Colloidal photonic crystals and materials derived from colloidal crystals can exhibit distinct structural colors that result from incomplete photonic band gaps. Through rational materials design, the colors of such photonic crystals can be tuned reversibly by external physical and chemical stimuli. Such stimuli include solvent and dye infi ltration, applied electric or magnetic fi elds, mechanical deformation, light irradiation, temperature changes, changes in pH, and specifi c molecular interactions. Reversible color changes result from alterations in lattice spacings, fi lling fractions, and refractive index of system components. This review article highlights the different systems and mechanisms for achieving tunable color based on opaline materials with closepacked or non-close-packed structural elements and inverse opal photonic crystals. Inorganic and polymeric systems, such as hydrogels, metallopolymers, and elastomers are discussed.

Poly(methyl methacrylate) (PMMA)-based colloidal photonic crystals have an incomplete photonic band gap (PBG) and typically appear iridescent in the visible range. As powders, synthetic PMMA opals are white, but when infiltrated with carbon black nanoparticles, they exhibit a well-defined color that shows little dependence on the viewing angle. The quantity of black pigment determines the lightness of the color by controlling scattering. The combined effects of internal order within each particle and random orientation among the particles in the powder are responsible for this behavior. These pigments were employed as paints, using a mixture of polyvinyl acetate as a binder and deionized water as the solvent, and were applied to wood and paper surfaces for color analysis.

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The thermal-induced changes in molecular magnets based on Prussian blue analogues, M(3)[Fe(CN)(6)](2).xH(2)O (M = Mn, Co, Ni, Cu, Zn, and Cd), were studied from infrared, X-ray diffraction, thermo-gravimetric, Mössbauer, and magnetic data. Upon being heated, these materials loose the crystalline water that enhances the interaction between the metal centers, as has been detected from Mössbauer spectroscopy data. At higher temperatures, a progressive decomposition process takes place, liberating CN(-) groups, which reduces the iron atom from Fe(III) to Fe(II) to form hexacyanoferrates(II). The exception corresponds to the cobalt compound that undergoes an inner charge transfer to form Co(III) hexacyanoferrate(II). In the case of zinc ferricyanide, the thermal decomposition is preceded by a structural transformation, from cubic to hexagonal. For M = Co, Ni, Cu, and Zn the intermediate reaction product corresponds to a solid solution of M(II) ferricyanide and ferrocyanide. For M = Mn and Cd the formation of a solid solution on heating was not detected. The crystal frameworks of the initial M(II) ferricyanide and of the formed M(II) ferrocyanide are quite different. In annealed Mn(II) ferricyanide samples, an increasing anti-ferromagnetic contribution on heating, which dominates on the initial ferrimagnetic order, was observed. Such a contribution was attributed to neighboring Mn(II) ions linked by aquo bridges. In the anhydrous annealed sample such interaction disappears. This effect was also studied in pure Mn(II) ferrocyanide. The occurrence of linkage isomerism and also the formation of Ni(III), Cu(III), and Zn(III) hexacyanoferrates(II) were discarded from the obtained experimental evidence.

The hydrogen storage in zeolite-like hexacyanometalates with different exchangeable alkali metals within the cavities was studied. The H 2 adsorption isotherms were recorded at 75 and 85 K in order to estimate the involved adsorption heats using the isosteric method. The electric field gradient within the porous framework favors the hydrogen adsorption in the materials under study but also could lead to kinetic effects for the pore filling. Such effects were particularly pronounced for sodium among the studied compositions: Zn 3 A 2 [Fe-(CN) 6 ] 2 (A ) Na + , K + , Rb + , Cs + ) and Zn 3 [Co(CN) 6 ] 2 . For Na + , a strong interaction with the H 2 molecule takes place, where appreciable kinetic effects even at 258 K are observed. For Zn 3 [Co(CN) 6 ] 2 (rhombohedral phase) where the cavities are free of exchangeable metal and, in consequence, have a weak electric field gradient on their surface, the largest hydrogen storage capacity, close to 12 H 2 molecules per cavity (1.82% by weight), was observed. The hydrogen adsorption in these materials involves adsorption heats in the 6-8.5 kJ/mol range, following the order K > Rb > Cs ≈ Zn 3 [Co(CN) 6 ] 2 . The porous framework of this family of materials is formed by ellipsoidal cavities communicated by elliptical windows. The alkali metals are sited close to the windows. The pore accessibility and pore volume were evaluated from CO 2 adsorption isotherms recorded at 273 K. The free volume was found to be accessible to the CO 2 molecule for all of the studied compositions. According to the obtained isotherms the stabilization of the CO 2 molecule within the pores is caused by the electrostatic interaction between the electric field gradient at the cavity and the adsorbate quadrupole moment. The estimated strength for the guest-host interaction and the accessible pore volume follow the order Na > K > Rb > Cs. The largest accessible pore volume was found for Zn 3 [Co(CN) 6 ] 2 , close to 8 CO 2 molecules per cavity (28% by weight), but with the weaker guest-host interaction. The materials under study were characterized from X-ray diffraction, thermo-gravimetric, infrared, and Mo ¨ssbauer data. The obtained results shed light on the role of the electric field gradient at the cavity for the hydrogen adsorption.

The hydrogen adsorption in porous Prussian blue analogues shows the highest value for copper, suggesting the possibility that a direct interaction between the copper atom and the hydrogen molecule is established. The bonding of copper (2+) to the CN group of cyanometallates shows a unique behavior. The trend of copper to receive electrons in its 3d hole to adopt an electronic configuration close to 3d 10 is complemented by the ability of the CN group to donate electrons from its 5σ orbital, which has certain antibonding character. Because of this cooperative effect, the electronic configuration of the copper atom at the cavity surface is close to Cu(+). The resulting large availability of electron density on the copper atom favors its interaction with the antibonding σ* orbital of the hydrogen molecule. The charge removed from the metal t 2g orbitals is compensated (donated) by H 2 through a side-on σ interaction. From these combined mechanisms, where H 2 behaves as an acceptor-donor ligand for the copper atom, the high ability that copper hexacyanometallates show for the hydrogen storage could be explained. This hypothesis is supported by the obtained hydrogen adsorption data for Cu 3 [Ir(CN) 6 ] 2 , Cu 3 [Fe(CN) 6 ] 2 , Cu 2 [Fe(CN) 6 ], Cu[Pt(CN) 6 ], and Cu 3-x Mn x [Co(CN) 6 ] 2 , where 0 e x e 3, and also by the estimated values for the involved adsorption heats. The studied samples were previously characterized using X-ray diffraction, thermogravimetry, and infrared and Mo ¨ssbauer spectroscopies.