Entropy in multiple equilibria, compounds with different sites.

The influence of entropy in multiple chemical equilibria is investigated for systems with different types of sites for the condition that the binding enthalpy of the species is the same within each type of sites and independent of those species that are already bonded. This allows splitting of the free reaction enthalpy into the particle distribution term and all other contributions for each type of sites separately and, hence, to evaluate this entropy contribution to the free reaction enthalpy. The situations for which this applies can be chemically very different, e.g. acid base, ligand exchange, isomerisation, conformational change, rearrangement of a ligand, ion exchange, adsorption of a species on the surface of a particle or a dendrimer, insertion of charged or neutral species into the cavities of a microporous or mesoporous host. We provide physical insight by discussing Xrc1{n1ABn2}Xrc2 systems. The number of coordination sites A and B are n1 and n2, respectively. The indices rc1 = 1, 2,…,n1 and rc2 = 1, 2,…,n2 count the number of X bonded to sites A and sites B, respectively. An important result is that the large number of equilibrium constants needed to describe those situations can be expressed as a function of two constants only. This allows studying systems quantitatively by experimental and theoretical means which otherwise might be difficult to handle. It has also implication for theoretical studies in the sense that it is sufficient to model only two reactions instead of many in order to describe a system. The results remain valid for systems with more than two types of different sites. The description of the entropy driven development of the fractional equilibrium coverage of the sites provides a new tool for understanding adsorption and ion exchange isotherms. The fractional equilibrium coverage of the sites can be described as a linear combination of individual Langmuir isotherms despite of the fact that such a linear combination has never the shape of the original Langmuir isotherm. This is remarkable and very useful. It provides us with new tools for describing and testing isotherms based on well defined, transparent physical ideas. Explicit solution for systems with 2, 3, 4, 5, 6, and 12 coordination sites are reported. Applications to a system with 12 coordination sites serve to illustrate information that can be obtained for complex situations.

Entropy in multiple equilibria, theory and applications.

Analysis of multiple equilibria where the binding enthalpy of the species is the same for all sites and independent of those that are already bonded delivers insight on the evolution of the entropy as the reactions proceed and explains why the equilibrium constants always decrease with increasing number of occupied sites. The validity of the results is independent of the nature of bonding. Consequences are explained by discussing dicarboxylic acids, cation exchange of zeolite A, and the insertion of guests into the one-dimensional channels of zeolite L, by cation exchange for cationic guests and from the gas phase for neutral molecules. The quantitative link between the description of multiple equilibria and Langmuir's isotherm equation provides new insight. We express the concentration dependence of fraction filling of guests on the length of the channels, a result which is important for understanding observations and planning the synthesis of new host-guest composites. The free reaction enthalpy can be split in the particle distribution term and all other contributions. The results are therefore relevant for analyzing and interpreting experimental data also of systems where the condition of constant binding enthalpy is not or only partially fulfilled.

Supramolecular Organization of Dye Molecules in Zeolite L Channels: Synthesis, Properties, and Composite Materials.

Sequential insertion of different dyes into the 1D channels of zeolite L (ZL) leads to supramolecular sandwich structures and allows the formation of sophisticated antenna composites for light harvesting, transport, and trapping. The synthesis and properties of dye molecules, host materials, composites, and composites embedded in polymer matrices, including two- and three-color antenna systems, are described. Perylene diimide (PDI) dyes are an important class of chromophores and are of great interest for the synthesis of artificial antenna systems. They are especially well suited to advancing our understanding of the structure-transport relationship in ZL because their core fits tightly through the 12-ring channel opening. The substituents at both ends of the PDIs can be varied to a large extent without influencing their electronic absorption and fluorescence spectra. The intercalation/insertion of 17 PDIs, 2 terrylenes, and 1 quaterrylene into ZL are compared and their interactions with the inner surface of the ZL nanochannels discussed. ZL crystals of about 500 nm in size have been used because they meet the criteria that must be respected for the preparation of antenna composites for light harvesting, transport, and trapping. The photostability of dyes is considerably improved by inserting them into the ZL channels because the guests are protected by being confined. Plugging the channel entrances, so that the guests cannot escape into the environment is a prerequisite for achieving long-term stability of composites embedded in an organic matrix. Successful methods to achieve this goal are described. Finally, the embedding of dye-ZL composites in polymer matrices, while maintaining optical transparency, is reported. These results facilitate the rational design of advanced dye-zeolite composite materials and provide powerful tools for further developing and understanding artificial antenna systems, which are among the most fascinating subjects of current photochemistry and photophysics.

Structure of nanochannel entrances in stopcock-functionalized zeolite†L composites.

