Laser Techniques in the Study

Chapter 8

Laser Techniques in the Study

of the Photochemistry of Carbonyl Compounds

Containing Lignin like Moieties

A. B. Berinstain, M. K. Whittlesey, and J. C. Scaiano

Department of Chemistry, Ottawa—Carleton Chemistry Institute, University of Ottawa, Ottawa KIN 6N5, Canada

The photochemistry and photophysics of some lignin-related compounds has been studied. The triplet state of these compounds is relatively low at 60 kcal/mol, and is π- π * in nature. The π - π * nature is due to methoxy substitution on the chromophore and results in relatively long triplet lifetimes and low reactivity, compared to π - π * triplets like that of benzophenone. The a-phenoxy group of α-guaiacoxyacetoveratrone provides this lignin model compound with a β-phenyl ring for triplet deactivation as well as a source of a phenoxy radical after cleavage. The intersystem crossing quantum yield is significantly less than unity which is consistent with evidence for cleavage from the singlet state.

Thermomechanical pulps (TMP) and chemithermomechanical pulps (CTMP) are mainly used today for manufacturing of newsprint, catalog, and directory papers and have the advantage of being produced in high yield from wood. This method of production involves relatively little treatment by harmful chemicals and has become a popular choice as a source of pulp for paper products. TMP and CTMP still contain most of the lignin that was present in the original wood and for this reason they suffer from the limitation that photo-induced yellowing of the products made from these pulps occurs (1). Chemical pulps have most of their lignin removed and, at the cost of wastage of wood and harsh chemical treatment, are more widely produced for long-term usage due to their photostability and higher strength.

It is believed that yellowing occurs predominantly through oxidation of phenoxy radicals supplied by the phenolic hydroxy groups of lignin, although yellowing still occurs to a certain extent with a fully alkylated lignin molecule (2, 3). Hence there must be another source of yellowing in addition to the phenolic hydroxy groups in the natural product. It has been shown that upon irradiation, phenacyl aryl

ether moieties in the lignin molecule are also an efficient source of phenoxy radicals (4). Studies by Castellan et al. have suggested that α-guaiacoxyacetoveratrone (I) represents a suitable lignin model compound with which to model these phenacyl aryl ether moieties of lignin. (5, 6, 7) While this model compound does not have all the features of lignin, it does contain a carbonyl group, is heavily substituted with methoxy groups, and it contains a guaiacoxy moiety, which upon photodegradation, will produce the corresponding phenoxy radical.

This article presents an account of laser photolysis studies on α-guaiacoxyacetoveratrone (I) and acetoveratrone (II). We summarize our own results and try to place mem in the context of our understanding of other substrates which have structural features in common with I and IL We conclude that the products from I arise largely from the singlet manifold and that the decay of its triplet state is dominated by intramolecular interactions involving the guaiacoxy group. Finally, we comment briefly on other, largely heterogeneous systems, where the photochemistry of I may eventually enlighten our understanding of related processes in paper products.

Phosphorescence Studies.

For most carbonyl compounds, we expect to have two near-lying excited states in the triplet manifold, which are either π- π * or π - π * in character. The π - π * states frequently show radical-like behavior. Benzophenone is an example of such a molecule which has an π - π * triplet state in which we see occurrences of hydrogen abstraction and very efficient intersystem crossing. When the lowest state is the π - π * state, largely centered on the aromatic part of the molecule, as in the case of p-methoxyacetophenone, the reactivity decreases significantly (8, 9). With the nature of lignin and the nature of the model we have chosen, we are mostly interested in molecules which have this type of behavior.

The low temperature phosphorescence spectrum of benzophenone (see Figure 1) has a well-resolved structure, in which the splitting corresponds to the carbonyl vibrational frequencies. From this structure, one can determine that the triplet energy of benzophenone is approximately 69 kcal/mol. (10)

By comparison, for triplet states which are π - π * in nature, this type of structural resolution is absent from the phosphorescence spectra (Figure 1). Hence for the types of molecules in this study, the exact triplet energy becomes more difficult to determine. The spectra of these molecules are shifted to longer wavelengths and the triplet energy can be estimated at about 60 kcal/mol.

