Symposium: Applications of Inorganic Photochemistry
Symposium: Applications of Inorganic Photochemistry
Organometallic Photochemistry: Basic Principles and Applications to Materials Chemistry David R. Tyler Department of Chemistry, University of Oregon, Eugene, OR 97403-1253 The field of organometallic photochemistry has matured considerably during the past decade. Comprehensive studies on virtually all types of organometallic compounds have led to a fundamental understanding of the excited states, primary photoprocesses, kinetics, and mechanistic pathways associated with these molecules. (Note that organometallic molecules are broadly defined as molecules having metal–carbon bonds.) A growing number of textbooks summarize the wealth of data and fundamental principles that pertain to the field (1–4), and numerous reviews summarize the relevant literature published each year. As organometallic photochemistry has matured, researchers have shifted their attention from fundamental studies of photophysics and mechanisms to applications of those fundamental principles. Of these applied studies, a vast majority focus on materials chemistry because organometallic compounds provide a convenient way to introduce metals into a system. In order to understand the role of photochemistry in these applications, we start with a discussion of excited states and primary photoprocesses. Excited States and Primary Photoprocesses A variety of low-energy excited states are found in organometallic complexes. It is important to know the type of excited state involved in a photoreaction because each type of excited state typically leads to a characteristic reactivity. Excited states are classified by the types of orbitals involved in the electronic transition from
ground to excited state. As a model to facilitate the discussion of these excited states, the simple molecular orbital diagram in Figure 1 is presented. This diagram shows the interaction of a metal atom with n ligands. As shown, the ligands also have empty π orbitals, which is typical for many ligands found in organometallic complexes. Note that the interaction of n ligand σ-type orbitals with the metal orbitals forms n M–L σ bonding molecular orbitals (which are predominantly ligand in character) and n M–L σ* (antibonding) molecular orbitals, which are predominantly metal in character. The 9-n remaining valence orbitals on the metal are used to π bond to the empty π orbitals on the ligands, a type of bonding called π-backbonding because it involves electron donation from the metal to the empty ligand π orbitals. As shown in the figure, π-backbonding stabilizes the 9-n metal orbitals and increases the energy of the ligand π orbitals. The various excited states encountered in organometallic complexes will be explained by reference to this molecular orbital scheme. Ligand field excited states (labeled 1 in Fig. 1) involve electronic excitation between d orbitals (or between orbitals that are primarily d in character); for that reason they are also known as d–d excited states and the excitations as d–d transitions. Generally, in d–d transitions, a π-bonding or nonbonding d orbital is deoccupied and a metal–ligand antibonding orbital is occupied (Fig. 1). The result is a weakening of the metal–ligand bond, which typically leads to metal–ligand bond dissociation. A question of considerable interest concerns how the bond breaks: does it cleave heterolytically or homolytically (5, 6)? In general, heterolysis occurs when metal–ligand dative bonds are weakened by d–d excitation. Typical examples of this behavior include the photodissociation of metal–CO bonds in most metal carbonyl complexes. For example: hν
MLm(CO)n → MLm(CO) n–1 + CO
(1)
MLm(CO)n = Mo(CO)6, Fe(CO)5, Re(CO)5Cl, CpV(CO)4, (η6-mesitylene)Mo(CO)3, and numerous others
In another example, d–d excitation of CpFe(η6-arene)+ labilizes one bond to the arene and the initial product is CpFe(η4-arene)+ (eq 2) (7, 8). (Subsequent reactions lead to cleavage of the other Fe-arene bonds and the formation of CpFe(solvent)3+ .)
Figure 1. Molecular orbital diagram for the interaction of a metal with n ligands. The electronic transitions are: 1, d–d; 2, MLCT; 3 , LMCT; 4, intraligand.
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(2)
Symposium: Applications of Inorganic Photochemistry
Figure 2. Mechanism of the oxidation of ML 6 (L = CNAr; Ar = 2,6-diisopropylphenyl) following MLCT excitation.
