Photochromism and electrochemistry of a dithienylcyclopentene electroactive polymer

Electrochemical Ring-Opening and -Closing of a Spiropyran

The bistability of molecular switches is an essential characteristic in their use as functional components in molecular-based devices and machines. For photoswitches, light-driven switching between two stable states proceeds via short-lived changes of the bond order in electronically excited states. Here, bistable switching of a ditertbutyl-substituted spiropyran photoswitch is instead demonstrated by oxidation and subsequent reduction in an overall four-state cycle. The spiropyran structure chosen has reduced sensitivity to the effect of secondary electrochemical processes such as H⁺ production and provides transient access to a decreased thermal Z–E isomerization barrier in the one electron oxidized state, akin to that achieved in the corresponding photochemical path. Thus, we show that the energy needed for switching spiropyrans to the merocyanine form on demand, typically delivered by a photon, can instead be provided electrochemically. This opens up further opportunities for the utilization of spiropyrans in electrically controlled applications and devices.

Noncommutative Switching of Double Spiropyrans

The spiropyran family of photochromes are key components in molecular-based responsive materials and devices, e.g., as multiphotochromes, covalently coupled dyads, triads, etc. This attention is in no small part due to the change in properties that accompany the switch between spiropyran and merocyanine forms. Although the spiropyran is a single structural isomer, the merocyanine form represents a family of isomers (TTT, TTC, CCT, etc.) and protonation states. Combining two spiropyrans into one compound increases the number of possible structures dramatically and the interaction between the units determines, which are impeded due to intramolecular quenching of excited states. Here, we show that the coupling of two spiropyran photochromes through their phenol units yields favorable interactions (crosstalk) between the components that provides access to species inaccessible with the component monospiropyran alone. Specifically, the ring opening of one spiropyran unit, which is thermally stable at −30 °C, prevents ring opening of the second spiropyran unit. Furthermore, whereas protonated E- and Z-monomerocyanines were previously shown to undergo thermal- and photo-equilibration, the corresponding protonated E- and Z- bimerocyanines are thermally stable and show one-way photoisomerization from the Z,Z- to an emissive E,E-bimerocyanine form. Subsequent deprotonation at room temperature resets the system to the bispiro ring-closed form, but deprotonation at −30 °C yields the otherwise inaccessible bimerocyanine form. This form is photochemically inert but undergoes a two-step thermal relaxation via the merocyanine-spiropyran form, showing that the connection at the phenol units provides sufficient intramolecular interaction to fine-tune the complex isomerization pathways of spiropyrans and demonstrating noncommutability in photo- and pH-regulated multistep isomerization pathways.

Diarylethene-based conjugated polymer networks for ultrafast photochromic films

Two new diarylethene-based conjugated polymers were synthesized, and their films exhibited ultrafast photochromism properties and excellent fatigue resistance.

Enhanced Two-Photon Photochromism in Metasurface Perfect Absorbers

Light switchable materials are essential to optoelectronic applications in photovoltaics, memories, sensors, and communications. Natural switchable materials suffer from weak absorption and slow response times, preventing their use in low-power, ultrafast applications. Integrating light switchable materials with metasurface perfect absorbers offers an innovative route to achieving desirable features for nanophotonic devices, such as directional emission, low-power and broadband operation, high radiative quantum efficiency, and large spontaneous emission rates. Here we show an enhanced two-photon photochromism based on a metasurface perfect absorber: film-coupled colloidal silver nanocubes. The photochromic molecules, spiropyrans, are sandwiched between the silver nanocubes and the gold substrate. With nearly 100% absorption and an accompanying large field enhancement in the molecular junction, the transformation of spiropyrans to merocyanines is observed under excitation with 792 nm laser light. Fluorescence lifetime measurements on the merocyanine form reveal that large Purcell enhancement in the film-coupled nanocubes leads to large enhancements of the spontaneous emission rate and a high quantum efficiency. An averaged incident power as low as 10 μW is enough to initiate the two-photon isomerization of spiropyran in the film-coupled nanocubes, and a power of 100nW is able to excite the merocyanines to emit fluorescence. The power consumption is orders of magnitude lower than bare spiropyran thin films on silicon and gold, which is highly desirable for the writing and reading processes relevant to optical data storage. By sweeping the plasmonic resonance of the film-coupled nanocubes, wavelength specificity is demonstrated, which opens up new possibilities for minimizing the cross talk between adjacent bits in nanophotonic devices.

