Azulene effect

In addition,<1,48,48) it was noted that whereas the azulene effect on the sensitized reaction is sensitive to changes in solvent viscosity, the azulene effect on the direct photoreaction was independent of solvent viscosity, as would be predicted for Forster-type energy transfer. The inescapable conclusion is that cis-trans isomerization upon direct irradiation of stilbene takes place in the singlet manifold. 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

The only recent example of Forster transfer of photochemical importance is the demonstration by Saltiel163 that the ability of azulene to increase the photostationary transjcis ratio in direct photoisomerization of the stilbenes is due entirely to radiationless transfer of excitation from traw.y-stilbene singlets to azulene. As expected for Forster transfer, this azulene effect did not depend upon solvent viscosity. The experimental value of R0, the critical radius of transfer in Forster s formula,181 was 18 A, in good agreement with the value calculated from the overlap of stilbene emission and azulene absorption. 

Stilbene 1 undergoes isomerization in the singlet excited state on direct irradiation [13,16-23]. The mechanism was established by J. Saltiel et al. from the experiments of the azulene effect on the photostationary state isomer composition [13,24,25]. Thus, usually, quenching experiments as well as observation of the transient spectra are necessary to distinguish the singlet and triplet mechanisms of isomerization of olefin on direct irradiation. Furthermore, triplet sensitization is necessary to study the isomerization in the triplet manifold. 

In several excellent surveys, Saltiel and co-workers have presented the advances in interpretation of the azulene effect [25, 26, 105, 194, 195]. The evaluation of results from comparative quenching measurements of fluorescence and cis trans photoisomerization under direct and sensitized excitation conditions confirms a singlet pathway for stilbene. For bromo-substituted stilbenes, however, participation of the triplet state has been... 

The azulene effect is sensitive to substituents introduced into stilbene. A comparison of slope/intercept ratios is given for stilbene and several bromostilbenes (Table 8). The results show clearly that the triplet state is... 

Other quenchers that have been used in the benzophenone-sensitized cis trans isomerization of stilbene are /9-carotene, oxygen, and di-tert-butyl nitroxide. /9-Carotene shifts the photostationary state to the trans side similar to the azulene effect [237]. In the presence of di-tert-butyl nitroxide, a radical quencher, the photostationary state is slightly shifted to the cis side Caldwell and Schwerzel [226] have suggested the involvement of the twisted triplet state and a quenching mechanism other than energy transfer, probably vibrational relaxation to the ground state caused by spin exchange. 

Figure 5. Azulene effect on the benzophenone-sensitized stilbene photoisomerization in benzene line 1, degassed line 2, O2 atmosphere line 3, degassed with 2.90 x 10 M p-C. From Ref. 41, with permission of North-Holland Publishing.

Azulene does have an appreciable dipole moment (0.8 The essentially single-bond nature of the shared bond indicates, however, that the conjugation is principally around the periphery of the molecule. Several MO calculations have been applied to azulene. At the MNDO and STO-3G levels, structures with considerable bond alternation are found as the minimum-energy structures. Calculations which include electron correlation effects give a delocalized n system as the minimum-energy structure. ... 

Azulene (XI) possesses a transannular bond which has the same effect as those of bowtiene (Fig. 3). The splittings of the top filled and bottom empty degenerate orbitals of cyclododecapentaene in this case are half the corresponding splitting in the case of bowtiene, and are not large enough to produce an effective vibronic interaction between the ground and lowest excited states of the resultant azulene molecule. 

Eq. (4.4 a) is satisfied in the position 1 of azulene. Eq. (4.4b) is valid in position 6 of fulvene, position 6 of azulene, position 3 of fluoranthene, and position 5 of acenaphthylene. Even in a few exceptional cases where the previous relations do not hold, a consideration of the coulombic effect of attacking reagents leads to a conclusion favorable to the hypothesis of frontier density growth. An example of such cases is position 3 of... 

