Through functionalization with enantiomerically pure substituents, semiconducting dye molecules such as squaraines can be directed into homochiral arrangements, in which molecular chirality is transferred to a supramolecular level resulting in excitonic circular dichroism. Replacing the sterically demanding CMe 2 $_2$ groups in indolenine-derived squaraines with sulfur atoms promotes aggregation and structural order in otherwise homogeneous thin films of benzothiazole-derived squaraines. Here, we investigate solvent-processed and thermally annealed thin films of a benzothiazole-derived squaraine functionalized with homochiral citronellyl groups. The crystalline arrangement of the thin films does not correspond to the single crystal structure found, which consists of a solvent-containing pseudo-polymorph. The spectral shape of the excitonic circular dichroism in spin-cast thin films is sensitive to the processing solvent and annealing temperature. Uniaxial anisotropy, evidenced by X-ray diffraction and spectroscopic ellipsometry, shows excitonic transitions that are exclusively polarized either in the plane or perpendicular to the plane of the thin film. We interpret this in terms of a Davydov splitting into spatially separated upper (in-plane) and lower (out-of-plane) Davydov components. This rarely documented arrangement changes our reading of optical spectra, including circular dichroism, recorded under standard normal incidence conditions, which probe only transitions with an in-plane dipole-moment component.

Squaraine thin films are emerging as functional optoelectronic elements because of their anisotropic optical properties in the visible to near-infrared spectral range, which include characteristic properties such as large Davydov splitting, hybrid Frenkel-charge transfer excitons, and giant circular dichroism. The prototypical squaraine 2,4-bis-[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-squaraine (SQIB) condenses into two different polymorphs (orthorhombic and monoclinic unit cells), both with distinct optical, electronic, and morphological properties. Polarized Raman microscopy spectra can distinguish between these polymorphs and indicate their crystallographic alignment, similar to polarized transmission spectroscopy, while atomic force microscopy precisely maps all topographical features. During crystallization, periodic nanodunes with cracks and protrusions along the local c-axis can form from the orthorhombic SQIB polymorph without any lithographic steps. The full dielectric tensor is known for this polymorph, and the components of the real part are strongly negative near the absorption bands. For metallic nanoparticles, it is known that a negative dielectric function can lead to localized surface plasmons and field confinement. In this study, we investigate which of the morphological featuresnanodunes, cracks, or protrusionshave the potential to influence the excitonic properties in a similar enhancing fashion.

Organic semiconductors are emerging as a promising class of photovoltaic materials for neural interfaces, offering high power conversion, mechanical flexibility, biocompatibility, and tunable optoelectronic properties. In this study, the application of a D18:Y6-based organic photovoltaic (OPV) electrode is investigated for subretinal stimulation in a model of retinal degeneration. It is demonstrated that OPV devices can be engineered to electrically stimulate the retina via network-mediated pathways, in a manner comparable to established neurostimulation approaches. The presented OPV electrodes reliably activate retinal ganglion cells, eliciting consistent spike responses to light pulses within the near-infrared range. The stimulation onset, spike latency, and response profiles suggest effective faradaic charge injection as a key mechanism for neuronal activation, particularly for longer pulse durations. Moreover, it is shown that both light intensity and pulse duration can be used to finely tune the neural response, offering a high degree of control over retinal ganglion cell activation. Finally, it is proven that D18:Y6 blend is biocompatible in vitro when in direct contact with human induced pluripotent stem cell (iPSC)-derived retinal organoids and mouse explants. These results validate the photo-electrical performance and biocompatibility of D18:Y6 OPVs and position them as a viable candidate for next-generation, minimally invasive retinal prosthetics.

A theoretical study is conducted to unveil the nature of intermolecular interactions in orthorhombic 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (SQIB) crystalline thin films. The orthorhombic polymorph of SQIB crystallizes in the Pbcn space group with four equivalent molecules per unit cell (Z = 4), leading to three orthogonally polarized Davydov components and a dark state. The remarkable agreement between experimental polarized absorbance spectra and the simulated spectra utilizing the Frenkel-Holstein Hamiltonian allows one to reliably assign the observed spectroscopic features to three Davydov components originating from intermolecular Coulomb interactions. Additionally, the energetic location of the dark state, which plays an important role in nonradiative relaxation, is predicted to lie roughly midway between the lower and upper Davydov components.