alkyl → metal charge transfer states. The alkyl orbital in question is the metal–alkyl bonding orbital (analogous to the σ orbital in Fig. 1), and thus depopulation of this orbital will weaken and break the metal–alkyl bond. (This type of transition corresponds to transition 3 in Fig. 1.) A classic example of this type of photoreactivity is provided by coenzyme B12 and its numerous model complexes:
(4) Metal–ligand bond homolysis following d–d excitation is rare. Most examples of homolysis involve ligand-to-metal charge-transfer excitations. These excited states are discussed below. In charge-transfer transitions, electrons are transferred from one part of a molecule to another. For example, in a metal-to-ligand charge-transfer (MLCT; transition 2 in Fig. 1), an electron moves from an orbital that is primarily metal in character to one that is mainly ligand in character. Other common types of charge transfer include ligand-to-metal charge transfer (LMCT; transition 3 in Fig. 1) and charge transfer to solvent (CTTS). As is the case for classical coordination complexes, charge-transfer excited states of organometallics typically lead to redox reactions. Of the various types of charge transfer, MLCT transitions are the most important in organometallic photochemistry because the types of ligands found in organometallic complexes typically have low energy π-acceptor orbitals and the transitions are therefore at lower energy than the other types of charge transfer. Also, the metal centers in organometallic compounds are generally in low oxidation states, which makes them easier to oxidize than reduce. An example of MLCT reactivity is the following (9): hν(MLCT)
{
ML 6 → ML6 + Cl + [? CHCl2] CHCl 3
+
(3)
In this case, LMCT excitation forms R? and a Co(II) metal center. Again, note this is a redox reaction: LMCT leads to a reduction of the Co center and oxidation of the ligand from R{ to R?. Another interesting example of LMCT reactivity is discussed in ref 11. Many organometallic complexes have a framework of metal-metal bonds, and several types of electronic transitions are associated with orbitals contained in this framework. For example, excitation from a metal-metal bonding to metal-metal antibonding orbital is denoted as a σM–M → σM–M* transition. (For simplicity, the transition is usually just denoted as σ → σ*.) Transitions from nonbonding d orbitals are also possible in these complexes, and they are referred to as dπ → σ* transitions because the d orbital being depopulated typically has πtype symmetry with respect to the metal–metal bond. In both transitions, a metal–metal antibonding orbital is occupied, which should weaken the metal–metal bonding. In fact, it is observed experimentally that both σ → σ* and dπ → σ* excited states lead to homolytic cleavage of the metal–metal bond and consequently to the formation of metal radicals (eq 5) (12): hν ? LnM–ML n → ← 2MLn
(5)
MLn = Mn(CO)5, Re(CO)5, Co(CO)4, CpFe(CO)2, CpMo(CO)3, CpW(CO)3, etc.
The mechanism for this reaction is shown in Figure 2. Note that the essential feature in the mechanism is quenching of the MLCT excited state by electron transfer to chloroform, a step facilitated by the fact that, in the MLCT excited state, the “reduced” ligand is susceptible to electrophilic attack. Also note the net oxidation of the metal in eq 3. Although MLCT excitation will generally not dissociate metal–ligand bonds, ligand substitution can occur by associative attack of an entering nucleophile on the oxidized metal center. Another interesting example of MLCT reactivity is discussed in ref 10. One important exception to the generalization that LMCT transitions occur at high energy in organometallic complexes is in complexes with nondative (covalent) metal–ligand bonds. For example, complexes with M–alkyl bonds oftentimes have photochemically accessible
In the absence of a metal-radical trap, the two metal radicals can recombine and no net photolysis will take place. Many ligands have characteristic electronic transitions, which are still present when the ligands are coordinated to a metal. For example, Re2(CO)8L2 (L 2 = 1,10phenanthroline) has a phenanthroline-centered π → π* transition at essentially the same wavelength as uncoordinated 1,10-phenanthroline. Such transitions are often referred to as intraligand transitions. Rydberg transitions are another type of excitation found in organometallic complexes. Located in the far ultraviolet or the vacuum UV, these transitions involve electronic excitations to atomic orbitals higher in energy than the valence shell molecular orbitals. These transitions are frequently high enough in energy to dissociate multiple ligands when the molecules are irradiated in the gas phase (where the molecules have no competing mechanisms for dissipating the energy). These excited states are particularly useful in the photochemical deposition of metallic thin films where the loss of all ligands is required.
