Azobenzene undergoes trans → cisisomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis →transisomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Photoisomerization in different classes of azobenzene

The Maxwell displacement current generated from single fatty acid monolayers containing azobenzene by photoisomerization was investigated in the context of the phase diagram of monolayers on a water surface. Transient displacement‐current pulses were found to be produced when successive isomerizations were induced in monolayers by irradiation with ultraviolet and visible light. In contrast, the displacement current was never generated in the so‐called solid phase by the application of surface pressure or photoirradiation, probably because of the lack of free volume for the isomerizing chromophore in single monolayers and as well as the aggregation of molecules in the monolayer. The displacement current generated during monolayer compression was also investigated in connection with phase transitions.

Generation of Maxwell displacement current from spread monolayers containing azobenzene

Interest in the dynamic behaviour of carriers in organic materials is motivated by possible applications that include organic thin-film transistors, organic electro-luminescent devices and organic photoconductors. It can also provide an insight into the modelling of carrier transport and trapping in organic semiconductors and insulators. Here, we use an advanced second-harmonic generation technique to probe and visualize real carrier motion in organic materials. This is a time-resolved microscopic optical second-harmonic generation technique that allows direct and selective probing of dynamic carrier motion in organic materials. Experiments making use of this technique and using pentacene field-effect transistors have revealed dynamic changes of second-harmonic-generation intensity profiles arising from pentacene. Carrier velocity in organic solids is thus determined from the visualized carrier motion. We anticipate that this direct visualization technique will find wide application in the illustration of space-charge field formation in organic and inorganic materials, including biomaterials and polymers.

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Direct imaging of carrier motion in organic transistors by optical second-harmonic generation

Installing Dynamic Molecular Photomechanics in Mesopores: A Multifunctional Controlled-Release Nanosystem

have been successfully used to direct theformation of ordered fullerene arrays. In this way theinterfullerene distance and symmetry of the arrangementaresolelydeterminedbythemoleculartemplateandcanbechanged by adjusting its structure. More importantly, inter-actions between fullerenes can also be modified with thisstrategy.Wehavedesignedandsynthesizedatetraacidicazoben-zene molecule NN4A (Figure1a) that exclusively formsKagomopennetworkswithtwotypesofcavitiesthathavedifferent size and symmetry at the liquid–solid interface.These cavities are capable of accommodating fullerenemolecules as guest species. Herein, we examine the siteselectivityofthenetworksfordifferentfullerenes,afeaturewhichhasnotbeenobservedforothersystemsatliquid–solidinterfaces.Azobenzenederivativesaretypicalphotochromiccompoundsthathaveawiderangeofpotentialapplications,including for optical switching, holographic storage, lightharvesting, long-term energy storage, and nonlinear opticalmaterials.

Site‐Selective Fabrication of Two‐Dimensional Fullerene Arrays by Using a Supramolecular Template at the Liquid‐Solid Interface

INTEREST in the behaviour of Langmuir–Blodgett (LB) films1 is motivated by possible applications that include molecular electronics, biosensors and modelling of biological membranes We recently2–5 reported an electrical technique that allows molecular motions in LB films to be probed by measuring the Maxwell displacement current6 generated across electrodes above and below the film: this current results from changes in the orientation of the molecular dipoles Here we use the technique to study the effect of photoisomerization in an LB film consisting of an azobenzene derivative, which can be switched photochemically between cis and trans isomers Films of this sort are of particular interest because of their possible use in information-storage devices7–11 Successive isomerizations induced by irradiation with ultraviolet and visible light produced transient displacement-current pulses between the electrodes We anticipate that this technique will become widely used for dynamical studies of LB films

Generation of Maxwell displacement current across an azobenzene monolayer by photoisomerization

The optical molecular switching in single monolayers containing azobenzene derivatives at the air water interface due to photoinduced cis‐trans isomerization was detected by using a displacement‐current‐measuring technique. The reversible displacement current generation was found to be sustained by the alternative applications of ultraviolet light and visible light, whereas, the current generation was never initiated by the successive irradiation of ultravilolet or visible light. In order to clarify the physical mechanism relevant to the displacement current generation, the optical molecular switching in single monolayers deposited on solid substrates was also examined by the displacement‐current‐measuring technique using a (indium tin oxide)/monolayer/air‐gap/(indium tin oxide) structure. As a result, the displacement‐current generation from single monolayers deposited on the solid substrates was found to be initiated in a similar manner as that from monolayers formed on the water surface. Finally, we co...

Investigation of photoinduced molecular switching in a single monolayer by a displacement‐current‐measuring technique

The optical molecular switching in single monolayers containing azobenzene derivatives deposited on solid substrates due to photo-induced cis-trans isomerization was detected by using a displacement current-measuring technique. The reversible displacement current generation was found to be sustained by the alternative applications of UV light and visible light, whereas the current generation was never initiated by the successive irradiation of UV or visible light. Finally, we concluded that the technique developed here will be used in future molecular electronic technologies.

Hiromasa Fuwa

Papers

Generation of Maxwell displacement current across an azobenzene monolayer by photoisomerization

Investigation of photoinduced molecular switching in a single monolayer by a displacement‐current‐measuring technique

Displacement Current Generated by Photo-Induced Molecular Switching in a Single Monolayer