Three Pinaceae resins originating from trees of high industrial significance-European larch, European spruce, and Atlas cedar-were examined in this work. These resins exhibited ease of processing using ethyl alcohol solutions, exceptional film formation, and great dielectric qualities with measured breakdown fields in the range of 5-7.3 MV cm-1. Because their film surface was essentially trap-free, it was possible to fabricate organic field effect transistors that are hysteresis-free and have outstanding stability under 12-hour bias stress at working voltages below 10 V, with current retention approaching 90% of the original value and transfer curve recovery occurring within 90 minutes. These environmentally friendly materials, which are freely available, are a great option for applications aiming to produce sustainable electronics.

In amorphous organic semiconductors, charge transport typically takes place via slow hopping processes, but it is known that molecular aggregation can lead to enhanced exciton and charge transport through coupling of the transition dipole moments. In this work, we investigate the optical, morphological, and electronic properties of thin films of a merocyanine dye, which aggregates easily due to its dipolar character. Firstly, in spin-coated thin films the degree of aggregation can be tuned by thermal annealing, leading to strong spectral shifts alongside with Davydov splitting of >800 meV. At the same time, the mobility increases by approximately three orders of magnitude. We combine variable angle spectroscopic ellipsometry and polarization-resolved transmission spectroscopy with density functional theory to demonstrate that the aggregated molecules are oriented in an upright, standing configuration relative to the substrate surface. This arrangement involves a co-facial orientation of the molecular pi-systems, which is advantageous for lateral charge transport. Secondly, by utilizing highly oriented pyrolytic graphite as an ordered substrate and low-rate vacuum deposition, we are able to template the growth of the merocyanine layer and to substantially improve the in-plane morphological order. By combining atomic force microscopy and photoluminescence microspectroscopy we observe large oriented domains hundreds of mu m(2) in size, emitting linearly polarized light, whereby maintaining the edge-on molecular arrangement. This promises a further significant enhancement of lateral charge carrier mobility.

Although squaraine dyes are commonly praised as candidates for light-based applications, little is known about their excited state landscape beyond the low-energy visible light region. Our work aims for an improved understanding of the photophysical properties of squaraines at the example of N-isobutyl substituted anilino-squaraine (SQIB) by extending ground-state and excited-state absorption spectroscopy of the molecule into the ultraviolet up to 6.5 eV. In addition, we distinguish the relative transition dipole moments of the excited state absorption peaks with the help of transient absorption anisotropy experiments. To relate experimental features to specific states, we employ a set of ab initio methods including time-dependent density functional theory (TDDFT), the Bethe-Salpeter equation (BSE) and n-electron valence perturbation theory on top of a self-consistent complete active space (CASSCF/NEVPT2). Our assignment is complemented by vibronic simulations and a discussion of two-photon absorption measurements. Through this joint effort, we are able to provide a consistent picture of the optical behavior of SQIB across the visible and ultraviolet light regime, and assign a total of twelve electronically excited states to our experimental data.

ABSTRACTWhile chirality is a prevalent character of numerous biological and synthetic organic molecules, its selective absorption of circularly polarized light, known as circular dichroism (CD), is typically small due to intrinsically weak coupling between magnetic and electric dipoles. However, thin films of aggregated, enantiopure prolinol‐derived squaraine molecules (ProSQ‐C16) exhibit an unusually large excitonic CD signal, although the underlying mechanism is not yet known. In this study, we employ steady‐state and ultrafast transient absorption spectroscopy to investigate the nature and dynamics of excitons in aggregates of enantiopure and racemic ProSQ‐C16 thin films. Highly resembling transient responses of enantiopure thin films under excitations at different photon energies strongly indicate that a single type of aggregate dominates the linear optical response, that is, a strong red‐shifted (J‐like) and weak blue‐shifted (H‐like) absorption band. On the other hand, the transient properties of the racemic thin film deviate from this pattern and remain largely ambiguous. The short lifetime of excited states and coherent oscillations present in the dynamics of the transient absorption signal indicate that the early time dynamics are governed by a transition towards a dark intermediate state, which might arise from intermolecular charge transfer with potential contributions from the coupling of excitons to the vibrations. This non‐radiative relaxation pathway explains the unusually weak fluorescence of the predominately J‐like behaving aggregate. Our findings conclusively show that the chiral aggregate structure has a strong impact on the optical and dynamic response of the excitons and underline the significance of non‐Frenkel exciton states for the optical properties of anilino squaraine dyes.