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Symposium: Applications of Inorganic Photochemistry Applications: Photochemical Deposition of Thin Films One of the major applications of organometallic photochemistry to materials chemistry is in the photochemical deposition of thin films (13, 14). In the photochemical film deposition process, a thin metal film is deposited on a substrate surface by using light to dissociate the ligands from an organometallic complex. During the irradiation, the organometallic molecule can either be adsorbed to the substrate surface (eq 6) or it can be in the gas phase above the substrate (eq 7). hν
MLn(adsorbed) → M(s) + nL hν
MLn(g) → M(s) + nL
(6) (7)
The specific compounds used in these studies depend on the type of thin film desired. Thus, in the semiconductor industry, molecules such as Ga(CH3) 3 and AsH3 are irradiated. If thin films of transition metals are needed, then molecules such as Fe(CO) 5, W(CO)6, or Cr(CO)6 are used. In each case, the photochemical principle is the same: the organometallic molecule is irradiated and the ligands are dissociated, leaving behind the naked metal atom to form the thin film on the substrate surface. As discussed previously, the types of excited states leading to ligand dissociation include d–d, LMCT, and Rydberg excited states. In modern photochemical thin-film deposition processes, the light source for the process is typically a laser. The light beam can be perpendicular to the substrate surface or parallel to the surface (Fig. 3). The perpendicular orientation is used in maskless direct writing processes (e.g., for circuits) because it gives sharply defined deposition areas; the parallel irradiation method is used to deposit films over a large area. One complication that results from using lasers is that not all deposition reactions occur as the result of a photochemical reaction; some reactions are pyrolytic—that is, energy from the laser simply heats the molecules, causing the ligands to dissociate. Mechanistic methods have been developed for distinguishing between the two deposition pathways. One final note concerns the fate of the ligands in these photoreactions. Ideally, the organic material making up the ligands is simply vaporized and not deposited with the metals. In practice, carbon and sometimes other elements are often deposited and one of the prime areas of technological research is to find ways to minimize these co-depositions. Specific organometallic molecules used in these photochemical deposition studies include SiH4 and Si2H6 for the deposition of silicon, SiO2, or Si3N4; AlMe3, GeMe3, and InMe3 are used to prepare the respective metal films. In combination with group 15 alkyls and hydrides (e.g., PH3, AsH3, PMe3), these same group 13 alkyls are used to prepare 13–15 semiconductors such as GaP, InP, and GaAs. Group 12 alkyls and group 16 alkyls are used to make 12–16 semiconductors (e.g., CdMe2 with TeEt2 will produce CdTe). One of the advantages of using a photodeposition method to prepare these materials is that materials with various compositions can be prepared by varying the starting materials and by varying the amounts present. For example, ternary materials such as AlGaAs are produced by adding a third material to the starting mixture of gases. Alloys are produced in a similar fashion.
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Figure 3. Deposition of a thin film by irradiation (top) perpendicular to a surface; (bottom) parallel to a surface.
In addition to forming thin films of metals or semiconductor materials on the surface of a substrate, photolytic degradation of organometallics can also be used to form metal particles within a transparent substrate (15). For example, one technique for doing surfaceenhanced Raman spectroscopy (SERS) is to adsorb molecules on silver or gold particles (colloids) contained within silica gel or a xerogel. Standard techniques for depositing the gold or silver within the gel generally include some heating, which has numerous disadvantages. Recently, however, it was demonstrated that UV irradiation of Au(CH3)2(tfac) or Au(CH3)2(hfac) dissolved in the gel materials led to gold clusters within the gel and that these materials were suitable for SERS.
In this case, the advantages of the photochemical deposition process over the thermal methods include the ability to pattern the deposit and to control the gold particle size by controlling the photolysis time. In summary of this section, for each reaction discussed above, irradiation is used to photodissociate all the ligands from metal complexes using d–d, LMCT, or Rydberg excited states with the subsequent deposition of a metal film, colloid, or particle. Applications of Organometallic Photochemistry to Polymers Organometallic molecules have been used as photoinitiators in cationic and radical (chain) polymerization reactions. Examples of each type abound, but only selected examples will be discussed here because the fundamental principle is the same in each case: irradiation is used to generate an intermediate species (a cation or radical species) that can initiate a polymerization reaction. The complexes used to initiate cationic polymerization have photolabile ligands (16, 17). Among the most commonly studied initiators are those that have photodissociable arene ligands. An example is the CpFe(C6H6)+ complex. This molecule and related ones have relatively low energy ligand field absorption bands. Irradiation into
Journal of Chemical Education • Vol. 74 No. 6 June 1997
Symposium: Applications of Inorganic Photochemistry these bands leads to loss of the arene ligand (eq 8, via a mechanism in which the first step is reaction 2). The coordinatively unsaturated species thus produced acts as a Lewis acid to bond monomers. Following coordination of a monomer, polymerization occurs by the (generally accepted) mechanism in eq 10.
