Synthesis, crystal structures, and optical/electronic properties of sphere-rod shape amphiphiles based on a [60]fullerene-oligofluorene conjugate

Abstract

A series of sphere-rod shape amphiphiles, in which a [60]fullerene (C60) sphere was connected to the center of an oligofluorene (OF) rod through a rigid linkage (OF-C60), were designed and synthesized. Alkyl chains of various lengths were attached onto the OFs on both sides of the C60 spheres. These compounds, denoted as alkyl-OF-C60, were fully characterized by (1)H NMR, (13)C NMR, and FTIR spectroscopy and by MALDI-TOF mass spectrometry. The morphologies and structures of their crystals were elucidated by wide-angle X-ray diffraction (WAXD) and by electron diffraction in transmission electron microscopy (TEM). Butyl-OF-C60 forms a monoclinic unit cell (a=1.86, b=3.96, c=2.24 nm; α=γ=90°, β=68°; space group P2), octyl-OF-C60 also forms a monoclinic unit cell (a=2.21, b=4.06, c=1.81 nm; α=γ=90°, β=75.5°; space group C2m), and dodecanyl-OF-C60 forms a triclinic structure (a=1.82, b=4.35, c=2.26 nm; α=93.1°, β=94.5°, γ=92.7°; space group P1). The inequivalent spheres and rods were found to pack into an alternating layered structure of C60 and OF in the crystals, thus resembling a "double-cable" structure. UV/Vis absorption spectroscopy revealed an electron perturbation between the two individual chromophores (C60 and OF) in their ground states. Fluorescence spectroscopy exhibited complete fluorescence quenching of their solutions in toluene, thus suggesting an effective energy transfer from OF to C60. Cyclic voltammetry indicated that the energy-level profiles of C60 and OF remained essentially unchanged. This work has broad implications in terms of understanding the self-assembly and molecular packing of conjugated materials in crystals and has potential applications in organic field-effect transistors and bulk heterojunction solar cells.