PHOTOPHYSICAL PROPERTIES OF 3 METHANOFULLERENE DERIVATIVES

The [6,6]-ring bridged methanofullerenes utylbenzoate]-1,2-dihydro-1,2 -methanofullerene[60] (1), ethyl)phenyl]-1,2-dihydro-1,2-methanofuller ene[60] (2), and lcarboxylate]-1,2-dihydro-1,2-methanofullerene[60] (3 ) were synthesized by the Diederich method (3) or the Wudl method via tosylhydrazone salts (1 and 2). Groundstate absorption spectra of the methanofullerenes in toluene and cyclohexane solutions are presented a nd possible assignments discussed. The sharp, 430 nm transition is ass ociated with the existence of less than 60 pi electrons, whereas a bro ad band peaking at 495 nm as well as weak features in the 700 nm regio n could be related to forbidden transitions of C-60. The allowed trans itions of C,, in the UV region are modified relatively little in the m ethanofullerenes. Laser flash photolysis and pulse radiolysis techniqu es were used to obtain triplet-triplet absorption spectra between 400 and 1100 nm and determine photophysical properties. The three methanof ullerenes have very similar T-T spectra, with a strong peak at 720 nm whose molar absorption coefficient is of the order of 14000 M(-1)cm(-1 ) somewhat lower than the 20200 M(-1)cm(-1) measured for the correspon ding 750 nm band of C-60 Values near unity were determined for Phi(Del ta) the quantum yield for O-1(2)((1) Delta(g)) production by energy tr ansfer from the triplet state of the three methanofullerenes, implying that the quantum yield of triplet production of each is approximate t o 1. The results indicate that it is possible for the spectroscopic an d photophysical properties of methanofullerenes to vary little with th e nature of the methano adduct and to undergo only slight modification s with respect to the corresponding properties of C-60. UP to now, thi s has been found valid only when the functionalization does not involv e an electron donor, The biological photosensitization efficiencies of these methanofullerenes are therefore expected to be similar to those of C-60 but with their hydrophobicity and intracellular site delivery modulated by the nature of the methano-adduct.