Bucky Light Bulbs: White Light Electroluminescence from a Fluorescent C60 Adduct−Single Layer Organic LED

Abstract

Recently it was reported that a C60 adduct, Th-hexapyrrolidine (THP, Figure 1a), has a substantial yellow-green fluorescence.1 The photophysical properties of THP are atypical of C60 and its derivatives.2 The fluorescence of C60 and its monoadducts is very weak, with reported quantum yields on the order of 10-4.3-6 The origin of the fluorescence properties of THP may be due to a reduction of conjugation within the fullerene core leading to higher excitation energies and larger single-triplet gaps. The result is a displacement of the spectrum to the blue, a small reduction of the excited-state S1 f T1 intersystem crossing, an increase in the rate of singlet state radiative decay (fluorescence rate), and a decrease in the rate of triplet decay to the ground state (T1 f S0) by thermal deactivation.1 Due to the photophysical properties of THP, we have utilized it as a chromophore in the fabrication of a white light organic LED. Though lightly n-doped as well as pristine fullerene C60 was shown to exhibit electroluminescence with apparent decomposition,7 to the best of our knowledge, this is the first time electroluminescence has been obserVed from a C60 deriVatiVe, an unprecedented result in light of the fact that C60 is universally an efficient luminescence quencher.8 Traditionally, organic electroluminescent devices are multilayered, with the electron-transport, hole-transport and emissive materials segregated.9 In contrast, we fabricated the organic LEDs by blending THP (emissive material) with a hole transport material, poly(9-vinylcarbazole) (PVK), and an electron-transport material, 2,5-bis-(4-naphthyl)-1,3,4-oxadiazole (BND), to form a single-layer device (Figure 1). This molecularly-doped polymer device configuration has been used by others.10-14