Multifunctional hybrid materials are obtained by modifying zeolite L (ZL) with stopcock molecules, consisting of a tail group that can enter the ZL nanochannels and a head group too large to pass the channel opening. However, to date no microscopic-level structural information on modified ZL materials has been reported. Herein we draw atomistic pictures of channel openings and stopcock-functionalized ZL based on first-principles calculations. We elucidate the interactions of the tail group with the inner surface of ZL channels and the space-filling properties of the stopcocks, revealing cork- or lid-sealing modes. Water is essential to obtain stable modifications. AlOH groups are the preferred modification sites, bipodal modifications suffer from strain, and tripod binding is ruled out. Our results suggest the viability of recursive functionalization by cross-linking.

Formation of two homo-chromophoric H-aggregates in DNA-assembled alternating dye stacks.

The understanding and description of collectively excited multichromophores is of crucial importance for many areas of basic and applied research. DNA has been used for the construction of well-defined heterochromophoric stacks. Electronic coupling among non-adjacent chromophores of the same type leads to the co-existence of PDI and pyrene H-aggregates in hybrids composed of alternating chromophore stacks.

Luminescence enhancement after adding stoppers to europium(III) nanozeolite†L.

Stopper molecules attached to nanozeolite L (NZL) boost the luminescence of confined Eu(3+)-ß-diketonate complexes. The mechanism that is responsible was elucidated by comparing two diketonate ligands of different pK(a) and two aromatic imines, and by applying stationary and time resolved spectroscopy. The result is that the presence of the imidazolium based stopper is favorable to the sustainable formation of Eu(3+)-ß-diketonate complexes with high coordination by decreasing the proton strength inside the channels of NZL. A consequence is that strongly luminescent transparent films can be prepared using aqueous suspension of the stopper modified composites.

Host-guest interactions and orientation of dyes in the one-dimensional channels of zeolite L.

A combined experimental and modeling study of methylacridine (MeAcr(+)) dye-zeolite L composites unravels the microscopic origin of their functional properties. The anisotropic orientation of the cationic dye inside the ZL channel is unambiguously determined and understood. The most stable orientation of MeAcr(+), which features both its long and short molecular axes nearly perpendicular to the channel axis, is mainly determined by dye-ZL electrostatic interactions but also depends on the cosolvent water. In ZL, MeAcr(+) is not hydrogen bonded to water or ZL framework oxygens and is hydrophobically solvated by water molecules. These findings further support the hypothesis that the cosolvent can importantly influence properties of dye-zeolite composites. Of relevance for a deeper comprehension of the physical chemistry of these hybrids is the observation that trivial energy transfer processes (self-absorption) are often playing a significant role in the optical properties of the composites.

First-principles simulation of the absorption bands of fluorenone in zeolite L.

The absorption spectrum of fluorenone in zeolite L is calculated from first-principles simulations. The broadening of each band is obtained from the explicit treatment of the interactions between the chromophore and its environment in the statistical ensemble. The comparison between the simulated and measured spectra reveals the main factors affecting the spectrum of the chromophore in hydrated zeolite L. Whereas each distinguishable band is found to originate from a single electronic transition, the bandwidth is determined by the statistical nature of the environment of the fluorenone molecule. The K(+)···O=C motif is retained in all conformations. Although the interactions between K(+) and the fluorenone carbonyl group result in an average lengthening of the C[double bond, length as m-dash]O bond and in a redshift of the lowest energy absorption band compared to gas phase or non-polar solvents, the magnitude of this shift is noticeably smaller than the total shift. An important factor affecting the shape of the band is fluorenone's orientation, which is strongly affected by the presence of water. The effect of direct interactions between fluorenone and water is, however, negligible.

Formation of two-dimensional supramolecular polymers by amphiphilic pyrene oligomers.

Reading the bands: Amphiphilic pyrene trimers self-assemble into two-dimensional, supramolecular polymers in aqueous medium. Folding and aggregation processes are accompanied by simultaneous development of J- and H-bands and significant changes in the fluorescence properties. The formation of sheet-like nano-structures is confirmed by AFM.

Nanochannels: hosts for the supramolecular organization of molecules and complexes.