Hemoglobin is another heme-containing protein, which has been shown to be active towards PAH, oxidation in presence of peroxide [420], This protein was also modified via PEG and methyl esterification to obtain a more hydrophobic protein with altered activity and substrate specificity. The modified protein had four times the catalytic efficiency than that of the unmodified protein for pyrene oxidation. Several PAHs were also oxidized including acenaphthene, anthracene, azulene, benzo(a)pyrene, fluoranthene, fluorene, and phenanthrene however, no reaction was observed with chrysene and biphenyl. Modification of hemoglobin with p-nitrophenol and p-aminophenol has also been reported [425], The modification was reported to enhance the substrate affinity up to 30 times. Additionally, the solvent concentration at which the enzyme showed maximum activity was also higher. Both the effects were attributed to the increase in hydrophobicity of the active site. 

The arylation of electron-rich arenes, such as azulene (55)206 and heteroarenes, has been sporadically described. Under similar conditions phenols undergo arylation, which is preferably directed at the ort/zo-positions, probably due to the involvement of palladium phenolate intermediates.188,207 Polysubstitution occurs readily.208 The para-position can be attacked only with the sterically hindered 2,6-di-t-butylphenol.209 Similar ortho-diarylation of arenes bearing carbonyl groups (acetophenone, anthrone, benzanilide, etc.) shows that the or//zo-di reeling effect of the substituent is more important than its other electronic effects.189... 

The Friedel-Crafts acylation at the 3-position of the azulene ring was possible due to the effect of the electron-withdrawing 1-methoxycarbonyl group. 183 has been prepared previously in an eight-step synthetic route in an unsatisfactory reaction yield156. 

We have reported the synthesis of a series of azulene-substituted methyl cations, i.e., tri(l-azulenyl)methyl, di(l-azulenyl)phenylmethyl, and (1-azul-enyl)diphenylmethyl cations (2a+, 3a+, and 4a+) (Figure 6) (6). In order to examine substituent effects on the azulene rings and to enhance their stabilities, a series of the cations (2b-d+, 3b-d, and 4b-d+) bearing tert-buty groups on each azulene ring were also synthesized (7). 

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

Heteroatom-containing derivatives of pentalene and azulene are 8- and 10-jr-electron systems, respectively, and have been extensively studied by Hafner and co-workers.301-304 A few examples are presented in Schemes 30, 47, and 86. Electron-rich azapentalenes 247—249305 306 and various aza-azu-lenes 250—252300-302 have intense colors and are stable but oxygen-sensitive compounds. Azulenes with nitrogen heteroatoms placed in positions with high electron density, such as position 5, or positions 5 and 7, present a hypsochromic effect, while a heteroatom in position 6, with low electron density, exerts a bathochromic effect. When both types of positions are substituted by heteroatoms, their effect is canceled out.307 The bond lengths in 252 (Scheme 86), as determined by X-ray, indicate delocalization as in 252a and 252b.308 309... 

The reactions of 534 with substituted quinones produced mixtures of regioisomers. The substituent effect on the regioselectivity of the [8 + 2] cycloaddition reactions was said to be dependent on steric as well as electronic effects. Equation 156 shows the reaction between 534 and 2-methylbenzoquinone (539). The reaction afforded a mixture of two regioisomeric adducts 540 and 541, which were transformed to azulenes 542-545 under the reaction conditions applied318. 

The interpretation of these effects as the formation of a proton addition complex was further supported by Plattner et al. (1952) by means of spectroscopic and conductimetric investigations. In these interactions the change of the absorption spectrum is characteristic, since the blue colour of the azulene in organic solvents is changed to a yellow colour in... 

Long and Schulze (1961) determined spectroscopically, in perchloric acid, the Hp-value for which the concentration ratio Cah+/ca = 1- This value can be regarded as an approximate uncorrected pAg-value. These investigations were extended to other azulene derivatives, permitting a study of the effect of polar substituents on the basicity (Long and Schulze, 1964). 

In calculations on proton addition complexes of azulene and methyl-azulenes by means of the HMO method, Heilbronner and Simonetta (1952) took the effect of methyl groups into aecount by adjustment of the a and j8 values, as had already been done by PuUmann and collaborators (1950) in calculating the spectra of the methylazulenes. These calculations correctly reproduce the effeet of a methyl group on the basicity of azulene. In contrast to the ease of naphthalene, the position of addition of the proton remains independent of the position of the methyl group ... 

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