Fullerene Chromophore Systems

Quantum Yield Efficiency of fluorescence percentage of incident energy emitted after absorption. The higher the quantum yield, the greater the intensity of the fluorescence, luminescence, or phosphorescence. See Papp, S. and Vanderkooi, J.M., Tryptophan phosphorescence at room temperature as a tool to study protein structure and dynamics, Photochem. Photobiol. 49, 775-784, 1989 Plasek, J. and Sigler, K Slow fluorescent indicators of membrane potential a survey of different approaches to probe response analysis, J. Photochem. Photobiol. 33, 101-124, 1996 Vladimirov, Y.A., Free radicals in primary photobiological processes, Membr. Cell Biol. 12, 645-663, 1998 Maeda, M., New label enzymes for bioluminescent enzyme immunoassay, J. Pharm. Biomed. Anal. 30, 1725-1734, 2003 Imahori, H., Porphyrin-fullerene linked systems as artificial photosynthetic mimics, Org. Biomol. Chem. 2, 1425-1433, 2004 Katerinopoulos, H.E., The coumarin moiety as chromophore of fluorescent ion indicators in biological systems, Curr. Pharm. Des. 10, 3835-3852, 2004. 

The moderate visible absorption of Ceo-donor dyads (see above) prompts to C o-chromophore systems as suitable models for the mimicry of the primary events in natural photosynthesis [330-332]. To enhance the absorption of the fullerene containing dyads in the visible region strong chromophoric units, such as (i) ruth-enium(II) polypyridyl complexes or (ii) metalloporphyrins have been implemented as antenna systems. In these C6o-chromophore systems the role of the fullerene changed dramatically, namely, it only accepts an electron or energy from the pho-toexcited chromophore. 

Fullerene containing systems are also of interest and have been covered in some detail in terms of applications and properties." Strong photoenergy transfer has been observed between pendant fullerene groups in PPV copolymers and main chain chromophores, especially at low temperatures." In PPV/fullerene composite films, an increase in the substituted side-chain length has been found to lower the energy of the emission bands while the fullerene dopant also quenched the emissions. In this way the electronic affinity of the PPVs can be controlled." Conjugated polyelectrolytes also have applications as fluorescent sensors." ... 

Wrobel, D. and Graja, A. Photoinduced electron transfer processes in fullerene-organic chromophore systems. Coord. Chem. Rev. 255, 2555-2577, 2011. 

Thus, it has been shown that the porphyrin chromophores act as an antenna system for transmitting its excited energy to the noncovalently associated fullerene moieties. Furthermore, the examination of excited-state characteristics revealed significant energy transfer properties of these complexes upon photoexcitation. 

The covalent assembly of functional Jt-systems is a general synthetic principle and in some cases they can even be achieved in a multi-component fashion. One of the most impressive examples is the very elegant access to covalently linked donor-fullerene arrangements by 1,3-dipolar cycloadditions with in situ-generated azomethine ylids [59]. However, here only the multi-component de novo synthesis of the chromophore structures will be considered. The major developments have been achieved in condensation-based and cross-coupling strategies. 

Nevertheless, other chromophores have been investigated and they have provided interesting insights, particularly porphyrin and Cso groups, since these serve as useful mimics of the cofactors present in the photosynthetic reaction center (Figure 37). Electron transfer involving porphyrins and fullerenes will be presented in more detail elsewhere in this Handbook Volume III, Part 2, Chapter 2 and Volume II, Part 1, Chapter 5 respectively), and so only a brief discussion is presented here. An excellent overview of photoinduced ET processes in Cso-based multichromophoric systems has been produced previously [116]. 

Photoinduced electron transfer in fullerene based supramolecular systems has been described in Section 7.3.5 as an example of process that can be followed both by time-resolved fluorescence and transient absorption, and reaction time on the picosecond time scale was calculated from fluorescence decay measurements. Transient absorption with subpicosecond or picosecond resolution allows the characterization of the product formed while fluorescence is quenched. On a longer time-scale it provides information on the lifetime of the photoproduct. The porphyrin-fullerene (H2P-C60) diads in which the two chromophores are linked... 

Porphyrin-fullerene conjugates attract wide attention for their intramolecular energy and electron transfer properties [87, 88]. By attachment of the porphyrin to two points on the Cgo surface, the interchromophoric spatial relationship in the cyclophane-type molecular dyads trans-2 ( )-52 [67] and trans-1 54 [68] (Scheme 7-8), which controls both energy and electron transfer, is rigorously defined. In the two systems, as well as in the fullerene-porphyrin conjugate 58 [75] (Scheme 7-9), the close proximity between fullerene and porphyrin chromophore leads to a nearly complete quenching of the porphyrin luminescence, presumably as a result of efficient energy transfer between the porphyrin donor and the fullerene acceptor. 

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