Chapter 9
Photodegradation of α-Guaiacoxyacetoveratrone Triplet-State Reactivity Induced by Protic Solvents Downloaded by UNIV ILLINOIS URBANA-CHAMPAIGN on October 12, 2016 | http://pubs.acs.org Publication Date: June 11, 1993 | doi: 10.1021/bk-1992-0531.ch009
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J. A. Schmidt , E. Goldszmidt , Cyril Heitner , J . C. Scaiano , A. B. Berinstain , and L. J. Johnston 2
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Pulp and Paper Research Institute of Canada, 570 Boulevard St. Jean, Pointe Claire, Quebec H9R 3J9, Canada Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada 2
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α-Guaiacoxyacetoveratrone, an aromatic ketone and lignin model compound, reacts exclusively from the excited singlet state in aprotic solvents dioxane and acetonitrile, with quantum yields < 0.1. Addition of a protic co-solvent (water, methanol or ethanol) at 5 mole percent and higher increases the quantum yield of photodegradation to > 0.2, and increases the triplet lifetime. Most of the enhanced yield can be quenched in the presence of 0.1 M sorbic acid, a triplet quencher. This behaviour, highly unusual for aromatic ketones, can be qualitatively understood as a consequence of hydrogen bonding on the rate ofβ-phenylquenching of the triplet state. Mechanical and ultra-high yield pulps are used primarily for low-value, short-life grades of paper such as newsprint and advertising, catalog, and directory papers. Their use in more valuable paper products is severely restricted by the rapid yellowing which occurs upon exposure to daylight, a phenomenon which is attributed to the photooxidation of lignin. α-Guaiacoxyacetoveratrone (I, SCHEME 1) is a model of one type of photolabile lignin structure: aryloxyacetophenones, which are C units with a carbonyl group adjacent to the aromaticring,and an O-aryl linkage at the αcarbon (throughout this paper we will use the IUPAC nomenclature, which labels atoms as α,β,γ,etc. beginning with the carbon adjacent to the ketone function). In the excited state, the carbon O-aryl bond in I is labile, and can cleave to give phenacyl (II) and phenoxyl (III) radicals. These radicals can recombine to form I, or to give lowmolecular weight degradation products (1,2). In addition, the phenoxyl radical can form an oligomeric material (1), which exhibits a strong and stable paramagnetism (Barclay, L.R.C., Mount Allison University, personal communication, 1991). In an earlier paper (3), we reported that the quantum yield of photodegradation and the triplet state lifetime of I are both significantly higher in a protic solvent. As well, we presented evidence that significant reaction can occurfromthefirstexcited singlet state of I, a phenomenon previously unobserved for aromatic ketones. Here 9
0097-6156/93/0531-0122$06.00/0 © 1993 American Chemical Society Heitner and Scaiano; Photochemistry of Lignocellulosic Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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9.
SCHMIDT ET A L .
Photodegradation of a-Guaiacoxyacetoveratrone
OMe
123
Scheme 1.
we present evidence that the presence of a protic solvent in the range of 5-40 mole percent induces reaction from the triplet state, which is unreactive in aprotic solvents. Experimental a-Bromoacetoveratrone was prepared using the procedure of Landucci et al. (4) and was then condensed with guaiacol in dry acetone in the presence of I^CC^ (5) to give a-guaiacoxyacetoveratrone (I). The quantum yield of photodegradation, , is defined as follows: 9
d
, 9,
moles of I consumed =
1
moles of photons absorbed The number of photons absorbed was determined using a commercially-available electronically integrating actinometer (QuantaCount, PTI, Deer Park N.J.), which was calibrated using azoxybenzene (6). Solutions of I (3.0 mL, 0.01 M) were irradiated with monochromatic light (325 nm) in a long-necked quartz cell, l x l cm . The cells 2
Heitner and Scaiano; Photochemistry of Lignocellulosic Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
PHOTOCHEMISTRY OF U G N O C E L L U L O S I C MATERIALS
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were sealed with rubber septa and deoxygenated with a stream of nitrogen for about 20 min. before irradiation. Disappearance of I was monitored by HPLC (Varian Vista 5500) using a Whatman Protesil 300 Octyl column, a mobile phase of 40% acetonitrile and 60% acetate buffer (10 mM, pH 4), and UV detection (275 nm). Before analysis, the solutions of I were diluted 1:25, and benzophenone (Aldrich Gold Label) was added as an internal standard. Triplet quenching experiments in aqueous solutions were done by preparing solutions of I containing 100 mM sorbic acid (fra/t£,;raft£-2,4-hexadienoic acid). In non-aqueous solvents naphthalene and cyclohexadiene were also used as triplet quenchers. Results Singlet State Participation. Upon absorption of light energy, I is converted from its ground electronic state, S , to its first excited state S . From S 1 can cross to the triplet excited state, T a process known as intersystem crossing. It is well-known that in benzophenone and acetophenone the quantum yield of intersystem crossing, tyte is 1 (7) (i.e. all reactions occur from T^, and this is often assumed as true for aromatic ketones in general. However, as the results below indicate, this is not the case for I. 0
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T A B L E I:
Quantum Yield of Photodegradation („) and Triplet Lifetime (x) for a-Guaiacoxyacetoveratrone
Solvent
4>< (unquenched)
* 0.2, compared with values < 0.1 in the pure aprotic solvent (Table I). Most of this additional yield was quenched in the presence of 0.1 M sorbic acid, which suggests that the additional degradation occursfromthe triplet state. Thus, addition of a protic co-solvent has apparently caused the triplet state to become reactive. As Figure 1 shows, in dioxane
