Alfredo Ceccarelli

Some photo-reactions of substituted poly(acrylophenones)

Abstract Poly(p-methoxyacrylophenone) (PPMA) and poly(3′,4′-dimethoxyacrylenephenone) (PDMA) were exposed in the form of thin films to long-wave u.v. radiation (λ ⩾ 300 nm) under high vacuum. The low molecular mass products from both were ethane and methane, the quantum yields (Φi) for their formation being in the range 1 × 10−4 π → π ∗ (S 0 → S 1 ) transition of the phenyl group is sufficiently red-shifted, by the presence of the methoxy substituents, to cause absorption in the long-wave region (λ > 300 nm). In addition, there is a considerable interaction between n → π ∗ and π → π ∗ orbitals, and this produces a composite chromophore. For the purposes of comparison, the plausible model compound 3, 4 dimethoxybenzylalcohol (DMBA) was irradiated under identical conditions. Not only were the gaseous products identical and the quantum yields similar, but DMBA underwent similar spectral changes, including coloration. In this case the S0 → S1 absorption is the chromophore. The results have some bearing on the coloration of lignin, which contains similar structural elements.

Photodegradation of lignin model compounds: part 2—substituted stilbenes

Three stilbenes—St 1 (3,5-dimethoxy-4-hydroxystilbene), St 2 (3,3′,5,5′-tetramethoxy-4-hydroxystilbene) and St 3 (3,3′,5-trimethoxy-4-hydroxystilbene)—were exposed under high vacuum conditions and in the form of thin films to long-wave UV (λ ≥ 300 nm) radiation. In all cases very rapid changes occurred in the UV and emission spectra. In particular, intensities of both π → π∗ absorptions and fluorescences decreased while new long-wave chromophores and fluorophores were produced. Rapid colouration (yellow) accompanied these changes. Methane and ethane were the only detectable low molecular weight products, and quantum yields for their formation were estimated (±20%). It was concluded that OCH3 fission was occurring with concomitant formation of phenoxyl radicals. These, in turn, are relatively long-lived and can undergo further photolysis to yield precursors of o-quinones, for which other spectroscopic evidence (NMR, infrared and UV/visible) was obtained and to which the colouration was ascribed. Gel permeation chromatography indicated the presence of products of cyclization (i.e. substituted dihydrophenanthrenes) and of higher molecular weight material, and it was suggested that the latter is the product of phenoxy radical interactions. Plausible reaction schemes which account for the experimental data are given.

Photodecomposition of lignin model compounds: Part I

The plausible lignin model compounds, 3,4-dimethoxy-α-(2-methoxyphenoxy)-β-hydroxy-propiophenone (I) and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-propandiol-1,3 (II) were irradiated in the form of thin films with long-wave ultraviolet (λ ≥ 300 nm) radiation under high vacuum. Methane and ethane were the only detectable low molecular mass products, the quantum yields for their formation being low (around 10−4 mol einstein−1). The compounds also underwent coloration and a number of other changes, as indicated by spectroscopy, and these were attributed to the formation of quinonoid species, their precursors being phenoxy radicals formed by direct photolysis of the OCH3 bonds in the benzene rings. In the case of I, the long-wave chromophore is composite, involving both the carbonyl (n→π∗ transition) and the phenyl group (π→π∗ transition). In II the absorption due to the S0→S1 transition of the benzene ring extends into the long-wave region (λ > 300 nm). It is suggested that OCH3 fission and subsequent reactions of the phenoxy radicals contribute to the coloration of lignin, which contains identical structural elements.

Long-wave photochemistry of di-methoxylated compounds related to lignin

Abstract Four methoxylated compounds which resemble moieties either found in lignin or produced in lignin, i.e. poly(3,4-dimethoxyacrylophenone) (Poly 34), guaiacylacetoveratrone (34Keto), 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propan-1,3-diol (34Hydroxy) and 3,3′,5,5′-tetramethoxy-4-hydroxytilbene (ST2), were irradiated in the form of thin films under vacuum, wavelengths being restricted to 300 nm and greater. In all cases new absorptions were formed in the long-wave UV and visible regions and the samples became coloured (yellow), ST2 being particularly reactive. In all cases methane and ethane were the only two low molecular weight products, and it was proposed that OCH 3 bond fission was occurring, all of the compounds having appreciable long-wave absorptions before irradiation. 13 C NMR spectra showed that the depletion of the methoxy C-atoms was accompanied by the formation of new carbonyls. Further evidence for OCH 3 fission was obtained from the photolysis of 3-ethoxy-4-hydroxystilbene in which ethane, ethylene and n -butane were formed. Spectral observations indicated that the new carbonyls were quinones and o -quinones, these being formed from the phenoxy radicals in turn formed by OC fissions, and the coloration was attributed to quinonoid species. The work demonstrates that lignin-type compounds can undergo coloration reactions, which are independent of oxidation, and are totally photo-induced. Plausible reaction mechanisms are proposed.

