The Diversity of Linear Conjugated Polyenes and Colours in Nature: Raman Spectroscopy as a Diagnostic Tool

This review is centered on the linear conjugated polyenes, which encompasses chromatic biomolecules, such as carotenoids, polyunsaturated aldehydes and polyolefinic fatty acids. The linear extension of the conjugated double bonds in these molecules is the main feature that determines the spectroscopic properties as light-absorbing. These classes of compounds are responsible for the yellow, orange, red and purple colors which are observed in their parent flora and fauna in nature. Raman spectroscopy has been used as analytical tool for the characterization of these molecules, mainly due to the strong light scattering produced by the delocalized pi electrons in the carbon chain. In addition, conjugated polyenes are one of the main target molecular species for astrobiology, and we also present a brief discussion of the use of Raman spectroscopy as one of the main analytical tools for the detection of polyenes extra-terrestrially.     

Comments on the optical properties of fully conjugated polymers: Analogy between polyenes and polydiacetylenes

The similarity of the π-electron backbones of the polyenes and the polydiacetylenes suggests an analogy between these two fully conjugated, linear polymers. The extent of the analogy is discussed through the examination of a series of model compounds: the polyenes and the polyenynes. The optical band gaps and the polarizabilities of polyenynes are compared to those of the polyenes and to the predictions of a modified free-electron model. It is concluded that the chain-dependent electronic properties of the polyenes may be extrapolated with some confidence to the polydiacetylenes.     

Efficient syntheses of the keto-carotenoids canthaxanthin, astaxanthin, and astacene

Three keto-carotenoids were prepared by the oxidation of the stable C(40) trisulfone 6, which has been used as the key compound in our beta-carotene synthesis. The first allylic oxidation to the unsaturated ketone and the second oxidation to the alpha-hydroxyketone produced the C(40) trisulfones 7 and 10, respectively. The Ramberg-Backlund reaction of the oxidized C(40) trisulfone was efficiently effected by the use of a mild base, NaOMe, in the presence of CCl(4) as a halogenating agent to give the C(40) disulfones 8 and 11. Base-promoted dehydrosulfonation reaction of the disulfone compounds produced the fully conjugated polyenes of canthaxanthin (1), astaxanthin (2), and astacene (3).     

Dienes and trienes in the low temperature oxydation of branched polyethylene

The low temperature (20–80°) oxidation of a sample of low density polyethylene, containing initially dienic and trienic conjugated sequences, is characterized by the rapid disappearance of these structures in a first order process with activation energy of 150 kJ/mol. The primary product of polyene-oxygen interaction is decomposed in a slow process having a low apparent activation energy, and characterized by the formation of at least two carbonyl compounds absorbing respectively at 1745 and 1710 cm^?1 in i.r. The latter, probably an αβ unsaturated aldehyde absorbing also in u.v. at 228 nm, is produced in a process disfavoured by increase of temperature. Study of film photostability vs storage time suggests that the polyenes and their primary oxidation products play a significant part in the photoinitiation of fresh samples.     

Bimolecular hole transfer from the trimethoxybenzene radical cation in the excited state

Bimolecular hole transfer quenching of the 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) by hole quenchers (Q) such as biphenyl (Bp), naphthalene (Np), anisole (An), and benzene (Bz) with higher oxidation potentials than that of TMB was directly observed during the two-color two-laser flash photolysis at room temperature. From the linear relationships between the inverse of the transient absorption changes of TMB*+ during the second 532-nm laser excitation versus the inverse of the concentration of Q, the rate constant of the hole transfer from TMB*+* to Q was estimated to be (8.5 +/- 0.4) x 10(10), (1.4 +/- 0.7) x 10(11), (1.3 +/- 0.6) x 10(11), and (6.4 +/- 0.3) x 10(10) M(-1)s(-1) for Bp, An, Np, and Bz, respectively, in acetonitrile based on the lifetime of TMB*+*. The estimated rate constants are larger than the diffusion-controlled rate constant in acetonitrile. Short lifetime, high energy, and high oxidation potential of TMB*+* cause the lifetime-dependent quenching process or static quenching process as the major process during the quenching of TMB*+* by Q as indicated by the Ware's theoretical model. The subsequent hole transfer from Q*+ to TMB, giving TMB*+, was found to occur at the diffusion-controlled rate for Bp and An as Q. For Q such as Np and Bz, the dimerization of Q*+ with Q to give dimer radical cation (Q2*+) occurred competitively with the hole transfer from Q*+ to TMB.     