CCl3 radical thus produced can then react with a vinyl monomer to initiate the polymerization reaction. ? Re(CO)5 + CCl 4 → Re(CO)5Cl + ?CCl3
(14)
Heterolysis of an M–L Bond Somewhat surprisingly, even M–L heterolyses can lead to initiation of radical chain reactions. In these reactions, the key feature is again the formation of CCl3 radicals (eq 15). hν
(8)
S = solvent or monomer
(9)
(10) Note that only a relatively small number of alkenes and heterocycles undergo cationic polymerization. For example, epoxides will generally polymerize with cationic initiation and alkenes with electron donating groups will also polymerize. It is also noteworthy that the arene ligand in CpFe(C6H 6)+ can be varied and derivatized so as to change the absorption spectrum. This allows one to control the wavelength of light used in the photochemical polymerization process and also the extinction coefficient of the absorbing band. In principle, any organometallic molecule that loses a dative ligand in a photochemical heterolysis reaction is a potential catalyst for this type of polymerization. Other common catalysts are (η 5-C6H 7)Fe(CO)3 + and CpFe(CO) 2L+ (L = various phosphines or other ligands). The former molecule, in particular, is exceptionally reactive. When these molecules are irradiated, an M–CO bond is dissociated; the coordinatively unsaturated species thus produced reacts according to the polymerization scheme in eqs 9 and 10. Molecules that produce radicals upon excitation are capable of initiating radical chain polymerization reactions of vinyl monomers (18). Organometallics produce radicals in numerous ways, and several of these methods are outlined in the following reactions.
CCl4
W(CO)6 → W(CO)5 → ? CCl3 + [W(CO)5Cl] (15) – CO
In addition to initiating polymerization reactions, organometallic molecules can play a key role in the degradation of polymers. Photodegradable polymers have numerous real and potential applications, including uses as degradable plastics, photoresists, biomedical materials, and precursors for ceramic materials. Of these, the largest application is as degradable plastics for environmental and agricultural applications. The environmental uses of photodegradable plastics fall into two categories: (i) bulk reduction of solid waste, and (ii) litter control. In agriculture, these materials find use primarily as mulches. Although most polymers will photodegrade slowly over time, the process is too slow and the rate too unpredictable to use unmodified polymers for applications where photodegradable plastics are necessary. Thus it is necessary to make plastics photodegradable by modifying their composition. This is accomplished by one of two methods (19). The first is to add light-activated radical initiators to the plastic. Examples of light-activated radical initiators include metal oxides (e.g., TiO2, ZnO), metal chlorides (e.g., FeCl3), metal acetylacetonate complexes, metal stearates, and organic radical initiators such as peroxides and benzophenones. The second method is to incorporate a light-absorbing chromophore into the polymer backbone. Common chromophores include organic functionalities such as the carbonyl group and nitroaromatic groups, but polymeric materials having organometallic metal–metal bonds along the backbone have been reported recently (20). This leads to reactivity because metal–metal bonds are cleaved homolytically with visible light (e.g., eq 5). Polymers with metal– metal bonds along the backbone thus react as follows:
Homolysis of an M–C Bond hν
Mn(CO)5CH3 → ?Mn(CO)5 + ?CH3 hν
MR4 → ? MR3 + ? R
(11) (12)
M = Ti, Zr; R = neopentyl
The photochemistry above has been demonstrated with a variety of polymers including polyurethanes, polyureas, vinyl polymers, and polyamides. A sample polymer is shown below:
In each case, it is likely that an LMCT (σligand → M) transition is responsible for the M–C bond homolysis.
Homolysis of an M–M Bond hν
Re2(CO)10 → 2 ?Re(CO)5
(13)
In the reaction shown in eq 13 above, a metal radical is produced by homolysis of the metal–metal bond. A typical behavior of metal radicals is halogen atom abstraction from an alkyl halide, such as CCl4 (eq 14). The
In the solid state, irradiation of thin films of these materials led to polymer fragmentation as a result of metal–metal bond cleavage. Note that it is necessary to capture the metal radicals produced in these photolysis reactions or the metal–metal bonds will simply recombine and no net reaction will occur. The oxygen molecule is an efficient scavenger of metal radicals, and in the
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Symposium: Applications of Inorganic Photochemistry examples shown here, the metal radicals formed by photolysis were captured with ambient oxygen. (Also note, the polymers were synthesized with either Cp2Mo2(CO)6 units or Cp2Fe2(CO)4 units. For environmental purposes, the Fe-containing polymers are more benign [rust would be the final metal-containing product]. In addition, iron is cheaper than Mo.) Related metal–metal bond photolyses were also utilized in a clever photoimaging system designed by Meyer, Sullivan, and O’Toole (21). In this system, illustrated below, dark-green films of poly-[(vbpy)Re(CO)3] 2 were prepared on electrode surfaces by reductive electropolymerization of [(vbpy)Re(CO)3Cl]. Irradiation of this material in the presence of CCl4 formed yellow poly[(vinylbipyridine)Re(CO)3Cl] by photolysis of the Re–Re bond followed by metal–radical capture with CCl4. The green to yellow image can be reversed in an electrochemical reduction step that reforms the metal–metal bonded dimer. Methods were described for permanently developing the image by reacting the polymer/electrode material with organic oxidants.
