Artificial Photosynthesis

One-dimensional channel materials, such as zeolites and mesoporous silicas, are very attractive hosts for the preparation and investigation of hierarchically organized structures, presenting a successive ordering from the molecular up to macroscopic scale. The focus of this article is on artificial photonic antenna systems and on photocatalytically active layers that have been built by incorporating organic dyes, complexes, metal cations and clusters into 1-D nanochannel materials. We show that zeolite L as a host material allows for the design and preparation of a large variety of highly organized host-guest systems. The combination of a tuneable host morphology and the possibility of obtaining highly organized molecular patterns of guests leads to a variety of potential optical and photoelectronic applications. Strongly absorbing systems exhibiting efficient FRET along the c-axis of the zeolite crystals are accessible by sequential inclusion of multiple types of dyes. These new light-harvesting materials offer unique possibilities as building blocks for solar-energy conversion devices. A complementary approach consists in integrating photochemically active substances into zeolite monolayers coated on an electrode and taking advantage of intrazeolite processes for designing a reversible electrode for photocatalytic water oxidation. The photoelectrochemical water splitting capability of systems based on Ag+/AgCl/Agn-zeolite photoanodes are discusse

Zeolite Microcrystals as Hosts for Supramolecular Organization of Dye Molecules

Zeolite microcrystals can act as host for supramolecular organization of molecules, complexes, clusters, and quantum-size particles. They allow the design of precise and reversible functionalities. Techniques for arranging zeolite microcrystals of good quality and narrow size distribution as dense monograin layers on different substrates can be used to realize specific properties. The chemical reactivity between the intercalated molecules offers possibilities for in situ synthesis of molecular chains, clusters, and quantum-size particles, which might not be accessible otherwise. In some cases, guest-host reactivity must be considered. The reactivity of intercalated compounds with (small) molecules penetrating from the outside is an option for changing the composition of a material, i.e., molecules intercalated as monomers in a first step can be linked to form chains. New electronic structures are accessible either by specific geometrical arrangements made possible by the structure of the host and/or by explicitly involving its electronic properties. Some systems meet the conditions necessary for the occurrence of intrazeolite charge transport (ionic and electronic), realized by the guests in their ground state and in electronically excited states under high-vacuum conditions or in the presence of a solvent, depending on the composition and the structure of the material. In this article, we focus on organic dye molecules in the one-dimensional channels of zeolites with a hexagonal framework. This system consists of supramolecularly organized dye molecules. It is shown to provide fascinating possibilities for building an artificial antenna device which consists of highly concentrated monomeric dye molecules of up to 0.4M with a large Förster energy-transfer radius and a high luminescence quantum yield in an ideal geometrical arrangement of optimal size. Extremely fast electronic excitation-energy transport has been demonstrated by us in oxonine- and pyronine-dye-loaded zeolite L microcrystals. Many other highly organized dye-zeolite materials can be prepared, and they are expected to show a wide variety of challenging properties. We report on methods to distinguish between dye molecules which are inside of a microcrystal and those adsorbed on its outer surface, and we explain a demonstration experiment illustrating the intercalation of thionine into zeolite L and the thus resulting improved chemical stability of this dye

Artificial antenna systems

The work describes experimental and theoretical results on the water oxidation in absence of an externally added potential, on the charge transport in organized microporous media and on the transport of excitation energy in an antenna system. Water oxidation to O2 takes place at the solid/water phase boundary of a thin AgCl layer in the presence of a small excess of Ag+. This water oxidation step shows self-sensitization as the reaction proceeds, the sensitivity is extended from the near-UV-visible towards the red range. The quantum yield per redox equivalent for O2 evolution upon illumination with near UV light (340-390 nm) is ∼ 0·8 and it is the same upon illumination with blue light (420-480 nm). In the green range it is ∼ 0·5. We discuss parameters controlling these reactions. Zeolite microcrystals are investigated as hosts for supramolecular organization of clusters, complexes and molecules. The possibility to arrange zeolite microcrystals of good quality and narrow size distribution as dense monograin layers on different types of substrates allows the discovery of specific properties. In the present context, three functionalities are of special importance intrazeolite ion transport, intrazeolite charge transport and intrazeolite excitation energy transport. All of them have been clearly demonstrated experimentally although there are still some controversies going on. Highly concentrated dyes have the tendency to form aggregates which generally show very fast radiationless decay. In natural antenna systems the formation of aggregates is prevented by fencing the chlorophyll molecules in polypeptide cages. A similar approach is possible by enclosing dyes inside a microporous material such that the volume of the cages and channels is able to uptake monomers only, but not aggregates. We know a number of materials bearing linear channels running through the whole microcrystal which allow the formation of highly anisotropic, monomeric dye assemblies. A few cases based on zeolite L as a host and the cationic dye molecules pyronine and oxonine have been investigated experimentally to some extent for this purpose. While the molecules can penetrate the channels, the geometrical constraints of this system excludes aggregation and therefore self-quenching up to very high concentrations, namely 0·2 M. Microcrystals with cylinder morphology and a size in the range of 100 nm have been found to be optimal for realizing collection efficiencies in order of 99

