Experimental observation of C_(60) LUMO splitting in the C_(60)~(2-) dianions due to the Jahn-Teller effect. Comparison with the C_(60)~(·-) radical anions

摘要

New fullerene salts (TMP~+)2·(C_(60)~(2-))·(C6H4Cl2)2 (1), {DB-18-crown-6·[Na~+]·(C6H5CN)2}2·(C_(60)~(2-))·C6H5CN·C6H4Cl2 (2),{cryptand[2,2,2]·(Na~+)}2·(C_(60)~(2-)) (3) and (PPN~+)2·(C_(60)~(2-)·(C6HCl2)2 (4) were obtained as single crystals.Their crystal structures were solved and their optical and magnetic properties were analyzed. The spectra of the salts in the IR and UV-visible-NIR ranges indicate the formation of C_(60)~(2-) dianions in 1-4. These salts show similar behavior in EPR measurements, explained by the diamagnetic ground state of the C_(60)~(2-) dianions and the thermal population of the excited triplet state, which is separated by an energy gap of 487-540 cm~(-1). The magnetic susceptibility of 4 also increased above 130 K due to the population of the excited triplet state. The observed splitting of the C_(60) LUMO is attributed to the Jahn-Teller (JT) effect. We analyzed the splitting by an extended Hlickel method using the single-crystal structural data for the compounds containing neutral, mono- and dianions of C_(60) The splitting of the initially triply degenerated C_(60) LUMO produces three molecular orbitals. The gap between the lowest and highest orbitals is very small in neutral C_(60) (128-140 cm~(-1)), it increases in C_(60)~(·-) (500-710 cm~(-1)) and increases further in C_(60)~(2-) (1080-1670 cm~(-1)). It was found that the splitting of the C_(60) LUMO is realized in different ways for the mono- and dianions. The ground and first excited state are separated in C_(60)~(·-) by a small gap of 55-180 cm~(-1) only. This gap is noticeably larger in the C_(60)~(2-) dianions and falls into the 760-1390 cm~(-1) range.