Comparative Study of the Host−Guest Complexes of [60]- and [70]-Fullerenes with N,N‘-Dibenzyl-1,4,10,13-tetraoxa-7,16-diaza-cyclooctadecane in Different Solvents

Abstract

Charge-transfer absorption bands of the host−guest complexes of [60]- and [70]-fullerenes with the aza crown ether N,N‘-dibenzyl-1,4,10,13-tetraoxa-7,16-diaza-cyclooctadecane (1) in π-donor solvents (toluene and o-xylene) and also in the nondonor solvent (CCl4) have been detected. The CT absorption band of the [70]-fullerene−aza crown complex appears at a longer wavelength than that of the [60]-fullerene complex in each solvent, thereby indicating that [70]-fullerene has a higher electron affinity than [60]-fullerene. In analyzing these CT absorption bands, some other electron acceptors have also been used; this has led to the estimation of the hitherto undetermined vertical electron affinities of N-bromophthalimide and tetrachlorophthalimide and also the vertical ionization potential of the aza crown ether. Absorption spectrometric data established a 1(fullerene):2 (crown ether) stoichiometry of the complexes in solution having a crown ether concentration of ∼10-2 mol·dm-3. By determining the formation constants at four different temperatures, the enthalpies and entropies of complexation have been determined. A plot of TΔSf 0 against ΔHf0 shows that the extent of desolvation of the crown ether cavity due to the inclusion of the host fullerenes is very nearly equal for the two complexes. It has also been found that the formation constant of the C70·1 complex is greater than that of the C60·1 complex in each solvent at all four temperatures studied. This observation is on the order of the electron affinities of the two fullerenes (EA of C70 > EA of C60), which means that CT interaction plays an important role in the formation of the complexes.