One-dimensional self-assembly of perylene-diimide dyes by unidirectional transit of zeolite channel openings

Confined supramolecular architectures of chromophores are key components in artificial antenna composites for solar energy harvesting and storage. A typical fabrication process, based on the insertion of dye molecules into zeolite channels, is still unknown at the molecular level. We show that slipping of perylene diimide dyes into the one-dimensional channels of zeolite L and travelling inside is only possible because of steric-interaction-induced cooperative vibrational modes of the host and the guest. The funnel-like structure of the channel opening, larger at the entrance, along with a directionally asymmetric entrance\u2013exit probability, ensures a favorable self-assembly process of the perylene units

Synthesis of Zeolite L. Tuning Size and Morphology

Summary.: A convenient synthesis of zeolite L is presented. The size of the crystals can be tuned between 30 and 6000 nm, spanning a volume range of seven orders of magnitude. The zeolite L crystals, which typically feature a cylindrical morphology, are synthesized with various aspect ratios ranging from elongated to disc-shaped. The importance of obtaining zeolite crystals with well-defined size and morphology is discussed in view of potential applications of zeolite L containing organic dye molecules as guest

Entropy in multiple equilibria : argon and nitrogen adsorption isotherms of nonporous, microporous, and mesoporous materials

Analysis of multiple equilibria of compounds with different coordination sites is extended to the description of adsorption isotherms with focus on the low relative pressure range. The entropy evolution is described using the particle distribution theory which also holds for adsorbents consisting of materials bearing more than one type of sites and applies for the condition that the adsorptive-adsorbent binding strength is larger than the adsorptive- adsorbate binding strength, so that monolayer coverage is favored. This allows to accurately determine the adsorption enthalpy. No assumption concerning the growth mechanism and specifics regarding the structure of the surface is needed. We find on a rigorous basis that this leads to Langmuir’s equation for each site inde-pendently, that the total fractional amount of bound adsorptive can be described as a linear combination of individual Langmuir isotherms, and that such a linear combination has never the shape of the original Langmuir isotherms. The results are successfully applied to argon and nitrogen adsorption isotherms of nonporous, microporous, and mesoporous adsorbents which allows comparing systems for which the properties of the active surface span a large range. We observe that all experimental data can accurately be described by means of a linear combination of two Langmuir isotherms in the low relative pressure range up to a coverage of 60%–95%. This means that the shape of all adsorption isotherms is essentially determined by the entropy decrease with increasing coverage. The two site interactions involved exhibit substantially different adsorption enthalpies. Interestingly the Ar enthalpy of adsorption ΔadsH∅1 of the sites 1 for the St¨ober-type silica and of the three investigated MCM-41 adsorbents (with pore size of 2.7 nm, 4.1 nm, and 4.4 nm) are similar, namely 11 kJ/mol. The situation is analogous for the enthalpy of adsorption ΔadsH∅2 for the sites 2, which amounts to 8 kJ/mol. A significantly larger enthalpy of adsorption ΔadsH∅1 for the sites 1, namely 14.3 kJ/mol, and ΔadsH2∅ = 11.7 kJ/mol for the sites 2 is measured for potassium zeolite L thus reflecting the more polar nature of this adsorbent. The measured specific surface area for these samples ranges from 14 m2/g for the St¨ober-type silica up to 1100 m2/g for the MCM-41(4.1 nm) adsorbent. The information provided by the lc2-L (linear combination of 2 Langmuir functions) analysis allows calculating the evolution of the coverage of site 1 and of site 2 as a function of increasing pressure. The inflection points of the isotherms, which mark the point where the curvature changes sign, were determined by numerically evaluating the second derivatives which vanish at this point and are compared with values obtained using BET analysis