The surprising nonlinear optical properties of conjugated polyyne oligomers

Polyynes represent a unique class of conjugated organic compounds. The third-order nonlinear optical response of polyynes has been extensively modeled theoretically, and it is generally believed that the increase in molecular second hyperpolarizability (gamma) as a function of length for polyynes should be lower than that for polyenes. Experimental evidence to test this prediction, however, has been absent. We have synthesized conjugated polyynes that contain up to 20 consecutive sp-hybridized carbons, and we have determined their nonresonant gamma-values as a function of the number of acetylene repeat units (n). These gamma-values demonstrate a power-law behavior versus n(gamma approximately n(4.28+/-0.13)), with an exponent that is both larger than theoretically predicted for polyynes and substantially higher than that observed for polyenes or polyenynes. Furthermore, no saturation of the linear or nonlinear optical properties is observed.     

One-electron oxidation of 2-(4-methoxyphenyl)-2-methylpropanoic and 1-(4-methoxyphenyl)cyclopropanecarboxylic acids in aqueous solution. the involvement of radical cations and the influence of structural effects and pH on the side-chain fragmentation reactivity

A product and time-resolved kinetic study on the one-electron oxidation of 2-(4-methoxyphenyl)-2-methylpropanoic acid (2), 1-(4-methoxyphenyl)cyclopropanecarboxylic acid (3), and of the corresponding methyl esters (substrates 4 and 5, respectively) has been carried out in aqueous solution. With 2, no direct evidence for the formation of an intermediate radical cation 2*+ but only of the decarboxylated 4-methoxycumyl radical has been obtained, indicating either that 2*+ is not formed or that its decarboxylation is too fast to allow detection under the experimental conditions employed (k > 1 x 10(7) s(-1)). With 3, oxidation leads to the formation of the corresponding radical cation 3*+ or radical zwitterion -3*+ depending on pH. At pH 1.0 and 6.7, 3*+ and -3*+ have been observed to undergo decarboxylation as the exclusive side-chain fragmentation pathway with rate constants k = 4.6 x 10(3) and 2.3 x 10(4) s(-1), respectively. With methyl esters 4 and 5, direct evidence for the formation of the corresponding radical cations 4*+ and 5*+ has been obtained. Both radical cations have been observed to display a very low reactivity and an upper limit for their decay rate constants has been determined as k < 10(3) s(-1). Comparison between the one-electron oxidation reactions of 2 and 3 shows that the replacement of the C(CH3)2 moiety with a cyclopropyl group determines a decrease in decarboxylation rate constant of more than 3 orders of magnitude. This large difference in reactivity has been qualitatively explained in terms of three main contributions: substrate oxidation potential, stability of the carbon-centered radical formed after decarboxylation, and stereoelectronic effects. In basic solution, -3*+ and 5*+ have been observed to react with -OH in a process that is assigned to the -OH-induced ring-opening of the cyclopropane ring, and the corresponding second-order rate constants (k-OH) have been obtained. With -3*+, competition between decarboxylation and -OH-induced cyclopropane ring-opening is observed at pH >or=10, with the latter process that becomes the major fragmentation pathway around pH 12.     

Photooxydation du polyethylene—2. Role des dienes et trienes conjugues dans l'amorçage de la photooxydation du PEbd

Study of the u.v. spectrum of a LdPE sample reveals the presence of conjugated dienes and trienes which react easily with oxygen photochemically or thermally. We studied the oxidation of these structures during storage in the dark at ambient temperature, and the consequences of this process on the photooxidative stability of the samples. The primary products of interaction between the polyenes and oxygen (adduct or charge transfer complex) are important in the photoinitiation of the oxidation.     

