Molecular chirality and the inherently connected differential absorption of circular polarized light (CD) combined with semiconducting properties offers great potential for chiral opto-electronics. Here we discuss the temperature-controlled assembly of enantiopure prolinol functionalized squaraines with opposite handedness into intrinsically circular dichroic, molecular J-aggregates in spincasted thin films. By Mueller matrix spectroscopy we accurately probe an extraordinary high excitonic circular dichroism, which is not amplified by mesoscopic ordering effects. At maximum, CD values of 1000 mdeg/nm are reached and, after accounting for reflection losses related to the thin film nature, we obtain a film thickness independent dissymmetry factor g = 0.75. The large oscillator strength of the corresponding absorption within the deep-red spectral range translates into a negative real part of the dielectric function in the spectral vicinity of the exciton resonance. Thereby, we provide a new small molecular benchmark material for the development of organic thin film based chiroptics.
Organic semiconductors are emerging as promising candidates for novel electrically self-sufficient photovoltaic prosthetics for neurostimulation, especially for restoration of light sensitivity in degenerate retina. Considering future applications, it is essential to gain fundamental insight into the signaling mechanisms at the organic photosensor-electrolyte-neuron interface. Particularly, targeting voltage-gated ion channels by a pure photo-capacitive stimulation is a preferred therapeutic approach as it avoids redox reactions involved in faradaic charge injection. The present study investigates whether single neuroblastoma (N2A) cells, grown on a photosensor based on a small molecular squaraine:fullerene photoactive layer blend, optionally covered with silicon dioxide, can be activated by photo-capacitive stimulation. Indeed, upon pulsed illumination, a rapid transient photocurrent strongly depolarizes the membrane potential and subsequently activates fast-responding voltage-gated sodium channels. The dielectric top coating on the organic layer ensures sufficient capacitive charge injection efficiency while maintaining the rapid response of the device. Due to the high irradiance level required for photo-capacitive stimulation, another slower, independent and unintended, non-electrical signaling pathway is identified. This activates voltage-gated potassium channels, presumably by photothermal effects. The present study provides the basis for further improvements on standalone photovoltaic neuro-stimulating platforms based on organic photoactive layers.
As a step toward the realization of neuroprosthetics for vision restoration, we follow an electrophysiological patch-clamp approach to study the fundamental photoelectrical stimulation mechanism of neuronal model cells by an organic semiconductor-electrolyte interface. Our photoactive layer consisting of an anilino-squaraine donor blended with a fullerene acceptor is supporting the growth of the neuronal model cell line (N2A cells) without an adhesion layer on it and is not impairing cell viability. The transient photocurrent signal upon illumination from the semiconductor-electrolyte layer is able to trigger a passive response of the neuronal cells under physiological conditions via a capacitive coupling mechanism. We study the dynamics of the capacitive transmembrane currents by patch-clamp recordings and compare them to the dynamics of the photocurrent signal and its spectral responsivity. Furthermore, we characterize the morphology of the semiconductor-electrolyte interface by atomic force microscopy and study the stability of the interface in dark and under illuminated conditions.
H2O2 plays a significant role in a range of physiological processes where it performs vital tasks in redox signaling. The sensitivity of many biological pathways to H2O2 opens up a unique direction in the development of bioelectronics devices to control levels of reactive‐oxygen species (ROS). Here a microfabricated ROS modulation device that relies on controlled faradaic reactions is presented. A concentric pixel arrangement of a peroxide‐evolving cathode surrounded by an anode ring which decomposes the peroxide, resulting in localized peroxide delivery is reported. The conducting polymer (poly(3,4‐ethylenedioxythiophene) (PEDOT), is exploited as the cathode. PEDOT selectively catalyzes the oxygen reduction reaction resulting in the production of hydrogen peroxide (H2O2). Using electrochemical and optical assays, combined with modeling, the performance of the devices is benchmarked. The concentric pixels generate tunable gradients of peroxide and oxygen concentrations. The faradaic devices are prototyped by modulating human H2O2‐sensitive Kv7.2/7.3 (M‐type) channels expressed in a single‐cell model (Xenopus laevis oocytes). The Kv7 ion channel family is responsible for regulating neuronal excitability in the heart, brain, and smooth muscles, making it an ideal platform for faradaic ROS stimulation. The results demonstrate the potential of PEDOT to act as an H2O2 delivery system, paving the way to ROS‐based organic bioelectronics.
Local polarized surface photovoltage (SPV) spectroscopy is used to characterize an organic squaraine‐fullerene photovoltaic layer blend. The anisotropic optical response of the textured photoactive layer is dominated by the largely Davydov‐split absorption within the deep red of the crystallized donor compound. Through knowledge of the crystallographic texture, the local molecular in‐plane orientation is deduced. Kelvin probe force microscopy (KPFM) maps the differential SPV of the active layer on the nanoscale without complications by interfaces, which is spatially correlated with the pleochroic optical response of the thin film. The SPV shows a wavelength‐dependent, bichromatic change upon rotating the polarization axis of the illuminating light. With that subtler, nanoscaled optoelectronic sensing or actuating platforms become possible.
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