Nanochannels: Hosts for the Supramolecular Organization of

Invited Feature Article pubs.acs.org/Langmuir

Nanochannels: Hosts for the Supramolecular Organization of Molecules and Complexes Gion Calzaferri* Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland ABSTRACT: 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. atoms or 1.75 × 107 water molecules. This changes dramatically when crossing the 10 nm limit, where a sphere can host 30 880 gold atoms or 17 500 water molecules, but the number reduces to 31 gold atoms or 17 water molecules in a 1 nm sphere. Understanding the size range below 10 nm is especially fascinating and challenging. The diameter of the nanochannels that we shall explore is mainly in this range, with special emphasis on the 1 nm world, and their length can be somewhere between 10 nm and a few thousand nm. Nanochannels can assemble to nanometer- or micrometer-sized packages, and they can act as hosts for the supramolecular organization of guests, namely, molecules and complexes. A higher level of organization can be realized by interfacing the particles with other objects and by assembling them into specific structures, thus realizing successive ordering from the molecular up to the macroscopic scale. This makes guest−nanochannel composites appealing for utilization in nano/microelectronics, optics, photonics, sensing, biology, and diagnostics. Nanochannels can be 1D, meaning that one channel does not have more than two entrances that can be opened or closed

1. INTRODUCTION Materials bearing channels of uniform diameter in the range of a few nanometers play an important role in nature, science, and technology. They are found as single tubes of different lengths, as fragile assemblies of single tubes, and also as robust arrangements of many tubes. The latter can exist as noncrystalline or crystalline objects of different morphologies and variable size. Assemblies always show a degree of polydispersity in their size, shape, surface structure, charge, and functionality that is manifest in the achievable degree of their structural perfection and the nature and population of defects in the assembled system. Perfect monodispersity is a privilege of molecules. But even molecules, the size and connectivity of which are exactly defined, give rise to a huge variety of fluctuating states, especially when in contact with a solvent or another environment, as has been beautifully visualized in single-molecule spectroscopy and in advanced computer simulations. The extent of importance of the polydispersity of the objects depends on the properties that we are studying. We shall not often explicitly refer to this aspect, but it is advisible to keep it in mind.1−15 The prefix nano has its origin in the Greek word nanos, meaning dwarf. In science, it denotes a factor of 10−9. The diameter of a gold atom is about 0.3 nm (nm). A sphere of 100 nm diameter can host a huge number of atoms or molecules, for example, 3.09 × 107 gold © 2012 American Chemical Society

Received: January 6, 2012 Revised: February 16, 2012 Published: February 28, 2012 6216

dx.doi.org/10.1021/la3000872 | Langmuir 2012, 28, 6216−6231

Langmuir

Invited Feature Article

Scheme 1. Simplified Illustration of Nanochannel Propertiesa

a

(A) Important difference among 1D, 2D, and 3D nanochannels. (B) Situation used to calculate the number of molecules, drawn as spheres, at the inner and outer surfaces of radius r and r + δ, respectively. The spheres do not match perfectly, to give a better impression of real situations.

Table 1. Examples of Cationic Dyes That Have Been Inserted into ZL Channels

and that they have no nodes or branching. Two-dimensional nanochannels have nodes, and branching can span a plane or a surface. They may have many but at least three openings. The 3D nanochannels cannot be spread out on a plane. Networks can emerge when these basic structures are extended in space. I do not try to give a systematic view of the numerous possibilities but restrict my remarks to the simplest possible aspects sketched in Scheme 1A, which is intended to display an intuitive impression of what changes with increasing dimensionality. The possibilities for building complex structures based on 2D and especially 3D nanochannel hosts are obviously larger than those based on 1D nanochannel hosts. The latter are, however, much easier to control. They still give rise to a rich world of fascinating objects with very distinguished properties and have therefore attracted much interest. In the next chapter we first outline some perhaps trivial but important consequences emerging from very simple geometrical reasoning. Then we discuss the supramolecular organization of guests in 1D and 2D nanochannels. The insertion of luminescent guests and modifications of the host have recently resulted in remarkable

organizational patterns. This is explained in detail, especially regarding 1D guest−nanochannel composites. The richness of the objects that have been synthesized so far and the many more that are still waiting to be made leads to fascinating scientific challenge and to many options for application in different fields. The structures and abbreviations of some molecules of interest are collected in Tables 1, 2, and 3.

2. SIMPLE GEOMETRICAL REASONING The number of molecules that can be placed inside a sphere of diameter d, inside a tube of diameter d and length l, or on their inner and outer surfaces can be easily estimated. The following examples apply to ambient conditions and are intended to develop an intuitive feeling for the nanoworld. The number of molecules nmol in a volume V is calculated using the fact that the ratio between nmol and Avogadro’s number NA is equal to the ratio between V and the molar volume Vm: nmol = 6217

NA V Vm

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Table 2. Examples of Neutral Dyes That Have Been Inserted into ZL Channels

For a sphere of radius r and for a tube of length l and radius r, we write eq 2: nmol(sphere) =

We treat the molecules as spheres of radius rmol, a useful simplification that we shall abandon later. The inner and outer surfaces of a sphere of radius r and thickness δ of the shell, which are effective for monolayer coverage for molecules of radius rmol, are

NA 4π 3 N r and nmol(tube) = A πr 2l Vm 3 Vm

A sph,in = 4π(r − rmol)2 and

(2)

A sph,out = 4π(r + δ + rmol)2

Using this, we find that the maximum number of water molecules or gold atoms in a sphere of 1 nm diameter is 17 or 30, respectively. It is 1000 times larger for a 10-nm-diameter sphere. This illustrates that we somehow cross a boundary when diving into the size range below 10 nm. The numbers of water molecules and gold atoms in a cylinder of 1 nm diameter and length are 26 and 46, respectively, and they are s times these values if the length is s times larger. The number nmol,A of molecules that can be on a surface A as a monolayer depends on their shape and extension, but it cannot be larger than the ratio of the area to be covered and the van der Waals area of the molecules Amol plus the void area Avoid caused by their shape. nmol, A =

A A mol + A void

(4)

In the description of water molecules and gold atoms as spherical particles, which as an average is not unreasonable, their radius rmol can be estimated using the molar volume. The numbers are rH2O = 0.19 nm and rAu = 0.16 nm. The maximum number of spherical particles nmol,sph that can be placed on a surface of area A can be estimated by means of eq 5. nmol,sph ≃

A (2rmol)2

(5)

Using this, we find for a sphere of 1 nm diameter and negligible thickness δ, which is a limiting case, that 8 water molecules or 14 gold atoms form a monolayer on the inner surface and 40 or 53, respectively, make a monolayer on the outer surface. Hence, the difference between the capacities of

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Table 3. Examples of Stopcocks That Have Been Used to Plug the ZL Channel Entrances

the inner and the outer surfaces is remarkable. For spheres of 10 nm diameter, the difference between the inner and outer surface is less important; for example, for gold we find 2900 atoms on the inner surface and 3300 atoms on the outer surface. This again illustrates that some aspects change dramatically when diving below the 10 nm range. The surface of a cylinder consists of two different parts, namely, the base and the coat. They nearly always have different physical and chemical properties. Whereas the extension of the coat scales with the length of the cylinder, the extension of the base remains constant. This is trivial but has important consequences. The number of objects, molecules, or atoms that forms a monolayer on the inner surface of the coat and on the base can be estimated by

means of eq 6. We note that the coat and the two bases of the inner surface share 2[π(r − rmol)/(2rmol)] objects. nmol,coat ≃

2π(r − rmol)l 2

(2rmol)

and nmol,base ≃ 2

πr 2 (2rmol)2 (6)