Applications of Organometallic Photochemistry to Surface Derivatization Photoinduced metal–ligand bond dissociation can be used as a way to derivatize surfaces. To exploit M–L dissociation for this purpose, a metal complex is attached to a surface and then irradiated. Loss of a ligand produces a vacant coordination site that can be filled by a variety of appropriately tailored ligands that lend a desired property to the surface. For example, in one study (22), Wrighton and coworkers used CpMn(CO) 3 attached to SiO2, Si, or Au surfaces as a means to put electroactive ferrocene on a surface. Attachment was accomplished by derivatizing the Cp ligand appropriately. In the case of the Au surface, the Mn complex was attached by reacting [HS(CH2)11C5H4]Mn(CO)3 with the surface. Irradiation of the attached complex in the presence of a phosphine led to phosphine substitution for a photodissociated CO:
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What makes these results particularly interesting is that the surface bound CpMn(CO)3 species are not thermally reactive; the phosphine substitutions only occur photochemically. This allowed Wrighton to pattern the surface by selectively irradiating portions of the surface in the presence of a ligand. He showed that substitution by the PPh2(CH2)11Fc ligand (Fc = ferrocenyl) yielded redox-active ferrocene on the surface. These redox-active patterned surfaces should be useful in tailoring microfabricated structures and circuits. Acknowledgment The support of my work described herein by the National Science Foundation and the Donors of the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged. Literature Cited 1. Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic: New York, 1979; Geoffroy, G. L. J. Chem. Educ. 1983, 60, 872. 2. Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes; Plenum: New York, 1994. 3. Ferraudi, G. J. Elements of Inorganic Photochemistry; Wiley-Interscience: New York, 1988. 4. Horvath, O.; Stevenson, K. L. Charge Transfer Photochemistry of Coordination Compounds; VCH: New York, 1993. 5. Armentrout, P. B.; Simons, J. J. Am. Chem. Soc. 1992, 114, 8627–8633. 6. Male, J. L.; Davis, H. B.; Pomeroy, R. K.; Tyler, D. R. J. Am. Chem. Soc. 1994, 116, 9353–9354. 7. Gill, T. P.; Mann, K. R. Inorg. Chem. 1980, 19, 3007. 8. Chrisope, D. R.; Park, K. M.; Schuster, G. B. J. Am. Chem. Soc. 1989, 111, 6195. 9. Mann, K. R.; Gray, H. B.; Hammond, G. S. J. Am. Chem. Soc. 1977, 99, 306–307. 10. Geiger, D. K.; Ferraudi, G. Inorg. Chim. Acta 1985, 101, 197. 11. Geoffroy, G. L.; Bradley, M. G. Inorg. Chem. 1978, 17, 2410. 12. Meyer, T. J.; Caspar, J. V. Chem. Rev. 1985, 85, 187–218. 13. Sato, H. Appl. Organometal. Chem. 1989, 3, 363–382. 14. Herman, I. P. Chem. Rev. 1989, 89, 1323–1357. 15. Akbarian, F.; Dunn, B. S.; Zink, J. I. J. Phys. Chem. 1995, 99, 3892–3894. 16. Hendrickson, W. A.; Palazotto, M. C. In Photosensitive Metal-Organic Systems; Kutel, C.; Serpone, N., Eds.; American Chemical Society: Washington, DC, 1993; Chapter 21. 17. Klingert, B.; Riediker, M.; Roloff, A. Comments Inorg. Chem. 1988, 7, 109–138. 18. Curtis, H.; Irving, E.; Johnson, B. F. G. Chem. Br. 1986, 327. 19. Rabek, J. F. Photostabilization of Polymers; Elsevier: New York, 1990. 20. Tenhaeff, S. C.; Tyler, D. R. Organometallics 1992, 11, 1466–1473. 21. O’Toole, T. R.; Sullivan, B. P.; Meyer, T. J. J. Am. Chem. Soc. 1989, 111, 5699–5706. 22. Kang, D.; Wollman, E. W.; Wrighton, M. S. In Photosensitive Metal-Organic Systems; Kutel, C.; Serpone, N., Eds.; American Chemical Society: Washington, DC, 1993; Chapter 3.
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