bk-2003-0847.ch029

Chapter 29

Downloaded by GEORGETOWN UNIV on August 29, 2015 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch029

Harvesting the Fields of Inorganic and Organometallic Photochemistry for New Photoinitiators 1

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C h a r l e s Kutal , Y o s h i k a z u Yamaguchi , W e i Ding , C y n t h i a Τ. Sanderson , X i n y o n g Li , G a r y G a m b l e , a n d I. J o n a t h a n Amster 1

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Department of Chemistry, University of Georgia, Field Street, Athens, G A 30602 JSR Corporation, Tsukuba Research Laboratory, 25 Miyukigaoka, Tsukuba, Ibaraki 305-0841, Japan

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Several iron(II) metallocenes are effective photoinitiators for ionic polymerization reactions. Photoexcitation of ferrocene and 1,1'-dibenzoylferrocenes in solutions of ethyl αcyanoacrylate produces anionic species that initiate polymerization of the electrophilic monomer. Irradiation of [CpFe(η -arene)] (Cp is η -C H ) in epoxide-containing media generates several cationic species capable of initiating ring-opening polymerization. The diversity of photoinitiation mechanisms exhibited by these iron(II) metallocenes is discussed in terms of their electronic structures. 6

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I. Introduction Photoinitiated polymerization is a process that employs light and a photosensitive compound, termed a photoinitiator, to initiate the production of a polymeric substance from a lower molecular-mass precursor (monomer, oligomer, crosslinkable polymer). While details of the mechanism of photoinitiation will vary from system to system, all processes of this type can be

332

© 2003 American Chemical Society

In Photoinitiated Polymerization; Belfield, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


333 dissected into two essential steps - one photochemical and the other thermal. In the photochemical step, the absorption of a photon by the photoinitiator, PI, produces one or more reactive species, IN (eq. 1). In some systems, this transformation requires the participation of a second component termed a coinitiator. Following its formation, IN undergoes a thermal reaction with the precursor that initiates the polymerization process (eq. 2). For the majority of reported photoinitiators, IN is a radical or a strong acid (1-4). Common radicalgenerating photoinitiators include benzoin and its derivatives, benzil ketals, substituted acetophenones, aromatic ketone/amine combinations, acylphosphine oxides, dye-borate systems, and fluorinated titanocene derivatives. Popular acidgenerating photoinitiators include 2-nitrobenzyl esters, triazines, cationic iron(II)-sandwich complexes, and onium salts belonging to the diaryliodonium and triarylsulfonium families. PI IN + monomer

(1)

IN

(2)

polymer

Inspection of the above list reveals that most of the common photoinitiators are organic compounds. The small number of inorganic and organometallic entries is rather surprising, because compounds belonging to these families undergo a fascinating array of photochemical reactions, many of which yield reactive species capable of initiating useful chemistry (2,5,6). Beginning in 1986, we embarked upon a program aimed at expanding the selection of inorganic and organometallic photoinitiators (7). Our primary goals have been to discover new photoinitiators that equal or exceed the performance of their purely organic counterparts, to elucidate the detailed mechanisms by which photoinitiation occurs in these new systems, and to exploit this chemistry in the design of novel photosensitive materials. This chapter reviews our recent studies of four closely related types of iron(II)-containing metallocene photoinitiators (Figure 1). We begin with a brief discussion of the structures and reactivities of the electronic excited states present in these compounds. We then consider the roles played by the metallocenes in several examples of photoinitiated anionic and cationic

A

Β

C

D

Figure 1. Structures ofseveral types of iron(II) metallocenes.


334 polymerizations. A particularly intriguing aspect of our results is the change in the mechanism of photoinitiation that follows from variations in metallocene structure.


