REDOX AND EXCITATION STUDIES WITH C-60-SUBSTITUTED MALONIC-ACID DIETHYL ESTERS

Flash photolytic formation of excited triplet states, their consecutiv e reductive quenching with diazabicyclooctane (DABCO), and pulse radio lytic formation of pi-radical anions of various Cao-derivatives have b een recorded. The fullerenes were functionalized via single, double, a nd triple cyclopropylation of C-60 with bromomalonic acid diethyl este r. Flash photolytic irradiation at 308 nm of these C-60 derivatives in toluene solution yielded triplet excited states which exhibited a str ong blue shift of lambda(max) by nearly 100 nm as compared to plain C- 3(60). This is rationalized in terms of a gradual destruction of the f ullerene's pi-system with an increasing number of bis(ethoxycarbonyl)m ethylene groups. The blue shift coincides with a significant slow down for the rate of reductive quenching of the excited triplet states by DABCO, i.e., 1.3 x 10(6) M(-1) s(-1) vs 2.5 x 10(9) M(-1) s(-1) for th e quenching of equatorial-(C-3(60))[C(COOEt)(2)](3) and C-3(60), respe ctively. The radical-induced reduction of functionalized C-60 has been studied in a toluene/acetone/2-propanol mixture by means of time-reso lved pulse radiolysis with measurements being conducted in the charact eristic near-IR region. An almost linear dependence is obtained betwee n the energy of the most significant IR-pi-radical anion band versus t he number of bis(ethoxycarbonyl)methylene groups at the fullerene core , with the respective lambda(max) ranging from 1080 nm for C-60(.-) to 1015 nm for equatorial-(C-60(.-))[C(COOEt)(2)](3). A corresponding tr end emerges from cyclic voltammetry measurements on the redox potentia l in toluene/2-propanol. They show a difference of 330 mV between the formation of C-60(.-) (E(1/2) = -0.55 V vs SCE) and the first reductio n of equatorial-C-60-[C(COOEt)(2)](3) (E(1/2) = -0.86 V vs SCE). It ap pears that all these physicochemical parameters very sensitively refle ct the site and degree of functionalization of C-60.