The numbers of water molecules and gold atoms that form a monolayer on the inner surface of the coat of cylinder of 1 nm diameter and length are 13 and 21, respectively, whereas 10 and 15 are on the two base surfaces. The numbers of molecules shared by the coat and the base are 5 and 6, respectively. This reflects the importance of the shape of the host. We remind the reader, as a comparison, that the diameter of a human hair is on the order of 6219


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Figure 1. Guests in channels of nanosized diameter. (Left) One-dimensional channels. (A) Guests enter and exit a channel so that an equilibrium situation can be established. (B) Channels are plugged on both sides by means of a stopcock, so that no guests can enter or leave. (C) Sequential insertion of guest of different types leads to sandwich structures. (D) Channels are plugged on both sides by a stopcock, so that no guests can enter or leave. (E) Entrance and exit of guests of a channel that has been plugged on one side. (F) The same as in E for a sandwich, which means that a noncentrosymmetrical arrangement results. (G) The noncentrosymmetrical arrangement is stabilized by plugging the channel with an additional stopcock. (Right) Two-dimensional channel system. This scheme illustrates that guests can enter and exit in a similar way as for the 1D situation (A).

side before adding the second type of guest, illustrated in Figure 1E−G, as has been realized for the first time experimentally with zeolite L (ZL) as a host.23 The transport of molecules and ions in nanochannels has been studied for decades, often in connection with catalysis or membranes, and the diffusion of molecules in mesoporous channel materials, carbon nanotubes, zeolites, and other channel materials has been extensively discussed in the literature.1,24−26 The transport of guests of a size that does not allow them to pass each other inside the channels is of special interest in achieving a high degree of organization. The channels may contain coguests, which are small molecules or ions such as water, methanol, oxygen, alkali ions, protons, and others. Different properties of the guest can be influenced by them considerably. One is the transport. The situations illustrated in Figure 2 can be distinguished to describe the diffusion of guests that cannot pass each other inside a nanochannel. (A) Guests traveling in an empty channel may be considered to be the most trivial case as long as their mutual interactions remain unimportant. In this case, the square root law (eq 7) can be used to illustrate that filling the channels completely can be time-consuming for long channels; x,̅ t, and D denote the average distance of diffusion, the time, and the diffusion coefficient, respectively.

40 000 nm and therefore is beyond the nanoworld addressed in this article.

3. SUPRAMOLECULAR ORGANIZATION OF GUESTS Nanochannels can act as hosts for supramolecularly organizing guests. The possibility to realize complex structures increases with increasing dimensionality. We discuss 1D and 2D situations but not 3D situations, for which the complexity surpasses the scope of this account, and we focus on guests of size and shape that do not allow them to pass each other inside the channels. Figure 1 illustrates situations where guests can enter and exit the channels of the host and where equilibrium situations can be established. A specific grouping can be frozen by adding stopcock molecules so that guest can neither exit nor enter the channels anymore. Perfect closure is much easier to realize for 1D than for 2D hosts and becomes inherently difficult for 3D hosts. It is possible to establish complex arrangements, by the sequential insertion of guests that cannot pass each other inside the channels, and also to freeze them. This is relatively easy for 1D hosts, difficult for 2D hosts, and inherently complex for 3D hosts because of the many different pathways to be controlled. We therefore discuss the 1D case, for which examples for all situations illustrated in Figure 1 (left) have been realized in the laboratory based on mainly inorganic−organic host−guest composites10−19 but also for some based on organic−organic host−guest composites.20−22 The sequential insertion of guests of different types shown in Figure 1C leads to sandwich structures. As soon as a second type of guest has entered on both sides, the first set is locked up by steric hindrance and hence revoked from equilibrium. This situation can be frozen by adding stopcock molecules (Figure 1D). It is possible to realize noncentrosymmetric structures by plugging the 1D channels on one

x̅ =

2Dt

(7)

An interesting influence regarding the size of the coguest on diffusion is illustrated in Figure 2B, where it has been reported for the molecule ResH in ZL in the presence of different solvents.27 Other complications may arise with guests that can take two different orientations, as illustrated in Figure 2C, where the horizontal orientation h allows them to travel with no hindrance but the vertical orientation v may slow down or even completely block the diffusion. 6220


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Figure 2. Different guest pathways. (A) Guests traveling in an empty channel. (B) Guests and coguests can travel only if the much smaller coguests can pass the larger guests inside the channels; otherwise, both are locked in their current position. (C) Guests can take two different orientations, where the horizontal position h allows them to travel with no hindrance but the vertical position v may slow down or even completely block the diffusion. (D) Asymmetrical head−tail guests, with a strong preference for entering the channels with their head first, lead to a distinguished ordering. (E) Insertion of asymmetrical head−tail guests, all with the same orientation, into channels that are open on one side only. See also Figure 6.

OH groups present on the coat can be used to perform targeted functionalization and hence fine tune the properties of the particles.10−16,23,29,37−41 4.1. Structure of the Host. Structure and morphology of ZL are explained in Figure 3. The primary building unit of the hexagonal ZL framework consists of TO4 tetrahedrons where T represents either Al or Si. They form 1D channels running parallel to the crystals c axis. The composition of ZL is (M+)9[(AlO2)9(SiO2)27]·nH2O, where M+ represents the monovalent cations compensating for the negative charge caused by the aluminum atoms. The number n of water molecules is 21 in fully hydrated materials and 16 for crystals equilibrated at about 22% relative humidity.10,34 It is useful to imagine ZL as consisting of a bunch of strictly parallel channels as shown in Figure 3A. The channels have a smallest free diameter of 0.71 nm, and the largest diameter inside is 1.26 nm. The distance between the centers of two neighboring channels is 1.84 nm.10 Each ZL crystal consists of a large number of channels, nch, which can be estimated by means of eq 8, where dZ is the diameter of the crystal in nanometers. For example, a crystal with a diameter of 600 nm features nearly 100 000 strictly parallel channels.

Asymmetric head−tail guests may show a strong preference for entering the channels with their head first, which can lead to a distinguished ordering, as illustrated in Figure 2D,E. This can result in materials displaying remarkable nonlinear optical properties.8,28 It should, however, be stressed that the understanding and control of the parameters for fine tuning the preference of molecules for entering the channels with their head first so that a high packing of oriented molecules with few defects is obtained are not well understood. Detailed kinetic studies are needed to explore this interesting option. An extension of the pathways illustrated for 1D channel hosts to 2D is fascinating, but today still very little is known concerning these more complex systems. We concentrate on 1D channel host−guest composites in the next section because they appear to be mature for entering different fields of application and also because they can serve to develop ideas and strategies for exploring other 1D host−guest composites and the more complex 2D host−guest composites.