Excited States.

Upon absorption of light by the compounds used in this study, an excited singlet state is formed. Rapid intersystem crossing then takes place into the π-π* triplet manifold, at about 60-63 kcal/mol. Usually carbonyl compounds will go through chemistry strictly from the triplet state, although in the case of molecules such as I, reaction is also observed from the excited singlet state. In any case, the reactions involve free radicals and we expect that the products formed from the triplet reaction will arise from random encounters by the free radicals. In the case of the singlet, random radical-radical reactions can occur between radicals that have escaped the primary solvent cage, but geminate processes arising from radical pair reactions within the solvent cage are also expected since they are formed with the appropriate electronic spin to directly lead to products.

Figure 1: Phosphorescence spectra of benzophenone, a-guaiacoxyacetoveratrone (I), and acetoveratrone (II) in ethanol glass at 77K.

Acetoveratrone Photochemistry.

All of the molecules in this study have triplet states which are easily detectable by the technique of nanosecond transmission laser flash photolysis. (11) The triplet state of acetoveratrone has a lifetime in excess of 15 μs in ethanol (Figure 2); under conditions of laser excitation the decay involves a mixture of first and second order kinetics, with the latter dominating at high laser powers. This second order decay demonstrates that the triplet state is decaying at least partly by triplet-triplet annihilation.

By contrast, the triplet lifetime of benzophenone in ethanol is < 100 ns and decays via hydrogen abstraction (12). Molecules with p-methoxy substitution do not abstract hydrogen readily, due to the π - π * nature of their low-lying triplet state. The long triplet lifetime of acetoveratrone reflects the unreactivity of this molecule towards

ethanol in this case. Similar results were obtained in hydrocarbon solvents such as cyclohexane.

Figure 2: Decay trace of acetoveratrone in ethanol at room temperature monitored at 400 nm after 308 nm laser excitation.

Although acetoveratrone does not react with ethanol, it will readily abstract a hydrogen atom from phenols (Figure 3). This reaction may be important since a large number of phenoxy groups are present in lignin. In the presence of phenol, the acetoveratrone triplet lifetime is shortened The reaction occurs with a rate constant of 3.0 x 108 M-1 s-1 in acetonitrile and produces a transient absorption spectrum which can be assigned to the phenoxy radical (λmax ~400 nm) and the ketyl radical of acetoveratrone (λmax ~380 nm).

Figure 3: Transient absorption spectra produced from reaction of triplet acetoveratrone with phenol. Spectra recorded 220 ns (A), 700 ns (B), 1.3 μs (C) and 2.3 μs (D) after 308 nm laser excitation.

The transient spectrum of the phenoxy radical can be readily generated by reaction of phenol with tert-butoxy radicals, formed by the photodecomposition of di-terr-butyl peroxide. This is a very efficient reaction which takes place with a quantum yield of approximately 0.7-0.9 (13, 14). The transient spectra of the phenoxy and guaiacoxy radicals are shown in Figure 4.

Figure 4: Transient absorption spectra of independently-generated phenoxy and guaiacoxy radicals, after 308 nm laser excitation of guaiacol (A) and phenol (B) in acetonitrile-di-tert-butyl peroxide, monitored at λ < 460 nm.

The effect of β-phenyl Rings.

α-Guaiacoxyacetoveratrone not only contains the same chromophore as the one present in acetoveratrone, but it also contains an aromatic ring in the beta position. β-phenyl rings are known to greatly affect the photochemistry of these compounds. For example, acetophenone has a triplet lifetime of ~2 μs in benzene solution; the triplet lifetime is determined by interactions with the solvent.

β-Phenylpropiophenone which now incorporates a β-phenyl ring, has a triplet lifetime 3 orders of magnitude shorter, Figure 5 {15). Much work has been done on β-phenylpropiophenones and what is known is that a charge transfer interaction will occur between the β-phenyl ring and the carbonyl group, and the β position for the phenyl ring is critical (15, 16). If the ring is in the alpha position the molecules undergo fragmentation, while in the gamma position, the phenyl ring is too far away to induce intramolecular quenching, and this structure leads to a Norrish Type II reaction (17).

