1.4- dimethoxybenzene absorption

Photoinduced oxidation of 1,4-dimethoxybenzene (DMB) and tetrahydrofuran (THF) by [Au(C N N-dpp)Cl]+ in acetonitrile upon UV/Vis irradiation have been observed. The time-resolved absorption spectrum recorded 12 (xs after excitation of [Au(C N N-dpp)Cl] with a laser pulse at 35 5 nm showed the absorption band of the DMB radical cation at 460nm, whereas upon excitation at 406 nm in the presence of THF, a broad emission characteristic of the protonated salt of 2,9-diphenyl-l,10-phenanthroline (Hdpp ) developed at 500 nm. 

Fig. 15 (a) Absorption spectrum of dimethoxybenzene cation radical (--)obtained... 

Soma et al. (9) studied the interaction between a Ru - and CuZ -exchanged montmorillonite and p-dimethoxybenzene (DMBO) under desiccating conditions at room temperature. On the basis of Raman, IR, and VIS absorption spectra, they demonstrated that DMOB is stably adsorbed on Cu + and Ru +-exchanged montmorillonites as a radical cation. Oxidation by the metal cation was found to be reversible in the presence of water vapor ... 

I. 4-methoxyacetophenone (30 //moles) was added as an internal standard. The reaction was stopped after 2 hours by partitioning the mixture between methylene chloride and saturated sodium bicarbonate solution. The aqueous layer was twice extracted with methylene chloride and the extracts combined. The products were analyzed by GC after acetylation with excess 1 1 acetic anhydride/pyridine for 24 hours at room temperature. The oxidations of anisyl alcohol, in the presence of veratryl alcohol or 1,4-dimethoxybenzene, were performed as indicated in Table III and IV in 6 ml of phosphate buffer (pH 3.0). Other conditions were the same as for the oxidation of veratryl alcohol described above. TDCSPPFeCl remaining after the reaction was estimated from its Soret band absorption before and after the reaction. For the decolorization of Poly B-411 (IV) by TDCSPPFeCl and mCPBA, 25 //moles of mCPBA were added to 25 ml 0.05% Poly B-411 containing 0.01 //moles TDCSPPFeCl, 25 //moles of manganese sulfate and 1.5 mmoles of lactic acid buffered at pH 4.5. The decolorization of Poly B-411 was followed by the decrease in absorption at 596 nm. For the electrochemical decolorization of Poly B-411 in the presence of veratryl alcohol, a two-compartment cell was used. A glassy carbon plate was used as the anode, a platinum plate as the auxiliary electrode, and a silver wire as the reference electrode. The potential was controlled at 0.900 V. Poly B-411 (50 ml, 0.005%) in pH 3 buffer was added to the anode compartment and pH 3 buffer was added to the cathode compartment to the same level. The decolorization of Poly B-411 was followed by the change in absorbance at 596 nm and the simultaneous oxidation of veratryl alcohol was followed at 310 nm. The same electrochemical apparatus was used for the decolorization of Poly B-411 adsorbed onto filter paper. Tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte when methylene chloride was the solvent. 

In a subsequent study using diphenylnitrenium ion, several intermediates were detected. With 1,3,5-trimethoxybenzene or 1,3-dimethoxybenzene, the decay of the nitrenium ion occurred concurrently with the appearance of sigma adducts (141, Fig. 13.69). These were characterized on the basis of their absorption maxima and their behavior toward pyridine bases. On the other hand, when readily oxidized arenes, such as AW-dimethylanihne were employed, the characteristic ion radicals were detected (Fig. 13.70). ... 

When the reaction was conducted in O-deuteriated HFIP, (CF3)2CHOD, as a solvent, the deuterium was found exclusively at the 2-position of the 1,3-dimethoxybenzene 312. Laser flash photolysis at 248 nm irradiation carried out with 10 3 M solutions of substrates 311 with different silyl groups (SiMe3, SiMe2Ph, SiMePh2, SiMe2C6H4X-4 X = OMe, Me, F, I) show absorptions of the transient cations 313 with maxima at 380-390 nm and a weak band at 300 nm. The absorption spectra are similar to that of the parent 2,6-dimethoxybenzenium ion 314 (R = H) (kmax = 410 nm) but show a slight hyp-sochromic shift. 

In fact, the addition of 1,4-dimethoxybenzene (DMB) (0.13 equiv.), easier to oxidize than 21, appreciably quenches the reaction. The laser flash photolysis with a nitrogen-pulsed laser (337 nm) of a TPP+BF4 (1.6 x 10-4 M) dichloromethane solution, in the presence of 21 (6.2 x 10 2 M), conditions in which nearly 80% of the singlet excited sensitizer (Ered = 0.29 V vs SCE) is quenched by 21, gives a transient absorption (A 550 nm) assigned to a pyranil radical TPF. 

Dendrimer 1 (Fig. 5) is a classical example of a dendrimer built around a metal complex core. In this compound, the 2,2 -bipyridine ligands, that constitute the first coordination sphere of the Ru ion, carry branches containing 1,3-dimethoxybenzene and 2-naphthyl chromophoric imits separated by aliphatic connectors (10). Since the interchromophoric interactions are weak, the absorption spectrum of 1 is substantially equal to the summation of the spectra of [Ru(bpy)s], which is characterized by a broad spin-allowed Ru bpy metal-to-ligand (MLCT) band around 450 nm (11), and of the chromophoric groups contained in the branches, which show very intense bands in the near UV region. 

It should be noted that the 1,3-dimethoxybenzene and 2-naphthyl chromophoric units contained in the branches of the dendrimer are not involved in metal coordination. In some way, they belong to a second coordination sphere. If is considered a large metal complex, the absorption and emission bands of the 1,3-dimethoxybenzene and 2-naphthyl chromophoric units can formally be classified as LC. However, can be more properly viewed as a supramolecular (multicomponent) species (12). In such species, each chromophoric imit displays its own absorption spectrum since there is no appreciable interactions among them in the ground state, but in the excited state even weak interactions can cause intercomponent energy or electron-transfer processes. This kind of reasoning can also be applied to all the other systems discussed in this chapter. 

The three components of the self-assembled structure have complementary properties so that new functions emerge from their assembly. Dendrimer 5 has a very high molar absorption coefficient in the UV spectral region because of 12 dimethoxybenzene and 16 naphthyl units, but it is unable to sensitize the emission of an Nd ion placed in its cyclam core. The [Ru(bpy)2(CN)2] complex can coordinate (by the cyanide ligands) and sensitize the emission of Nd ions. Self-assembly of the three species leads to a quite unusual Nd complex which exploits a dendrimer and an Ru complex as ligands. Such a system behaves as an antenna that can harvest UV to VIS light absorbed by both the... 

Radical cations of methoxybenzenes efficiently oxidize phenols and other reductants. For example, the radical cations of anisole, 1,3-dimethoxybenzene (DMB), and 1,3,5-trimethoxybenzene (TMB), produced in >90% yield by reaction of OH with the methoxybenzenes at pH 1, can oxidize phenols and other reductants. The product radicals were identified in most cases by their known absorption spectra and extinction coefficients. The rate constants, determined by monitoring the buildup of the product radical and/or the decay of the radical cation as a function of the concentration of reductant, are summarized in Table 2. The rate constants are high for phenols bearing electron-donating substituents and much lower for phenols bearing strong electron-withdrawing substituents. 

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