4. ZEOLITE L IS AN IDEAL 1-NM-DIAMETER CHANNEL HOST The most important 1D 1 nm channel hosts for preparing guest− host composites have so far been zeolite-based, and among them ALPO4-5, zeolite L (ZL), MFI silicalite-1, and ZSM-5 have been the most successful.8−19,28−33 The MFIs are actually not 1D channel hosts but have often been handled as such because of the different diameters of the two types of perpendicularly oriented channels that can accept guests. The largest variety of 1D guest− zeolite composites with impressive photochemical, photophysical, and optical properties has been prepared by using ZL as a host. The reasons for this are the possibility of adjusting the size of the particles in the range of about 30 nm up to 10 000 nm and the morphology ranging from elongated to barrel-shaped to discs34−36 but also the possibilities of inserting neutral moleculs and cations and for allowing the preparation of rare earth complexes inside by means of the ship-in-a-bottle procedure. Many different dyes have so far been inserted into the channels of ZL (Tables 1 and 2). Furthermore, the channel entrances can be selectively functonalized and completely blocked, if desired, but also the

nch ≅ 0.267(dZ)2

(8)

The number nuc of unit cells of a ZL crystal of equal length and diameter expressed in nanometers can be estimated from this as nuc = 0.356nchdZ. The void space, provided by the channels with respect to the total volume of a crystal, is about 26%. 4.2. Guest−ZL Composites. ZL allows, through geometrical constraints, the realization of extremely high concentrations of well-oriented molecules that behave essentially as monomers or as very weakly interacting monomers. A 30 nm × 30 nm crystal can host 5000 guests that occupy 2 unit cells, and a 60 nm × 60 nm crystal can host up to 40 000 guests. It is convenient to introduce a parameter bearing information on the dye concentration but based on the purely geometrical (space-filling) properties of ZL as a host (i.e., showing to what extent the ZL channels are filled with dye 6221


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Figure 3. Zeolite L (ZL). (A) Schematic view of the channels. (B) Top view illustrating the hexagonal structure of ZL. The lines represent oxygen bridges between Si and Si/Al. Each channel is surrounded by six neighbors. The center-to-center distance between two channels is 1.84 nm. (C) Top and side views of a channel. The charge-compensating cations are seen as isolated spheres, and the surface is saturated with hydroxyl groups. (D) van der Waals representation with a dye molecule (Th+) entering one of the channels and side view of a channel that consists of 0.75-nm-long unit cells with van der Waals openings of 0.71 nm at the smallest and 1.26 nm at the widest places. The double arrow indicates an example of the orientation of the electronic transition dipole moment (ETDM) μS1←S0 of a dye. (E) Scheme illustrating the number of channels per surface area, nCH/cm2 = 4.3 × 1013. (F) SEM image of ZL crystals with a diameter of about 800 and a length of 1000 nm (upper). (Lower) Oriented monolayer prepared with disk-shaped ZL crystals of about 800 nm diameter on a glass plate.

molecules). The loading or occupation probability p of a guest− ZL composite is defined by eq 9. p=

number of occupied sites total number of sites

that it occupies about three unit cells and therefore has no other option than to align parallel to the channel axis. 4.4. Oscillator Strength. A way to get a feeling for the intensity of light absorption and the luminescence of dye−ZL composites is to express it in terms of the oscillator strength f, which can be calculated from the absorption spectrum according to eq 11, where ν̅ is the wavenumber in cm−1 of the electronic transition S0 → S1 and ε(ν̅) is the extinction coefficient in M−1 cm−1 at wavenumber ν̅.46

(9)

The symbol ns represents the number of unit cells occupied by a guest. It can, for example, be equal to 1, 2, or 3, but ns is not necessarily an integer. The loading ranges from 0 for an empty ZL to 1 for a fully loaded one. The concentration c(p) of a guest−ZL composite can be expressed as a function of the occupation probability (eq 10): c(p) = 0.752

p ⎛ mol ⎞ ⎜ ⎟ ns ⎝ L ⎠

f = 4.319 × 10−9 M·cm 2

∫ ε(υ̅) dυ̅

(11)

The magnitude of the oscillator strength f of organic dyes is often on the order of 1 but can also be larger. For cyanine dye PC21+, one finds a value of 1.4, and for perylene dyes DXP and PR149, it is 0.76. Assuming a ZL particle of 30 nm size that is fully loaded with a dye with f = 1 that occupies three unit cells, we find an oscillator strength of up to 3000. This is a very large value. It can be compared with an oscillator strength of 11, as reported for a GaAs quantum-sized particle of about 7 nm diameter consisting of roughly 20 000 atoms.47 This simple reasoning does not apply to dye−ZL particles larger than about 100 nm because then the saturation phenomena and inner filter effects complicate the situation. 4.5. Packing. The orientation and packing of the molecules inside the channels influence the material properties. So-called

(10)

4.3. Orientation of the Guest. Dyes usually adopt a specific orientation inside the channels. The structure of methylviologen− ZL has been studied be means of powder X-ray and vibrational spectroscopy,10 and fluorenone−ZL was studied using advanced first principles calculations.42 Most investigations rely on optical microscopy observations of small single crystals.31,43−45 A doublecone-type distribution was found to be a useful description regarding the orientation guest inside the 1D channels of ZL. We explain this in Figure 4, where we show images of MeAcr+, which has a size and shape such that it fits into one unit cell of ZL and aligns perpendicular to the channel axis. In contrast, PR149 is so long 6222


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two involved molecules being large with respect to the length of the ETDM. It can be shown that the length lμ* of the ETDM can be expressed as follows, where the value of the parameter const is equal to 3.036 × 10−6cm0.5 and the oscillator strength f is dimensionless:16 lμ* = const

f ΔE

(13)

From eq 13, we find, as an example, the length of the ETDM of an organic molecule absorbing light at 500 nm with oscillator strength one to be 0.215 nm. This means that the distance between the ETDM of two adjacent molecules in a nanochannel of interest here is usually large enough, so that eq 12 can be considered to be a good approximation. We show in Figure 5 schematically differ-

Figure 4. Orientation of dye molecules inside a channel. (A) Distribution of the molecule on a double cone with a half cone angle β. (B) Polarization of the luminescence of a single crystal, examined by means of a polarizer. (C) Relative intensity of the observed fluorescence as a function of the observation angle ε with respect to the c axis for different half cone angles β. (D−F) Optical fluorescence microscopy images of dyes with different orientations. (D) The orientation of the ETDM is perpendicular to the channel axis, β = 90°. The double-cone distribution of the dye molecules reduces to a plane. (E) Hardly any angle dependence is seen in the images. This result is expected for magic angle orientation β = 54.7°. (F) The orientation is parallel to the channel axis β = 0°. The doublecone distribution reduces to a line.15,43

Figure 5. Different orientations and arrangements of molecules in 1D channels. The double arrows indicate the direction of the ETDM of the first allowed electronic transition. (A) Representative orientations of molecules. Orientation of molecules that align their ETDMs (B) parallel and (B′) perpendicular to the channel axis and that have no electronic interaction because optically inert spacers keep them sufficiently large distances apart. Orientation of molecules that align their ETDMs (C) parallel and (C′) perpendicular to the channel axis and that are so close that Davydov coupling is important.

H dimers can form if the molecules are sufficiently small.48 Recently, J-type Davydov coupling has been observed in several dye−ZL composites.15−18,49−51 The Davydov coupling strength βc between two molecules (A*...A)↔(A...A*), with A* denoting an electronically excited molecule A, can be calculated by using eq 12.16,52

κ f 1 βc = AD 3 R ΔE n2

ent orientations and the packing of dye molecules in a 1D channel. The double arrows indicate the direction of the ETDM of the first allowed electronic transition. The inverse power to the third distance dependence of the coupling strength βc allows us to deduce that molecular engineering can be applied to fine tune the distance between the chromophors, for example, by covalently binding optically inert spacers to the end of the chromophores. From eq 12, we know that the value of βc decreases by a factor of 3.375 if the distance between two interacting ETDMs is augmented from 2/3R to R. This possibility of fine tuning has not yet been explored systematically for dye−nanochannel composites. 4.6. Surface Properties. The base and the coat surfaces of ZL crystals feature different reactivities. This property can be used to synthesize objects with selectively functionalized surfaces. The fact that the channel entrances are exclusively located on the base surfaces is the reason for their significantly different reactivity compared to that of the coat. This is the basis of the stopcock principle,10,37,39 as has been recently discussed in detail.13,41 The utilization of these different chemical properties to bind different chromophors selectively to the base and the coat leads to composites with intriguing properties. We illustrate an example of

