydov components can be resolved. Rotating by 90° shifts intensity between the upper and lower components, a signature of orthogonal polarization states.
However, polarized microscopy alone captures only the projected polarization in the sample plane. For a full picture—including out-of-plane components—one must recover the complete dielectric tensor. This requires ellipsometry.
Imaging Mueller Matrix Ellipsometry extends classical ellipsometry by capturing either the full 4×4 or the partial 3×4 Mueller matrix for each pixel of the image. In Dr. Schiek’s work, the latter was done using a Nanofilm EP4 system equipped with a single rotating compensator on the incident side and high-magnification objectives providing ~2 μm spatial resolution. Measurements in both reflection and transmission geometries at multiple incidence and azimuthal angles are combined to reconstruct the dielectric tensor for multiple rotational platelet domains.

Fig. 2. On the left is a sketch of the EP4 imaging ellipsometer and a partial Mueller matrix (last row missing). On the right is the m13 element of a platelet structure consisting of several subdomains, for which several regions of interest (ROIs) must be selected for data fitting. The angle of incidence (AOI) for this reflection measurement was 50° and the selected wavelength for imaging was 598 nm.

Extracting the Dielectric Tensor

The orthorhombic platelet presents an ideal test case: its crystal axes are orthogonal, and the dielectric tensor is diagonal in the crystallographic frame. Using a combination of fixed-wavelength Mueller matrix maps and azimuthal rotation (“theta scans”) at multiple incidence angles, Dr. Schiek’s team determines the in-plane crystal orientation, particularly the c-axis direction. Once the orientation is fixed, full spectroscopic ellipsometry can be performed. The fitting procedure—developed in-house—uses a phenomenological oscillator model constrained by Kramers–Kronig relations to determine the complex dielectric tensor components εa(ω), εb(ω), and εc(ω) in spectral resolution. The results reveal:

• Multiple Davydov-split excitonic resonances corresponding to the four molecules in the orthorhombic unit cell.
• A dark excitonic state—predicted by group theory but invisible to linear optical spectroscopy—whose presence is inferred from the tensor structure and excitonic polarization analysis.
• Negative real permittivity in spectral ranges close to the excitonic resonances, indicating strong intermolecular coupling, which gives rise to the metallic luster of the platelets.
There are three visible Davydov resonances, whereby in the projected view normal to the (110) surface—as is usual in conventional microscopy/spectroscopy—the two lower Davydov components overlap due to their close spectral positions and similar polarization orientations. These tensor measurements directly link the measured optical anisotropy to the molecular packing geometry—something impossible to extract from conventional microscopy.

Fig. 3. The real (upper) and imaginary (lower) parts of the dielectric tensor for orthorhombic SQIB plates are shown on the left. UDC = Upper Davydov Component along crystallographic c-axis. LDC = Lower Davydov Component along the a-axis (LDC₂) and the b-axis (LDC₁). The polarized absorbance spectra on the bottom right for azimuthal rotation angles from 0° to 90° were measured under normal incidence. At 0° UDC is at its maximum while at 90° the projected LDC is at its maximum. The projected LDC is the superposition of LDC₁ and LDC₂.

The Role of Crystal Symmetry and Polymorphism
An intriguing feature of the SQIB material is its polymorphism. At excessive annealing temperatures, the orthorhombic platelets partially transform to a greenish monoclinic phase, which is thermodynamically more stable despite its lower symmetry.

The monoclinic polymorph has only two molecules per primitive unit cell. Consequently, its Davydov splitting is simpler: just two components (upper and lower), without the dark state present in the orthorhombic form. However, the monoclinic system’s oblique lattice means that the polarization of these components is no longer aligned with principal crystal axes. Only one Davydov component lies along the monoclinic b-axis (the principal axis), while the upper component is rotated by an angle other than 90° in the projection.
Using the EP4 system in transmission intensity mode and user-defined measurement routines via the Python interface, Dr. Schiek’s group confirms the predicted polarization angles experimentally, with measured orientations matching the calculated non-orthogonal rotational offset for the other Davydov component.

