M@C(60) and related endohedral metallofullerenes comprise a significant portion of the metallofullerene yield in the traditional arc synthesis, but their chemistry and potential applications have been largely overlooked because of their sparse solubility. In this work, procedures are described to solublize Gd@C(60) species for the first time by forming the derivative, Gd@C(60)[C(COOCH(2)CH(3))(2)](10), and its hydrolyzed water-soluble form, Gd@C(60)[C(COOH)(2)](10). Imparting water solubility to Gd@C(60) permits its evaluation as a magnetic resonance imaging (MRI) contrast agent. Relaxometry measurements for Gd@C(60)[C(COOH)(2)](10) reveal it to possess a relaxivity (4.6 mM(-1) s(-1) at 20 MHz and 40 degrees C) comparable to that of commercially available Gd(III) chelate-based MRI agents. An in vivo MRI biodistribution study in a rodent model reveals Gd@C(60)[C(COOH)(2)](10) to possess the first non-reticuloendothelial system (RES) localizing behavior for a water-soluble endohedral metallofullerene species, consistent with its lack of intermolecular aggregation in solution as determined by light-scattering measurements. This first derivatization and use of a M@C(60) species suggests new potential for metallofullerene technologies by reducing reliance on the chromatographic purification procedures normally employed for the far less abundant M@C(82) and related endohedrals. The recognition that water-soluble fullerene derivatives can be designed to avoid high levels of RES uptake is an important step toward fullerene-based pharmaceutical development.

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Thermally activated delayed fluorescence (TDF) emission from C 70 and 1,2-C 70 H 2 has been time-resolved to provide thermodynamic and kinetic information on excited electronic states. The energy gap between S 1 and T 1 states was deduced from the temperature dependence of initial TDF intensities and independently from the ratio of TDF intensity to time-integrated prompt fluorescence. S 1 -T 1 gaps were found to be approximately 2470 cm -1 for C 70 and 2180 cm -1 for 1,2-C 70 H 2 , with relative uncertainties of 2-3%. Time-resolved TDF measurements from fullerene samples immobilized in PMMA films revealed lifetimes for triplet state decay unaffected by bimolecular deactivation processes. The intrinsic triplet lifetimes at 298 K were found from both TDF and transient absorption measurements to be 24.5 ( 1.5 ms for C 70 and 1.95 ( 0.1 ms for 1,2-C 70 H 2 .

Photophysical and photochemical properties of C120, the [2+2]-dimer of C60, have been investigated. In toluene solution, the 327 nm ground-state absorption peak has a molar absorptivity of 113 000 dm3 mol-1 cm-1, somewhat more than twice that of the analogous 336 nm peak of C60. The S1 and T1 origin energies in C120 are found to be lower than those in C60 by approximately 1070 and 625 cm-1, respectively. The T1 state's intrinsic exponential lifetime at 296 K is 42 ± 2 μs, or a factor of 3.4 times shorter than for C60. The T n ← T1 absorption spectrum of C120 shows bands at 710 and 1060 nm. The former is similar to those seen in [6,6]-adducts of C60, but the latter may reflect electronic coupling between the two halves of the dimer molecule. In degassed room-temperature toluene solution, C120 photodissociates to C60 with a quantum yield of 2.3 × 10-3. This dissociation occurs from the T1 state and is strongly suppressed by dissolved oxygen. The rate constant for T1 dissociation increases from ca. 55 s-1 at 297 K to 800 s-1 at 333 K, implying an Arrhenius activation energy near 64 kJ mol-1 (5360 cm-1) and a prefactor in the range of vibrational frequencies.

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Photophysical measurements have been made on 1,2-C70H2, the most stable isomer of the simplest C70 derivative. This dihydride's electronic absorption spectrum is more diffuse than that of C70. Red shifts in the fluorescence and phosphorescence emission spectra show that excitations to the lowest singlet and triplet electronic states require approximately 8% less energy in 1,2-C70H2 than in C70 and that the S1−T1 gap of 1,2-C70H2 is ca. 310 cm-1 smaller than that of the parent fullerene. The dominant peak in the dihydride's T n ← T1 near-infrared absorption spectrum falls at 1050 nm. Compared to the corresponding transition of C70, this absorption feature is broadened, red-shifted by 75 nm, and reduced in peak molar absorptivity by a factor of 2. Derivatization of C70 to form 1,2-C70H2 accelerates T1 decay by more than an order of magnitude, giving an intrinsic triplet lifetime of 2.0 ms at room temperature. The rate constant for triplet deactivation through ground state self-quenching is near 7.6 × 107 M-1 s-1, or approximately twice that of C70.