Dimeric electronic absorption spectra

Phthalic anhydride forms a persistent anion-radical on reduction, exemplified by the isobenzofuran-l,3-semidione (20). The ESR spectrum of 20 was observed by several groups at about the same time and analyzed with comparable results. The splittings indicated for 20 are those of Sioda and Koski. Lasia has examined by polographic means the consequences of ion-pairing of 20 for its dimerization electronic absorption spectra of 20 and its derivatives and their MO interpretation have been studied by Shida et al ° ... 

In an earlier work, we have proposed a theoretical procedure for the spectroscopy of antiferromagnetically (AF) coupled transition-metal dimers and have successfully applied this approach to the electronic absorption spectrum of model 2-Fe ferredoxin. In this work we apply this same procedure to the [Fe2in - 82) P o - CeH48)2)2 complex in order to better understand the electronic structure of this compound. As in our previous work" we base our analysis on the Intermediate Neglect of the Differential Overlap model parameterized for spectroscopy (INDO/S), utilizing a procedure outlined in detail in Reference 4. 

A number of these have been prepared using the synthetic procedures shown in Scheme 2. Of particular interest is [Os2(pyz)(NH3)10]5+ which has a radically different electronic absorption spectrum from either [Os 2(pyz)(NH3)10]4+ or [Osin2(pyz)(NH3)10]6+, and is believed to be a class III delocalized mixed-valence dimer, as is the dinitrogen analogue [Os2(N2)(NH3)10]s+ (see p, 556). A similar situation probably obtains for [Os2 (pyz)Cl(NH3), ]4+.1 l4a... 

In the decay of the radical cation of 4-methyldiphenylamine (44), for which an electronic absorption spectrum with A.max = 689 nm was observed, the main reaction route is the formation of a benzidine-type dimer, similarly to that in the case of 39+. The dimerization rate constant is 2.3 x 104 M 1 s 1. However, in the presence of a large excess of parent molecules acting as a base, the formation of cyclized dimers was also suggested. In contrast, the formation of cyclic structures was found to be characteristic of the radical cations of 3 -substituted (45) and 3,3 -disubstituted (46) derivatives of diphenylamine. On the basis of CV measurements, the formation of dihydrodiphenylphenazine derivatives may be anticipated and, consequently, 47 is assumed to be the product of dimerization of 45. In both cases the rate constants were as fast as ca 1 x 107 M 1 s-1 the 3-methyl substituent promotes visibly the reaction between the 6-position of the phenyl ring of... 

Willett, R. D., Liles, 0. L. (1967) The electronic absorption spectrum and bonding in the CugClg dimer. Inorg. Chem. 6, 1668. 

The main qualitative predictions of this model are strikingly demonstrated by the low-temperature RRS spectra of crystalline (TTF)Br whose structure consists of almost isolated (TTF+)2 dimers. The electronic absorption spectrum of polycrystalline (TTF)Br at 12 K is shown in Fig. 4a. The long wavelength absorption, displaying a doublet structure around 700 nm,is due to the CT transition of the dimers. The vertical bars indicate the laser wavelengths used to excite the Raman spectra. Those obtained with excitation wavelength at Ao = 530.9 nm and at A = 647.1 nm, resonant with LE and CT transition respectively, are shown in Fig. 4b. 

Karraker measured the electronic absorption spectrum of dimeric uranium penta-ethoxide and the weak narrow bands observed at 5405, 5680, 6622, 6934, 10200, 11 690, and 14 490 cm" have been assigned to f-f transitions for a distorted octahedral f system with a spin-orbit coupling constant of 1905 cm. ... 

The reaction is carried out in a solvent with a high dielectric constant such as DMSO, and the final product is the dimer [( -cp)Fe(CO)2)]2. No formation of free halogen is detected, and isobestic points are observed in the electronic absorption spectrum when the photolysis is carried out at wavelengths > 400 nm. Photolysis... 

Derivatives of the linear polyacenes, naphthalene,46 anthracene,41,90,93 naphthacene,91,92 and pentacene,92 form stable photodimers M2 when irradiated in concentrated Oa-free solution or (exceptionally) in the crystalline state.94,95 Transannular a-bonding of the molecular dimer components results in a folding of the aromatic planes about the bonded atoms and a reduced -electron delocalization reflected in a shift of the absorption spectrum to much higher frequencies.46,92,96... 

It is unlikely in such cases where different species exist on the surface that the photocurrent spectrum coincides with the absorption spectrum, since the efficiency of charge transfer will be different for different species. Memming 52> has concluded that for a cyanine dye adsorbed on a SnO 2-electrode the monomer seems to be more effective for charge injection than the dimer or higher aggregates. Hauffe and co-worker S3> have found that chelating dyes are especially efficient for sensitized electron injection into ZnO-electrodes which is seen in the variation of the photocurrent spectrum. 

Subsequently, the radical cation structure 137 and that of its (exo-) stereoisomer were confirmed independently by an analysis of their electronic absorption spectra [390]. The large energy gap (AE = 1.67 eV) observed for the iyw-dimer radical cation is incompatible with the photoelectron spectrum of the parent molecule (AE = 0.15 eV), but can be explained by ring-opened structures, such as 137 (Fig. 27). 

The products of the (e q + RCH=CH2) reaction are RCH—CH2 earbanions. Some of these have been identified by their chemical reactivity. Others have been observed through their absorption spectra by means of pulse-radiolysis techniques. The carbanion of acrylamide, for instance, has been shown to dimerize, to react with other free radicals, inducing anionic polymerization, and to react with oxygen, Ag+ and Fe(CN) - ions, presumably by electron-transfer reactions (Chambers et al., 1967). The absorption spectrum of the product of the (dimethyl fumarate + ey5) reaction has been observed in alkaline solution. The rate... 

A second illustrative example of the utility of TRPES and TRCIS for studying complex molecular photodissociation dynamics that involve multiple electronic state is the case of the weakly bound cis-planar C2V nitric oxide dimer [174], The weak (Do = 710cm-1) 1 A ground-state covalent bond is formed by the pairing of two singly occupied ji orbitals, one from each NO(X2II) monomer. The very intense UV absorption spectrum of the NO dimer appears... 

If there is a nonnegligible electronic interaction between adjacent chromophoric units already in the ground state, the absorption spectrum of the species may substantially differ from the sum of the absorption spectra of the component units. When the units have the same chemical nature, the interaction leads to formation of dimers. When the two units eu-e different, the interaction is usually CT in nature with formation of CT complexes. Excitation of such dimers leads to an excited state that is substantially the same as the corresponding excimers, and excitation of the CT ground state complexes leads to an excited state that is substantially the same as that of the corresponding exciplexes. 

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