Kinetics of oxidation of benzyl alcohols by the dication and radical cation of ABTS. Comparison with laccase-ABTS oxidations: an apparent paradox

Laccase, a blue copper oxidase, in view of its moderate redox potential can oxidise only phenolic compounds by electron-transfer. However, in the presence of ABTS (2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) as a redox mediator, laccase reacts with the more difficult to oxidise non-phenolic substrates, such as benzyl alcohols. The role of ABTS in these mediated oxidations is investigated. Redox interaction with laccase could produce in situ two reactive intermediates from ABTS, namely ABTS++ or ABTS*+. These species have been independently generated by oxidation with Ce(iv) or Co(iii) salts, respectively, and their efficiency as monoelectronic oxidants tested in a kinetic study towards a series of non-phenolic substrates; a Marcus treatment is provided in the case of ABTS++. On these grounds, intervention of ABTS++ as a reactive intermediate in laccase-ABTS oxidations appears unlikely, because the experimental conditions under which ABTS++ is unambiguously generated, and survives long enough to serve as a diffusible mediator, are too harsh (2 M H2SO4 solution) and incompatible with the operation of the enzyme. Likewise, ABTS*+ seems an intermediate of limited importance in laccase-ABTS oxidations, because this weaker monoelectronic oxidant is unable to react directly with many of the non-phenolic substrates that laccase-ABTS can oxidise. To solve this paradox, it is alternatively suggested that degradation by-products of either ABTS++ or ABTS*+ are formed in situ by hydrolysis during the laccase-ABTS reactions, and may be responsible for the observed oxidation of non-phenolics.     

The rearrangement of the cubane radical cation in solution

The rearrangement of the cubane radical cation (1*+) was examined both experimentally (anodic as well as (photo)chemical oxidation of cubane 1 in acetonitrile) and computationally at coupled cluster, DFT, and MP2 [BCCD(T)/cc-pVDZ//B3LYP/6-31G* ZPVE as well as BCCD(T)/cc-pVDZ//MP2/6-31G* + ZPVE] levels of theory. The interconversion of the twelve C2v degenerate structures of 1*+ is associated with a sizable activation energy of 1.6 kcalmol(-1). The barriers for the isomerization of 1*- to the cuneane radical cation (2*+) and for the C-C bond fragmentation to the secocubane-4,7-diyl radical cation (10*+) are virtually identical (deltaH0++ = 7.8 and 7.9 kcalmol(-1), respectively). The low-barrier rearrangement of 10*+ to the more stable syn-tricyclooctadiene radical cation 3*+ favors the fragmentation pathway that terminates with the cyclooctatetraene radical cation 6*+. Experimental single-electron transfer (SET) oxidation of cubane in acetonitrile with photoexcited 1,2,4,5-tetracyanobenzene, in combination with back electron transfer to the transient radical cation, also shows that 1*+ preferentially follows a multistep rearrangement to 6*+ through 10*+ and 3*+ rather than through 2*+. This was confirmed by the oxidation of syn-tricyclooctadiene (3), which, like 1, also forms 6 in the SET oxidation/rearrangement/electron-recapture process. In contrast, cuneane (2) is oxidized exclusively to semibullvalene (9) under analogous conditions. The rearrangement of 1*+ to 6*+ via 3*+, which was recently observed spectroscopically upon ionization in a hydrocarbon glass matrix, is also favored in solution.     

Photoageing of rigid PVC—II. Degradation thickness profiles

Bulk samples of non-photostabilized PVC were exposed in a photochemical reactor equipped with a fluorescent lamp (λ ≥ 300 nm) at 70 °C. After 500 h of exposure, thin (20 μm) slices parallel to the surface were cut with a microtome and analyzed by FTIR and UV spectrophotometry and steric exclusion chromatography. The results reveal the existence of at least three distinct zones: (a) The superficial zone of thickness lower than 100 μm, which is characterized by the predominance of oxidation products and chain scissions; (b) The subcutaneous zone, in the 100–400 μm depth interval characterized by the predominance of the products of HC1 zip elimination (conjugated polyenes) and crosslinking (increase of M_w; and (c) The undegraded core zone beyond 400 μm depth. These characteristics can be partially explained in terms of diffusion controlled oxidation kinetics, but with some peculiarities linked to conjugated polyenes. Among these are the consequences of their photoreactivity and screen effect, and the fact that they are concentrated in fractions of high molecular weight.     

