[6-6]-Closed versus [6-5]-Open Isomers of Imino- and Methanofullerenes: A Comparison with Pristine C60 and C59N

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 phosphorescence indicate that the excited-state energy levels of the [6-5]-open (2) and [6-6]-closed (1) iminofullerene isomers follow a seemingly surprising, reverse trend relative to the [6-5]-open (4) and [6-6]-closed (3) methanofullerene isomers. Specifically, the [6-5]-open iminofullerene (2) reveals almost identical characteristics to that of the [6-6]-closed methanofullerene (3). On the other hand the [6-6]-closed iminofullerene (1) is observed to resemble the [6-5]-open methanofullerene (4). Transient absorption spectra, i.e., singlet-singlet, triplet-triplet, and pi-radical anion absorption are in accord with the emission studies. These effects have been rationalized in terms of an efficient engagement between the nitrogen electron pair and the fullerene ct-system. The [6-5]-open methanofullerene (4) is, therefore, 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 with the former two. In contrast, the [6-6]-closed isomer (3) is characterized by a 58 pi-electron system, and the [6-5]-open isomer (2) is characterized by a 62 pi-electron system. In terms of optimizing the electron acceptor properties and photostability, the most promising monofunctionalized fullerene derivative for the design of multicomponent donor acceptor arrays is unequivocally the [6-6]-closed iminofullerene (1).