Fullerene absorption characteristics

The hypsochromic shift of the maximum values of the characteristic fullerene absorption bands is directly proportional to the number of the adduct substituents, and is independent of the type and nature of the solvent (chloroform, acetonitrile) (Fig. 1.3). 

As the number of substituents increases, molar absorption coefficients determined in the maxima of the characteristic fullerene absorption bands decrease exponentially (Fig. 1.4). The maximum decrease in the extinction coefficient is observed in transiting from Cg to mono- and di-substituted products, slightly changing at > 3. 

The dendritic effect evidenced for 1-8 might be useful to optimize the optical hmiting properties characteristic of fullerene derivatives. Effectively, the intensity dependant absorption of fuUerenes originates from larger absorption cross sections of excited states compared to that of the ground state [32], therefore the... 

Fullerenes can be easily chemically reduced by the reaction with electropositive metals [1, 97-99], for example, alkali- and alkaline earth metals. The anions Cjq"" (n = 1-5) can be generated in solution by titrating a suspension of in liquid ammonia with a solution of Rb in liquid ammonia [100], whereupon the resulting anions dissolve. Monitoring of this titration is possible by detecting the characteristic NIR absorption of each anion by UV/Vis/NIR spectroscopy. The solubility of the alkali metal fullerides in the polar solvent NHj demonstrates their salt character. 

In contrast to highly stable and prolific fullerene anionic species, fullerene cations are rare. The first fullerene cation was prepared in 1996 by Reed and co-workers500 by single-electron oxidation of C76 to form radical cation C76 + isolated in solid form as the CBnH6Br6 salt [Eq. (3.56)]. The cation was identified in solution by a characteristic visible-near-infrared absorption (Amax = 780 nm), FT-IR and EPR spectroscopy. C60 + was generated in an analogous way later.501 Reed et al.501 also succeded in... 

A thiochroman-fused fullerene shows an absorption band at 441 nm in hexane which is characteristic of a 1 1 cycloadduct of It is shifted to 435 nm in the derived sulfoxide <1995TL6899>. The bands associated with 1,2-bis(thioxanthen-9-ylidene)ethene 237 are red shifted by 40-70 nm in its 1 1 complex with 0 in keeping with the interaction between the 7i-systems of the component parts. The IR spectrum is essentially a combination of the spectra of the two individual molecules. In the crystal, layers of Cgo alternate with layers of the alkene <2005MI711>. 

Abstract. Water-soluble polymers of acrylamide and acrylic acid with high extent ( 90%) of Ceo consumption are obtained by technique of low-temperature radiation living radical polymerization. In absorption spectra of these copolymers one can see gradually descended unstructured absorption in the range 240-700 mn, characteristic for fullerene covalent-bound or its klasters. The way of radiation initiation is used to obtain the products of high purity, because it is not necessary to embed into the system any initiators or catalyst. Latter is very important in the case of synthesis of polymers for medical purposes. Also at radiation initiation a rate of initiation reaction does not depend on the temperature and the sterilization of products takes place simultaneously. 

Thus, water - soluble polymers of acrylamide and acrylic acid containing covalent bound fullerene are obtained. These products demonstrate in the optical absorption spectra the unstructured gradually descending absorption in the region from 240 nm to 700 nm characteristic for covalent bound fullerene or its aggregations. 

Consistently, the PIA spectra of toluene solutions containing MP-Ceo and OPVn (n = 2, 3 or 4) in a 1 1 molar ratio, recorded using selective photoexcitation of MP C60 at 528 nm (Fig. 1.28b), invariably exhibit an absorption at 1.78 eV with an associated shoulder at 1.54 eV, characteristic of MP-C6o(7i) [103]. The monomolecular decay (—AT oc Ip, p = 0.89-0.96) with lifetime 150-260 ps associated with these PIA bands supports this assignment. Furthermore, weak fullerene fluorescence at 1.73 eV (715 nm) is observed under these conditions for all three mixtures. No characteristic PIA bands of OP Vw+ radical cations or MP-Cg0 radical anions are discernible under these conditions. From these observations we conclude that electron transfer from the ground state of the OPVn molecules to the singlet or triplet excited state of MP-Cgo does not occur in toluene solution. 

Figure 2.32 Absorption spectra of some higher fullerenes. The pure compounds also show very characteristic color In solution. The spectra of and C were obtained after enriching by HPLC the respective substances from a mixed fraction containing both ( JCS 1991).

Addition of base or acid had, however, a significant impact on the stability and yield of the characteristic 1080 nm absorption band. A semilogarithmic correlation between the proton concentration and the intensity of the fullerene 7t-radical anion band (1080 nm) is observed in anaerobic aqueous solutions. This observation has been ascribed to a reversible protonation of Ceo /Y-CD. Experimental proof for this assumption was brought forward by the fact that the 1080 nm absorption, in an alkaline solution (pH 10), diminished upon acidifying (pH 3) and was completely restored upon addition of base (pH 10). The reversible protonation process gives rise to a pKa of 4.5. 

Detection of an NIR band for DODAB, lecithin, and DHP solutions of [60]fullerene confirmed our earlier report, which was based on differential absorption changes in the Vis region. Interestingly, the characteristic NIR-band of Ceo " within these lipid materials reveals a blue shift with a maximum centered around 1020 nm as compared to organic solutions of [60]fullerene and aqueous Ceo/Y CD where the... 

Addition of various concentrations of [60]fullerene, for example, to a ZnTPP solution, resulted in an accelerated decay of the 7t-radical anion (ZnTPP "). The observed rate was linearly dependent on the [60]fullerene concentration, which, in turn, has led to the assumption that the ZnTPP tt-radical anion reacts with [60]fullerene. To confirm a probable electron transfer, the formation of the characteristic C60 absorption in the NIR ( ax = 1080 run) was also monitored. The grow-in rate of the C o " absorption at various wavelengths in the 980-1060 nm range was nearly identical to the decay rate of the MP absorption at 650-750 nm. For example, in the case of ZnTPP 7t-radical anion (ZnTPP "), a bimolecular rate constant of (2.5+1.0) x 10 M s was derived from the ZnTPP " decay (720 nm) and (1.4 1.0) x 10 M s from the Ceo formation (970 nm). These two values are in reasonable agreement and confirm unmistakably the electron transfer from the one-electron reduced metalloporphyrin (ZnTPP) to the singlet ground state of the fullerene ... 

Tin-(IV) porphyrins (Sn P) are very easily reduced to their long-lived 7t-radical anions. Because their reduction potential is only slightly more negative than that of [60]fullerene, it was expected that the electron transfer between these two species, if sufficiently rapid as compared with the decay of the radicals, may lead to the observation of equilibrium conditions. Indeed, we found such an equilibrium with a tin-(IV) porphyrin (Sn" (Ph)3(Py)P). Reduction of this porphyrin resulted in formation of the characteristic 7t-radical anion with absorption in the 700-800 nm range. 

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