Fullerenes fluorescence

Similar to the fullerene ground state the singlet and triplet excited state properties of the carbon network are best discussed with respect to the tliree-dimensional symmetry. SurjDrisingly, the singlet excited state gives rise to a low emission fluorescence quantum yield of 1.0 x 10 [143]. Despite the highly constrained carbon network,... 

In a recent study, poly(aryl ether) dendritic branches terminated with triethyleneglycol chains were attached to Cgg [66] dendrimer 32 represents the fourth generation. The photophysical properties of these fullerodendrimers have been systematically investigated in three solvents, namely toluene, dichloromethane, and acetonitrile. On increasing dendrimer generation, it has been found that in each solvent (i) the maximum of the fullerene fluorescence band is red-shifted... 

The fluorescence emitted by fullerene (C6o) is intrinsically totally depolarized because of its almost perfect spherical symmetry (fi,). 

Fig. 7.2 Effect of fullerene on the surfaces (FoS) (10pg/cm2 of C60) on the fluorescence of 2, 7 -dichlorofluorescein in the presence of different concentrations of H202. (Incubation time -lOmin, - 20min, - 30min 1, 2, 3 - concentration of H202 97, 194, and 970 pM). p < 0.05 ...

However, fullerene C60-modified surface is adequate for the adhesion and normal growth of cells in culture. Cells of the line MA-104 in the Eagle-MEM medium formed on fullerene film a normal monolayer. Cellular viability was assessed with the resazurin (Alamar Blue) reduction test. The dye resazurin is reduced by mitochondrial dehydrogenases of viable cells into the fluorescent product resorufin (maximum X = 530 nm, max X = 590 nm). The intensity of... 

The picture was absolutely different when cells grown on FoS (10pg/cm2) were irradiated 30 min with intensive visible light. In this condition fullerene C60 has revealed a powerful phototoxic action a relative fluorescence of irradiated control cells (MA-104) was 1.80 0.05 and of the cells, grown on fullerene surface - 0.96 0.06 (p < 0.001), so the intensity of cellular metabolism has dropped to the level of cell-free control (1.0). ft means that practically all cells were dead (Fig. 7.3). 

Ma B, Sun YP (1996). Fluorescence spectra and quantum yields of 60-70 fullerenes under different solvent conditions. J. Chem. Soc. Perkin. Trans. 12 2157. 

Figure 22. Human embryonic kidney cells (A), rat vascular smooth muscle cells (B, C) and human osteoblast-like MG 63 cells (D) in cultures on micropattemed surfaces. A, B PTFE irradiated with UV light produced by a Xe2 -excimer lamp for 30 min in an ammonia atmosphere through a mask with holes 100 pm in diameter and center-to-center distance 300 pm C PE irradiated with Ar ions (energy 150 keV, ion dose lO ions/cm ) through a mask with holes 100 pm in diameter and center-to-center distance 200 pm fullerenes Qo deposited through a mask with rectangular holes with an average size of 128 3 pm per 98 8 pm on glass coverslips. Day 7 after seeding. A native cells in an inverted phase-contrast microscope B, C cells stained with hematoxylin and eosin, Olympus microscope IX 50 D cells stained with fluorescence-based LIVE/DEAD viability/cytotoxicity kit (Invitrogen), Olympus microscope IX 50. Bars 300 pm (A), 200 pm (B, D), Imm (C) [10,11].

A similar approach was used in grafting Cjq onto a pregenerated lithiated polyethylene surface [121]. A polyethylene film with terminal diphenylmethyl groups was deprotonated with BuLi to yield an anionic polyethylene surface that was treated with Cg0 and quenched with methanol. The incorporation at the polyethylene surface was determined by XPS, UV/Vis and fluorescence spectroscopy. This reaction also works for polyisopropene, polybutadiene [69], poly(vinylbenzyl chloride) or poly(N-vinylcarbazole) PVK [54] with BuLi or NaH as a base. Charge-transfer interactions in the soluble fullerene-PVK derivative between the positively charged carbazole and Cjq lead to an enhanced photoconductivity compared with PVK [54]. 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

The pseudorotaxane self-assembles through hydrogen bonding, and the binding constant, calculated using steady-state fluorescence, is 1.4 x 10. The zinc phthalocyanine derivative exhibited an excited-state lifetime of approximately 3.1 ns. The fast decay of the excited state of 1 ZnPc upon complexation with the fullerene... 

