rXXXX American Chemical Society 1592 dx.doi.org/10.1021/jz200586v | J. Phys. Chem. Lett. 2011, 2, 1592–1600 LETTER pubs.acs.org/JPCL Dimerization of Radical-Anions: Nitride Clusterfullerenes versus Empty Fullerenes Alexey A. Popov,* ,†,‡ Stanislav M. Avdoshenko, § Gianaurelio Cuniberti, §,|| and Lothar Dunsch † † Department of Electrochemistry and Conducting Polymers, The Center of Spectroelectrochemistry, Leibniz Institute of Solid State and Materials Research (IFW) Dresden, Helmholtzstr. 20, D-01069 Dresden, Germany ‡ Chemistry Department, Moscow State University, 119992 Moscow, Russian Federation § Institute for Materials Science and Max Bergmann Center of Biomaterials, Dresden University of Technology, D-01062 Dresden, Germany ) Division of IT Convergence Engineering and National Center for Nanomaterials Technology, POSTECH, Pohang 790-784, Republic of Korea b S Supporting Information O ne of the important properties of fullerenes for their broad applications is the ease with which they can accept elec- trons. Endohedral metallofullerenes, and, in particular, nitride clusterfullerenes (NCFs) also exhibit such properties 1À3 (gas- phase electron affinity of Sc 3 N@C 80 is even higher than that of C 60 4 ). Moreover, in a number of recent studies, NCFs have been shown to be superior to C 60 in photovoltaic applications. 5À8 Despite the importance of the electron transfer reactions of NCFs in this context, there is still a challenging question that has not been clearly understood up to now. Whereas empty fullerenes exhibit perfectly reversible reduction steps in solution at room temperature, the majority of NCFs exhibit irreversible reduction behavior at standard voltammetric scan rates. 9À17 For Sc 3 N@C 80 , Echegoyen has shown that electrochemical reversibility can be reached at fast scan rates (5 V/s was reported for the first re- duction step). 9 Extended studies of Dy 3 N@C 78 and Dy 3 N@C 80 have shown that for these NCFs electrochemically reversible reductions were not achieved up to the scan rates of 80 V/s. 14 Likewise, irreversible reduction steps are also observed for Sc 3 N@C 68 , 15 Sc 3 N@C 78 , 18 and a series of M 3 N@C 2n (M = lanthanide), 2 the only exclusion from this rule being M 3 N@C 88 , which exhibited reversible reductions. 17 Dedicated studies have shown that despite electrochemical irreversibility, reduction steps of NCFs are chemically reversible. 9,10,14 That is, the follow- up reaction accompanying electron transfer is reversible and pristine NCFs can be recovered at the end of a voltammetric cycle. Thus, irreversible reactions with the solvent and the sample decomposition can be excluded. (This was shown by multiple cycling as well as mass-spectrometric studies of the electrolysis products, which showed no traces of NCFs’ derivatives.) To explain this peculiar reduction behavior of NCFs, we suggested a double-square schema in which structural rearrangement of the cluster (e.g., pyramidalization) in radical anions was proposed; 14,16 however, the nature of this rearrangement remained unclear. Extended theoretical studies of the charged NCFs did not reveal any structural changes that might be responsible for the irrever- sibility of electrochemical reduction. 19À22 Chemical derivatization of NCFs also changes their electro- chemical properties. It was shown that trifluoromethylated Sc 3 N@C 80 derivatives tend to exhibit electrochemically rever- sible reduction steps. For Sc 3 N@C 80 (CF 3 ) 2 , three reversible re- duction steps have been observed, and stability of the radical Received: May 2, 2011 Accepted: June 7, 2011 ABSTRACT: In contrast with empty fullerenes, nitride clusterfullerenes usually exhibit irreversible reduction steps at moderate electrochemical scan rates. However, these reduction steps are chemically reversible, indicating that reversible follow-up reaction takes place. To explain this phenomenon, we analyze in this work if anion-radicals of nitride clusterfullerenes are more prone to dimerization than anion-radicals of empty fullerenes. Extensive DFT computa- tions are performed to find the most stable dianionic dimeric structures of Sc 3 N@C 68 , Sc 3 N@C 80 , Sc 3 N@C 80 (CF 3 ) 2 , [5,6] and [6,6] pyrrolidine adducts of Sc 3 N@C 80 and Y 3 N@C 80 , a series of Y 3 N@C 2n (2n = 78, 80, 84, 86, 88), as well as those of empty fullerenes C 60 ,C 70 , and C 84 . Dimerization energies of the most stable isomers are computed in the gas phase, with the use of van der Waals corrections, and in solution. It is found that dianionic dimers of nonderivatized nitride clusterfullerenes are substantially more stable than those of empty fullerenes, which can be an explanation of the electrochemical irreversibility of the former. SECTION: Molecular Structure, Quantum Chemistry, General Theory