Adding a methoxy group to acetophenone (III) to form p-methoxyacetophenone (V) lengthens the triplet lifetime with respect to acetophenone since the methoxy group changes the lowest lying triplet state from π-π* to a less reactive π-π* state. Adding a p-phenyl ring to give p-methoxy-p-phenylpropiophenone (VI) again shortens the lifetime of the triplet (Figure 5) although it is enhanced with respect to the p-phenylpropiophenone (IV) with no methoxy substituent(16). This is true because the mechanism of quenching of the p-phenyl ring probably prefers an π-π* state (15).

Figure 5: Effect of ß-phenyl rings and methoxy substituents on triplet lifetimes.

The presence of an Oxygen Atom in the ß Position.

The molecular conformation involved in ß-phenyl quenching in ß-phenylpropiophenones requires eclipsing the methylene hydrogens on the alpha and beta carbons which is energetically unfavorable (see Figure 6). When the ß carbon is

replaced with an oxygen atom, these methylene hydrogens are no longer present and the quenching conformation lies at lower energy. The consequence of this is that the α-phenoxyacetophenones have shorter triplet lifetimes than their p-phenylpropiophenone analogs (18).

Figure 6: Molecular conformation leading to triplet deactivation via β -phenyl quenching.

The quantum yield of decomposition (Φr) for the β-phenylpropiophenone (IV) is essentially zero, but α-phenoxyacetophenone (VII) shows Φr, ~ 0.01 (18). This quantum yield is not large, but in terms of the decomposition, it is an important reaction. The molecule goes through β -cleavage to produce the phenoxy radical (18) in good chemical yields, as shown in Figure 7.

Figure 7: Effect of oxygen atom in β position and methoxy substituents on triplet lifetimes and degradation quantum yields.

As in the case above, adding a methoxy group increases the triplet lifetime since the molecule goes from an π-π* state to a π-π* state. Φr also increases slightly

which perhaps suggests that the inhibiting effect of π,π* states on the fragmentation reaction is less severe than on intramolecular quenching. Phcnoxy radicals are stabilized by methoxy substitution (19). Comparing (VII) and (I X) in Figure 7 shows that the triplet lifetime is not greatly affected, but Φr quadruples. In this case, we now produce a more stabilized phenoxy radical. Therefore it can be observed that the presence of an oxygen atom in the β position induces fragmentation.

The triplet-triplet absorption spectra for the molecules shown above are very similar to that recorded for acetoveratrone and related lignin models, in spite of the fact that there are significant differences (almost 10 kcal/mol) in the triplet energies.

Laser Power Effects.

There have been reports in the literature that the lignin model, α-guaiacoxyacetoveratrone, shows wavelength dependent photochemistry (5). However, we have found that these observations arise from the fact that as one changes laser wavelength, one normally changes other experimental parameters. For example, the laser power, laser pulse duration, and substrate concentration are normally changed. Changes in concentration are required to maintain a reasonable absorption at the excitation wavelength. While we reproduced without difficulty the reported wavelength effects, we find that when ground state concentrations and laser energy are kept constant in the different experiments, the rate of decay of the signal due to the transient is independent of the laser wavelength. Using 100% of our laser power at 308 nm (~ 30mJ/pulse) appears to induce multiphoton chemistry, as we observe some as yet unidentified transients which are very long lived and very easy to misinterpret as triplets of the model compound. This is not believed to be relevant to the lignin modeling system, since it it unlikely that we are dealing with multiphoton processes in the photoreversion of paper. All the transient results reported in this paper are from experiments carried out under flow conditions and in which the laser power and substrate concentration were carefully controlled

Triplet Quenching Studies.

The triplet state of ct-guaiacoxyacetoveratrone is quenched by sorbic acid with a second order rate constant of 2.8 x 109 M-1 r-1. Sorbic acid is a conjugated diene and is an excellent quencher of the triplet state through energy transfer. It is a very versatile quencher since it has good solubility in a wide range of solvents including alcohols and water. The quenching rate constant quoted above is a bit slower than most triplet-diene quenching constants; this may reflect the fact that the donor triplet has an unusually low triplet energy for a carbonyl compound.