Kinetics of the radical scavenging activity of beta-carotene-related compounds

To clarify the non-enzymatic radical-scavenging activity of beta-carotene-related compounds and other polyenes, we used differential scanning calorimetry to study the kinetics of radical polymerization of methyl methacrylate (MMA) by 2,2'-azobisisobutyronitrile (AIBN) or benzoyl peroxide (BPO) in the absence or presence of polyenes under nearly anaerobic conditions at 70 degrees C, and analyzed the results with an SAR approach. The polyenes studied were all-trans retinol, retinol palmitate, calciferol, beta-carotene and lycopene. Polyenes produced a small induction period. The stoichiometric factor (n) (i.e. the number of radicals trapped by each inhibitor molecule) of polyenes was close to 0. Tetraterpenes (beta-carotene, lycopene) suppressed significantly more of the initial rate of polymerization (R(inh)) than did diterpenes (retinol, retinol palmitate). The inhibition rate constants (k(inh)) for the reaction of beta-carotene with AIBN- or BPO-derived radicals were determined to be 1.2-1.6x10(5) l/mol s, similar to published values. A linear relationship between (k(inh)) and the kinetic chain length (KCL) for polyenes was observed; as (k(inh)) increased, KCL decreased. KCL also decreased significantly as the number of conjugated double bonds in the polyenes increased. Polyenes, particularly beta-carotene and lycopene, acted as interceptors of growing poly-MMA radicals.     

Molecular mechanics (MM3) calculations on conjugated hydrocarbons

The MM3 molecular mechanics program has been extended to conjugated systems. A VESCF method is applied to the pi-system to calculate bond orders, from which various stretching and torsional parameters are obtained. The procedure gives somewhat better results than the analogous MM2 calculations. It has been applied to a study of 81 compounds of aromatic and other conjugated hydrocarbons, as well as 45 alkenes and unconjugated polyenes. The structures calculated are generally in good agreement with experiment, and the heats of formation of these compounds can be calculated with a rms value of 0.62 kcal/mol, which may be compared with the average experimental error of 0.61 kcal/mol. In addition, vibrational frequencies for five representative conjugated model structures are calculated, with an rms value of 46 cm^?1, and from these, other properties such as entropy can be calculated.     

Practical synthesis of 1,5-dimethyl substituted conjugated polyenes from geranyl acetate

A protocol to synthesize 1,5-dimethyl substituted conjugated polyenes via dehydrogenation of geranyl acetate was established. C₅ unit elongation to multi-1,5-dimethyl substituted conjugated polyenes was also achieved via Horner-Wadsworth-Emmons olefination in good yields and good selectivities.     

Assessment of density functionals for pi systems: Energy differences between cumulenes and poly-ynes; proton affinities, bond length alternation, and torsional potentials of conjugated polyenes; and proton affinities of conjugated Shiff bases

Woodcock et al. [J. Phys. Chem. A 2002, 106, 11923] pointed out that no density functional was able to obtain the correct sign of the relative energies of the allene and propyne isomers of C3H4 and that density functional theory (DFT) predicts that poly-ynes are insufficiently stabilized over cumulenes for higher homologues. In the present work, we show that the recent M05 density functional predicts the correct ordering of allene and propyne and gives a mean unsigned error (MUE) of only 1.8 kcal/mol for the relative energies of the two isomers of C3H4, C5H4, and C7H4. Two other recent functionals, M05-2X and PWB6K, also give reasonably low MUEs, 2.7 and 3.0 kcal/mol, respectively, as compared to 6.2 kcal/mol for the popular B3LYP functional. Another challenging problem for density functionals has been a tendency to overpolarize conjugated pi systems. We test this here by considering proton affinities of conjugated polyenes and conjugated Schiff bases. Again M05-2X performs quite well, with MUEs of 2.1 and 3.9 kcal/ mol, respectively, as compared to 5.8 and 5.9 kcal/mol for B3LYP. Averaged over the three problems, M05-2X has a MUE of 3.0 kcal/mol, the BMK functional of Boese et al. has an MUE of 3.2 kcal/mol, and M05 has an MUE of 5.1 kcal/mol. Twenty-two other tested functionals have MUEs of 5.2-8.1 kcal/mol averaged over the three test problems. Both M05 and M05-2X do quite well, compared to other density functionals, for torsion potentials in butadiene and styrene, and M05 does very well for bond length alternation in conjugated polyenes. Since the M05 functional has broad accuracy for main group and transition metal chemistry and M05-2X has broad accuracy for main group chemistry, we conclude that significant progress is being made in improving the performance of DFT across a wide range of problem types.     