Nanochannels have been used as hosts for supramolecular organization for a large variety of guests. The possibilities for building complex structures based on 2D and especially 3D nanochannel hosts are larger than those based on 1D nanochannel hosts. The latter are, however, easier to understand and to control. They still give rise to a rich world of fascinating objects with very distinguished properties. Important changes are observed if the channel diameter becomes smaller than 10 nm. The most advanced guest-nanochannel composites have been synthesized with nanochannels bearing a diameter of about 1 nm. Impressive complexity has been achieved by interfacing these composites with other objects and by assembling them into specific structures. This is explained in detail. Guest-nanochannel composites that absorb all light in the right wavelength range and transfer the electronic excitation energy via FRET to well-positioned acceptors offer a unique potential for developing FRET-sensitized solar cells, luminescent solar concentrators, color-changing media, and devices for sensing in analytical chemistry, biology, and diagnostics. Successful 1D nanochannel hosts for synthesizing guest-host composites have been zeolite-based. Among them the largest variety of guest-zeolite composites with appealing photochemical, photophysical, and optical properties has been prepared by using zeolite L (ZL) as a host. The reasons are the various possibilities for fine tuning the size and morphology of the particles, for inserting neutral molecules and cations, and for preparing rare earth complexes inside by means of the ship-in-a-bottle procedure. An important fact is that the channel entrances of ZL-based composites can be functonalized and completely blocked, if desired, and furthermore that targeted functionalization of the coat is possible. Different degrees of organizational levels and prospects for applications are discussed, with special emphasis on solar energy conversion devices.

On the significance of the anchoring group in the design of antenna materials based on phthalocyanine stopcocks and zeolite L.

The synthesis of stopcocks based on zinc phthalocyanine for selective adsorption at the channel entrances of zeolite L is reported. The introduction of either an inert SiMe3 moiety, an imidazolium cation or a reactive isothiocyanate (NCS) group allows attachment to the channel entrances of zeolite L through van der Waals interactions, electrostatic interactions, or covalent binding, respectively. Stopcocks that rely on van-der-Waals-driven adsorption require careful selection of the solvent used for the deposition onto the zeolite surface to avoid a nonspecific distribution of the molecules. Regarding the design of photonic antenna systems, a stopcock with a cationic tail was found to be the most convenient, based on the observation that efficient energy transfer from molecules located in the zeolite nanochannels is more readily obtained than in the other cases.

Designing dye-nanochannel antenna hybrid materials for light harvesting, transport and trapping.

We discuss artificial photonic antenna systems that are built by incorporating chromophores into one-dimensional nanochannel materials and by organizing the latter in specific ways. Zeolite L (ZL) is an excellent host for the supramolecular organization of different kinds of molecules and complexes. The range of possibilities for filling its one-dimensional channels with suitable guests has been shown to be much larger than one might expect. Geometrical constraints imposed by the host structure lead to supramolecular organization of the guests in the channels. The arrangement of dyes inside the ZL channels is what we call the first stage of organization. It allows light harvesting within the volume of a dye-loaded ZL crystal and also the radiationless transport of energy to either the channel ends or center. One-dimensional FRET transport can be realized in these guest-host materials. The second stage of organization is realized by coupling either an external acceptor or donor stopcock fluorophore at the ends of the ZL channels, which can then trap or inject electronic excitation energy. The third stage of organization is obtained by interfacing the material to an external device via a stopcock intermediate. A possibility to achieve higher levels of organization is by controlled assembly of the host into ordered structures and preparation of monodirectional materials. The usually strong light scattering of ZL can be suppressed by refractive-index matching and avoidance of microphase separation in hybrid polymer/dye-ZL materials. The concepts are illustrated and discussed in detail on a bidirectional dye antenna system. Experimental results of two materials with a donor-to-acceptor ratio of 33:1 and 52:1, respectively, and a three-dye system illustrate the validity and challenges of this approach for synthesizing dye-nanochannel hybrid materials for light harvesting, transport, and trapping.

Self-assembling zeolite crystals into uniformly oriented layers.

We report important progress made in the synthesis of oriented functional layers of nanochannel materials by using coordination chemistry as a tool. Zeolite L (ZL) crystals have been arranged into oriented layers through the coordinative interactions between a functional organic linker (L) and metal cations used for connecting the different parts. As organic linker we used a terpyridyl ligand bearing a urea group and a reactive siloxane part. Two strategies that lead to monolayers with different properties are described. The first consists of reacting the siloxane group of ligand L with OH groups of the substrate (S), and selectively reacting the siloxane group of L with OH groups located at the base of the ZL crystals. Next, metal cations M(n+), for example, Zn(2+) or Cu(2+), are coordinated to the terpy group on the modified substrate. To this the modified ZL is added and coordinatively bound by the terpy(Mn(n+))terpy interaction, leading to oriented ZL layers. The second method consists of reacting substrate S and ligand L in the presence of a metal cation. A layer with reactive siloxane groups is formed on S to which the ZL crystals are bound by the reaction of the hydroxyl groups of their base. Zn(2+), Cu(2+), and lanthanide ions Eu(3+) and Tb(3+)have been tested successfully, all of them leading to high-quality ZL monolayers with open channels, accessible for accepting guests, oriented perpendicularly with respect to the surface of S.

Surprising properties of a furo-furanone.

