Ultrafast Dynamics of Photochromic Systems

Redox-conjugated reversible isomerization of ferrocenylazobenzene with a single green light

A new reversible isomerization cycle for meta-ferrocenylazobenzene accomplished by combination of a single green light (546 nm) and redox change between Fe(II) and Fe(III) was discovered. In the Fe(II) state, trans-to-cis isomerization proceeded upon the green light irradiation exciting the metal-to-ligand charge transfer (MLCT) in a high quantum yield (Phit-->c = 0.51) which exceeds that of azobenzene (Phit-->c = 0.12 (313 nm excitation)). The cis molar ratio reached 35% in the photostationary state. The oxidation to the Fe(III) state followed by irradiation with the same green light led to the cis-to-trans back-reaction to recover almost all of the trans-form. The "on-off switching" of the MLCT character played an important role in the redox-dependent response to the green light for the isomerization. The photoisomerization behavior of ferrocenylazobenzenes was strongly dependent on the substitution position of the ferrocenyl moiety on the benzene ring. The MLCT excitation was not effective for the trans-to-cis conversion in para-ferrocenylazobenzene. Time-dependent density functional theory (TD-DFT) calculations for meta- and para-ferrcenylazobenzene showed that the origin of the visible band (MLCT band) is different. The initial orbital for the MLCT in meta-ferrocenylazobenzene is delocalized over Fe and the Cp ring, while that in para-ferrocenylazobenzene is localized on the iron.

Single-molecule optomechanical cycle

Light-powered molecular machines are conjectured to be essential constituents of future nanoscale devices. As a model for such systems, we have synthesized a polymer of bistable photosensitive azobenzenes. Individual polymers were investigated by single-molecule force spectroscopy in combination with optical excitation in total internal reflection. We were able to optically lengthen and contract individual polymers by switching the azo groups between their trans and cis configurations. The polymer was found to contract against an external force acting along the polymer backbone, thus delivering mechanical work. As a proof of principle, the polymer was operated in a periodic mode, demonstrating for the first time optomechanical energy conversion in a single-molecule device.

All-optical processing with molecular switches

A gradual transition from electrical to optical networks is accompanying the rapid progress of telecommunication technology. The urge for enhanced transmission capacity and speed is dictating this trend. In fact, large volumes of data encoded on optical signals can be transported rapidly over long distances. Their propagation along specific routes across a communication network is ensured by a combination of optical fibers and optoelectronic switches. It is becoming apparent, however, that the interplay between the routing electrical stimulations and the traveling optical signals will not be able to support the terabit-per-second capacities that will be needed in the near future. Electrical inputs cannot handle the immense parallelism potentially possible with optical signals. Operating principles to control optical signals with optical signals must be developed. Molecular and supramolecular switches are promising candidates for the realization of innovative materials for information technology. Binary digits can be encoded in their chemical, electrical, or optical inputs and outputs to execute specific logic functions. We have developed a simple strategy to gate optical signals with optical signals by using a photoactive molecular switch. We have demonstrated that NAND, NOR, and NOT operations can be implemented exclusively with optical inputs and optical outputs coupling from one to three switching elements. Our remarkably simple approach to all-optical switching might lead to the development of a new generation of devices for digital processing and communication technology.

Synthesis, characterization, and photochemical properties of azobenzene-conjugated Ru(II) and Rh(III) bis(terpyridine) complexes

We synthesized azobenzene-conjugated bis(terpyridine) Ru(II) and Rh(III) mononuclear and dinuclear complexes and investigated their photochemical properties on excitation of the azo pi-pi band upon 366 nm light irradiation. The Ru mononuclear complex underwent trans-to-cis photoisomerization to reach the photostationary state with only 20% of the cis form, while the Ru dinuclear complex did not isomerize at all photochemically. On the other hand, the mononuclear and dinuclear Rh complexes showed almost complete trans-to-cis photoisomerization behavior. Cis forms of the Rh complexes thermally returned to the trans form at a much slower rate than those of organic azobenzenes, but they did not isomerize photochemically. The reduction potential of the cis forms was 80 mV more negative than that of the trans forms. The photoisomerization quantum yields of the Rh complexes were strongly dependent on the polarity, viscosity, and donor site of the solvents as well as the size of the counterions. We investigated the photoisomerization process of these complexes using femtosecond absorption spectroscopy. For the Rh complexes, we observed S(n) <-- S(2) and S(n) <-- S(1) absorption bands similar to those of organic azobenzenes. For the Ru complexes, we observed very fast bleaching of the MLCT band of the Ru complex, which indicated that the energy transfer pathway to the MLCT was the primary cause of the depressed photoisomerization. The electronic structures, which were estimated from ZINDO molecular orbital calculation, supported the different photochemical reaction behavior between the Ru and Rh complexes.