Could you explain the significance of your article to the non-specialist?

There is an ongoing search for materials where ligands can be relocated by light irradiation. Using such reversible photochemical processes we can design specific novel metastable complexes, which can be used, e.g., for fast optical switching. In [Pt(NH₃)₄Cl(NO)]Cl₂ we can switch the NO ligand from the Pt-N-O into the Pt-O-N configuration by illumination with red light, such that the material changes its optical and chemical properties. The original structure can be restored by illumination with infrared light. The use of red light for the purpose of ligand switching has great advantages for potential applications, e.g., phototherapy, where the light should penetrate tissue.

What has motivated you to conduct this work?

We performed this investigation as fundamental research in the field of reversible photochemistry, such that one day we might be able to prepare complexes by optical design, which cannot be prepared by standard chemical synthetic routes. With respect to applied research we are motivated by the fact that these compounds are suitable candidate materials for fast optical switching and holographic data storage, since the switching of the ligands takes only a few picoseconds and leads to a change of the refractive index of the order of 10^-2. Due to the fact that in [Pt(NH₃)₄Cl(NO)]Cl₂ the switching is performed by red light this material has also a high potential for phototherapeutic applications, since blood and tissue are sufficiently transparent in this spectral range. Especially we can make use of the fact that by the generation of the linkage isomer the bonding of NO to Pt is softened by 1.5 eV, such that the NO can be released concertedly by a further small amount of energy and may hence be used for cancer reduction purposes.

Where do you see this work developing in the future?

An interesting issue will be the investigation of the excitation process of the linkage isomers with time-resolved methods, ranging from optical femtosecond pump-probe spectroscopy to direct structural investigation of the molecular switching on a subpicosecond timescale by using, e.g., time-resolved EXAFS. This will give the information about the driving force for the light-induced molecular switching process. Knowing all the excited and relaxed intermediate states during generation of the linkage isomers, one has the possibility to manipulate the NO-ligand systematically, e.g. by using several time-delayed laser pulses. In this manner a targeted release of NO seems achievable by simply using two laser pulses in the red spectral range. E.g., the first one excites the NO-linkage isomers, while the second one then has enough energy to set the NO free.

Are there any particular challenges facing future research in this area?

One particular challenge will be the actual transfer from fundamental research to applications, i.e. the implementation of these materials in real devices such as phototherapeutic agents, optical switches or holographic data storage units.