The advanced development of optoelectronic devices requires a methodical knowledge of the fundamental material properties of the key active components. Systematic investigations and correlations of such basic optical properties can lead to new insights for the design of more potent materials. In this perspective, we provide a systematic overview of the uniaxial anisotropic complex refractive indices and the absorption coefficients obtained by ellipsometry as well as the optical band gap energies derived from Tauc plots of six selected solution-processed polymer thin films. While the optical band gap energies are intentionally distributed over the visible spectral range, we found that the absorption strength of all polymer samples are grouped in a random distribution within a rather uniform range of values.

The demand for information processing at ultrahigh speed with large data transmission capacity is continuously rising. Necessary building blocks for on‐chip photonic integrated circuits (PICs) are reconfigurable integrated low‐loss high‐speed modulators and switches. Phase change materials (PCMs) provide unique opportunities for integration into PICs. Here, the investigation of layered gallium monosulfide (GaS) as a novel low‐loss PCM from infrared to optical frequencies is pioneered, with high index contrast (Δ n ≈0.5) at the optical telecommunication band. The GaS bandgap switches from 1.5 ± 0.2 eV for the amorphous state to 2.1 ± 0.1 eV for the crystalline state. It is demonstrated that the reversible GaS amorphous‐to‐crystalline phase transition can be operated thermally and by picosecond green (532 nm) laser irradiation. The design of a reconfigurable integrated optical modulator on‐chip based on Mach‐Zehnder Interferometers (MZI) with the GaS PCM cell deposited on one of the arms for application is presented at the telecommunication wavelength of λ = 1310 nm, where the standard single mode optical fiber exhibits zero chromatic dispersion, and at λ = 1550 nm, where a minimum optical loss of 0.22 dB km −1 is obtained. This opens the route to applications such as reconfigurable modulators, beam steering using phase modulation, and photonic neural networks.

Transparent conductive oxides such as indium tin oxide (ITO) are standards for thin film electrodes, providing a synergy of high optical transparency and electrical conductivity. In an electrolytic environment, the determination of an inert electrochemical potential window is crucial to maintain a stable material performance during device operation. We introduce operando ellipsometry, combining cyclic voltammetry (CV) with spectroscopic ellipsometry, as a versatile tool to monitor the evolution of both complete optical (i.e., complex refractive index) and electrical properties under wet electrochemical operational conditions. In particular, we trace the degradation of ITO electrodes caused by electrochemical reduction in a pH-neutral, water-based electrolyte environment during electrochemical cycling. With the onset of hydrogen evolution at negative bias voltages, indium and tin are irreversibly reduced to the metallic state, causing an advancing darkening, i.e., a gradual loss of transparency, with every CV cycle, while the conductivity is mostly conserved over multiple CV cycles. Post-operando analysis reveals the reductive (loss of oxygen) formation of metallic nanodroplets on the surface. The reductive disruption of the ITO electrode happens at the solid-liquid interface and proceeds gradually from the surface to the bottom of the layer, which is evidenced by cross-sectional transmission electron microscopy imaging and complemented by energy-dispersive X-ray spectroscopy mapping. As long as a continuous part of the ITO layer remains at the bottom, the conductivity is largely retained, allowing repeated CV cycling. We consider operando ellipsometry a sensitive and nondestructive tool to monitor early stage material and property changes, either by tracing failure points, controlling intentional processes, or for sensing purposes, making it suitable for various research fields involving solid-liquid interfaces and electrochemical activity.

Organic molecular beam deposition (OMBD) of para-hexaphenylene (p6P) on polycrystalline platinum results in the formation of unique nanoaggregates, predominantly as nanofibers and nanoribbons. These aggregates exhibit distinct morphological properties, as revealed by atomic force microscopy (AFM). Grazing incidence X-ray diffraction (GIXD) confirms the p6P herringbone structure as partially oriented aggregates with a bias of previously observed contact planes parallel to the substrate. The optical properties of the aggregates are analyzed using polarization microscopy, fluorescence microscopy, and Raman microscopy to distinguish three different aggregate types with a focus on aspects such as molecule orientation within the aggregates, including those lying and standing upright. Polarized microscopy indicates that the molecular orientation within the fluorescing, fiber-like aggregates is generally perpendicular to the long fiber axis and parallel to the substrate, which seems not to be the case for the other two types. This finding is crucial for applications utilizing p6P's polarized emission, such as in photonic and optoelectronic devices.

Four pinaceae pine resins analyzed in this study: black pine, shore pine, Baltic amber, and rosin demonstrate excellent dielectric properties, outstanding film forming, and ease of processability from ethyl alcohol solutions. Their trap-free nature allows fabrication of virtually hysteresis-free organic field effect transistors operating in a low voltage window with excellent stability under bias stress. Such green constituents represent an excellent choice of materials for applications targeting biocompatibility and biodegradability of electronics and sensors, within the overall effort of sustainable electronics development and environmental friendliness.

