Semiconducting Supramolecular Organic Frameworks Assembled from a Near-Infrared Fluorescent Macrocyclic Probe and Fullerenes

We report here a
new extended tetrathiafulvalene (exTTF)-porphyrin
scaffold, 2, that acts as a ball-and-socket receptor
for C60 and C70. Supramolecular interactions
between 2 and these fullerenes serve to stabilize 3D
supramolecular organic frameworks (SOFs) in the solid state formally
comprising peapod-like linear assemblies. The SOFs prepared via self-assembly
in this way act as “tunable functional materials”, wherein
the complementary geometry of the components and the choice of fullerene
play crucial roles in defining the conductance properties. The highest
electrical conductivity (σ = 1.3 × 10–8 S cm–1 at 298 K) was observed in the case of the
C70-based SOF. In contrast, low conductivity was seen for
the SOF based on pristine 2 (σ = 5.9 × 10–11 S cm–1 at 298 K). The conductivity
seen for the C70-based SOF approaches that seen for other
TTF- and fullerene-based supramolecular materials despite the fact
that the present systems are metal-free and constructed entirely from
neutral building blocks. Transient absorption spectroscopic measurements
corroborated the formation of charge-transfer states (i.e., 2δ+/C60δ− and 2δ+/C70δ−, respectively) rather than fully charge separated states (i.e., 2•+/C60•– and 2•+/C70•–, respectively) both in solution (toluene and benzonitrile) and in
the solid state at 298 K. Such findings are considered consistent
with an ability to transfer charges effectively over long distances
within the present SOFs, rather than, for example, the formation of
energetically trapped ionic species.