Excited states and electron transfer reactions of fullerenes

Abstract

Since the discovery of carbon clusters in the laser ablated graphite vapours, fullerenes C₆₀ and C₇₀ have provided a constant source of excitement for the experimental and theoretical scientists alike. Their brilliant colours in solutions and three dimensional aromaticity has fascinated the photochemists. Our group has recently characterised singlet and triplet states of fullerenes using complementary twin techniques of laser f~ash ph~tolysis and electron pulse radiolysis. Singlet - singlet (S _l→S_n) absorption spectra were studied in picosecond time domain and the rates of intersystem crossing were established. Triplet states were produced directly by photoexcitation in laser flash photolysis experimen~s and by energy transfer in pulse radiolysis. Triplet-triplet (T_l→T_n) spectra were characterised and rate constants for various triplet decay channels were measured. Fullerenes have low reduction potentials and form charge transfer (CT) complexes with good donors like aliphatic and aromatic amines. Exciting photochemical behaviour was observed on photoexcitation of the CT complexes which undergo ultrafast charge separation (CS) and charge recombination {CR) reactions. The dynamics of CS and CR reactions was studied in picosecond time scales in e60-diphenylamine and C₆₀-triphenylamine systems. Using external heavy atom effect and solvent polarity variations, various steps in the CS and CR processes generating fullerene triplet on CR were elucidated. Recent results of complexing fullerenes with suitable hydrophilic cage like γ-cyclodextrin (CD) making C₆₀ water soluble will also be presented. Excited state behaviour of fullerene complex in CD cage would be discussed.