Advances in Photochemistry / Edition 1 available in Hardcover
About the Author
David H. Volman, professor emeritus, chemistry, was born in 1916 in Los Angeles California. He received his BS and MS degrees in chemistry from UCLA in 1937 and 1938 and his Ph.D. from Stanford University in 1940. In 1940 he joined UC Davis as an instructor and junior chemist but left during World War II to work as research chemist for the U.S. Office of Scientific Research and Development. George Simms Hammond was an American scientist and theoretical chemist who developed "Hammond's postulate", and fathered organic photochemistry - the general theory of the geometric structure of the transition state in an organic chemical reaction.
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Advances in Photochemistry, Volume 27
John Wiley & SonsCopyright © 2002 John Wiley & Sons, Inc.
All right reserved.
Chapter OneSUPRAMOLECULARLY ORGANIZED LUMINESCENT DYE MOLECULES IN THE CHANNELS OF ZEOLITE L
Gion Calzaferri, Huub Maas, Marc Pauchard, Michel Pfenniger, Silke Megelski, and André Devaux
Department of Chemistry and Biochemistry, University of Bern, CH-3000 Bern 9, Switzerland
I. Introduction II. The System A. Geometrical Constraints B. Inner- and Outer-Surface of the Zeolite Nanocrystals C. The Dyes D. Three-Dye Antenna E. The Stopcock Principle III. Transfer of Electronic Excitation Energy A. Radiationless Energy Transfer B. Förster Energy Transfer in Dye Loaded Zeolite L C. Spectral Overlap IV. Elegant Experiments for Visual Proof of Energy Transfer and Migration A. Energy Transfer B. Intrazeolite Diffusion Monitored by Energy Transfer C. Energy Migration V. Conclusions Acknowledgments References
An important aim of photochemistry is to discover or to design structurally organized and functionally integrated artificial systems that are capable of elaborating the energy and information input of photons to perform useful functions such as transformation and storage of solar energy, processing and storage of information, and sensing of the microscopic environment on a molecular level. The complexity and beauty of natural systems have encouragedchemists to study the structure and properties of organized media like molecular crystals, liquid crystals and related regular arrangements, and to mimic some of their functions. Microporous structures containing atoms, clusters, molecules, or complexes provide a source of new materials with exciting properties. For this purpose, zeolites are especially appealing crystalline inorganic microporous materials. Some of them occur in nature as a component of the soil. Natural and synthetic zeolites possess a large variety of well-defined internal structures such as uniform cages, cavities, or channels. A useful feature of zeolites is their ability to host molecular guests within the intravoid space. Chromophore loaded zeolites have been investigated for different purposes such as interfacial electron transfer, microlasers, second harmonic generation, frequency doubling, and optical bistabilities giving rise to persistent spectral hole burning. The role of the zeolite framework is to act as a host for realizing the desired geometrical properties and for stabilizing the incorporated molecules. Incorporation of chromophores into the cavities of zeolites can be achieved in different ways, depending on the used substances and on the desired properties: from the gas phase, by ion exchange if cations are involved by crystallization inclusion, or by performing an in situ synthesis inside the zeolite cages.
Plants are masters of efficiently transforming sunlight into chemical energy. In this process, every plant leaf acts as a photonic antenna in which photonic energy is absorbed in the form of sunlight and transported by chlorophyll molecules for energy transformation. In natural photosynthesis, light is absorbed by an antenna system of a few hundred chlorophyll molecules arranged in a protein environment. The antenna system allows a fast energy transfer from an electronically excited molecule to unexcited neighbor molecules in a way that the excitation energy reaches the reaction center with high probability. Trapping occurs there. It has been reported that the anisotropic arrangement of chlorophyll molecules is important for efficient energy migration. 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 and by choosing conditions such that the volume of the cages and channels is able to uptake only monomers but not aggregates.
