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 radicallike 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 wellresolved 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 radicalradical 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-pphenylpropiophenone (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 αguaiacoxyacetoveratrone 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 aguaiacoxyacetoveratrone 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 aguaiacoxyacetoveratrone 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 βaryl 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 αguaiacoxyacetoveratrone 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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RECEIVED April 12, 1993