Reversible Charge Trapping in Bis-Carbazole-Diimide Redox Polymers with Complete Luminescence Quenching Enabling Nondestructive Read-Out by Resonance Raman Spectroscopy

The coupling of substituted carbazole compounds through carbon-carbon bond formation upon one-electron oxidation is shown to be a highly versatile approach to the formation of redox polymer films. Although the polymerization of single carbazole units has been proposed earlier, we show that by tethering pairs of carbazoles double sequential dimerization allows for facile formation of redox polymer films with fine control over film thickness. We show that the design of the monomers and in particular the bridging units is key to polymer formation, with the diaminobenzene motif proving advantageous, in terms of the matching to the redox potentials of the monomer and polymer film and thereby avoiding limitations in film thickness (autoinsulation), but introduces unacceptable instability due to the intrinsic redox activity of this moiety. The use of a diimide protecting group both avoids complications due to p-diamino-benzene redox chemistry and provides for a redox polymer in which the photoluminescence of the bis-carbazole moiety can be switched reversibly (on/off) with redox control. The monomer design approach is versatile enabling facile incorporation of additional functional units, such as naphthalene. Here we show that a multicomponent carbazole/naphthalene containing monomer (APCNDI) can form redox polymer films showing both p- and n- conductivity under ambient conditions and allows access to five distinct redox states, and a complex electrochromic response covering the whole of the UV/vis-NIR spectral region. The highly effective quenching of the photoluminescence of both components in poly-APCNDI enables detailed characterization of the redox polymer films. The poly-APCNDI films show extensive charge trapping, which can be read out spectroscopically in the case of films and is characterized as kinetic rather than chemical in origin on the basis of UV/vis-NIR absorption and resonance Raman spectroscopic analyses. The strong resonantly enhanced Raman scattering for the various oxidized and reduced states of APCNDI enables nondestructive "read-out" of the state of the polymer, including that in which charges are trapped kinetically at the surface, making poly-APCNDI highly suitable for application as a component in organic nonvolatile memory devices.

Switching Effects in Molecular Electronic Devices

The creation of molecular electronic switches by using smart molecules is of great importance to the field of molecular electronics. This requires a fundamental understanding of the intrinsic electron transport mechanisms, which depend on several factors including the charge transport pathway, the molecule-electrode coupling strength, the energy of the molecular frontier orbitals, and the electron spin state. On the basis of significant progresses achieved in both experiments and theory over the past decade, in this review article we focus on new insights into the design and fabrication of different molecular switches and the corresponding switching effects, which is crucial to the development of molecular electronics. We summarize the strategies developed for single-molecule device fabrication and the mechanism of these switching effects. These analyses should be valuable for deeply understanding the switching effects in molecular electronic devices.

Isolation and crystal structure of a dithiophene dication: controlling covalent connection and disconnection with temperature and phase

A dithienylethene (1o) undergoes a two-electron chemical oxidation to a singlet diradical as an open-isomer (1o(2+)) in solution, which cyclizes to a closed-form (1c(2+)) upon cooling. The latter crystallizes out and its structure is analyzed by single crystal X-ray diffraction. Equilibrium between 1o(2+) and 1c(2+) in solution is observed by NMR and UV spectroscopy at various temperatures and is further supported by reduction reactions with Zn powder.

Light switching of molecules on surfaces

Smart surfaces, surfaces that respond to an external stimulus in a defined manner, hold considerable potential as components in molecular-based devices, not least as discrete switching elements. Many stimuli can be used to switch surfaces between different states, including redox, light, pH, and ion triggers. The present review focuses on molecular switching through the electronic excitation of molecules on surfaces with light. In developing light-responsive surfaces, investigators face several challenges, not only in achieving high photostationary states and fully reversible switching, but also in dealing with fatigue resistance and the effect of immobilization itself on molecular properties. The immobilization of light-responsive molecules requires the design and synthesis of functional molecular components both to achieve light switching and to anchor the molecular entity onto a surface. This review discusses several demonstrative examples of photoswitchable molecular systems in which the photochemistry has been explored in the immobilized state under ambient conditions and especially on electroactive surfaces, including self-assembled monolayers, bilayers, and polymer films.