Theoretical, thermodynamic, spectroscopic, and structural studies of the consequences of one-electron oxidation on the Fe-X bonds in 17- and 18-electron Cp*Fe(dppe)X complexes (X = F, Cl, Br, I, H, CH3)

The compounds Cp*Fe(dppe)X ([Fe]X) and the corresponding cation radicals [Fe*]X*+ are available for the series X = F, Cl, Br, I, H, CH3. This has allowed for a detailed investigation of the dependence of the nature of Fe-X bonding on the identity of X and the oxidation state (charge) of the complex. Cyclic voltammetry demonstrates that the electrode potentials for the [Fe]X0/+ couples decrease in the order I > Br > Cl > H > F > CH3. An "inverse halide order" is seen, in which the most electronegative X leads to the most easily oxidized complex. This suggests that F is the best donor among the halides. The halide trend is also reflected in NMR spectroscopic data. M?ssbauer spectroscopy data also suggest that the F ligand is a strong donor (relative to H and CH3) in [Fe*]X*+. DFT calculations on CpFe(dpe)X ([Fe]X) model complexes nicely reproduce the trend in the electrode potentials for the [Fe*]X0/+ couples. Analysis of the theoretical data within the halogen series indicates that the energy of the [Fe]X HOMO does not correlate with the extent of its Fe(d(pi))-X(p(pi)) antibonding character, which varies in the order I > Br > Cl > F, but rather depends on the destabilizing electrostatic effect caused by X. This effect varies in the order F > Cl > Br > I. A thermochemical cycle that incorporates the [Fe*]X0/+ and [Fe*]0/+ electrode potentials was used to investigate the effect of the oxidation state of the complex on the homolytic bond dissociation energy (BDEhom), defined for the processes Fe-X --> Fe* + X* and Fe-X*+ --> Fe*+ + X*. For all X, it was found that a one-electron oxidation leads to a weakening of the Fe-X bond. This trend was reproduced by the DFT calculations. On the other hand, IR nu(Fe-X) spectroscopy data showed an increase in the stretching frequencies for X = H and Cl upon oxidation. X-ray crystallographic data showed a shortening of the Fe-Cl bond upon oxidation. The trends in IR and Fe-Cl bond distances were reproduced in the DFT calculations. The combined data therefore suggest that oxidation leads to weaker, but shorter, Fe-X bonds. A second thermochemical cycle was applied to investigate the effect of the one-electron oxidation on the heterolytic bond dissociation energies (BDEhet), defined for the processes Fe-X --> Fe+ + X- and Fe-X*+ --> Fe2+ + X-. In this case, the oxidation led to bond strengthening in all cases. The computed BDE values have been analyzed within Ziegler's transition state methodology and decomposed into two components, one electrostatic and one covalent, describing the interaction between the unrelaxed fragments. In all the computed BDEhom and BDEhet values of the [Fe]X models the electrostatic component is important. This helps to understand their respective variations upon oxidation.     

Carbon-13 nuclear magnetic resonance spectroscopy of conjugated polyynes

The ¹³C NMR spectra of several conjugated polyyne-aldehydes and ketones are compared with those of the corresponding alcohols. All data show considerable effects similar to those observed in conjugated polyenes.     

Carotenoid synthesis by way of sulfones: synthesis of beta-carotene]

Sulfones, prepared from conjugated arylsulfonyl ylids and allylic halides, eliminate corresponding aryl sulfinates with formation of conjugated polyenes. Using this method β-carotene ( 8 ) has been synthesized.     

Non-linear polamzabilities of conjugated chains: regular polyenes,solitons, and polarons

The hyperpolarizabilities of linear polyenes are shown to be extremely sensitive to confonnational changes and to the presence of charges along the conjugated chain. Geometries corresponding to those of regular polyenes and soliton and polaron defects are examined.