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

Abstract

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 π−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 π-system. The [6-5]-open methanofullerene ( 4 ) is, therefore, considered to be a true isoelectronic analogue of pristine C 60 (60 π-electrons). Although the [6-6]-closed iminofullerene ( 1 ), constitutes a quasi 60 π-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 π-electron system, and the [6-5]-open isomer ( 2 ) is characterized by a 62 π-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 ).