The electronic absorption, fluorescence, and excitation spectra of furo[3,4-c]furanone (1) have been measured in different solvents at different concentrations. We observed a complex dependence of absorption and excitation spectra as a function of the concentration in CH(2)Cl(2) and THF due to aggregate formation. Interestingly, the fluorescence spectra were not affected. Resolving the puzzle was made possible by the fact that 1 fits perfectly into the channels of zeolite L (ZL) microcrystals to form 1-ZL guest-host composites. The geometry of the ZL channel system ensures a well-defined orientation of the embedded dye molecules, thereby leading to a preferred orientation of their electronic transition dipole moment (ETDM) and thus to objects with pronounced optical anisotropy properties. This enabled us to understand that in solution the monomers that are present at low concentration form an aggregate in which the molecules sit on top of each other and arrange into a J-type aggregate configuration at higher concentrations. The signature of the latter is observed in the 1-ZL composites. This seems to be the first example in which the insertion of molecules into a nanochannel microcrystal has helped in understanding the weak intermolecular interactions that take place in solution.

Self-assembled nanofibers of fluorescent zeolite L crystals and conjugated polymer.

Through this work, we present self-assembled structures which can be obtained by mixing surface modified dye loaded zeolite L crystals and cationic precursors of a conjugated polymer. The zeolite crystals are modified with anionic end groups which give the former a polyanionic character and allow a polyelectrolytic assembly. Microfluidic forces, introduced during the drying of a drop of water containing both polyelectrolytes casted on a clean glass substrate, and localized adsorption on the single zeolite crystal lead to the formation of micro- and nanofibers of highly ordered zeolite nanocrystals. As a result, the fibers display a very well polarized emission from the organic dye included into the nanochannels of the inorganic crystal. Playing on the relative concentrations of polycation and zeolites, and after thermal conversion of the polymer precursor, rigid and insoluble fibers with diameters ranging from less than 200 nm to a few micrometers are obtained.

Weak forces at work in dye-loaded zeolite materials: spectroscopic investigation on cation-sulfur interactions.

The interaction between sulfur-containing chromophores and cationic species (K(+)) has been investigated in dye-loaded zeolite materials by means of photoluminescence spectroscopy. A red-shift in the emission spectra of the host-guest compounds (HGCs) has been detected and unambiguously connected to the close proximity between a conjugated moiety and nearby free charges, suggesting a specific role played by sulfur lone pair electrons. Quantum-chemical calculations on model compounds have been performed to support this hypothesis.

Multilevel organization in hybrid thin films for optoelectronic applications.

In this work we report two simple approaches to prepare hybrid thin films displaying a high concentration of zeolite crystals that could be used as active layers in optoelectronic devices. In the first approach, in order to organize nanodimensional zeolite crystals of 40 nm diameter in an electroactive environment, we chemically modify their external surface and play on the hydrophilic/hydrophobic forces. We obtain inorganic nanocrystals that self-organize in honeycomb electroluminescent polymer structures obtained by breath figure formation. The different functionalizations of the zeolite surface result in different organizations inside the cavities of the polymeric structure. The second approach involving soft-litography techniques allows one to arrange single dye-loaded zeolite L crystals of 800 nm of length by mechanical loading into the nanocavities of a conjugated polymer. Both techniques result in the formation of thin hybrid films displaying three levels of organization: organization of the dye molecules inside the zeolite nanochannels, organization of the zeolite crystals inside the polymer cavities, and micro- or nanostructuration of the polymer.

Time, space, and spectrally resolved studies on J-aggregate interactions in zeolite L nanochannels.

Temporally and spectrally resolved confocal microscopy has been used to explore the behavior of pyronine intercalated zeolite L crystals at different loadings. The low pyronine loading of 0.6% exhibits photophysical behavior similar to that of the free molecule in solution, indicating molecules are isolated from each other in the crystal channels. The higher loading of 20% results in a dye gradient along the channel axis, and the presence of an additional red-shifted spectroscopic transition, with shorter lifetimes. The new band is assigned to an inline arrangement of the molecules undergoing a J-aggregate-type coupling, a process so far not observed in subnanometer channels.

Novel phthalocyanine-based stopcock for zeolite L.

We report the first phthalocyanine-based stopcock for selective adsorption to the channel entrances of zeolite L and realisation of a new electronic dipole moment coupling situation.

Fabrication of oriented zeolite L monolayers employing luminescent perylenediimide-bridged silsesquioxane precursor as the covalent linker.

Oriented zeolite L (LTL, (M)(9)[Al(9)Si(27)O(72)].nH(2)O (M = K(+) and Na(+))) monolayers with dense packing and high coverage degree has been obtained by using a luminescent perylenediimide-bridged silsesquioxane precursor as the novel molecular binder.