In the absence of quencher, the guaiacoxy group reduces the lifetime of the triplet state from > 15 μs in acetoveratrone to ~500 ns in the case of a-guaiacoxyacetoveratrone; thus, at least 97% of the triplets must decay by processes (deactivation or cleavage) that involve the guaiacoxy group.

In the presence of 0.016 M sorbic acid, and given the rates of quenching and lifetimes measured, this concentration of diene will quench 96% of the triplets. This implies that the decay of the triplet occurs almost exclusively through energy transfer. (See Figure 8) The surprising result is that in the presence of this large concentration of sorbic acid, the transient absorption spectrum of the guaiacoxy radical with its characteristic band in the red part of the spectrum is observed. This demonstrates that the guaiacoxy radical must be produced from the singlet state, since virtually all the triplets have been quenched by the diene.

Figure 8: Transient absorption spectra; left: α-guaiacoxyacetoveratrone in ethanol in the presence of 0.016M sorbic acid 0.2 μs after 337 nm excitation; right: reaction of ferr-butoxy radicals generated by 308 nm excitation, with guaiacol in acetonitrile leading to guaiacoxy radical; recorded 450 ns after laser excitation.

In the presence of moderate amounts of sorbic acid (e.g., 0.0015 M), the triplet lifetime of I is shortened by can be readily time-resolved. Under these conditions we find that the characteristic guaiacoxy radical long-wavelength band is mostly present immediately following the laser pulse as opposed to growing in as the triplet decays.

In order to confirm the identity of the guaiacoxy radical, we also generated this species by reaction of rm-butoxy radicals with the phenol, as indicated above (see Figure 4). These experiments also yield a band in the 600 nm region (Figure 8), therefore supporting the formation of guaiacoxy radicals in a singlet process from I. Small differences between the two spectra of Figure 8 are presumably due to the presence of other carbon centered radicals produced in the cleavage of α-guaiacoxyacetoveratrone and to differences in the solvent employed.

Intersystem Crossing Quantum Yields.

Carbonyl compounds usually have intersystem crossing quantum yields, Φisc, of very close to unity. In the case of α-guaiacoxyacetoveratrone, we have shown that some of the chemistry is coming from the singlet state, which necessarily implies that the intersystem crossing quantum yield is no longer 1.

Photoacoustic spectroscopy was used to determine intersystem crossing quantum yields, Φisc, by a method previously described elsewhere (20). These values of Φisc show a solvent dependence and are 0.4 in ethanol and 0.6 in dioxane. This is consistent with singlet involvement in the chemistry of a-guaiacoxyacetoveratrone.

Experiments in Solid and Microheterogeneous Systems

Practical applications of the knowledge acquired with model compounds such as a-guaiacoxyacetoveratrone require the understanding of their photochemistry in microheterogeneous environments and in particular solid systems, in which the mobility of the substrates is limited. To this end we have started performing experiments exploring the photochemistry of a-guaiacoxyacetoveratrone incorporated in micelles, in zeolites, as well as in the pure crystalline material. In both, sodium dodecyl sulfate micelles and in the zeolite Na-X the triplet state of this model compound is very long lived (several microseconds). While a decrease in the efficiency of β-aryl quenching may reflect the reduced mobility, it is not clear why the fragmentation reaction does not take over as a dominant triplet decay path. In polycrystalline samples of α-guaiacoxyacetoveratrone all our attempts to detect the triplet state using time resolved diffuse reflectance techniques (21)were unsuccessful. In contrast, for the methoxy derivatives in Figure 5 detection of the triplet state under the same conditions was straightforward. The reasons for these differences are unclear at the present time.

While the results of the paragraph above raise many questions for which no answers are available, they serve to outline some of the challenges that lay ahead on the way to understanding, and hopefully solving, the problem of photoyellowing of pulp and paper products

Acknowledgements

This work has been generously supported by the Mechanical and Chemimechanical Wood-Pulps Network, which is part of Canada's Networks of Centres of Excellence.

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