Experimental observation of C60 LUMO splitting in the C602− dianions due to the Jahn–Teller effect. Comparison with the C60˙− radical anions

Abstract

New fullerene salts (TMP⁺)₂·(C₆₀²⁻)·(C₆H₄Cl₂)₂ (1), {DB-18-crown-6·[Na⁺]·(C₆H₅CN)₂}₂·(C₆₀²⁻)·C₆H₅CN·C₆H₄Cl₂ (2), {cryptand[2,2,2]·(Na⁺)}₂·(C₆₀²⁻) (3) and (PPN⁺)₂·(C₆₀²⁻)·(C₆H₄Cl₂)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 C₆₀²⁻ dianions in 1–4. These salts show similar behavior in EPR measurements, explained by the diamagnetic ground state of the C₆₀²⁻ 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 C₆₀ LUMO is attributed to the Jahn–Teller (JT) effect. We analyzed the splitting by an extended Hückel method using the single-crystal structural data for the compounds containing neutral, mono- and dianions of C₆₀. The splitting of the initially triply degenerated C₆₀ LUMO produces three molecular orbitals. The gap between the lowest and highest orbitals is very small in neutral C₆₀ (128–140 cm⁻¹), it increases in C₆₀˙⁻ (500–710 cm⁻¹) and increases further in C₆₀²⁻ (1080–1670 cm⁻¹). It was found that the splitting of the C₆₀ LUMO is realized in different ways for the mono- and dianions. The ground and first excited state are separated in C₆₀˙⁻ by a small gap of 55–180 cm⁻¹ only. This gap is noticeably larger in the C₆₀²⁻ dianions and falls into the 760–1390 cm⁻¹ range.