(12) −18

The value of the constant AD is equal to 1.615 × 10 m cm−1 if we express βc in cm−1. The magnitude of the interaction βc caused by exciton coupling and hence the resulting splitting of the levels depend on the oscillator strength f, the relative orientation κ of two neighboring electronic transition dipole moments (ETDMs), and the distance R between the interacting ETDMs. The expression for κ can be simplified in the present case to κ = 1 − 3 cos2(θ), where θ is the angle between two adjacent ETDMs. The coupling is largest for in-line orientation (θ = 0°, κ = −2), leading to J coupling, whereas H coupling occurs in an essentially parallel arrangement (θ = 90°, κ = 1). βc further depends on the electronic excitation energy ΔE, expressed in cm−1, and on the refractive index n of the environment. The validity of eq 12 depends on the condition of the distance between the ETDMs of 2

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Scheme 2. Functionalizationa

a

(A) Complex functionalization of a ZL crystal.40 (B) Scheme and examples of linkers.23,29,38

shows channels filled with donor molecules in the middle part and an acceptor at the ends of the channels. Antenna materials of this type have also recently been known as ZeoFRET.57 A donor molecule D is excited by the absorption of a photon to become D*. D* transfers its electronic excitation energy to an unexcited neighbor. After series of such steps, the electronic excitation reaches a luminescent trap (acceptor molecule A) and is then released as luminescence. The acceptors are thought to mimic the entrance to the reaction center of the natural antenna. I restrict this discussion to one-photon processes taking place under ambient temperature conditions. This means that processes in which two or more electronically excited molecules interact with each otherleading to phenomena such as annihilation processes, superradiance, lasing, and others but also very low temperature situations where shallow traps may be importantare not discussed. The expression derived by Förster for the energy-transfer rate constant applies under these conditions (eq 1458,59). The Förster resonance energy transfer (FRET) rate constant kFRET and the exciton coupling strength βc (eq 12) depend on the packing density and on the relative orientation of the chromophores in the channels.

complex functionalization in Scheme 2A, where first the desired structure inside the crystals is built by sequentially inserting two different dyes. This is followed by a modification of the channel entrances in the desired way and finally by a functionalization of the coat.40 The functionalization must not necessarily be made with fluorescent dyes. Some steps may also be directed toward the appropriate sealing of the channels, influencing the solubilization properties, matching the refractive index, changing the reactivity, and focusing on other targets.53 4.7. Monolayers. The different chemical properties of the base and the coat have often been used to prepare stable, dense, oriented monolayers of ZL, in most cases with the c axis of the crystals perpendicular to the surface, on different substrates, often quartz or glass. Stable, oriented ZL monolayers with dense packing can be realized by covalently binding the crystals to a substrate, by using ionic linkage, by hydrogen bond fixation, or by microcontact transfer printing.23,54−56 It is possible to bind the crystals so that the channels remain open on the side that is not bonded, thus resulting in c-oriented open-channel monolayers (c-ocMLs). This leads to the first unidirectional antenna system for light harvesting and a number of other interesting composites.23,29,38 We explain the principle of a successfully used method in Figure 6. Many different linker molecules have been used. Their choice depends on the nature of the substrate and on the desired properties. We illustrate in Scheme 2B that linkers can be symmetric (X = Y) or non symmetric (X ≠ Y). For example, linker 4 has additional functionality by being capable of self-assembling on a surface via hydrogen bonding and furthermore by coordinating and sensitizing lanthanide ions Ln3+ after the ZL monolayer has been formed. 4.8. Antenna Systems. The following design mimicking the key functionality of green plants' antenna system has been inspired by the experience I had with different zeolite materials. The reasoning was that a host consisting of 1D channels into which dye molecules can be inserted has the advantage of being conceptually the simplest possible choice.14 I illustrate this in Figure 7A−C, which

kFRET = TF

κ2 1 JD*A, 4 G where G = τ0, D * n R6

ϕ0, D *

(14)

Geometrical expression G in eq 14 describes the dependence of the rate constant on the distance R between the ETDMs of the electronically excited donor D* and the acceptor A and on the angle expressed by the orientation factor κ2. The decay time τ0,D* and the luminescence quantum yield of the donor ϕ0,D* are values in the absence of energy transfer. JD*A is the spectral overlap between the luminescence spectrum of the donor and the absorption spectrum of the acceptor. n is the refractive index of the environment. It is convenient to use the Theodor Förster constant TF, which is equal to 8.785 × 10−25 mol. The largest energy-transfer rate constant is observed if the ETDMs are oriented parallel to the channel axis, according to 6224


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Figure 6. Supramolecular organization achieved by preparing c-oriented open-channel ZL monolayers (c-ocMLs) on a substrate. (A) A substrate bearing reactive OH groups is modified with a linker molecule. The thus-modified substrate is reacted with ZL crystals, leading to B, where we illustrate that the channels remain open on one side and can be filled (1), (1′) with guests and in the next step (2) are plugged with a stopcock. (C) SEM image showing a typical quality of ZL monolayers and a scheme illustrating the unidirectional organization of dyes achieved by first filling the channels with one kind of molecule (blue rectangles) and subsequently with the next kind (green rectangles) and finally by plugging the channel entrances with stopcocks.23,29,38

⟨κ2⟩β=90° (eq 17). See Figure 8 for an explanation of the variables used in eqs 15−17.

Förster’s theory. It is therefore advisible to choose donors that have their EDTMs oriented parallel to the channel axis, as illustrated in Figure 7A−C. This also applies to the acceptors. The experimental data reported in Figure 7F, however, illustrate that impressive transfer is also observed for an acceptor that does not fulfill this condition. The experimentally observed angle of acceptor Ox+, used in both examples, is larger than 70°.43,45 To understand this, it is necessary to analyze the geometrical constraints imposed by the ZL host in more detail. Two situations are illustrated in Figure 7C: one where the orientation of the acceptor is parallel to the channel axis (right) and one where it is oriented at a steep angle. We discuss the ∥ and with respect to rate constants along the channel kFRET ⊥ neighboring channels kFRET . The dependence of the latter on the angle and the distance is illustrated in more detail in Figure 8A. The parameter κ2 that describes the angle dependence is averaged as expressed in eqs 15 and 16. This is reasonable because as a result of the hexagonal symmetry of the host each channel is surrounded by six neighbors (Figure 3) and also because it is currently not possible to observe individual donor−acceptor pairs in such systems. We analyze two limiting situations that allow us to gain a sufficient understanding. The one in which the acceptors are oriented perpendicular to the channel axis is denoted as ⟨κ2⟩β=90°, and the other is denoted as

⟨κ 2⟩ =

1 2π [sin(θ) sin(β − θ) cos(ϕ) dϕ 2π 0

∫

− 2 cos(θ) cos(β − θ)]2 ⟨κ 2⟩ =

(15)

1 [sin 2(θ) sin 2(β − θ) + 8 cos2(θ) cos2(β − θ)] 2 (16)

⟨κ 2⟩β= 90 ° =

9 sin 2(2θ) 8

⟨κ 2⟩β= 0 ° = 4 cos4(θ) + ⎛z ⎞ θ = arccos⎜ 12 ⎟ ⎝ R12 ⎠

1 4 sin (θ) 2

(17)

Calculated values of the geometrical factor G in FRET (eq 14) are reported in Figure 8B. It is obvious that parallel-orientated acceptors located in the same channel as the donors result in the maximum values of G and therefore the largest FRET rate constants. The value of G is considerably smaller for acceptors 6225


