Tetramethoxybenzene is a Good Building Block for Molecular Wires: Insights from Photoinduced Electron Transfer

Two
donor bridge–acceptor molecules with terminal triarylamine
and Ru­(bpy)32+ (bpy = 2,2′-bipyridine)
redox partners were synthesized and investigated by cyclic voltammetry,
optical absorption, luminescence, and transient absorption spectroscopy.
The two dyads differ only by the central bridging unit, which was
tetramethoxybenzene (tmb) in one case and unsubstituted phenylene
(ph) in the other case. Photoirradiation of the Ru­(bpy)32+ complex of the two dyads triggers intramolecular electron
transfer from the triarylamine to the 3MLCT-excited metal
complex, and this process occurs with time constants of 1.5 and 6.8
ns for the tmb- and ph-bridged dyads, respectively. Thermal electron
transfer in the reverse direction then leads to disappearance of the
photoproduct with a time constant of 10 ns in both dyads. The faster
rate of photoinduced charge transfer in the tmb-bridged dyad can be
understood in the framework of a hole-tunneling model in which the
electron-rich tmb bridge imposes a more shallow barrier than the less
electron-rich ph spacer. Until now tmb-based molecular wires have
received very little attention, and alkoxy substituents have been
mostly used for improving the solubility of oligo-p-phenylene vinylene (OPV) and oligo-p-phenylene
ethynylene (OPE) wires. Our study illustrates how four alkoxy-substituents
on a phenylene backbone can have a significant influence on the charge-transfer
properties of a molecular wire, and this is relevant in the greater
context of a future molecular electronics technology.