Detection of molecular spin-state changes in ultrathin films by photonic methods
Résumé
Ultrathin films of molecular spin crossover materials exhibit very appealing properties for a variety of photonic applications because the spin-state switching is accompanied by a spectacular change of the complex refractive index in a wide spectral range. After examining different optical spectroscopic approaches for the detection of spin-state changes in nanometric films, we found that conventional light absorption measurements can be used down to the nanometer thickness if the oscillator strength of the transition is high, which is often the case for charge transfer transitions in the ultraviolet range. Methods based on fluorescence energy transfer provide a straightforward means for detecting spin-state changes in films in the visible wavelength range, even if photobleaching may be a problem for certain luminophores. Alternatively, changes in the refractive index accompanying the spin transition can be conveniently determined by surface plasmon resonance spectroscopy, which can also provide very accurate film thickness determination. Plasmonic effects were also used to investigate spin-crossover films by means of surface-enhanced Raman spectroscopy. We found that this technique can provide information not only on the spin state of the molecules in very thin layers, but also on their chemical composition and structure.