Long-wave photodegradation of poly(3,5-dimethoxyacrylophenone) in the solid state

Abstract The photolysis of poly(3′,5′-dimethoxyacrylophenone) was studied in the solid state, thin films being exposed to long-wave (i.e. λ ⩾ 300 nm) u.v. radiation under high vacuum at 25 °C. The polymer underwent colouration (initially yellow) and new carbonyl species were formed. The degraded polymer reacted with o -phenylenediamine to produce a phenazine derivative which exhibited strong fluorescence at 350 nm. The intensities of NMR signals due to the methoxy carbons decreased, and, at the same time, methane and ethane were formed (quantum yields in the 10 −4 region). It was concluded that OCH 3 bond scission was occurring, the methyl radicals producing ethane and methane and the phenoxy radicals being transformed into quinonoid compounds to which the colouration was attributed. The polydispersity of the polymer increases during photodegradation, and this was ascribed to simultaneous chain scission and crosslinking, the former being associated with a type II decomposition and the latter being brought about by combination reactions of macro-phenoxy radicals. A plausible reaction scheme is presented.

Photodegradation of poly(3,5-dimethoxyacrylophenone) in the long-wave UV region

The photophysics and long-wave (λ ≥ 300 nm) photochemistry of poly(3,5-dimethoxyacrylophenone) (P3,5DMAP) were investigated. The quantum yield for fluorescence is low (< 10−4) and the principal excited species is the triplet carbonyl. Emission (phosphorescence) occurs at 450 nm, the triplet energy (i.e. ET(0-0)) is 270 kJ mol−1 and the phosphorescence lifetime is long (23 μs). These observations are consistent with the formation of a low-lying π,π∗ triplet which, in turn, is the result of substitution of the aromatic ketone by electron donating methoxy groups. The principal photoprocess is a Norrish type II decomposition which results in random chain scission. The low value of the quantum yield, i.e. Φcs = 10 4, is further evidence for a π,π∗ triplet which is typically unreactive in abstraction reactions. In the case of P3,5DMAP, H-abstraction is a prerequisite for biradical formation (and for chain scission). Inhibition of chain scission was investigated, using naphthalene and cis-1,3-pentadiene as quenchers, and the data could be represented by Stern-Volmer kinetics. However, the quenching rate constants (kO) were lower than the corresponding diffusion controlled values, and it was proposed that the actual encounter of triplet and quencher is subject to adverse steric interactions associated with the proximity of the polymer chains.

CHROMOPHORE PRODUCTION IN LIGNIN MODEL COMPOUNDS - ANAEROBIC REACTIONS

The anaerobic photochemistry of a number of plausible lignin model compounds (i.e.I: 3,4-Dimethoxy-α-(2-methoxyphenoxy)-β-hydroxypropiophenone;II: 1-(3,4-Dimethoxyphenyl)-2-(2-methoxyphenoxy)-propan-1,3-diol;Pol A: Poly(4-methoxyacrylophenone);Pol B: Poly(3,4-dimethoxyacrylophenone);St 1: 3,5-Dimethoxy-4-hydroxystilbene; andSt 2: 3,5,3′,5′-Tetramethoxy-4-hydroxystilbene) was studied, thin films of these materials being exposed to long-wave (λ≥300 nm) radiation under high vacuum conditions (10−6 torr). In all cases, the only low molecular weight products formed were methane and ethane, and quantum yields were estimated for these reactions. All materials underwent colouration (yellow) and a number of changes were also observed in both the absorbing and emitting characteristics. The colouration was attributed to the presence ofo-quinones which were formed (by further photolysis) from the phenoxy radicals, which were, in turn, produced by O−CH3 fission, the resulting methyl radicals being the precursors of methane and ethane. The stilbenes were in all cases much more reactive (by a factor of about 100); however, they also absorbed higher intensities of radiation in the 300<λ<350 nm region on account of the greater extent of red-shifting of the longest-wave π-π′ aromatic transitions. Gel permeation data indicate the formation of products of cyclization and isomerization of stilbenes and the dimerization of phenoxy radicals while new absorbances in the infrared and13C NMR confirm the presence ofo-quinones in all the models.