An artificial photonic antenna system is an organized multicomponent arrangement in which several chromophoric molecular species absorb the incident light and transport the excitation energy (not charges) to a common acceptor component. Imaginative attempts to build an artificial antenna different from ours have been presented in the literature. Multinuclear luminescent metal complexes, multichromophore cyclodextrins, Langmuir-Blodgett films, dyes in polymer matrices, and dendrimers have been investigated. Some sensitization processes in silver halide photographic materials and also the spectral sensitization of polycrystalline titanium dioxide films bear in some cases aspects of artificial antenna systems. The system reported in is of a bidirectional type, based on zeolite L as a host material, and able to collect and transport excitation over relatively large distances. The light transport is made possible by specifically organized dye molecules that mimic the natural function of chlorophyll. The zeolite L crystals consist of a continuous one-dimensional (1D) tube system. We have filled each individual tube with successive chains of different joint but noninteracting dye molecules. Light shining on the cylinder is first absorbed and the energy is then transported by the dye molecules inside the tubes to the cylinder ends.
A schematic view of the artificial antenna is illustrated in Figure 1.1. The monomeric dye molecules are represented by rectangles. The dye molecule, which has been excited by absorbing an incident photon, transfers its electronic excitation to another one. After a series of such steps, the electronic excitation reaches a trap that we have pictured as shaded rectangles. The energy migration is in competition with spontaneous emission, radiationless decay, and photochemically induced degradation. Very fast energy migration is therefore crucial if a trap should be reached before other processes can take place. These conditions impose not only spectroscopic but also decisive geometrical constraints on the system.
In this chapter, we describe the design and important properties of supra-molecularly organized dye molecules in the channels of hexagonal nanocrystals. We focus on zeolite L as a host. The principles, however, hold for other materials as well. As an example, we mention ZSM-12 for which some preliminary results have been reported. We have developed different methods for preparing well-defined dye-zeolite materials, working for cationic dyes, neutral dyes, and combinations of them. The formula and trivial names of some dyes that so far have been inserted in zeolite L are reported in Section II.C. The properties of natural and commercially available zeolites can be influenced dramatically by impurities formed by transition metals, chloride, aluminiumoxide, and others. This fact is not always sufficiently taken care of. In this chapter, we only report results on chemically pure zeolites, the synthesis of which is described in.
II. THE SYSTEM
Favorable conditions for realizing a device as illustrated in Figure 1.1 are a high concentration of monomeric dye molecules with high luminescence quantum yield, ideal geometrical arrangement of the chromophores, and an optimal size of the device. Dyes at high concentration have the tendency to form aggregates that in general show very fast radiationless decay. The formation of aggregates can be prevented by fencing dyes inside a microporous material and by choosing conditions such that the volume of the cages and channels is only able to uptake monomers but not aggregates. Linear channels running through microcrystals allow the formation of highly anisotropic dye assemblies. Examples of zeolites bearing such channels large enough to uptake organic dye molecules are reported in Table 1.1. Our investigations have concentrated on zeolite L as a host. The reason for this is that neutral dyes as well as cationic dyes can be inserted into the channels of zeolite L and that synthesis procedures for controlling the morphology of zeolite L crystals in the size regime from 30 to ~ 3000 nm are available. Many results obtained on zeolite L are valid for other nanoporous materials as well. In Figure 1.2, we show a scanning electron microscopy (SEM) picture of a zeolite L material with nice morphology. The hexagonal shape of the crystals can easily be recognized. For simplicity, we often describe them as crystals of cylinder morphology.
A space-filling top view and a side view of the zeolite L framework is illustrated in Figure 1.3. The primitive vector c corresponds to the channel axis while the primitive vectors a and b are perpendicular to it, enclosing an angle of 60º.
We distinguish between three types of dye molecules. (1) Molecules small enough to fit into a single unit cell. Examples we have investigated so far are biphenyl, hydroxy-TEMPO, fluorenone, and methylviologen (M[V.sup.2+]). Structural details of the latter are known based on vibrational spectroscopy, Rietveld refinement of X-ray data, and molecular modeling. We found that the M[V.sup.2+] lies along the channel wall, and that the angle between the main M[V.sup.2+] axis and the c axis of the zeolite is 27º. (2) Molecules with a size that makes it hard to guess if they align along the c axis or if they are tilted in the channel. Oxonine, pyronine, and thionine are molecules of this type, as we will see later. (3) Molecules that are so large that they have no other choice but to align along the c axis. Many examples fit into this category. The POPOP illustrated in Figure 1.4 is one of them. It is important to know if molecules can occupy at least part of the same unit cell, so that they can interact via their [pi]-system or if they can "only touch each other" so that their electronic coupling is negligible.