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Figure 7. Illustration of typical dye−ZL antenna systems, energy states, and experimental data. (A, B) Chromophors are embedded in the channels of the host with their ETDMs oriented parallel to the channels axis. The orientation of the ETDM is indicated by double arrows. Acceptor molecules are on both sides at the two ends of each channel. The orientation of their ETDMs may be (A) the same or (B) different with respect to the donors. (C) Detailed comparison between acceptors with their ETDMs oriented (right) parallel and (left) nonparallel to the channel axis. The center-tocenter distance between two neighboring channels is 1.84 nm. (D) Illustration of the energy states of the donors and the electronic excitation energy transport to an acceptor located at the end of a channel. (E) The same as in D but this time the acceptor is located somewhere inside the channel. (F) Absorption (−) and luminescence spectra (---) of two A,D−ZL antenna systems embedded in a thin foil of PMMA on top of a glass plate. Acceptor A was in both cases Ox+ located on both ends of the channels, as indicated in B, and the emission was measured upon excitation of D at 450 nm. (Left) Donor D was HY3G, and the D/A ratio was 33. The emission was measured upon excitation of D at 450 nm. (Right) Donor D was LYF083, and the D/A ratio was ratio was 52.17

Figure 8. Angle dependence of the FRET rate constant kFRET. (A) Relative orientation of the ETDMs of donor 1 and acceptor 2 located in a neighboring channel. (B) Calculated geometrical factor G (see eqs 14 and 17 as a function of the distance R12 in nm) between donor 1 and acceptor 2. () 1 and 2 are in the same channel and have their EDTMs oriented parallel to the channel axis. This reflects the distance dependence of the rate ∥ in Figure 7C (right). The acceptor, molecule 2, is in a neighboring channel with its EDTM oriented (···) perpendicular and (-·-) constant kFRET ⊥ . parallel to the channel axis, which means that the values refer to kFRET

organization of dyes inside the ZL channels can be regarded as the first stage of organization. It allows light harvesting within the volume of a dye-loaded ZL crystal and radiationless energy transport by means of FRET to either the cylinder ends or to another desired position. The second stage of organization is obtained by positioning 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 results by interfacing the material to an external device via a stopcock intermediate. Higher levels of organization can be achieved by the controlled assembly of guest−ZL composites into oriented structures. This has been explored by us and by others with remarkable success.15,19,23,38,44,54−56 The usually strong light scattering of ZL and dye−ZL particles can be suppressed or at

located in the neighboring channels but has a respectable value at a distance of 2 nm. G and therefore also the rate constant are zero for acceptors oriented perpendicular and located in the same channel as the donors. This is not true for acceptors in a neighboring channel, where the value of G at a 2 nm distance is only 13 times smaller. A factor in favor of transport to neighboring channels is that each channel is surrounded by six neighbors, which means that the probability of FRET to a neighboring channel can still be important. This explains the efficiency of the antenna systems reported in Figure 7F, which is at first surprising. 4.9. Organization. It is useful to distinguish between different stages of organization that have been achieved with luminescent dye−ZL composites.10 The supramolecular 6226


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Figure 9. Selection of supramolecularly organized functional dye−ZL composites. The gray bars represent the channel walls of the ZL host. The colored rectangles represent dye molecules inside the channels. (A) Dye−ZL composites. (B) Orientation of the ETDMs of stopcock molecules. (C) Interface with the external world. (D) Supramolecular organization of the dye−ZL crystals.15

least strongly reduced by refractive index matching and the avoidance of microphase separation.60 Figure 9 provides an impression regarding the many different dye−ZL composites and organizational patterns that have been realized in different laboratories. The gray bars represent the channel walls of the ZL host. The colored rectangles represent dye molecules. We see

in Figure 9A (left) an example of a crystal containing one kind of dye and an example containing two different arbitrarily mixed dyes. The two examples in the middle represent a higher organizational level. These composites are prepared by sequential insertion of the different molecules, a synthesis technique that has been used to synthesize systems with artificial antenna 6227


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properties.10,15 The stopcock-plugged composites shown on the right result in either bidirectional (upper) or monodirectional (lower) antenna. Representative orientations of the ETDMs of stopcock molecules are illustrated in Figure 9B, where we show on the left side selected values of the orientation factor κ2 of the donor−acceptor pairs and then orientations of the stopcock ETDM (v) perpendicular to the crystals c axis, (vv) in plane, (p) parallel to the channel axis, (pv) parallel and perpendicular orientations, and (vvp) a spherical orientation. The typical cone-shape distribution of a guest molecule’s ETDM is indicated in green. The communication of guests with an external reaction center via FRET is illustrated in Figure 9C. Nonfluorescent stopcock molecules can be used if they act as insulating or protecting chemical linkers between the dyes inside the channels and the chromophores located at the reaction center, (a, b). We illustrate in c and d that a thin insulating part between the luminescent stopcock and the reaction center can be added if the stopcocks are luminescent molecules in order to avoid direct contact that can lead to electron transfer or other reactions. The insulating part can be covalently bound to either the stopcock or the reaction center. The distance between the donors and acceptors should be shorter than the Förster radius if efficient FRET is desired. Examples of hierarchically organized structures, presenting successive ordering from the molecular up to the macroscopic scale, as realized by dye−ZL composites are illustrated in Figure 9D. Monolayers with oriented crystals play an important role, but crystals can also be linked, resulting in chains. Furthermore, hexagonal and nematic phase ordering, embedding into nanofibers, and other patterns have been demonstrated. Random embedding in a polymer or, more interestingly, the embedding of oriented dye−ZL crystals are of practical relevance because fully transparent devices can be obtained by applying refractive index matching technology. A fascinating field is also the interfacing with biological objects. To summarize, we can say that dye−ZL composites are ideal objects for establishing and studying the relationship between the arrangement of molecules inside the nanochannels and the macroscopic world. Many of the presented results and much of the reasoning are not limited to luminescent objects, which, however, are especially easy to characterize, to understand, and hence to control.

(LSC),17 and color-changing media10 but also for sensing in analytical chemistry, biology, and diagnostics.11,15,16,62 I have described how artificial photonic antenna systems can be synthesized by incorporating dyes into a nanoporous material. Apparently, the most successful host has so far been ZL. A nanometer-sized ZL crystal consists of many thousands of strictly parallel arranged 1D channels. These can be filled with suitable guest molecules (e.g., neutral and cationic dyes and rare earth complexes). Geometrical constraints imposed by the host structure have been used to enforce the supramolecular organization of the guests. The channel openings have been plugged with stopcocks. All dyes shown in Tables 1 and 2 have been inserted into the channels of ZL, most of them at elevated temperature. For details, see the references mentioned above, especially refs 10, 16−18, 23, and 51. It is important to realize that if the loading of a 500-nm-long zeolite takes 1 day at 200 °C then we must expect that according to eq 7 about 16 days are needed for the same loading of a 2000-nm-long ZL crystal. Fortunately, for many interesting applications crystals of less than 1000 nm length are needed. This can be understood when considering that a single ZL channel of 450 nm length can host up to 200 dyes that occupy 3 unit cells. The supramolecular organization of fluorescent dyes allows light harvesting within the volume and radiationless transport of the excitation energy via FRET to the channel ends. The second stage involves the coupling to an external acceptor stopcock at the end of the ZL channel, which can trap the electronic excitation energy. The third stage of organization is attained by interfacing the objects to an external device via a stopcock mediator. Electronic excitation-energy transfer occurs mainly along the channel axis. This implies that macroscopically organized unidirectional stuctures can be prepared for optimized energy-transfer purposes. They have been synthesized by first preparing oriented ZL monolayers, filling the channels with luminescent guests in the next step, and after this plugging them with stopcocks on one side only.23 A new generation of FRET-sensitized solar cells works by first absorbing light over a broad spectral range in the dye−zeolite antenna composite. The excitation energy migrates by radiationless transfer among the inserted dye. It travels in one direction only because of the specific ordering of the dyes, as illustrated on the left side of Figure 10. FRET to the semiconductor takes place across a very thin insulating layer once the excitation energy has reached the stopcock. The injected electronic excitation energy can now be used to drive the charge-separation process in the active medium, as illustrated in Figure 10 middle for a thin semiconducture layer. This option is interesting for developing thin layer photovolatic cells with indirected bandgap semiconductors such as silicon, for which normally thick layers are needed in order to reach sufficient light absorptivity. The option has also been considered for improving the performance of organic solar cells (Figure 10 right), which usually have high absorption coefficients only in a limited spectral range.61 An LSC is a waveguiding plate containing luminescent chromophores.63 Light enters the face of the plate, is absorbed, and is subsequently re-emitted at longer wavelength. The luminescent light is trapped by total internal reflection and guided to the edges of the plate, where it can be converted to electricity by a photovoltaic cell. Because the edge area of the plate is much smaller than the face area, the LSC operates as a concentrator of light. It has been well understood for more than 30 years that a major reflection loss in such a device is caused by the overlap between the absorption and emission spectra of the chromophores.63−67 A way to minimize this loss is to use specific antenna material that has now become available in the form of the