TTF-based D-A systems have been extensively used in recent years to play around photoinduced electron transfer processes. Typically, when an electron acceptor moiety that emits fluorescence intrinsically is linked to TTF (D), the fluorescence due to the A moiety may be quenched because of a photoinduced electron transfer process (Scheme 15.1). Accordingly, these molecular systems are potentially interesting for photovoltaic studies. For instance, efficient photoinduced electron transfer and charge separation were reported for TTF-fullerene dyads.6,7 An important added value provided by TTF relies on the redox behavior of this unit that can be reversibly oxidized according to two successive redox steps. Therefore, such TTF-A assemblies allow an efficient entry to redox fluorescence switches, for which the fluorescent state of the fluorophore A can be reversibly switched on upon oxidation of the TTF unit. 

Preparation of mono-adducts of fullerene - for studies on electrostatic interactions - was undertaken by cyclopropanation of fullerene with appropriately functionalised malonic esters 1 (Bingel reaction) to form 2. Coupling with the tert-butyl protected oligoamide-amino-dendron 3 and subsequent hydrolysis lead to the water-soluble fullerene dendron 5, which can carry up to nine negative charges after depro to nation. After association with the zinc complex of cytochrome C, photoinduced electron transfer (PET) from the redox protein to the fullerene can be accomplished, which was studied by fluorescence spectroscopy. 

Figure 3 Comparison of absorption and fluorescence (inset) spectra of different classes of C6oderivatives themethano-C6oderivative l -carboxy-l,2-methano[60]fullerene the pyrrolidino-C6o derivative N-ethyl-trans-2, 5 -dimethylpyrrolidino[3, 4 l,2][60] fullerene 33 (Fig. 24) (- - -),[B14al the C60 derivative 2-hydroxy-tetrahydrofu-ran[4, 5 l,2] [60]fullerene (—), and the amino-C60 derivative 3 (Fig. 4) AW -dimethyl-piperazine [2, 3 l,2][60]fullerene (-). (From Ref. 65.)...

The increasing pertubation of the fullerene ir-type electron system leads to a reduction of the symmetry, e.g., from /h to C2v or Cs, and converts some vibronic-forbidden states to allowed states. Besides the absorptivity in the UV-Vis, the fluorescence properties of the adducts are also influenced, as well as the fluorescence lifetime, the emission band positions, and especially the quantum yield, which increases compared with that of C6o [61,65-67,75,88-90,92]. 

The lifetimes of the singlet excited states are only partly influenced the values range from 1.1 to 1.8ns [61,66,84,85,89-92], and for C6o a value ofabout 1.3 ns was estimated. As for the fluorescence quantum yields, there is also a solvent dependence of the fluorescence lifetimes for fullerene derivatives with electron-... 

Table 2 Fluorescence Properties of Fullerene Derivatives Compared to Those of C oK-io...

Unlike the reversed shift of the emission band compared to the dihydrogen addend type, the singlet lifetime in the bis- and tris(bis-(ethoxycarbonyl)-methylene) derivatives is increased comparable to the former multiple adducts. The values range from 1.7 to 3.1 ns (tris-adduct), depending on the distorted T7-electron system of the fullerene core [67,108], In comparison to C6o, the fluorescence quantum yield for the malonic ester hexaadduct is increased by the factor 10 (30 X 10 4) [67,111], In the case of both pyrrolidino hexa-adducts (Th 14 and D3 15, Fig. 13), the effect is remarkably higher. The fluorescence quantum yields are increased about 100-fold (-0.02) compared to C6o. On the other hand, the singlet lifetime is only partly increased with -3.5 ns [111,112],... 

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