While for molecules of type (1) not only translational but also large amplitude modes can be activated, the latter are severely or even fully restricted for molecules of types (2) and (3). This finding has consequences on their stability and on their luminescence quantum yield, which generally increases. An example that we have investigated, is the very light sensitive DPH, which is dramatically stabilized when inserted into zeolite L, because there is not sufficient space available for trans to cis isomerization. In other cases, a strong increase of stability is observed because reactive molecules that are too large or anions such as hypochlorite have no access because they cannot enter the negatively charged channels. It is not surprising that the fluorescence quantum yield of cationic dyes is not or is only positively affected by the zeolite L framework. More interestingly, the fluorescence quantum yield of neutral dyes also seems to be positively influenced by the zeolite L framework despite the very large ionic strength inside the channels. Only one case of an anionic organic dye in the anionic zeolite L framework has been reported so far, namely, the resorufin, which is also the only case where severe luminescence quenching has been observed. Most results reported here refer to dye loaded zeolite L material that contains several water molecules per unit cell, see. If the water molecules are completely removed from the main channel, the spectroscopic properties may change.
A. Geometrical Constraints
The geometrical constraints imposed by the host determines the organization of the dyes. We focus on systems consisting of dye molecules in hexagonally arranged linear channels. Materials providing such channels are reported in Table 1.1. We investigate a cylindrical shape as illustrated in Figure 1.5. The primitive vector c corresponds to the channel axis while the primitive vectors a and b are perpendicular to it enclosing an angle of 60º. The channels run parallel to the central axis of the cylinder. The length, and the diameter of the cylinder are [l.sub.Z], and [d.sub.Z], respectively. The following concepts and definitions cover situations we found to be important. They refer to systems as illustrated in Figure 1.1.
1. The number of parallel channels n.sub.ch of a hexagonal crystal that can be approximated by a cylinder of diameter d.sub.Z is given by
[Mathematical Expression Not Reproducible In ASCII]
which can be approximated for zeolite L as
(2) [n.sub.ch] [??] 0.268(d.sub.Z]).sup.2] where [d.sub.Z] is in units of nanometers (nm). This means that a zeolite L of 500-nm diameter gives rise to ~67,000 parallel channels.
2. The dye molecules are positioned at sites along the linear channels. The length of a site is equal to a number [n.sub.s] times the length of c, so that one dye molecule fits into one site. Thus [n.sub.s] is the number of unit cells that form a site we name the [n.sub.s.sup.-site.] The parameter [n.sub.s] depends on the size of the dye molecules and on the length of the primitive unit cell. As an example, a dye with a length of [approximately equal to]1.5 nm in zeolite L requires two primitive unit cells, therefore [n.sub.s] = 2 and the sites are called 2-site. The sites form a new (pseudo) Bravais lattice with the primitive vectors a, b, and [n.sub.s] · c in favorable cases.
3. Different types of sites exist. Those occupied with luminescent dye molecules are marked with small letters. Capital letters are reserved for traps that may or may not be luminescent. Per crystal, the number of sites available for dye molecules is [i.sub.max] and the number of sites available for traps is [I.sub.max]. Equivalent [n.sub.s]-sites have the same geometrical properties. Dye molecules in equivalent sites i are assumed to be equivalent. The same is valid for traps.
4. In general, only dye molecules with a large electronic transition dipole moment µ[S.sub.1] [left arrow] [S.sub.0] are considered in this account, which means that the [S.sub.1] [left arrow] [S.sub.0] transition is of [pi] [left arrow] [pi] type.
5. Equivalent [n.sub.s]-sites i have the same probability [p.sub.i] to be occupied by a dye molecule. The occupation probability p is equal to the ratio between the occupied and the total number of equivalent sites. The number of unit cells [n.sub.uc] is controlled by the host while [n.sub.s
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Table of ContentsPhotochemistry of Triarylmethane Dye Leuconitriles (V. Jarikov & D. Neckers).
Structure and Reactivity of Organic Intermediates as Revealed by Time-Resolved Infrared Spectroscopy (J. Toscano).
Semiconductor Photocatalysis for Organic Synthesis (H. Kisch).
Photophysical Probes of DNA Sequence-Directed Structure and Dynamics (C. Murphy).
Cumulative Index, Volumes 1-26.
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