5. DISCUSSION AND CONCLUSIONS Assemblies, polymers, crystals, and biological structures bearing nanochannels have been studied for decades. Examples of nanochannel materials are zeolites, zeotypes, cyclodextrins, urea-based assemblies, mesoporous silica materials, collagens, metal organic frameworks, and organic, carbon, and metaloxide channels. All have been investigated to some extent as hosts for molecules, complexes, ions, or clusters. The possibility of building complex structures based on 2D and especially 3D nanochannels is obviously larger than that based on 1D nanochannels. The latter are, however, easier to understand. They still give rise to a rich world of fascinating objects with very distinguished properties and have therefore attracted much interest. 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. A high level of organization has been achieved by interfacing these composites with other objects and by assembling them into specific structures. Materials that absorb all light in the right wavelength range and transfer the electronic excitation energy via FRET to wellpositioned acceptors offer a unique potential for developing FRET-sensitized solar cells,61 luminescent solar concentrators 6228


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Figure 10. Schematic explanation of FRET-sensitized solar cells. (Left) The dye−zeolite antenna composite consist of nanochannels containing two types of chromophores (blue and green) and stopcocks (red). Light absorbed by the blue and green dyes travels to the stopcocks by FRET. The electronically excited stopcocks transfer their energy via FRET to the active part: a semiconductor, an active polymer, or other. (Middle and right) Principle of a FRET-sensitized solar cell. Arranging crystals of appropriate size and morphology with their channel axes perpendicular to the surface of the active layer allows transport of the excitation energy to the interface by FRET. The active layer can be very thin because the electron hole (n-p) pairs form near the surface. The transfer of electrons from the antenna to the active layer is prevented by introducing a thin insulating layer. The scheme in the middle shows the principle of operation of thin-layer semiconductor devices, and the scheme on the right represents that of organic or plastic solar cells. The white area on top of the stopcock heads is an insulating part directly integrated into these molecules. The antennas are enlarged with respect to the rest of the device.

Figure 11. Schematic explanation of a ZeoFRET-LSC. (Left) Light absorbed by the acceptors travels to the donors by radiationless transfer, which then emit at longer wavelengths. The double arrows in the channel on the left indicate the orientation of the ETDM of the chromophores. (Middle) Reduction of reflection losses are achieved by orienting the crystals: (upper) scheme of an oriented monolayer; (lower) SEM image of an oriented monolayer. (Right) Small test LSC (2 × 1 cm2). Light falling on the top surface of the flat plate is absorbed by the embedded fluorescent ZeoFRET, emitted at longer wavelengths, and transported by waveguiding to the edges of the flat plate.

devices require efforts at the interface of chemistry, physics, and engineering, and it will be interesting to see how this field develops in the near future.

ZeoFRET composites that I have described. Absorption and emission spectra are separated by employing a large amount of a strongly absorbing dye and very little emitting dye (Figure 7). A further important improvement of LSC devices is possible by reducing the reflection losses.67,68 With ZeoFRET, the waveguiding layers can be made strongly anisotropic as illustrated in Figure 11, thus reducing these reflection losses considerably.69 The wavelength range that has been covered so far extends from the near-UV to about 700 nm. Extension to 850 nm or even 900 nm seems to be possible but remains challenging. The photostability of dyes is considerably improved by embedding them into the ZL channels and plugging the latter with stopcocks, thus protecting the guests because of the confinement. The new ZeoFRET building blocks are ready to be tested in devices.57 Their size, morphology, and optical properties will need to be tailored to the specific task envisaged. Many challenging possibilities for preparing guest−ZL composites and for organizing them in different ways remain to be explored. Problems to be solved for achieving the desired performance of

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Web page: www.dcb-server. unibe.ch/groups/calzaferri. Notes

The authors declare no competing financial interest.

■ ■

DEDICATION Dedicated to Professor Roald Hoffmann on the occasion of his 75th birthday. REFERENCES

(1) Ozin, G. A.; Arsenault, A. C. Nanochemistry: A Chemical Approach to Nanomaterials; Royal Society of Chemistry: Cambridge, U.K., 2005; ISBN 0-85404-664-X.