II. Electronic Excited States of Iron(II) Metallocenes We shall follow the conventional practice (2,8) of classifying an electronic excited state in terms of its dominant molecular orbital configuration. Transitions between states are then conveniently labeled according to the orbitals that undergo a change in electron occupancy. Ligand field excited states result from transitions between valence d orbitals primarily localized on the metal. Typically, such a transition involves the promotion of an electron from a d orbital that undergoes a π-bonding (or antibonding) interaction with the attachedringsto a higher lying d orbital that is strongly σ-antibonding with respect to the metal-ring bonds. This angular redistribution of electron density does not alter the formal oxidation state of the metal, but it does weaken the metal-ring bonding and thereby enhances the likelihood ofringloss. Charge transfer excited states arise from the radial redistribution of electron density between the components (metal andrings)of the metallocene or between the metallocene and the surrounding medium. Transfer of an electron from a ligand-centered orbital to a metal d orbital generates a ligand-to-metal charge transfer (LMCT) excited state. Electron flow in the opposite direction produces a metal-to-ligand charge transfer (MLCT) excited state. A transition that results in the movement of electron density from the metallocene to the surrounding solvent gives rise to a charge-transfer-to-solvent (CTTS) excited state. Each of these transitions occurs with a change in the charge and/or formal oxidation state of the species involved (metal, ligand, solvent). Consequently, charge transfer excited states are susceptible to oxidation-reduction reactions and, in cases where the redox process creates a substitutional^ labile metal center, to accompanying ligand dissociation.

III. Photoinitiated Anionic Polymerization A. Ferrocene 5

Ferrocene (FeCp , where Cp denotes r) -C H ; see structure A in Figure 1) is the original and most celebrated metallocene. The compound is photoinert in solvents such as methanol, acetone, and cyclohexane (9). When dissolved in a strong electron-accepting medium such as carbon tetrachloride, however, FeCp2 2

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forms a ground-state donor-acceptor complex with the solvent that gives rise to a charge-transfer-to-solvent absorption band in the near ultraviolet region. Irradiation into this band induces a redox reaction that yields the ferricenium cation, FeCp , and the unstable CC1 radical anion, which rapidly dissociates to the CC1 radical and chloride ion (eq. 3) (JO). This type of chemistry has been used as a source of radicals for the photoinitiated polymerization of vinyl monomers (11). +

2

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3

FeCp + C C l 2

4

^

» [FeCp ,Ca ] 2

h

v

4

» FeCp + + C d /

(3)

2


complex We have discovered that photooxidation of FeCp also occurs in neat ethyl α-cyanoacrylate (CA) to produce an active initiating species for anionic polymerization of this electrophilic monomer (12). Figure 2 (spectrum a) displays the room-temperature electronic absorption spectrum of the metallocene in tetrahydrofuran (THF). The weak bands above 300 nm correspond to spin-allowed ligand field transitions predominantly localized on the d metal (13,14). Switching the solvent to a 2:3 (v:v) mixture of CA and THF results in the appearance of a band at 355 nm (spectrum b and inset), which is indicative of donor-acceptor complex formation between FeCp and the monomer. Given the ease of oxidizing FeCp and the strong electronaccepting character of CA, we assign this new feature as a CTTS (Cp Fe-*CA) transition of the complex. Solutions of CA containing millimolar concentrations of FeCp undergo no discernible change in viscosity for at least 24 h when stored in the dark at room temperature. Upon exposure to light, however, these solutions polymerize to a hard, plastic-like solid. Neither molecular oxygen nor hydroquinone, known radical scavengers, influences the rate of photoinitiated polymerization. In contrast, methanesulfonic acid exerts a strong inhibiting effect on this process. Thesefindingsindicate that polymerization proceeds by an anionic rather than a radical mechanism, with protons serving as an inhibitor by scavenging the photogenerated initiating species and/or reactive anionic sites on growing polymer chains. We propose that photoinduced charge transfer within the metallocene-CA complex leads to the species responsible for initiating CA polymerization. By analogy to the behavior described in eq. 3, irradiation into the CTTS absorption band of this complex (Figures 2) should produce the ferricenium cation and CA radical anion (eq. 4). Attack of the latter species on the monomer then initiates polymerization (eq. 5). Consistent with this charge transfer mechanism, we find that irradiating FeCp in neat CA results in the appearance of the characteristic 617-nm absorption band of FeCp (12). 2

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+

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FeCp + CA

[FeCp ,CA] -2^— FeCp

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2

+ 2

+ CA*

(4)

complex r

CA + nCA — - poly-CA


(5)


Figure 2. Electronic absorption spectrum of 25 mMferrocene: (a) in pure THF; (b) in a 2:3 (v:v) mixture of CA and THF. Inset shows the difference spectrum of (a) and (b).



