Hideki Kohn

Chiroptical switch based on azobenzene-substituted polydiacetylene LB films under thermal and photic stimuli

The azobenzene-substituted diacetylene (NADA) LB films were fabricated and their chiroptical properties were investigated in detail. It was found that the NADA LB films showed supramolecular chirality, although the NADA molecule itself was achiral. Overcrowded packing of azobenzene chromophores were believed to be responsible for the supramolecular chirality formation of NADA LB films. When irradiated by left- and right-handed circular polarized ultra-violet light (CPUL), obviously opposite CD signals for azobenzene chromophores and polydiacetylene (PDA) chains were observed from the resulting polymerized NADA (PNADA) LB films. Reversible changes of CD signals for PDA chains could be observed in response to thermal stimulus. When irradiated by left- and right-handed circular polarized lasers (CPL, 442 nm), a reversible chiroptical switch between two enantiometric structures of PDA chains could be observed by alternating the stereoregular packing of azobenzene chromophores in the side chains. Strong collective non-covalent interactions (π–π stacking) and rapid conformational rearrangement of azobenzene chromophores under CPUL or CPL treatment were believed to be responsible for the supramolecular chirality formation and reversible chiroptical switches upon thermal and photic stimuli. This research provides a novel model system for understanding the detailed mechanism of the chiroptical introduction and modulation in the PDA backbone.

Control and modulation of chirality for azobenzene-substituted polydiacetylene LB films with circularly polarized light.

Herein, photo-controlled rearrangement of the azobenzene chromophore in the side chains was used to realize enantioselective polymerization and light-triggered reversible switching between opposite chirality for the PDA backbone.

Direct observation of trapped carriers in polydiacetylene films by optical second harmonic generation

Trapped carriers in polydiacetylene (PDA) films were directly observed by the electric field induced second harmonic generation (EFISHG) using field effect transistor (FET) structure. Response of EFISHG signal from PDA-FET with applying voltage depended strongly on the polarity of gate voltage. For negative bias, which promotes hole injection from source electrode, EFISHG signal was not observed during bias application, whereas it was enhanced after turning off the bias. Electric field formed by trapped holes in PDA activated the EFISHG signal for the negative bias condition.

Direct observation of space charge field in tetracene field-effect transistor using time-resolved microscopic optical second harmonic generation

Using electric field-induced second harmonic generation (EFISHG) measurements, we directly probed a time-varying space charge field responsible for carrier injection followed by radiative recombination (electroluminescence; EL). Experiments using tetracene as the active layer of a top-contact field-effect transistor with application of a pulsed voltage showed that a space charge field remained around the source electrode—caused by injected holes, but not by electrons. Observing the time-varying EFISHG intensity at the electrodes clearly showed that the space charge field formed in the vicinity of the electrodes made a dominant contribution to the opposite-polarity carrier injection, directly leading to EL around the electrodes by carrier recombination. We concluded that EFISHG measurement is useful for probing carrier behavior in luminescent materials, by eliminating two-photon excited photoluminescence from the material induced using an incident laser.

Observation of Electron Injection into Organic Field-Effect Transistor with Au Electrodes using Electroluminescence under AC Electric Field

The electroluminescence (EL) from a tetracene field-effect transistor was examined by applying an AC voltage to elucidate electron injection from the source and drain Au electrodes. The EL intensity was proportional to the AC frequency up to 500 kHz, but it decayed rapidly with elapsed time when applying the AC voltage. The EL spectrum corresponded well with the photoluminescence spectrum. These results indicate that holes and electrons are injected into the tetracene alternately in accordance with the AC applied voltage, and that the EL is radiated due to the recombination of the injected holes and electrons, while the space charge formation leading to the gradual decay of the EL proceeds owing to the accumulation and trapping of excessive injected carriers.

Observation of Carrier Behavior in Organic Field-Effect Transistors with Electroluminescence under AC Electric Field

The carrier behavior in a tetracene thin film under AC electric field was examined by electroluminescence (EL) using the structure of organic field-effect transistors (OFETs). A square wave voltage was applied to the source electrode, whereas the drain and gate electrodes were connected to the ground. Electrons and holes were injected from the Au source electrode alternately, and they recombined at the interface between the tetracene thin film and the source electrode. A series of EL pulses was observed in accordance with the applied AC voltage, i.e., a transient EL was observed in a half cycle of AC. This result indicates that alternating carrier injection causes EL. Furthermore, EL intensity increased with the frequency and amplitude of AC voltage. It was suggested that the amount of electrons and holes injected from the source electrode in a unit time is controlled by the frequency and amplitude of AC voltage.

Induced Optical Chirality of Poly(diacetylene) Film by Circularly Polarized Light and Its Control by Changing Substrate Temperature

The strength of chirality of a blue-phase poly(diacetylene) (PDA) film polymerized from achiral monomers using circularly polarized light (CPL) was effectively controlled by changing substrate temperature (313 and 273 K). The obtained absorption spectra did not show a difference between PDA films polymerized using left- and right-CPL, whereas the chiral PDA films with opposite chirality were selectively produced by left- and right-CPL irradiations. The PDA film prepared at 313 K showed stronger chirality than that prepared at 273 K. Microscopy observation clearly indicates that the strength of chirality depends on the domain size.

Direct Observation of Carrier Behavior Leading to Electroluminescence in Tetracene Field-Effect Transistor

By using electric-field-induced second-harmonic generation (EFISHG) measurements, we directly probed carrier motion in tetracene field-effect transistors (FETs), followed by electroluminescence (EL). Under a pulse voltage application, positive charges were injected from the source electrode and accumulated on the gate insulator. After the voltage was removed, positive charges remained and formed a space charge field. As a result, electrons were pulled into the FET to recombine with the remaining holes for radiating EL. The EFISHG measurement provides a way to directly probe the electric field transient that contributes to the EL mechanism of light-emitting transistors.