Laser Techniques in the Study of the Photochemistry of Carbonyl

Chapter 8

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.ch008

Laser Techniques in the Study of the Photochemistry of Carbonyl Compounds Containing Ligninlike Moieties A. B. Berinstain, M . K. Whittlesey, and J. C. Scaiano Department of Chemistry, Ottawa—Carleton Chemistry Institute, University of Ottawa, Ottawa K1N 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 isπ-π*in nature. Theπ-π*nature is due to methoxy substitution on the chromophore and results in relatively long triplet lifetimes and low reactivity, compared to n-π* triplets like that of benzophenone. Theα-phenoxygroup of αguaiacoxyacetoveratrone 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 usedtodayfor 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 longterm usage duetotheir photostability and higher strength. It is believed that yellowing occurs predominantly through oxidation of phenoxy radicals supplied by the phenolic hydroxy groups of kgnin, 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 additiontothe phenolic hydroxy groups in the natural product It has been shown that upon irradiation, phenacyl aryl 0097-6156/93/0531-0111S06.00/0 © 1993 American Chemical Society Heitner and Scaiano; Photochemistry of Lignocellulosic Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


112

PHOTOCHEMISTRY OF LIGNOCELLULOSIC MATERIALS

ether moieties in the lignin molecule are also an efficient source of phenoxy radicals (4). Studies by Castellan et al. have suggested that a-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 ocguaiacoxyacetoveratrone (I) and acetoveratrone (II). We summarize our own results and try to place them in the context of our understanding of other substrates which have structural features in common with land 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.

OCH

3

I: a-Guaiacoxyacetoveratrone Phosphorescence

OCH3

II: Acetoveratrone

Studies.

For most carbonyl compounds, we expect to have two near-lying excited stares in the triplet manifold, which are either n-n* or TT-TT* in character. The n-7r* states frequently show radical-like behavior. Benzophenone is an example of such a molecule which has an n-ft* triplet state in which we see occurrences of hydrogen abstraction and very efficient intersystem crossing. When the lowest state is the 7I-7T* state, largely centered on the aromatic part of the molecule, as in the case of /Mnethoxyacetophenone, 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. (70) By comparison, for triplet states which are 7r-7r* 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.

Heitner and Scaiano; Photochemistry of Lignocellulosic Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

8. BERINSTAIN ET AL.

Carbonyl Compounds Containing Ligninlike Moieties 113


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 7T-7T* triplet manifold, at about 60-63 kcal/mol. Usually carbonyl compounds will go through chemistry strictly from the triplet stare, 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 arisingfromradical pair reactions within the solvent cage are also expected since they are formed with the appropriate dectronic spin to directly lead to products.

I

360

1

mr 420

i i 480 540 Wavelength, nm

J

600

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 us 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 15 us in acetoveratrone to -500 ns in the case of


120

PHOTOCHEMISTRY OF LIGNOCELLULOSIC MATERIALS

Photoacoustic spectroscopy was used to determine intersystem crossing quantum yields, isc, by a method previously described elsewhere (20). These values of isc 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 ocguaiacoxyacetoverarrone 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 perfonriing experiments exploring the photochemistry of a-guaiacoxyacetoverafrone 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 P-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 noncrystalline samples of a-guaiacoxyacetoveratrone all our attempts to detect the triplet state using time resolved diffuse reflectance techniques (2 7)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 servetooutline 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 Chermrnechanical Wood-Pulps Network, which is part of Canada's Networks of Centres of Excellence. Literature Cited (1) (2) (3) (4) (5) (6) (7)

Forman, L.V. Paper Trade J. 1940, 111, 34. Leary, G.J. Tappi 1968, 51, 1257. Grier, J.; Lin, S.Y. Svensk Papperstidning 1972, 75, 233. Kringstad, K.P.; Lin, S.Y. Tappi 1970, 53, 2296. Vanucci, C.; Fornier de Violet, P.; Bouas-Laurent, H.; Castellan, A. J. Photochem.Photobiol.A: Chem. 1988, 41, 251. Castellan, A.; Colombo, N.; Vanucci, C.; Fornier de Violet, P.; Bouas-Laurent, H. J. Photochem. Photobiol., A: Chem. 1990, 51, 451. Castellan, A.; Zhu, J.H.; Colombo, N.; Nourmamode, A.; Davidson, R.S.; Dunn, L. J. Photochem. Photobiol. A: Chem. 1991, 58, 263.



8.

BERINSTAIN

ET

AL.

Carbonyl Compounds Containing Ligninlike Moieties 121

(8) Scaiano, J.C. J. Photochem. 1973, 2, 81. (9) Wagner, P.J.; Nakahira, T. J. Am. Chem. Soc. 1973, 95, 8474. (10) Carmichael,I.;Hug, G.L. "Spectroscopy and Intramolecular Photophysics of Triplet States", Handbook of Organic Photochemistry. Scaiano, J. C. ed. 1989 CRC Press. Boca Raton, Florida. (11) Hadel, L.M. "Laser Flash Photolysis", Handbook of Organic Photochemistry. Scaiano, J. C. ed. 1989 CRC Press. Boca Raton, Florida. (12) Lissi, E.A.; Encinas, M.V. "Representative Kinetic Behavior of Selected Reaction Intermediates: Triplet States", Handbook of Organic Photochemistry. Scaiano, J. C. ed. 1989 CRC Press. Boca Raton, Florida. (13) Lissi, E. Can. J. Chem. 1974, 52, 2491. (14) Burkey, T.J.; Majewski, M.; Griller, D. J. Am. Chem. Soc. 1986, 108, 2218. (15) Netto-Ferreira, J.C.; Leigh, W.J.; Scaiano, J.C. J. Am. Chem. Soc. 1985, 107, 2617. (16) Boch, R.; Bohne,C.;Netto-Ferreira, J.C.; Scaiano, J.C. Can. J. Chem. 1991, 69, 2053. (17) Wagner, P.J.; Kelso, P.A.; Kemppainen, A.E.; Haug, A.; Graber, D.R. Mol. Photochem. 1970, 2, 81. (18) Netto-Ferreira, J.C.; Avellar, I.G.J.; Scaiano, J.C. J. Org. Chem. 1990, 55, 89. (19) Das, P.K.; Encinas, M.V.; Steenken, S.; Scaiano, J.C. J. Am. Chem. Soc. 1981, 103, 4162. (20) Berinstain, A.B. In Singlet State Participation and Solvent Effects on the Photodegradation ofα-guaiacoxyacetoveratrone,a Lignin Model Compound; Concordia University, Institute for Co-operative Education, Department of Chemistry and Biochemistry; Work Term Report; 1990. (21) Wilkinson, F.; Kelly, G. "Diffuse Reflectance Flash Photolysis", Handbook of Organic Photochemistry. Scaiano, J. C. ed. 1989 CRC Press. Boca Raton, Florida. RECEIVED April

12, 1993


Laser Techniques in the Study of the Photochemistry of Carbonyl

Recommend Documents