6229


Langmuir


(2) Ramamurthy, V., Inoue, Y., Eds. Supramolecular Photochemistry: Controlling Photochemical Processes; John Wiley & Sons, Hoboken, NJ, 2011; ISBN 978-0-470-23053-4. (3) Hisaki, I.; Murai, T.; Yabuguchi, H.; Shigemitsu, H.; Tohnai, N.; Miyata, M. Deoxycholamide Crystalline Frameworks as a Platform of Highly-Efficient Fluorescence Materials. Cryst. Growth Des. 2011, 11, 4652−4659. (4) Patzke, G. R.; Krumeich, F.; Nesper, R. Oxidic Nanotubes and NanorodsAnisotropic Modules for a Future Nanotechnology. Angew. Chem., Int. Ed. 2002, 41, 2446−2461. (5) Khlobystov, A. N.; Britz, D. A.; Briggs, G. A. D. Molecules in Carbon Nanotubes. Acc. Chem. Res. 2005, 38, 901−909. (6) Maggini, L; Bonifazi, D. Hierarchised Luminescent Organic Architectures: Design, Synthesis,Self-Assembly, Self-Organisation and Functions. Chem. Soc. Rev. 2012, 41, 211−241. (7) Scholes, G. D.; Fleming, G. R.; Olaya-Castro, A.; van Grondelle, R. Lessons from Nature about Solar Light Harvesting. Nat. Chem. 2011, 3, 763−774. (8) Caro, J.; Marlow, F.; Wübbenhorst, M. Chromophore-Zeolite Composites: The Organizing Role of Molecular Sieves. Adv. Mater. 1994, 6, 413−416. (9) Schulz-Ekloff, G.; Woehrle, D.; van Duffel, B.; Schoonheydt, R. A. Chromophores in Porous Silicas and Minerals: Preparation and Optical Properties. Microporous Mesoporous Mater. 2002, 51, 91−138. (10) Calzaferri, G.; Huber, S.; Maas, H.; Minkowski, C. Host−Guest Antenna Materials. Angew. Chem., Int. Ed. 2003, 42, 3732−3758. (11) Devaux, A.; Cucinotta, F.; Kehr, S.; De Cola, L. Functionalization and Assembling of Inorganic Nanocontainers for Optical and Biomedical Applications. Functional Supramolecular Architectures: For Organic Electronics and Nanotechnology; Samori, P., Cacialli, F., Eds.;Wiley-VCH: Weinheim, Germany, 2011; Vol. 1, pp 283−342. (12) Bein, T. Host-Guest Interactions in Zeolites and Periodic Mesoporous Materials. Introduction to Zeolite Science and Practice, 3rd revised ed.; Cejka, J., van Bekkum, H., Corma, A., Schüth, F., Eds.; Elsevier: Amsterdam, 2007; ISBN 978-0-444-53063-9, Chapter 18, pp 611−657. (13) Brühwiler, D.; Calzaferri, G.; Torres, T.; Ramm, J. H.; Gartmann, N.; Dieu, L.-Q.; López-Duarte, I.; Martínez-Díaz, M. V. Nanochannels for Supramolecular Organization of Luminescent Guests. J. Mater. Chem. 2009, 19, 8040−8067. (14) Gfeller, N.; Calzaferri, G. Energy Migration in Dye-Loaded Hexagonal Microporous Crystals. J. Phys. Chem. B 1997, 101, 1396− 1408. (15) Calzaferri, G.; Lutkouskaya, K. Mimicking the Antenna System of Green Plants. Photochem. Photobiol. Sci. 2008, 7, 879−910. (16) Calzaferri, G.; Devaux, A. Manipulation of Energy Treansfer Processes within the Channels of L-Zeolite. Supramolecular Photochemistry: Controlling Photochemical Processes; Ramamurthy, V., Inoue, Y., Eds.;John Wiley & Sons, Hoboken, NJ, 2011; ISBN 978-0-47023053-4, Chapter 9, pp 285−387. (17) Calzaferri, G.; Méallet-Renault, R.; Brühwiler, D.; Pansu, R.; Dolamic, I.; Dienel, T.; Adler, P.; Li, H.; Kunzmann, A. Designing Dye−Nanochannel Antenna Hybrid Materials for Light Harvesting, Transport and Trapping. ChemPhysChem 2011, 12, 580−594. (18) Gartzia-Rivero, L.; Bañuelos-Prieto, J.; Martínez-Martínez, V.; López-Arbeloa, I. Versatile Photoactive Materials Based on Zeolite L Doped with Laser Dyes. ChemPlusChem 2012, 77, 61−70. (19) Vohra, V.; Bolognesi, A.; Calzaferri, G.; Botta, C. Self-Assembled Nanofibers of Fluorescent Zeolite L Crystals and Conjugated Polymer. Langmuir 2010, 26, 1590−1593. (20) Winkler, R.; Berger, R.; Manca, M.; Hulliger, J.; Weber, E.; Loi, M. A.; Botta, C. All Organic Host-Guest Crystals Based on a DumbBell-Shaped Conjugated Host for Light Harversting through Resonant Energy Transfer. ChemPhysChem 2012, 13, 96−98. (21) Viani, L.; Poulsen, L.; Tolbod, M.; Jazdzyk, M.; Patrinoiu, G.; Cordella, F.; Mura, A.; Bongiovanni, G.; Botta, C.; Beljonne, D.; Cornil, J.; Hanack, M.; Egelhaaf, H.-J.; Gierschner, J. Spatial Control of 3D Energy Transfer in Supramolecular Nanostructured Host-Guest Architectures. J. Phys. Chem. B 2009, 113, 10566−10570.

(22) Poulsen, L.; Jazdzyk, M.; Communal, J.-E.; Sancho-Garcia, J. C.; Mura, A.; Bongiovanni, G.; Beljonne, D.; Cornil, J.; Hanack, M.; Egelhaaf, H.-J.; Gierschner, J. Three-Dimensional Energy Transport in Highly Luminescent Host-Guest Crystals: A Quantitative Experimental and Theoretical Study. J. Am. Chem. Soc. 2007, 129, 8585−8593. (23) Zabala Ruiz, A.; Li, H.; Calzaferri, G. Organizing Supramolecular Functional Dye−Zeolite Crystals. Angew. Chem., Int. Ed. 2006, 45, 5282−5287. (24) Ruthven, D. M., Diffusion in Zeolite Molecular Sieves. Introduction to Zeolite Science and Practice, 3rd revised ed.; Cejka, J., van Bekkum, H., Corma, A., Schüth, F., Eds.; Elsevier: Amsterdam, 2007; ISBN 978-0-444-53063-9; Chapter 21, pp 737−785. (25) Jung, Ch.; Kirstein, J.; Platschek, B.; Bein, T.; Budde, M.; Frank, I.; Müllen, K.; Michaelis, J.; Bräuchle, Ch. Diffusion of Oriented Single Molecules with Switchable Mobility in Networks of Long Unidimensional Nanochannels. J. Am. Chem. Soc. 2008, 130, 1638−1648. (26) Feil, F.; Naumov, S.; Michaelis, J.; Valiullin, R.; Enke, D.; Kärger, J.; Bräuchle, Ch. Single-Particle and Ensemble Diffusivities Test of Ergodicity. Angew. Chem., Int. Ed. 2012, 51, 1152−1155. (27) Brühwiler, D.; Gfeller, N.; Calzaferri, G. Resorufin in the Channels of Zeolite L. J. Phys. Chem. B 1998, 102, 2923−2929. (28) Kim, H. S.; Sohn, K. W.; Jeon, Y.; Min, H.; Kim, D.; Yoon, K. B. Aligned Inclusion of n-Propionic Acid Tethering Hemicyanine into Silica Zeolite Film for Second Harmonic Generation. Adv. Mater. 2007, 19, 260−263. (29) Cao, P.; Li, H.; Zhang, P.; Calzaferri, G. Self-Assembling Zeolite Crystals into Uniformly Oriented Layers. Langmuir 2011, 27, 12614− 12620. (30) Pham, T. C. T.; Kim, H. S.; Yoon, K. B. Growth of Uniformly Oriented Silica MFI and BEA Zeolite Films on Substrates. Science 2011, 334, 1533−1538. (31) Roeffaers, M. B. J.; Ameloot, R.; Baruah, M.; Uji-i, H.; Bulut, M.; De Cremer, G.; Müller, U.; Jacobs, P. A.; Hofkens, J.; Sels, B. F.; De Vos, D. E. Morphology of Large ZSM-5 Crystals Unraveled by Fluorescence Microscopy. J. Am. Chem. Soc. 2008, 130, 5763−5772. (32) van der Veen, M. A.; Valev, V. K.; Verbiest, T.; De Vos, D. E. In Situ Orientation-Sensitive Observation of Molecular Adsorption on a Liquid/Zeolite Interface by Second-Harmonic Generation. Langmuir 2009, 25, 4256−4261. (33) Moissette, A.; Lobo, R. F.; Vezin, H.; Bremard, C. Influence of Gallium Isomorphous Substitution in the Acidic MFI Zeolite Framework on Hole Formation, Transfer, and Trapping upon Incorporation of Anthracene. J. Phys Chem. C 2011, 115, 6635−6643. (34) International Zeolite Association, http://www.iza-structure.org. (35) Ohsuna, T.; Slater, B.; Gao, F.; Yu, J.; Sakamoto, Y.; Zhu, G.; Terasaki, O.; Vaughan, D. E.; Qiu, S.; Catlow, C. R. Fine Structures of Zeolite-Linde-L (LTL): Surface Structures, Growth Unit and Defects. Chem.Eur. J. 2004, 10, 5031−5040. (36) Zabala Ruiz, A.; Brühwiler, D.; Ban, T.; Calzaferri, G. Synthesis of Zeolite L. Tuning Size and Morphology. Monatsh. Chem. 2005, 136, 77−89. (37) Calzaferri, G. Dye Loaded Zeolite Material and Dye Loaded Zeolite Material Containin Devices, Patents EP 1335879, US 6932919, US 7327012. (38) Wang, Y.; Li, H.; Feng, Y.; Zhang, H.; Calzaferri, G.; Ren, T. Orienting Zeolite L Microcrystals with a Functional Linker. Angew. Chem., Int. Ed. 2010, 49, 1434−1438. (39) Maas, H.; Calzaferri, G. Trapping Energy from and Injecting Energy into Dye-Zeolite Nanoantennae. Angew. Chem., Int. Ed. 2002, 41, 2284−2288. (40) Busby, M.; Kerschbaumer, H.; Calzaferri, G.; De Cola, L. Orthogonally Bifunctional Fluorescent Zeolite-L Microcrystals. Adv. Mater. 2008, 20, 1614−1618. (41) López-Duarte, I.; Dieu, L.-Q.; Dolamic, I.; Martínez-Díaz, M. V.; Torres, T.; Calzaferri, G.; Brühwiler, D. On the Significance of the Anchoring Group in the Design of Antenna Materials Based on Phthalocyanine Stopcocks and Zeolite L. Chem.Eur. J. 2011, 17, 1855−1862. 6230