Multiple equilibria describe the complete adsorption isotherms of nonporous, microporous, and mesoporous adsorbents

The adsorption of simple gases begins with the formation of a monolayer on the pristine surface, not always followed by the formation of a second or more monolayers. Subsequently, cluster formation or cavity filling occurs, depending on the properties of the surface. The characteristically different shape of the isotherms related to these processes allows to clearly differentiate them. We analyzed argon and N2 adsorption isotherms quantitatively over the entire relative pressure range for adsorbents bearing different properties: the nonporous Stöber-type particles, the microporous zeolite L (ZL) and zeolite L filled with indigo (Indigo-ZL), and three mesoporous silica adsorbents of different pore size. The formal equilibria involved in cluster formation and in cavity filling have been derived and successfully applied to quantitatively describe the isotherms of the adsorbents. No indication regarding formation of a second monolayer on top of the first one was observed for the Stöber-type particles. Instead, cluster generation, which minimizes surface tension, starts early. The behavior of microporous ZL and of Indigo-ZL is different. A second monolayer sets up and cluster formation starts with some delay. The enthalpy of cluster formation is, however, practically identical with that seen for the Stöber-type particles. The difference between the experimental and the calculated inflection points is very small. The shapes of the isotherms seen for the mesoporous adsorbents differ significantly from those seen for the nonporous and for the microporous adsorbents. The quantitative analysis of the data proves that formation of a second monolayer is followed by filling of cavities which ends as soon as all cavity sites are filled. The sum of the individual fractional contributions, namely the monolayer formation ΘmL, the appearance of a second monolayer Θ2L on top of the first one, and the cavity filling, yields a calculated adsorption isotherm Θcalc which describes the experimental data Θexp well. The experimental and the calculated first inflection points are in excellent agreement, which is also the case for the second inflection points. The value of the cavity filling enthalpy is roughly 10% larger than that for the cluster formation seen in the nonporous and the microporous adsorbents. The volume for cavity filling is significantly smaller than the monolayer volume for the mesoporous adsorbent with a pore diameter of 2.7 nm, while it is the same or larger for pore diameters of 4.1 nm and 4.4 nm, respectively. We conclude that understanding the adsorption isotherms as signature of several sequential chemical equilibrium steps provides additional information data for clusters, cavities, and position of the inflection points, not accessible by means of the conventional models. The theory reported herein covers type I, II, IV and to some extent also type VI isotherms

Vibrations of H 8 Si 8 O 12 , D 8 Si 8 O 12 , and H 10 Si 10 O 15 As Determined by INS, IR, and Raman Experiments †

A detailed study of the vibrational structure of the silasesquioxanes H 8 Si 8 Based on the msd's, the lowest internal torsional frequency was estimated to be 41 ( 7 cm -1

Multiple equilibria description of type H1 hysteresis in gas sorption isotherms of mesoporous materials

We report argon adsorption/desorption isotherms of MCM-41 and of SBA-15. The shape of all hysteresis loops we have observed corresponds to type H1. The data have been analyzed quantitatively using the multiple equilibria description and applying the notion of metastable thermodynamic equilibrium. It is remarkable and important that for both mesoporous materials the desorption process can be understood according to this description but with a corresponding equilibrium constant. This procedure therefore allows determining the thermodynamic values for the enthalpy and the free enthalpy of cavity desorption and thus to obtain information not available so far. We observed that the MCM-41 adsorption isotherms show first an increase with the characteristic Langmuir shape, followed by the almost instantaneous filling of cavities that ends as soon as all cavities are completely filled. The SBA-15 isotherms show a characteristic Langmuir shape at low relative pressure. The pressure slightly below the inflection point at prel, infl = 0.312 marks the beginning of the growth of a second layer, before an almost instantaneous filling of cavities takes place. It is interesting to observe that the values of the enthalpy and the free enthalpy for cavity filling and cavity desorption differ by about 0.1 kJ/mol for both, MCM-41 and SBA-15. This means that the driving force for developing a hysteresis is small. The hysteresis loop is therefore driven by delicate changes occurring within the cavities, partially or completely filled by the adsorbate. The monolayer coverage and monolayer desorption processes are decoupled from the cavity filling and cavity emptying processes. Knowing thermodynamic parameters for the hysteresis loop ultimately helps to better characterize and understand the experimental observations