Cation absorption spectra

Deussen M, Bassler H (1993) Anion and cation absorption-spectra of conjugated oligomers... 

Therefore, a molecule in the TICT state can be regarded as a rigidly linked radical anion-radical cation pair. Experimental proof for this expectation can be gained from transient absorption spectroscopy. In the simplest case, the absorption spectrum of the TICT excited state is expected to be the sum of the individual ion spectra. This was indeed found in a few cases, with some perturbations which can be explained by the interaction of the closely-spaced radical ions. Thus, the transient absorption spectra of DMABN [135], of DMABK (or DMABA) [137] and of BA [58] resemble the spectrum of benzonitrile and acetophenone radical anion (the absorption of the dimethyl-amino radical cation is expected to be situated in the UV region and could not be observed) and to the sum of anthracene anion and cation absorption spectra, respectively. 

When biphenyl was added to polyethylene [174], the biphenyl anion and cation absorption spectra were obtained after irradiation, together with an apparently unchanged trapped-electron absorption. QJ and Qj were both removed by photobleaching. On warming, Qj starts to decay at —120°C, the temperature which corresponds to the first transition in polyethylene. This suggests that the decay is due to diffusion and reaction of the Q2 ion rather than to thermal ejection of the attached electron. 

A molecular ion-dominant spectrum has been observed for 2,3-dimethyl-1,3-butadiene (DB) excited with a 1.4-pm and 130 fs pulse, as shown in Fig. 2.5a, whereas many fragment ions were produced with a 0.8-pm femtosecond pulse of the same pulse width and intensity as shown in Fig. 2.5b. The cation absorption spectrum shown in Fig. 2.5c revealed that the non-resonant conditions hold with an excitation wavelength of 1.4 pm [4]. 

Fig. 2.6. a TOF mass spectra of anthracene at 1.4- and 0.8-p.m excitations, and b the cation absorption spectrum in a low-temperature matrix with two spectra of the excitation pulses... 

In Pedersen s early experiments, the relative binding of cations by crown ethers was assessed by extraction of alkali metal picrates into an organic phase. In these experiments, the crown ether served to draw into the organic phase a colored molecule which was ordinarily insoluble in this medium. An extension and elaboration of this notion has been developed by Dix and Vdgtle and Nakamura, Takagi, and Ueno In efforts by both of these groups, crown ether molecules were appended to chromophoric or colored residues. Ion-selective extraction and interaction with the crown and/or chromophore could produce changes in the absorption spectrum. Examples of molecules so constructed are illustrated below as 7 7 and 18 from refs. 32 and 131, respectively. 

Like acridine, phenanthridine and dimethyl acetylenedicarboxylate in methanol give a high yield of 1 1 1 molar adduct. Ultraviolet absorption spectrum comparisons show that this is best formulated as 9,10-dihydro-9-methoxy-10- (tran.s-l,2-dimethoxycarbonylvinyl) phenanthridine (142) rather than the corresponding phenanthridinium methoxide (143) under neutral conditions acidification changes the spectrum to that characteristic of the phenanthridinium cation. Crystallization of the adduct (142) from methanol containing 5-15% of water gave the betaine [(144) the positions of the ester and carboxylate groups have not been established], while in the presence... 

As yet no quinazoline has been discovered which has any appreciable amount (say, 1%) of hydrated species in the neutral molecule,but several quinazolines were shown to contain a mixture of anhydrous and hydrated species in the cations. Anhydrous neutral molecules and anhydrous cations have an ultraviolet absorption spectrum of the general type C (Fig. 3) and hydrated cations, the type... 

Kira and coworkers25 found that in deaerated DMSO solution of frans-stilbene both the solute cation and anion are produced and the anions are eliminated by aeration. Since they found26 that the absorption spectra of the anthracene cation and anion are quite similar, they suggested25 that the absorption spectrum observed by Hayon for anthracene solution in DMSO is a superposition of the spectra of the solute cation and anion. This observation casts a serious question on the yield of solvated electrons found by Hayon23. 

A supramolecular assembly of macromolecules bearing antenna dendron has been reported. Pyrazole-anchored PBE dendrons were synthesized to examine the coordination behavior to transition-metal cations (Cu, Au, Ag) [31]. Self-assembled metallacycles were found. The Cu-metallacycle further formed luminescent fibers about 1 pm in diameter. The luminescence (605 nm) occurred by the excitation of the dendron (280 nm) and the excitation spectrum was coincident with the absorption spectrum of the dendron, suggesting the antenna effect. Interestingly, the luminescence of the Cu-metallacycle fiber disappeared when the fiber was dissociated into the individual metallacycles in C2H2. 