Langmuir


(42) Fois, E.; Tabacchi, G.; Calzaferri, G. Interactions, Behavior, and Stability of Fluorenone inside Zeolite Nanochannels. J. Phys. Chem. C 2010, 114, 10572−10579. (43) Megelski, S.; Lieb, A.; Pauchard, M.; Drechsler, A.; Glaus, S.; Debus, C.; Meixner, A. J.; Calzaferri, G. Orientationof Fluorescent Dyes in the Nano Channels of Zeolite L. J. Phys. Chem. B 2001, 105, 25−35. (44) Hashimoto, S.; Samata, K.; Shoji, T.; Taira, N.; Tomita, T.; Matsuo, S. Preparation of Large-Scale 2D Zeolite Crystal Array Structures to Achieve Optical Functionality. Microporous Mesoporous Mater. 2009, 117, 220−227. (45) Gasecka, A.; Dieu, L.-Q.; Brühwiler, D.; Brasselet, S. Probing Molecular Order in Zeolite L Inclusion Compounds Using TwoPhoton Fluorescence Polarimetric Microscopy. J. Phys. Chem. B 2010, 114, 4192−4198. (46) Kuhn, H.; Försterling, H.-D. Principles of Physical Chemistry; John Wiley & Sons, LTD: Chichester, England, 2000. (47) Alén, B.; Bosch, J.; Granados, D.; Martínez-Pastor, J.; García, J. M.; González, L. Oscillator Strength Reduction Induced by External Electric Fields in Self-Assembled Quantum Dots and Rings. Phys. Rev. B 2007, 75, 045319−1−045319−7. (48) Hashimoto, S.; Hagiri, M.; Matsubara, N.; Tobita, S. Photophysical Studies of Neutral Aromatic Species Confined in Zeolite L: Comparison with Cationic Dyes. Phys. Chem. Chem. Phys. 2001, 3, 5043−5051. (49) Busby, M.; Blum, Ch.; Tibben, M.; Fibikar, S.; Calzaferri, G.; Subramaniam, V.; De Cola, L. Time, Space, and Spectrally Resolved Studies on J-Aggregate Interactions in Zeolite L Nanochannels. J. Am. Chem. Soc. 2008, 130, 10970−10976. (50) Calzaferri, G.; Brühwiler, D.; Meng, T.; Dieu, L.-Q.; Malinovskii, V. L.; Häner, R. Surprising Properties of a Furo-Furanone. Chem.Eur. J. 2010, 16, 11289−11299. (51) Busby, M.; Devaux, A.; Blum, C.; Subramaniam, V.; Calzaferri, G.; De Cola, L. Interactions of Perylene Bisimide in the OneDimensional Channels of Zeolite L. J. Phys. Chem. C 2011, 115, 5974− 5988. (52) Davydov, A. S. The Theory of Molecular Excitons. Usp. Fiz. Nauk 1964, 82, 393−448. (53) Albuquerque, R. Q.; Calzaferri, G. Proton Activity inside the Channels of Zeolite L. Chem.Eur. J. 2007, 13, 8939−8952. (54) Yoon, K. B. Organization of Zeolite Microcrystals for Production of Functional Materials. Acc. Chem. Res. 2007, 40, 29−40. (55) Cao, P.; Wang, Y.; Li, H.; Yu, X. Transparent, Luminescent, And Highly Organized Monolayers of Zeolite L. J. Mater. Chem. 2011, 21, 2709−2714. (56) Cucinotta, F.; Popovic, Z.; Weiss, E. A.; Whitesides, G. M.; De Cola, L. Microcontact Transfer Printing of Zeolite Monolayers. Adv. Mater. 2009, 21, 1142−1145. (57) Optical Additives GmbH, Switzerland, http://www.opticaladditives.com. (58) Förster, Th. Energiewanderung und Fluoreszenz. Naturwissenschaften 1946, 6, 166−175. (59) Fö rster, Th. Zwischenmolekulare Energiewanderung und Fluoreszenz. Ann. Physik (Leipzig) 1948, 2, 55−75. (60) Suárez, S.; Devaux, A.; Bañnuelos, J.; Bossart, O.; Kunzmann, A.; Calzaferri, G. Transparent Zeolite-Polymer Hybrid Materials with Adaptable Properties. Adv. Funct. Mater. 2007, 17, 2298−2306. (61) Köppe, R.; Bossart, O.; Calzaferri, G.; Sariciftci, N. S. Advanced Photon-Harvesting Concepts for Low-Energy Gap Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2007, 91, 986−995. (62) Li, Z.; Luppi, G.; Geiger, A.; Josel, H.-P.; De Cola, L. Bioconjugated Fluorescent Zeolite L Nanocrystals as Labels in Protein Microarrays. Small 2011, 7, 3193−3201. (63) Goetzberger, A.; Greubel, W. Solar Energy Conversion with Fluorescent Collectors. Appl. Phys. 1977, 14, 123−139. (64) Batchelder, J. S.; Zewail, A. H.; Cole, T. Luminescent Solar Concentrators. 1: Theory of Operation and Techniques for Performance Evaluation. Appl. Opt. 1979, 18, 3090−3110.

(65) Kittidachachan, P.; Danos, L.; Meyer, T. J.; Alderman, N.; Markvart, T. Photon Collection Efficiency of Fluorescent Solar Collectors. Chimia 2007, 61, 780−786. (66) Dienel, T.; Bauer, C.; Dolamic, I.; Brühwiler, D. Spectral-Based Analysis of Thin Film Luminescent Solar Concentrators. D. Sol. Energy 2010, 84, 1366−1369. (67) Debije, M. G.; Verbunt, P. P. Thirty Years of Luminescent Solar Concentrator Research: Solar Energy for the Built Environment. Adv. Energy Mater. 2012, 2, 12−35. (68) Mulder, C. L.; Reusswig, P. D.; Velázquesz, A. M.; Kim, H.; Rotschild, C.; Baldo, M. A. Dye Alignment in Luminescent Solar Concentrators: I. Vertical Alignment for Improved Waveguide Coupling. Opt. Exp. 2010, 18, A79−A90. (69) Calzaferri, G.; Brühwiler, D.; Kunzmann, A. Luminescence Concentrators and Luminescence Dispersers on the Basis of Oriented Dye Zeolite Antennas. Patent 2008, CH 698333 and WO 2010/ 009560

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