An extensive experimental study of a family of chiral aggregates of proline derived squaraines is presented and discussed with reference to two theoretical models and to MD simulations to shed light on the chiroptical properties of these systems.

We demonstrate simultaneous strong light-matter coupling of J- and H-like transitions of molecular aggregates of a merocyanine dye placed in a planar microcavity. The molecules form a uniaxial crystal with the transition dipole moments of the two transitions oriented along and normal to the cavity mirrors. Both transitions are spectrally narrow, allowing one to reach the strong-coupling regime between the excitons and cavity photons of the 3 lambda/2- and 1 lambda modes of the cavity. Our work explores the strong light-matter coupling of H-transitions, which has so far not been properly studied. We show how the number of coherently coupled molecules in the molecular aggregates influences the strong-coupling allowing us to relate the degree of aggregation to the observed Rabi energy. Our polaritonic system forms a platform to study the photophysics of complex strong-coupling phenomena.

The impact on chiral aggregation in solution processed and thermally annealed thin films of two indolenine and one anilino squaraines with chiral (S)-citronellyl functionalization at the nitrogen of the squaraine backbone is investigated. A pseudo polymorphic crystal structure is obtained for one of the indolenine squaraines, but thin films of both compounds are basically non-aggregated and truly isotropic as evidenced by spectroscopic ellipsometry. The anilino squaraine coalesces and readily aggregates to circular dichroic but discontinuous thin films. The extent of circular dichroism correlates with the morphology of the samples, which is quantified by Mueller matrix polarimetry in combination with atomic force microscopy. The shape of the CD spectra reinforces the hybrid Frenkel and charge-transfer excitonic nature of the characteristic double hump signatures within unpolarized absorbance spectra of previously investigated non-chiral anilino squaraines with linear alkyl chain functionalization. Such excitonic CD offers an additional design parameter for next-generation opto-electronic devices.

Controlling the polymorph formation in organic semiconductor thin films by the choice of processing parameters is a key factor for targeted device performance. Small molecular semiconductors such as the prototypical anilino squaraine compound with branched butyl chains as terminal functionalization (SQIB) allow both solution and vapor phase deposition methods. SQIB has been considered for various photovoltaic applications mainly as amorphous isotropic thin films due to its broad absorption within the visible to deep-red spectral range. The two known crystalline polymorphs adopting a monoclinic and orthorhombic crystal phase show characteristic Frenkel excitonic spectral signatures of overall H-type and J-type aggregates, respectively, with additional pronounced Davydov splitting. This gives a recognizable polarized optical response of crystalline thin films suitable for identification of the polymorphs. Both phases emerge with a strongly preferred out-of-plane and rather random in-plane orientation in spin-casted thin films depending on subsequent thermal annealing. By contrast, upon vapor deposition on dielectric and conductive substrates, such as silicon dioxide, potassium chloride, graphene, and gold, the polymorph expression depends basically on the choice of growth substrate. The same pronounced out-of-plane orientation is adopted in all crystalline cases, but with a surface templated in-plane alignment in case of crystalline substrates. Strikingly, the amorphous isotropic thin films obtained by vapor deposition cannot be crystallized by thermal postannealing, which is a key feature for the spin-casted thin films, here monitored by polarized in situ microscopy. Combining X-ray diffraction, atomic force microscopy, ellipsometry, and polarized spectro-microscopy, we identify the processing-dependent evolution of the crystal phases, correlating morphology and molecular orientations within the textured SQIB films.

Enlarging exciton coherence lengths in molecular aggregates is critical for enhancing the collective optical and transport properties of molecular thin film nanostructures or devices. We demonstrate that the exciton coherence length of squaraine aggregates can be increased from 10 to 24 molecular units at room temperature when preparing the aggregated thin film on a metallic rather than a dielectric substrate. Two-dimensional electronic spectroscopy measurements reveal a much lower degree of inhomogeneous line broadening for aggregates on a gold film, pointing to a reduced disorder. The result is corroborated by simulations based on a Frenkel exciton model including exciton-plasmon coupling effects. The simulation shows that localized, energetically nearly resonant excitons on spatially well separated segments can be radiatively coupled via delocalized surface plasmon polariton modes at a planar molecule-gold interface. Such plasmon-enhanced delocalization of the exciton wave function is of high importance for improving the coherent transport properties of molecular aggregates on the nanoscale. Additionally, it may help tailor the collective optical response of organic materials for quantum optical applications.