337 Real-time monitoring of photoinitiated polymerization in CA samples containing FeCp2 was accomplished with rapid scan Fourier transform infrared spectroscopy. Representative data in Figure 3 reveal that the reaction (plot b) exhibits an induction period, then accelerates rapidly, and finally approaches a plateau at -90% conversion. We attribute the induction period to the presence in the commercial monomer of 5-10 ppm of strong acid, which serves as a scavenger for adventitious traces of basic impurities. Polymerization is inhibited until sufficient anionic species are photochemically generated to consume this acid, whereupon rapid consumption of monomer commences. As expected, the addition of extra acid to a sample lengthens the induction period and slows the ensuing polymerization process (plot c). Precedent exists for the involvement of a photosensitive donor-acceptor complex in the generation of an initiator for anionic polymerization (15). In this earlier work, the solvent served as the donor and was present in considerable molar excess over the acceptor monomer. In contrast, our results demonstrate that very low (millimolar) concentrations of FeCp are sufficient for the photoinitiated polymerization of neat CA. The latter type of system is particularly attractive for the increasing number of applications in the coatings, adhesives, and reprographic industries that require solvent-free photosensitive formulations. 2

B. Benzoyl-Substituted Ferrocenes Dramatic changes in spectral and photochemical properties result from substituting a benzoyl group for a hydrogen atom on each cyclopentadienyl ring of ferrocene (structure Β in Figure 1) (16). As seen in Figure 4, higher intensity bands that extend farther out into the visible region replace the weak ligand field bands appearing in the electronic absorption spectrum of the parent compound. These intense bands arise from transitions to excited states containing appreciable metal-to-ligand charge transfer character. We can represent this MLCT contribution by a resonance structure of the type shown in Figure 5, where the formal charges on iron and oxygen signify a shift of electron density from the metal to the ligand. Conjugation between the π orbitals of the cyclopentadienyl ring and the adjacent carbonyl group allows the transferred charge to be spread over several atoms. Charge derealization stabilizes the resulting excited states and lowers transition energies (17). Moreover, the mixing of charge transfer character into the transitions relaxes the Laporte selection rule and increases band intensities.



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100

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2400

Time (sec)

Figure 3. Plots of percentage polymerization vs. time for samples of CA containing 10 mMferrocene: (a) unirradiated, (b) irradiated, no acid added, (c) irradiated, 150 ppm of methanesulfonic acid added. Samples (b) and (c) were irradiated with 110 mW/cm o polychromatic lightfrom a 200- W high-pressure mercury lamp. 2



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Wavelength (nnï)

Figure 4. Electronic absorption spectra in room-temperature methanol: (a) ferrocene, (b) 1,1 '-bis(o-chlorobenzoyl)ferrocene, (c) 1,1'dibenzoylferrocene.

Figure 5. Resonance structure representing the charge transfer charact that resultsfromconjugation between the cyclopentadienide ring and the carbonyl group of electronically-excited 1,1'dibenzoylferrocene. The oxidation state of the metal is indicated by a Roman numeral, while formal charges on atoms are circled. While ferrocene is photoinert in methanol, Ι,Γ-dibenzoylferrocene readily undergoes heterolytic metal-ring bond cleavage in this solvent to yield the benzoyl-substituted cyclopentadienide ion and the corresponding half-sandwich cationic complex (eq. 6; S is solvent) (16). This change in photochemical behavior reflects the MLCT character of the low-energy excited states of the benzoyl-containing derivative. Referring again to the resonance structure in Figure 5, we note that these states possess reduced hapticity (η -»η ) of a 5


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cyclopentadienyl ring and enhanced susceptibility of the metal center to nucleophilic attack. The first factor weakensring-metalbonding, while the second assists the formation of bonds to incoming ligands. Collectively, the two factors facilitate the substitution of a cyclopentadienide ligand by the surrounding solvent. Disappearance quantum yield data summarized in Table I reveal that Ι,Γ-dibenzoylferrocene and several analogues containing substituents on the phenyl ring undergo this process very efficiently. Interestingly, the substituents exert relatively little influence on the quantum efficiency.

O F e ^ ^ ^ l T ô ^ ^ ^ 1.7 mm, we observed two main series: [(H 0)Fe(CHO) _i ] and [X(CHO) .s] (Figure 8). A third series, [(H 0)CpFe(CHO)i^] (inset to Figure 8), appeared with D < 0.5 mm. This behavior indicates that [(H 0)CpFe(CHO) ] possesses a short lifetime (< 50 ms) in solution and only can be observed when generated near the tip end. The water present in the Fe-containing products most likely originated from traces of moisture introduced to the rigorously dried solvent 3

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bk-2003-0847.ch029

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