We have successfully proposed the application of transition metal compounds in holographic recording media. Such compounds feature an ultra-fast light-induced linkage isomerization of the transition-metal–ligand bond with switching times in the sub-picosecond regime and lifetimes from microseconds up to hours at room temperature. This article highlights the photofunctionality of two of the most promising transition metal compounds and the photophysical mechanisms that are underlying the hologram recording. We present the latest progress with respect to the key measures of holographic media assembled from transition metal compounds, the molecular embedding in a dielectric matrix and their impressive potential for modern holographic applications.

Phototriggered NO and CN release from [Fe(CN)₅NO]^{2 −} (NP) molecular monolayers is studied by a combination of electrochemistry, infrared spectroscopy, and mass spectrometry under light irradiation at temperatures of 80 K and 294 K. The NP molecular monolayers were electrostatically attached to thin films of mesoporous TiO₂ deposited on silicon. Irradiation of the surfaces results in NO and CN release, which is verified using mass spectrometry. The kinetic trace of the light driven NO release of the [Fe(CN)₅NO]^{2 −} is determined by inspection of the ν_NO stretching mode as a function of exposure to light in the violet/green spectral range. The decrease of the ν_NO-amplitude can be modeled considering the NO release as a two-step process with an intermediate state between the attached and the released state. According to literature, the intermediate state may be related to the light-induced linkage NO isomerization of the NP.

Photosensitive properties of [Ru(bpy)₂(OSO)] ∙ PF₆ dissolved in propylene carbonate originating from photoinduced linkage isomerism have been studied by temperature and exposure dependent transmission and UV/Vis absorption spectroscopy. An exceeding photochromic photosensitivity of S = (63 ± 10) × 10³ cm l J⁻¹ mol⁻¹ is determined. It is attributed to a maximum population of 100% molecules in the photoinduced isomers, a unique absorption cross section per molecule and a very low light exposure of Q ₀ = (0.25 ± 0.03) Ws cm⁻² for isomerism. Relaxation studies of O-bonded to S-bonded isomers at different temperatures verify the existence of two distinct structures of linkage isomers determined by the activation energies of E _A,1 = (0.76 ± 0.08) eV and E _A,2 = (1.00 ± 0.14) eV.

Permanently fixed noisily recorded refractive index gratings are discovered in single crystals of $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3:\mathrm{Fe}$ upon long-term exposure to a single laser beam with high intensities of $I=100\mathrm{kW}∕{\mathrm{m}}^2$ and ambient temperature. The fixing process is explained by considering the effect of laser-induced heating of the sample and the well-known simultaneous thermal fixing procedure.

The influence of ligand substitution on the photochromic properties of [Ru(bpy)₂(OSOR)]∙PF₆ compounds (bpy = 2,2'-bipyridine, OSO = 2-methylsulfinylbenzoate) dissolved in propylene carbonate is studied by UV/VIS laser-spectroscopy as a function of temperature and exposure. The group R is either Bn (CH₂C₆H₅), BnCl or BnMe. The photochromic properties originate from a phototriggered linkage isomerization located at the SO ligand. It is shown that the thermal stability of the studied compounds can be varied by ligand substitution in the range of 1.6 × 10³ s to 3.9 × 10⁴ s. In contrast, absorption spectra of ground and metastable states as well as the characteristic exposure of the photochromic response remain unchanged. The results are explained in the frame of photoinduced linkage isomerization located at the SO ligand.