[6-6]-closed versus [6-5]-open isomers of imino- and methanofullerenes: A comparison with pristine C-60 and (C59N)(center dot)

Two sets of [6-6]-closed and [6-5]-open isomers, namely, those of a methano - and iminofullerene series, are probed in light of their excited state and electron acceptor properties. Steady-state emission, fluorescence, and pho sphorescence indicate that the excited-state energy levels of the [6-5]-ope n (2) and [6-6]-closed (1) iminofullerene isomers follow a seemingly surpri sing, reverse trend relative to the [6-5]-open (4) and [6-6]-closed (3) met hanofullerene isomers. Specifically, the [6-5]-open iminofullerene (2) reve als almost identical characteristics to that of the [6-6]-closed methanoful lerene (3). On the other hand the [6-6]-closed iminofullerene (1) is observ ed to resemble the [6-5]-open methanofullerene (4). Transient absorption sp ectra, i.e., singlet-singlet, triplet-triplet, and pi-radical anion absorpt ion are in accord with the emission studies. These effects have been ration alized in terms of an efficient engagement between the nitrogen electron pa ir and the fullerene ct-system. The [6-5]-open methanofullerene (4) is, the refore, considered to be a true isoelectronic analogue of pristine C-60 (60 pi-electrons). Although the [6-6]-closed iminofullerene (1), constitutes a quasi 60 ct-electron system, it appears to bear only a weak resemblance wi th the former two. In contrast, the [6-6]-closed isomer (3) is characterize d by a 58 pi-electron system, and the [6-5]-open isomer (2) is characterize d by a 62 pi-electron system. In terms of optimizing the electron acceptor properties and photostability, the most promising monofunctionalized fuller ene derivative for the design of multicomponent donor acceptor arrays is un equivocally the [6-6]-closed iminofullerene (1).