Photodecomposition of polystyrene hydroperoxide: Part I—Reactions in dilute solution

Abstract The long-wave ( λ > 300 nm) photodecomposition of polystyrene hydroperoxide (PSOOH) in dilute solution (benzene) at 20 ± 1°C was studied under high-vacuum conditions. Although water was by far the most abundant low-molecular-weight product, trace amounts of a number of others, such as CO, O 2 , CO 2 , benzaldehyde and acetophenone, were also formed. New absorbances due to macromolecular ketonic and hydroxylic species were detected by a number of spectroscopic techniques. Molecular weight ( M n ) and gel-permeation chromatography measurements indicated that although random chain scission predominates in the earlier stages of the degradation, some crosslinking, as indicated by a broadening of the molecular weight distribution, also occurs. The kinetics of the reaction were investigated from the points of view of PSOOH concentration and chain scission, and found to be complex, particularly in the former case. Decomposition rates of PSOOH accelerate with reaction time, and the quantum yield is 1·6 ± 0·2 mol einstein −1 . These observations are consistent with sensitized decomposition, the initially formed hydroxy and alkoxy radicals from the PSOOH and, more importantly, the ketonic reaction products interacting with the PSOOH in a number of ways, to increase the overall rate of decomposition. Mechanisms of the sensitized decomposition, chain scission and crosslinking are discussed in the light of experimental observations.

Photoreactions of poly(o-butyrylstyrene) in the long-wave region

Abstract Poly( o -butyrylstyrene) (POBS) was exposed in the form of thin films to long-wave UV radiation (λ⪰ 300 nm) under high vacuum at 25 ± 1 °C. Ethylene was the principal low-molecular-weight product, but much smaller amounts of CO, methane, ethane and propane were also formed. Quantum yields for gaseous product formation were low, indicating that the Norrish type I and II reactions, which lead to their production, are subject to competition from the relatively favourable photoenolization. Transient spectra indicate the presence of the syn - and anti -enols, which have lifetimes of 140 ns and 4.7 μs respectively. Molecular weight data indicate that cross-linking occurs, but to a much lesser extent than in the isomeric p -acylstyrene polymers, both the Norfish type II reaction and photoenolization competing with free radical production. Other spectroscopic measurements indicate a minimal depletion of the carbonyl concentration, which is consistent with the predominance of these reactions. In terms of photodegradation, POBS is a relatively photostable polymer.

Photodegradation of poly(4-acetyl-α-methylstyrene) in solution

Abstract Dilute solutions of poly(4-acetyl-α-methylstyrene) (PMVAP) were exposed to long-wave ( λ ⩾ 300 nm) u.v. under high vacuum conditions at 25 ± 1°. The principal gaseous low molecular weight product was ethane, but a smaller amount of methane and a trace of CO were also detected, indicating that PMVAP undergoes simultaneous α-cleavages. Chain scission also occurs and was attributed to β-scission of the radicals formed by H-abstraction at the secondary C-atoms, both methyl radicals and carbonyl triplets (photoreduction) being involved. Chain scission is inhibited by the addition of naphthalene, and the diffusion-controlled quenching of the triplet can be represented by Stern-Volmer kinetics. The rate of chain scission is reduced when good H-donors, such as isopropanol, are present; they compete with the polymer in photoreduction reactions with the carbonyl triplet. The effects of less active H-donors such as methanol are more complex at low concentrations. They appear to act as transfer agents but, as the concentration increases, solvent quality plays a more important role. Tighter coiling of the polymer leads initially to more photoreduction and, with it, more chain scission. However, as the concentration of nonsolvent increases, the diffusive separation and disentanglement of macrofragments becomes more difficult and competition from intramolecular cyclization and perhaps also cross-linking increase, resulting in decreased rate of chain scission.