Licht et al. [17] developed a method of numerical analysis to describe the above-quoted equilibria of the 11 participating species (including alkali metal cations) in aqueous polysulfide solution, upon simple input to the algorithm of the temperature and initial concentration of sulfur, alkali metal hydroxide, and alkali metal hydrosulfide in solution. The equilibria constants were evaluated by compensation of the polysulfide absorption spectrum for the effects of H8 absorption and by computer analysis of the resultant spectra. Results from these calculations were used to demonstrate that the electrolyte is unstable, and that gradual degradation of polysulfide-based PECs (in the long term) can be attributed to this factor (Chap. 5). 

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. 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

Fig. 14 Transient absorption spectrum of anthracene cation radical (ANT+ ) obtained upon 30-ps laser excitation of the [ANT, OsOJ charge-transfer complex in dichloro-methane. The inset shows the authentic spectrum of ANT+ obtained by an independent (electrochemical) method. Reproduced with permission from Ref. 96b.

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

The vibronic coupling model has been applied to a number of molecular systems, and used to evaluate the behavior of wavepackets over coupled surfaces [191]. Recent examples are the radical cation of allene [192,193], and benzene [194] (for further examples see references cited therein). It has also been used to explain the lack of structure in the S2 band of the pyrazine absorption spectrum [109,173,174,195], and recently to study the photoisomerization of retinal [196],... 

Upon irradiation of an IPCT band in degassed condition (Xex> 365 nm), the colour of both LB films changed from pale yellow to blue. The UV/vis absorption spectrum after irradiation is shown in Figure 25, which is characteristic of 4,4 -bipyridinium radical cation monomer[108]. Coloured species photogenerated in mixed LB films of AV2+/AA or HV2+/AA systems decayed almost exponentially in the dark in vacuo with a lifetime of about 4 h at 20 °C [93,94]. The lifetime of 4,4 -bipyridinium radical cations in LB films was almost... 

The structure of HRP-I has been identified as an Fe(IV) porphyrin -ir-cation radical by a variety of spectroscopic methods (71-74). The oxidized forms of HRP present differences in their visible absorption spectra (75-77). These distinct spectral characteristics of HRP have made this a very useful redox protein for studying one-electron transfers in alkaloid reactions. An example is illustrated in Fig. 2 where the one-electron oxidation of vindoline is followed by observing the oxidation of native HRP (curve A) with equimolar H202 to HRP-compound I (curve B). Addition of vindoline to the reaction mixture yields the absorption spectrum of HRP-compound II (curve C) (78). This methodology can yield useful information on the stoichiometry and kinetics of electron transfer from an alkaloid substrate to HRP. Several excellent reviews on the properties, mechanism, and oxidation states of peroxidases have been published (79-81). 

Fig. 5 (A) Typical time-resolved picosecond absorption spectrum following the charge-transfer excitation of tropylium EDA complexes with arenes (anthracene-9-carbaldehyde) showing the bleaching (negative absorbance) of the charge-transfer absorption band and the growth of the aromatic cation radical. (B) Temporal evolution of ArH+- monitored at Amax. The inset shows the first-order plot of the ion...

Fig. 9 (A) Transient absorption spectrum of the cation radical from anthracene (AnH) in CH2C12 at about 35 ps following the 532 nm charge-transfer excitation of the 0s04 complex with 30-ps (FWHM) laser pulse. The inset shows the steady-state spectrum of AnH+- obtained by spectroelectrochemical generation. (B) The decay of the charge-transfer transient by following the absorbance at Amax = 742 nm. The inset shows the first-order plot of the absorbance decay subsequent to the maximum...

Fig. 12 Typical time-resolved absorption spectrum following the charge-transfer excitation of nitrosonium-EDA complexes with arene (hexamethylbenzene) showing the bleaching of charge-transfer absorption and growth of the donor cation radical...

Shinkai and Kunitake (1977b) prepared a hydrophobic flavin analogue, 3-hexadecyl-10-butylisoalloxazine [56]. Its absorption spectrum in CTAB micelles showed distinct shoulders at 420 nm and 460-470 nm, as in the flavin spectrum in organic solvents. This indicates that [56] is located in the hydrophobic region of the micelle. Isoalloxazine [56] bound to a cationic micelle readily oxidizes 2-mercaptoethanol, 1,4-butanedithiol, and thiophenol (Shinkai and Kunitake, 1977b Shinkai et al., 1977a). In non-micellar... 

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