Beyond Orthorhombic: The Challenge of Complex Tensors
For the orthorhombic crystal, the dielectric tensor aligns neatly with crystallographic axes, simplifying analysis. The monoclinic system is more challenging: its tensor components—except for one—are rotated relative to the crystal frame, and this rotation can also depend on the spectral range. In triclinic systems, no tensor component is aligned with the crystal lattice but all tensor components may be coupled. Schiek notes that extracting the full tensor for monoclinic or triclinic crystals remains an open challenge for the next stage of this research. Developing robust analysis methods for such systems will be crucial to extending Mueller matrix ellipsometry to a wider class of complex organic materials.

Why This Matters: Linking Molecular Design to Optical Function
This work is more than an exercise in ellipsometry—it points to a powerful feedback loop for material design. Organic semiconductors are increasingly used in light-harvesting, sensing, and optoelectronic devices, where excitonic structure determines performance. By quantitatively linking molecular packing, excitonic splitting, and dielectric anisotropy, Mueller matrix ellipsometry offers a tool to:
• Characterize new materials without requiring single-crystal growth or destructive sectioning.
• Identify polymorphs and phase transitions through their optical signatures.
• Support theoretical modeling such as prediction of dark states by providing experimentally validated tensor data.
Moreover, determining dielectric tensors and exciton transitions is not limited to organic materials but can also be applied to inorganic semiconductors and low-dimensional patterned materials.

Conclusion: Quant. Optical Probe for Complex Mol. Systems
Dr. Schiek’s work demonstrates that Imaging Mueller Matrix Ellipsometry can go beyond traditional optical microscopy, delivering quantitative dielectric tensors and revealing excitonic structures in complex organic crystals. For the orthorhombic polymorph of SQIB, the technique resolves multiple Davydov-split excitons, including a dark state. For the monoclinic form, it confirms the polarization geometry of upper and lower excitonic components. These results not only expand our understanding of excitonic coupling in organic crystals but also point the way toward broader application of IMME in nanomaterials research. As Dr. Schiek concluded, “Imaging ellipsometry is really cool—you can do something quantitative with it. You can get tensors.” In her work, that enthusiasm is backed by rigorous data and careful interpretation, showing how a sophisticated optical technique can turn molecular-scale order into measurable, meaningful parameters.

**About Dr. Manuela Schiek**

Dr. Manuela Schiek is a Senior Researcher and Lecturer at the Center for Surface and Nano Analytics (ZONA) at the Johannes Kepler University Linz, Austria, and also works in the Optical Nanometrology group at the National Metrology Institute (PTB) in Brunswick, Germany. Her research focuses on (chiral) molecular excitonics, functional organic thin films, and solid–liquid interfaces, with emphasis on polarization-resolved microscopy and spectroscopy. She is particularly interested in ellipsometry in various forms. Dr. Manuela Schiek (center) with key-publication collaborators Dr. Frank Balzer (left) and Dr. Matthias Duwe (right). For more information visit: https://orcid.org/0000-0002-0108-2998.

References

S. Funke, M. Duwe, F. Balzer, P. H. Thiesen, K. Hingerl, M. Schiek. Determining the Dielectric Tensor of Micro-Textured Organic Thin Films by Imaging Mueller Matrix Ellipsometry. J. Phys. Chem. Lett. 12 (2021) 3053–3058. DOI: 10.1021/acs.jpclett.1c00317
F. Balzer, T. Breuer, G. Witte, M. Schiek. Template and Temperature-Controlled Polymorph Formation in Squaraine Thin Films. Langmuir 38 (2022) 9266–9277. DOI: 10.1021/acs.langmuir.2c01023
D. Giavazzi, R. Schwarzl, M. Koch, M. Schiek, A. Painelli, F. C. Spano. Modeling the Electronic Coupling in Squaraine Thin Films: The Unusual Case of Three Davydov Components. Submitted 2025.

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